POWER SUPPLY
CHALLENGES
Solutions for Integrating Renewa Renewables bles
"A imely book o demonsrae how o balance he power supply and demand under very complex condiions, such as renewables inegraion, emperaure variaion and demand uncerainy." Chongqing Kang Proessor Deparmen o Elecrical Engineering singhua Universiy China
"Te auhor does a nice job o describe he changing characer o elecric power generaion while respecing he undamenals and working principles princ iples o an inegraed grid." Tomas Key Senior echnical Execuive Elecric Power Research Insiue EPRI USA
Solutions for Integrating Renew Renewables ables
Solutions for Integrating Renewables
Jacob Klimstra
Table of Contents: Foreword.............................................................................................................................................. 8 Noe o he reader.............................................................................................................................. 9 1. How o secure he elecriciy supply in a changing world .......................................10 1.1. An affordable, reliable and susainable supply o elecriciy .....................................12 1.2. Challenges o renewable energy sources .......................................................................18 1.3. Fauls and ailures in elecriciy supply sysems .......................................................... 21 1.4. Balancing supply and demand in energy markes ....................................................... 22 1.5. Conclusions..........................................................................................................................26 2. Balancing he elecriciy supply in case o calamiies .....................................................28 2.1. Maching elecriciy supply and demand ......................................................................30 2.2. Primary conrol reserves compensaing or he ailure o a power plan ...............31 2.4. Conclusions..........................................................................................................................49 3. Balancing power demand and supply when condiions change ...................................50
3.1. Elecriciy supply differs depending on local siuaions ............................................52 3.2. Power demand patern in Finland exempliying an indusrialised naion .............52 3.3. Power demand in he epublic o Ireland, exempliying a sysem wih much wind-based power ...................................................................................................56 3.4. Te 50Herz ransmission sysem operaor region in Germany, a region wih much solar-based power ..........................................................................67 3.5. Effecs o phoovolaics on oher power plans in exas and Caliornia ...............72 3.6. Conclusions..........................................................................................................................75 4. Acive and reacive power ...........................................................................................76 4.1. eacive power analogies ..................................................................................................78 4.2. Te hree basic load elemens in alernaing curren sysems .................................. 78 4.3. Te power acor cos φ ......................................................................................................84 4.4. Impedance o elecriciy ransmission sysems ............................................................ 86 4.5. Volage change over a power ransmission line ........................................................... 89 4.6. isks creaed when insufficien reacive power is supplied by renewable energy sources............................................................................................93 4.7. Conclusions..........................................................................................................................95 © Jacob Klimstra & Wärtsilä Finland Oy Editorial work: Jussi Laitinen Graphic design: Jiipee Mattila Printing house: Arkmedia, Vaasa 2014
Publisher: Wärtsilä Finland Oy 1st edition ISBN 978-952-93-3634-0 ISBN 978-952-93-3635-7 (pdf)
5. Energy sorage ..............................................................................................................96 5.1. Te enormous challenge o energy sorage...................................................................98 5.2. Basic properies o energy sorage devices..................................................................100 5.3. Applicaions or energy sorage devices ......................................................................101 5.4. Mehods and coss o energy sorage ...........................................................................104 5.7. Discussion on energy sorage.........................................................................................117 5.6. Conclusions........................................................................................................................119 6. Coss o producing elecriciy ..................................................................................120 6.1. Challenges in deermining kWh coss .........................................................................122 6.2. Varying condiions or generaing elecriciy .............................................................122 6.3. Cos analysis or differen generaing echniques......................................................124 6.4. Te oal coss o producing elecriciy .......................................................................134 6.5. Te elecriciy price or consumers ..............................................................................139 6.6. Discussion regarding elecriciy producion coss ....................................................140 6.7. Conclusions........................................................................................................................140 7. Fuure power supply sysems ....................................................................................142 7.1. Te road owards an opimum power supply sysem...............................................144 7.2. An opimised generaing porolio wihou renewables ..........................................145 7.3. An opimised power plan porolio design wih renewable energy sources......149 7.4. Discussion regarding he suggesed opimum power supply porolio ................157 7.5. An opimum elecriciy generaion porolio or emerging economies ...............159 7.6. Conclusions........................................................................................................................160 8. Power supply challenges – A review ........................................................................162 8.1. ealism is needed in he energy debae ......................................................................164 8.2. Low capaciy acors escalae balancing issues ...........................................................165 8.3. Flexible local generaors o limied size offer excellen backup or renewables .....167 8.4 Naural gas is ideal or backup capaciy........................................................................167 8.5. Inegraing power demand and hea demand offers good perspecives ...............168 8.6. Agile, flexible power plans help o ensure a reliable and cos-effecive power supply ....................................................................................169 8.7. Oulook or he uure .....................................................................................................170
Appendix 1 ......................................................................................................................................172 Appendix 2 ......................................................................................................................................176 Biograph...........................................................................................................................................182 eerences .......................................................................................................................................183 Glossary ...........................................................................................................................................185
Foreword Hans en Berge Secreary General o Eurelecric
Europe's elecriciy markes are changing. Hisorically, elecriciy markes were based on generaion capaciy wih comparaively low fixed coss and high variable (ossil) uel coss. Bu he raio o variable and fixed coss is shifing, as renewable generaion based on solar and wind wih litle o no variable cos increasingly eners he marke. Despie echnological advances and efficiency gains, even he mos modern, sae-o-he-ar ossil uel capaciy is finding i increasingly difficul o compee in a marke where subsidised capaciy is able o generae a zero variable cos. Ye firm capaciy will be needed o back up variable generaion. In his conex, many argue he curren marke environmen is no longer fi or purpose. Te peneraion o low-carbon, inermiten, generaion capaciy is a posiive sep owards less carbon-inensive elecriciy sysems. However, he change in generaion porolios does pose a number o challenges or he markes. How do we ensure ha sufficien capaciy is available when he wind doesn' blow and he sun doesn' shine? Currenly subsidy schemes remove variable renewable capaciy rom marke price signals. Tis canno and should no be a long-erm opion – or any ype o elecriciy generaion. Tis is no academic debae. aher, he European elecriciy indusry is already eeling he effecs o he recen changes on he ground. Companies have o mohball recenly buil gas plans, or insance, or pu invesmen projecs on hold. Te unavourable marke condiions are also discouraging exernal invesors rom puting heir money ino he elecriciy secor. Meanwhile, policy suppor coss, or insance or renewables or energy efficiency, are increasing he price ha end cusomers pay or heir elecriciy. In shor: he challenges are big and he soluions are as ye unclear. A resh look a marke design and a a 'smarer' energy sysem in general is needed. Tis book conribues o ha discussion, wih a paricular ocus on he effecs or generaors. In describing and analysing he curren environmen and he way ha some o he challenges can be addressed, i addresses key concerns o he European – and global – elecriciy indusry oday.
Note to the reader Jacob Klimsra
Many readers o his book have o make imporan decisions abou elecrical energy supply in differen regions. Elecriciy is crucial or creaing wealh and comor in a modern sociey. Wih ever-growing demand or elecrical energy, is generaion has a huge impac on global uel consumpion and he relaed emissions. Tis demand is expeced o double over he nex weny years or so. During he pas hundred years, scieniss and engineers have acquired a horough knowledge o he power supply sysem. Modern power plans achieve high uel efficiencies, and heir emissions have been drasically reduced. Excellen ransmission and disribuion sysems ensure a high degree o reliabiliy in he power supply o consumers. However, measures need o be aken o make he elecriciy supply more susainable. Ulimaely, he depleion o ossil uels will occur and he issue o global warming rom greenhouse gases canno be negleced. Tereore, he power secor has o adjus accordingly and find a new pah. Decisions made now will have a long-erm impac and opimum soluions have o be chosen. Te purpose o his book is o explain he challenges arising rom he adven o a large volume o inermiten renewable energy sources. In addiion, innovaive soluions are offered or keeping he power supply sysem reliable and affordable. Te book also discusses he effecs o renewable energy on he cos o elecriciy per kilowat-hour. Low coss are very imporan since energy is so inerwined wih he economy – any increase in he price o elecriciy has a significan impac on he cos o producs and services. Power sysems are no based on eelings and opinions, bu on scienific and echnical acs. Tereore, some mahemaics and physics are presened in his book. Neverheless, readers wihou a echnical background should also be able o undersand he issues displayed. Wriing his book ook much more effor han iniially expeced. Large levels o inermien power sources in a sysem have an impac on he requiremens or coningency reserves, on balancing elecriciy producion and demand, on supplying reacive power and on coss. Tis required searching or acual daa on he oupu o solar panels and wind urbines, in combinaion wih acual power demand paterns. Forunaely, mos European and USA-based ransmission sysem operaors make he necessary inormaion available on he inerne, alhough ofen heavy number crunching was required o ransorm his daa ino a workable orma. Also, he Inernaional Energy Agency always provides useul inormaion. Ye, his book does no preend o cover every aspec o he issues a sake. Hopeully i helps he reader o gain more insigh ino he mater and serve owards achieving he bes soluions. I am indebed o Wärsilä or offering he possibiliy o wrie his book. Te coninuous suppor rom a number o co-workers during is preparaion is highly appreciaed. In alpha beical order, I would paricularly like o menion Chrisian Hulholm, Jaime Lopez, Jiipee Matila, Jussi Laiinen, Kär Aavik, Kenneh Engblom, Kimi Arima, Mas Ösman, Niklas Wägar and Svane Behlehem . I am especially graeul o my wie Anna Marha who allowed me o dedicae so much ime o wriing o his book.
1 How to secure the electricity supply in a changing world
Te economy is largely buil on a reliable supply o cheap elecriciy. A challenge is o keep he supply sysem sable and affordable wih he rapid expansion o inermiten renewable energy sources. Te new sysem canno jus be buil on op o he old one. o make he inegraion successul and o ensure prosperiy in he uure, new echnical soluions and marke condiions are needed. Business as usual is no an opion or he power secor.
12 Power supply challenges
1.1. An affordable, reliable and sustainable supply of electricity Elecriciy is all around us. Wihou elecriciy, communicaions, indusrial aciviies and services come o a hal. Households suffer badly when he power supply sops. Tanks o elecriciy, lie in ho regions is bearable. Agriculural producs can be reaed and sored or exended periods o ime wih elecrical chilling. Soon, elecric vehicles will be common on he roads. Beore long, elecriciy will be he major energy carrier or energy consumers. Elecriciy producion is sill mosly based on ossil uels. Because o he emissions ha hese uels produce during combusion, legislaors and sociey in general are demanding more renewable energy sources. However, he adven o renewable energy based on, or example, solar radiaion and wind is creaing challenges in mainaining he delicae balance beween elecriciy producion and demand. Power plans charged wih he balancing ask have o adap heir oupu aser and more requenly han beore. Errors in orecasing elecriciy producion rom renewable sources add up o errors in demand predicion. Consequenly, more reserve capaciy, wih a much more responsive characer han in he pas, is needed. radiional seam-based power plans lack his flexibiliy. However, agile generaing echniques exis ha have he abiliy o assis in accommodaing vas amouns o renewable elecriciy sources and help, in so doing, reducing use o ossil uels. According o he media, energy sorage, smar grids, huge ransconinenal power ransmission lines and demand side managemen can solve he issues arising rom he inermitency o renewable elecriciy sources. Such news should be judged wih grea care. I would be convenien o have affordable sorage sysems playing a major role in balancing elecriciy supply wih demand. Soring elecrical energy direcly as elecriciy is no ye possible in pracice. Energy has o be sored chemically as in uel and in bateries, or mechanically as in flywheels, compressed air and raised waer-reser voir levels. Hea rom concenraed solar power sysems can be emporarily sored in molen sals or seam generaion a a laer poin in ime. Te challenge is o find economic sorage sysems ha have he righ properies o serve he balancing o elecriciy generaion and demand. Te required sorage properies depend on he ype o balancing required. Flywheels migh help or shor-erm requency regulaion in ime spans o a ew seconds, while bateries can help o cover unbalances up o an hour and pumped hydro can ake care o smoohing in 24-hour inervals. However, no sorage sysems exis ye ha can subsiue he use o uels such as naural gas and coal in covering, or example, seasonal lacks in power oupu rom renewable sources, such as hose occurring wih solar PV oupu during he darker seasons. High winds can occur in coninen-wide areas, so smoohing wind-urbine oupu wih long ransmission lines is no an effecive opion. Using excess elecriciy during he peak oupu periods o solar panels and wind urbines or waer heaing and chilling is a beter opion. Smar appliances and smar meers in households
1. How to secure electricity supply in a changing worldy
Gross Domestic Product (PPP) per capita per year in 2009 (year 2000 US$) 40 000
North America
35 000
30 000
25 000
Europe
20 000
15 000
World average China Latin America
10 000
5 000
Middle East
Asia (ex China) Africa
0 0
2 000
4 000
6 000
8 000
10 000
12 000
14 000
Electricity use per capita per year (kWh)
Figure 1.1. There is a direct relationship between the amount of electrical energy used (kWh) and wealth levels, as expressed in gross domestic product based on purchasing power parity (PPP).
appear o offer only very limied possibiliies or balancing he supply o elecriciy wih he demand. Te use o elecrical energy is direcly linked wih economic value, as can be seen in figure 1.1 In conras o common belie, he domesic use o elecriciy in households is, on a global average, less han a quarer o he oal elecriciy use. Te large remaining porion is consumed by indusrial users and by commercial users o creae economic value. Elecriciy is, hereore, primarily a value creaor. A number o conclusions can be drawn rom he relaionship beween gross domesic produc and elecriciy use as shown in figure 1.1 Simply said, i he power supply in Arica would increase by a acor o five, he economy migh poenially also grow by a acor o five and much povery would disappear. In addiion, i Norh America would lower he inensiy o elecriciy used in is economy o he European level, elecriciy consumpion migh be lowered by some 20% wihou losing any wealh. Te use o more efficien appliances, beter building insulaion, and a largescale inroducion o LED lighing are expeced o conribue o reduced elecriciy use in he USA. In Europe, by conras, he replacing o gas-uelled heaing wih elecric hea pumps and he adven o elecric vehicles migh lead o some increase in elecriciy use. China appears o closely ollow he global rend line beween elecriciy use and GDP. Te Middle Eas is clearly an oulier: cheap uel, ho climae, and relaively low indusrial oupu resul in low GDP creaion per uni o elecric
13
14 Power supply challenges energy. Neverheless, alhough economic boundary condiions differ rom counry o counry, elecriciy use and economic welare are closely relaed. Despie is excellen value o sociey, elecriciy has 50 watt o be affordable. High elecriciy prices can be a reason or energy-inensive indusries o move o a counry wih lower prices. Disinguishing beween he cos, price and value o an economic commodiy such as elecrical energy, Factor 40 higher prohelps in beter undersanding he mechanisms leading o ductivity with electricity affordabiliy. Te basic coss o elecriciy consis o he cos o he capial invesmen or he generaing uni, he cos o elecriciy ransporaion and disribuion aciliies, he cos o uel and he cos or operaion and mainenance. Te price cusomers pay or elecrical energy normally conains a leas hese basic coss, wih profi margins and 2000 watt governmen-imposed axes added. Te ulimae economic value or he cusomer per kilowat-hour delivered should naurally be higher han he price ha he cusomer pays. A domesic consumer generally pays more per kWh Figure 1.2. An example of han an indusrial user. Tis is parly because o higher where electricity substantially disribuion and reail coss, bu also because profi mar- increases productivity. gins and levies are generally higher in he case o privae cusomers. For an aluminium smeler, he value, price, and cos o elecriciy are basically close ogeher since energy coss heavily deermine he end-produc coss. For a scienis, banker, or amily member using a deskop compuer, he cos, price and especially he value o elecriciy can be acors differen. Compuers raise produciviy so much ha he price o he elecriciy o run hem is almos irrelevan
Cost, price and value of electricity for an aluminium smelter
Cost, price and value of electricity for a household
120
120
h100 W k r 80 e p s 60 t n e 40 c o r u E 20
h100 W k r 80 e p s 60 t n e 40 c o r u E 20
0
0 Cost
Figure 1.3.
Price
Value
Cost
The cost, price and value of electricity compared
Price
Value
1. How to secure electricity supply in a changing worldy
Figure 1.4. The densely populated and polluted environment were created in the new industrial cities during the Industrial Revolution (1760–1840).
or he user. In households, a 2 kW vacuum cleaner has he same power as 40 people using duspans and brushes. One hour o vacuum cleaning migh cos 0.50 € or he elecriciy, bu hiring 40 cleaning people insead migh cos a leas 500 € in wealhy economies. Te social coss o elecriciy can cause he real coss o be higher han he sum o he coss or capial, uel and operaions plus mainenance. Such social coss include, among oher hings, he value o he environmenal damage caused as a resul o polluion rom he uel producion and rom he emissions. Subsidies or mining jobs also have o be included in he social coss. Poliicians migh claim he creaion o a subsanial number o jobs conneced wih he inroducion o renewable energy, bu such jobs can also be seen, a leas parly, as social coss as long as subsidies dominae he marke or renewables. Ulimaely, he inegral economic value o a produc such as elecriciy should a leas exceed all he coss o making ha produc. I he cos o elecriciy exceeds is value, using elecriciy will be a luxury and a burden on he economy, wihou creaing wealh. Neverheless, he accepabiliy o neglecing social coss depends o a large exen on he acual wealh level o he paricular counry. When people are sarving, iems such as ood and waer are urgenly needed, and in such cases some conneced
15
16 Power supply challenges
Pure sine wave
Sine wave distorted with 3rd and 5th harmonics
1
1
e g a 0.5 t l o v 0 e v i t a–0.5 l e R
e g a 0.5 t l o v 0 e v i t a–0.5 l e R
–1
–1
0
10
20
Time (ms)
30
0
10
20
30
Time (ms)
Figure 1.5. A clean 50 Hz sine wave and a sine wave distorted with harmonics.
environmenal damage is jus aken or graned. Te indusrial revoluion in he 18h and 19h cenuries had desrucive effecs on he environmen, bu he resuling increase in he level o prosperiy ulimaely released money or repairs and improvemens. Enorcing he same environmenal sandards globally or elecriciy producion, regardless o wheher i is in emerging economies or in he affluen areas o he world is, hereore, no air i he associaed coss are high. While many people use power as a synonym or elecrical energy, his book will disinguish beween power and energy. I is scienifically incorrec o sae ha a machine or a power plan can produce power, since power is he capaciy o deliver energy. A car can have an engine wih a maximum power capaciy o 125 kW (kilowats), bu as long as he engine is no running, no energy is sen o he wheels. Driving he car or one hour a ull power means ha he engine delivers an amoun o energy equalling 125 kW ∙ 1 h = 125 kWh (kilowat-hours). An elecric power saion o 360 MW (megawats) consanly running a ull oupu during 4380 hours, equalling hal a year, produces 360 ∙ 4380 = 1576800 MWh (megawat-hours), or almos 1.6 Wh (erawat-hours) o elecrical energy. High reliabiliy in supplying qualiy elecriciy is obviously imporan o energy consumers, who generally require ha heir need or elecric energy is ulfilled a any ime. Failure o supply elecriciy will a leas be a nuisance, and generally also resuls in financial losses. Users in commercial and indusrial environmens expec a power supply sysem reliabiliy o a leas 99.99%, meaning ha he supply ails on average in oal only 53 minues per year. For applicaions where a consan availabiliy o elecriciy is crucial, uninerrupable power supply sysems and backup generaors are common pracice. For some applicaions, such as daa cenres and hospial operaing heares, a supply reliabiliy o over 99.999% is required. A reliable supply o elecriciy also requires ha he volage and requency are mainained wihin narrow limis. In addiion, he delivered volage should be clean and no excessively superseded by harmonic or random disorions, i.e. volage variaions wih a requency oher han he basic requency o 50 Hz or 60 Hz. Disorions are caused by conrol elecronics and by lighing sysems such as LEDs. I he
1. How to secure electricity supply in a changing worldy
volage deviaes oo much in value and shape rom he sandards, he perormance o he users’ equipmen will be derimenally affeced, and he equipmen migh even be damaged. Qualiy elecriciy means high supply reliabiliy o he proper volage. Te elecriciy supply should also be susainable. Te burden imposed on he environmen should be accepable, while naural resources have o be used as efficienly as possible. echnologies are available nowadays o achieve very low emissions o polluans, including nirogen oxides (NO X ) and sulphur oxides (SO X ). Boh have negaive impac on air qualiy, and cause acidificaion o waer basins and soil. As an example o emission reducions, he power secor in he USA was responsible or 6.2 Monnes o NO X in 1995, bu or only 2.2 Monnes in 2009, hanks o exhaus gas cleaning and cleaner uels. Ye, oal ossil uel consumpion in he USA, meaning oil, gas and coal ogeher, was roughly he same in 2009 as i was in 1995. Anoher issue is global warming. Globally, he power secor is responsible or roughly a quarer o anhropogenic CO 2 emissions. Te European Union aims o reduce greenhouse gas emissions by some 85% rom he 1990 levels by he year 2050. Te power secor should be emiting zero greenhouse gases by ha ime. o achieve his, a reducion in energy consumpion, he large-scale inroducion o renewable energy sources, and carbon capure and sequesraion (CCS) or ossil uel applicaions are seen as being he major measures. In his conex, i is imporan o know ha power plans are long-erm invesmens wih a echnical lie exceeding 40 years. Te EU policy means ha newly buil power plans ha are no prepared or CCS migh ace early reiremen. However, an excessively abrup weaning rom ossil uel usage in order o bring down CO2 emissions will disrup he economy. Te reason behind his is ha per uni o delivered energy mos renewable energy sources are more expensive han
30 r i a e h t 25 n i s n o 20 i t a r t ) b n e p 15 p c ( n o c 2 10 O S d n 5 a
SO2
NOX
X
O N
0 1980
1990
2000
2010
Year
Figure 1.6. The substantial decline in average concentrations of NOX and SO2 in the USA’s ambient air (source EPA).
17
18 Power supply challenges ossil uels. Furhermore, energy sources based on wind, solar radiaion, idal flows, and wave energy are by naure variable in oupu. Te Inernaional Energy Agency (IEA) has esimaed ha jus 3.7 %, or 0.8 PWh, o he oal global elecrical energy demand was derived rom renewable sources in 2010, excluding hydropower. A large par o his is based on biomass, primarily wood. Wood is ofen used in exising coal-fired power plans via co-firing or supplemenary firing. Burning wood in power saions is heavily subsidized in some counries, bu he posiive effec on reducing greenhouse gas emissions is quesionable. Esimaes are ha oresry aciviies and he ransporaion coss involved migh already resul in 200 g/kWh in CO 2 emissions, i.e. almos he same amoun o CO2 ha a naural-gas-fired cogeneraion plan emis. I hydropower is included in he renewables, some 19.7 % o elecriciy is currenly derived rom renewable sources. Te effor required o increase he amoun o renewable energy is huge. Ineviably, ossil-based power plans will sill be needed or many decades. In any case, ully absaining rom he use o ossil uels is difficul, since hese energy sources can easily be sored in large quaniies. In paricular, naural gas can serve as a versaile, cheap and relaively low-carbon backup batery or balancing he inermiten elecriciy supply coming rom wind, solar radiaion and idal-flow generaors. Neverheless, ossil uel resources are ulimaely finie. Expecaions are ha he global demand or elecrical energy will almos double over he coming 20 years. Tereore, maximum uel efficiency is required and any wasing and flaring o uels should be avoided. Te goal should ulimaely be o achieve a gradual shif o affordable renewable energy sources wih maure equipmen having sufficien warranies rom reliable manuacurers.
1.2. Challenges of renewable energy sources. Elecriciy demand has always shown variabiliy. Shor-erm variaions in demand occur because elecriciy consumers swich heir appliances on and off a random. Te ne effec o his on demand is small and convenional generaors can adap heir oupu accordingly. Moreover, here are daily paterns caused by ypical socieal behaviour, where people go o work or school in he morning and reurn home in he evening, and finally go o bed. Tese daily paterns are affeced by he seasons, since in he colder regions more lighing and hea are required in he winerime. In ho regions, air condiioning is needed in he summer. In many areas, seasonal paterns can clearly be disinguished. Figure 1.7 has 17.520 daa poins o illusrae how he power demand varies in he norh-wesern par o Germany during a ull year. Each weekend, here is a sharp drop in demand. Te variabiliy in oupu o uel-based power plans is much higher even han he variabiliy in demand shown in figure 1.7 Tis is because o he inermiten oupu o a subsanial amoun o renewable elecriciy sources in he sysem. I is noable ha a he end o he year, when he labour orce sops work or he Chrismas
1. How to secure electricity supply in a changing worldy
Power supply data Tennet region, Germany, year 2012
15 000
) W M ( y10 000 l p p u s r 5 000 e w o P 0
Figure 1.7. An example of the dynamic pattern in the electricity demand in the Tennet region of Germany (data from Tennet).
holidays, he demand is very low. During his holiday period, srong winds were prevalen over Germany, resuling in excess elecriciy ha had o be expored o neighbouring counries or a negaive ee o up o 200 €/MWh. Elecriciy generaors based on renewable energy sources such as wind, sunshine and idal flows are generally graned unresriced eed-in ino he elecriciy grid. Teir oupu however depends heavily on he weaher and he ime o day. Moreover, heir oupu is never ully predicable and is someimes even close o zero. Figure 1.8 illusraes he variabiliy in oupu o wind urbines and solar PV panels in he German 50Herz SO (ransmission Sysem Operaor) region during week Week 26, 2012, 50 Hertz TSO are, Germany 125 000
Others 100 000
) W M ( r e w o P
75 000
Wind 5 000
Solar 2 500
0 Mon
Tue
Wed
Thu
Fri
Sat
Sun
Figure 1.8. Wind and solar -based power output, and the remaining supply from other sources in the German 50Hertz TSO region, week 26, 2012 (cumulative curve, the black arrows give the extremes for ‘others’).
19
20 Power supply challenges 26, 2012. Early on Monday, wind urbines generaed almos all he power ha was needed. On Tursday morning a 8 am, however, he oupu rom wind and solar sources was so low ha 10.4 GW had o be derived rom oher sources. Since elecriciy ransmission and disribuion grids have virually no energy sorage capaciy, he producion and consumpion o elecriciy have o be precisely mached. I he driving power or he generaors exceeds he elecriciy consumpion + sysem losses, he generaors will increase heir speed and, simulaneously, he requency in he sysem will go up. Alernaively, i demand is higher han supply, he requency will drop. For he sysem o operae properly, and or many sensiive applicaions, he requency has o remain wihin narrow limis. Tereore, generaors are equipped wih conrollers ha can correc heir oupu depending upon he deviaion rom he desired sysem requency. Variaion in power plan oupu is hereore necessary, bu i is no economic o run a power plan consising o a single generaing uni in a wide load range. echnical resricions also limi a single generaor rom having a wide oupu range. A low loads, he uel efficiency is low while he mainenance coss per kWh are high. Tereore, generaors are swiched off i heir load is below a cerain hreshold. Conversely, i he generaors ha are online canno mee an increase in demand, addiional generaors have o be swiched on. Wih much inermiten renewable capaciy in he sysem, he balancing ask o he uel-based and hydro-based generaors is rapidly increasing. When he sun ses, he oupu rom phoo-volaic cells (PV) drops o zero, while elecriciy demand generally increases. Tis resuls in he dispachable generaing capaciy having o ramp up is oupu significanly. Figure 1.9 gives an example o he rapidly changing oupu rom all he wind urbines in he German Amprion SO region during he 24 hours o April 28, 2012. Tis illusraes a ypical example o he passage o a depression. wo subsanial increases in he wind-power oupu o up o 1 GW per hour were
2 500
s e n i 2000 b r u t d n ) 1 500 i w W t M u ( p 1 000 t u o r e 500 w o P
1.2 GW/hour decline in output
0 0
6
12
18
24
Hours of April 28, 2012
Figure 1.9. Large differences in power output from wind turbines in the German Amprion TSO region on April 28, 2012.
1. How to secure electricity supply in a changing worldy
observed. Te large decline in wind-power oupu, rom 1.8 GW o 0.6 GW, in he ime span rom 11 am o 12 am required a large amoun o as backing-up by power plans. Even worse siuaions occur when he wind reaches gale orce and wind ur bines have o be sopped in order o avoid physical damage. Because o his, backup power plans have o be increasingly flexible. ransmission sysem operaors (SOs) ry o inroduce Demand Side Managemen (DSM) or balancing. Someimes, i is called Demand Sysem esponse. ypical elecric appliances, such as rerigeraors and air-condiioners, can be swiched off or a while. Te use o washing machines and laundry dryers can ofen be posponed o when he general demand or elecriciy is dropping. Tis requires smar appliances ha respond o a signal rom he grid operaor. Variable pricing o elecriciy migh also help, and or his smar meers wih a momenary ariff indicaor are needed. Neverheless, such demand managemen measures can only be par o he soluion o keep he sysem sable. A huge number o appliances would have o be conrolled o have any noiceable effec. As an example, using DSM o compensae or he 1.2 GW decrease in wind urbine oupu, as shown in Figure 1.9, requires swiching off he equivalen o 450000 laundry dryers. A laundry dryer runs on average or some 100 hours per year. Te probabiliy ha a laundry dryer is running a any paricular ime is, hereore, only 1%. o sum up, i is no expeced ha domesic elecriciy demand can be shifed by more han a ew percen hrough he use o smar appliances and smar meers. Beter DSM opporuniies migh be presen wih indusrial users o elecriciy.
1.3. Faults and failures in electricity supply systems In addiion o he need or normal balancing, auls and ailures can and do occur in elecriciy supply sysems. Tese maluncions are also called coningencies, and hey affec he balance beween supply and demand. A ailing power plan resuls in an insananeous loss o elecriciy supply. Immediaely upon he occurrence o such a loss in generaing capaciy, he roaing ineria in he sysem helps o avoid an abrup change in requency. A consequen drop in requency is unavoidable, bu sysem operaors allocae spare oupu rom he generaors ha are online o compensae or he los uni. Tis spare oupu is called primary reserve. Afer applicaion o he primary reserves, addiional generaing capaciy is rapidly acivaed o resore he requency o he desired value so ha he primary reserves are available again or he nex coningency. Tis addiional capaciy is called secondary reserve. Wih a growing racion o he power capaciy derived rom non-dispachable generaors in he sysem, i becomes increasingly difficul o have adequae backup capaciy wihin he sysem. Having jus a ew large power plans online is very risky, since hen he relaive effec o one power plan ailing on he dispachable capaciy is large. Modern coningency reserves have o consis o smaller agile power plans ha are well disribued across he area o be served. Cogeneraion unis are an example o such disribued power plans. Cogeneraion o hea and power (CHP) is an effecive means o improving uel efficiency and
21
Figure 1.10. The typical electric power of a laundry dryer is 2.7 kW.
22 Power supply challenges reducing greenhouse gas emissions. Wih an adequae number o such local generaors in a sysem, hese unis can also be uilised or requency conrol and balancing. Again, highly flexible power plans wih as ramping raes and shor saring and sopping imes will be needed or balancing elecriciy producion and demand. Fauls in he ransmission and disribuion sysem canno be avoided. rees may all on high volage lines, and icy rain in combinaion wih high winds can damage he wires. Excavaions requenly cause damage o underground cables. Decenralized generaion is beneficial in his respec, since i reduces he dependence on a ew disan generaors and long power lines. Because o heir increased conribuion o elecric generaing capaciy, decenralized generaors should also be able o comply wih he grid codes se by ransmission and disribuion sysem operaors or large power plans. Being able o ride hrough a shor circui is an example one o he new requiremens or local generaors (see appendix 2).
1.4. Balancing supply and demand in energy markets When elecriciy was supplied only by ully inegraed uiliies, all coss in he sysem were supposed o be covered by he ariff charged o he cusomers. In such a sysem, any profi goes o he sysem owner, which is ofen he sae, a province or a municipaliy. Te sysem owners are also responsible or any financial losses. In some counries, elecriciy is even subsidised. Planning expansions o, and renewals o, he generaion
Figure 1.11. An integrated power company producing and delivering electricity to end users.
1. How to secure electricity supply in a changing worldy
Politicians
Emission limits, emission tax and renewable subsidies
Energy tax and renewable subsidies Government
Competing generating company 1
Transmission system operator
Competing generating company 2
Subsidised intermittent renewable generation
Large industrial customers
Regulator
Large industrial customers with self generation
Distribution system operator
Small industrial customers
Commercials Competing electricity retailer A
Competing electricity retailer B Households
Competing electricity retailer C
Variable rules
Variable emission limits, tax and subsidies
Variable electricity flow
Figure 1.12. An unbundled power sector affected by many players.
and disribuion sysem is easy in such circumsances. For his reason some governmens and poliicians preer a siuaion whereby he elecriciy supply is ully conrolled by inegraed uiliies, wih perhaps a ew independen elecriciy producers eeding ino he grid. However, he ree-marke hinking a he end o he wenieh cenury advocaed economic liberalisaion, wih privaisaion and deregulaion in all segmens o all markes. By having privae invesors ake over he role o he public secor, produciviy was supposed o increase and he coss o consumers would hen be reduced. A power supply run by he public secor was, and ofen is, considered o be bureaucraic, ineffecive and less cusomer riendly. Is his ruly he case? Ta is he big quesion. Currenly, he liberalised and unbundled power secor complains o permanen inererence rom policy makers wih ever-changing rules and high subsidies or some ypes o generaion, making i difficul o inves in new power plans. Ye, exensive lobbying coninues simulaneously by he differen sakeholders or
23
24 Power supply challenges geting preerenial rules or heir ypical aciliy or echnology. Consan changing o he rules creaes difficulies or long-erm invesmens. Power saions have a very long echnical lie, and ransmission and disribuion lines las even longer. Tis is he reason ha some counries, especially in Asia, have decided no o adop he liberalised marke model, which is generally dominaed by shor-erm profi making and quarerly resuls. Moreover, in many counries wih a liberalised elecriciy marke, he governmen sill derives much income by axing energy use and by charging value added ax. Te ulimae price o elecriciy o domesic consumers has, in general, no decreased as a resul o he new open markes. Grid requency, volage levels and reliabiliy all have o be guaraneed, even in an open elecriciy marke. Tereore, independen ransmission sysem operaors (SOs) are charged wih he conrol o requency and volage, and wih seting rules or mainaining grid sabiliy and supply reliabiliy. SOs esimae he power needs or he near uure wih elaborae predicion models, and use a marke mechanism o ensure ha sufficien generaing capaciy will be available. Wih he inroducion o much inermiten generaing capaciy rom renewables, he uncerainy in predicing he oupu required rom non-renewable power plans is growing. As an example, he large changes in oupu rom wind urbines as shown in Figure 1.9, were no prediced by he orecasing models, as can be seen rom figure 1.13. In he simples open marke approach, a power plan is remuneraed only or he energy delivered. Te producer ha offers he cheapes elecriciy would be firs in he meri order in a naional or regional energy marke. In his simple marke model, he SO requires elecriciy producers o include all relevan services, including 2 500
s e n 2 000 i b r u t d n ) 1 500 i w W t M u ( p 1 000 t u o r e w 500 o P
Actual
Forecast
0 0
6
12
18
24
Hours of April 28, 2012
Figure 1.13. An example of a large deviation between the predicted and the actual power output from wind turbines in the German Amprion TSO region, April 28, 2012.
1. How to secure electricity supply in a changing worldy
1
) W 0.5 G ( e c 0 n a l a b–0.5 m I –1 0:00
3:00
6:00
9:00
12:00
15:00
18:00
21:00
24:00
Time
50.10 50.08 50.06 50.04 ) 50.02 z H50.00 ( ƒ
49.98 49.96
June 2003
49.94
June 2006 49.92 49.90
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 0 : 0 1 : 0 2 : 0 3 : 0 4 : 0 5 : 0 6 : 0 7 : 0 8 : 0 9 : 1 0 : 1 1 : 1 2 : 1 3 : 1 4 : 1 5 : 1 6 : 1 7 : 1 8 : 1 9 : 2 0 : 2 1 : 2 2 : 2 3 : 0
The 24 hours of April 10, 2013 50.15 ) z 50.10 H ( y c n e u q e r f d i r G
50.05 50.00 49.95 49.90 49.85
Figure 1.14. Examples of imbalances caused by electricity trading (data from KEMA report 74100846-ETD/SDA 12-00079, Swissgrid and TENNET.
25
26 Power supply challenges backup or ailing power plans and requency conrol in heir energy delivery offering. In a more exended marke model, power plans can be remuneraed or he availabiliy o reserve power and or heir capabiliy o achieve as ramping up or down o heir oupu. Even acors such as saring up reliabiliy and supply reliabiliy, migh be worh rewarding. Manuacurers o energy sorage echnologies are aiming or financial compensaion or he balancing capabiliies o heir producs. Apar rom pumped-hydro sorage, mos echnologies or shor and medium-erm sorage are sill under developmen, and researchers are eager o promoe heir echnologies o subsidy providers. Elecriciy supply markes generally operae by offering energy in fixed ime spans, such as in hourly or even 15 minue inervals. Tis approach gives rise o periodic deviaions in grid requency. Tis is illusraed in figure 1. 14. Frequency sabiliy has, hereore, decreased since he inroducion o open elecriciy markes. Each ime a rading ime span ends, or begins, power plans increase or decrease heir power oupu. Tis has o be compensaed or by he requency regulaion capaciy o he power plans, which was originally inended or occasional coningencies, such as he loss o a power plan. Sluggishly reacing power plans have difficuly in resoring he grid requency o wihin is required range. In chaper 7, his book will show how a properly seleced generaing porolio in an elecriciy supply sysem can improve sysem sabiliy wih reduced coss and higher reliabiliy. Wih a proper approach, his sabiliy can even be reached wih a high proporion o inermiten renewable generaion in he sysem. Much inermien generaion ineviably reduces he uilisaion acor o he oher power plans. Te consequence o a low uilisaion acor is higher specific capial coss (€/MWh) or uel-based power plans. Low invesmen coss will, hereore, be a key elemen or new power generaing capaciy. Te shif owards more renewable generaion in he sysem will cerainly reduce ossil uel consumpion. However, he consequen decrease in he uilisaion acor o he oher power plans will ineviably increase he capial coss per kWh produced. Moreover, uel-based power plans will have o be ar more flexible in he uure, wih requen sars and sops and high ramping raes in oupu.
1.5. Conclusions A seady growh in elecriciy use, coinciding wih concerns or susainabiliy, creaes subsanial challenges. Policy makers inerere increasingly wih markes and use subsidies and levies o achieve heir arges. Invesors ace uncerainy o profiabiliy because o requenly changing boundary condiions. Dispachers o power supply sysems have o live wih he challenges o variable oupus o renewable energy sources and he uncerainies rom orecasing errors. o compensae or he unpredicabiliy o he markes and o backup he inermiten oupu o renewables, a new level o flex ibiliy is needed in power sysems.
Balancing the 2 electricity supply in case of calamities
Sabiliy in elecriciy supply sysems has o be mainained even during disurbances such as a major shor circui, generaor ailure or losing a large load. In keeping he sysem sable, he role o roaing ineria is essenial. When inegraing renewables wih no or low ineria o he sysem, he balancing becomes more difficul. o avoid risks o requency collapses and blackous, new soluions are needed or he uel-based backup generaion.
30 Power supply challenges
2.1. Matching electricity supply and demand he main ask o an elecric power supply operaor is o coninuously mach elecriciy generaion wih elecriciy demand. Coninuous maching is needed since he supply sysem as such canno sore elecrical energy. I elecriciy demand sysemaically exceeds he power delivered by he machines ha drive he generaors, he generaing unis will respond by decreasing heir roaional speed. Consequenly, he grid requency will drop and he sysem will collapse in a mater o seconds, resuling in a blackou. Forunaely, blackous won’ occur i he unbalance in demand and supply is shor-erm, since generaing unis and elecric moor drives have energy sored in heir roaing mass, he so-called ineria. Ta buffer limis he rae o change in requency in he case o an unbalance beween generaion and demand. Te energy sored in his ineria creaes ime or he engines or urbines ha drive he generaors o adjus heir oupu in order o resore balance. Te sabiliy o he requency in alernaing curren sysems is a good measure o balance. Frequency is by definiion Balance he number o imes ha a ull sine wave occurs per second in he grid. Te inernaional uni denoing requency is herz (Hz). Te roaional speed o he generaors deermines Inertia his requency, and he so-called nominal value o he requency depends on he global locaion. America and Japan operae using 60 Hz, while mos oher areas o he world have 50 Hz. In realiy, he grid requency varies somewha around he desired value. Figure 2.2 shows he grid requency in Te Neherlands during a shor ime span o 5 minues afer 5.00 am on April 12, 2013. Cusomer demand is never ully consan and generaors also have some variabiliy in heir oupu. Figure 2.1. An illustration of the delicate balance between electricity demand Ye, on average he requency has o mach he desired value. I he acual requency deviaes or jus a small racion and production, with rotating inertia as a buffer with some energy stored. rom he desired value, no acion is aken o change he oupu seting o he generaors. Tere are a number o reasons or ha. Each measuremen sysem is affliced wih some inaccuracy, while conrol sysems also have some insensiiviy. Endeavours o keep he requency wihin very narrow limis, meaning real isochronous operaion, would resul in overacive conrol o he machines ha drive he generaors, which in urn would lead o unnecessary wear. Figure 2.2 shows an example o a sysem wih a permited measuremen error range o +/– 10 mHz, wih an addiional zone o +/– 10 mHz where no acion rom he generaor is required. Consequenly, he resul is a oal dead band o +/– 20 mHz. Neverheless, he average requency over a prolonged ime span should be exacly 50.000 Hz, and grid operaors ake acion i he cumulaive deviaion rom his desired value becomes excessive. In Figure 2.2, he grid requency exceeds he dead band a 20 seconds afer 5.00 am on April 10, 2013. A ha momen, he so-called primary conrol reserve power Generation
Demand
2. Balancing the electricity supply in case of calamities
50.05
50.04
Frequency regulation action
50.03 ) z H ( 50.02 y c n e u 50.01 q e r f 50.00 d i r G
Additional dead zone Allowed measurement error
49.99
Additional dead zone
49.98
49.97 0
60
120
180
240
300
Seconds after 5:00 am on April 10, 2013
Figure 2.2. Example of the rules for frequency control, with an example of actual frequency variations (frequency data source: TENNET)
plans sar o auomaically slighly reduce heir oupu setings. Some one hundred seconds laer, he requency is back wihin is allowed limis and he conrol acion o he generaors ceases. Tis requency regulaion acion is handled auomaically by he so-called primary reserves. Auomaic acion is he only opion because o he as response required o keep he requency wihin is narrow band. Te primary conrol reserves are also known as requency conainmen reserves (FC). Subsanial changes in requency will occur i a large cusomer disconnecs, or i a large power saion suddenly ails. In modern power supply sysems, many generaors are inerconneced via he ransmission grid. Te number o online generaors should be sufficien o ensure ha a ailure o he larges uni can be absorbed, o a large exen, by he spare capaciy o he oher generaing unis.
