A simple, low-cost coating can change color like a chameleon to stay cool when it’s hot outside and warm up when it is cold. It could one day help bring down the energy use of heating and cooling buildings.
The coating, which is simple and easy to make, could save up to 20 percent more energy use per year than previously reported temperature-regulating coatings that do not change color, researchers write in a study published in the journal Nano Letters.
Today, buildings consume about a third of the world’s energy, according to the International Energy Agency. As the world’s population increases and climate change makes the weather more extreme, building energy use and carbon footprint is expected to double by 2050.
To put a dent in heating and cooling related energy use, researchers are developing a plethora of passive technologies that do not require energy. These include straightforward approaches such as white, and even colorful paints that reflect sunlight and infrared heat to keep homes cool. Other energy-free heating and cooling approaches include radiative cooling materials that send heat into space, sunlight-scattering films to keep food cold for longer, and thermal fabrics that help regulate temperature.
Researchers from the Harbin Institute of Technology in China looked at the Namaqua chameleon for inspiration. This lizard is the only desert chameleon in the world, found in the deserts of Southwestern Africa, and it changes color based on surrounding temperature to regulate its body heat. The chameleon makes its skin a light gray to reflect light and stay cool when outside temperatures are hot, but at night when it gets cooler, it takes on a heat-absorbing dark brown color.
The researchers made a temperature-adaptive cooling coating by mixing tiny microscopic capsules into a binder that they can spray or brush onto a metal foil. The microcapsules are filled with polyvinylidene fluoride, a chemical that changes color in response to temperature changes. Laboratory tests showed that the coating begins changing color from dark brown to light grey when it reached 20°C. When it got to 30°C, the now light-colored film reflected up to 93% solar radiation.
For a more thorough test, the researchers put the coating on small building models along with white paint, blue steel tiles, and a passive radiative cooling paint. They placed the buildings outside for four seasons.
The new coating stayed warmer in winter weather than the passive radiative cooling paint. In the summer, it was cooler than the tiles and white paint. During spring and fall months, it was the only material that adapted to fluctuating outdoor temperatures during the day and switch between heating and cooling states.
According to the researchers, the coating should increase the number of suitable hours for human comfort by 55 percent and reduce household energy consumption by 20 percent compared to the radiative cooling paint. “Furthermore, [it] exhibits excellent application performance with the advantages of low cost, easy preparation, and simple construction,” they write.
Source: Yan Dong et al. “Warm in Winter and Cool in Summer”: Scalable Biochameleon Inspired Temperature-Adaptive Coating with Easy Preparation and Construction. Nano Letters, 2023.