As the world heats up, keeping people and things cool will consume more and more energy. And that energy will add more and more carbon emissions to the atmosphere, which will in turn make the world even hotter. One persistent problem in urban areas is the so-called “heat island” effect. Buildings and other man-made objects get so hot during the day they can’t cool down completely at night.
But scientists at the Columbia School of Engineering say they have come up with a new polymer coating that can be applied like paint that will help buildings shed heat, keeping them cooler and reducing the amount of cooling needed for the people and things inside.
Passive Daytime Radiative Cooling
The key to keeping buildings cool is what is known as passive daytime radiative cooling. If a surface reflects the sun’s rays and has a high thermal emittance — which maximizes radiational heat loss — the surface will lose heat to the surrounding atmosphere even in bright sunshine.
Until now, the best solution has been to paint the exterior surfaces of buildings white, but the pigments used to make white paint do not reflect all the wave lengths found in sunlight and can absorb UV radiation. Ordinary paint cannot be applied easily to things like roof tops, meaning overall it only helps surfaces shed heat by a modest amount. The researchers at Columbia wanted to find a coating that works better.
A High Performance Polymer Coating
What they came up with is a high performance polymer coating that can be applied like paint to any surface. The polymer forms a foam-like surface with lots of air pockets or voids. Those voids scatter and reflect sunlight, while the white color of the polymer resists solar heating.
Polymers and solvents are already used in paints. The researchers substituted air voids that reflect all wavelengths of sunlight — from UV to infrared — for the white pigment typically used in paint. “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 Jyotirmoy Mandal, lead author of the study which was published in the journal Science on September 27.
The researchers found that their coating lowered surface temperatures by 6º C in the hot, dry conditions of Arizona and 3º C in the foggy, tropical environment of Bangladesh. “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,” says Yuan Yang, assistant professor of materials science and engineering at Columbia.
The team also created colored polymer coatings with cooling capabilities by adding dyes. “Achieving a superior balance between color and cooling performance over current paints is one of the most important aspects of our work,” says professor Nanfang Yu. “For exterior coatings, the choice of color is often subjective, and paint manufacturers have been trying to make colored coatings, like those for roofs, for decades.”
“Polymers are an amazingly diverse class of materials, and because this technique is generic, additional desirable properties can be conveniently integrated into our PDRC coatings, if suitable polymers are available,” Mandal adds.
Technology For Today
“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,” says Claes-Göran Granqvist, a pioneer in the field of radiative cooling. He is a professor of physics at Uppsala University and was not involved with the research.
“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.”
“Now is a critical time to develop promising solutions for sustainable humanity,” Yang notes. “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.”
Any Color You Want, Including White
Professor Yu adds that he used to think white was the most hardest color to achieve. “When I studied watercolor painting years ago, white paints were the most expensive. Cremnitz white or lead white was the choice of great masters, including Rembrandt and Lucian Freud.
“We have now demonstrated that white is in fact the most achievable color. It can be made using nothing more than properly sized air voids embedded in a transparent medium. Air voids are what make snow white and Saharan silver ants silvery.”
Earlier this year, we reported on a study from the University of Birmingham that warns the energy needed to keep us all cool as the planet gets warmer could consume every kilowatt of renewable energy available in the entire world by 2050. Even the Trumpies now admit the world will probably warm by 4º C by the end of this century.
Thank An Immigrant
Finding ways to cool man-made structures to reduce the need for air conditioning may not seem like a huge deal, but it could be an important part of helping humans cope with the consequences of making fossil fuels the basis of civilization. America is fortunate to have immigrants like Jyotirmoy Mandal, Yuan Yang, and Nanfang Yu to do such basic research that will benefit us all.
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