2.2. Primary control reserves compensating for the failure of a power plant I is ineresing o analyse wha happens when a power plan in an elecriciy supply sysem ails. o simpliy such an analysis, we presume a supply sysem wih en power plans o he same power capaciy. Each o he en power plans has a nominal power capaciy o 500 MW, and hey are all running a 90% o heir capaciy o provide 10% o primary conrol reserves. Ta would, a leas in heory, be sufficien o compensae or a ailure o one o he en power plans. Nominal power means he nameplae power o he generaing uni, while nominal speed means he generaing uni’s normal amoun o revoluions per minue. In his example, he elecriciy demand ha he en power plans
31
32 Power supply challenges
1
2
3
4
5
6
7
8
9
10
Transmission lines
Demand by customers
Figure 2.3. Ten power plants initially supplying the required electricity demand when one of them, number 7, fails.
supply amouns o 90% o 10 · 500 MW = 4500 MW. I will now be shown wha happens i, or example, power plan number 7 suddenly ails and he sysem immediaely lacks 450 MW o he required power supply. Tis example may appear o be somewha exaggeraed since a sudden loss o 10% o he dispachable generaion is no common. However, wih much renewable capaciy in a sysem, such occurrences are becoming increasingly realisic. In addiion, he effecs o unbalance in a sysem can be clearly shown wih his example. Afer he ailure o one plan, he nine remaining power plans canno insananeously ramp up he power oupu o he machines ha drive he generaors rom he iniial 450 MW o he newly required 500 MW in order o supply he oal sysem demand o 4500 MW. Power plans need some ime o reac o a newly desired oupu value. Tereore, i no energy was available rom he roaing mass (he roaional ineria) in he sysem, he unbalance would immediaely sop all generaors wih a resuling blackou. Te amoun o energy sored in a roaing generaing se
Rotational energy 2 E r =½ l r ω Energy in
ω=2 π f = 2 π n/60
Energy out
Figure 2.4. The energy stored within the power supply system as a result of the flywheel effect of the spinning generating units. (f = rotational speed in revolutions per second, I r = moment of inertia).
2. Balancing the electricity supply in case of calamities
is linearly proporional wih he momen o ineria I , and he square o he running speed n. Ineria is a propery characerising he flywheel effec o he roaing mass. Te running speed n gives he number o revoluions per minue o he generaor roor. In a 50 Hz sysem, a generaor wih a single pole pair runs a 3000 rpm. In his case, he requency equals n/60. Te amoun o energy Er sored in he roaing ineria o a generaing se is generally expressed as a racion o he nominal power capaciy Pnominal o ha generaing se. Tis racion is called he ineria consan τ I o a generaing se: Er I r ½ I r ω2 τ I= = =½ (2π f )2 Pnominal Pnominal Pnominal
Equation 2.1
Te dimension τ I o he ineria consan is he same as ha o ime, and is expressed in joule/wat = J/(J/s) = s (second). Te ineria consan o a large generaing uni lies in he range beween 5 and 10 s. Tis means ha when a 500 MW generaing uni is running a is nominal speed, is roaing pars have 2500 MJ o roaional energy in he case o a τ I o 5 s, and 5000 MJ or a τ I o 10 s. Tis is also he amoun o energy ha has o be ranserred o he roaing pars when he generaing uni is sared up and acceleraed o is nominal speed. I he 5000 MJ o energy during his acceleraion is supplied o he roaing ineria wih a machine ha uses naural gas or uel, we can calculae he amoun o gas required. I he uel efficiency o he driving machine is 40% and he naural gas has a lower heaing value o 36 MJ/m 3 , i requires 5000/36 · 100/40) · 350 m 3 o gas o bring he roor up o nominal speed. Such an amoun o gas provides enough energy o hea up 35000 lires o waer rom 20 °C o 100 °C, or preparing 280000 cups o ea. A cluser o 3000 car bateries can also deliver he required amoun o energy. Tis illusraes ha alhough he energy sored in he roaing ineria o a generaing se is no elecrical, i is neverheless an impressive amoun o energy.
2.2.1. The usefulness of the inertia constant τ I Te ineria consan τ I is very helpul in geting a firs impression as o how as a generaing uni will change speed in case o an unbalance beween he power supply o he generaor and power demand. Unbalance occurs when he elecrical load changes while he power supplied o he generaor shaf rom is prime mover, i.e. he driving engine or urbine, remains he same. For an ineria consan o 5 s, he amoun o energy in he roaing pars is enough o supply he nominal load o he generaor or 5 s wihou any energy inpu rom is prime mover. Afer ha ime span, he generaor will come o a sandsill. Furhermore, i he prime mover were o supply 90% o he nominal load o he generaor, while he load o he generaor equals he nominal load, he roaional energy would be enough o cover he unbalance or 50 s. Tus, i akes en imes longer han in he case o no power supply rom he prime mover beore sandsill is reached. However, even in he later case, he requency will rapidly reach a value ouside he permissible range.
33
34 Power supply challenges Equaion 2.1 reveals ha he roaional energy o he generaing se is proporional wih he square o he insananeous running speed. Tis means ha a higher speeds, here is much more energy in he ineria han a lower speeds. Tereore, he requency will no decrease linearly wih ime i here is a fixed unbalance beween he power supply o he generaor and he generaor load. Figure 2.5 shows how he requency o he grid served by he nine remaining generaors (Figure 2.3) decreases i each generaor receives 450 MW rom is prime mover while he combined load remains 4500 MW. Each o he nine running generaors should receive 500 MW o avoid an unbalance. Tereore, each generaing uni ‘eels’ a power supply defici o 50 MW. Te requency curve in Figure 2.5 is a represenaion o equaion 2.2. Te derivaion o equaion 2.2 requires some considerable mahemaical manipulaion. Te ineresed reader can find he derivaion o equaion 2.2 in Appendix 1.
f(t)= ( f
2 nominal
+
P Pnominal
f 2nominal
t )
τ i
Equation 2.2.
Prominal=500 MW, P= –50 MW, Pinitial=450 MW 60
50 ) z H (
40
y c n e u 30 q e r f d i 20 r G
10
0 0
20
40
60
80
100
Time (seconds)
Figure 2.5. The decline in speed for a generator where the generator load constantly exceeds its driving shaft power by 50 MW (inertia constant τ I = 10 s, constant load presumed).
Te drop in requency during he very firs seconds ollowing a major coningency even is a perec indicaor o he amoun o unbalance. In he beginning, he roaing requency o he generaor se is sill very close o he nominal requency (50 Hz or 60 Hz) and he approximaion can, hereore, be made ha he iniial drop in requency per uni o ime d/d equals: df dt
=
P f 2τ i Pnominal nominal
Equation 2.3.
2. Balancing the electricity supply in case of calamities
Equaion 2.3 reveals ha in our example o a τ I o 10 s, a power defici o 50 MW per generaor wih a nominal power o 500 MW a a requency o 50 Hz, resuls in a change in requency o exacly 0.25 Hz per second a he sar o he occurrence. Tis is indicaed in Figure 2.6 by he brown line. Would he power defici per generaor have been only 25 MW, he decline in requency would have been halved o 0.125 Hz/s. Tis simple relaionship beween unbalance size and he iniial requency change is very convenien, especially in island operaion where jus a ew generaors have o mainain grid sabiliy. Based on he value o he inclinaion in requency versus ime, he uel supply o he machine driving he generaor can be immediaely and adequaely adaped so ha no ime is los in resoring he generaor requency. Te horizonal red lines in Figure 2.6 give he maximum allowed dynamic requency limis in he Coninenal Europe synchronous area. Te green line in Figure 2.6 represens he nominal grid requency o 50.000 Hz.
Frequency decline without control action 51
) 50.5 z H ( y c n e u q e r F
50
49.5
49 0
0.5
1
1.5
2
2.5
3
3.5
4
Time (seconds) Figure 2.6. A close-up of the first 4 seconds of figure 2.5; the thick brown line gives the decline in grid frequency for a 10% unbalance in the system due to the tripping of a power plant (τ I = 10 s).
2.2.2. Self-regulating power in an electricity grid I he grid requency decreases, elecriciy demand auomaically goes down slighly. Tis is primarily caused by he synchronous elecric moors in he grid demanding less power, since heir load declines along wih heir running speed. Tis is called he selregulaing power o he supply sysem. Te power requiremen o synchronous moordriven pumps can decrease by 6% per Hz requency drop. For moor-driven applicaions wih a consan orque, power demand can decrease 2% per Hz. Since synchronous moors orm jus a racion o he oal load, he sel-regulaing power o he sysem in indusrialised counries is generally presumed o be 1% per Hz deviaion rom he
35
36 Power supply challenges nominal requency. In areas wih minor indusrial aciviies, he sel-regulaing power o he grid will be close o zero. Te posiive effec o sel-regulaing power should no be overesimaed. I he grid requency drops rom he desired value o 50 Hz o 49.8 Hz, he sel-regulaing power lowers demand by only 0.2 · 1% = 0.2%. Tis 0.2% is only a 9 MW reducion rom he 4500 MW oal load in our en generaor sysem example. I, however, he grid requency would drop rom 50 Hz all he way down o 40 Hz, he decrease in elecriciy demand because o he sel-regulaing power would be 10 Hz · 1% / Hz = 10%, i.e. 450 MW in our example. Tis renders a 50 MW reducion in demand or each o he nine generaors ha remained online ollowing he rip o one machine. Consequenly, a a requency o 40 Hz, generaion and demand are mached again i he nine power plans keep heir power oupu seting a he iniial 450 MW. In realiy, i will be difficul or he generaors o keep heir power oupu consan when he requency decreases so much. Te power oupu o he prime mover ha drives he generaor is proporional wih he produc o orque M and running speed n. I he grid requency decreases rom 50 Hz o 40 Hz, he driving orque M has o increase by a acor o 50/40 = 1.25 in order o deliver he same power o he generaor. Tis can easily resul in mechanical overload. A he same ime, urbo machinery in paricular is burdened wih naural requencies o he roor sysem ha limi is range in running speeds. Generaors suffer when running a low requencies because o he so-called magneic over fluxing, which resuls in possibly harmul overheaing. In pracice, he sel-regulaing power o a grid hardly offers any help in balancing.
P nominal per generator 500 MW, P =50 MW, 1%/Hz
self regulation of grid load
60
50 ) z H ( 40 y c n e u 30 q e r f d i 20 r G
Load with self regulation
Fixed load
10
0 0
25
50
75
100
125
150
Time (seconds)
Figure 2.7. Mitigation of the decrease in frequency due to self regulation of grid load in the case of a 50 MW initial unbalance between generator output and load.
2. Balancing the electricity supply in case of calamities
2.2.3. Explanation of the droop function of generator sets A decrease in grid requency rom 50 Hz o 40 Hz is excessive and unaccepable or many applicaions and generaors. Te maximum deviaion rom he nominal requency during dynamic evens, such as he loss o a generaor or he loss o major load is, hereore, se a +/– 800 mHz in he Coninenal Europe synchronous conrol area. I sel-regulaion o he load alone would be presen, his requency range would be heavily exceeded in he case o calamiies. Tereore, each elecriciy supply sysem has generaors offering primary conrol reserves ha are acivaed auomaically i he grid requency exceeds is ighly defined limis. Te change in oupu rom he primary conrol reserves depends on he exen o he deviaion in grid requency rom he nominal requency. In oher words, he desired oupu o a generaor acing as primary reserve depends on he acual grid requency; he lower he requency, he higher he oupu o he primary conrol reserves. Tis dependence o he oupu se poin on he grid requency is generally called he droop s generaor (Equaion 2.4). Te minus sign in equaion 2.4 indicaes ha a decrease ∆ in grid requency resuls in an increase ∆P in he power oupu rom he generaor providing primary reserves. P generator
1 =
s generator
–f P generator nominal f nominal
Equation 2.4.
Tis is ofen re-writen in erms o he regulaing power Pregulaing o he sysem: P generator
Pregulating f ,
= –
Equation 2.5.
in which: Pregulating
=
P generator nominal s generator f nominal
Equation 2.6.
Te droop value seting o he conrol sysem lies generally wihin a range o 2 o 8%. A droop s generaor o 4% means, or insance, ha he power oupu o he generaor increases rom 0% o 100% i he requency decreases by 4%, say rom 50 Hz o 48 Hz. Te relaionship beween oupu change and requency deviaion is linear. In measuremen and conrol echnology language, his is called a proporional acion, indicaed wih he leter P. I means ha he exra power rom he primary conrol reserves is only presen as long as he deviaion rom he desired nominal requency exiss.
2.2.4. The function of primary reserves As explained above, primary reserves are inended o avoid excessive deviaions in requency during a major occurrence affecing he balance in he elecriciy supply sysem. However, he primary reserves only arres he requency emporarily. Tey canno reurn he requency o is nominal value. Figure 2.8 illusraes how he power oupu
37
38 Power supply challenges rom a 500 MW generaor running a 250 MW, and acing as a primary reserve, varies wih requency or hree differen droop setings. For a grid requency o exacly 50Hz, he oupu o he generaor equals 250 MW. When he grid requency decreases, he oupu rom he generaor will increase linearly along wih i. Conversely, i he grid requency increases, he oupu rom he generaor will decrease by ollowing he droop line. Figure 2.8 reveals ha he lower he droop percenage is, he hefier he reacion o he generaor will be on deviaions rom he nominal grid requency. A firs sigh, oping or a very low droop value migh be he bes opion o keep he grid requency as close as possible o he desired 50 Hz. However, i he droop is very small, meaning ha he gain acor is very high, he primary reserves may reac fiercely o deviaions in requency. Tis resuls in exra wear o he machinery, while increasing he risk o oscillaions and sysem insabiliy. Some radiional power plans suffer heavily rom rapid changes in oupu. High-emperaure seam boilers can experience caviaion and hermal shock during sudden load changes. Moreover, all generaors acing as primar y conrol reserves do no have he same dynamic properies in pracice. Each uni has is own delay ime when reacing o an increase in he oupu se poin, and each uni will have is own ypical ramp up rae. Unil now, a close o sepwise increase in oupu was no possible. Neverheless, some modern generaing echniques can reac much aser han radiional unis. Te ypical ramp up rae or primary conrol reserves in he Coninenal Europe synchronous area sysem is 100% wihin 30 seconds (Figure 2.9). Afer a shor delay o, say 2 seconds, he primary conrol reserves are supposed o increase heir oupu a a fixed rae. 500 MW generator running at 250 MW 500
) 400 W M ( t u 300 p t u o r o 200 t a r e n e G 100
0 48.0
48.5
49.0
49.5
50.0
50.5
51.0
51.5
52.0
Grid frequency (Hz) 4% droop
3% droop
5% droop
Frequency limits
Figure 2.8. Output from a 500 MW generator acting as a primary control reserve, running at 250 MW at a nominal grid frequency of 50 Hz, with three different droop settings.
2. Balancing the electricity supply in case of calamities
100
) % ( 80 s e v r e s 60 e r y r a m 40 i r p t u p t u 20 O 0 0
5
10
15
20
25
30
35
Time after ramping up command (seconds)
Figure 2.9.
Typical ramp-up rate of primary control reserves.
Le us now reurn o our example o he elecriciy supply sysem illusraed in Figure 2.3, whereby en idenical generaors were each carrying a load o 450 MW when suddenly one generaor ripped. Wihou any acion being aken wih respec o he se poin o he prime mover ha drives each generaor, he grid requency would drop o 40 Hz, as shown in Figure 2.7. However, i each o he nine remaining generaors is also used parly or primary requency conrol, each wih a droop o 4% as depiced in Figure 2.10, he grid requency will no decrease all he way down o 40 Hz. Due o he droop seting, he se poin or ull oupu o each power plan will be reached already a a grid requency o 49.8Hz, as shown in Figure 2.10. Figure 2.6 reveals ha afer he coningency o one ailing power plan, his 49.8 Hz will already be reached in abou 0.5 seconds afer he rip o power plan number 7. Tereore, he new se poin ha asks or ull oupu o he generaors can be presumed o be presen almos immediaely afer he loss o one o he en generaors in he sysem o our example. However, nowihsanding he quick change in heir se poins, he nine power plans ha use heir addiional available capaciy or primary conrol reserves will no immediaely reach heir ull oupu. I we presume ha he oupu o hese power plans ollow he prescribed ramping up as shown in Figure 2.9, he load rom he grid and he power supplied o he generaors will equal each oher afer abou 25 seconds (see Figure 2.11). A ha poin, he grid requency reaches is minimum. According o Figure 2.9, he addiional 50 MW needed per generaor ha was los when one o he original en generaors ripped is ully available rom he primary reserves afer 30 seconds. Tis 50 MW per generaor is slighly more han he grid load requires or saying balanced a he menioned minimum in requency. Tis is because o he reducion in load creaed by he sel-regulaing power. Te excess
39
40 Power supply challenges energy delivered is hen used o accelerae again he roaing masses, he ‘flywheels’, in he sysem. Neverheless, he nominal 50 Hz requency canno be reached wih primary conrol reserves ollowing a droop line. In our example, as soon as he grid requency exceeds 49.8 Hz, he oupu o he primary reserves once again decreases (see Figure 2.10). Ta would creae anoher mismach beween power supply and load. Tereore, a deviaion beween he acual requency and he nominal grid requency o slighly less han 0.2 Hz will remain where only primary conrol reserves are used o resore balance. Tis deviaion beween he requency ulimaely reached wih he help o primary reserves and he desired requency o 50.000 Hz is called he quasi seady sae deviaion. ransmission sysem operaors define he permissible minimum and maximum o his deviaion. Exra capaciy, he so-called secondary conrol reserve, is needed or providing he exra power in he sysem so as o resore he grid requency o he required narrow band around he nominal requency. As menioned earlier, his narrow requency band equals only +/– 20 mHz around 50 Hz in he Coninenal Europe synchronous area. Te applicaion o secondary conrol reserves increases he requency urher, so ha he primary reserves can reduce heir oupu and reurn o heir iniial load seting a 50 Hz. In oher words, secondary reserves release he primary reserves rom heir duy. Tis enables primary reserves o be ready or he nex major occurrence, such as he sudden loss o a generaor or he losing or receiving o a large load. Te previous example whereby primary conrol reserves equalled exacly he amoun o los generaing capaciy, is obviously an excepion. In realiy, he power 500 MW generator running at 450 MW 500
) 400 W M ( t u 300 p t u o r o 200 t a r e n e G 100
0 48.0
48.5
49.0
49.5
50.0
50.5
51.0
51.5
52.0
Grid frequency (Hz)
Figure 2.10. The droop function of a generator having a 500 MW nominal power, again with a droop setting of 4%, but now running at 450 MW at 50 Hz.
2. Balancing the electricity supply in case of calamities
41
plans in a sysem do no all have he same Only self regulation capaciy. Primary reserves should be large Typical point for load shedding enough o compensae or a leas he loss With primary reserves activated o he larges power plan in a sysem. In ) 50 pracice, power plans no dedicaed as z H ( 48 primary reserves will also provide some y c n compensaing power or resoring he grid e 46 u q requency o is nominal value. Neverhe e r 44 f less, our example illusraes he way con d i r 42 ingencies are handled in a grid sysem. G ransmission sysem operaors 40 0 25 50 75 100 125 150 speciy he maximum allowed dip in Time (seconds) requency, officially called he minimum insananeous requency afer loss o generaion. In he Coninenal Europe Figure 2.11. Primary control reserves prevent the system synchronous area, his value equals 49.2 from decreasing too much in frequency after a loss in generHz. I he grid requency drops below his ating capacity (inertia constant 10 s, 1%/Hz self regulation, 50 MW unbalance for a generator with a nominal capacity of minimum allowed requency, load shed- 500 MW). ding will be used o avoid oo deep deviaions rom he nominal requency. Tis means ha a group o consumers will have no access o elecriciy or a while. Te example wih he 10 power plans o equal size, where one o he unis rips, shows ha losing 10% o generaing capaciy gives a requency dip way below 49.2 Hz. Hence, here should be enough power plans in a sysem o ensure ha he loss o one power plans does no reduce he online generaing capaciy by more han abou 3%. In paricular, sysems having a large racion o renewable elecriciy sources need dispachable power plans o a limied size, and consequenly more o hem han in he case o large power plans only.
2.2.5. The consequences of a lower inertia constant Should he ineria consan o he combined generaors in a sysem decrease, or example, due o he inroducion o a large amoun o renewable energy sources ha are indirecly conneced o he grid via requency converers, he grid requency can drop o quie low values during a coningency. Te red line in Figure 2.12 shows a deep dip in requency i he ineria consan o he sysem is 5 s insead o 10 s. Te oher condiions are he same as in Figure 2.11, wih 1%/Hz sel regulaion, 50 MW unbalance per generaor and a nominal generaor capaciy o 500 MW. Te 46 Hz minimum in he red curve occurs 18 seconds afer he calamiy ha ripped one o he 10 power plans in he example. A ha poin, he oupu o he primary reserves has no ye reached is maximum, since ha occurs 30 seconds afer he rip. Afer 18 seconds, he oupu o he primary reserve per generaor is hereore only 18/30 · 50 MW = 30 MW. However, he sel-regulaing power o he grid equals 4 · 1/100 · 500 MW = 20 MW or a requency drop o 4 Hz and a sel-
42 Power supply challenges
Parts of Manhattan were left in the dark after hurricane Sandy in 2012.
Figure 2.11.
regulaing power sensiiviy o he load o 1% per Hz. Tis means ha power demand and power supply are ully mached again a 46 Hz so ha he requency will no decrease urher. Te oupu o he primary reserves coninues o increase afer he minimum in requency has been reached. Tis addiional power supply will again accelerae he ineria. Tis will go aser or an ineria consan o 5 s han or one o 10 s, since less energy is needed o bring he roors back o nominal speed in case o lower ineria. Te deep dip in requency observed or he lower ineria value is unaccep-
Inertia constant 5 s
Inertia constant 10 s
51 ) z H ( y c n e u q e r f d i r G
50 49 48 47 46 45 0
25
50
75
100
125
150
Time (seconds)
Figure 2.12. The effect of lowering the inertia constant on the frequency dip in case of a calamity (further conditions as in Figure 2.11.).
2. Balancing the electricity supply in case of calamities
able is mos cases. I i is no possible o increase he ineria, he amoun o primary reserves has o be increased or he primary reserves have o be made aser wih a smaller iniial delay.
2.2.6. The effect on frequency deviations of more powerful or faster primary reserves. Increasing he amoun o primary conrol reserves and having a higher ramp rae in he reserves help miigae he effecs o disurbances. Boh measures will reduce he undesired deep dip in requency, and will shoren he ime needed o reurn o 50 Hz ollowing he loss o a generaor. Doubling he power capaciy o he primary conrol reserves, which resuls in wice as much capaciy as he los oupu o a ailing generaor in our previous example, reduces he dip in requency by almos a acor o wo. Tis is illusraed by he dark red line in figure 2.13. Te reason ha he reducion in dip is no exacly a acor o wo is ha he sel-regulaing power decreases he load o a lesser exen when grid requencies come closer o 50 Hz. Anoher observaion is ha wih a higher amoun o primary reserves, he ulimae quasi seady sae requency will be closer o 50 Hz han in he case where he primary reserves equal jus he loss in power rom he ripped generaor. Tis is because even above 48.8 Hz, where he oupu reducion o he primary reserves kicks in because o he chosen droop curve, more power han he power los rom he ailing generaor is available now. An addiional advanage o higher primary reserves is he shorer ime ha he grid requency subsanially deviaes rom he nominal requency o 50 Hz. Figure 2.13 clearly illusraes ha he cumulaive, or inegral, loss in grid requency over ime is much lower or he red line han or he blue line. Tis cumulaive loss is he area beween each curve and he 50 Hz line in figure 2.13. Te dimension o cumulaive loss is Hz s, because i is he resul o a deviaion in Hz during a given number o seconds. For he blue line, he cumulaive requency deviaion equals 146 Hz s. Doubling he primary reserves brings he cumulaive requency deviaion in he example o Figure 2.13 down o only 41 Hz s. Tereore, doubling he primary reserves decreases he cumulaive requency deviaion by a acor o 3.5 in his case. A grid operaor has o ensure ha, a he end o he day, he average requency is back o 50.000 Hz, o avoid or insance, deviaions o synchronous clocks. Tis means ha afer a requency dip, all generaors have o run a a higher requency han 50 Hz or a while o compensae or he dip. Wih a lower cumulaive requency deviaion, less effor is required o compensae or his deviaion. Should more primary reserve capaciy be made available han he nominal oupu o he larges generaor in a sysem in order o avoid excessive requency dips in case o coningencies, a larger amoun o generaing capaciy mus be run below nominal load. Tis has a negaive impac on capial coss, uel consumpion, and operaion and mainenance coss.
43
44 Power supply challenges Doubling he power-up ramp rae o he primary conrol reserves, rendering ull oupu in 15 s insead o 30 s, resuls in almos he same posiive effec on he requency dip as doubling he capaciy o primary conrol reserves. Tis is illusraed in figure 2.14. Te only sligh difference is ha he ulimae requency beore he secondary conrol reserves sep in will no be slighly above 48.8 Hz. Tis is he logical consequence o he 4% droop in he example: i he grid requency rises above 48.8 Hz, he power oupu o he primary conrol reserves auomaically decreases again. However, i he droop seting o he more agile primary conrol reserves is changed o 2%, he exac same posiive curve as or doubling he primary conrol reserves will resul. A major conclusion here is ha aser primary conrol reserves offer an effecive opion or reducing he requency deviaion caused by a coningency. I he relaive amoun o he sysem’s roaing ineria decreases, such as when much indirecly coupled renewable elecriciy sources ha do no add o he ineria are inroduced, he decline in requency afer a major loss in generaion will be aser. In ha case, more primary conrol reserves are required o keep he sysem sable, or he primary conrol reserves need o have higher ramping raes and a shorer iniial delay.
Primary reserves 2x lost generator output Typical point for load shedding Primary reserves equal to lost generator output 50.5 ) z H ( y c n e u q e r f d i r G
50.0 49.5 49.0 48.5 48.0 47.5 47.0 0
25
50
75
100
125
150
Time (seconds) Figure 2.13. Frequency dip in the case of primary reserves with two different power capacities, otherwise the same conditions as in Figure 2.11. apply. 30 s response time, double amount primary reserves
50
) z 49.5 H ( y c n e 49 u q e r f d i r 48.5 G
Only 15 s response time, minimum primary reserves
48 0
10
20
30
40
50
60
Time (seconds) Figure 2.14. The almost identical effect of a faster ramp-up rate of primary control reserves compared with a doubled primary control response capacity (otherwise same conditions as in Figure 2.13).
2.2.7. The solution for delivering faster primary reserves Nowadays, agile and flexible generaor ses are available ha have a very as response o sepwise changes in he desired power oupu. Such smar generaors can change a cerain amoun o heir oupu rapidly, wih a delay o less han 1 s, he acual response ime depending o some exen on heir running speed. Naurally, he allowed increase
2. Balancing the electricity supply in case of calamities
45
in oupu depends on he power oupu se Best in class response for poin beore he reques or a change. As primary frequency control Aeroderivative an example, a machine ha runs already a gas turbine 100 80% load can never accep a load increase Combustion engine o 40% o he nominal oupu. ) 95 t l u a Industrial gas turbine Figure 2.15 shows he response o a n p i t combined cycle 90 u m number o differen power generaing o o Steam-based r n e f 85 power plant echniques o a sepwise change in w o o desired oupu. Te daa are based on P % ( 80 bes-in-class machines. Less agile ypes 75 migh be slower by more han a acor o 0 10 20 30 wo. Te response curve or he combusTime (seconds) ion-engine-driven power plan applies or smar power generaion sysems based on urbocharged engines wih Figure 2.15. Response of different generating techniques to elecromagneic gas injecion valves per a stepwise change in the power output set point . cylinder. Faser primary reserves resul in smaller deviaions rom he nominal Synchronous area grid requency during calamiies, even where here is less ineria in he sysem. One imporan aspec o primary Control Control Block Block Control reserves is heir locaion in he sysem. area I a large power plan o 1500 MW ails, he primary reserves canno be locaed 1000 km away since i would cause he inerconnecing ransmission lines o overload. Tis is why large synchronous areas are spli up ino conrol blocks and conrol areas. Each conrol area should be able o resolve mos o he consequences o is own coningencies. Figure 2.16. Control areas and control blocks interconnected Neighbouring conrol areas in he same with high-voltage transmission lines in a synchronous area. block are allowed o offer some suppor, The blue blocks represent power plants. provided he inerconnecors can carry he load. As wih mos hings in lie, local problems should preerably be solved locally.
2.2.8. Conclusions regarding primary reserves and inertia levels Occurrences such as he loss o an acive power plan or he loss or arrival o a major load, will always creae a disurbance in requency. Te iniial change rae in requency is deermined by he size o he unbalance beween power supply and demand and by he ineria consan o he sysem. Primary conrol reserves, i.e. power plans running
46 Power supply challenges a par-load, adap heir oupu auomaically when he grid requency changes since heir desired oupu is deermined by he grid requency via heir droop curve. Primary reserves are preerably allocaed in such a way ha a conrol area can resolve a large par o is own coningencies. I is clear ha oping or jus a ew large power plans o supply he required elecriciy or an area is no ideal. In a sysem having a large number o generaors in a single conrol area, i is easier o compensae or he ailure o one generaor wih he oher generaors. Te oupu o an individual uni is hen jus a small racion o he combined oupu. Wih a large number o generaors acive as primary reserves, here is also no need o operae hem a a relaively low load. In addiion, he ailure o one o hem has only a minor effec on he combined reserve capaciy. In oher words, muliple generaors in a sysem improve he reliabiliy o primary reserves and reduce he impac o a ailing uni. Fas responding primary reserves can compensae or less roaing ineria wihou he need o having more primary reserves available. Wihou as primary reserves, more power plan capaciy has o operae a par-load, i.e. below is raed oupu. Operaing a par-load increases he uel consumpion (MJ/kWh), as well as he capial and mainenance coss per kWh (Figure 2.17). unning a generaing uni a 50% load doubles he mainenance and capial coss per kWh.
2.3. Secondary and tertiary control reserves Afer acivaion o he primary conrol reserves, secondary and eriary reserves (also known as requency resoraion reserves (F), and replacemen reserves (), respecively) are needed. Tere are wo reasons or his. Firsly, when primary reserves
4.5
8.0
n o i t p m u s ) n h o c W / l k e j u ( f M c fi i c e p S
7.5
4.0
7.0
3.5
6.5
3.0
6.0
2.5
5.5
2.0
5.0
1.5 100
150
200
250
300
350
e c n a n e t n ) h i a W k m / s + t l c a t ( i s p t s a c o c c fi i c e p S
400
Load (MW)
Figure 2.17. Example of fuel consumption, and capital and maintenance costs of a 400 MW power plant according to load.
2. Balancing the electricity supply in case of calamities
have been ully acivaed, no spare requency conrol capaciy is available or anoher ripping power plan or a loss o major load. Te risk o anoher power plan ailing or load disurbances aking place is always higher during a major even in he sysem han when everyhing is running smoohly. Secondly, due o he droop characerisic o he deployed primary reserves, a deviaion rom he nominal requency o 50 Hz will remain. Acivaion o he secondary conrol reserves will supply exra power o he sysem so ha he grid requency can reurn o is nominal value. As a consequence, he droop-based primary conrol reserves will auomaically reurn o heir original se poin and be released rom heir acion unil he nex disurbance occurs. In he Coninenal Europe synchronous area, he secondary conrol reserves in he relevan conrol area auomaically commence delivering oupu wihin 30 seconds ollowing a major disurbing occurrence. Tis happens when he primary conrol reserves are ully acive. Afer 15 minues, he ull capaciy o he secondary conrol reserves has o deliver is power o he sysem. Tis approach is illusraed in figure 2.18. Unil recenly, such a shor deploymen ime required all secondary conrol reserves o be spinning all he ime. Large power plans can never provide ull oupu rom sandsill wihin 15 minues. As a consequence, secondary reserves based on such power plans would always be running a a level below heir nominal oupu. Tis again causes higher uel consumpion and higher capial and operaional coss.
Secondary reserves 15 min
Delay 30 sec
50.000 Hz reference
f +
Dead band
Ramp up
30 sec
Power output
–
Ramp up
Actual grid frequency f
+ P
P
–
Other generators
System dynamics System load + loss
Figure 2.18. A possible setup for primary control reserves and secondary control reserves in the system.
47
48 Power supply challenges
2.3.1. Engine-driven power plants can provide secondary reserves from standstill Nowadays some engine-driven power plans are able o deliver ull oupu rom sandsill wihin abou 5 minues (Figure 2.19). Tis opens up possibiliies or non-spinning secondary conrol reserves. eaching ull speed ollowing he sar command and reaching synchronisaion wih he grid akes abou 30 seconds or such plans. Tis more han complies wih he Coninenal Europe synchronous area’s requiremen or secondary reserves o sar 30 seconds afer he even ha riggered he primary reserves, and o ramp up o ull oupu wihin 15 minues. Te abiliy o reach ull oupu in 5 minues is aser by a acor o 3 han wha is 100 d a ) Running speed Power output demanded in he Coninenal Europe syn o e l u 80 d l chronous area. n a v a l A quick-saring power plan has o d a 60 n e i e be consanly preheaed o provide he p m s o 40 n g f as perormance shown in figure 2.19. n i o n 20 However, such power plans generally n % u ( R consis o muliple idenical generaors 0 0 60 120 180 240 300 360 420 operaing in parallel. unning one o Time from start command (seconds) he muliple unis online or elecrical energy producion releases sufficien hea o keep a leas 30 idenical Figure 2.19. The fast ramping up in power output from generaing unis preheaed. Anoher standstill to full load of a smart power generator. advanage o having muliple secondary conrol reserve unis in parallel is he low risk o losing much allocaed reserve capaciy. In case a single uni ails, only a racion o he allocaed power or secondary reserves is los. In some conrol areas in compeiive markes, power plan operaors can bid in he ahead markes or offering secondary reserves. Te power plans offering he lowes price or heir service will normally be seleced o provide he reserves. In oher sysems, he power balance in he relevan area is coninuously measured wih energy flow meers, and secondary balancing is acivaed by a compuerised conrol sysem ha sends ou se-poin changes o seleced power plans. In hese cases, as non-spinning secondary reserves also offer subsanial advanages. Non-spinning means using no uel, suffering no wear, and producing no emissions. eriary conrol reserves are acivaed o ree he secondary reserves or he nex coningency. Tis can be done auomaically (direcly acivaed) or manually (schedule acivaed) by he ransmission sysem operaor. Par o he eriary conrol reserves can be non-spinning. Tis is possible when he power plan has he rapid response capabiliy as depiced in figure 2.19.
2. Balancing the electricity supply in case of calamities
Combined output 100
Primary reserves 80
Scheduled tertiary reserves
s f e o ) v r t s e u e s p v 60 r e r t e s t u u o e p k r t a y u e r o p a 40 r f i m e o r w p o % ( P
Secondary reserves
Direct tertiary reserves
20
0 0
10
20
30
40
50
60
Time (minutes)
Figure 2.20 Example of the sequence in utilising primary, secondary, and tertiary control reserves following a major occurrence.
2.4. Conclusions Tis chaper has explained he delicae balance beween elecriciy generaion and demand and he consequences o rapid disurbances in he sysem. oaing ineria and he dedicaed conrol reserves play an imporan role in keeping he sysem balanced. Wih he inroducion o a subsanial amoun o renewable elecriciy sources, balancing becomes more challenging. Agile, as-reacing generaors appear o offer excellen balancing duies during coningencies, even in he case o less ineria and reserve capaciy in he sysem.
49
3 Balancing power demand and supply when conditions change
Te demand or power changes coninuously according o weaher and human behaviour. Tereore, he combined oupu rom elecriciy generaors in a synchronous sysem has o vary all he ime o mach he changing demand. Te inroducion o variable wind and solar energy creaes addiional dynamics in maching power demand and supply. Counry cases are used in his chaper o explain he challenges o his balancing ac.
52 Power supply challenges
3.1. Electricity supply differs depending on local situations Firs o our counry examples is Finland. I represens a counry wih a relaively seady power demand and hardly any variable renewable power. As an indusrialised counry in a cold climae, Finland shows a demand patern compleely differen rom ha o a counry wih ho summers or a service-based economy. Ireland serves as an example o a service-based economy wih an increasing amoun o wind power capaciy. Germany is an indusrialised counry wih large amouns o boh wind and solar power. exas is a sae wih ho summers and subsanial poenial or solar and wind power. Finally, Caliornia, a sae wih high number o solar insallaions – and much more o come – is discussed briefly.
3.2. Power demand pattern in Finland exemplifying an industrialised nation Finland is a relaively sparsely populaed Norhern European counry ha has considerable energy-based indusry, such as paper mills and smelers. Winers can be severely cold. Elecriciy is he mos common heaing sysem or homes, albei nowadays ofen wih energy-efficien hea pumps. Larger ciies use disric heaing based on he wase hea rom power plans. Finland has no indigenous gas reserves and he low populaion densiy ouside he major ciies excludes building an exensive gas disribuion sysem. Finland has some 5.4 million inhabians, wih an average annual elecriciy consumpion o 15.7 MWh/capia, he highes in he world apar rom Norway wih 23.2 MW h/capia (daa IEA, year 2011). anking hird and ourh in annual elecriciy consumpion per capia are Canada wih 15.7 MWh and he USA wih
15 000
d n a l n 12 500 i F d n a 10 000 m e d ) r W 7 500 e M ( w o p 5 000 a t a d y 2 500 l r u o H 0
Midsummer celebration
0
5
10
15
20
25
30
35
40
45
50
Week numbers year 2012
Figure 3.1.
Hourly data of electric power demand in Finland for the year 2012 (hourly data from ENTSO-E).
3. Balancing power demand and supply when conditions change
13.4 MWh. By comparison, he world average in annual elecriciy consumpion per capia is only 2.9 MWh. Figure 3.1 shows how elecriciy demand in Finland varied during he year 2012. Te curve is blurred because o daily variaions in elecriciy demand. An anomaly in he curve is he deep dip in power demand in he las week o June. Tis is caused by he amous Midsummer Day celebraions. Only pubs, resaurans, and hospials are uncioning hen. A he end o he year, beween Chrismas and he New Year, mos indusrial and commercial aciviies are also down. A ha ime o he year, Finns are ypically all eaing ham and drinking glögi, a spicy glühwein (mulled wine). Figure 3.1 reveals ha Finland had a minimum elecriciy demand o abou 6.5 GW in 2012. Finland has a policy aimed a reducing greenhouse gas emissions and decreasing he counry’s dependence on ossil uels. Te plan is o reduce greenhouse emissions by 80% o he 1990 level by he year 2050. Biomass, wind power and geohermal hea should be he major sources o renewable energy. Because o he high baseload level, he Finnish auhoriies have decided o suppor building addiional nuclear power plans. Nuclear power will replace some old coal-fired power plans and pro vide addiional capaciy o saisy he growing demand or elecriciy. Nuclear power plans have low uel coss and very low carbon dioxide emissions. Te high invesmen coss, however, require running hem or a large par o he ime a ull oupu. Te Olkiluoo 3 power plan, which has been under consrucion since 2005, will have a power capaciy o 1600 MW and he planned Olkiluoo 4 plan a capaciy o beween 1000 and 1800 MW. Te discussions abou primary and secondary reserves in Chaper 2 made clear ha insalling such large power plans requires a subsanial amoun o primary, secondary and eriary conrol reserves or mainaining sabiliy in case o a rip. Te hourly daa o elecric power demand in Finland, as shown in Figure 3.1 is ploted in a disribuion curve in figure 3.2. Such a disribuion curve is based on a rearranged daa series, saring wih he highes value and ending wih he lowes. I gives a good indicaion as o wha racion o he oal ime a cerain power demand is required. Apparenly, he power demand was higher han 6.5 GW or 99% o he ime. Demand exceeded 10 GW or only 40% o he ime. Peaks in demand higher han 12 GW occurred only 10% o he ime. Tereore, power plans responsible or generaing such peaks in demand will have a very low uilisaion acor. A uilisaion acor o 1 (= 100%) means ha he power plan is producing is nominal oupu coninuously hroughou he year. In pracice, however, power plans require mainenance and hey someimes rip. In addiion, hey have o provide reserve capaciy. A uilisaion acor o 0.9 or a baseload power plan is, hereore, already quie good. Te shor-duraion absolue peak in demand rom 14.0 GW o 14.5 GW shown in Figure 3.2 occurs only in he case o exremely cold weaher. Te capial coss per kWh o power plans providing such a peak are very high. I would be beter o offer Finns a good price or reducing demand during very cold days. Maximising he use o available wood burners and avoiding heaing elecric saunas would reduce elecriciy consumpion during spells o very cold weaher.
53
54 Power supply challenges
15 000
d n a m 12 500 e d r e ) 10 000 w o W p M ( e d 7 500 v r n a u l c n n i o F 5 000 i t u b i r 2 500 t s i D
P > 12 GW, 10% of the time P > 10 GW, 40% of the time
Base load 6.5 GW 99% of the time
0 0
10
20
30
40
50
60
70
80
90
100
% of the time, year 2012
Figure 3.2. Power demand in Finland distributed over the 8784 hours of the year 2012, with maximum demand on the left-hand side and minimum demand on the right.
Te dynamics in elecriciy demand in Finland are such ha convenional power plans, in combinaion wih impors and expors, can easily handle hem. I is no expeced ha Finland will insall many solar panels. However, here are plans o insall 3 GW o wind urbines. Using biomass as a renewable uel is a good opion, since here are an esimaed 5 000 o 10 000 rees per capia in he counry. Biomass based elecriciy producion is conrollable and can, hereore, be seen as a dispachable supply source or he elecriciy grid. Figure 3.3 combines he daily consumpion o elecrical energy (GWh) and he daily maximum and minimum emperaures a Helsinki–Vanaa airpor in 2012. Low emperaures subsanially increase he use o elecriciy. On February 3, he coldes day o he year, he minimum emperaure min a Helsinki airpor was –31 °C and elecriciy consumpion reached he absolue peak o 330 GWh. Demand varied ha day beween 12.5 and 14.5 GW. Alhough here is a relaionship beween elecriciy consumpion and he number o dayligh hours, space heaing is primarily responsible or he high peak demand or elecriciy in Finland. A cold spell in he beginning o December 2012 also creaed a peak in elecriciy demand. Figure 3.4 relaes he use o elecriciy o he average daily emperaure deermined rom ( T max +T min)/2 as given Typical Finnish landscape.
3. Balancing power demand and supply when conditions change
Year 2012 February 3 350
) h W G ( e s u y t i c i r t c e l e y l i a d d n a l n i F
35
Tmax (deg C) 300
25
250
15
200
5
Electricity use
150
–5
100
–15
Tmin (deg C)
50
–25
0
–35 1
Figure 3.3.
