Stanford University engineers have invented an ultrathin multilayered material which can reflect sunlight from, say, buildings and thereby lower the temperature of the building.
Though the technology is still young, the inventors believe that this can provide a solution to air conditioning rooms, which now costs a lot of energy. Published in Nature, a paper by Aaswath Raman et al. describes the new material and its properties.
At a thickness of about 1.8 microns, the material is thinner than an aluminium foil. It is made up of seven layers of hafnium oxide and silicon dioxide, each of differing thicknesses, on top of a thin layer of silver.
It is designed to reflect both infrared light (which cannot be seen) and visible sunlight.
What is special is that the material reflects light at the frequency which is not absorbed by atmospheric gases. This is known as the atmospheric window.
When light with a frequency between 8 and 13 microns is beamed into the atmosphere, it goes unabsorbed and escapes into outer space. This is the frequency window at which the material reflects the infrared and visible light which strikes it. By this method, the material can cool the interior by almost 5 degrees Celsius.
Though radiative cooling happens in the 8-13 micron window, atmospheric gases do absorb radiation at about 10 microns. Since absorption at 10 micron band can also warm up the atmosphere, of what advantage is this process? According to Dr Aaswath Raman, “It is from the perspective of the radiative surface (the radiative cooler) that there is a benefit due to this process.
Certainly most surfaces exposed to the sky radiate their heat out as thermal radiation — one could also call [it] ‘thermal light’ since this is the heat all objects emit away as light at wavelengths that correspond to the object’s temperature. The entire goal of our work is to harness this effect on the surface, specifically during the day, and not to cool the atmosphere itself in anyway.”
