Windows that can switch from transparent to opaque, potentially saving energy by reducing air-conditioning costs on hot days, have been developed by researchers at the Massachusetts Institute of Technology.
While other systems for causing glass to darken do exist, the new method offers significant advantages including rapid response times, low power needs and greater change in opacity.
"It's this combination of these two – of a relatively fast switching time and a nearly black colour – that has really got people excited," says Mircea Dinca, professor of chemistry at MIT, who headed up the research.
The technology, Dinca suggests, could lead to significant energy savings, by drastically reducing the need for air conditioning in buildings with many windows during hot weather. "You could just flip a switch when the sun shines through the window, and turn it dark," or even automatically make that whole side of the building go dark all at once, he says.
Preliminary testing has shown that once the glass is switched from clear to dark, or vice versa, it requires little to no power to maintain its new state, and only requires electricity when it's time to switch back again.
The new system uses electrochromic materials, which change their colour and transparency in response to an applied voltage. Many existing products, such as eyeglasses that become darker when the light gets brighter, use photochromic materials – which tend to have much slower response times and to undergo a smaller change in colour.
Previous uses of electrochromic materials also fall short when it comes to response time – Boeing 787 aircraft have electrochromic windows that darken in response to a change in voltage to prevent sunlight glare, but the change takes place gradually over a few minutes.
The drawbacks arise because such materials rely on a movement of electrons that gives the whole window a negative charge, with positive ions then moving through the material to restore the electrical balance and create the colour-changing effect. The overall reaction speed is limited as protons move much more slowly than electrons.
The MIT team overcame this issue by using sponge-like materials called metal-organic frameworks (MOFs), which conduct both electrons and ions at very high speeds – significantly improving the overall reaction time.
The new material is made by combining two chemical compounds, an organic material and a metal salt, which, once mixed, self-assemble into a thin film of the switchable material.
For now, the material needs to go through further testing to prove that it can be utilised outside of a lab setting. However, in the future Dinca predicts that the material could go on to be implemented in some types of low-power displays, such as electronic ink which is used in Kindles.