Direct thermal-to-electric energy conversionengines based on thermoelectric devices and thermophotovoltaic (TPV) energy converters provide 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 and electricity, including power generation, refrigeration, and heat pumping.
A thermoelectric power generator has a maximum efficiency given by
, where Th,Tc = temperatures at the hot and cold sides Tm = mean temperature , Z = measure of the electronic power produced by the thermal gradient, divided by the thermal conductivity.
A large thermal conductivity would degrade performance. The product of Z and the working temperature 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 with reasonable ZT are often heavily doped semiconductors and some semimetals. The ZT value of a given material is temperature dependent; it usually peaks at certain temperature and drops off at higher temperatures.
     Thermoelectric materials can be used in all-solid-state devices to produce electricity from hot sources. Left figure schematically represents how electricity can be generated for a heat source heated by a solar concentrator. With an appropriate thermal storage scheme, this could provide a 24-hour source of power. Efficient thermoelectric (TE) materials are usually semiconductors that possess simultaneously high electronic conductivity (), high thermoelectric power, and low thermal conductivity (). These properties define the thermoelectric figure of merit ZT = (S2/)T; where T is the temperature. The S2 product is often called the power factor. The quantities S2 and are transport quantities and therefore are determined by the details of the crystal and electronic structure and scattering of charge carriers. Generally they cannot be controlled independently, however, the combination of new theories and experimental results suggests that they may be able to be decoupled to a significant degree. This raises potential new research opportunities for huge improvements in the figure of merit. State-of-the-art thermoelectric materials have ZT ~ 1. Recent developments on superlattices and nanostructured materials have led to the demonstration of ZT values of up to 2.4 (right figure 2). These nanostructured materials possess significantly lower thermal conductivity than their bulk counterparts, while having a power factor comparable to that of their bulk counterparts. With further research and development on thermoelectric materials and understanding of electron and phonon transport mechanisms (to achieve ZT>3), thermoelectric converter efficiency up to 35% could be achieved.
     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 significantly reduced with only a small reduction in the electronic power factor. Bi2Te3-based alloys have a peak ZT around 1 near room temperature. Thus, these materials are not optimal for solar power production, where the operating temperatures are higher. Bi2Te3-based materials, used in some power generation applications, have a module efficiency that is limited to 5%. The U.S. National Aeronautics and Space Agency used SiGe alloys (and PbTe-based alloys) to make radioisotopepowered thermoelectric power generators operating in the temperature range of 300900C (and 300600C for PbTe-based alloys), with a system conversion efficiency ~67%. These materials all have a maximum ZT less than but close to 1.
Commercial thermoelectric materials, with a maximum ZT~1, were mostly discovered in 1950s. Little progress was made in the subsequent years. In the 1990s, the possibility of improving the thermoelectric figure of merit based on electron band gap engineering and phonon engineering in nanostructures was investigated (Hicks and Dresselhaus 1993). These ideas have lead to a resurgence in thermoelectric research and significant progress in improving ZT, particularly based on nanostructured materials (Tritt 2001; Chen et al. 2003). Venkatasubramanian et al. (2001) reported that Bi2Te3/Sb2Te3-based p-type superlattices have a room-temperature ZT of 2.4. Harman et al. (2002) reported that PbTe/PbTeSe superlattices with nanodots formed by strain have a room-temperature ZT of 2.0. Hsu et al. (2004) reported bulk nanostructures of AgPb2SbTe2+m. with a ZT of 2.2 at 527C. Meanwhile, several research projects aiming at improving device efficiency based on more mature materials are under way. The Jet Propulsion Laboratory reported a segmented thermoelectric unicouple with an efficiency of ~14% with the hot side at 975K and cold side at 300K. Solar thermoelectric power generators made of materials with ZT~4 operating between room temperature and 1000C would reach an efficiency of 35%. Given the impressive development made in the field of thermoelectrics over the past decade, the development of such materials seems to be a realizable goal