Scientists develop paintable window coating using photonic crystals

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University of California, Berkeley Lab scientists are developing a polymer heat-reflective coating that can be painted onto windows to improve their energy efficiency. The coating, which contains photonics crystals to reflect light selectively at different frequencies, could reduce the need for heating and air-conditioning and therefore decrease carbon emissions.

The Berkeley team is receiving part of a $3.95 million award from the US Department of Energy’s Advanced Research Projects Agency­–Energy (ARPA-E) to develop this product. The multi-institutional team includes scientists from the University of Colorado Boulder, Caltech, and Netherlands-based materials company, Materia.

Although energy efficient window coatings exist commercially, they require a professional contractor to install them, a barrier for many building owners. The current market cost for these commercially-installed retrofit window coatings is about $15 per square foot; Berkeley scientists are trying to reduce this to $1.50.

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 five million cars off the road.

Berkeley Lab’s paintable window coating is based on brush block copolymers that rapidly self-assemble to photonic crystals, which are easily tunable across the entire spectrum of solar energy. The material is called a bottlebrush polymer, and has an unusual molecular architecture consisting of one main rigid chain of molecules with bristles coming off the sides, which lends it some unique properties, one being that it doesn’t entangle easily.

‘Imagine spaghetti versus gummy worms,’ explained Berkeley Lab scientist Raymond Weitekamp. ‘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 to commercialise these coatings and other related polymer-based technologies.

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.

‘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,’ said Berkeley Lab’s Steve Selkowitz, a leading expert on building science and window technologies. ‘In the development phase, all that optical testing becomes a feedback loop to the chemistry.’

To develop a cost-effective and scalable product, Weitekamp will also be working with Arman Shehabi, an expert in analysing energy use of buildings, who 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 maximised. He’ll also use technoeconomic models to look at things like manufacturing considerations and payback period.

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Further information 

Advanced Research Projects Agency­–Energy (ARPA-E) funding for energy efficient window coatings