Glass-polymer hybrid cools in the sun

Materials World magazine
13 Feb 2017
The material can keep buildings warm by reflecting sunlight. Image: University of Colorado at Boulder

A new metamaterial film could provide a passive 'air conditioning' system.

Engineers at the University of Colorado Boulder, USA, have developed a scalable glass-polymer hybrid metamaterial that acts as an 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 film cools the object underneath by efficiently reflecting incoming solar energy while allowing the surface to shed its own heat in the form of infrared thermal radiation. It could provide an eco-friendly means of supplementary cooling for thermoelectric power plants, which currently require large amounts of water and electricity to ensure machines maintain operating temperatures.

The researchers' glass-polymer hybrid material measures just 50 micrometres thick 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, co-director of the research, published in Science.

The material makes use 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 night-time cooling and is used for residential cooling in some areas, but daytime cooling has historically been more difficult. For a structure exposed to sunlight, even a small amount of directly absorbed solar energy is enough to negate passive radiation.

The challenge was to create a material that could reflect incoming solar rays 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 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, also a professor of mechanical engineering and a Fellow of the American Society of Mechanical Engineers.

‘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 in the University of Wyoming's Department of Civil and Architectural Engineering and a co-author of the paper.

As well as cooling buildings and power plants, the material could also help improve the efficiency and lifetime of solar panels, which can overheat to temperatures that hamper their ability to convert solar rays into electricity.