A team of University of Colorado Boulder engineers has developed a scalable manufactured metamaterial - an engineered material with extraordinary properties not found in nature - to act as a kind of air conditioning system for structures. It has 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 space while simultaneously allowing the surface to shed its own heat in the form of infrared thermal radiation.
The new material, which is described today in the journal Science, 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 researchers' glass-polymer hybrid material measures just 50 micrometers thick - slightly thicker than the aluminum 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.
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 CU Boulder researchers, then, 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, 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.
During field tests in Boulder, Colorado and Cave Creek, Arizona, the metamaterial successfully demonstrated its average radiative cooling power larger than 110W/m2 for continuous 72 hours and larger than 90W/m2 in direct, noon-time sunlight. That cooling power is roughly equivalent to the electricity generated using solar cells for similar area, but the radiative cooling has the advantage of continuous running both day and night.