Boffins Perfect Power Capture From Wasted Heat

From complex data centres to humble computers, heat is often regarded as something of a nuisance, but researchers believe they have now found a much more efficient way to harvest that wasted heat and turn it into usable electricity.

This is according to Peter Hagelstein, an associate professor of electrical engineering at MIT, and co-author of a paper on the new concept which will appear this month in the Journal of Applied Physics. 

“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,” Hagelstein told MIT news.

According to Hagelstein, existing solid-state devices to convert heat into electricity are not very efficient. He therefore carried out new research with graduate student Dennis Wu, to find how close realistic technology could come to achieving the theoretical limits for the efficiency of such conversion.

Conventional wisdom states 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 said.

Hagelstein teamed up with Yan Kucherov, now a consultant for the Naval Research Laboratory, for experiments involving a different new technology, thermal diodes. They demonstrated efficiency as high as 40 percent of the Carnot Limit.

And even more good news is that their calculations show that this new kind of system could ultimately reach as much as 90 percent of that ceiling.

Apparently Hagelstein, Wu and others 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 said that with present systems it is 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 tradeoff. You either get high efficiency or high throughput,” said Hagelstein. But the team found that using their new system, it would be possible to get both at once, he said.

A key to the improved throughput was reducing the separation between the hot surface and the conversion device.

According to Hagelstein, a company called MTPV Corp is already working on the development of “a new technology closely related to the work described in this paper. Founder of the company, Robert DiMatteo is hoping to eventually commercialise Hagelstein’s new idea.

In the meantime, he said the technology now being developed by his company, which he expects to have on the market next year, could produce a tenfold improvement in throughput power over existing photovoltaic devices, while the further advance described in the new paper could make an additional tenfold or greater improvement possible. The work described in this paper “is potentially a major finding,” he said.

Worldwide 60 percent of all the energy produced by burning fuels or generated in powerplants is wasted, mostly as excess heat, and that this technology could “make it possible to reclaim a significant fraction of that wasted energy,” said DiMatteo.