Highly efficient thermoelectric generation, but only in the cold

The ideal crystalline structure of graphene is a hexagonal grid. (credit: CORE-Materials / Flickr)
This being Ars, I expect that many readers are familiar with Peltier coolers, which make use of something called the thermoelectric effect and use electrical power to pump heat. The nice thing about the thermoelectric effect is that it works both ways: a temperature difference can generate electrical power. The problem, of course, is that generating electricity this way is about as efficient as a teenager cleaning house.
But if you choose the right materials, it doesn't have to be that way. Recent research shows that it might be possible to convert heat energy to electrical energy with an efficiency in the range of 15-20 percent. If that can actually be realized, it would be a truly incredible advance.
Putting the heat on efficiency
What makes thermoelectric conversion so inefficient? It boils down to how heat and electricity are moved through a material. Heat is carried through a material in two different ways. One route is via the vibrations of atoms—essentially heat is carried by sound waves called phonons. But electrons can also carry heat. In an insulator, the electrons don't travel very far, so heat is only carried by phonons. In an electrical conductor, however, heat transfer is more likely to be dominated by electron motion.
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