Mirror, Mirror, On The Roof

An ultrathin reflective material that can redirect sunlight and heat to cool buildings and other objects to a temperature that’s 5 °C below their ambient temperature has been developed by Shanhui Fan, Aaswath P. Raman, and colleagues at Stanford University (Nature 2014, DOI: 10.1038/nature13883). A device made using the material, which includes seven alternating layers of silicon dioxide and hafnium oxide atop a layer of silver, achieves passive cooling without electricity in two ways. First, the device reflects 97% of incoming sunlight, preventing solar heating. Second, it channels surrounding heat into outer space. The new material’s layers of varying thickness radiate infrared rays at a frequency that lets them pass through the atmosphere—it essentially soaks up the surrounding heat and sends it away. The prototype device is the size of a personal pizza, but Fan says the technology could be scaled up to create large panels necessary for cooling buildings.