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Solar pond and its application to desalination Article · July 2012
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Asian Transactions on Science & Technology (ATST ISSN: 2221-4283) Volume 02 Issue 03
Solar pond and its application to desalination A.Z.A. Saifullaha*, A.M. Shahed Iqubala and Anirban Sahab a
Department of Mechanical Engineering, IUBAT – International University of Business Agriculture and Technology, Dhaka 1230, Bangladesh Department of Electrical and Computer Engineering, the University of Texas at San Antonio, Tx – 78249, USA
b
Abstract This paper discusses the solar pond technology and how it is applied to desalination. A solar pond is a shallow body of water which acts as a solar collector with integral heat storage for supplying thermal energy. Solar ponds are mainly two types: convective solar ponds and non-convective solar ponds. The shallow solar pond and the deep saltless pond are the examples of convective type. There are three types of non-convective solar ponds: salinity gradient solar pond (SGSP), membrane solar pond and polymer gel layers solar pond. A SGSP is a pool of water about 1-5 m deep, which contains dissolved salts to establish a stable density gradient. There are three layers in a SGSP: upper convective zone (UCZ), lower convective zone (LCZ) and salinity gradient non-convective zone (NCZ) in the middle. Incident solar energy is collected and stored which may be delivered at temperature near 100C. The SGSP is the most eco-friendly and environment-friendly among all the solar energy systems for electricity generation, desalination, hot water applications in agriculture, green house heating, domestic hot water production and space heating and cooling of buildings. Nevertheless, a SGSP is more cost-effective since its collection cost per square meter is only one-fifth of that of a liquid flat plate collector, and cost of 1KWh of electricity production by a SGSP is only one-fifth of that produced by photovoltaic cells. A solar pond multi-stage flash distillation system (SPMSF) is very promising for Bangladesh. MSF plants can produce 6-60 L/m2/day, whereas for typical solar stills it is 3-4 L/m2/day. Keywords: solar pond, salinity gradient, sodium chloride, desalination, SPMSF built open water reservoir commonly called as solar
Introduction
pond [4]. Solar pond is a convenient and effective Study and research have been made for a low cost means which collects solar radiation and stores its collection and storage system of solar energy in thermal energy for a relatively longer period of time. various countries [1]-[3]. Simultaneous collection Remarkable research effort and publications started and storage of solar energy is feasible in a purposely
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in 1960’s, mostly in Israel. Then going slowly effort
pump, solar vapour compression refrigeration and
on research speeded throughout the world after the
solar pond [14]. Solar pond is a simple and low cost
energy crisis in 1970’s [5]. Research has been done
solar energy system. Solar pond is now an attractive
on solar pond for about 50 years. It is now used in
means which can be used for electric power
Israel, USA, India and Australia [6] – [10]. China
generation, desalination, salt production, grain
has done remarkable progress in study and
drying, fruit and vegetable drying, fruit and
application of solar pond technology to various
vegetable canning industry, aquaculture, dairy
applications [11], [12]. Some other countries like
industry, green house heating, domestic hot water
Iran, Turkey and Libya are also actively engaged in
production and space heating and cooling of
research on solar ponds [18], [26], [32]. Simulation
buildings.
has been performed for heat and mass transfer in a The objective of this paper is to describe the SGSP by several researchers [26], [60]-[63]. technology of solar pond and its application to Besides, experimental research in SGSP is also there desalination. [26], [64]. Scope and Limitations The thermo-nuclear reaction in the sun originates solar energy. Solar energy covers the entire
This paper gives a detailed concept on SGSP with its
electromagnetic
surface
working principle, thermal behavior, and stability
receives about 47% of the total energy reaching the
criterion. Besides, management of a SGSP has been
earth. This amount only is the usable energy [13].
described with its construction, salt used, site
Solar energy can be utilized directly by two
selection, soil character, salinity gradient formation,
technologies – solar thermal and solar photovoltaic.
obstruction and remedies and heat extraction. Solar
Solar thermal technology results in solar collectors,
desalination has been discussed
solar water heater, solar passive space heating and
emphasis on SPMSF.
cooling
system,
wave
solar
spectrum.
