| Passive daytime radiative cooling(PDRC)technology is an emerging and environment-friendly(real zero energy consumption and zero pollution)cooling technology,that can strongly reflect the solar irradiance and radiate infrared heat to outer space through the “atmospheric window” in the form of electromagnetic waves,thus inducing a sub-ambient temperature drop without consuming any energy.PDRC technology can be widely used in zero-energy buildings,efficient solar cells,human thermal management and other related fields.However,most of PDRC materials were facing a huge challenge in terms of scale expansion due to the low cooling performance,complex preparation technology,high cost,limited-service life,and easily contaminated.PDRC coatings were considered to be one of the most promising forms of utilization for large-scale promotion of PDRC technology in the future because of its good scene applicability,easy preparation and versatility.At present,although many studies had been carried out on PDRC coatings,in order to achieve the spectral selection characteristics,most of PDRC coatings were designed with multi-layer structures,which brought inconvenience to the subsequent preparation and practical application of coatings.So,it was necessary to develop a single-layer PDRC coating with selected properties.The ingenious photonic structure of biology in nature provided new ideas for the design of PDRC coatings.And coating design inspired by the coupling of surface structural parameters and particle parameters need to be investigated further.There was a lack of energy-saving data on materials used for long-term operation of outdoor buildings due to the limitations of the scale and cost of PDRC material preparation,so it was an urgent need to conduct experimental research on radiative cooling materials in the field of building energy-saving.This paper systematically studied the theoretical design,the regulation of spectral radiative properties,preparation and cooling performance of PDRC coatings.The main research contents include:Based on the Mie theory combined with the Monte Carlo ray tracing method,a differential-integral calculation method for the radiative characteristics of a particle cloud containing mixed heterogeneous particles was developed,and the structural parameters of the single-layer PDRC coating containing mixed heterogeneous particles were optimized.The comparison between the model and the experimental results verified the accuracy and feasibility of the model and algorithm.Taking the mixed particles of Ti O2 and Si O2 as an example,the effects of particle size,volume fraction,coating thickness,and substrate on the radiative characteristics of the PDRC coating were studied.Combined with the atmospheric transmission model,the effect of atmospheric water vapor on the spectral effective emissivity of the PDRC coating was studied.The results showed that the spectral emissivity and spectral characteristic parameters of the coatings increased with increasing particle volume fraction and coating thickness,and decreased with increasing particle size.The average effective emissivity in the “atmospheric window” band of the PDRC coating decreased significantly with the increase of water vapor.Inspired by the enhanced radiation emission force of human skin wrinkle structure,a radiative cooling coating with a biomimetic skin natural wrinkle structure was proposed.The Bio-RC coatings were designed and optimized by using the FDTD method,and the effects of surface shape(pyramid,moth-eye,natural wrinkle,smooth plane),wrinkle height,wrinkle angle and particle parameters on the spectral radiative properties of the Bio-RC coatings were studied.The optimum structure combination parameters of the coating were obtained,and the design method for the radiative cooling coating containing the particles with the surface microstructure was established.The results showed that the naturally wrinkled surface enhances the control of the coating’s dual-band radiative properties in the solar band and the “atmospheric window” band.The emissivity of the Bio-RC coating in the “atmospheric window” band first increased and then decreased with the increase of the height and apex angle of the natural wrinkle structure.The optimal values were 5 μm and 60°,respectively.Based on the optimized parameters,the Bio-RC coating with the natural wrinkle structure was prepared,and the forming mechanism of the natural wrinkle on the surface of the Bio-RC coating was analyzed.The surface morphology,internal structural characteristics and spectral characteristics of the Bio-RC coating samples were characterized and tested,and compared with the performance of the PDRC coating with the smooth surface.The different colors Bio-RC coatings were expanded.The properties of the coatings required for practical application were tested and the material production costs were calculated.The results showed that the surface of the Bio-RC coating presented randomly distributed wrinkle with an average roughness of ~5 μm;The BioRC coating could reflect 94.7% of solar irradiance,and the emissivity in the “atmospheric window” band was 96.2%,which was obviously higher than that of the coating with smooth surface.All performance of the Bio-RC coating can meet outdoor application standards.In order to study the cooling performance of Bio-RC coatings,an outdoor test platform for RC coatings was designed and built.The heat dissipation and cooling power tests of the Bio-RC coating were carried out,including the tests in different typical climate time periods,different placement angles and different colors.The effects of environmental factors such as solar radiation and wind speed impact on the radiative cooling power were analyzed.A 12-month test of the cooling effect of the Bio-RC coating applied to outdoor buildings and an air-conditioning power adjustment experiment within a period of time was carried out in Weihai,Shandong.The results showed that the BioRC coating achieved the cooling effect of an average effective cooling of 5.9 ℃ and a cooling power of 85.3W/m2 at noon.In summer,the average summer temperature difference of the roof/indoor air between two outdoor buildings was 10.4 ℃ and 4.4 ℃,respectively.More than 20% of the cooling power consumption of the air conditioning can be saved.In order to study the effect of different climatic environments and geographical regions on the energy-saving effect of PDRC technology,the energy-saving effect of the application of Bio-RC coatings for buildings was analyzed by using building energy analysis software combined with national meteorological data.The map of the application potential of PDRC technology in China was drawn. |
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