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Layered radiation-cooled film based on cellulose acetate prevents snow and ice from melting in sunlight

Time : 2022-03-07 Hits : 34

Preventing snow and ice from melting in the sun in a sustainable way is an important issue in many areas of human life, from daily cold-chain foods to ice sports to melting icebergs at high altitudes/latitudes. According to statistics, 40% of the world's conserved food (about 400 million tons) is preserved through the cold chain, which consumes 11% of the world's electricity and contributes to about 2.5% of global greenhouse gas emissions.

The melting of ice in the sun is linked to changes in the energy flow in the ice system. Figure 1 summarizes the typical heat flow of ice at different latitudes. As can be seen from the figure, solar radiation (wavelength 0.3~2.5μm) is the main heat load, causing the rise in ice temperature and consequent melting. Meanwhile, mid-infrared radiation (wavelength 2.5~18μm) is the main energy flow to counteract this trend. Therefore, it is of great practical significance to explore a sustainable way to balance this energy flow so as to achieve passive protection of various ice systems under sunlight.


Figure 1. Representative heat flow of ice at different latitudes

The recent boom in daytime radiation cooling offers a strategy for balancing the energy flow. Researchers have developed a variety of special materials and structures, such as multilayer/patterned photonic structures, nanoparticle/porous polymer based films, cooled wood, and ultra-white coatings (solar reflectivity greater than 0.95). The experimental cooling power of the above method can reach 40~100 W·m-2 under clear daylight, and the sub-ambient cooling temperature is 3~13°C. However, in order to preserve ice in the sun, there are several strict requirements in addition to traditional considerations. First of all, in practical applications the ice needs to be at a lower temperature, so it needs very high radiative cooling performance. For example, it has been calculated that increasing the net radiated power from 70 W·m-2 to 110 W·m-2 can prevent the melting of ice/frozen food without the need for additional refrigeration units. In addition, for the special requirements of cryopreservation, radiative cooling materials must be abundant, mass-produced and have a low impact on the environment.

For this reason, Jia Zhu and Minghuai Wang of Nanjing University have developed a cellulose-acetate (CA) based layered radiation-cooled thin film that can be mass-produced, ecologically friendly, and provides effective passive protection for various forms/sizes of ice in the sun. This work provides an important way to develop effective, mass-produced and sustainable snow and ice conservation, and provides inspiration for conservation approaches for other key elements of the ecosystem. The research is published in Science Advances in a paper titled "Protecting Ice from Melting Under Sunlight via Radiative Cooling."

Design and characterization of radiative cooling thin films

The intrinsic vibration of molecules enables CA films to have broadband and high infrared emissivity, which is conducive to high performance large-scale cooling. Customized pores act as effective scattering centers for incoming solar radiation, giving the CA film high solar reflectivity. Therefore, the layered design of the film minimizes the thermal load of the ice in sunlight and achieves effective passive protection of the ice system at different latitudes. At the end of the life cycle, layered CA films can be digested by microorganisms in nature and decomposed into CA. Layered CA films are made from natural cellulose, which is widely found in plant cell membranes and can be obtained from the cell embryos of natural plants.


FIG. 2 Layered design and life cycle of porous CA membrane

The researchers achieved the desired optical properties of the CA film through a layered design, and measured the cooling temperature and cooling power of the CA film. The cooling temperature is defined as the temperature reduction of the film from the ambient temperature. Under direct sunlight, the layered CA film achieves cooling power up to 110 W·m-2 and cooling temperature of about 12℃, which confirms the good radiative cooling ability of the layered CA film.


Figure 3 Outdoor ice/chilled food preservation in low/mid-latitude areas

Radiation-cooled film for ice and snow protection

Ice and snow surfaces (tiny ice crystals) are two typical landforms. They have different solar reflectance, but similar mid-infrared emissivity. The researchers demonstrated the passive cooling effect of layered CA membranes on ice and snow surfaces. As shown in the figure below, ice melting at high latitudes is dominated by radiative energy transfer processes resulting from the output of incoming solar radiation and mid-infrared radiation. Since both incident solar radiation and mid-infrared radiation output are independent of surface area size, the researchers used small-scale experiments to evaluate the cooling effect of layered CA films on ice at high latitudes. The results show that the temperature difference between CA films with and without layered design is as high as 6.3℃. As shown in Figure C below, even at an average solar radiation of about 700 W·m-2, the surface temperature of ice with layered design CA films is about 7℃ cooler than the ambient temperature.In addition to ice, layered CA film also provides good protection for snow surface. The researchers conducted field tests on snow surfaces with and without layered CA membranes. The results showed that snow with layered CA material had 50 percent more residue than bare snow. Further results directly confirm that the radiative cooling film can effectively slow the melting of local glaciers.


FIG. 4. Protection of snow and ice by layered design of CA film

The cooling effect of ice in high latitude area is modeled and analyzed

The researchers used a model to assess the expected cooling effect of the film at scale. The researchers used thermodynamic models to study the melting of an iceberg (a chunk of ice that breaks away from its parent glacier) that measures dozens of square kilometers (the size of a large ski resort). It turned out that the iceberg had lost 1m of its thickness through melting during the summer. In contrast, layered CA membranes add 1m to the iceberg's thickness. In addition, the researchers used climate model experiments to extend layered CA films to sea ice within the Beaufort Current region. In these specific areas, the concentration of sea ice protected by CA film increased by 5% to 40% and the thickness increased by 0.5 to 2.5m, indicating that the method can effectively protect the ice system in high latitude area/target.

Finally, the biodegradability of the layered CA membrane at the end of its service life was fully considered. Soil exposure tests (at 31 ° N in Nanjing) have shown that layered CA membranes exhibit the best biodegradability in various polymers for the development of radiative cooling materials. Therefore, it is considered to provide environmentally friendly protection for snow and ice at different latitudes.


Figure 5. Protection of ice domains at high latitudes

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