WO2018112542A1

WO2018112542A1 - A multi compressor refrigeration system and control thereof

Info

Publication number: WO2018112542A1
Application number: PCT/AU2017/051436
Authority: WO
Inventors: Jean Francois Tuyau
Original assignee: Avance Energy Solutions Pty Ltd
Priority date: 2016-12-22
Filing date: 2017-12-21
Publication date: 2018-06-28

Abstract

There is provided herein a system which optimises control of multi-compressor refrigeration systems by utilising dual variable speed drive (VSD) compressors to more accurately meet refrigeration demand and reduced energy wastage. Specifically, the present system comprises a lead VSD compressor and a secondary VSD compressor. The system further comprises a controller controlling the VSD compressors. During a first stage, the controller controls only the lead VSD compressor according to refrigeration demand. However, for a second stage wherein refrigeration demand exceeds that of the first stage, the controller additionally operates the secondary VSD compressor and controls the operational speeds of the VSD compressors according to refrigeration demand.

Description

A multi compressor refrigeration system and control thereof Field of the Invention

[1] This invention relates generally to the control of refrigeration racks and, more particularly, this invention relates to multi compressor refrigeration system and control thereof, especially for use for parallel-compression refrigeration racks.

Background of the Invention

[2] Multi-compressor refrigeration racks are utilised in industrial and retail settings wherein compressors are sequentially brought online as refrigeration demand increases.

[3] Figure 3 shows a prior art multi-compressor refrigeration rack control strategy wherein direct online (DOL) compressors are brought online sequentially as demand increases.

[4] Specifically, figure 3 shows a refrigeration demand trend 20, given in the example as a linear trend for illustrative convenience. As such, as the refrigeration demand 20 increases, 5 DOL compressors are sequentially brought online to meet such demand through stages 1 - 5. However, as can be seen, at each stage, there is shown excess supply 21 wherein supply exceeds the refrigeration demand 20.

[5] For example, at stage 1, a first DOL compressor is brought online wherein the compressor may operate at, for example, 50 Hz. However, as is evident, the fixed supply of the first DOL compressor exceeds demand until such time that the demand 20 increases to meet the output of the first DOL compressor at which time the next DOL compressor is required to be brought online at stage 2 wherein supply again exceeds demand 20.

[6] The excess supply 21 such control strategy wastes energy.

[7] In an attempt to reduce such excess supply and therefore energy wastage, the prior art control strategy as a substantially shown in figure 4 is sometimes utilised wherein mechanical unloaders are utilised. For example, for a DOL compressor having dual pistons, and mechanical unloader may be selectively operated to selectively vent/decompress one of the pistons.

[8] Specifically, for the first stage 1 shown in figure 4, the first DOL compressor initially has the unloader in operation such that only the second piston is in operable and, as such, the excess compressive supply is approximately half that as compared to that shown in figure 3. However, as the demand 20 reaches the compressive supply of the first piston of the first DOL compressor, at control point 22, the unloader is deactivated such that the second piston of the first the compressor becomes operable. However, as can be appreciated, when bringing the second piston back into operation, the compressive supply of the first DOL compressor again exceeds demand 20.

[9] As such, while the utilisation of unloaders may reduce the excess supply and therefore energy wastage by approximately half as compared to the DOL control strategy of figure 3, it is desirous to further reduce such energy wastage.

[10] The present invention seeks to provide a control system wiring and methodology for controlling multi-compressor refrigeration racks, which will overcome or substantially ameliorate at least some of the deficiencies of the prior art, or to at least provide an alternative.

[11] It is to be understood that, if any prior art information is referred to herein, such reference does not constitute an admission that the information forms part of the common general knowledge in the art, in Australia or any other country.

Summary of the Disclosure

[12] There is provided herein a system which optimises control of multi-compressor refrigeration systems by utilising dual variable speed drive (VSD) compressors to more accurately meet refrigeration demand and reduced energy wastage.

[13] Specifically, the present system comprises a lead VSD compressor and a secondary VSD compressor. The system further comprises a controller controlling the VSD compressors.

[14] During a first stage, the controller controls only the lead VSD compressor according to refrigeration demand.

[15] However, for a second stage wherein refrigeration demand exceeds that of the first stage, the controller additionally operates the secondary VSD compressor and control the operational speeds of the VSD compressors according to refrigeration demand.

[16] The controller may operate the VSD compressors between lower and upper speed threshold. As such, when transitioning from the first stage to the second stage, the controller may drop the operational speed of the lead VSD compressor down to the lower speed threshold whereafter operational speeds of both VSD compressors are increased from lower to upper speed thresholds to meet demand.

