Self-shading window shifts from transparent to opaque with new tech

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With summer heat comes summer air conditioning bills.

Air conditioning is crucial to people living in hot climates during the summer months, but it is an energy hog and can send electricity costs straight to the sun.

Installing awnings, or drawing curtains or blinds over windows, can block the sun and the heat that comes with it, but engineers are making progress in creating windows that can darken themselves.

So called "self-shading" windows have coatings that can change color when a condition changes — say when someone flips a switch. There are a few different varieties of these materials — some are photochromic, meaning they change when the light around them changes. Some brands of eyeglass lenses are made from this material. Others are thermochromic, meaning they change with heat (such as mood rings).

Then there are those that change when an electrical charge is applied, and these are useful because they can be changed on demand. Some companies market "privacy windows" that can turn from clear to a frosted glass with the flip of a switch.

But these electrochromic windows have some limitations, according to Mircea Dinca, a professor of chemistry at the Massachusetts Institute of Technology. They can take several minutes to change color, and current designs also cannot darken to a fully opaque shade.

But Dinca says he and a team of researchers have developed a design that can fully darken at a much faster speed.

Electrochromic windows are made up of several layers of different compounds. There is the electrochromic layer itself and a layer full of ions. When the voltage is applied, the ions move into the electrochromic layer and react with the material, forcing the color to change. This part of the process is what slows down the transition.

But the new design uses metal-organic frameworks, which can transmit the ions much faster, according to a news release. The team was also able to blend coatings of two different colors — green and red — that combine to make a shade that is almost completely impossible to see through, and different settings can produce various shades.

The team published its results Thursday in the journal Chem.

The research was funded by the Masdar Institute, based in the United Arab Emirates, and the U.S. Department of Energy.

Solutions such as these could greatly reduce the demand for air conditioning, which is currently booming around the world, as more countries become prosperous enough to afford systems. Americans spend $11 billion a year on air conditioning alone, and that releases 100 million tons of carbon emissions into the atmosphere.

The use of air conditioners can actually raise temperatures outside, contributing to the phenomenon known as urban heat island effect. This creates a kind of self-reinforcing cycle, where the heat from air conditioners is released outside, warming temperatures and furthering the need for air conditioning.

This phenomenon is exactly what is happening in cities such as New York and Abu Dhabi, where the Masdar Institute is based. The group has a collaboration with MIT to develop technologies that can reduce its energy needs.