A team of University of Colorado Boulder engineers has developed a scalable manufactured metamaterial—an engineered material with 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 no energy or water consumption.
When applied to a surface, the metamaterial film cools the object underneath by reflecting incoming solar energy back into space while also allowing the surface to shed its own heat in the form of infrared thermal radiation.
The material, which is described in the journal Science, could provide an eco-friendly means of supplementary cooling for thermoelectric power plants. These currently require large amounts of water and electricity to maintain the operating temperatures of their machinery.
The researchers’ glass-polymer hybrid material measures 50 micrometers thick—slightly thicker than the aluminum foil found in a kitchen—and can be manufactured 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 to achieve maximum spectral reflectance.
According to the research, 10 - 20 square meters of this material on the rooftop could cool a single-family house in summer.
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.
Applying this material to the surface of a solar panel, researchers believe that they can cool the panel and recover an additional 1-2% of solar efficiency.
The engineers have applied for a patent for the technology and are working with CU Boulder’s Technology Transfer Office to explore potential commercial applications. They plan to create a 200-square-meter “cooling farm” prototype in Boulder in 2017.