Solar cells work more efficiently when they are cooled, but sitting the the sun all day, it is not always easy to achieve this.
Stanford School of engineering graduate students Linxiano Zhu, Shanhui Fan, a Professor of Electrical Engineering and grad student Aaswath Raman on Friday, October 10, 2014. ( Norbert von der Groeben )
Researchers at Stanford University in the US have come up with a transparent overlay material that increases efficiency by cooling the cells even in full sunlight.
The material works by radiating heat away from the solar cells.
The researchers used a patterned silica material laid on top of a traditional solar cell. The material is transparent to the visible sunlight that powers solar cells, but captures and emits thermal radiation, or heat, as infrared rays.
According to Shanhui Fan, a professor of electrical engineering at Stanford: “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.”
When it was tested on a custom-made solar absorber, a device that mimics the properties of a solar cell without producing electricity, it cooled the underlying absorber by as much as 23 degrees Fahrenheit.
“For a typical crystalline silicon solar cell with an efficiency of 20%, 23 deg F of cooling would improve absolute cell efficiency by over 1%, a figure that represents a significant gain in energy production,” said the researchers.
The same technology could be applied to other systems that need to be cool but also exposed to the visible spectrum of sunlight.
“Say you have a car that is bright red,” said Linxiao Zhu, co-first-author of the paper. “You really like that colour, 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 optimises use of the thermal portions of the electromagnetic spectrum without affecting visible light,” Zhu said, “so you can radiate heat efficiently without affecting colour.”
The work by Shanhui Fan, a professor of electrical engineering at Stanford, research associate Aaswath P. Raman and doctoral candidate Linxiao Zhu is described in the current issue of Proceedings of the National Academy of Sciences.
