METHOD FOR CONTROLLING A VAPOUR COMPRESSION SYSTEM DURING LOAD SHEDDING

Abstract

A method for controlling a vapour compression system (1) including two or more evaporators (5, 12), each evaporator (5, 12) being arranged in thermal contact with a refrigerated volume, the refrigerated volumes storing goods of various types, and each evaporator (5, 12) receiving refrigerant via an expansion device (6, 13) is disclosed. In response to receipt of a load shedding command originating from a power grid (17), the vapour compression system (1) reduces a compressor capacity of the compressor unit. The refrigerated volumes are divided into at least two prioritized categories of refrigerated volumes, where a first category (18) includes refrigerated volumes storing goods of a temperature critical type, and a second category (19) includes refrigerated volumes storing goods of a temperature non-critical type. Refrigerant supply to the evaporator(s) (5, 12) being in thermal contact with the refrigerated volume(s) of the second category (19) is discontinued, and refrigerant supply to the evaporator(s) (5, 12) being in thermal contact with the refrigerated volume(s) of the first category (18) is continued. Thereby the vapour compression system (1) is capable of providing load shedding services for an extended period of time without compromising temperature critical storage.

Claims

1. A method for controlling a vapour compression system, the vapour compression system comprising a compressor unit comprising at least one compressor, a heat rejecting heat exchanger, and two or more evaporators, each evaporator being arranged in thermal contact with a refrigerated volume, the refrigerated volumes storing goods of various types, and each evaporator receiving refrigerant via an expansion device, the method comprising the steps of: the vapour compression system receiving a load shedding command originating from a power grid, in response to receipt of the load shedding command, the vapour compression system reducing a compressor capacity of the compressor unit, dividing the refrigerated volumes into at least two prioritized categories of refrigerated volumes, where a first category includes refrigerated volumes storing goods of a temperature critical type, and a second category includes refrigerated volumes storing goods of a temperature non-critical type, discontinuing refrigerant supply to the evaporator(s) being in thermal contact with the refrigerated volume(s) of the second category, and continuing refrigerant supply to the evaporator(s) being in thermal contact with the refrigerated volume(s) of the first category.

2. The method according to claim 1, wherein the step of discontinuing refrigerant supply comprises closing the expansion device(s) which supply refrigerant to the evaporator(s) being in thermal contact with the refrigerated volume(s) of the second category.

3. The method according to claim 1, wherein the step of reducing the compressor capacity comprises stopping all compressors of the compressor unit.

4. The method according to claim 1, wherein the vapour compression system is a refrigeration system and the refrigerated volumes are display cases.

5. The method according to claim 1, wherein the step of dividing the refrigerated volumes into at least two prioritized categories of refrigerated volumes comprises evaluating thermal capacity of the goods stored in the display cases.

6. The method according to claim 1, further comprising the step of, in response to receipt of the load shedding command, activating a thermal storage and supplying cooling to at least some of the refrigerated volumes from the thermal storage.

7. The method according to claim 1, wherein the step of dividing the refrigerated volumes into at least two prioritized categories (18, 19) of refrigerated volumes is performed on the basis of operation history of the refrigerated volumes.

8. The method according to claim 1, wherein the vapour compression system comprises a medium temperature (MT) part and a low temperature (LT) part, and wherein the method further comprises the step of supplying refrigerant from the MT part of the vapour compression system to the LT part of the vapour compression system, in response to receipt of the load shedding command.

9. The method according to claim 1, further comprising the step of pumping refrigerant out of the evaporator(s) being arranged in thermal contact with the refrigerated volume(s) of the second category.

10. The method according to claim 1, wherein the load shedding command originating from the power grid is provided by an aggregator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The invention will now be described in further detail with reference to the accompanying drawings in which:

[0050] FIGS. 1-7 are diagrammatic views of vapour compression systems being controlled in accordance with methods according to seven different embodiments of the invention, and

[0051] FIG. 8 is a block diagram illustrating a number of vapour compressions systems being administered by an aggregator in accordance with a method according to an embodiment of the invention.

