CO2 transcritical two-stage compression cycle refrigerator system and its control method

Abstract

The application discloses a CO2 transcritical two-stage compression cycle refrigerator system and a control method thereof, and belongs to the technical field of refrigeration. In the system, a compressor is provided with a first-stage compression cavity and a second-stage compression cavity; a gas cooler inlet is communicated with a second-stage compression cavity exhaust end, and an outlet is communicated with an economizer high-pressure side inlet; an economizer high-pressure side outlet is divided into a branch and a trunk, the branch is connected to an economizer low-pressure side inlet through an adjusting valve, and the trunk is connected to a liquid storage tank inlet through a high-pressure valve; the economizer low-pressure side outlet and the first-stage compression cavity exhaust end are jointly communicated with a second-stage compression cavity suction end; a liquid storage tank outlet is connected to an evaporator inlet through an electronic expansion valve, and an evaporator outlet is communicated with a first-stage compression cavity suction end; a sensing detection unit collects system operation parameters and transmits the system operation parameters to a control unit, and the control unit adjusts valve opening degrees according to the collected parameters. The application can realize self-adaptive adjustment of intermediate cooling flow according to working conditions and dynamic optimization of high-pressure side pressure in transcritical working conditions, and can improve operation safety and energy efficiency levels of the system under all working conditions.

Description

Technical Field

[0001] This invention belongs to the field of refrigeration technology, and in particular relates to a CO2 transcritical two-stage compression cycle refrigerator system and its control method. Background Technology

[0002] In the commercial refrigeration sector, traditional fluorinated refrigerants are gradually being restricted due to environmental impacts, making refrigeration technology using natural refrigerants an important development direction for the industry. CO2, as a natural refrigerant, possesses characteristics such as GWP=1, ODP=0, non-toxicity, non-flammability, and chemical stability, making it a highly promising alternative working fluid for commercial refrigeration. Currently, CO2 refrigeration technology has been applied in large-scale centralized refrigeration systems, but technical bottlenecks still exist in small and medium-sized distributed commercial refrigerators, making it difficult to achieve efficient and stable operation.

[0003] The critical temperature of CO2 is 31.1℃, and the critical pressure is 7.37MPa. When the ambient temperature is higher than its critical temperature, the refrigeration system needs to operate in transcritical cycle mode, where the high-pressure side working pressure can reach 9-10MPa, and the exhaust temperature can exceed 100℃. Under these conditions, the compression ratio of a single-stage compression system is too high. On the one hand, excessively high exhaust temperatures can easily cause lubricating oil carbonization and compressor valve seal failure, shortening the compressor's lifespan; on the other hand, compression process losses increase, significantly reducing the system's energy efficiency ratio.

[0004] Two-stage compression technology is a common approach to alleviate the aforementioned problems, but existing two-stage compression CO2 refrigeration systems still have several shortcomings: First, they often employ two independent compressors connected in series, resulting in a large overall structure, complex piping connections, and insufficient compactness; second, the efficiency of intermediate cooling methods is limited, with existing intermediate heat exchangers, flash tanks, and regenerators exhibiting problems such as insufficient cooling effect, structural complexity, and limited cooling capacity; third, under transcritical operation mode, the exhaust pressure is difficult to precisely control, failing to match the optimal exhaust pressure for the corresponding ambient temperature to maximize system energy efficiency. Therefore, there is an urgent need to develop a compact, highly efficient, and energy-optimizable transcritical two-stage compression CO2 refrigeration system to meet the full-condition operation requirements of commercial freezers. Summary of the Invention