29
57
85
11 3
14 1
169
1 97
2 25
2 53
2 81
3 09
3 37
3 65
Daily electricity use and daily temperatures in Finland in 2012.
in Figure 3.3. Te relaionship proves o be quie significan, winess he high correlaion coefficien o 0.85. I shows ha a variaion o beween 175 and 325 GWh in he daily elecriciy consumpion in Finland relaes o he weaher. Te peculiar circular deviaion in Figure 3.4 a he minimum elecriciy consumpion o close o 150 GWh per day is again caused by he Midsummer celebraions. Figure 3.5 illusraes he elecriciy demand patern based on he hourly daa rom he week commencing Ocober 8, 2012, when he weaher was sill no exremely cold and he holiday season is over. Te baseload o abou 8 GW ) deermines he bulk o demand during h 350 W ha whole week, confirming ha con G ( 300 e inuous indusrial aciviies dominae s u 250 y elecriciy use. Comparing he patern t i Black trend line c i y = –3.6804x + 250.61 200 or week days wih ha o weekends r t R = 0.8529 c e reveals ha only par o he inerme l e 150 y diae load is relaed o work aciviies. In l i a 100 d paricular, he peaks a he end o every d 50 n day are caused by domesic aciviies. a l n i ypical Friday evening and Saurday F 0 –30 –20 –10 0 10 20 30 evening peaks are caused by he almos Average temperature Helsinki-Vantaa airport (°C) compulsory weekly amily sauna. Tere are around 3 million saunas in Finland and he majoriy o hem are elecrically Figure 3.4. Correlation of the average daily temperature at Helsinki airport with daily electricity use. heaed. 2
55
56 Power supply challenges Figure 3.6 gives he power demand patern or Monday Ocober 8, 2012. I reveals ha Finns sar heir aciviies early in he morning and go o bed early in he evening. Alhough an increase in demand rom 8 GW o 10 GW beween 7 am and 9 am looks small in he diagram, i is sill a subsanial amoun o addiional power. Te maximum ramp-up rae in power demand is abou 1 GW/hour. Saring already rom noon, he power demand slowly decreases on weekdays. Te inermediae load level beween 7 am and 8 pm covers a coninuous ime span o 15 hours. Te demand patern in Figure 3.6 does no show subsanial peaks. In conclusion, Finland is a counry wih a relaively high season-dependen baseload. Te daily variaions in elecriciy demand are abou 10% o he average demand. Daily cycling in demand is quie predicable and he ramping up and ramping down can be easily covered by early preparaion o he power plans. Finland has abou 3.3 GW o hydropower, which helps in shaving he peaks and in ramping elecriciy producion up and down.
3.3. Power demand in the Republic of Ireland, exemplifying a system with much wind-based power Ireland has been chosen as an example because i has a noable amoun o wind power in is elecriciy supply sysem. Like Finland, Ireland is a counry wih a relaively small number o inhabians, bu i is represenaive o hose regions wih a moderae climae and a limied amoun o indusry. An ineresing aspec is ha Ireland aims o insall subsanially more wind
Modern Finnish sauna.
Week 41, 2012 12 000
d n 10 000 a l n i F 8 000 d ) n W a 6 000 M m e ( d 4 000 r e w o 2 000 P 0 Mo
Tu
We
Th
Fr
Sa
Su
Figure 3.5. Electric power demand in Finland during the week of October 8–14, 2012 (hourly data from ENTSO-E).
Monday, October 8, 2012 d 12 000 n a 10 000 l n i F 8 000 d ) n W a 6 000 M m e ( d 4 000 r e 2 000 w o P 0 0
2
4
6
8
10
12
14
16
18
20
22
24
Hour of the day Figure 3.6. Electric power demand in Finland on Monday, October 8, 2012.
3. Balancing power demand and supply when conditions change
57
power. A arge o beween 5.5 and 6.3 GWh is menioned in he Irish Power plant capacity Finland in 2010: 17 GW Gae 3 projec or covering ulimaely 40% o he elecriciy demand rom renewables by he year 2040. Te counry has some 4.5 million Renewables inhabians and an elecriciy consumpion o 4.5 MWh per capia. and peat Figure 3.8 shows ha he dynamics in elecriciy demand in Ireland 15% are proporionally much higher han hose in Finland. Tis is ypical Fossil Nuclear fuels or a service-based economy. Te baseload is only abou 2 GW, com16% 49% pared wih a leas 7 GW in Finland. Te daily variaions in demand are also close o 2 GW, abou he same as in Finland. Te peaks and valHydro 20% leys in elecriciy demand appear o ollow a well defined patern. Te only anomaly, i.e. he dip in demand a he end o he year, is apparenly caused by Chrismas and he relaed holiday week. Te disribuion curve o power demand in Ireland (Figure 3.9) Figure 3.7. Finland has a substantial shows ha he peak beween 4 GW and 4.6 GW occurs or less han amount of hydropower, which creates 2.5% o he ime and is, hereore, very uneconomic rom a view- flexibility. poin o capial coss or he insalled generaing capaciy. A base demand o almos 2 GW means ha having jus a ew large power plans o cover his baseload creaes a high risk o blackous in case o a calamiy. Figure 3.10 gives he power plan porolio in Ireland or he year 2010. Te oal generaing capaciy slighly exceeds 6.2 GW, which is enough o cover he maximum demand. Seven gas-fired power plans dominae he porolio. Te larges power plan has a capaciy o 463 MW. I ha one rips during he nigh and is ull oupu is los, abou a quarer o he 2 GW online generaing capaciy disappears. I is close o impossible o correc such a coningency wih primary conrol reserves. Te addiion o flexible power plans based on muliple unis in parallel wih an inrinsically quick response o demand changes migh offer a soluion here. Power demand in Ireland is clearly seasonal, wih he dark seasons requiring more power han when here is more ligh. Home heaing in Ireland is radiionally based ) 5 000 W M ( d 4 000 n a l e r 3 000 I d n a m 2 000 e d r e 1 000 w o P 0
0
5
10
15
20
25
30
35
40
45
50
Weeks year 2012
Figure 3.8.
Power demand in the Republic of Ireland in 2012 (data from Eirgrid)
58 Power supply challenges on oil, alhough naural gas is replacing oil in some areas. Te Gul Sream in he ) 5 000 W Alanic Ocean has a moderaing effec on P > 4 GW, 2.5% of the time M ( 4 000 d he climae. Te average daily emperaure n a l varies beween 5 and 16 °C hroughou e r 3 000 I he year, a much smaller range han he d n a 2 000 –20 o +20 °C in Finland. Base demand ≈ 2GW m e d Daily power demand variaions in r 1 000 e Ireland have he same characerisics as w o P 0 in oher counries having service econo0 10 20 30 40 50 60 70 80 90 100 mies and moderae climaes, such as Bel% of the time gium and Te Neherlands. Demand is always lower on Saurdays and Sundays han i is on weekdays. Figure 3.9. Distribution curve of the power demand in the On a weekday, power demand sars Republic of Ireland during 2012. o rise rom around 6 am and reaches a seady level by abou 10 pm. Te black curve in Figure 3.13 shows he demand patern on Monday February 27 only. Tis was a day wih high elecriciy demand. In Figure 3.11 he red curve shows ha he maximum rise in power demand per hal hour was abou 260 MW beween 7 am and 8 am. A slighly smaller rise rae in power demand occurred around 7 pm, when people arrive home, swich on he lighs and prepare heir evening meal. Te ases ramping down ook place beween 10 pm and 11 pm, a a rae o 200 MW/30 min. Wih Alanic Ocean winds, Ireland is suiable or wind urbines, especially a is wes coas. Te amoun o insalled wind urbine capaciy in 2012 averaged approximaely 1650 MW. Te combined power oupu rom Irish wind urbines shows high levels o variabiliy, nowihsanding he ac ha he capaciy is spread over some 125 wind ) W 500 M ( 450 y t i 400 c a 350 p a 300 c r 250 e w 200 o p 150 l a 100 n i 50 m o 0 N
Irish fuel-based electricity generation capacity (MW)
Gas Gas Gas Gas Gas Gas Peat
l i O e t a l l i t s i D
Reeks1
258
Figure 3.10.
90
90
90
432 403 118 111
l i O l e u F y v a e H
l i O l e u F y v a e H
54
54
l i O l e u F y v a e H
Gas Gas Peat Gas Coal Coal Coal Gas Gas Gas
108 342 400
l i O e t a l l i t s i D
91
85
283 283 283 163 104 463
52
l i O e t a l l i t s i
Gas Gas
D
52
The fuel-based power plant portfolio in Ireland (year 2010).
81
81
l i O l e u F y v a e H
l i O l e u F y v a e H
l i O l e u F y v a e H
l i O l e u F y v a e H
54
54
241 241 52
l i O e t a l l i t s i D
l i O e t a l l i t s i
Gas Peat Gas
D
52
384 137 445
3. Balancing power demand and supply when conditions change
parks. Figure 3.14 reveals ha he power oupu rom he combined wind urbines varied beween 1 500 MW and zero, wih a endency owards less energy producion during he middle par o he year. Ye, in 2012 oupu equal o he insalled power o 1650 MW was never reached. Alhough he variabiliy in wind power oupu is subsanial, he higher oupu during he darker seasons when more power is needed han in he summer is a posiive hing. Neverheless, he com bined oupu rom he wind urbines is also ofen close o zero. Figure 3.16 gives he cumulaive elecriciy producion o he Irish wind parks or 2012. Te inclinaion o he curve is lower in he middle o he year han early and lae in he year. Tis confirms he lower elecrical energy producion rom wind power in he summer ime. Te oal amoun o elecriciy produced by he combined windmills during he leap year 2012 was 4.1 PWh. A leap year has 8 784 hours and he average insalled wind ur bine capaciy was 1650 MW. Tis renders a capaciy acor o 4100000 MWh/(1 650 MW · 8784 h) = 0.28, or 28%. Tis is quie good or wind energy. In comparison, he capaciy acor o he German wind parks was jus 17.5% in 2012. oal elecriciy demand in Ireland was 25.6 PWh = 25600 GWh in 2012, so he wind urbines covered 4.1/25.6 · 100% = 16% o demand. Tis, however, does no mean ha less power capaciy rom he oher generaors was required, since here are imes when he combined windmills have hardly any oupu a all. Figure 3.16 shows ha he wind-based power oupu was lower han 165 MW. i.e. 10% o he insalled capaciy, during 23% o he ime, or or 84 days, in 2012. Tis means ha
s r 20 u o h t h g15 i l y a d d10 n a s e r u 5 t a r e p m 0 e T
59
3.0 ) 2.5 2.0 1.5
0
60 120 180 240 300 January 1 – December 31, 2012
Min temp (°C)
Max temp (°C)
1.0 360
h W T ( e s u y t i c i r t c e l e y l h t n o M
Electricity use Daylight hours
Figure 3.11. Monthly electricity use in Ireland compared with the number of daylight hours and the average minimum and maximum temperatures at Dublin airport. ) W5 000 M ( d4 000 n a m e3 000 d r e w2 000 o p d1 000 n a l e 0 r I
February 27 – March 4, 2012
Figure 3.12. The electricity demand pattern in Ireland from Monday February 27 to Sunday March 4, 2012. ) W M ( 4 000 d n a 3 500 m e d 3 000 r e w2 500 o p d 2 000 n a l e r I
300 150 0
Ramping up 260 MW/30 min
–150 –300
0
2
4
6
8
10 12
) n i m 0 3 / W M ( t d / P d
14 16 18 20 22 24
Hour of February 27, 2012
Figure 3.13. Electricity demand pattern (black) in Ireland on February 27, 2012, with the half-hourly change rate dP/dt in demand (red).
60 Power supply challenges
Year 2012, 1 650 MW wind turbine capacity in Ireland t u 1 500 p t u 1 250 o r e 1 000 w o ) p W 750 e M n ( i 500 b r u t 250 d n i 0 W
0
5
10
15
20
25
30
35
40
45
50
Weeks of 2012 Figure 3.14. Eirgrid).
The combined output from wind turbines in Ireland during 2012 (data from
uel-based capaciy was largely responsible or covering demand during ha racion o he ime. Te oupu rom wind urbines exceeded 1 000 MW or only 10% o he ime, or 36 days. Te insalled uel-based elecriciy generaing capaciy in Ireland is 6.2 GW, which would render a capaciy acor o 25 600/(6.2 · 8784) = 47% i he power plans had o produce he ull 25.6 PWh in demand. Because wind produced 4.1 GWh, he power plans delivered only 21.5 GWh. Teir acual capaciy acor was hereore 39%. Te blue curve in Figure 3.17 shows ha due o wind power, he baseload or he power plans is some 1.5 GW, considerably lower han he 2 GW
Figure 3.15.
Ireland plans to double the amount of wind energy by 2020.
3. Balancing power demand and supply when conditions change
61
base demand. Ye, he peak oupu rom Wind power output distributed curve for 2012 power plans sill equals peak demand: 1 650 MW installed, capacity factor 28% 4.6 GW. Since he same amoun o power t 1 500 Power only 10% of u the time > 1 000 MW p plan capaciy is needed as when here is t 1 250 u o no wind power, he consequence is ha r e 1 000 he uilisaion acor o he power plans w o ) p W750 decreases by a acor o 47/39 = 1.2. Tis Power 23% of e M n ( the time < 165 MW i means ha he capial coss or he power b r 500 u t plans per kWh produced are 20% higher d 250 n han in he case o no wind power. i W 0 Te ac ha wind urbines produced 0 10 20 30 40 50 60 70 80 90 100 16% o he elecrical energy needs in Ire% of the time land in 2012 does no mean ha he uel consumpion or covering he elecriciy Figure 3.16. Distribution curve of the output from the comneeds was 16% lower han i here were bined wind turbines in Ireland during the 8 784 hours of 2012. no wind power. Because o he volaile and unpredicable characer o wind Ireland 2012: effect of wind power output on power plant output distribution oupu, uel-based power plans will, on 5 000 average, run a a relaively lower rac ) 4 000 ion o heir nominal capaciy han hey Power demand W would i here were no wind urbines M ( 3 000 r Power plant output e in he sysem. Te consequence o his w 2 000 o is higher uel consumpion and higher P 1 000 mainenance and operaional coss per 0 kWh. Te 2012 load-ollowing patern o 0 10 20 30 40 50 60 70 80 90 100 he power plans is shown in figure 3.18. % of time Tis is a compleely differen picure han he power demand curve in Figure 3.8, Figure 3.17. Comparison of the distribution curves of power since he variabiliy is much higher. Ire- demand and demand minus the contribution from wind turbines. land’s ypical base demand o 2 GW has ranslaed ino a base oupu o only 1.5 GW rom he power plans. Consequenly, many more sars and sops and higher ramping up and ramping down raes occur or he power plans ha provide inermediae and peak load han wihou wind power in he sysem. Tis variabiliy increases he wear rae o he power plans. o illusrae he level o increased volailiy imposed on power plans due o wind energy, he change in power demand per hal hour and he change in power supply rom he uel-based generaors have been ploted in figure 3.19. A firs sigh, he difference beween he wo diagrams is no so large. Boh diagrams are sill dominaed by he weekly demand paterns. However, one can noice ha he maximum in change rae or he power plans is 450 MW/30 min, while power demand has a maximum change rae o 350 MW/30 min. amping up he elecriciy supply is in boh cases higher han ramping down. However, since less uel-based power plan capaciy is online because o he oupu rom wind power, he proporion o volailiy
62 Power supply challenges
Power demand minus the output from wind turbines 5 000 s t n a l p 4 000 r e w 3 000 o ) p W y M l p ( 2 000 p u s r e 1 000 w o P 0 0
5
10
15
20
25
30
35
40
45
50
Weeks of 2012
Figure 3.18. Half-hourly values of the power demand minus the wind power output during 2012, representing the contribution from non-wind power plants to power demand.
per he level o power plan oupu is much higher. Neverheless, deriving jus 16% o he elecrical energy rom wind power having a capaciy acor o 28% does no creae serious balancing problems. When he oupu o power plans has o increase 300 MW wihin a ime span o hal an hour, i makes considerable difference when only 1000 MW is online han when 2000 MW is online. eserve capaciy or coningencies and orecasing errors will help in ramping up oupu, bu where only litle power plan capaciy is online in cases where as ramping up is needed, his hardly helps. Wihou wind urbines, he change in power demand rom power plans would never have had ramping up 1 vertical unit = 200 MW/30 min Demand s e g ) n n a i h m c y 0 l r 3 / u W o h - M f l ( a H
Power plants
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
January 2012 Figure 3.19. Comparison of the half-hourly change rates in power demand and supply from the fuel-based power plants (situation year 2012).
3. Balancing power demand and supply when conditions change
63
Double wind power compared with 2012 5 000 4 000 m o s r f t 3 000 y n l a ) l p p p 2 000 r W u M s e ( r w 1 000 e o p w o 0 P –1 000 0
5
10
15
20
25
30
35
40
45
50
Weeks of 2012
Figure 3.20. Half-hourly values of the calculated power supply from Irish power plants with double the wind power capacity of 2012.
values higher han 14.5% o he online power-plan capaciy per 30 minues in 2012. amping down would never have been aser han 9.6% o he online power-plan capaciy per 30 minues. For he wind power capaciy in 2012, he power plan ramping up showed a maximum o 23% o he online power-plan capaciy and ramping down 16.3%. However, or a doubling o wind power, he power plans will ofen be urher pushed rom he grid. In ha case, he relaive changes in power requesed rom hem will be high, exceeding 500% o he online capaciy. Consequenly, a limied amoun o wind power in a sysem is acors easier o accommodae han a large racion. In he case o a doubling o wind urbine capaciy in Ireland compared wih ha o 2012, he hal-hourly changes in power supply or he uel-based generaors would occasionally exceed 550 MW. Figure 3.20 illusraes ha he backup patern o he Effect of doubling wind power output on power plant output distribution uel-based power plans would be oally 5 000 differen rom he power demand patern 4 000 shown in figure 3.8. Te baseload par ) Power W demand or he power plans almos disappears. 3 000 M ( Power plant r Tis disappearance is a consequence o output e w2 000 deriving more han 25% o he elecrical o Demand P doubled wind energy rom wind. Te power oupu 1 000 rom double he amoun o wind urbines 0 would exceed demand a leas 15 imes 0 10 20 30 40 50 60 70 80 90 100 per year. % of the time Peak demand rom uel-based power capaciy does no, in pracice, decrease Figure 3.21. Distribution curves for 2012 showing the impact when he insalled capaciy o wind ur- of wind power output on the annual distribution of fuel-based bines increases rom 1.65 GW o 3.3 GW, power plant output.
64 Power supply challenges
Table 3.1. Effect of wind power on fuel-based power plants in Ireland, based on year 2012 data. Power plant max output
Power plant min output
Power plant average output
Power plant crest factor
Fraction demand covered by wind
Utilisation factor power plants
MW
MW
MW
max/ average
%
%
No wind power
4588
1624
2917
1.57
0
47
2012 wind power
4446
913
2450
1.81
16
39
Doubled 2012 wind power
4384
–207
1980
2.21
32
32
Situation
winess figure 3.21. In paricular, he relaive volailiy in oupu o he uel-based power plans will increase and many more sars and sops will be experienced. In addiion, he capaciy acor o he exising power plans will decrease urher o 32%. able 3.1 summarises he effecs o wind power on he uel-based power plans in Ireland. Te hree scenarios o no wind urbines, he 2012 insalled power o wind urbines, and a doubling o he 2012 insalled power o wind urbines are shown. Te power plan cres acor is he peak load o he power plans divided by heir average load. Te cres acor is an indicaion o he volailiy in oupu ha he power plans experience. Te minimum power plan oupu can never be less han zero; he ‘– 207 MW’ in he hird column o able 3.1 only indicaes ha he wind urbines will occasionally produce more elecriciy han is requesed by demand. Beore he inroducion o large levels o wind and solar-based elecriciy supply, he convenional power plans in Ireland were generally able o ollow demand because o is high predicabiliy. Sophisicaed models could predic he load quie well based on hisoric daa 42.23 and weaher orecasing. Local winds 40 and sunshine are ar harder o predic, ) and even orecasing he average wind % ( 30 n 20.67 speed or a counry such as Ireland does o 20.29 i t 20 u no always give accurae resuls. Figure b i r t 3.22 gives he deviaion beween he s 6.94 i 10 5.12 D 2.43 orecas or he oal wind power oupu 0.89 0.80 0.52 0.13 in Ireland and he acual oupu, based 0 –550 –450 –350 –250 –150 –50 50 150 250 350 450 550 on daa rom Eirgrid or he year 2012. Wind power forecast minus actual wind power (MW) Te deviaions have been arranged in 11 classes o 100 MW each (Figure 3.22). In 42% o he hal-hourly daa, Figure 3.22. Distribution of differences between day-ahead he deviaions amouned o beween +50 forecasted wind power and actual wind-power output in Ireland MW and –50 MW. For almos 7% o he for 2012 (classes of 100 MW).
3. Balancing power demand and supply when conditions change
65
ime, he orecas is beween 150 and 250 MW oo high. Tere are even a ew cases t 1 600 u where he orecas exceeds he acual y=0.8676x + 50.291 p 1 400 t u r 2 =0.8975 wind oupu by more han 450 MW. o 1 200 r e 1 000 Tis means ha addiional generaing ) w o W p 800 capaciy has o be reserved o fill he gap. d M ( n 600 i Tis reserve capaciy has o be as, pro w l 400 viding oupu changes o beween + and a u t 200 c –100 MW in 30 minues, wih someimes A 0 exremes o –550 MW and +350 MW. 0 200 400 600 800 1 000 1 200 1 400 1 600 Such excursions in unexpeced oupu Forecast wind power output (MW) require special generaing machinery. Figure 3.23 illusraes he difference beween he orecased wind power Figure 3.23. Actual wind power output versus day-ahead oupu and he acual oupu rom he forecasted output (data from Eirgrid). combined wind urbines in a differen way. Te daa poins or he acual oupu o he combined wind urbines are ploted agains he orecased daa poins. Te red rend line shows ha he orecas is on average 10% higher han he acual oupu. Occasional large deviaions o around 500 MW beween he orecased oupu and he real oupu are clearly visible in his diagram. In a sysem where more han 25% o he elecriciy demand is covered by wind power, he porolio o power plans has o be flexible, able o provide as ramping up and down, and be capable o enduring requen sars and sops. Te challenge or balancing increases when he capaciy acor o wind parks decreases. Te reason or his is ha he raio beween peak oupu and average oupu is higher or a lower capaciy acor. In order o comply wih he rules or coningency reserves, he uni size o he power plans should be limied. Alhough he median value o he supply rom power plans or a doubling o he wind capaciy in he Irish sysem in 2012 is sill 2 GW, power plans ofen have o deliver less han 1 GW. Tis 1 GW should never be provided by less han 10 generaing unis, since oherwise a ailure o one o he unis would require an excessive load sep or he remaining unis. Elecriciy rom an onshore wind park coss abou 10 €cn/kWh, given a 5% ineres on he invesmen, a echnical lie o 20 years, and a capaciy acor o abou 30%. I, in he Irish case, he 2012 wind park capaciy is doubled o pro vide 32% o he elecriciy demand, he kWh coss o he non-wind power plan porolio migh increase by abou 3 €cn due o a lower uilisaion acor, higher uel consumpion per kWh, and a higher rae o wear. In order o show he acual inegraed coss o wind energy, hese 3 €cn/kWh should be allocaed o he wind energy coss via he racion o he power plan elecriciy producion relaive o he wind urbine producion (100–32)/32 = 2.1. Te real coss o he Irish wind elecriciy would, hereore, be abou 10 + 2.1 · 3 = 16.3 €cn/kWh should he 2012 insalled capaciy be doubled. Tis does no ake ino accoun he coss o
66 Power supply challenges
Wind output major wind energy countries Europe 40 000 ) 35 000 W M 30 000 ( t u p 25 000 t u o 20 000 r e w 15 000 o p d 10 000 n i W 5 000
Spain Germany rest DK east DK west Germany NW Germany NE Ireland
0
31 days of January 2012
Figure 3.24. Interconnecting the European wind turbines does not help to smooth their electricity production, because atmospheric conditions are often continent-wide.
he exra reserve power required because o deviaions beween he prediced and acual wind-power oupu. However, i he wind ur bines are produced and mainained by he Irish hemselves, hey will also be job creaors. Moreover, an ineres rae o 5% is nowadays quie atracive or privae invesors. Te benefi or he burden on he economy o a large-scale applicaion o wind urbines hereore depends on he boundary condiions in he counry. A basic negaive aspec o wind parks is he variabiliy in oupu. Te widespread idea ha inerconnecing European wind parks wih high-volage lines, he so-called super grids or elecriciy highways, will smooh he peaks o wind-power can be shown o be incorrec. Figure 3.24 gives he combined oupu rom all he wind urbines in Germany, Denmark, Ireland and Spain in 30 minue inervals in January 2012. Te com bined oupu is very peaky. Te reason or his is ha weaher sysems in Europe generally cover a large area o he coninen. In summary, a ransiion o an elecriciy sysem where wind power covers more han 25% o he demand means ha he large convenional power plans canno alone ensure he delicae bal-
Figure 3.25. An example of a weather system dominating a continent.
Figure 3.26. Adjustment of wind turbine output by pitching of the blades and yawing of the nacelle.
3. Balancing power demand and supply supply when conditions change
67
ancing o demand and producion. Fas hydropower and pumped hydro migh help bu such acil aciliies iies are ofen no presen in suffic sufficien ien levels. Shavi Shaving ng peaks in oupu rom wind urbines wih elecrical heaers in ho waer boilers migh help o some exen. Anoher mehod is o curail high oupus rom wind urbines by yawing and piching o he propellers. Yawing means roaing he nacelle away rom he posiion where i caches mos o he wind energy, whi while le piching means chang changing ing he angle o he blades or he same purpose. Many counries, however, allow renewable energy sources unresriced supply ino he grid. Ulimaely, flexible, agile power saions are needed or load balancing, or coningency reserves, and or providing reserve power should he acual wind power deviae rom he orecased value.
3.4. The 50Hertz transmission system operator region in Germany, 3.4. a region with much solar-based power Elecriciy produced by phoovolaic panels irradiaed by he sun shows variabiliy, bu he naure o he variaions is very differen rom ha o wind-based energy. Te oupu rom solar PV panels is always zero during he nigh, bu an overcas sky will also heavily reduce heir oupu. During he day, he oupu o a single PV panel can drop rapidly rom 100% o 10% should a cloud suddenly block he solar radiaion. Tis means ha clusers o solar PV panels can show much more drasic changes in oupu han wind arms. PV applicaions on smaller islands and auonomic municipal elecriciy supply sysems in paricular can suffer rom his. However, combining he oupu rom solar PV panels covering a widespread area will smooh he variaions caused by clouds. Figure 3.27 shows he power oupu rom he combined solar PV panels in he area operaed by he 50Herz ransmission Sysem Operaor in Germany in he week commencing May 20 (week 21), 2013. In early 2013, he insalled PV capaciy was Solar PV output, 50 Hertz Transmission, already 7.2 GW, ollowing an increase o week 21, 2012 3 GW during 2012. Te insalled hermal 5 000 week 21, 2013 forecast power plan capaciy was 17.8 GW and week 21, 2013 actual ) W4 000 he wind-urbine capaciy 12.4 GW. Te M ( pumped-hydro capaciy was 2.8 GW. Te t 3 000 u p 50Herz ransmission Sysem Operaor t u o 2 000 has a relaively large share o PV capaciy r e w insalled in is region, and he 15-minue o P 1 000 inerval oupu daa are available via he inerne. 0 Mo Tu We Th Fr Sa Su Clouds are he cause o he daily dierences in Figure 3.27. Te oupu can be differen d ifferen by a acor o our. Clouds are Figure 3.27. Power output from the combined solar PV also he reason why he oupu curves are panels in the 50Hertz Transmission GmbH area in Germany no always smooh. Te orecas or he during the week commencing May 20, 2013.
68 Power supply challenges
) h W 40 G ( 35 n o i t 30 c u d 25 o r 20 p y t i 15 c i r 10 t c e 5 l e y l i 0 a D
Solar PV output, 50 Her tz TSO, Germany
0
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Figure 3.28. The daily electricity production from the solar PV panels in the 50Hertz transmission system operator area in Germany in 2012.
combined oupu o he PV panels does no deviae excessively rom he acual value. Alhough he insalled i nsalled PV capaciy c apaciy (peak capaciy capaciy)) in i n he he region mus have been close o 8 GW in week 21, 2013, ha value was never reached. Te major reason or his is he non-opimum posiion o many panels and he presence o clouds in he area. Since he laiude o he 50Herz ransmission sysem operaor is beween 50 and 54 norh, solar irradiaion rom Ocober o February is quie low. Figure 3.28 gives he daily elecrical energy producion rom he solar panels in ha area expressed in GWh or he 366 days o 2012. Te diagram does no represen he acual solar irradiaion since he insalled PV panel capaciy almos doubled during he year. Neverheless, he oupu per day can varyy by a acor o 500. In he darker var 50 Hertz Transm Transmission ission region Germany, year 2012 seasons, he conribuion rom he solar 5 000 panels is negligible. Tis means ha he May-25 ) capaciy o he oher power plans canno W 4 000 M be reduced as long as no economic long ( t u 3 000 erm energy sorage aciliy or solar p t u power is available. o V 2 000 Figure 3.29 compares hree very P Jun-21 r a l differen days o solar PV elecriciy o 1 000 S producion. June 21 could have shown Jan-02 0 he highes oupu rom he solar panels, 0 4 8 12 16 20 bu apparenly he weaher siuaion was Time of the day no avourable or PV. Te flucuaions in PV oupu in he ime span around noon on June 21 and May 25 are also caused Figure 3.29. Three days in 2012 having substantial differences in PV panel output. by clouds. c louds.
24
3. Balancing power demand and supply supply when conditions change
Solar PV panels in areas in laiudes above 40° have a very low oupu in he darker seasons. Solar panels can be uel savers in he ligher seasons, bu he capaciy o he oher generaors has o be large enough o cover he peak elecriciy demand in he darker season. During he course o a year, he panels have no oupu a all hal he ime, as shown in he disribuion curve o Figure 3.30. Tis is caused by he absence o sunshine during he nigh. As a consequence, he capaciy acor o solar panels in he 50Herz SO area does no even reach 10%. Te reason in par is he posiioning o he panels. Panels insalled on roos acing wes or eas, as well as panels wih an inerior inclinaion, will never reach heir nominal oupu. Opimum oupu is obained i he PV panels are mouned on a racking sysem ha auomaically selecs he bes posiion o he panels wih respec o he sun. However, municipal planning rules ofen prohibi he insallaion o such sysems since hey are ground based, while no planning resricions exis or roo mouned PV panels. Germany also has an enormous number o wind urbines insalled. Te counry’s combined insalled wind urbine capaciy amouned o some 37 GW by he end o 2012. In he 50Herz SO area, he wind power capaciy increased rom 11.8 GW o 12.4 GW in 2012. I would be ideal i he lack o sunshine in he darker seasons could be compensaed or by increased oupu rom he wind urbines. However, alhough he peaks in wind urbine oupu are acually higher in he winer, here are sill many days wih very litle wind. Te correlaion coefficien coefficien beween he black rend line and he acual daa o wind energy in Figure 3.31 has he
69
Statistical distribution solar PV output, 50 Hertz TSO, Germany in 2012
) W5 000 M ( t u 4 000 p t u 3 000 o r e w2 000 o p V 1000 P r a 0 l o 0 S
Average installed capacity ≈ 6.2 GW Maximum output: 4.5 GW Energy produced: 5.13 TWh Capacity factor: 9.5%
Output only 10.8% of the time > 1/3 of the installed capacity
10
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% of the time Figure 3.30. The distribution of solar PV output in the 50Hertz TSO area in 2012.
Day-to-day energy output wind 50 Her tz TSO y 250 g r e 200 n e ) c h150 i r t c W e G ( 100 l e y l 50 i a D 0
y = 0.0812x 2 – 4.7933x + 101.86 R2 = 0.1786
0
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40
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Figure 3.31. Daily electric energy output (GWh) from wind turbines in the 50Hertz TSO area during 2012.
50 Hertz transmission area, Germany in 2012 40 d n 35 i w 30 t ) u h p W 25 t u G o ( s 20 y e g n 15 r i e b 10 n r u e t y 5 l i a 0 D
y = –0.0804x + 18.069 R2 = 0.1285
0
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% of the time Figure 3.32. Daily energy output from the solar PV panels plotted against that of the wind turbines (data points of 366 days, year 2012, 50Hertz TSO region, Germany).
70 Power supply challenges
50 Hertz transmission, Germany, May 25, 2012
10 000
Demand 8 000
) W M ( r e w o P
Others 6 000
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Figure 3.33. Power demand, output from solar panels and wind turbines, and the resulting balancing by the other generators.
low value o only 0.18. Tis means ha he volailiy in wind-based elecriciy producion is so high ha he rend line gives no proper indicaion o he acual insananeous producion. Even when elecriciy rom wind and solar is combined, here are many occasions when oher generaors have o cover he ull elecriciy demand. During imes o really high wind urbine oupu, here is generally litle sunshine. Sormy weaher and heavy clouds usually come ogeher. Figure 3.32 shows ha i he daily elecriciy producion rom wind exceeds 150 GWh, he PV panels generally deliver relaively litle energy. During days wih much sunshine, wih a combined PV panel oupu higher han 25 GWh, he wind-based elecriciy producion does no exceed 80 GWh. Te blue rend line in Figure 3.32 has a very low correlaion wih he acual daa, meaning ha no real relaionship beween solar PV oupu and windurbine oupu is presen. wo examples will now show he impac o solar panels and wind urbines on daily load ollowing in he 50Herz SO area. May 25, 2012, was a day wih very much sunshine and moderae winds. Te blue curve in Figure 3.33 shows he elecriciy demand or he whole day in 15 minue inervals. Minimum demand equalled 7 GW. Maximum demand was 10.5 GW. Tis is a ypical patern or an indusrialised counry. Te maximum rise rae in demand was abou 1 GW per hour beween 5.30 am and 7 am. May 25 was a Friday, and apparenly many indusr indusrial ial and commercial aciviies closed early on ha day, hence he decline in demand in he afernoon. Saisying such a demand patern wih convenional nuclear and coalfired power plans is no difficul because o he high predicabiliy o he demand patern and he relaively high baseload.
3. Balancing power demand and supply when conditions change
50 Hertz transmission, Germany, May 12, 2012
10 000
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8 000
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Solar
Others
Wind
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0 0
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Figure 3.34. The impact of much wind and solar-based electricity on the other power plants in the 50Hertz TSO area.
However, he power supply rom he solar panels and wind urbines covered a subsanial proporion o he demand. Te red line in Figure 3.33 shows ha he minimum load o he oher power providers was jus 4 GW while hey had o cover wo subsanial peaks. Tis subsanially reduced he seady load, which has negaive consequences or nuclear and coal-based power plans. Te radiionally raher profiable dayime power supply almos halved or he power saions. For he whole day, he renewable energy sources covered some 30% o he elecric energy demand. Figure 3.34 shows an even larger impac ha he power supply rom wind urbines and solar panels has on he oupu rom he power plans. On May 12, 2012, he wo renewable sources covered almos 70% o he oal elecriciy demand. Beween 2 pm and 5 pm, he power plans needed o supply only 1 GW o he grid. Again, i jus wo 500 MW power plans would hen have o fill he demand no covered by he renewables, a ripping o one o hem would mean losing so much dispachable capaciy ha primary reserves could never fill he gap. Te impac o he solar PV panels and wind urbines on elecriciy producion in he German 50 Herz SO area shows ha he ypical baseload ofen disappears. Te need or a leas a cerain amoun o dispachable power in he sysem means ha occasionally elecriciy has o be expored o neighbouring areas. Even he curailmen o wind urbine oupu is needed now and hen o avoid an unconrollable siuaion, such as here being limied expor opporuniies hrough ransmission resricions or insufficien demand.
71
72 Power supply challenges
3.5. Effects of photovoltaics on other power plants in Texas and California he exas ransmission operaor ECO provides hourly daa on he daily elecriciy demand in is region. CAISO rom Caliornia also provides he oupu rom he dieren elecriciy sources. Tis opens up opporuniies or discussing he effec o phoovolaics on uel-based generaors. Figure 3.35 illusraes how power demand in he ECO region varied during 2012. In conras o Finland, Ireland and Germany, he demand or power is maximised during he summer. Tis is because o he high ambien emperaures ha require a subsanial amoun o cooling or households, commercial buildings and acories. Te variaions in demand during he dayime are apparenly much higher in he summer. Te power demand can vary beween 35 GW and 65 GW on a ho day. Figure 3.36 shows power demand ploted agains he average daily ambien
80 000
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d n a m 60 000 e d ) r e W40 000 w o M ( p T 20 000 O C R E
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Figure 3.35.
Hourly data of power demand in the ERCOT region during 2012.
80 000
d n a m e d ) r e W w o M ( p T O C R E
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Average daily temperature in Texas (°C) Figure 3.36. Power demand in the ERCOT region plotted against the daily average temperature in Texas (year 2012).
3. Balancing power demand and supply when conditions change
73
emperaure in exas. Te correlaion ) beween he emperaure and demand is W70 000 M obvious. For relaively low emperaures, ( 60 000 d n 50 000 elecriciy demand in he sae also a P > 40 GW only 29% of the time m ends o increase. Te overhauling o e 40 000 d r 30 000 power plans normally akes place in e w he winer when he elecriciy demand o 20 000 p T is much lower. Occasionally, however, O 10 000 C during exremely cold spells, he power R 0 E 0 10 20 30 40 50 60 70 80 90 100 demand can reach 50 GW. In he pas % of the time, year 2012 rolling black-ous have occurred because o insufficien capaciy, also caused by rozen cooling sysems. Figure 3.37. Distribution curve of power demand in the Alhough peak elecriciy demand ERCOT TSO region for 2012. in he ECO region approached 70 GW in 2012, demands higher han 40 GW occur or less han 2500 hours per year (figure 3.37). Tis means ha a large racion o he generaing capaciy in exas will have a low uilisaion acor. Tis requires generaing equipmen wih a relaively low capial invesmen per kW. Due o he variabiliy in demand, hese power plans should be able o experience many sars and sops. In addiion, hey should have August 7, 2012, Texas high power oupu ramping up and down ) 70 45 ) W Power demand (GW) C raes. G ( 60 40 ° ( d e Figure 3.38 gives an example o he r n 50 35 u a t a m patern in power demand and ambien r e 30 e d40 p Austin temperature °C emperaure during a ho day in Augus r e 30 25 m e w 2012. During he nigh, less han 40 GW T o P 20 20 is required, while a around 4 pm when 0 4 8 12 16 20 24 he emperaure reaches is peak, some Time(hours) 65 GW is needed o saisy demand. Te maximum ramp up rae is 8.5 GW Figure 3.38. Hourly power demand in the ERCOT region and per hour. Figure 3.39 clearly illusraes the hourly temperature in Austin, Texas on August 7, 2012. ha here is almos no ime lag beween power demand and emperaure. Apparenly, he insulaion o buildings in exas is poor. Power demand could be subsanially reduced during high and very low emperaures wih beter insulaion. Te direc relaionship beween power demand and ambien emperaure migh offer ineresing opporuniies or saving uel consumpion wih solar phoovolaics. Figure 3.39 gives simulaed resuls or a case where he maximum oupu rom solar PV equals 10 GW and a case wih a peak PV oupu o 20 GW. Solar PV clearly provides smoohing o he power plan oupu during he dayime. However, as in Germany, a high ramping up o power plans is needed where here is a subsanial amoun o PV capaciy when he sun ses. In addiion, he uilisaion acor o he
74 Power supply challenges peaking plans will be urher reduced by PV capaciy. Tis siuaion clearly Power demand requires generaing equipmen ha can quickly sar and rapidly ramp up in Case 1 generator output oupu. In addiion, as in he case o he oher areas discussed in his book, he Case 2 generator ) output W amoun o insalled power plan capaciy G ( r e canno be reduced as a resul o he w o inroducion o solar phoovolaics. Nev P Case 2: Max 20 GW of solar PV output erheless, a PV sysem wih a maximum oupu o 20 GW would have produced 113 GWh in exas on Augus 7, 2012. Case 1: Max 10 GW Ta would have saved some 900 J o of solar PV output uel on a single day or a power plan efficiency o 45%. Te 900 J equals 25 Time of the day (hours) million sandard m 3 o naural gas wih a lower calorific value o 36 MJ/ m 3. Tis Figure 3.39. Simulated cases where photovoltaic capacity is enough uel or one million passenger has been added in the ERCOT region. cars o drive a leas 300 km each. Te Caliornian ransmission sysem operaor CAISO makes available hourly daa on power demand and generaion. Generaion is spli ino he differen elec70
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Figure 3.40. Although wind and solar energy compensate each other to some extent, back-up capacity is always needed.
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3. Balancing power demand and supply when conditions change
75
riciy sources, such as hose based California, October 16, 2013 on uel, wind and phoovolaics. Tis makes i easy o deermine he effec o renewable energy on he oupu rom uel-based power plans. Te daa o he ) W solar PV oupu in figure 3.41 show ha Thermal power plants M ( he maximum oupu o PV panels was r Solar PV outputs e w Thermal if doubling of PV 2 GW on Ocober 16, 2013. Te ligh o P Thermal if no PV blue curve represening he oupu rom he hermal power plans is quie fla during he dayime. A relaively small peak in he oupu o he hermal power Time of the day (hours) plans occurs in early evening when he sun ses. I he insalled PV capaciy was doubled, he green curve would apply Figure 3.41. The effect of the output of solar PV panels on or he hermal power plans. Te green the output of thermal power plants in California on October curve clearly ends o peak more han he 16, 2013. ligh blue and he dark blue curves. Tis requires flexible backup generaors. Doubling he solar panel capaciy in Caliornia would have saved he equivalen o 3.6 million m 3 o naural gas on Ocober 16, 2013. 18 000 16 000 14 000 12 000 10 000 8 000 6 000 4 000 2 000 0
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3.6. Conclusions Maching power demand and supply becomes more complicaed wih variable and nondispachable renewable energy sources. In areas where wind and solar energy have an insalled capaciy close o or higher han hal o he average power demand, baseload generally disappears. Te capaciy acor o hermal power plans decreases – bu heir insalled capaciy has o be almos he same as when here are no renewables in he sysem. Wha is obviously needed is flexible backup power. Backup power plans should be capable o rapid, coninuous saring and sopping. Tey should ramp oupu quickly up and down and hey should be uel-efficien a any load. Where hydropower is no presen, he bes soluion appears o be gas-fired plans based on muliple combusion engines. Tey can ac as reserve capaciy, answer o wind orecasing errors and ollow he oupu o renewables as closely as possible. In oher words, agile gas-based power plans are able o mach power demand and supply once again.
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4 Active and reactive power Elecrical engineers disinguish beween acive and reacive power in elecriciy supply sysems. Acive power reers o he energy ranser rom an elecric generaor o a load via conducing wires. Reacive power resuls rom currens creaed by alernaing volage in cerain elemens o he sysem wihou a ne release o energy. Reacive power affecs he ransmission capabiliy o high-volage lines and he volage in he sysem. Large shares o renewable energy sources, long ransmission lines, and reliance on a ew large power plans can cause a lack in he reacive power supply and problems wih he reliabiliy o he supply sysem. Noe rom he auhor: Te relaionship beween an increasing amoun o renewable energy in a sysem and problems wih reacive power is no well covered in available publicaions. Tis is why hese issues are addressed here, even hough he conen becomes raher echnical. Sudens, researchers, consulans, business managers, and policy makers should benefi rom his chaper. Readers no ineresed in echnical deails can jump o secions 4.6. and 4.7. where he consequences o renewable elecriciy or he reacive power supply are summarized.