The
refrigeration
and
air-
conditioning system, solar cooker, solar furnace, solar greenhouse, solar dryer, solar distillation, and
with special
There is no data and recommendations on site selection,
linear
establishment,
heat
selection, extraction,
salt
gradient
environmental
solar thermo-mechanical systems. Solar thermomechanical system includes solar thermal water
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protection and cost analysis of a SGSP, because no
storage of heat and transport of thermal energy, at
research project on experimental
temperature 40-50 C above normal, out of the system [15]-[17].
solar pond has been operated in Bangladesh until today.
There are three types of non-convective solar ponds in terms of the methods of maintaining layered
Solar Ponds structure. One is SGSP where density gradient is A solar pond is a shallow body of water which acts
maintained by salt water. The other is membrane
as a solar collector with integral heat storage for
solar pond which uses horizontal and vertical
supplying thermal energy. There are two types of
membranes. The third one is polymer gel layers solar
solar ponds – convective solar pond and non-
pond [16].
convective solar pond. The shallow solar pond is a A SGSP is a system for solar energy collection and convective solar pond. It consists of a large bag that storage. It uses solar radiation to heat water; stores prevents evaporation but permits convection. The sensible heat in dense saline water; establishes bag has blackened bottom with foam insulation density gradient to prevent convective heat flow and below, and two types of glazing (sheets of plastic or thus stores thermal energy. Fig. 1 shows the glass) on top. Solar energy heats the water in the bag schematic view of a SGSP [17], [18]. during the day and at night the hot water is pumped into a large heat storage tank to minimize heat loss.
A SGSP has 3 main layers. These are UCZ (Upper
Another type is the deep, saltless pond. Double
Convective Zone): top layer; NCZ (Non-convective
glazing covers deep saltless pond. When solar
Zone): middle layer and LCZ (Lower Convective
energy is not available or at night placing insulation
Zone): bottom layer.
on the top of the glazing reduces heat loss [15]. UCZ is of almost low salinity and at close to ambient A non-convective solar pond is a large shallow body
temperature. This zone is the result of evaporation,
of water 1 to 5 m deep, but 3-4 m on the average,
wind mixing, surface flushing and nocturnal cooling.
which is arranged in a way so that the temperature
Generally this layer is maintained as thin (0.3 – 0.5
gradient is reversed from the normal. This allows
m) as possible by the use of wave-suppressing
collection of radiant energy into heat (up to 95 C),
meshes or by placing wind-breaks near the ponds.
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NCZ is a gradient which is much thicker and
to this the temperature of the bottom layer may rise
occupies 1.5 m or more than half of the depth of the
up to 95 C making the SGSP a unique energy trap
pond.
with added advantage of built-in long-term heat
In NCZ,
both
salt concentration and
temperature increases with depth. The vertical
storage capacity [15], [19].
salinity gradient in NCZ holds back convection and Thermal Performance thus offers the thermal insulation effect. Temperature gradient is formed due to absorption of solar
The thermal performance of a SGSP which is similar
radiation at the pond base. LCZ is a zone of almost
to that of a conventional flat plate solar collector has
constant relatively high salinity (20-30% by weight)
been shown by Srinivasan [20]. Assuming steady
at nearly constant high temperature. Heat is stored in
state condition,
Qu Qa Qe
LCZ, which should be sized to supply energy throughout the year. It is almost as thick (usually 1
where
Qu useful heat extracted, Qa solar
m) as the NCZ [15], [19]-[21]. This is the heat energy absorbed and collector, heat storage and heat removal medium. The bottom boundary is a black body [22].
Qe heat losses.
The thermal efficiency of a SGSP is defined as
Qu / I Working Principle where I is the solar energy incident on the pond. When solar radiation falls on the surface of the
0 Qe / I
SGSP, most of it penetrates and absorbed at the bottom of the pond. The temperature of the dense
where 0
salt layer thereby increases. If there were no salt, the
Again,
Qe U 0 (Ts Ta )
where
Ta ambient temperature
bottom layer would become less dense than the top layer and the buoyancy effect would cause this water
and rise up and thus the layers would mix. Heat from the surface of the pond is then rapidly dissipated to the surroundings. But the denser salt layer at the bottom of a SGSP prevents the heat to be transferred to the
Qa / I optical efficiency of the pond.
