In a new study, researchers at the US Department of Energy’s National Renewable Energy Laboratory (NREL) have combined two technologies, developing windows that automatically change colour when heated and act as solar panels as well. The thermochromic photovoltaic tech can block glare and reduce the need for cooling, while also harvesting the energy from the light that can chip in for electricity. The windows are made from a thin film of an emerging solar cell material called perovskite. This is wedged between two panes of glass, with a solvent vapour injected into the gap. When the humidity is low, the perovskite remains transparent and natural light is let in as normal. But when the glass reaches temperatures between 35 and 46 degree Celsius, the vapour causes the perovskite crystals to rearrange themselves into different shapes, each one changing the colour of the glass in about seven seconds.