These Windows Tint With a Flick of a Switch

Stanford engineers are developing electric windows that block glare without blocking your view

A smart window prototype dims in response to electricity. Barile et al./Joule 2017

When the sun is beaming through your window and into your eyes, the solution nowadays is the same as it's long been—blinds. But what if instead, you could simply press a button and instantly make your window tinted? A new technology promises a future where that could be possible.

"I think it's going to be a game changer," Stanford University engineer Michael McGehee says of his dynamic windows. He compares how humans respond to glare now from windows as akin to being as crude as wearing glasses with small metal slits that can open or close to shield our eyes. Blinds may date back to ancient Persia, but the first models of them were developed in Europe in the 18th century. In 1841, it was American John Hampson who actually received a patent for a method to keep the slats of a blind in place by turning a rod or pulling a cord, creating blinds as we know them.

"So many people have beautiful views out of their offices and their homes and they lose their views" when they have to pull blinds or curtains over a gorgeous sunset or a bright, clear day, McGehee says.

Smart or dynamic glass that can go from transparent to tinted and back is not a new innovation—it's existed for decades in various forms, and has even made its way onto airplanes. But these existing smart windows haven't been perfect, McGehee says, noting that they are expensive, slow (often taking 20 minutes or more to fully tint, depending on their size) and the light passing through them is often tinted blue when they're transparent.

Photochromic lenses, which are often seen in the sunglasses than can "transition" from clear to tinted automatically, have also been around for a long time. Donald Stookey, the inventor of CorningWare, first patented these glasses in 1950, and they were released commercially starting in 1965. The lenses can easily and cheaply change in response to UV light, McGehee says, but they wouldn't make practical smart windows because users would have no control over when they wanted tint and when they didn't.

"The products that are available are almost good enough, but just not quite there," McGehee says. Inspired by the idea of a graduate from his lab who suggested he take this on, McGehee set off to see if he could improve things without knowing where he would end up.

In a recent study published in the journal Joule, McGehee describes windows that could make blinds a relic of the past. These windows use a mixture of transparent metals within glass that can conduct electricity pumped through it, moving ions around within the metals that can then block light in response to the electricity. The window glass appears as clear as normal glass in its dormant state, but flip a switch and with 30 seconds it goes dark, blocking up to 95 percent of light. They've tested this process more than 5,000 times on prototypes, and each time the glass has gone back and forth without a problem.

Energy-saving 'smart window'

McGehee and his collaborators have applied for a patent for their invention and are now working to improve it to make it marketable. So far, their smart window has only been tested at relatively small sizes of about four square inches, and they hope to scale it up to window sizes to see how fast it will switch back and forth. This can be an issue, since larger windows mean more ions that have to be moved around to block the light. They also are working with window manufacturers on how they could make this device, which hasn’t been produced yet commercially, cheap enough to be affordable for consumers.

More than just pure vanity, McGehee intends for his smart windows to save energy in homes and businesses by blocking UV light that can heat up a building in the summer, and letting it in in the winter. These electric windows could even be programmed to have their tint change automatically.

“From my perspective, what's interesting about this particular technology is that it cuts across a number of the different performance outcomes that we’re searching for when we talk about green building design,” Brendan Owens, an engineer who oversees ratings with the U.S. Green Building Council, said about dynamic window creations like McGehee’s invention. While he stresses that USGBC does not endorse or promote specific technologies in its quest to make buildings as environmentally efficient as possible, he likes dynamic windows because they can easily deal with a number of problems architects and engineers must contend with at once, including avoiding glare, providing adequate lighting and keeping the space comfortable, all with an easy to use interface for people actually living and working in the buildings.

“All those things make this technology an interesting evolution in the way we’re going to make buildings going forward,” he says.

McGehee also sees a lot to come in the near future.

"It's surprising how little most people know about this [technology]," McGehee says. "I think it's going to be spectacular."

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