Transparent film cools buildings by rejecting 70 per cent of solar heat

A transparent film that can be placed over windows to reflect up to 70 per cent of the sun’s incoming heat has been developed by MIT engineers.

To battle the summer heat, office and residential buildings tend to crank up the air conditioning, sending energy bills soaring. It’s estimated that air conditioners currently use about 6 per cent of all the electricity produced in the United States, at an annual cost of $29bn.

The new heat-rejecting film is able to remain highly transparent below 32 degrees Celsius but when the temperature rises beyond this acts as an autonomous system to reject heat.

The researchers estimate that if every exterior-facing window in a building were covered in this film, the building’s air conditioning and energy costs could drop by 10 per cent.

The film is similar to transparent plastic wrap, and its heat-rejecting properties come from tiny microparticles embedded within it.

These microparticles are made from a type of phase-changing material that shrinks when exposed to temperatures of 30 degrees Celsius or higher. In their more compact configurations, the microparticles give the normally transparent film a more translucent or frosted look.

Applied to windows in the summer, the film could passively cool a building while still letting in a good amount of light.

Nicholas Fang, a professor of mechanical engineering at MIT, says the material provides an affordable and energy-efficient alternative to existing smart window technologies.

“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,” he said. “We thought there might be room for new optical materials and coatings, to provide better smart window options.”

After some research into reducing the energy consumption of buildings, Fang’s team found that a significant portion of a building’s heat comes through windows, in the form of sunlight.

“It turns out that for every square metre, about 500 watts of energy in the form of heat are brought in by sunlight through a window,” Fang said. 

The team uses a material made from poly (N-isopropylacrylamide)-2-Aminoethylmethacrylate hydrochloride microparticles.

These microparticles resemble tiny, transparent, fibre-webbed spheres and are filled with water. At higher temperatures the spheres essentially squeeze out all their water and shrink into tight bundles of fibres that reflect light in a different way, turning the material translucent.

“It’s like a fishnet in water,” Fang says. “Each of those fibres making the net, by themselves, reflects a certain amount of light. But because there’s a lot of water embedded in the fishnet, each fibre is harder to see. But once you squeeze the water out, the fibres become visible.”

In previous experiments, other groups had found that while the shrunken particles could reject light relatively well, they were less successful in shielding against heat. Fang and his colleagues realised that this limitation came down to the particle size: The particles used previously shrank to a diameter of about 100 nanometres—smaller than the wavelength of infrared light—making it easy for heat to pass right through.

Instead, Fang and his colleagues expanded the molecular chain of each microparticle, so that when it shrank in response to heat, the particle’s diameter was about 500nm, which Fang says is “more compatible to the infrared spectrum of solar light.”

The researchers created a solution of the heat-shielding microparticles, which they applied between two sheets of 12-by-12-inch [30x30cm] glass to create a film-coated window. They shone light from a solar simulator onto the window to mimic incoming sunlight and found that the film turned frosty in response to the heat. When they measured the solar irradiance transmitted through the other side of the window, the researchers found the film was able to reject 70 per cent of the heat produced by the lamp.

Going forward, the team plans to conduct more tests of the film to see whether tweaking its formula and applying it in other ways might improve its heat-shielding properties.

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