U 0 over-all heat-loss coefficient
Neglecting heat losses from the bottom and sides of the pond and assuming the temperature of the upper mixed layer to be the same as the ambient,
top layer of fresh water by natural convection. Due
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(dT / dZ ) ( I / K )[( (Z ) ]
K w thermal conductivity of water
where
b thickness of the gradient zone
and
A steady state analysis of a SGSP that includes the effect of solar radiation absorption in the gradient zone on the temperature profile has been given by
The effect of ground-heat losses on the performance of a SGSP has been analyzed by Hull et al [20], [24]. They have expressed the ground heat-loss coefficient as
Nielsen [20].
U g K (1/ D bP / A)
In steady state, the energy equation can be written as
K (d 2T / dZ 2 ) I (d / dZ ) K thermal conductivity of water
where and
fraction
reaching a depth
Z.
of solar radiation I
where
K ground conductivity, D
depth of water table, P pond perimeter, A surface area
b a constant whose value is around
and On integration this equation gives
0.9 (depending upon the side slope).
(dT / dZ ) ( I / K )[ (Z ) (Z1 ) (dT / dZ ) Z2 ]
The thermal efficiency of a steady state solar pond now becomes
Z1 interface between the upper
where
convective zone and the gradient zone
Z 2 interface between the gradient
and
K T U T 1 z2 ( Z )dZ w g z Z 2 Z1 1 I I
zone and storage zone If
is the fraction of the incident solar energy
where
T temperature difference
between the storage zone and the upper mixed layer. which is extracted from the system as heat, including heat losses, then the energy balance of the storage
Stability of Solar Pond
zone gives A solar pond is said to be statically stable if its
(dT / dZ ) z2 ( I / K )[ (Z ) )]
density increases with height from the top. Wind
Combining the above two equations the temperature
blowing at the top surface and heating of the side
profile in the gradient zone is obtained as
walls, etc. cause disturbance to a solar pond. The
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criterion of dynamic stability is rather tough and is
conservation of mass, momentum and energy [20].
S T Z theoritical S
The stability criterion is written as
The thickness of the gradient zone can be reduced by
obtained by perturbation analysis of the basic laws of
T
T S Sc 1 S Z Z Pr 1
T
where
1 p thermal T
expansion coefficient
Pr 1 T Sc 1 Z
the development of internal convective zones or erosion of the boundaries of the gradient zone. Wind-induced mixing is primarily responsible for the erosion of the gradient zone-surface zone interface and can be minimized by using floating
S
1 S
salinity
plastic nets or pipes. The density and temperature gradients at the gradient zone -storage zone interface
expansion coefficient
causes the erosion of the gradient zone-storage zone
Pr Prandtl number
interface. The gradient zone -storage zone interface
Sc Schmidt number
remains stationary, which was experimentally found
For typical conditions, this result is simplified to
S T 1.19 Z Z
by Nielsen [20], if the salinity and temperature gradients satisfy the following relationship
S Z AT Z
0.63
where S is in kg/m3 and T in C.
where A = 28 (kg/m4)(m/K)0.63.
The above criterion has to be satisfied at all points within the gradient zone in order to prevent development of internal convective zones within the gradient zone. A safety margin of 2 is essential as
Management of Solar Pond Typical Construction:
recommended by Hull et al [20], [25]. Safety margin (SM) is defined as
Size of a SGSP ranges from hundreds to thousands square meters in surface area. These are 1-5 m deep.
S S SM Z actual Z theoritical where
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Typically these are lined with a layer of sand insulation and then a dark plastic or rubber impermeable liner material [17].
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layers (10-20 cm thick) of decreasing salinity
Salt used:
stacked on top of the storage layer using horizontal Sodium
chloride
(NaCl)
is
used
normally. diffuser. Lastly fresh water is the final layer pumped
Magnesium
chloride
(MgCl2),
sodium
nitrate on the surface [17], [27]. Fig. 2 shows the formation
(NaNO3), sodium carbonate (Na2CO3), sodium of the salinity gradient. sulfate (Na2SO4), ammonium nitrate (NH 4NO3), fertilizer salts as urea (NH2CO.NH2) satisfy the
Maintenance of Salinity Gradient:
stability criterion and thus considered suitable for a The concentration gradient existing in SGSP leads to solar pond [17], [26]. steady diffusion of salt from higher to lower concentration, that is, from bottom to top through the
Site selection:
gradient zone. The transport of salt through the Since solar ponds are horizontal collectors, sites gradient zone by diffusion is expressed as should be at low to moderate northern latitudes, that is, latitudes between -40 to + 40 degree [16].
