Energy efficiency, that is, the amount of usable energy in a given fuel, is based off a number of factors. Hydrocarbon fuels, for example, have a given amount of chemical energy, that, when ignited, can be used to generate electricity, or propel locomotives and passenger vehicles.
Even in the most efficient motor vehicle, more than 50% of the chemical energy contained in gasoline is wasted as heat when it is ignited. Hot exhaust simply flows out of the tailpipe, leaving less than 50% of the available energy to run the engine, propel the vehicle, and power accessories like heaters and audio-visual systems. The same waste occurs in municipal power generation, heat rising from stacks, wasted energy that could be harvested.
Thermoelectric devices have the potential to recover some of that wasted heat energy. Thermoelectric materials are semiconductors that block the flow of heat while allowing electrons to flow, and the more heat it absorbs, the more electricity it can produce. They can also work in reverse, generating heat from the flow of electricity. Thermoelectric devices, for the most part, are not efficient or cheap enough to warrant their installation.
Mercouri Kanatzidis, a professor of chemistry at Northwestern University, led the research into a new material that could be a viable solution to both the efficiency and cost concerns. Using a naturally occurring compound, lead telluride [PbTe], and by combining a number of modern techniques, the new thermoelectric device is about 20% more efficient converting heat energy into electricity.
“Our system is the top-performing thermoelectric system at any temperature,” said Kanatzidis, “The material can convert heat to electricity at the highest possible efficiency. At this level, there are realistic prospects for recovering high-temperature waste heat and turning it into useful energy.”
It functions best about 1,200 °F [650 °C], which is about the temperature of car exhaust cruising at highway speed. If successful, such a device could be used to recover wasted heat to recharge vehicle batteries and run accessories, reducing the load on the engine and increasing fuel efficiency. The more efficient device could also be used to recover heat lost by power generating facilities.
The new material is a great advance in thermoelectrics, but there could be challenges to commercializing it. Part of the problem is that there are two types of thermoelectric devices, positive and negative [p-type and n-type], that work together to generate electricity from heat. The new PbTe material is a p-type, so further research is going to be needed to come up with a viable n-type before commercialization is possible.