DETAILED DESCRIPTION

[0052] FIG. 1 is a diagrammatic view of a vapour compression system 1 being controlled in accordance with a method according to a first embodiment of the invention. The vapour compression system 1 comprises a compressor unit comprising a number of compressors 2, one of which is shown, a heat rejecting heat exchanger 3 and a number of evaporators 5, three of which are shown, arranged in a refrigerant path. The evaporators 5 are arranged fluidly in parallel, and the refrigerant supply to each evaporator 5 is controlled by means of an expansion device 6.

[0053] Refrigerant circulating in the refrigerant path is compressed by means of the compressor 2 before being supplied to the heat rejecting heat exchanger 3. When passing through the heat rejecting heat exchanger 3, heat exchange takes place between the refrigerant and the ambient, in such a manner that heat is rejected from the refrigerant.

[0054] The refrigerant leaving the heat rejecting heat exchanger 3 is distributed among the expansion devices 6, where it undergoes expansion before being supplied to the respective evaporators 5. The refrigerant being supplied to the evaporators 5 is thereby in a mixed gaseous and liquid state.

[0055] When passing through the evaporators 5, the liquid part of the refrigerant is evaporated, while heat exchange takes place between the refrigerant and the ambient, in such a manner that heat is absorbed by the refrigerant. Finally, the refrigerant is once again supplied to the compressor 2.

[0056] Each evaporator 5 is arranged in thermal contact with a refrigerated volume. Thereby, the heat exchange taking place in a given evaporator 5 provides cooling to a corresponding refrigerated volume. Furthermore, the refrigerant supply to each evaporator 5 is controlled by means of its corresponding expansion device 6 in such a manner that the temperature inside the corresponding refrigerated volume is maintained within a specified temperature range.

[0057] In the method according to the first embodiment of the invention, the vapour compression system 1 receives a load shedding command originating from a power grid. In response thereto, the vapour compression system 1 reduces the compressor capacity of the compressor unit in order to fulfil the load shedding needs of the power grid. This could, e.g., include stopping one or more compressors 2 completely and/or by reducing the capacity of one or more variable capacity compressors 2. Thereby the available refrigerant in the vapour compression system 1 is reduced.

[0058] Furthermore, the vapour compression system 1 divides the refrigerated volumes into at least two prioritized categories. A first category includes refrigerated volumes storing goods of a temperature critical type, and a second category includes refrigerated volumes storing goods of a temperature non-critical type.

[0059] Finally, the refrigerant supply to the evaporators 5 being in thermal contact with refrigerated volumes of the second category is discontinued, e.g. by closing the corresponding expansion devices 6, while the refrigerant supply to the evaporators 5 being in thermal contact with the refrigerated volumes of the first category is continued. Accordingly, the evaporators 5 being in thermal contact with the refrigerated volumes storing goods of a temperature critical type continue operating in a normal manner, while the evaporators 5 being in thermal contact with refrigerated volumes storing goods of a temperature non-critical type no longer provide cooling to the corresponding refrigerated volumes.

[0060] Thereby it is ensured that the reduced amount of available refrigerant is distributed to the refrigerated volumes with temperature critical storage, at the expense of the refrigerated volumes with temperature non-critical storage. Accordingly, it is ensured that the vapour compression system 1 is capable of providing load shedding services to the power grid for an extended period of time, without compromising the quality or safety of any temperature critical storage.

[0061] FIG. 2 is a diagrammatic view of a vapour compression system 1 being controlled in accordance with a method 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.

[0062] The vapour compression system 1 of FIG. 2 further comprises a receiver 7 arranged in the refrigerant path between the heat rejecting heat exchanger 3 and the expansion devices 6, via a high pressure valve 4. In the receiver 7, the refrigerant is separated into a liquid part and a gaseous part. At least part of the gaseous part of the refrigerant is supplied directly to the compressor 2, via a bypass valve 8, without passing through the expansion devices 6 and the evaporators 5. Thereby the gaseous part of the refrigerant is not subjected to the pressure drop taking place in the expansion devices 6, and the energy required in order to compress this part of the refrigerant is therefore reduced. The liquid part of the refrigerant in the receiver 7 is supplied to the expansion devices 6 in the manner described above with reference to FIG. 1.

[0063] In response to receiving a load shedding command originating from a power grid, the vapour compression system 1 is operated essentially in the manner described above with reference to FIG. 1. Furthermore, the bypass valve 8 may be closed, thereby ensuring that all refrigerant is directed towards the expansion devices 6 and the evaporators 5.

