Brilliant white roofs have been used to reflect sunlight and cool homes since ancient times in Greece. Now, researchers in California have added a modern spin to the strategy. They’ve created a new multilayer coating that cools using a one-two punch. First, it reflects nearly all the incoming energy in sunlight. Second, it absorbs heat (say, from a building below) and radiates it away without warming the nearby air. Because the new technology doesn’t require any electricity, it could slash the cost of air conditioning buildings—an application that consumes nearly 15% of all electricity used in the United States.
The first part of the new cooling technology, reflecting, is easy to grasp: Just look in a mirror. The second part, radiating away heat, is less intuitive. Buildings, trees, and people all radiate heat in the form of infrared light—the eerie glow you see in night vision goggles. Typically this infrared radiation occurs over a broad range of wavelengths between 6 and 30 micrometers. Because molecules in the air can absorb at the top and bottom of that range, the radiation heats up its surroundings. Wavelengths between 8 and 13 micrometers, however, pass right through the air into the cold vastness of space.
So researchers led by Shanhui Fan, an electrical engineer at Stanford University in Palo Alto, California, set out to create a mirrored coating that was also good at radiating energy at long wavelengths. “There is no natural material that reflects solar wavelengths but is also radiative in thermal wavelengths” just the right length to escape the atmosphere, he says.
To design such a material, Fan and his team created a seven-layer stack of alternating silicon dioxide (SiO2) and hafnium dioxide (HfO2). They grew the layers atop a silicon wafer the size of a pie plate, topped with a thin silver layer that acted as an initial good reflector. The first four layers of SiO2 and HfO2 were ultrathin and designed to reflect nearly all the incoming light not reflected by the silver. Together those five layers reflected 97% of the incoming energy. The top three layers—two thicker SiO2 layers separated by a thick HfO2 layer—acted as the radiator, absorbing heat from below and reemitting the energy at wavelengths between 8 and 13 micrometers. In various tests, the coating cooled surfaces below it by 5°C, even in full sunlight, Fan and his team report online today in Nature.
Fan notes that nearly all energy technologies work by moving heat from a hot place to a cold one. In the case of the coating, he says, the dumping ground is space—a place readily accessible from anywhere on Earth.
“This is a very novel and simple idea,” says Eli Yablonovitch, a photonics engineer at the University of California, Berkeley. But whether the coating can be applied cheaply enough over large areas remains to be shown. Fan says existing technology can do the job. But if coating rooftops doesn’t pan out, he says, the technology might still yield better air conditioning systems by enabling them to flush their waste heat into space.