78 Power supply challenges
4.1. Reactive power analogies eacive power is like he oam on op o a glass o beer, where he liquid in he glass represens acive power. Te oam akes space in he glass and hereore reduces he beer-conaining capaciy. By comparison, reacive power reduces he elecriciy ranspor capaciy o a power ransmission line. Anoher simplified analogy or acive and reacive power is a rain. Te passengers carried by he rain can be compared wih acive power, while he unavoidable rain’s mass and volume can be seen as a burden, like reacive power. eacive power resuls rom curren in he sysem ha ransers no energy. eacive power affecs no only he acive power capaciy o ransmission lines, bu also volages in he sysem.
Figure 1.1.
Analogies for active and reactive power.
4.2. The three basic load elements in alternating current systems eacive power and is consequences or a power supply sysem can only be properly undersood wih some basic knowledge o elecrical sysems. In he ollowing secions, he hree basic elemens in elecric circuis will be explained. Tese elemens are he resisors, he capaciors and he inducors. Te inenion is no o repea any elecrical engineering exbook, bu o use illusraions ogeher wih some basic mahemaics o provide an inroducion o he naure o acive and reacive power.
4.2.1. Resistive load Acive power resuls when a volage is applied o a resisive load. A resisive load R is an elemen ha passes an elecric curren wihou any delay when a volage is applied o i, while i urns elecric energy ino hea or work. One example o a pure resisor is an elec-
4. Active and reactive power
79
ric heaer. Te curren I R hrough he resisor is direcly proporional o he applied volage V R . Te resisance R is expressed in he uni ohm (Ώ): V R
R
=
I
Equation 4.1.
R
Te power P released in he resisive load is he produc o volage V and curren I . Te uni o volage is vol (V) and he uni o curren is ampere (A). Te uni o power, or energy flow, is wat (W), equalling joule/second (J/s). P
=
V I
Equation 4.2.
Figure 4.2. An electric heater, an example of a resistive load.
I he volage applied o a resisor is a sinusoidal wave, he resuling curren hrough he resisor is also a sine wave. A I sinusoidal curren is basically an alernaing curren (ac). Figure 4.3 gives a represenaion o a sine-wave volage generaor wih a resisor as he load. Te curren I R is a resul o he volage V R . Figure 4.4 is an illusraion where a sinusoidal volage wih Generator R a peak value o 325 vol is applied o a resisor o 2.5 ohm. Te ~ peak value o a sine wave is also called ampliude. Te ampliude expresses he maximum excursion o he sine wave rom zero. Since he curren hrough a resisive load ollows he volage immediaely, a sinusoidal curren wih a peak value o Figure 4.3. A schematic representation of an ac generator with a 325 V/ 2.5 Ώ = 130 A resuls (he black curve in Figure 4.4). resistive load. Since boh he curren and volage waves in he example have heir peaks and valleys a he same ime, hey are reerred o as being in phase. Te produc o he insananeous values o volage and curren gives he power dissipaed in he resisor. Tis produc is illusraed by he solid red line wih a peak value o 325 V · 130 A = 42250 wat ≈ 42.3 kW. Te volage and he curren oscillae around an average value o zero; hal o he ime hey are posiive and hal o he ime hey are negaive. Te resuling solid red power curve appears always o be above zero. Te reason or his is simple: basic mahemaics ells us ha muliplying wo negaive values resuls in a posiive value. Te power curve oscillaes around an average value o 21.1 kW and apparenly has he double requency o he volage. Te offse o he power curve rom he horizonal axis (he dashed dark red line) equals he effecive averaged power dissipaed in he resisor. I is ineresing o see rom he solid red line in Figure 4.4 ha he insananeous energy delivered o a resisor is never consan in an alernaing curren sysem. Tis generally does no affec he user because o he high requency. Te ligh coming rom an incandescen lamp will ollow he oscillaions in power o some exen, bu he human eye is hardly sensiive o ligh variaions wih requencies higher han 50 Hz. I he periodic variaions in volage level or power are a R
V
R
80 Power supply challenges problem or he applicaion, a recifier is used o ranser he alernaing volage ino direc volage. A direc volage does no have he periodical variaions o alernaing volage. Compuers and audio equipmen are examples o elecric appliances ha use direc volage. We will now mahemaically deermine he average power dissipaed in a resisive load. Tis will also reveal he concep o he effecive value or roo mean square value (rms) o an alernaing volage or curren. Te volage and curren values o alernaing elecriciy sysems are normally always given as rms values. Te resul o muliplying wo sine waves can be ound by using he basic mahemaical relaionship: sin x · sin x
= sin2x = ½ – ½ cos 2x , Equation 4.3.
1 000
50 000
800
40 000
I
t n e 600 r r u c 400 d n a
30 000 20 000
V 200
e g a t l o V
10 000
0 –200
–10 000
–400
–20 000 0
10
20
30
40
50
60
Time (millisecond) Voltage
Current
Instaneous power Effective power
Figure 4.4. A sinusoidal voltage with a peak value of 325 volt inducing a sinusoidal current with a peak value of 130 ampere in a resistor of 2.5 ohm (Ώ), resulting in a cosine-shape power curve around a positive offset of 21.2 kW equalling the effective power delivered.
Equation 4.4.
Tis shows mahemaically ha he produc o wo in-phase sine waves resuls in a consan equalling he produc o he ampliude (= peak value) divided by √2 o each sine wave, and in a varying componen. I we use he ampliude values o our example o 325 V or volage and o 130 A or curren, he consan urns ino 325/√2 · 130/√2 = 21 125. Tis is exacly he value o he effecive power as shown by he dashed line in figure 4.4. Te variable componen in he produc o wo sine waves clearly has double he requency o he iniial volage wave. Tis is indicaed by he 2x in he cosine uncion. Since he average dissipaed power in he resisive load equals he produc o he volage ampliude divided by √2 imes he curren ampliude divided by √2, we call he peak value o a sine wave divided by √2 is ‘effecive’ value or ‘roo mean square’ (rms) value:
V rms =V peak /
r e w o P
0
We can now give each sine wave an ampliude o A and B , so ha he equaion becomes: A sin x · B sin x = A B – ½ AB cos 2x
P
Equation 4.5.
4. Active and reactive power
81
In he case o a sine wave wih a peak value o 325 vol, he effecive value V rms = 325/√2 = 230 vol. Tis is a amiliar number or many people. Again, in elecrical engineering, wha is always mean when alking abou volage and curren, unless oherwise saed, is he rms value o an alernaing volage or curren. Since he rms value o he curren passing a resisor o 2.5 Ώ or an rms volage o 230 vol is 230/2.5 = 92 ampere. Te power P dissipaing in his resisor equals hen: . . P = V rms I rms = 230 92 = 21125 W 21.1 kW
Equation 4.6.
Tis resuling average produc o volage and curren is called acive power. Te visualisaion o he produc o he wo sine waves, as given in figure 4.4, is inended o help he reader o undersand beter he resul o he dry mahemaical manipulaion. Te reader should now be able o undersand he concep o acive power, where volage and curren are in phase and heir rms values deermine he power dissipaed in a resisive load.
4.2.2. Capacitive load Capaciors are elecrical componens ha sore an elecric charge on plaes or conducors ha are separaed by a non-conducive layer, he so-cal led dielecricum. Tis dielecricum can be air, as is he case or power ransmission lines in parallel. Unlike resisors, capaciors do no dissipae energy. Te relaionship beween he volage V beween he plaes and he elecric charge Q on one o he plaes is given by he capaciance C: C=
Q
Equation 4.7.
V
Wihou a difference in charge beween he plaes, here will be no volage beween he plaes. A capacior can be compared o a waer ank; a sream o waer has o fill he ank in order o creae he waer level. In order o creae a volage over a capacior, an elecric curren has o charge a plae o ha capacior. Tis means ha he curren has o be earlier han he volage. In oher words, he curren leads – – – – – – – he volage. Te capaciy C is expressed in arad (F) and he charge is expressed in coulomb (C). In alernaing curren sysems wih only a capacior conneced o he generaor, he curren will lead a Dielectrium sinusoidal volage by 90 degrees. Te relaionship beween volage and curren is he reacance X C o he capacior: V C 1 = X C = I C j2πfC
Equation 4.8.
in which ƒ is he requency o he volage wave in herz. Te j is a so-called operaor indicaing he 90° phase shif beween volage and curren. Te reacance o a capacior appears o be inversely proporional o he volage requency, meaning ha he reacance
+
+
+
+
+
+
+
–
–
–
+
+
+
Figure 4.5. A basic representation of a capacitor. In ac systems, the charge on the plates oscillates between positive and negative.
82 Power supply challenges o a capacior wih a given capaciance I 500 25 000 C diminishes wih increasing requency. t n 400 20 000 e Figure 4.6 shows he volage, curren, and r 300 15 000 I r × u 200 10 000 he produc o volage and curren or a c V 100 5 000 d t n c 0 0 peak volage o 325 vol and a capaciance a u –5 000 d V –100 o 1500 μ F or a requency o 50 Hz. Te –10 000 o e –200 r g P –300 –15 000 a produc o volage and curren has again t l –400 –20 000 o –25 000 V –500 double he requency o he volage, bu i 0 10 20 30 40 50 60 has on average no offse rom zero. TereTime (millisecond) ore, alhough he curren reaches peaks o Voltage Current Voltage × current 153 A, on average no power is delivered Average product o he capacior, as indicaed by he green dashed line. Te produc o volage and curren is, hereore, called reacive power, Figure 4.6. Reactive power as the product of voltage and current in the case of a purely capacitive load wih he uni vol ampere (VAr). Te naural capaciance in mos elecriciy supply sysems is relaively small. Underground elecric cables have more capaciance han overhead cables due o he closer proximiy o he conducive wires. Also, some modern elecronic devices have a capaciive elemen nex o a resisive one. Such an example is he LED ligh. We will now inroduce he phasor or vecor concep ha is commonly used in elecrical engineering. A sine wave is acually a projecion o he posiion o a poin moving along a circle a a consan speed. Poin A in Figure 4.7 sars a 0° and moves in an ani-clockwise direcion along a circle wih a radius r. Te verical disance o poin A rom he saring poin is ploted agains a verical axis, while he angle covered rom he saring poin is ploted agains a horizonal axis. Te ampliude, i.e. he maximum excursion rom he saring poin, equals he radius o he circle. Te dark green arrow in he circle is called he phasor o poin A. I poin A arrives a 90°, he phasor poins in a verical direcion. I he phasor covers he ull 360° o he circle in 20 milliseconds, he so-called period ime o he sine wave equals 20 ms. A period ime o 20 ms means ha poin A complees 50 90° revoluions in one second. Te requency o he sine wave is in ha case 50 herz Amplitude (Hz). 180° Figure 4.8 gives he symbolic represenaion o a generaor wih a purely capaciive load. As menioned earlier, 270° he curren leads he volage over he 0 30 6 0 90 120 1 50 18 0 210 240 270 300 330 36 0 Degrees capacior by 90°. Tis is indicaed by he verical curren phasor and he horizonal volage phasor on he righ-hand Figure 4.7. The sine wave as the projection of a point (A), moving anti-clockwise along a circle side o he illusraion. r
A
4. Active and reactive power
o summarize, because o a capaciive elemen in an AC elecriciy supply sysem, he volage and curren are no longer in phase. For a purely capaciive elemen, he curren wave leads he volage wave by 90 degrees. On average, no energy is dissipaed in a capaciive elemen.
4.2.3. Inductive load
I C I C ~
V C
90°
V C
Figure 4.8. Representation of a generator with capacitive load, with the voltage phasor and the current phasor leading the voltage by an angle of 90 degrees
Inducors consis o coils o wound conducive wire ha creae a magneic field when an elecric curren flows hrough hem. Tis magneic field atracs iron. Elecric moors derive heir moive power rom his propery. An inducor can be compared o a car ha needs a driving orce in order o accelerae. o give moion o he mass o a car, a orce has o be applied o he mass beore speed resuls. Te speed o he car, hereore, always lags behind he driving orce. By comparison, he curren hrough a pure inducor lags behind he driving volage. I he volage across he inducor is a sine wave, he curren hrough he inducor lags behind he volage by 90° resuling in a 180° shifed cosine wave. Te 90° lagging equals 5.0 ms in a 50 Hz sysem and 4.17 ms in a 60 Hz sysem. Many elemens in an elecriciy supply sysem, such as elecric moors, fluorescen lighs and ransormers, have inducance. Even a simple sraigh wire has some inducance and hus long overhead ransmission lines can have a significan inducance. Figure 4.10 represens a generaor wih a purely inducive load. Te phasor diagram shows ha he curren is lagging he volage by 90°. Te relaionship beween volage V L and curren I L or an inducor wih an inducance L equals: V L = X L = j2πfL I L
83
Figure 4.9. An electric coil.
Equation 4.9.
in which L is he inducance expressed in henri (H). Te j again indicaes a 90° phase shif beween he volage and curren, while f is he requency o he volage. Te reacance o an inducor is direcly I L proporional o he requency, meaning ha he reacance o a given inducance increases wih increasing requency. Figure 4.11 gives he volage and curV 90° V ~ L L ren, as well as heir produc, in he case I L o an inducive load, where he yellow curve is volage and he blue curve is he resuling curren. Te averaged produc Figure 4.10. Diagram of a generator with a purely inductive load and the voltage phasor with the lagging current o volage and curren, he dashed purple phasor. curve, is zero as in he case o a capacior.
84 Power supply challenges Neverheless, he insananeous value o he produc o volage and curren reaches high values. Te produc o volage and curren has again double he requency o ha o he basic volage wave. In he example o figure 4.11, he peak in curren reaches 130 A or a peak volage o 325 V and an inducance o 8 millihenri (mH) or a requency o 50 Hz. Te produc o volage and curren is again he reacive power.
4.3. The power factor cos φ
500
25 000
I 400 t n 300 e r r 200 u c 100 d n 0 a
20 000 15 000 10 000
I
5 000
V
×
t c u d –5 000 o r –10 000 P 0
V–100
e g–200 a t l –300 o V–400
–15 000 –20 000 –25 000
–500 0
10
20
30
40
50
60
Time (millisecond) Voltage
Current
Voltage × current Average product
Tis secion will explain he power acor cos φ. Knowledge o he background o he Figure 4.11. Reactive power as the product of voltage and power acor is imporan in undersanding current in the case of a 50 Hz voltage with a peak value of is effec on reacive power demand in 325 V acting upon a purely inductive load of an inductance of 8 millihenri. power supply sysems and on mainaining he proper volage level. eal elecriciy consuming elemens, such as elecric moors, ovens and lighs, can have resisance, inducance and capaciance a he same ime. Te elecric moor shown in Figure 4.12 is a ypical example o a device wih resisance and inducance. Figure 4.13 is a schemaic represenaion o such a device. Te curren I Z flows hough he inducor as well as hrough he resisor, since hese wo elemens are in series. Tis curren is ploted in Figure 4.13 as an arrow, a phasor, along he horizonal axis. Te previous secion has shown ha he volage over an inducance leads he curren by 90°. Tereore, we have o plo he volage over he inducive elemen as shown by he green V L phasor. Te volage V R over he resisor is in phase wih he curren, so i is ploted along he horizonal axis. Te resuling volage across he combinaion o resisor and inducor equals: V Z = (V L2 + V R 2 ) = I Z ((2πƒL)2 + R 2 ) Figure 4.12. An electric motor has resistive and inductive elements
Equation 4.10.
Te quoien V Z/ I Z is called he apparent resistance or impedance Z. Te word impedance has been derived rom he Lain impedio, meaning hindrance. Lierally, i means ha one’s ee, he pedes, are wrapped. In he example o Figure 4.13, he rms volage V Z equals 230 V . Since he impedance Z equals √ ((2π · 50 · 0.04) 2 + 20 2) = 23.65 Ώ, he resuling curren I Z equals 230 /23.65 = 9.73 A. Te volage V Z leads he curren I Z by he angle φ. Te angle φ can easily be ound rom cosine φ = V R / V Z . In his case, cosine φ equals 194/230 = 0.84, which is he power acor cos φ.
4. Active and reactive power
85
Te angle φ equals 32.9° in his example, since cos 32.9 = 0.84. Te power acor indicaes which racion o he volage L = 40 mH Impedance over he impedance is acually available φ = 230 V R = 20Ω or he resisor or dissipaing energy. ƒ = 50 Hz Knowledge o he power acor is indispensible when designing and operaing Power factor cos φ = Z = √((2πƒL) + R ) = 23.65 Ω 194/230 = 0.84 power supply sysems. I = V /Z = 230/23.65 = 9.73 A Te elecrical load, consising o an inducance L and a resisance R , can be Figure 4.13. Example of an electrical load with inductance replaced by he impedance Z (Figure 4.14). and resistance, resulting in a phase shift between voltage and Te phasor diagram can be redrawn wih current. he volage V Z a he horizonal axis and he curren I Z lagging behind he volage by an angle φ. Te curren can hen be spli ino a phasor I acive = I Z cos φ and a phasor I reacive = I Z sin φ. Only he real par o he curren will conribue o energy release in he impedance. Tereore, he acive power o he impedance equals: V L
122 V
I Z
V Z
V L
230 V
Z
V R
V Z
V R
2
Z
Pactive = V Z · I Z cos φ
I Z
194 V
2
2
I
Z
Equation 4.11.
I
= 8.17 A
active
and he reacive ‘power’ o he impedance equals: Preactive = V Z · I Z sin φ
9.73 A
φ V
~
Z
Z
I Z = 9.73 A
Generator
I
= 5.28 A
reactive
Equation 4.12.
Te uni o acive power is wat (W) and he uni o reacive power is VAr. In he examples shown in figures 4.13 and 4.14, he acive power is 230 · 8.17 = 18.8 kW and he reacive power is 230 · 5.28 = 12.1 VAr. Due o he reacive par o he impedance, he elecric curren rom he generaor o he acive load is a acor o 1/cos φ, or 9.73/8.17 = 1.19 here, higher han in he case o no reacive par. I means ha boh he generaor and he ransmission line have o be able o accep a higher curren han he acive curren delivered o he load. Te blue curve in Figure 4.15 gives he resuling curren or an rms volage o 230 V or an impedance o 23.65 Ώ and a power acor cos φ o 0.84 as per he examples in figures 4.13 and 4.14. Te
V = 230 V Z
Figure 4.14. Electric current to an electrical impedance split into a an active-real-component and a reactive – imaginary – component.
400
20
Voltage (V)
300
15
Current
200
10
Z
V 100
5
e g 0 a t l o–100 V
0 –5
–200
–10
–300
–15
–400
–20 0
5
10
15
20
25
30
35
40
45
Z I
t n e r r u C
50
Time (millisecond)
Figure 4.15. Diagram of voltage and current for V rms = 230 V and I rms = 9.73 A for an impedance of 23.65 Ώ and a power factor cos φ = 0.84
86 Power supply challenges blue curren curve ollows he yellow volage curve by he angle φ, equalling 32.9° here, since cos 32.9 = 0.84. For a 50 Hz sysem, an angle o 32.9° resuls in a ime shif beween volage and curren o 32.9/360 · 20 ms = 1.83 ms, since a complee 50 Hz sine curve o 360° covers 20 ms. o summarize, he power acor in an ac elecrical sysem indicaes he phase shif beween he volage wave and he curren wave. In sysems wih a power acor lower han 1, volage and curren are no longer in phase. In addiion, he elecric curren in a sysem wih a power acor lower han 1 is always higher han in he case o a power acor o 1 , provided he supply volage is he same.
4.4. Impedance of electricity transmission systems When a generaor supplies boh acive and reacive power o is load, he ransmission sysem beween he generaor and he load has o ranspor boh componens o he curren. ransmission and disribuion lines also have heir own elecrical impedance. Te generaor ‘eels’ he sum o he ransmission sysem impedance and he load impedance. Since reacive power increases he oal curren in he power lines, more energy will be dissipaed in he resisance o he power lines, and his resuls in a urher reducion in volage over he line resisance. In addiion, he reacance o a power ransmission line isel in combinaion wih a highly reacive load can cause a subsanial volage drop over he line.
4.4.1. Transmission line resistance Te elecrical resisance o a ransmission line depends upon he lengh o he cable, is diameer, is maerial properies and he emperaure. Copper has a low elecrical resisance, bu i lacks he ensile srengh o seel. ensile srengh is needed when wires are bridged beween ransmission owers. Te mechanical load on he wires will increase in case o gales or when reezing rain causes ice o orm on he wires. Copper is relaively expensive and hereore aluminium is ofen used as a conducor. Again, aluminium lacks he needed mechanical srengh. Ta is why a modern ransmission line consiss o an inner seel cable wih aluminium conducors wrapped around i. Aluminium has roughly double he ensile srengh o copper, while seel is wice as srong as aluminium. Te acual elecrical resisance o a conducor can be ound by muliplying he resisiviy ρ by he lengh l o he conducor and dividing i by he area A o he conducor: R=
ρl A
Equation 4.13.
For a conducor diameer d o 25 mm, equalling an area A o 491 mm 2 , an aluminium ransmission line has a specific resisance o abou 2.82 · 10 –8/ (491 · 10–6) = 5.74 · 10 –5 Ώ per meer. Tis seems o be a small number, bu or a ransmission line o 250 km he resisance o each o he hree wires o a hree-phase sysem is 250 · 103 · 5.74 · 10 –5 = 14.3 Ώ. For a curren o 500 A per wire, such a resisance
4. Active and reactive power
87
resuls in a power loss o I 2R o 5002 · 14.3 = 3.57 MW per wire, or 10.7 MW or he oal hree-wire sysem. I he sysem capaciy is 500 MW, one loses slighly more han 2% o he energy over a disance o 250 km. Should he ransmission line have only resisance and no reacive componens, he volage loss would be I · R = 500 · 14.3 = 7.15 kV. For a 380 kV line, such a volage loss is relaively small. In realiy, ransmission-ower-based power lines have considerable inducance ha Figure 4.16. Two double three-phase high-voltage transmissubsanially affecs he volage drop. Tis sion lines with transmission towers carrying the cables will be shown in secion 4.4.3. Firs, we have o check i he energy loss in he ransmission line will no overhea he wires. A higher emperaure will increase he resisiviy ρ and reduce he mechanical srengh o he wires. Generally, an increase ∆T in he operaing emperaure o 50 K wih respec o 20 °C, i.e. an operaing emperaure o 70 °C is considered as being he limi. As a rule o humb, an overhead aluminium cable wih is wire area A expressed in mm 2 can accep a curren o: I max = 1.14 · A¾ · T
Equation 4.14.
In he example above, wih a conducor area o 491 mm 2 , he maximum allowed curren equals 838 A. Te presumed curren o 500 A is hereore no a problem when he ambien emperaure is low. Te resisiviy isel depends on he emperaure: ρactual = ρ0 [1 + α (T actual – T 0)]
Equation 4.15.
in which α is he emperaure coefficien o he maerial. For aluminium, α equals 0.0039/K. Tis means ha a emperaure rise o 50 K increases he resisance o a power line made rom aluminium by almos 20%. Tis acor has o be considered in ho areas when elecriciy demand peaks because o air condiioning.
4.3.2. Transmission line inductance Te inducance o a se o wires ransmiting elecric energy is relaively low per meer lengh. However, ac ransmission lines can cover disances o up o a housand kilomeres. Ten he inducance plays an imporan role in he energy ranspor capaciy o he ransmission sysem. Te specific inducance o a high-volage ransmission wire depends on is inner radius r and he disance s rom he oher wires:
Table 4.1. The electrical resistivity ρ of some conductive materials. Material
Resistivity ρ at 20 °C (Ώ m)
Copper
1.70 10 –8
Aluminium
2.82 10 –8
Steel
14.3 10 –8
88 Power supply challenges
Lspecific = π ( ¼ + ln r s )
Equation 4.16.
in which μ is he magneic permeabiliy in a ree space, equalling 4π · 10 –7. For s = 2 m and r = 12.5 mm, Lspecific equals 2.13 μH/m. Teoreically, ree space means a vacuum, bu ambien air as a medium around he wires does no make much difference. For a line requency o 50 Hz, a specific inducance o 213 μH/m renders a specific inducive reacance X l (spec) o: X l(spec) = j2 · π · 50 · 2.13 = j 670 /m
Equation 4.17.
4.4.3. Transmission line capacitance An overhead power ransmission line has a relaively low capaciance compared o ground cables. Ye, is capaciance canno be negleced since he curren needed o charge and discharge he wires has o be provided by he generaor and ransmited hrough he wires. In elecrical engineering, he capaciance per meer o wire lengh o a single phase sysem is approached by: Cspecific =
π s ln r
Equation 4.18.
Here, ε is he permitiviy, which has a ree space value o 8.85 pF/m (p = pico = 10 –12). Again, ambien air can be considered as ree space here. Te disance beween he wires is s again and he radius o he wire is r . A larger disance beween he wires reduces he capaciance, while a larger wire diameer increases he capaciance. Using he same line dimensions as or he inducance, we find ha Cspecific = π · 8.85/ln (2/0.0125 ) = 5.48 pF/m. Tis resuls in a specific capaciive reacance X C(spec) = 1/j2πƒC = –j 581 MΏ/m (M = 10 6 and he produc o he operaors j · j is by definiion –1).
4.4.4. Transmission line impedance In principle, a power line can be represened by a chain o muliple small secions o, say, one meer lengh having some resisance, inducance and capaciance. Acually, some conduciviy beween he lines and he earh should also be aken ino accoun since elecrical charge ends o leak away via insulaors and he air. Especially in misy weaher, he ypical crackling noise caused by he discharging can be heard. Figure 4.17 gives a represenaion o a small secion o a ransmission line. Te shun resisance Gspec is a measure o he conduciviy beween he line and he earh.
Lspec
Rspec
C spec
Gspec
Figure 4.17. A diagram of a small section of a power transmission system with resistance R spec, inductance Lspec, capacitance C spec and shunt resistance Gspec
4. Active and reactive power
89
Deermining he oal impedance o a power ransmission line consising o a large number o infiniesimal secions as shown in Figure 4.17, requires advanced mahemaical knowledge. Forunaely, he impedance o a ransmission line o medium lengh l , up o 250 km, can be approached by a so-called π secion as shown in figure 4.18.
4.5. Voltage change over a power transmission line Tis chaper aims o explain how he volage a he end o a ransmission line is affeced by he impedance o he line and by he naure o he load. Secion 4.5.3 will reveal how he naure o he load affecs he power carrying capaciy o he ransmission line. As an example, a power line o 200 km will be used, wih a resisance o R spec o 50 μΏ/m, an inducance o Lspec o 2 μH/m, a capaciance o Cspec l · Lspec o 5 pF/m and a negleced shun resisance G spec. l · Rspec Wih hese daa, he raio o he volage V in a he beginning o he line rom he generaor and he volage V out a he end o he line can be V in ½ · C spec ½ · C spec Gspec deermined. Tis can be done wihou a load, wih a purely resisive load, or wih any impedance. In his example, a single phase sysem will be used. Alhough real AC ransmission sysems are hreeFigure 4.18. Diagram of a replacement for a power transmission line of up to 250 km. phase, he simplified example is used o show he mechanisms involved.
4.5.1. Case without load at the end of the transmission line Normally, overhead high-volage elecriciy ransmission lines are disconneced rom he supply sysem when hey do no carry any load. However, wih a subsanial amoun o varying renewable elecriciy sources in a sysem, disconnecing is less easy since he power available via he ransmission line can serve as a backup or he renewable generaors. Moreover, indirecly coupled renewable energy sources do no ofen provide reacive power. Tereore, i is ineresing o see how he volage V out a he end o such a ransmission line differs rom he volage V in a is beginning when here is no load. Tis exreme case shows how even a non-loaded ransmission line inroduces a change in volage. Figure 4.19 shows he replacemen scheme o a 200 km long ransmission line, again wih he resisance o R spec o 50 μΏ/m, an inducance o Lspec o 2 μH/m and a capaciance o Cspec o 5 pF/m. Since here is no load a he end o he ransmission line, curren I line passes hree elemens o he line, i.e. he inducor o j 126 Ώ, he resisor o 10 Ώ and he final capacior o – j 6 366 Ώ. Te ne reacive par o he impedance Zline = V in / I line equals – j 6 366 + j 126 = – j 6 240. Tereore, Zline = √ (10 2 + 6 240 2) ≈ 6 240 Ώ. Tis shows ha he resisance o only 10 Ώ and he inducance o 126 Ώ have pracically no effec on he impedance in his case, so ha he impedance is dominaed
90 Power supply challenges j 126 Ω by he capacior. Te line curren I line equals V in 10 Ω l generator l line /6 240, and hereore he volage V ou equals 6 366 · V in /6 240 = 1.013 V in . Te volage a he end o he ransmission line is, hereore, higher han V in V out – j 6366 Ω – j 6366 Ω he volage a he beginning. Te generaor curren I generat or is slighly more han wo imes higher han he line curren I line in his case because o he wo impedances o – j 6 366 and – j 6 240 in Figure 4.19. Model of a 200 km long singlephase transmission line. parallel. Te generaor curren leads he generaor volage by almos 90° because o he capaciive characer o he load. For readers wihou a background in elecrical engineering, he ac ha he volage a he end o a ransmission line is higher han a he beginning migh appear as a miracle. However, a purely capaciive load on an overhead ransmission line wih is ypical inducive properies always increases he volage a he end o such a line. A raised volage a he end o a ransmission line can creae problems or appliances and local generaors.
4.5.2. Case with a purely resistive load at the end of a transmission line Te nex example is a case whereby a 100 kV, 200 km long, ransmission line is loaded wih a purely resisive load, as represened in figure 4.20. Te volage a he end o he ransmission line is shared by he resisance o he load and he shun capacior o he ransmission line o –j 6366 Ώ. In he case o a low resisance value R load , he curren induced by he volage V out is dominaed by he curren hrough he resisor. Where he value o R load is high, he curren approaches ha o he case wihou any load. I he resisance a he end o he power line is decreased o 10 Ώ, he line curren I line reaches a value o 800 A (0.8 kA), which we presume o be he maximum allowed curren o avoid overheaing. In Figure 4.21, he line curren is shown as a uncion o resisive load by he blue line. Te yellow line shows ha or his resisive load o 8 Ώ, he volage V ou a he end o he ransmission line has dropped o 8% o he volage V in . Tis is way oo high. A volage drop o 90% o V in is generally considered as being ulimaely accepable. A volage drop o 90% is in his case j 126 Ω 10 Ω l generator l line reached or a load resisance o 281 Ώ. Te line curren is hen 321 A, while he power V out dissipaed in he load equals 29 MW. For V in – j 6366 Ω – j 6366 Ω Rload his load, he power loss in he ransmission line equals I line2 · R line = 3302 · 10 = 1.09 MW, equalling some 3.8% o he power dissipaed in he load. Te power acor cos φ Figure 4.20. Model of a 200 km overhead transmission line with a purely resistive load. or he 29 MW load is 0.93 (inducive), as
4. Active and reactive power
91
shown in figure 4.22. Tereore, or his 200 km ransmission line he load limi d 50 1.0 a caused by volage drop is reached long o l ) n ) 40 beore he limiing line curren o 800 A 0.8 A i k ( d W e is reached. t M 30 0.6 I a ( p r ; I he load resisance is increased i o s s t s i i V 20 0.4 / o 1 000 Ώ, he power dissipaed in he d e r r e V resisive load equals 10 MW. Figure 4.22 w 10 0.2 o P shows ha or a load o 10 MW he power 0 0 acor o he line curren I line reaches 1. 0 200 400 600 800 1 000 Te volage V out hen roughly equals V in . Load resistance value R () Te line curren is hen 0.1 kA. For a load resisance higher han 1000 Ώ, and consequenly a load lower han 10 MW, Figure 4.21. Ratio of ingoing and outgoing voltage of a 100 he line curren sars o lead V in . V ou is kV transmission line, with the line current and the power dissihen slighly exceeding V in as in he ear- pation in the load depending upon the resistance of the load. lier example o no load a all, as can be seen rom he yellow line in figure 4.21. For a load resisance lower han 100 Ώ, he volage over he load decreases so much ha he power dissipaion in he load also decreases. Te maximum power dissipaed in he load or he given ransmission line dimensions and line volage is, hereore, 37 MW wih a curren o 0.5 kA. Only shorening he ransmission line and hereby reducing is impedance will help in ransporing more power o he load. e n i l
n i t u o
load
4.5.3. Case with a mixed load at the end of a transmission line In realiy, he load a he end o a ransmission line is generally mixed, wih a resisive and an inducive par. Many renewable energy sources, such as wind urbines and solar panels, provide only acive power and do no paricipae in providing reacive power, so ha he generaors supplying he remaining 1.0 load will sense a highly reacive load. Tis t 0.9 subsanially lowers he capaciy o a rans n e r r 0.8 mission line beween he load and he gen u Capacitive Inductive c e eraors, as will be shown wih an example. 0.7 n i l Figure 4.22 shows he model o he power φ s 0.6 supply sysem o be used or his. o C In he case o a load consising o 0.5 0 5 10 15 20 25 30 35 a resisor o 100 Ώ and an inducive Load (MW) reacance o j100 Ώ, he impedance o he capacior a he end o he line is so high compared wih he load ha is Figure 4.22. Power factor cos φ of the line current I as a conribuion o he line curren can be function of resistive load for the transmission system shown in negleced. Te impedance Zload o he figure 4.20. e n i l
l
line
92 Power supply challenges load equals √ (R load2 + X load2) = √ (10000 + j 126 Ω 10 Ω I generator I line 10000) = 141 Ώ. Te power acor cos φ o V out he load equals R load/Zload = 0.71. Tis value Rload o cos φ can be quie realisic locally when V in – j 6 366 Ω – j 6 366 Ω here are many acive-power-only providing jX load renewables in he sysem. Because he capacior a he load end o he line has been negleced in his case, he line impedance and he load impedance can Figure 4.23. Model of a high-voltage 200 km overhead power transmission line with a load having resist be presumed o share he same curren I line . ance and inductance Te impedance o he capacior a he beginning o he line is also quie high, so we will also neglec ha elemen. Te combined inducive reacance o he sysem is hen j126 + j100 = j226 Ώ and he combined resisance 10 + 100 = 110 Ώ. Tis renders a com bined impedance o √ (1102 + 2262) = 251 Ώ. For a volage V in o 100 kV, he resuling curren I line equals 100000/251 = 398 A. Te volage V out over he load can hen be deermined rom Zload · I line = 141 Ώ · 398 A= 56.12 kV. Tis represens a dramaic volage drop over he ransmission line, since he volage a he beginning o he line is 100 kV. Ye, he power dissipaed in he resisor o he load is only 398 2 · 100 = 15.8 MW. Tis example illusraes ha long high-volage overhead ransmission lines have severe difficuly in ransporing reacive power. Even or a relaively low acive load, he volage a he end o he line easily collapses in he case o a load having a high inducive componen. Figure 4.24 illusraes wih phasors why he volage over he load collapses so easily in he case o a parly reacive load wih a cos φ value o 0.7. Te high line
V in = 100 kV
V L(line) I generator I line j 126Ω
V R(line) V L (line) = 50 kV
10 Ω 100Ω V r(load)
V in
V L (load) = 40 kV
V out V L(load)
j 1 00Ω
V out = 56 kV
φload V r(load) = 40 kV
I line
V r(line) = 4 kV
Figure 4.24. Phasor diagram of a 200 km 100 kV transmission line with a load of impedance Z of √ (1002 + 1002) = 251 Ώ and of cos φ = 0.7
4. Active and reactive power
93
inducance compared o he load induc200 800 ance drasically reduces he volage over 150 600 he load. Te line curren I line lags behind I line Vin 400 he volage V in a he beginning o he line ) 100 ) Vout V A 200 k 50 ( ( by φline = 64°, since cos φ line = (40+4)/100 t e n 0 0 g e = 0.44. r a r t l –50 –200 u o Te volage drop over he ransmis C V –100 –400 sion line decreases i he values o he –150 –600 resisor and he inducor o he load are –200 –800 increased wih respec o he values given 0 10 20 30 40 in figure 4.24. Tis auomaically reduces Time (ms) he power ransmited over he line. For a value o R load as well as X load o 625 Ώ, he volage over he ransmission line Figure 4.25. Phase shift between the voltage Vin at the drops by only 10%. Such a volage drop beginning of the transmission line and the voltage Vout at the is generally considered as he maximum end of the line, and the phase shift with the line current Iline for the conditions shown in figure 4.24. accepable value. Te power dissipaed in R load equals 6.5 MW in ha case. By comparison, or a purely resisive load he power dissipaed in he resisor equals 26 MW or a volage drop o 10%. Figure 4.26 illusraes ha he energy ranserring capaciy o a high-volage overhead ransmission line heavily decreases when he load has a relaively low power acor. In he example given, he maximum amoun o energy ha can anyhow be ) ranserred or a cos φ o 0.7 is almos 16 % ( 100 n i MW (see Figure 4.26). I he impedance V cos φ = 1 / 90 t o he load is urher lowered, he power u o dissipaed decreases since he volage over V o i t 80 he load urher decreases. I will be clear cos φ = 0.7 a r e ha i he lengh o he line is shorened, cos φ = 0.9 g 70 a t l he volage drop will be smaller. Te line o V inducance and resisance are direcly 60 0 10 20 30 40 proporional o he lengh. Active power of load (MW)
4.6. Risks created when insufficient reactive power is supplied by renewable energy sources
Figure 4.26. Ratio of ingoing and outgoing voltage over a 200 km 100 kV transmission line as a function of active power demand for different power factors cos φ of the load.
Tis chaper has explained ha power supply sysems have o provide boh acive and reacive power. Elecric loads have an apparen resisance, or impedance. Te impedance consiss o a resisive (real) par and a reacive (imaginary) par. Te reacive pars originae rom he capaciive and inducive elemens in he sysem. Te same applies or ransmission and disribuion lines. High-volage overhead ransmission lines have a ne
94 Power supply challenges inducance, while underground disribuion cables have a ne capaciance. On average, he combined elecric loads in a power supply sysem have a slighly inducive characer wih a power acor close o 0.85. Ta means ha he real par o he apparen resisance or impedance has a value ha is 85% o he impedance, while he inducive par equals 53% o he impedance. Te square roo o he sum o he real par squared and he reacive par squared makes 100% again: √ (85 2 + 53 2) = 100. Tis has been explained in more deail in he previous secions. Overhead ransmission lines have difficuly in ransmiting reacive power o an inducive naure. eacive power ends o drasically decrease he volage over an overhead ransmission line. radiionally, where he elecriciy supply has come rom power plans only, he plans were buil in close proximiy o he locaions where he energy was consumed. Tis mean ha he ransmission lines were quie shor. Tereore, volage reducion and energy losses over he ransmission lines were small. In he case o remoe locaions, ransormers wih a conrollable ranser raio could adjus he volage o he end users o he desired nominal value. Large consumers wih loads having a low inducive power acor could be asked o insall adjusable capacior unis. Capaciors help o compensae or he reacive par o he load. Also, grid operaors ake measures o compensae or reacive power. So-called SVCs (saic VAr compensaor) and Sacoms (saic synchronous compensaors) can provide as-acing reacive power or high-volage elecriciy ransmission neworks. Someimes, grid operaors insall so-called roaing synchronous condensers. Such machines are synchronous generaors running wihou a prime mover, such as an engine or urbine, o drive hem. By adjusing he magneic field o he spinning generaor, a capaciive load, or even an inducive load, can be creaed. Cerain ransormer ypes can also compensae or highly inducive loads. Neverheless, large power saions carried he bulk o he reacive power in he pas. Currenly however, wih he adven o many local generaors based on renewable energy sources, such as solar radiaion and wind, he relaive number o acive power saions is ofen drasically reduced. Chaper 3 shows ha even wih an average elecriciy supply o 20% rom wind urbines, occasional high winds can push almos all power saions rom he grid. Te same applies or areas where more han 15% o he elecriciy is produced rom solar PV panels. Solar panels in paricular are generally no equipped wih auomaic power-acor conrol sysems, bu synchronous generaors are. I relaively litle power in a sysem is provided by large power saions, jus a ew power saions need be online. Consequenly, he running power saions are ofen locaed much urher rom he cusomers han in he pas. Tereore, i hese ew power saions have o provide he bulk o he reacive power via overhead ransmission lines, here is a high risk ha he volage o he remoely locaed cusomers will collapse over he long lines. Volage collapse due o a long-disance reacive power supply is ypically a local phenomenon. I does no mean ha he power plans a he generaor end o he ransmission line will rip because o overloading. Te long ransmission lines simply
4. Active and reactive power
canno ransmi he acive and reacive power in case o a highly reacive load. Consumers using he same ransmission line, bu a shorer disance rom he power plans, may no see he volage drop ha disan consumers experience. Te problem o volage collapse and energy ransmission resricions can be avoided by insalling more local power capaciy based on synchronous generaors. Modern moderaely sized power plans offer excellen backup capaciy. Tey can help o provide reacive power where here is a high peneraion o renewable sources wih varying oupu.
4.7. Conclusions ransporing large levels o reacive power via alernaing curren ransmission lines over long disances leads easily o volage collapse a he end o he line. In such cases, he acive power ranspor capaciy is drasically reduced. Tis can cause a blackou. Tereore, uure elecriciy supply sysems wih a high proporion o wind urbines and solar panels canno only use large power plans ar away rom he load or load balancing and requency conrol. Tis would have a negaive impac on supply reliabiliy and adequacy. As a soluion, smaller power saions based on modular generaing unis in parallel should be posiioned a relaively shor disances rom he load cenres. Tis solves he problem o local volage collapses caused by reacive power.
95
5 Energy storage I would be very convenien i elecrical energy could be easily and cheaply sored in large quaniies. Sorage would help o shave peaks in elecriciy demand and o provide reserve capaciy in he supply sysem. Excess elecriciy rom wind urbines migh be used when here is no wind. Dayime power rom solar panels could be sored or he evening and abundan solar energy in he summer could be used in he winer. Unorunaely, convering elecrical energy ino chemical or mechanical energy creaes significan coss and losses o energy. o suppor renewable power, he mos promising soluion is he inegraion o elecriciy and hea use, including hea sorage.
