A Study On Resistance Characteristics And Calculation Method Of Solar Hot Water Collector Fields

For the intensive and intensive isolated counties in the Western solar energy enrichment area,compared with distributed solar heating,solar heating has the advantages of low investment cost per unit area,convenient operation and maintenance management,high safety and reliability.However,the large load and high intensity of solar energy central heating lead to a large increase of heat collection area,which makes the resistance balance of the solar collector field difficult and the process of hydraulic optimization complex.Compared with the relatively stable supply and return water temperature of the traditional central heating,the working medium temperature in the solar collector is greatly affected by the solar radiation intensity,outdoor temperature and other meteorological factors,and is always in a state of change.However,in the western plateau area with strong solar radiation and large temperature difference between day and night,the temperature fluctuation of fluid in the solar collector field is intensified and the temperature change rate is increased sharply,which results in uneven flow distribution,large resistance change and violent pressure fluctuation in the solar collector field.At present,the calculation of resistance in the design of the solar collector field refers to the calculation method of resistance in heating system,which is based on qualitative temperature.Its applicability and accuracy need to be studied.Through the analysis of the flow and thermal process of the large-scale solar collector field,the influencing factors of resistance characteristics of the solar collector field are determined,the numerical model of flow distribution of the solar collector field is established,and the evaluation indexes of resistance characteristics are put forward.Under the typical influence factors,the resistance characteristics of the large-scale collector field are calculated,and the resistance characteristics of different configurations,flow rates per unit heat collecting area,inlet temperatures and fluids are analyzed.Combined with the heat collection efficiency index,the performance of the solar collector field is evaluated,and the influence relationship between the change of influence factors and the performance of the solar collector field is mastered.An experimental test system for the resistance characteristics of the solar collector field is built.The resistance under the condition of temperature change in the actual operation of the solar collector field under different configurations,flow rates and typical parameters is studied experimentally,and the numerical calculation model is verified.Finally,according to the research results,the selection principle of the solar collector field design parameters is given,the calculation method of the solar collector field resistance correction is proposed,the reference values of specific frictional resistance of flat plate solar collector in series / parallel are given,the correction coefficient of resistance temperature difference of the solar collector field is proposed,and the reference value is given.The following suggestions are put forward for the resistance balance of the largescale solar collector field:(1)In the design,the configuration of the solar collector field with a smaller ratio of the number of branch lines to the number of collectors in series is selected,and the flow rate per unit collector area should be small.(2)The inlet temperature of the fluid in the solar collector field should be reduced to below 40?.(3)Under the economic conditions,the larger pipe diameter should be selected for the water supply and return pipe,and the fluid with low viscosity should be selected for the solar collector field.(4)In order to make the flow distribution of the solar collector field more uniform and avoid the phenomenon of local high temperature and overheating,the resistance imbalance rate of branch of the solar collector field should be controlled within 9%,and the resistance ratio of water supply and return pipe to the most unfavorable branch should be less than 11%.Through theoretical analysis,numerical simulation and experimental verification,this paper analyzes the thermal process and flow process of large-scale solar collector fields,determines the influencing factors of the resistance of the solar collector field,grasps the resistance characteristics of the solar collector field under different influencing factors,and puts forward the calculation method of the solar collector field resistance correction,which lays a foundation for optimizing the resistance characteristics of the solar collector field,improving the operation stability and improving the performance of the collector.