A method for controlling a vapour compression system with a heat pump evaporator

Abstract

A method for controlling a vapour compression system (1) comprising a heat recovery system, at least one refrigeration evaporator and a heat pump evaporator (9) is disclosed. The vapour compression system (1) comprises a first suction line (13) connected to an outlet of each of the at least one refrigeration evaporator (8), and a second suction line (14) connected to an outlet of the heat pump evaporator (9), the first suction line (13) and the second suction line (14) being arranged fluidly in parallel. The first suction pressure is controlled by adjusting a compressor capacity of the at least one refrigeration compressor (3, 4), and in order to match a first suction pressure setpoint value. A compressor capacity of the at least one heat pump compressor (4, 16) is controlled based on a heat recovery request from the heat recovery system. The superheat of refrigerant leaving the heat pump evaporator (9) is controlled by adjusting a supply of refrigerant to the heat pump evaporator (9) via the associated expansion device (11), and in order to obtain minimal superheat.

Description

[0001] A METHOD FOR CONTROLLING A VAPOUR COMPRESSION SYSTEM WITH A HEAT PUMP EVAPORATOR

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a method for controlling a vapour compression system with a heat recovery system, at least one refrigeration evaporator and a heat pump evaporator. The method according to the invention ensures appropriate and optimal operation of the at least one refrigeration evaporator as well as of the heat recovery system. The invention further relates to a vapour compression system being configured to be controlled in accordance with the method.

[0004] BACKGROUND OF THE INVENTION

[0005] Vapour compression systems may have a primary function of providing refrigeration to one or more refrigerated volumes, via at least one refrigeration evaporator. Such vapour compression systems may further be provided with a heat recovery system, in which heat is recovered from the refrigerant before the refrigerant passes through the heat rejecting heat exchanger. The recovered heat may be applied for heating rooms or tap water, e.g. at the premises where the vapour compression system is located, and / or be supplied to a district heating network. In order to allow for boosting of the recovered heat, the vapour compression system may be provided with a separate heat pump evaporator, which can be activated if the operation of the at least one refrigeration evaporator is unable to provide a desired heat recovery.

[0006] In vapour compression systems being provided with a heat recovery system and a separate heat pump evaporator, conflicts may arise between the control of the refrigeration part of the vapour compression system and the control of the heat recovery system. In particular, the suction pressure prevailing in the suction line interconnecting the outlet of the respective evaporators and the inlet of the compressor unit may be adversely affected by such control conflicts.

[0007] DESCRIPTION OF THE INVENTION

[0008] It is an object of embodiments of the invention to provide a method for controlling a vapour compression system with a heat pump evaporator in which maximum heat recovery is obtained without compromising control of the refrigeration part of the vapour compression system. It is a further object of embodiments of the invention to provide a method for controlling a vapour compression system with a heat pump evaporator in which maximum heat recovery is obtained while maintaining an appropriate suction pressure.

[0009] It is an even further object of embodiments of the invention to provide a vapour compression system with a heat pump evaporator, which is controllable to obtain maximum heat recovery without compromising control of the refrigeration part of the vapour compression system.

[0010] According to a first aspect the invention provides a method for controlling a vapour compression system, the vapour compression system comprising a compressor unit with at least two compressors, at least one compressor being configured to operate as a refrigeration compressor and at least one compressor being configured to operate as a heat pump compressor; a heat recovery system; a heat rejecting heat exchanger; at least one refrigeration evaporator, each with an associated expansion device and each arranged in thermal contact with a refrigerated volume; and a heat pump evaporator with an associated expansion device, the vapour compression system further comprising a first suction line interconnecting an outlet of each of the at least one refrigeration evaporator and an inlet of the at least one refrigeration compressor, the first suction line defining a first suction pressure; and a second suction line interconnecting an outlet of the heat pump evaporator and an inlet of the at least one heat pump compressor, the second suction line defining a second suction pressure, the first suction line and the second suction line being arranged fluidly in parallel, the method comprising the steps of:

[0011] - controlling the first suction pressure by adjusting a compressor capacity of the at least one refrigeration compressor, and in order to match a first suction pressure setpoint value,

[0012] - adjusting a compressor capacity of the at least one heat pump compressor based on a heat recovery request from the heat recovery system, and

[0013] - controlling the superheat of refrigerant leaving the heat pump evaporator by adjusting a supply of refrigerant to the heat pump evaporator via the associated expansion device, and in order to obtain minimal superheat.