Qm [(Sl Su ) D] / b
Soil character:
where b = thickness of gradient zone, D = mass Evaluation of geological soil character is necessary diffusion coefficient and because the underline earth should be free from stresses, strain and crack, which could cause
S l , S u salinity in lower
and mixed layers respectively [20].
differential thermal expansions, resulting in earth
So, stability is to be maintained by introducing salt at
movement if the structure is not homogeneous [16].
the bottom while the top is frequently washed with
As thermal conductivity of soil increases greatly with moisture content the water table of the site must be at least a few meters below the bottom of the
fresh water. When solar radiation fall on the pond, the part which is transmitted to the bottom heats the lower layer and as a result inverse temperature gradients are set in. These are temperature gradients
pond to minimize the heat loss [16].
that are reversed from the normal. Inverse Forming the Salinity Gradient:
temperature gradients are maintained to eliminate
First the storage layer is formed with high
convection currents that occur due to temperature
concentration brine solution mixed in bottom. Then
difference during normal temperature gradient [16].
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than
Obstructions and Remedies
chlorine
and
is
therefore
recommended chemical for algae control 1.
Cleanliness of pond since transmittance in solar ponds. Chlorine is more corrosive can be reduced due to contaminants. than Cupricide due to the acidic effect it Filtration
can
remove
contaminants. has on the pH [50].
Construction
of
larger
ponds
can 4.
Horizontal temperature gradient created
minimize the effect of contaminants. by salt solution and removal. 2.
Increase of UCZ caused by surface waves Injecting and removing salt solutions and evaporation. very slowly can minimize horizontal Use of floating nets and wind barriers can temperature gradient [16], [20], [23]. reduce surface waves and mixing of UCZ.
3.
Heat extraction
Algae and bacterial growth. For extracting energy stored in the bottom layer, hot Algae growth can be controlled by water is removed continuously from the bottom by a adding bleaching powder. Alternatively, pump, passed though an external heat exchanger or algae growth can be minimized by adding an evaporator and then returned so as to heat this 1.5 mg CuSO4 per liter of water. If the water again. Another method of heat removal is water used is alkaline, CuSO4 will not extracting heat with a heat transfer fluid by pumping dissolve. it through a heat exchanger placed on the bottom of The pond clarity can be maintained and the pond [15], [22]. the thermal efficiency of the solar pond
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can be improved by using a combination
Heat is extracted from a SGSP conventionally by
of chemical and biological treatment
drawing the heat from the LCZ only. This is done
methods. Hydrochloric acid could be
with the help of an in-pond heat exchanger located in
used initially as a shock treatment to kill
the LCZ. A heat transfer fluid is circulating in a
all the algae and then introduction of
closed cycle extracts heat from the internal heat
brine shrimps would control the growth
exchanger and transfers its thermal energy through
of algal and maintain transparency [49].
an external heat exchanger. Fig. 3 shows this method
Cupricide is found to be more effective
of heat extraction used for heating application [28].
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The 3000 m2 solar pond at Pyramid Hill, Australia
The El Paso Solar Pond project was initiated by the
used this method.
University of Texas at El Paso in 1983. It is a research, development and demonstration project. It
Fig. 4 shows another conventional method of heat has been operating since May 1986 showing extraction by pumping the hot brine from the top of electricity, process heat and fresh water can be the LCZ through an external heat exchanger and then produced successfully in the Southwestern United sending back the brine at a lower temperature to the States using solar pond technology [55]. bottom of the LCZ. The velocity of the brine circulated is to be regulated in order to prevent
Pyramid Hill Solar Pond:
erosion of the gradient layer [28]. An example of this A consortium of RMIT University, Geo-Eng type of heat extraction is the sodium chloride SGSP Australia Pty Ltd and Pyramid Salt Pty Ltd has of 3000 m2 constructed at El Paso, USA in 1983 finished a project by the use of a solar pond located [29]. at the Pyramid Hill salt works in Northern Victoria. A novel system of heat extraction for improved
Its purpose is to capture and store solar energy using
efficiency is to extract heat from the non-convective
pond water which can go up to 80 0C [56]. The heat
gradient layer of a SGSP as well as, or instead of,
produced by the pond will be used for commercial
from the lower convective zone (LCZ). This method
salt
has been analyzed theoretically and compared with
specifically producing brine shrimps for stock feed.
the experimental results at Bundoora East, RMIT.
