switchable photovoltaic window
Dr Lance Wheeler (front) has developed a switchable photovoltaic window along with (from left) Nathan Neale, Robert Tenent, Jeffrey Blackburn, Elisa Miller and David Moore. Image: Dennis Schroeder/NREL

US researchers have created a smart glass that provides shading and energy generation when the sun is shining, and a clear window when there’s no sun.

Developed by the US Department of Energy’s National Renewable Energy Laboratory (NREL), the device darkens when heated, allowing it to generate electricity from a broad spectrum of light.

With current solar windows there’s a tradeoff between the light available to solar cells, and that provided to building occupants for daylighting. Allowing more visible light to go through to occupants means less available for solar energy generation; allowing less means more opaque windows and a loss of views and daylight.

“There is a fundamental tradeoff between a good window and a good solar cell,” NREL scientist Lance Wheeler said.

“This technology bypasses that. We have a good solar cell when there’s lots of sunshine and we have a good window when there’s not.”

When not generating electricity, the window allows an average of 68 per cent of visible light to pass through. When the window turns dark after being heated by sunlight – a process that took about three minutes during testing – only three per cent of light is allowed through.

The research, published in Nature Communications, found a solar power conversion factor of 11.3 per cent.

Dr Wheeler said that while thermochromic technology currently existed, there was nothing that converted energy into electricity.

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The NREL technology is based on methylamine molecules that are selectively absorbed into the device. When heated by solar, the molecules are driven out and the device darkens. When cooled, the methylamine re-absorb into the device, which becomes transparent.

Other advanced materials in the device include perovskites and single-walled carbon nanotubes.

However, more testing is required as performance declined over the course of 20 cycles due to restructuring of the switchable layer.

Dr Wheeler said the technology could be integrated into buildings, vehicles and other products such as smart phones. It could even power integrated technology in glass, such as rain sensors and motors, that could open and close windows.

NREL has begun to assess commercialisation pathways, teaming up with industry to develop a marketplace for the product, which is being dubbed SwitchGlaze.

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