Cooling system and method of limiting input power

Abstract

A method of controlling a refrigeration system includes measuring a current (or power) drawn by the refrigeration system 902; comparing the measured value to a predetermined limit 903; if the measured

Description

BACKGROUND

[0001] The present disclosure relates to a chiller or air handler used, for example, in a refrigeration or air conditioning system and a method of controlling one or more aspects of the system.

[0002] In one example, a chiller is used in a refrigeration system to remove heat from a liquid coolant. The liquid coolant can be used to cool computer systems in a data center and in other applications, the liquid coolant can be used to cool industrial machinery. In another example, a computer room air handler is used to remove heat from, for instance, a data center. The hot air in the computer room can move via fans over coils that are cooled by, for instance, a water chiller system. SUMMARY

[0003] According to embodiments disclosed herein, a method of controlling a refrigeration system includes obtaining a measured current drawn by the refrigeration system; comparing the measured current to a predetermined current limit; calculating a reduced cooling demand value compared to a cooling demand of the refrigeration system if the measured current is greater than or equal to the predetermined current limit; and reducing an output capacity of a fluid handling motor based on the reduced cooling demand value to reduce the current drawn by the refrigeration system.

[0004] The fluid handling motor may be part of a compressor.

[0005] The fluid handling motor may be part of a fan.

[0006] The refrigeration system may cool a cooling medium configured to cool a remote area, the cooling medium may have a measured return temperature as it is delivered from the remote area to the refrigeration system, and the cooling demand of the refrigeration system may be based on a comparison of the measured return temperature to a predetermined return temperature.

[0007] The cooling medium may have a measured supply temperature as it is delivered from the refrigeration system to the remote area, and the cooling demand of the refrigeration system may be further based on a comparison of the measured supply temperature to a predetermined supply temperature.

[0008] The method may further comprise after reducing the output capacity of the fluid handling motor, obtaining an updated measured current drawn by the refrigeration system, comparing the updated measured current to the predetermined current limit, if the updated measured current is greater than or equal to the predetermined current limit, calculating an updated reduced cooling demand value that is lower than the reduced cooling demand value, and further reducing the output capacity of the fluid handling motor based on the updated reduced cooling demand value.

[0009] The method may further comprise if the updated measured current is lower than the predetermined current limit by a predetermined current differential value, increasing the output capacity of the fluid handling motor based on the cooling demand of the refrigeration system.

[0010] Calculating the reduced cooling demand value may include scaling the reduced cooling demand value based on a magnitude of a difference between the measured current and the predetermined current limit.

[0011] If the measured current is equal to the predetermined current limit, the reduced cooling demand value may be 90% of the cooling demand.

[0012] If the measured current is greater than the predetermined current limit by at least a predetermined current differential value, the reduced cooling demand value may be 0% of the cooling demand and the fluid handling motor is shut down.

[0013] According to embodiments disclosed herein, a refrigeration system includes a fluid handling motor and a control system. The control system includes an energy meter and a controller. The energy meter measures a current drawn by the refrigeration system. The controller is in communication with the energy meter and the fluid handling motor. The controller receives a command to limit current to a predetermined current limit; compares the measured current to a predetermined current limit; calculates a reduced cooling demand value compared to a cooling demand of the refrigeration system if the measured current is greater than or equal to the predetermined current limit; and reduces the output capacity of the fluid handling motor based on the reduced cooling demand value to reduce the current drawn by the refrigeration system.

[0014] The fluid handling motor may be part of a compressor.

[0015] The compressor may include a screw compressor.

[0016] The compressor may include a centrifugal compressor.

[0017] The fluid handling motor may be part of a fan.

[0018] The controller may be further configured to after reducing the output capacity of the fluid handling motor, compare an updated measured current obtained by the energy meter to the predetermined current limit, if the updated measured current is greater than or equal to the predetermined current limit, calculate an updated reduced cooling demand value that is lower than the reduced cooling demand value, and further reduce the output capacity of the fluid handling motor based on the updated reduced cooling demand value.

[0019] The controller may be further configured to increase the output capacity of the fluid handling motor based on the cooling demand of the refrigeration system if the updated measured current is lower than the predetermined current limit by a predetermined current differential value.

[0020] The reduced cooling demand value may be scaled based on a magnitude of a difference between the measured current and the predetermined current limit.

[0021] According to embodiments disclosed herein, a method of controlling a refrigeration system includes obtaining a measured current drawn by the refrigeration system; comparing the measured current to a predetermined current limit; determining a cooling demand offset based on a comparison of the measured current to the predetermined current limit; reducing a cooling demand value of the refrigeration system by the cooling demand offset; and reducing an output capacity of at least one of a first fluid handling motor and a second fluid handling motor of the refrigeration system.

