US team develops more efficient flexible thermoelectric generator

Penn State researchers have been working to improve the performance of thermoelectric generators that convert differences in temperature to electricity. When the devices are placed near a heat source, electrons moving from the hot side to the cold side produce an electric current.

“A large amount of heat from the energy we consume is essentially being thrown away, often dispersed right into the atmosphere,” said Shashank Priya, associate vice president for research and professor of materials science and engineering at Penn State. “We haven’t had cost-effective ways with conformal shapes to trap and convert that heat to useable energy. This research opens that door.”

In prior work, the team created rigid devices that were more efficient than commercial units in high-temperature applications. Now the team has developed a new manufacturing process to produce flexible devices that offer higher power output and efficiency.

“These results provide a promising pathway toward widespread utilisation of thermoelectric technology into waste heat recovery application,” said Wenjie Li, assistant research professor at Penn State. “This could have a significant impact on the development of practical thermal to electrical generators.”

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Flexible devices better fit the most attractive waste heat sources, like pipes in industrial and residential buildings and on vehicles, the scientists said. Furthermore, they do not have to be glued on surfaces like traditional, rigid devices, which further decreases efficiency.

In tests being conducted on a gas flue, the new device exhibited 150 per cent higher power density than other units, the scientists reported in Applied Materials & Interfaces. A scaled-up version, just over three-inches squared, maintained a 115 per cent power density advantage. That version exhibited a total power output of 56.6W when placed on the hot surface, the scientists said.

“Think about an industrial power plant with pipes hundreds of feet long,” Priya said in a statement. “If you can wrap these devices around an area that large, you could generate kilowatts of energy from wasted heat that’s normally just being thrown away. You could convert discarded heat into something useful.”

Thermoelectric devices are made up of small couples connected together to form a flat, square device.

In creating the new device, scientists placed six couples along a thin strip. They then used flexible metal foil to connect 12 of the strips together, creating a device with 72 couples. Liquid metal was used between the layers of each strip to improve device performance.

“As you scale up these devices, you often lose power density, making it challenging to fabricate large-scale thermoelectric generators,” said Bed Poudel, associate research professor at Penn State. “This illustrates the extraordinary performance of our 72-couple device.”

The 72-couple device exhibited the highest reported output power and device power density from a single thermoelectric generator, the scientists said.

The gaps between the strips provide the flexibility to fit around shapes like pipes. The gaps also allow for flexibility in altering the fill factor, or the ratio between the area of thermoelectric material and the area of the device, which can be used to optimise thermoelectric devices for different heat sources, the scientists said.