A major advance in daytime radiative cooling.

The past decade has seen a recasting of air conditioning as an ironic villain. This technology that we use to live in defiance of heat is itself, as a nearly unparalleled energy hog, contributing to the very heat it exists to dispel. We turn up the AC and the climate responds. According to the United States Department of Energy, air conditioning in the United States accounts for 117 million metric tons of carbon dioxide released into the atmosphere every year.

As described in the current issue of Science, researchers at the University of Colorado and the University of Wyoming have developed a new metamaterial (a material engineered to have extraordinary properties) that offers at least a partial potential solution in the form of daytime radiative cooling. That's the process by which incoming thermal energy from the Sun is exchanged for outgoing energy in the form of infrared radiation. 

While efficient nighttime radiative cooling systems are pretty reasonable, achieving the same thing during daylight hours has been hampered by a fundamental problem: Absorbing even just a few percent of the incoming radiation from the Sun easily washes away any potential cooling benefits. What's needed is a material that strongly emits infrared radiation, but just barely absorbs energy from the Sun.  

Materials scientists have accomplished this previously by using very complicated and difficult-to-produce nanomaterials. As the current paper explains, these prior attempts are all hampered by the fact that they require exotic and impractical fabrication techniques. The challenge here was to make something that could actually be scaled to real-world use. 

The resulting material is composed of a layer of visibly transparent polymers randomly embedded with tiny spheres of glass and then covered over by a thin layer of silver. Basically, incoming light of many different wavelengths gets caught up in and then reemitted by the spheres. The randomization of these spheres is part of what accounts for the wide range (96 percent) of reflectivity across the spectrum of sunlight.