Energy-saving smart windows darken in under a minute

Stanford University engineers have developed dynamic windows that can switch from transparent to opaque or back again in under a minute, a significant improvement over dimming windows currently being installed to reduce cooling costs in some buildings.

The newly designed “smart” windows consist of conductive glass plates outlined with metal ions that spread out over the surface, blocking light in response to an electrical current. The results are described in a study published Aug. 9 in the journal Joule.

“We’re excited because dynamic window technology has the potential to optimize the lighting in rooms or vehicles and save about 20 percent in heating and cooling costs,” said Michael McGehee, a professor of materials science and engineering at Stanford and senior author of the study. “It could even change the way people wear sunglasses.”

The researchers have filed a patent for the new technology and have entered into discussions with glass manufacturers and other potential partners.

However, more research is needed to make the surface area of the windows large enough for commercial applications. The prototypes used in the study are only about 4 square inches (25 square centimeters) in size. The researchers also want to reduce manufacturing costs to be competitive with dynamic windows already on the market.  

“This is an important area that is barely being investigated at universities,” McGehee said. “There’s a lot of opportunity to keep us motivated.”

Fast and durable

Commercially available smart windows are made of materials, such as tungsten oxide, that change color when charged with electricity. But according to McGehee, these materials tend to be expensive, have a blue tint, can take more than 20 minutes to dim and become less opaque over time. 

The Stanford prototype blocks light through the movement of a copper solution over a sheet of indium tin oxide modified with platinum nanoparticles.

When transparent, the window is clear and allows about 80 percent of surrounding natural light to pass through. When dark, the transmission of light drops below 5 percent. It only takes about 30 seconds to change from transparent to dark or vice versa.

To test durability, the researchers switched the windows on and off more than 5,000 times and saw no degradation in the transmission of light.

“We’ve had a lot of moments where we’ve thought, how is it even possible that we’ve made something that works so well so quickly?”  McGehee said. “We didn’t tweak what was out there. We came up with a completely different solution.”

Other Stanford co-authors of the study are former postdoctoral scholar Christopher Barile (now at the University of Nevada-Reno), graduate research assistant Daniel Slotcavage, and graduate students Jingye Hou, Michael Strand and Tyler Hernandez.

The research was funded by the  Precourt Institute for Energy with additional support from Stanford Graduate Fellowships in Science & Engineering and the National Science Foundation Graduate Research Fellowship Program.

This article is based on a news release published by the media office of Cell Press/Elsevier.