Automotive thermoelectric generator

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An automotive thermoelectric generator (ATEG) is a device that converts some of the waste heat of an internal combustion engine (IC) into electricity using the Seebeck Effect. A typical ATEG consists of four main elements: A hot-side heat exchanger, a cold-side heat exchanger, thermoelectric materials, and a compression assembly system. ATEGs can convert waste heat from an engine's coolant or exhaust into electricity. By reclaiming this otherwise lost energy, ATEGs decrease fuel consumed by the electric generator load on the engine. However, the cost of the unit and the extra fuel consumed due to its weight must be also considered.

Contents

Operation principles

In ATEGs, thermoelectric materials are packed between the hot-side and the cold-side heat exchangers. The thermoelectric materials are made up of p-type and n-type semiconductors, while the heat exchangers are metal plates with high thermal conductivity. [1]

The temperature difference between the two surfaces of the thermoelectric module(s) generates electricity using the Seebeck Effect. When hot exhaust from the engine passes through an exhaust ATEG, the charge carriers of the semiconductors within the generator diffuse from the hot-side heat exchanger to the cold-side exchanger. The build-up of charge carriers results in a net charge, producing an electrostatic potential while the heat transfer drives a current. [2] With exhaust temperatures of 700 °C (≈1300 °F) or more, the temperature difference between exhaust gas on the hot side and coolant on the cold side is several hundred degrees. [3] This temperature difference is capable of generating 500-750 W of electricity. [4]

The compression assembly system aims to decrease the thermal contact resistance between the thermoelectric module and the heat exchanger surfaces. In coolant-based ATEGs, the cold side heat exchanger uses engine coolant as the cooling fluid, while in exhaust-based ATEGs, the cold-side heat exchanger uses ambient air as the cooling fluid.

Efficiency

Currently, ATEGs are about 5% efficient. However, advancements in thin-film and quantum well technologies could increase efficiency up to 15% in the future. [5]

The efficiency of an ATEG is governed by the thermoelectric conversion efficiency of the materials and the thermal efficiency of the two heat exchangers. The ATEG efficiency can be expressed as: [6]

ζOV = ζCONV х ζHX х ρ

Where:

Benefits

The primary goal of ATEGs is to reduce fuel consumption and therefore reduce operating costs of a vehicle or help the vehicle comply with fuel efficiency standards. Forty percent of an IC engine's energy is lost through exhaust gas heat. [7] [8] Implementing ATEGs in diesel engines seems to be more challenging compared to gasoline engines due to lower exhaust temperature and higher mass-flow rates. [9] [10] This is the reason most ATEG development has been focused on gasoline engines. [6] [11] [12] However, there exist several ATEG designs for light-duty [13] and heavy-duty [14] [15] diesel engines.

By converting the lost heat into electricity, ATEGs decrease fuel consumption by reducing the electric generator load on the engine. ATEGs allow the automobile to generate electricity from the engine's thermal energy rather than using mechanical energy to power an electric generator. Since the electricity is generated from waste heat that would otherwise be released into the environment, the engine burns less fuel to power the vehicle's electrical components, such as the headlights. Therefore, the automobile releases fewer emissions. [4]

Decreased fuel consumption also results in increased fuel economy. Replacing the conventional electric generator with ATEGs could ultimately increase the fuel economy by up to 4%. [16]

The ATEG's ability to generate electricity without moving parts is an advantage over mechanical electric generators alternatives. [1] In addition, it has been stated that for low power engine conditions, ATEGs may be able to harvest more net energy than electric turbogenerators. [9]

Challenges

The greatest challenge to the scaling of ATEGs from prototyping to production has been the cost of the underlying thermoelectric materials. Since the early-2000s, many research agencies and institutions poured large sums of money into advancing the efficiency of thermoelectric materials. While efficiency improvements were made in materials such as the half heuslers and skutterudites, like their predecessors bismuth telluride and lead telluride, the cost of these materials has proven prohibitive for large-scale manufacturing. [17] Recent advances by some researchers and companies in low-cost thermoelectric materials have resulted in significant commercial promise for ATEGs, [18] most notably the low-cost production of tetrahedrite by Michigan State University [19] and its commercialization by US-based Alphabet Energy with General Motors. [20]

Like any new component on an automobile, the use of an ATEG presents new engineering problems to consider, as well. However, given an ATEG's relatively low impact on the use of an automobile, its challenges are not as considerable as other new automotive technologies. For instance, since exhaust has to flow through the ATEG's heat exchanger, kinetic energy from the gas is lost, causing increased pumping losses. This is referred to as back pressure, which reduces the engine's performance. [7] This can be accounted for by downsizing the muffler, resulting in zero net or even negative total back-pressure on the engine, as Faurecia and other companies have shown. [21]

To make the ATEG's efficiency more consistent, coolant is usually used on the cold-side heat exchanger rather than ambient air so that the temperature difference will be the same on both hot and cold days. This may increase the radiator's size since piping must be extended to the exhaust manifold, and it may add to the radiator's load because there is more heat being transferred to the coolant. [16] Proper thermal design does not require an upsized cooling system.

