Small Thermoelectric Generators by G. Jeffrey Snyder

hermoelectric generators are all solid-state devices that convert T heat into electricity. Unlike traditional dynamic heat engines, thermoelectric generators contain no moving parts and are completely silent. Such generators have been used reliably for over 30 years of maintenance-free operation in deep space probes such as the Voyager missions of NASA.1 Compared to large, traditional heat engines, thermoelectric generators have lower efficiency. But for small applications, thermoelectrics can become competitive because they are compact, simple (inexpensive) and scaleable. Thermoelectric systems can be easily designed to operate with small heat sources and small temperature differences. Such small generators could be mass produced for use in automotive waste heat recovery or home co-generation of heat and electricity. Thermoelectrics have even been miniaturized to harvest body heat for powering a wristwatch.

Thermoelectric Power

A thermoelectric produces electrical power from heat flow across a temperature gradient.2 As the heat flows from hot to cold, free charge carriers (electrons or holes) in the material are also driven to the cold end (Fig. 1). The resulting voltage (V) is proportional to the temperature difference (∆T) via the Seebeck coefficient, α, (V = α∆T). By connecting an electron conducting (n-type) and hole conducting (p-type) material in series, a net voltage is produced that can be driven through a load. A good thermoelectric material has a Seebeck coefficient between 100 µV/K and 300 µV/K; thus, in order to achieve a few volts at the load, many thermoelectric couples need to Fig. 1. Schematic of a thermoelectric generator. Many thermoelectric couples (top) of be connected in series to make the n-type and p-type thermoelectric semiconductors are connected electrically in series and thermoelectric device (Fig. 1). thermally in parallel to make a thermoelectric generator. The flow of heat drives the free electrons (e-) and holes (h+) producing electrical power from heat. (Copyright Nature A thermoelectric generator converts Publishing Group,2 reprinted with permission.) heat (Q) into electrical power (P) with efficiency η.

P = ηQ (1) may take precedence over maximum efficiency greater than that of a Carnot efficiency. In this case the temperature cycle (∆T/ Th). The efficiency of a The amount of heat, Q, that can difference (and therefore thermoelectric thermoelectric generator is typically be directed though the thermoelectric efficiency as described below) may be defined as materials frequently depends on the size only half that between the heat source of the heat exchangers used to harvest and sink.3 η (2) the heat on the hot side and reject it on The efficiency of a thermoelectric the cold side. As the heat exchangers converter depends heavily on the temperature difference ∆T = T – T are typically much larger than the h c Where the first term is the Carnot across the device. This is because thermoelectric generators themselves, efficiency and ZT is the figure of merit the thermoelectric generator, like when size is a constraint (or high P/V is for the device. While the calculation of desired) the design for maximum power all heat engines, cannot have an