[0022] The first fluid handling motor may be part of a first compressor and the second fluid handling motor may be part of a second compressor. 10023] The first fluid handling motor may be part of a first fan and the second fluid handling motor may be part of a second fan.

[0024] The cooling demand offset may be greater as the amount by which measured current exceeds the predetermined current limit is greater.

[0025] The method may further comprise, after reducing the output capacity of at least one of the first fluid handling motor and the second fluid handling motor, obtaining an updated measured current drawn by the refrigeration system, comparing the updated measured current to the predetermined current limit, and if the updated measured current is greater than or equal to the predetermined current limit, determining a second cooling demand offset based on a comparison of the updated measured current to the predetermined current limit; reducing the cooling demand value of the refrigeration system by the second cooling demand offset; and further reducing the output capacity of at least one of the first fluid handling motor and the second fluid handling motor of the refrigeration system.

[0026] The method may further comprise maintaining the output capacity of one of the first fluid handling motor and the second fluid handling motor to operate the refrigeration system at the cooling demand value reduced by the cooling demand offset.

[0027] The method may further comprise reducing the output capacity of both the first fluid handling motor and the second fluid handling motor to operate the refrigeration system at the cooling demand value reduced by the second cooling demand offset.

[0028] Reducing the output capacity of at least one of the first fluid handling motor and the second fluid handling motor may include first reducing the output capacity of the shorter running of the first fluid handling motor and the second fluid handling motor before reducing the output capacity of the longer running of the first fluid handling motor and the second fluid handling motor. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 is schematic view of a chiller according to embodiments disclosed herein.

[0030] FIG. 2 is a schematic view of another chiller according to embodiments disclosed herein.

[0031] FIG. 3 is a schematic view of another chiller according to embodiments described herein, particularly, a CRAH unit.

[0032] FIG. 4 is a schematic view of another chiller according to embodiments described herein, particularly, a CRAC unit.

[0033] FIG. 5 is a schematic view of a control system of the chiller of any of FIGS. 14.

[0034] FIG. 6 is a table illustrating exemplary operation stages of the compressors of any of FIGS. 1, 2, and 4 including, for instance, concurrent mechanical and free cooling modes.

[0035] FIG. 7 is a table illustrating further exemplary operation stages of the compressors of any of FIGS. 1, 2, and 4 including, for instance, a mechanical cooling only mode.

[0036] FIG. 8 is a flowchart illustrating an exemplary method of operating the chiller of any of FIGS. 14

[0037] FIG. 9 is a schematic view of another control system of the chiller of any of FIGS. 1- 4. JL—M.„ j JL j~ jl J—M j M.„ j IS IS .i—j kJ JL X- J—1 JL JL J.

[0038] Before any constructions of the disclosure are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. Fhe invention is capable of other constructions and of being practiced or of being carried out in various ways.

[0039] FIG. 1 illustrates a refrigeration system 100 including a chiller 102 having one or more compressors 104 and a control system 106. Each of the compressors 104 includes a fluid handling motor. In some embodiments, the refrigeration system 100 includes a single compressor 104, but other embodiments may include two, three, four, five, or more compressors. Fhe refrigeration system 100 further includes the control system 106, an evaporator 108, an expansion valve 110, and a condenser 112. FIG. 2 illustrates an embodiment of the refrigeration system 100 that is identical to that shown in FIG. 1, but it further includes a cooling tower 122.

[0040] Fhe chiller 102 includes a refrigerant loop 114 that circulates refrigerant from the compressor(s) 104 to the condenser 112, the expansion valve 110, and the evaporator 108. The evaporator 108 defines a cooling medium loop 116 that circulates a cooling medium (e.g.. a fluid such as air, coolant, or water). The cooling medium is in a heat exchange relationship with the refrigerant of the chiller 102 and so that the refrigerant removes heat from the cooling medium. That is, heat from the cooling medium is transferred to the refrigerant. The cooling medium is directed from the evaporator 108 to a remote area (e.g., a data center or building) 118 and is used to cool computer systems, related systems, and the like and / or the cooling medium is used in an air conditioning system to cool the air inside the building. The cooling medium returns the heat removed from the remote area 118 to the evaporator 108. The heat carried by the cooling medium is transferred to the refrigerant, which is delivered back to the compressors 104. Through operation of the fluid handling motors of each of the compressors 104, refrigerant is compressed and directed toward the condenser 112, where the heat in the refrigerant is transferred to ambient in the case of an air-cooled chiller 102 or to a second cooling medium through a second cooling medium loop 120 in the case of a liquid-cooled chiller 102. 10041] In embodiments including a cooling tower 122 (FIG. 2), the condenser 112 is configured to circulate the second cooling medium (e.g., coolant or water). The second cooling medium is also in a heat exchange relationship with the refrigerant of the chiller 102, such that the refrigerant heats the second cooling medium. That is, heat from the refrigerant is transferred to the second cooling medium in the condenser 112. The second cooling medium is directed from the condenser 112 to the cooling tower 122. The cooling tower 122 removes the heat from the second cooling medium before the second cooling medium is returned to the condenser 112. The refrigerant is directed from the condenser 112 to the expansion valve 110, which expands the refrigerant to cool the refrigerant before the refrigerant is transferred to the evaporator 108.