98 Power supply challenges
5.1. The enormous challenge of energy storage Everywhere in naure and hroughou sociey in general, he sorage o energy is essenial. Plans sore energy as carbohydraes. Animals sore energy in a layers or use in imes when ood is scarce. Mankind learned o sore crops rom he erile seasons. Har vesing energy rom he sun, which is he basic energy provider or he world, naurally occurs in baches. A ypical example is he poao. Poaoes are seasonal plans, and he poao crop normally peaks during he second hal o he summer. aking he complee poao crop o he consumer marke a once would resul in very low prices emporarily, while also creaing a high risk o having no poaoes on he marke long beore he new crop arrives. Tis is why sorage is essenial. A warehouse owner can buy poaoes a harves ime while he cos is low, sore hem and sell a a higher price laer. raders call he conneced profi making mechanism arbirage. Where here is compeiion beween warehouse owners, he price increases can be limied. A reliable energy supply or susaining he economy in modern sociey has much in common wih a sable ood supply or susaining lie. In boh cases, sorage is indispensable. Te previous chapers o his book have shown ha energy is difficul o sore as elecriciy. radiional power saions use uel or medium-erm energy sorage. Fuel is an excellen example o sored energy. Figure 5.1. A reliable and affordable food supply Hydropower-based generaors require cheap and efficient means of storage. use eiher he poenial energy Same applies for energy supply. sored in elevaed waer levels or he kineic energy rom waer sreams. able 5.1. gives some ypical values or he volumeric energy densiy o common energy sources or elecriciy producion. Te values given are approximaions, since hey depend on he composiion o he energy source. Coal, wood and oil can be sored in bulk nex o he power saion. Gas is ed o he saion via a pipeline or sored locally as liquefied gas. As an example, a coal-fired power plan wih a nominal elecrical oupu o 500 MW and a uel efficiency o 40% consumes 500/0.4 = 1 250 MW o uel. Tis equals 1.25 GJ/s. Coal has a volumeric energy densiy o around 19 GJ/m 3 , he exac value depending on is composiion. Te 500 MW power plan uses, hereore, 1.25/19 ∙ 3 600 ≈ 237 m 3 o coal per hour. Tis is a subsanial volume. Neverheless, a sorage area o 100 m by 100 m and a heigh o 10 m conains almos enough coal or 18 days o he power plan’s ull oupu. o illusrae his coal demand, he
5. Energy storage
99
load capaciy o a ypical Panamax bulk carrier ship o Table 5.1. Approximate volumetric energy 75000 onnes is needed or hese 18 days o power plan density of common energy sources for elec9 operaion. Knowing ha a 500 MW elecrical oupu is tricity generation (G = 10 ). he equivalen o he average power oupu rom 5 million Volumetric energy Energy source density manual labourers brings hese figures ino perspecive. I is an enormous uel sream ha is needed o generae a conCoal 19 GJ/m inuous elecric energy flow o 500 MW. Forunaely, uels Fuel oil 35 GJ/m such as coal, oil, and high-pressure naural gas conain a significan amoun o energy per cubic mere. Natural gas at 80 bar 3 GJ/m I is ineresing o noe ha a human being needs abou Liquefied natural gas 21 GJ/m 3000 kcal o ood per day in order o provide 8 hours o Animal fat 33 GJ/m hard manual labour. Tese 3000 kcal equal 12.5 MJ, since 1 kcal = 4.1868 kJ. Tree daily shifs o 5 million workers Wood 10 GJ/m each, alogeher 15 million workers, would be needed o Water at 400 m elevation 0.004 GJ/m coninuously roae a generaor producing 500 MW. Te Uranium 235 1500000000 GJ/m oal ood consumpion by hese workers would be 225 J/ day, or 6 800 m 3 o animal a wih a calorific value o 33 Compressed air at 80 bar 0.033 GJ/ m GJ/m3. Te power plan uses abou 108 J per day when Hydrogen at 80 bar 0.85 GJ/ m i is running a ull oupu on coal, bu he workers would need in any case some 135 J o susain heir bodies when hey do no work. Tereore, he uel efficiency o he workers and ha o power plans are no so differen. Neverheless, his example illusraes he high energy supply needed o keep a 500 MW power plan running. For hydropower, considerably larger sorage volumes are required han or uels. Te 4 MJ/m 3 o poenial energy E potent ial given or waer a a 400 meer elevaion in able 5.1. has been derived rom: 3
3
3
3
3
3
3
3
3
3
E potential = ρ · g · h
Equation 5.1.
in which densiy ρ = 1000 kg/m3 , g = acceleraion o graviy ≈ 10 m/s 2 and h = 400 m. I he efficiency o he urbine-generaor combinaion o a hydropower plan equals 80%, one needs 500 MW/0.80 ∙ 4 MJ/m 3 = 156 m 3 o waer per second flowing rom an elevaion o 400 m o deliver 500 MW o elecriciy. By comparison, he river hine has an average waer flow o 2300 m 3/s when i arrives a he border beween Germany and Te Neherlands. Te river Elbe has an average waer flow o 711 m 3/s when meeing he sea, while he Danube’s flow is abou 1500 m 3/s when i leaves Germany. Te average elecriciy demand in Germany in 2010 was 67 GW, which is a acor 134 higher han he 500 MW in he example. By comparison, average elecriciy demand was 473 GW in he USA and 450 GW in China in he same year. Looking back o he example given using coal, o run he 500 MW hydropower plan a ull oupu or 18 days, one needs 18 ∙ 24 ∙ 3600 ∙ 156 ≈ 245 million m 3 o waer a a 400 m elevaion. Tis equals an area o 5 km by 2 km wih he waer level saring 25 m above he 400 m elevaion.
100 Power supply challenges Te example shows ha deriving energy rom raised waer levels in order o mee he elecriciy consumpion akes a huge amoun o waer. Te energy densiy o pipeline naural gas expressed in GJ/m 3 is roughly a acor 7 500 higher han ha o waer a an elevaion o 400 m. For coal, i is a acor o almos 50 000. Te idea o having 100% elecriciy producion based on wind and solar power wih energy sored in raised waer levels sounds ineresing bu appears difficul o realize. ime spans wihou wind over large areas can las more han a week, while solar panels provide hardly any elecriciy during he dark winer monhs. Covering elecriciy demand or prolonged imes wihou uels remains, hereore, a major challenge. Neverheless, he sorage o elecriciy as elecrical, mechanical, chemical or even hermal energy migh be economically atracive. Ta can be he case in deregulaed elecriciy markes so as o make a profi rom arbirage, and in regulaed markes so as o lower coss or consumers. Te economics depend on he applicaion. I makes a grea difference wheher he sorage is used or: • Short-term applications, such as reserve capacity and frequency regulation; • Medium-term applications, such as peak shaving with daily charg ing and
discharging; • Long-term applications for bridging seasonal dierences in renewable
oupu. Te nex secions o his chaper will discuss a number o sorage mehods.
5.2. Basic properties of energy storage devices Energy sorage devices have a number o properies ha deermine heir applicabiliy and value in elecriciy supply sysems. Basic properies are he amoun o energy (MWh) o be sored and he speed o energy charging and discharging (MW). Figure 5.2 gives a schemaic represenaion o an energy sorage device in an elecriciy supply sysem. Te concep is such ha any elecriciy produced bu no consumed will immediaely be sen o he sorage device. By combining a converer and a charger, elecrical energy is urned ino he ype o energy ha can be sored. An elecric moor can urn elecrical energy ino mechanical energy direcly by, or example, acceleraing a flywheel or indirecly by driving a waer pump or an air compressor. An AC o DC converor urns an alernaing curren ino a direc curren or charging a batery or or he elecrolysis o waer or hydrogen producion. Te power capaciy o he charger is generally deermined by wo hings: he size o he charger and he power ha he sorage device can accep. In he case o pumped hydro, he waer basin isel is no a limiing acor in he speed o filling i, bu he size o he combined elecric moor and pump is. For bateries however, he charge curren is limied. Charging oo quickly overheas he batery and causes a permanen loss o sorage capaciy.
5. Energy storage
Direct delivery of electricity
Electricity production
Converter/ charger MW
Converter/ discharger MW
Electricity consumption
Energy storage MWh/GWh/TWh
Energy storage device
Figure 5.2. A representation of an energy storage device
Faser charging can also reduce he lie o a batery. Convenional lead-acid baeries used in cars require slow recharging bu can susain a large shor-erm discharge curren when he engine sars. In oher words, he charging device can have a differen power capaciy han he discharging device. Te energy sorage capaciy depends on he size o he sorage device. Bateries consis o chemical cells. A large number o cells can be mouned in parallel o creae more capaciy. For pumped hydro, simply a larger waer reservoir is needed o creae more capaciy. Energy sored as hea or chill canno be effecively convered back o elecriciy. Such energy sorage is raher useul or demand-side managemen o elecriciy supply. Elecric heaing coils in ho waer reservoirs can be swiched on during imes o high oupu o wind urbines and low elecriciy demand. erigeraed warehouses can be exra cooled a imes when elecriciy prices are low. A limiaion here is ha many edible producs have o be sored wihin a narrow emperaure range o preserve heir qualiy. Energy sored in uels shows pracically no decay. Mos sorage devices, however, lose energy wih ime. Chemical bateries lose a leas 1% o heir ull charge per day and flywheels some 5%. By conras, open waer reservoirs can mainain heir energy. Te evaporaion can be more han compensaed by rainall, alhough his depends on he local climae and he season.
5.3. Applications for energy storage devices Te perormance requiremens or sorage devices depend on heir inended use. Since sorage devices can boh ake in and give ou energy, hey can in principle be used in all balancing asks needed or securing he supply o elecriciy. I is he ime scale especially ha deermines he economic opporuniy or a sorage device. Such an opporu-
101
102 Power supply challenges
Table 5.2.
Balancing tasks in electricity supply systems.
Seconds
Minutes
Hourly
Daily
Weekly
Seasonal
Frequency control
Load following
Ramp rate smoothing
Peak shaving
Wind output smoothing
Summer/winter pattern smoothing
Voltage support
Predicton error compensation
Load following
Solar output smoothing
Weekend storage
Summer solar use in winter
Primary reserves
Secondary reserves
Demand response
Arbitrage
niy exiss only i marke rules allow remuneraion or operaing a sorage aciliy. able 5.2 summarises he balancing asks or differen ime-scales. Te deerral o grid expansion and o peaking-power generaor invesmens can be an addiional benefi o energy sorage. Figure 5.3 illusraes he possible asks or sorage sysems wihin a 24-hour ime span. I a sorage sysem is used or coninuous requency conrol, i is acually permanenly in operaion. Te amoun o energy exchange is in his case relaively small compared wih he power capaciy. For daily peak shaving, energy can be accumulaed during he nigh. Charging can ake place rom 11 pm o 6 am when demand is low, and discharging can ake place rom 5 pm o 9 pm when demand is high. Charging would ake 7 hours and discharging 4 hours in his example. Tis means ha he sorage device charger can have a power capaciy ha is a acor o 7/4 = 1.75 lower han ha o he discharger. Oher opporuniies or sorage sysems include compensaing or differences beween orecased and real demand, and beween orecased and real renewable oupu. Elecriciy demand isel can be prediced quie well based on hisoric demand paterns and weaher orecass. Large emporary deviaions can, however, occur beween he prediced and acual wind power. Tis is especially rue when he wind speed changes rapidly rom high o low or vice versa. A difference o fifeen minues beween he orecased wind speed and he acual wind speed can make a huge difference in he oupu o a wind arm. Te predicion error migh be posiive or negaive, so he size o he charger and he discharger has o be he same. Frequency conrol migh be a special applicaion or shor-erm energy sorage. In marke-based supply sysems, requency deviaions occur especially each ime a new rading ime begins, say every 15 or 60 minues. Te injecion or absorpion o energy a hose momens can minimise requency deviaions. Wind park oupu can remain high or a number o consecuive days, and hen be ollowed by some 10 days o litle or no wind a all. Where 20 % o he average elecriciy demand in an area has o be covered by wind energy, he insalled wind urbine capaciy should be 80% o he average power demand i he capaciy acor o he combined wind urbines is 25%. Tis can be simply calculaed: wih a capaciy acor o 25%, on average 0.25 ∙ 80% = 20% o elecriciy demand is produced by he wind urbines. In he case o high winds covering a large area, he combined oupu
5. Energy storage
Peak shaving
Wind prediction error compensation
Peak load
d n a m e d r e w o P
Intermediate load
+ Frequency control + Voltage control + Contingency reserve + Arbitrage
Base load
0
2
4
6
8
Ramp rate smoothing
10
12
14
16
18
20
22
24
Time of the day (hours) Figure 5.3.
Possible energy storage applications in time scales up to 24 hours.
o he wind urbines can provide more power han he insananeous demand, especially during he nigh. Imagine ha he winds are so high ha 90% o he insalled wind urbine capaciy is running a ull oupu. Ta means ha 0.90 ∙ 80% = 72% o he average power demand is produced by wind. During he nigh, power demand can easily be less han 72% o he average demand. emporary sorage o par o he oversupply o wind power oupu would help o leave space or some dispachable power generaors in he grid, and he energy sored could be used laer during imes o litle wind. Te charging power o he sorage device would be higher han he discharge power in his case. I he excess summerime oupu rom solar panels would be sored or use in he winer, he sorage capaciy would have o be large. Enough energy sorage or a leas 3 monhs is needed in a case like his. Sysems such as flywheels, bateries and compressed air can immediaely be discarded or such applicaions because heir sorage capaciy is limied. Also, hydrogen canno be used or long-erm sorage because o is low volumeric energy densiy. Some sorage sysems are by naure bi-direcional. Tis means ha hey can immediaely swich rom charging o discharging. Lead-acid bateries have his abiliy. For pumped-hydro and compressed-air sorage, i akes some minues o swich mode. Te bi-direcional capabiliy sops when a sorage sysem is ull or empy. Idenical resricions apply or ho waer and chill sorage sysems. I he emperaure limis or ho waer reservoirs or rerigeraed producs are reached, such emperaure-based sysems lose heir sorage or dispach capabiliy. Demand-side managemen sysems generally have a lower capaciy during he nigh han during working hours, simply because here is less elecriciy demand during he nigh.
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104 Power supply challenges
5.4. Methods and costs of energy storage 5.4.1. Lead-acid batteries Price saemens or energy sorage sysems can be conusing. A lead-acid batery will be used o illusrae his. Lead-acid bateries are a proven echnology and require a low invesmen compared wih, or insance, lihium-ion bateries. Our example is a 12 V and 80 Ah batery, ypically used in cars. Based on is nameplae, one migh presume ha a 12 V and 80 Ah batery has a maximum amoun o 12 ∙ 80 = 960 Wh energy sored, equalling almos 1 kWh. Te reail price o such a baery is abou 75 €. A firs sigh, he sorage componen o he lead-acid batery-based sorage sysem would hereore cos abou 75 €/kWh. Tis is however no he case, as will be explained below. Te recommended charge curren or a 12 V 80 Ah batery is some 8 A, so an AC o DC charger o 8 A ∙ 12 V ≈ 100 W is needed. Such a charger migh cos 25 €. Full charging wih 8 A akes roughly 11 hours, since he charging efficiency is 90%. Charging he batery wih oo high a curren will release hydrogen and oxygen, resuling in a loss o liquid in he batery. oo high a curren leads also o overheaing and shorened batery lie. Figure 5.4. A conventional lead-acid battery. Te sandard maximum discharge curren o a leadacid batery expressed in ampere is abou hree imes he nameplae capaciy in Ah, in his case 3 ∙ 80 = 240 A. Te discharging power o he batery is hereore 240 A ∙ 12 V = 2.7 kW. A DC o AC discharger o such a capaciy migh cos 50 €. Te given discharge capaciy o 2.7 kW does no mean however ha he batery can use is ull charge o energy o 0.96 kWh o supply his power. I ha would be he case, he 2.7 kW could be delivered during 0.96 kWh/2.7 kW ≈ 36/100 hour = 21 minues. Te nameplae capaciy in Ah is, however, only valid when discharging he batery more slowly, during a ime span o 20 h. Ta would mean a discharge curren o 4 A insead o he maximum 240 A. However, here is anoher resricion, since discharging he batery deeper han 50% o is maximum sorage capaciy will drasically reduce is lie. Tereore, he pracical sorage capaciy o a 12 vol, 80 Ah lead-acid Table 5.3. Investment in a 12 V 80 Ah batery in dynamic operaion is less han 0.5 kWh. Tis lead-acid battery-based storage device. means ha he real specific capial invesmen in a lead€ Item acid batery or sorage use in an elecriciy supply sysem Battery 75 urns rom he iniial purchase price o € 75/kWh o more Charger 25 han € 150/kWh. Tis leads us o sorage sysem coss. Te batery plus Discharger 50 he charger and discharger require an invesmen o some Total 150 75 € + 25 € + 50 € = 150 € or an effecive 0.5 kWh sorage
5. Energy storage
Direct delivery of electricity
Electricity production
AC/DC converter charger 100 W
DC/AC converter discharger 2.7 kW
Electricity consumption
Energy storage 1 kWh = < 0.5 kWh effective Car size 12 V 80 Ah lead-acid accumulator
Figure 5.5.
Example of a lead-acid energy battery for use in electricity supply systems.
(see able 5.3. and Figure 5.5). Tis definiively does no mean ha he user has o pay € 150 or each kWh o elecriciy rom he batery. I he sysem can susain a maximum o 2 000 charging/discharging cycles, and no ineres on invesed capial is used, he cos per cycle o delivering he 0.5 kWh during each cycle equals 150 €/(2 000 ∙ 0.5 kWh) = 0.15 € /kWh. Tis has o be added o he price o purchasing he elecriciy. I he elecriciy or charging coss 5 €cn/kWh, one has o add 1 €cn o his because o he 90% efficiency o he charging and o he discharging processes. Te oal price o elecriciy rom he sorage device is hen 15 + 5 + 1 = 21 €cn/kWh, i.e. more han 4 imes he purchase price o 5 €cn/kWh. As menioned beore, his price presumes ha he capial invesed in he sorage sysem is ineres ree and ha no mainenance and operaion coss are incurred. Presuming a discoun rae o 5% and a sysem lie o 5 years, he annual capial coss equal 23.1% o € 150 = 34.65 €, or abou 173 € or he 5 year ime span. For 2 000 ull cycles in his ime span, meaning again a combined energy oupu o 2 000 ∙ 0.5 kWh = 1 000 kWh, his renders specific invesmen coss o 17.3 €cn/ kWh. Ta resuls in an oupu elecriciy price o 17.3 + 5 + 1 = 23.3 €cn/kWh. A lead-acid batery also needs regular inspecions o he level o he liquid covering he elecrodes. Ta migh ake wo man hours over he lie span o he batery a a cos o say 75 €. Tis adds 7 500 €cn/1 000 kWh = 7.5 €cn o he elecriciy coss. In addiion, a lead-acid batery loses charge a a rae o 10% o is ull charge per monh. For a ime span o 5 years, his equals 600% o is ull charge, or 6 kWh. In our example o 5 €cn purchase coss per kWh, his decay loss amouns o only 60 €cn in 5 years which is close o negligible. o summarize, he applicaion o his batery or some 2 000 cycles wih a deph o discharge o 50% will add almos 26 €cn o he original kWh price o 5 €cn. Afer 5 years o use, bateries are hazardous wase and heir reamen may creae addiional coss. And, o course, afer every 5 years new bateries would have o be acquired.
105
106 Power supply challenges Using he lead-acid batery or requency conrol, providing raher shor-erm power delivery and shor-erm power absorpion based on a very small deph o discharge, he echnical lie o he batery migh be 10 years. During ha ime span, some deerioraion o he batery will occur due o inernal corrosion. Te nominal charging power o he batery in he example was shown o be 100 W and he price o equipmen 150 €. Tis means ha he invesmen in absorbing power rom he grid is 150 €/100 W or 1 500 €/ Table 5.4. Summary of the total costs of kW. Te maximum emporary discharging power equals electricity from a lead-acid battery with 2000 2.4 kW, resuling in a discharge power invesmen o only cycles and an effective storage capacity of 0.5 kWh. €150/2.4 = 62.5 €/kW. Tese are figures o consider when using sorage echnology or shor-erm balancing such as Costs item €cnt/kWh requency conrol. By comparison, a gas-uelled generaor Capital costs 17.3 can do he same job and migh cos abou € 600/kW. Maintenance costs 7.5 Te power ramp-up and ramp-down imes also play a role in esimaing he value o energy sorage in Charging energy costs 5 grid sysems. Elecro-chemical batery sysems respond Efficiency loss costs 1 immediaely wih maximum capaciy, which makes hem Total costs 30.8 very suiable or primary reserves. Tis is especially rue in island operaion and emergency supply sysems. For shor-erm balancing, he size o he bateries seems less imporan. However, heir use is resriced o he poin where a he batery has reached is minimum charge level, or o he poin where he batery is ully charged. Te key perormance indicaors o lead-acid-batery based energy sorage sysems are summarized in able 5.5. Tese fig ures are indicaions and deailed specificaions differ, depending on equipmen size and supplier. Te key perormance indicaors (KPIs) are needed o invesigae he realisic applicaions o a sorage echnology. Te KPIs given in able 5.5. reveal immediaely ha soring solar-PV based energy accumulaed in he summer, May – Augus, o be used in he winer, November – February, is never economic wih lead-acid baeries. I would mean using he sorage aciliy or only one cycle per year wih an average sorage ime o six monhs. Hal o he sored energy would be los due o decay. Wih a discoun rae o 5% and a maximum sysem lie o 10 years, he coss per kWh delivered would be a leas € 45.
Table 5.5. Example of the key performance indicators of a small lead-acid battery based energy storage system. Effective energy storage investment
Turnaround efficiency
Energy decay
€/kWh
%
%/month
150
80
10
Maximum depth of discharge
Charge powerbased investment
Ramp-up time
cycles
%
€/kW
min
2000
50
1500
0
System life
Discharge. Ramp powerdown time based investment €/kW 56
min 0
Operations + maintenance
costs % of capital 10
Turnaround time min 0
5. Energy storage
As shown above, using he batery or daily peak shaving wih energy accumulaed during he nigh would add “only” abou 25 €cn o he kWh price. Frequency conrol on a per-minue basis using he high discharge power migh offer a more economic opporuniy, bu he low charging power o lead-acid bateries is a botleneck. One convenional applicaion o such bateries in elecriciy supply sysems is o pro vide energ y or he saring moors o engine-driven generaor ses. Anoher common use is in emergency applicaions in cases o grid ailure. Bateries are needed during he inerim period when uel-based generaors are saring up. A proven applicaion o lead-acid batery-based sorage is or off-grid use in combinaion wih solar panels a locaions wihou an elecriciy grid. In elecriciy supply sysems, he bes applicaion migh be or primary conrol reserves in he case o power plan rips. Te bateries could immediaely supply heir maximum power when needed, while he sorage capaciy needs only o be enough or abou 10 minues unil he secondary reserves sar working. Anoher applicaion could be o compensae or he loss o ineria caused by many indirecly coupled renewable energy sources in he sysem.
5.4.2. Lithium-ion batteries Lihium-ion bateries are bes known or heir use in hybrid and ully elecric cars. In hese bateries, one elecrode is made o graphie while he oher one is lihium-meal oxide. Teir as charging rae when compared wih lead-acid bateries and heir acor seven higher energy conen per kg make hem suiable or mobile applicaions. When recovering kineic energy rom a vehicle during deceleraion, as charging is necessary. Boh he charging and discharging power o a Li-ion batery relaive o is energy sorage capaciy is abou 1 W/Wh. In comparison, or a lead-acid batery he opimum charging power is abou 0.1 W/Wh and he shor-erm discharging power is 2.7 W/Wh. Tese are average values and may differ in individual sysems. Jus as in lead-acid bateries, he energy sored in Li-ion bateries decays wih ime. Te decay in Li-ion bateries depends very much on emperaure; a 20 °C i is abou 8% per monh bu a 50 °C i is already 23%. Teir allowed deph o discharge (DOD) is some 80%, which is beter han he 50% DOD o lead-acid bateries. Te price o Li-ion bateries is currenly around 500 €/kWh. Bu he price o a uiliy-scale sorage sysem appears o be much higher. A UK projec launched in 2013 in Bedordshire o 10 MWh capaciy inended or peak shaving, renewable energy accommodaion and grid-expansion deerral, is said o require an invesmen o £ 18.7 M. Tis equals 2 175 €/kWh o sorage capaciy. Apparenly, he charging and discharging equipmen connecing he bateries o he high-volage grid, ogeher wih he conrol and proecion devices, requires his level o invesmen money. As wih lead-acid bateries, Li-ion bateries can accep a limied number o deep discharging cycles. Currenly, replacemen is needed afer some 1 500 o 6 000 cycles. Again, afer his, new bateries will have o be insalled and he old ones reaed as hazardous wase.
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108 Power supply challenges
5.4.3. Pumped hydro According o he World Energy Oulook 2013 publicaion rom he Inernaional Energy Agency, hydropower-based generaion produced 3 566 Wh o elecrical energy in he year 2011. I is, hereore, by ar he larges renewable energy source in he world, meeing 16% o global elecriciy needs. Te combined insalled power capaciy equalled abou 1 W. Te average uilisaion acor o hydropower is hereore 3 566 Wh/8 760 h ∙ 1 W = 41%. One o he reasons or his relaively low uilisaion acor is he variaion in demand over he course o a day and varying producion over he seasons. In some regions, no waer is available during par o he year. Te Tree Gorges Hydro aciliy in China is an example o an impressive 22.5 GW projec inended o produce 100 Wh per year, reaching a uilisaion acor o 51%. Figure 5.6 shows he eigh counries ha have he larges elecriciy producion rom hydropower in he world. In Norway, Venezuela, Brazil and Canada, a subsanial racion o he oal elecriciy demand is me by hydropower. Hydropower aciliies are concenraed in areas wih high precipiaion and subsanial differences in elevaion. In counries where much elecriciy is generaed by hydropower, pumped sorage is generally no needed. Convenional hydropower is excellen or balancing elecriciy producion and demand. Pumped sorage, in com binaion wih hydropower, migh only help o raise he waer level in basins during he rainy season when here is excess energy rom run-o-he-river generaors. Providing aciliies or pumped hydro sorage in counries ha have o depend primarily on flucuaing renewable sources such as solar and wind, or reducing ossil uel consumpion is no easy. Such counries do no have much hydropower, a leas no ye, and finding space or subsanially large elevaed waer level reservoirs
1 000
r e w o p o r d y h ) h m W o r T ( f y t i c i r t c e l E
100
800
80
600
60
400
40
200
20
0
0
a a i n a d h n C C a
l i a z r B
A U S
i a a y s s w r u R N o
e s u y t i c i r t c e l e l a n o i t a n l a t o t f o %
a d i I n
l a e u z n e e V
Figure 5.6. The eight countries with the highest electricity production from hydropower and its fraction of total electricity demand (approximate figures, data for the year 2009).
5. Energy storage
109
is ofen problemaic. Currenly, he globally Upper reservoir insalled capaciy or pumped hydro sorage is Dam esimaed o be beween 90 GW and 127 GW, according o diverse lieraure sources. Tis equals jus 3% o he world’s insalled power t e generaion capaciy. r a e Tunnel e n In 2009, he members o Eurelecric G penstock (comprising almos all he European energy powercompanies) produced abou 550 W h rom house p Lower m reservoir hydropower, equalling 16% o he elecriciy P u demand in he area. Te capaciy was 198 GW. Typical pumped Pumpturbine storage developement Pumped sorage in he Eurelecric area is currenly some 35 GW, wih a sored energy o 2.5 Wh. Tis amoun o sored energy is only Figure 5.7. A typical pumped storage facility. sufficien o deliver he 35 GW or hree days. Te averaged power demand in he Eurelecric area was abou 400 GW in 2009. Te hydropower plans are primarily in Scandinavia, he Alps and he Pyrenees. Imagine now Germany, wih an elecric energy demand o 590 Wh in 2010. Tis convers ino an average power demand o 590 Wh/8760 h ≈ 67 GW. I wind power would cover 50% o Germany’s elecriciy demand in he winer and pumped hydro could fill a gap o 10 windless days, i would require a leas 67 GW ∙ 10 ∙ 24 h∙ 0.5 = 8040 GWh o energy sored in pumped hydro. Wih waer levels a heir maximum, Germany currenly has 30 GWh o pumped hydro sorage. Tis means ha Germany would need more han 250 imes he curren sorage volume. Te conneced power oupu capaciy should be around 35 GW, which is only a acor o 5 higher han he curren oupu capaciy o 7 GW. Tis reveals ha he curren capaciy is only inended o provide shor-erm power. Wih 7 GW o oupu capaciy, he 30 GWh o sored energy is consumed in jus 4 hours. As a consequence, i Germany wans o smooh wind power wih pumped hydro, he logical soluion is o purchase sorage volume rom is neighbouring counries Norway, Sweden and Ausria. Te scale issue is eviden also in soring solar power wih pumped hydro. In he summer o 2013 Germany had abou 40 GW o insalled solar PV. Wih a capaciy acor o 9% over he year, he cells produced 31.5 Wh. I 10 Wh o summer solar power would be sored or he winer, a sorage capaciy higher by a acor o 333 han he curren capaciy o 30 GWh would be needed. Depending on he local siuaion, he invesmen or he energy sorage componen o pumped hydro ranges beween 30 €/kWh and 60 €/kWh. Te required pumps, elecric moors, urbines and generaors cos beween 500 and 750 €/kW. Te energy consulancy DNV-KEMA has revealed plans or an arificial energy sorage island off he coas o Te Neherlands. Te power capaciy would be 1.5 GW wih maximum o 20 GWh energy sored. Tis convers o a maximum ull-load duraion o 20 GWh/1.5 GW, equalling abou 13 hours. Tereore he purpose is
110 Power supply challenges primarily daily balancing. Te esimaed oal Current pumped hydro storage: 30 GWh coss o he island are beween 1.3 and 1.6 billion Euros. I we use he price o 500 €/kW menioned above or he energy converers, he share o he charging and dis charging equipmen or 1.5 GW would be 0.75 billion Euros. Belgium is also considering building such an Capacity of pumped island. Again, because o he lack o sorage hydro needed to store one third of capaciy hese islands will no help in backingsolar energy in Germany up several windless days. Te echnical lie o waer sorage basins can exceed a cenury, while he charging and discharging equipmen migh las 30 o 40 years. Te urn-around efficiency o pumped hydro is abou 75 % or loads over 70%, and he deph o discharge is abou 90%. Annual operaion and mainenance coss migh amoun o 3% o he invesed capial. In he case ha he Figure 5.8. To store 10 TWh of the 31.5 TWh of solar energy island’s 20 GWh sorage would be ully energy produced in Germany annually, the pumped used every day or peak shaving, he added hydro storage capacity would have to be a factor 333 coss per kWh would only be some 3.5 €cn greater. or a discoun rae o 5%. Some evaporaion will occur depending on he waer emperaure and he relaive humidiy o he air, bu rainall will compensae. Te acual energy loss is probably negligible. Swiching beween charging and discharging or pumped hydro akes around 4 minues, while ramping up and down he ull range o some 20% o 100% capaciy akes up o 1 minue. I he sorage island would be used or only a single charging and discharging cycle per year, as in he case o moving solar energy rom summer o winer, he coss per kWh delivered would rise o abou 750 €cn. I will be clear by now ha even large pumped-hydro sorage aciliies are primarily inended or daily peak shaving and no or longerm energy sorage.
5.4.4. Flywheels Flywheel sorage uilises as roaing elemens or accumulaing energy. Te roor is acceleraed, or ‘charged’, by an inegraed elecric moor ha acs as a generaor when he sysem discharges. Te ypical roaing speed is 20000 rpm. o reduce ricion losses, he roors are posiioned in a vacuum
Figure 5.9. Image of a planned energy island in the North Sea, the so-called Plan Lievense. Courtesy of DNV GL, Lievense CSO and Gebroeders Das.
5. Energy storage
chamber and magneic bearings are used. ypically, he amoun o energy sored is 25 kWh wih a discharge capaciy o 100 kW. Te ypical dimensions o such a flywheel would have a heigh o 2 m and a diameer o 1.2 m. Te ramp up ime o ull capaciy is abou 1 second. Flywheels as sorage sysems are sill under developmen. Teir properies could be suiable or requency regulaion and spinning reserve. Unlike bateries, flywheels do no have a limied number o charging cycles. In ha respec hey migh be a preerred opion or requency regulaion. However, heir curren size is oo small or large-scale elecriciy supply applicaions. Mos probably, flywheel-based energy Upper sorage will be limied o smaller, nichevacuum chambers ype applicaions.
5.4.5. Compressed air
Lower vacuum chambers Carbon fiber composite flywheel
111
Axial electromagnet Upper radial electromagnet Patented molecular vacuum sleeve
Pressurised air can release is poenial MotorSynchronous energy by expanding over a urbine or generator reluctance stator 4 pole m-g reciprocaing expander. Pressurised air can rotor also supply a combusion urbine wih air 2” thick steel Lower radial housing or he combusion chamber. Te energy electromagnet densiy o compressed air a a pressure o 70 bar (7 MPa) is abou 29 MJ/m 3 , which is quie low compared o mos oher Figure 5.10. Illustration of a flywheel energy storage device. Image source: Powerthru. means o energy sorage as shown in able 5.1. For soring an amoun o energy ha migh be useul or elecriciy supply sysems, large underground caverns or aquiers are required. One problem wih compressing ambien air is ha unless he air is cooled during compression, i can reach a emperaure o 700 °C when being compressed o 70 bar. Tis drasically increases he power demand or compression. I he air cools off during sorage, he pressure drops in proporion o he absolue emperaure. Cooling rom 700 °C o 100 °C reduces he pressure by a acor 2.6. Tis is why pracical insallaions use inercoolers during he compression process. Te hea rom he inercoolers is hen released o he amosphere. Wih so-called adiabaic sorage sysems, he hea released during he compression process is also sored and is used or heaing he air o creae more air volume beore i expands. Te sorage volume has preerably o be filled agains a fixed pressure creaed by a waer column, oherwise he compression and expansion equipmen has o operae across a wide pressure range. Figure 5.11 is an illusraion o a compressed-air energy sorage sysem (CAES). Tere are wo compressed-air energy sorage insallaions (CAES) is he world: in Hundor, Germany and McInosh, Alabama, USA. Te 110 MW oupu McInosh plan, buil in 1991, requires 0.69 kWh o elecriciy or compression and 1.17 kWh o uel energy o produce 1 kWh o elecrical oupu. Te energeic efficiency o he plan hereore equals 54%. However, proponens o he echnology sae ha he
112 Power supply challenges
Ai r intake
Fuel input Heater
Compressed air 70 bar Water at 70 bar
Figure 5.11.
Diagram of a CAES energy storage system.
energy sored in uel can never be ully convered ino elecric energy. Tey use by convenion a sandard 50% efficiency or urning uel energy ino elecriciy, hus ending up wih an alernaively defined urn-around efficiency o 78.5%. Te 1978 buil, 290 MW oupu Hundor plan has a urn-around efficiency o only 62.5%, even wih his posiive definiion. In he example o a CAES he size o he McInosh plan, providing 110 MW oupu requires a uel supply o 117 MW and a compressed-air supply o 69 MW. I he air is supplied a a pressure o 70 bar and a consequen energy air densiy o 29 MJ/m 3 , he conneced air flow equals roughly 69/29 = 2.4 m 3 per second. o run he plan during 4 hours or peaking applicaions requires a sored volume o 4 ∙ 3600 ∙ 2.4 ≈ 34560 m 3. Tis is a sorage volume o abou 33 m ∙ 33 m ∙ 33 m. I will be clear ha only underground caverns and geological srucures are large enough o hold he required volumes. Apparenly, using CAES or long-erm sorage o large quaniies o energy is no realisic. Te main applicaion migh be ound or use in ime spans less han one day or peak shaving. Compressed-air energy sorage in he discharging mode has sar-up and ramping-up imes equal o ha o an open-cycle gas urbine. Tis would allow i o be used or secondary reserve conrol, bu no or primary reserves as in he case o bateries. Also, is oupu load range is comparable wih ha o a gas urbine, wih a rapidly decreasing efficiency below 70% load. When saring he charging mode, he orque on he driving moors has o be gradually increased wih a compressor bypass. Is power absorpion capaciy ully depends on he size o he compressor. I a caviy o 34560 m 3 has o be filled during 8 hours o sore 34540 m 3 ∙ 29 MJ/
5. Energy storage
m3 = 1001660 MJ ≈ 1 J, he energy flow ino he caviy equals 1001660 MJ/ (8 ∙ 3600) = 34.8 MW. For a presumed isenropic 85% efficiency o he compression process and a moor efficiency o 96%, he power consumpion during charging will be 42.5 MW. Te coss o such a CAES sysem depend heavily on he availabiliy o sufficien sorage volume. Te compression equipmen migh coss some 500 €/kW, he ur bine, hea exchanger and generaor some 1000 €/kW, and he high-pressure valves and conrol equipmen some 10 M€. Tese coss are esimaes or a well-esablished produc; es sies wih iniially unique equipmen migh be acors more expensive. Te above menioned coss add up o a oal o almos 150 M€, or 1360 € per kW o oupu power. o ha has o be added he developmen work or he sorage volume, which depends o a large exen on he availabiliy o a suiable sie. I he sysem is used during 1000 hours a year and he echnical lie is esimaed a 30 years, he annual capial coss (Fixed Charge ae FC) o 150 M€ invesmen or a discoun rae o 5% are 0.065 ∙ 150 M€ = 9.75 M€, since here: (1+0.05)30
FCR = 0.05 ·
30
= 0.065
Equation 5.2.
(1+0.05) –1
Te discoun rae is he ineres rae o be paid or he invesed capial. able 5.6 summarises he coss o he 110 MW CAES plan example operaing or 1000 ull oupu hours per year. Te coss per kWh o delivered elecriciy are abou 15 €cn/kWh. I should be sressed ha no invesmen and operaion coss or he sorage space are included here. Also, any emission charges are excluded. I he elecriciy purchase price during charging would be a ypical 6 €cn/kWh insead o he very cheap 3 €cn/kWh, his would add anoher 2.1 €cn o he coss per kWh. In comparison, a peaking plan running on naural gas can produce elecriciy or less han 10 €cn/kWh or he same boundary condiions as in able 5.6. When we compare he coss or daily peak shaving o a CAES wih ha o pumped hydro, CAES can be more expensive by a acor o 5 o 20.
Table 5.6. Example of the estimated electricity output costs of a 110 MW CAES in the case of 1000 full load running hours per year and an electricity purchase price of 3 €cnt/kWh Costs item
Condition
Capital costs
5% discount, 30 years life
8.9 €cnt/kWh
6 €/GJ fuel, 1.17 kWh fuel/kWh
2.5 €cnt/kWh
3 € cnt/kWh input, 0.69 kWh/kWh
2.1 €cnt/kWh
Fuel costs Input electricity Operation/maintenance Total costs output electricity
Costs
1.5 €cnt/kWh (excluding storage volume costs)
15 € cnt/kWh
113
114 Power supply challenges
5.4.6. Power to gas Te naural gas secor in Europe recenly venilaed ideas on using he large pipelines currenly used or ransporing naural gas, or soring excess elecrical energy as hydrogen. Hydrogen can be produced by elecrolysis o waer. Elecrolysis is barely used or bulk hydrogen producion in indusry because o he poor energeic efficiency and he high coss o purchasing elecriciy. Hydrocarbons, primarily naural gas and oil, are used or producing hydrogen or eriliser acories and refineries. Te heoreical maximum efficiency o be reached by he curren echnologies in producing hydrogen wih elecrolysis is only abou 70 %, despie exensive research carried ou his ar. I hydrogen were o be injeced ino high-pressure gas ransmission pipes, a ypical pressures o 6 o 8 MPa, he process o compression would require a subsanial amoun o energy. Hydrogen is always said o have a high specific energy conen, bu ha reers o is mass based energ y densiy o 120 MJ/kg. Te volumeric energy conen o hydrogen a 0 °C and 101.325 kPa pressure is only 10.78 MJ/m 3 , which is on average only abou a quarer o ha o naural gas. Te poin now is ha compressors are volume-based machines. Compressing 1 kg o hydrogen rom is ambien pressure o 6 MPa or pipeline injecion akes abou 7 MJ o compression energy in a muli-sage process wih inercoolers. Te elecric moor driving he compressor hereore consumes an amoun o elecriciy equal o 6 % o he energy sored in he compressed gas. Ta urher reduces he efficiency o he process. Also, ransporing hydrogen by pipeline consumes more energy han ransporing naural gas. Te reason is again he low volumeric energy densiy o hydrogen, meaning ha close o our imes more hydrogen gas has o be ranspored or he same amoun o energy han in he case o naural gas. Clearly, he ranspor and sorage capaciies o naural-gas pipeline sysems are drasically reduced i hydrogen would make up a large racion o he gas mixure. I he inenion is o conver he sored hydrogen back o elecriciy, he heoreically maximum conversion efficiency is 60 %. Te heoreical maximum urn-around efficiency would be 70 ∙ (1- 0.06) ∙ 0.60 = 0.04 = 40 %. In pracice, however, his efficiency migh be as low as 25 %. Should hydrogen be produced only in imes o peak elecriciy rom renewable sources wih associaed low elecriciy prices, he uilisaion acor o he required equipmen migh be as low as 4%. Invesing in equipmen wih such a low uilisaion can hardly be profiable. I he produced hydrogen were o be injeced ino naural gas sreams, he resuling blend should no show large variaions in composiion. Almos all gas applicaions suffer rom variable gas composiion, because i becomes more difficul o opimise he combusion process wih respec o uel efficiency and emissions. A he same ime, gas meers measure volume sreams and, hereore, a misreading o he amoun o delivered energy will occur i he volumeric energy conen varies. Neverheless, he world’s economy needs much hydrogen as eedsock or he chemical indusry. According o markesandmarkes.com, 53 million onnes o
5. Energy storage
115
hydrogen were produced globally in ) 45 2010. Te energy chemically sored in m Upper calorific value / J 40 ha amoun o hydrogen is 6.35 PJ (or M ( e 35 6.35 million GJ), or 151 million onnes u l Lower calorific value a v 30 o oil equivalen (oe). Tis is roughly f i c i 1.2 % o he global energy supply in r 25 o l 2010, and represens a grea deal o a 20 c C 0 10 20 30 40 50 energy. Abou hal o he hydrogen is Volumetric percentage of hydrogen in a used or making ammonia or erilisers, reference natural gas (%) while he oher hal is used mainly in refineries o build ligher componens Figure 5.12. Reduction in the calorific value of natural gas rom crude oils. Mos hydrogen is through blending with hydrogen. derived rom naural gas, oil, and coal via seam reorming. Te equaions ha govern he process are: 3
HC + H O –> CO + 2H 2
CO + H O –> CO + H 2
2
2
Equation 5.3.
2
Seam reorming has an energy-efficiency o 65–75 %. As menioned earlier, elecrolysis is barely used or bulk hydrogen producion because o he poor ullcycle energy efficiency o uel o elecriciy and elecriciy o hydrogen. However, he chemical indusry could urn heir hydrogen producion ino a hybrid process wih an opion o swich rom naural gas o elecriciy, hereby profiing rom very low elecriciy prices during imes o excess oupu rom renewable elecriciy sources. Such an approach could also reduce he chemical indusry’s greenhouse gas emissions. In summary, inermixing variable amouns o hydrogen wih naural gas or energy sorage would have very low energy efficiency and would deeriorae he gas qualiy. Mehanisaion o hydrogen would produce a higher qualiy gas, bu such a process has even lower energy efficiency han he producion o hydrogen. A beter applicaion or hydrogen produced wih excess elecriciy migh be ound in he chemical indusry.
5.4.7. Heat and chill Convering elecrical energy ino hea or chill or energy sorage reduces hermodynamically he exergeic value o he energy. Elecrical energy has an exergeic value acor o 1, since heoreically i can be convered back ino mechanical energy wih 100% efficiency. Even hough producing hea wih elecriciy reduces he exergeic value o elecriciy, i is ofen worh i. Te need or low emperaure hea in he world is large, or indusrial processes as well as or he heaing o buildings, or cooking and or saniary waer. Heaing waer wih naural gas also means using an energy source o high exergeic value. Furhermore, i an elecric hea pump is used, he amoun o hea energy produced can be a acor hree o six higher han he energy inpu rom elecriciy.
