Adding a white, reflective coating to a rooftop can help keep a house cooler in the summer and save on air-conditioning bills. But if you live in a city with cold winters, the same coating leads to you needing to use more heat when it’s freezing outside. A new color-changing rooftop coating is designed to help solve that problem by automatically adjusting to help control the indoor temperature.
A team of engineers at China’s Harbin Institute of Technology were inspired by a desert-dwelling reptile, the Namibian chameleon, which changes color to adjust its own body temperature. On a hot day, the chameleon turns light gray, so it can reflect sunlight. When the weather is colder, it turns dark brown to absorb heat.
The rooftop coating uses chemistry to change color, with “thermochromic” microparticles that react naturally as outdoor temperatures rise or fall. The mix of microparticles can be sprayed or brushed on a roof, and when it heats up to 68 degrees Fahrenheit, the coating starts to change from dark gray to light gray. When it reaches 86 degrees, it can reflect as much as 93% of solar radiation. In cold weather, the coating absorbs heat and gets warmer. In spring and fall, if there are huge temperature swings throughout the day, the coating may change color multiple times.
In outdoor tests on miniature houses in the summer, the team found that their coating remained much cooler than ordinary white paint. In the winter, it was warmer than other coatings that are highly reflective. In climates with hot summers and cold winters, the scientists estimate the coating could help save an additional 20% on energy costs compared to other reflective coatings.
That’s a big deal: Buildings are responsible for around 40% of global carbon emissions, and a large chunk of that comes from heating and cooling their interiors. Researchers now plan to commercialize their technology.
The product should be affordable because it builds on low-cost cooling materials that already exist. “This temperature-adaptive radiative cooling coating only requires adding thermochromic materials to it,” says Cheng Ziming from Harbin Institute of Technology’s Center for Energy and Environmental Engineering. “The most important thing is that its benefits far outweigh its costs.”
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