[0014] Thus, according to the first aspect, the invention provides a method for controlling a vapour compression system. In the present context the term 'vapour compression system' should be interpreted to mean a system in which a flow of fluid medium, such as refrigerant, circulates and is alternatingly compressed and expanded, thereby providing either refrigeration or heating of a volume. Thus, the vapour compression system may be a refrigeration system, an air condition system, a heat pump, etc. The vapour compression system being controlled by means of the method according to the first aspect of the invention is preferably a refrigeration system in which refrigeration is provided to at least one volume.

[0015] The vapour compression system comprises a compressor unit, a heat recovery system, a heat rejecting heat exchanger, at least one refrigeration evaporator, and a heat pump evaporator, arranged in a refrigerant path.

[0016] The compressor unit comprises at least two compressors, where at least one compressor is configured to operate as a refrigeration compressor and at least one compressor is configured to operate as a heat pump compressor. The compressors may be dedicated refrigeration compressors or heat pump compressors, respectively. However, it is not ruled out that at least one of the compressors is configured to operate as a refrigeration compressor as well as to operate as a heat pump compressor, so that the capacity of that compressor may be selectively applied as refrigeration compressor capacity or heat pump compressor capacity. This will be described in further detail below.

[0017] The heat recovery system comprises a heat recovery heat exchanger arranged in the refrigerant path in such a manner that heat exchange takes place between the refrigerant leaving the compressor unit and a heat recovery fluid and heat is rejected from the refrigerant. The heat recovered from the refrigerant in this manner is then applied for heating purposes, as described above.

[0018] Refrigerant leaving the heat recovery heat exchanger passes through the heat rejecting heat exchanger, where heat exchange also takes place with a secondary fluid flow in such a manner that heat is rejected from the refrigerant.

[0019] Each of the at least one refrigeration evaporator is associated with an expansion device, and is arranged in thermal contact with a refrigerated volume. Accordingly, the refrigerant supply to a given refrigeration evaporator can be controlled by appropriately operating the expansion device associated therewith, in order to obtain a desired temperature in the corresponding refrigerated volume. The refrigeration evaporators may be arranged in thermal contact with separate refrigerated volumes, in which case the temperature in the various refrigerated volumes can be controlled independently of each other by individually operating the expansion devices of the respective refrigeration evaporators. However, it is not ruled out that two or more refrigeration evaporators are arranged in thermal contact with the same refrigerated volume. Similarly, the heat pump evaporator is associated with an expansion device, and the refrigerant supply to the heat pump evaporator can, thus, be controlled by appropriately operating the associated expansion device.

[0020] The vapour compression system further comprises a first suction line and a second suction line. The first suction line interconnects an outlet of each of the at least one refrigeration evaporator and an inlet of the at least one refrigeration compressor, and the first suction line defines a first suction pressure. Similarly, the second suction line interconnects an outlet of the heat pump evaporator and an inlet of the at least one heat pump compressor, and the second suction line defines a second suction pressure. The first suction line and the second suction line are arranged fluidly in parallel. Accordingly, the at least one refrigeration evaporator and the heat pump evaporator are connected to separate compressors of the compressor unit via separate suction lines. Furthermore, the second suction pressure, prevailing in the second suction line, is completely independent of the first suction pressure, prevailing in the first suction line. Moreover, the first suction pressure is unaffected by operation of the heat pump evaporator and the at least one heat pump compressor, and the second suction pressure is unaffected by operation of the at least one refrigeration evaporator and the at least one refrigeration compressor. Accordingly, the first suction line and the second suction line are completely separated from each other, and the suction pressure prevailing in the respective suction lines can be controlled completely independently of each other.

[0021] In the method according to the first aspect of the invention, the first suction pressure is controlled by adjusting a running compressor capacity of the at least one refrigeration compressor, and in order to match a first suction pressure setpoint value. Accordingly, an appropriate suction pressure can be maintained in the first suction line, while the at least one refrigeration evaporator is operated in an appropriate manner ensuring that the refrigeration demand of the at least one refrigerated volume is met. Thus, the refrigeration part of the vapour compression system is operated in an efficient and optimal manner.

[0022] Furthermore, a running compressor capacity of the at least one heat pump compressor is adjusted based on a heat recovery request from the heat recovery system. Thus, it is ensured that the total running capacity of the compressors of the compressor unit, i.e. the combined running capacity of the at least one refrigeration compressor and the at least one heat pump compressor, is sufficient to meet the heat recovery request from the heat recovery system, by adjusting the running compressor capacity of the at least one heat pump compressor, independently of the operation of the refrigeration part of the vapour compression system. It should be noted that, since the outlet of the at least one refrigerant compressor as well as the outlet of the at least one heat pump compressor is connected to the heat recovery heat exchanger, the running compressor capacity of both contributes to the heat recovery.

