Powering Your Car with Waste Heat
New thermoelectric materials will be tested in BMW, Ford, and Chevrolet vehicles by the end of summer.
Power from heat: A thermoelectric generator that converts waste heat from a car’s exhaust system into electricity could improve fuel economy.
General Motors
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."
A peek inside: An artist’s rendering of a Chevrolet Suburban shows the muffler-like thermoelectric generator inserted into the exhaust system.General Motors
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.