Patent Publication Number: US-11656016-B2

Title: Cooling system that comprises multiple cooling apparatus and reduces power consumption

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is based on Japanese Patent Application No. 2019-209956 filed on Nov. 20, 2019, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a cooling system which includes a first cooling apparatus and a plurality of second cooling apparatuses to which a heat transfer medium is supplied from the first cooling apparatus. 
     Description of the Related Art 
     Conventionally, there has been known a cooling system which includes a plurality of cooling apparatuses for cooling a plurality of heat generating elements, respectively, piping connected to the plurality of cooling apparatuses, and a pump connected to the piping and in which a refrigerant is supplied through the piping to cooling pipes extending through the interiors of the plurality of cooling apparatuses (see Japanese Patent Application Laid-Open (kokai) No. 2015-183993). In this cooling system, the flow rate of the refrigerant supplied to each cooling apparatus is optimized so as to prevent excessive operation of the pump, thereby reducing the power consumption thereof. 
     Although the cooling system disclosed in Japanese Patent Application Laid-Open No. 2015-183993 reduces the power consumption of the pump, the cooling system is not designed in consideration of power consumption of an apparatus for cooling the refrigerant, and therefor has room for improvement, in terms of reducing the power consumption of the entire cooling system. 
     SUMMARY 
     One or more embodiments of the present invention provide a cooling system which includes a first cooling apparatus and a plurality of second cooling apparatuses to which a heat transfer medium is supplied from the first cooling apparatus and which can reduce the power consumption of the entire cooling system. 
     One or more embodiments provide a cooling system which includes a first cooling apparatus, a plurality of second cooling apparatuses, and a setting section (i.e., processor). The first cooling apparatus is electrically driven so as to supply a first heat transfer medium cooled to a first set temperature or lower. Each second cooling apparatus includes a heat exchange section for heat exchange between the first heat transfer medium supplied from the first cooling apparatus and a second heat transfer medium. Each second cooling apparatus is electrically driven so as to supply a third heat transfer medium cooled to a second set temperature or lower, the second set temperature being changed individually with lapse of time. The setting section obtains the second set temperatures of the plurality of second cooling apparatuses and variably sets the first set temperature on the basis of the plurality of obtained second set temperatures. 
     According to the above-described configuration, the first cooling apparatus is electrically driven so as to supply the first heat transfer medium cooled to the first set temperature or lower. Therefore, the first cooling apparatus consumes electrical power when cooling the first heat transfer medium to the first set temperature or lower. 
     Each second cooling apparatus is electrically driven so as to supply the third heat transfer medium cooled to the second set temperature or lower, the second set temperature being changed individually with lapse of time. Therefore, each second cooling apparatus consumes electrical power when cooling the third heat transfer medium to the second set temperature or lower. Meanwhile, each second cooling apparatus includes a heat exchange section for heat exchange between the first heat transfer medium supplied from the first cooling apparatus and the second heat transfer medium. Therefore, when cooling the third heat transfer medium, each second cooling apparatus can use thermal energy supplied from the first heat transfer medium to the second heat transfer medium through the heat exchange section. 
     The lower the first set temperature, the larger the power consumption of the first cooling apparatus. Meanwhile, the lower the first set temperature, the smaller the power consumptions of the plurality of second cooling apparatuses. The sum of the power consumption of the first cooling apparatus and the power consumptions of the plurality of second cooling apparatuses changes depending on the relation between the first set temperature and the individual second set temperatures of the plurality of second cooling apparatuses at each point in time. In view of this, the setting section obtains the second set temperatures of the plurality of second cooling apparatuses and variably sets the first set temperature on the basis of the obtained second set temperatures. Accordingly, the first set temperature can be changed appropriately in accordance with the plurality of second set temperatures at each point in time, whereby the power consumption of the entire cooling system can be reduced. 
     The greater the amounts by which the second set temperatures are lower than the first set temperature, the greater the power consumptions of the second cooling apparatuses. In addition, when the differences between the second set temperatures and the first set temperature increase, the power consumptions increase in a quadratic curve. 
     In view of this, in a second means, the setting section variably sets the first set temperature to the average of the plurality of obtained second set temperatures. By virtue of such a configuration, it is possible to prevent the differences between the second set temperatures and the first set temperature from becoming extremely large, and none of the power consumptions of the second cooling apparatuses become extremely large. Accordingly, the power consumption of the entire cooling system can be reduced. 
     In a third means, each of the second cooling apparatuses includes a compression section which is electrically driven so as to compress the second heat transfer medium in gas state, a bypass flow passage through which the second heat transfer medium flows while bypassing the compression section, and an on-off valve for opening and closing the bypass flow passage. The setting section variably sets the first set temperature to a stop temperature which is the first set temperature which allows a target second cooling apparatus, which is a second cooling apparatus whose second set temperature is the highest among the plurality of obtained second set temperatures, to supply the third heat transfer medium cooled to the highest second set temperature or lower in a state in which the compression section is stopped and the on-off valve is opened, and the setting section stops the compression section and opens the on-off valve in the target second cooling apparatus. 
