New device could increase energy efficiency of engines

A new semiconductor device being developed by US scientists generates electricity from wasted heat, and could substantially increase energy efficiency of motors or generators by as much as 30%, cutting costs and emissions.

Associate Professor Peter L Hagelstein of the Massachusetts Institute of Technology (MIT) Department of Electrical Engineering and Dr Yan Kucherov of ENECO Inc, based in Salt Lake City, have invented a semiconductor technology which uses wasted heat energy and converts it into electrical energy.

The technology is based on thermionics, a technology that originated nearly a century ago with a vacuum tube containing a parallel cathode and anode separated by a vacuum gap, where heated electrons left the cathode and were absorbed into the colder anode. The conversion of heat to electricity “occurs as the electrons transport ‘uphill’ against an electric field in the gap region”, said Hagelstein.

The MIT device replaces the vacuum gap with a multi-layer semiconductor structure, first suggested by Professor Gerald D Mahan of the University of Tennessee.

With regard to a standard vehicle engine, typically about only one third of the energy does any work, Leroy H Becker, Marketing Director for ENECO Inc, explained to edie. One third of the energy is lost as heat through a vehicle’s radiator, and the final third is lost as heat out of the exhaust, so that the device could be applied to either of these places, he said. The advantage from an environmental point of view is that any energy that is recovered could be used in any other part of the engine, for example, by replacing the alternator, said Becker.

The most likely application for the technology is to raise the efficiency of micro-turbines, said Becker. “A typical micro-turbine system will put out about 20% electricity – we think we can increase that by 30%,” he said. However, he emphasised that the technology is still in the early stages of its development, and so practical, commercial applications may still be some way off.

The new technology is twice as efficient as previous attempts by scientists to solve the problem of efficiency. “That such good results were obtained in the first generation of the new device technology indicates that the general approach has great promise for improved performance in more mature implementations,” say Hagelstein and Kucherov.

“Thermocouples and thermopiles have been with us for over a century,” said MIT Professor of Electrical Engineering, Emeritus, Louis D Smullin. “I believe that these new devices represent the first big step in performance of these devices. In the 50s there was much hope that direct conversion of heat to electricity would open up a new era, but it was not to be. With these new devices, maybe these dreams will come true.”

Among other advantages, say the researchers, the device would be virtually silent, vibration-free, and require low maintenance costs. The technology also produces no emissions, points out Hagelstein. “Solid state thermal to electric energy conversion converts energy due to how electrons transport in the conductor, a process that generates no pollution,” he said. However, he notes that some of the materials used in the present generation of devices are toxic, which will affect the eventual disposal of the devices.

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