Capturing Wasted Energy
Each time you drive your car, about one-third of the gasoline your engine burns is not used to propel the vehicle forward. Instead, that energy just goes out the exhaust pipe in the form of heat. In fact, MU Professor of Physics David Singh says about half of all of the energy consumed in the United States results in waste heat. Singh and his colleagues have been trying to find ways to harness that waste heat and convert it into electricity using thermoelectric materials.
Thermoelectric materials are materials that produce usable electrical power when heat flows through them. This solid-state energy conversion does not involve moving parts, and instead thermoelectrics are heat engines that use electrons as their working fluid. They can be used for generating power from heat sources, in reverse for cooling applications, or in a mixed mode for precise temperature controllers. Singh has been studying thermoelectrics for more than a decade, beginning with his research for the Navy, which was seeking cooling systems for submarines.
“The problem with thermoelectrics is that they are not very efficient, so the amount of electricity you get is not that large,” Singh says. “The big goal in thermoelectrics research is to find materials that would give you better efficiency.” Singh and his fellow researchers have just published an article in Advanced Materials showing that very low amounts of sulfur doping in a material (bismuth telluride selenide) can improve both the conductivity and the thermal power of a thermoelectric material.
“The challenge in thermoelectrics is you need a material that produces a high voltage and at the same time has a low resistance or a high conductivity,” Singh says. “You need both of those things, but usually when you do something to a material, you increase one at the expense of the other. Our article reports a way to increase both.”
Singh says thermoelectrics are used in a variety of applications, such as power supplies for the Voyager and Pioneer spacecraft. Those craft carried plutonium pucks that produced heat as they decayed, and that heat was converted to electricity with thermoelectric materials. Some high-end wine coolers, which do not vibrate because there is no compressor, use thermoelectrics. He says that lack of vibration is also important in certain military applications.
Singh says he and his colleagues are now trying to find other materials that operate in different temperature ranges, and they also are exploring different classes of materials such as oxides and new kinds of semiconductor materials. He says the key to thermoelectrics is that they are scalable.
“That’s the special thing,” he says, “If you want to make a very small device, thermoelectrics becomes the only way to go. As you scale down most technologies for energy conversion they stop working. You can’t make a tiny air conditioner by scaling down, for example, but you can make a thermoelectric device that small.”