Simulation And Seasonal Performance Research Of Transcritical CO₂ Heat Pump System

As a new generation of refrigerants,CO₂ has become more and more important in the fields of refrigeration,air conditioning and heat pumps due to its chemical stability,non-toxicity and environmental friendliness.Improving the operating efficiency of the transcritical CO₂ heat pump hot water system is of great significance for solving energy problems as well as mitigating the greenhouse effect.Lots of scholars from both domestic and abroad are working on this field.A combination of experimental research and simulation method has been used to carry out a seasonal operation experiment of transcritical CO₂ heat pump unit,studying and analyzing the impact of different parameters on operating performance.An optimized operation plan has been proposed based on the existing research results;A system dynamic model has been established to study the dynamic operating characteristics of trans-critical carbon dioxide heat pump system,which can lay a good foundation for system optimization design and optimization control in the future.Firstly,A practical trans-critical CO₂ heat pump unit was tested to study the influence of ambient temperature on energy efficiency.The results show that under the conditions of summer and transitional seasons,the effect of ambient temperature on heating capacity and COP is limited.If hot water with the same temperature is produced,the higher ambient temperature corresponds to higher heating efficiency,but as the ambient temperature increases,the heating efficiency Lifting decreases.For emample,ambient temperature increased from 24°C to 28°C,COP increased by 3.75%,and from 28°C to 32°C,COP increased by 2.34%.Under winter conditions,the ambient temperature has a greater impact on heating capacity and COP.the lower the ambient temperature,the more the heating efficiency decreases.Heating efficiency decreases rapidly with decreasing ambient temperature;For emample,the ambient temperature decreases from 14°C to 10°C,COP Decreased by 12.8%;the ambient temperature decreases from 10°C to 6°C,the heating capacity reduced by 31.8%,and the COP decreased by 32.9%;After that,the influence of the outlet water temperature on the energy efficiency of the trans-critical CO₂ heat pump unit is studied.The results show that,rising of the outlet hot water temperature may lead to the decrease of heating capacity and increase of the power consumption.In winter conditions,the outlet hot water temperature increased from 60°C to 90°C,the heating capacity decreased by 38.9%;in summer,it was 10.96%.Secondly,a dynamic simulation model was established for the actual CO₂ heat pump unit.Distributed parameter models are established for air coolers and evaporators.Quasi-steady-state models are used for compressors and expansion valves.Each single component is integrated into a complete system according to the conservation of mass,conservation of energy,and momentum equations.After examing the reliability of the model,a simulation study of the dynamic characteristics of transcritical CO₂ heat pum system was carried out.internal parameters（compressor speed）and external parameters（inlet water flow）are selected as step inputs to simulate the dynamic response characteristics of the system.The results show that changing the compressor speed can effectively adjust the high and low pressure of the system as well as the outlet water temperature,and the system gives a fast response speed,which requires higher control accuracy.At the same time,System changes caused by changing the influent flow are less noticeable and slower to respond.Finally,a seasonal operation experiment on an actual CO₂ heat pump unit was performed based on the climatic conditions of Nanjing,and the monthly heating energy efficiency distribution of the system was obtained.The optimization strategy of compressor frequency conversion and heating time controlling is adopted to enhance economic benefits.The results show that when the compressor frequency conversion strategy is adopted,the COP in summer,transition season,and winter are increased by 10.6%,4.9%,and 6.4%;as heating time controlling strategy is adopted,COP of the direct thermal mode is increased by 4.5%in winter.During summer and transition season,the COP decreased by 1.7%and 3.7%,but the total cost decreased by 31.5%and 29.6%as well,which account 22%of the total cost saving.The economic benefits were significantly improved.By carrying out the work above,the operating performance and dynamic response of transcritical CO₂ heat pump system will be understood more deeply.The research results provide ideas for guiding the optimal control and efficient operation of the system.