When it's hot out, buildings have a hard time staying cool: bombarded with ambient heat and generating yet more inside, their air conditioning systems have to work hard to keep temperatures down. Now, a new super-thin coating developed at Stanford could be applied to buildings to help them cool themselves more effectively.
The new material, developed by researchers from the University's electrical engineering division, both manages to reflect sunlight and radiate internal building heat, directing both out into the atmosphere to reduce temperature. The researchers refer to it as photonic radiative cooling, or "a one-two punch that offloads infrared heat from within a building while also reflecting the sunlight that would otherwise warm it up."
The material—which is just 1.8 microns thick, thinner than the thinnest aluminum foil—is made up of eight layers. There are seven of silicon dioxide and hafnium oxide which sit on top of a thin layer of silver. Crucially, these layers are all different thicknesses, tuned to match the frequencies of the infra-red radiation of the building which accounts for much of its internal heat. That means it allows the radiation—and heat—to pass out of the building and into the atmosphere with ease. "Think about it like having a window into space," explains Shanhui Fan, one of the researchers behind the work.
But the material is also highly mirrored, reflecting 97 percent of the sun's rays from its surface. Combined, it can provide a highly efficient means of helping cool a building passively—which is why the researchers are now developing a way to spray it on to surfaces in bulk. If they can achieve that, and make it cost-effective, the new material could start appearing on the roof of many a building to keep them cool—and the AC turned down. [Stanford]