Scientists have developed a thin, inexpensive material with extraordinary properties not found in nature - to act as a kind of air conditioning system for structures with the ability to cool objects even under direct sunlight with zero energy and water consumption.
When applied to a surface, the metamaterial film cools the object underneath by efficiently reflecting incoming solar energy back into the space while simultaneously allowing the surface to shed its own heat in the form of infrared thermal radiation.
The new material could provide an eco-friendly means of supplementary cooling for thermoelectric power plants, which currently require large amounts of water and electricity to maintain the operating temperatures of their machinery.
The glass-polymer hybrid material measures just 50 micrometres thick - slightly thicker than the aluminium foil found in a kitchen - and can be manufactured economically on rolls, making it a potentially viable large-scale technology for both residential and commercial applications.
"We feel that this low-cost manufacturing process will be transformative for real-world applications of this radiative cooling technology," said Xiaobo Yin, assistant professor at University of Colorado Boulder in the US.
The material takes advantage of passive radiative cooling, the process by which objects naturally shed heat in the form of infrared radiation, without consuming energy.
Thermal radiation provides some natural nighttime cooling and is used for residential cooling in some areas, but daytime cooling has historically been more of a challenge.
For a structure exposed to sunlight, even a small amount of directly-absorbed solar energy is enough to negate passive radiation.
The challenge for the researchers was to create a material that could provide a one-two punch: reflect any incoming solar rays back into the atmosphere while still providing a means of escape for infrared radiation.
To solve this, they embedded visibly-scattering but infrared-radiant glass microspheres into a polymer film. They then added a thin silver coating underneath in order to achieve maximum spectral reflectance.
"Both the glass-polymer metamaterial formation and the silver coating are manufactured at scale on roll-to-roll processes," said Ronggui Yang, a professor at CU Boulder.
"Just 10 to 20 square meters of this material on the rooftop could nicely cool down a single-family house in summer," said Gang Tan, an associate professor at University of Wyoming.
In addition to being useful for cooling of buildings and power plants, the material could also help improve the efficiency and lifetime of solar panels. In direct sunlight, panels can overheat to temperatures that hamper their ability to convert solar rays into electricity.
"Just by applying this material to the surface of a solar panel, we can cool the panel and recover an additional one to two per cent of solar efficiency," Yin added.
The research was published in the journal Science.
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