Researchers at Columbia Engineering have invented a high-performance exterior polymer coating that can be applied like paint to act as a spontaneous air cooler. The innovation may represent an important tool in efforts to address the Earth’s changing climate.

"Now is a critical time to develop promising solutions for sustainable humanity," said Yuan Yang, an assistant professor of materials science and engineering assistant. "This year, we witnessed heat waves and record-breaking temperatures in North America, Europe, Asia and Australia. It is essential that we find solutions to this climate challenge, and we are very excited to be working on this new technology that addresses it."

The polymer coating was created with a solution-based phase-inversion technique that gives it a porous foam-like structure, with nano- to microscale air voids that have a different refractive index than the surrounding polymer. The design is based on the passive daytime radiative cooling (PDRC) phenomenon, which makes a surface spontaneously cool by reflecting sunlight and radiating heat to the colder atmosphere.

Developing practical PDRC designs has been challenging, in part because of the difficulty in applying them to rooftops and buildings with different shapes and textures. White paints have been the benchmark for PDRC, but their UV light-absorbing pigments and poor reflectiveness of longer solar wavelengths have limited their performance.

The new polymer coating, however, essentially replaces the pigments in white paint with air voids that reflect all wavelengths of sunlight, from UV to infrared. It turns white to avoid solar heating, efficiently losing heat to the sky and staying cooler than the ambient air. And it can be fabricated, dyed and applied to anything that can be painted: rooftops, buildings, water tanks, vehicles, even spacecraft.

Add that to the fact that it can be used in a variety of environments — such as developing countries where summer heat can be extreme and is projected to get worse. In those areas, common cooling methods such as air conditioners are expensive and require ready access to electricity — not to mention the fact that they consume significant amounts of energy and often require coolants that deplete ozone or have a strong greenhouse effect.

Yang and his colleagues are refining their design in terms of applicability, while exploring possibilities such as the use of completely biocompatible polymers and solvents. They are in talks with industry about next steps.

The study is published online today in Science.