A system that harvests heat from an engine's exhaust to generate electricity and reduce a vehicle's fuel consumption is under development at Purdue University.

A university team is collaborating with General Motors Co. on the development of a prototype that uses thermoelectric generators, or TEGs, says Xianfan Xu, Purdue professor-mechanical engineering and electrical and computer engineering.

TEGs generate an electric current to charge batteries and power a vehicle’s electrical systems, reducing the engine's workload and improving fuel economy.

The research at the university is funded with a $1.4 million, 3-year grant from the National Science Foundation and the U.S. Department of Energy.

The prototype, to be installed in the exhaust system behind the catalytic converter, will harvest heat from gases that are nearly 1,300° F (700° C).

Xu says current thermoelectric technology cannot withstand the temperatures inside catalytic converters, where gases are about 1,830° F (1,000° C). But researchers are working on new thermoelectrics capable of withstanding these high temperatures, a step that would allow greater fuel savings.

The project begins Jan.1, and the initial prototype aims to reduce fuel consumption by 5%. Xu says future systems capable of working at higher temperatures could make possible a 10% reduction.

The thermoelectric material is contained in chips a few inches square that will be tailored for their specific location within the system. “They are optimized to work best at different temperatures, which decrease as gas flows along the system,” Xu says in a statement.

The researchers are tackling problems associated with the need to improve efficiency and reliability, integrate a complex mix of materials that might expand differently when heated and extract as much heat as possible from the exhaust gases.

Thermoelectric materials generate electricity when there is a temperature difference.

“The material is hot on the side facing the exhaust gases and cool on the other side, and this difference must be maintained to continually generate a current,” says Xu, who has been collaborating with GM in thermoelectric research for about a decade.

A critical research goal is to develop materials that are poor heat conductors.

“You don't want heat to transfer rapidly from the hot side to the cool side of the chip,” Xu says. “You want to maintain the temperature difference to continuously generate current.”

Researchers at GM are using a thermoelectric material called skutterudite, a mineral made of cobalt, arsenide, nickel or iron, he notes.

“The biggest challenge is system-level design – how to optimize everything to get as much heat as possible from the exhaust gas,” Xu says. “The engine exhaust has to lose as much heat as possible to the material.”

Rare-earth elements, such as lanthanum, cesium, neodymium and erbium, reduce the thermal conductivity of skutterudite. The elements are mixed with skutterudite inside a furnace.

Because using pure rare-earth elements is costly, researchers also are working to replace them with alloys called mischmetals.