Nanocrystal-coated fibers might reduce wasted energy

April 18, 2012

Glass fibers coated with a thermoelectric material that generates electrical current when exposed to heat (credit: Purdue University/Scott W. Finefrock)

Purdue University 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 percent 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.”

The researchers 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. 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.

Capturing wasted energy

Such coated 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.

The fibers also could be used to create a solid-state cooling technology that does not require compressors and chemical refrigerants. The fibers might  also be woven into a fabric to make cooling garments.

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.

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 nanocrystals are a critical ingredient, in part because the interfaces between the tiny crystals serve to suppress the vibration of the crystal lattice structure, which reduces 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.”

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. ”Of course, the fact that our process uses such a small quantity of material – a layer only 300 nanometers thick – minimizes the toxicity issue,” Wu said. “However, we also are concentrating on materials that are non-toxic and abundant.”

Ref.: Daxin Liang et al., Flexible Nanocrystal-Coated Glass Fibers for High-Performance Thermoelectric Energy Harvesting, Nano Letters, 2012 [DOI: 10.1021/nl300524j]