CO2 supercritical servo system and control method thereof

Abstract

The invention provides a CO2 supercritical servo system which comprises a main system, an auxiliary system and a control unit, and the main system comprises a CO2 screw compressor, a CO2 horizontal oil separator, a CO2 precooler, a CO2 condenser and a CO2 floating ball valve which are connected in sequence; the auxiliary system comprises a CO2 supercritical piston compressor, a CO2 supercritical oil separator and a CO2 piston compressor oil reservoir which are connected in sequence; the exhaust side of the auxiliary system is divided into three passages through an automatic control valve, wherein the first passage is connected into a CO2 oil separator exhaust pipe of the main system and enters a CO2 condenser of the main system through a CO2 precooler; the second passage is connected to a defrosting inlet of the tail end evaporator; the third channel completes internal circulation between a low circulation barrel and a precooler of the system, and the pressure of the system is kept stable; the control unit controls the automatic control valve to be switched among the three channels. According to the high-efficiency servo refrigerating system, the high-efficiency servo refrigerating system integrating pressure maintaining, intelligent defrosting and dynamic load supplementing is constructed.

Description

Technical Field

[0001] The present invention belongs to the technical field of refrigeration equipment, and in particular relates to a CO2 supercritical servo system and a control method thereof. Background Art

[0002] Carbon dioxide (CO2), a natural refrigerant, boasts non-toxicity, non-flammability, and low global warming potential (GWP). In recent years, it has garnered widespread attention in refrigeration, heat pump, and supercritical applications. In particular, CO2, with its excellent heat transfer properties and environmental friendliness, has become a leading alternative to traditional CFCs in transcritical refrigeration systems. However, CO2 systems also face numerous technical challenges, including high operating pressures and complex control. The operating pressure of CO2 in the supercritical state typically reaches 7-12 MPa, making the system extremely sensitive to pressure fluctuations. Any instability can lead to a sharp drop in energy efficiency or equipment overpressure. Severe frosting on the terminal evaporator compromises stability, especially in low-temperature environments or under frequent start-stop conditions. Traditional electric heating or hot gas bypass defrost solutions suffer from slow response, high energy consumption, and significant system disturbances. Single-unit systems also suffer from poor load regulation. When user demand fluctuates significantly, traditional systems require frequent startups and shutdowns of the main unit or the use of variable-frequency compressors to adjust the load. This increases system complexity and cost and can also cause the main unit to lose life due to frequent startups.

[0003] Existing public technologies, such as patent CN204373252U, provide a conversion-type CO2 transcritical cycle refrigeration system, which realizes single / double-stage cycle conversion by switching the flow direction of the refrigerant in different heat exchangers. Although it improves energy efficiency, it still has the following deficiencies in terms of system flexible adjustment, rapid defrosting and efficient pressure maintenance: the system structure relies on a complex heat exchanger layout and mechanical valve switching, and the adjustment response is slow; it is unable to realize the functions of "dynamic loading" and "servo pressure control" during operation; there is a lack of a rapid defrost path facing the evaporation end, and it needs to rely on external auxiliary heating means.

[0004] In summary, existing CO2 transcritical systems struggle to simultaneously meet the comprehensive requirements of high energy efficiency, rapid response, and stable operation. Therefore, a CO2 supercritical servo system with a more integrated structure, more intelligent control, and more complex functions is urgently needed to address the problems of inaccurate pressure control, delayed defrost, and poor system flexibility in traditional solutions. Summary of the Invention

[0005] In view of the above technical problems existing in the prior art, the present invention proposes a CO2 supercritical servo system and a control method thereof to solve the above technical problems.

[0006] According to a first aspect of the present invention, a CO2 supercritical servo system is provided, comprising a main system, an auxiliary system and a control unit;

[0007] The main system includes a CO2 screw compressor, a CO2 horizontal oil separator, a CO2 precooler, a CO2 condenser and a CO2 float valve connected in sequence;

[0008] The auxiliary system includes a CO2 supercritical piston compressor, a CO2 supercritical oil separator and a CO2 piston compressor oil reservoir connected in sequence;

[0009] The exhaust side of the auxiliary system is divided into three paths through an automatic control valve: the first path is merged into the exhaust pipe of the CO2 oil separator of the main system, and enters the CO2 condenser of the main system through the CO2 precooler; the second path is connected to the defrost inlet of the terminal evaporator; the third path completes the internal circulation between the system CO2 circulation barrel and the CO2 precooler to maintain system pressure stability;

[0010] The control unit controls the automatic valves switching between the three paths. By integrating the main and auxiliary compressor systems and the three-path automatic valves, a three-in-one servo function—"pressure maintenance, intelligent defrost, and load replenishment"—is established. This allows a single auxiliary unit to flexibly respond to three operating conditions, simplifying the complex structure of traditional multi-unit parallel or bypass systems and improving system integration and response speed.