54 The Electrochemical Society Interface • Fall 2008 a thermoelectric generator efficiency generator will extract waste heat to a small region to maintain a high can be complex,4 use of the average from the exhaust that will deliver DC source temperature while the exhaust material figure of merit, zT, can provide electrical power to recharge the battery. is directed through a counter flow heat an approximation for ZT. By reducing or even eliminating the exchanger to preheat the incoming 6 2 need for the alternator, the load on the gasses (Fig. 3). The thermal efficiency α T zT = (3) engine is reduced thereby improving of such combustors can be 80-95%. ρκ fuel efficiency by as much as 10%. Perhaps the best way to achieve high Here, Seebeck coefficient α ( ), electrical Instead of recovering waste heat, efficiency with such a device is to resistivity (ρ), and thermal conductivity Co-generation recovers some of the include a fuel cell just before the catalytic 7 (κ) are temperature (T) dependent useful work wasted on heat. Often a combustion. A single chamber fuel cell materials properties. high energy content fuel with a high produces electricity when placed in a Recently, the field of thermoelectric flame temperature (such as natural gas) hot fuel-air mixture. The unreacted materials is rapidly growing with the is used for low ∆T heating (e.g. home fuel could then be combusted by the discovery of complex, high-efficiency heating or hot water). In electricity–heat catalyst for use by the thermoelectric. materials. A diverse array of new cogeneration, electricity is produced approaches, from complexity within with nearly 100% efficiency (as opposed Harvesting MicroPower the unit cell to nanostructured bulk, to ~40% for power plants) because the nanowire and thin film materials, have remaining energy is used for heating The ability to fabricate exceedingly all lead to high efficiency materials.2 instead of being wasted. In applications small semiconducting thermoele- such as home co-generation, the desire ments has enabled the possibility of Heat to Electricity for silent, vibration, and maintenance harvesting very small amounts of heat free operation will favor thermo- for low power applications such as electrics. Residential co-generation and For large electrical power generation wireless sensor networks, mobile devices, automotive waste heat recovery are two 8 applications traditional dynamic and even medical applications. Various examples where “small” systems could thin film techniques have been utilized thermal to electric generators (e.g. have an impact on the global energy Rankine, Brayton, or Stirling engine) to produce small thermoelectric devices consumption if implemented on a large 9 have several times the efficiency of a including electrochemical MEMS, scale. 10 11,12 thermoelectric system. However, such CVD and sputtering with at least dynamic systems are expensive and do three companies (Micropelt, Nextreme, not scale easily for small applications. Portable Power ThermoLife) recently established When high quality combustible fuel is to produce devices (Fig. 4). Devices available, internal combustion engines For small portable applications, power utilizing the thermoelectric material are cost effective and reasonably sources that are smaller and lighter in a direction parallel to the deposition 12 efficient in the 100 W to 100 kW range than conventional batteries are of great plane lose some efficiency due to but tend to be noisy. For applications commercial interest. The energy density the thermal shorting of the substrate, requiring less than 100W, the scalability of a combustible fuel is 10 to 100 times but gain from the density and length of thermoelectrics gives them a clear that of a battery. Thus in principle, even advantage. for low thermal-to-electric conversion efficiencies (e.g. a few percent), a small combustor supplying heat to a Power from Waste Heat thermoelectric generator could provide greater energy density than a battery. Efforts are already underway to However, a 10% efficient generator can demonstrate waste heat recovery in 5 require at least 500°C. In addition, to automobiles (Fig. 2). In this ~1 kW ensure that the heat is directed through range, even relatively inefficient the thermoelectric and not lost in the thermoelectrics can be competitive exhaust, the heat exchangers must be for use with such waste heat sources carefully designed. One such design (e.g. automobile exhaust) when design, is that of a “Swiss roll” combustor fabrication, and maintenance cost where catalytic combustion is localized are factored in. The thermoelectric

Fig. 3. Single-chamber solid-oxide fuel cell with Swiss roll combustor.6 The high power density fuel cell produces electricity from a mixture of fuel and air. The unreacted mixture is then catalytically combusted at the center of Fig. 2. Conceptual design of thermoelectric generator producing electricity from the Swiss roll heat exchanger that could waste heat in the engine exhaust. Copyright BMW, reprinted by permission. be placed between two thermoelectric generators for additional electrical power.

The Electrochemical Society Interface • Fall 2008 55 Snyder (continued from previous page)