[0042] In some embodiments, such as the one shown in FIG. 3, the refrigeration system 100 may be an air-cooled refrigeration system (including, for example, a computer room air handler -CRAH 102), in which each fan 105a, 105b includes the fluid handling motor. Stated another way, the air-cooled refrigeration system 100 may utilize fans 105a, 105b rather than compressors. The CRAH unit 102 would be located within a data center or in some other appropriate structure, and the cooling tower or chiller 122 would be located outside the structure. A single chiller 122 may serve multiple CRAH units 102. The fans 105a, 105b bring air into the CRAH unit 102, across the cooling coil 112, through the fans 105a. 105b, and out of the CRAH unit 102 into the room. Of course, this embodiment is not limiting, as the fans 105a, 105b could be located upstream of the cooling coil 112 or in other appropriate locations. A cooling medium loop 120 runs from the cooling coil 112 to the chiller 122 and back to the cooling coil 112 in order to remove the heat from the air moved by the fans 105a, 105b. The air-cooled refrigeration system 100 includes the cooling coil 112 having a round tube plate fin or microchannel style heat exchanger that expels heat using one or more fans to pass ambient-temperature air over the coil. This air-cooled refrigeration system embodiment operates in largely the same manner as the other described refrigeration systems herein. Cooling demand, as described below, applies to the air-cooled refrigeration system embodiment in the sense that the fluid handling motor speed of the fan(s) corresponds to the air volume demand, which can itself be considered the cooling demand. Such fans could, for instance, include DC fluid handling motors that may be proportionally turned down.

[0043] With reference to FIG. 4, some embodiments of the chiller 102 may be a computer room air conditioner (CRAC). The CRAC unit 102 may include the same components as the chiller 102 of FIG. 1, but the CRAC unit 102 includes the condenser 112 outside the unit itself.

[0044] The control system 106 is in communication with a display unit 124, a building management system (BMS) 126, or both. Either or both of the display unit 124 or the BMS 126 is configured to deliver an input to the control system 106. With reference to FIG. 5, the control system 106 includes, among other features, a controller 128 and an energy meter 130. The controller 128 is in communication with the energy meter 130. In the illustrated embodiment, the energy meter 130 is configured to measure an actual, or live, current 132 drawn by the chiller 102 (e.g., by the fluid handling motor(s) of the compressor(s) 104, pump(s), and / or fan(s), as well as by other components such as valves, controls system devices, and the like). The energy meter 130 is further configured to send current measurement data to the controller 128. In some embodiments, the controller 128 may also be in communication with one or more of the fluid handling motors of the compressors 104, the evaporator 108, the condenser 112, or the expansion valve 110, thereby receiving signals relating to the chiller operation 134. In the illustrated embodiment of FIG. 5, however, the fluid handling motors of the compressors 104 do not communicate information to the controller 128. In some embodiments, the controller 128 is further configured to receive a user input current limit 136 specifying the available current via user interaction with, for instance, the display unit 124, the BMS 126, or both. In some embodiments, the user input current limit 136 is selectable by a user as a power limit value (e.g., in kilowatts), which is interpreted by the controller 128. In some embodiments, a user may determine the power limit value by, for instance, observing the value of one of more fuses of the system. Stated another way, the power or current limit may be dictated by the electrical infrastructure available, e.g., the maximum allowed current of the mains fuses.

