A heat-rejecting film was developed that could be applied to a building’s windows to reflect up to 70 percent of the Sun’s incoming heat. The film remains highly transparent below 32 °C (89 °F). Above that temperature, the film acts as an “autonomous system” to reject heat. If every exterior-facing window in a building were covered in this film, air conditioning and energy costs could drop by 10 percent.

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.

The coating rejects up to 70 percent of incoming solar heat. (Courtesy of MIT)

The thermochromic material is made from poly (N-isopropylacrylamide)-2-Aminoethyl methacrylate hydrochloride microparticles. These microparticles 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. When the molecular chain of each microparticle shrinks in response to heat, the particle’s diameter is about 500 nanometers, which is more compatible to the infrared spectrum of solar light.

A solution of the heat-shielding microparticles was applied between two sheets of 12 × 12” glass to create a film-coated window. A light was shown from a solar simulator onto the window to mimic incoming sunlight; the film turned frosty in response to the heat. When the solar irradiance transmitted through the other side of the window was measured, the film was able to reject 70 percent of the heat produced by the lamp.

A small calorimetric chamber was lined with the heat-rejecting film and the temperature inside the chamber was measured as a light was shone from a solar simulator through the film. Without the film, the inner temperature heated to about 102 °F. With the film, the inner chamber stayed at a more tolerable 93 °F.

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