     According to the above-described configuration, each second cooling apparatus includes a compression section which is electrically driven so as to compress the second heat transfer medium in gas state, a bypass flow passage through which the second heat transfer medium flows while bypassing the compression section, and an on-off valve for opening and closing the bypass flow passage. Therefore, when the first set temperature is sufficiently lower than the second set temperature of a second cooling apparatus, the second cooling apparatus can supply the third heat transfer medium cooled to the second set temperature or lower by means of so-called free cooling by stopping the compression section and opening the on-off valve. 
     In view of this, the setting section variably sets the first set temperature to a stop temperature which is the first set temperature which allows a target second cooling apparatus, which is a second cooling apparatus whose second set temperature is the highest among the plurality of obtained second set temperatures, to supply the third heat transfer medium cooled to the highest second set temperature or lower in a state in which the compression section is stopped and the on-off valve is opened. Subsequently, the setting section stops the compression section and opens the on-off valve in the target second cooling apparatus. Therefore, the target second cooling apparatus can supply the third heat transfer medium cooled to the corresponding second set temperature or lower by means of free cooling, whereby the power consumption of the target second cooling apparatus can be reduced greatly. For example, the stop temperature is a temperature obtained by subtracting a necessary temperature difference required for free cooling from the highest second set temperature. 
     As described above, the lower the first set temperature, the larger the power consumption of the first cooling apparatus. Therefore, in the case where the highest second set temperature is lower than a predetermined temperature, if the first set temperature is set to the stop temperature and the target second cooling apparatus is caused to perform free cooling, there arises a possibility that the power consumption of the first cooling apparatus becomes excessively large, and the power consumption of the entire cooling system cannot be reduced. 
     In order to avoid such a possibility, in a fourth means, on the condition that the highest second set temperature among the plurality of obtained second set temperatures is higher than a predetermined temperature, the setting section sets the first set temperature to the stop temperature, and stops the compression section and opens the on-off valve in the target second cooling apparatus. Namely, in the case where the highest second set temperature is higher than the predetermined temperature, the target second cooling apparatus performs free cooling, and, in the case where the highest second set temperature is lower than the predetermined temperature, the target second cooling apparatus does not perform free cooling. Accordingly, the power consumption of the entire cooling system can be reduced. 
     In a fifth means, each of the second cooling apparatuses includes a compression section which is electrically driven so as to compress the second heat transfer medium in gas state, a bypass flow passage through which the second heat transfer medium flows while bypassing the compression section, and an on-off valve for opening and closing the bypass flow passage. The setting section variably sets the first set temperature to a stop temperature which is the first set temperature which allows target second cooling apparatuses, which are second cooling apparatuses whose second set temperature is found in the largest number among the plurality of obtained second set temperatures, to supply the third heat transfer medium cooled to the largest-number second set temperature or lower in a state in which the compression section is stopped and the on-off valve is opened, and the setting section stops the compression section and opens the on-off valve in each of the target second cooling apparatuses. 
     According the above-described configuration, as in the third means, when the first set temperature is sufficiently lower than the second set temperature of a second cooling apparatus, the second cooling apparatus can perform free cooling. In view of this, the setting section variably sets the first set temperature to a stop temperature which is the first set temperature which allows target second cooling apparatuses, which are second cooling apparatuses whose second set temperature is found in the largest number among the plurality of obtained second set temperatures, to supply the third heat transfer medium cooled to the largest-number second set temperature or lower in a state in which the compression section is stopped and the on-off valve is opened. Subsequently, the setting section stops the compression section and opens the on-off valve in each of the target second cooling apparatuses. Therefore, the target second cooling apparatuses can supply the third heat transfer medium cooled to the corresponding second set temperature or lower by means of free cooling, whereby the power consumptions of the target second cooling apparatuses can be reduced greatly. For example, the stop temperature is a temperature obtained by subtracting a necessary temperature difference required for free cooling from the largest-number second set temperature. 
     As described above, the lower the first set temperature, the larger the power consumption of the first cooling apparatus. Therefore, in the case where the largest-number second set temperature is lower than a predetermined temperature, if the first set temperature is set to the stop temperature and the target second cooling apparatuses are caused to perform free cooling, there arises a possibility that the power consumption of the first cooling apparatus becomes excessively large, and the power consumption of the entire cooling system cannot be reduced. 
     In order to avoid such a possibility, in a sixth means, on the condition that the largest-number second set temperature among the plurality of obtained second set temperatures is higher than a predetermined temperature, the setting section sets the first set temperature to the stop temperature, and stops the compression section and opens the on-off valve in each of the target second cooling apparatuses. Namely, in the case where the largest-number second set temperature is higher than the predetermined temperature, the target second cooling apparatuses perform free cooling, and, in the case where the largest-number second set temperature is lower than the predetermined temperature, the target second cooling apparatuses do not perform free cooling. Accordingly, the power consumption of the entire cooling system can be reduced. 
     The power consumption of the cooling system changes depending on the combination of the first set temperature and the plurality of second set temperatures. The relation among the first set temperature, the plurality of second set temperatures, and the measured value of the power consumption of the cooling system can be obtained beforehand through, for example, an experiment. 
