| Renewable energy application is an important mean of building energy conservation.Among them,solar energy,as a clean,universal and long-term green energy,has incomparable advantages over conventional energy.The flat-plate solar collector has strong pressure bearing capacity,is easy to integrate with the building,and has a high proportion in the solar heating utilization market.At the same time,the temperature of outer space is close to absolute zero,so it is a very ideal natural cold source.The nocturnal radiant cooler can use the 8～14μm band"atmosphere window"to exchange radiant heat with the outer space,so as to realize the passive refrigeration,which has the strong energy conservation potential.Due to the structural similarity between flat-plate solar collector and nocturnal radiant cooler,and the complementarity in working hours,the two devices can be integrated:collecting solar energy heat in the daytime to provide domestic hot water for users,using the sky as a cold source at night to realize radiative cooling,and reducing the energy consumption of air conditioning in summer.In order to realize the radiative cooling function,the transparent cover plate of the existing integrated device mostly adopts PE film with high transmittance for the whole band.In the daytime,the cover plate only plays the role of"wind screen",and the long wave radiant heat loss is very large,which cannot achieve high heat collection temperature and efficiency.This paper is devoted to solve the contradiction between the radiant heat loss in the daytime and the radiative cooling at night,and to develop an integrated device of solar energy collection and nocturnal radiative cooling.Firstly,the paper analyzes the radiation characteristics of the sun and the night sky,and the basic heat transfer characteristics of the solar collector/radiant cooler.On the basis of the traditional flat-plate solar collector,the device takes the glass cover plate as the radiative cooling surface,and the air interlayer between the cover plate and the collector core as the cooling medium flow channel,so as to realize the radiative cooling function.In order to improve the cooling power,a transparent radiative cooling film is pasted on the outer surface of the glass cover plate,and a"wind screen"is stamped on the top surface of the device.Among them,the ideal radiative cooling film should have high emissivity in the"atmosphere window"band,high reflectivity in the mid far infrared band outside the"atmosphere window"and high transmittance in the solar energy heat utilization band;the ideal"wind screen"should have high transmittance in the solar energy heat utilization band and the"atmosphere window"band,and high reflectivity in the mid far infrared band outside the"atmosphere window".In this paper,PET film is used as the radiative cooling film and PE film as the"wind screen".Then,the theoretical model of transient one-dimensional heat transfer of the integrated device is established,and the heat balance equations of glass cover plate,cooling medium and collector plate core are itemized.The solution method of the theoretical model is proposed:the theoretical heat transfer model is discretized,and the boundary conditions and initial conditions are the inlet temperature,flow rate of working medium,environmental parameters,structural parameters and surface temperature of the device.Under the condition,the heat balance equations are solved iteratively by MATLAB software,and the outlet temperature and effective cooling power of the device are calculated.Among them,the paper focuses on the calculation method of the effective radiation term of the model.It is considered that the radiative cooling surface of the device has obvious spectral selectivity only in the"atmosphere window"band,and its emissivity in the"atmosphere window"band and other mid far infrared band is regarded as a fixed value,which simplifies the calculation of the effective radiation and ensures the certain accuracy.Then,according to the design scheme,the test platform of the integrated device is built to test the performance of the solar energy collection and radiative cooling.The theoretical model of transient heat transfer is verified.Two groups of static comparative tests were designed to analyze the influence of single and double PE film structure and whether to lay PET film on the radiative cooling performance of the device:if the temperature difference between the surface of glass cover plate and the environment under the condition of no ventilation was taken as the index to measure the radiative cooling performance of the device,compared with the single PE film structure,the radiative cooling performance of the device was improved by 10%by using the double PE film structure;compared with the device without PET film,When a layer of PET film is laid on the surface of the glass cover plate,the radiative cooling performance of the device is improved by 36%.Two groups of continuous ventilation tests are designed.Under the test conditions,the temperature of the air through the device can be reduced by2.4～5.6℃,and the cooling power per unit area of the device is 6.4～15.2w/m~2.The test results of the device outlet temperature and cooling power per unit area are in good agreement with the model calculation results,among which,the absolute error between the theoretical value and the measured value of the device outlet temperature is between0.07℃～0.73℃,while the relative error between the theoretical value and the measured value is between 0.43%and 25.2%,and the root mean square error is 12.2%.The experimental results show that the transient intercept heat collection efficiency of the integrated device is increased by 15.3%,and the total heat loss coefficient is reduced by32%.Finally,the theoretical heat transfer model of the device is used to analyze the outlet temperature and cooling power per unit area of the device under different working medium types,different environmental parameters(ambient dry bulb temperature,ambient humidity,etc.),design parameters(thickness of air insulation layer,height of air passage,radiation property of key components,etc.)and operation parameters(inlet temperature of working medium,mass flow of working medium,etc.)The numerical calculation is carried out to analyze the influence of various factors on the radiative cooling performance of the device,and then the corresponding optimization scheme of the device is put forward.After the optimization of the device,the cooling power per unit area is significantly increased,which is about 3 times of the cooling power per unit area before the optimization.Four cities,Urumqi,Beijing,Shanghai and Chongqing,were selected as representatives to analyze the radiative cooling performance of the device in different regions.It can be seen that in Urumqi,the radiative cooling performance is the best,the temperature difference between the air inlet and outlet is about 4.1℃,and the radiative cooling power per unit area can reach 51.8w/m~2,followed by Beijing,Shanghai and Chongqing.The integrated device is more suitable for Urumqi,Hohhot,Lanzhou and other areas with low air moisture content in summer and a certain demand for cooling. |
