Throughout most of their history, human beings have had to devise strategies to cool down foods and building without the convenience of an AC system or a fridge. Indeed, the oldest and most basic system to escape from solar radiation is the shade of a tree. Another solution, found in the villages dotting the Mediterranean basin since the dawn of history, is the use of white paint to reflect solar radiation. Based on the latter, the current passive daytime radiative cooling (PDRC) techniques leverage the reflective properties of materials, while adding infrared emitters, i.e, able to release more heat than they receive. To achieve such outcome, highly sophisticated materials are required, frequently resorting nanotechnology, that can reflect certain solar wavelengths and send them back to the outer space. At the MIT, however, they have turned to extremely simple materials to achieve passive cooling. And the first results of this technological project look encouraging indeed.
In fact, the system created by Bikram Bhatia and the rest of his team is a high-tech version of the classic parasol. The key lies in combining a base that emits radiation in the mid-infrared range and a reflector that blocks direct sun rays. The sunlight- blocking strip has been designed to provide shade at different hours without the need for an active sun-tracking system. Under it, the researchers placed a box with several elements for different purposes. Firstly, there is an infrared emitter, which is a copper sheet painted in white and placed on top of an insulator, Secondly, there are two layers of polyethylene with a small gap that minimize diffuse solar radiation. Finally, the remaining perimeter of the box is covered with silver foil to prevent the transfer of parasitic heat loads, i.e., the heat transferred from one surface to another. With all those elements working together, the first tests achieved a 6 C cooling.
However, the trial was carried out in the Boston area, a coastal city with high humidity in the atmosphere, which blocks the emission of infrared radiation. The researchers believe that the efficiency of this technological project would be much higher in dry areas, with a maximum theoretical cooling of 40 C. This simple device could be very helpful in areas lacking power supply or added to existing cooling systems to reduce electric energy consumption.
In one of our previous articles about innovative energy-saving projects, we mentioned the striking properties of the Egyptian blue, an old pigment able to reflect infrared radiations and cool down buildings passively. One of the latest developments in this field is the polymer-based paint announced by Columbia Engineering, which belongs to the University of Columbia. Their nanoporous material can be spread over buildings, water deposits, and other structures, achieving a similar efficiency to the passive cooling system created by Bhatia at the MIT. In their own tests, they managed to reduce the temperature of the surface by 6 C. Currently, the team is exploring the commercial applications and the scalability of their paint. It looks like AC systems are facing some serious competition.