     In view of this, in a seventh means, the setting section variably sets the first set temperature, on the basis of a previously obtained relation among the first set temperature, the plurality of second set temperatures, and a measured value of power consumption of the cooling system, and the plurality of obtained second set temperatures, such that the power consumption of the cooling system becomes the minimum. By virtue of such a configuration, since the relation among the first set temperature, the plurality of second set temperatures, and the measured value of the power consumption of the cooling system is obtained beforehand, it is possible to reduce the power consumption of the entire cooling system while reducing the processing load of the setting section. 
     The setting section may be configured to learn a predetermined relation among the first set temperature, the plurality of second set temperatures, and the measured value of the power consumption of the cooling system through operation of the cooling system. 
     In view of this, in an eighth means, the setting section successively obtains the first set temperature, the plurality of second set temperatures, and a measured value of power consumption of the cooling system; learns a predetermined relation among the first set temperature, the plurality of second set temperatures, and the measured value of the power consumption of the cooling system; and variably sets the first set temperature on the basis of the learned predetermined relation and the plurality of obtained second set temperatures such that the power consumption of the cooling system becomes the minimum. By virtue of such a configuration, the setting section can learn the predetermined relation through operation of the cooling system. Therefore, as the learning progresses, the accuracy of the predetermined relation increases, whereby the power consumption of the entire cooling system can be reduced further. 
     The power consumption of the first cooling apparatus changes in accordance with the first set temperature. The relation between the first set temperature and the measured value of the power consumption of the first cooling apparatus can be obtained beforehand through, for example, an experiment. Also, the power consumptions of the second cooling apparatuses change depending on the combination of the first set temperature and the second set temperatures. The relation among the first set temperature, the plurality of second set temperatures, and the measured values of the power consumptions of the second cooling apparatuses can be obtained beforehand through, for example, an experiment. 
     In view of this, in a ninth means, the setting section variably sets the first set temperature, on the basis of a previously obtained first relation between the first set temperature and a measured value of the power consumption of the first cooling apparatus, a previously obtained second relation among the first set temperature, the second set temperatures, and measured values of power consumptions of the second cooling apparatuses, and the plurality of obtained second set temperatures, such that the sum of the power consumption of the first cooling apparatus and the power consumptions of the plurality of second cooling apparatuses becomes the minimum. By virtue of such a configuration, the power consumption of the first cooling apparatus and the power consumptions of the second cooling apparatuses can be calculated more accurately, whereby the power consumption of the entire cooling system can be reduced further. 
     The setting section may be configured to learn the first relation between the first set temperature and the measured value of the power consumption of the first cooling apparatus through operation of the cooling system. Also, the setting section may be configured to learn the second relation among the first set temperature, the second set temperatures, and the measured values of the power consumptions of the second cooling apparatuses through operation of the cooling system. 
     In view of this, in a tenth means, the setting section successively obtains the first set temperature and a measured value of power consumption of the first cooling apparatus; learns a first relation between the first set temperature and the measured value of the power consumption of the first cooling apparatus; successively obtains the first set temperature, the second set temperatures, and measured values of power consumptions of the second cooling apparatuses; learns a second relation among the first set temperature, the second set temperatures, and the measured values of the power consumptions of the second cooling apparatuses; and variably sets the first set temperature on the basis of the learned first and second relations and the plurality of obtained second set temperatures such that the power consumption of the cooling system becomes the minimum. By virtue of such a configuration, the setting section can learn the first and second relations through operation of the cooling system. Therefore, as the learning progresses, the accuracies of the first and second relations increase, whereby the power consumption of the entire cooling system can be reduced further. 
     In an eleventh means, the first cooling apparatus includes a first compression section which is electrically driven so as to compress a fourth heat transfer medium in gas state, a first heat exchange section for heat exchange between the fourth heat transfer medium and a fifth heat transfer medium cooled by atmospheric air, a first bypass flow passage through which the fourth heat transfer medium flows while bypassing the first compression section, and a first on-off valve for opening and closing the first bypass flow passage. In the case where the first cooling apparatus can supply the first heat transfer medium cooled to the first set temperature or lower in a state in which the first compression section is stopped and the first on-off valve is opened, the setting section stops the first compression section and opens the first on-off valve. 
     According to the above-described configuration, the first cooling apparatus includes a first compression section which is electrically driven so as to compress the fourth heat transfer medium in gas state, a first heat exchange section for heat exchange between the fourth heat transfer medium and the fifth heat transfer medium cooled by atmospheric air, a first bypass flow passage through which the fourth heat transfer medium flows while bypassing the first compression section, and a first on-off valve for opening and closing the first bypass flow passage. Therefore, when cooling the first heat transfer medium, the first cooling apparatus can use thermal energy supplied to the fourth heat transfer medium, through the first heat exchange section, from the fifth heat transfer medium cooled by atmospheric air. Accordingly, when the atmospheric temperature is sufficiently lower than the first set temperature, the first cooling apparatus can supply the first heat transfer medium cooled to the first set temperature or lower by means of free cooling by stopping the first compression section and opening the first on-off valve. 