[0005] In view of the shortcomings of the related technologies, the purpose of this invention is to provide a CO2 transcritical two-stage compression cycle refrigerator system and its control method to solve the problems mentioned in the background technology.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A CO2 transcritical two-stage compression cycle refrigerator system includes a compressor, a gas cooler, an economizer, a high-pressure valve, a liquid receiver, an electronic expansion valve, an evaporator, a regulating valve, a sensing and detection unit, and a control unit. The compressor consists of a primary compression chamber and a secondary compression chamber; The inlet of the gas cooler is connected to the exhaust end of the secondary compression chamber, and the outlet of the gas cooler is connected to the high-pressure side inlet of the economizer. The high-pressure side outlet of the economizer is divided into a branch line and a main line. The branch line is connected to the low-pressure side inlet of the economizer via a regulating valve, and the main line is connected to the inlet of the storage tank via a high-pressure valve. The low-pressure outlet of the economizer and the exhaust end of the first-stage compression chamber are connected together to the intake end of the second-stage compression chamber; The outlet of the liquid storage tank is connected to the inlet of the evaporator via an electronic expansion valve, and the outlet of the evaporator is connected to the suction end of the primary compression chamber. The sensing and detection unit is used to collect the pressure, temperature and mass flow parameters of the system and transmit them to the control unit. The control unit adjusts the opening of each valve according to the collected parameters to realize the dynamic adjustment of the intermediate cooling flow of the branch and the control of the high pressure side of the system.

[0007] In some embodiments, the sensing and detection unit includes four pressure sensors, which are respectively located at the outlet of the evaporator, the intake end of the secondary compression chamber, the exhaust end of the secondary compression chamber, and the outlet of the liquid storage tank.

[0008] In some embodiments, the sensing and detection unit further includes thirteen temperature sensors, which are respectively disposed at the inlet of the evaporator, the outlet of the evaporator, the suction end of the first-stage compression chamber, the exhaust end of the first-stage compression chamber, the suction end of the second-stage compression chamber, the exhaust end of the second-stage compression chamber, the inlet of the gas cooler, the outlet of the gas cooler, the high-pressure side inlet of the economizer, the high-pressure side outlet of the economizer, the low-pressure side inlet of the economizer, the low-pressure side outlet of the economizer, and the outlet of the high-pressure valve.

[0009] In some embodiments, the sensing unit further includes two mass flow meters, one located at the outlet of the gas cooler and the other at the inlet of the electronic expansion valve.

[0010] In some embodiments, the control unit includes a PLC controller and a property calculation module, which are connected via RS485 communication. The property calculation module encapsulates a refrigerant property calculation program, which can calculate the corresponding saturation temperature based on pressure parameters and the refrigerant enthalpy based on pressure and temperature parameters. The PLC controller calculates the refrigerant superheat at the corresponding measuring point based on the calculated saturation temperature and temperature parameters. It also calculates the heat transfer capacity and coefficient of performance (COP) of the system's heat exchange components based on the calculated enthalpy and mass flow rate parameters. Finally, it adjusts the opening of the regulating valve, high-pressure valve, and electronic expansion valve according to the superheat, exhaust pressure, and COP.

[0011] In some embodiments, the PLC controller uses a PID algorithm to perform closed-loop regulation of the opening of the regulating valve, the high-pressure valve, and the electronic expansion valve, respectively.

[0012] A control method for a CO2 transcritical two-stage compression cycle refrigerator system is provided, applied to the aforementioned CO2 transcritical two-stage compression cycle refrigerator system, which has subcritical and transcritical operating modes. The control method includes the following steps: In subcritical operation mode, the high-pressure valve is kept fully open, the opening of the regulating valve is adjusted according to the intake superheat of the secondary compression chamber to adjust the intermediate cooling flow of the branch, and the opening of the electronic expansion valve is adjusted according to the outlet superheat of the evaporator to control the liquid supply of the evaporator. In transcritical operation mode, the superheat of the regulating valve and the electronic expansion valve is maintained in a closed loop. At the same time, the opening of the high-pressure valve is dynamically adjusted according to the exhaust pressure of the secondary compression chamber and the coefficient of performance of refrigeration, so as to adjust the high-pressure side pressure of the system to the target value corresponding to the current operating condition.

[0013] In some embodiments, the superheat calculation steps are as follows: the pressure and temperature parameters of the corresponding measuring point are collected by the sensing and detection unit, the control unit calculates the corresponding saturation temperature based on the pressure parameters, and then the difference between the temperature parameter of the measuring point and the saturation temperature is calculated to obtain the refrigerant superheat of the measuring point.

