NREL Develops Switchable Solar Window
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 colour 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% 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 colour—a process that took about 3 minutes of illumination during testing—only 3 percent is allowed through the window. 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 Lance Wheeler, a scientist at 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.”
The proof-of-concept paper published in Nature Communications established a solar power conversion efficiency of 11.3 percent. “There are thermochromic technologies out there but nothing that actually converts that energy into electricity,” Wheeler said. He is the lead author of the paper, “Switchable Photovoltaic Windows Enabled by Reversible Photothermal Complex Dissociation from Methylammonium Lead Iodide.”
His co-authors, all from NREL, are David Moore, Rachelle Ihly, Noah Stanton, Elisa Miller, Robert Tenent, Jeffrey Blackburn, and Nathan Neale.