     In view of this, in the case where the first cooling apparatus can supply the first heat transfer medium cooled to the first set temperature or lower in a state in which the first compression section is stopped and the first on-off valve is opened, the setting section stops the first compression section and opens the first on-off valve. Therefore, the first cooling apparatus can supply the first heat transfer medium cooled to the first set temperature or lower by means of free cooling, whereby the power consumption of the first cooling apparatus can be reduced greatly. For example, the first cooling apparatus performs free cooling in the case where the atmospheric temperature is lower than a temperature obtained by subtracting the necessary temperature difference required for free cooling from the first set temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present disclosure will be apparent from the following description made with reference to the accompanying drawings. 
         FIG.  1    is a schematic diagram of a cooling system according to a first embodiment; 
         FIG.  2    is a schematic diagram showing the relation among a first set temperature, second set temperatures, and power consumptions; and 
         FIG.  3    is a time chart showing the set temperatures and power consumptions of a high-capacity refrigerator and chillers. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     A first embodiment will now be described with reference to the drawings. The first embodiment is embodied as a cooling system including a plurality of chillers which supply heat transfer mediums of different set temperatures to a plurality of control targets, respectively. 
     As shown in  FIG.  1   , a cooling system  100  includes a high-capacity refrigerator  10 , chillers  40 A to  40 C, a setting section  70 , etc. 
     The high-capacity refrigerator  10  (first cooling apparatus) includes a compressor  11 , a condenser  16 , an expansion section  19 , an evaporator  21 , a bypass flow passage  27 , an on-off valve  28 , etc. 
     The compressor  11  (first compression section) is driven by a motor  12  and compresses a fourth heat transfer medium in gas state. The fourth heat transfer medium is, for example, a hydrofluorocarbon (HFC)-based refrigerant or a hydrofluoroolefin (HFO)-based refrigerant. The motor  12  is driven by electric power supplied thereto. Namely, the compressor  11  is electrically driven so as to compress the fourth heat transfer medium in gas state. The compressor  11  and the condenser  16  are connected to each other by a flow passage  13 . 
     The condenser  16  (first heat exchange section) condenses the gas-sate fourth heat transfer medium compressed by the compressor  11 . Specifically, the condenser  16  has a flow passage  17  with which the gas-state fourth heat transfer medium is brought into contact. One end (first end) of the flow passage  17  and the outlet port of a cooling tower  30  are connected to each other by a flow passage  31 . The other end (second end) of the flow passage  17  and the inlet port of the cooling tower  30  are connected to each other by a flow passage  32 . The cooling tower  30  cools a fifth heat transfer medium flowing thereinto through the flow passage  32  by using the atmospheric air and discharges the cooled fifth heat transfer medium through the flow passage  31 . The fifth heat transfer medium is, for example, water. Namely, in the condenser  16 , heat exchange is performed between the fourth heat transfer medium and the fifth heat transfer medium cooled by the atmospheric air. 
     The fourth heat transfer medium condensed by the condenser  16  is suppled to a flow passage  18 . The expansion section  19  is provided in the flow passage  18 . The expansion section  19  is an expansion valve or a capillary which expands the fourth heat transfer medium in liquid state, thereby atomizing the fourth heat transfer medium. The fourth heat transfer medium atomized by the expansion section  19  is supplied to the evaporator  21  through the flow passage  18 . 
     The evaporator  21  includes a flow passage  22 , etc. In the evaporator  21 , the atomized fourth heat transfer medium evaporates. A common flow passage  35  is connected to one end (first end) of the flow passage  22 , and a common flow passage  36  is connected to the other end (second end) of the flow passage  22 . A first heat transfer medium in liquid state flows through the flow passage  22  and the common flow passages  35  and  36 . The first heat transfer medium is, for example, water. By virtue of the configuration described above, the first heat transfer medium flowing through the flow passage  22  is cooled. The fourth heat transfer medium evaporated in the evaporator  21  is supplied to the compressor  11  through a flow passage  26 . 
     The bypass flow passage  27  (first bypass flow passage) causes the fourth heat transfer medium to flow from the flow passage  26  to the flow passage  13  while bypassing the compressor  11 . An on-off valve  28  (first on-off valve) for opening and closing the bypass flow passage  27  is provided in the bypass flow passage  27 . 
     The high-capacity refrigerator  10  cools the first heat transfer medium flowing through the flow passage  22  to a first set temperature Tw and supplies the cooled first heat transfer medium to the common flow passage  35 . A pump  39  is provided in the common flow passage  35 . The pump  39  causes the first heat transfer medium to circulate through the flow passage  22  and the common flow passages  35  and  36 . The operating state of the high-capacity refrigerator  10  is controlled by the setting section  70 . 
     The setting section  70  is a microcomputer including a CPU, a ROM, a RAM, an input/output interface, etc. The setting section  70  controls the drive states of the compressor  11  and the pump  23  such that the first heat transfer medium cooled to the first set temperature Tw is supplied to the common flow passage  35 . 
