Method for controlling a co2 refrigeration system in transcritical operation

Abstract

A method for controlling a CO2 refrigeration system (1) in sub- and transcritical operation is described, the refrigerant of which is subcooled via at least one subcooler circuit (2, 3, 4, 5, 6, 21) that can be switched on demand to increase the useful cooling capacity and / or the COP related to the useful cooling capacity. In order to improve the energy efficiency of the system at high ambient temperatures with comparatively simple implementation and without the need for complex approval procedures, it is proposed that at least one subcooler circuit is activated when the gas cooler outlet temperature of the CO2 refrigeration system (1) is exceeded, via a gas cooler outlet temperature switch-on value and / or the control signal of the medium-pressure valve (13), which represents the flash gas fraction in the refrigeration circuit of the CO2 refrigeration system (1), preferably as a voltage or current value.

Description

[0001] The invention relates to a method for controlling a CO2 refrigeration system in sub- and transcritical operation, the refrigerant of which is subcooled via at least one subcooling circuit that can be switched on as needed to increase the useful cooling capacity and / or the useful cooling-related COP or the coefficient of performance.

[0002] Against the backdrop of regulatory provisions prohibiting the use of certain fluorocarbons in refrigeration systems, efforts have been made in recent decades to develop energy-efficient systems with alternative refrigerants that have the lowest possible global warming potential. Refrigeration systems based on CO₂ are now particularly prevalent, not least because of the easy availability of this refrigerant. However, at outside temperatures above approximately 27 °C, CO₂ refrigeration systems enter a so-called transcritical operating mode, which means that the removal of waste heat generated during operation significantly reduces the effective cooling capacity and thus the coefficient of performance (COP).

[0003] It is known from the prior art to compensate for any potential drop in the performance of CO₂ refrigeration systems operating in transcritical mode by incorporating gas coolers into the refrigeration circuit, which allow for supercritical cooling of the refrigerant after compression. However, a disadvantage of this approach, apart from the higher operating pressures, the associated higher component requirements, and the increased installation and maintenance costs, is that efficiency decreases at high ambient temperatures, which, due to climate change, are increasingly occurring even in temperate climates during the summer months. This ultimately results in increased energy consumption.

[0004] To further increase the efficiency of CO₂ refrigeration systems operating in transcritical mode, devices such as ejectors, parallel compressors, or mechanical subcoolers are used. Propane is one of the refrigerants used in mechanical subcoolers. However, the use of propane requires complex approval processes due to the higher charge quantities of the flammable refrigerant R₂90 required.

[0005] The invention is therefore based on the objective of designing a method of the type described above in such a way that the energy efficiency of the plant is improved at high ambient temperatures with comparatively simple implementation and without the need for complex approval procedures.

[0006] The invention solves the stated problem by activating at least one subcooler circuit when the gas cooler outlet temperature of the CO2 refrigeration system is exceeded, via a gas cooler outlet temperature switch-on value and / or the control signal of the medium pressure valve representing the flash gas content in the refrigeration circuit, in particular as an analog, preferably as a voltage or current value, via a control signal switch-on value.

[0007] In general, several subcooler circuits that can be switched on as needed are preferably provided. Each subcooler circuit can be assigned to its own subcooler module, with the subcooler modules being connected in parallel to form a subcooler module group. This subcooler module group, comprising several subcooler modules, is in particular integrated in series into the refrigeration circuit of the CO₂ refrigeration system, such that, with respect to the refrigerant flow of the CO₂ refrigeration circuit of the refrigeration system, the subcooler module group is located downstream of the gas cooler on one side and upstream of the high-pressure valve and the medium-pressure separator or medium-pressure collector on the other.

