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More-Efficient Thermoelectrics

An advance makes the conversion of heat to electricity practical.

By improving the electronic properties of a common thermoelectric material–a type of semiconductor that converts heat into electricity–researchers have doubled its performance, making it more practical for generating electricity from waste heat such as that produced in power plants and car engines.

Electric heat: A small sample of a new material for converting heat into electricity is attached to electronic leads and a tiny heater for testing.

Thermoelectrics haven’t been widely used to generate electricity because they are expensive and inefficient. To increase the efficiency, the researchers, including Joseph Heremans, a professor of mechanical engineering and physic at Ohio State University, added trace amounts of thallium to lead telluride, a thermoelectric material that’s been generating electricity onboard deep space probes for decades. The added thallium doubled the material’s ability to convert heat into electricity by increasing the voltage that it produces. Heremans says that the improved efficiency could translate into a 10 percent increase in the fuel economy of cars if the devices are used to replace alternators in automobiles by generating electricity from the heat in exhaust. The new materials are described in this week’s issue of the journal Science.

The new work is important for several reasons, says Gang Chen, a professor of mechanical engineering at MIT who was not involved in the work. First, it’s a “quite impressive” increase in the efficiency of one kind of thermoelectric material, he says. Conventional lead telluride thermoelectrics convert about 6 percent of the energy in heat into electricity. Once it’s incorporated into a thermoelectric generator, the more efficient thallium-enhanced material could increase this to 10 percent, once losses, such as those from making electrical connections, are taken into account.

More important, Chen says, Heremans’s work gives researchers a new way to improve thermoelectric materials that could increase the efficiency of a wide variety of experimental materials. Thermoelectric materials are good electronic conductors but poor thermal conductors: the heat difference within the material largely accounts for the thermoelectric properties. Almost all the recent improvements to thermoelectric materials–and there have been significant improvements in the past few years–have come with a decrease in their thermal conductivity. Heremans and his colleagues have tried a different approach, increasing the voltage that the materials create. “That is,” Heremans says, “we get the electrons to do more work.”

Techniques employed to cut the thermal conductivity could be used to complement the new techniques developed by Heremans and his colleagues. That would allow the researchers to double the performance of the materials yet again, suggests Heremans. And that, in turn, would start to make thermoelectric devices competitive with conventional generators, says Jeffrey Snyder, a materials-science researcher at Caltech and one of the other researchers involved with the Science paper.

One drawback to the new materials is that thallium is extremely toxic, so it would require safeguards during manufacturing and disposal. (During use, the materials are encapsulated and therefore pose less of a danger.) However, Heremans says that the devices could be removed from old cars and put on new ones since they could easily last the lifetime of several vehicles, decreasing waste-disposal problems.

Heremans is optimistic that the new materials can be quickly commercialized, since engineers already have years of experience working with lead telluride. He says that the first products, likely thermoelectric generators that convert automobile exhaust into electricity, could be ready in three to four years.