[0064] FIG. 3 is a diagrammatic view of a vapour compression system 1 being controlled in accordance with a method 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.

[0065] The vapour compression system 1 of FIG. 3 further comprises a low pressure liquid buffer 9 arranged in the refrigerant path between the evaporators 5 and the compressor 2, in such a manner that gaseous refrigerant being supplied from the receiver 7 to the compressor 2 via the bypass valve 8 enters the refrigerant path between the low pressure liquid buffer 9 and the compressor 2. Accordingly, refrigerant leaving the evaporators 5 is collected in the low pressure liquid buffer 9. By flooding the evaporators 5 during normal operation of the vapour compression system 1, liquid refrigerant is thereby accumulated in the low pressure liquid buffer 9.

[0066] In the low pressure liquid buffer 9, the refrigerant is separated into a liquid part and a gaseous part, and the gaseous part is supplied to the compressor 2. A pump 10 is arranged to pump liquid refrigerant from the low pressure liquid buffer 9 to the receiver 7.

[0067] In response to receiving a load shedding command originating from a power grid, the vapour compression system 1 is operated essentially in the manner described above with reference to FIGS. 1 and 2. Furthermore, the pump 10 may be activated, in order to supply the previously accumulated liquid refrigerant from the low pressure liquid buffer 9 to the receiver 7, thereby ensuring that liquid refrigerant is available for the evaporators 5 being in thermal contact with the refrigerated volumes of the first category, even if the compressor 2 is completely stopped. In addition, the refrigerant being present in the evaporators 5 being in thermal contact with the refrigerated volumes of the second category may thereby be pumped out and supplied to the evaporators 5 being in thermal contact with the refrigerated volumes of the first category.

[0068] FIG. 4 is a diagrammatic view of a vapour compression system 1 being controlled in accordance with a method 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. 3, and it will therefore not be described in detail here.

[0069] The vapour compression system 1 of FIG. 4 further comprises a thermal storage 11 arranged to, upon request, perform heat exchange with refrigerant in the low pressure liquid buffer 9.

[0070] In response to receiving a load shedding command originating from a power grid, the vapour compression system 1 is operated essentially in the manner described above with reference to FIGS. 1, 2 and 3. Furthermore, the thermal storage is activated in the sense that heat exchange is performed between the thermal storage 11 and the gaseous part of the refrigerant in the low pressure liquid buffer 9. Thereby the gaseous part of the refrigerant in the low pressure liquid buffer 9 is at least partly condensed, thereby providing a larger amount of liquid refrigerant which can be supplied to the receiver 7. This allows the refrigerant supply to the evaporators 5 being in thermal contact with the refrigerated volumes of the first category to be upheld for an extended period of time, thereby extending the period during which the vapour compression system 1 is capable of providing load shedding services to the power grid.

[0071] FIG. 5 is a diagrammatic view of a vapour compression system 1 being controlled in accordance with a method according to a fifth embodiment of the invention. The vapour compression system 1 of FIG. 5 is very similar to the vapour compression system 1 of FIG. 1, and it will therefore not be described in detail here.

[0072] In the vapour compression system 1 of FIG. 5, the compressor 2, the heat rejecting heat exchanger 3, the evaporators 5 and the expansion devices 6 which are also illustrated in FIG. 1 constitute a medium temperature (MT) part of the vapour compression system 1. The vapour compression system 1 of FIG. 5 further comprises a low temperature (LT) part, comprising a number of LT evaporators 12 arranged fluidly in parallel, and each receiving refrigerant via an expansion device 13, and an LT compressor 14. The refrigerant leaving the LT evaporators 12 is supplied to the LT compressor 14. The LT compressor 14 compresses the refrigerant and supplies it to the MT compressor 2 for further compression.

[0073] In response to receiving a load shedding command originating from a power grid, the vapour compression system 1 is operated essentially in the manner described above with reference to FIG. 1. For instance, the vapour compression system 1 may be operated in such a manner that available refrigerant is directed towards the LT evaporators 12 to a greater extent than towards the MT evaporators 5.