[0023] Finally, the superheat of refrigerant leaving the heat pump evaporator is controlled by adjusting a supply of refrigerant leaving the heat pump evaporator via the associated expansion device, and in order to obtain minimal superheat. Accordingly, the heat pump evaporator is operated by means of a standard minimal superheat control strategy.

[0024] In summary, the refrigeration part of the vapour compression system is operated in an optimal manner, essentially without taking into account that the vapour compression system comprises a heat recovery system. In particular, the expansion device of the at least one refrigeration evaporator is operated in such a manner that a desired temperature is obtained in each refrigerated volume, and possibly in order to obtain a specified superheat level at the outlet of each refrigeration evaporator. Simultaneously, the first suction pressure is kept stable and at an appropriate level by adjusting the compressor capacity of the at least one refrigeration compressor.

[0025] Furthermore, in order to also optimize heat recovery, in particular in order to meet a heat recovery request from the heat recovery system, the compressor capacity of the at least one heat pump compressor is adjusted in such a manner that the total running compressor capacity of the compressor unit matches the heat recovery request. The refrigerant supply to the heat pump evaporator is then adjusted by appropriately operating the associated expansion device, and in order to obtain minimal superheat at the outlet of the heat pump evaporator, and thus in accordance with the running capacity of the at least one heat pump compressor. This will cause the second suction pressure to settle such that the temperature difference between refrigerant flowing through the heat pump evaporator and a secondary fluid flowing across the heat pump evaporator is just sufficient to obtain the minimal required superheat at the outlet of the heat pump evaporator. Thus, the second suction pressure is kept stable by the adjustment of the refrigerant supply to the heat pump evaporator, and this is done in an energy optimal manner, due to the minimal superheat at the outlet of the heat pump evaporator.

[0026] Thus, the refrigeration part of the vapour compression system and the heat recovery part of the vapour compression system are controlled essentially independently of each other, in the sense that there is no risk of control conflicts. In order to obtain this, it is important that the first suction line, connected to the outlet of the at least one refrigeration evaporator, and the second suction line, connected to the outlet of the heat pump evaporator, are separate suction lines arranged fluidly in parallel, and connected to separate compressors. This ensures that operation of the heat pump evaporator has no impact on the first suction pressure, thus efficiently preventing control conflicts. Accordingly, maximum heat recovery is obtained without compromising control of the refrigeration part of the vapour compression system, in particular while maintaining an appropriate and stable suction pressure in the first suction line.

[0027] The compressor unit may comprise at least one compressor being configured to selectively operate as a refrigeration compressor or as a heat pump compressor, and the method may further comprise the step of switching at least one compressor from operating as a refrigeration compressor to operating as a heat pump compressor, or vice versa.

[0028] According to this embodiment, at least one of the compressors of the compressor unit is neither a dedicated refrigeration compressor, nor a dedicated heat pump compressor. Instead, the compressor may be selectively connected to the first suction line, thus operating as a refrigeration compressor, or to the second suction line, thus operating as a heat pump compressor. This allows at least part of the compressor capacity available at the compressor unit to be selectively applied as refrigeration compressor capacity or heat pump compressor capacity, depending on the current need. Thus, sufficient compressor capacity is available for either purpose, without requiring excessive installed compressor capacity.

[0029] Alternatively or additionally, at least one of the compressors of the compressor unit may be a dedicated refrigeration compressor and / or at least one of the compressors of the compressor unit may be a dedication heat pump compressor.

[0030] The step of switching at least one compressor may comprise operating a three-way valve. According to this embodiment, the three-way valve connects the at least one compressor to the first suction line and to the second suction line. The three-way valve may be switchable between a first position where it establishes a fluid connection between the first suction line and the at least one compressor and prevents fluid flow from the second suction line towards the at least one compressor, and a second position where it establishes a fluid connection between the second suction line and the at least one compressor and prevents fluid flow from the first suction line towards the at least one compressor. Accordingly, the at least one compressor can be switched from operating as a refrigeration compressor to operating as a heat pump compressor by switching the three-way valve from the first position to the second position, and vice versa.