[0045] The chiller 102 is controlled by the control system 106, and specifically the controller 128, to adjust, among other operating parameters, a cooling capacity delivered by the chiller 102 to the remote area 118 via the cooling medium. The controller 128 is configured to operate the chiller 102 at a cooling capacity in a manner that targets the cooling medium returning to the evaporator 108 at a predetermined return temperature value (e.g., 53 degrees Fahrenheit). In some embodiments, the controller 128 is further configured to operate the chiller 102 at a cooling capacity in a manner that also targets the cooling medium leaving the evaporator 108 at a predetermined supply temperature value (e.g., 45 degrees Fahrenheit). The controller 128 correlates the cooling capacity to a cooling demand value (e.g., a cooling demand percentage), which represents a comparison between a measured return temperature of the cooling medium and the targeted predetermined return temperature value. In some embodiments, the cooling demand value is also based on a comparison between a measured supply temperature of the cooling medium and the targeted predetermined supply temperature value. The controller 128 then determines the cooling demand value required to maintain the return temperature of the cooling medium at the targeted predetermined return temperature value. In some embodiments, the controller 128 further determines the cooling demand value required to maintain the supply temperature of the cooling medium at the targeted predetermined supply temperature value (e.g., 45 degrees Fahrenheit). For example, a cooling demand of 100% is the most cooling demand to maintain the cooling capacity of the chiller 102 (e.g., maintain at least one of the predetermined supply and return temperatures of the cooling medium), while a cooling demand of 0% is the least amount of cooling demand. Cooling demand represents the amount of cooling capacity required. One way to adjust (e.g., reduce) the cooling capacity is to adjust the output of the fluid handling motor(s) of the compressor(s) 104. A current limiting function 138, which will be discussed in greater detail below, may override or limit the output of the fluid handling motor(s) of the compressor(s) 104 such that one or more of the supply temperature and the return temperature of the cooling medium may exceed the targeted predetermined values.

[0046] Because the chiller 102 illustrated in FIGS. 1, 2, or 4 can have two or more compressors 104a, 104b, (or comparably, the fans 105a, 105b of FIG. 3) each of the fluid handling motors of the compressors 104a, 104b is controlled by the controller 128 based on the cooling demand value. The initial compressor 104a, 104b to run may be determined by run time. For instance, if both compressors 104a, 104b are off in a given moment, the initial compressor 104a, to start running will be the compressor 104a, 104b with the lowest run hours. As an example (FIG. 6), when the cooling demand value is set or increased, the first compressor 104a may initially start. The first fluid handling motor of the first compressor 104a may then load from 0% to 100%. Next, the second fluid handling motor of the second compressor 104b may start. The second fluid handling motor of the second compressor 104b may then load from 0% to 100%. Once both compressors 104a, 104b are running, a first-in-last-out rotation is utilized. When the cooling demand value decreases in this example, the second fluid handling motor of the second compressor 104b will reduce the load from 100% to 0% and then shut off. The first fluid handling motor of the first compressor 104a will then reduce the load from 100% to 0% and then shut off (if the cooling demand value is zero). In contrast, FIG. 7 illustrates an alternative staging in which the fluid handling motors of the first and second compressors 104a, 104b load and unload differently. For example, the first fluid handling motor of the first compressor 104a is loaded from a 25% output capacity to a 75% output capacity. Then, the fluid handling motors of the first compressor 104a unloads to 50% while the second fluid handling motor of the second compressor 104b loads to 50%. In subsequent stages, the fluid handling motors of the compressors 104a, 104b alternate loading until both reach 100% capacity. For example, the first fluid handling motor of the first compressor 104a remains at 50% while the second fluid handling motor of the second compressor 104b loads to 75%, the first fluid handling motor of the first compressor 104a loads to 75% while the second fluid handling motor of the second compressor 104b remains at 75%, and so on. The examples shown in FIGS. 6 and 7 may be utilized when the particular system already has multiple set points for other functionality. The control system 106, 206 can use these already-available stages to operate according to the needs of the cooling demand values. Other embodiments may include, for instance, fine-controllable compressors 104 (e.g., proportionally controlled compressors, variable speed compressors, etc.), which may be adjusted to any value (e.g., percentage) of the total cooling capacity.

[0047] As shown in FIGS. 1-4, the chiller 102 is configured to be powered by a primary power source 162 under normal circumstances. The primary power source 162 is three-phase power source from a utility grid, for example. The backup power source 164 is a three-phase generator (e.g., a diesel generator or UPS). Although, the primary power source 162 and the backup power source 164 are shown as only powering a single chiller 102, it should be understood that both may supply more than one chiller 102. The energy meter 130 can monitor and measure the voltage and frequency of the electricity supplied to the chiller 102 from the respective power source, which indicates the amount of current drawn from the respective power source by the chiller 102. In some embodiments, the energy meter 130 can monitor and measure the voltage and frequency of the electricity provided to the chiller 102 and the amount of current drawn by the chiller 102 from the respective power source. Should the chiller 102 draw more current from the respective power source than a maximum current limit, the chiller may trip a fuse or circuit breaker to avoid overheating or other damage. In order to avoid this outcome, a user may indicate a current limit (e.g., a user current limit 136). Regardless of the power source, the chiller 102 is controlled to operate within the user current limit 136.