[0074] FIG. 6 is a diagrammatic view of a vapour compression system 1 being controlled in accordance with a method according to a sixth embodiment of the invention. The vapour compression system 1 of FIG. 6 is very similar to the vapour compression systems 1 of FIGS. 2 and 5, in the sense that it comprises the receiver 7 and bypass valve 8 of FIG. 2 and the LT part of FIG. 5. The vapour compression system 1 of FIG. 6 is therefore controlled essentially as described above with reference to FIGS. 1 and 5, and it will therefore not be described in detail here.

[0075] FIG. 7 is a diagrammatic view of a vapour compression system 1 being controlled in accordance with a method according to a seventh embodiment of the invention. The vapour compression system 1 of FIG. 7 is very similar to the vapour compression system 1 of FIG. 6, and it will therefore not be described in detail here.

[0076] The vapour compression system 1 of FIG. 7 further comprises a valve 15 arranged in the refrigerant path between the outlets of the LT evaporators 12 and the outlets of the MT evaporators 5.

[0077] In response to receiving a load shedding command originating from a power grid, the vapour compression system 1 is operated essentially as described above with reference to FIGS. 1, 2, 5 and 6. Furthermore, the valve 15 may be opened. Since the pressure prevailing in the MT part of the vapour compression system 1 is higher than the pressure prevailing in the LT part of the vapour compression system 1, this causes the refrigerant leaving the MT evaporators 5 to flow towards the LT part of the vapour compression system 1.

[0078] The large evacuated volume of the LT-evaporators 12 can thereby be utilized to dump the evaporated gas from the MT part of the vapour compression system 1, while the LT compressor 14 is at a standstill as well. This can continue until the pressures of the MT part and the LT part of the vapour compression system 1 are balanced out. This enables the vapour compression system 1 to provide load shedding for a prolonged period of time without jeopardizing the temperature in the refrigerated volumes of the MT part of the vapour compression system 1. To this end, goods stored in the refrigerated volumes of the LT part of the vapour compression system 1 are frozen, and may hence be considered as non-temperature sensitive, as long as it is ensured that no melting takes place.

[0079] As the pressure in the LT part of the vapour compression system 1 increases, the temperature of the goods stored in the refrigerated volumes of the LT part of the vapour compression system 1 might become lower than the condensation temperature of the gaseous refrigerant in the MT part of the vapour compression system 1. Thereby the gaseous refrigerant in the MT part of the vapour compression system 1 may start to condensate, while transferring heat to the goods stored in the refrigerated volumes of the LT part of the vapour compression system 1, i.e. extracting cooling from these goods. Thereby the load shedding may be prolonged even further.

[0080] FIG. 8 is a block diagram illustrating a number of vapour compression systems 1, three of which are shown, being administered by an aggregator 16. The vapour compression systems 1 may each be in the form of one of the vapour compression systems 1 illustrated in any of FIGS. 1-7.

[0081] The aggregator 16 receives load shedding requests from a power grid 17, the load shedding requests indicating a total load shedding need by the power grid 17 to be met by the vapour compression systems 1 being administered by the aggregator 16. Based thereon, the aggregator 16 generates load shedding commands for each of the vapour compression systems 1. The load shedding commands request each of the vapour compression systems 1 to provide load shedding in such a manner that the load shedding contributions from all of the vapour compression systems 1 add up to meet the load shedding need of the power grid 17 which was specified by the load shedding request received by the aggregator 16.

[0082] Each vapour compression system 1 comprises two or more refrigerated volumes. Upon receipt of a load shedding command from the aggregator 16, the vapour compression systems 1 reduce the compressor capacity, thereby reducing their power consumption. Furthermore, each vapour compression system 1 divides its refrigerated volumes into two prioritized categories, i.e. a first category 18 including refrigerated volumes storing goods of a temperature critical type, and a second category 19 including refrigerated volumes storing goods of a temperature non-critical type. Then the vapour compression systems 1 are operated in such a manner that the refrigerated volumes of the first category 18 are prioritized over the refrigerated volumes of the second category 19 when the available refrigerant is distributed among the evaporators arranged in thermal contact with the refrigerated volumes. This could, e.g., be performed in the manner described above with reference to FIGS. 1-7.

[0083] While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.