As anyone who’s ever paid an electric bill knows, cooling is costly. The second law of thermodynamics dictates that energy can’t spontaneously flow from a colder object to a warmer one, all else being equal. Cooling machines such as air conditioners and refrigerators therefore require an energy input to create and sustain an inside temperature lower than that of the outside air.
The second law is inviolate, but it allows for some counterintuitive effects. Earth’s atmosphere—greenhouse gases and all—is nearly transparent to IR radiation between 8 and 13 µm, which also happens to be the peak wavelength range of thermal radiation at typical terrestrial temperatures. If a material radiates especially strongly into the atmospheric transparency window, it can shed thermal energy directly to outer space, almost as if the atmosphere weren’t there.
Passive radiative cooling below the ambient air temperature has been demonstrated since the 1970s, but only at night. Daytime cooling is trickier: The cooling material must emit strongly in the IR while absorbing essentially none of the Sun’s visible light. Absorption of just a few percent negates the IR radiative cooling power. In 2014 Stanford University’s Shanhui Fan and his colleagues found a way to achieve daytime radiative cooling using a nanophotonic material with alternating layers of silicon dioxide and hafnium dioxide. The composite, though, could be made only in small amounts under clean-room conditions.
Now Ronggui Yang, Xiaobo Yin (both of the University of Colorado Boulder), and their colleagues have come up with an effective cooling material that can be produced in large quantities by standard industrial roll-to-roll methods. The material, illustrated here, consists of micron-sized SiO2 beads randomly distributed in a transparent polymer film. Over the course of their experiment, the researchers made hundreds of square meters of the film. Day or night, the material’s cooling power hovers around 100 W/m2, so a 10–20 m2 sheet rivals the power of a residential air conditioner. (Y. Zhai et al., Science, in press, doi:10.1126/science.aai7899.)