Radiative cooling: protecting ice from melting under sunlight

Researchers have developed a biodegradable film made of cellulose acetate which, thanks to its radiative cooling properties, effectively protects ice subjected to solar radiation from melting, with possible applications for iced food and glaciers. 

Ice plays a significant role in many aspects of life, ranging from food preservation to ice sports and ecosystems, thereby providing incentive to protect ice from melting under solar radiation. Fundamentally, ice melts under sunlight due to the imbalance in energy flow from incoming sunlight and outgoing thermal radiation. 

The bioengineering development in daytime radiative cooling offers an interesting strategy for balancing the energy flows. Scientists have used a range of materials and structures in these promising works, including multilayered or patterned photonic structures, porous poly film based on nanoparticles, cooling wood and super-white paints with solar reflectivity greater than 0.95. 

Radiative cooling can therefore balance the energy flows without energy consumption to sustainably protect ice. 

To preserve ice under sunlight, several stringent requirements must be met. For example, an increase of net radiation power from 70 to 110 Wm-2 can prevent ice or frozen food from melting without additional refrigeration. 

In a study published in Science Advances (1), a team of scientists presents a hierarchically designed radiative cooling film developed using abundant and biodegradable cellulose acetate (CA) molecules. The film uses connected nanofibers to form multiple pores of varying sizes to ideally scatter and reflect sunlight. 

They monitored the temperature evolution of iced food wrapped in different materials under natural outdoor sunlight. This results in longer preservation of iced food covered with the CA film below 0 degrees Celsius for 5.5 hours owing to optical properties of the packing materials. The material provided highly effective passive preservation with 98% integrity after 80 minutes of sunlight. 

Using climate model experiments, the scientists also showed how the CA film effectively protected ice systems such as glaciers at high latitudes. Its design and excellent radiative cooling capability made it possible to lower the surface temperature by 4°C at high latitudes.


(1) https://phys.org/news/2022-02-radiative-cooling-ice-sunlight-iced.html