Harnessing waste heat to produce electricity
That heat emanating from your computer as you sit reading this article amounts to nothing more than wasted energy. And your computer is not alone. More than half of the energy consumed worldwide is wasted, most of it in the form of excess heat. But new research at the Massachusetts Institute of Technology (MIT) indicates it might be possible to harvest much of the wasted heat produced by everything from computer processors to car engines and electric powerplants, and convert it into usable electricity. This kind of waste-energy harvesting might lead to mobile phones with double the talk time, laptop computers that can operate twice as long before needing to be plugged in to mains power, or energy plants that produce 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 (energy). But current commercial thermoelectric devices only achieve about one-tenth of that limit. However, in experiments involving a new technology, thermal diodes, an MIT research team has been able to demonstrate efficiency as high as 40 percent of the Carnot Limit, with calculations showing that this new system could ultimately reach as much as 90 percent of that ceiling.
Instead of trying to improve the performance of existing devices, the research team, including Peter Hagelstein, associate professor of electrical engineering, graduate student Dennis Wu and Yan Kucherov, a consultant for the Naval Research Laboratory, started from scratch. The men 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 its new system, it would be possible to get both at once, he says.
One of the keys to achieving the improved throughput lay in reducing the separation between the hot surface and the conversion device. Building on a recent finding by MIT professor Gang Chen that showed heat transfer could take place between very closely-spaced surfaces at a rate that is orders of magnitude higher than predicted by theory, the researchers were able to show that heat can not only be transferred, but can also be converted into electricity.
The new technology depends on quantum dot devices, a specialized type 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. In the meantime, however, new technology closely related to the work detailed in the MIT research team’s paper could see a tenfold improvement in throughput power over existing photovoltaic devices being developed by a company called MPTV Corp that could be on the market as early as next year.
Via MIT news