Turning Heat to Electricity  

Renewable Energy World has an article n harvesting waste heat and turning it into power - Turning Heat to Electricity.

In everything from computer processor chips to car engines to electric powerplants, the need to get rid of excess heat creates a major source of inefficiency. But new research points the way to a technology that might make it possible to harvest much of that wasted heat and turn it into usable electricity.

That kind of waste-energy harvesting might, for example, lead to cellphones with double the talk time, laptop computers that can operate twice as long before needing to be plugged in, or power plants that put out more electricity for a given amount of fuel, says Peter Hagelstein, co-author of a paper on the new concept appearing this month in the Journal of Applied Physics.

Hagelstein, an associate professor of electrical engineering at MIT, says existing solid-state devices to convert heat into electricity are not very efficient. The new research, carried out with graduate student Dennis Wu as part of his doctoral thesis, aimed to find how close realistic technology could come to achieving the theoretical limits for the efficiency of such 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. But current commercial thermoelectric devices only achieve about one-tenth of that limit, Hagelstein says. In experiments involving a different new technology, thermal diodes, Hagelstein worked with Yan Kucherov, now a consultant for the Naval Research Laboratory, and coworkers to demonstrate efficiency as high as 40 percent of the Carnot Limit. Moreover, the calculations show that this new kind of system could ultimately reach as much as 90 percent of that ceiling.