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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. The material could signify a paradigm shift. 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 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).
Waste heat temperatures in these areas can range from 400 to 600 degrees Celsius (750 to 1,100 degrees Fahrenheit), the sweet spot for thermoelectrics use. The new material, based on the common semiconductor lead telluride, is the most efficient thermoelectric material known. It exhibits a thermoelectric figure of merit (so-called "ZT") of 2.2, the highest reported to date. Chemists, physicists, material scientists and mechanical engineers at Northwestern and Michigan State University collaborated to develop the material.
Scientists rate the performance of a thermoelectric material by a measure known as ZT, which accounts not only for the stuff's ability to produce a voltage, but also its ability to conduct electricity (which should be high) and its ability conduct heat (which should be low). The best ZT values researchers had achieved were between 1.6 and 1.8, but researchers hoped to reach a value of 2, at which point applications of thermoelectrics would become more practical. "A ZT in the range of 2 and above represents an overall heat-to-electricity conversion efficiency in the 12 to 17% range, similar to what you'd see with industrial photovoltaics," Kanatzidis says. (A ZT of 2 is almost twice as efficient as a ZT of 1.)
The new thermoelectric material consists primarily of lead and tellurium; past studies found that lead telluride was the best thermoelectric system at the kind of high temperatures one might find in engines and other hot spots. The researchers adopted three different techniques for soaking up energy from phonons. Within the material, grains of semiconducting lead telluride that are hundreds to thousands of nanometers wide absorb phonons of longer wavelengths. Also, precipitates of strontium telluride 2 to 10 nanometers wide target shorter wavelengths. Finally, trace amounts of sodium injected within the material's crystalline structure go after the shortest wavelengths. As a result, the material achieves a world-record ZT of 2.2. "That's conservatively between 15 and 30% more efficient than the previous record-holder," Kanatzidis says. The scientists detailed their findings online today in Nature.