Cycle Optimization Of CO₂ Mixed Refrigerant Two-stage Compression Heat Pump Based On Pinch Point Temperature Difference

The air source heat pump applied in cold areas has some problems such as too high compressor exhaust temperature and serious attenuation of heating performance coefficient,which restrict its further application.Therefore,aiming at the adaptability of air source heat pump at low temperature,this project proposed the solution idea of CO₂ mixed working medium combined with two-stage compression technology,carried out studies on mixed working medium screening,mass ratio optimization,comparison of various cycle heating performance and exergic loss analysis of the system.Furthermore,a CO₂ mixed working medium two-stage compression heat pump cycle suitable for large temperature cross heat source in cold region is proposed,which lays a theoretical foundation for further optimization of heat pump system.Firstly,based on the theoretical thermodynamic cycle analysis,the variation of performance parameters of CO₂ mixed working medium two-stage compression heat pump with the composition and component ratio of the mixed working medium was explored.The results show that the CO₂/R32 mixture with CO₂ mass fraction between 20%and 90%has the best heating performance coefficient COP_hot,and can significantly reduce the exhaust pressure of the CO₂transcritical heat pump system.In addition,the optimal CO₂/R32 component ratio of COP_hot is almost independent of the evaporation ambient temperature.Therefore,it is suggested that CO₂/R32 can be used as the optimal mixing medium for heat pump circulation with large temperature span characteristics.Secondly,based on the pinch pointtemperature difference principle,combined with the compressor,heat exchanger and throttle mechanism and other component models,the steady state model of CO₂ mixed working medium two-stage compression cycle system was constructed.The relationship between the heating performance of CO₂/R32 two-stage compression heat pump system and the maximum exhaust pressure p _H is simulated and analyzed.The results show that with the increase of p _H,the heat transfer pinch point is transferred from the inside of the air cooler to the outlet,and the rapidly decreasing zone of the fluid temperature difference on both sides has a trend of shortening first and then lengthening.At the same time,the optimal p _H occurs at the heat transfer pinch point both inside the air cooler and at the outlet.At this time,the system COP_hot and interstage performance is optimal,the rapid drop zone of temperature difference in the air cooler is the shortest,and the growth rate of heat production begins to decline.In addition,when the mass fraction of CO₂ is larger,the evaporation temperature is lower,the capacity ratio of high and low pressure machine is smaller than R_V,and the optimal p _H is larger.Thirdly,based on the simulation model,the influence of CO₂/R32 mixed working medium allocation ratio on the performance of two-stage compression system was analyzed.The results show that when the evaporation temperature is 20℃and CO₂/R32（0.10/0.90）,the COP_hot reaches the maximum,but the heat Q_H is only 38.6%of that of the pure CO₂ system,and the exhaust temperature T_H,dis of the system is 7℃higher than that of the pure CO₂ system.When CO₂/R32（0.9,0.1）,compared with pure CO₂,COP_hot of the system increased by 6.5%,T_H,disdecreased by 4.5℃,and Q_H decreased by 15.7%.Based on the three performance indexes,the optimal CO₂/R32 component ratio is 0.90/0.10.Finally,the performance comparison and exergy analysis of CO₂/R32 mixed working medium with three proportions were carried out.The results show that with the increase of compressor capacity ratio R_inj,intermediate injection ratio R_inj and high/low pressure capacity ratio R_V,the total compressor power and heating Q_H increase gradually,while COP_hot will increase first and then decrease.When COP_hot is at its maximum,there exists an optimal combination of R_inj and R_V,which is affected by the ratio of working medium and evaporation temperature.In addition,due to temperature slippage in the evaporation process of intercooler,heat absorption of low temperature heat source in CO₂/R32（0.90,0.10）and CO₂/R32（0.10,0.90）system increases with the increase of R_inj,but the growth rate decreases,which is different from that of CO₂ pure working medium.Finally,the exergic loss rate of CO₂/R32 was different with different proportions.The exergic loss rate of CO₂/R32（0.10,0.90）was the highest in the system at low level compressor,which was about 28%.Exergic loss rate of pure CO₂ and CO₂/R32（0.90,0.10）throttle valve TV before evaporator was the highest,about 35%and 33%,respectively.