116 Power supply challenges Hea and chill can be excellenly sored in waer a relaively low cos. Examples exis o heaing he conens o underground waer reservoirs wih hea rom solar collecors in he summer or use in he winer. A disric heaing sysem in Ausria owned by EVN uses a 50000 m 3 ank or hea sorage o balance he hea oupu rom a large combined hea and power plan wih hea demand. Since he specific hea capaciy o waer equals 4.185 kJ/(kg K), cooling he conens o he ank rom 94 °C o 60 °C releases 50 000 000 kg ∙ (94 – 60)K ∙ 4.185 kJ/(kg K) = 7114500000 kJ ≈ 7.1 J. Heaing he conens again wih excess elecriciy requires abou 2 GWh, since 1 kWh equals 3.6 MJ so ha 2 GWh is 7.2 J. Dissipaing 2 GWh during a ime span o 4 hours means ha he ank can absorb 2000 MWh/4 h = 500 MW o elecriciy during 4 hours while heaing he conens again rom 60 °C o 94 °C. Te capial invesmen or such a ank is abou 100 €/ m3 , or abou 3 € per kWh sorage capaciy. Many local combined hea and power (CHP) insallaions o 2 o 20 MW o elecrical oupu are equipped wih hea sorage anks. Applicaions are primarily ound in disric heaing sysems and greenhouses. Modern home heaing sysems also use hea sorage, someimes in combinaion wih solar hea collecors. In Denmark, hea sorage anks are increasingly being equipped wih elecrical heaing coils or acceping cheap excess elecriciy rom wind urbines. Hea sorage is an excellen way o smoohing he variable oupu rom renewable energy sources. Tis reduces boh ossil uel consumpion and greenhouse gas emissions. Hea pumps help improve he elecriciy o hea conversion effeciveness when low oupu emperaures are required. Underfloor heaing in combinaion wih hea pumps can yield a so called coefficien o perormance (COP) ha easily exceeds
Figure 5.13. A 50000 m3 heat storage tank owned by EVN in Theiss, Austria, with a capacity of up to 7.2 TJ.
5. Energy storage
hree. A COP o hree means ha each kWh o elecriciy supplied o he sysem resuls in hree kWh (10.8 MJ) o heaing. Tis is a higher by a acor o hree han direc elecrical heaing. Elecrical hea pumps are hereore considered as he preerred providers o heaing in modern wellinsulaed buildings. A huge number o warehouses in he world use chilling or preserving he qualiy o perishable producs. Meling ice o 0 °C ino waer o he same emperaure consumes 334 kJ/kg o hea, equal o he amoun o energy needed or heaing waer rom 10 °C o 90 °C. Tis is he so-called laen hea o meling. Tereore, reezing an amoun o waer wih excess elecriciy is again an excellen way o soring energy. urning a liquid ino a solid, and vice versa, is called phase Figure 5.14. Melting ice changing. consumes heat. One example o phase changing is he cooling power o a popsicle, a low-calorie alernaive o ice cream. Te specific sensible hea o ice is 2.1 kJ/ (kg K). I we presume a mass o 100 g and an iniial emperaure o –10 °C, he consumed ice akes 10 ∙ 0.1 ∙ 2.1 = 2.1 kJ o hea rom he human body o warm up o 0 °C. Meling he popsicle akes 0.1 ∙ 334 = 33.4 kJ. Heaing he consumed ice o a body emperaure o 37 °C akes 37 ∙ 0.1 ∙ 4.2 = 15.5 kJ. Te oal hea inpu rom he body o he popsicle is, hereore, 2.1 + 33.4 + 15.5 = 51 kJ. Tis equals a hea consumpion o 12.2 kcal, or only 0.6% o an average daily energy inake wih ood. Alhough meling ice akes relaively much energy, slimming by eaing popsicles is apparenly no very effecive. I he popsicle would conain 5 gram o sugar wih an energy conen o 84 kJ, he slimming effec is even negaive. A daily inake o 17 kg o pure ice a –10 °C would be beter. Tis amoun is abou 25% o he average mass o a human body and consumes 100 % o he daily ood inake or he heaing.
5.7. Discussion on energy storage All energy sorage sysems discussed here are suiable only or shor and medium erm sorage. Ta also applies or sorage opions no deal wih in his chaper, such as sodium-sulphur bateries, mass-based graviy sysems and ocean botom balloons. Demand response sysems wih smar meers and smar appliances are also soluions or shor-erm balancing only. Economically and pracically, soring energy in bateries, compressed air, flywheels, hydrogen and even pumped hydro is no realisic or ime spans exceeding a week. In realiy, here are prolonged ime spans where renewable energy sources have a limied oupu, even over large geographical areas. Laiudes
117
118 Power supply challenges above 45° have hardly any sunshine in he ) winer season. J 100 M / Te large-scale applicaion o renew g ( 80 able energy sources can drasically reduce n o i s 60 he use o ossil uels. However, wih he s i m curren sae-o-he-ar energy sorage e 40 o echnology, a ull wihdrawal rom radi c c 20 fi ional uels is economically no possible. i c e Te associaed coss would be excessive. p 0 S l l l a a o i n e n e However, even wihou energy sorage, a g e g a s ) o o l a a a c c e r l t e h p i k t e n u o a n f c e w l i g subsanial decrease in he use o ossil P r A v t u r l a M v y r o ( B a a B n H e uels can be achieved. Backup can be provided wih he radiional uels: gas, oil, coal and nuclear uel, which offer easy Figure 5.15. Emissions of CO2 per fuel type, based on the long-erm sorage. In paricular, gaseous lower calorific value. uels seem o offer good possibiliies, since much gas is available as naural gas, shale gas, coal-bed mehane and biogas. Te CO2 emission o mehane, he main consiuen o gaseous uels, is 54.8 g/MJ, which is low compared o coal and oil (see Figure 5.15). Te Bergermeer aciliy under consrucion near Bergen in Te Neherlands will be an example o a ypical gas sorage sie. Te gas field can sore 8.4 billion normal cubic meres o gas, o which 4.1 billion m 3 is is working volume. Te res 2
10 000
) W k / o r 1 000 u E ( t n e m 100 t s e v n i r 10 e w o P 1 0.01
Natural gas cavern Storage as heat Compressed air Pumped hydro Lead-acid battery NA-S battery Flywheel Li-ion battery
0.1
1
10
100
1 000
Energy storage investment (Euro/kWh) Figure 5.16. General impression of investment levels for different storage techniques. The costs per kWh of electric energy delivered depend on the life of the storage method and its utilisation factor.
5. Energy storage
119
o he volume is made up o cushion gas. For a lower calorific value o 36 MJ/m3 , he working energy sorage capaciy equals 4.1 · 109 · 36 = 147600000000 MJ = 41 Wh o uel. For an injecion ime o 110 days, he average charging uel flow equals 16.3 GW. Te equipmen or filling he field resrics he injecion flow o 19.75 GW. For a wihdrawal ime o 90 days, he average discharge uel flow equals 19.9 GW. Te echnically maximum discharge flow is 26.8 GW. I he maximum discharge uel flow o 26.8 GW is convered ino elec- Figure 5.17. Artist's impression of the Bergermeer gas storage riciy wih 48% efficiency, i renders facility. The site is under construction in the Netherlands. an elecric power o 12.9 GW. Ta is slighly more han hal he maximum oupu o he large Tree Gorges hydro-power plan in China. Te esimaed oal coss o he aciliy are some 800 M€. I a pracical efficiency o flexible sysems or convering gas ino elecriciy o 48% is presumed, he invesmen or he Bergermeer aciliy will be 0.04 € per kWh sorage volume or elecriciy. For a discoun rae o 5% and a echnical lie o 30 years, his renders annual addiional coss o only 0.25 €cn/kWh delivered, even i he aciliy would be charged and discharged only once per year. Such low coss challenge every shor-erm sorage alernaive.
5.6. Conclusions For mos energy sorage echnologies, requency regulaion and peak shaving seem o be he bes applicaions. Bu soring energy or more han a ew days can easily more han double elecriciy coss. Combining renewable energy sources wih hea producion in CHP sysems could offer an effecive means or using excessive renewable energy. Tis would reduce ossil uel consumpion subsanially. Full absinence rom ossil uels appears o be uterly uneconomic wih curren sae-o-he-ar sorage echnology. Naural gas seems o offer low-cos soluions or soring energy and balancing elecriciy supply and demand wih a relaively low burden on he environmen.
6 Costs of producing electricity Knowing he producion coss o elecrical energy is crucial or power producers operaing in compeiive markes. Many cos iems canno be conrolled by he owner o a power plan. Tereore, profiabiliy o an invesmen is very difficul o predic. Tis is challenging or elecriciy producers. Te cos figures given in his chaper are examples ha can be modified by he reader or paricular siuaions. Te inenion is o gain an insigh ino he underlying mechanisms ha deermine he kWh coss.
122 Power supply challenges
6.1. Challenges in determining kWh costs Low elecriciy coss are crucial or he economy. Tis is especially rue when a counry has o impor he generaing equipmen and he required primary energy. Power plans, as well as elecriciy ransmission and disribuion sysems, are cosly long-erm capial invesmens. Te echnical lie o hese sysems can exceed ory years and he payback imes are long. Elecriciy supply sysems have o comply wih a large number o echnical rules and regulaions ha change coninuously. Tis adds coss o he already high basic invesmens in equipmen. Tereore, even in a compeiive elecriciy marke, owners o elecriciy supply sysems need a cerain amoun o confidence ha heir invesmen will pay off and ulimaely provide hem wih a decen profi. Te shor-erm inererence wih marke condiions by policy makers urher complicaes he mater. One migh even wonder i open markes are really he bes soluion or long-erm invesmens ha have such a crucial impac upon sociey. Free markes are obviously he bes soluions or simple commodiies ha can easily be shipped and sored, bu elecriciy is no a simple commodiy. o cope wih he uncerainy, a new keyword is emerging wihin he energy secor: flexibiliy.
6.2. Varying conditions for generating electricity Tere is no universal soluion available or providing reliable and affordable elecriciy in a susainable way because boundary condiions differ compleely rom one locaion o anoher, and hey change ofen. Some counries can go or easy soluions. For example, Norway has such vas resources o hydro-power ha he demand or elecriciy can easily be covered in all seasons. France and Swizerland rely on nuclear power plans ha produce elecriciy a low marginal coss per kWh. Ye, he Fukushima nuclear disaser has shown ha he ulimae lie-cycle coss o nuclear power plans can be excessive, and he effecs on he environmen negaive. Power supply based on solar radiaion can be effecively used in areas wih much sunshine during all seasons, provided shor-erm sorage is available o cover he nighly demand. As seen in chaper 5, long-erm sorage o elecriciy is prohibiively expensive. Tereore, in mos cases i is no easible o obain a sable and low-cos elecriciy supply rom renewables wihou using ossil uels as backup. In paricular, or areas wih subsanial seasonal differences in weaher condiions, ossil uels seem o be he only energy sorage mehod ha is pracically and economically available. I is no likely ha affordable long-erm-sorage sysems will be developed soon. Ye, some policy makers have decided ha he use o ossil uels or elecriciy producion mus be oally curailed wihin he coming ew decades. Te coss and consequences o such a arge may no always be undersood. Fuel prices differ subsanially depending upon locaion. In Norh America, an abundan availabiliy o naural gas resuling rom he producion o shale gas has lowered gas prices since 2012. In Asia, many counries depend on gas impors and
6. Costs of producing electricity
123
are, hereore, capive cusomers ha ENDEX TFF Gas Future Reference Price, ICE Brent Index (BINDEX) Daily ace high gas prices a levels almos equal Price & Henry Hub Natural Gas Future Settlement Price o oil per MJ. Alhough indigenous gas resources in Europe canno ully cover Brent oil ) J he demand, heir presence keeps gas G / $ prices lower han in Asia. I here were S U ( no issues wih emissions, coal migh e APX-ENDEX gas c i mainain is role as he major provider o r p l cheap energy or elecriciy producion. e u F Coal resources are widespread and abunHenry Hub gas dan in he world. Furhermore, marke condiions or Weeks from January 1, 2004 till April 1, 2014 elecriciy differ considerable rom counry o counry. Verically inegraed elecriciy supply companies wih a monopoly are less Figure 6.1. The development of oil and natural gas prices in vulnerable wih respec o economic sur- the world. vival han companies ha have o compee in open markes. Te monopoly companies are ofen owned by he sae, a province or a municipaliy. Teir goal is o serve he communiy wih affordable and reliable elecriciy, even in remoe areas. Tus, heir cusomers indirecly own he company. Managemen in such companies has o ensure ha he elecriciy supply mees he rules se by he policy makers. All coss are recovered by charging cusomers wih a fixed connecion ee, a capaciy ee relaed o he maximum elecric power o he connecion, and an energy ee or each kWh consumed. Also privae energy companies can be verically inegraed and monopolisic. In ha case, a governmen appoined regulaor migh be employed o oversee heir processes and use, or insance, benchmarking o esablish low consumer prices or o ensure maximum ax revenues. In ully compeiive markes, balancing elecriciy demand and producion in he supply sysem is conrolled by a ransmission sysem operaor (SO) who requess a cerain amoun o producion wihin a cerain ime span based on demand predicion models. Such models use hisorical demand paterns in combinaion wih weaher orecass. Where here are many wind urbines and solar PV panels in he sysem, he SO also uses weaher predicions or orecasing he renewable elecriciy producion. Independen power producers can offer heir oupu o an energy exchange, boh on a day-ahead basis as well as in hourly inervals, or even five-minue inervals. In some marke sysems, elecriciy producers are compensaed no only or pro viding elecric energy, bu also or he so-called ancillary services ha are needed o keep he sysem sable. Tese ancillary services are requency conrol, coningency reserves in case o plan ailures, load ollowing, as ramping up and down, reacive power supply, and orecasing error compensaion. Ulimaely, ree markes or he producion, ransporaion, and reailing o elecriciy can only survive i he sysem is sufficienly profiable or he invesors. 30
25
20
15
10
5
0
0
52
104
156
208
260
312
364
416
468
510
124 Power supply challenges
Vertically integrated electricity supply
Unbundled, free market electricity supply
Generators Transmission Distribution
Independent electricity generators
Transmission system operator
Retail
Distribution system operator
Electricity retail company
Customers
Electricity retail company
Customers
Electricity retail company
Customers
Customers
Figure 6.2. Two extremes: a fully integrated electricity supply system and a completely free market electricity supply system.
Volailiy in boundary condiions resuling rom unpredicable uel prices, changing environmenal resricions, non-dispachable elecriciy sources, variable subsidy schemes, and requen inererence rom policy makers has compleely changed he elecriciy supply secor. Mos power indusry leaders are pleading or more sabiliy in he rules and regulaions; oherwise i becomes unatracive o inves in power supply sysems. Ye, open markes have creaed a subsanial lobby circui ha marke players use o influence Government Competition actions Public poliicians and bureaucras. Opinions opinion someimes have more impac on decision Emission makers han echnical and scienific argu- TSO rules legislation Electricity menaion. generator Taxes Government
6.3. Cost analysis for different generating techniques
supported competition Economy dependent Fuel costs demand Shareholder Market profits rules
Elecriciy generaors, ransmission lines, and disribuion sysems cos money and Figure 6.3. The many boundary condiare, hereore, capial invesmens. Monop- tions of an electricity generator in a free olisic public uiliies can collec ees rom market. cusomers o build up capial resources so ha no money has o be borrowed rom a bank or invesmens. As a consequence, he money paid in advance canno be used by he cusomers or privae invesmens or bank accoun savings. Tis cusomer money will render lower capial coss han when he power supplier borrows capial and pays shareholders a dividend. Addiional coss are imposed by he mainenance o equipmen and he salaries o operaors. Fuel-based generaors are burdened wih uel coss, he exen o which depends upon heir efficiency and he price o he uel. eserve capaciy is needed or coningency reserves and or guaraneeing proper balancing in case o demand orecasing errors and he oupu rom renewable resources. Te ulimae producion coss per kWh can differ grealy depending on circumsances. I a power supplier uses an exising power plan, he invesmen or which
6. Costs of producing electricity
has been ully depreciaed, i can produce elecriciy a almos marginal coss. A new-buil plan burdened wih bank loans and shareholder invesmens can have difficuly in compeing wih an old exising plan, alhough is uel efficiency migh be much higher. Tis seems unair bu i is he consequence o ree markes.
6.3.1. Capital costs
Maintenance costs
Fuel costs
Operation costs
Demolition costs
POWER PLANT
Project lead costs
125
Capital costs
Electricity
Emission costs
Te capial coss per kWh produced are deermined by hree acors: he paymen rae or capial, he lie o he equipmen, Figure 6.4. The various costs related to electricity production. and he uilisaion acor o he equipmen. Te paymen rae or capial is by definiion called he discoun rae R : R = share holder capital action · profit rate + bank loan action · interest rate Equation 6.1.
As an example, i he shareholder racion o he capial invesmen equals 25% and a profi rae o 10% is required, while he bank provides 75% o he capial a an ineres rae o 6%, he resuling discoun rae R equals: R = 0.25 · 0.10 + 0.75 · 0.06 = 0.07
Equation 6.2.
For an equipmen lie expecancy o n years, he so-called fixed charge rae FC equals: (1 + R )n FCR = R ·
(1 + R )n –1
Equation 6.3.
Te fixed charge rae is hen 0.075 or a discoun rae o 0.07 and an equipmen lie o 40 years, while or a lie o jus 20 years, he FC will be 0.094. Te equaions allow he readers o deermine he FC or heir own possible applicaion. able 6.1. gives examples o ypical capial invesmens in differen power plan echniques. Te equipmen invesmen price can differ rom counry o counry, depending upon, or example, equipmen coss, local labour coss, and inrasrucural requiremens. Te addiional coss caused by he lead ime o each projec have been esimaed by using a fixed charge rae based on a discoun rae o 0.07 and an average invesmen o hal he equipmen and insallaion coss during he lead ime unil commissioning. echnically, he lieime o generaing equipmen can be very long, bu spare pars migh become obsolee and cumulaive changes in regulaions migh prove he echnique o be inadequae in he long run. Te capial coss per kWh produced or he differen generaing mehods as menioned in able 6.1., depend o a large exen on he uilisaion acor o he gen-
126 Power supply challenges
Table 6.1.
Examples of typical capital investments for different generating methods.
Generating set type
Typical equipment + installation investment
Project lead time
Investment including lead time costs
Equipment life
€/kW
months
€/kW
years
Hard coal
1500
40
1687
40
Nuclear
3000
60
3562
40
750
24
806
40
500
12
519
40
Wind onshore
1500
6
1528
20
Wind offshore
3000
12
3112
20
Solar PV cell
1700
3
1716
20
Hydro
1000
50
1175
60
Gas turbine comb. cycle Gas engine simple cycle
eraors. A uilisaion acor o 100% means coninuously running a 100% capaciy hroughou he lie o he equipmen. Tis is physically impossible, since every echnique needs mainenance, while occasionally ailures occur resuling in addiional downime. A he same ime, he consan need or balancing elecriciy demand and producion means ha no all generaors can always run a maximum oupu. Someimes hey run on par-load and someimes hey are even swiched off because o lack o demand. Non-dispachable generaors based on wind and solar radiaion, or insance, are no able o produce heir maximum oupu all he ime. Wind urbines and solar panels ofen have a legislaion-based prioriy or eeding heir elecriciy
Figure 6.5.
Every machine needs regular maintenance resulting in downtime.
6. Costs of producing electricity
o he grid. However, due o he variabiliy o wind speed and solar irradiaion heir maximum oupu is available only during a small racion o he ime. Tis is expressed in heir capaciy acor, which shows how many kWh hey produce in realiy compared wih he kWh hey would produce by running a ull oupu. Chaper 3 shows ha he capaciy acor o solar PV panels can be below 10% in counries wih a relaively dark winer season. In very sunny counries, i migh reach 30%. Onshore wind urbines have a capaciy acor rom 15 % o 35 %. Offshore wind urbines migh have a capaciy acor ranging beween 20 o 45 %. However, he highes percenage applies only or opimum locaions. Wih an unresriced eed-in possibiliy, he capaciy acor immediaely ranslaes ino he uilisaion acor. I is ineresing o noe rom able 6.1. ha onshore wind ur bines currenly require almos he same capial invesmen per kW as solar panels. In counries wih a moderae amoun o sunshine, such as Germany, he elecriciy producion in kWh per insalled kW is abou wice as high or onshore wind ur bines han or solar panels. Tis means ha he capial coss per kWh produced are wice as high or he solar panels as or he wind urbines, under he assumpion ha heir liecycles are o equal lengh. Te capial coss expressed in €cn/kWh or he differen generaing echniques given in able 6.1. are shown in Figure 6.6 as a uncion o he uilisaion acor. I should be kep in mind ha he coss shown are based on he invesmen daa in able 6.1. or a discoun rae o 7%. A ully depreciaed power plan ha remains a remnan o a monopolisic uiliy migh heoreically have zero capial coss. Privae invesors in solar PV panels migh be happy wih a discoun rae o 3%, especially in imes when ineres raes on savings are low and subsanial subsidies or insalling panels are available. Tis can resul in arificially low capial coss or renewable energy sources. I seems air o compare he differen generaing echniques or he same financial boundary condiions, because subsidies ulimaely originae rom axpayer money. I is also imporan o know ha every generaing echnique needs backup power, he amoun o which depends upon is reliabiliy and mainenance needs. Fuel-based and hydro-elecric power plans can have an availabiliy o a leas 95% and hereore require backup capaciy. Tey can share his reserve power resuling in 5 % addiional capial coss per power plan. Solar panels and wind urbines need close o 100% backup capaciy. Ta easily adds 1 o 2 €cn/kWh o he capial coss o he menioned renewable energy sources, even when using relaively cheap gas engines or gas urbines or backup. Te lines in Figure 6.6 or he capial coss o generaors based on wind and solar radiaion do no cover he same uilisaion acor range as hose o oher generaing echniques. Te reason is ha he maximum capaciy acor or solar panels is 30%, or onshore wind 35%, and or offshore wind 45 %. In mos cases, he acual capaciy acors will be much lower han hese maximum values. Generaors based on wind, solar radiaion, and nuclear energy have on he one hand he highes specific capial coss, bu on he oher hand hey need no uel or
127
128 Power supply challenges
20
) h W k / s t c € ( r o t c a f l a t i p a c c i fi i c e p S
18
Offshore wind Nuclear Solar PV Onshore wind Hard coal Hydro Gas turbine c.c. Gas engine s.c.
16 14 12 10 8 6 4 2 0 0
10
20
30
40
50
60
70
80
90
100
Utilisation factor (%)
Figure 6.6. Capital costs per kWh depending on the utilisation factor, based on data from table 6.1 and a discount rate of 7%.
he uel cos is very low. Te high invesmen coss o nuclear and coal-based power plans mean ha he plans should preerably run a ull oupu as much as possible. In addiion, he seam-based echniques used or hese power plans are no very suiable or requen sars and sops and or rapid changes in oupu. Seam boilers and seam urbines suffer less wear i hey operae under consan physical condiions. Gas engine and gas urbine-based echniques are he bes soluions or peaking power and inermediae power, since heir specific capial coss say relaively low, even in he case o a low uilisaion acor.
6.3.2. Primary energy costs Fuel-based power plans require primary energy or producing elecriciy. Te price o uel depends on producion coss, profi raes, ransporaion ees, sorage coss and subsidies, as well as axes. able 6.2. gives some indicaive prices per uni o primary energy as valid in Norh America in 2013. Figure 6.1 showed already ha large differences in he price o uel can occur rom counry o counry. Te price o Ausralian hermal coal was abou 0.3 €cn/MJ in 2013. Five years earlier, he price was abou hree imes higher. When cheap shale gas reached he Norh American markes, i became cheaper o use gas or elecriciy producion han coal. Gas has also less emission issues. Te decline in coal use in Norh America resuled in low inernaional coal prices and, consequenly, impored coal is currenly he cheapes primary energy source or power plans in Europe. In 2013, many brand new highly efficien gas-uelled power plans
6. Costs of producing electricity
could no compee because o he low Table 6.2. Indicative global prices of priprice o coal. A he same ime, many mary energy in North America in 2013. mohballed coal-fired plans were pu Fuel type Fuel price (€cnt/MJ) back ino operaion. Thermal coal 0.15 Te efficiency o convering uel ino elecrical energy is also a Nuclear fuel 0.10 deermining acor or he uel coss Natural gas 0.24 expressed in €cn/kWh. able 6.3. Light fuel oil 1.20 gives an indicaion o he uel efficiencies o differen elecriciy generaing echniques running a ull oupu. Tese are power plan efficiencies and no he efficiencies relaed o delivering he energy o cusomers. ransmission and disribuionlines and ransormers have heir losses, which means he ulimae uel efficiency o supplying cusomers wih elecriciy rom a disan large power plan can be less efficien han rom a smaller local power plan. Te wear and ear o machinery also lower uel efficiency, bu his can parly be resored hrough mainenance and ser vicing. All power plans – excep hose based on modular generaing unis in parallel – show an increase in specific uel consumpion i heir oupu decreases rom maximum oupu. Tis is caused by a higher impac o parasiic losses, such as pump drives, and o sysem losses such as bearing ricion and he blow by o urbine blades when running a reduced load. By conras, modular power plans based on muliple engines can mainain heir ull load uel efficiency down o very low loads by swiching off individual generaing unis. Te unis ha remain online can always run close o heir maximum oupu. Figure 6.7 gives he ypical uel efficiencies o differen elecriciy generaing echniques versus load. Mos power plans will no run below 50% o nominal oupu because o echnical and economic resricions. Fuel efficiency in combinaion wih uel price deermines he uel coss expressed in €cn/kWh. Since uel prices largely depend on marke orces, i is impossible or power plan projecs o predic he uure uel coss. Figure 6.8 gives examples o uel coss or a coal price o 0.24 €cn/ MJ, a gas price o 0.35 €cn/MJ, and a Table 6.3. Some values of power plant effinuclear uel price o 0.10 €cn/MJ. I ciencies at nominal output. should be noed ha a large par o he Fuel efficiency Power plant type nuclear uel price is deermined by he at nominal output processing coss and less by he comNuclear 33% modiy coss o raw uranium oxide. Hard coal 40% Te volailiy in nuclear uel coss is, Gas turbine simple cycle 38% hereore, relaively small. However, in he case o many new build nuclear Gas turbine combined cycle 55% power plans, uel scarciy would drive Gas engine simple cycle 47% up he price. Currenly, nuclear power
129
130 Power supply challenges plans have by ar he lowes specific uel coss o all uel-based power plans. Bu he low gas price o 0.24 €cn/MJ in he USA in 2013 rendered specific uel coss or gas-uelled plans o close o 1.5 €cn/ kWh. For such boundary condiions, gas-based power plans can even push nuclear power plans ou o he marke or baseload. A complicaing acor in deermining uel coss are he effecs on uel efficiency o rapid load variaions and requen sars and sops o he generaing equipmen. Seam boilers suffer rom rapid changes in emperaure and heir saring-up and sopping processes are, hereore, relaively slow. Preheaing and a slow ramping up o power plans consume exra uel. Combusion engines and aero-derivaive simple-cycle combusion urbines are no affliced wih his problem. As seen in chaper 3, large racions o wind and solar-based generaors in a sysem mean ha as ramping up and down and requen sars and sops o uel-based generaors become common pracice. Te inroducion o wind and solar increases, hereore, he specific uel coss o he oher power plans.
60 55
GT CC
) 50 % ( y 45 c n e i c 40 ffi e l 35 e u F 30
Modular gas engine
Coal GT SC Nuclear
25 20 0
20
40
60
80
100
Relative power plant output (%)
Figure 6.7. Examples of the fuel efficiencies for different electricity generating techniques as a function of relative power output.
Coal 0.24 €cnt/MJ Gas 0.35 €cnt/MJ Nuclear fuel 0.10 €cnt/MJ
4.5
) h W k / t n c € ( s t s o c l e u f c fi i c e p S
4.0
GT SC
3.5 3.0
Modular gas engine
GT CC
2.5
Coal
2.0 1.5
Nuclear
1.0 0.5 0.0
6.3.3. Emission costs
0
20
40
60
80
100
Relative power plant output (%)
In some counries, a ee is charged or emiting greenhouse gases such as CO 2 , Figure 6.8. Examples of the fuel costs for different powerpolluans such as NO X , SO2 , and paricu- plant techniques fuel efficiencies as in Figure 6.7, and for given laes. In oher counries, CO 2 emiters have boundary conditions of fuel prices. o purchase emission cerificaes. Tere are mehods available ha can reduce he emissions o polluans, bu carbon capure and sorage rom exhaus gas sreams (CCS) is no common pracice. Te equipmen is expensive, capuring gases consumes addiional energy, and here are conroversies abou where o sore he capured CO 2. Proposals or minimising greenhouse gas emissions rom uel use via rade and cap cerificaes have ailed because o an excess o
6. Costs of producing electricity
Table 6.4. Examples of CO2 emissions depending on power plant type and fuel (nominal conditions). CO 2 from fuel
Nominal fuel efficiency emission
Specific CO 2
g/MJ
%
g/kWh
Black coal/steam
98
40
882
Lignite/steam
109
36
1090
Gas/simple cycle gas turbine
56
38
530
Gas/comb. cycle gas turbine
56
55
366
Gas/combustion engine
56
47
429
Gas/engine with cogeneration
56
47 + 40
232
Power plant type
cerificaes, resuling in a very low CO 2 price o abou 5 €/onne as o early 2014. Such a low price is clearly insufficien o finance CCS. A CO 2 price o beween 60 € and 80 € per onne would be needed or making CCS possible. Te CO2 emissions expressed in g/kWh depend on he composiion o he uel and he uel efficiency. able 6.4 gives some examples or seady-sae ull-load condiions o differen power generaing echniques. I he CO2 price would rise o 80 €/onne, he emission coss or coal-based power plans would be more han 7 €cn/kWh. Tis is due o he high CO 2 emissions rom coal and he relaively low uel efficiency o coal power plans. Tis is illusraed in Figure 6.9. For a decreasing load, he uel efficiency decreases and
8
) h 7 W k / t 6 n c € ( 5 s t s 4 o c 2 O 3 C c fi i 2 c e p S 1
Coal CO2 80 €/tonne GT SC Mod gas engine GT CC Coal GT SC Mod gas engine GT CC
CO2 20 €/tonne
0 0
20
40
60
80
100
Relative power plant output (%)
Figure 6.9. Costs of CO2 emissions per kWh with two different CO2 price scenarios (fuel efficiencies as in Figure 6.7).
131
132 Power supply challenges hereore he CO2 coss rise. Even or a CO 2 price o 20 €/onne, he CO 2 coss o a coal-fired plan reach a level close o is capial coss as shown in Figure 6.6. Apparenly, he impac o pricing o CO 2 emissions is so high ha i can easily increase or decrease he marke viabiliy o a generaing echnique. Gas-based plans clearly have an advanage because gas releases less CO 2 per MJ han coal, while gas plans also have higher uel efficiency. Gas urbine combined cycles and power-plans based on muliple engine-driven generaors have he lowes CO 2 coss per kWh. Where engines are used in a combined hea and power plan, heir CO2 coss will be halved.
6.3.4. Operation and maintenance costs unning echnical equipmen, such as a power plan, causes wear. urbine blades, bearings, valves, air filers, lubricaing oil and exhaus-gas-cleaning caalyss deeriorae during he operaion o a power plan. Te machinery housing and adminisraive offices need regular cleaning and mainenance. In addiion, power plans need personnel or operaing and supervising he machinery, despie a high degree o auomaion in conrolling and monioring he equipmen. A disincion can be made beween 1.4 ) fixed operaion and mainenance h W 1.2 (O&M) coss and variable O&M coss. k / Nuclear t n Fixed coss, such as hose or personnel c € ( 1.0 and insurance, occur independenly o s t Coal + GTs s o he uilisaion acor o he generaing c 0.8 M sysem. By conras, variable coss are & O direcly dependen on he uilisaion e 0.6 l b acor. Boilers and urbines also experi a i Modular gas engine plant r a 0.4 ence addiional wear due o sopping, v c fi saring and rapid load changes. Such i c 0.2 e evens are expressed in equivalen run p S ning hours. eciprocaing engines, how0,0 ever, can experience requen sars and 0 20 40 60 80 100 sops wihou causing addiional wear. Power plant load(%) Variable mainenance coss are generally based on running hours wihou aking ino accoun he ac ha he load Figure 6.10. An example of variable O&M costs for different level migh affec he wear rae. Variable power plant types depending on the load of the power plant operaional coss are anyhow based only (100% = nominal output). on he number o running hours, since running a reduced oupu does no decrease he required operaional aciviies. Figure 6.10 is an atemp o relae he variable O&M coss wih he load or differen power plan ypes. Te effec o requen sars and sops on mainenance needs has
6. Costs of producing electricity
no been included in his. Each sar and sop o a supercriical boiler or urbine adds equivalen running hours o a power plan’s mainenance requiremen. By conras, he mainenance coss per kWh o power plans based on muliple combusion engine-driven generaors do no depend on he load since individual generaing unis will be swiched off when he load decreases, meaning ha he wear acor consequenly ceases. Tis is anoher advanage o modular plans in siuaions where he need or uel-based elecriciy is reduced by a subsanial inpu rom renewable elecriciy sources. In real lie, he variable O&M coss can differ subsanially rom hose shown in Figure 6.10. Coss depend on he equipmen qualiy, he uel qualiy, as well as he operaing condiions, which can vary rom easy o difficul. As an example, poor qualiy black coal burns so slowly ha boilers suffer rom hermal overheaing o he burner graes. When he uilisaion acor o power plans is reduced, eiher because o poor marke condiions or hrough swiching rom ull-ime producion o becoming a backup plan or renewables, he fixed O&M coss per kWh oupu increas. able 6.5 gives an overview o he published average fixed O&M coss. Also, hese values can differ subsanially rom case o case. A wind urbine wih a capaciy acor o 20% produces 0.2 ∙ 8760 h = 1752 kWh per year per insalled kW. In ha case, he fixed annual O&M coss o 13 €/kW become 1300/1752 = 0.74 €cn/kWh. A nuclear power plan wih a uilisaion acor o 95% has fixed O&M coss o only 0.25 €cn/ kWh. I ha nuclear power plan would run only 4 monhs o he year a an average oupu o 80% during he winer ime o cover he lack o elecriciy producion rom solar panels, he fixed O&M coss would be some 12/4 ∙ 100/80 ∙ 0.25 = 0.93 €cn/ kWh. A modular plan based on gas engines used or peaking and secondary reserves
Table 6.5.
Examples of fixed O&M costs for different power-plant techniques. Power plant type
Fixed O&M costs per year € per kW installed capacity
Hard-coal/steam
16
Lignite/steam
18
Nuclear/steam
21
Gas GT comb. cycle
10
Gas engine modular
15
Wind turbine
13
Solar PV
8
Hydro
8
133
134 Power supply challenges only migh have a uilisaion acor o 30%, resuling in fixed O&M coss o 15 ∙ 100/ (0.3 ∙ 8760) = 0.57 €cn/kWh.
6.4. The total costs of producing electricity I would be grea i he previous secions o his chaper would have provided a simple able revealing he coss per kWh or producing elecriciy wih he differen echniques. Ta appears o be impossible since he boundary condiions or each echnique differ, depending on he coss o financing, uel, consrucion, emissions, plus he uilisaion and load acors. A ully depreciaed nuclear power plan can produce elecriciy a very low marginal coss, bu as soon as he capial coss are aken ino accoun and he uilisaion acor is low, he kWh coss can become very high. A coal-fired power plan migh bea a gas-uelled power plan in kWh coss, bu i he price o CO 2 emissions rises subsanially, coal-based plans canno compee anymore. Figure 6.11 gives he oal kWh cos or a number o generaing mehods or he boundary condiions given in figures 6.12, 6.13 and ables 6.4 and 6.5. I is imporan o know ha any addiional coss, such as he need or reserve power and requency 20
) h W k / s t c € ( s t s o c h W k l a t o T
18 16 14 12 10 8 6 4 2 0
Hard coal
Nuclear
GTCC
Mod. gas engine
Onshore wind
Offshore wind
Solar
Hydro
Total
6.06
5.34
4.68
4.72
9.34
12.17
19.61
1.4
CO2
1.6
0
0.73
0.85
0
0
0
0
Fixed O&M
0.2
0.25
0.13
0.19
0.74
0.57
0.61
0.1
Var. O&M
0.5
0.6
0.76
0.5
0.4
0.5
0.6
0.4
Fuel
2.16
1.09
2.3
2.68
0
0
0
0
Capital
1.6
3.4
0.76
0.5
8.2
11.1
18.4
0.9
Costs:
Figure 6.11. Total production costs of electricity where the CO2 price is 20 €/tonne, the utilisation factor is 90% (except wind and solar) and the plant is running at full output [coal price 0.24 €cnt/MJ, gas price 0.35 €cnt/MJ, nuclear fuel 0.10 €cnt/MJ, other conditions from table 6.4 and 6.5 and Figure 6.10].
6. Costs of producing electricity
20
) h W k / s t c € ( s t s o c h W k l a t o T
18 16 14 12 10 8 6 4 2 0
Hard coal
Nuclear
Gas engines
Onshore wind
Offshore wind
Total
10.86
5.34
6.85
7.27
9.34
12.17
19.61
1. 4
CO2
6.4
0
2.9
3.4
0
0
0
0
Fixed O&M
0.2
0.25
0.13
0.19
0.74
0.57
0.61
0. 1
Var. O&M
0.5
0.6
0.76
0.5
0.4
0.5
0.6
0. 4
Fuel
2.16
1.09
2.3
2.68
0
0
0
0
Capital
1.6
3.4
0.76
0.5
8.2
11.1
18.4
0. 9
Costs:
GTCC
Solar
Hydro
Figure 6.12. Production costs per kWh with a CO2 price of 80 €/tonne [other boundary conditions as in Figure 6.11.
conrol, are no included in he producion coss. Te coss given are purely or he elecric energy. For he condiions applying in Figure 6.11, he kWh coss o he uel based power plans are around 5 €cn/kWh and muual compeiion is a good possibiliy. Hydropower beas all oher sources in coss and ha is why counries blessed wih such resources are very orunae. enewable energy based on wind and solar radiaion is cosly because o low capaciy acor. Again, he coss or backup capaciy or wind and solar-based elecriciy sources have no been aken ino accoun. Fuel efficiencies, O&M coss, and he price o equipmen or uel-based generaors and hydro-elecric plans are no expeced o change drasically in he near uure. However, major uncerainies exis or he coss o CO 2 emissions and uel. Should he price o coal be 0.15 €cn/MJ insead o 0.24 €cn/MJ, he coss per kWh o he coal-based plan would drop rom 6.06 €cn/kWh o 5.25 €cn/kWh. I he price o CO2 is 5 €/onne insead o 20 €/onne, he cos o coal-based elecriciy is only 4 €cn per kWh insead o 6.06 €cn/kWh. I he gas price would be 0.7 €cn/ MJ insead o 0.35€cn/MJ, which is currenly he case in Europe, combined cycle and modular gas engine-based plans would have elecriciy coss o abou 7 €cn/ kWh insead o around 4.7 €cn/kWh.
135
136 Power supply challenges
5
s t s o c ) h h W W k k l / s a t c n € i ( g r a M
4 3 2 1 0
Hard coal
Nuclear
GTCC
Gas engines
Onshore wind
Offshore wind
Solar
Hydro
Costs:
Marginal
4.16
1.69
3.53
4.05
0.4
0.5
0.6
0.4
CO2
1.6
0
0.73
0.85
0
0
0
0
Var. O&M
0.5
0.6
0.5
0.5
0.4
0.5
0.6
0.4
Fuel
2.16
1.09
2.3
2.68
0
0
0
0
Figure 6.13. Marginal kWh costs for a utilisation factor of 90% and a nominal load for the boundary conditions of Figure 6.11.
Tese changes in boundary condiions reflec he siuaion in Wesern Europe in 2013 and explain why coal-fired power plans had he larges marke share in ha year. In Norh America, he low gas price o close o 0.24 €cn/MJ pushed coalfired power plans ou o he marke. Tereore, alhough he deerminaion o he compeiiveness o power plans is difficul, modiying Figure 6.11 o projec-specific boundary condiions can be very helpul when esimaing he coss o producing elecriciy. I CO2 coss would rise o 80 €/onne, coal-fired power plans would be compleely pushed ou o he marke, even wih avourable condiions, i.e. a 90% uilisaion acor and running a ull oupu. Tis is illusraed by Figure 6.12. Tereore, he consequences o high CO 2 emission charges or counries depending on coal based power plans will be severe. One should no make he misake o believing ha, based on Figure 6.12, on-shore wind urbines are becoming compeiive wih coal-fired power plans. Wind urbines always need backup capaciy. Tey can only push oher generaors rom he marke in imes o sufficien wind. In open elecriciy producion markes where power plans have o compee by offering elecrical energy in shor-ime inervals, marginal producion coss are ofen used. Te compeing power plans are presen anyhow, and i hey can sell elecriciy a a price higher han heir marginal coss hey receive a leas an income sream. Te difference beween producion coss and marke price migh no be sufficien o cover he fixed coss, bu geting some income is always beter han no income a all. Ulimaely however, a power plan should recover he oal coss o producing elecriciy.
6. Costs of producing electricity
Figure 6.13 gives only he marginal coss o producing elecriciy or he boundary condiions o Figure 6.11. In his case, he renewable elecriciy sources bea all oher power generaors. Fully depreciaed power plans, or example many old coal plans, have a subsanial advanage over new gas-based power plans. And again, i he coal price would be jus 0.15 €cn/MJ insead o 0.24 €cn/MJ, he CO2 price 5 €cn/onne insead o 20 €/onne, and he gas price 0.7 €cn/MJ insead o 0.35 €cn, he gas-based plans would be ully pushed rom he marke. Te coal-based marginal elecrical energy coss would be only 2.4 €cn/kWh, while he gas-based marginal coss would be abou 5.7 €cn/kWh. Some owners o power plans even leave he mainenance coss ou when offering elecriciy o he marke. Tey will run heir power plan unil he momen a major overhaul becomes necessar y and hen decide i i is worhwhile o carry ou he necessar y mainenance acions or o close down he power plan. In ree elecriciy markes, everyhing is possible. Te sory his ar illusraes he difficuly power plan invesors ace in making he proper choice or a power plan. Te siuaion ges even more complicaed when many renewable elecriciy sources are aken ino he sysem. A uilisaion acor
16
) h W k / s t c € ( e c i r p h W k l a t o T
14 12 10 8 6 4 2 0
Costs:
Hard coal
Nuclear
GTCC
Mod gas engine
Large scale hydro
Total
11.73
15.95
7.54
6.64
4.31
CO2
1.76
0
0.83
0.86
0
Fixed O&M
0.76
1
0.48
0.71
0.34
Var. O&M
0.83
1
0.83
0.52
0.6
Fuel
2.38
1.2
2.55
2.68
0
6
12.75
2.85
1.87
Capital
3.37
Figure 6.14. kWh costs where the utilisation factor is 24% and the plant is running at a generator output of 60% [other boundary conditions as in Figure 6.11.
