US researchers have demonstrated zero-carbon air conditioning that doesn’t need an electricity supply.
The technology is based on the principle that warm objects on Earth radiate energy mainly at mid-infrared wavelengths. The atmosphere is transparent in a specific window of the mid-infrared, so radiation from warm objects escapes through the atmosphere into the cold of space.
By engineering a surface to select these wavelengths that it emits and reflects, it’s theoretically possible to allow the heat from a hot surface to escape, while at the same time preventing the sun’s energy from heating it up.
Based on these assumptions, Dr Aaswath Raman of Stanford University and his colleagues invented a way to encourage buildings to dump their heat without the need for pumps and compressors - they simply radiate it into outer space.
The material invented reflects 97 per cent of sunlight while itself radiating at a wavelength of between eight and 13 microns (or millionths of a metre), which is where the atmosphere is most transparent.
The material consists of four layers of silicon dioxide interspersed with three of hafnium dioxide. Each of these seven layers is of a different, precisely defined thickness, ranging from 13 to 688 nanometres (or billionths of a metre). It is backed by a layer of silver 200 nanometres thick, to act as a mirror.
The result, a sheet with a total thickness of less than two microns, is the photonic equivalent of a semiconductor: it does to light what a semiconductor does to electricity, namely manipulates its energy levels.
The sun emits significantly more radiation than warm bodies on Earth, so, to cool in the sun, a surface must transmit most radiation in the transparency window, while reflecting over 94 per cent of sunlight.
Previous attempts at radiative coolers, such as those by materials physicist Claes-Gorän Granqvist and colleagues at Chalmers University of Technology in Sweden, have achieved net cooling only at night.