Converting heat directly into electricity is nothing new; it has been possible since 1821. But thermoelectric materials have been too inefficient to make them practical for anything but a few niche uses, such as in deep space probes.
Recent advances using nanotechnology, however, have revived this moribund field, and have car makers such as General Motors and BMW taking notice, hoping to increase fuel efficiency and eventually replace alternators and possibly even internal combustion engines with thermoelectric generators.
“I think right now that thermoelectrics have a good chance of succeeding,” General Motors senior analyst Francis Stabler reported last week at the Materials Research Society meeting in Boston.
As much as 70 percent of the fuel energy burned up in car engines doesn’t go toward moving the vehicle along or powering the CD player, he said. Instead, it’s dissipated as waste heat. Stabler says a new generation of thermoelectric materials can convert heat to electricity well enough to be used for taking the burden of electricity generation off the engine, thereby saving fuel.
Researchers still need to find ways to make these materials cheaply and consistently, however, before they can be widely deployed. But certain niche uses could help the technology get established. Already, Amerigon, a Deerborn, MI manufacturer, has sold well over a million car seat heating and cooling units that use an older version of the technology. When electricity is applied to thermoelectrics materials they transfer heat, cooling an area or heating it depending on the direction of the current.
If the next generation of thermoelectric materials can be manufactured inexpensively, they could be used in more demanding applications. Wrapped around a car’s exhaust pipe, for instance, they could harvest waste heat to produce electricity. Initially, this electricity might be used to supplement the electricity generated by the vehicle’s alternator, making it possible to run more electrical devices without adding more strain to the engine.
If the technology proves to be reliable, Stabler says, it could eventually replace alternators altogether and run electrical water and oil pumps, relieving the extra work for the engine, boosting performance, and saving fuel. John Fairbanks, technology development manager at the U.S. Department of Energy, suggests that if all GM cars alone used this technology, it would save roughly 100 million gallons of gas per year.
Thermoelectric materials, which are most often made of semiconductors, need to conduct electricity well, allowing electrons to move away from a heat source and thereby generate an electrical current. But the material also has to conduct heat poorly, or else it will heat up and the temperature difference that drives the electrons will disappear. The challenge is that when electrical conductivity goes up, heat conductivity tends to go up as well.
The growing knowledge of how to structure materials on a nanoscale could provide a solution. For example, researchers have created materials with molecular lattices that interrupt vibrations from heat, keeping the heat from thermally conducting, while allowing electrons to move freely.
Stabler believes thermoelectric generators can beat out near-term competitors for improving fuel efficiency, such as turbo-chargers and turbo-generators, which also harvest energy from exhaust. “Thermoelectrics is something that seems to give a better efficiency gain long term,” he says, adding that “there’s always going to be waste heat.”
According to the DOE’s Fairbanks, there is an even chance that thermoelectric generators could one day beat out internal combustion engines.
While GM’s Stabler agrees this could happen, he cautions that it’s a long way off. A new technology has to be well-proven before it can be implemented in essential systems like power generation. Even after researchers have succeeded in making materials that can be manufactured, it could be an additional three to eight years, he says, before the industry is willing to use them to completely replace the alternator in production vehicles.
But don’t be surprised if cars start appearing that have extra power skimmed from exhaust heat. It’d be “environmentally friendly,” Stabler says. “Being able to generate some power from waste heat certainly will attract some attention.”
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