0929ctt solar coating lo res

Solar is hot.

In addition to a reference to solar’s growing popularity as a renewable energy source, solar cells are literally hot—no wonder, really, since they spend their life soaking in the sun.

While this may seem like no problem, the efficiency of solar cells decreases with increasing temperature. So keeping panels cool is kind of a big deal.

A trio of Stanford University researchers has responded to this problem by developing a clear and protective cooling coating for solar cells that allows them to shunt away excess heat.

The team’s coating is a thin silica photonic crystal with a micron-scale pattern that lets visible light pass through undisturbed, but collects and releases thermal radiation.

“Solar arrays must face the sun to function, even though that heat is detrimental to efficiency,” lead researcher Shanhui Fan says in a Stanford news report. “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.”

The work is the latest from the same group that brought us a multilayered oxide mirror earlier this year that keeps buildings cooler by reflecting heat into space as infrared radiation.


The Stanford trio of researchers with their multilayered oxide mirror. Credit: Norbert von der Groeben

Testing the new silica coating on a solar absorber, the team found that it could cool the device by up to 23°F.

“For a typical crystalline silicon solar cell with an efficiency of 20%, 23°F of cooling would improve absolute cell efficiency by over 1%, a figure that represents a significant gain in energy production,” the Stanford report states.

The report says the coating works best in “dry, clear environments,” where many large-scale solar arrays are already located.

As far as scaling up the technology, the team suspects it is quite feasible using techniques such as nanoprint lithography to pattern the coating.

0929ctt solar coating lo res

When laid over a solar cell, the transparent material shown here can radiate heat away from solar cells, allowing them to produce electricity more efficiently. Credit: Fan lab; Stanford University

In addition to coating solar panels to boost efficiency, the coatings could help keep other things cool in the hot sun, too.

“Say you have a car that is bright red,” co-first-author Linxiao Zhu says in the report. “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.”

“Our photonic crystal thermal overlay optimizes use of the thermal portions of the electromagnetic spectrum without affecting visible light, so you can radiate heat efficiently without affecting color,” Zhu adds.

Other technologies have arisen lately to help keep surfaces from overheating, including this glass paint that helps metal surfaces chill out under the sun.

The future has never been so cool.

The paper, published in Proceedings of the National Academy of Sciences of the U.S.A., is “Radiative cooling of solar absorbers using a visibly transparent photonic crystal thermal blackbody” (DOI: 10.1073/pnas.1509453112).