     Also, in the case where the high-capacity refrigerator  10  can supply the first heat transfer medium of the first set temperature Tw or lower in a state in which the compressor  11  is stopped and the on-off valve  28  is opened, the setting section  70  stops the compressor  11  and opens the on-off valve  28  (free cooling). For example, since the atmospheric temperature is low in winter, the temperature of the fifth heat transfer medium supplied from the cooling tower  30  to the condenser  16  becomes low. Accordingly, even in a state in which the compressor  11  is stopped, the first heat transfer medium flowing through the flow passage  22  of the evaporator  21  can be cooled, and the high-capacity refrigerator  10  can supply the first heat transfer medium cooled to the first set temperature Tw or lower. Notably, when the temperature of the fifth heat transfer medium supplied from the cooling tower  30  to the condenser  16  is lower than the first set temperature Tw by about 10° C. (necessary temperature difference) or more, the high-capacity refrigerator  10  can perform free cooling. 
     The chillers  40 A to  40 C (second cooling apparatuses) have the same configuration. Here, the chiller  40 A will be described. 
     The chiller  40 A includes a compressor  41 , a condenser  46 , an expansion section  49 , an evaporator  51 , a bypass flow passage  57 , an on-off valve  58 , etc. 
     The compressor  41  (compression section) is electrically driven so as to compress a second heat transfer medium in gas state. The second heat transfer medium is, for example, a hydrofluorocarbon (HFC)-based refrigerant or a hydrofluoroolefin (HFO)-based refrigerant. The compressor  41  and the condenser  46  are connected to each other by a flow passage  43 . 
     The condenser  46  (heat exchange section) condenses the gas-state second heat transfer medium compressed by the compressor  41 . Specifically, the condenser  46  has a flow passage  47  through which the gas-state second heat transfer medium flows. A flow passage  43  is connected to one end (first end) of the flow passage  47 , and a flow passage  48  is connected to the other end (second end) of the flow passage  47 . The condenser  46  has a flow passage  45  through which the liquid-state first heat transfer medium flows. A branch flow passage  37  is connected to one end (first end) of the flow passage  45 , and a branch flow passage  38  is connected to the other end (second end) of the flow passage  45 . The branch flow passage  37  is branched from the common flow passage  35 . The branch flow passage  38  is branched from the common flow passage  36 . In the condenser  46 , heat exchange is performed between the first heat transfer medium flowing through the flow passage  45  and the second heat transfer medium flowing through the flow passage  47 . 
     The second heat transfer medium condensed by the condenser  46  is supplied to the flow passage  48 . The expansion section  49  is provided in the flow passage  48 . The expansion section  49  is an expansion valve or a capillary which expands the second heat transfer medium in liquid state, thereby atomizing the second heat transfer medium. The second heat transfer medium atomized by the expansion section  49  is supplied to the evaporator  51  through the flow passage  48 . 
     The evaporator  51  includes a flow passage  52 , a flow-through section  53 , etc. The atomized second heat transfer medium flows through the flow passage  52 . The flow passage  48  is connected to one end (first end) of the flow passage  52 , and a flow passage  54  is connected to the other end (second end) of the flow passage  52 . The flow passage  52  and the compressor  41  are connected to each other by the flow passage  54 . The flow passage  52  is accommodated in the flow-through section  53 . Flow passages  55  and  56  are connected to the flow-through section  53 . A third heat transfer medium flows through the flow-through section  53 , the flow passage  55 , and the flow passage  56 . The third heat transfer medium is, for example, a liquid composed of ethylene glycol (60%) and water (40%). A pump  59  is provided in the flow passage  55 . The pump  59  causes the third heat transfer medium to circulate through the flow-through section  53  and the flow passages  55  and  56 . By virtue of the configuration described above, the second heat transfer medium flowing through the flow passage  52  evaporates, and thus the third heat transfer medium flowing through the flow-through section  53  is cooled. The second heat transfer medium evaporated in the evaporator  51  is supplied to the compressor  41  through the flow passage  54 . 
     The bypass flow passage  57  (bypass flow passage) causes the second heat transfer medium to flow from the flow passage  54  to the flow passage  43  while bypassing the compressor  41 . An on-off valve (on-off valve) for opening and closing the bypass flow passage  57  is provided in the bypass flow passage  57 . 
     The chiller  40 A cools the third heat transfer medium flowing through the flow-through section  53  to a second set temperature Ta and supplies the cooled third heat transfer medium to the flow passage  55 . The operation state of the chiller  40 A is controlled by the setting section  70 . The setting section  70  controls the drive state of the compressor  41  such that the third heat transfer medium cooled to the second set temperature is supplied to the flow passage  55 . 
     Also, in the case where the chiller  40 A can supply the third heat transfer medium of the second set temperature Ta or lower in a state in which the compressor  41  is stopped and the on-off valve  58  is opened, the setting section  70  stops the compressor  41  and opens the on-off valve  58  (free cooling). Notably, when the temperature of the first heat transfer medium supplied from the common flow passage  35  to the condenser  46  is lower than the second set temperature Ta by about 10° C. (necessary temperature difference) or more, the chiller  40 A can perform free cooling. 
     Similarly, the chiller  40 B cools the third heat transfer medium flowing through the flow-through section  53  of the chiller  40 B to a second set temperature Tb and supplies the cooled third heat transfer medium to the flow passage  55  of the chiller  40 B, and the chiller  40 C cools the third heat transfer medium flowing through the flow-through section  53  of the chiller  40 C to a second set temperature Tc and supplies the cooled third heat transfer medium to the flow passage  55  of the chiller  40 C. Also, in the case where the chiller  40 B ( 40 C) can supply the third heat transfer medium of the second set temperature Tb (Tc) or lower in a state in which the compressor  41  is stopped and the on-off valve  58  is opened, the setting section  70  stops the compressor  41  and opens the on-off valve  58  (free cooling). 