[0008] The invention is based on the insight that, in a particularly simple and therefore advantageous way, information can be obtained from the gas cooler outlet temperature of the CO₂ refrigeration system and / or the control signal of the intermediate pressure valve, which serves as a balancing valve to maintain a constant collector pressure in the refrigeration circuit, as to when at least one subcooler circuit must be activated or deactivated in order to achieve a consistently high coefficient of performance (COP) or high cooling capacity, even at high ambient temperatures, despite any inherent control inertia and taking into account minimal wear of the system components. The control signal represents, via the corresponding voltage resolution (e.g., 0 to 10 V) or current resolution (e.g., 4 to 20 mA), the value required to maintain the specified intermediate pressure, which depends on the flash gas fraction.Due to their high efficiency, the measures according to the invention make it possible not only to reduce the size of components of the CO₂ refrigeration circuit, such as compressors, gas coolers, medium-pressure separators or collectors, and piping by approximately 10–15%, but also to design the respective subcooler circuits in a correspondingly compact manner. This has the decisive advantage that, particularly when using propane (R290) as the subcooler refrigerant, the charge quantities of the respective subcooler circuits can be limited to 150 g in accordance with OVE EN 60335-2-89:2018, thus eliminating the need for potentially complex approval procedures and safety precautions. Furthermore, the measures according to the invention also allow the infrastructure necessary for operating the refrigeration system to be smaller, for example, by requiring fuses to have a lower current-carrying capacity.Due to the compact dimensioning of the components made possible by the control measures according to the invention and the reduced CO₂ mass flows resulting from the parallel connection of the modules, excessively high loads are not transferred to the subcooler circuits, thus protecting them from excessively high evaporation temperatures and ensuring that each component of a subcooler module or subcooler module group operates within its operating range. This allows load fluctuations in the refrigeration circuit of the CO₂ refrigeration system, which lead to varying loads on the subcooler circuits, to be reliably compensated.

[0009] Initial activation, i.e., the first activation of a subcooler circuit when subcooler circuits are initially deactivated, occurs exclusively when the gas cooler outlet temperature of the CO₂ refrigeration system is exceeded via a gas cooler outlet temperature switch-on threshold. The gas cooler outlet temperature switch-on threshold is preferably at least 25 °C.

[0010] Particularly favorable control conditions with high energy efficiency and low component wear are achieved by considering both predefined on / off waiting times, the gas cooler outlet temperature of the CO₂ refrigeration system, and the respective subcooler circuit temperatures when activating or deactivating the individual subcooler circuits. In the case of a subcooler module group with multiple subcooler circuits or modules, the common subcooler inlet temperature measured on the inlet side of the subcooler module group is generally used. Preferably, the gas cooler outlet temperature of the CO₂ refrigeration system corresponds to this subcooler inlet temperature. Conversely, the common subcooler outlet temperature measured on the outlet side of the subcooler module group can be used as the subcooler outlet temperature.

[0011] For example, the gas cooler outlet temperature of the CO₂ refrigeration system, the subcooler outlet temperature of an already activated subcooler circuit, and / or the control signal of the intermediate pressure valve can be determined before, during, and / or after a predetermined shutdown waiting period. This subcooler circuit is deactivated if, on the one hand, the gas cooler outlet temperature falls below a predetermined gas cooler outlet temperature shutdown value, and / or, on the other hand, the subcooler outlet temperature falls below a predetermined subcooler outlet temperature shutdown value and / or the control signal of the intermediate pressure valve falls below a predetermined control signal shutdown value. The gas cooler outlet temperature shutdown value is preferably at most 24 °C.

[0012] With a view to high energy efficiency and reduced wear or increased service life of the subcooler circuit components, this subcooler circuit is only deactivated if either the gas cooler outlet temperature falls below a predefined gas cooler outlet temperature cut-off value, or if the subcooler outlet temperature and / or the control signal of the medium-pressure valve fall below a predefined control signal cut-off value. This is achieved through the parallel monitoring of a power-dependent variable, i.e.,The subcooler circuits remain activated only as long as necessary, taking into account the subcooler outlet temperature and / or the control signal of the medium-pressure valve, on the one hand, and the gas cooler outlet temperature, which is influenced by the ambient temperature and thus dependent on the time of day, on the other hand, whereby preferably only a power-dependent quantity or the gas cooler outlet temperature is used as the deactivation criterion.

[0013] Since the use of subcooler circuits introduces additional noise sources, quiet hours, particularly in residential areas and due to official regulations, must be observed, and noise emissions must be reduced accordingly during these times. Because the gas cooler outlet temperature regularly falls below the gas cooler outlet cut-off value along with the ambient temperature during the evening and night hours, the control measures described above according to the invention offer the additional advantage that subcooler circuits are deactivated even when the subcooler outlet temperature is still above the cut-off value. Consequently, noise control requirements during the evening and night hours can be met without any noticeable loss of efficiency.