Plan is there to generate electricity from the heat
An in-pond heat exchanger made of polyethylene
stored in the pond in a subsequent stage [57].
production
as
well
as
for
aquaculture,
pipe has been used to extract heat for over 2 months. Bhuj Solar Pond: Heat extraction from the gradient layer increases the overall efficiency of the SGSP by up to 55%,
The first large-scale solar pond in industrial
compared
environment to cater actual user demand is the 6000
with conventional
method
extraction solely from the LCZ [28], [30].
of
heat
m2 solar pond in Bhuj, India. It supplied about 15 million litres of hot water in total to the dairy at an
Examples of Solar ponds
average temperature of 750C
El Paso Solar Pond: between September 1993 and April 1995 [58].
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desalination is very costly compared to the use of
Ohio Solar Ponds:
fresh water from rivers or ground water as it Four SGSP have been designed, built and operated in typically uses extremely huge amount of energy as Ohio by Ohio State University.. Two solar ponds well as specialized, expensive infrastructure. But, were constructed in Columbus for physical studies, beside recycled water this is one of the non-rainfall one solar pond was constructed at the Ohio dependent water sources particularly countries like Agriculture Research and Development Center at Australia which traditionally have depended on Wooster and one solar pond was constructed in rainfall in dams to supply their drinking water. As Miamisburg to heat a community swimming pool scarcity of water has appeared as a major problem all and
recreational
building.
Data
and throughout the world desalination is getting priority
recommendations have been developed from these to meet the increasing demands for fresh water. One research efforts on site selection, linear selection, salt of the most important international health issues is gradient
establishment,
heat
extraction
and clean potable water. The warm arid countries in the
environmental protection. Sodium chloride was used Middle East and North Africa (MENA) and Southern as the stabilizing salt for each pond. The costs of 2
2
Asia within the latitudes 15-350N are the areas with
building solar ponds varied from $38/m to $60/m
the severest water shortages. The increase in ground [59]. water salinity and infrequent rainfall characterize SGSP applied to Desalination
these areas. Simultaneous increase in industrial and
Desalination is termed to be any of several processes which remove some amount of salt and other minerals from water. Generally speaking, desalination refers to extraction of salts and minerals, as in soil desalination. Desalination of water is done for converting salt water to fresh water to make it suitable for human consumption or irrigation. Sometimes the process gives out table salt as
a
by-product.
submarines
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use
Many
seagoing
desalination.
ships
and
Large-scale
agricultural activities with the increasing world population growth all throughout the world leads to the depletion and pollution of fresh water resources [38], [41]. During the last couple of years desalination technologies have been significantly developed. Large energy consumption occurs in the major commercial desalination processes using oil and natural gas as heat and electricity, while emission of harmful CO2. Kalogirou estimated that the production of 1000 m3/day of fresh water
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requires 10,000 of oil/year [51]. This is of high
(MED). Thermal desalination is an energy-intensive
significance as it needs a recurrent energy expense
process. During the last several years substantial
which only a few of the water-short areas can afford.
amount of research work has been done on
Alternative sources energy, especially, renewable
desalination using solar energy. For sea water
energy sources have been drawn attraction to sea
desalination there are mainly two approaches for
water desalination [42]. However, today only 0.02%
solar energy utilization. The solar distillation plant
of the global desalination is run by renewable energy
may consist of integrated or separated systems for
systems
an
the solar collector and the distiller. Integrated
ecologically advantageous means of the use of
systems are termed as direct solar desalination which
renewable energy [39]. Due to the diffuse nature of
involves different types of solar stills. Separated
solar energy, large-scale desalination plants using
systems are called indirect solar distillation. The first
solar energy have the problems of relatively low
approach is suitable for small production systems,
productivity rate, the low thermal efficiency rate and
such as, utilization of green house effect to evaporate
the considerable land area requirement. But the very
salty water in a simple solar still [42]. Solar stills are
nature of solar desalination plants is free energy and
used in regions where the fresh water demand is less
insignificant operation cost. Besides, this technology
than 200 m3/day [52].
is suitable for small-scale production, especially in
involves more than one subsystem: one for energy
remote arid areas and islands, where there is scarcity
collection, another for energy storage and the third
of supply of conventional energy [54].
subsystem for energy utilization in the desalination
[46].
