AIR conditioning is a transformative technology. It has made the world’s torrid climes pleasanter to live in, and enabled the siesta-free working habits of the temperate regions to move closer to the equator. But cooling buildings takes a lot of energy. Heat must be pumped actively from their interiors to their exteriors. Fully 15% of the electricity used by buildings in the United States is devoted to this task. 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.

The idea, described in this week’s Nature, is both cunning and simple. Outer space is very cold (about 3°C above absolute zero) and very big, so it is the perfect heat sink. Earth radiates heat into it all the time. But this is compensated for by the heat the planet receives from the sun. 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. 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. Since, optically speaking, energy levels correspond to wavelengths, such an arrangement can be tweaked to reflect some wavelengths and preferentially emit others. And that, in choosing the layers’ pedantically exact dimensions, is just what Dr Raman and his colleagues have done.

Polythene plan

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. Mounted on a silicon wafer to keep it flat, held in a specially designed box made of Mylar, polythene, polystyrene, acrylic and wood, to minimise the conduction of heat into it from its surroundings, and then left outside on a sunny, albeit rather wintry Californian day, the photonic sheet settled down to a temperature 4.9°C cooler than its surroundings. If it were thermally connected to those surroundings, rather than isolated from them, that temperature difference would disappear—but the result would be to cool the surroundings slightly.

Turning this discovery into a useful device will be a journey down a long road. Dr Raman and his colleagues have, however, taken the first step by working out that they should be able to replace the hafnium dioxide (which is expensive) with titanium dioxide (which is cheap). They will probably need to replace the silver, too—though the cost of silicon dioxide, also known as sand, is not so much of a problem.

The process will also have to be scaled up. And it will work only on those parts of a building (mainly the roof) that have a clear view of the sky, and thus of outer space, so it will not replace air conditioning completely. But the idea of even part of a building’s cooling system being electricity-free is an attractive one, so this may be the start of something really cool.