[0014] In some embodiments, under transcritical operation mode, the system's coefficient of performance (COP) is calculated in real time. By gradually adjusting the opening of the high-pressure valve, different high-pressure side pressure conditions of the system are traversed within a preset pressure range. Based on the changing trend of the COP, the target value of the system's high-pressure side pressure under the current operating condition is determined.

[0015] In some embodiments, the calculation steps for the coefficient of performance (COP) are as follows: the refrigerant mass flow rate parameter and the pressure and temperature parameters at the inlet and outlet of the system heat exchange components are collected by the sensing and detection unit; the control unit calculates the refrigerant enthalpy value at the corresponding location based on the pressure and temperature parameters; the system heat exchange is calculated by combining the mass flow rate parameter; and the system COP is calculated by combining the compressor operating power.

[0016] Compared with the prior art, the beneficial effects of the present invention are: 1. The CO2 transcritical two-stage compression cycle refrigerator system provided by the present invention adopts a two-stage compression combined with an economizer branch structure with a regulating valve, which can achieve efficient intermediate cooling, reduce the suction temperature of the second stage compression, and ensure the safe operation of the compressor; the high-pressure side pressure is independently regulated by the high-pressure valve, which is adapted to the characteristics of CO2 transcritical cycle, and the pipeline layout is clear and reasonable, which is easy to integrate into the refrigerator equipment, taking into account both operational reliability and energy efficiency improvement potential.

[0017] 2. The control method of the CO2 transcritical two-stage compression cycle refrigerator system provided by the present invention includes subcritical and transcritical dual operation modes, which can adapt to different ambient temperatures and load conditions; the system's basic operation is ensured by superheat closed-loop control, and the high-pressure side pressure target value is dynamically adjusted in combination with the coefficient of performance (COP) to achieve adaptive energy efficiency optimization under all operating conditions, effectively balancing operational safety and energy efficiency. Attached Figure Description

[0018] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate exemplary embodiments of the invention and, together with their description, serve to explain the invention and do not constitute an undue limitation thereof. In the drawings: Figure 1 This is a system schematic diagram of an embodiment of the CO2 transcritical two-stage compression cycle refrigerator system and its control method of the present invention.

[0019] In the picture: 1. Compressor; 11. Primary compression chamber; 12. Secondary compression chamber; 2. Gas cooler; 3. Economizer; 4. High-pressure valve; 5. Liquid receiver; 6. Electronic expansion valve; 7. Evaporator; 8. Regulating valve. Detailed Implementation

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0021] In the description of this invention, it should be understood that the terms "center", "lateral", "longitudinal", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0022] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0023] Example 1: See appendix Figure 1 This paper presents an illustrative embodiment of the CO2 transcritical two-stage compression cycle refrigerator system proposed in this invention. The CO2 transcritical two-stage compression cycle refrigerator system includes a compressor 1, a gas cooler 2, an economizer 3, a high-pressure valve 4, a liquid storage tank 5, an electronic expansion valve 6, an evaporator 7, a regulating valve 8, a sensing and detection unit, and a control unit.

[0024] The compressor 1 includes a primary compression chamber 11 and a secondary compression chamber 12; the inlet of the gas cooler 2 is connected to the exhaust end of the secondary compression chamber 12, and the outlet of the gas cooler 2 is connected to the high-pressure side inlet of the economizer 3; the pipeline of the high-pressure side outlet of the economizer 3 is divided into a branch and a main line, wherein the branch is connected to the low-pressure side inlet of the economizer 3 via a regulating valve 8, and the main line is connected to the inlet of the liquid storage tank 5 via a high-pressure valve 4; the low-pressure side outlet of the economizer 3 and the exhaust end of the primary compression chamber 11 are connected to the suction end of the secondary compression chamber 12; the outlet of the liquid storage tank 5 is connected to the inlet of the evaporator 7 via an electronic expansion valve 6, and the outlet of the evaporator 7 is connected to the suction end of the primary compression chamber 11; the sensing and detection unit is used to collect the pressure parameters, temperature parameters and mass flow parameters of the system operation and transmit them to the control unit. The control unit adjusts the opening of each valve according to the collected parameters to realize the dynamic adjustment of the intermediate cooling flow of the branch and the control of the high-pressure side pressure of the system.