Solar
distillation
presents
The other approach often
process. The desalination process may be MSF, Desalination involves desalting a variety of raw MED, vapor compression (VC), reverse osmosis waters (sea water, brackish ground water or (RO), membrane distillation (MD) or electrodialysis industrial waste-water) through suitable treatment [38], [40]. One significant problem that affect the and obtaining fresh water for drinking and irrigation. still performance is the direct contact between the Solar energy has been used for distillation of collector and the saline water which causes corrosion brackish or saline water for a considerably long time. and scaling in the still and thus reduces the thermal The current leading desalination process is thermal efficiency desalination
which
includes
multistage
[43].
The
water
desalination
with
flash humidification-dehumidification (HD) uses air as the
distillation (MSF) and Multi-effect distillation
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working fluid, which eliminates this problem. The
for fiscal year 1998… The act is based on the
overall efficiency of the desalination plant increases
fundamental need in the US and world-wide for
by combining the principle of humidification-
additional sources of potable water.
dehumidification with solar desalination, and thus SGSP enables the most convenient and least seems to be the best method for water desalination expensive
option
compared
to
other
solar
by solar energy [44]. A HD system consists of a desalination technologies to store heat for daily and compact unit containing two heat exchangers for seasonal cycles. For steady and constant water evaporation and condensation respectively. The production, this is very important from the view constructions
are
lightweight
usually
and point of operational advantage and economic benefit.
inexpensive, and work at atmospheric pressure. As The heat storage enables SGSP to power desalination desalination capacity is relatively low, the system during night time and cloudy days. SGSP used performance
must
be
improved
to
make
it desalination for a 24-hour a day operation needs only
economically competitive [42], [44]. Solar ponds half the size to produce same quantity of water and parabolic troughs are the most frequently used compared to other solar desalination options. For solar thermal technology for desalination [53]. desalination SGSP can make the use of reject brine Thermal desalination using SGSP is one of the most
as a basis to build it. This advantage is very
promising solar desalination technologies. Solar
important when SGSP is considered for inland
ponds
a
desalting for fresh water production or brine
desalination plant has been investigated by Tabor
concentration to be used in salinity control and
[33], Tleimat and Howe [34], [35], Guy and Ko [36]
environmental clean-up applications.
generated
thermal
energy
to
drive
and Posnansky [37]. The United States government At present the most common and simple technique used solar pond technology especially for this for large-scale desalination is MSF, which produces purpose. The Water Desalination Research and fresh water a total amount of about 10 million Development (DesalR&D) Program was authorized ton/day globally [42]. by Congress under the Water Desalination Act (Act) of 1996. The Act authorized program funding began
A solar pond multi-stage flash distillation system
in October 1997 for a six year period. To start the
(SPMSF) comprises a set of evaporative condensers
program, funding was appropriated at $3.7 million
and heat exchanger for extracting heat from the
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SGSP.
Repetitive
cycles
of
evaporation
and
condensation using low temperature heat from the
annual O & M equivalent to 3% of the investment costs, 85% plant factor) [47].
SGSP produces fresh water. Fig. 5 shows the Szacsvay et al. has describes a desalination system schematic diagram of solar pond desalination [15], consisting of a solar pond as the heat source and an [22]. Atlantis autoflash multistage desalination unit. The Several medium scale MSF desalination plants using
Atlantis Company developed an adapted MSF
solar energy have been recently implemented.
system called “Autoflash”, since the standard MSF
Block’s findings show that MSF plants can produce
process was not able to operate couple to any heat
6-60 L/m2/day, whereas for typical solar stills it is 3-
source. The basis of autoflash process is multistage
4 L/m2/day [45].
flash process concept. Computer simulation and experimental results of a small-size solar pond and
A SGSP is one of the most common types of solar desalination subsystem in Switzerland which had collectors. The SGSP driven desalination plant in been in operation for 9 years were done for Margarita de Savoya, Italy, has a capacity of 50-60 m3/day and that in El Paso, Texas has the capacity of 19 m3/day. Parabolic trough collector is another
performance and layout data. It was found that the cost of distillate could be reduced from $ 5.48/m3 for small desalination system with a capacity of 15 to
frequently occurring source for solar energy which is 2.39 m3/day for desalination systems with a capacity used in a MSF plant in Kuwait with a production rate of 300 m3/day [48].
