New rainbow films offer eco-friendly option for AC units, can cool your house and car

Researchers have developed an aesthetically pleasing film that reflects sunlight and radiates heat, reducing the need for air conditioning in buildings and cars.
Kavita Verma
Researchers develop plant-based film.
Researchers develop plant-based film.

Qingchen Shen  

Researchers at Cambridge University have developed an eco-friendly alternative to air conditioning units that can keep buildings and cars cool without requiring external power. The team created a plant-based film that can get cooler when exposed to sunlight and comes in a range of textures and iridescent colors.

This innovative material uses passive daytime radiative cooling, which means that it emits its heat into space without being absorbed by the air or atmosphere. The result is a surface that can be several degrees cooler than the air around it without using any electrical power.

A bi-layered, aesthetic, and effective film

The researchers layered colorful cellulose nanocrystals (CNCs) with a white-colored material made from ethyl cellulose, creating a bi-layered film that was both robust and effective. The films were made in vibrant colors, including blue, green, and red. When placed under sunlight, the films were an average of nearly 40°F cooler than the surrounding air and generated over 120 Watts of cooling power, rivaling many types of residential air conditioners.

The most challenging aspect of this research was finding a way to make the two layers stick together. The CNC films were brittle, and the ethyl cellulose layer had to be plasma-treated to get good adhesion. The result, however, was robust and could be prepared several meters at a time in a standard manufacturing line.

Potential future improvements

The team plans to improve the aesthetic appearance of the cellulose-based cooling films and find ways to make the films even more functional. The CNC materials can be used as sensors to detect environmental pollutants or weather changes, which could be useful if combined with the cooling power of their CNC-ethyl cellulose films.

The researchers acknowledge support and funding from Purdue University, the American Society of Mechanical Engineers, the European Research Council, the Engineering and Physical Sciences Research Council, the Biotechnology and Biological Sciences Research Council, the European Union, and Shanghai Jiao Tong University.

As stated by the release, the researchers presented their results at the spring meeting of the American Chemical Society (ACS), which is held virtually and in person from March 26-30, 2023.

The development of this plant-based film offers a promising solution to the issue of energy-intensive air conditioning units. With this innovative technology, buildings and cars can be kept cool without requiring external power, potentially reducing energy consumption and greenhouse gas emissions.

Study abstract:

Daytime radiative cooling (DRC) materials offer a sustainable approach to thermal management by exploiting net positive heat transfer to deep space. While such materials typically have a white or mirror-like appearance to maximize solar reflection, extending the palette of available colors is required to promote their real-world utilization. However, the incorporation of conventional absorption-based colorants inevitably leads to solar heating, which counteracts any radiative cooling effect. In this work, efficient sub-ambient DRC (Day: −4 °C, Night: −11 °C) from a vibrant, structurally colored film prepared from naturally derived cellulose nanocrystals (CNCs), is instead demonstrated. Arising from the underlying photonic nanostructure, the film selectively reflects visible light resulting in intense, fade-resistant coloration, while maintaining a low solar absorption (~3%).  Additionally, a high emission within the mid-infrared atmospheric window (>90%) allows for significant radiative heat loss. By coating such CNC films onto a highly scattering, porous ethylcellulose (EC) base layer, any sunlight that penetrates the CNC layer is backscattered by the EC layer below, achieving broadband solar reflection and vibrant structural color simultaneously. Finally, scalable manufacturing using a commercially relevant roll-to-roll process validates the potential to produce such colored radiative cooling materials at a large scale from a low-cost and sustainable feedstock.

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