Wednesday, September 19, 2012
A practical, cheap device able to convert heat directly into electricity could transform the energy use of everything from cars to power plants. Moving a step closer to making such a device practical, researchers have made a material that produces about 20 percent more electricity from heat than previous thermoelectric materials. What's more, it doesn't require any difficult or expensive fabrication techniques, and it's made of lead telluride, which isn't prohibitively expensive.
We currently waste huge amounts of heat. It's spewed into the atmosphere from vehicle exhaust pipes and power plant smokestacks. Thermoelectric materials can be used to generate electrical current from that heat, but so far they've been too expensive and inefficient to be widely used.
Thermoelectrics have found some niche commercial applications. In addition to generating electricity, they can do the reverse—using electric current to move heat around for portable coolers and seat heaters in cars. They've also been used as generators on space missions (see "Nuclear Generator Powers Curiosity Mars Mission").
Unlike previous thermoelectric materials, the new one, described in the journal Nature, could be efficient enough to make thermoelectric generators practical. The material works best at high temperatures, about 650 °C, which is close to the temperature of exhaust gases for a car cruising down the highway at 65 miles per hour. At that temperature, it could convert about 20 percent of the energy in that exhaust into electricity. That could be used, for example, to charge a battery in a hybrid vehicle or reduce the load on a car's alternator and improve fuel economy (see "Powering Your Car with Waste Heat").
A thermoelectric material blocks heat from flowing through it while allowing electrons to flow, creating an electric current. The new material excels at blocking heat flow by using microscopic interruptions, or boundaries, within the material. At the smallest size, the researchers added impurities into the material that interrupt its regular crystalline structure at the scale of individual atoms. For disruptions at a slightly larger scale, they mixed in nanoscale pieces of a similar material, each two to 10 nanometers wide. Finally, by controlling how the material crystallizes as it cools, they created microscopic grains that are a few hundred nanometers across. Researchers had previously done each of these things by themselves. "We're the first to put it all together," says Mercouri Kanatzidis, the professor of chemistry at Northwestern University who led the work.
One key to making this work was ensuring that the interruptions in the material didn't also block the flow of electrons. The researchers did this by adding impurities to the material that increase the number of electrons in the material and by choosing nanostructures that automatically orient themselves within the larger material in a way that creates a clear path for the electrons to flow.
John Fairbanks, a technology development manager at the U.S. Department of Energy, says the new material is "a great advance," but warns that there could be challenges to commercializing it. A thermoelectric device needs both positive and negative versions of the material (p- and n-type). The new one is only a positive type, so it needs a partner, he says. Also, regulators in the United States and the European Union will balk at including a lead-based material in a vehicle, even though it would be far less lead than what's found in a typical vehicle battery, he says. The material could also be used in industrial settings or power plants to capture wasted heat.