This Sunlight Blocking Window Film Can Greatly Reduce Energy Costs

The summer is here, and offices and homes are cranking up the air-conditioning to beat the heat. The US Department of Energy estimates that air-conditioning uses six percent of all the energy produced and costs $29 billion annually to cool homes. There should be a way to cool buildings down without hurting the environment and our pockets.

Researchers at the Massachusetts Institute of Technology (MIT) may have found a way to do just that. That’s because they have developed a heat-rejection window film that reflects up to 70 percent of the sun’s heat.

The film is very similar to plastic wrap, according to an MIT news release, and the heat rejecting properties come from tiny microparticles that are embedded in the film. The film is transparent below 32C (89F), and if the temperature goes above that, the film acts to reject heat and passively helps to cool buildings.

The engineers estimate that if the new film covers every outside window, the building’s air-conditioning cost will go down by 10 percent. The window film provides a cost-efficient – and energy efficient – alternative to using smart windows according to  Nicholas Fang, a professor of mechanical engineering at MIT.

“Smart windows on the market currently are either not very efficient in rejecting heat from the sun, or, like some electrochromic windows, they may need more power to drive them, so you would be paying to basically turn windows opaque,” Fang said in the news release. “We thought there might be room for new optical materials and coatings, to provide better smart window options.”

In 2017, Fang and his team began working with researchers at the University of Hong Kong who were very interested in finding a way to reduce the energy use of buildings in the city’s hot summers. “Meeting this challenge is critical for a metropolitan area like Hong Kong, where they are under a strict deadline for energy savings,” said Fang, referring to Hong Kong’s commitment to reduce its energy use by 40 percent by the year 2025.

Fang’s engineering students calculated that a large amount of a building’s heat comes from sunlight entering through windows, “It turns out that for every square meter, about 500 watts of energy in the form of heat are brought in by sunlight through a window,” Fang said. “That’s equivalent to about five light bulbs.”

The researchers began by looking at thermochromic – temperature sensitive –  materials that turn colors when exposed to heat as a way to passively reflect a portion of a buildings incoming heat from sunlight. They created a solution of the heat shielding microparticles between two sheets of glass to essentially create a film-coated window, and the result was a film that shrinks and becomes opaque (frosty) when exposed to temperatures above 32C.

The researchers also tested a calorimetric chamber to measure the temperature difference between treated and untreated windows. There was almost a 13C difference which could be a distinction between having to run an air-conditioner or not. The research was published in the journal Joule.

The team is still doing further research to see if changing the formula will increase the heat shielding properties to produce more comfort and savings.  Right now, a ten percent energy saving makes sense for large office buildings. Hopefully, further research will make it more attractive for homeowners too.