Running on Waste Heat

It’s estimated that more than half of U.S. energy — from vehicles and heavy equipment, for instance — is wasted as heat.  Mostly, this waste heat simply escapes into the air.  But that’s beginning to change, thanks to thermoelectric innovators such as MIT’s Gang Chen.

Thermoelectric materials convert temperature differences into electric voltage.  About a decade ago, Chen, the Carl Richard Soderberg Professor of Power Engineering and head of MIT’s Department of Mechanical Engineering, used nanotechnology to restructure and dramatically boost the efficiency of one such material, paving the way for more cost-effective thermoelectric devices.

Using this method, GMZ Energy, a company co-founded by Chen and collaborator Zhifeng Ren of the University of Houston, has now created a thermoelectric generator (TEG) — a one-square-inch, quarter-inch-thick module — that turns waste heat emitted by vehicles into electricity to lend those vehicles added power.

“Everybody recognizes the great potential of waste heat, but the challenge has always been that not many think seriously about systems that can turn that heat into power,” Chen says.  “It’s not just waste heat, it’s wasted potential to do useful work.”

In a TEG, electricity is generated when heat enters the top of the module, and then moves through the semiconductor material — packed into the TEG — to the cooler side.  The resulting motion of electrons in the semiconductor under this temperature difference creates a voltage that’s extracted as electricity.

However, in many TEGs atomic vibrations in the material can also leak heat from the hot to the cold side.  GMZ’s method essentially slows the heat leakage, leading to a 30 to 60 percent increase in performance across many thermoelectric materials.

The company’s TEG can withstand temperatures of roughly 600 degrees Celsius on its hot side (top surface), while maintaining a temperature of 100 C on its cold side (bottom surface).  With this gradient of 500 C, a module that’s 4 centimeters squared can produce 7.2 watts of power.  Installed near a car’s exhaust pipe, for instance, this converted electricity could power the car’s electrical components, essentially reducing the load on the vehicle’s alternator, reducing fuel costs and overall emissions.

Running on Waste Heat