Scientists develop transparent cooling coating for solar cells

WASHINGTON, Sep 21, 2015 -- Xinhua

Researchers at the Stanford University have developed a new coating that allows sunlight to hit solar cells while shunting away the heat they generated to improve their work efficiency.

The discovery, published Monday in the U.S. journal Proceedings of the National Academy of Sciences, may address a problem that has long bedeviled the solar industry: The hotter solar cells get, the less efficient they become at converting the photons in light into useful electricity.

The work by Shanhui Fan, a professor of electrical engineering at Stanford, research associate Aaswath Raman and doctoral candidate Linxiao Zhu, was based on a thin, patterned silica material laid on top of a traditional solar cell.

The material was transparent to the visible sunlight that powers solar cells, but can capture and emit thermal radiation, or heat, from infrared rays, they said.

"Solar arrays must face the sun to function, even though that heat is detrimental to efficiency," Fan said. "Our thermal overlay allows sunlight to pass through, preserving or even enhancing sunlight absorption, but it also cools the cell by radiating the heat out and improving the cell efficiency."

Their experiments showed that the overlay allowed visible light to pass through to the solar cells, but that it also cooled the underlying absorber by as much as 13 degrees Celsius.

For a typical crystalline silicon solar cell with an efficiency of 20 percent, 13 degrees Celsius of cooling would improve absolute cell efficiency by over 1 percent, a figure that represents a significant gain in energy production.

The researchers said the new transparent thermal overlays work best in dry, clear environments, which are also preferred sites for large solar arrays.

They believed they can scale things up so commercial and industrial applications are feasible, perhaps using nanoprint lithography, which is a common technique for producing nanometer-scale patterns.

Zhu said the technology has significant potential for any outdoor device or system that demands cooling but requires the preservation of the visible spectrum of sunlight for either practical or aesthetic reasons.

"Say you have a car that is bright red," Zhu said. "You really like that color, but you'd also like to take advantage of anything that could aid in cooling your vehicle during hot days. Thermal overlays can help with passive cooling, but it's a problem if they're not fully transparent."

That's because the perception of color requires objects to reflect visible light, so any overlay would need to be transparent, or else tuned such that it would absorb only light outside the visible spectrum.

"Our photonic crystal thermal overlay optimizes use of the thermal portions of the electromagnetic spectrum without affecting visible light," Zhu said. "So you can radiate heat efficiently without affecting color."

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