A full 15% of the electricity used by buildings in the US is devoted to air conditioning. The electrical demand during daytime peak cooling hours is immense. The use of pumps and compressors to rid buildings of heat uses a lot of energy, at a great cost. However, if the ideas of Dr. Aaswath Raman of Stanford University become widely implemented, that may all change. He and his colleagues have invented a way to allow buildings to dump their heat by simply radiating it into outer space.

The principle behind this is deceptively simple. Outer space is very cold, and very large, and Earth radiates heat into it constantly, though this is offset by the heat of the Sun. So to cool down one area, like a building, you would simply need to reflect the sun’s heat back into outer space. The difficult part is making the theory work in practice.

The previous attempts at radiative cooling have only been successful for nighttime cooling, so to have this experiment succeed is no mean feat.

A New Invention

To achieve this method of cooling, Dr. Raman and his colleagues invented a highly reflective material that can reflect 97% of sunlight back into space, while radiating itself into the area of the atmosphere where it is most transparent. The material consists of seven microscopically thin layers of silicon dioxide and hafnium dioxide. Each layer varies in thickness from 13 to 688 nanometers (billionths of a meter), and all is backed with a super-thin layer of silver to reflect like a mirror.

The precise measurements of each layer have been computed to reflect the entire solar spectrum of light, while shedding the infra-red light at the proper wavelength of between 8 and 13 microns to easily escape into outer space without interference.

The resulting sheet of material is a thickness of less than 2 microns, and acts like a semiconductor, manipulating the energy levels of light. The team tested it using a wafer of silicon to keep it flat, and a specially designed box, and found that the sheet settled to a temperature 10 degrees Fahrenheit cooler than the surrounding climate.

The Long Process to Production

There will be a long process to figure out if any substitutions can be made to replace any expensive substances to make this material viable for widespread use. Replacing the expensive hafnium dioxide with the much cheaper material of titanium dioxide, for example, would increase that viability. The production would have to be done on a much larger scale, as well.

The use of such a material could reduce the costs of building maintenance greatly, though it would not replace traditional air conditioning entirely. The material must be used on the parts of a building facing the sky, namely, the roof, and it would work to passively cool the building during the day, saving a great deal in air conditioning costs. Even having part of the cooling process be electricity-free is an extremely attractive option, and the global impact of this invention could have a huge impact on global energy use.