Even when shut, windows let in the sun’s light and warmth, which we usually welcome. But sunlight can fade our carpets or wake us up when we’d rather nap; the sun’s heat is sometimes too much in summer; and windows can be peeped into.
People have long handled these downsides with curtains, blinds and other window treatments. Some modern windows can block light and heat with the touch of a button, but such “smart windows” usually require electricity, making them expensive and hard to retrofit.
Now scientists at Princeton University have invented a technology aimed at smartening up the millions of windows already in place. They coated a piece of glass with a laminate containing transparent solar cells and a network of invisible electronics that will turn the panel dark blue on command, thereby blocking most of the sun. The result could be akin to sunglasses for your house, available whenever you want them. The scientists, who share a patent on the technology with Princeton, hope to make a version of their laminate into a sheet of film to be applied to the inside of existing windows, making them easier to retrofit.
A few similar films are already on the market, but they must be hard-wired or plugged into an electrical outlet, which can make installation complicated and leave windows and glass doors hard to open and close. Other scientists have developed transparent solar cells, but those usually produce power from the infrared portion of the electromagnetic spectrum, which can leave the cells less capable of controlling the entry or exit of heat through the glass. The Princeton team relied on the near-UV portion of the electromagnetic spectrum, which means that their window laminate works better as a thermal barrier.
The scientists say that such windows could cut our energy bills. They could also improve privacy with a version that would get so dark that people couldn’t see in or out. “This new technology is actually smart management of the entire spectrum of sunlight,” says Yueh-Lin Loo, a Princeton engineering professor who co-wrote a recent scientific paper on the research. The electrical charge generated by the solar cells darkens or lightens the laminate by causing a reaction in two polymer layers that are sandwiched around a conducting layer. Instead of requiring a continual flow of energy to keep the laminate dark or light, the system needs only an occasional jolt, she says. The invisible solar cells produce 10 times the energy needed to keep the panels dark or light or trigger a change, Dr. Loo says, and a small battery attached to the laminate should handle the job once the sun has set. She notes that the solar cells can still produce power when the laminate is set to dark, as long as they get at least some sun.
Windows that change color in response to an electric charge are known as “electrochromic.” Unlike “photochromic” glass, which makes the lenses in some eyeglasses automatically turn darker in the sun, electrochromic glass gives users the power to control the transparency—in this case, with a smartphone app that communicates with the laminate wirelessly via Bluetooth. That could also let users schedule their windows to shift from light to dark during the day or change them over remotely.
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