The device, developed by the US Department of Energy’s National Renewable Energy Laboratory (NREL), responds to heat by transforming from transparent to tinted. As the window darkens, it generates electricity.
The color change is driven by methylamine molecules that are reversibly absorbed into the device. When solar energy heats up the device, the molecules are driven out, and the device is darkened. When the sun is not shining, the device is cooled back down, and the molecules re-absorb into the window device, which then appears transparent.
It allows an average of 68 per cent of light in the visible portion of the solar spectrum to pass through when it’s in a transparent state. When the window changes color, only 3 per cent is allowed through the window.
“There is a fundamental tradeoff between a good window and a good solar cell,” said Lance Wheeler from NREL.
“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.”
In testing under 1-sun illumination, the 1-square-centimeter demonstration device cycled through repeated transparent-tinted cycles, but the performance declined over the course of 20 cycles due to restructuring of the switchable layer. Ongoing research is focused on improving cycle stability.
In a proof-of-concept paper published in Nature Communications, the device's solar power conversion efficiency was 11.3 per cent.
“There are thermochromic technologies out there but nothing that actually converts that energy into electricity,” Wheeler said.
[NREL's Lance Wheeler holding a sample of their switchable photovoltaic glass. Photo: NREL]