Theoretical And Experimental Research On Subcritical CO₂ Refrigeration System

With the increasing awareness of national environmental protection,natural working fluid CO₂has received more and more attention as an environmentally friendly refrigerant.To improve the safety of the unit and reduce the pressure of the refrigeration system,the heat load of the CO₂experimental unit will be transfered to the surrounding soil by the buried pipe as the condenser.Because the underground soil temperature is constant and lower than the critical temperature of CO₂,the CO₂experimental unit can be assured to operate safely throughout the year.The stable underground low temperature environment makes the CO₂refrigeration unit avoid the influence of external temperature changes,so that the purpose of high efficiency,steady,environmentally friendly and safe operation of the refrigeration system is realized.In this paper,the numerical modeling of the subcritical CO₂refrigeration system has been built and the refrigeration process has been analyzed in theory.The effects of condensation temperature,dryness and subcooled on the performance of subcritical CO₂refrigeration system have been analyzed as well.In order to study the effect of refrigeration compressor on the performance of subcritical CO₂refrigeration system,the theoretical analysis of compressor efficiency at different condensation temperature for subcritical CO₂refrigeration system COP,compressor exergy loss,the rate of compressor exergy loss and the coefficient of refrigeration compressor exergy loss have been also analyzed in theory.The theoretical analysis results indicate that the higher the condensation temperature,the lower the system exergy efficiency,COP,compressor exergy loss rate.When the evaporation temperature is-15°C,the condensing temperature changes from 12°C to30°C,the exergy efficiency of the refrigeration system reduces 47.83%,from 0.92 to 0.48,the system COP decreases 47.64%,from 5.94 to 3.11.The dryness of the outlet of the buried pipe condenser has a great influence on the exergy loss rate of the compressor.When the dryness is 0.3,the increase of the degree of subcooled is 1°C,the compressor exergy loss rate increases by 0.005 k W and the lower the dryness,the greater the effect of subcooled on the compressor exergy loss rate.In this paper,a subcritical CO₂refrigeration experimental unit has been built,and the effects of frequency,suction pressure and suction pressure on compressor performance have been analyzed.In order to study the change of underground soil temperature,when the long-term operation of the refrigeration experimental unit continuous transfer heat to the underground soil,the subcritical CO₂refrigeration experimental unit have been run for a long time under the design experimental conditions,consecutively,and the underground temperature has bee recorded,In order to further study the performance of subcritical CO₂refrigeration experimental unit,the effect of evaporation pressure on the performance of subcritical CO₂refrigeration system under different outdoor temperatures have been studied,the experimental results are as follows:（1）The performance of the compressor at different frequencies have been studied.The system COP decreases with the increase of the compressor frequency.When the cold storage heat load is low,the low frequency operation of the compressor can save energy.The volumetric efficiency and electrical efficiency of the compressor increased first and then decreased with the increase of the compressor frequency.The volumetric efficiency and electrical efficiency of the compressor reach the maximum at the compressor frequency of 55 Hz,which are 0.795 and 0.62,respectively.（2）The performance of the compressor has been studied under different suction pressures.When the suction pressure of the compressor increases,the coefficient of performance COP increases,and when the suction pressure of the compressor is higher than15 bar,the rate of COP increase decreases.When the compressor frequency is 50 Hz,the experimental efficiency of the compressor is close to the theoretical value of the AUSTIN model.The average absolute error of the theoretical and experimental values is 0.02,and the average relative error is 3.12%.（3）The performance of the compressor has been studied under different suction superheats,it is found that the system COP gradually decreases,when the suction superheat of the compressor increases.When the suction superheat increases from 0°C to 24°C at30Hz,the system COP decrease from 1.86 to 1.68,down 9.68%.Compressor superheat on the compressor has little effect on system cooling capacity and volumetric efficiency.The exhaust temperature of the compressor increases with the increase of the suction superheat of the compressor.The exhaust superheat increases from 5.8 liters to 23°C when the exhaust temperature rises from 97.0°C to 122.4°C and the compressor frequency is 50 Hz.（4）The effect of underground soil temperature when the heat load of the CO₂experimental unit transfered to the surrounding soil has been studied,taking the temperature of the liquid supply pipe at a distance of 10m from the ground as an example:in the initial stage of the start-up,the soil temperature rises sharply with the increase of the running time.After that,underground soil temperature gradually stabilizes.Finally,the underground temperature reached equilibrium,and the temperature fluctuates slightly at around 21°C.In the initial stage of shutdown,the underground soil temperature dropped sharply,and then underground soil temperature gradually stabilizes.Finally,the underground temperature returns to equilibrium,the temperature fluctuates slightly around14.3°C.The compressor discharge temperature rises rapidly to 101.2°C after 1 h of start-up.When the experimental unit is running stably,the compressor discharge temperature fluctuates slightly above and below 122.7°C.（5）The influence of outdoor temperature and condensing temperature on system performance has been studied,taking the outdoor temperature as 9.2°C as an example,when the system evaporation pressure increases,the system COP shows an upward trend,and the system exergy efficiency increases first and then decreases.There is an optimum evaporation pressure（20.2 bar）,which makes the system exergy efficiency reach the maximum（0.465）.When the evaporation pressure rises,the flowmeter exergy loss at the compressor inlet increases rapidly.The evaporation pressure is increased from 18.6 bar to24.3 bar under the outdoor temperature of 15.7°C.The system exergy loss increases by0.057k W,when the evaporation pressure increases by1 bar.（6）The proportion of exergy loss distribution of various components of the system has been studied.When the system evaporation pressure is 16.2 bar,and the outdoor temperature is 9.2°C,the compressor exergy loss accounts for the largest proportion of system exergy loss,accounting for 42.82%,followed by the condenser,the proportion at18.03%,the loss ratio of the plate-changing electronic expansion valve is at least 6.16%.The exergy loss of the flowmeter at the suction port of the compressor accounts for 7.17%of the exergy loss of the system.The exergy loss of the flowmeter can be avoided in some way.The flowmeter can be replaced in the future,so that the exergy loss of the system can be reduced,and the exergy efficiency of the system can be improved.