The added weight of ATEGs causes the engine to work harder, resulting in lower gas mileage. Most automotive efficiency improvement studies of ATEGs, however, have resulted in a net positive efficiency gain even when considering the weight of the device. [22]

History

Although the Seebeck effect was discovered in 1821, the use of thermoelectric power generators was restricted mainly to military and space applications until the second half of the twentieth century. This restriction was caused by the low conversion efficiency of thermoelectric materials at that time.

In 1963, the first ATEG was built and reported by Neild et al. [23] In 1988, Birkholz et al. published the results of their work in collaboration with Porsche. These results described an exhaust-based ATEG which integrated iron-based thermoelectric materials between a carbon steel hot-side heat exchanger and an aluminium cold-side heat exchanger. This ATEG could produce tens of watts out of a Porsche 944 exhaust system. [24]

In the early 1990s, Hi-Z Inc designed an ATEG which could produce 1 kW from a diesel truck exhaust system. The company in the following years introduced other designs for diesel trucks as well as military vehicles

In the late 1990s, Nissan Motors published the results of testing its ATEG which utilized SiGe thermoelectric materials. Nissan ATEG produced 35.6 W in testing conditions similar to the running conditions of a 3.0 L gasoline engine in hill-climb mode at 60.0 km/h.

Since the early-2000s, nearly every major automaker and exhaust supplier has experimented or studied thermoelectric generators, and companies including General Motors, BMW, Daimler, Ford, Renault, Honda, Toyota, Hyundai, Valeo, Boysen, Faurecia, Tenneco, Denso, Gentherm Inc., Alphabet Energy, and numerous others have built and tested prototypes. [25] [26] [27]

In January 2012, Car and Driver named an ATEG created by a team led by Amerigon (now Gentherm Incorporated) one of the 10 "most promising" technologies. [28]

Related Research Articles

A nuclear electric rocket is a type of spacecraft propulsion system where thermal energy from a nuclear reactor is converted to electrical energy, which is used to drive an ion thruster or other electrical spacecraft propulsion technology. The nuclear electric rocket terminology is slightly inconsistent, as technically the "rocket" part of the propulsion system is non-nuclear and could also be driven by solar panels. This is in contrast with a nuclear thermal rocket, which directly uses reactor heat to add energy to a working fluid, which is then expelled out of a rocket nozzle.

<span class="mw-page-title-main">Four-stroke engine</span> Internal combustion engine type

A four-strokeengine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:

  1. Intake: Also known as induction or suction. This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing a partial vacuum in the cylinder through its downward motion.
  2. Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.
  3. Combustion: Also known as power or ignition. This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. the compressed air-fuel mixture is ignited by a spark plug or by heat generated by high compression, forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft.
  4. Exhaust: Also known as outlet. During the exhaust stroke, the piston, once again, returns from B.D.C. to T.D.C. while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust port.
<span class="mw-page-title-main">Combined cycle power plant</span> Assembly of heat engines that work in tandem from the same source of heat

A combined cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined cycle gas turbine (CCGT) plant. The same principle is also used for marine propulsion, where it is called a combined gas and steam (COGAS) plant. Combining two or more thermodynamic cycles improves overall efficiency, which reduces fuel costs.

Internal combustion engine cooling uses either air or liquid to remove the waste heat from an internal combustion engine. For small or special purpose engines, cooling using air from the atmosphere makes for a lightweight and relatively simple system. Watercraft can use water directly from the surrounding environment to cool their engines. For water-cooled engines on aircraft and surface vehicles, waste heat is transferred from a closed loop of water pumped through the engine to the surrounding atmosphere by a radiator.

<span class="mw-page-title-main">Cogeneration</span> Simultaneous generation of electricity and useful heat

Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time.

<span class="mw-page-title-main">Energy transformation</span> Process of changing energy

Energy transformation, also known as energy conversion, is the process of changing energy from one form to another. In physics, energy is a quantity that provides the capacity to perform work or moving or provides heat. In addition to being converted, according to the law of conservation of energy, energy is transferable to a different location or object, but it cannot be created or destroyed.

