Homes in Greece and other sunbaked countries are regularly painted white to reflect as much sunlight as possible. Researchers are building on that age-old strategy with a new wave of "passive radiative cooling" materials that shed sunlight and heat. Most are not easy to apply to existing roofs and walls, but a team of U.S. researchers has now created a cooling paint that can coat just about any surface, lowering its temperature by 6°C.
The advance underscores "terrific progress in this field," says Xiaobo Yin, a materials scientist at the University of Colorado in Boulder whose team has developed a passive radiative cooling plastic film and has formed a startup company, Radi-Cool, to commercialize it. The new materials, Yin says, could drop cooling costs by up to 15% in some climates. "It's quite a big number," he says. And with 17% of all residential electricity use in the United States going toward air conditioning, the savings could be substantial.
White paints typically reflect only about 80% of visible light, and they still absorb ultraviolet (UV) and near-infrared (near-IR) rays, which warm buildings. To do better, the new materials start by incorporating materials or structures that reflect nearly all the sun's incoming rays, including near-IR heat and, in some cases, UV as well. They also contain polymers or other substances that, because of their chemical makeup, radiate away additional heat as mid-IR light, at wavelengths of 8 to 13 micrometers. The atmosphere does not block these wavelengths, effectively allowing the materials to shed excess heat into space without warming the surrounding air.
In 2014, researchers led by Shanhui Fan, an electrical engineer at Stanford University in Palo Alto, California, reported in Nature that by alternating layers of silicon dioxide and hafnium dioxide, they could create a highly reflective surface that stayed 5°C cooler than the surrounding air. Last year, Fan and his colleagues used another material—a polymer and silver film combo—to cool water for use in air conditioning. The team showed that it had the potential to save 21% of air conditioning costs over the summer. Fan's team has since formed its own startup, SkyCool Systems.
Yin and his colleague Ronggui Yang jumped in last year with their material: a plastic film embedded with tiny glass beads that cooled surfaces by up to 10°C. And in Australia, applied physicists Angus Gentle and Geoff Smith of the University of Technology Sydney reported in 2015 that a cool roofing material made of a pair of polymers kept a commercial roof 3°C and 6°C cooler than ambient air in the midday sun and at night, respectively.
Applying these coatings to roofing and siding materials remains a sticking point. Highly reflective compounds can be integrated in traditional-looking shingles and clay tiles for new construction or renovation. But it's been harder to come up with options for existing buildings.
That's where the new passive cooling paint comes in. Researchers led by Yuan Yang and Nanfang Yu, applied physicists at Columbia University, had been experimenting with making highly reflective materials by adding air voids to plastics. Then one of their students, Jyotirmoy Mandal, stumbled on an easy method for creating a similar texture. Mandal was studying ways of curing a common polymer called polydimethylsiloxane from a liquid to a solid thin film when he noticed that under certain conditions, the material turned from transparent to white as it dried. Under the microscope he saw that an array of interconnected air voids had formed in the dried film, causing it to strongly reflect light. (The same effect explains why snow is white even though ice cubes are transparent.)
The researchers then found they could re-create the effect with other polymers. Eventually, they settled on a highly durable commercial polymer called PVDF-HFP. The polymer starts as a solution in acetone, to which the researchers add a small amount of water. When painted on a surface, the acetone quickly evaporates, and the polymer separates from the water, creating a network of water droplets. Finally, the water also evaporates, leaving a spongelike arrangement of interconnected voids that reflect up to 99.6% of light, including IR, visible, and UV. The dried film also emits heat primarily in the desirable mid-IR range.
Under the relentless midday sun of Phoenix, painted surfaces remained 6°C cooler than the surrounding air, the researchers report in a paper published online in Science this week. And for good measure, they also showed that they could dye the paint, varying its appearance, although the colored paint sacrificed some cooling.
"It looks very good for being widely applicable," Gentle says. Though durable, PVDFHFP is roughly five times as expensive as traditional acrylics used in paints, says Ronnen Levinson, a cool roof expert at Lawrence Berkeley National Laboratory in California. But the added cost carries a benefit. Gentle says: "All these very cool coatings make it like nighttime in the middle of the day."