[0025] Furthermore, the specific specifications and connection methods of each component in this embodiment are as follows: Compressor 1 is a DC inverter rotor compressor with a frequency adjustment range of 80-140Hz. The inverter structure can match the dynamic load changes of the freezer, reducing the losses caused by frequent start-stop of compressor 1; Regulating valve 8 and high-pressure valve 4 both have a pressure-bearing capacity of 14MPa, suitable for CO2 transcritical high-pressure conditions, wherein regulating valve 8 has a diameter of 1.0mm and high-pressure valve 4 has a diameter of 1.4mm; Electronic expansion valve 6 is a CO2 electronic expansion valve with a pressure-bearing capacity of 9MPa and a diameter of 0.8mm, precisely matching the low-pressure liquid supply requirements of evaporator 7, used to control the liquid supply and outlet superheat of evaporator 7; Liquid storage tank 5 has a pressure-bearing capacity of 14MPa and a capacity of 1.2L, which can stably store liquid refrigerant and buffer... Flow fluctuations; Economist 3 is a shell-and-tube heat exchanger with a 6mm diameter thin tube and a 9mm diameter thick tube. The thin tube flows through the main CO2 refrigerant, and its smaller diameter makes it more pressure-bearing, suitable for high-pressure operation of the main circuit; the thicker tube flows through the branch CO2 refrigerant, suitable for low-pressure operation after throttling and pressure reduction of the branch circuit. The two exchange heat fully within the shell-and-tube structure, resulting in high intermediate cooling heat exchange efficiency; Gas cooler 2 and evaporator 7 are both tube-fin heat exchangers with a 7mm diameter and 0.8mm wall thickness, ensuring sufficient heat exchange and a compact structure; The system components are connected and combined by welding, with a single two-stage compressor replacing the dual-compressor series structure, simplifying the piping and reducing the number of welded joints, which can significantly reduce the risk of refrigerant leakage. The overall size of the equipment is smaller, making it suitable for distributed installation scenarios of small and medium-sized commercial refrigerators.

[0026] The sensing unit includes four pressure sensors, located at the outlet of evaporator 7, the suction end of secondary compression chamber 12, the discharge end of secondary compression chamber 12, and the outlet of liquid receiver 5. The pressure at the outlet of evaporator 7 corresponds to the first-stage suction pressure of compressor 1. All these pressure points are refrigerant-side points, installed inside the refrigerant flow pipeline, directly detecting the pressure state of the CO2 refrigerant within the system. This provides the basic data for calculating subsequent refrigerant properties such as saturation temperature and enthalpy.

[0027] The sensing and detection unit also includes thirteen temperature sensors, which are respectively located at the inlet and outlet of evaporator 7, the suction end and discharge end of primary compression chamber 11, the suction end and discharge end of secondary compression chamber 12, the inlet and outlet of gas cooler 2, the high-pressure side inlet, high-pressure side outlet, low-pressure side inlet and outlet of economizer 3, and the outlet of high-pressure valve 4. All of these temperature measuring points are refrigerant-side measuring points, correspondingly arranged within the refrigerant flow pipeline at each location, directly detecting the temperature state of the CO2 refrigerant itself. In conjunction with pressure measuring points at the same locations, they can be used to calculate thermodynamic parameters such as refrigerant superheat and enthalpy at the corresponding measuring points.

[0028] The sensing and detection unit also includes two mass flow meters, which are respectively installed at the outlet of the gas cooler 2 and the inlet of the electronic expansion valve 6.

[0029] The aforementioned multi-point pressure, temperature, and flow sensors enable the system to collect full-dimensional operating status data. By relying on the fusion of multiple sensors to collect complete operating condition data, it provides complete data support for subsequent PLC real-time optimization control and safety protection mechanisms, solving the shortcomings of traditional single-point monitoring that cannot identify multi-dimensional operating condition fluctuations.

