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.
The best commercial materials are alloys of Bi 2 Te 3 with Bi 2 Se 3 (n-type) and with Sb 2 Te 3
(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. Bi 2 Te 3 -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. Bi 2 Te 3 -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 radioisotope-
powered thermoelectric power generators operating in the temperature range of 300–900°C (and
300–600°C for PbTe-based alloys), with a system conversion efficiency ~6–7%. 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 Bi 2 Te 3 /Sb 2 Te 3 -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
AgPb 2 SbTe2+m. with a ZT of 2.2 at 527°C. 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 1000°C 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.
Solar Thermophotovoltaics. Solar TPVs are similar to solar cells in that they convert photon
energy into electricity. The fundamental difference from other PVs is that the photon source
comes from a terrestrial thermal radiation emitter rather than directly from the sun. The radiation
emitter can be heated by thermal sources such as fuel combustion or by concentrated solar
radiation. Solar TPVs have a theoretical system efficiency of >30% for a concentration ratio of