Thermal Performance Of U-pipe Evacuated Tube Solar Collector With Phase Change Material

The U-pipe evacuated tube solar collector with the characteristics of high thermal efficiency,fast start-up and strong frost resistance,has been widely used in solar thermal utilization system.However,due to the intermittent and instability of solar energy,there is a serious mismatch in time between the solar collector's heat collection and the user's heat.At the same time,when the solar radiation is strong,problems such as high temperature and overheating are prone to occur,which even damages the collector and the whole system.Phase change material has the advantage of a large energy storage density and small temperature range in thermal storage and release process,which is the main technical way to solve the energy mismatch in the process of solar thermal utilization.Therefore,a U-pipe evacuated tube solar collector with phase change material is proposed in this paper.Through theoretical analysis,numerical simulation and experimental test,the collector's thermal performance is studied.Based on the theoretical analysis of the structure and energy conservation relationship of U-pipe evacuated tube solar collector,the useful calculation formulas of heat collection and heat loss in the process of energy transfer are obtained.The mathematical models of U-pipe evacuated tube solar collector with phase change material and traditional U-pipe evacuated tube solar collector without phase-change are established respectively.The temperature distribution,liquid phase ratio of phase-change material and inlet and outlet temperature of the collector are simulated by CFD software.The thermal performance test platform of U-pipe evacuated tube solar collector with phase change material is built,and the accuracy of the mathematical model is verified by the experimental test.The thermal performance of U-pipe evacuated tube solar collector with or without phase change is compared and analyzed under the typical meteorological conditions of four seasons in Lhasa.The effects of physical parameters such as melting point,thermal conductivity,density and latent heat value of phase change materials,as well as the velocity of working fluid on the inlet and outlet temperature,effective heat collection and operation time of the collector were studied.The main conclusions are as follows:(1)Compared with the traditional U-pipe evacuated tube solar collector without phase change material,the process of temperature rise and drop of U-pipe evacuated tube solar collector with phase change material is more gentle,the internal temperature distribution is more uniform,the peak value of outlet temperature is reduced,and the peak value of outlet temperature is delayed,and the effective operation time of the collector is delayed to varying degrees.Under the four typical meteorological conditions in Lhasa,the peak temperature at the outlet of the collector with phase change material decreased by 8.9 K,6.9 K,8.2 K and 8.5 K respectively,and the operation time prolonged by 2.1 h,2 h,2.3 h and 1.6 h respectively.Due to the better heat preservation effect of U-pipe evacuated tube solar collector,although the effective operation time of the collector is delayed,the total effective heat collection has not changed much.(2)With the increase of the melting point of the phase change material,the peak value of the outlet temperature of the collector with phase change material decreases first and then increases.Compared with the collector with phase change material,when the temperature of the phase change material is 333 K,the peak value of the outlet temperature of the U-pipe evacuated tube solar collector with phase change material decreases the most,which is 8.2 K.When the thermal conductivity of the phase change material is more than 0.5 W/(m~2?K),the increase of the thermal conductivity of the phase change material has little effect on the peak value of the collector outlet temperature;with the increase of the density and latent heat value of the phase change material,the peak value of the collector outlet temperature gradually decreases and then tends to remain unchanged,and the peak value of the collector outlet temperature decreases by 10.9 K at most.With the increase of working fluid velocity,the peak value of collector outlet temperature decreases gradually.(3)The latent heat is released when the solar radiation is weak in the phase change collector,which prolongs the operation time of the collector during the peak period of user's heat consumption.In the range of parameter selection,with the increase of the melting point of phase change material and the velocity of working medium,the liquid phase ratio of phase change material decreases gradually,and the effective operation time of the collector decreases gradually;with the increase of latent heat value and density of phase change material,the operation time of the collector increases gradually,while the thermal conductivity of phase change material has little effect on the operation time of the collector.Based on the above research,the influence of phase change materials on the outlet temperature,operation time and effective heat collection of U-pipe evacuated tube solar collector is obtained,and the physical parameters and working medium operation parameters of phase change materials are optimized.The results provide theoretical basis for the application and promotion of U-pipe evacuated tube solar collector with phase change material in solar thermal utilization.