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sustainability news: Saving money with wasted heat



From: Megan Fellman, Northwestern University
Published April 18, 2014 08:18 AM

Saving money with wasted heat

Nearly two-thirds of energy input is lost as waste heat. Now Northwestern University scientists have discovered a surprising material that is the best in the world at converting waste heat to useful electricity. This outstanding property could be exploited in solid-state thermoelectric devices in a variety of industries, with potentially enormous energy savings.

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An interdisciplinary team led by inorganic chemist Mercouri G. Kanatzidis found the crystal form of the chemical compound tin selenide conducts heat so poorly through its lattice structure that it is the most efficient thermoelectric material known. Unlike most thermoelectric materials, tin selenide has a simple structure, much like that of an accordion, which provides the key to its exceptional properties.

The efficiency of waste heat conversion in thermoelectrics is reflected by its figure of merit, called ZT. Tin selenide exhibits a ZT of 2.6, the highest reported to date at around 650 degrees Celsius. The material’s extremely low thermal conductivity boosts the ZT to this high level, while still retaining good electrical conductivity.

The ZT metric represents a ratio of electrical conductivity and thermoelectric power in the numerator (which needs to be high) and thermal conductivity in the denominator (which needs to be low).

Potential areas of application for the high-temperature thermoelectric material include the automobile industry (a significant amount of gasoline’s potential energy goes out of a vehicle’s tailpipe), heavy manufacturing industries (such as glass and brick making, refineries, coal- and gas-fired power plants) and places where large combustion engines operate continuously (such as in large ships and tankers).

"A good thermoelectric material is a business proposition -- as much commercial as it is scientific," said Vinayak P. Dravid, a senior researcher on the team. "You don't have to convert much of the world's wasted energy into useful energy to make a material very exciting. We need a portfolio of solutions to the energy problem, and thermoelectric materials can play an important role."

Dravid is the Abraham Harris Professor of Materials Science and Engineering at the McCormick School of Engineering and Applied Science. 

Details of tin selenide, probably among the world’s least thermally conductive crystalline materials, are published today (April 17) by the journal Nature.

The discovery comes less than two years after the same research group broke the world record with another thermoelectric material they developed in the lab with a ZT of 2.2.

"The inefficiency of current thermoelectric materials has limited their commercial use," said Kanatzidis, Professor of Chemistry at Northwestern.  "We expect a tin selenide system implemented in thermoelectric devices to be more efficient than other systems in converting waste heat to useful electricity."

Read more at Northwestern University newsroom.

Coal power plant image via Shutterstock.

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