A team of engineers from Columbia University has created a polymer coating that uses nano-to-microscale air voids to reflect sunlight and cool down buildings.
Passive daytime radiative cooling (PDRC) is a phenomenon where a surface spontaneously cools by reflecting sunlight and radiating heat to a cooler atmosphere. PDRC works best if a surface has high solar reflectance—meaning that most of the sun’s radiation is reflected away—and a high thermal emittance—meaning that it very easily radiates heat into the atmosphere.
To find a coating with better PDRC than white paint (the current industry standard), the researchers wanted to develop a polymer coating with air voids (pockets of air between coated aggregate particles) inside of it. These micro-and-nano-scale “bubbles” scatter and reflect sunlight to avoid solar heating, and also mean that the polymer loses intrinsic heat easily. To create this coating, the researchers started by preparing a precursor solution of polymer and water in acetone, then applied a film of the solution onto a surface and left it to dry. When the acetone evaporated, the polymer phase-separated from the water, making micro and nano droplets of water. When that water evaporated, the researchers were left with “bubbles” in the polymer.
Previous research had shown that polymers were capable of PDRC, but there was no consensus on how to get transparent polymers to reflect sunlight, and there was no easy method of applying polymers to a surface. The research team solved the second problem by creating paint with its polymer in it instead of using a traditional white pigment. By making this substitution, the researchers were able to create a “white paint” that doesn’t absorb UV radiation. "This simple but fundamental modification yields exceptional [solar reflectance] and [thermal emittance] that equal or surpass those of state-of-the-art PDRC designs, but with a convenience that is almost paint-like," said doctoral student Jyotirmoy Mandal, the study’s lead author.
The paint worked in multiple hot climates, from the tropical to the desert, and the researchers were able to dye it different colors without losing its passive cooling abilities. The team is currently working on how to make its paint usable on a wide scale, and is in talks with larger industrial players. The researchers hope that their paint could become a useful tool in the battle against global climate change.
"Now is a critical time to develop promising solutions for sustainable humanity," said Yuan Yang, assistant professor of material sciences and engineering, and also one of the study’s authors. "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."
Read the original article, published in the Science journal, here.