| Nowadays the building energy consumption occupies about one third of the social total energy consumption in China,in which heating and cooling represente the largest portion.In seasons when or regions where the climate is hot,the cooling energy demand is much higher than that of the heat,and the solar thermal collector is thus of little value.Additionally,a traditional solar collector remains idle at nighttime in the absence of solar irradiation.Objects on the ground are usually able to gather cooling power through radiative cooling,that is,by radiating heat to the cold outer space and the upper atmosphere through the atmospheric window(8～13μm).Similarly,a traditional radiative cooler is not quite suitable for seasons when or areas where the climate is cold.In addition,the cooling power of a radiative cooling device is usually one order of magnitude less than that of solar irradiation.Therefore,such a device cannot easily and economically produce a cooling effect at daytime,and call for a long payback period.Since the flat plat solar collector and radiative cooling collector operate at opposite time period and provide opposite function,while have the similar components such as the cover,collecting plate,insulation material and frame.If nocturnal radiative cooling can be integrated with diurnal solar heating into a single unit using same components,the initial cost of this hybrid collector will be reduced significantly.What is more,this collector can provide heat energy at daytime and cooling energy at nighttime to achieve a multi-functional and all day operation.Inspire by this,the present thesis carried out significant research on the comprehensive utilization of diurnal solar heating and nocturnal radiative cooling.The main achievements of this thesis are outlined as follows:1.The idea of comprehensive utilization of diurnal solar heating and nocturnal radiative cooling is proposed along with the spectral selectivity principle of the key components of the solar heating and radiative cooling collector(short for SH-RC collector or PT-RC collector).An ideal PT-RC collecting plate should exhibit high spectral absorptivity(emissivity)in both solar radiation and atmospheric window bands so that it could obtain a favorable heating efficiency during daytime and a large cooling power at nighttime.Furthermore,the plate should exhibit low spectral absorptivity(emissivity)in other bands aside from solar radiation and atmospheric window wavelengths,allowing it to reduce daily radiant heating loss and nightly radiant cooling loss when exchanging heat with the sky and the surroundings.Correspondingly,an ideal PT-RC transparent cover should possess high spectral transmittance in both solar radiation and atmospheric window bands,and own high reflectance in other bands aside from solar radiation and atmospheric window wavelengths.2.A real PT-RC collecting plate is designed and manufactured.It consists of an aluminum substrate,a layer of Ti-based solar absorbing coating and a layer of 30μm-thick PET film.The new composite plate after natural cooling was called the PT-RC collecting plate.The collecting plate has high spectral absorptivity of approximately 0.92 in the solar radiation band and high emissivity of approximately 0.8 in the atmospheric window spectrum.In other bands aside from the solar radiation and atmospheric window wavelengths the plate exhibit relatively high reflectance of approximately 0.45.3.A PT-RC collector is designed and fabricated.Also,a PT-RC experimental set-up was built to investigate the solar heating and radiative cooling performance of the PT-RC collector under different working modes and conditions.In diurnal collector testing mode,thermal efficiency of the SH-RC collector at zero-reduced temperature was 62.7%,which was about 86.4%of that of the traditional flat-plate solar heating collector.In diurnal system testing mode,the daily average thermal efficiency of the PT-RC system at zero-reduced temperature was 38.6%.In nocturnal collector testing mode,the PT-RC collector had net radiative cooling powers of 50.3 W/m2 on a clear night and 23.4 W/m2 on an overcast night.4.A quasi-steady-state mathematic model that considers the spectral radiant distribution of the sky,the transparent cover and the collecting surface was established.The RMSD between the experimental and simulation results are only 4.59%in solar heating mode and 4.90%in radiative cooling mode,which proves that the established mathematic model could predict the thermal performance of the PT-RC system in both modes.5.Based on the validated mathematic model,the effects of different working parameters on the thermal performance of the PT-RC system are investigated.Furthermore,the annual performances of the PT-RC system in four cities of different climates are performed.It is found that the system operated in Beijing occupies both the optimum solar heating and radiative cooling performance,the system exhibits the weakest solar heating performance worked in Hefei and the worst radiative cooling performance run in Fuzhou.6.On basis of the research of comprehensive utilization of solar heating and radiative cooling,a new idea of integrated utilization photovoltaic,photothermic and radiative cooling(PV-PT-RC)is proposed in the study.A combined PV-PT-RC system can obtain electricity and heat energy through photovoltaic and photothermic conversions in the daytime and gather cooling energy in the nighttime through radiative cooling.The proposed system presents a multifunctional advantage compared with the traditional PV panel,solar collector,PV/T collector,and RC device.A PV-PT-RC module is designed and fabricated to investigate its thermal performance.Besides,a quasi-steady-state mathematic model is established to theoretically evaluate the performance of the module under different working conditions. |
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