turn more heat to power
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
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
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
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;
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.
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