Scientists, including one of Indian origin, have invented a polymer coating that acts as a spontaneous air cooler and can be applied like paint on rooftops, buildings, water tanks, vehicles and even spacecraft.
The researchers from Columbia University in the US used a solution-based phase-inversion technique that gives the high-performance exterior passive daytime radiative cooling (PDRC) polymer a porous foam-like structure. PDRC is a phenomenon where a surface spontaneously cools by reflecting sunlight and radiating heat to the colder atmosphere.
Developing practical PDRC designs has been challenging: many recent design proposals are complex or costly, and cannot be widely implemented or applied on rooftops and buildings, which have different shapes and textures, said researchers, including Jyotirmoy Mandal, a doctoral student at Columbia University. Up to now, white paints, which are inexpensive and easy to apply, have been the benchmark for PDRC, they said.
White paints, however, usually have pigments that absorb Ultraviolet (UV) light, and do not reflect longer solar wavelengths very well, so their performance is only modest at best. The air voids in the new porous polymer, described in the journal Science, scatter and reflect sunlight, due to the difference in the refractive index between the air voids and the surrounding polymer.
The polymer turns white and thus avoids solar heating, while its intrinsic emittance causes it to efficiently lose heat to the sky. The team built upon earlier work that demonstrated that simple plastics and polymers, including acrylic, silicone, and PET, are excellent heat radiators and could be used for PDRC.
The challenges were how to get these normally transparent polymers to reflect sunlight without using silver mirrors as reflectors and how to make them easily deployable. The researchers decided to use phase-inversion because it is a simple, solution-based method for making light-scattering air-voids in polymers.
Polymers and solvents are already used in paints, and the new method essentially replaces the pigments in white paint with air voids that reflect all wavelengths of sunlight, from UV to infrared. "This simple but fundamental modification yields exceptional eflectance and thermal emittance that equal or surpass those of state-of-the-art PDRC designs, but with a convenience that is almost paint-like," said Mandal.
"The researchers found their polymer coating's high solar reflectance and high thermal emittance kept it significantly cooler than its environment under widely different skies, eg by six degrees Celsius in the warm, arid desert in Arizona and three degrees Celsius in the foggy, tropical environment of Bangladesh," said Yuan Yang, an assistant professor at Columbia.
"The fact that cooling is achieved in both desert and tropical climates, without any thermal protection or shielding, demonstrates the utility of our design wherever cooling is required," Yang said. The team also created coloured polymer coatings with cooling capabilities by adding dyes.
"Achieving a superior balance between colour and cooling performance over current paints is one of the most important aspects of our work," said Nanfang Yu, an associate professor at Columbia. "For exterior coatings, the choice of colour is often subjective, and paint manufacturers have been trying to make coloured coatings, like those for roofs, for decades," Yu said.