[0030] The control unit includes a PLC controller and a property calculation module, which are connected via RS485 communication. The property calculation module encapsulates a refrigerant property calculation program, built on the Python open-source programming language and the CoolProp open-source fluid property calculation library. It can calculate the corresponding saturation temperature based on pressure parameters and the refrigerant enthalpy based on pressure and temperature parameters. Real-time calculation of various thermodynamic parameters is achieved by calling built-in functions, eliminating the need for additional external detection equipment. The calculation response is fast, providing underlying data support for real-time calculation of superheat, heat transfer, and coefficient of performance (COP).

[0031] The PLC controller calculates the refrigerant superheat at the corresponding measuring point based on the calculated saturation temperature and temperature parameters. It also calculates the heat transfer capacity and coefficient of performance (COP) of the system's heat exchange components based on the calculated enthalpy and mass flow rate parameters. Finally, it adjusts the opening of regulating valve 8, high-pressure valve 4, and electronic expansion valve 6 according to the superheat, exhaust pressure, and COP.

[0032] The PLC controller uses a PID algorithm to perform closed-loop regulation of the opening of regulating valve 8, high-pressure valve 4 and electronic expansion valve 6 respectively.

[0033] The refrigeration cycle process of this system is divided into subcritical operation mode and transcritical operation mode: In subcritical operation mode, the high-pressure, high-temperature CO2 refrigerant compressed by compressor 1 enters gas cooler 2 for cooling. The cooled high-pressure CO2 enters economizer 3 through the high-pressure side inlet and branches off at the high-pressure side outlet of economizer 3. The CO2 entering the branch is throttled and cooled by regulating valve 8, and then enters economizer 3 through the low-pressure side inlet. The temperature of the branch CO2 refrigerant after throttling and depressurization is lower than that of the main high-pressure CO2 refrigerant, and the two exchange heat within economizer 3. The heat-exchanged branch CO2 mixes with the first-stage exhaust gas from compressor 1. The CO2 then enters the suction end of the secondary compression chamber 12. In subcritical operation mode, the high-pressure valve 4 remains fully open and does not participate in regulation. The CO2 in the main circuit enters the liquid storage tank 5 through the high-pressure valve 4 to stably supply liquid to the evaporator 7. The CO2 refrigerant is throttled by the electronic expansion valve 6 and then sent to the evaporator 7 to complete evaporation and heat absorption. After the low-temperature and low-pressure CO2 refrigerant evaporates and absorbs heat in the evaporator 7, it enters the primary compression chamber 11 to complete the primary compression. The primary exhaust mixes with the CO2 refrigerant at the low-pressure side outlet of the economizer 3 and is cooled before entering the secondary compression chamber 12 to complete the secondary compression, thus completing the entire refrigeration cycle.

[0034] In the transcritical operation mode, the basic cycle process is the same as that in the subcritical operation mode. The difference is that the high-pressure valve 4 participates in the regulation, and achieves dynamic control of the discharge pressure of compressor 1 by changing its own opening degree. The optimization goal is to maximize the coefficient of performance (COP) and adapt to the operating conditions under different high ambient temperatures.

[0035] In the above illustrative embodiment, the CO2 transcritical two-stage compression cycle refrigerator system uses a single two-stage compressor instead of a dual-compressor series configuration, reducing equipment size and simplifying piping structure. This effectively reduces the risk of refrigerant leakage and is suitable for the distributed installation needs of small and medium-sized commercial refrigerators. Using natural CO2 refrigerant, it is non-greenhouse-economy, environmentally friendly, and safe, aligning with the environmental policy development trend of the refrigeration industry. Furthermore, the system incorporates an economizer with an intermediate cooling structure, effectively reducing compressor exhaust temperature and energy loss during compression. By relying on multi-sensor fusion to collect comprehensive operating data and combining it with PLC intelligent control, a robust safety protection mechanism is constructed, significantly mitigating compressor overheating and wear issues and extending compressor lifespan.