[0031] At least one compressor may be switched from operating as a heat pump compressor to operating as a refrigeration compressor in the case that the refrigeration compressors operate at maximum capacity and the first suction pressure exceeds a predefined threshold value. In the case that the refrigeration compressors operate at maximum capacity, and the first suction pressure exceeds a predefined threshold value, this is an indication that the currently available refrigeration compressor capacity is insufficient to maintain the first suction pressure at an appropriate level. Therefore, in order to ensure appropriate operation of the refrigeration part of the vapour compression system, at least one of the compressors, which is currently operating as a heat pump compressor, is switched to operate as a refrigeration compressor, regardless of the current demand of the heat recovery system. Thus, the primary function of the vapour compression system, i.e. the refrigeration part, is prioritized over the heat recovery system, which may be regarded as a secondary function of the vapour compression system, by ensuring that sufficient refrigeration compressor capacity is available, at the cost of the available heat pump compressor capacity.

[0032] At least one compressor may be switched from operating as a refrigeration compressor to operating as a heat pump compressor in the case that a request for increased heat recovery is received from the heat recovery system. According to this embodiment, in the case that an increase in heat recovery is required, the available heat pump compressor capacity is increased by switching at least one compressor from operating as a refrigeration compressor to operating as a heat pump compressor, thus ensuring that sufficient heat pump compressor capacity is available to meet the increased demand for heat recovery. However, it should be noted that such a switch should preferably only be performed if the refrigeration compressor capacity available after the switch is sufficient to ensure that the first suction pressure is maintained at a suitable level, e.g. by increasing the running capacity of the remaining refrigeration compressors.

[0033] The request for increased heat recovery may, e.g., originate from a power grid. For instance, a surplus of electrical energy may be available in the power grid, and therefore it may be desirable that the vapour compression system increases its electrical energy consumption. This can be obtained by increasing the total running compressor capacity of the compressor unit in such a manner that the heat recovery is increased.

[0034] Operation of the at least one heat pump compressor may be stopped, and / or start of the at least one heat pump compressor may be prevented, in the case that a running capacity of the at least one refrigeration compressor is sufficient to meet the heat recovery request from the heat recovery system.

[0035] According to this embodiment, the at least one heat pump compressor and the heat pump evaporator are only applied if the normal operation of the vapour compression system, i.e. the operation of the refrigeration part of the vapour compression system, does not result in heat recovery at the heat recovery system which is sufficient to meet the heat recovery request. Thus, the operation of the at least one heat pump compressor and the separate heat pump evaporator may be regarded as a supplement to the normal operation of the vapour compression system, resulting in increased heat recovery, without interfering with the normal operation of the vapour compression system.

[0036] The method may further comprise the steps of receiving a load shedding request from a power grid, and, in response to the load shedding request, decreasing the running capacity of the at least one heat pump compressor before decreasing the running capacity of the at least one refrigeration compressor.

[0037] There may be periods where there is a deficiency of electrical energy in a power grid. This could, e.g., be during peak load periods and / or during periods where power production is low, for instance power production provided by renewable power sources. During such periods the power grid may issue load shedding requests to power consumers. According to this embodiment, in response to such a load shedding request, the vapour compression system will seek to reduce its energy consumption by decreasing the running compressor capacity. When doing so, the running capacity of the at least one heat pump compressor is decreased before decreasing the running capacity of the at least one refrigeration compressor. Thereby it is ensured that the primary function of the vapour compression system, i.e. the refrigeration part of the vapour compression system, is unaffected by the load shedding, or at least that a refrigeration demand of the at least one refrigerated volume can be met. This is at the cost of the heat recovery, possibly to an extent where a heat recovery request of the heat recovery system may not be met.

[0038] According to a second aspect, the invention provides a vapour compression system comprising :

[0039] - a compressor unit comprising at least one compressor being configured to operate as a refrigeration compressor and at least one compressor being configured to operate as a heat pump compressor,

[0040] - a heat recovery system,

[0041] - a heat rejecting heat exchanger, at least one refrigeration evaporator, each with an associated expansion device, and each being arranged in thermal contact with a refrigerated volume, and a heat pump evaporator with an associated expansion device, wherein the compressors, the heat recovery system, the heat rejecting heat exchanger, the at least one refrigeration evaporator and the heat pump evaporator are arranged in a refrigerant path, and wherein the refrigerant path comprises a first suction line interconnecting an outlet of each of the at least one refrigeration evaporator and an inlet of the at least one refrigeration compressor, the first suction line defining a first suction pressure, and a second suction line interconnecting an outlet of the heat pump evaporator and an inlet of the at least one heat pump compressor, the second suction line defining a second suction pressure, the first suction line and the second suction line being arranged fluidly in parallel.