[17] For a third stage wherein refrigeration demand exceeds that of the second stage, the controller may bring online conventional non-variable speed direct online (DOL) compressors wherein the dual VSD controllers are able to smooth supply step changes caused thereby.

[18] In embodiments, the upper speed threshold of the lead VSD compressor is at least double that of the lower speed thresholds of both VSD compressors. As such, when transitioning from the first edge the second stage, the controller may be configured for controlling the lead VSD compressor to the upper speed threshold whereafter, at the start of the second stage, the controller may slow the lead VSD compressor to the lower speed threshold and additionally operate the secondary VSD compressor at a respective lower speed threshold. In this way, excess supply and associated energy wastage can be effectively eliminated.

[19] As is evidenced in Figure 10, the present dual-VSD control system may offer average power consumption savings of between 1% and 12% compared to conventional DOL compressor control systems.

[20] Furthermore, in embodiments, the controller comprises a single analogue control output controlling respective analogue speed control inputs of both the lead and secondary VSD compressors. Furthermore, the controller may comprise a really output for controlling the operation of the secondary VSD compressor.

[21] Such configuration allows for the retrofit of the present system to conventional multi- compressor refrigeration rack systems.

[22] As such, with the foregoing in mind, in accordance with one aspect, there is provided a multi compressor refrigeration system comprising: a lead variable speed drive (VSD) compressor; a secondary VSD compressor; a controller controlling the VSD compressors, wherein the controller is configured for: for a first stage: operating only the lead VSD compressor; and controlling the operational speed of the lead VSD compressor according to refrigeration demand; for a second stage wherein refrigeration demand exceeds that of the first stage: additionally operating the secondary VSD compressor; and controlling the operational speeds of the VSD compressors according to refrigeration demand. [23] The controller may be configured for transitioning from the first stage to the second stage by decreasing the speed of the lead VSD compressor.

[24] The controller may be configured for: at the end of the first edge, controlling the lead VSD compressor to a lead VSD compressor upper speed threshold; at the start of the second stage: controlling the lead VSD compressor to a lead VSD compressor lower speed threshold; and controlling the secondary VSD compressor to a secondary VSD compressor lower speed threshold.

[25] The capacity of the lead VSD compressor when operating at the lead VSD compressor upper speed threshold may be approximately twice the combined capacities of the lead VSD compressor and the second VSD compressor when the lead VSD compressor is operating at the lead VSD compressor lower speed threshold and the secondary VSD compressor is operating at the secondary VSD lower speed threshold.

[26] The system may further comprise a direct online (DOL) compressor and wherein the controller may be configured for: for the second stage, not operating the DOL compressor; and for a third stage wherein refrigeration demand exceeds that of the second stage, operating the DOL compressor.

[27] During the third stage, the controller may be configured for controlling the operational speeds of the VSD compressors according to refrigeration demand.

[28] The system may further comprise a further DOL compressor and wherein the controller may be configured for: for the third stage, not operating the further DOL compressor; and for a fourth stage wherein refrigeration demand exceeds that of the third stage, operating the further DOL compressor.

[29] The controller may comprise: a single analogue speed control output for controlling analogue speed control inputs of both the lead and secondary VSD compressors; and a control relay output for controlling the secondary VSD compressor.

[30] During the first stage, the control relay output may be low.

[31] During the second stage, the control relay output may be high.

[32] Other aspects of the invention are also disclosed. Brief Description of the Drawings

[33] Notwithstanding any other forms which may fall within the scope of the present invention, preferred embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which:

[34] Figure 1 shows a control system for controlling multi-compressor refrigeration racks in accordance with an embodiment;

[35] Figure 2 shows a control methodology for controlling multi-compressor refrigeration racks using the system of Figure 1 in accordance with an embodiment;

[36] Figure 3 shows a prior art DOL compressor only control methodology;

[37] Figure 4 shows a prior art DOL compressor only control methodology utilising unloaders;

[38] Figure 5 shows a single VSD control methodology in accordance with an embodiment;

[39] Figure 6 shows a dual VSD control methodology in accordance with an embodiment;

[40] Figure 7 shows a "speed boost" dual VSD control methodology in accordance with an embodiment;

[41] Figure 8 shows exemplary VSD compressor wiring in accordance with an embodiment;

[42] Figure 9 shows an exemplary controller wiring in accordance with an embodiment;

[43] Figure 10 shows comparative energy consumption of different control strategies at Trial Site #1;

[44] Figure 11 shows energy savings of the various control strategies of figure 10; and [45] Figure 12 shows comparative energy consumption of different control strategies at Trial Site #2.

