Thermoelectric Electricity Generation From Low Temperature Flue Gas Stream
Abstract:
Almost 20-50% of industrial energy input is lost to environment in the form of waste heat as hot exhaust gases, cooling water, and heat lost from hot equipment surfaces and heated products contributing 1/3 rd to greenhouse gas emissions. This waste heat arises due to both equipment inefficiencies and thermodynamic limitations of the equipment and processes. This lost or waste energy is estimated to be over 10 quads/yr., an amount equipment to more than 1.72 billion barrels of oil or 127 days worth of imported crude oil supply. supply. As the industrial sector continues continues efforts to improve improve its energy efficiency, recovering waste heat losses provides an attractive opportunity for an emission free and less costly energy resource. The waste heat recovery market is expected to grow at a CAGR of 6.5% over the next five years to reach $53,120 million by 2018. Various technologies, including technologies, are being considered to recover process waste energy to useful electrical energy. Thermoelectric (TE) materials produce electric current when joined together and subjected to a temperature difference across the junction. This property makes it possible to produce direct current electricity electricity by waste heat on one side of a TE material, while exposing the other side to lower or ambient temperature surroundings.
Keywords:
Thermoelectric generator, Waste low grade heat, Seebeck effect,
Introduction:
Non-renewable electricity generation and a number of manufacturing processes reject large quantities of energy into the atmosphere each year in the form of waste heat. This waste heat arises due to both equipment inefficiencies and thermodynamic limitations of the equipment and processes and is typically unrecoverable using conventional technologies because of the relatively low temperatures (usually below 450 °F or 232 °C ) that the heat is rejected at. This 60% of uncovered waste low quality heat has less thermal and economic value than high temperature heat, it is ubiquitous and available in large quantities. Waste heat has lower utility that is having a lower exergy or higher entropy. This large volume of low grade waste heat presents significant opportunity for new and more effective energy recovery technologies to capture interest and market share, especially as energy prices climb and concern climate change affects regulatory policy on process efficiencies. One potential option for the converting low grade waste heat into electricity is to use a thermoelectric generator, or TEG. A TEG is solid semiconductor state device that converts heat into electricity by the Seebeck effect. TEGs have recently emerged as viable electricity generators because of improved thermodynamic efficiencies and higher survivable operating temperatures.
TE materials available prior to about 1995 produced thermal-to-electric conversion efficiencies in the 2% to 5% range and were only used in small niche applications. However, recent significant advances in the scientific understanding of quantum well and nanostructure effects on TE properties and modern thin layer and nano-scale manufacturing technologies have combined to create the opportunity of advanced TE materials with potential conversion efficiencies of over 15%. The advent of these advanced TE materials offers new opportunities to recover waste heat more efficiently and economically with highly reliable a relatively passive systems that produce no noise and vibration.
Uses Of Waste Flue Gas:
Low grade waste heat can be used for a number of processes, and common uses include preheating combustion gases, heating and cooling buildings, and providing heat to industrial reactions or manufacturing processes. Typically recycling waste for processes is more valuable than producing electricity, but recycling can require significant changes and in many cases simply is not practical. Electricity production from waste, on the other hands, is a stand-alone process that produces a universally tradable community. Thus, electricity production is smart alternatives when process recycling cannot be implemented. Organic Rankine cycles (ORC), Striling cycles, or other thermodynamic engines, such as a TEG, can be used to convert waste heat into electricity.
Thermoelectric Principles:
A) Thermoelectric Power Generation:
Fig. 1 : Seebeck Effect
Thermoelectric Power Generation is based on Seebeck effect. The Seebeck effect is a phenomenon in which a temperature differences between two dissimilar electrical conductors or semiconductor produes a voltage difference between the two substances. When is applied to one of the two conductors or semiconductors, heated electrons flow toward the cooler one.
The simplest TEG consists of a
thermocouple consisting of n-type (materials with excess electrons) and p-type (materials with deficit of electrons) elements connected electrically in series and thermally in parallel. Heat is input on one side and rejected from the other side, generating a voltage across the TE couple. The magnitude of the voltage produced is proportional to the temperature gradient.