[0014] Conversely, before, during and / or after a predetermined switch-on waiting time, the gas cooler outlet temperature of the CO2 refrigeration system, the subcooler outlet temperature of an already activated subcooler circuit and / or the control signal of the medium pressure valve can be determined, whereby a further subcooler circuit is activated when a. if, on the one hand, the gas cooler outlet temperature exceeds a predetermined gas cooler outlet temperature cut-off value and / or the subcooler outlet temperature exceeds a predetermined subcooler outlet temperature cut-off value and / or the control signal of the intermediate pressure valve exceeds the control signal cut-off value, and if b. the subcooler outlet temperature exceeds a predetermined subcooler outlet temperature threshold and / or the control signal of the intermediate pressure valve exceeds a control signal threshold.

[0015] Preferably, that additional subcooler circuit is only activated when a. if both the gas cooler outlet temperature exceeds a predetermined gas cooler outlet temperature cut-off value, and the subcooler outlet temperature exceeds a predetermined subcooler outlet temperature cut-off value and / or the control signal of the intermediate pressure valve exceeds a control signal cut-off value, and if b. the subcooler outlet temperature exceeds a predetermined subcooler outlet temperature threshold and / or the control signal of the intermediate pressure valve exceeds a control signal threshold.

[0016] In general, a "determination" of the corresponding temperatures or signals within the meaning of the invention can also be understood as, for example, a "tapping".

[0017] The on / off waiting times are preferably chosen so that a stable operating point of the entire CO2 refrigeration system is reached again after activation or deactivation.

[0018] Against this background, and considering the need for reduced wear, particularly on actuators such as valves, it is recommended that the on / off waiting times be at least 5 minutes. This prevents the compressors in the respective subcooler circuits from being switched on and off more than 6 to 8 times per hour, which would otherwise lead to increased component wear. The on / off waiting times of different subcooler modules or subcooler module groups can, in principle, begin and end at the same times.

[0019] The invention also relates to a subcooler module for a method according to the invention, as well as to a device for carrying out such a method, wherein the device comprises a CO₂ refrigeration system forming a refrigeration circuit and having at least one gas cooler, and at least one subcooler module that can be switched on and off as required. A device can have a subcooler module group comprising several subcooler modules that can be switched on and off as required. Each subcooler module comprises a subcooler circuit, which has at least one evaporator, at least one compressor, at least one condenser, and at least one expansion valve, wherein propane is provided as the refrigerant for the subcooler circuit, and wherein the amount of propane is preferably at most 150 g.

[0020] An improvement in the refrigerant charge-related cooling capacity is achieved by designing the liquid line between the condenser and the expansion valve of a subcooler module according to the invention without a filter drier. Alternatively, a drier can be provided in the suction line located between the evaporator and the compressor. It has been shown that these measures allow the refrigerant charge to be reduced while maintaining a constant cooling capacity. This is because, due to the lower density of the refrigerant in the suction line, comparatively less refrigerant is required. It is also possible to provide a filter at this location instead of a filter drier in the liquid line between the condenser and the expansion valve.

[0021] The invention is illustrated in the drawing as an example. It shows Fig. 1 a schematic circuit diagram of a device according to the invention and Fig. 2 a schematic subcooler circuit of a subcooler module according to the invention.

[0022] Based on the schematic circuit diagram of a device according to the invention made of Fig. 1 The following describes a method according to the invention for controlling a CO₂ refrigeration system 1 in sub- and transcritical operation. The circuit diagram serves only to generally illustrate the principle according to the invention and therefore, for the sake of clarity, does not explicitly show any further components of a device according to the invention.

[0023] The refrigerant of a CO₂ refrigeration system 1 is subcooled via subcooler circuits 2, 3, 4, 5, 6, which can be activated as needed, to increase the cooling capacity and coefficient of performance (COP). The subcooler circuits 2, 3, 4, 5, 6 are each assigned to corresponding subcooler modules, which are schematically indicated by blocks. The subcooler modules together form a subcooler module group 7 connected in parallel, which in turn is integrated in series into the refrigeration circuit of the CO₂ refrigeration system 1. Thus, with respect to the refrigerant flow of the CO₂ refrigeration system, the subcooler module group 7 is located downstream of a gas cooler 8 and upstream of a high-pressure valve 9 and a medium-pressure separator 10. The dashed line 11 indicates an imaginary system boundary between an outdoor installation above line 11 and an arrangement of the components shown below line 11 in a machine room.