[0036] Example 2: See appendix Figure 1 This paper presents an illustrative embodiment of the control method for a CO2 transcritical two-stage compression cycle refrigerator system proposed in this invention. The method is applied to the CO2 transcritical two-stage compression cycle refrigerator system of Embodiment 1. This system has subcritical and transcritical operating modes. The control method includes the following steps: In subcritical operation mode, the high-pressure valve 4 is kept fully open, the opening of the regulating valve 8 is adjusted according to the intake superheat of the secondary compression chamber 12 to adjust the intermediate cooling flow of the branch, and the opening of the electronic expansion valve 6 is adjusted according to the outlet superheat of the evaporator 7 to control the liquid supply of the evaporator 7. In the transcritical operation mode, the superheat closed-loop regulation of regulating valve 8 and electronic expansion valve 6 is maintained. At the same time, the opening of high pressure valve 4 is dynamically adjusted according to the exhaust pressure of secondary compression chamber 12 and the coefficient of performance of refrigeration, so as to adjust the high pressure side pressure of the system to the target value corresponding to the current operating condition.

[0037] The calculation steps for superheat are as follows: the pressure and temperature parameters of the corresponding measuring point are collected by the sensing unit, the control unit calculates the corresponding saturation temperature based on the pressure parameters, and then the difference between the temperature parameter of the measuring point and the saturation temperature is calculated to obtain the refrigerant superheat of the measuring point.

[0038] Specifically, the PID closed-loop control logic for each valve is as follows: The regulating valve 8 adjusts its opening based on the superheat feedback from the suction side of the secondary compression chamber 12: the pressure sensor collects the pressure data at the suction end of the secondary compression chamber 12 and sends it to the PLC controller. The PLC controller transmits the pressure data to the property calculation module via RS485 communication to calculate the corresponding saturation temperature. The difference between the calculated temperature and the actual temperature at the measuring point is used to obtain the superheat. If the superheat is less than the set value, the opening of the regulating valve 8 is reduced; if the superheat is greater than the set value, the opening of the regulating valve 8 is increased. The adjustment process is achieved by adjusting the proportional, integral, and derivative parameters of the PID controller. The opening of the regulating valve 8 is controlled by the analog signal output by the PLC controller.

[0039] The high-pressure valve 4 adjusts its opening based on the feedback of the exhaust pressure of the secondary compression chamber 12: if the measured exhaust pressure is greater than the set value, the opening of the high-pressure valve 4 is increased to reduce the exhaust pressure; if the measured exhaust pressure is less than the set value, the opening of the high-pressure valve 4 is decreased to increase the exhaust pressure. The adjustment process is achieved through a PID algorithm.

[0040] The electronic expansion valve 6 adjusts its opening based on the superheat feedback from the outlet of the evaporator 7: the pressure sensor collects the pressure data at the outlet of the evaporator 7 and sends it to the PLC controller. The PLC controller transmits the pressure data to the property calculation module via RS485 communication to calculate the corresponding saturation temperature. The difference between the calculated temperature and the actual temperature at the measuring point is used to obtain the superheat. If the superheat is less than the set value, the opening of the electronic expansion valve 6 is reduced; if the superheat is greater than the set value, the opening of the electronic expansion valve 6 is increased. The adjustment process is achieved by adjusting the proportional, integral, and derivative parameters of the PID controller. The opening of the electronic expansion valve 6 is controlled by the analog signal output by the PLC controller.

[0041] In transcritical operation mode, the system's coefficient of performance (COP) is calculated in real time. By gradually adjusting the opening of high-pressure valve 4, different high-pressure side pressure conditions within a preset pressure range are traversed. Based on the changing trend of the COP, the target value of the high-pressure side pressure under the current operating condition is determined. This optimization logic addresses the deficiency of existing two-stage CO2 systems in dynamically matching the optimal exhaust pressure with ambient temperature. Relying on real-time COP calculation as the evaluation criterion, it automatically traverses the high-pressure side pressure range, locks in the energy-efficient operating condition under the current load and ambient temperature, and maximizes the COP under all operating conditions.

