USA: Scientists at the US National Renewable Energy Laboratory (NREL) claim to have developed a thermochromic window that not only reduces the need for air conditioning but simultaneously generates electricity.
The technology, dubbed thermochromic photovoltaic, is said to allow the window to change colour to block glare and reduce unwanted solar heating. This colour change also leads to the formation of a functioning solar cell that generates on-board power.
The research builds upon earlier work at NREL into a thermochromic window that darkened as the sun heated its surface. As the window shifted from transparent to tinted, perovskites embedded within the material generated electricity. Perovskites are a crystalline structure shown to have remarkable efficiency at harnessing sunlight.
“A prototype window using the technology could be developed within a year,” claimed Bryan Rosales, a postdoctoral researcher at NREL and lead author of the paper, Reversible Multicolor Chromism in Layered Formamidinium Metal Halide Perovskites, which appears in the journal Nature Communications.
The first-generation solar window was able to switch back and forth between transparent and a reddish-brown color, requiring temperatures between 150ºF (65ºC) and 175ºF (80ºC) to trigger the transformation. The latest iteration allows a broad choice of colours and works at 95ºF to 115ºF (35ºC to 46ºC), a glass temperature easily achieved on a hot day.
By using a different chemical composition and materials, the researchers were also able to rapidly speed up the colour transformation. The time was reduced to about seven seconds from the three minutes it took during the proof-of-concept thermochromic photovoltaic window demonstrated in 2017.
The scientists sandwiched a thin perovskite film between two layers of glass and injected vapour. The vapour triggers a reaction that causes the perovskite to arrange itself into different shapes, from a chain to a sheet to a cube. The colours emerge with the changing shapes. Lowering the humidity returns the perovskite to its normal transparent state.
Additional research is planned. One area to be explored is the number of times the thermochromic window can be cycled into an electricity-generating operating mode and return to transparent. The efficiency of converting sunlight into electricity will also be explored.
This research is funded by the US Department of Energy’s Building Technologies Office. NREL is the US Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by the Alliance for Sustainable Energy.