[0042] The vapour compression system according to the second aspect of the invention is configured to be controlled by means of the method according to the first aspect of the invention, and the remarks set forth above with reference to the first aspect of the invention are therefore equally applicable here. In particular, the first suction line and the second suction line arranged fluidly in parallel allow for the independent control of the refrigeration part of the vapour compression system and the heat recovery part of the vapour compression system described above. Furthermore, any feature described below in combination with the second aspect of the invention may also be combined with the first aspect of the invention.

[0043] The vapour compression system may comprise at least two refrigeration evaporators arranged fluidly in parallel. According to this embodiment, at least two refrigeration evaporators, controlled essentially independently of each other, are fluidly connected to the first suction line. In this case it may be difficult to keep the first suction pressure stable at an appropriate level by adjusting the refrigerant supply to the respective refrigeration evaporators. Therefore, in this case it is very appropriate that the first suction pressure is controlled by adjusting a compressor capacity of the at least one refrigeration compressor.

[0044] The at least two refrigeration evaporators may be arranged in thermal contact with separate refrigerated volumes, in which case the control of the respective refrigeration evaporators is completely independent of each other. As an alternative, at least some of the at least two refrigeration evaporators may be arranged in thermal contact with the same refrigerated volume. An example of a vapour compression system with at least two refrigeration evaporators is a supermarket refrigeration system with multiple display cabinets constituting refrigerated volumes.

[0045] At least one of the compressors may be configured to selectively operate as a refrigeration compressor or as a heat pump compressor. For instance, the compressor may be connected to the first suction line and the second suction line via a three-way valve. This has already been described in detail above with reference to the first aspect of the invention.

[0046] The heat recovery system may be thermally connected to a district heating system. According to this embodiment, the recovered heat is supplied to the district heating system, and the heat recovery request may originate from the district heating system. Alternatively or additionally, the recovered heat may be applied locally at the premises where the vapour compression system is located.

[0047] The vapour compression system may further comprise a bypass refrigerant path arranged in parallel with the heat rejecting heat exchanger. According to this embodiment, the heat rejecting heat exchanger may be bypassed, and the heat rejection from the refrigerant may take place solely via the heat recovery heat exchanger of the heat recovery system.

[0048] The vapour compression system may further comprise a receiver arranged in the refrigerant path between an outlet of the heat rejecting heat exchanger and an inlet of the at least one refrigeration evaporator. In the receiver, liquid refrigerant is separated from gaseous refrigerant. The liquid refrigerant may be supplied to the expansion devices of the at least one refrigeration evaporator and the heat pump evaporator, while at least part of the gaseous refrigerant may be supplied directly to the compressors of the compressor unit.

[0049] BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The invention will now be described in further detail with reference to the accompanying drawings in which

[0051] Figs. 1-4 are diagrammatic views of vapour compression systems according to four embodiments of the invention, and

[0052] Fig. 5 is a flow chart illustrating a method according to an embodiment of the invention.

[0053] DETAILED DESCRIPTION OF THE DRAWINGS

[0054] Fig. 1 is a diagrammatic view of a vapour compression system 1 according to a first embodiment of the invention. The vapour compression system 1 comprises a compressor unit 2 with a number of compressors 3, 4, three of which are shown. Two of the compressors 3 are in the form of dedicated refrigeration compressors, and one of the compressors 4 is of a kind which can be switched between operating as a refrigeration compressor and a heat pump compressor. This will be described in further detail below.

[0055] The vapour compression system 1 further comprises a heat recovery heat exchanger 5, forming part of a heat recovery system, a heat rejecting heat exchanger 6, a receiver 7, a refrigeration evaporator 8 and a heat pump evaporator 9, all arranged in a refrigerant path. An expansion device 10 is associated with the refrigeration evaporator 8, and another expansion device 11 is associated with the heat pump evaporator 9.

[0056] Refrigerant flowing in the refrigerant path is compressed by the compressors 3, 4 of the compressor unit 2. The refrigerant leaving the compressor unit 2 passes through the heat recovery heat exchanger 5, where heat exchange takes place with a secondary fluid in such a manner that heat is rejected from the refrigerant, so as to recover heat for the heat recovery system. Next, the refrigerant passes through the heat rejecting heat exchanger 6, where heat exchange also takes place with a secondary fluid in such a manner that heat is rejected from the refrigerant, thus cooling the refrigerant further.