[0011] In a specific embodiment, a CO2 circulation tank is also included for collecting refrigerant returned from the main system and auxiliary system. The addition of a CO2 circulation tank collects refrigerant returning from the main and auxiliary systems, balances the liquid / gas phase cycle, stabilizes storage pressure, and ensures a more stable and reliable medium supply when switching between modes, effectively avoiding control instability caused by circuit fluctuations.

[0012] In a specific embodiment, a pressure sensor is also provided on the CO2 circulation barrel, and a temperature sensor is provided on the terminal evaporator. Placing pressure and temperature sensors at the circulation barrel and the terminal evaporator allows real-time collection of key operating condition data, providing an accurate basis for the next step of automatic mode determination, ensuring that the servo unit starts or stops at the appropriate time.

[0013] In a specific embodiment, the pressure sensor, temperature sensor, and automatic control valve are electrically connected to the control unit. This arrangement enables centralized control of the entire gas circuit and the start and stop of the compressor, giving the system programmable, visual, and fully automatic operation capabilities, reducing human intervention and misoperation.

[0014] In a specific embodiment, the control unit automatically switches the passage of the automatic control valve and starts and stops the CO2 supercritical piston compressor according to the signals collected by the temperature sensor and the pressure sensor, and is configured to operate in at least one of the following modes:

[0015] Maintain unit mode, based on real-time pressure monitoring of the CO2 circulation barrel, start the CO2 supercritical piston compressor when the pressure exceeds the preset upper limit, and put the CO2 supercritical piston compressor into standby mode when the pressure drops to the lower limit;

[0016] In defrost mode, in response to the defrost demand signal of the terminal evaporator, the exhaust gas of the CO2 supercritical piston compressor is directly delivered to the terminal evaporator to defrost the evaporator. After the defrost process is completed, the refrigerant is discharged into the CO2 circulation barrel to wait for subsequent recycling;

[0017] In refrigeration load replenishment mode, the exhaust gas from the CO2 supercritical piston compressor is mixed with the refrigerant from the main system via an automatic valve, dynamically adjusting the cooling capacity to supplement the main system's load. The control unit automatically switches between three modes based on real-time pressure and temperature signals, ensuring the system remains in the optimal operating range while enabling rapid defrosting and dynamic load replenishment on demand, achieving a balance between energy conservation and reliability.

[0018] In a specific embodiment, a CO2 oil cooling plate is also included, which is installed between the CO2 horizontal oil separator and the CO2 screw compressor. This arrangement further cools and separates the lubricating oil and refrigerant, improving oil separation efficiency, reducing unit temperature, extending compressor life, and reducing maintenance efforts.

[0019] According to a second aspect of the present invention, a control method for the CO2 supercritical servo system as described above is proposed, comprising:

[0020] S1: Real-time detection of the pressure data of the CO2 circulation barrel of the refrigerant returned from the main system and the auxiliary system, as well as the temperature data of the terminal evaporator;

[0021] S2: In response to the pressure of the CO2 circulation barrel exceeding the preset upper limit, the maintenance unit mode is started, and the exhaust gas of the auxiliary system is introduced into the CO2 condenser of the main system through the automatic control valve until the pressure drops to the lower limit;

[0022] S3: In response to the terminal evaporator temperature being lower than the defrost threshold, the defrost mode is activated, and the exhaust gas of the auxiliary system is directed to the terminal evaporator through the automatic control valve to defrost the evaporator. After the defrost process is completed, the refrigerant is discharged into the CO2 circulation barrel to wait for subsequent recycling;

[0023] S4: In response to the main system load demand exceeding a preset threshold, the cooling load supplement mode is started, and the mixing ratio of the auxiliary system exhaust and the main system refrigerant is adjusted by the automatic control valve to dynamically supplement the cooling capacity.

