In a sunny Shanghai test, a small enclosure behind a new smart window stayed about 3.4 degrees Celsius cooler during the day than one covered with quartz glass. The window did more than shade the room: a coating on its outer surface helped heat escape, while a silver layer could switch the pane from clear to opaque.
Windows bring daylight indoors, but they can also let in heat that makes rooms uncomfortable and increases the need for air conditioning. Simply darkening glass is not always enough. Dark surfaces can absorb sunlight and warm up. The researchers set out to combine several jobs in one design: keep much of the sun’s heat out, let trapped heat radiate away, and offer adjustable privacy.
The team from the Chinese Academy of Sciences, Northeast Forestry University and collaborating institutions built a layered window with a thin, changeable silver film. When a small voltage is applied, silver moves from a gel and forms a reflective layer on an inner surface. The pane becomes opaque, adding shade and privacy. Reversing the voltage removes the silver and restores transparency.
Even when the window is clear, its layers reflect a substantial share of near-infrared sunlight. These rays are not visible to the eye, but they carry heat. A silicone coating on the outside lets heat in longer infrared wavelengths escape toward the atmosphere. That heat-releasing layer keeps working whether the silver film is switched on or off.
In one design, visible-light transmission dropped from about 44 percent to below 5 percent when the pane turned opaque. At the same time, reflection of near-infrared light rose from roughly 54 percent to 83 percent. The glass can also be made in different tints. By changing layer thickness during production, the researchers created light yellow, pale cyan, gray-purple and rose-red samples. Those colors are built into the design; the window cannot change to any color on demand.
Outdoor tests in Shanghai compared small enclosures covered with the prototype, quartz glass and commercial smart windows. In one test, the prototype’s enclosure averaged about 3.4 degrees Celsius cooler than the quartz-glass enclosure during daylight, with the largest gap reaching about 7 degrees. In a separate model-house test under simulated sunlight, the prototype kept the interior an average of 4.3 degrees cooler than quartz glass when clear, and 5.9 degrees cooler when opaque.
The results are promising, but they come from small experimental structures, not occupied buildings. The researchers also found little change in measured reflectance after 350 switching cycles, though that does not establish how the window would hold up over years outdoors. Its heat-releasing feature is designed for windows that face the sky. Vertical windows may need a changed surface to send more heat in that direction.
That makes skylights a natural place to explore the idea first. They admit daylight from above, where strong sun can also bring unwanted heat. A single pane that shifts between clear daylight and stronger shade, while helping release heat, could offer building designers another way to balance comfort and energy use. The study was published in Nano Letters in 2026; real-world building trials will show how well the design performs at larger scale.