Researchers have unveiled a sustainable substitute for air conditioning systems, which consume high levels of energy and release harmful greenhouse gases. The alternative is a plant-based film that cools down upon exposure to sunlight and is available in different vivid colors and textures. This innovative material has the potential to cool down buildings, cars, and other constructions without needing external electricity.
The researchers will present their results at the spring meeting of the American Chemical Society (ACS). ACS Spring 2023 is a hybrid meeting being held virtually and in-person March 26–30.
“To make materials that remain cooler than the air around them during the day, you need something that reflects a lot of solar light and doesn’t absorb it, which would transform energy from the light into heat,” says Silvia Vignolini, Ph.D., the project’s principal investigator. “There are only a few materials that have this property, and adding color pigments would typically undo their cooling effects,” Vignolini adds.
Passive daytime radiative cooling (PDRC) refers to the capability of a surface to emit its own heat into space without it being absorbed by the air or atmosphere. Such surfaces can become several degrees colder than the surrounding air without using any electrical power. Materials that promote this effect can be used in buildings and other structures to reduce the use of energy-intensive cooling methods like air conditioning.
Although some paints and films have been developed to achieve PDRC, most of them are either white or have a mirrored finish. According to Qingchen Shen, Ph.D., who is presenting the research at a meeting, a building owner who wants to use a blue-colored PDRC paint would not be able to find one. Colored pigments absorb specific wavelengths of sunlight and only reflect the colors we see, which can cause undesirable warming effects.
However, there is a way to achieve color without using pigments. Structural color, which is the phenomenon where different thicknesses of a film interact with light to create a prism of colors, can be seen in soap bubbles. Professor Silvia Vignolini’s research focuses on identifying the causes behind different types of structural colors found in nature. Her group has found that cellulose nanocrystals (CNCs), derived from plant cellulose, can be transformed into iridescent, colorful films without any added pigment.
As it turns out, cellulose is also one of the few naturally occurring materials that can promote PDRC. Vignolini learned this after hearing a talk from the first researchers to have created a cooling film material. “I thought wow, this is really amazing, and I never really thought cellulose could do this.”
Shen and Vignolini have developed a bi-layered PDRC film that combines colorful CNC materials with a white ethyl cellulose material. They created films with vivid blue, green, and red colors that were nearly 40 F cooler than the surrounding air when exposed to sunlight. A square meter of the film produced over 120 Watts of cooling power, making it comparable to many residential air conditioners. However, getting the two layers to adhere to each other was the biggest challenge, requiring plasma treatment of the ethyl cellulose layer. The researchers have since improved the films’ aesthetics by making them glittery and colorful, and by altering the texture of the ethyl cellulose layer.
The researchers are now focusing on making the films more functional by incorporating sensors made from CNC materials. These sensors could detect environmental pollutants or changes in weather, enhancing the usefulness of their CNC-ethyl cellulose films. For instance, a cobalt-colored PDRC on a building facade in a densely populated urban area could provide cooling while also detecting higher levels of smog-causing molecules in the air and alerting officials.
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