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Researchers
are developing a technique that uses nanotechnology to harvest energy
from hot pipes or engine components to potentially recover energy wasted
in factories, power plants and cars.
“The
ugly truth is that 58% of the energy generated in the United States is
wasted as heat,” said Yue Wu, a Purdue University assistant professor of
chemical engineering. “If we could get just 10 percent back that would
allow us to reduce energy consumption and power plant emissions
considerably.”
Researchers
have coated glass fibers with a new “thermoelectric” material they
developed. When thermoelectric materials are heated on one side
electrons flow to the cooler side, generating an electrical current.
Coated
fibers also could be used to create a solid-state cooling technology
that does not require compressors and chemical refrigerants. The fibers
might be woven into a fabric to make cooling garments.
The
glass fibers are dipped in a solution containing nanocrystals of lead
telluride and then exposed to heat in a process called annealing to fuse
the crystals together.
Such
fibers could be wrapped around industrial pipes in factories and power
plants, as well as on car engines and automotive exhaust systems, to
recapture much of the wasted energy. The “energy harvesting” technology
might dramatically reduce how much heat is lost, Wu said.
Findings were detailed in a research paper appearing last month in the journal Nano Letters.
The paper was written by Daxin Liang, a former Purdue exchange student
from Jilin University in China; Purdue graduate students Scott Finefrock
and Haoran Yang; and Wu.
Today’s high-performance thermoelectric materials are brittle, and the devices are formed from large discs or blocks.
“This sort of manufacturing method requires using a lot of material,” Wu said.
The
new flexible devices would conform to the irregular shapes of engines
and exhaust pipes while using a small fraction of the material required
for conventional thermoelectric devices.
“This
approach yields the same level of performance as conventional
thermoelectric materials but it requires the use of much less material,
which leads to lower cost and is practical for mass production,” Wu
said.
The new approach promises a method that can be scaled up to industrial processes, making mass production feasible.
“We’ve
demonstrated a material composed mostly of glass with only a
300-nanometer-thick coating of lead telluride,” Finefrock said. “So
while today’s thermoelectric devices require large amounts of the
expensive element tellurium, our material contains only 5 percent
tellurium. We envision mass production manufacturing for coating the
fibers quickly in a reel-to-reel process.”
In
addition to generating electricity when exposed to heat, the materials
also can be operated in a reverse manner: Applying an electrical current
causes it to absorb heat, representing a possible solid-state
air-conditioning method. Such fibers might one day be woven into cooling
garments or used in other cooling technologies.
The
researchers have shown that the material has a promising thermoelectric
efficiency, which is gauged using a formula to determine a measurement
unit called ZT. A key part of the formula is the “Seebeck coefficient,”
named for 19th century German physicist Thomas Seebeck, who discovered
the thermoelectric effect.
ZT
is defined by the Seebeck coefficient, along with the electrical and
thermal conductivity of the material and other factors. Having a low
thermal conductivity, a high Seebeck coefficient and electrical
conductivity results in a high ZT number.
“It’s
hard to optimize all of these three parameters simultaneously because
if you increase electrical conductivity, and thermal conductivity goes
up, the Seebeck coefficient drops,” Wu said.
Most
thermoelectric materials in commercial use have a ZT of 1 or below.
However, nanostructured materials might be used to reduce thermal
conductivity and increase the ZT number.
The
Purdue researchers have used the ZT number to calculate the maximum
efficiency that is theoretically possible with a material.
“We
analyze the material abundance, the cost, toxicity and performance, and
we established a single parameter called the efficiency ratio,” Wu
said.
Although
high-performance thermoelectric materials have been developed, the
materials are not practical for widespread industrial applications.
“Today’s
higher performance ones have a complicated composition, making them
expensive and hard to manufacture,” Wu said. “Also, they contain toxic
materials, like antimony, which restricts thermoelectric research.”
The
nanocrystals are a critical ingredient, in part because the interfaces
between the tiny crystals serve to suppress the vibration of the crystal
lattice structure, reducing thermal conductivity. The materials could
be exhibiting “quantum confinement,” in which the structures are so tiny
they behave nearly like individual atoms.
“This
means that, as electrons carry heat through the structures, the average
voltage of those heat-carrying electrons is higher than it would be in
larger structures,” Finefrock said. “Since you have higher-voltage
electrons, you can generate more power.”
This confinement can raise the ZT number.
A U.S. patent application has been filed for the fiber-coating concept.
Future
work could focus on higher temperature annealing to improve efficiency,
and the researchers also are exploring a different method to eliminate
annealing altogether, which might make it possible to coat polymer
fibers instead of glass.
“Polymers could be weaved into a wearable device that could be a cooling garment,” Wu said.
The
researchers also may work toward coating the glass fibers with a
polymer to improve the resilience of the thermoelectric material, which
tends to develop small cracks when the fibers
are bent at sharp angles.
Researchers
demonstrated the concept with an experiment using a system containing
tubes of differing diameters nested inside a larger tube. Warm water
flows through a central tube and cooler water flows through an outer
tube, with a layer of thermoelectric material between the two.
The
Purdue researchers also are exploring other materials instead of lead
and tellurium, which are toxic, and preliminary findings suggest these
new materials are capable of a high ZT value.
“Of
course, the fact that our process uses such a small quantity of
material—a layer only 300 nm thick—minimizes the toxicity issue,” Wu
said. “However, we also are concentrating on materials that are
non-toxic and abundant.”
The work has been funded by the National Science Foundation and U.S. Department of Energy.
Flexible Nanocrystal-Coated Glass Fibers for High-Performance Thermoelectric Energy Harvesting
Source: Purdue University
