power may soon hit automotive sector
Up to 60 percent of each gallon of gasoline
in a vehicle is wasted, lost as heat that pours out of the exhaust
pipe. Technology to collect that heat and convert it back into
electricity that can recharge the battery, power the lights,
wipers, power steering, and even the electric motor in a hybrid
vehicle is now a work in progress.
The solution lies in thermoelectric devices, and engineers at
the A. James Clark School of Engineering, University of Maryland,
are challenging previous assumptions about the behavior of the
nanoscale materials used to build them. Create better materials,
they say, and cars will make much better use of that expensive
But contrary to the common assumption in nanotechnology, “better”
in this case may not always mean “smaller.” That realization
may change the way engineers develop future thermoelectric devices.
A material whose response to a change in temperature generates
electric potential, or vice versa, exhibits what is known as
the thermoelectric effect. Thermoelectric devices can generate
electricity when heated by an external source, or quickly cool
or heat their environment when powered with electricity.
“The reason thermoelectric devices have so far been limited to
niche markets is that their efficiency is still too low,” explained
graduate student Jane Cornett, Department of Materials Science
and Engineering. “The goal of our work is to design thermoelectric
materials that convert energy from one form to another more efficiently
so we can promote the widespread use of products that recycle
waste heat and effectively reduce our consumption of fossil fuels.”
For example, cars manufactured or retrofitted with a thermoelectric
device placed around the exhaust pipe can use waste heat to generate
electricity, improving their overall miles per gallon, especially
with a power-draining system like air conditioning. If the device
is too bulky and inefficient, however, it will consume more energy
than it contributes.
To tackle the problem, Cornett and her advisor, Professor Oded
Rabin, Department of Materials Science and Engineering and Institute
for Research in Electronics and Applied Physics, had to challenge
some popular theories.
“Previous models told us that the use of nanomaterials at small
dimensions would lead to an improvement in power generation efficiency,”
said Cornett. “The models also predicted that the smaller the
nanostructure, the more significant the improvement would be.
In practice, people weren’t seeing the gains they thought they
should when they designed thermoelectric devices with nanoscale
components, which indicated to us that there might be an issue
with the interpretation of the original models.”
Cornett and Rabin have presented a revised thermoelectric performance
model that confirms that smaller is not always better.
Using advanced computer modeling to investigate the potential
of thermoelectric nanowires only 100 to 1000 atoms thick (about
1,000 times smaller than a human hair), they demonstrate that
in the set of the tiniest nanowires, measuring 17 nanometers
or less in radius, decreasing their radii does result in the
increased thermoelectric performance previous models predict.
In nanowires above 17 nanometers in radius, however, an improvement
is seen as the radius increases.
“The surprising behavior in the larger size range demonstrates
that a different physical mechanism, which was overlooked in
previous models, is dominant,” said Cornett.
“People were looking for solutions in the wrong places,” said
Rabin. “We’ve created a better understanding of how to search
for the best new materials.”
Thermoelectric devices are currently used in a few consumer products,
including refrigerators and CPU coolers in computers. They could
eliminate the need for fluorocarbon refrigerants, giving rise
to fluid and compressor-free cooling systems that pose fewer
health and environmental hazards.
Cornett and Rabin’s research is supported in part by the Minta
Martin Foundation and the ARCS Foundation.