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September 12th, 2008

Advance makes thermoelectrics twice as efficient

Published on September 12th, 2008 | By: pwray@ceramics.org
Joseph Heremans (photo: OSU)

Joseph Heremans (photo: OSU)

Ohio State University researchers say they have invented a new material that “will make cars even more efficient by converting heat wasted through engine exhaust into electricity” and with “twice the energy efficiency” of any thermoelectric material currently on the market. Lead researcher Joseph Heremans, an OSU professor of mechanical engineering and physics, reports on the new material’s invention in the July 25, 2008, edition of Science. Heremans explains that a thermoelectric material’s efficiency is determined and rated on the basis of how much heat it converts into electricity at a given temperature. Until now, he says, the most efficient material used commercially in thermoelectric power generators has been an alloy called sodium-doped lead telluride, which has a rating of 0.71. By adding trace amounts of thallium to lead telluride, Heremans’ team has created a new material thallium-doped lead telluride – which has a rating of 1.5, double that of its predecessor. The improved efficiency, he says, could translate into a 10 percent boost in a car’s fuel economy, if devices made from the new material are used to replace car alternators. Such devices could then capture waste heat from auto exhaust systems, convert it to electricity and use the electricity to power cars or their accessories. Such devices would also be more practical because they would have no moving parts to wear out or break down, he contends. Heremans says the material does all the work.

“It produces electrical power just like conventional heat engines, steam engines, gas or diesel engines that are coupled to electrical generators, but it uses electrons as the working fluids instead of water or gases, and makes electricity directly.”

Heremans credits new ideas in quantum mechanics and a 2006 paper published by other researchers in Physical Review Letters as the inspiration for this new approach.

“It comes down to a peculiar behavior of an electron in a thallium atom when it has tellurium neighbors. We’d been working for 10 years to engineer this kind of behavior using different kinds of nanostructured materials, but with limited success. Then I saw this paper, and I knew we could do the same thing we’d been trying to do with nanostructures, but with this bulk semiconductor instead.”

Heremans’ team learned that, at 450°F, the new TE material converts heat to electricity with an efficiency rating of about 0.75, close to that of sodium doped telluride. As temperatures rose, so did the efficiency of the new material, peaking at 950°F and a rating of 1.5. Heremans believes, however, this rating can be boosted by another factor of two. “That’s what we’re shooting for now,” he reveals, acknowledging the help of his research team: Vladimir Jovovic, an OSU doctoral student; Ken Kurosaki, Anek Charoenphakdee, and Shinsuke Yamanaka of Osaka University in Japan, and G. Jeffrey Snyder, Eric S. Toberer and Ali Saramat at the California Institute of Technology. According to MIT’s Technology Review, one of the team’s challenges is that thallium is extremely toxic and, so, requires safeguards during manufacture and disposal. Heremans points out, however, that materials are encapsulated during use and, so, pose less of a danger. He also says devices containing the material can be removed from old cars and installed on new ones since they will easily survive several vehicles’ lifetimes and, therefore, decrease waste-disposal problems. Heremans is optimistic the new material can be commercialized quickly because of the scientific community’s years of experience working with lead telluride. He predicts the first products – probably thermoelectric generators that convert automobile exhaust into electricity – will be ready for market within the next three to four years.


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