Chips turn more heat to power

By Kimberly Patch, Technology Research News

The technology to cleanly and quietly turn heat into electricity without the use of a turbine or generator has existed for nearly a century. The trouble is, it has never been efficient enough for widespread practical use.

A pair of scientists at the Massachusetts Institute of Technology and Eneco Inc. have made a device that nearly doubles the amount of electricity that can be extracted from heat. The researchers' thermal diode converts about 18 percent of thermal energy to electricity, while current thermoelectric generators convert about 10 percent.

The technology could be used to generate additional electricity from power plants, which throw off enormous amounts of waste heat, and to generate electricity using the heat from automobile engines. The technology could also produce electricity in conjunction with devices that concentrate sunlight.

In the thermal diode, heat causes electrons to flow from one semiconductor layer to another. The device operates at about 18 percent efficiency when the heat source is between 200 and 300 degrees Celsius, said Yan Kucherov, chief scientist at Eneco, Inc.

The diodes also have the potential to be more efficient. "We should be able to improve [the] existing devices to 20 to 23 percent efficiency at the same temperatures," said Kucherov. In addition, making the thermal diodes from different materials could increase the efficiency to 25 to 33 percent at 450 to 500 degrees Celsius, he said.

The researchers made their thermal diodes more efficient than existing thermoelectric devices by making two basic changes. "We demonstrated two physical mechanisms that can greatly enhance the ability of a thermoelectric to convert heat into electricity," said Peter Hagelstein, an associate professor of electrical engineering and computer science at MIT.

First, the researchers found an efficient way to move electrons across the device. Second, they found a way to block the electrons from returning, which effectively increased the voltage of the forward current, making the energy conversion more efficient. "We blocked the ohmic return current by reducing the electron concentration near the collector, so that it took more voltage to generate the same return current," said Hagelstein.

The initial results were surprising enough that the scientists did not at first understand exactly what was happening, said Hagelstein. "We had thought for a long time that these devices could be understood based on the usual semiconductor equations. The experimental results were not in agreement with the predictions of such models," he said. The results at first seemed to be in violation of the laws that govern thermoelectrics, but the scientists eventually resolved the apparent conflicts. "It became clear that we had found something new," said Hagelstein.

The principles involved increase the efficiency of thermal-to-electrical energy conversion by about eight times, and can apply to any semiconductor material, said Hagelstein. "We believe that the mechanisms are applicable to all semiconductor thermoelectrics," he said.

If the work is correct, it proves that a substantial improvement in thermal-to-electrical energy conversion is possible, said George Nolas, an assistant professor of physics at the University of South Florida. "This would open new commercial markets and consumer demand" for this type of energy, and foster further research into using energy extracted from heat sources, he said.

The research is interesting but a key challenge of using thermal diodes to generate electricity is to keep the temperature different at the two ends of the device, said Wenmin Qu, a senior engineer at the Hydac Group in Germany.

Although it is difficult to predict when these thermoelectric devices could become practical, given the resources, "we could see prototype devices within a year," Hagelstein said.

The same mechanisms that allow the diodes to generate electricity from waste heat could eventually be employed to make more efficient refrigerators, said Yan Kucherov, chief scientist at Eneco. Thermoelectric generators can be run in reverse so that electricity running through them cools the surrounding air. "It definitely can be adapted to refrigeration. Our model shows [a] possibility of going to liquid nitrogen temperatures with a single stage," said Kucherov. Nitrogen gas turns to liquid at -196 degrees Celsius.

Hagelstein and Kucherov presented the research at the Materials Research Society Fall meeting in Boston on November 27, 2001. The research was funded by Eneco and the Defense Advanced Research Projects Agency (DARPA).

Timeline:   1 year
Funding:   Corporate, Government
TRN Categories:  Energy; Materials Science and Engineering; Semiconductors
Story Type:   News
Related Elements:  Technical paper, "Enhancement of Thermal to Electrical Energy Conversion with Thermal Diodes," presented at the Materials Research Society Fall meeting in Boston, November 27, 2001.


December 19/26, 2001

Page One

LED fires one photon at a time

Chips turn more heat to power

Data protected on unlocked Web sites

Surgeons gain ultrasonic vision

Temperature changes laser color


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