The vast majority of government-sponsored research in the field of thermoelectrics over the past 10-15 years has been in the area of thermoelectric power generation. The driving force behind most of this research seeks ways to improve our utilization of energy. Consider that less than a fourth of the energy content in the gasoline in your car actually goes into useful work to move the vehicle. The majority of the energy escapes as heat loss to the ambient primarily through the vehicle exhaust and radiator. Likewise, the U.S. manufacturing industry discharges roughly one-third of the energy consumed as thermal losses to the atmosphere or to cooling systems. This heat loss is measured in Quads (1015 BTU) and represents a huge opportunity for thermoelectrics to someday impact national energy consumption and our dependence on foreign fuel.
Thermoelectric waste heat recovery is the process of recapturing this lost heat and converting it to electrical power. This is the primary focus of most DOE, DARPA and DoD research for new, more efficient power generator materials and devices.
For any thermoelectric power generator (TEG), the voltage (V) generated by a TEG is directly proportional to the number of couples (N) and the temperature difference (ΔT) between the top and bottom sides of the TEG and the Seebeck coefficients of the n- and p-type materials (αp and αp, respectively).
Power output from a TEG is a function of the temperatures, the materials (and device effective) figure of merit (ZT) and also a function of how well the generator resistance (R) matches the resistance of the attached electrical load (RLoad).
To convert waste heat at reasonable efficiencies, one needs a) large temperature differences (hundreds of degrees C), b) high figure of merit (ZT) materials (ZT=1 or higher), and c) the ability to match the electrical loads with the thermoelectric resistance. In addition, any high ZT material must be capable of being incorporated into a device without significant losses that would degrade the device effective ZT in order to achieve the efficiencies described in the equations above. Heat flow from the exhaust stream must be extracted and conducted through the TEG in order to be converted. As depicted in figure 1, this heat must then be exhausted at a lower temperature to maintain the desired temperature difference across the TEG.
While waste heat recovery is the driving force for much of the thermoelectric power generation research, other application areas could utilize many of the same material and device advancements, namely direct generation, co-generation and energy harvesting.