Simulation Study Onoptimization Of Collector Structure For Direct Expansion Solar-air Source Heat Pump Water Heater

Direct-expansion solar-air source heat pump water heater(DX-SASHPWH)has been received attracting attention.The new collector/evaporator in this system combines the solar heat absorbing plate with the spiral finned tube.It can absorb both solar radiant energy and heat in the air,which makes up the disadvantage of low flow density and large fluctuations of solar radiation.The collector/evaporator is the core component of the heat pump water heater,and its thermal performance has a great influence on the performance of the heat pump hot water system.In this paper,the influence of the structural parameters of the finned tube on the thermal performance has been studied by simulation,and the structure of the collector/evaporator is optimized to further improve the heating efficiency and stability of the system.In this paper,based on the DX-SASHPWH system test bench,the one-dimensional steady-state heat transfer mathematical model of the collector/evaporator was developed.By coupling its structural parameters,inlet parameters,optical parameters and the environmental parameters,the characteristic factors such as the efficiency factor,the heat transfer factor,the photothermal efficiency and the total fin efficiency was calculated.Then the mathematical model of the heat pump system was developed to calculate the heating time of the system,the power consumption of the compressor,the average COP and the average heat collection efficiency.Multiple sets of experimental data and simulated data were compared under different environmental conditions.The maximum error of the average COP was 8.85%,which verified the accuracy of the mathematical model.Furthermore,the model was used to analyze the influence of fin structure parameters on the thermal performance of the collector/evaporator.According to the simulation results,the fin height,the fin spacing and the inner diameter had a great influence on the heat collecting performance,and the influence of the fin thickness and the finned tube spacing was relatively slight.By comparing and analyzing the heat collecting performance of different heat exchange areas,the heat capacity per unit fin area was the largest when the heat exchange area on the air side was 13.22 m2,which can increase the heat collection of the collector/evaporator and meet certain economic conditions.The original structure was used as the control group,and the nine groups of structures were selected for simulation calculation under the same conditions of the total heat exchange area on the air side.The heat capacity of the unit length of the two-phase zone and the annual energy consumption efficiency were used as evaluation indexes to determine the optimal structure.The optimum structure was then determined that the inner diameter was 9 mm,the fin pitch was 1.7 mm and the fin height was 10.3 mm.Combined with the meteorological parameters of the typical meteorological year in Nanjing,the performance of the system during annual operation after structural optimization are compared with that before optimization.The results showed that after optimization,the average COP of the system could be increased by 10.17%,and the energy consumption efficiency of the whole year was increased by 9.36%.Finally,according to the solar resource zoning,the optimal structure and annual operation performance of Xiamen and Chengduwere compared and analyzed.The results showed that the optimal structure of the collector/evaporator will be different in different regions.When the system was operated in Xiamen,the average COP can be increased by 5.49% and the annual energy consumption efficiency is increased by 8.99%.When the system in Chengdu was operated with the best structure for the whole year,the average COP can be increased by 11.08%,and the annual energy consumption consumption is increased by 13.61%.The system can take advantage of the combined heat source collector/evaporator in areas where solar radiation intensity is insufficient and the ambient temperature is high,and the structure is optimized to have greater benefits,which can improve energy utilization efficiency.