Powering Your Car with Waste Heat

At least two-thirds of the energy in gasoline used in cars and trucks is wasted as heat. Thermoelectrics, semiconductor materials that convert heat into electricity, could capture this waste heat, reducing the fuel needs of the vehicle and improving fuel economy by at least 5 percent. But the low efficiency and high cost of existing thermoelectric materials has kept such devices from becoming practical in vehicles.

Now researchers are assembling the first prototype thermoelectric generators for tests in commercial cars and SUVs. The devices are a culmination of several advances made independently at thermoelectric device-maker BSST in Irwindale, California, and at General Motors Global R&D in Warren, Michigan. Both companies plan to install and test their prototypes by the end of the summer—BSST in BMW and Ford cars, and GM in a Chevrolet SUV.

BSST is using  new materials. Bismuth telluride, a common thermoelectric, contains expensive tellurium and works at temperatures of only up to 250 °C, whereas  thermoelectric generators  can reach 500 °C. So BSST is using another family of thermoelectrics—blends of hafnium and zirconium—that work well at high temperatures. This has increased the generator efficiency by about 40 percent.

At GM, researchers are assembling a final prototype based on a promising new class of thermoelectrics called skutterudites, which are cheaper than tellurides and perform better at high temperatures. The company's computer models show that in its Chevrolet Suburban test vehicle, this device could generate 350 watts, improving fuel economy by 3 percent.

Fabricating skutterudites, which are cobalt arsenide compounds that are doped with rare earth elements such as ytterbium, is a time-consuming, complicated process, and incorporating them into devices is difficult, says GM scientist Gregory Meisner. The crucial challenge is making good electrical and thermal contacts. The large temperature gradient across the device puts mechanical stress on the contact-thermoelectric interface. Plus, joining the different materials introduces resistance that heats up the contact, degrading the device. "By a suitable choice of materials, you can affect resistance," he says. "The challenge is in arriving at the right formula for materials—both the semiconductor thermoelectric and the contact."

Another key challenge will be integrating the device into vehicles. The researchers have already tested a bismuth telluride generator in an SUV. "Right now, the device is just inserted into the exhaust system," Meisner says. "A section of pipe is cut out and the device, which looks like a muffler, is inserted. We need to design something that's more integrated into the vehicle system rather than an add-on device."

Both BSST and GM researchers also need to find ways to make larger volumes of the new materials cheaply. Meisner cautions that it might be at least another four years before thermoelectric generators make it into production vehicles.