     The temperatures of control targets A to C to which the chillers  40 A to  40 C supply the third heat transfer medium are controlled by control sections A to C (not shown), respectively. The second set temperatures Ta to Tc are set by the control sections A to C, respectively. The second set temperatures Ta to Tc are individually changed by the control sections A to C, respectively, with lapse of time. 
       FIG.  2    is a schematic diagram showing the relation among the first set temperature Tw, the second set temperatures Ta to Tc, and power consumptions. The area of each arrow shows the magnitude of power consumption. Temperature Tt 2  is the temperature of the fifth heat transfer medium flowing from the high-capacity refrigerator  10  into the cooling tower  30 . Temperature Tt 1  is the temperature of the fifth heat transfer medium flowing from the cooling tower  30  into the high-capacity refrigerator  10 . 
     As shown in  FIG.  2   , the lower the first set temperature Tw, the larger the power consumption Po of the high-capacity refrigerator  10 . Meanwhile, the lower the first set temperature Tw, the smaller the power consumptions Pa to Pc of the chillers  40 A to  40 C. The chiller  40 C can perform free cooling and does not consume electrical power (i.e., power consumption Pc=0). The sum of the power consumption Po of the high-capacity refrigerator  10  and the power consumptions Pa to Pc of the chillers  40 A to  40 C changes depending on the relation between the first set temperature Tw and the individual second set temperatures Ta to Tc of the chillers  40 A to  40 C at each point in time. 
     In view of the foregoing, the setting section  70  obtains the second set temperatures Ta to Tc of the chillers  40 A to  40 C from the above-described control sections A to C, and variably sets the first set temperature Tw on the basis of the obtained second set temperatures Ta to Tc. 
       FIG.  3    is a time chart showing the set temperatures and power consumptions of the high-capacity refrigerator  10  and the chillers  40 A to  40 C. 
     At time t 1 , the first set temperature Tw of the high-capacity refrigerator  10  is 7° C., and the power consumption Po of the high-capacity refrigerator  10  is 20 kW. The second set temperatures Ta to Tc of the chillers  40 A to  40 C are −20° C., −10° C., −15° C., respectively, and the power consumptions Pa, Pb, and Pc of the chillers  40 A to  40 C are 10 kW, 6 kW, and 8 kW, respectively. The power consumption of the entire cooling system  100  is 44 kW, which is the sum of the power consumption Po and the power consumptions Pa to Pc. At that time, the first set temperature Tw of 7° C. is not a first set temperature Tw (stop temperature Tf) which allows the chiller  40 B (target chiller, target cooling apparatus), whose second set temperature Tb (=−10° C.) is the highest among the second set temperatures Ta to Tc, to supply the third heat transfer medium of the second set temperature Tb (=−10° C.) or lower in a state in which the compressor  41  is stopped and the on-off valve  58  is opened. The stop temperature Tf is a temperature equal to or lower than a temperature obtained by subtracting the above-described necessary temperature difference (for example, 10° C.) from the highest second set temperature. The stop temperature Tf in this case is about −20° C. or lower. 
     In the case where the highest second set temperature Tb is lower than a predetermined temperature (for example, 0° C.), if the first set temperature Tw is set to the stop temperature Tf and the chiller  40 B performs free cooling, there arises a possibility that the power consumption Po of the high-capacity refrigerator  10  becomes excessively large, and the power consumption Pt of the entire cooling system  100  cannot be reduced. In order to avoid such a possibility, on the condition that the highest one of the second set temperatures Ta to Tc obtained from the control section A to C is higher than the predetermined temperature, the setting section  70  sets the first set temperature Tw to the stop temperature Tf, and stops the compressor  41  and opens the on-off valve  58  in the target chiller (free cooling). At time t 1 , the first set temperature Tw is not set to the stop temperature Tf and free cooling is not performed. At time t 1 , the first set temperature Tw is set to 7° C. at which the operation efficiency of the high-capacity refrigerator  10  becomes the maximum. Notably, the first set temperature Tw at which the operation efficiency of the high-capacity refrigerator  10  becomes the maximum changes depending on, for example, the atmospheric temperature and thus the temperature of the fifth heat transfer medium supplied from the cooling tower  30  to the high-capacity refrigerator  10 . 
     At time t 2 , the second set temperature Tb is changed to 25° C. Therefore, the first set temperature Tw of 7° C. becomes the first set temperature Tw (stop temperature Tf) which allows the chiller  40 B (target chiller), whose second set temperature Tb (=25° C.) is the highest among the second set temperatures Ta to Tc, to supply the third heat transfer medium of the second set temperature Tb (=25° C.) or lower in the state in which the compressor  41  is stopped and the on-off valve  58  is opened. Notably, the stop temperature Tf in this case is about 15° C. or lower. The highest second set temperature Tb (=25° C.) among the second set temperatures Ta to Tc is higher than the predetermined temperature (for example, 0° C.). In view of this, the setting section  70  stops the compressor  41  and opens the on-off valve  58  in the chiller  40 B, so that the chiller  40 B performs free cooling. As a result, the power consumptions Pa to Pc of the chillers  40 A to  40 C are 10 kW, 0 kW, and 8 kW, respectively, the power consumption Po of the high-capacity refrigerator  10  is 18 kW, and the power consumption Pt of the cooling system  100  is 36 kW. 