[0042] The calculation steps for the coefficient of performance (COP) are as follows: The refrigerant mass flow rate parameter and the pressure and temperature parameters at the inlet and outlet of the system heat exchange components are collected by the sensing and detection unit. The control unit calculates the refrigerant enthalpy value at the corresponding location based on the pressure and temperature parameters. The system heat exchange is calculated by combining the mass flow rate parameter. Finally, the COP of the system is calculated by combining the operating power of compressor 1, providing a quantitative evaluation standard for pressure optimization.

[0043] The pressure and temperature parameters are input to the PLC controller in the form of analog signals. The PLC controller transmits the parameters to the property calculation module via RS485 communication and calls the function to calculate the corresponding enthalpy value. The system heat exchange is calculated by multiplying the refrigerant mass flow rate by the enthalpy difference between the inlet and outlet of the heat exchange component. The operating power of compressor 1 is collected by the power meter, and the power meter transmits the data to the PLC controller via RS485 communication.

[0044] In the above illustrative embodiment, the control method of the CO2 transcritical two-stage compression cycle refrigerator system relies on multi-sensor fusion to collect comprehensive operational status data from the system, and uses a PLC as the control core to achieve closed-loop regulation throughout the entire process. In subcritical mode, a dual superheat closed-loop system ensures the basic operational stability of the system, while in transcritical mode, the system dynamically adjusts the exhaust pressure and optimizes energy efficiency by combining real-time refrigeration performance coefficients. This method can adaptively match complex operating conditions such as compressor frequency changes, ambient temperature fluctuations, and refrigerator load changes, and automatically adjusts throughout the process without manual intervention, effectively balancing equipment operational safety and system energy efficiency under all operating conditions.

[0045] Finally, it should be noted that the various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0046] The above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them; although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications can still be made to the specific implementation of the present invention or equivalent substitutions can be made to some technical features without departing from the spirit of the technical solutions of the present invention, and all such modifications and substitutions should be covered within the scope of the technical solutions claimed in the present invention.

Claims

1. A transcritical two-stage compression cycle CO2 refrigerator system, characterized in that, It includes a compressor, gas cooler, economizer, high-pressure valve, liquid receiver, electronic expansion valve, evaporator, regulating valve, sensor detection unit, and control unit; The compressor includes a primary compression chamber and a secondary compression chamber; The inlet of the gas cooler is connected to the exhaust end of the secondary compression chamber, and the outlet of the gas cooler is connected to the high-pressure side inlet of the economizer; The high-pressure side outlet of the economizer is divided into a branch line and a main line. The branch line is connected to the low-pressure side inlet of the economizer via the regulating valve, and the main line is connected to the inlet of the storage tank via the high-pressure valve. The low-pressure outlet of the economizer and the exhaust end of the first-stage compression chamber are connected together to the intake end of the second-stage compression chamber. The outlet of the liquid storage tank is connected to the inlet of the evaporator via the electronic expansion valve, and the outlet of the evaporator is connected to the suction end of the primary compression chamber; The sensing and detection unit is used to collect the pressure, temperature and mass flow parameters of the system and transmit them to the control unit. The control unit adjusts the opening of each valve according to the collected parameters to realize the dynamic adjustment of the intermediate cooling flow of the branch and the control of the high-pressure side pressure of the system.

2. The CO2 transcritical two-stage compression cycle refrigerator system according to claim 1, characterized in that, The sensing and detection unit includes four pressure sensors, which are respectively located at the outlet of the evaporator, the intake end of the secondary compression chamber, the exhaust end of the secondary compression chamber, and the outlet of the liquid storage tank.

