New polymer coating keeps buildings cool

Researchers from Columbia University have created a passive daytime radiative cooling polymer coating with air voids that act as a spontaneous air cooler.

In an article1 published in Science in September, a team of researchers at the Columbia School of Engineering say they have come up with a new polymer coating which uses passive daytime radiative cooling (PRDC), the process by which objects naturally reflect sunlight and radiate heat to the colder atmosphere.

This kind of technique can help reduce the air conditioning air load in buildings. Until now, white paints were commonly used as PDRC technologies. But if they reflect about 80% of visible light, they also absorb ultraviolet and near-infrared rays due to their pigments. It prevents paints to reflect longer solar wavelength. Columbia researchers used phase-inversion technique to replace the pigments in white paint with air voids. By incorporating this design, any incoming solar rays were backed into the atmosphere.

The process could cool down structures to sub-ambient temperatures. The researchers found their polymer coating’s high solar reflectance (R > 96%) and high thermal emittance (? ~ 97%) kept it significantly cooler than its environment under widely different skies, eg by 6°C in the warm, arid desert in Arizona and 3°C in the foggy, tropical environment of Bangladesh.

“Nature offers many ways for heating and cooling, some of which are extremely well known and widely studied and others that are poorly known. Radiative cooling – by using the sky as a heat sink – belongs to the latter group, and its potential has been strangely overlooked by materials scientists until a few years ago," said Uppsala University Physics Professor Claes-Göran Granqvist, who was not involved in the study. "The publication by Mandal et al. highlights the importance of radiative cooling and represents an important breakthrough by demonstrating that hierarchically porous polymer coatings, which can be prepared cheaply and conveniently, give excellent cooling even in full sunlight.

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1 MANDAI, J., OVERVIG, A., JIA, M., et al. Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling. Science [online]. 2018, September 27. Available following this link.