[0057] The refrigerant leaving the heat rejecting heat exchanger 6 is supplied to the receiver 7, via a high pressure valve 12. In the receiver 7, the refrigerant is separated into gaseous refrigerant and liquid refrigerant. The liquid refrigerant is supplied to the refrigeration evaporator 8 and to the heat pump evaporator 9, via the respective associated expansion devices 10, 11. Due to the expansion taking place in the expansion devices 10, 11, the refrigerant entering the refrigeration evaporator 8 and the heat pump evaporator 9 is in the form of a mixture of gaseous and liquid refrigerant.

[0058] In the refrigeration evaporator 8, the liquid part of the refrigerant is at least partly evaporated, while heat exchange takes place in such a manner that heat is absorbed by the refrigerant. The refrigeration evaporator 8 is arranged in thermal contact with a refrigerated volume, and the heat exchange taking place in the refrigeration evaporator 8 therefore provides cooling to the refrigerated volume. The refrigerant leaving the refrigeration evaporator 8 enters a first suction line 13 defining a first suction pressure.

[0059] In the heat pump evaporator 9, the liquid part of the refrigerant is also at least partly evaporated while heat exchange takes place in the manner described above. However, the heat pump evaporator 9 is not arranged in thermal contact with a refrigerated volume. Instead, the heat pump evaporator 9 may, e.g., be arranged to perform heat exchange with the ambient, or it may be arranged in thermal contact with the heat rejecting heat exchanger 6. The refrigerant leaving the heat pump evaporator 9 enters a second suction line 14 defining a second suction pressure. The first suction line 13 and the second suction line 14 are arranged fluidly in parallel, i.e. they are completely separated from each other. The gaseous part of the refrigerant in the receiver 7 is supplied directly to the first suction line 13.

[0060] The dedicated refrigeration compressors 3 are permanently connected to the first suction line 13. Accordingly, the dedicated refrigeration compressors 3 suck refrigerant from the first suction line 13, and thus from the outlet of the refrigeration evaporator 8. The compressor 4 can be selectively connected to the first suction line 13 or to the second suction line 14 by appropriately operating a three-way valve 15. When the compressor 4 is connected to the first suction line 13 it operates as a refrigeration compressor, and when it is connected to the second suction line 14 it operates as a heat pump compressor.

[0061] The vapour compression system 1 of Fig. 1 may be operated in the following manner. The supply of refrigerant to the refrigeration evaporator 8 is controlled by means of the associated expansion device 10 in order to ensure a desired temperature in the refrigerated volume, and possibly so as to obtain a specified superheat of the refrigerant leaving the refrigeration evaporator 8. The first suction pressure, i.e. the suction pressure prevailing in the first suction line 13, is controlled by adjusting a compressor capacity of the refrigeration compressors 3, 4, in order to match a first suction pressure setpoint. This may include adjusting a running capacity of a given compressor 3, 4 and / or operating the three-way valve 15 so as to switch the compressor 4 from being connected to the first suction line 13 to being connected to the second suction line 14, or vice versa. Thus, an appropriate and stable first suction pressure is maintained in the first suction line 13, regardless of the operation of the refrigeration evaporator 8.

[0062] Upon receipt of a heat recovery request from the heat recovery system, a compressor capacity of the heat pump compressor 4 is adjusted in order to ensure that the heat recovery request can be met. This could, e.g., include adjusting the running capacity of compressor 4 and / or operating the three-way valve 15 so as to switch the compressor 4 from being connected to the first suction line 13 to being connected to the second suction line 14, or vice versa.

[0063] Finally, the superheat of refrigerant leaving the heat pump evaporator 9 is controlled by operating the associated expansion device 11, so as to adjust the supply of refrigerant to the heat pump evaporator 9, and in order to obtain minimal superheat. Thus, the operation of the heat pump evaporator 9 is in accordance with the running capacity of the heat pump compressor 4, and thus in accordance with the heat recovery request from the heat recovery system. Accordingly, operation of the heat pump evaporator 9 does not affect the first suction pressure. Thus, a heat recovery request from the heat recovery system can be met without compromising the control of the refrigeration evaporator 8, and thus the primary function of the vapour compression system 1.

[0064] Fig. 2 is a diagrammatic view of a vapour compression system 1 according to a second embodiment of the invention. The vapour compression system 1 of Fig. 2 is very similar to the vapour compression system 1 of Fig. 1, and it will therefore not be described in detail here.

