A team of researchers at MIT has developed a new way of making windows that can switch from transparent to opaque, potentially saving energy by blocking sunlight on hot days and thus reducing air-conditioning costs. While other systems for causing glass to darken do exist, the new method offers significant advantages by combining rapid response times and low power needs.
Once the glass is switched from clear to dark, or vice versa, the new system requires little to no power to maintain its new state; unlike other materials, it only needs electricity when it’s time to switch back again.
The new discovery uses electrochromic materials, which change their color and transparency in response to an applied voltage. These are quite different from photochromic materials, such as those found in some eyeglasses that become darker when the light gets brighter. Such materials tend to have much slower response times and to undergo a smaller change in their levels of opacity.
Existing electrochromic materials suffer from similar limitations and have found only niche applications. For example, Boeing 787 aircraft have electrochromic windows that get darker to prevent bright sunlight from glaring through the cabin. The windows can be darkened by turning on the voltage, the researcher says, but “when you flip the switch, it actually takes a few minutes for the window to turn dark. Obviously, you want that to be faster.”
The reason for that slowness is that the changes within the material rely on a movement of electrons — an electric current — that gives the whole window a negative charge. Positive ions then move through the material to restore the electrical balance, creating the color-changing effect. But while electrons flow rapidly through materials, ions move much more slowly, limiting the overall reaction speed.
The MIT team overcame that by using sponge-like materials called metal-organic frameworks (MOFs), which can conduct both electrons and ions at very high speeds. Such materials have been used for about 20 years for their ability to store gases within their structure, but the MIT team was the first to harness them for their electrical and optical properties.