[0048] An amount of the cooling demand value of the chiller 102 corresponds to the current drawn by the chiller 102. Other components or factors may also increase the current draw. As noted above, one way to adjust the cooling demand value of the chiller 102, and therefore adjust the amount of current drawn from the respective power source, is by adjusting the output capacity of the fluid handling motor(s) of the compressor(s) 104 with the current limiting function 138. Adjusting the output capacity of the fluid handling motor(s) of the compressor(s) 104 is an effective way to limit the current drawn by the chiller 102, because the fluid handling motor(s) of the compressor(s) 104 draw the greatest amount of current to operate of all the components of the chiller 102. Therefore, when the chiller 102 must operate within the user current limit 136, the control system 106 is configured to adjust a cooling demand value such that the chiller 102 limits the amount of current drawn from the respective power source. The control system 106 is, therefore, configured to adjust the cooling demand value via the current limiting function 138 by adjusting the output capacity of the first fluid handling motor of the first compressor 104a, the second fluid handling motor of the second compressor 104b, or both.

[0049] FIG. 8 illustrates a method of controlling the chiller 102 according to a user current limit 136. In some embodiments, the user current limit 136 may be set due to the sizing of the electrical mains infrastructure on site. In further embodiments, the user current limit 136 may depend on the respective power source being utilized in a given moment. For instance, the backup power source 164 may require a user current limit 136 that is lower than what is permissible with the primary power source 162.

[0050] At step 901, a predetermined current limit is set by a user as the user current limit 136. In some embodiments, the predetermined current limit may be based at least in part on whether a backup power source 164 is in use. The user can set the predetermined current limit manually using the display 124 of the chiller 102. Alternatively, the predetermined current limit can be set by the BMS 126.

[0051] At step 902, the current drawn by the chiller 102 is measured to determine a measured current. In the illustrated embodiment, the energy meter 130 of the control system 106 measures the current drawn by the chiller 102. Moreover, in the illustrated embodiment, since the power source 162, 164 is a three-phase power source, a current of each of the three phases is measured by the energy meter 130. The highest current of the three phases is used to determine the maximum current drawn by the chiller 102. This current selection feature protects the on-site infrastructure against peak current. In other embodiments, the controller 128 may calculate an average current based on the current of each of the three phases. Then, the calculated average may be used to determine the current drawn by the chiller 102.

[0052] At step 903, the measured current is compared, via the controller 128, to the predetermined current limit. The controller 128 determines a cooling demand offset based on comparing the measured current to the predetermined current limit. The cooling demand offset correlates to how much the cooling demand must be reduced and is based on the value of the measured current relative to the predetermined current. In the illustrated embodiment, the cooling demand offset is expressed as a percentage. Stated another way, the controller 128 determines a reduced cooling demand value, which is the cooling demand reduced by the cooling demand offset.

[0053] At step 904, if the controller 128 determines that the measured current is at least equal to the predetermined current limit, the cooling demand of the chiller 102 is reduced by the controller 128 by the cooling demand offset to the reduced cooling demand value. In the illustrated embodiment, the controller 128 reduces the cooling demand, while the chiller 102 is currently operating, by a first cooling demand offset to reduce the current drawn by the chiller 102 to a first reduced cooling demand value. In the illustrated embodiment, the first cooling demand offset is the smallest cooling demand offset relative to the current cooling demand. The controller 128 also determines the change in the fluid handling motor(s) of the compressor(s) 104 required to achieve the required reduction in cooling demand (e.g.. the cooling demand offset). For example, if the chiller is operating at the 100% cooling demand, the first cooling demand offset is 10% such that the cooling demand value will drop from 100% to 90%.

[0054] At step 905, the controller 128 also reduces an output capacity of at least one of the fluid handling motors of the compressors 104 to achieve the required reduction in cooling demand. In other words, the controller 128 reduces an output capacity of at least one of the fluid handling motors of the compressors 104 to reduce the cooling demand by the first cooling demand offset. The controller 128 uses the required change in the fluid handling motor(s) of the compressor(s) 104 to determine how to unload the fluid handling motor(s) of the compressor(s) 104. In embodiments with two compressors 104, based on the required change, the controller 128 reduces the output capacity of the first fluid handling motor of the first compressor 104a, the second fluid handling motor of the second compressor 104b, or both from a first load to a second load to achieve the required reduction in cooling demand to meet the reduced cooling demand value.

