This Thinner, Light Radiative Cooling Paint Could Replace Your AC

Engineer Robert Bean is a fan of radiative cooling, where buildings or objects radiate long-wave radiation out to the coldness of space. "There will come a time when we won’t use compressors for the cooling of people and buildings. It is simply not necessary," wrote Bean for HPAC magazine. "The heat sinks we need to reject heat to, or absorb heat from, are literally within our reach and there are some very smart people who will show us how to get very good at accessing them."

One of those very smart people is Xiulin Ruan, a professor at Purdue University. Everyone seems to want the radiative paint he developed a few years ago. “I’ve been contacted by everyone from spacecraft manufacturers to architects to companies that make clothes and shoes," he told Purdue University News. "They mostly had two questions: Where can I buy it, and can you make it thinner?”

Ruan didn't have a great answer to the first question, only saying: “We are in discussions right now to commercialize it.” However, he addressed the second question with a new lighter and thinner version. It is made with hexagonal boron nitride (h-BN) as the pigment—h-BN is currently used as a lubricant in race cars and in cosmetics.

We previously noted that most infrared radiation is blocked or absorbed by carbon dioxide or water molecules in the atmosphere. But there is a "sky window" or "atmospheric window" where infrared radiation with wavelengths of between 8-13 micrometers (8,000-13,000 nm) can escape.

The newer paint is more effective at bouncing back solar radiation of these wavelengths and incorporates voids of air which makes it 80% lighter. The thickness is reduced from 0.4 to 0.15 millimeters. This makes it useful for the exteriors of airplanes or cars, but it also probably makes it less expensive.

Ruan said the key points that make radiative cooling so exciting. “Using this paint will help cool surfaces and greatly reduce the need for air conditioning,” he said. “This not only saves money, but it reduces energy usage, which in turn reduces greenhouse gas emissions. And unlike other cooling methods, this paint radiates all the heat into deep space, which also directly cools down our planet. It’s pretty amazing that a paint can do all that.”

According to the paper published in Cell Reports Physical Science, "ultrawhite hBN-acrylic paints that achieve solar reflectance of 97.9% and sky window emissivity of 0.83 with only 150 μm thickness and 0.029 g/cm2 weight, representing significant reductions from previous radiative cooling paints." And the key point: "Field tests show full daytime cooling under direct sunlight, reaching 5–6°C [~9-11°F] below ambient temperature on average during daylight hours."

Traditional air conditioning uses electricity, compressors, and refrigerants to move heat from the inside to the outside of buildings, heating up the outdoors while it cools the indoors. Bean has said that "regardless of an HVAC appliance’s efficiency score, even the highest rated cooling products when connected to a combustion-based power system, contradict the philosophies of sustainability."

All that air conditioning stuff is connected back to a power plant, burning fuel and creating heat to make electricity to cool us down. Instead, As Ruan said, radiative cooling sends the heat right out into space using no power at all. Imagine if every building, every surface parking lot, and every car sitting in every parking lot was painted with this reflective radiative paint—that's a lot of heat that could be sent right back to where it came from, right off the planet.

Forget painting the town red; it's time to paint the world white.