MIT researchers have developed a coating for windows that rejects up to 70% of incoming solar heat. Photo courtesy of the researchers.
MIT researchers have developed a coating for windows that rejects up to 70% of incoming solar heat. Photo courtesy of the researchers.

To battle the summer heat, office and residential buildings tend to crank up the air conditioning, sending energy bills soaring. Indeed, it's estimated that air conditioners use about 6% of all the electricity produced in the US, at an annual cost of $29 billion – an expense that's sure to grow as the global thermostat climbs.

Now, engineers at Massachusetts Institute of Technology (MIT) have developed a heat-rejecting film that could be applied to a building's windows to reflect up to 70% of the sun's incoming heat. The film is able to remain highly transparent below 32°C (89°F). Above this temperature, the researchers say, the film acts as an ‘autonomous system’ to reject heat. They 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%.

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 85°F 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," Fang says. "We thought there might be room for new optical materials and coatings, to provide better smart window options."

Fang and his colleagues, including researchers from the University of Hong Kong, have reported their results in a paper in Joule.

Just over a year ago, Fang began collaborating with researchers at the University of Hong Kong. They were keen on finding ways to reduce the energy usage of buildings in the city, particularly in the summer months, when the region grows notoriously hot and air-conditioning usage is at its peak.

"Meeting this challenge is critical for a metropolitan area like Hong Kong, where they are under a strict deadline for energy savings," says Fang, referring to Hong Kong's commitment to reduce its energy use by 40% by the year 2025.

After some quick calculations, Fang's students found that a significant portion of a building's heat comes through the windows, in the form of sunlight. "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 says. "That's equivalent to about five light bulbs."

Fang, whose group studies the light-scattering properties of exotic, phase-changing materials, wondered whether such optical materials could be fashioned for windows, to passively reflect a significant portion of a building's incoming heat.

The researchers looked through the literature for ‘thermochromic’ materials – temperature-sensitive materials that temporarily change phase, or color, in response to heat. They eventually landed on microparticles of poly(N-isopropylacrylamide)-2-aminoethylmethacrylate hydrochloride, which resemble tiny, transparent, fiber-webbed spheres and are filled with water. At temperatures of 85°F or higher, the spheres essentially squeeze out all their water and shrink into tight bundles of fibers that reflect light in a different way, turning the material translucent.

"It's like a fishnet in water," Fang says. "Each of those fibers making the net, by themselves, reflects a certain amount of light. But because there's a lot of water embedded in the fishnet, each fiber is harder to see. But once you squeeze the water out, the fibers 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 realized that this limitation came down to particle size. The particles used in previous studies shrank to a diameter of about 100nm – smaller than the wavelength of infrared light – making it easy for heat to pass right through.

To prevent this happening, Fang and his colleagues expanded the molecular chain of each microparticle, so that in response to heat they only shrank to a diameter of 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 glass to create a film-coated window. They shone light from a solar simulator onto this 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% of the heat produced by the lamp.

The team also lined a small calorimetric chamber with the heat-rejecting film and measured the temperature inside the chamber as they shone light from a solar simulator through the film. Without the film, the inner temperature heated to about 102°F – "about the temperature of a high fever," Fang notes. With the film, the inner chamber stayed at a more tolerable 93°F.

"That's a big difference," Fang says. "You could make a big distinction in comfort."

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.

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.