The Economist is following Stanford researchers who are (literally) making some really cool stuff:
Fully 15% of the electricity used by buildings in the United States is devoted to [air conditioning]. If an idea dreamed up by Aaswath Raman of Stanford University and his colleagues comes to fruition, that may change. Dr Raman has invented a way to encourage buildings to dump their heat without the need for pumps and compressors. Instead, they simply radiate it into outer space.
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To encourage one part of Earth’s surface (such as an individual building) to cool down, all you need do in principle is reflect the sunlight which falls on it back into space, while also encouraging as much radiative cooling from it as possible.
To try to turn principle into practice Dr Raman has made a material which reflects 97% 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. Production of the material is made possible with modern manufacturing methods. It consists of four layers of silicon dioxide interspersed with three of hafnium dioxide.
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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.
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They worked out on a computer how thick those layers needed to be to reflect pretty much the entire solar spectrum while, at the same time, shedding infra-red light at the frequency which can most easily escape from Earth into outer space. And then they made it, to see if it works.
It does.
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