Solid-state devices get energy boost

Researchers at the Massachusetts Institute of Technology (MIT) are working on a way to boost the energy efficiency of solid-state devices by harvesting the currently wasted heat they generate for conversion into usable electricity.

The organisation has published a paper on the subject this month in the Journal of Applied Physics and hopes to see its so-called quantum-dot devices commercialised in the next couple of years. They are initially likely to be used in high-value systems such as computer chips but, as they become cheaper, could be used in everything from solar panels to transportation technology.

“There’s a gold mine in waste heat, if you could convert it,” said co-author of the paper, Peter Hagelstein, an associate professor of electrical engineering at MIT. “A lot of heat is generated to go places and a lot is lost. If you could recover that, your transportation technology is going to work better.”

Work on the initiative started in 2002 when Hagelstein began undertaking research with graduate student Dennis Wu as part of his doctoral thesis, although the team has now expanded to include others. The aim was to discover how closely existing technology could come to reaching the theoretical limits of converting heat into electricity.

The theory is that energy conversion can never exceed a specific value called the Carnot Limit, which is based on a formula devised in the 19th century. Existing devices can currently only hit about one tenth of that limit, however.

But quantum-dot devices have been proved to covert energy at up to 40 per cent of the Carnot Limit and calculations demonstrate that this figure could rise to 90 per cent in future. The devices comprise a type of semiconductor, in which the electrons and holes that carry electrical charges, are tightly co nfined in all three dimensions.

Because the gap between the hot surface and the conversion device is reduced, they are able to generate high levels of high-throughput power, which can be converted into electricity and harnessed.

Like existing systems, quantum-dot devices also convert heat into electricity efficiently. Where they differ from them, however, is that the latter are unable to generate high-throughput power, which means that they need to be relatively large and expensive to do their job.

MTPV (Micron-gap Thermal Photo-Voltaics) is already trying to develop technology based on related research undertaken by MIT professor Gang Chen, which demonstrated that heat transfer could take place between closely spaced surfaces at a rate that is orders of magnitudes higher than predicted by the theory.

The goal is to have photovoltaic equipment based on such work available by next year, but the new finding is potentially “major”, said MPTV founder Robert DiMatteo.