If heat is not your thing, rejoice: A thin plastic sheet may soon provide some relief from the intense summer sun. The film, made from transparent plastic embedded with tiny glass spheres, absorbs almost no visible light, yet pulls in heat from any surface it touches. Already, the new material, when combined with a mirrorlike silver film, has been shown to cool whatever it sits on by as much as 10°C. And because it can be made cheaply at high volumes, it could be used to passively cool buildings and electronics such as solar cells, which work more efficiently at lower temperatures.
During the day most materials—concrete, asphalt, metals, and even people—absorb visible and near-infrared (IR) light from the sun. That added energy excites molecules, which warm up and, over time, emit the energy back out as photons with longer wavelengths, typically in the midrange of the infrared spectrum. That helps the materials cool back down, particularly at night when they are no longer absorbing visible light but are still radiating IR photons.
In recent years, researchers have tried to goose this “passive cooling” effect by making materials that absorb as little visible light as possible yet continue to emit mid-IR light. In 2014, for example, researchers led by Shanhui Fan, an electrical engineer at Stanford University in Palo Alto, California, created a sandwichlike film of silicon dioxide (glass) and hafnium dioxide that reflected almost all the light that hit it while strongly emitting mid-IR light, a combination that allowed it to cool surfaces by as much as 5°C. Still, Fan and his colleagues had to use clean room technology to make their films, a costly process that doesn’t work well on a large scale.
When Xiaobo Yin, a materials scientist at the University of Colorado in Boulder, saw Fan’s paper, he noticed the material worked in part by encouraging infrared photons to bounce back and forth between the layers of the film in a manner that made it a stronger IR emitter. Yin wondered whether there was a simpler way to do this. From previous work, Yin knew that spherical objects can act like tiny resonance chambers—much as the sound box of a guitar encourages sound waves of a particular frequency to bounce back and forth inside. He and his colleagues calculated that glass beads about 8 micrometers in diameter—little bigger than a red blood cell—would make powerful IR resonators and thus strong IR emitters.
So they bought a batch of glass powder from a commercial supplier and mixed it with the starting material for a transparent plastic called polymethylpentene. They then formed their material into 300-millimeter-wide sheets and backed them with a thin mirrorlike coating of silver. When laid across objects in the midday sun, the bottom layer of silver reflected almost all the visible light that hit it: The film absorbed only about 4% of incoming photons. At the same time, the film sucked heat out of whatever surface it was sitting on and radiated that energy at a mid-IR frequency of 10 micrometers. Because few air molecules absorb IR at that frequency, the radiation drifts into empty space without warming the air or the surrounding materials, causing the objects below to cool by as much as 10°C. Just as important, Yin notes that the new film can be made in a roll-to-roll setup for a cost of only $0.25 to $0.50 per square meter.
“This is very nice work demonstrating a pathway toward large-scale applications of the concept of radiative cooling,” says Fan, who did not work on the current project. Yin says that he and his colleagues are already working on one such application, chilling water that could then be used to cool buildings and other large structures. That could be particularly useful in electricity-generating power plants, where cooling water even a few degrees can increase energy production efficiency by a percentage point or two, a “big gain,” Yin says. And without the silver backing, he adds, the plastic film could also increase the power generation from solar cells, which operate more efficiently at lower temperatures.