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US engineers develop cooling ‘metamaterial’

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The engineered glass-polymer hybrid material can cool roofs and other structures with zero energy consumption.

Engineers at the University of Colorado Boulder have developed a scalable manufactured metamaterial to act as a kind of air conditioning system for structures, that has the ability to cool objects 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 space while simultaneously allowing the surface to shed its own heat in the form of infrared thermal radiation.

The 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 50μm thick and can be manufactured economically on rolls, making it a potentially viable large-scale technology for both residential and commercial applications.

Xiaobo Yin, co-director of the research, said: “We feel that this low-cost manufacturing process will be transformative for real-world applications of this radiative cooling technology.”

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 reflect any incoming solar rays back into the atmosphere while still providing a means of escape for infrared radiation. To solve this, the researchers 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.

Gang Tan, an associate professor in the University of Wyoming’s department of civil and architectural engineering, said: “Just 10-20m2 of this material on the rooftop could cool down a single-family house in summer.”

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.

The engineers have applied for a patent for the technology and are exploring potential commercial applications. They plan to create a 200m2 ‘cooling farm’ prototype in Boulder in 2017.

Ronggui Yang, a professor of mechanical engineering at the university, said: “The key advantage of this technology is that it works 24/7 with no electricity or water usage. We’re excited about the opportunity to explore potential uses in the power industry, aerospace, agriculture and more.”

The study was published in the journal Science.
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