Hitting the Theoretical Roof of Converting Waste Heat to Electricity

Wednesday, November 18 2009

The need to get rid of excess heat creates a major source of inefficiency in everything from computer processor chips to car engines. According to Peter Hagelstein, an associate professor of Electrical Engineering at MIT, existing solid-state devices to convert heat into electricity are not very efficient. Experimenting with thermal diodes, an MIT team has come closer to the theoretical limitations for the efficiency of such a conversion.

Theory says that such energy conversion can never exceed a specific value called the Carnot Limit, based on a 19th-century formula for determining the maximum efficiency that any device can achieve in converting heat into work. Current commercial thermoelectric devices only achieve about one-tenth of that limit, Hagelstein says. In experiments involving thermal diodes, the MIT team demonstrated efficiency as high as 40 percent of the Carnot Limit. The calculations also show that this new kind of system could ultimately reach as much as 90 percent of that ceiling.

Graduate student Dennis Wu and Yan Kucherov, now a consultant for the Naval Research Laboratory, were part of the research team. They carried out their analysis using a simple system in which power was generated by a single quantum-dot device — a type of semiconductor in which the electrons and holes (which carry the electrical charges in the device) are very tightly confined in all three dimensions. By controlling all aspects of the device, they hoped to better understand how to design the ideal thermal-to-electric converter.