A Purdue University team is collaborating with General Motors, which is developing a prototype using thermoelectric generators, or TEGs. The prototype, to be installed in the exhaust system behind the catalytic converter, will harvest heat from gases that are about 700 degrees Celsius, or nearly 1,300 degrees Fahrenheit, Xu said. The project begins Jan.1. The first prototype aims to reduce fuel consumption by 5 percent, and future systems capable of working at higher temperatures could make possible a 10 percent reduction, said Xu, whose work is based at the Birck Nanotechnology Center in Purdue’s Discovery Park.
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 said.
The researchers are tackling problems associated with the need to improve efficiency and reliability, to integrate a complex mix of materials that might expand differently when heated, and to 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,” said 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 said. “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.
“The biggest challenge is system-level design – how to optimize everything to get as much heat as possible from the exhaust gas,” Xu said. “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.”