B) Thermoelectric Heating And Cooling
Fig. 2: Peltier Effect
Thermoelectric Heating And Cooling is based on the Peltier effect. The Peltier effect is a temperature difference created by applying a voltage between two electrodes connected to sample of semiconductor material. When electric input is applied to a thermocouple, electrons move from p-type material to n-type material absorbing thermal energy at the cold junction. The electrons dump their extra energy at the hot junction as they flow from n-type back to the p-type material through the electrical connector. Removing heat from the hot side will drop the temperature on the cold side rapidly, the magnitude of the drop depending on the electric current applied.
Efficiency Of Thermoelectric Materials: Figure Of Merit (ZT)
Good TE materials should have the following characteristics: 1) High electrical conductivity to minimize Joule heating (rise in temperature from resistance to electric current flowing through it) 2) Large Seebeck coefficient for maximum conversion of heat to electrical power or electrical power to cooling performance 3) Low thermal conductivity to prevent thermal conduction through the material. These three properties are commonly called ‘Figure Of Merit (Z)’ which is defined as 2
Z
=α σ / λ
Where -1
α is the Seebeck coefficient of the material (volt ⋅kelvin ), -1
-1
σ is the electrical conductivity of the material (amp ere⋅volt ⋅meter ), and -1
-1
λ is the thermal conductivity of the material (watt ⋅meter ⋅kelvin )
Fig. 3 : Figure Of Merit
Efforts To Increase Figure Of Merit:
Fig. 4:Efficiency as function of temperature difference
Most efforts to increase ZT have centered about finding a way to decrease the thermal Conductivity. The thermal conductivity of any solid is a sum of the thermal conductivity of the lattice, due to phonon transport, and the electric thermal conductivity, due to the transport of charge carriers. Most thermoelectric materials are also semiconductors, however. For many semiconductor materials, the electric thermal conductivity, which is directly proportional to the electric conductivity, is a substantial portion of the overall thermal conductivity.
Though the thermal and electric conductivity are not entirely independent, the ratio of thermal to electric conductivity can still be decreased by scattering the phonons responsible for heat transport through the lattice. Phonon scattering can be achieved without altering the ability of the electrons to pass through, thereby lowering the σ / λ ratio.
Both theoretical and experimental research is exploring phonon scattering as an effective method of reducing the thermal conductivity of thermoelectric materials, and thereby increasing ZT. Research is ongoing to increase thermoelectric performance in bulk materials. In addition, nanostructured materials have been found to have increased thermoelectric performance compared to their bulk alloys.
Thermoelectric Materials:
A) Bismuth chalcogenides and their nanostructures: Materials such as Bi2 Se3 and Bi2 Te3comprise some of the best performing room temperature
thermoelectrics with a temperature-independent thermoelectric effect, ZT, between 0.8 and 1.0. Nano structuring these materials to produce a layered superlattice structure of alternating Sb2 Te3 and Bi2 Te3 layers produces a device within which there is good electrical conductivity but perpendicular to
which thermal conductivity is poor. The result is an enhanced ZT (approximately 2.4 at room temperature for p-type). B) Lead telluride Thallium doped lead telluride alloy (PbTe) achieves a ZT of 1.5 at 773 K. Sodium doped PbTe have ZT
value of 1.4 at 750 K and ZT 1.8 at 850 K in sodium-doped PbTe1-xSex alloy. Lead telluride to convert 15 to 20 percent of waste heat to electricity, reaching a ZT of 2.2
C) Inorganic clathrates Inorganic clathrates have a general formula A xByC46-y (type I) and A xByC136-y (type II), in these formulas B and C are group III and IV atoms, respectively, which form the framework where “guest” atoms A
(alkali or alkaline earth metal) are encapsulated in two different polyhedra facing each other.
D) Silicides
Higher silicides display ZT levels with current materials. They are mechanically and chemically strong and therefore can often be used in harsh environments without protection. E) Tin selenide
Tin selenide (SnSe) has a ZT of 2.6. This is the highest value reported to date. F) Nanomaterials and superlattices
Nanostructured Bi2 Te3 /Sb2 Te3 superlattice thin films, other nanomaterials show potential in improving thermoelectric properties. Another example of a superlattice involves a PbTe/PbSeTe quantum dot superlattices provides an enhanced ZT (approximately 1.5 at room temperature) that was higher than the bulk ZT value for either PbTe or PbSeTe (approximately 0.5).
