Preparation And Cooling Performance Of Passive Daytime Radiation Cooling Films

To reduce the huge energy consumption and the resulting environmental problems such as global warming,a green and energy efficient cooling technology is urgently needed.Passive radiative cooling is a new type of cooling technology that does not consume any energy but only uses its own special structure and properties to transfer heat to the cold outer space in the form of thermal radiation through infrared atmospheric windows.The ideal passive radiative cooling material requires two requirements,namely a high emissivity in the atmospheric window（8μm-13μm）and a high reflectivity in the solar wavelength band（0.2μm-2.5μm）.With this objective in mind,a series of composite films based on polymer compounds with inorganic nanoparticles as additives were prepared and their morphology and radiation cooling properties were investigated in this paper to systematically investigate the relationship between material components,microstructure and cooling performance,and to propose a possible passive radiation cooling mechanism.The main studies include the following:（1）Three-dimensional porous composite films were obtained by added TiO₂nanoparticles to cellulose acetate（CA）through a simple solvent replacement strategy.Due to the different volatility of the solvents,numerous cavities were formed in the cellulose substrate during the volatilisation process,the cavity size was regulated by controlling the rate of solvent volatilisation to an average distribution of about 5μm,at which size the cavities reflect sunlight strongly according to the Mie scattering theory.Meanwhile,TiO₂,a transition metal oxide with semiconducting properties,exhibits properties such as high refractive index and high infrared emissivity.The synergistic effect of the introduced TiO₂nanoparticles and the cavities can effectively increase the sunlight reflectivity up to 97%.Actual outdoor test results show that the composite film exhibits a cooling effect of approximately 10°C even under high density solar radiation（897 W/m²）conditions.（2）The hollow TiO₂（H-TiO₂）was obtained by structural modulation of TiO₂nanoparticles with high refractive index using the template method.The nanoparticles with hollow structure can obtain high emissivity in the mid-infrared and far-infrared wavelengths,while porous PLA/H-TiO₂ composite films were prepared by electrostatic spinning using biodegradable polylactic acid as the substrate.A series of flexible porous films were prepared by regulating the voltage to achieve an inter-fibre pore size distribution between 0.8μm-5.0μm and by controlling the spinning time to produce 1 mm thick composite films,varying the content of inorganic nanoparticles.The results show that the reflectance,emissivity and cooling performance of the PLA/H-TiO₂ composite films increase gradually with increasing H-TiO₂ content;the PLA/H-TiO₂ composite film with 8 wt%H-TiO₂and a thickness of 1 mm has the best cooling performance,with 98%reflectance in the solar region and 94%emissivity in the far infrared region,and achieves a cooling of almost 14°C at a solar irradiance of 988 W/m².The series of composites prepared have a unique microstructure,exhibit excellent cooling performance,and can be produced on a large scale to reduce energy consumption,which has a broad development prospect.