| HOUSTON-Your
car's engine loses 70 percent of its energy as waste
heat-but Australian and Oregon scientists may have figured
out an efficient way not only to recover that lost energy,
but to at long last capture the power-producing potential
of geothermal heat.
The trick is to
convert it to electricity-and a promising way to accomplish
this, the researchers have discovered, involves using
extremely thin nanowires to potentially more than
double the efficiency of thermoelectric materials.
"If all goes
well, nanostructured thermoelectric devices may be
practical for applications such as recycling of waste
heat in car engines, on-chip cooling of computer microprocessors
and silent, more compact domestic refrigerators,"
says Heiner Linke, a University of Oregon assistant
professor of physics associated with ONAMI, the Oregon
Nanoscience and Microtechnologies Institute.
Linke and Tammy
Humphrey, an Australian Research Council fellow currently
visiting the University of California at Santa Cruz,
presented their findings today (Tuesday, April 5)
at the Nanoscale Devices and System Integration Conference
in Houston. A review of their study in the online
version of the journal Nature Materials described
their results as "dramatic" and "a
phenomenal enhancement relative to current bulk thermoelectrics."
The pair discovered
that two objects can have different temperatures yet
still be in equilibrium with each other at the nanoscale-a
fact that may blow right past a non-physicist but
which is crucial in order to attain the kind of performance
needed for widespread application of thermoelectric
technology in power generation and refrigeration.
Imagine a hot
cup of coffee sitting on a bench. The coffee will
quickly cool because molecules in the cup spontaneously
ferry heat from hot to cold in a rush to reach equilibrium
with the temperature of the bench. The same effect
happens with electrons in the materials studied by
Humphrey and Linke. In physics, this is the law of
thermodynamics: that heat will always flow from hot
to cold. Of course, the energy expended by those electrons
is normally lost.
Thermoelectric
materials try to recover this energy by converting
it to electricity, but they don't work very well if
the flow of heat is uncontrolled. The breakthrough
presented by Humphrey and Linke involves controlling
the motion of electrons using materials that are structured
on the nanoscale.
"The idea
is to play one type of non-equilibrium (the temperature
difference) against another one," Linke explains.
Humphrey and Linke
have shown that if an electrical voltage is applied
to an electrical system in addition to a temperature
difference, it is possible to harness electrons having
a specific energy. This means that if a nanostructured
material is designed to only allow electrons with
this particular energy to flow, a novel type of equilibrium
is achieved in which electrons do not spontaneously
ferry heat from hot to cold.
"This delicate
balance may have huge practical importance because
it means that thermoelectric devices, which use electrical
contact between hot and cold regions in a semiconductor
to transform heat into useful electrical energy, can
be operated near equilibrium," says Humphrey.
"This is a key requirement for cranking up their
efficiency toward the Carnot limit, the maximum efficiency
possible for any heat engine."
Because the system
is in a state of equilibrium, the flow of electrons
is reversible, Humphrey explains, noting that reversibility
allows the device to reach maximum possible efficiency.
Until now, the
efficiency of such devices, which have no moving parts
and can be small enough to fit on a microchip, has
been too low (less than 15 percent of the Carnot limit
for power generation) for use in all but a few specialized
applications.
However, Linke
and Humphrey say implementation of their design principle
is possible by tailoring the electronic bandstructure
in state-of-the-art thermoelectric materials made
up of a huge number of nanowires. If all goes well,
nanostructured thermoelectric devices with efficiencies
close to 50 percent of the Carnot limit may be realized,
Linke says.
Such materials
could make possible the generation of electricity
from geothermal sources-or from the waste heat of
engines in hybrid cars, he explains.
The study was
funded by the Australian Research Council and Linke's
CAREER Award from the National Science Foundation.
Editors Note:
Heiner Linke may be reached in Houston through 11:30
a.m. on Wednesday, April 6, at (713) 526-1991. His
name is pronounced HEYE ner LINK ee.
Contact:
Melody Ward Leslie, (541) 346-2060, mleslie@uoregon.edu
Sources:
Heiner Linke, (541) 346-4583, linke@uoregon.edu
Tammy Humphrey, (831) 459-1292, tammy.humphrey@unsw.edu.au
Links:
Heiner Linke: http://darkwing.uoregon.edu/~linke/
Tammy Humphrey: http://www.humphrey.id.au
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