Solar panels

The hotter solar cells turn out to be, the less efficient they’re at converting sunlight to electrical energy, an issue that has long vexed the solar trade. Now, Stanford engineers have developed a clear transparent overlay that will increase efficiency by cooling the cells even in full direct sunlight.

When laid over a photo voltaic cell, the clear materials shown here can radiate heat away from photo voltaic cells, allowing them to produce electricity extra efficiently.

Every time you stroll outside you emit energy into the universe: Heat from the top of your head radiates into space as infrared light.

Now three Stanford engineers have developed a technology that improves on solar panel efficiency by exploiting this basic phenomenon. Their invention shunts away the heat generated by a photo voltaic cell under sunlight and cools it in a manner that enables it to convert more photons into electrical energy.

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.

The group’s discovery, tested on a Stanford rooftop, addresses a problem that has long bedeviled the solar industry: The warmer photo voltaic cells get, the less efficient they become at converting the photons in light into useful electrical energy.

The Stanford solution relies on a thin, patterned silica material laid on top of a conventional solar cell. The material is transparent to the visible light that powers solar cells, but captures and emits thermal radiation, or heat, as infrared rays.

“Photo voltaic arrays should face the sun to perform, despite the fact that that heat is detrimental to efficiency,” Fan stated. “Our thermal overlay permits 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.”

A cool method to enhance solar efficiency

In 2014, the same trio of inventors developed an ultrathin material that radiated infrared heat directly back towards space with out warming the atmosphere. They presented that work in Nature, describing it as “radiative cooling” because it shunted thermal power straight into the deep, chilly void of space.

In their new paper, the researchers applied that work to improve solar array efficiency when the sun is thrashing down.

The Stanford group tested their technology on a customized solar absorber – a device that mimics the properties of a solar cell with out producing electricity – coated with a micron-scale pattern designed to maximise the capability to dump heat, in the form of infrared light, into space. Their experiments confirmed that the overlay allowed visible light to go through to the photo voltaic cells, however that it also cooled the underlying absorber by as much as 23 degrees Fahrenheit.

For a typical crystalline silicon solar cell with an efficiency of 20%, 23 F of cooling would enhance absolute cell efficiency by over 1%, a figure that represents a significant gain in power production.

The researchers said the brand new clear thermal overlays work best in dry, clear environments, that are also preferred sites for large photo voltaic arrays. They believe they’ll scale things up so commercial and industrial applications are possible, perhaps utilizing nanoprint lithography, which is a typical approach for producing nanometer-scale patterns.

“That is not necessarily the only method,” mentioned Raman, a co-first-author of the paper. “New techniques and machines for manufacturing these sorts of patterns will continue to advance. I am optimistic.”

Cooler vehicles

Zhu stated the technology has a lot of 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.

heat reflection
It works in a similar way to heat reflective glass or paint, like the one illustrated above

“Say you’ve got a car that’s bright red,” stated Zhu, co-first-author of the paper. “You actually like that colour, however you’d also wish to benefit from anything that might assist in cooling your car during scorching days. Thermal overlays will help with passive cooling, however it’s an issue if they are not absolutely transparent.”

That is because the perception of colour requires objects to reflect visible light, so any overlay would must be clear, or else tuned such that it will absorb only light outside the visible spectrum.

“Our photonic crystal thermal overlay optimizes use of the thermal portions of the electromagnetic spectrum with out affecting visible light,” Zhu said, “so you’ll be able to radiate heat effectively with out affecting colour.”




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