US Dept. of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) scientists are developing a new paint-on coating that will help building owners improve the energy efficiency of their windows, without the need for trained installers.
“Instead of hiring expensive contractors, a homeowner could go to the local hardware store, buy the coating, and paint it on as a DIY retrofit—that’s the vision,” said Berkeley Lab scientist Raymond Weitekamp. “The coating will selectively reflect the infrared solar energy back to the sky while allowing visible light to pass through, which will drastically improve the energy efficiency of windows, particularly in warm climates and southern climates, where a significant fraction of energy usage goes to air conditioning.”
There are retrofit window films on the market now that have spectral selectivity, but a professional contractor is needed to install them, a barrier for many building owners and homeowners. A low-cost option could significantly expand adoption and result in potential annual energy savings of 35 billion kilowatt-hours, reducing carbon dioxide emissions by 24 billion kilograms per year, the equivalent of taking 5 million cars off the road.
The Berkeley Lab technology relies on a type of material called a bottlebrush polymer, which, as its name suggests, has one main rigid chain of molecules with bristles coming off the sides. This unusual molecular architecture lends it some unique properties, one being that it doesn’t entangle easily.
“Imagine spaghetti versus gummy worms,” Weitekamp explained. “Spaghetti can be tied up in knots. If you want to rearrange cooked spaghetti back to its uncooked alignment, you would have to put significant energy into unwinding it. But with gummy worms you can line them all up easily because they’re pretty rigid.”
Weitekamp is collaborating with Berkeley Lab’s Steve Selkowitz, a leading expert on building science and window technologies, and Arman Shehabi, an expert in analyzing energy use of buildings, to develop a cost-competitive and scalable product. Their target cost is $1.50 per square foot, one-tenth the current market cost for commercially installed energy efficient retrofit window coatings.
One of the technical challenges remaining is to improve the fidelity of the material, so that while infrared light is strongly reflected, visible light is not scattered or hazy. This will allow the coating to reflect the majority of the sun’s energy, reducing the amount of heat passing into a building, while still appearing clear to the eye. Taking advantage of the cutting-edge windows testing facilities at Berkeley Lab, Selkowitz will be analyzing the performance of the coating.
“In the development phase, all that optical testing becomes a feedback loop to the chemistry,” Selkowitz said. “Additionally we can model and measure thermal comfort, which is important because what will motivate people to buy this coating is comfort in addition to energy savings.”
Shehabi will be developing building simulation models and life cycle assessment models to understand how this technology would impact energy use in buildings and how energy savings could be maximized. He’ll also use technoeconomic models to look at things like manufacturing considerations and payback period.