Description of Embodiments

[46] Figure 5 shows a multi-compressor refrigeration rack control strategy utilising at least one variable speed drive (VSD) compressor in conjunction with at least one DOL compressor.

[47] As is shown in figure 5, during stage 1, the VSD compressor is brought online wherein, for example, the VSD compressor may operate at, for example, 30 Hz.

[48] However, during the initial stages of stage 1, the compressive supply of the VSD compressor exceeds demand 20. However, when the compressive supply of the VSD compressor meets demand 20 at control point 23, the speed of the VSD compressor is gradually increased such that the compressive supply output of the VSD compressor follows demand 20 until such time that, at control point 24, the VSD compressor may be operating at, for example, 50 Hz.

[49] At control point 24, a secondary DOL compressor (such as one operating at 50 Hz) is brought online and the speed of the VSD compressor is dropped back to 30 Hz. However, as can be seen from stage 2, during the initial period of the second stage, the combined compressive supply of the VSD and the DOL compressor (operating at 30 Hz and 50 Hz respectively) exceeds demand 20 until such time that, at control point 25, demand matches supply at which time the speed of the VSD compressor is increased again to follow demand until point 25 wherein the VSD compressor is again operating at 50 Hz at which time the process is repeated by bringing on further DOL compressors.

[50] As such, the utilisation of one VSD controller in conjunction with other DOL compressors can reduce excess supply by approximately half as compared to the DOL compressor only methodology of figure 3.

[51] Turning now to figure 6, there is shown a control methodology utilising dual VSD compressors referred to herein as a lead VSD compressor and a secondary VSD compressor.

[52] As can be seen, in the first half of stage 1, the first VSD compressor is brought online and wherein, for example, the lead VSD compressor may initially operate at a lower speed threshold of, for example, 30 Hz. As such, as demand 20 increases, at control point 27, demand 20 matches the supply of the lead VSD compressor operating at 30 Hz. As such, the speed of the lead VSD compressor is increased to follow demand 20 such that, at control point 28, the lead VSD compressor is operating at speed threshold of, for example 50 Hz.

[53] At control point 28, the speed of the lead VSD compressor is dropped down again to the lower speed threshold of 30 Hz and wherein the secondary VSD compressor is brought online.

[54] As such, after control point 28, both the lead and secondary VSD compressors are operating at the lower speed threshold of 30 Hz each. It should be noted that in embodiments, the lead and secondary VSD compressors may operate at differing lower speed thresholds.

[55] However, as can be seen, after control point 28, with both the lead and secondary VSD compressors operating at a combined 60 Hz, the supply at this combined rate slightly exceeds the demand 20 until such time that at control point 29, the 60 Hz combined supply exactly matches the demand 20 whereafter the speed of the VSD compressors may be increased to follow demand.

[56] From stage three and onwards, extra DOL compressors may be brought online.

[57] As can be appreciated from figure 6, the control of dual VSD is in this manner greatly reduces the excess supply 21 and therefore energy wastage.

[58] Turning now to figure 7, there is shown a "speed boost" dual VSD control methodology wherein excess supply 21 may be substantially eliminated.

[59] Specifically, the control methodology shown in figure 7 is characterised in that the supply capacity of a single lead VSD is sufficient such that the supply smoothly transitions the subsequent speed drop-down and bringing online of the secondary VSD. In other words, when operating at the upper speed threshold, the lead VSD compressor has a supply capacity approximately twice that of the lead and secondary VSD compressors operating at lower speed thresholds.

[60] Specifically, there is shown in figure 7 stage 1 wherein, initially, the lead VSD is brought online and, for example, when operating at the lower speed threshold of, for example, 30 Hz, supply initially exceeds demand as is evident from the initial period of stage 1.

[61] However, at control point 30, as the supply of the lead VSD matches demand 20, the speed of the lead VSD is gradually increased to follow demand 20.

[62] At control point 31, the supply of the lead VSD is able to smoothly transition the subsequent drop down in speed and bringing online of the secondary VSD.

[63] Specifically, for example, at control point 31, the lead VSD is operated at, for example, 60 Hz, as opposed to 50 Hz as described above.

[64] As such, after control point 31, the speed of the lead VSD drops down to the lower speed threshold of 30 Hz and wherein the secondary VSD is brought online and initially operable at the lower speed threshold of 30 Hz.

