The new windows rely on advanced materials such as perovskites and single-walled carbon nanotubes. The technology responds to heat by transforming from transparent to tinted and as the window darkens, it generates electricity. The colour change is driven by molecules of methylamine that are reversibly absorbed into the device. When solar energy heats the device up, the molecules are driven out and the device is darkened. When the sun isn’t shining, the device is cooled back down and the molecules are re-absorbed into the window, thus turning it transparent.
The demonstration device developed by the NREL allows an average of 68 percent light in the visible portion of the solar spectrum to pass through when it’s in a transparent, or bleached, state. This changes to only 3 percent when the window changes colour, a process that took only 3 minutes during testing. Existing solar window technologies are static, which means they are designed to harness a fraction of the sunlight without sacrificing too much visible light transmission needed for viewing or the comfort of building occupants.
“There is a fundamental tradeoff between a good window and a good solar cell” said NREL scientist Lance Wheeler. “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. There are thermochromic technologies out there but nothing that actually converts that energy into electricity.”
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.
The path to commercialisation of the technology was explored last year during a two-month programme called Energy I-Corps, funded by the Emerging Technologies programme within the Department of Energy's Building Technologies Office.
The proof-of-concept paper “Switchable Photovoltaic Windows Enabled by Reversible Photothermal Complex Dissociation from Methylammonium Lead Iodide” published in Nature Communications established a solar power conversion efficiency of 11.3 percent. Wheeler is the lead author and the co-authors, all from NREL, are David Moore, Rachelle Ihly, Noah Stanton, Elisa Miller, Robert Tenent, Jeffrey Blackburn, and Nathan Neale.
Wheeler added that the 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.
Image: NREL scientist Lance Wheeler (front) along with (from left) Nathan Neale, Robert Tenent, Jeffrey Blackburn, Elisa Miller, and David Moore. (Photo by Dennis Schroeder/NREL)