[0055] If the controller 128 determines that the measured current is greater than or equal to the predetermined current limit by a predetermined current differential value, then the reduced cooling demand value will be set to 0% (e.g., a cooling demand offset of 100%) and the fluid handling motor(s) of the compressor(s) 104 will shut off (e.g. an output capacity of 0%). In other words, the sum of the predetermined current limit and the predetermined current differential value sets a maximum value at which the chiller 102 will not operate because the fluid handling motor(s) of the compressor(s) 104 will shut off. In such case the cooling demand offset is 100%.

[0056] The current drawn by the chiller 102 is continually measured by the energy meter 130. Therefore, once the cooling demand value is reduced, at step 906, the current drawn by the chiller 102 is again measured to determine the measured current. The measured current is again compared to the predetermined current limit at step 907. At step 908, if the measured current has increased, the controller 128 determines an updated cooling demand offset (e.g., a second cooling demand offset), further reduces the cooling demand of the chiller by the second cooling demand offset to an updated reduced cooling demand value (e.g., a second reduced cooling demand value), and further reduces the output capacity of the fluid handling motor(s) of the compressor(s) 104 to reach the updated reduced cooling demand value. While a second cooling demand offset is discussed above, the cooling demand offset is a dynamic value which changes continually depending on the current draw of the chiller 102, so embodiments disclosed herein may include a series of updating cooling demand offset values (e.g., second, third, fourth, fifth, and so on). This cooling demand offset value may continuously change according to the live current draw measured by, for instance, a power meter.

[0057] At step 909, if the measured current has not increased but has not decreased by the predetermined current differential value, the controller maintains the cooling demand value of the chiller 102 and maintains an output capacity of the fluid handling motor(s) of the compressor(s) 104.

[0058] At step 910, if the measured current has decreased by a predetermined current differential value, the cooling demand offset is reduced, the cooling demand value of the chiller 102 is increased, and the output capacity of the respective fluid handling motor(s) of the compressor(s) 104 is increased.

[0059] For example, if the predetermined current limit is set to 800 Amperes and the measured current drawn by the chiller 102 is determined to be at least 800 Amperes, the controller 128 determines that the cooling demand value must be reduced by a first (minimum) cooling demand offset of 10%. Accordingly, a 10% reduction in the cooling demand value (e.g., cooling demand offset) would require the fluid handling motor(s) of the compressor(s) 104 to reduce the cooling demand to 90%. To achieve this cooling demand, the output capacity of one of the fluid handling motors of the compressors 104 would be reduced incrementally according to the first-in-last-out methodology described above. The fluid handling motors of the compressors 104 will continue to reduce the output capacity incrementally as the drawn current reaches 800 or more Amperes. Of course, this example assumes the cooling demand is initially 100%, but other values may be contemplated herein.

[0060] In some embodiments, once the cooling demand value has been reduced, the current drawn by the chiller 102 is again measured to determine the measured current. The measured current is again compared to the predetermined current limit. If the measured current has increased beyond 800 Amperes, for example, the controller 128 determines a second cooling demand offset proportional to the value of the current as measured between 800A and 880A, further reduces the cooling demand value of the chiller by the second cooling demand offset, and further reduces the output capacity of at least one of the fluid handling motor(s) of the compressor(s) 104. Otherwise, if the measured current has not increased but has not decreased by the predetermined current differential value of 80 Amperes, the controller 128 maintains the cooling demand value of the chiller at 90% and maintains an output capacity of the fluid handling motor(s) of the compressor(s) 104 to maintain the reduced cooling demand value of the chiller 102 at 90%. Otherwise, if the measured current has decreased by a predetermined current differential value, the cooling demand value of the chiller 102 is increased back to the previous cooling demand value of 100%, and the output capacity of the fluid handling motor(s) of the compressor(s) 104 is increased.

