Better way to harness waste heat

New MIT research points the way to a technology that might make it possible to harvest much of the wasted heat produced by everything from computer processor chips to car engines to electric powerplants, and turn it into usable electricity.

More than half of the energy consumed worldwide is wasted, most of it in the form of excess heat. This new technology would allow conversion of waste heat into electricity with an efficiency several times greater than existing devices. That kind of waste-energy harvesting might, for example, lead to mobile phones 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.

Theory says that 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, says Peter Hagelstein, Associate Professor of Electrical Engineering. In experiments involving a different new technology, thermal diodes, Hagelstein worked with Yan Kucherov, a consultant for the Naval Research Laboratory, and coworkers to demonstrate efficiency as high as 40% of the Carnot Limit. The calculations show that this new kind of system could ultimately reach as much as 90% of that ceiling.

How they did it

Hagelstein and his team started from scratch rather than trying to improve the performance of existing devices. They carried out their analysis using a very 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.

Hagelstein says that with present systems it’s possible to efficiently convert heat into electricity, but with very little power. It’s also possible to get plenty of electrical power - what is known as high-throughput power - from a less efficient and therefore larger and more expensive system. “It’s a trade-off. You either get high efficiency or high throughput,” says Hagelstein. But the team found that using their new system, it would be possible to get both at once, he says.
The new technology depends on quantum dot devices, a specialised kind of chip in which charged particles are very narrowly confined to a very small region. Such devices are under development, but still a few years away from commercial availability.

Written by David Chandler, MIT News Office.