3
of 100 m /day [46]. The average daily solar energy incident in India is 5 Desalination using solar troughs was tested mainly in kWh/m2. India is in advancement in solar pond the USA. Small-scale units are commercially research and applications [2], [20]. Like India available. These combine the MSF process with Bangladesh is also in the tropics. It is located steam generating parabolic troughs. A typical plant between 20.30-26.38 North Latitude and 88.04-92.44 produces 450 L/day in three stages by using 48 kW. The current cost of the collectors (about 45 m2) is
degrees East Latitude. This is an ideal location for solar energy utilization. The daily average solar
about US $ 10,000, which translates into production 3
radiation varies between 4 to 6.5 kWh/m2. Solar
costs of 7.9 US$/m (5% interest, 20 years life time, pond appears to be highly promising for Bangladesh
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[31]. Bangladesh has 68 districts in total. The 9
Conclusions
districts, namely, Khulna, Satkhira, Bagerhat, 1. Solar pond is an efficient source of Barguna, Patuakhali, Bhola, Pirozpur, Jhalkathi and renewable heat energy. Cox’sbazar lie in the coastal belt. The ground water 2. Solar pond is environmentally sustainable. available there for drinking purpose is salty. This 3. The great advantage of solar pond it water must have to be desalted in order to make possesses built-in long term thermal energy fresh water for drinking supplies. For a developing storage, which no other country like Bangladesh SGSP used desalination solar collection device match. method is the most suitable. 4. Solar pond can be economically constructed if there is plenty of inexpensive salt, flat land and
Advantages
easy access to water. 1.
Diffuse radiation (cloudy days) can be 5.
A great factor in the future of solar
used. pond operation is the implementation of 2.
It is a unique energy trap with built-in an acceptable means of long term energy storage capacity. salt recycling. This is a major point
3.
For low grade heat (below 100 C) when a solar pond is to be used on a collection cost/m2 of collector area of farm or private land. On-farm use will SGSP is 1/5th that of flat plate collector predominate in the more northerly [15]. latitudes, because of availability of
4.
1 kg of salt as salt-water concentrate can land, machinery and labor. produce energy 3 times more than the 6. Solar pond can complement the use of heat produced by burning the same fossil fuel in industries to generate thermal energy amount of coal in the combustion and chamber [15]. commercial electricity.
5.
Pollution free.
6.
In Germany, cost of producing 1kWh of
7.
Solar pond is highly promising for the tropics and the lower latitudes for
electricity by a SGSP is only 21% of that electric power generation. produced by photovoltaic cells [23]. 8. Solar pond is eco-friendly.
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9. Solar pond desalination is the most
4.
Golding, 2008: ’Solar Ponds’ in Solar
effective for Bangladesh. 10. A
Akbarzadeh, A., J. Andrews and P.
solar pond multi-stage flash
distillation system appears
Energy Conversion and Photoenergy
to be
Systems in Encyclopedia
of Life
promising for the coastal districts of
Support Systems (EOLSS), developed
Bangladesh for fresh water production.
under the Auspices of the UNESCO,
11. Solar pond projects are yet to be started
EOLSS Publishers, Ozfodr, UK. Vol. 1
in
Bangladesh.
Therefore,
– Solar Ponds.
an
experimental project on solar pond
www.Toodoc.com/solar-ponds-
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Figure Captions Figure 1
Schematic view of a SGSP
Figure 2
The formation of the salinity gradient
Figure 3
Heat extraction using an internal heat exchanger by conventional method
Figure 4
Heat extraction using an external heat exchanger by conventional method
Figure 5
Schematic diagram of solar pond desalination
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Figure 1 Schematic view of a SGSP
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Figure 2 The formation of the salinity gradient
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Figure 3 Heat extraction using an internal heat exchanger by conventional method
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Figure 4 Heat extraction using an external heat exchanger by conventional method
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Figure 5 Schematic diagram of solar pond desalination
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