Thermoelectric Materials Could Lead to Energy Savings
Institute of Technology (MIT) Institute Professor Mildred S.
Dresselhaus and co-workers are developing innovative materials for
controlling temperatures that could lead to substantial energy savings
by allowing more efficient car engines, photovoltaic cells and
electronic devices, university officials announced on Nov. 20.
Novel thermoelectric materials already have resulted in a new
consumer product: a simple, efficient way of cooling car seats in hot
climates. The devices, similar to the more-familiar car seat heaters,
provide comfort directly to the individual rather than cooling the
entire car, saving on air conditioning and energy costs.
The research is based on the principle of thermoelectric cooling and
heating, which was first discovered in the early 19th century and was
advanced into some practical applications in the 1960s by MIT professor
(and former president) Paul Gray, among others.
Dresselhaus and colleagues are now applying nanotechnology and other
cutting-edge technologies to the field. Thermoelectric devices are
based on the fact that when certain materials are heated, they generate
a significant electrical voltage. Conversely, when a voltage is applied
to them, they become hotter on one side, and colder on the other. The
process works with a variety of materials, and especially well with
semiconductors -- the materials from which computer chips are made. But
it always had one big drawback: it is very inefficient.
The fundamental problem in creating efficient thermoelectric
materials is that they need to be very good at conducting electricity,
but not heat. That way, one end of the apparatus can get hot while the
other remains cold, instead of the material quickly equalizing the
temperature. In most materials, electrical and thermal conductivity go
hand in hand. So researchers had to find ways of modifying materials to
separate the two properties.
The key to making it more practical, Dresselhaus explained, was in
creating engineered semiconductor materials in which tiny patterns have
been created to alter the materials' behavior. This might include
embedding nanoscale particles or wires in a matrix of another material.
These nanoscale structures -- just a few billionths of a meter across
-- interfere with the flow of heat, while allowing electricity to flow
freely. "Making a nanostructure allows you to independently control
these qualities," Dresselhaus said.
She and her MIT collaborators started working on these developments
in the 1990s, and soon drew interest from the U.S. Navy because of the
potential for making quieter submarines (power generation and air
conditioning are some of the noisiest functions on existing subs).
"From that research, we came up with a lot of new materials that nobody
had looked into," Dresselhaus said.
After some early work conducted with Ted Harman of MIT Lincoln Labs,
Harman, Dresselhaus, and her student Lyndon Hicks published an
experimental paper on the new materials in the mid 1990s. "People saw
that paper and the field started," she said. "Now there are conferences
devoted to it."
Her work in finding new thermoelectric materials, including a
collaboration with MIT professor of Mechanical Engineering Gang Chen,
invigorated the field, and now there are real applications like seat
coolers in cars. Last year, a small company in California sold a
million of the units worldwide.
The same principle can be used to design cooling systems that could
be built right into microchips, reducing or eliminating the need for
separate cooling systems and improving their efficiency.
The technology also could be used in cars to make the engines
themselves more efficient. In conventional cars, about 80 percent of
the fuel's energy is wasted as heat. Thermoelectric systems could
perhaps be used to generate electricity directly from this wasted heat.
Because the amount of fuel used for transportation is such a huge part
of the world's energy use, even a small percentage improvement in
efficiency can have a great impact, Dresselhaus explained. "It's very
practical," she said, "and the car companies are getting interested."
The same materials might also play a role in improving the
efficiency of photovoltaic cells, harnessing some of the sun's heat as
well as its light to make electricity. The key will be finding
materials that have the right properties but are not too expensive to
Dresselhaus and colleagues are continuing to probe the
thermoelectric properties of a variety of semiconductor materials and
nanostructures such as superlattices and quantum dots.
Mildred S. Dresselhaus: http://web.mit.edu/physics/facultyandstaff/faculty/millie_dresselhaus.html