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Thermoelectric material puts waste heat to use

Posted: 26 Sep 2012     Print Version  Bookmark and Share

Keywords:thermoelectric  heat  electricity 

Northwestern University scientists claim to have developed a thermoelectric material that can convert 15 to 20 per cent of waste heat to useful electricity. The material, which boasts a figure of merit of 2.2, makes it a world's best at converting waste heat to electricity.

The inefficiency of current thermoelectric materials has limited their commercial use. Now, with a very environmentally stable material that is expected to convert 15 to 20 per cent of waste heat to useful electricity, thermoelectrics could see more widespread adoption by industry. Possible areas of application include the automobile industry (much of petrol's potential energy goes out a vehicle's tailpipe), heavy manufacturing industries (such as glass and brick making, refineries, coal- and gas-fired power plants) and places were large combustion engines operate continuously (such as in large ships and tankers). Waste heat temperatures in these areas can range from 400 to 600°C (750 to 1,100°F), the sweet spot for thermoelectrics use.

The new material, based on the common semiconductor lead telluride, exhibits a thermoelectric figure of merit (so-called "ZT") of 2.2, the highest reported to date, claim the researchers. Chemists, physicists, material scientists and mechanical engineers at Northwestern and Michigan State University collaborated to develop the material.

"Our system is the top-performing thermoelectric system at any temperature," said Mercouri G. Kanatzidis, who led the research and is a senior author of the paper. "The material can convert heat to electricity at the highest possible efficiency. At this level, there are realistic prospects for recovering high-temperature waste heat and turning it into useful energy." Kanatzidis is Charles E. and Emma H. Morrison Professor of Chemistry in Northwestern's Weinberg College of Arts and Sciences. He also holds a joint appointment at Argonne National Laboratory.

"People often ask, what is the energy solution?" said Vinayak P. Dravid, one of Kanatzidis' close collaborators. "But there is no unique solution—it's going to be a distributed solution. Thermoelectrics is not the answer to all our energy problems, but it is an important part of the equation." Dravid is the Abraham Harris Professor of Materials Science and Engineering at the McCormick School of Engineering and Applied Science and a senior author of the paper.

"Improving the ZT never stops, the higher the ZT, the better," Kanatzidis said. "We would like to design even better materials and reach 2.5 or 3. We continue to have new ideas and are working to better understand the material we have."

The efficiency of waste heat conversion in thermoelectrics is governed by its figure of merit, or ZT. This number represents a ratio of electrical conductivity and thermoelectric power in the numerator (which need to be high) and thermal conductivity in the denominator (which needs to be low). "It is hard to increase one without compromising the other," Dravid said. These contradictory requirements stalled the progress towards a higher ZT for many years, where it was stagnant at a nominal value of 1.

Kanatzidis and Dravid have pushed the ZT higher and higher in recent years by introducing nanostructures in bulk thermoelectrics. In January 2011, they published a report in Nature Chemistry of a thermoelectric material with a ZT of 1.7 at 800°K. This was the first example of using nanostructures (nanocrystals of rock-salt structured strontium telluride) in lead telluride to reduce electron scattering and increase the energy conversion efficiency of the material. The performance of the new material is nearly 30 per cent more efficient than its predecessor. The researchers achieved this by scattering a wider spectrum of phonons, across all wavelengths, which is important in reducing thermal conductivity.

"Every time a phonon is scattered the thermal conductivity gets lower, which is what we want for increased efficiency," Kanatzidis said. A phonon is a quantum of vibrational energy, and each has a different wavelength. When heat flows through a material, a spectrum of phonons needs to be scattered at different wavelengths (short, intermediate and long). In this work, the researchers show that all length scales can be optimised for maximum phonon scattering with minor change in electrical conductivity. "We combined three techniques to scatter short, medium and long wavelengths all together in one material, and they all work simultaneously," Kanatzidis said. "We are the first to scatter all three at once and at the widest spectrum known. We call this a panoscopic approach that goes beyond nanostructuring."

In particular, the researchers improved the long-wavelength scattering of phonons by controlling and tailoring the mesoscale architecture of the nanostructured thermoelectric materials. This resulted in the world record of a ZT of 2.2. The successful approach of integrated all-length-scale scattering of phonons is applicable to all bulk thermoelectric materials, the researchers said.

- Julien Happich
  EE Times





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