Intense summer days with the sun beating down on you call for air conditioning at homes and offices but result in high utility bills which can have a devastating impact on your wallet. The use of air conditioners not only increases fuel consumption which releases carbon dioxide in the air, but also contributes to global warming due to the greenhouse gases emitted by the hydrofluorocarbons present in them.
Scientists at the University of Colorado (CU) Boulder have engineered a material that can cool down buildings without any energy consumption. The material, which has been made in the form of a thin film, works in two ways: it can reflect sunlight falling on its surface back into the atmosphere while also purging the heat of the surface it is placed on. The film has the potential to cool surfaces such as rooftops by as much as 10°C.
As described in the journal Science, researchers made the material by embedding visibly-scattering but infrared-radiant glass microspheres into a polymer film. A layer of silver coating was added underneath in order to get maximum spectral reflectance. The hybrid material is only 50 micrometers thick – equivalent to the thickness of a sheet of aluminum foil that is commonly used to wrap food in.
With the silver layer reflecting all the sunlight that falls on it, the glass-polymer hybrid material is able to make use of the principle of passive radiative cooling – the process by which a body loses energy in the form of long wave infrared radiation. Certain wavelengths of infrared radiation can escape from the earth’s atmosphere without any hindrance. Keeping this in mind, the researchers had added glass spheres with a diameter of eight microns to the material. These microspheres convert the energy being emitted from an object such as a rooftop into the right wavelength which can then pass through the atmosphere. This has the added effect of cooling the object.
The metamaterial made by the team of researchers has a radiative cooling power of 93 watts per square metre under direct sunshine and is even more effective at night.
According to the press release, the hybrid material could be used for the cooling of thermoelectric power plants, buildings, houses and even solar panels.
“Just by applying this material to the surface of a solar panel, we can cool the panel and recover an additional one to two percent of solar efficiency,” said Xiaobo Yin, co-director of the research and an assistant professor at CU Boulder. “That makes a big difference at scale.”
The film can be made using roll-to-roll manufacturing methods at a cost of only 50 cents per square metre, making it both economical and easy to produce.
The research team have already applied for a patent for their technology. They are also working with CU Boulder’s Technology Transfer Office to explore potential commercial applications for it in the power industry, aerospace and agriculture among others.
The film was developed as a result of a $3 million grant given to the research team in 2015 by the US Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E) which invests in high-potential and high-impact energy technologies that are too early for private-sector investment.