Micro combined heat and power, micro-CHP, µCHP or mCHP is an extension of the idea of cogeneration to the single/multi family home or small office building in the range of up to 50 kW. Usual technologies for the production of heat and power in one common process are e.g. internal combustion engines, micro gas turbines, stirling engines or fuel cells.

<span class="mw-page-title-main">Thermal efficiency</span> Performance measure of a device that uses thermal energy

In thermodynamics, the thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc.

<span class="mw-page-title-main">Waste heat</span> Heat that is produced by a machine that uses energy, as a byproduct of doing work

Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utility than the original energy source. Sources of waste heat include all manner of human activities, natural systems, and all organisms, for example, incandescent light bulbs get hot, a refrigerator warms the room air, a building gets hot during peak hours, an internal combustion engine generates high-temperature exhaust gases, and electronic components get warm when in operation.

<span class="mw-page-title-main">Energy recovery</span>

Energy recovery includes any technique or method of minimizing the input of energy to an overall system by the exchange of energy from one sub-system of the overall system with another. The energy can be in any form in either subsystem, but most energy recovery systems exchange thermal energy in either sensible or latent form.

<span class="mw-page-title-main">Thermoelectric generator</span> Device that converts heat flux into electrical energy

A thermoelectric generator (TEG), also called a Seebeck generator, is a solid state device that converts heat directly into electrical energy through a phenomenon called the Seebeck effect. Thermoelectric generators function like heat engines, but are less bulky and have no moving parts. However, TEGs are typically more expensive and less efficient.

<span class="mw-page-title-main">Waste heat recovery unit</span> Energy recovery heat exchanger

A waste heat recovery unit (WHRU) is an energy recovery heat exchanger that transfers heat from process outputs at high temperature to another part of the process for some purpose, usually increased efficiency. The WHRU is a tool involved in cogeneration. Waste heat may be extracted from sources such as hot flue gases from a diesel generator, steam from cooling towers, or even waste water from cooling processes such as in steel cooling.

A liquid nitrogen vehicle is powered by liquid nitrogen, which is stored in a tank. Traditional nitrogen engine designs work by heating the liquid nitrogen in a heat exchanger, extracting heat from the ambient air and using the resulting pressurized gas to operate a piston or rotary motor. Vehicles propelled by liquid nitrogen have been demonstrated, but are not used commercially. One such vehicle, Liquid Air, was demonstrated in 1902.

<span class="mw-page-title-main">Thermal wheel</span> Type of energy recovery heat exchanger

A thermal wheel, also known as a rotary heat exchanger, or rotary air-to-air enthalpy wheel, energy recovery wheel, or heat recovery wheel, is a type of energy recovery heat exchanger positioned within the supply and exhaust air streams of air-handling units or rooftop units or in the exhaust gases of an industrial process, in order to recover the heat energy. Other variants include enthalpy wheels and desiccant wheels. A cooling-specific thermal wheel is sometimes referred to as a Kyoto wheel.

<span class="mw-page-title-main">Applications of the Stirling engine</span> Practical uses for Sterling engine technology

Applications of the Stirling engine range from mechanical propulsion to heating and cooling to electrical generation systems. A Stirling engine is a heat engine operating by cyclic compression and expansion of air or other gas, the "working fluid", at different temperature levels such that there is a net conversion of heat to mechanical work. The Stirling cycle heat engine can also be driven in reverse, using a mechanical energy input to drive heat transfer in a reversed direction.

<span class="mw-page-title-main">Alphabet Energy</span>

Alphabet Energy was a startup company founded in 2009 at the University of California, Berkeley by thermoelectrics expert Matthew L. Scullin and Peidong Yang. The company uses nanotechnology and materials science applications to create thermoelectric generators that are more cost effective than previous bismuth telluride-based devices. The company is based in Hayward, California. It started with a license to use silicon nanowire developed at Lawrence Berkeley National Laboratory. They moved from UC Berkeley to offices in San Francisco in 2011, and later to Hayward.

<span class="mw-page-title-main">Exhaust heat recovery system</span>

An exhaust heat recovery system turns waste heat energy in exhaust gases into electric energy for batteries or mechanical energy reintroduced on the crankshaft. The technology is of increasing interest as car and heavy-duty vehicle manufacturers continue to increase efficiency, saving fuel and reducing emissions.