     At time t 3 , the second set temperature Tc is changed to 28° C. The setting section  70  sets the first set temperature Tw to a first set temperature Tw (=15° C.) (second stop temperature Tf 2 ) which allows the chiller  40 B, whose second set temperature Tb (=25° C.) is the second highest among the second set temperatures Ta to Tc, to supply the third heat transfer medium of the second set temperature Tb (=25° C.) or lower in the state in which the compressor  41  is stopped and the on-off valve  58  is opened. Notably, the second stop temperature Tf 2  in this case is about 15° C. or lower. The second highest second set temperature Tb (=25° C.) among the second set temperatures Ta to Tc is higher than the predetermined temperature (for example, 0° C.). Moreover, the chiller  40 C whose second set temperature Tc (=28° C.) is the highest among the second set temperatures Ta to Tc also satisfies the condition for performing free cooling. Therefore, the setting section  70  stops the compressor  41  and opens the on-off valve  58  in the chiller  40 B, so that the chiller  40 B performs free cooling, and the setting section  70  stops the compressor  41  and opens the on-off valve  58  in the chiller  40 C, so that the chiller  40 C performs free cooling. As a result, the power consumptions Pa to Pc of the chillers  40 A to  40 C are 13 kW, 0 kW, and 0 kW, respectively, the power consumption Po of the high-capacity refrigerator  10  is 16 kW, and the power consumption Pt of the cooling system  100  is 29 kW. 
     The present embodiment having been described in detail above has the following advantages. 
     The setting section  70  obtains the second set temperatures Ta to Tc of the chillers  40 A to  40 C and variably sets the first set temperature Tw on the basis of the obtained second set temperatures Ta to Tc. Accordingly, the first set temperature Tw can be changed appropriately in accordance with the second set temperatures Ta to Tc at each point in time, whereby the power consumption Pt of the entire cooling system  100  can be reduced. 
     The setting section  70  variably sets the first set temperature Tw to the stop temperature Tf which is a first set temperature Tw which allows a target chiller, which is a chiller whose second set temperature is the highest among the obtained second set temperatures Ta to Tc, to supply the third heat transfer medium of the highest second set temperature or lower in the state in which the compressor  41  is stopped and the on-off valve  58  is opened. Subsequently, the setting section  70  stops the compressor  41  and opens the on-off valve  58  in the target chiller. Therefore, the target chiller can supply the third heat transfer medium cooled to the corresponding second set temperature or lower by means of free cooling, whereby the power consumption of the target chiller can be reduced greatly. 
     On the condition that the highest second set temperature among the obtained second set temperatures Ta to Tc is higher than a predetermined temperature, the setting section  70  sets the first set temperature Tw to the stop temperature Tf, and stops the compressor  41  and opens the on-off valve  58  in the target chiller. Namely, in the case where the highest second set temperature is higher than the predetermined temperature, the target chiller performs free cooling, and, in the case where the highest second set temperature is lower than the predetermined temperature, the target chiller does not perform free cooling. Accordingly, the power consumption of the entire cooling system  100  can be reduced. 
     The setting section  70  variably sets the first set temperature Tw to the second stop temperature Tf 2  which is a first set temperature Tw which allows a second target chiller, which is a chiller whose second set temperature is the second highest among the obtained second set temperatures Ta to Tc, to supply the third heat transfer medium of the second highest second set temperature or lower in the state in which the compressor  41  is stopped and the on-off valve  58  is opened. Subsequently, the setting section  70  stops the compressor  41  and opens the on-off valve  58  in the second target chiller. Therefore, the second target chiller can supply the third heat transfer medium cooled to the corresponding second set temperature or lower by means of free cooling, whereby the power consumption of the second target chiller can be reduced greatly. 
     Second Embodiment 
     A second embodiment will now be described. In the following description, the difference between the second embodiment and the first embodiment will be mainly described. Notably, portions identical with those of the first embodiment are denoted by the same reference numerals, and their description will not be repeated. 
     The greater the amounts by which the second set temperatures Ta to Tc are lower than the first set temperature Tw, the greater the power consumptions of the chillers  40 A to  40 C. In addition, when the differences between the second set temperatures Ta to Tc and the first set temperature Tw increase, the power consumptions Pa to Pc increase in a quadratic curve. Therefore, setting the first set temperature Tw to an excessively high temperature is not desired. 
     In view of this, the setting section  70  variably sets the first set temperature Tw to the average of the second set temperatures Ta to Tc obtained from the control sections A to C. For example, at time t 3  of  FIG.  3   , the setting section  70  sets the first set temperature Tw to the average (=11° C.) of the second set temperatures Ta to Tc obtained from the control sections A to C. In this case as well, the chillers  40 B and  40 C satisfy the condition of performing free cooling. Therefore, the setting section  70  stops the compressor  41  and opens the on-off valve  58  in each of the chillers  40 B and  40 C, so that the chillers  40 B and  40 C perform free cooling. 