[65] As such, prior control point 31, the lead VSD is operative at an upper speed threshold of 60 Hz and wherein after the control point 31, both the lead and secondary VSD operate at a lower speed threshold of 30 Hz, being a combined 60 Hz. As such, there is a smooth refrigeration supply transition between stages as is evident from figure 7.

[66] It should be noted that the operational frequencies of the VSDs described herein is utilised primarily for illustrative convenience and that the dual VSD control strategy of figure 7 is characterised only in that the supply of the lead VSD sufficient so as to be able to match the combined supply of both the lead and secondary VSDs when the secondary VSD is brought online and both brought down to lower operational speed thresholds. In this manner, VSD compressors of differing capacities may be mixed and matched and operated at differing lower and upper operational speed threshold so as to achieve the control methodology exemplified in figure 7.

[67] Turning now to figure 1, there is shown an exemplary control system wiring installation 1 for implementing the control strategy as a substantially shown in figure 7. The system wiring installation of figure 1 allows the utilisation of existing refrigeration input/output (10) control cards, so as to allow the dual VSD control methodology to be retrofitted for existing installations or be implemented using common hardware.

[68] Specifically, the system 1 comprises a controller 2 which, for example, may be the Danfoss AK-SC55, Danfoss AK-SC255, Danfoss AK-SM 800 series or Emerson e2 series of controllers or the like.

[69] The controller 2 comprises a CPU 8 for processing digital data. The controller 2 further comprises a memory device 6 for storing digital data including computer program code instructions and associated data. The memory device 6 is in operable communication with the CPU 8 across a system bus 5. As such, during use, the CPU 8 fetches these computer code instructions and associated data from memory 6 for interpretation and execution.

[70] In the embodiment shown in figure 1, the dual VSD control methodology is shown as a computer code instruction control algorithm software module 7 residing within memory 6.

[71] The controller 2 has a demand input 3 for reading the refrigeration demand of the multi compressor refrigeration rack.

[72] As can be seen, the controller 2 controls both the lead VSD 17 and the secondary VSD 18. The controller 2 may further control a at least one additional DOL compressors 19.

[73] The wiring installation 1 is characterised in that a single analogue output is utilised for speed control of both the lead and secondary VSDs 17, 18 in the manner which will be described in further detail below with reference to figure 2.

[74] Furthermore, at least the secondary VSD 18 may be brought online utilising a secondary relay output 10. [75] As such, the controller 2 is able to use the secondary relay output 10 to selectively bring the secondary VSD 18 online and wherein the single analogue output is able to therefore control the speed of either the lead VSD 17 alone or both of the lead and secondary VSDs 17, 18 together.

[76] Specifically, there is shown the controller 2 comprising an analogue output 9 which, for example, may be a 0 - 10 V analogue output which is wired to analogue speed control inputs 13, 14 of the respective VSDs.

[77] For example, 5 V output may control the respective VSDs at lower speed thresholds of, for example 30 Hz and wherein 10 V may control the VSDs to operate at the upper speed thresholds, such as 50 or 60 Hz.

[78] Each VSD 17, 18 further comprises a power on input 12, 15 and wherein the controller 2 comprises respective primary and secondary relay outputs 4, 10 for controlling such. As such, primary relay output 4 may be utilised for bringing the lead VSD compressor 17 online and the secondary relay output 10 may be utilised for bringing the secondary VSD compressor 18 online.

[79] The controller 2 may further comprise a plurality of DOL relay outputs 11 for controlling the respective power on inputs 16 of a plurality of additional DOL compressors 19.

[80] Turning now to figure 2, there is shown a control methodology 20 for controlling the lead and secondary VSD compressors and additional DOL compressors in the manner substantially described above with reference to figure 7.

[81] As can be seen, the methodology 20 comprises stage 1 wherein the lead VSD primary relay output 4 is high such that the lead VSD compressor 17 is operational.

[82] Now, the methodology 20 initiate at control point 30 of figure 7 wherein the analogue speed control output 9 is gradually increased to increase the operational speed of the first VSD 17 to follow demand.

[83] When reaching stage 2, the secondary relay output 10 is actuated to bring the second VSD compressor 18 online and wherein the analogue output 9 goes low such that both the first and secondary VSD compressors 17, 18 operate at a lower speed threshold (either at the same lower speed threshold or respectively lower speed thresholds). As such, during stage 2, the analogue output 9 is gradually brought high so as to gradually increase the speeds of both the first and secondary VSD compressors 17, 18 to meet demand 20. [84] At stage 3, the secondary relay output 10 goes low to deactivate the secondary VSD compressor and wherein, simultaneously the DOL relay output 11 goes high so as to bring a first DOL compressor 19 online. At this time also, the analogue output 9 goes low.

