摘要
In this study,based on the energy balance for different components of a double-layered vacuum-tube solar collector with a U-tube,the thermal performance of the collector unit is investigated separately using an analytical and quasi-dynamic method.The model used in this study determines the temperature distribution in longitudinal and radial directions.In this research,the effects of physical parameters and heat transfer including the size of the collector,thermal-loss coefficient,absorption coefficient,mass flow and thermal resistance of the air layer under different climate conditions have been evaluated on the performance of the vacuum-tube collector.The results showed that by increasing the diameter of the tube with constant length,the annual thermal efficiency of the collector increased.Also,in a fixed-diameter tube,with increasing tube length,the annual efficiency increases,but this increase is meagre for lengths of>1.5 m.The optimal mass flow rate for maximum efficiency has been obtained for cities with different climates.According to the results,the optimal flow for different climates has different values that can be optimized as a relationship between the average solar radiation annually as a symbol of temperature and flow.