[0024] The medium-pressure separator 10 is connected on one side to a refrigerant line 12 leading to cooling points not shown in detail, and on the other side via a medium-pressure valve 13 to a flash gas outlet line 14.

[0025] The device also includes several sensors, among them for measuring various temperatures. Specifically indicated are a gas cooler outlet temperature sensor 15, as well as, for each subcooler module group 7, a common subcooler inlet temperature sensor 16 and a common subcooler outlet temperature sensor 17. The temperature measured by the gas cooler outlet temperature sensor 15 generally corresponds to the temperature measured by the subcooler inlet temperature sensor 16. In addition, corresponding pressure sensors 18, 19, and 20 are also present.

[0026] The control system operates such that, with subcooler circuits 2, 3, 4, 5, 6 initially deactivated, a first subcooler circuit 2 is activated for the first time when the gas cooler outlet temperature of the CO₂ refrigeration system is exceeded, via a gas cooler outlet temperature activation threshold. For example, a first subcooler circuit 2 is activated when the gas cooler outlet temperature of the refrigeration circuit, as determined by the gas cooler outlet temperature sensor 15, exceeds a predefined gas cooler outlet temperature activation threshold of, for example, 25 °C. Therefore, none, only one, or several of the subcooler circuits 2, 3, 4, 5, 6 can be activated or deactivated as needed.The subsequent control scheme can proceed as follows: After a waiting period of at least 5 minutes, it is checked whether both the gas cooler outlet temperature determined by the gas cooler outlet temperature sensor 15 and the subcooler outlet temperature determined by the subcooler outlet temperature sensor 17 are greater than or equal to a predefined gas cooler outlet temperature cut-off value and subcooler outlet temperature cut-off value, respectively. The gas cooler outlet temperature cut-off value can be, for example, 24 °C, and the subcooler outlet temperature cut-off value, for example, 7 °C. Alternatively, the control signal of the intermediate pressure valve 13 can also be used, in which case the control signal must be greater than a corresponding control signal cut-off value.Once these conditions are met, a further condition is added: the subcooler outlet temperature must exceed a predefined subcooler outlet temperature threshold and / or the control signal of the medium-pressure valve 13 must exceed a control signal threshold. The subcooler outlet temperature threshold can, for example, be 22 °C. If all conditions are met, the next subcooler circuit 3 is activated for at least 5 minutes. If the activation conditions continue to be met, this sequence is repeated until all subcooler circuits 2, 3, 4, 5, 6 are activated or the corresponding threshold is undershot.

[0027] Conversely, to deactivate an already activated subcooler circuit 2, 3, 4, 5, 6, only one condition must be met, i.e. either the gas cooler outlet temperature must fall below a predetermined gas cooler outlet temperature cut-off value or the subcooler outlet temperature must fall below a predetermined subcooler outlet temperature cut-off value and / or the control signal of the medium pressure valve must fall below a control signal cut-off value.

[0028] As this is in Fig. 2As shown, a subcooler module for a method according to the invention has a subcooler circuit 21 comprising an evaporator 22, a compressor 23, a condenser 24, and a throttling device 25. The throttling device can be, for example, a capillary, an expansion valve, or the like. A filter 27 is located in the liquid line 26, which is arranged between the condenser 24 and the throttling device 25 and is designed without a filter drier. A drier 29 is provided in the suction line 28, which is arranged between the evaporator 22 and the compressor 23. The subcooler circuits 2, 3, 4, 5, 6 described above can be configured accordingly.

Claims

1. Method for controlling a CO2 refrigeration system (1) in sub- and transcritical operation, the refrigerant of which is subcooled via at least one subcooling circuit (2, 3, 4, 5, 6, 21) that can be switched on as required to increase the useful cooling capacity and / or the useful cooling-related COP, characterized by the fact that at least one subcooler circuit is activated when the gas cooler outlet temperature of the CO2 refrigeration system (1) is exceeded via a gas cooler outlet temperature switch-on value and / or the control signal of the medium pressure valve (13) representing the flash gas content in the refrigeration circuit of the CO2 refrigeration system (1), preferably as a voltage or current value, via a control signal switch-on value.

2. Method according to claim 1, characterized by the fact thatSeveral subcooler circuits (2, 3, 4, 5, 6, 21) are provided which can be switched on as needed, wherein, with subcooler circuits (2, 3, 4, 5, 6, 21) initially deactivated, a first subcooler circuit (2, 3, 4, 5, 6, 21) is activated for the first time only when the gas cooler outlet temperature of the CO2 refrigeration system (1) is exceeded via a gas cooler outlet temperature switch-on value.