[0065] However, the vapour compression system 1 of Fig. 2 comprises a plurality of refrigeration evaporators 8, three of which are shown, each with an associated expansion device 10, and arranged fluidly in parallel with each other between the receiver 7 and the first suction line 13. The refrigerant supply to the respective refrigeration evaporators 8 can be adjusted independently of each other, and it is therefore particularly advantageous that the first suction pressure is controlled by adjusting the capacity of the refrigeration compressors 3, 4, and that the operation of the heat pump evaporator 9 does not affect the first suction pressure.

[0066] Fig. 3 is a diagrammatic view of a vapour compression system 1 according to a third embodiment of the invention. The vapour compression system 1 of Fig. 3 is very similar to the vapour compression system 1 of Fig. 2, and it will therefore not be described in detail here.

[0067] However, the compressor unit 2 of the vapour compression system 1 of Fig. 3 comprises two dedicated refrigeration compressors 3 being permanently connected to the first suction line 13, and one dedicated heat pump compressor 16 being permanently connected to the second suction line 14. Accordingly, none of the compressors 3, 16 of the compressor unit 2 can be switched between operating as a refrigeration compressor and as a heat pump compressor. Thus, the refrigeration compressor capacity can only be adjusted by adjusting the running capacity of the dedicated refrigeration compressors 3, and the heat pump compressor capacity can only be adjusted by adjusting the running capacity of the dedicated heat pump compressor 16.

[0068] Fig. 4 is a diagrammatic view of a vapour compression system 1 according to a fourth embodiment of the invention. The vapour compression system 1 of Fig. 4 is very similar to the vapour compression system 1 of Fig. 2, and it will therefore not be described in detail here. However, the vapour compression system 1 of Fig. 4 comprises a bypass refrigerant path 17 arranged in parallel with the heat rejecting heat exchanger 6. Accordingly, refrigerant leaving the heat recovery heat exchanger 5 may be passed through the bypass refrigerant path 17, rather than passing through the heat rejecting heat exchanger 6, by appropriately operating three-way valve 18. When the refrigerant is passed through the bypass refrigerant path 17, heat rejecting heat exchange takes place only in the heat recovery heat exchanger 5, and thus maximum heat recovery is obtained, since essentially all rejected heat is recovered.

[0069] Fig. 5 is a flow chart illustrating a method for controlling a vapour compression system according to an embodiment of the invention. The vapour compression system being controlled may, e.g., be one of the vapour compression systems illustrated in Figs. 1-4.

[0070] The process is started at step 19. At step 20 it is investigated whether or not the first suction pressure, i.e. the suction pressure prevailing in the first suction line, is equal to a first suction pressure setpoint value. If this is not the case, the process is forwarded to step 21, where the refrigeration compressor capacity is adjusted in order to control the first suction pressure so as to approach the first suction pressure setpoint. The process is then forwarded to step 22, where a heat recovery request is received from the heat recovery system.

[0071] In the case that step 20 reveals that the first suction pressure is equal to the first suction pressure setpoint, it is concluded that the first suction pressure is stable and at an appropriate level, and that adjustment of the refrigeration compressor capacity is therefore not required. The process is therefore forwarded directly to step 22.

[0072] At step 23 it is investigated whether or not the total compressor capacity, i.e. the combined capacity of the refrigeration compressors and the heat pump compressors, matches the heat recovery request. If this is not the case, the process is forwarded to step 24, where the heat pump compressor capacity is adjusted in order to ensure that the total compressor capacity matches the heat recovery request, and that the heat recovery request of the heat recovery system can therefore be met. The process is then forwarded to step 25, where the heat pump evaporator is controlled in accordance with a standard superheat control strategy.

[0073] In the case that step 23 reveals that the total compressor capacity matches the heat recovery request, the process is forwarded directly to step 25. Finally, the process is returned to step 20.

