US researchers have developed a metamaterial they say could be used to help cool buildings without the use of energy or water.

The glass-polymer hybrid film, developed by engineers at the University of Colorado Boulder, cools objects it is applied to by reflecting incoming solar energy while also allowing the surface to emit its own heat in the form of infrared thermal radiation.

While passive thermal radiation often occurs during the night, during the daytime a small amount of solar energy would be enough to negate any cooling effect.

So the researchers worked to create a material that would enable thermal radiation during the day, in the form of a film that would both reflect solar rays back into the atmosphere while also allowing infrared radiation a means of escape.

The solution was to embed “visibly scattering but infrared-radiant glass microspheres into a polymer film”. A thin silver coating was also added underneath to “achieve maximum spectral reflectance”.

While it isn’t the first example of using radiative cooling during the day (see High-tech mirrors could slash cooling needs), what makes the research particularly exciting is that the film is just 50 micrometres thick and can be economically manufactured on rolls, increasing the potential for a large-scale rollout.“We feel that this low-cost manufacturing process will be transformative for real-world applications of this radiative cooling technology,” co-director of the research Xiaobo Yin said.

The product could be applied on everything from residential buildings, to commercial buildings, data centres and power stations.

“Just 10 to 20 square metres of this material on the rooftop could nicely cool down a single-family house in summer,” co-author Gang Tan said.

It could also help improve the efficiency of solar panels.

“Just by applying this material to the surface of a solar panel, we can cool the panel and recover an additional one to two percent of solar efficiency,” professor Yin said. “That makes a big difference at scale.”

Another co-author, Ronggui Yang, said the key advantage was that the technology required no energy or water.

“We’re excited about the opportunity to explore potential uses in the power industry, aerospace, agriculture and more,” Dr Yang said.

The engineers have applied for a patent and are currently exploring potential commercial applications.

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