Daytime Radiative Cooling — Passive Cooling During Day
By Judhajit Chakraborty, Assoc. AIA
First of all I would like to wish all the readers a very happy 2015.
We all know or have heard about nighttime radiative cooling. It is a type of cooling to the night sky based on the principle of heat loss by long-wave radiation from the earth’s surface to the atmosphere. Clear starry skies help in achieving more radiative cooling. However, higher levels of humidity will slow down the process. This passive cooling strategy has been used a long time. Green roofs, roof ponds, etc use this strategy to keep houses cool at night. But what about daytime, especially in arid/semi-arid regions?
Not long ago, an urban heat island effect study in downtown Phoenix, AZ showed building façade temperatures at a whopping 170-180°F, about 60°F higher than the ambient air temperature and much of the radiation and re-radiation from the building façade stays there, with no where to go as the air temperature is hot too and it cools down only at night. Unless, there is a material smart enough to reflect all the radiation to the outer space which is very cold, 3°F above absolute zero or about -270°F. Dr. Aaswath Raman and his team from Stanford University came up with such a material. They invented a material that would radiate all sorts of radiation (direct, indirect, infra-red) to the outer space during daytime thereby resulting a surface temperature of about 10°F cooler than the ambient air temperature. The other cool part of it: the material radiates sunlight at a wavelength between 8-13 microns and that is basically a transparent portal to the outer space. So, in a nutshell, this smart material directly radiates sunlight to the outer space and without being obstructed by anything, atmosphere, clouds, nothing.
So, what is that material? It consists of four layers of silicon dioxide which is sandwiched with three layers of hafnium dioxide. Each of the seven layers has a precisely defined thickness ranging from 13 to 688 nanometers (billionths of a meter). It is also backed by a layer of silver 200 nanometers thick, which will act as a mirror.
Dr. Raman’s team computed the thickness of those layers for reflecting the entire solar spectrum while, at the same time, getting rid of infrared light at the frequency which can most easily escape from Earth into outer space. And then they built a mock up, to see if their theory works.
Well, it did. They mounted the sandwiched material on a silicon wafer to keep it flat. It was then held in a specially
designed box made of Mylar, polythene, polystyrene, acrylic and wood, to minimize the conduction of heat into it from its surroundings, and then left outside on a sunny, but a cold ,California day. The photonic sheet settled down to a temperature 10°F cooler than its surrounding air temperature. Thermal isolation was necessary as without that, the temperature difference would disappear—but the result would have been a slightly cooler surrounding.
A long journey still awaits for turning this discovery to a commercially available building material. Dr. Raman and his team, however, have taken the first cost reduction step by working out that hafnium dioxide (which is expensive) can be replaced with titanium dioxide (which is cheap). They will probably need to replace the silver, too—though the cost of silicon dioxide, or sand, is cheap. The material 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.
Though it will not replace air conditioning completely the idea of cooling down the building mass (roof) passively during daytime will save a lot on air conditioning costs when in USA, more than 15% of a buildings electricity is devoted to cool a building. Passive cooling now has become a little cooler! Cheers to Dr. Raman and his team at the Stanford University. ■