A solid-state heat engine which employs the electron gas as a working fluid. It directly converts heat energy into electrical energy using the Seebeck effect. This phenomenon can be demonstrated using a thermocouple which comprises two legs (thermoelements) of dissimilar conducting materials joined at one end to form a junction. If this junction is maintained at a temperature which differs from ambient, a voltage is generated across the open ends of the thermoelements. When the circuit is completed with a load, a current flows in the circuit and power is generated. In practice the thermocouples are fabricated generally from n- and p-type semiconductors, and several hundred are connected electrically in series to form a module which is the active component of a thermoelectric generator. Provided a temperature difference is maintained across the device, it will generate electrical power. Heat is provided from a variety of sources depending on the application, and they include burning fossil fuels in terrestrial and military applications, decaying long-life isotopes in medical and deep-space applications, and waste heat. The performance of the thermoelectric generator, in terms of efficiency, output power, and economic viability, depends upon its temperature regime of operation; the materials used in the module construction; its electrical, thermal, and geometrical design; and the generator load. The power output spectrum of thermoelectric generators spans 14 orders of magnitude and ranges from nanowatt generators fabricated using integrated circuit technology to the nuclear-reactor-powered 100-kW SP-100 generator intended to provide electrical power to orbiting space stations. See also Nuclear battery; Radioactivity and radiation applications; Seebeck effect; Space power systems; Space probe; Thermocouple; Thermoelectricity.
Historically the use of thermoelectric generators has been restricted to specialized applications where combinations of their desirable properties, such as the absence of moving parts, reliability, and silent operation, has outweighed their low overall conversion efficiency, typically 5%. In these applications, fuel cost or weight is a major consideration, and improving the conversion efficiency is the main research target. Improving the figure of merit has been regarded as the most important factor in increasing the conversion efficiency of a thermoelectric generator.
Selection of thermocouple material depends upon the generator's temperature regime of operation. The figures of merit of established thermoelectric materials reach maxima at different temperatures, and semiconductor compounds or alloys based on bismuth telluride, lead telluride, and silicon germanium cover the temperature ranges up to 150°C (300°F), 650°C (1200°F), and 1000°C (1830°F) respectively, with the best materials capable of generating electrical power with an efficiency of around 20%. Material research is focused on improving the figure of merit and to a lesser, though an increasing, extent, the electrical power factor.
The emergence of thermoelectrics as a technology for application in waste heat recovery has resulted in a successful search for materials with high electrical power factors and cheap materials. The rare-earth ytterbium-aluminum compound YbAl3 has a power factor almost three times that of bismuth telluride, the established material for low-temperature application, while magnesium tin (MgSn) has almost the same performance as lead telluride but is available at less than a quarter of the cost.
