New way to harness waste heat

Tuesday, April 13, 2010 @ 03:04 PM gHale

Wasted heat produced by everything from computer processor chips to car engines to electric power plants is just that: Wasted heat. The trick is how to safely turn it into usable electricity.

There is now a new technology that can turn that wasted heat into electricity with an efficiency several times greater than existing devices. That kind of waste-energy harvesting could lead to cellphones with double the talk time, laptop computers that can operate twice as long before needing a charge, or power plants that put out more electricity for a given amount of fuel.

A long posted theory states conversion of heat into electricity 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, said Peter Hagelstein, associate professor of electrical engineering at MIT. In experiments involving a different new technology, thermal diodes, Hagelstein worked with Yan Kucherov, a consultant for the Naval Research Laboratory to demonstrate efficiency as high as 40% of the Carnot Limit. The calculations show this new kind of system could ultimately reach as much as 90% of that ceiling.

Instead of trying to improve upon existing devices, Hagelstein and his team decided to start from scratch. They carried out their analysis using a very simple system in which power comes from a single quantum-dot device — a type of semiconductor in which the electrons and holes, which carry the electrical charges in the device, remain 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.

With present systems it’s possible to efficiently convert heat into electricity, but with very little power, Hagelstein said. It’s also possible to get plenty of electrical power — or high-throughput power — from a less efficient, and therefore larger and more expensive system. “It’s a tradeoff. You either get high efficiency or high throughput,” Hagelstein said. But the team found that using their new system, it would be possible to get both at once, he said.

The next step for the new technology depends on quantum dot devices, a specialized kind of chip in which charged particles remain narrowly confined to a very small region. Such devices are under development, but still a few years away from commercial availability.

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