A team at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) have developed a novel perovskites and CNTs-based demonstration device that responds to sunlight by transforming from transparent to tinted while converting sunlight into electricity.

NREL develops thermochromic windows from CNTs and perovskites imageA switchable photovoltaic window

The thermochromic windows technology responds to heat, as was said, 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 again appears transparent.

It was reported that 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 color (a process that took about 3 minutes of illumination during testing) only 3% 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 trade-off between a good window and a good solar cell," said a scientist at NREL involved in this project. "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 established a solar power conversion efficiency of 11.3%. "There are thermochromic technologies out there but nothing that actually converts that energy into electricity," the researchers said.

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. Current research is focused on improving cycle stability.

NREL reported that the path to commercialization of the technology was explored last year during a two-month program called Energy I-Corps in which teams of researchers were paired with industry mentors to learn what customers want of the technology and develop viable ways to reach the marketplace. The NREL team teamed up to develop a market strategy for a product they called SwitchGlaze. The effort was funded by the Emerging Technologies program within the Department of Energy's Building Technologies Office.

The team expects this technology to possible be integrated into vehicles, buildings, and more. 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.