Researchers at Columbia Engineering (New York, New York, USA) developed a high-performance polymer coating that acts as a spontaneous air cooler and can be fabricated, dyed, and applied like paint. This coating features nano-to-microscale air voids that gives it a porous foam-like structure and enables it to scatter and refract sunlight. It then turns white and thus avoids solar heating while simultaneously emitting radiative heat to the sky.
The polymer coating operates via a process known as passive daytime radiative cooling (PDRC). With PDRC, a surface spontaneously cools by reflecting sunlight and radiating heat to the colder atmosphere—the process works best when the surface has both a high solar reflectance (R) that minimizes solar heat and a high thermal emittance (Ɛ) that maximizes radiative heat loss, thereby resulting in a net heat loss.
PDRC-based solutions have numerous practical applications, particularly in developing countries with extreme heat. Yet while PDRC is potentially a more cost effective and environmentally friendly option than, for instance, air conditioning, no one had been able to develop a PDRC design that could be used widely.
The Columbia Engineering team had previously conducted research that proved simple plastics and polymers could be used for PDRC due to their ability to radiate heat. Building upon that earlier research, they used a solution-based phase-inversion technique to make light-scattering air voids in polymers, thereby replacing the pigments in white paint as a method of reflecting sunlight.
According to Jyotirmoy Mandal, a member of the Columbia Engineering team and a doctoral student in materials science and engineering, this newest insight was a game changer. “This simple but fundamental modification yields exceptional reflectance and emittance that equal or surpass those of state-of-the-art PDRC designs, but with a convenience that is almost paint-like,” says Mandal.
The results of the Columbia Engineering study were published in the September 27 edition of Science magazine.
Source: Columbia Engineering, www.engineering.columbia.edu.