HomeTrending NewsColor which keep cars and building cool when exposed to sunlight

Color which keep cars and building cool when exposed to sunlight

When temperatures rise, cool air from air conditioning can be a welcome relief, but “air conditioning” units use a lot of energy and emit harmful greenhouse gases. Scientists have discovered an environmentally friendly alternative: a vegetable film that cools when exposed to sunlight and has different textures and bright iridescent colors. The material may one day be able to keep buildings, cars and other structures cool without the need for external power.

The researchers will present their results at the spring meeting of the American Chemical Society (ACS).

“To make materials that stay cooler than the air around them during the day, you need something that reflects a lot of sunlight and doesn’t absorb it, which would transform the energy from light to heat,” says Silvia Vignolini, Ph. D., the main researcher of the project. “There are only a few materials that have this property, and the addition of color pigments would usually nullify their cooling effects,” added Vignolini.

Passive diurnal radiative cooling (PDRC) is the ability of a surface to emit its own heat into space without being absorbed by the air or atmosphere. The result is a surface that, without the use of electricity, can be several degrees cooler than the air around it. When used on buildings or other structures, materials that support this effect can help reduce the use of air conditioning and other energy-intensive cooling methods.

Some paints and films currently being developed can achieve PDRC, but most are white or have a mirror finish, says Qingchen Shen, Ph.D., who is presenting work at the meeting. Both Vignolini and Shen are at Cambridge University (UK). But a building owner who wanted to use PDRC blue would be out of luck—color pigments, by definition, absorb specific wavelengths of sunlight and reflect only the colors we see, causing unwanted warming effects in the process.

However, there is a way to achieve color without using pigments. Soap bubbles, for example, display a prism of different colors on their surfaces. These colors result from the way light interacts with the varying thickness of the bubble film, a phenomenon called structural color.

Part of Vignolini’s research focuses on identifying the causes of different types of structural colors in nature. In one case, her group found that cellulose nanocrystals (CNCs), which are derived from cellulose found in plants, can be made into iridescent, colorful films without any added pigment.

As it turns out, cellulose is also one of the few naturally occurring materials that can support PDRC. Vignolini learned this after hearing a lecture by the first researchers who created the chilling film material. “I was like, wow, that’s really amazing, and I never thought cellulose could do that.”

In a recent work, Shen and Vignolini layered colored CNC materials with a white-colored material made of ethyl cellulose to form a colored two-layer PDRC film. They produced films with vibrant blues, greens, and reds that, when placed under sunlight, averaged nearly 40 F cooler than the surrounding air.

 A square meter of foil generated more than 120 watts of cooling power, which rivals many types of residential air conditioners. The most challenging aspect of this research, Shen says, was finding a way to bond the two layers together—the CNC films themselves were fragile, and the ethyl cellulose layer had to be plasma treated to get good adhesion. However, the result was films that were robust and could be prepared several meters at a time on a standard production line.

Since the creation of these first films, researchers have been improving their aesthetic appearance. Using a method adapted from approaches previously explored by the group, they produce cellulose-based cooling films that are glittery and colorful. They also modified the ethyl cellulose film to have different textures, such as the differences between the types of wood finishes used in architecture and interior design, Shen says. These changes would give people more options when incorporating PDRC effects into their homes, businesses, cars and other structures.

The researchers now plan to find ways to make their films even more functional. According to Shen, the CNC materials can be used as sensors to detect environmental pollutants or weather changes, which could be useful if combined with the cooling performance of their CNC-ethylcellulose films. For example, a cobalt-colored PDRC on the facade of a building in a car-filled urban area could one day keep the building cool and contain detectors to alert officials to higher levels of smog-causing molecules in the air.

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