Converting Heat to Electricity Gets Easier

Development promises to enable plants to produce electricity directly from heat.

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By dispersing nanocrystals of rock salt into lead telluride, researchers at Northwestern University, Evanston, Ill., have created a material that can generate electricity directly from heat far more efficiently than previously possible.

The researchers say the material boasts a high "thermoelectric figure of merit" -- the material can convert up to 14% of heat waste to electricity.

"It has been known for 100 years that semiconductors have this property that can harness electricity," explains Mercouri Kanatzidis, professor of chemistry at Northwestern's Weinberg College of Arts and Sciences. "To make this an efficient process, all you need is the right material, and we have found a recipe or system to make this material."

Chemical plants producing high grade heat could make their systems more efficient by using this new technology, Kanatzidis  notes. However, applicability isn't limited to industrial sites. "Thermoelectric technology is scalable. So you can make small and portable generators. It can be very attractive where other technologies are not suitable, e.g, [for handling] vehicle exhaust."

Past attempts at nanoscale inclusions increased the scattering of electrons, which reduced overall conductivity. The Northwestern team overcame this problem by using, for the first time, nanostructures in lead telluride (PbTe) to reduce electron scattering, while still increasing the material's energy conversion efficiency. More details appear in a recent article in Nature Chemistry by Kanatzidis and his co-workers.

"We can put this material inside of an inexpensive device with a few electrical wires and attach it to something like a light bulb," says Vinayak Dravid, professor of materials science and engineering at Northwestern's McCormick School of Engineering and Applied Science and a co-author of the paper. "The device can make the light bulb more efficient by taking the heat it generates and converting part of the heat, 10 to 15%, into a more useful energy like electricity."

Within the next year the team hopes to boost the thermoelectric figure of merit to 2 from its current 1.7, and that, ultimately, it might reach 2.5 to 3, notes Kanatzidis.

In addition, the team already is working with private industry to commercialize the material, a process which Kanatzidis expects should take two to four years. Industrially, the technology could be implemented via direct contact with a hot waste stream or contact with a heat-transfer fluid heated by the waste stream.

"One challenge remaining is the full scale-up on the multi-kilogram and ton scale. Another is the demonstration of long-term stability … to high temperatures and thermal cycling," says Kanatzidis. "It is stable over the timescale of months and years, but we do not know about tens of years."

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