BERKELEY, CA - It’s estimated that 10 percent of all the energy used in buildings in the United States can be attributed to window performance, costing building owners about $50 billion annually, yet the high cost of replacing windows or retrofitting them with an energy efficient coating is a major deterrent. U.S. Dept. of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) researchers are seeking to address this problem with creative chemistry — a polymer heat-reflective coating that can be painted on at one-tenth the cost.

“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.”

A team of Berkeley Lab scientists is receiving part of a $3.95 million award from the Department of Energy’s Advanced Research Projects Agency­–Energy (ARPA-E) to develop this product. The multi-institutional team is led by researcher Garret Miyake at the University of Colorado Boulder, and also includes Caltech and Materia Inc.

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. 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.”

As a graduate student at Caltech, Weitekamp worked on understanding and controlling how bottlebrush polymers self-assemble into nanostructures behaving as photonic crystals, which can selectively reflect light at different frequencies. Last year he came to Berkeley Lab as part of Cyclotron Road, a program for entrepreneurial researchers, to commercialize these coatings and other related polymer-based technologies. He has been working on the development of polymeric materials as a user at the Molecular Foundry, a DOE Office of Science User Facility at Berkeley Lab.

“We were very compelled by the potential impact of [Weitekamp’s] technology across a number of industries,” said Cyclotron Road Director Ilan Gur. “His ideas aligned with the Foundry’s expertise in polymer chemistry, and the window application fit squarely into Berkeley Lab’s existing strengths in buildings technology and energy analysis.”

For the ARPA-E award, 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.

“ARPA-E invests in high-risk, high-reward projects,” Shehabi said. “The high reward in this project isn’t in the performance improvement. It’s transformative in how windows could be retrofitted—it’s something you can do yourself. The market need is very large, and there’s nothing low-cost out there that meets that need.”

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.

“We have a well-equipped optics lab where we can do detailed optical measurements of any coating on any glass substrate, looking at the optical and spectral properties, which can provide feedback to the chemical synthesis process,” Selkowitz said. “In the development phase, all that optical testing becomes a feedback loop to the chemistry. 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 lifecycle 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.

“We didn’t have any need to use particularly cost-effective materials and feedstocks when we were making this in the lab, but to scale this up, we’ll have to think through the technoeconomics,” Weitekamp said. “This was originally an exploratory synthetic chemistry project, but having the deep windows and building expertise on the applied side here at Berkeley Lab, we thought, we can do this in a bigger and better way.”