137
138 Power supply challenges o 90% and running only a 100% load will seldom be possible anymore or power plans. Chaper 3 shows ha even wihou renewable elecriciy sources, he average capaciy acor o a real power-plan porolio is hardly higher han 50%. Te Irish example shows ha when wind energy provided 16% o he elecrical energy, he uilisaion acor o he oher power plans was only 39%. Wihou wind power, he uilisaion acor would have been abou 47%. I he Irish wind-based capaciy were o double, he uilisaion acor o he power plans drops o 32%. Ye, he power plans need o be able o supply peak demand as long as no energy sorage sysem is available or covering a lack o wind oupu. Te German case shows ha solar panels here have a lower capaciy acor han wind urbines. Tereore, he backup capaciy or solar panels generally has a higher uilisaion acor han he backup capaciy or wind. However, he daily variaions are larger or solar han or wind energy. Tis makes invesing in power plans even more complicaed. In he hypoheical case ha a power plan will only run a a fixed load o 60% and be online or only 40% o he ime, is uilisaion acor is only 0.6 ∙ 40 = 24%. Te elecriciy producion coss will hen be much higher han when running 90% o he ime a 100% load. Te producion coss per kWh or he 60% load case are given in Figure 6.14. Te low uilisaion acor means ha hose power plans ha require high invesmens per kW canno compee anymore because o he burden
End user electricity price (€cnt/kWh), year 2011 30
Households Industry 25
20
15
10
5
0
m u i g l e B
k r a m n e D
d n a l n i F
e c n a r F
y n a m r e G
y l a t I
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y a w r o N
d n a l o P
n i a p S
n e d e w S
y e k r u T
m o d g n i K d e t i n U
A S U
a n i h C
Figure 6.15. End user electricity prices in different countries (sources: EIA, Eurelectric, Eurostat, www.europe.eu).
6. Costs of producing electricity
139
o he capial coss. Tis reveals ha new nuclear power and coal plans have no uure in counries such as IreAdditional VAT land and Germany because hey have a lo o renewable charges 0.6 15.9 Energy Measurement energy sources. I he price o CO 2 were o quadruple o 24.1 levy 2.5 80 €/onne compared wih he 20€/onne in Figure 6.14, Concession levy 6.4 backup elecriciy rom coal would cos 17 €cn/kWh. Electricity use Lower coal prices will no really help in his case. levy 7.9 Grid charge 20.6 In realiy, power plans wih a uilisaion acor o only Retail 8.2 24% will never run consanly a 60% load. Te increasing Renewables levy 13.8 need o ulfil he asks o requency conrol, load ollowing, reserve power, and orecas-error compensaion means ha heir oupu will change all he ime. Nuclear Figure 6.16. Percentages of the different and coal-based power plans are less suied o changing items making up the 26.1 €cnt/kWh price of oupu because hey suffer i seam condiions vary. Ta electricity for households in Germany (Data from year 2012). also applies or he seam par o gas urbine combined cycles. Tereore, heir variable mainenance coss will be even higher han indicaed in Figure 6.14 while heir uel efficiency will be lower. Currenly, some owners o combined cycle power plans disconnec he seam cycle rom he gas urbine. Tis is o avoid wearing o he hea recovery seam generaor and seam urbine, and o creae more flexibiliy. Te seam par o he cycle responds relaively slowly o changes in oupu demand.
6.5. The electricity price for consumers As seen in chaper 1, he price o a produc normally differs rom he coss o produce i. Generally, he overall producion coss o elecriciy vary beween 2 €cn/kWh or exising hydro-elecric power plans as a minimum, and 20 €cn/kWh rom solar panels as a maximum. Occasionally however, even higher coss migh occur in rural locaions where small perol-uelled generaors are he only source o elecriciy. Prices or elecriciy differ considerably beween counries, even in an area such as he European Union. Figure 6.15 gives he approximae elecriciy prices in a number o counries. A Danish household pays wice as much per kWh han a domesic cusomer in France or Finland, or example. Germany ranks second in kWh price or domesic users. Denmark and Germany are he EU counries wih he highes share o renewable elecriciy sources. Indusry in he USA pays he lowes kWh raes. Tis cerainly enhances he compeiiveness o energy-inensive indusries. Domesic consumers generally pay considerably more per kWh han indusrial users. Tis is parly due o higher disribuion coss or homes han or large consumers. However, levies, axes and disribuion charges play a significan role here. Tey can resul in domesic cusomers paying our imes as much as indusry. Figure 6.16 shows he composiion o he price ha households paid in Germany in 2012. Te domesic reail price is 26.1 €cn/ kWh, so i is eviden ha solar
140 Power supply challenges panels producing elecriciy or 20 €cn/kWh are compeiive. However, i appears ha only 6.3 €cn/kWh o he reail price o elecriciy is or he energy. Compeiion beween elecriciy producers has, hereore, a close o negligible effec on he price ha a domesic consumer pays in counries such as Germany, Denmark, Te Neherlands, Ialy, Spain and Sweden. In hese counries, domesic elecriciy use is heavily charged wih levies and axes. High elecriciy prices or indusry can resul in energy-inensive companies leaving or cheaper regions.
6.6. Discussion regarding electricity production costs A firs sigh, building up an elecriciy generaion porolio based on minimum coss seems simple. echniques wih low uel prices combined wih relaively high capial coss are he mos suiable or baseload. echniques wih low capial invesmens bu higher uel coss are he bes opion or inermediae and peak load. Hydropower based on a sufficien waer supply in reservoirs is he cheapes opion or all applicaions bu is no available in mos counries. However, he basic producion coss o elecriciy are no he only issue here. Elecriciy supply sysems also need coningency reserves, requency-conrol capaciy, load-ollowing capaciy, reacive power supply, and orecasing-error reserves. Tese ancillary services cos money. In radiional verically inegraed power supply sysems, hey were an inegral par o he asks o a power plan. unning a cerain racion o he online power plans below heir nominal oupu could generally provide hese services. Even in compeiive power markes, sysem operaors may charge elecriciy producers wih ancillary services as par o he deal o supply energy. In some markes, power suppliers can receive compensaion or offering ancillary services. Bu no all generaing echniques have similar capabiliies or providing hese services. Wih elecriciy sources o inermiten oupu, he need or ancillary services, including as ramping up and down and even energy sorage, drasically increases. In open elecriciy markes, such services should cerainly be rewarded financially. I is quie a challenge o find he economically and echnically opimum or building up a generaing porolio as condiions can change in an unpredicable way. However, his chaper has shown ha hydropower and gas-based power end o offer he cheapes soluions when significan levels o flexibiliy are required. Power plans based on muliple unis in parallel have he addiional advanage o low mainenance coss and high uel efficiency, even a low loads.
6.7. Conclusions Tis chaper has shown ha he coss o producing elecriciy wih generaors operaing in compeiive markes depend heavily on unpredicable boundary condiions.
6. Costs of producing electricity
Tis makes i exremely difficul o predic he profiabiliy o exising power plans, and especially ha o uure power plans. Poliical inererence, flucuaing uel prices, and he increasing amoun o subsidised renewable elecriciy sources heavily affec he ulimae cos o elecriciy. Despie he many uncerainies, new power plans are definiively needed over he coming decades because o he crucial role o elecriciy in he economy. enewables alone canno effecively cover he oal elecriciy demand in he oreseeable uure. Flexibiliy is he key o coping wih he uncerainies in power generaion sysems. Where hydropower is no available, he bes means or achieving flexibiliy appears o be modular gas power plans based on combusion engine echnology.
141
7 Future power supply systems Fuure power supply sysems require firs and oremos flexible power generaion o ensure sysem reliabiliy and opimisaion a he lowes possible cos. Flexible generaion enables susainabiliy by allowing large amouns o renewable energy – while mainaining grid sabiliy. Agile, as-reacing generaing unis offer opimum soluions or he many challenges ha power supply sysems ace. Tis chaper shows how power plan porolios can be opimized wih flexible generaing unis in differen supply sysems.
144 Power supply challenges
7.1. The road towards an optimum power supply system radiionally, power plans were buil o las or a leas ory years. However, radical changes caused by he inroducion o subsidised renewable elecriciy sources and by open compeiion in energy markes can make even brand new power plans obsolee. In 2013, a one-year-old combined-cycle plan near oterdam was dismanled and sold in pars o regions wih beter boundary condiions. Major power suppliers have been conroned wih ully unexpeced circumsances. Te previous chapers have shown ha mainaining sabiliy in elecriciy supply sysems is becoming increasingly complicaed. Planning and designing a generaion and ransmission porolio has never beore been so difficul. Te crucial role o elecriciy in he economy requires a highly reliable supply o qualiy elecriciy wih, a he same ime, minimum coss. Te large-scale inroducion o renewable elecriciy sources wih variable and uncerain oupu requires compleely new approaches. Flexibiliy in power-plan oupu, as well as as ramping up and down, is much more needed now han in he pas. Sakeholders also alk abou he need or expanding grids, demand-side response and energy sorage. Long-erm profiabiliy is needed bu is very hard o predic because o coninuously changing suppor schemes or renewables, emission reducion echniques and he charges or emission righs. Tis book canno give a single soluion or achieving he opimum power supply sysem, since local boundary condiions deermine wha he bes opion is. However, his chaper will show ha flexible and agile generaors subsanially help o increase reliabiliy and sabiliy in very differen power sysems. Furhermore, hey appear o do his wih good uel efficiency and minimum invesmen coss.
30 minute interval demand year 2012, 50 Hertz transmission 15 000 12 500
) W10 000 M ( r 7 500 e w o 5 000 P 2 500 0 0
10
20
30
40
50
Weeks of the year 2012
Figure 7.1. Power demand pattern in the German 50 Hertz Transmission area during the year 2012.
7. Future power supply systems
7.2. An optimised generating portfolio without renewables
145
Demand distribution year 2012, 50 Hertz transmission 15 000
)12 500 radiionally, in imes when he flow o W 10 000 M elecrical energy was almos enirely one ( r 7 500 e direcional, rom he power plan o he w5 000 o consumers, elecriciy demand paterns, P 2 500 and consequenly elecriciy producion 0 0 10 20 30 40 50 60 70 80 90 100 paterns, were quie predicable. Te effec % of the time o daily aciviy paterns, weekends, public holidays and he weaher on elecriciy consumpion is always clearly disinguish- Figure 7.2. Power demand distribution in the German 50 able in power demand paterns and is, Hertz Transmission area during the year 2012. hereore, easy o orecas. Figure 7.1 gives a ypical example o a power demand patern. I is based on daa a 30 minue iner vals or he 50Herz ransmission Sysem Operaor area in Germany or he year 2012. Apparenly, demand here was never higher han 14 GW and never lower han 5 GW. Te daa poins o figure 7.1 have also been ranserred ino he power demand disribuion curve shown in figure 7.2. We will now make a hypoheical case o creaing a power-plan porolio ha can ulfil he power demand as given in figure 7.1 in an opimised way. Chaper 3 shows ha in realiy, exensive wind and solar based capaciy is presen in he 50Herz SO area, bu ha will be presumed no o exis in his example. In addiion, ransmission and disribuion losses will be negleced, since hose losses only ac as exra load. Power plans will be needed o cover he baseload, inermediae load, and he peak load. In addiion, generaing capaciy is needed or primary, secondary and eriary reserves, as well as or plans ha are undergoing mainenance. Figure 7.3 summarises he Replacement for tertiary reserves differen modes o generaing capaciy ha have o be Tertiary reserves covered. Secondary reserves Power demand in he 50Herz SO area is always Primary reserves higher han 7 GW, apar rom very ew downward excurMaintenance reserves sions. In an idealised siuaion, en power plans each o 700 Peak load MW can carry his baseload. However, power plans require regular mainenance so ha an addiional reserve plan o Intermediate load 700 MW is needed as backup o guaranee sufficien availabiliy. In addiion, sufficien primary reserves have o be available in case one or more power plans are down. One Base load opion or his is o run 11 power plans, each o a 700 MW nominal capaciy a a load o 636 MW, o supply he 7 GW. unning a 91% load has no severe negaive consequences or uel efficiency and increases he capial coss per kWh Figure 7.3. The different modes of gener by only 10%. When one o he baseload plans ails, he ating capacity in which has to be invested
146 Power supply challenges remaining ones have o ramp up o 700 MW wihin 30 seconds when ollowing he ypical Coninenal European rule or primary reserves. Secondary reserves are needed o add exra power o he sysem so as o resore he requency o 50 Hz so ha he primary reserves can reurn o he preconingency oupu. Tereore, 636 MW should be allocaed or secondary reserves. An alernaive is o add anoher 700 MW power plan o he baseload porolio and run each o he welve power plans in parallel a 583 MW, equalling 83% load. A much beter soluion, however, is o use a quick-sar non-spinning power plan o 636 MW. Engine-based generaing unis can deliver ull oupu wihin some five minues afer he sar command, as has been shown in chaper 3. Te benefi o his is ha he eleven online baseload plans ha saisy he 7 GW demand can remain running a 91% load wih, hereore, beter uel efficiency han when running a 83% load. Moreover, a non-spinning power plan does no accumulae wear while waiing o perorm is ask as a secondary reserves provider. In addiion, such a power plan requires a considerably lower invesmen han a dedicaed baseload power plan, (see figure 6.6). Te ulimae oupu reliabiliy o a non-spinning reserve power plan based on, or insance, 25 idenical unis in parallel, is much higher han ha o a single uni. In addiion o he secondary reserves, anoher 636 MW o non-spinning eriary reserves has o be presen o release he secondary reserves ollowing a coningency. ypical seam-based power plans can never provide ull oupu rom sandsill
Figure 7.4.
Photograph of an agile engine-based multiple-unit power plant
7. Future power supply systems
700 MW
10.36 GW
700 MW
61.5 TWh
400 MW
4.4 GW
200 MW
2.2 GW
700 MW
700 MW replace reserve
400 MW
18 TWh
200 MW
6 TWh
700 MW 700 MW
700 MW mainten. reserve
700 MW 700 MW 700 MW
630 MW agile tertiary. reserve
700 MW 700 MW 700 MW
700 MW agile second. reserve
Base load generators
400 MW
200 MW
400 MW
200 MW
400 MW
200 MW
400 MW
200 MW
400 MW
200 MW
400 MW
200 MW
400 MW 400 MW
400 MW mainten. reserve
Intermediate load generators
200 MW 200 MW
200 MW mainten. reserve
Peak load generators
Figure 7.5. An optimum power plant portfolio to satisfy the demand pattern of figure 7.1.
wihin he required ime span o one hour, as required or eriary reserves. More agile power plans offer a soluion in his case. o ulfil he power demand as shown in figure 7.1, inermediae generaion is also needed in he demand range beween 7 GW and 11 GW. Tis requires anoher 4 GW o capaciy o be regularly is online. Inermediae demand is characerised by power ramping up in he morning and down in he lae evening, especially on weekdays. Te 4 GW can be supplied by 10 unis o 400 MW each. Te reserves or inermediae power generaion can be warraned by he reserves allocaed or he baseload plans. Ye, a reserve plan or inermediae power is needed because he oher generaors need regular mainenance. Peak demand, occurring less han 30% o he ime, requires 2 GW o agile power plans plus a reserve uni o cover mainenance. Again, in a simple approach, 11 plans each o 200 MW are required. One o hese power plans can also be used or compensaing or errors in orecasing demand. Figure 7.5 gives he resuling power plan porolio o cover he demand shown in figure 7.1. In 2012, he oal elecric energy demand was 85.5 Wh in he 50Herz SO region. Wih he power plan porolio proposed in he previous paragraph, he fieen plans dedicaed o baseload including he reserves would produce 7 GW · 7884 h = 61.5 Wh rom an insalled capaciy o 13 ∙ 700 MW + 2 ∙ 636= 10.37 GW. Tis yields a uilisaion acor o 61.5 W h/(10.37 GW· 8784 h) · 100% = 67.5% or he baseload plans.
147
148 Power supply challenges Figure 7.2 shows ha he inermediae load, beween 7 GW and 11 GW, will be needed abou 51 % o he ime. Ta can be convered ino 4 GW · 8784/2 h = 18 Wh. Tereore, he 4.4 GW o he eleven inermediae load plans would have a uilisaion acor o 46.6%. Te 2.2 GW o he eleven peaking unis would produce he remaining 85.5 – (61.5 +18) = 6 Wh wih a uilisaion acor o 31.0%. Te combined porolio has a uilisaion Optimised generation portfolio acor o 57.4%. In real lie, he uilisaion acor o a 100 100 power plan porolio is generally lower 80 67.8 han he 57.4% o his example. Te main 61 57.4 60 reason is ha many counries have an 46.6 annual demand patern wih a relaively 31 40 26 13 lower baseload han in figures 7.1 and 20 7.2. In counries wih ho climaes, he 0 Interm. load Peak load Total Base load baseload in he mild seasons is lower Installed capacity (%) Utilisation factor (%) han in ho seasons. In cold counries, baseload is highes in winer. ecenly, in counries wih significan Figure 7.6. Subdivision of installed generating capacity with levels o wind and solar-based generaion, the connected utilisation factor such as Denmark (DK), Germany (DE), Ialy (I) and Spain (ES), he uilisaion acor o he uel-based power plans has decreased drasically. In addiion, new-buil power plans wih high uel efficiency ofen ake on he role o he old power plans, while hese old plans remain and ac o provide reserve capaciy. Figure 7.7 gives he uilisaion acor o he power plans o 22 EU counries in he year 2012. Te highes value is almos 50% and he lowes is 13.7%. Te average uilisaion acor o he power plans or he 22 counries is only 37.3%, as shown by he red line. Tis shows ha he example in figure 7.5 is clearly an idealised case.
Power plant utilisation factor 22 EU countries, year 2012 ) % ( r o t c a f n o i t a s i l i t U
50 40 30 20 10 0 AT BE CZ DE DK ES
FI
FR GB GR HU IE
IT
LT LU LV MT NL PL PT RO SE
Figure 7.7. The utilisation factor of the power plant portfolio of 22 EU countries in 2012 (data from Eurelectric).
7. Future power supply systems
149
For selecing he bes generaing echniques or he power plan porolio shown in figure 7.5, chaper 6 o his book clearly shows he bes opions rom an economic poin o view. Te hireen 700 MW baseload power plans in our example could be nuclear or coal-fired plans. Te remaining wo 636 MW plans in he baseload porolio have o be more agile and flexible, since hey have o provide he secondary and eriary coningency reserves. Te inermediae plans could be gas urbine combined cycles and he peaking unis could be simple-cycle gas engines and gas urbines. Hydropower, i abundanly available, would be he firs choice or all; baseload, inermediae load, as well as peak load. Te oal coss o producing elecriciy wih he power plan porolio proposed in figure 7.5 can be deermined wih he mehods shown in chaper 6. Planners and economic analyss or power plans have radiionally ollowed he mehodology described here and used invesmen coss and variable boundary condiions or such iems as uel prices o deermine he compeiiveness o a ypical power plan ype. In he old days, he only opion or secondary reserves was spinning power plans. Te curren availabiliy o agile power plans ha offer non-spinning reserves means ha uel consumpion, mainenance coss and invesmen coss can be decreased.
7.3. An optimised power plant portfolio design with renewable energy sources When renewable elecriciy sources are added o he sysem, power supply sysem planners canno simply use a power demand disribuion curve such as ha in figure 7.2 anymore. Such an approach was only possible or a highly predicable load patern or he power plans. Tis will be shown by using again daa rom he 50 Herz SO area in Germany. Germany is pioneering he large-scale inroducion o renewable elecriciy sources. In 2012, he 50Herz SO area had on average 12 GW o wind power insalled wih a capaciy acor o 17.6 %, and an average 8 GW o solar PV wih a Average values, 50 Hertz TSO, year 2012 capaciy acor o 9.5 %. Tese wo renew60 able sources produced 18.5 Wh and 5.1 50 Wh o elecriciy respecively. Te oal 40 elecric energy demand or he area was 30 85.5 Wh in 2012. Te wind urbines 20 and solar panels produced, hereore, 10 27.6% o he elecric energy demand in 0 he 50Herz SO region. Wind Solar Others Someimes high winds occur on Capacity (GW) Production (TWh) sunny days. Ten he oupu rom windurbines can be as high as 90% o he insalled wind-based capaciy, while solar- Figure 7.8. Installed generation capacities and their output, panel oupu can be 80% o he insalled 50 Hertz TSO.
150 Power supply challenges solar-based capaciy. Te oal oupu Demand distribution year 2012, 50 Hertz transmission rom he renewable sources is hus 0.9 ∙ 15 000 12 + 0.8 ∙ 8 = 17.2 GW. Tis oupu even exceeds he 14 GW maximum demand 12 500 Total demand o he 50Herz SO area. Chaper 3 ) 10 000 W shows ha such a siuaion occurred M ( 7 500 r only a ew imes in 2012. However, he e Demand minus w 5 000 renewable contribution expansion o renewables capaciy has o P 2 500 no ended. Tereore, high oupus rom 0 renewable elecriciy sources can easily push all oher generaors rom he grid i –2 500 0 10 20 30 40 50 60 70 80 90 100 heir oupu is no curailed, expored or % of the time sored. Te real-lie disribuion curve o ha par o he power demand ha had o be Figure 7.9. Distribution curves of total power demand and delivered by elecriciy sources oher han demand minus renewable contributions in the German 50Hertz he renewables in he German 50Herz TSO region during the year 2012 . In the case of no imports/ exports, power plants have to supply the power requirement SO region in 2012 is given by he according to the thick red curve. hick red line in figure 7.9. Tese oher elecriciy sources can be power plans, impors or energy sorage. Te hick red line has been deermined by subracing he hal-hourly oupu rom wind and solar rom he hal-hourly power demand curve. Te hick red curve indicaes ha a subsanial load o some 5 GW remained or power plans during 80 % o he ime. A firs sigh his migh offer opporuniies or ypical baseload power plans. However, he wind urbines and solar panels creae much volailiy in he sysem. Figure 7.9 illusraes ha or some 20% o he ime, siuaions occurred ha required power plans o produce less han 5 GW. A real seady baseload was no longer presen. elying solely on a power demand disribuion curve is, hereore, inadequae or deermining he required power plan porolio where here is much wind and solar-based power in he sysem. Neverheless, ineresing conclusions can be drawn rom he hick red line in figure 7.9. For almos 2% o he ime, he power demand rom power plans was less han 1 GW, while or almos 1% o he ime he oupu rom renewables exceeded even he oal elecriciy demand. Ye, here were imes ha solar and wind generaion had close o zero oupu so ha he power plans combined wih impors sill had o be able o cover peak demand. Tis can be seen rom he overlap o he dashed power demand curve and he hick red line in he region o peak demand. Figure 7.9 urher shows ha power plans plus impors had o cover elecriciy demand higher han 10 GW only during 10% o he ime. I he renewable generaors had no been presen, his would have happened during some 40% o he ime. Because o he very shor ime during which power plans have o deliver peak demand o beween 12 and 14 GW, peak-demand reducion via incenives or cusomers (demand side response) migh be a good opion here.
7. Future power supply systems
Demand minus renewables output (30 minute interval data) 15 000 12 500 10 000
) W M ( r e w o P
7 500 5 000 2 500 0
–2 500 0
10
20
30
40
50 50
Weeks of the year 2012
Figure 7.10. Power demand minus the contribution from renewable energy sources contribution in the 50Hertz TSO region during the year 2012
Te consequence o he wind urbines and solar panels in he 50Herz SO area in Germany is a subsanial variabiliy in he difference beween oal power demand and he conribuion rom he renewable sources. Tis difference, i.e. he amoun o elecriciy demand o be covered by power plans and possibly impors, is shown in figure 7.10. Comparing figure 7.10 wih figure 7.1 reveals ha designing a power plan porolio is now much more complicaed han beore he inroducion o renewables. From weeks 4 o 6, renewables hardly conribued owards he saisying o demand. For example, in he beginning and he end o he year, as well as during week 13, heir oupu someimes exceeded power demand. Baseload power plans could sill be applied occasionally up o a capaciy o 5 GW, bu hey had o be requenly swiched off i all he renewable elecriciy was o be accommodaed in he area. A major di difference fference bew beween een he pater paterns ns in power demand rom power plan planss in figures 7.1 and 7.10 is he much higher variabiliy in figure 7.10. In week 3, 2012, an example o a rapid increase in he required power supply rom power plans can be seen. Figure 7.11 is a sample o he relevan daa in a narrow ime span. Wind urbine oupu decreased rapidly rom 5 GW o 2 GW in he morning, while power demand increased subsanially. Power plans had o add some 6 GW in oupu wihin a ime rame o 5 hours rom a saring poin o 4.5 GW. During a period o wo hours, he required increase was even 1.5 GW per hour. For nuclear and coal-based power plans, such increases in oupu afer a lengh o ime a sandsill are close o impossible. Te issue is also ha or his occurrence, he iniial demand or power rom he power plans was quie low. I seam-based power plans had o cover he balance, i would only have been possible i hey were ru running nning a a dras drasical ically ly reduced oupu
151
152 Power supply challenges wih much seam hrot hrotling ling a he sar sar Sample of the effect of drastic renewable output reduction o he occurrence. occu rrence. on required power plant output in week 3, 2012 I should also be menioned ha he s 10 e precise orecasing o subsanial changes ch anges l b a 9 in wind-relaed power is no easy. A shif w e n o hal an hour beween a prediced e 8 r n change in wind srengh and he realiy o n ) 7 can easily occur. Te dashed curves in W m G o ( 6 r figure 7.11 show ha a 30 minues earlier f y l or laer han prediced change in wind p 5 p u power can make more han 1 GW differDemand minus renewable output s r Renewable decrease 30 minutes earlier 4 e ence in he required power plan oupu. w Renewable decrease 30 minutes later o Te increased variabiliy and higher ore P 3 casing errors mean ha power plans 0 50 100 150 200 250 300 350 have o be able o ramp up much aser Time (minutes) han orecased or delay heir increase in power oupu. Tis is no easy or ypical Figure 7.11. A sample of an occurrence event in the 50 Hertz TSO area in Germany, when the wind turbine output dropped baseload power plans. amping up and down he oupu drastically while power demand increased rom power plans is becoming a serious issue where here are many wind urbines and solar panels in he sysem. In paricular, i is he relaive change in oupu, i.e. he required change in oupu divided by he acual oupu, ha causes difficulies. As an example, ramping up wihin hal an hour rom an iniial oal combined power plan oupu o 3 GW o 6 GW is much more difficul or a porolio o seam-based plans han ramping up rom 9 GW o 12 GW. Figure 7.12 shows ha where here is no wind and solar based power supply in he sysem, he relaive change in demand rom power plans per 30 minues is generally lower han + or –15% o heir oupu. Wih renewable sources in he sysem, he change lies mos o he ime wihin + or –50% per 30 minues, a acor hree higher han in he case o no renewables. As a consequence, power plans have o be much more flexible and agile han in he pas. I should be menioned here ha he relaive changes in demand minus renewables shown in figure 7.12 have been calculaed based on he presumpion ha power plans always provided a leas 1 GW o he power demand, even hough he renewables someimes produced more han he oal demand. I his presumed minimum limi o 1 GW would no be applied or he diagram, here would appear occasional excursions o very high percenages in he relaive change in power demand minus ha o renewables. As a resul, he figure would become unreadable. However, in realiy he relaive changes in load or he power plans will occasionally be even much higher han hose indicaed in figure 7.12. I is clear ha a power plan porolio having significan baseload capaciy, as shown in figure 7.5, is no he proper opion where here is a subsanial amoun o
7. Future power supply systems
100
s u 75 n i ) m % 50 d ( n n i a m m e 0 25 d 3 n r e i 0 s p e s e g l n b –25 a a h c w –50 e e v n i t e a r l –75 e R –100 –20
–15
–10
–5
0
5
10
15
20
Relative changes in power demand per 30 min (%)
Figure 7.12. Comparison of ramping-up and ramping-down rates in power output from non- renewables with and without renewable sources in the system (based on data from 50 Hertz TSO, year 2012)
power based on wind and solar radiaion in he sysem. In his example rom he 50 Herz SO area in Germany, only a moderae 27.6% o he required elecric energy came rom wind and solar. Te inenion in Germany and in many oher counries oo, is o op or much higher percenages o renewables, meaning ha he ypical baseload or power plans wi willll no longer exi exis. s. Consequenly, convenional nuclear and coal-based power plans are ulimaely becoming obsolee.
7.3.1. Curtailment or storage of peaks in renewable output Anoher poin is wha o do when he power oupu rom renewables renewables exceeds exceeds he power demand. Exporing emporary excess elecrical energy o neighbouring areas is one possibiliy. Te marginal producion coss o elecriciy rom wind urbines and solar panels is very low, less han 0.5 €cn/kWh, as has been shown in chaper 6. When wind energy is expored or such a low price, he power plans in he imporing areas canno compee wih i. As a resul, hey also will experience higher flucuaions in oupu and end up wih a lower uilisaion acor. Te sorage o excessive renewable oupu is anoher possibiliy. A close inspecion o figure 7.9 reveals, however, ha he acual energy conen o he occasional excessive oupu rom renewables is no ye very high. I he oupu o he renewable sources were o be curailed so ha a leas 1 GW o dispachable power plans remained in he sysem, only 139 GWh o renewable elecriciy would be rejeced in an enire year. I a minimum dispachable power o 2.5 GW was chosen, 545 GWh
153
154 Power supply challenges
Table 7.1. The effect of curtailing renewable power output on rejected renewable output (year 2012 data). Minimum dispatchable power
Renewable energy rejected
Fraction of renewable output rejected
2.5 GW
545 GWh
2.3%
1 GW
139 GWh
0.6%
rom he renewables would have been rejeced. 139 GWh is only 0.6% o he 23.6 Wh o elecric energy produced by wind urbines and solar panels in he 50Herz SO area in 2012. Te 545 GWh is sill only 2.3%. Ta’s why policy makers and decision makers really have o consider i i is worhwhile capuring he occasional high oupus rom renewable sources. Sorage aciliies or capuring he excess renewable energy in he case o he 2.5 GW minimum dispachable power would need an inpu power capaciy o 2.5 + 2 = 4.5 GW, since he elecriciy producion rom renewables occasionally exceeded demand by 2 GW. Since he excess energy o be sored is 521 GWh, a uilisaion acor o 521 GWh/(4.5 GW ∙ 8784 h) ∙ 100% = 1.3% would resul or he sorage sysem. An invesmen in bateries, pumped hydro, compressed air, or hydrogen or such a low uilisaion acor will never be economic. Even an invesmen in new ransmission lines o expor he excess elecric energy would no be economic. Exporing in combinaion wih sorage o he excess energy migh be an opion i sufficien exising ransmission capaciy is available, and i sorage capaciy could be offered by already exising hydro capaciy rom neighbouring Norway, or example.
7.3.2. Converting excess electric energy into heat is a good solution In he uure, much more solar and wind based generaing capaciy will be insalled, so elecriciy producion rom renewables will ofen subsanially exceed demand. Measures have o be aken o deal wih he bulk o he excess energy, alhough i is beter again o curail he high peaks ha occur only sporadically. Te cheapes opion or accommodaing and uilising excess elecriciy appears o be he insallaion o elecric heaing coils in gas-fired ho waer boilers, and in he Table 7.2.
Heat and electricity demand in Germany in 2010. Energy application
Energy use
Hot water
118 TWh
Space heating
681 TWh
Process heat
536 TWh
Total heat use
1 335 TWh
Electricity use
582 TWh
7. Future power supply systems
hea-recovery sysems o cogeneraion insallaions. Ho waer is needed in large quaniies or such appliHot water caions as disric heaing sysems, acories, hospiElectrical heating als, hoels, and public swimming pools. In 2010, hea element demand exceeded elecriciy demand in Germany by a acor o 2.3 (see able 7.2). Cold water According o Öko-Insiu e.V, some 194 Wh or 14% o he oal hea demand o 1335 Wh in GerBurner many was supplied by cogeneraion insallaions in Gas supply Grid 2010. Te bulk o hea demand is covered by burning naural gas, heaing oil, coal and wood. Te esimaed insalled elecrical power o he cogeneraion insalla- Figure 7.13. A hybrid hot-water boiler for ions in Germany was 18 o 23 GW, or roughly 10% accommodating excess electricity. o he oal insalled capaciy o power saions. echnically, here are possibiliies or having a subsanially larger amoun o cogeneraion insallaions. When here is excess elecriciy rom wind urbines and solar panels, he generaors o he cogeneraion plans can be swiched off while he required hea is produced by elecric heaing coils in he hea recovery boilers. Tis is ar more economic han invesing in alernaive sorage echniques. Te engine-generaor Geothermal power
Coal-based until obsolete
Hydro power Bio waste
Gasifier
Wind turbines Solar PV panels Gas-based smart cogen Biomass (co-)firing Pumped hydro
Natural gas resource & storage
Heat use and storage
12 GW
Nuclear-based until obsolete
Bio waste
Gasifier
8 GW 4 GW
Agile power generator
4 GW
Gas-turbine combined cycle
3 GW
1 GW
1 GW 250 GW 3 GW
Demand management system
Natural gas resource & storage
1 GW
Export Import
GRID
Figure 7.14. Suggestion for an optimum power supply portfolio for the 50Hertz TSO case in 2012. The red arrows indicate energy flows towards and from the grid, while the green arrows indicate fuel gas flows.
155
156 Power supply challenges combinaions o he cogeneraion unis will operae as dispachable and flexible uel based generaors, should here be insufficien elecriciy rom renewables. Denmark is currenly inroducing his hybrid soluion using elecrical heaing in combined hea and power insallaions. I migh also be a good idea o sar insalling elecrical heaing coils o gas-fired domesic heaing sysems and waer boilers. Tis is no expensive. Insalling elecric hea pumps in combinaion wih a hea sorage sysem is anoher promising opion. Wih urher expansion o volaile renewable elecriciy sources, much more shorerm excess elecriciy will ener he supply sysem and ha can be pu o excellen use o reduce he consumpion o gas, oil, and coal or heaing waer. Apparenly, he German 50Herz ransmission sysem operaor has been able o keep he elecriciy sysem sable during 2012 wih he help o exising power plans and hrough expors and impors wih neighbouring areas. A brand new dedicaed elecriciy supply sysem would, however, be differen rom he curren one. A suggesion or such a sysem is given in figure 7.14. Te maximum elecriciy demand in he region is 14 GW, as shown in he demand disribuion curve o figure 7.2. In addiion o he insalled 12 GW o wind-urbine power and 8 GW o solar, 1 GW o biomass-based power is presen in figure 7.14 as a renewable resource. Furhermore, he 3 GW o agile cogeneraion insallaions have been equipped wih elecric heaing coils in heir hea-recovery sysems. Also, 3 GW o elecriciy can be absorbed by domesic and commercial boilers and hea pumps. Pumped hydro and impors/expors are boh given a capaciy o 1 GW. An addiional 250 MW o demand reducion has been made possible wih smar appliances and similar savings. Tis migh mean emporarily swiching off 125,000 washing machines or umble dryers. Apparenly, mainaining grid balance wih smar domesic appliances requires quie an effor. A beter soluion migh be o reduce he demand rom large users, such as cooled warehouses. A oal o 4 GW o combined cycle gas urbines has been added or covering he inermediae load during imes o low wind and sunshine. A urher 4 GW based on flexible and agile machinery has been added o provide power or as ramping up and down, o compensae or orecasing errors, and or non-spinning secondary and eriary reserves. Te maximum size o an individual power plan in he sysem should no exceed 300 MW. Oherwise, he ripping o a generaor canno be compensaed or properly by he primary reserves offered by he oher generaors. Te oal capaciy including he demand managemen sysem as shown in figure 7.14, is sufficien o cover maximum demand and o absorb he bulk o he excess oupu rom he renewable sources. Te sysem can easily be prepared or a uure wih double he amoun o wind and solar-based generaion by insalling more elecric heaing using boilers and hea pumps. A flee o hybrid and ully elecric vehicles will offer addiional flexibiliy in elecriciy demand. Te uel-based power plans will ineviably have a low uilisaion acor, bu heir invesmen and operaing coss will be low due o he applicaion o relaively cheap gas-based echniques and nonspinning reserves.
7. Future power supply systems
50 Hertz transmission, Germany, May 12, 2012
50 Hertz transmission, Germany, May 25, 2012
10 000
) W M ( r e w o P
8 000
Load 6 000
Others 4 000 2 000
Wind
0
Solar 0
4
8
12
16
20
Time of the day (hours)
24
0
4
8
12
16
20
24
Time of the day (hours)
Figure 7.15. Power demand and supply in the 50Hertz TSO area for two different days in May 2012.
7.4. Discussion regarding the suggested optimum power supply portfolio o give more deailed reasons or he suggesed power supply porolio in he previous secion, we use wo more figures rom chaper 3. Te siuaion on May 12, 2012, as depiced on he lef side o figure 7.15, involves high oupu rom wind urbines and a moderae solar panel oupu. Te red curve shows he elecriciy producion ha has o be covered by oher sources. Te minimum in he red curve is a very low 1 GW. I we exclude imporing and exporing, his 1 GW has o be supplied by uel-based power plans. Tese plans also have o provide primary reserves and requency conrol. In order o guaranee sysem sabiliy during maluncions, he size o each power plan should be resriced o a maximum o 100 MW. Wih hese premises, he load o 1 GW can be provided by 11 power plans, each running a 91 MW. Each o he 100 MW power plans should be insalled close o load cenres and disribued hroughou he SO area in order o avoid problems wih he supply o reacive power. Direcly ollowing he dip in he red curve ending a 6 pm on May 12, he power plans have o quickly ramp up heir oupu by abou 1 GW per hour. Tis is because he oupu rom boh wind and solar declined rapidly. Forunaely, his happened a a momen when he oal demand also decreased. Te as downward slope in wind power oupu could also have happened 30 minues earlier or laer. In he case o an earlier decline in wind oupu, he upward slope in he red curve would have been much aser. Combined cycles require a leas one hour o reach ull oupu, so hey reac oo slowly or such an uncerain even. By conras, flexible power plans driven by as-reacing engines can reach ull oupu in less han 5 minues. Hydropower migh do he same job, bu i is no abundanly available in he area.
157
158 Power supply challenges
Electricity grid connection
1
2
3
4
5
6
7
8
9
10
Flexible fuel supply
Figure 7.16. A flexible power plant based on multiple agile engines in parallel.
Te suggesed 4 GW o agile flexible generaion in figure 7.14 can easily provide he as ramping required. I he weaher orecas predics ha he oupu rom he renewable sources will remain low or eigh hours or so, he suggesed 4 GW o combined cycles could be sared up and a large number o he engine-driven unis could be gradually swiched off. Some flexible power plans should say online or requency conrol, load ollowing, primary reserves, and or compensaing or orecasing errors. For saisying he power demand on May 25, 2012, here is a sufficienly long ime span or he insalled capaciy o combined cycles o reach heir ull oupu o 4 GW. Te peaks in demand o up o 8 GW can nicely be covered by agile power generaion. Even i more combined cycles were available, saring hem up or hese peaks would no be worhwhile because o he shor ime ha hey are needed. 50 Any residual demand can be covered r 48 a by he he projeced cogeneraion plan e y 46 / engines ha would oherwise be uilised a t i 44 p litle in May because o he low hea a 42 c / demand. r 40 u o 38 Using bateries or oher sorage h t t a 36 devices migh help a litle or smoohing w o 34 l he load paterns indicaed in red in i K 32 figure 7.15, bu he energy required o 30 charge he sorage device has anyway o 2003 2004 2005 2006 2007 2008 2009 2010 Time (year) be generaed by uel-based generaors in his case. Invesing in sorage aciliies can only be considered i he renewable Figure 7.17. Example of an emerging economy: the develsources produce a subsanial amoun o opment of the per person electricity use per capita in Ethiopia excess energy during he day or a large (data from IEA).
7. Future power supply systems
racion o he year. Ta migh be he case i he renewable porolio o he 50Herz SO area will be double ha o he siuaion in 2012. Even hen, prolonged periods o no wind and sunshine will occur, so backup power based on uels is required anyhow. I should be menioned ha agile flexible power plans do no require smoohing o heir oupu o achieve opimum efficiency. When here is a high share o wind and solar power in he sysem, he uel-based power plans should be: • Agile to provide faster ramping up and ramping down than has traditionally
been he case • Fast so as to compensate for forecasting errors in the output from wind and
solar • Not too big so that the tripping of a single plant can be compensated for by a
minimum amoun o primary reserves • Fast so that the amount of primary reserves can be limited, even with a low
amoun o ineria in he sysem • Fast to oer non-spinning secondary reserves • Close to the load to avoid voltage collapse by the reactive power supply over
long ransmission lines • Inexpensive with low capital costs since their utilisation factor will be low.
7.5. An optimum electricity generation portfolio for emerging economies Te World Bank esimaes ha some 1.3 billion people are sill wihou elecriciy. Tese people are primarily locaed in developing counries. o pull such counries ou o povery and make hem economically compeiive wih he res o he world, heir per capia elecriciy use should reach a leas some 1000 kWh/year by he year 2025. Tis would drasically increase heir produciviy. Forunaely, many emerging economies currenly show an annual growh in power demand o up o 10% per year. Figure 7.17 is an example rom Ehiopia, wih a seady rise in elecriciy use per capia per year. Ye, his per capia elecriciy use is sill exremely low compared o he world average o 2500 kWh/year. Developing counries generally have a poor inrasrucure or supplying elecriciy. Many remoe locaions have o rely on small porable generaors running on expensive perol or on bateries o provide a small amoun o elecriciy. I is a chicken and egg problem; he elecriciy demand is oo small o make a proper disri buion sysem economic, while wihou elecriciy insufficien money can be earned o build he required sysem. Te bes opion appears o be o sar in he same way as Europe did a cenury ago. Small, municipal power plans can serve a local communiy, hus avoiding he coss o large ransmission sysems. Such power plans should be based on muliple flexible unis running in parallel o provide he required
159
160 Power supply challenges reliabiliy and efficiency. Tis soluion also offers he possibiliy o expand he size o he power plans gradually o mee growing demand. apid economic growh can be expeced as soon as a communiy ges access o reliable and affordable elecriciy. Te power plans should iniially be able o run on differen uels. Many remoe locaions do no ye have access o naural gas, so heavy uel oil (HFO) is ofen he only easible energy source unil such ime ha demand is high enough o allow he consrucion o gas pipelines. Dual-uel engines can easily run on boh ypes o uel. Moreover, i is oreseen ha bio uels, wind urbines, geohermal power, hydropower, and especially solar panels will play an increasing role in emerging economies. Sunshine is ofen abundan and he coss o solar panels are decreasing. Power plans consising o muliple agile unis in parallel ha can susain many sars and sops offer he proper backup or renewable sources. Mos probably, emerging economies will ulimaely bypass he concep o using large convenional uel-based power plans.
Figure 7.18. A typical village in an emerging economy
7.6. Conclusions Tis chaper has discussed possible power plan porolios or indusrialised counries, or counries wih ambiious arges or renewable energy sources and or emerging economies. I appears ha in all cases a high share o agile, flexible and reliable power plans is needed. Flexible power generaion will be much more imporan han smar appliances, smar meers, and smar grids. Flexible power plans are a good soluion or balancing elecriciy supply sysems and are needed o bridge periods o low oupu rom renewables. Tey are essenial or developing affordable, reliable and susainable power supply sysems.
8 Power supply challenges – A review Te previous chapers describe he challenges o mainaining a sable , reliable and affordable elecriciy supply wih much inermiten renewable power in he sysem. Reduced roaing ineria requires power plans wih high agiliy. Large racions o reacive power canno be supplied via ransmission lines. A disappearing baseload demands more flexible power plans ha are capable o saring and sopping requenly. Te uilisaion acors o uel-based power plans will decrease. Te crucial decisions required or obaining a more susainable global energy supply have o be based on a horough knowledge o all he aspecs described in his book. In any case, uure power plans have o be agile and flexible. Tis chaper reviews he issues so as o remind he reader ha opimum soluions depend o a large exen on variable boundary condiions.
