Research On Performance Of CO₂ Refrigeration System Used In Large And Medium-sized Cold Storage

Carbon dioxide has been used as the primary refrigerant in the refrigeration system for more than 100 years and is the most commonly used environment-friendly natural refrigerant.However,the high condensation pressure of carbon dioxide also restricts the development and application of carbon dioxide refrigerant for a long time.The critical pressure of carbon dioxide(7.3MPa)and critical temperature(31.3℃)determine the operation mode of carbon dioxide refrigeration system,which can be divided into subcritical cycle,transcritical cycle and supercritical cycle.Carbon dioxide refrigeration system is more used in commercial supermarket and building air conditioning system,and it is mostly used in trans-critical circulation.However,it is less used in large cold storage system.On the one hand,it is due to system performance reasons,and on the other hand,it will also involve the initial investment of refrigeration system.Combined with the above two points,this paper proposes a carbon dioxide refrigeration system applied to large and medium-sized cold storage.The carbon dioxide refrigeration system takes the buried pipe as the condenser,and makes full use of the characteristic that the underground soil temperature is low and keeps stable all the year round,so that the carbon dioxide circulation can be kept in the subcritical range when the system is running.The ejector is added between the compressor and the buried pipe in the system.When the internal pressure of the system is too high,the gaseous carbon dioxide in the liquid storage tank can be ejected back to the buried pipe for condensation to ensure the stability of the system pressure.The article first introduces the carbon dioxide buried pipe system and carbon dioxide buried pipe ejector system.The mathematical model of the two systems is established,and the performance of the two systems under different working conditions is compared by theoretical calculation.The suction and exhaust pressure changes of the compressor at different frequencies are compared by experiments.And the performance of the cold storage refrigeration system is analyzed by long-term operation of the buried pipe system.The following conclusions can be drawn:(1)The COP of buried pipe system and buried pipe ejector system increases with the increase of evaporation temperature,and decreases with the increase of condensation temperature.The change of evaporation temperature has a greater impact on the system performance.When the evaporation temperature is-15℃,the COP of the two systems is equal.The rise of evaporation temperature makes the difference between the two systems decrease from 9.3% to-14%,and the rise of condensation temperature makes the difference between the two systems increase from 3.3% to 9.5%.Therefore,it can be concluded that the buried pipe ejector system is more suitable for the operating conditions of high compressor pressure ratio,lower evaporation temperature and higher condensation temperature.(2)In a buried pipe ejector system,increasing ejector ratio will increase the exhaust pressure of the system and the ejector inlet/outlet pressure ratio.When the ejector fluid pressure is 5MPa,the exhaust pressure and the ejector inlet/outlet pressure ratio can be increased by 18.75%.At the same time,the increase of gas-liquid mass flow ratio(ejection ratio)will make the compressor power consumption of the two systems rise,COP decreases,and the difference between the two systems increases with the increase of gas-liquid mass flow ratio(ejection ratio).When the gas-liquid mass flow ratio increases from 0.04 to 0.4,the COP difference increases from 4.5% to 11.2%.The compressor power difference increases from-4.8% to-12.6%.(3)With the increase of supercooling degree,the COP of the buried pipe ejection system has a significant increase,up to 13.7%.However,the degree of supercooling has no obvious effect on COP of the buried pipe system,and the difference between the two systems increases from-0.4% to 12%.When the undercooling degree and the gas-liquid mass flow ratio change simultaneously,COP decreases slightly with the increase of the undercooling degree in the buried pipe system when the gas-liquid mass flow ratio is small(0-0.2).When the gas-liquid mass flow rate is relatively large(0.2-0.4),COP increases slightly with the increase of undercooling degree.However,for a buried ejector system,COP will increase with the increasing of supercooling degree and the decreasing direction of ejector ratio.(4)In the case of compressor starting frequency of 25 hz,30 hz and 35 hz,the exhaust pressure stability time of the buried pipe system is about 75 s,50s and 30 s,and the stable time of the system operation is about 270 s,260s and 200 s.The peak exhaust pressure of the buried pipe ejection system at the corresponding frequency is about 50 s,40s and 30 s,and the stable operation time of the system is about 250 s,125s and 100 s.After the system runs steadily,the exhaust pressure of the buried pipe system is about 54 bar,while that of the buried pipe ejector system is 52 bar.Under the condition of constant compressor starting frequency,the suction and exhaust pressures of the buried pipe ejector system are lower than those of the buried pipe system,but the compressor pressure ratio is much higher than that of the buried pipe system.(5)when the temperature of the cold storage remains constant at-10℃,the internal pressure,flow rate and temperature of the buried pipe system can be stabilized within a certain range.Among them,the exhaust pressure was maintained around 60 bar,the suction pressure was maintained around 14 bar,the inlet and outlet pressure of the buried pipe was maintained around 59 bar and 54 bar,the pressure in the reservoir was maintained around53 bar,the evaporation pressure was maintained around 22 bar,and the compressor pressure ratio was maintained around 4.3.The pressure difference between the inlet and outlet of the buried pipe is about 5bar,and the difference between the evaporation pressure and the suction pressure is about 8bar due to the existence of the mass flow meter.The suction flow of the compressor in the system is maintained at 174kg/h,and the liquid and gas outlet flow of the plate heat exchanger is maintained at 128kg/h and 50kg/h.The exhaust temperature and suction temperature of the compressor in the buried pipe system are maintained near 90℃and-16℃,the outlet temperature of the buried pipe is maintained at 18℃,the temperature of the liquid reservoir is maintained at 17℃,and the difference between the storage temperature and suction temperature is 6℃.The soil temperature below 10 m increased slightly with the increase of depth,and maintained at 14.8℃ as a whole.However,the soil temperature at 3m underground is easily affected by the ambient temperature,which varies greatly from 12.5℃ to 13.7℃ during the measurement period.(6)The accumulative power consumption of the unit within 48 hours of continuous operation is 241.55KW·h,and the average power consumption per cubic meter per day is2.4KW·h.The actual COP of the unit is maintained around 1.5 during operation.The electricity cost during the operation time is about 202.12 yuan.(7)The temperature of the measured points in the warehouse is affected by the fan and the electric heating device.Point 1 and 2 change around-9.45℃,and point 3 and 4 change around-9.75℃.The temperature at point 1 fluctuated the most,which is 0.21℃.The temperature fluctuation at point 4 is the smallest,which is 0.05℃.