New device could aid production of electricity

An MIT scientist and a colleague have invented a semiconductor technology that could allow efficient, affordable production of electricity from a variety of energy sources without a turbine or similar generator.

Many researchers have worked to convert heat to electricity directly without the moving parts of a generator. Such a device would be virtually silent, vibration-free, and low in maintenance costs. Until now, however, the efficiency of such devices has been a problem because the amount of electricity they produce from a given amount of energy has been low.

The new, unnamed technology could have major implications for the recovery of waste heat from power plants and cars. The heat lost through engine exhausts might be captured by the technology and converted into electricity to augment or replace a vehicle’s electrical and air conditioning systems. It could also be important in the primary generation of electrical power.

The technology is based on thermionics, which originated nearly a century ago with the basic vacuum tube, a device that consisted of two parallel conductive plates (cathode and anode) separated by a vacuum gap.

In this high temperature tube, electrons boiled off the cathode, traversed the gap and then 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 Peter L. Hagelstein of MIT’s Department of Electrical Engineering and Computer Science.

These early ‘vacuum gap’ designs had prohibitive manufacturing costs and high operating temperatures – above 1,000 degrees Celsius, which has limited the technology to nuclear-powered converters in space probes, satellites and special military systems.

The new technology essentially replaces the traditional vacuum gap with a multi-layer semiconductor structure. Hagelstein and colleague Dr. Yan Kucherov of ENECO, Inc. demonstrated two basic enabling physical mechanisms that allow this technology to be implemented practically.

Louis D. Smullin, MIT Professor of Electrical Engineering, Emeritus, said of the new work: ‘Thermocouples and thermopiles have been with us for over a century. 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.’

By careful selection of materials, ENECO scientists are creating highly efficient, solid state conversion devices, called ‘thermal diodes,’ that will operate from 200 to 450 degrees Celsius, which are typical temperatures for waste heat and for concentrated solar radiation.

The technology is also said to be environmentally friendly. ‘Solid state thermal to electric energy conversion converts energy due to how electrons transport in the conductor, a process that generates no pollution,’ Hagelstein said. He noted, however, 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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