Windows are great for letting in light, but during the summer months there is the unwanted downside of the heat, of having to use the air conditioner all the time. Now, researchers have developed windows that can change color automatically when heated by sunlight, to keep buildings cool – and they double as solar panels.

Color-changing glass has been around for a long time. More recent developments have made it electronic and switchable on demand, adapting it to the size of the window. At the same time, transparent (or semi-transparent) solar cells are becoming more efficient, so much so that they can be installed on windows.

In this new study, researchers at the US Department of Energy’s National Renewable Energy Laboratory (NREL) combined the two technologies into one window. “Thermochromic PV” technology, as it is called, can change color when heated by sunlight to block rays and reduce the need for cooling, and when it does, it also begins to produce. energy from that light.

The windows are made of a thin film of perovskite – a new material used in solar cells – wedged between two glass panels, with solvent vapor injected into the space.

When humidity is low, the perovskite remains transparent, allowing the window to let in light normally. But at certain temperatures, the steam causes the perovskite crystals to rearrange, first into a chain, then into a sheet, then into a cube. Each new shape changes color, blocking light to different degrees and hopefully cooling the room a few degrees. When colored, electricity production begins.

When the glass reaches between 35 and 46 ° C, the windows can change between several different colors, from transparent to yellow, to orange, red and brown, in about seven seconds. This is a huge improvement over an earlier prototype developed by the NREL team, which could only switch between a transparent color and a reddish-brown color when it hit 65.5-79.4 ° C, and it took three minutes to do it.

Ideally, thermochromic PV windows could help reduce the need for air conditioning, which can be a huge consumer of energy in hotter periods and climates, and even bring in some extra electricity to help keep things running. The team says a window prototype using the new technology could be developed within a year.

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