Study On Optimal Control Of A R744 Two-temperature Supermarket Refrigera-tion System

Due to the destruction of the ozone layer,chlorofluorocarbons(CFCs)and hydro-chloro fluorocarbons(HCFCs)have been banned.In order to reduce the impact of chem-ical refrigerants on the environment,R744,as a natural refrigerant with zero ODP and a GWP value of 1,has become an important substitute for synthetic refrigerants because its performance is similar to that of synthetic refrigerants.However,R744 is a working medium with a lower critical temperature(31.1℃)and a higher critical pressure(73.8 bar).Under the high temperature environment,R744 will inevitably enter transcritical operation,causing a large amount of throttling loss,resulting in a low system COP.For this study,in order to reduce the throttling loss of R744 refrigeration system in the high temperature environment,a high-pressure parallel compressor circuit and an ejector circuit are added on the basis of the conventional compression refrigeration system to form the parallel compression refrigeration system(PCR)and the ejector compression refrigeration system(ECR).To this end,the thermodynamic models of PCR systems and ECR systems are established for theoretical research.Firstly,the re-lationship between gas cooler outlet pressure,intermediate throttling pressure and am-bient temperature is optimized.Secondly,the performance of the two systems under the same operating condition is compared and analyzed,and the critical point where two systems have equal COP is found.Finally,according to the critical point,a set of control strategy for switching between two refrigeration modes is added to form the improved R744 multi-mode optimized control refrigeration system(MMOCR).It can switch ejection compression mode(ECM)and parallel compression mode(PCM)by automatic control valve according to the change of ambient temperature,so that the R744 refrig-eration system can operate with the highest efficiency under any ambient temperature.Under the guidance of theoretical research results,the MMOCR system experimental platform is designed and constructed,and experimental research is conducted.Through theoretical analysis and experimental research,the following main conclusions are drawn:(1)At a certain ambient temperature,when the R744 refrigeration system is oper-ating transcritically,the optimal gas cooler outlet pressure exists so that the system has a maximum COP.The relationship between the gas cooler outlet pressure and the am-bient temperature is written into the controller program for experimental testing.The results show that the error between the actual outlet temperature and pressure of the gas cooler and the theoretical calculation results are within 13%and 17%,respectively.(2)The simulation results show that as the increase of ambient temperature,the ejector pressure ratio increases from 1.3 1 to 1.65,the ejector efficiency increases from 0.57 to 0.63,the outlet pressure increases from 36.85bar to 46.47bar,but the entrain-ment ratio decreases from 0.56 to 0.44.The experimental results show that as the in-crease of ambient temperature,the ejector pressure ratio increases from 1.19 to 1.32,the outlet pressure increases from 33.43bar to 37bar,but the entrainment ratio decreases from 0.32 to 0.27.In addition,at an ambient temperature of 30℃,the cooling capacity ratio of the medium-low temperature display cabinet is increased from 1 to 3.5,the entrainment ratio is increased from 0.26 to 0.39,and the pressure ratio is increased from 1.25 to 1.32,so the ambient temperature and the ratio of medium to low temperature cooling capacity have a great influence on the performance of the ejector.(3)The simulation results show that when the ambient temperature increases from-5℃ to 35℃,the MMOCR system COP decreases from 4.52 to 1.22.During transcriti-cal operation,when the ambient temperature is 21.1℃ to 35℃,the ECR system COP is 19.5%to 32.6%higher than that of the PCR system.However,when the ambient temperature is lower than 17.32℃,the PCR system COP began to be higher than that of the ECR system.The experimental results showed that when the ambient temperature decreased from-5℃ to 35℃,the MMOCR system COP decreased from 4.52 to 1.22.During transcritical operation,when the ambient temperature is 21℃ to 35℃,the COP of the ECM mode is 5.5%to 13.1%higher than that of the PCM mode.In the high temperature environments,the ejector has a significant effect on improving system per-formance.