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.”