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charon writes:

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.

[...] So they [Xiaobo Yin and his team] 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.

An abstract of the paper is available online.

Yao Zhai, et al. Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling, Science, 09 Feb 2017, DOI: 10.1126/science.aai7899

Original Submission