Smart Windows

The natural light that streams through a sunny window is great — until you’re squinting and wiping sweat off your brow. You can shut the shades or turn on the air conditioning, but you’re left with a dark room or a rising electric bill. 

A solution to this conundrum was thought up 40 years ago: smart windows, tunable to your preferences. By using special materials that block specific wavelengths of light, these windows adapt to the weather or to your personal comfort. Too hot? The windows can become tinted to block light and keep rooms cooler. Too cold? The tint disappears, letting warm natural light heat your space. Want some privacy? Some glass can cloud at the flick of a switch. 

Over the past few decades, advancements in smart window research have moved the tech well beyond the lab: The market for smart glass is expected to reach $7.5 billion by 2028. Types of smart windows can already be found in boats, cars and airplanes and are making their way into buildings such as offices and airport terminals. And, assuming they overcome a few key challenges, they could play an important role in making homes more energy efficient one day, proponents say.

Buildings account for a whopping 39 percent of energy consumption in the U.S., and 35 percent of that is associated with heating, ventilation and air conditioning. Because smart windows can selectively block heat or let it in, they can bring down these energy needs: Roughly 35 percent of a typical building’s energy is lost through windows.

But for smart windows to go mainstream — and trim everyone’s energy use — the price needs to go down. Current devices can cost up to 10 times the price of standard energy-efficient windows. Making the tech cheaper, and maybe even smarter, are areas of keen interest for researchers.

In the future, says materials physicist Claes-Göran Granqvist, “there’s no reason really to have any other windows than these smart windows.”

Smart windows originated long before phones and televisions were considered “smart.” In the early 1980s, scientists from Chalmers University of Technology in Sweden and the Lawrence Berkeley National Laboratory in California were brainstorming new ways of making energy-efficient building materials. The researchers came up with the idea for a responsive window that would dynamically change its tint. Granqvist, who was part of that early research, employed the phrase “smart windows” on a grant application. The name stuck — and the first smart window came to fruition in 1984.

The original prototypes spanned mere centimeters and featured glass that reversibly changed from transparent to darkened, a feat accomplished by sandwiching together glass, several layers of materials and transparent conductors that supplied small jolts of electricity. The voltage altered how the materials interacted with light — changing the wavelengths that were reflected or absorbed.

As the tech advanced, researchers explored other substances that manipulate light in response to other prompts, such as heat, ultraviolet light and magnetism. Today, a range of special “smart materials” is used, and researchers continue to investigate new ones.

Those early prototypes used “electrochromic” materials, meaning they reversibly change in response to electrical current or voltage. Electrochromic windows typically feature five layers overall, including two layers that serve as electrodes (like the positive and negative poles of a battery) and an inner electrolyte layer that contains ions. When voltage is applied, positively charged ions are driven into one layer while electrons move into the other. The reaction creates a tint that blocks some visible light as well as the heat-packed infrared light. The tint remains until another round of voltage triggers a reverse reaction that extracts the electrons and ions, thus turning the window transparent once again.

Smithsonian