Research On Radiative Cooling Based On Multilayer Dielectric Films

Since the industrial revolution,the greenhouse effect has been intensified,and the energy consumption caused by the cooling demand of human beings has been increasing day by day.In order to reduce energy consumption,a passive cooling mechanism,namely radiative cooling,which is independent of external electric power,is proposed.Realizing radiative cooling under sunlight is challenging because it relies on specific materials or structures that are highly reflective of sunlight and highly emitting of infrared heat.In this thesis,several kinds of radiation cooling devices based on multi-layer dielectric films were investigated in both theory and experiment.The main research contents are as follows:(1)A multilayer radiative cooler based on silicon dioxide(SiO2),titanium dioxide(TiO2)and silver(Ag)was designed,achieving a high reflectivity in the solar radiation band(0.3 μm-2.5 μm)and a high emissivity in the atmospheric window band(8 μm-14μm).The theoretical calculation of the cooling power showed that the multi-layer film structure could achieve a cooling power of 95 W/m2 and a cooling amplitude above 10℃.The SiO2/TiO2/Ag multilayer film was prepared,and the typical setup for the cooling test was designed.The measured results showed that the multilayer film prepared according to the design could continuously keep cooler than the ambient during the day,and the cooling peak could reach more than 10℃.(2)Given the defect of insufficient reflection of metallic silver reflector in the ultraviolet band in the current radiation cooler,a multilayer film structure based on photonic crystal was designed by using the finite element method.The photonic bandgap is expanded by the superposition of photonic crystals,which not only improves the reflectivity in the solar band but also achieves nearly ideal selective radiation.The results showed that the average reflectivity of the designed photonic crystal composed of SiO2 and Si3N4,together with Ag reflector,could reach 98.88%in the solar irradiation band,and its emissivity in the atmospheric window is as high as 90%.As the ambient temperature is 315 K,the radiative cooler can achieve a cooling power of up to 106 W/m2 and a cooling effect exceeding 11℃.(3)Although photonic crystal radiators have excellent radiation cooling performance,the number of layers is relatively large.In order to reduce the layer number of the multilayer film and decrease the fabrication difficulty,the thesis employed the genetic algorithm and differential evolution algorithm to optimize the multilayer film based on SiO2 and Si3N4 materials.The final optimization results show that a cooling power of 87 W/m2 can be achieved through a eight-layered multilayer structure.According to the optimized structure parameters,the multilayer films were prepared by plasma chemical vapor deposition,and the outdoor cooling test was carried out.The experiment showed that the film structure could achieve a cooling peak of 8℃,and an average cooling effect of more than 5℃ during the day with a sunny condition.In addition,the cooling amplitude is closely related to the solar radiation intensity and atmospheric humidity,among which atmospheric humidity has a greater impact on the cooling effect of the film,and the greater the humidity,the worse the cooling effect.