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 sunlight. Fundamentally, ice melts under sunlight due to the imbalance of energy flow of incoming sunlight and outgoing thermal radiation. Radiative cooling can therefore balance the energy flows without energy consumption to sustainably protect ice.
In a new report now published in Science Advances, Jinlei Li and a team of scientists in advanced microstructures, engineering, atmospheric sciences, and fine mechanics and physics, in China and the US, developed a hierarchically designed radiative cooling film using abundant and eco-friendly cellulose acetate molecules. The versatility of the materials provided effective and passive protection to ice in various forms and scales, under sunlight. The outcomes can effectively provide a scalable and tuneable route to preserve ice and other critical elements of ecosystems.
The process of ice preservation can impact daily ice or iced food, iced sports and iceberg levels at high altitudes or latitudes. Research shows how cold chain logistics alone consume 11 percent of global electricity, and approximately 2.5 percent of the world’s greenhouse gas emission to preserve 40 percent of the world’s food. Resource limitations have clearly arisen due to imbalanced energy flow due to ice melting under sunlight, and it is of great practical significance to balance this and create a sustainable path for passive preservation of ice systems under sunlight. The bioengineering development in daytime radiative cooling offers a promising strategy to balance the energy flows. Materials scientists have used a range of materials and structures in these promising works, including multilayer or patterned photonic structures, porous poly film based on nanoparticles, cooling wood and super-white paints with solar reflectivity greater than 0.95. To preserve ice under sunlight, several stringent requirements must be in place. For example, a calculated increase of net radiation power from 70 to 110 Wm-2 can prevent ice or ice food from melting without additional refrigeration. In this work, Li et al. designed a hierarchical film based on abundant and eco-friendly cellulose acetate (CA) to achieve high cooling performance.
Biodegradable cellulose acetate (CA) materials for enhanced cooling performance
The material showed favourable traits for high-performance, large-scale cooling applications due to broadband and high mid-infrared emissivity. The tailored pores functioned as effective scattering centres for incoming solar radiation to minimize thermal load on ice under sunlight to realize passive protection for ice systems. Li et al. chose CA to construct the film due to its eco-friendly abundance as a biodegradable film that can undergo degradation in nature. The team derived raw cellulose acetate (CA) from natural cellulose, which exists in the plant cytoderm. To design, develop and characterise the cellulose acetate film, the team showed how the impact of broadband and the effective reflection of sunlight allowed the realisation of cooling under sunlight. Based on the theoretical model, Li et al. showed a cellulose acetate-based film with a porous structure, with multiple pore sizes to support strong scattering and reflection of sunlight. To accomplish this, they developed a CA molecule-based scalable film using roll-to-roll electrospinning. They presented the microscopic appearance of the product with nanofibers connected to form multiple pores with varying sizes, to ideally scatter sunlight.
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Hierarchically designed and eco-friendly CA film for passive protection of ice under sunlight via radiative cooling. Energy transfer process of ice systems at (A) low/middle and (B) high (>70.5°N) latitudes, respectively, in a unit of watts per square meter. The solar irradiation and mid-infrared emission are the dominant energy input and output for both scenarios. The unbalanced energy flows lead to the melting of ice. (C to F) Hierarchical designs and life cycle of the porous CA film for realizing passive ice protection via radiative cooling. (C) The intrinsic molecular vibrations and (D) porous structure endow the CA film with high mid-infrared emissivity and solar reflectivity, respectively. Therefore, the thermal loads on the ice systems are substantially reduced with the hierarchically designed film. (E) At the end of the life cycle, the hierarchically designed CA film can be decomposed by the natural microorganism to reproduce CA (right). (F) The abundant raw materials of the hierarchically designed CA film can be derived from the cytoderm of natural plants. Image credit: Science Advances