Study On Characteristics And Optimization Of Phase Change Thermal Storage Unit In Solar Water Heating System

Solar energy is a very common clean energy source with broad application prospects.However,due to the intermittent and unstable characteristics of solar energy in the process of utilization,in order to overcome these characteristic,people use heat storage devices to store solar energy in the form of heat energy,and then convert thermal energy into other energy when using.Solar water heating system is the earliest and most widely used solar energy heat storage system.Research on medium and low-temperature heat storage technology has an important impact on improving the performance of solar water heating system.(1)A physical model of phase change heat exchanger is established based on a small solar water heating system,and a phase change heat storage unit is selected to study the model.The physical and mathematical model of the phase change thermal storage unit is established.The heat storage characteristics of phase change thermal storage unit with and without natural convection are analyzed and researched;the effects of phase change heat storage unit size and fluid temperature on the storage characteristics of phase change heat storage unit are studied;The influence of inlet flow rate of the phase change thermal storage unit on the heat storage/release characteristics was studied.The influence of the initial temperature of the phase change thermal storage unit on heat release characteristics was studied.It was concluded that the best condition is when the radius of the thermal storage unit is 60 mm,the temperature of the hot fluid is 373 K,and the inlet velocity was 0.07m/s.(2)The simulation model of solar water heating system is established and simulated under a year-round operation.The simulation results of the typical summer day and the typical winter day were selected and compared.The useful energy absorbed by the collector,the efficiency of the collector and the solar energy guaranteed rate under the condition of phase change or not are compared and studied.It is concluded that the useful energy absorbed by collector,the efficiency of the collector and the solar energy guarantee rate in typical day in summer are higher than those of the typical day in winter;the useful energy absorbed by collector,the efficiency of the collector and the solar energy guarantee rate with phase change are higher than those without phase change.(3)The influence of different inlet velocity on the inlet and outlet temperature of heat transfer fluids and the changes of collector efficiency,solar energy guarantee rate and inlet and outlet temperature of heat exchanger fluids in typical winter and summer weather were simulated and analyzed.It was concluded that with the increase of inlet velocity,the inlet temperature of water tank decreased slightly and the outlet temperature increased gradually.The optimum inlet velocity is 0.07m/s by comprehensive analysis.With the progress of the exothermic process,both the collector efficiency and solar energy guarantee rate remain unchanged at first and then decrease;the temperature that can be reached during the exothermic process of the hot water storage tank is about 353 K,which proves the rationality of the conclusion of the third chapter.