Gentherm Incorporated, formerly called Amerigon, is an American thermal management technologies company. Gentherm created the first thermoelectrically heated and cooled seat system for the automotive industry. Called the "Climate Control Seat" system, it was first adopted by the Ford Motor Company and introduced as an option on the model year 2000 Lincoln Navigator in 1999. Today it is available on more than 50 vehicles sold by Ford, General Motors, Toyota (Lexus), Kia, Hyundai, Nissan (Infinity), Range Rover and Jaguar Land Rover.

<span class="mw-page-title-main">Internal combustion engine</span> Engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber

An internal combustion engine is a heat engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is typically applied to pistons, turbine blades, a rotor, or a nozzle. This force moves the component over a distance, transforming chemical energy into kinetic energy which is used to propel, move or power whatever the engine is attached to.

<span class="mw-page-title-main">Thermogalvanic cell</span> Electrochemical cell in which a temperature difference produces a voltage

In electrochemistry, a thermogalvanic cell is a kind of galvanic cell in which heat is employed to provide electrical power directly. These cells are electrochemical cells in which the two electrodes are deliberately maintained at different temperatures. This temperature difference generates a potential difference between the electrodes. The electrodes can be of identical composition and the electrolyte solution homogeneous. This is usually the case in these cells. This is in contrast to galvanic cells in which electrodes and/or solutions of different composition provide the electromotive potential. As long as there is a difference in temperature between the electrodes a current will flow through the circuit. A thermogalvanic cell can be seen as analogous to a concentration cell but instead of running on differences in the concentration/pressure of the reactants they make use of differences in the "concentrations" of thermal energy. The principal application of thermogalvanic cells is the production of electricity from low-temperature heat sources. Their energetic efficiency is low, in the range of 0.1% to 1% for conversion of heat into electricity.