[85] As such, during stage 3, the analogue output 9 gradually goes high so as to increase the operational speed of the lead VSD compressor 17 until reaching stage 4 wherein the secondary relay output 10 goes high so as to bring the secondary VSD compressor 18 online and wherein the analogue output 9 simultaneously goes low so as to bring the first and secondary VSD compressors 17, 18 down to lower speed thresholds wherein, during stage 4, the analogue output 9 gradually goes high so as to gradually increase the operational speeds of both the first and second VSD compressors 17, 18 so as to follow demand.

[86] The methodology 20 may repeat for any number of additional DOL compressors 19.

[87] Figure 8 shows an exemplary wiring diagram for the VSD compressor headers wherein, as can be seen, a common analogue input is utilised for speed control for both of the VSD compressors 17, 18.

[88] Figure 9 shows an exemplary wiring diagram for the controller 2.

[89] Figure 10 shows a graph of the weather-normalised relative power consumption performance over an extended test period of 11 months showing from Trial Site #1, in order from top to bottom, DOL control, single VSD control, and "speed boost" dual VSD control.

[90] As can be appreciated, the regression model reveals that the average power consumption of the "speed boost" dual VSD control methodology is less at every ambient operating temperature as compared to the other control methodologies, with the present dual-VSD control solution having an average power consumption savings of 1% to 6% versus single-VSD control and an average power consumption savings of 1% to 12% versus DOL control (with relative efficiency gains dependant on ambient operating conditions).

[91] Figure 11 shows a dollarised value comparison and energy savings for the various control methodologies of figure 10.

[92] Figure 12 shows a graph of the weather-normalised relative power consumption performance at Trial Site #2, where the present dual-VSD solution was implemented as a step- change retrofit, altering the refrigeration rack from DOL control directly to the present dual- VSD control solution. The regression model reveals that the present dual-VSD control solution yields energy efficiency savings of 2% - 18% versus DOL control. [93] The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that specific details are not required in order to practice the invention. Thus, the foregoing descriptions of specific embodiments of the invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed; obviously, many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, they thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the following claims and their equivalents define the scope of the invention.

Claims

1. A multi compressor refrigeration system comprising:

a lead variable speed drive (VSD) compressor;

a secondary VSD compressor;

a controller controlling the VSD compressors, wherein the controller is configured for:

for a first stage:

operating only the lead VSD compressor; and

controlling the operational speed of the lead VSD compressor according to refrigeration demand;

for a second stage wherein refrigeration demand exceeds that of the first stage: additionally operating the secondary VSD compressor; and

controlling the operational speeds of the VSD compressors according to refrigeration demand.

2. A system as claimed in claim 1, wherein the controller is configured for transitioning from the first stage to the second stage by decreasing the speed of the lead VSD compressor.

3. A system as claimed in claim 1, wherein, the controller is configured for:

at the end of the first edge, controlling the lead VSD compressor to a lead VSD compressor upper speed threshold;

at the start of the second stage:

controlling the lead VSD compressor to a lead VSD compressor lower speed threshold; and

controlling the secondary VSD compressor to a secondary VSD compressor lower speed threshold.

4. A system as claimed in claim 3, wherein the capacity of the lead VSD compressor when operating at the lead VSD compressor upper speed threshold is approximately twice the combined capacities of the lead VSD compressor and the second VSD compressor when the lead VSD compressor is operating at the lead VSD compressor lower speed threshold and the secondary VSD compressor is operating at the secondary VSD lower speed threshold.

5. A system as claimed in claim 1, further comprising a direct online (DOL) compressor and wherein the controller is configured for:

for the second stage, not operating the DOL compressor; and

for a third stage wherein refrigeration demand exceeds that of the second stage, operating the DOL compressor.

6. A system as claimed in claim 5, wherein, during the third stage, the controller is configured for controlling the operational speeds of the VSD compressors according to refrigeration demand.

7. A system as claimed in claim 5, further comprising a further DOL compressor and wherein the controller is configured for:

for the third stage, not operating the further DOL compressor; and

for a fourth stage wherein refrigeration demand exceeds that of the third stage, operating the further DOL compressor.

8. A system as claimed in claim 1, wherein the controller comprises:

a single analogue speed control output for controlling analogue speed control inputs of both the lead and secondary VSD compressors; and

a control relay output for controlling the secondary VSD compressor.

9. A system as claimed in claim 8, wherein, during the first stage, the control relay output is low.

10. A system as claimed in claim 9, wherein, during the second stage, the control relay output is high.