This smart window uses electricity to quickly change from clear to dark

In a world where even mundane devices like water bottles and toothbrushes have become smart and connected, it’s easy to scoff at yet another attempt at smartifying practical gadgets. But new technology for smart windows, described today in the journal Joule, might actually be an intelligent idea. That’s thanks to their energy-saving potential: Dynamic windows that darken when the sun is shining on them could help reduce cooling costs in the summer, just like putting down the blinds does. And with the help of an internet connection and an intelligent schedule, these could be automatic, so you wouldn’t have to remember when to turn them on and off.

Windows that tint on demand already exist—one prominent class of them is known as electrochromic windows. In fact, if you’ve flown on a Boeing 787 Dreamliner, you’ve seen electrochromic windows on the fuselage, which dim with a button’s press, either by the passenger or the flight crew. But a group of researchers at Stanford are aiming for a better, more dynamic one: Their prototype goes from transparent to opaque in less than three minutes. And it does it using an innovative approach.

Electrochromic technology in general isn’t perfect, says Ioannis Kymissis, an associate professor of electrical engineering at Columbia University. “They’re not amazing, but they’re not terrible,” he says. “For privacy applications, they’re not as high performing as people would like.” In other words, they might not get dark enough to be totally opaque, and the transition time can be slow. (One brand, SageGlass, says their windows take between 7 to 12 minutes to transition.)

But this new approach is clever, says Kymissis, who wasn’t involved in the new research. Their method involves using ions from two metals, like copper and silver, in an electrolyte gel on the window. The glass also needs to have indium tin oxide in it, a transparent conductor that’s ubiquitous in television and smartphone screens. By applying a negative electrical voltage, the window becomes dark because the ions form elemental, solid metal, which is opaque. A positive voltage causes the metal to dissolve back into ion form, allowing the light to come through.