[0061] In some embodiments, the compressor(s) 104 include an oil-free centrifugal type compressor. In embodiments utilizing an oil-free centrifugal type compressor, controlling the cooling demand value of the chiller 102 can be done by varying the speed of the fluid handling motor(s) of the compressor(s) and / or changing the position of the inlet guide vanes of the compressor(s). The compressor(s) of these embodiments may include a built-in current limiting function. By changing the value of this variable, the compressor(s) dynamically adjust fluid handling motor speed and compressor inlet guide vane position to achieve a maximum cooling capability within a maximum current limit. 10062] Such embodiments may include a control system 206 like the one in FIG. 9. The control system 206 includes, among other features, a controller 228 and an energy meter 230. The controller 228 is in communication with the energy meter 230. In the illustrated embodiment of FIG. 9, the energy meter 230 is configured to measure an actual, or live, power 232 drawn by the chiller 102 (in, for instance, kW). In some embodiments, the controller 228 may also be in communication with one or more of the first compressor 104a, the second compressor 104b, the evaporator 108, the condenser 112, and the expansion valve 110, thereby receiving signals relating to the chiller operation 234. For instance, the on-board controllers of the compressor(s) may report to the controller 228 their current draw or power consumption. In some embodiments, the input power of the compressor(s) is provided dynamically to the chiller control system 206 by a compressor interface 266.

[0063] A user may specify a power limit setpoint 236 for the chiller 102 in Kilowatts. The value of the power limit setpoint 236 can be determined by the limits of the power source 162, 164 and can be provided to the chiller control system 206 by the BMS 126. The control system 206 is able to limit the power draw 232 of the chiller 102. The controller 228 calculates the maximum current draw allowed for each compressor to remain within the total input power limit in view of the number of active compressors and the power limit setpoint 236, which sets the speed adjustment value 238 of each of the compressors. In some embodiments, the compressors determine internally what speed to perform within the given current limit (e.g., limit fluid handling motor speed, adjust IGV position, etc.), and the controller 228 only indirectly causes that speed change. Power limiting in this manner allows the control system 206 to compensate for environmental changes that can affect input power. The operation of this algorithm can be enabled or disabled by the BMS 126 such that it only runs when the backup power source 164 is in use.

[0064] In some embodiments, the power used by the chiller 102 is measured, but the power used by each compressor 104 of the system 100 is also measured. The power used by the compressors 104 is subtracted from the total power used by the chiller 102 in order to determine the amount of power used by the non-compressor components of the chiller 102 (e.g., a fixed power). Fixed Power ~ Total Input Power - (Comp 1 Power + Camp 2 Power + + Comp n Power)

[0065] The amount of power available to each compressor 104 (compressor power limit) is determined by subtracting the fixed power from the power limit setpoint 236 specified by the user and dividing that difference by the number of active compressors 104. Power Limit Setpoint - Fixed Power Comp Power Limit - ...................,..............■............................................................ Number 0( Active Compressors

[0066] This compressor power limit is then converted into a compressor current limit with the formula below: 1000 x Comp Power Limit Cmnp Current Limit ~ ---------------=------------- Input Voltage x x Power Factor

[0067] Once the current limit for each compressor 104 is determined, the per-compressor current limit can be input into the controller aboard each compressor 104. In embodiments having compressors 104 with their own controllers, the on-board controllers operate to prevent a current draw of the particular compressor 104 from exceeding a predetermined maximum value (e.g., the current limit).

[0068] Various additional features and advantages of the invention are set forth in the following claims.

[0069] When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

Claims

What is claimed is:

1. A method of controlling a refrigeration system, the method comprising:obtaining a measured current drawn by the refrigeration system;comparing the measured current to a predetermined current limit;calculating a reduced cooling demand value compared to a cooling demand of the refrigeration system if the measured current is greater than or equal to the predetermined current limit; andreducing an output capacity of a fluid handling motor based on the reduced cooling demand value to reduce the current drawn by the refrigeration system.

2. The method of claim 1, wherein the fluid handling motor is part of a compressor.

3. The method of claim 1, wherein the fluid handling motor is part of a fan.

4. The method of claim any preceding claim, whereinthe refrigeration system cools a cooling medium configured to cool a remote area,the cooling medium has a measured return temperature as it is delivered from the remote area to the refrigeration system, andthe cooling demand of the refrigeration system is based on a comparison of the measured return temperature to a predetermined return temperature.

5. The method of claim 4, whereinthe cooling medium has a measured supply temperature as it is delivered from the refrigeration system to the remote area, andthe cooling demand of the refrigeration system is further based on a comparison of the measured supply temperature to a predetermined supply temperature.

6. The method of any preceding claim, further comprisingafter reducing the output capacity of the fluid handling motor, obtaining an updated measured current drawn by the refrigeration system,comparing the updated measured current to the predetermined current limit,if the updated measured current is greater than or equal to the predetermined current limit, calculating an updated reduced cooling demand value that is lower than the reduced cooling demand value, andfurther reducing the output capacity of the fluid handling motor based on the updated reduced cooling demand value.

