Energy-Saving Windows Get Smarter

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The windows testing facility at Lawrence Berkeley National Lab. (Photo: LBNL)

The window testing facility at Lawrence Berkeley National Lab. (Photo: LBNL)

Windows may not be as sexy as solar panels or electric cars, but they play a major role in energy efficiency. Buildings are responsible for 40% of the country’s energy use, which is why researchers at Lawrence Berkeley National Laboratory are trying to improve windows by making them smarter.

As Berkeley Lab engineer Howdy Goudey demonstrates in his lab, studying windows involves some pretty complex physics.

“So we use an infrared camera to study heat transfer in windows,” he says, pointing to a normal-looking video camera that senses heat instead of visible light. Goudey uses the camera to study how windows lose energy.

For the most part, windows simply aren’t good insulators. They leak heat in the winter when we want a warm house and they let heat in during the summer. Many homes still have single-pane windows, which were the name of the game in the 1940s and 50s when California was booming.


That changed when energy prices sky-rocketed in the 1970s. Double-pane windows became common. And then came double-pane windows with invisible coatings, which are twice as efficient. Today, they make up more than half of windows sold.

Measuring Low-e Windows

Goudey demonstrates how they work by turning on two heat lamps. “You’ve seen them in a diner keeping food warm," he says, putting them behind two identical-looking double-pane windows.

We stand in front of one window, which feels like standing in the sun. “But if you hold your hand to other one, compared to this one, it’s very dramatic,” Goudey says.

An infrared image of two windows during winter conditions, as seen from the inside of a room. The window on the right has a low-e coating while the window on the left doesn't. Warmer temperatures mean a better insulating window. (Image: LBNL)

The second window is cooler because it has a low-emissivity coating, or low-e, as its known. It’s an invisible layer of metal on the glass that acts as an insulator. And it does one more thing.

When sunlight shines directly through a window, it provides both light and heat. Most of us want light coming in, but heat is the last thing we want on a hot summer day. So, the coating on the window blocks the heat from the sun (in the form of infrared light), while letting in the visible light. This is known as solar gain. (Check out this guide for more on what to look for when buying windows.)

“If you have a few windows in a room with direct sun on them, its equivalent to running a little space heater. So it’s significant energy,” says Goudey.

However, on a cold winter day, the extra heat from sun would be helpful. “You’d actually like that solar energy to come in and help heat the space,” he says.

That’s why researchers are working to develop a “smart” or dynamic window that can change based on the weather or temperature.

Using Nanotechnology to Make Windows Smarter

At Berkeley Lab’s Molecular Foundry, Delia Milliron grows tiny nanocrystals that will eventually become a window coating.

“Nanocrystals are very small,” says Milliron. “Way smaller than you can see with your eyes. And so that’s why when we spread them out in a coating on the window, you don’t see anything.”

Milliron’s coating is dynamic. In one setting, it lets in both the light and heat from the sun. But, apply an electric charge of a couple volts and the window blocks the heat from the sun, while still letting light in.

Ideally, these windows would be controlled by your heating and cooling system, which could adjust them based on the weather. Milliron and her team are currently working on the coating itself. Their next step is to build a full-scale prototype. Other companies also have similar kinds of dynamic windows in the works.

Windows as Energy Suppliers

This changes the conversation about windows, says Stephen Selkowitz, head of building technologies at Berkeley Lab. Before, windows were energy losers. Now, windows could actually make buildings more efficient. And that means big cost savings.

“If we add up all the energy and economic impact of windows in the US, it costs building owners about $40 billion a year. And I’d rather have the $40 billion in my pocket than sort of sending it out the window,” says Selkowitz.

Smart windows could start appearing in larger projects like office buildings next year and should be more widely available to homeowners in three to five years. But they could be twice as expensive as today's windows. Selkowitz expects the cost coming down as manufacturing ramps up.

“The biggest expense in replacing windows is often the labor of replacing the window. And if you already decided to put a new window in, the marginal cost of going to a much better window is almost always worth it,” he says.

So, while it may be only a few tech-geeks that spring for smart windows at first, Selkowitz says that leads the way for the rest of us – and for new buildings codes, where technology can have a much broader impact.