SOLAR THERMOELECTRIC POWER GENERATORS

Direct thermal-to-electric energy conversionengines based on thermoelectric devices and thermophotovoltaic (TPV) energy convertersprovide new opportunities for medium power ranges that may rival direct photovoltaic (PV)power conversion and involve no moving parts.
Thermoelectric energy conversion technology, based on the Peltier effect and the Seebeck effect,exploits the thermal energy of electrons (and holes) for the energy conversion between heat andelectricity, including power generation, refrigeration, and heat pumping.A large thermal conductivity would degrade performance. The product of Z and the workingtemperature T forms a nondimensional figure of merit, ZT. With a value of ZT between 3 and 4,thermoelectric devices would have an efficiency approaching that of an ideal heat engine. Thus,the key for the thermoelectric technology is to find materials with ZT>3. Materials withreasonable ZT are often heavily doped semiconductors and some semimetals. The ZT value of agiven material is temperature dependent; it usually peaks at certain temperature and drops off athigher temperatures.
The best commercial materials are alloys of Bi2Te3 with Bi2Se3 (n-type) and with Sb2Te3(p-type). The alloys are used because the phonon thermal conductivity can be significantlyreduced with only a small reduction in the electronic power factor. Bi2Te3-based alloys have apeak ZT around 1 near room temperature. Thus, these materials are not optimal for solar powerproduction, where the operating temperatures are higher. Bi2Te3-based materials, used in somepower generation applications, have a module efficiency that is limited to 5%. The U.S. NationalAeronautics and Space Agency used SiGe alloys (and PbTe-based alloys) to make radioisotopepoweredthermoelectric power generators operating in the temperature range of 300–900°C (and300–600°C for PbTe-based alloys), with a system conversion efficiency ~6–7%. These materialsall have a maximum ZT less than but close to 1.