| Low-temperature outer space is the most important cold source of the Earth and also the crucial guarantee for maintaining the energy balance of the Earth.The radiative sky cooling(RSC)technology makes full use of the low-temperature cold source characteristic of outer space by dissipating heat from the Earth's surface to outer space in the form of thermal radiation via the atmospheric window that has a high transparency within the wavelength range of 8-13?m;Thus passive cooling can be achieved.This unique passive characteristic has greatly broadened its application background and attracted extensive attention from research scholars.The key to RSC is the spectral characteristics of the emitter.The emitter needs to have a high emissivity in the mid-infrared wavelength band(especially the atmospheric window band).If the emitter is to achieve passive cooling below the ambient temperature under solar radiation,the emitter also needs to have a low solar absorption,so the RSC has relatively strict requirements on the spectral characteristics of the emitter.Besides,considering the characteristics of single function and low cooling power of RSC,it is necessary to systematically analyze the heat transfer process of RSC and the effect mechanism of various factors on the performance of RSC to further explore the methods of improving RSC performance.At the same time,considering the characteristics of the muti-energy demand in practical applications,the homologous characteristics of the solar radiation and thermal radiation,and the objective fact that the solar radiation power is an order of magnitude greater than the net cooling power of RSC,if RSC and solar energy utilization process are combined to the same device,not only the dual-function characteristics of the device be realized,but also the energy yield of the device can be improved.Based on the above analyses of RSC,and considering that the current photovoltaic conversion(PV)technology has been relatively mature,this dissertation is focused on the RSC and the comprehensive utilization of RSC and PV(RSC-PV).The detailed research content is as follows:1.A thermal analysis model of RSC is established.The cooling potential of RSC is systematically analyzed and predicted,and the effect of different environmental and structural parameters on the performance of RSC is investigated.The results show that solar irradiance,cooling loss of the emitter,and transmittance of the atmospheric window are key environmental parameters that affect RSC.Therefore,reducing the solar absorption of the emitter,enhancing the thermal insulation performance of the RSC device,and dry/sunny weather are beneficial to improve the cooling performance of RSC.2.A spectrally selective emitter composed of silicon dioxide and silver film is designed and fabricated.Its reflectivity in the solar radiation band is approximately 0.96,and the average emissivity in the atmospheric window band is about 0.85.Besides,the emitter also exhibits high thermal emission characteristic in the remaining wavelength band.Experimental results show that the emitter can achieve an obvious cooling effect that is about 5.9? lower than the ambient temperature under solar radiation in Beijing,but it cannot achieve a cooling effect lower than the ambient temperature in a similar time and environment in Hefei,which indicate that the performance of the RSC will be restricted by region or environment.3.A strategy that keeps emitter away from the sun for RSC is proposed and experimentally verified.The results show that the emitter shaded from sunlight can easily achieve a cooling effect lower than the ambient temperature,even a mid-infrared selective emitter with a solar reflection of only 0.66 can achieve an average temperature of 3.2? lower than the ambient temperature.However,the emitter that faces the sun is difficult to achieve passive cooling,even a highly reflective mirror with a solar reflection of 0.93 can't achieve a cooling effect below ambient temperature.4.A new idea of comprehensive utilization of radiative sky cooling and photovoltaic conversion(RSC-PV)is proposed.A set of RSC-PV comprehensive utilization experiment system is designed and constructed.This system uses photovoltaic conversion to generate electricity during the daytime and uses radiative sky cooling to obtain cooling at night.The experiment results show that the maximum radiative cooling power of the system during nighttime exceeds 70 W/m2 and the average photovoltaic conversion efficiency during daytime is approximately 14.9%,which indicates that the system has obvious dual function characteristic of cooling and power generation.5.From the optical point of view,a broadband spectrally selective surface composed of a one-dimensional multilayer film and a two-dimensional periodic microstructure of air holes is designed for the RSC-PV comprehensive utilization.Spectral analysis shows that the designed surface has a high transmittance in the photovoltaic response band of silicon solar cells(0.3-1.1?m)with an equivalent transmittance of about 0.9,it has a high reflectance in the remaining solar radiation band(1.1-4 ?m)with an equivalent reflection of approximately 0.6,and it has a high emissivity in the mid-infrared band with an equivalent emissivity of approximately 0.9.Besides,the combined thermal and electrical evaluation shows that the designed surface can improve the comprehensive utilization performance of the RSC-PV.6.Based on the idea of spectrally selective utilization,a discussion is made on the methods of spectrally passive thermal management of solar cells,including solar spectrum splitting,enhanced radiative sky cooling,and full-spectrum thermal management.Simulation analysis shows that all three spectrally passive thermal management methods can reduce the temperature of the solar cells.The cooling effect of the full-spectrum thermal management is the best.The cooling effect of enhanced radiative sky cooling is independent of the properties of solar cells,while the cooling effect of solar spectrum splitting is related to the properties of solar cells. |
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