3. The CO2 transcritical two-stage compression cycle refrigerator system according to claim 1, characterized in that, The sensing and detection unit also includes thirteen temperature sensors, which are respectively installed at the inlet of the evaporator, the outlet of the evaporator, the suction end of the first-stage compression chamber, the exhaust end of the first-stage compression chamber, the suction end of the second-stage compression chamber, the exhaust end of the second-stage compression chamber, the inlet of the gas cooler, the outlet of the gas cooler, the high-pressure side inlet of the economizer, the high-pressure side outlet of the economizer, the low-pressure side inlet of the economizer, the low-pressure side outlet of the economizer, and the outlet of the high-pressure valve.

4. The CO2 transcritical two-stage compression cycle refrigerator system according to claim 1, characterized in that, The sensing and detection unit also includes two mass flow meters, which are respectively installed at the outlet of the gas cooler and the inlet of the electronic expansion valve.

5. The CO2 transcritical two-stage compression cycle refrigerator system according to claim 1, characterized in that, The control unit includes a PLC controller and a physical property calculation module, which are connected via RS485 communication. The property calculation module encapsulates a refrigerant property calculation program, which can calculate the corresponding saturation temperature based on the pressure parameters, and calculate the refrigerant enthalpy based on the pressure and temperature parameters. The PLC controller calculates the refrigerant superheat at the corresponding measuring point based on the calculated saturation temperature and the temperature parameters, calculates the heat exchange capacity and refrigeration performance coefficient of the system's heat exchange components based on the calculated enthalpy value and the mass flow rate parameters, and adjusts the opening of the regulating valve, high-pressure valve, and electronic expansion valve according to the superheat, exhaust pressure, and refrigeration performance coefficient.

6. The CO2 transcritical two-stage compression cycle refrigerator system according to claim 5, characterized in that, The PLC controller uses a PID algorithm to perform closed-loop regulation of the opening of the regulating valve, high-pressure valve and electronic expansion valve respectively.

7. A control method for a CO2 transcritical two-stage compression cycle refrigerator system, characterized in that, The control method, applied to the CO2 transcritical two-stage compression cycle refrigerator system according to any one of claims 1-6, wherein the system has a subcritical operating mode and a transcritical operating mode, comprises the following steps: In subcritical operation mode, the high-pressure valve is kept fully open, the opening of the regulating valve is adjusted according to the intake superheat of the secondary compression chamber to adjust the intermediate cooling flow of the branch, and the opening of the electronic expansion valve is adjusted according to the outlet superheat of the evaporator to control the liquid supply of the evaporator. In transcritical operation mode, the superheat of the regulating valve and the electronic expansion valve is maintained in a closed loop. At the same time, the opening of the high-pressure valve is dynamically adjusted according to the exhaust pressure of the secondary compression chamber and the coefficient of performance of refrigeration, so as to adjust the high-pressure side pressure of the system to the target value corresponding to the current operating condition.

8. The control method for the CO2 transcritical two-stage compression cycle refrigerator system according to claim 7, characterized in that, The calculation steps for the superheat are as follows: the pressure and temperature parameters of the corresponding measuring point are collected by the sensing and detection unit, the corresponding saturation temperature is calculated by the control unit based on the pressure parameters, and the difference between the temperature parameter of the measuring point and the saturation temperature is calculated to obtain the refrigerant superheat of the measuring point.

9. The control method for the CO2 transcritical two-stage compression cycle refrigerator system according to claim 7, characterized in that, In the transcritical operation mode, the system's coefficient of performance (COP) is calculated in real time. By gradually adjusting the opening of the high-pressure valve, different high-pressure side pressure conditions of the system are traversed within the preset pressure range. Based on the changing trend of the COP, the target value of the high-pressure side pressure of the system under the current operating condition is determined.

10. The control method for the CO2 transcritical two-stage compression cycle refrigerator system according to claim 9, characterized in that, The calculation steps for the coefficient of performance (COP) are as follows: the refrigerant mass flow rate parameter and the pressure and temperature parameters at the inlet and outlet of the system heat exchange components are collected by the sensing and detection unit. The control unit calculates the refrigerant enthalpy value at the corresponding location based on the pressure and temperature parameters, calculates the system heat exchange capacity based on the mass flow rate parameter, and then calculates the COP of the system based on the compressor operating power.

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