Claims

CLAIMS1. A method for controlling a vapour compression system (1), the vapour compression system (1) comprising a compressor unit (2) with at least two compressors (3, 4, 16), at least one compressor being configured to operate as a refrigeration compressor (2, 3) and at least one compressor being configured to operate as a heat pump compressor (4, 16); a heat recovery system; a heat rejecting heat exchanger (6); at least one refrigeration evaporator (8), each with an associated expansion device (10) and each arranged in thermal contact with a refrigerated volume; and a heat pump evaporator (9) with an associated expansion device (11), the vapour compression system (1) further comprising a first suction line (13) interconnecting an outlet of each of the at least one refrigeration evaporator (8) and an inlet of the at least one refrigeration compressor (3, 4), the first suction line (13) defining a first suction pressure; and a second suction line (14) interconnecting an outlet of the heat pump evaporator (9) and an inlet of the at least one heat pump compressor (4, 16), the second suction line (14) defining a second suction pressure, the first suction line (13) and the second suction line (14) being arranged fluidly in parallel, the method comprising the steps of:- controlling the first suction pressure by adjusting a compressor capacity of the at least one refrigeration compressor (3, 4), and in order to match a first suction pressure setpoint value,- adjusting a compressor capacity of the at least one heat pump compressor (4, 16) based on a heat recovery request from the heat recovery system, and- controlling the superheat of refrigerant leaving the heat pump evaporator (9) by adjusting a supply of refrigerant to the heat pump evaporator (9) via the associated expansion device (11), and in order to obtain minimal superheat.

2. A method according to claim 1, wherein the compressor unit (2) comprises at least one compressor (4) being configured to selectively operate as a refrigeration compressor or as a heat pump compressor, and wherein the method further comprises the step of switching at least one compressor (4) from operating as a refrigeration compressor to operating as a heat pump compressor, or vice versa.

3. A method according to claim 2, wherein the step of switching at least one compressor (4) comprises operating a three-way valve (15).

4. A method according to claim 2 or 3, wherein at least one compressor (4) is switched from operating as a heat pump compressor to operating as a refrigeration compressor in the case that the refrigeration compressors (3, 4) operate at maximum capacity and the first suction pressure exceeds a predefined threshold value.

5. A method according to any of claims 2-4, wherein at least one compressor (4) is switched from operating as a refrigeration compressor to operating as a heat pump compressor in the case that a request for increased heat recovery is received from the heat recovery system.

6. A method according to any of the preceding claims, wherein operation of the at least one heat pump compressor (4, 16) is stopped, and / or start of the at least one heat pump compressor (4, 16) is prevented, in the case that a running capacity of the at least one refrigeration compressor (3, 4) is sufficient to meet the heat recovery request from the heat recovery system.

7. A method according to any of the preceding claims, further comprising the steps of receiving a load shedding request from a power grid, and, in response to the load shedding request, decreasing the capacity of the at least one heat pump compressor (4, 16) before decreasing the capacity of the at least one refrigeration compressor (3, 4).

8. A vapour compression system (1) comprising :- a compressor unit (2) comprising at least one compressor being configured to operate as a refrigeration compressor (3, 4) and at least one compressor being configured to operate as a heat pump compressor (4, 16),- a heat recovery system,- a heat rejecting heat exchanger (6),- at least one refrigeration evaporator (8), each with an associated expansion device (10), and each being arranged in thermal contact with a refrigerated volume, and- a heat pump evaporator (9) with an associated expansion device (11), wherein the compressors (3, 4, 16), the heat recovery system, the heat rejecting heat exchanger (6), the at least one refrigeration evaporator (8) and the heat pump evaporator (9) are arranged in a refrigerant path, andwherein the refrigerant path comprises a first suction line (13) interconnecting an outlet of each of the at least one refrigeration evaporator (8) and an inlet of the at least one refrigeration compressor (3, 4), the first suction line (13) defining a first suction pressure, and a second suction line (14) interconnecting an outlet of the heat pump evaporator (9) and an inlet of the at least one heat pump compressor (4, 16), the second suction line (14) defining a second suction pressure, the first suction line (13) and the second suction line (14) being arranged fluidly in parallel.

9. A vapour compression system (1) according to claim 8, wherein the vapour compression system (1) comprises at least two refrigeration evaporators (8) arranged fluidly in parallel.

10. A vapour compression system (1) according to claim 8 or 9, wherein at least one of the compressors (4) is configured to selectively operate as a refrigeration compressor or as a heat pump compressor.

11. A vapour compression system (1) according to any of claims 8-10, wherein the heat recovery system is thermally connected to a district heating system.

12. A vapour compression system (1) according to any of claims 8-11, further comprising a bypass refrigerant path (17) arranged in parallel with the heat rejecting heat exchanger (6).

13. A vapour compression system (1) according to any of claims 8-12, further comprising a receiver (7) arranged in the refrigerant path between an outlet of the heat rejecting heat exchanger (6) and an inlet of the at least one refrigeration evaporator (8).

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