[0024] In a specific embodiment, S2 specifically includes: in response to the CO2 circulation barrel pressure exceeding the preset upper limit, the control unit starts the CO2 supercritical piston compressor and switches the automatic control valve to the third passage to introduce the auxiliary system exhaust into the CO2 condenser of the main system.

[0025] In a specific embodiment, S3 specifically includes: in response to the temperature falling below the defrost threshold for a preset duration, the control unit activates the CO2 supercritical piston compressor and switches the automatic control valve to the second path, directing high-pressure exhaust gas to the defrost inlet of the terminal evaporator to defrost the evaporator. After the defrost process is completed, the refrigerant is discharged into the CO2 circulation drum for subsequent recycling. This setting clarifies the control logic of over-limit startup and return shutdown, allowing the auxiliary unit to more accurately maintain system pressure and more controllable energy consumption.

[0026] In a specific embodiment, S4 specifically includes: in response to the main system load demand exceeding a preset threshold, the control unit starts the CO2 supercritical piston compressor and switches the automatic control valve to the first path, so that the auxiliary system exhaust and the main system refrigerant are mixed in proportion; the mixed refrigerant is cooled by the CO2 condenser and then transported to the downstream system until the load demand drops below the threshold. The closed-loop definition of startup conditions, flow path switching, defrost completion, and unit shutdown can quickly and controllably complete the terminal defrost process, avoiding overheating or prolonged occupation of the refrigeration circuit. The system can automatically replenish cooling capacity based on real-time load, improving the host's response speed and reducing frequent starts and stops.

[0027] The CO2 supercritical servo system proposed in this paper integrates a CO2 supercritical piston compressor with a semi-hermetic screw compressor in a servo-controlled manner. Combined with a three-way automatic valve, a circulating drum, sensors, and automated control, it creates a high-performance servo refrigeration system integrating pressure maintenance, intelligent defrosting, and dynamic reloading. This system not only significantly simplifies traditional multi-machine or bypass circuit structures but also enables real-time closed-loop monitoring and automatic switching of critical operating conditions. It balances energy efficiency, response speed, and operational reliability, offering multiple advantages such as rapid defrost, seamless reloading, and precise pressure control, meeting the application requirements of modern high-performance refrigeration equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The accompanying drawings are included to provide a further understanding of the embodiments and are incorporated into and constitute a part of this specification. The accompanying drawings illustrate the embodiments and, together with the description, serve to explain the principles of the present invention. Other embodiments and many of the expected advantages of the embodiments will be readily apparent as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale with respect to each other. Like reference numerals designate corresponding similar parts.

[0029] Figure 1A schematic diagram of the system framework of a CO2 supercritical servo system according to an embodiment of the present invention is shown;

[0030] Figure 2 A flow chart of a control method for a CO2 supercritical servo system according to an embodiment of the present invention is shown.

[0031] The meaning of the numbers in the figure: 1. CO2 screw compressor; 2. CO2 horizontal oil separator; 3. CO2 oil cooling plate shell; 4. CO2 supercritical piston compressor; 5. CO2 supercritical oil separator; 6. CO2 piston compressor oil reservoir; 7. CO2 precooler; 8. CO2 condenser; 9. CO2 float valve; 10. CO2 piston system defrost outlet; 11. CO2 piston compressor suction port; 12. CO2 circulation barrel. DETAILED DESCRIPTION

[0032] The present invention will be further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are intended only to illustrate the invention and are not intended to limit the invention. It should also be noted that, for ease of description, only portions relevant to the invention are shown in the accompanying drawings.

[0033] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments of the present invention can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0034] While the above describes specific embodiments of the present invention, the scope of protection of the present invention is not limited thereto. Any modifications or substitutions that can be readily conceived by a person skilled in the art within the technical scope disclosed herein are intended to be encompassed within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

[0035] Figure 1 FIG. 4 shows a schematic diagram of a system framework of a CO2 supercritical servo system according to an embodiment of the present invention. Figure 1 As shown, the system consists of two main circulation paths: the main system and the auxiliary servo system. The main system comprises the primary refrigeration circuit, formed by the CO2 screw compressor 1. The auxiliary servo system, centered around the CO2 supercritical piston compressor 4, can be used in three modes: system pressure maintenance, terminal defrost, and load replenishment. Both systems achieve thermal coupling and circuit sharing through shared CO2 circulation barrels, condensers, precoolers, and other components.