Industrial Waste Heat Recovery:
The quality of waste heat differs from industries to industries. Waste heat stream composition is very important since it determines heat capacity, thermal conductivity, phase change temperatures, and corrosiveness. The potential industrial scenario is necessary since the application temperature limits the utility of th ermoelectric (TE) devices; the delta T (hot side vs. cold side) determines the TE device’s efficiency; and the waste heat source composition will determine corrosion, erosion, scaling, fouling and other effects, dictating the demands on the hot-side material composition and the heat transfer surface designs.
Table 1: Thermoelectric waste heat opportunities in the U.S. industries
Engineering Challenges:
Older bulk thermoelectric materials (i.e., Bi2Te3, PbTe, and SiGe materials) generally have low ZT value ~ 1 & very low conversion efficiencies of only 3-6% which cannot meet industrial waste energy recovery sufficiently. Hence, advanced TE materials must be developed & having characteristics like greater ZT, long term stability at industrial operating conditions, low material and fabrication costs, better thermal, chemical & structural stability, etc.
New device engineering and fabrication techniques must be developed that enhance efficiency, reliability, and power output or cooling capacity while maintaining low costs. The thermoelectric device (thermocouples, heat exchange attachment, wiring, interconnections, etc.) must be engineered in a manner that allows for low-cost large-quantity production. The TE device innovations must be compatible with the high-performance heat exchange/transfer technologies that provide the necessary high interface heat fluxes in miniaturized systems. TE device manufacturing encounters many of the same challenges as fuel cell development and microchannel heat exchanger and reactor development.
The thermoelectric generator system consists of the thermoelectric device, all heat exchange technology (hot- and cold-side) necessary to maintain the operating conditions appropriate for the device, and an electronic volt/ampere control module. The TEG system must be capable of installation and operation without economically offsetting modifications or new large capital requirements to existing plant equipment and processes.
Alternative Techniques:
There are significant waste heat opportunities at process flow temperatures near 150°C (~302 °F), such as in industrial water/steam boiler applications and ethylene furnaces. The largest waste energy opportunity is in water/steam boilers in the commercial and industrial applications (~1,170 TBtu/yr collectively). Unfortunately, these temperatures are generally too low to efficiently employ thermoelectric power generation. An alternative technology, called piezoelectric power generation (PEPG), is emerging as a low-temperature power generation technology that can directly convert heat into electrical energy. This technology operates on the theory of an oscillatory liquid-to-gas expansion within a closed chamber stressing a piezoelectric thin-film membrane, thereby creating a time-dependent voltage output. It operates most effectively in the 100°C to 150°C temperature range, so it would be best suited for waste heat recovery in water/steam boilers and ethylene applications (at 150°C). There are other configurations that are intended to capture mechanical energy directly and convert it into an electrical signal. PEPG technology is currently fielded in a thin-film membrane configuration that is only about 1% efficient in converting heat to electrical energy. There are many technical challenges associated with this early stage power generation technology including: •
Low conversion efficiency
•
High internal impedance
•
Requirement for oscillatory heat loads
•
Oscillatory electrical signals
•
Complex oscillatory fluid dynamics within the liquid/vapor chamber
•
Difficulties in obtaining high enough oscillatory frequencies
•
Long-term reliability and durability
•
Very high cost ($10,000 /W)
The technology usually employs a common piezoelectric oxide material, lead zironate titanate (PZT), in the active membrane. PZT is used in many microelectromechanical systems (MEMS) applications because of its high piezoelectric and electromechanical coupling coefficients. Different piezoelectric materials and device materials are being investigated to improve performance and manufacturability, and lower the cost of PEPG devices. Current devices can operate at ~100 Hz, but the real need is to operate near 1000 Hz. It is also clear that similar heat transfer interface challenges will exist at the hot- and cold-
sides in PEPG systems as in TEG systems. At the lower temperatures applications, there will be less temperature driving potential to transfer the large amounts of heat involved. This will lead to larger heat exchange systems than in the higher temperature heat recovery applications addressed by TEGs if gas flows are used on the PEPG system hot-side. In these applications (150°C), if liquid or two-phase flow streams can be used instead of gas flows, then much better heat transfer conditions on the PEPG hot side can be achieved, which will invariably lead to higher PEPG system performance. The low energy conversion efficiency of PEPGs can theoretically be overcome by cascading a single device into a multiple number of devices in series thermally and outputting electrically in parallel. However, PEPG cascading has not been demonstrated to date and there are technical challenges associated with this approach, not the least of which is controlling heat losses at each stage. If PEPG cascading can be accomplished and cost-effectively demonstrated, then energy conversion efficiencies of the multi-stage PEPG stack could be 8-10% or even higher. This provides a strong opportunity to recover a significant fraction of the large amount of waste energy available at ~150°C in industrial water/steam boilers and ethylene applications. Currently, Defense Advanced Research Projects Agency (DARPA) is conducting R&D to advance this technology and is certainly a viable energy conversion technology that DOE could invest in to accelerate its development and demonstration. Industrial interest in this technology should be fostered as much as possible.
