New material reflects light and radiates heat

MIT Technology Review : Passive radiative cooling is when an object emits heat at frequencies between 8 μm and 13 μm (a range known as the thermal window) so that the heat bypasses Earth’s atmosphere and is released into space. Materials capable of passive radiative cooling are of interest for use in cooling buildings, but have not been effective in sunlight. Now, Shanhui Fan of Stanford University and his colleagues have developed a material that radiates heat in the thermal window and also reflects 97% of sunlight. The material is made of alternating layers of silicon dioxide and hafnium oxide on a silicon wafer. The researchers placed an 8-inch square of the material on a building’s roof to test its reflection and cooling. The team next plans to test a square meter sized piece. They hope to produce larger pieces by adapting the technology used to make coated windows. Fan believes that a building with a roof covered in the material could eliminate the building’s need for air conditioning.