Thermoelectric Materials for Space and Automotive Power Generation
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Thermoelectric
Materials for Space and Automotive Power Generation Jihui Yang and Thierry Caillat
Abstract Historically, thermoelectric technology has only occupied niche areas, such as the radioisotope thermoelectric generators for NASA’s spacecrafts, where the low cooling coefficient of performance (COP) and energy-conversion efficiency are outweighed by the application requirements. Recent materials advances and an increasing awareness of energy and environmental conservation issues have rekindled prospects for automotive and other applications of thermoelectric materials. This article reviews thermoelectric energy-conversion technology for radioisotope space power systems and several proposed applications of thermoelectric waste-heat recovery devices in the automotive industry. Keywords: thermal conductivity, thermoelectricity.
Introduction Of the various static energy-conversion technologies considered for radioisotope power systems for space applications, thermoelectric (TE) energy conversion has received the most interest. Radioisotope thermoelectric generators (RTGs) generate electrical power by converting the heat released from the nuclear decay of radioactive isotopes (typically plutonium-238) into electricity using a TE converter (Figure 1). RTGs have been successfully used to power a number of space missions, including the Apollo lunar missions; the Viking Mars landers; Pioneer 10 and 11; and the Voyager, Ulysses, Galileo, and Cassini outer-planet spacecrafts. These generators have demonstrated their reliability over extended periods of time (tens of years) and are compact, rugged, radiation-resistant, and scalable. They produce no noise, vibration, or torque during operation. These properties have made RTGs suitable for autonomous missions in the extreme environment of space and on planetary surfaces. Converter units use TE materials, which, when operating over a temperature gradient, pro-
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duce a voltage called the Seebeck voltage1 (see the introductory article by Tritt and Subramanian in this issue). The system conversion efficiency for state-of-practice RTGs is about 6%. The most widely used TE materials, in order of increasing operating temperature, are bismuth telluride (Bi2Te3); lead telluride (PbTe); tellurides of antimony, germanium, and silver (TAGS); lead tin telluride (PbSnTe); and silicon germanium (SiGe). All of these materials, except Bi2Te3, have been used in RTGs on space missions. A wide variety of physical, thermal, and TE property requirements must be met for the design of reliable RTG converters. These properties are summarized in this article; the discussion will also include lifetime power-output degradation mechanisms in state-of-practice generators. The number of motor vehicles on U.S. roads and the number of miles driven by those vehicles continue to grow, resulting in increased air pollution and petroleum consumption, and reliance on foreign sources of that petroleum, despite improvements in vehicle emissions control
and fuel efficiency. To counter these trends, new v
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