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Coating Harvests Energy from Heat

Wednesday, April 25, 2012

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Purdue University researchers are developing a nanotech coating and process that harvest energy from hot pipes or engine components, potentially enabling the recapture of energy now 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 Dr. Yue Wu, a Purdue 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.”

Thermoelectric Material

Wu’s team coated glass fibers with a new “thermoelectric” material the group developed. When thermoelectric materials are heated on one side, electrons flow to the cooler side, generating an electrical current, the researchers say.

Purdue University / Scott W. Finefrock

Glass fibers coated with a thermoelectric material generate electrical current when exposed to heat. The technology might be used to harvest energy from hot pipes or engine components, potentially recovering energy wasted in factories, power plants and cars, Purdue University researchers say.

The process could also work in reverse, with coated fibers used to create a solid-state cooling technology that does not require compressors and chemical refrigerants.

The team has applied for a patent for the fiber coating, detailed in “Flexible Nanocrystal-Coated Glass Fibers for High-Performance Thermoelectric Energy Harvesting,” published recently in the journal NanoLetters.

According to Purdue, 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, or on car engines and automotive exhaust systems to recapture much of the wasted energy, the team says. The “energy harvesting” technology might dramatically reduce how much heat is lost, said Wu.

Making Mass Production Feasible

The process improves on today’s high-performance thermoelectric materials, which tend to be brittle. The devices are formed from large discs or blocks.

“This sort of manufacturing method requires using a lot of material,” Wu said.

By contrast, the new flexible coated fibers would conform to the irregular shapes of engines and exhaust pipes while using a small fraction of the material required for conventional thermoelectric devices, making the process less expensive and mass production more feasible.

“We’ve demonstrated a material composed mostly of glass with only a 300-nanometer-thick coating of lead telluride,” said Dr. Scott W. Finefrock. “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.”

Cooling Applications

In addition to generating electricity when exposed to heat, the material 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.

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.”

Future work could focus on higher-temperature annealing to improve efficiency or on methods to eliminate annealing altogether, which might make it possible to coat polymer fibers instead of glass.

In that case, says Wu, “polymers could be weaved into a wearable device that could be a cooling garment.”

Nontoxic Options

The researchers also are exploring alternative materials for toxic lead and tellurium.

“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.”

The work has been funded by the National Science Foundation and U.S. Department of Energy.