Research On Performance Of CO₂ Booster Refrigeration System In Supermarket

In recent years,the destruction of the ozone layer and global warming have attracted much attention,and the replacement of traditional HCFCs,HFCs and other refrigerants with high GWP and ODP causing these global environmental problems has become increasingly important,and natural refrigerant CO₂ is the refrigerant Alternative directions for the future.Supermarkets,as large energy consumers,are also places where the amount of refrigerant is large.From the perspective of indirect emissions of greenhouse gases or direct emissions from refrigerant leakage,supermarkets are important areas for refrigerant replacement in the future.Therefore,for the promotion and application of natural refrigerant CO₂ in the field of supermarket,it is very necessary to study the performance of CO₂ booster refrigeration system and the optimization scheme.In order to explore the performance of the CO₂ booster refrigeration system under different working conditions,this article uses Matlab to perform a thermodynamic analysis of the conventional CO₂ booster refrigeration system.The results show that when the ambient temperature increases from-3?to 40?,the COP is reduced from 5.24 to 1.06,which shows that when the ambient temperature increases,the performance of the system will quickly decline,which is not conducive to the promotion of the system in different temperature zones,but also shows that the upgradeability is very high,therefore,this paper studies the optimization direction of components of conventional CO₂ booster system through exergy analysis.The results show that the system's exergy efficiency is very low,only 19%?ambient temperature is 15??and 20%?ambient temperature is 30??.High pressure stage compressor and gas cooler are the most irreversible components of the system.Through advanced exergy analysis,it can be seen that the high pressure stage compressor is the most optimizable component,because it accounts for the most avoidable endogenous exergy destruction of the system,and the endogenous exergy destruction are higher than exogenous,so the focus is on improving its own structure and performance.The gas cooler can avoid the most exogenous exergy destruction,so this component is the easiest to reduce its irreversibility through the improvement of other components;most of the system damage is avoidable and endogenous?42.7%when the ambient temperature is 15?and 38.76%when the ambient temperature is 30??,indicating that most of the irreversibility of the system can be reduced by optimizing the system components.Most of the unavoidable damage of the system comes from endogenous?37.2%when the ambient temperature is 15?and 35.36%when the ambient temperature is 30??.In order to improve the performance of conventional CO₂ booster system,in this paper,three optimization schemes of parallel compression,mechanical supercooling and internal heat exchanger?IHX?were thermodynamic analysis to study the optimal variables control methods and performance of them.The results show that for the parallel compression booster system,as the auxiliary compressor suction ratio increases,the optimal intermediate pressure and optimal high pressure of the system decrease,and the COP increases.As the ambient temperature increases,the optimal intermediate pressure and optimal high pressure values increase.The optimal intermediate pressure and optimal high pressure are proportional to the ambient temperature.Choosing the appropriate intermediate pressure and high pressure can make the system obtain the maximum COP.For the mechanical subcooling booster system,when the ambient temperature is below 4?,the optimal subcooling degree of the system decreases as the ambient temperature increases,and the COP decreases slightly.When the ambient temperature is greater than 4?,the optimal subcooling degree increases with the increase of the ambient temperature,and the COP of the system also decreases rapidly.For the booster system with IHX,it is the best method to use the refrigerant at the inlet of the gas cooler and the refrigerant at the inlet of the high pressure stage compressor to exchange heat in IHX,but IHX can only be used under transcritical conditions to improve the COP,and IHX should be bypassed under subcritical conditions.This paper compares these systems under different working conditions.It is found that when the ambient temperature is-2?¹0?,the parallel compression system is the best.When the temperature is above 10?,the optimal system is a mechanical subcooling system.Comparing the performance of the system under study in Harbin,Beijing,Shanghai,Xiamen and other typical Chinese cities,it was found that from the perspective of performance improvement,mechanical supercooling is the biggest improvement,followed by the parallel compression system,and both are in Shanghai performed best.IHX can only improve in areas with high climatic environment temperature,but it is not obvious,and it is worth noting that the system with IHX performs better where the ambient temperature is higher,which is more conducive to improving the performance of system where the ambient temperature is high.However,it should be noted that no matter where the area is,when the working condition is subcritical,the IHX should be controlled to be in a no-load state to avoid a decline in system performance.