3. Method according to claim 1 or 2, characterized by the fact thatBefore, during and / or after a predetermined shutdown waiting time to ensure a minimum operating time, the gas cooler outlet temperature of the CO2 refrigeration system (1), the subcooler outlet temperature of an already activated subcooler circuit (2, 3, 4, 5, 6, 21) and / or the control signal of the medium pressure valve (13) are determined and this subcooler circuit (2, 3, 4, 5, 6, 21) is deactivated if, on the one hand, the gas cooler outlet temperature falls below a predetermined gas cooler outlet temperature switch-off value and / or, on the other hand, the subcooler outlet temperature falls below a predetermined subcooler outlet temperature switch-off value and / or the control signal of the medium pressure valve (13) falls below a control signal switch-off value.

4. Method according to claim 3, characterized by the fact thatThis subcooler circuit (2, 3, 4, 5, 6, 21) is only deactivated if either the gas cooler outlet temperature falls below a predetermined gas cooler outlet temperature cut-off value or the subcooler outlet temperature falls below a predetermined subcooler outlet temperature cut-off value and / or the control signal of the medium pressure valve (13) falls below a control signal cut-off value.

5. Method according to any one of claims 1 to 4, characterized by the fact thatThe gas cooler outlet temperature of the CO2 refrigeration system (1), the subcooler outlet temperature of an already activated subcooler circuit (2, 3, 4, 5, 6, 21), and / or the control signal of the intermediate pressure valve (13) are determined before, during, and / or after a predetermined activation waiting time, and a further subcooler circuit (2, 3, 4, 5, 6, 21) is activated if: a. the gas cooler outlet temperature exceeds a predetermined gas cooler outlet temperature cut-off value, and / or b. the subcooler outlet temperature exceeds a predetermined subcooler outlet temperature cut-off value and / or the control signal of the intermediate pressure valve (13) exceeds a control signal cut-off value; and b. the subcooler outlet temperature exceeds a predetermined subcooler outlet temperature threshold and / or the control signal of the intermediate pressure valve (13) exceeds a control signal threshold.

6. Method according to claim 5, characterized by the fact thatThat further subcooler circuit (2, 3, 4, 5, 6, 21) is only activated if a. both the gas cooler outlet temperature exceeds a predetermined gas cooler outlet temperature cut-off value, and the subcooler outlet temperature also exceeds a predetermined subcooler outlet temperature cut-off value and / or the control signal of the intermediate pressure valve (13) exceeds a control signal cut-off value, and if b. the subcooler outlet temperature exceeds a predetermined subcooler outlet temperature threshold and / or the control signal of the intermediate pressure valve (13) exceeds a control signal threshold.

7. Method according to any one of claims 3 to 6, characterized by the fact that The waiting times for switching on and / or off must be at least 5 minutes.

8. Method according to any one of claims 3 to 7, characterized by the fact that The on and / or off waiting times begin and / or end at the same times.

9. Subcooler module for a method according to one of claims 1 to 8, comprising a subcooler circuit (2, 3, 4, 5, 6, 21) comprising at least one evaporator (22), at least one compressor (23), at least one condenser (24) and at least one throttling device (25), characterized by Propane as a subcooler refrigerant for the subcooler circuit (2, 3, 4, 5, 6, 21), wherein the amount of propane is preferably not more than 150 g.

10. Subcooler module according to claim 9, characterized by the fact that the liquid line (28) arranged between the at least one capacitor (24) and the at least one throttling device (25, 26, 27) is designed without a filter drier.

11. Subcooler module according to claim 10, characterized by the fact thata dryer (29) is provided in the suction line (28) arranged between the evaporator (22) and the compressor (23) and / or a filter (27) is provided in the liquid line (26) arranged between the at least one condenser (24) and the at least one throttling device (25).

12. Device for carrying out a method according to one of claims 1 to 8, comprising a CO2 refrigeration system (1) forming a refrigeration circuit and having at least one gas cooler (8) and at least one subcooler module according to one of claims 9 to 11 that can be switched on to the refrigeration circuit as required.

13. Device according to claim 12, characterized by a subcooler module group comprising several subcooler modules that can be switched on as required according to one of claims 9 to 11.

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