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September 19, 2012

Commercializable Thermoelectric material could convert 20% of waste heat into Electricity

Phys Org - Northwestern University scientists have developed a thermoelectric material that is the best in the world at converting waste heat to electricity. This is very good news once you realize nearly two-thirds of energy input is lost as waste heat. This is a very environmentally stable material that is expected to convert 15 to 20 percent of waste heat to useful electricity, thermoelectrics could see more widespread adoption by industry.

Possible areas of application include the automobile industry (much of gasoline'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).

Even before the Northwestern record-setting material, thermoelectric materials were starting to get better and being tested in more applications. The Mars rover Curiosity is powered by lead telluride thermoelectrics (although it's system has a ZT of only 1, making it half as efficient as Northwestern's system), and BMW is testing thermoelectrics in its cars by harvesting heat from the exhaust system.

"Now, having a material with a ZT greater than two, we are allowed to really think big, to think outside the box," Dravid said. "This is an intellectual breakthrough." "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 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.

ZT of 2.2 means it is very good for working with 30-40% efficient car engines as a hybrid to make them more efficient

ZT 3.0 means that the material can replace some engines entirely in cars and replace cooling devices in refrigerators.

Technology Review indicates that the new material could make thermoelectric power practical.

Typical conversion systems become less efficient as they are scaled down to smaller sizes. This means there is a crossover point: below some power level thermoelectric technology will tend to be more efficient. Increasing ZT will move the crossover point to higher power levels, increasing the range of applications where thermoelectrics compete. Thus the ZT of 3 to compete with current best car size and refridgerator mechanical systems.

Heat engines typically operate at 30-40 percent efficiency, such that ~ 15 TW of heat is lost to the environment. To be competitive compared to current engines and refrigerators (efficiency 30-40 percent of Carnot limit), one must develop materials with ZT > 3. For the last 50 years, the ZT of materials has increased only marginally, from about 0.6 to 1, resulting in performance less than 10 percent of Carnot limit. There is no fundamental upper limit to ZT.









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