Conclusion:
Energy content of waste streams was evaluated based on reference temperatures of 77°F [25°C] and 300°F [150°C]. Calculations based on a 77°F [25°C] reference reflect maximum heat recoverable by cooling heat streams to atmospheric temperatures. The 300°F [150°C] reference reflects the typical practice of cooling exhaust gases to no less than 300°F (150°C) in order to prevent flue gas condensation. Based on a reference temperature of 77°F [25°C], waste heat losses via sensible and latent heat contained in exhaust gases studied in this report are about 1.5 quadrillion Btu/yr. Only about 160 TBtu/yr are estimated as potentially recoverable energy based on a refer ence temperature of 300°F [150°C]. Work potential based on Carnot efficiency for energy conversion (mechanical or electrical) was also evaluated in order to better compare waste heat with different exhaust temperatures. Based on a 77°F
[25°C] ambient reference temperature, the work potential of all the waste heat studied is about 600 TBtu/yr. Despite the very low Carnot efficiency for low temperature energy conversions, about 75% of the work potential is contained in low temperature waste heat streams (i.e., at less than 450°F [230°C]). This is a result of the very large mass flow rate of these low temperature waste heat streams.
Major issued about TEGs:
> ZT ~ 1 material will not provide the thermal efficiency and system costs to be the long term solution to industrial scale waste heat recovery. However, they can serve as prototype system components to demonstrate TEG waste heat recovery concepts and provide lessons learned for industrial applications where heat exchange degradation, thermal cycling, vibration and other deleterious operating conditions are commonly encountered.
> Advanced TE materials having ZT ~ 2 properties that have recently been developed and characterized and new advanced TE materials with ZT ~ 4 envisioned in the long-term future will strongly enhance TEG commercialization by providing the thermal conversion efficiencies needed to make TEG economically attractive. The 20% energy conversion efficiency appears possible in advanced TEG systems operating at hot-side temperatures of ~ 1000°K. > Waste heat applications with large mass flow rates, high temperatures and no on-site opportunities for thermal exchange with other fluids/solids have been identified (e.g., glass furnaces, primary aluminum cells, and aluminum furnaces). Integrating TEG systems into the exhaust manifolds of these processes is practical. The development of these applications can provide the foundation for a new industry dedicated to recovering the energy losses associated with industrial manufacturing.
> Enhanced TEG performance ( ZT ~ 2 and >15% thermal efficiency) will provide for a more attractive business case, both for the TEG developer and the end-user. Higher efficiency levels will produce more power output at higher power density, thereby creating a stronger value proposition for the industry (i.e., more power output will be possible from smaller devices and systems).
Reference:
1. Jacob G. Latcham, MIT: ‘Heat Exchanger Design for Thermoelectric Electricity Generation from Low Temperature Flue Gas Streams’, 2009.
2. http://en.wikipedia.org/wiki/Waste_heat_recovery_unit
3. http://en.wikipedia.org/wiki/Thermoelectric_materials
4. Engineering Scoping Study of Thermoelectric Generator Systems for Industrial Waste Heat Recovery, November 2006, US Department of Energy.
5.
Waste Heat Recovery: Technology & Opportunities in U.S. Industries, BCS, March 2008.
6. http://scholarworks.wmich.edu/cgi/viewcontent.cgi?article=1493&context=masters_theses
7. https://tu-dresden.de/die_tu.../2012_06_EEVC_II_Dresden_TEG.pdf