Fig. 4. Microfabricated thermoelectric elements (Copyright Micropelt). Completed device (RTI) next to a penny (USA 1 cent.) (Copyright RTI.) of the thermoelectric elements such social cost of energy production, small References that 5V can be produced with a 10K thermal to electric power sources for temperature drop (ThermoLife). cogeneration and waste heat recovery 1. L. A. Fisk, Science, 309, 2016 (2005). At the same time, manufacturers may someday play a significant role, 2. G. J. Snyder and E. S. Toberer, Nature of bulk thermoelectric devices, which however small. Materials, 7, 105 (2008). typically have thermoelectric elements 3. G. J. Snyder, “Thermoelectric Energy of 1-2 mm in length, can now reduce Acknowledgments Harvesting,” in Energy Harvesting the size of the thermoelectric elements13 Technologies, edited by S. Priya even to 100µm.14 (Springer, in press). The assistance of Sossina A good example of thermoelectric Haile, Jeongmin Ahn, Rama 4. G. J. Snyder, “Thermoelectric Power Generation: Efficiency and energy harvesting is the thermoelectric Venkatasubramanian (RTI), Joachim Compatibility,” in Thermoelectrics wristwatch, which utilizes thin bulk Nurnus (MicroPelt), Eric Toberer, Paul Handbook Macro to Nano, edited thermoelectric devices (Fig. 5). The Ronney, BSST, Andreas Eder (BMW), by D. M. Rowe (CRC, Boca Raton, watch is driven by body heat converted and Matsuo Kishi (Seiko) is gratefully into the electrical power by the 2006), Ch. 9. acknowledged. thermoelectric. At least two models have 5. K. Matsubara, in Twenty-first International Conference on been built, one by Seiko and another About the Author by Citizen. The Seiko watch14 (Fig. 5) Thermoelectrics, Proceedings, ICT’02, under normal operation produces 22 418 (2002). G. Jeffrey Snyder is a faculty associate µW of electrical power. With only a 1.5K 6. J. Ahn, Z. Shao, P. D. Ronney, and S. at the California Institute of Technology M. Haile, in The 5th International Fuel temperature drop across the intricately- (Caltech) in Pasadena, California. His Cell Science, Engineering & Technology machined thermoelectric modules, the interest is focused on thermoelectric Conference (Fuel Cell 2007), 250832 open circuit voltage is 300 mV, and engineering and advanced materials (2007). thermal to electric efficiency is about development such as complex Zintl phase 7 Z. P. Shao, S. M. Haile, J. Ahn, P. 0.1%. and nanostructured bulk thermoelectric D. Ronney, Z. L. Zhan, and S. A. Although no longer in production, materials (http://thermoelectrics.caltech. Barnett, Nature, 435, 795 (2005). the thermoelectric wristwatch edu/). He may be reached at jsnyder@ 8. J. A. Paradiso and T. Starner, IEEE demonstrates the viability of utilizing caltech.edu. Pervasive Computing, 4, 18 (2005). thermoelectric in small power sources. 9. G. J. Snyder, J. R. Lim, C.-K. Huang, As the cost of producing these and J.-P. Fleurial, Nature Materials, 2, devices drops with mass production 528 (2003). and the increasing need for remote 10. R. Venkatasubramanian, C. Watkins, power sources, viable applications will D. Stokes, J. Posthill, and C. Caylor, undoubtedly arise. In addition, with 2007 IEEE International Electron the ever increasing economic and Devices Meeting - IEDM ‘07, 367 (2007). 11. H. Böttner, J. Nurnus, A. Gavrikov, et al., J. Microelectromechanical Systems, 13, 414 (2004). 12. I. Stark and M. Stordeur, in Eighteenth International Conference on Thermoelectrics. Proceedings, ICT’99, 465 (1999). 13. G. J. Snyder, A. Borshchevsky, A. Zoltan, et al., in Twenty- first International Conference on Thermoelectrics, 463 (2002). 14. M. Kishi, H. Nemoto, T. Hamao, M. Yamamoto, S. Sudou, M. Mandai, and S. Yamamoto, in Fig. 5. Seiko Thermic, a wristwatch powered by body heat using a thermoelectric Eighteenth International Conference on 14 generator; (a) the watch, (b) cross-sectional diagram. (Copyright by Seiko Instruments Thermoelectrics Proceedings, ICT’99, Incorporated, reproduced with permission.) 301 (1999).

56 The Electrochemical Society Interface • Fall 2008