Think of this new tech as sunglasses for our windows

In hot parts of the world, it holds the potential to dramatically cut energy use

This clear window coating keeps rooms bright and cool. Most visible sunlight passes right through, as shown in the top left. At the same time, the coating absorbs invisible, heat-raising sunlight and radiates it into outer space.

Adapted from S. Kim et al/ACS Energy Letters 2022

This is another in our new series identifying technologies and actions that can slow climate change, reduce its impacts or help communities cope with a rapidly changing world.

It takes a lot of energy to keep buildings comfortably cool in hot parts of the world. Researchers hope to reduce that energy use with a new window coating. Unlike air conditioners and fans, it doesn’t need power to work.

As it comes through windows, sunshine is a big source of heat in buildings, explains Tengfei Luo. He hopes to change that. Luo is an engineer at the University of Notre Dame in Indiana. His team has just designed the new coating. They describe it in the Dec. 9, 2022 ACS Energy Letters.

Sunlight contains both the visible light that we can see ― and the light our eyes can’t see. Those unseen hues fall in the ultraviolet and near infrared wavelengths. All three types can pass through glass. Windows need to let visible light through, but not the other two, explains Luo. “Except for heating up the room, they really don’t do anything,” he says.

Luo and his team set out to design a clear coating that blocked the warming wavelengths. But they didn’t just want to keep it out of buildings. They wanted to keep it from contributing to climate change. These wavelengths of light can’t leave Earth’s atmosphere, so they end up heating Earth one way or another. (Other wavelengths can escape into space.)

The ideal coating would do three things. First, it should let in as much visible light as possible. At the same time, it should block all the ultraviolet and near-infrared light it can. Finally, it should radiate any heat energy at wavelengths likely to escape back into space.

Ultrathin layers of materials like silicon dioxide and titanium dioxide can interact with light in ways that help, says Luo. But none could do it all. So his team decided to layer such materials, like a stack of pancakes. But how many layers should there be — and in what pattern? This layering changes how the structure interacts with different wavelengths of light, explains Luo.

His group theorized that more layers would up their chance of optimizing the brightness of light coming in. That should keep heat out. But knowing how to optimize the layer and ordering of layers can get really complicated as the number of layers increases. Unless, that is, you have a quantum computer.

Quantum computers work with information in ways that traditional computers can’t. That lets them solve some types of problems — like this one — quite fast.

the side of a skyscraper is completely covered in windows
Windows count for a quarter of all of the energy used to heat and cool windows. To lower that energy, you could reduce the windows in a structure. Another option: Cut the energy moving into or out of window glass.Witthaya Prasongsin/Moment/Getty Images Plus

After considering billions of options …

The team chose four promising materials. Then they set up the problem like this: Stack those materials in very thin layers, and in any order. There can be up to 24 layers. Among all possible arrangements, tally which worked best at keeping heat out but letting in visible light?

It turns out there were hundreds of billions of possible arrangements, says Luo. A traditional computer would take millions of years to evaluate them. And a quantum computer: It can evaluate them in “just a fraction of a second,” Luo says. 

But it wasn’t enough to look at each possible arrangement once. “Finding the best solution is a bit like finding the smallest piece of sand on a beach,” Luo says. “As you find smaller and smaller sand, you need more precise rulers.”

Here, that “ruler” was a computer model that gets more accurate with more rounds of measurements. It can take thousands of cycles of measuring and refining the program to get to an answer, explains Luo. The team had its blueprint for the coating after about two days of computing.

They deposited that coating, layer by layer, on a glass slide. At the very top they added one last material, called PDMS. This layer radiates heat energy at wavelengths that can travel through Earth’s atmosphere. A bit of that light might still be absorbed by objects on Earth. Most, however, will reach space, says Luo.

The result was a surprisingly clear coating, his team found. Despite a mild orange tint, it was highly transparent to visible light.

New coating requires no fancy techniques

The coating uses common, inexpensive materials. It requires no exotic techniques. So it could be bonded to new windows as they’re made or possibly added to existing ones.

Luo’s team sent a sample of the window to Phoenix, Arizona, for testing. After a day and a half in that hot climate, it kept a test chamber 6 degrees Celsius (10.8 degrees Fahrenheit) cooler than an untreated window. Simulations suggest that in hot climates, this new coating could cut the energy used to cool buildings by close to one-third. (They haven’t tested how the coating would affect homes during cold winters.)

Luo sees potential for widespread use of this glass coating in hot climates.

To seriously address climate change, energy-efficient buildings are a must.

Windows can be part of that solution, according to Thomas Culp. He wasn’t involved in this research. But he works as an independent engineer who consults to the National Glass Association. It takes more energy to maintain comfortable temperatures in buildings with windows, he points out. Right now, he notes, “Windows account for 25 percent of the energy used to heat and cool buildings.”

Culp calls for researchers to tackle this problem with new technology, as Luo’s team did. Their work is an example of “using modern computer tools to design advanced materials that can help reduce energy use in buildings,” he says. That could ultimately reduce the carbon emissions that contribute to climate change, he says.

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