[0036] In a specific embodiment, the main system includes a CO2 screw compressor 1, a CO2 horizontal oil separator 2, a CO2 oil cooling plate 3, a CO2 precooler 7, a CO2 condenser 8, and a CO2 float valve 9, which are connected in sequence. The CO2 screw compressor 1 is the driving core of the main system and is used to absorb and compress low-pressure gaseous CO2. The CO2 horizontal oil separator 2 receives high-pressure gas from the CO2 screw compressor 1 and separates the lubricating oil therein. The CO2 oil cooling plate 3 is arranged between the CO2 horizontal oil separator 2 and the CO2 screw compressor 1 and is used to cool the separated lubricating oil and return it to the CO2 screw compressor 1, thus closing the oil circuit. The high-pressure gas in the main system flows through the CO2 precooler 7 and, under the operating conditions of the auxiliary servo system, also receives the servo exhaust mixed flow, thus having a precooling regulation function. The CO2 precooled by the CO2 condenser 8 is further cooled and partially condensed into liquid, providing a cold source for the subsequent refrigeration process. The CO2 float valve 9 connects the CO2 condenser 8 with the downstream low-pressure system (not shown in detail) to control the flow of liquid CO2 into the liquid storage circuit.

[0037] In a specific embodiment, the auxiliary servo system includes a CO2 supercritical piston compressor 4, a CO2 supercritical oil separator 5 and a CO2 piston compressor oil reservoir 6 connected in sequence. The CO2 supercritical piston compressor 4 is the driving core of the auxiliary system and can be started independently as a servo control source; the CO2 supercritical oil separator 5 receives high-pressure exhaust from the CO2 supercritical piston compressor 4 and separates the lubricating oil therein; the CO2 piston compressor oil reservoir 6 is connected to the CO2 supercritical oil separator 5 for storing and refluxing the separated lubricating oil to complete the auxiliary compressor oil circuit closed loop; the auxiliary servo system is also provided with a CO2 piston compressor intake port 11 for back-sucking low-pressure refrigerant from the circulation barrel 12.

[0038] In a specific embodiment, a CO2 circulation barrel 12 is further provided for receiving liquid or gaseous reflux from the main / auxiliary system for pressure balance or system buffering.

[0039] In a specific embodiment, the exhaust of the CO2 supercritical piston compressor 4 passes through the CO2 supercritical oil separator 5 and enters a three-way automatic control valve assembly (not individually numbered, but represented by multiple branches in the figure). The valve distributes the airflow into the following three paths:

[0040] The first path (pressure maintenance mode): automatic control valve → main system CO2 precooler 7 outlet 1 → CO2 circulation barrel 12 → CO2 condenser 8 inlet, used for the auxiliary system to participate in the main circulation condensation and maintain the system pressure stable;

[0041] Second path (defrost mode): automatic control valve → CO2 piston system defrost outlet 10 on the upper right, then sent to the end evaporator; the evaporator outlet gas (not shown) is recovered to the circulation tank 12.

[0042] The third path (load supplement mode): automatic control valve → CO2 precooler 7 → CO2 condenser 8 → floating CO2 ball valve 9, supplementing the refrigeration load.

[0043] In a specific embodiment, it also includes a pressure sensor arranged in the CO2 circulation barrel 12, a temperature sensor arranged in the terminal evaporator, and a control unit, and the pressure sensor, temperature sensor, and automatic control valve are electrically connected to the control unit. In a specific embodiment, the control unit can be an embedded hardware module (such as a processor such as a 32-bit ARM architecture microcontroller), including a signal input interface connected to the pressure sensor and the temperature sensor, and a drive output interface connected to the electromagnetic actuator of the automatic control valve. With this setting, centralized control of the entire gas circuit and the start and stop of the compressor can be achieved, so that the system has programmable, visual and fully automatic operation capabilities, reducing human intervention and misoperation. The control unit automatically switches the path of the automatic control valve and starts and stops the CO2 supercritical piston compressor 4 according to the signals collected by the temperature sensor and the pressure sensor, and automatically switches between the three servo modes.

