Direct thermal to electric power generation and refrigeration technologies, based on thermoelectric effects, are attractive for use in a wide range of applications because of their reliability, quiet operation, reduced complexity, reduced maintenance cost, and power scalability (e.g., from milliwatts to kilowatts and, potentially, to megawatts). Direct energy conversion between heat and electricity using thermoelectric effects for power generation, refrigeration, and heat pumping has been studied extensively. For example, devices such as thermoelectric (TE) coolers have been used for the cooling of semiconductor lasers, in compact refrigerators, and in analytical equipment. TE coolers can have limited efficiency, however, because of the limitations of the materials used in their fabrication. Thermoelectric power generators have been used to provide power in space missions and for terrestrial applications in remote areas.
The efficiency of thermoelectric devices, such as coolers and/or power generators, is determined at least in part by the material's figure of merit, Z=S2σ/k, where S is the Seebeck coefficient, σ the electrical conductivity, and k the thermal conductivity. Many known power generators, made of state-of-the art commercial materials, have relatively low efficiency, limiting them to only a few niche applications. Most of the efforts in thermoelectrics research have been in developing new materials, such as nanostructured materials. Significant progress has been made in recent years in materials research, leading to increases in the figure-of-merit. In addition to materials research, different device configurations have been explored with the hope of improving the energy conversion efficiency. Past studies have included the investigation of thermoelectric effects in pn junctions and minority carrier effects, thermionic refrigeration, and power generation based on single and multilayer structures.
Theoretical studies have shown that neither minority carrier nor thermionic emission-based devices can lead to thermal-electric energy conversion efficiency higher than that of pure thermoelectric devices. It has also been stated that the built-in potential of semiconductor structures does not have any effect on thermoelectric transport. Some recent experimental data, however, suggests that certain semiconductor diode structures can lead to increased output voltage and efficiency. Some researchers have recognized that the nonequilibrium between electrons and phonons can potentially be exploited to increase energy conversion efficiency. There have been few, if any, proposals to use novel device structures to improve efficiency, and the success of the few device structures that have been proposed has been quite limited.