References

  1. 1 2 Yang, Jihui; Stabler, Francis R. (13 February 2009). "Automotive Applications of Thermoelectric Materials". Journal of Electronic Materials . 38 (7): 1245–1251. Bibcode:2009JEMat..38.1245Y. doi:10.1007/s11664-009-0680-z. S2CID   136893601.
  2. ↑ Snyder, G. Jeffrey; Toberer, Eric S. (February 2008). "Complex Thermoelectric Materials". Nature Materials . 7 (2): 105–14. Bibcode:2008NatMa...7..105S. doi:10.1038/nmat2090. PMID   18219332.
  3. ↑ "TEGs –Using Car Exhaust To Lower Emissions". Science 2.0. 27 August 2014. Retrieved 23 September 2020.
  4. 1 2 Laird, Lorelei (16 August 2010). "Could TEG improve your car's efficiency?". Energy Blog. United States Department of Energy. Archived from the original on 19 July 2011. Retrieved 22 September 2020.
  5. ↑ Smith, Kandler; Thornton, Matthew (January 2009), Feasibility of Thermoelectrics for Waste Heat Recovery in Conventional Vehicles, National Renewable Energy Laboratory, doi:10.2172/951806
  6. 1 2 Ikoma K.; Munekiyo M.; Kobayashi M.; et al. (28 March 1998). Thermoelectric module and generator for gasoline engine vehicles. Seventeenth International Conference on Thermoelectrics. Proceedings ICT98 (Cat. 98TH8365). Nagoya, Japan: Institute of Electrical and Electronics Engineers. pp. 464–467. doi:10.1109/ICT.1998.740419.
  7. 1 2 Yu, C. “Thermoelectric automotive waste heat energy recovery using maximum power point tracking”. Energy Conversion and Management, 2008, VOL 50; page 1506
  8. ↑ Chuang Yu; Chau K.T. (July 2009). "Thermoelectric automotive waste heat energy recovery using maximum power point tracking". Energy Conversion and Management. 50 (6): 1506–1512. doi:10.1016/j.enconman.2009.02.015.
  9. 1 2 Fernández-Yáñez, P.; Armas, O.; Kiwan, R.; Stefanopoulou, A.G.; Boehman, A.L. (November 2018). "A thermoelectric generator in exhaust systems of spark-ignition and compression-ignition engines. A comparison with an electric turbo-generator". Applied Energy. 229: 80–87. doi:10.1016/j.apenergy.2018.07.107. ISSN   0306-2619. S2CID   116417579.
  10. ↑ Durand, Thibaut; Dimopoulos Eggenschwiler, Panayotis; Tang, Yinglu; Liao, Yujun; Landmann, Daniel (July 2018). "Potential of energy recuperation in the exhaust gas of state of the art light duty vehicles with thermoelectric elements". Fuel. 224: 271–279. doi:10.1016/j.fuel.2018.03.078. ISSN   0016-2361. S2CID   102527579.
  11. ↑ Haidar, J.G.; Ghojel, J.I. (2001). "Waste heat recovery from the exhaust of low-power diesel engine using thermoelectric generators". Proceedings ICT2001. 20 International Conference on Thermoelectrics (Cat. No.01TH8589). Institute of Electrical and Electronics Engineers. pp. 413–418. doi:10.1109/ict.2001.979919. ISBN   978-0780372054. S2CID   110866420.
  12. ↑ Friedrich, Horst; Schier, Michael; Häfele, Christian; Weiler, Tobias (April 2010). "Electricity from exhausts — development of thermoelectric generators for use in vehicles". ATZ Worldwide. 112 (4): 48–54. doi:10.1007/bf03225237. ISSN   2192-9076.
  13. ↑ Fernández-Yañez, Pablo; Armas, Octavio; Capetillo, Azael; Martínez-Martínez, Simón (September 2018). "Thermal analysis of a thermoelectric generator for light-duty diesel engines". Applied Energy. 226: 690–702. doi:10.1016/j.apenergy.2018.05.114. ISSN   0306-2619. S2CID   115282082.
  14. ↑ Wang, Yiping; Li, Shuai; Xie, Xu; Deng, Yadong; Liu, Xun; Su, Chuqi (May 2018). "Performance evaluation of an automotive thermoelectric generator with inserted fins or dimpled-surface hot heat exchanger". Applied Energy. 218: 391–401. doi:10.1016/j.apenergy.2018.02.176. ISSN   0306-2619.
  15. ↑ Kim, Tae Young; Negash, Assmelash A.; Cho, Gyubaek (September 2016). "Waste heat recovery of a diesel engine using a thermoelectric generator equipped with customized thermoelectric modules". Energy Conversion and Management. 124: 280–286. doi:10.1016/j.enconman.2016.07.013. ISSN   0196-8904.
  16. 1 2 Stabler, Francis. "Automotive Thermoelectric Generator Design Issues". DOE Thermoelectric Applications Workshop.
  17. ↑ "NSF/DOE Thermoelectrics Partnership: Thermoelectrics for Automotive Waste Heat Recovery | Department of Energy". energy.gov. Retrieved 1 May 2017.
  18. ↑ Media, BioAge. "Green Car Congress: Alphabet Energy introduces PowerModules for modular thermoelectric waste heat recovery; partnership with Borla for heavy-duty trucks". www.greencarcongress.com. Retrieved 1 May 2017.
  19. ↑ Lu, Xu; Morelli, Donald T. (26 March 2013). "Natural mineral tetrahedrite as a direct source of thermoelectric materials". Physical Chemistry Chemical Physics. 15 (16): 5762–6. Bibcode:2013PCCP...15.5762L. doi:10.1039/C3CP50920F. ISSN   1463-9084. PMID   23503421.
  20. ↑ "Alphabet Energy goes from B to C round · Articles · Global University Venturing". www.globaluniversityventuring.com. Retrieved 1 May 2017.
  21. ↑ "Emissions Control Technologies". Faurecia North America. Archived from the original on 5 August 2017. Retrieved 1 May 2017.
  22. ↑ Stabler, Francis. "Benefits of Thermoelectric Technology for the Automobile". DOE Thermoelectric Applications Workshop.
  23. ↑ A. B. Neild, Jr., SAE-645A (1963).
  24. ↑ Birkholz, U., et al. "Conversion of Waste Exhaust Heat in Automobile using FeSi2 Thermoelements". Proc. 7th International Conference on Thermoelectric Energy Conversion. 1988, Arlington, USA, pp. 124-128.
  25. ↑ Orr, B.; Akbarzadeh, A.; Mochizuki, M.; Singh, R. (25 May 2016). "A review of car waste heat recovery systems utilising thermoelectric generators and heat pipes". Applied Thermal Engineering. 101: 490–495. doi: 10.1016/j.applthermaleng.2015.10.081 .
  26. ↑ "Green Car Congress: Thermoelectrics". www.greencarcongress.com. Retrieved 1 May 2017.
  27. ↑ Thacher E. F., Helenbrook B. T., Karri M. A., and Richter Clayton J. "Testing an automobile thermoelectric exhaust based thermoelectric generator in a light truck" Proceedings of the I MECH E Part D Journal of Automobile Engineering, Volume 221, Number 1, 2007, pp. 95-107(13)
  28. ↑ “2012 10Best: 10 Most Promising Future Technologies: Thermal Juice”, Car & Driver , December 2011.
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