[0044] Continue to refer Figure 2 , Figure 2 FIG. 1 shows a flow chart of a control method for a CO2 supercritical servo system according to an embodiment of the present invention. Figure 2 As shown, the control method includes the following steps:

[0045] S1: Real-time detection of the pressure data of the CO2 circulation barrel of the refrigerant returned from the main system and the auxiliary system and the temperature data of the terminal evaporator.

[0046] S2: In response to the pressure of the CO2 circulation barrel exceeding the preset upper limit, the maintenance unit mode is started, and the exhaust gas of the auxiliary system is introduced into the CO2 condenser of the main system through the automatic control valve until the pressure drops to the lower limit.

[0047] In a specific embodiment, in response to the CO2 circulation barrel pressure exceeding a preset upper limit, the CO2 supercritical piston compressor is controlled to start, and the automatic control valve is switched to the third passage to introduce the auxiliary system exhaust into the CO2 condenser of the main system.

[0048] S3: In response to the terminal evaporator temperature being lower than the defrost threshold, the defrost mode is started, and the exhaust gas of the auxiliary system is directed to the terminal evaporator through the automatic control valve to defrost the evaporator. After the defrost process is completed, the refrigerant is discharged into the CO2 circulation barrel to wait for subsequent recycling.

[0049] In a specific embodiment, when the temperature falls below the defrost threshold for a preset duration, the CO2 supercritical piston compressor is activated and the automatic valve is switched to a second path, directing high-pressure exhaust gas to the defrost inlet of the terminal evaporator. Once defrost is complete, the second path is closed, the CO2 supercritical piston compressor is stopped, and the recovered gaseous refrigerant is returned to the CO2 circulation drum through the intake port. This configuration clarifies the control logic for over-limit startup and return shutdown, enabling the auxiliary unit to more accurately maintain system pressure and more controllable energy consumption.

[0050] S4: In response to the main system load demand exceeding a preset threshold, the cooling load supplement mode is started, and the mixing ratio of the auxiliary system exhaust and the main system refrigerant is adjusted by the automatic control valve to dynamically supplement the cooling capacity.

[0051] In a specific embodiment, in response to the main system load demand exceeding a preset threshold, the CO2 supercritical piston compressor is activated and the automatic valve is switched to the first path, allowing the auxiliary system exhaust to mix with the main system refrigerant in a proportional manner. The mixed refrigerant is cooled by the CO2 condenser and then transported to the downstream system until the load demand drops below the threshold. A closed-loop definition of startup conditions, flow path switching, defrost completion, and unit shutdown allows for rapid and controllable terminal defrost operations, avoiding overheating or prolonged occupancy of the refrigeration circuit. The system automatically replenishes cooling capacity based on real-time load, improving host response speed and reducing frequent starts and stops.

[0052] The CO2 supercritical servo system of this application is equipped with breakthrough CO2 supercritical servo compressor technology and constructs three core functional systems: through intelligent pressure control technology, it accurately maintains the stability of the CO2 system pressure to ensure that the system is always in an efficient operating range; innovative terminal intelligent defrost mechanism to ensure continuous and stable operation of the equipment; intelligent load supplement function, real-time perception of system needs, dynamic adjustment of the CO2 side load, significantly improving the system response speed and environmental adaptability, creating a stable, reliable, efficient and energy-saving user experience for users.

[0053] In the description of the present invention, it should be understood that the terms "upper", "lower", "inside", "outside" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. The word "comprising" does not exclude the presence of elements or steps not listed in the claims. The word "one" or "an" preceding an element does not exclude the presence of a plurality of such elements. The simple fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used for improvement. Any reference signs in the claims should not be interpreted as limiting the scope.

Claims

1. A CO2 supercritical servo system, characterized in that: Including main system, auxiliary system and control unit; The main system includes a CO2 screw compressor, a CO2 horizontal oil separator, a CO2 precooler, a CO2 condenser and a CO2 float valve connected in sequence; The auxiliary system includes a CO2 supercritical piston compressor, a CO2 supercritical oil separator and a CO2 piston compressor oil reservoir connected in sequence; The exhaust side of the auxiliary system is divided into three paths through an automatic control valve: the first path passes through the exhaust pipe of the CO2 oil separator incorporated into the main system, through the CO2 precooler and into the CO2 condenser of the main system; the second path is connected to the defrost inlet of the terminal evaporator; the third path completes the internal circulation between the system CO2 circulation barrel and the CO2 precooler to maintain system pressure stability; The control unit controls the automatic control valve to switch among the three paths.

