Transparent coating cools solar cells to boost efficiency

Stanford engineers, including one of Indian-origin, have developed a transparent coating that cools solar cells to boost efficiency.
The hotter solar cells get, the less efficient they become at converting the photons in light into useful electricity, a problem that has long vexed the solar industry.
Shanhui Fan, a professor of electrical engineering, research associate Aaswath P Raman and doctoral candidate Linxiao Zhu at Stanford University in US, have found a solution which is based on a thin, 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, from infrared rays.

"Solar arrays must face the sun to function, even though that heat is detrimental to efficiency," said Fan.
"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," Fan said.

In 2014, the same trio of researchers had developed an ultrathin material that radiated infrared heat directly back toward space without warming the atmosphere.
They presented that work in the journal Nature, describing it as "radiative cooling" because it shunted thermal energy directly into the deep, cold void of space.

In their new study, the researchers applied that work to improve solar array performance when the Sun is beating down.
The team tested their technology on a custom-made solar absorber - a device that mimics the properties of a solar cell without producing electricity - covered with a micron-scale pattern designed to maximise the capability to dump heat, in the form of infrared light, into space.
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 minus 5 degrees Celsius.
For a typical crystalline silicon solar cell with an efficiency of 20 per cent, minus 5 degrees Celsius of cooling would improve absolute cell efficiency by over 1 per cent, 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 believe they can scale things up so commercial and industrial applications are feasible, perhaps using nanoprint lithography, which is a common technique for producing nanometre-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.
The study was published in the Proceedings of the National Academy of Sciences.