To say that there is a lot of waste heat in society is an understatement. It is difficult to think of any electronic, or for that matter any industrial piece of equipment that does not generate waste heat. Not only is energy lost in the generation of this heat there is often additional energy consumed to remove it from the system.
Thermoelectric materials hold the promise of being able to convert waste heat into electricity. While the first applications will likely include integrated circuits and other electronic systems, a system that can be sufficiently scaled-up may some day approach the refrigeration and general cooling markets.
A thermoelectric material does not behave, as one would anticipate. Namely, it should demonstrate good electrical conductivity while maintaining a temperature difference i.e. poor thermal conductivity. This combination of properties is not normally found in a material. Consider, for example, copper where a high electrical and thermal conductivity go hand in hand, making it a poor thermoelectric material. Many of the thermoelectric systems today are manufactured to have the desired properties.
There are various approaches to the direct conversion of heat into electricity, including those focused on Huffman junctions, superlattice structures or metal oxides, to name but three. The concept of the Huffman junction was outlined in United States patent 3,169,200, which issued in 1965. As presented in Fig. 2 the thermoelectric device incorporates two surfaces separated by a small space. Electrons tunnel across this space or junction with a net flow from the hot to the cold surface. Today work is continuing with regard to the structure and fabrication of devices based on this concept. As might be anticipated it is no small task to provide a space with the required dimensions.
Superlattice systems present another area on interest, where the active theremoelectric component is based on a superlattice of thin (~1 nm) layers of material. Structures implementing p-type Bi2Te3/Sb2Te3 alloys (Venkatasubramanian R. et al. Thin-film thermoelectric devices with high room-temperature figures of merit. Nature 413, 597 (2001)) have demonstrated good thermoelectric properties. A structure based on a superlattice active is detailed in United States patent 6,300,150, for example.
With rising energy costs and advances in manufacturing techniques thermoelectric systems will continue towards commercialization. The payoff for a system that can generate electricity from waste heat is too large to ignore.
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