Research On Thermal Performance Of Straight-Through All-Glass Evacuated Collector

In the utilization of low-temperature heat in solar energy,the traditional U-shaped all-glass evacuated tube collector is easy to produce,low price,widely used.However,the flow of working medium in the tube is generated by buoyancy and thermosiphon,Hence,the heat exchange efficiency is low.This paper proposes a straight-through all-glass evacuated tube collector,which is a forced convection active heat exchange.It has good thermal performance and is more cost-effective.At the same time,the active flow in the pipeline helps to improve the water quality and is of great significance in the use of large-scale solar hot water systems.This paper focuses on the straight-through all-glass evacuated tube collector.Based on the MATLAB simulation calculation,FLUENT simulation and experimental verification method,the main content and conclusions are as follows:(1)First of all,according to the national standards and working requirements of all-glass vacuum heat collection tubes,referring to the traditional U-shaped all-glass vacuum heat collection tube design method,both ends of the design are fused together,and there is no free end straight-through all-glass vacuum heat collection tube.Considering the safety and service life of the collector during operation,the practical feasibility of the straight-through all-glass vacuum tube collector is given from theoretical and experimental perspectives.(2)In order to study the temperature distribution characteristics of the collector during operation,using the finite element method,taking the collector unit as the research object,and taking the energy flow density of the heat absorption tube simulated by SOLTRACE as the boundary condition,it is known by ANSYS thermal simulation simulation:The velocity field and temperature field of the collector tube are unevenly distributed,and have the characteristic of symmetrical distribution.The wall temperature of the heat absorption tube gradually increases in the axial direction,and decreases in the radial direction with the increase of the circumferential angle and gradually tends to be flat,respectively.The maximum and minimum values are reached when the circumferential anglesθ=0°andθ=180°,and the maximum change is between the circumferential angles 0°～100°.The temperature of the outer wall surface of the heat absorption tube is greatly affected by the inlet flow rate of the working medium and the intensity of solar radiation.The flow rate of the heat transfer medium is small.The temperature of the working medium is high at the exit section of the heat absorption tube,but the maximum temperature and temperature difference of the outer wall surface are large,so it is reasonably controlled.The inlet flow rate of the working medium is very important.(3)The thermal performance of the collector is further studied.According to the energy flow pattern,a one-dimensional heat transfer model of a single collector tube is established and its heat transfer process is analyzed.Derive the parameter expressions that characterize its thermal performance,such as heat loss coefficient U_L,instantaneous thermal efficiency,and temperature difference between inlet and outlet(T_o-T_i).With the help of MATLAB software programming and iterative solution using the hypothetical temperature method,the influencing factors of the thermal performance of the collector tube are analyzed:ambient temperature,wind speed and solar radiation intensity have a great influence on the thermal performance of the collector,and the meteorological parameters should be fully used to improve the thermal performance of the collector;The higher the inlet temperature of the working medium,the lower the thermal efficiency,and the smaller the temperature difference between the inlet and the outlet.When the inlet temperature of the working medium is equal to the ambient temperature,the instantaneous efficiency is a fixed value of 0.456;As the flow rate of the working fluid increases,the temperature difference between the inlet and the outlet decreases first and then tends to be flat.Therefore,in practical applications,appropriate operating parameters should be selected to maximize the thermal performance of the collector.(4)Build a field test platform for the thermal performance of vacuum collector tubes,collect data using dynamic testing methods,and compare and analyze the experimental results with the simulation results.The errors of thermal simulation and one-dimensional heat transfer simulation are within 7.4%and 11.80%,respectively to verify the reliability of theoretical modeling.It is known from the comparison test that the instantaneous thermal efficiency of the straight-through collector tube is about 25%higher than that of the traditional collector tube with one end closed.