Scientists convert heat into electricity

SALT LAKE CITY, June 4 (UPI) -- U.S. scientists have developed a technology that converts heat into sound and then into electricity.

University of Utah physicist Orest Symko said the technology also holds promise for harnessing solar energy and cooling computers and radars.

"We are converting waste heat to electricity in an efficient, simple way by using sound," said Symko. "It is a new source of renewable energy from waste heat."

Symko plans to test the technology within a year to produce electricity from waste heat at a military radar facility and at the university's hot-water-generating plant.

The research is funded by the U.S. Army, which is interested in "taking care of waste heat from radar and also producing a portable source of electrical energy which you can use in the battlefield to run electronics," said Symko.

The scientist and his colleagues said they expect the technology could be used within two years as an alternative to photovoltaic cells for converting sunlight into electricity, as well as a way to cool laptop and other computers.

Five of Symko's doctoral students will present the research Friday in Salt Lake City during the annual meeting of the Acoustical Society of America.

bump that thought... I’ve been following a company called Eneco that is making some progress.

first attempt at a semiconductor version of a thermal diode operates at the comparatively low temperature of 200 °C.

Microchip can turn heat into electricity

* 10:20 05 February 2002

* From New Scientist Print Edition.

A microchip that can transform heat into electric current is now working on a lab bench at the Massachusetts Institute of Technology, US. Its inventors say it could harness heat from a car’s engine and provide power for its electronics, charge laptop batteries by recycling heat from the computer’s microprocessor, or simply bask in the baking desert sun generating electricity.

The device may be clever, but it has a decidedly unprepossessing name: a thermal diode. Nonetheless, it marks an important step in thermal electronics - or thermionics - which has seen little innovation since the inventor Thomas Edison first observed the thermionic effect in 1883.

In a thermionic vacuum tube, an electrically heated electrode “boils” off free electrons, which jump across a gap, drawn by a voltage applied to another electrode. But there’s another type of vacuum tube that doesn’t need to have electricity fed into it. Instead, it generates electricity - albeit very inefficiently - by using heat from the environment.

The heat gives a few electrons enough kinetic energy to boil off and jump a tiny gap, creating a minuscule electric current. But the temperatures needed to generate this current are very high—around 1000 °C. “They are not very efficient, and tend to be expensive,” says Gao Min at the University of Cardiff. As a result thermal diodes have found only limited applications, such as making electricity from nuclear sources in space probes or satellites.

Better idea

Attempts to make semiconductor versions of these devices have always been foiled by the technical difficulties of creating a very narrow vacuum gap between chip layers. But Peter Hagelstein, a physicist at MIT, and Yan Kucherov at energy conversion start-up ENECO in Salt Lake City, Utah, have a better idea. Their first attempt at a semiconductor version of a thermal diode operates at the comparatively low temperature of 200 °C.

Hagelstein and Kucherov’s big idea is to replace the vacuum gap with layers of an electron-rich semiconducting material. They found that this significantly boosted the current generated.

In an experiment funded by the US military, they used an indium antimonide-based semiconductor. This comprised an electron “emitter” doped with electron-donating impurities to give a surplus of electrons. On the opposite side, they placed an electron “collector” doped with electron-deficient impurities. These have lots of missing electrons, effectively creating positive “holes”.

Highly doped

By placing an additional highly doped electron-rich layer between the emitter and collector, the team got more electrons to traverse the gap, though they are not quite sure how yet. “The details of the mechanism are still under discussion,” says Hagelstein.

They speculate that high-energy electrons reaching the electron-rich layer cause a scattering “chain reaction” whose overall effect is to turn more of the heat into current.

The pair are now refining their device to see if it can work at even lower temperatures. But their major challenge will be making them affordable, says Min.

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