Thermochromic windows capable of converting sunlight into electricity at a high efficiency have been developed by scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL).
Relying on such advanced materials as perovskites and single-walled carbon nanotubes, the new technology responds to heat by transforming from transparent to tinted. As the window darkens, it generates electricity. The color change is driven by molecules (methylamine) 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.
The NREL-developed demonstration device allows an average of 68 percent of light in the visible portion of the solar spectrum to pass through when it’s in a transparent, or bleached, state. When the window changes color—a process that took about 3 minutes of illumination during testing—only 3 percent is allowed through the window.
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.
This technology could be integrated into vehicles, buildings, and beyond. The electricity generated by the solar cell window could charge batteries to power smartphones or on-board electronics such as fans, rain sensors, and motors that would open or close the windows as programmed.