Research And Optimization On The Start-up Performance Of Vapor Compression Refrigeration Cycle

Refrigeration and air-conditioning systems account for up to 15% of the total energy consumption of the national economy.With the gradual advancement of "carbon peaking" and "carbon neutrality",the improvement of the energy efficiency of vapor compression refrigeration cycles has become more and more important.The energy efficiency of the vapor compression refrigeration system is reduced and the cooling capacity is attenuated during the start-up process.Therefore,researching the mechanism of the cooling capacity attenuation during the start-up process and exploring methods to improve the start-up performance will be helpful to improve the annual energy efficiency ratio of the refrigeration system,to reduce the loss in the food cold chain,to improved thermal comfort of air-conditioning systems,and to speed up the replacement of refrigerants in the refrigeration and air-conditioning industry.In this study,the mechanism of the attenuation of the cooling capacity during the startup process and the optimization of the startup performance were researched.First,theoretically analyzed the refrigeration system during the startup process,clarified the power of refrigerant migration and its influence mechanism on startup performance;constructed a refrigerant migration model,clarified the influence of the structural parameters of the refrigeration system on refrigerant migration;summarized the principles of startup performance optimization and discussed the scheme of improving the startup performance;finally,through visualization,the similarities and differences of the refrigerant migration law between different optimization schemes was compared,and the feasibility of the optimization principle and the optimization scheme were verified.The comparative study of the refrigeration system between steady-state and the start-up process shows that the heat transfer coefficient of the evaporator is largely changed,which causes the attenuation of the cooling capacity,and it is the weakness of the refrigeration system startup.Theoretical analysis of refrigerant distribution and migration law shows that the liquid refrigerant migration from evaporator to accumulator at the initial stage of startup directly leads to the attenuation of the heat transfer coefficient of the evaporator,and it is the driving force of liquid refrigerant migration that the rapid increase of "high momentum" gaseous refrigerant in the evaporator at the initial stage of startup.Both the structural parameters and operating conditions of refrigeration system influenced the startup performance by changing the refrigerant migration and the distribution law.Based on the model of refrigerant distribution at steady-state and the migration during on-off process,the influence of both the structural parameters and operating conditions on the start-up performance of the refrigeration system was analyzed.The results show that: reducing the volume of the condenser and increasing the volume of the evaporator can effectively reduce the amount of liquid refrigerant moving out of the evaporator during the startup process,and improve the startup performance;increasing the operating temperature span of the system leads to more liquid refrigerant moving out of the evaporator and the degradation of the start-up performance.The change of the structural parameters of the heat exchanger has a greater impact on the start-up performance than the operating conditions.This paper verifies the influence of the system structural parameters and operating conditions on the start-up performance by means of experiments,and proves the validity of the model.Through the theoretical analysis and comparison of the energy transfer between the steady-state and the start-up process,the mechanism of the cooling capacity and heating capacity loss during the start-up process were clarified.The loss of cooling capacity includes: thermal inertia loss caused by the gaseous refrigerant migration from high-pressure side to the low-pressure side during shutdown;the waste of cooling energy caused by the migration of liquid refrigerant from evaporator to accumulator during the start-up process;the cooling capacity loss caused by the mass flow rate attenuation of the throttling device during the start-up process.The heating capcity loss includes: thermal inertia loss caused by the gaseous refrigerant migration during shutdown;heating capacity loss caused by the attenuation of discharge amount of compressor in the middle and late stages of startup;heating capacity loss caused by the absence of a liquid seal at the outlet of condenser.Based on the analysis of the start-up performance degradation mechanism,the reasons for the start-up performance degradation are clarified: the lack of refrigerant participating in the cycle during start-up and the thermal inertia loss of the heat exchanger cause the start-up performance degradation.Based on this analysis,the start-up performance optimization principle is proposed: the improvement of the system start-up performance can be achieved by reducing the amount of liquid refrigerant in accumulator and accelerating the migration speed of the liquid refrigerant in accumulator.Based on this optimization principle,the system components,control strategies and refrigeration cycle were optimized.The experimental research on the start-up characteristics of the refrigeration system verifies the effect of the components and the control strategy optimization scheme.The results show that the optimization of the components improved the start-up performance by reducing the amount of liquid refrigerant migrating into the accumulator during start-up.The optimization of the control strategy improves the start-up performance by accelerating the liquid supply speed to the evaporator during start-up and preventing the migration of refrigerant during shutdown.Heat exchanger optimization has a better start-up performance improvement than the control strategy optimization;among the three types of throttling device,the startup performance of the electronic expansion valve is the best,and the start-up performance of the capillary is the worst.The experimental research on the system cycle optimization scheme shows that the system cycle optimization scheme can greatly improve both the start-up and steady-state performance of the vapor compression refrigeration system.The vapor bypass cycle improves the startup performance by preventing the migration of liquid refrigerant to accumulator;the regenerative cycle improves the startup performance by accelerating the migration speed of the liquid refrigerant in accumulator.The visualization study of the refrigerant migration law in the vapor bypass cycle and regeneration cycle verified the mechanism of the start-up performance improvement.Both the two new cycles can improve the start-up performance of the refrigeration system,and the improvement of the regeneration cycle is more obvious.The cooling and heating start-up time of the vapor bypass cycle were approximately saved for 36.1% and 51.22%,respectively;the maximum increasement of average cooling and heating COP were approximately 0.2 and 0.34,respectively.However,the cooling and heating start-up time of the regenerative cycle were saved for 54.76% and65.39%,respectively;and the average cooling and heating COP during the start-up process were increased by 0.43 and 0.34,respectively.All the performance improvement of regeneration cycle is better than vapor bypass cycle.At the same time,the startup performance of the regenerative cycle is higher than that of the existing optimization scheme(the valve-closing start-up scheme(VCS)).Compared to VCS,the regenerative cycle saves 50 s of startup time;the average cooling COP is increased by 0.19.The start-up process of the regenerative cycle is closer to the quasi-steady-state.The start-up perfectness characterizes the degree of the start-up process approaching to the quasi-steady-state.Compared with the conventional refrigeration cycle,the start-up perfectness of the vapor bypass cycle is increased by 12.36%,the start-up perfectness of the regeneration cycle is increased by 42.64%,and the start-up perfectness of regeneration cycle is 26.37% higher than that of VCS.The regenerative cycle improves the start-up performance by regenerating the heat between the bottom of accumulator and outlet of condenser.This process is soldem influenced by the environment,therefore,the regenerative cycle remains high stability.The startup performance of the regenerative cycle is isolated to the downtime.With the change of the evaporation temperature,the startup performance of the regenerative cycle fluctuates slightly.However,even though the vapor bypass cycle can effectively improve the start-up performance,the start-up performance improvement fluctuated under heating working conditions,due to under-or over-adjustment of the vapor bypass flux.Through the research and optimization of the start-up performance of the vapor compression refrigeration system,this paper verified the start-up mechanism of the refrigeration system,proposed an efficient start-up performance optimization program,and broadened the ways to improve the start-up performance of the refrigeration system.