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Heat waves are multiplying around the world, becoming more frequent and intense. Developing countries are the hardest hit: no only heat waves are more extreme than in other parts of the world, but cooling methods are also needed. more difficult to implement because of costs. In such situations, passive cooling – which does not require electricity or energy – is the way to go.
Plastics and other inexpensive polymers are in fact excellent heat radiators, which would make them perfect for passive daytime radiative cooling (PDRC) if scientists could determine how to get these normally transparent surfaces to reflect the light of the day. sun without using silver mirrors.
Researchers at Columbia University have finally been able to accomplish this. Report in the newspaper Science, the authors described a PDRC polymer coating with voids at the nanoscale and micro, serving as a passive air cooler. What is amazing is that the coating can be applied like paint on roofs, buildings, vehicles – almost everything.
Apply and cool
A passive cooling surface is effective when it has a high solar reflectance and emittance. In other words, if both of these variables are high enough, there will be a net effect of heat loss under intense sunlight.
The cheapest and often the most practical PDRC is white paint. If you have already changed from a black shirt to a white shirt on a hot summer day, you need to know how much it can make all the difference.
The problem with white paint, however, is that it does not reflect the longer wavelengths of light. Therefore, its cooling performance is modest.
Passive Day Radiative Cooling (PDRC) reflects sunlight and emits heat to generate net heat loss. In this way, a surface can reach temperatures below room temperature. Credit: Jyotirmoy Mandal.
Using phase inversion, the Columbia engineering team was able to introduce voids into the dispersion of light in the polymers. The process involves mixing the polymer with a solvent next to water, wherein the polymer is insoluble. In the end, the pigments of the white paint were replaced by air spaces reflecting all wavelengths of light, from UV to infrared. And even if there were no pigments, the polymer was still white.
What you get is a much better performance than conventional white paint and even better than some of the sophisticated and costly CRDPs. In addition, it is convenient for the polymer coating to be applied as a paint on virtually any surface.
During the tests, the coating kept the surfaces much colder than the environment in very different heavens. For example, the polymer paint cooled the surfaces by 6 ° C in the hot and arid desert of Arizona and 3 ° C in the misty and tropical environment of Bangladesh.
Another selling point is stability. For example, cellulose, which is the main component of paper, turns yellow over time. The porous films developed at Columbia, however, did not seem to change a bit during a month on the ground.
"The fact that cooling is achieved in desert and tropical climates, with no thermal protection or shielding, demonstrates the utility of our design wherever cooling is required," said Yuan Yang, an assistant professor of science and engineering. materials, in a statement.
"The moment is now critical for developing promising solutions for a sustainable humanity," added Yang. "This year, we have witnessed record heat waves and record temperatures in North America, Europe, Asia and Australia. It is essential that we find solutions to this climate challenge and we are very happy to work on this new technology that responds to it. "
It was not so long ago that white was thought to be the most difficult color to make. New research shows, however, that white may be the most accessible color.
"It can be made using only properly dimensioned air gaps, integrated into a transparent backing. It's the voids in the air that make Silvery Snow White ants and Silver Saharan Ants, "said Nanfang Yu, an associate professor of applied physics.
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