Performance Analysis Of CO₂ Transcritical Cycle With Mechanical Subcooling Unit

In recent years,the refrigeration industry has developed rapidly.Refrigerants have always been an important factor restricting the development of the industry.At present,HFC refrigerants have been included in the list of controlled substances by the Kigali Amendment due to their severe impact on the climatic environment.However,the complete ban of―ozone-friendly‖chlorine-free substances still takes time to complete the conversion.At the same time,it is also necessary to prepare an optional alternative refrigerant in advance.Therefore,the use of natural working fluids has become one of the mainstream directions of people’s research.As an excellent refrigerant,carbon dioxide（CO₂,R744）has attracted the attention of scholars at home and abroad.Meanwhile,the research on CO₂refrigeration system has been deepened.The performances of two-stage compression CO₂ transcritical cycle with R134a mechanical subcooling unit are analyzed.The overall COPs of the single-stage compression mechanical subcooling system（SMS）,one-throttling and two-stage compression low-pressure mechanical subcooling system（OTLMS）,one-throttling and two-stage compression low-pressure mechanical subcooling system（OTHMS）,two-throttling and two-stage compression low-pressure mechanical subcooling system（TTLMS）,and two-throttling and two-stage compression high-pressure mechanical subcooling system（TTHMS）system are discussed in detail.With the increase in subcooling degree and compressor discharge pressure,the COPs of the SMS,OTLMS,OTHMS,TTLMS,and TTHMS system increase first and then decrease.Under the working condition of optimal discharge pressure and optimal subcooling degree,TTHMS shows an obvious advantage in COP over the system with no subcooling under the same working condition.When the evaporation temperature is-30°C,the COP of TTHMS is improved by 18.31%.COP of the SMS system can be increased by up to 50%for S system.When the ambient temperature is40°C and evaporation temperature is-20°C,the COP of the TTHMS system is 45.36%higher than that of the no subcooling system.The COP of the SMS system is 45.28%higher than that of the no subcooling system with the-10℃evaporation temperature and40℃ambient temperature.The power consumption ratio of OTLMS is significantly higher than those of other systems.With the comprehensive analysis of four systems,the TTHMS system and SMS system get more benefits than others.In this stady,we build R134a mechanical subcooling CO₂ transcritical refrigeration system test.We stabilize the system by adjusting electric heater and electronic expansion valve opening.The effects of compressor discharge pressure and subcooling temperature on the coefficient of refrigeration performance（COP）are systematically studied.Moreover,the optimal operating conditions of the system are analyzed.Through the analysis of experimental data,it is concluded that the maximum COP is obtained at the discharge pressure of 8.5 MPa and subcooling temperature of 15 ^oC.The experimental results are in the same trend with the simulated results with the change of evaporation temperature and ambient temperature.The deviation between the experimental and the simulation is within25%.Some auxiliary piping arrangements are unreasonable,resulting in inevitable cold loss in the system.Further improvement of the system can reduce the deviation.