Theoretical Analysis And Experimental Study Of Transcritical CO₂ Refrigeration System With Two-stage Throttling

In recent years,with climate change remarkable and the global temperature increasing year by year,the world began to pay attention to the ozone hole and greenhouse effect on climate,environment,and human living environment.One of the biggest causes of the ozone hole and the greenhouse effect is the use and leakage of refrigerants such as CFCs and HCFCs.As a large user of refrigerants,the use of zero ODP and low GWP refrigerants is the inevitable trend of the future development of the field of refrigerants,in which the natural working medium CO₂ is an important direction of refrigerant replacement in the future.However,in China,the application of CO₂ refrigeration systems is still relatively few.To accelerate the promotion and application of CO₂ in the field of commercial refrigeration,it is necessary for theoretical analysis,experimental research,and optimization design of the transcritical CO₂ refrigeration system.To study the performance of the transcritical CO₂ refrigeration system with two-stage throttling,this paper established a system simulation model,carried out the thermodynamic analysis of transcritical CO₂ refrigeration system with two-stage throttling,and calculated the optimal charge mass of the system,we obtained the optimal exhaust pressure,system power consumption and optimal refrigerant charge mass of the system under different working conditions.The results show that when the ambient temperature increases from-5℃to 40℃,the COP of the system decreases from 6.06 to 1.19,and the power consumption of the compressor increases from 1.32k W to 6.72k W.As the ambient temperature increases,the system charge mass increases from 19.09kg at 0°C to 19.67kg at 40°C.Finally,20kg was selected as the total charge mass of the subsequent experimental system.After pressure evaluation,the system could operate safely and efficiently under this charging amount.According to the theoretical model,a transcritical CO₂ system with two-stage throttling test bench was designed and built,and the field operation situation and results were analyzed,and the actual operating performance of the system was obtained.Through analysis,the system can effectively control the cold storage temperature within 2-4℃,and the fluctuation of the temperature is very small.The pressure of the system is always lower than 12MPa,and it can operate safely and normally.The deviation between the daily COP obtained by the experiment and the simulated results is within 15%,and the variation trend of COP is consistent with good coincidence.Subsequently,this paper proposes three feasible optimization systems for the single-stage transcritical CO₂ system with two-stage throttling,and analyzes and compares the performance of the optimization system of internal heat exchanger,parallel compression,and mechanical subcooling.The results show that the IHX optimization system has the best performance improvement at 7-14℃,and the mechanical subcooling has the greatest performance improvement at other ambient temperatures.The parallel compression performance improvement is in the middle of the two.Finally,the seasonal and annual performance evaluation of the four systems in typical Chinese cities（Harbin,Beijing,Shanghai,Chongqing,Kunming and Xiamen）is carried out.The results show that the system can operate stably in the region with little fluctuation in ambient temperature.In the same season in different cities,the ambient temperature varies greatly,resulting in the performance difference of the system.As the climate environment temperature becomes higher and higher,the hours of high temperature become larger and larger,resulting in lower and lower APF of the system.However,the performance of the optimization system is improved more and more.The APF of the mechanical subcooling optimization system is always higher than that of the parallel compression optimization system,the IHX optimization system and the conventional single-stage compression system with two-stage throttling.Among the six cities,Xiamen has the largest annual total power consumption reduction ratio of mechanical subcooling and parallel compression optimization systems,and Kunming has the largest annual total power consumption reduction ratio of IHX optimization system.In mild regions,the optimization system using IHX has a greater advantage,and in other climates,the optimization system using mechanical subcooling has a greater advantage.