Study On The Influence Of Inter-stage Parameters And Cycle Optimization Of CO₂/R41 Two-stage Compression Heat Pump System

The application of air heat pump in cold regions is limited by its large suction capacity,low refrigerant circulation flow,high compression ratio and high discharging temperature.The combination of refrigerant replacement method and two-stage compression technology with air source heat pump shows good in cold regions.This paper takes the CO₂/R41 mixed refrigerant as the working fluid of the heat pump to improve and optimize the heating performance of the two-stage compression heat pump system and conducts in-depth research and discussion on the optimization of its circulation process and the improvement strategy of the system performance when the mechanical supercooling device is installed.Firstly,the experimental device of CO₂/R41 transcritical heat pump system is built and analyze the relationship between the heating performance of the heat pump system and the evaporation temperature and the mass fraction of the mixed refrigerant with corresponded thermodynamic model.The results show that heating capacity and COP are most affected by the outlet temperature of the gas cooler.Adding R41 can increase the heating capacity of the system and reduce the discharging temperature of the system.The outlet temperature of gas cooler increases by 5℃ and cop decreases by 0.6.When the mass fraction of R41 increases by 5%,the heating capacity can be increased by 0.2 kW,the discharging temperature can be reduced by about 5℃.Then,on account of theory and experiment of single-stage system,four thermodynamic and exergy analysis models of two-stage compression heat pump cycle using CO₂/R41 mixture are established,and the system performance is analyzed under various working conditions to explore the optimal cycle process.The results show that when under the working condition of minimum evaporation temperature of-30℃,compared with pure CO₂ system CO₂/R41 mixed refrigerant is used,the heating capacity and exergy efficiency increase by 31.03%（90 kJ/kg）and 4%respectively,and the optimal discharging pressure decreases by 4 MPa.The incomplete cooling heat pump system has better heating capacity and COP,but the complete cooling heat pump system has lower discharging pressure and compression power consumption.When the evaporation temperature is lower than-20℃ because the COP（maximum 2.78）,exergy efficiency（maximum15.01%）and heating capacity（maximum 274.8 k J/kg）of the two-stage throttling intermediate incomplete cooling flash tank system are higher than those of other circulation processes,it is determined as the preferred cycle form for heating CO₂/R41 heat pump in cold region.Finally,the coupling theoretical model of two-stage compression system with mechanical cooling device is established,and the influence of mechanical subcooling device position,circulating refrigerant,circulating subcooling degree and main loop CO₂ mass fraction on heat pump performance is analyzed.The results show that the front mechanical undercooling system has better heating performance.Among the seven refrigerants,R152a has the largest increase in the COP of the system,and R410A has the smallest increase.The increase of the heating capacity increases with the increase of the cooling degree.When the cooling degree of the system is 25℃,the heating capacity increases by 39.1%compared with that of the system without mechanical cooling device.The COP of the mechanical cooling increases with the increase of CO₂ mass fraction.When CO₂ mass fraction is 0 to 1,the COP of the system with mechanical undercooling is 5.83% to 19.35%higher than that of the two-stage compression system without mechanical undercooling.When the supercooling degree of refrigerant in the mechanical supercooling device is constant,the COP and heating capacity of the system can be increased by 2% and 3% respectively for each 30%increase of CO₂ mass fraction in the main circuit.