7. The method of claim 6, further comprisingif the updated measured current is lower than the predetermined current limit by a predetermined current differential value, increasing the output capacity of the fluid handling motor based on the cooling demand of the refrigeration system.

8. The method of any preceding claim, wherein calculating the reduced cooling demand value includes scaling the reduced cooling demand value based on a magnitude of a difference between the measured current and the predetermined current limit.

9. The method of claim 8, wherein, if the measured current is equal to the predetermined current limit, the reduced cooling demand value is 90% of the cooling demand.

10. The method of claim 8, wherein, if the measured current is greater than the predetermined current limit by at least a predetermined current differential value, the reduced cooling demand value is 0% of the cooling demand and the fluid handling motor is shut down.

11. A refrigeration system comprising:a fluid handling motor; anda control system includingan energy meter, the energy meter configured to measure a current drawn by the refrigeration system; anda controller in communication with the energy meter and the fluid handling motor, the controller configured toreceive a command to limit current to a predetermined current limit, compare the measured current to the predetermined current limit, if the measured current is greater than or equal to the predetermined current limit, calculate a reduced cooling demand value compared to a cooling demand of the refrigeration system, andreduce an output capacity of the fluid handling motor based on the reduced cooling demand value to reduce the current drawn by the refrigeration system.

12. The refrigeration system of claim 11, wherein the fluid handling motor is part of a compressor.

13. The refrigeration system of claim 12, wherein the compressor includes a screw compressor.

14. The refrigeration system of claim 12, wherein the compressor includes a centrifugalcompressor.

15. The refrigeration system of claim 11, wherein the fluid handling motor is part of a fan.

16. The refrigeration system of any one of claims 11 to 15, wherein the controller is furtherconfigured toafter reducing the output capacity of the fluid handling motor, compare an updated measured current obtained by the energy meter to the predetermined current limit,if the updated measured current is greater than or equal to the predetermined current limit, calculate an updated reduced cooling demand value that is lower than the reduced cooling demand value, andfurther reduce the output capacity of the fluid handling motor based on the updated reduced cooling demand value.

17. The refrigeration system of claim 16, wherein the controller is further configured to increase the output capacity of the fluid handling motor based on the cooling demand of the refrigeration system if the updated measured current is lower than the predetermined current limit by a predetermined current differential value.

18. The refrigeration system of any one of claims 11 to 17, wherein the reduced cooling demand value is scaled based on a magnitude of a difference between the measured current and the predetermined current limit.

19. A method of controlling a refrigeration system, the method comprising: obtaining a measured current drawn by the refrigeration system; comparing the measured current to a predetermined current limit; determining a cooling demand offset based on a comparison of the measured current to the predetermined current limit;reducing a cooling demand value of the refrigeration system by the cooling demand offset; andreducing an output capacity of at least one of a first fluid handling motor and a second fluid handling motor of the refrigeration system.

20. The method of claim 19, wherein:the first fluid handling motor is part of a first compressor and the second fluid handling motor is part of a second compressor, orthe first fluid handling motor is part of a first fan and the second fluid handling motor is part of a second fan.

21. The method of claim 19 or 20, wherein the cooling demand offset is greater as the amount by which measured current exceeds the predetermined current limit is greater.

22. The method of any one of claims 19 to 21, further comprising, after reducing the output capacity of at least one of the first fluid handling motor and the second fluid handling motor, obtaining an updated measured current drawn by the refrigeration system, comparing the updated measured current to the predetermined current limit, and if the updated measured current is greater than or equal to the predetermined current limit, determining a second cooling demand offset based on a comparison of the updated measured current to the predetermined current limit;reducing the cooling demand value of the refrigeration system by the second cooling demand offset; andfurther reducing the output capacity of at least one of the first fluid handling motor and the second fluid handling motor of the refrigeration system.

23. The method of claim 22, further comprising maintaining the output capacity of one of the first fluid handling motor and the second fluid handling motor to operate the refrigeration system at the cooling demand value reduced by the cooling demand offset.

24. The method of claim 23, further comprising reducing the output capacity of both the first fluid handling motor and the second fluid handling motor to operate the refrigeration system at the cooling demand value reduced by the second cooling demand offset.

25. The method of any one of claims 19 to 24, wherein reducing the output capacity of at least one of the first fluid handling motor and the second fluid handling motor includes first reducing the output capacity of the shorter running of the first fluid handling motor and the second fluid handling motor before reducing the output capacity of the longer running of the first fluid handling motor and the second fluid handling motor.

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