Experimental And Numerical Simulation Study On Operational Thermal Characteristics Of Solar Vacuum Tube Collector With Phase Change Energy Storage

With the rapid development of economy,the total energy consumption of China is also growing rapidly.China's energy development is facing many problems and challenges.In order to solve these problems,China's energy structure must be adjusted to increase the proportion of clean energy consumption.Solar energy resources are abundant in many regions of China,but the time-space asymmetry of solar energy has limited the further utilization of solar energy resources.The solar vacuum tube collector with phase change energy storage combined solar thermal utilization and phase change energy storage technology,which are perfectly complementary to solve the problem of the time-space asymmetry of solar energy.Based on the solar vacuum tube collector with phase change energy storage proposed by previous studies,this paper studied thermal performance of the solar vacuum tube collector with phase change energy storage experimentally and numerically.The main contents and conclusions of this research are as follows:(1)This paper analyzed the basic principles and formulas of solar radiation,and gave theoretical methods for calculating parameters such as solar irradiance.Based on the experimental requirements,the solar irradiance data collected in the experiment were modified to provide a reference for subsequent calculation of collector efficiency and calculation of heat flux in numerical simulation.At the same time,this paper analyzed the heat transfer characteristics of the solar vacuum tube collector with phase change energy storage based on the research of the solar vacuum heat collection tube.The heat storage amount of the phase change material in the vacuum tube was used as useful energy and the corrected average value of the solar irradiance was used as the calculation value in the calculation formula of the efficiency of the collector.In addition,the principle of phase change energy storage provided a reference for calculating the heat absorption and heat release of phase change materials in experiments.The calculation method used in the numerical simulation chapter is also given.(2)This paper studied the solar vacuum tube collector with energy storage by experimental methods.The research indicated that during sunny and cloudy conditions in the heat absorption phase,the calculated collector efficiency reached30% to 45%.However,under cloudy conditions,the solar irradiance is very low,the temperature rise of paraffin is also low and the paraffin is not completely melted.Theefficiency of the collector is only about 6%.At the same time,in the endothermic phase,due to the natural convection of water in the U-shaped copper tube and liquefied paraffin,the paraffin of the vacuum tube will have a temperature stratification phenomenon of high paraffin temperature at the top and low paraffin temperature at the bottom.In the exothermic stage,the amount of hot water that can be provided at 50? under sunny conditions has reached 2.89 L,and the amount of hot water at 40? has reached 4.44 L.In the experiment of phase-change energy storage solar vacuum collector tubes running in series,due to the natural convection of the water in U-shaped copper tubes,the average paraffin temperature in each vacuum tube is different.The average temperature of paraffin far from the water outlet of the system is lower.After the vacuum tubes are operated in series,the total heat that can be stored increased significantly,and the amount of hot water and hot water temperature provided by the system also increased.The series system can provide7.64 L of hot water at 60? and 13.07 L of hot water at 40? in a single daily experiment.(3)A numerical simulation method is used to simulate a two-dimensional physical model of a part of the solar vacuum tube collector with phase change energy storage to get the temperature distribution and phase change characteristics of paraffin during the phase change process.The simulation also studied the effect of fin size on the phase transition of paraffin.The determination of heat flux in the model refers to the corrected solar irradiance data,and on this basis,the temperature increment of paraffin in the experiment is considered.The simulation results show that during the temperature rise of the paraffin,temperature stratification occurs.The temperature of the paraffin at the top of the model is high and the temperature of the paraffin at the bottom is low.The middle paraffin has a uniform temperature due to the enhanced heat transfer by the copper tube and fins.During the phase change process,the paraffin in the top layer and near the wall of the copper tube and the fins melts first,and the phase interface generally shows a trend moving from the top to the bottom.The results of the study of the fin size on the paraffin phase transition process show that the increase in the length and thickness of the fins can effectively increase the average temperature of the paraffin in the model,but the shorter the time it takes to complete the entire phase change process.Although the increase in the length and thickness of the fins can effectively enhance heat transfer,in practical applications,it will also reduce the total amount of paraffin filled in the vacuum tube,which willreduce the total heat storage.Therefore,the design of the size of fins needs to be combined with actual needs.