2. The CO2 supercritical servo system according to claim 1, characterized in that: The system also includes a CO2 circulation barrel for collecting the refrigerant returned from the main system and the auxiliary system.

3. The CO2 supercritical servo system according to claim 2, characterized in that: It also includes a pressure sensor arranged on the CO2 circulation barrel and a temperature sensor arranged on the terminal evaporator.

4. The CO2 supercritical servo system according to claim 3, characterized in that: The pressure sensor, the temperature sensor, and the automatic control valve are electrically connected to the control unit.

5. The CO2 supercritical servo system according to claim 4, characterized in that: The control unit automatically switches the passage of the automatic control valve and starts and stops the CO2 supercritical piston compressor according to the signals collected by the temperature sensor and the pressure sensor, and is configured to operate in at least one of the following modes: Maintaining the unit mode, based on the real-time pressure monitoring of the CO2 circulation barrel, starting the CO2 supercritical piston compressor when the pressure exceeds a preset upper limit, and putting the CO2 supercritical piston compressor into standby mode when the pressure drops to a lower limit; In defrost mode, in response to a defrost demand signal from the terminal evaporator, the exhaust gas from the CO2 supercritical piston compressor is directly delivered to the terminal evaporator to defrost the evaporator. After the defrost process is completed, the refrigerant is discharged into the CO2 circulation barrel to wait for subsequent recycling; In the refrigeration load supplement mode, the exhaust gas of the CO2 supercritical piston compressor is mixed with the refrigerant of the main system through the automatic control valve, and the refrigeration capacity is dynamically adjusted to supplement the load of the main system.

6. The CO2 supercritical servo system according to claim 1, characterized in that: It also includes a CO2 oil cooling plate shell, which is arranged between the CO2 horizontal oil separator and the CO2 screw compressor.

7. A method for controlling a CO2 supercritical servo system according to any one of claims 1 to 6, characterized in that: include: S1: Real-time detection of the pressure data of the CO2 circulation barrel of the refrigerant returned by the main system and the auxiliary system and the temperature data of the terminal evaporator; S2: In response to the pressure of the CO2 circulation barrel exceeding a preset upper limit, starting the maintenance unit mode, introducing the exhaust gas of the auxiliary system into the CO2 condenser of the main system through the automatic control valve until the pressure drops to a lower limit; S3: In response to the terminal evaporator temperature being lower than the defrost threshold, the defrost mode is activated, and the exhaust gas of the auxiliary system is directed to the terminal evaporator through the automatic control valve to defrost the evaporator. After the defrost process is completed, the refrigerant is discharged into the CO2 circulation barrel to wait for subsequent recycling; S4: In response to the main system load demand exceeding a preset threshold, a refrigeration load supplement mode is started, and the mixing ratio of the auxiliary system exhaust and the main system refrigerant is adjusted by the automatic control valve to dynamically supplement the refrigeration capacity.

8. The control method according to claim 7, characterized in that: The S2 specifically includes: in response to the pressure of the CO2 circulation barrel exceeding a preset upper limit, the control unit starts the CO2 supercritical piston compressor and switches the automatic control valve to the third passage to introduce the exhaust gas of the auxiliary system into the CO2 precooler of the main system.

9. The control method according to claim 7, characterized in that: The S3 specifically includes: in response to the temperature being lower than the defrost threshold and lasting for a preset period of time, the control unit starts the CO2 supercritical piston compressor, switches the automatic control valve to the second path, and directionally delivers the high-pressure exhaust gas to the defrost inlet of the terminal evaporator, defrosts the evaporator, and discharges the refrigerant after the defrost process to the CO2 circulation barrel to wait for subsequent recycling.

10. The control method according to claim 7, characterized in that: The S4 specifically includes: in response to the main system load demand exceeding a preset threshold, the control unit starts the CO2 supercritical piston compressor and switches the automatic control valve to the first passage, so that the auxiliary system exhaust and the main system refrigerant are mixed in proportion; the mixed refrigerant is cooled by the CO2 condenser and then transported to the downstream system until the load demand drops below the threshold.

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