164 Power supply challenges
8.1. Realism is needed in the energy debate Energy is he driving orce behind produciviy, produc qualiy, and comorable living. Energy is, hereore, he engine o he economy. Energy will be supplied o end-users increasingly as elecrical energy, because elecriciy is so versaile and convenien. Ulimaely, he world’s energy supply has o be susainable, wih accepable shor- and longerm impacs on he environmen. Ye, a he same ime, a susainable energy supply has o be affordable and very reliable. Other In 2011, he share o elecriciy in he world’s oal primary Oil 4.5 energy supply (PES) was only 12%. Despie coninuous 4.8 improvemens in power plan efficiency, sill a considerable 34% o global PES was needed or elecriciy producion. Almos Nuclear 70% o he primary energy required or elecriciy producion 11.7 Coal 41.3 semmed rom coal, naural gas and oil. Hydropower and nuclear Hydro power were responsible or he bulk o he remainder, while 15.8 elecriciy sources based on solar radiaion, wind and biomass Natural gas produced less han 4.5% o he elecriciy demand. 21.9 o replace he 3 150 Moe (megaonne oil equivalen) o ossil uels needed or elecriciy producion in 2011 by renewable energy sources is a huge ask. A ull wihdrawal rom he use o Figure 8.1. Energy sources for ossil uels migh no be possible or many decades o come, espe- electricity production in 2011 (data cially since he demand or elecriciy in he world is expeced o from IEA Key World Energy Statistics 2013). double over he nex 20 years. Te world’s populaion will grow and elecriciy demand in emerging economies will increase rapidly. In indusrialized counries, elecric hea pumps will ofen replace radiional heaing sysems in homes and elecric vehicles will be common on he roads. Te world economy will, hereore, increasingly depend on elecriciy – and much more o i will be needed han now. Policy makers and ciizens canno be blamed or lacking a clear view o he elecriciy supply challenges ahead. Newspapers and he inerne bring exciing news abou wind arms and solar insallaions ha would produce enough power or, say, 100000 households. Te ac ha households consume on average only 20% o he elecriciy needs in a modern naion is no explained. Globally, indusry uses some 46% o he elecric energy supplied. Te remaining 34% is or services. Moreover, he unavoidable need or backup generaing capaciy or renewables is hardly ever discussed in he news. In many areas in he world, he capaciy acor o a wind arm is lower han 25%, meaning ha on average he oupu is less han 25% o he insalled power capaciy. Solar panels ofen have a lower capaciy acor han wind urbines; in Germany or example i averages 9.5%. Tis is parly caused by he naural absence o solar radiaion during he nigh, bu also by clouds. In very sunny places, as in he Sahara, solar panels migh reach a capaciy acor o 30%. We read news abou solar panels reaching grid pariy, meaning ha he coss o elecriciy rom a solar panel can
8. Power supply challenges – A review
Capital costs solar panels; investment 1 716 €/kW 50
) h W 40 k / t n c 30 € ( s t s o c 20 l a t i p 10 a C
DR = 2.5%; life = 20 years DR = 2.5%; life = 40 years DR = 10%; life = 20 years DR = 10%; life = 40 years
0 5
10
15
20
25
30
35
40
Capacity factor (%) Figure 8.2. Capital costs per kWh produced for solar panels depend on the boundary conditions (DR = discount rate).
compee wih he elecriciy coss rom he grid. However, such saemens can be made only i he boundary condiions are known. Firs o all, he coss o insallaion and grid connecions have o be included. Figure 8.2 gives an example o he capial coss o solar panels per kWh produced, depending on panel lie and he discoun rae D (he ineres rae) ha he owner requires. I a privae consumer is happy wih a discoun rae o 2.5 % and he panel lie is 20 years, he coss o elecriciy rom PV panels in a sunny locaion migh approach 5 €cn/kWh, which easily resuls in grid pariy. For a proessional invesor who desires a 10% discoun rae in an area where he capaciy acor o PV would only be 10%, elecriciy rom an idenical panel coss almos 25 €cn/kWh. Panel lie does no have a large effec on he capial coss when he discoun rae is high. Conversely, wih a low discoun rae he effec is considerable. In conclusion, o alk abou he grid pariy o renewable energy sources wihou giving he boundary condiions is misleading. When he coss o renewable energy are discussed, he coss o backup capaciy, eiher rom sorage sysems or generaors, are generally no aken ino accoun. Ye backup is always needed or solar panels and wind urbines. Chaper 6 has shown ha he acual price a consumer pays or elecrical energy can be higher by a acor o hree han he producion coss. Tis is because o profi raes, disribuion coss, governmen levies and axes. Tereore, he real cos o creaing a susainable elecriciy supply depends on many acors.
8.2. Low capacity factors escalate balancing issues One concern wih renewable elecriciy sources is he effec o prioriy eed-in, and even unconsrained eed-in, on supply sysem sabiliy. I a counry wans o cover one hird
165
166 Power supply challenges
Maximum
Average
Minimum
175
d n a m e d r e w o p e g a r e v a f o %
150 125 100 75 50 25 0
Power demand
Wind 20% cap factor
Wind 35% cap factor
Figure 8.3. Low capacity factors of renewable energy sources result in high variability in their output.
o is average elecriciy demand by wind power, he disurbing effec depends heavily on he capaciy acor o he wind urbines. Tis is illusraed in Figure 8.3. I he capaciy acor or wind parks is 20%, which is normal in a counry like Germany, and a hird o he elecriciy demand has o be covered by wind, he maximum wind oupu can exceed even he maximum demand or elecriciy. As a resul, he grid is heavily disurbed and curailmen o wind oupu is ofen required. However, i he capaciy acor is 35%, as is possible in opimum locaions, he maximum in wind oupu is slighly less han he average demand or elecriciy. In his case, he wind oupu exceeds demand only occasionally during imes when demand is below average. Figure 8.3. illusraes his. Idenical siuaions can occur wih solar power. Anoher effec o having a subsanial share o elecriciy rom renewables is ha he remaining uel-based generaors ofen have o very quickly ramp heir oupu up and down. Tey have o sar and sop requenly, and have o compensae or subsanial orecasing errors. A he same ime, hese power plans need o be much smaller in capaciy han in he pas in order o ensure grid sabiliy during coningencies. Solar panels in ho climaes, where peak demand or elecriciy coincides wih peak sunshine, can have an effecive smoohing impac on wha he uel-based generaors have o supply during he dayime. However, where a large racion o elecriciy demand is covered by solar panels, oher generaing capaciy will experience high ramping up raes in he evening when he sun ses.
8. Power supply challenges – A review
8.3. Flexible local generators of limited size offer excellent backup for renewables I has been suggesed ha he ask o balancing renewable energy could be handled by ransporing excess elecriciy rom one counry o anoher. Curren ransmission lines were no inended or his purpose. Te invesmen coss o such a sysem would be very high while is uilisaion acor would be low. Moreover, he energy loss during ranspor would be high. Te siuaion is worse in he case where renewable sources do no produce a proporional racion o reacive power. Long overhead ransmission lines beween generaion and demand lose heir capaciy should hey have o ranspor larger racions o reacive power. Tis resuls in a local collapse o volage, i.e. a black ou, especially when acive power demand is low. Moreover, high winds generally cover a subsanial par o coninens so ha peaks in wind urbine oupu in adjacen counries generally coincide. Building exensive ransmission sysems or ransporing elecriciy will hereore no help in solving he problems arising rom inermitency o renewable energy sources. In addiion, large disan power plans offer insufficien reserve capaciy in case o calamiies. Muliple smaller local generaors are needed insead – in ac hey are he soluion or all problems menioned.
8.4 Natural gas is ideal for backup capacity Te media and some researchers ofen give he impression ha here are high hopes or developing elecriciy sorage sysems o smooh demand flucuaions and even ou he varying oupu o renewable sources. Tis appears o be possible only in locaions wih guaraneed daily sunshine or wih winds wihou long doldrums – meaning he gaps in oupu are less han 24 hours. Ten bateries, pumped hydro and compressed air migh have sufficien capaciy. Neverheless, such sorage adds a leas 10 €cn per kWh o he cos o elecriciy. Covering a windless ime span o 10 days rom sorage will be prohibiively cosly, le alone soring solar energy rom he summerime o be consumed in he winer. Demand-side response managemen helps only or shor-erm balancing. Moreover, i is exremely difficul o find sufficien swichable load capaciy. o creae a demand response o 1 GW wih smar appliances in households, a leas 500 000 acive washing machines or laundry dryers o 2 kW each have o be conrolled. I heir uilisaion acor is 5%, en million smar appliances are needed or creaing he 1 GW demand response. Te pracical realisaion o demand response rom households seems o be wishul hinking, especially since households consume on average only 20% o all elecriciy produced. Only a large-scale hydropower sysem is capable o providing long-erm sorage. Norway is an example. Te so-called power-o-gas concep o producing hydrogen wih excess elecriciy and injecing i ino naural-gas pipelines or laer use, has an
167
168 Power supply challenges
Figure 8.4.
Electricity is all around us – even when we are unplugged.
energy efficiency as low as 25% and would add o he coss o elecriciy considerably. Moreover, hydrogen deerioraes he qualiy o naural gas. Naural gas in combinaion wih smar agile generaors offers an excellen opion as backup sysems or keeping elecriciy sysems balanced over exended ime spans. Using naural gas only as a ransien uel and burning i all as quickly as possible during he firs hal o he 21s cenury is no wise. One should save he gas as much as possible or creaing flexible backup over a much longer ime span.
8.5. Integrating power demand and heat demand of fers good perspectives A very effecive way o avoid grid insabiliy caused by excess elecriciy rom wind urbines and solar panels is an inegraed approach wih hea demand. Demand or hea in he world is sill much higher han he demand or elecriciy. Excess elecriciy rom renewables can easily be ed ino ho-waer boilers and sorage anks wih elecrical heaing coils. Disric heaing sysems are an ineresing applicaion. Where here is insufficien elecriciy producion rom renewable sources, combined hea and power (CHP) unis can provide he needed elecriciy and hea. In he case o excess elecriciy, he cogeneraion unis can swich he engines off while he waer is heaed by elecriciy. Te engine-driven gas-uelled generaing unis o cogeneraion sysems can also provide he necessary ancillary services, such as compensaing or orecasing errors and providing reserve capaciy.
8. Power supply challenges – A review
8.6. Agile, flexible power plants help to ensure a reliable and costeffective power supply Ideally, flexible power generaion is based on local power plans each having muliple idenical generaing unis in parallel. Te size o each individual generaor in such power plans is limied so as o creae sufficien flexibiliy in oupu and o guaranee a high combined availabiliy. Te invesmen coss in such a sysem are relaive low because o he uniormiy and series producion, while he lead ime needed or engineering, procuremen and consrucion is shor because o he sandardised unis. Mainenance coss per kWh and uel efficiency are independen o he power plan’s oupu. Te generaors can be swiched on and off wihou reducing he inerval beween mainenance work. Te generaors provide non-spinning reserves, as well as adequae reserves or orecasing errors since hey can sar up in less han a minue and deliver ull oupu wihin 5 minues. Mainenance is easy since i is ully sandardised. Te agiliy o hese power generaors makes hem suiable or lifing he burden o requency conrol and rapid up and down ramping rom less flexible power plans. Tey can provide as primary reserves ha are needed when ineria in he sysem is diminishing. Tey can also offer secondary reserves wihou spinning. Tese advanages will lower he oal coss o elecriciy producion. In sysems wih a subsanial amoun o renewable elecriciy sources, smar asreacing power generaors offer he opimum soluion o keep he sysem balanced
Figure 8.5. Power systems can be reliable, affordable and sustainable in the future – if we make the right choices now.
169
170 Power supply challenges in an affordable and reliable way. Furhermore, smar poweragile generaors are he ideal soluion in emerging economies, where power demand is iniially low bu likely o accelerae rapidly. Teir oupu flexibiliy and load-independen uel efficiency and mainenance coss make hem he cheapes opion or guaraneeing high reliabiliy in he elecriciy supply.
8.7. Outlook for the future Te goal o ataining a susainable, reliable, and affordable elecriciy supply involves a huge challenge. educing CO 2 emissions wih eiher renewable energy sources or carbon capure and sorage (CCS) will easily double elecriciy producion coss. In a closed economy his would no creae problems since any expenses or he equipmen and or he resources will say wihin ha economy. I will even creae jobs. Te inrinsic value o elecriciy is so high ha a doubling o elecriciy coss would sill leave hem well below he rue value o having elecriciy. In realiy however, economies are open and energy inensive producion will move o areas wih he lowes elecriciy coss i governmens do no creae special incenives or he indusry. Ye, even i a doubling o elecriciy producion coss is very likely in a more susainable world, acceping an even larger increase in coss should be avoided. In a desperae atemp o quickly reach prese goals or lower emissions and having a lower dependence on uel impors, decision makers migh op or economically unaracive soluions. Tis book has shown ha flexible and agile local power plans o a limied size can excellenly serve as a aciliaor or inegraing renewable energy and or improving he uel efficiency o less flexible power plans. Flexible and modular power plans are also he perec soluion or providing he world’s elecriciydeprived populaion wih his resource o help hem in creaing wealh. Naural gas is an ideal backup uel and should be saved or ha purpose as much as possible. Gas companies have o ensure a good qualiy o gas or elecriciy generaors, since he power secor will be heir larges cusomer in he uure.
172 Appendix 1
Appendix 1 Background physics and mathematics regarding the role of inertia in power systems
A car wih a mass m a sandsill a he op o a slope has so-called poenial energy due o graviy. Graviy is he orce wih which he earh ‘pulls’ a he car. Tis orce is proporional o he mass m o he car and he acor g. Te acor g is called he acceleraion o graviy. Te value o g depends on he posiion o an objec on earh, bu generally a value o 9.81 m/s2 is used. Te poenial energy Ep o he car a he op o he slope wih a heigh h equals: E p = m · g · h
Equation A1.0.
As soon as he brakes o he car are released, he saic orce o graviy sars he car moving down he slope. Wihou an exernal orce, he mass o he car would never move. Te mass o an objec can be seen as resisance agains change in moion: mass means ineria. Because o he orce o graviy, he car develops speed. Tis speed is called velociy v in physics. I road ricion and air resisance are negleced, all poenial energy will be convered ino speed-relaed energy, known as kineic energy in physics. Te kineic energy Ek o an objec depends on is mass and is speed: E k = ½mv 2
Equation A1.1.
I he heigh o he slope is 50 meres and he mass o he car is 1200 kg, he poenial energy E p o he car equals: E p = 1200 ∙ 9.81 ∙ 50 = 588600 kg m/s 2 m = 588600 Nm = 588600 J ≈ 0.59 MJ,
since 1 N (newon) = 1 kg m/s 2 and 1 J (joule) = 1 N m. Te inernaional sysem or unis uses he ollowing prefixes o shoren large numbers: k (kilo) = 1000 M (mega) = 1000,000 G (giga) = 1000,000,000 (era) = 1000,000,000,000 P (pea) = 1000,000,000,000,000 E (exa) = 1000,000,000,000,000,000 Z (zeta) = 1000,000,000,000,000,000,000 Y (yota) = 1000,000,000,000,000,000,000,000
Appendix 1
As soon as he car has reached he botom o he slope, all is poenial energy has been convered ino kineic energy. Equaion A1.2 reveals ha he car will have a speed o: v = √ (588600 ∙ 2/1200) = 31.3 m/s, equalling 112.8 km/h.
I he brakes o he car are hen applied, he kineic energy will be convered ino hea. Te 0.59 MJ o kineic energy is enough o hea up waer or only 13 cups o ea. Tis illusraes ha soring large amouns o energy as poenial energy via, or insance, ele vaing masses o pumped hydro, or as kineic energy in a flywheel, is no an easy ask. A physical objec ha can roae around a shaf also has ineria. I means ha wihou an exernal orce, or momentum in his case, he objec will no sar o roae. Conversely, i he objec is roaing, i will need an exernal orce o slow i down again. Te momenum applied delivers energy o he objec, and ha energy is again sored as kineic energy in he objec. Te ineria o objecs ha can roae is called he moment of inertia I . Te momen o ineria o an objec depends on is mass m and he disance r o ha mass o is shaf. A disk has a momen o ineria I equal o ½ m r 2 , and he I o a hin ring equals m r 2 . A disk:
Tin ring:
Te momen o ineria o a generaor ogeher wih he prime mover ha supplies he generaor wih energy closely resembles ha o a disk. Te kineic energy Ek o a roaing objec equals: Ek = ½ I (2 π f )2 = 2 π 2 I f 2
Equation A1.3.
in which is he number o revoluions per second in Hz (herz). For a small change in requency, he derivaive o equaion A1.3 gives he amoun o change in he roaing energy: dE = 4π 2 I f df
Equation A1.4.
173
174 Appendix 1 Or: dE = 4π 2 I f df
Equation A1.5.
For a change o he roaing energy in ime, we wrie: df dE = 4π 2 I f dt dt
Equation A1.6.
Te change in roaing energy o a generaor is he difference Δ P in power supply o he generaor by he prime mover and he power demand rom he connecors o he generaor. Tereore, we can wrie: P = 4π 2 I f
df dt
Equation A1.7.
and: df P = 2 dt 4π I f
Equation A1.8.
For power sysems, he ineria o a generaor is generally expressed in he ineria consan τ i. τ i =
2π 2 I f n2
Equation A1.9.
Pn
in which Pn = he nominal power o he generaor and n he nominal requency o he generaor. By combining he wo las equaions, or he insananeous change in requency due o unbalance in power we can wrie: df 1 P = I f n2 dt Pn 2fτ i
Equation A1.10.
A = 0, when he unbalance Δ P jus occurs, f = f n , so: df P f n at t = 0 = dt Pn 2τ i
Equation A1.11.
Tis means ha he iniial inclinaion in speed change or a sysem wih an ineria consan τ i is deermined by he racion o he unbalance in power Δ P o he nominal power Pn. Equaion A1.10 can be rewriten as: 2 P f n f df = dt Pn 2τ i
Equation A1.12.
Appendix 1
By inegraing his equaion, we ge: 2 P f n f = t+C Pn τ i 2
Equation A1.13.
A t = 0, f 2 = ( f n)2 , so ha C = ( f n)2. Tereore, he change in roaional requency versus ime equals: P f n2 2 f = (f n + Pn τ i t)
Equation A1.14.
Equaion A1.14 applies or a consan unbalance beween he power supply and he power demand, meaning ha he load is no affeced by he requency. In realiy, he sel regulaing power in he grid will ensure ha he unbalance is reduced when he requency changes. In ha case, equaion A1.10 has o be rewriten as: a df P – 100 Pinitial (f n – f) 2 1 = f n dt 2fτ i Pn
Equation A1.15.
In which a is he percenage o load change per Hz and Pinitial is he iniial load. As soon as he primary conrol reserves P pc sep in, he unbalance will be urher reduced and equaion A1.15 has o be rewriten as: a df P – 100 Pinitial ( fn – f) + P pc ) 2 1 = f n dt 2fτ i Pn
Equation A1.16.
Equaions A1.15 and A1.16 are quie complicaed o inegrae mahemaically, bu numerical inegraion wih small seps in ime gives a good approximaion. Figures 2.7, 2.11, 2.12 and 2.13 in Chaper 2 are he resuls o he numerical inegraion o equaions A1.15 and A1.16.
175
176 Appendix 2
Appendix 2 Power plant performance during a major fault in the system
Fauls and ailures are unavoidable and occur in any sysem. Fauls in elecriciy supply sysems generally mean deviaions rom a normal siuaion caused by exernal evens, such as a alling ree, lighning, animal acions and vandalism. So-called ailures resul rom equipmen maluncioning and weak grids. Human error is also a cause o undesired occurrences. A ailing generaor resuling in a power plan rip causes a sudden unbalance beween elecriciy producion and consumpion, as described in chaper 2. For a generaor, a shor circui in a ransmission line is a aul o which i will respond. Tis secion will describe in general erms wha happens wih a generaor in he case o a shor circui in he grid. A deailed echnical descripion o all he dynamic evens would, however, require a separae book. Here, i is imporan o undersand wha he consequences o a shor circui are or a power plan and he grid. Forunaely, grids are equipped wih insrumens ha deec anomalies. As soon as a major problem such as a shor circui is deeced, he relevan grid secion will be isolaed via breakers. Modern grid proecion equipmen is capable o aking such an acion wihin 150 milliseconds. Neverheless, during a limied ime, a shor circui can have a severe effec on grid volage, leading o a subsanial loss o load or generaors ha sense he volage. Wih a so-called boled shor circui, he volage drops o zero and he generaor loses is exernal load. A sepwise loss o load implies ha a generaor will accelerae Fuel supply
Speed/load controller n P
Prime mover
Coupling
GENERATOR V = C.N.Ф V,I
Flywheel Voltage/power factor controller
Figure A2.1. Control circuit of a prime mover driving a generator in a generating set. The prime mover cannot instantaneously reduce its power output should there be a loss of load caused by a short circuit in the grid.
Appendix 2
100 90 80 70 ) 60 % ( e g 50 a t l o V 40
Energinet dk German Transmission Grid Code (FRT to zero voltage depent on distance of the WT from PCC)
30
National Grid PSE-Operator REE
20 10 0 –500
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
Duration of short circuit (ms)
Figure A2.2. Examples of different fault-ride through rules in Europe prior to a uniform ENTSO-E rule being implemented.
is roaional velociy or a while. Te reason is ha he prime mover ha is driving he generaor canno reduce is power oupu immediaely. I he generaor has a higher speed han he grid requency, he volage o he generaor can be considerably urned ou o phase wih respec o he volage o he main grid. In ha case, a reurn o he grid volage afer he correcive acion by he grid proecion sysem can cause damage o he generaor se and lead o sysem insabiliy. Te risks o damage o he generaing se and insabiliy o he grid are he reasons ha in he pas local generaors, such as cogeneraion insallaions and wind urbines were disconneced rom he grid as soon as he volage exceeded specified limis. econnecion ook place afer he grid volage reurned o normal. However, wih a subsanial amoun o decenralised generaors in a sysem, his radiional approach migh resul in he available generaing capaciy immediaely afer clearance o he aul being insufficien. Te relaive number o large, high ineria generaing unis ha radiionally helped o run hroughou grid auls can be low when oupu rom wind urbines and solar PV panels is significan. Tis is why grid operaors nowadays speciy ha all generaors supplying energy o he grid should have aul-ride-hrough capabiliy, so ha hey can suppor he grid during and afer a calamiy. I means also ha he generaor has o provide reacive power o he grid during he occurrence. Aferwards, he generaor has o deliver he same power o he grid as beore. As menioned earlier, i he load o a generaor insananeously disappears, he uel supply o he prime mover ha drives he generaor canno immediaely be reduced. For urbines, i akes ime o decrease he opening o he uel supply valves
177
178 Appendix 2 or seam valves. oo drasic reducions Induction coils Rotor in he uel supply can cause flame ou in he combusors o gas urbines. For a S modern reciprocaing engine, he ypical Slip rings and N our-sroke process means ha almos contact brushes N wo revoluions o he engine shaf are Direct current for needed o consume he uel already supthe magnetic field S plied o he cylinders. wo revoluions AC voltage Drive shaft o an engine running a 750 rpm already occupy 160 ms. Te uel supply o a prime mover should no be drasically Electric energy cu off during a shor circui, since afer he occurrence he rules declare ha he generaing unis should again deliver Figure A2.3. Schematic representation of a generator with two pole pairs: m = 2. (picture taken from google images). heir iniial oupu. In a firs approach o sudy he effec o a shor circui on a generaor, a synchronous generaing uni can be seen as a prime mover, such as a urbine or an engine, which drives a roaing magne in a se o saionary coils. Te roaing magne is called he roor and he saionary par is called he saor. Te volage V generaed in he saionary coils is direcly proporional o he running speed n and he magneic flux Φ rom he roor: V = c · n· Φ
Equation A2.1
In grid-parallel operaion, he grid dominaes he requency and volage. For he generaor o produce elecric energy, he angle o he roor’s magneic field had o lead he phase angle o he volage o he saor by he so-called load angle δ. Simply said, he magne has o pull a he grid via a virual spring in order o ranser energy o he saionary coils. Te load angle δ increases wih he relaive power oupu o he engine-generaor combinaion. Under seady-sae condiions, he load-angle curve versus he generaor oupu has a roughly sine-wave shape: i he load angle exceeds 90°, he generaor has reached is pull-ou or ipping orque and he sysem becomes unsable. Te load angle also depends on he srengh o he magneic field, where a sronger field (= more magneisaion) leads o a smaller load angle. Simply said again, i he pulling spring is siffer, he spring expands less or a given orce. For commercial generaors, he elecrical load angle ranges beween 15° and 60°. Te physical, or mechanical, roor angle δ lm can be ound by dividing he elecrical load angle δ le by he number o pole pairs m. Tis pole-pair number m is 1 or an n = 3000/min machine in a 50 Hz grid, and m is 4 or a generaor running a 750/min in a 50 Hz grid. Wih a sronger magneic field rom he roor han is necessary or generaing he required volage, he generaor curren will lead he volage (capaciive characer). Tis means ha he generaor produces he reacive curren needed or he inducive loads o he grid. In his case, he load angle is smaller han in
Appendix 2
3.0
Pull-out torque or tipping torque
2.5
2.0 P
r e w1.5 o P 1.0
0.5
0.0 0
20
40
60
80
100
120
140
160
180
Electrical load angle (deg)
Figure A2.4. Example of steady-state generator output versus electrical load angle δle for a typical electrical load angle of 24° and a nominal power P of 1.
he case o a roor field ha is jus sufficien o produce he necessary volage. Wih lower exciaion han needed o produce he grid volage, so-called under-exciaion, he generaor needs reacive curren rom he grid and he load angle is larger. Wih a larger load angle, he sabiliy o he generaor is smaller. Te ‘spring’ is weaker in his case. I he load angle exceeds 90°, he so-called pull-ou or ipping orque is passed and he generaor will sar o spin aser han he grid. Tis is called pole slip. Wih a synchronous generaor uni in grid-parallel operaion, wo exreme siuaions can occur: insananeous disconnecion rom he grid by he generaor circui breakers, or a shor circui in he conneced grid. In boh cases, he load o he generaor is los. Loss o he elecric load means ha he driving energy rom he prime mover will sar o accelerae he generaor. A simplified example will now show how he speed o he generaor and he angle o is poles wih respec o ha o he grid migh change in he case o a ull loss o load. In realiy, he siuaion wih a shor circui is no so dramaic, since he grid volage generally does no drop o dead zero. We can re-wrie equaion A2.3 in erms o he angular roor speed ω = 2π f , and describe how he running speed o he generaing uni will increase in ime (= accelerae) wih a loss o power Δ P: dω πP = f dt τ i Pnominal nominal
Equation A2.6.
179
180 Appendix 2 Te acual angular speed increase Δω can be ound by inegraing equaion A2.6: ω =
πP f t τ i Pnominal nominal
Equation A2.7.
Te mechanical load angle increase Δφlm is ound by inegraing equaion A2.7 and equals in radians: φlm =
πP f t 2 2τ i Pnominal nominal
Equation A2.8.
Equaion A2.8 can be re-writen in degrees insead o in radians (π = 180°). For a ull loss o load, Δ P equals Pnominal, so ha: φlm =
180
2τ i
f nominal t 2
Equation A2.9.
Figure A2.5 illusraes he simplified case o how he running speed o a generaing se wih 4 pole pairs will increase i all o he ull load is los while he prime mover coninues eeding energy o he generaor as needed or ull load. In his example, an ineria consan τi o 1.5 s has been chosen. Te figure also shows how he mechanical load angle δlm and he elecrical load angle δle change in ime wih respec o he cenral grid o a consan requency o 50 Hz. I will be clear
100
) g e d ( e l g n a d a o l l a i c i r t c e l e + l a c i n a h c e M
790
Mech load angle Electr load angle Speed (rpm) 80
780
60
770
40
760
20
750
0
740 0
25
50
75
100
125
) m p r ( d e e p S
150
Time (ms)
Figure A2.5. Example of a change in running speed and load angles with respect to the central grid in the simplified case of a fully lost generator load.
Appendix 2
ha recurrence o he ull grid volage afer 150 ms will show a large mismach in running speed, volage and phase angle. Te siuaion when he grid reurns can be compared wih ha o connecing a generaor o he grid wih a mismach in synchronisaion. In realiy, he case is much more complex. Beween he locaion o a shor circui in he grid and he generaor, generally a line secion and a ransormer wih impedance are presen. In addiion, a generaor is equipped wih damping windings and has inernal impedance. Tis will miigae he exreme reacion as given in our example. Te real-lie siuaion is so complex ha only compuer modelling wih many parameers can approach wha will acually happen in pracice. In he conex o his book, i is imporan o undersand ha a rapid emporary reducion in power supply rom he prime mover o he generaor during a grid aul helps o avoid a severe mismach when he aul has been cleared. A flexible power generaor is able o sense he drop in volage caused by he shor circui, and o reac wih a emporary insananeous decrease in oupu. Combusion engines have he abiliy o reard he onse o combusion in he cylinders insananeously, resuling in a direc decrease in power oupu. When he grid volage reurns, he iniial iming will be immediaely resored. Such a rapidly reacing aciliy creaes he flexibiliy o ride hrough a grid aul.
181
Dr. Jacob Klimsra Senior Energy Specialis ‘I Hazzewâld’ Hazzeloane 3 Broekserwâld Te Neherlands
Jacob Klimsra sudied elecrical, elecronic and mechanical engineering. He worked 29 years a he research laboraory o N.V. Nederlandse Gasunie on pulsaing combusion, vibraion-based machinery diagnosics and reciprocaing machines. From 1993 o early 2000, Jacob was Head o Deparmen o Indusrial Gas Applicaions a Gasunie esearch. From 2000 unil he end o 2009, Jacob was employed by Wärsilä as a senior specialis or engine-driven power sysems. From 2010, Jacob serves he indusry as a consulan. Jacob received he ichard Way Memorial Prize or his Ph.D. hesis, he Van Oosrom Meyjes Prize rom Te oyal Neherlands Insiuion o Gas Engineers and 5 Oral Presenaion Awards a well as he Disinguished Speaker Award rom SAE. In Sepember 2000, he received he ICE Division Speaker Award rom he American Sociey o Mechanical Engineers. In 2010, he received a lieime recogniion award rom Cogen Europe. He is he co-auhor o he book Smar Power Generaion published in 2011. Jacob also eaches a shor courses on energy and engine echnology a universiies around he world. In addiion, he serves a he Advisory Board o PowerGen conerences and acs as moderaor and speaker a many inernaional conerences. Since he beginning o 2014, he is managing edior o he Cogeneraion and On Sie Power Producion magazine.
References Books, papers, reports and magazines:
Vuorinen, Asko, ‘Planning o Opimal Power Sysems, 2009 ediion, ISBN 978-95267057-1-2, Ekoenergo Oy, Espoo, Finland, 2009 Klimsra, Jacob and Markus Hoakainen, ‘Smar Power Generaion’, ISBN 978-951-692846-6, Avain Publishers, Helsinki, Finland, 2011 Dekker, G. and J. Frun, KEMA repor 74100846-ED/SDA 12-00079, Arnhem, Te Neherlands, March 16, 2012 Inernaional Energy Agency, ‘World Energy Oulook 2012’, ISBN 978-92-64-18084-0, Paris, France, 2012 Inernaional Energy Agency, ‘Key World Power Saisics’ (years 2000 – 2013) Te Economis, vol. 407, nr. 8830, April 2013 Chabo, Bernard, ‘enewable Energy or Elecriciy in Caliornia in 2012 and is Fuure ole’, htp://www.renewablesinernaional.ne/ Eurelecric, ‘Hydropower or a susainable Europe’, Eurelecric Fac Shees, February 2013 Appleyard, David, ‘racking he Price o U.S. Grid-conneced PV’ enewable Energy World Conerence, Mumbai, India, 6-8 May 2013 ENSO-E, ‘ Nework Code or equiremens or Grid Connecion Applicable o all Generaors, Brussels, Belgium, 8 March 2013 Marens, Deborah, edior, ‘ Join EASE/EER recommendaions or a European Energy Sorage echnology Developmen oadmap owards 2030’ European Associaion or Sorage o Energy, March 2013 Seamus Garvey, ‘Compressed Air Energy Sorage (CAES): Cycle Efficiency’, Pre-Conerence Workshop on Energy Sorage, Marcus Evans, Amserdam, 1s December 2010 Jacob P. Aho, Andrew D. Buckspan, Fiona M. Dunne and Lucy Y. Pao, ‘Conrolling Wind Energy or Uiliy Grid eliabiliy’, ASME Magazine MECHANICAL ENGINEEING, Sepember 2013
Websites:
www.eirgrid.com/operaions/sysemperormancedaa/ www.sepco-solarlighing.com/blog/bid/102981/enewable-Energy-and-he-Indusrial-evoluion-Par-1 htp://conen.caiso.com/green/renewrp/20130610_DailyenewablesWach.x htp://www.swissgrid.ch/swissgrid/de/home/reliabiliy.hml htp://www.enne.org/bedrijsvoering/xmldownloads/index.aspx htp://www.amprion.ne/en/grid-daa htp://www.50herz.com/cps/rde/xchg/rm_de/hs.xsl/Nezkennzahlen.hml htps://www.ensoe.eu/daa/daa-poral/ htp://www.ree.es/en/aciviies/realime-demand-and-generaion htp://www.power-hru.com/ htp://www.me.ie/ htp://en.ilmaieeenlaios.fi/ htp://www.we-a-sea.org/een-energie-eiland-voor-de-kus-van-nederland/ htp://www.erco.com/gridino/ htp://www.markesandmarkes.com www.europe.eu www.eia.gov www.eurelecric.org www.erco.com www.caiso.com
Glossary Acive power – Produc o volage and curren resuling in a ne release o elecrical energy energy.. Ancillary services – Services provided by elecriciy generaors o ransmission sysem operaors (SOs) oher han producing elecrical energy direcly, such as requency conrol, backup capaciy, and spinning and non-spinning reserve. Availabiliy Ava ilabiliy – Condiion in which a machine is ready o perorm he duy or which i is inended. Balancing – Conrolling elecriciy producion so ha i ully maches elecriciy demand. Baseload – Consan demand level or elecrical energy ha is presen during a prolonged period o ime.
maximum load, hus ensuring high uel efficiency and low mainenance coss. Cogeneraion – Effecive mehod or uilising he hea released during he producion o elecrical energy or process heaing, space heaing or cooling. Combined cycle – Process where he hea released by a gas engine or gas urbine is recovered and urned ino seam or a seam urbine driving an addiional generaor. Compressed air energy sorage – Sysem ha uses elecrical energy o drive an air compressor o sore compressed air in a caviy or laer use wih an expandergeneraor combinaion during imes o peak elecriciy demand, or or balancing elecriciy supply and demand. Compression hea pump – Machine ha uses a moor-driven compression and expansion process o bring hea rom Black sar capabiliy – Capabiliy o a generaingg uni o sar up wihou supgenerain one emperaure level o anoher, or por rom an exernal elecriciy source, heaing or or cooling. such as he elecriciy grid. Coningency – Abnormal condiion or siuaion leading o he close o sepCalorific value – Energy conen o a uel expressed per uni o volume or uni o wise increasing or decreasing o generaing power. mass. Capaciy acor – aio o acual elec- Cres acor – aio o he peak value o a variable quaniy and is avera average ge value. rical energy ha a generaing se can produce in a cerain ime span and Demand side managemen – Mehod or decreasing elecriciy demand by he amoun o elecrical energy ha i swiching off par o he elecriciy concould produce i running a ull oupu sumpion. during ha ime span. Carbon capure and sorage – Process o Deph o discharge – aio o he amoun capuring he carbon dioxide emissions o energy ha can be aken rom an rom he use o uels and soring hem energy sorage device and he sored indefiniely, or example in geological energy in ha device, wihou damaging he sorage device. ormaions. Cascading – Mehod or an array o elec- Discoun rae – Fracion o an invesed capial ha is desired as an annual riciy generaors in parallel o run yield. individual generaors only close o
– Capaciy o a generain generaingg Dispachabiliy – uni o deliver a cerain perormance as required by he generaor operaor or ransmission sysem operaor. Disribuion grid – Sysem ha disribues elecriciy or gas o households, commercial users, and small indusries. Droop – Dependence o he oupu rom a generaor on a deviaion rom a requency sepoin Elecriciy demand patern – Hourly, daily, weekly, monhly, or annual paern o elecriciy use, including baseload, inermediae load and peak load. Elecriciy highway – Buzzword or a high-capaciy sysem or ransporing elecrical energy over longer disances. Elecriciy inensiy – Amoun o elecrical energy needed o creae a cerain amoun o gross domesic produc, ofen expressed in kWh/€ or kWh/$. – Amoun o physical work sored Energy – or delivered o a process. Energy sorage – Soring energy or laer use, ofen in pumped hydro, bateries, flywheels, and compressed air, bu primarily in uels. Faul ride hrough – Capabiliy o remain conneced o he elecriciy grid in he case o a aul in he grid resuling in a shor circui. Final energy use – Energy used by end consumers, such as indusries, commercial users and households. Does no include he energy consumpion needed or processing uels or power plan losses. Fixed charge rae – ae o capial coss resuling rom a given discoun rae and a given lie o an insallaion. Forecasing error – Difference beween he orecased oupu rom e.g. wind
urbines and he acual required oupu. – Number o repeiive cycles Frequency – per second o a process, wih he uni Hz (herz). Gas engine – Machine ha convers chemical energy sored in uel gas ino mechanical energy. Gas urbine combined cycle – Combinaion o a gas or oil-fired urbine and a seam urbine. Te seam urbine uses hea rom he exhaus gas o he gas urbine. Generaing porolio – Combinaion o all he power plans in a given area, such as nuclear plans, coal-fired plans, gasfired plans, hydropower and renewable elecriciy genera generaors. ors. Gross domesic produc (GDP) – oal moneary value o he amoun o goods and services produced per year in a counry. Ofen, he GDP is expressed in he local purchasing power pariy (PPP) o he US$, since he buying power o he US$ differs rom counry o counry. High-volage ac – Tree-wire sysem or ransporing elecrical energy a high volage (> 35 kV) as alernain alernaingg curren. High-volage dc – wo-wire sysem or ransporing elecrical energy a high volage (> 300 kV) as direc curren. HVAC – Acronym or heaing, venilaion and air condiioning. Hydrogen economy – Te idea ha hydrogen serves as a major energy carrier in he world. Inermediae load – Elecric load ha is presen only during 10 o 18 hours per day, due o he increased demand rom indusry, commercial buildings, and households.
Ineria consan – Te energy sored in he roaing elemens o a generaing uni divided by is nominal power capaciy. Key perormance indicaors – Imporan numbers indicaing he perormance o machinery, such as he specific uel consumpion (MJ/kWh), specific invesmen coss (€/kW), and availabiliy (%). Load shedding – Swiching off elecriciy users or appliances in order o balance elecriciy producion and demand. Cogeneraion ion insal Microo cogenera Micr cogeneraion ion – Cogenera laion inended or homes wih an elecric oupu o up o a ew kW, where he hea released is uilised or heaing he building buildi ng and saniary saniary waer waer.. Mineral oil – Oil rom oil wells o ossil origin. Nominal power – Te nameplae power capaciy o a generaor or an elecric appliance, ofen equal o he maximum max imum power capaciy. Non-spinning reserves – Generaion capaciy ha runs only when secondary and eriary reserves are needed o compensaee or coningencies. compensa O&M – Operaion and mainenance. Operaional availabiliy – – Te ime ha a machine is available afer mainenance requiremens requireme ns and logisical delays have been aken ino accoun. Phase shif – Te angle ha wo sine waves are shifed wih respec o each oher. Phase change o maerials – Changing he physical condiion o a maerial, such as ice ino waer by meling, waer ino seam, or vice versa. Peak acor – See cres acor. Peak load – Eiher he load ha occurs above inermediae load, or he maximum load ha occurs during a cerain ime span.
Peak shaving – Decreasing peaks in elecriciy demand by reducing demand, using sored energy or generaing elecriciy wih flexible power plans. Phoovolaics – Dc elecriciy generaion direcly rom solar irradiaion wih ligh-sensiive elemens. Plaeau load – See inermediae load. Power – Te capaciy o perorm work wihin a cerain ime span (joule/ second = wat). Power capaciy – Te nameplae power oupu or consumpion o a machine. Primary energy supply – Te energy supply based on uels, nuclear power, or renewables bu which is no ye con vered ino ino,, or example, elecriciy or reaed, such as mineral oil ino perol. Prime mover – Machine ha can drive a process by supplying mechanical energy. Purchasing power pariy – PPP, he buying capaciy o he US$ in a cerain counry. Ramping down rae – Te speed wih which a genera generaor or or prime mover can decrease is power oupu (e.g. MW/s). Ramping up rae – Te speed wih which a generaor or prime mover can increase is power oupu. Reacive power – Produc o volage and curren when he wo quaniies are 90 degrees ou o phase so ha no ne energy is released. Reliabiliy – Te probabiliy, ofen expressed in he percenage o ime, ha a machine can saisically perorm is duy. – Energy no resuling Renewable energy – rom ossil uels or nuclear uel. Roaing ineria – Propery o a roaing mass resuling in conaining energy
in proporion wih he square o he number o revoluions per ime uni. Round-rip efficiency – Energy efficiency o processes, where he energy ulimaely ends up in he same shape as i sared, e.g. elecriciy rom a batery. Shoulder load – See inermediae load. Simple cycle – Termodynamic process where uel energy is convered ino mechanical or elecrical energy in a single process. Single cycle – See simple cycle. Smar Power Generaion – Arrangemen o a number o agile generaing unis in parallel ha can undergo muliple sars and sops wihou suffering rom exra wear, which resuls in load-independen uel efficiency and minimal mainenance coss per kWh. Solar irradiaion – adiaion received rom he sun a a cerain locaion a a cerain ime. Spaial impac – Space (volume and area) needed or insallaions such as a power plan. Specific capial coss – Capial coss per uni o a produced produc, such as €/ MWh. Specific uel coss – Capial coss per uni o uel energy, such as €/GJ.
Specific mainenance coss – Capial coss o carrying ou mainenance per uni o a produc, such as €/kWh. Spinning reserve – Generaing capaciy, generally expressed in MW, ha is synchronised wih he grid and ready o produce elecriciy when needed. Supergrid – See elecriciy highway. Synchronisaion – Process o ensuring ha a generaor has he same requency as he elecriciy grid, while he valleys and cress o he sine waves o he generaor and grid coincide. ransmission grid – High volage grid connecing generaors wih large consumers and he disribuion grids, also or he cross-border exchange o elecriciy. urn-around efficiency – See round rip efficiency. Unbundled power secor – Siuaion where elecriciy producion, ransmission, disribuion and reailing are carried ou by separae companies. Uilisaion acor – Fracion o use o he nameplae power capaciy o machinery during a cerain ime span. Verically inegraed uiliy – Company ha produces, ranspors, and disribues elecriciy (and/or gas, waer, disric heaing), and sells i o end users.