     The present embodiment having been described in detail above has the following advantages. Notably, only the advantages different from those of the first embodiment will be described. 
     Since it is possible to prevent the differences between the second set temperatures Ta to Tc and the first set temperature Tw from becoming extremely large, none of the power consumptions of the chillers become extremely large. Accordingly, the power consumption Pt of the entire cooling system  100  can be reduced. 
     Notably, the setting section  70  may set the first set temperature Tw to the average of the second set temperatures Ta to Tc obtained from the control sections A to C on the condition that the average of the second set temperatures Ta to Tc is higher than a predetermined temperature (for example, 0° C.). By virtue of such a configuration, the power consumption Po of the high-capacity refrigerator  10  can be prevented from becoming excessively large. 
     Also, the bypass flow passage  57  and the on-off valve  58  in each of the chillers  40 A to  40 C may be omitted. In such a case, the chillers  40 A to  40 C do not perform free cooling. 
     Third Embodiment 
     A third embodiment will now be described. In the following description, the difference between the third embodiment and the first embodiment will be mainly described. Notably, portions identical with those of the first embodiment are denoted by the same reference numerals, and their description will not be repeated. 
     In the present embodiment, as shown by broken lines at the right end of  FIG.  1   , the cooling system  100  includes several hundred or more chillers in addition to the chillers  40 A to  40 C. The branch flow passage  37  of each chiller  40  is connected to the common flow passage  35 , and the branch flow passage  38  of each chiller is connected to the common flow passage  36 . 
     The setting section  70  variably sets the first set temperature Tw to a stop temperature Tf which is a first set temperature Tw which allows target chillers (target second cooling apparatuses), which are chillers whose second set temperature is found in the largest number among the second set temperatures Ta, Tb, Tc, . . . obtained from the control sections A, B, C, . . . , to supply the third heat transfer medium of the largest-number second set temperature or lower in the state in which the compressor  41  is stopped and the on-off valve  58  is opened. Subsequently, the setting section  70  stops the compressor  41  and opens the on-off valve  58  in the target chillers. 
     As described above, the lower the first set temperature Tw, the larger the power consumption Po of the high-capacity refrigerator  10 . Therefore, in the case where the largest-number second set temperature is lower than the predetermined temperature (for example, 0° C.), if the first set temperature Tw is set to the stop temperature Tf (for example, lower than −10° C.) and the target chillers are caused to perform free cooling, there arises a possibility that the power consumption Po of the high-capacity refrigerator  10  becomes excessively large and the power consumption Pt of the entire cooling system  100  cannot be reduced. 
     In order to avoid such an issue, on the condition that the second set temperature which is found in the largest number among the second set temperatures Ta, Tb, Tc, . . . obtained from the control sections A, B, C, . . . is higher than the predetermined temperature, the setting section  70  sets the first set temperature Tw to the stop temperature Tf, and stops the compressor  41  and opens the on-off valve  58  in each of the target chillers. 
     The present embodiment having been described in detail above has the following advantages. Notably, only the advantages different from those of the first embodiment will be described. 
     The target chillers, which are chillers employing a second set temperature which is found in the largest number among the second set temperatures Ta, Tb, Tc . . . , can supply, by means of free cooling, the third heat transfer medium cooled to the corresponding second set temperature or lower. Also, the power consumptions of the target chillers can be reduced greatly. Moreover, the number of target chillers which perform free cooling can be increased. 
     On the condition that the second set temperature which is found in the largest number among the obtained second set temperatures is higher than the predetermined temperature, the setting section  70  sets the first set temperature Tw to the stop temperature Tf, and stops the compressor  41  and opens the on-off valve  58  in each of the target chillers. Namely, in the case where the largest-number second set temperature is higher than the predetermined temperature, the target chillers perform free cooling, and, in the case where the largest-number second set temperature is lower than the predetermined temperature, the target chillers do not perform free cooling. Accordingly, the power consumption Pt of the entire cooling system  100  can be reduced. 
     Fourth Embodiment 
     A fourth embodiment will now be described. In the following description, the difference between the fourth embodiment and the first embodiment will be mainly described. Notably, portions identical with those of the first embodiment are denoted by the same reference numerals, and their description will not be repeated. 
     In the present embodiment, as shown by broken lines at the right end of  FIG.  1   , the cooling system  100  includes several hundred or more chillers in addition to the chillers  40 A to  40 C. The branch flow passage  37  of each chiller  40  is connected to the common flow passage  35 , and the branch flow passage  38  of each chiller is connected to the common flow passage  36 . Each chiller does not include the bypass flow passage  57  and the on-off valve  58 . Namely, each chiller does not perform free cooling. 
     The power consumption Pt of the cooling system  100  changes depending on the combination of the first set temperature Tw and the plurality of second set temperatures. The relation among the first set temperature Tw, the plurality of second set temperatures, and the measured value of the power consumption Pt of the cooling system  100  can be obtained beforehand through, for example, an experiment. 
     In view of this, the setting section  70  variably sets the first set temperature Tw, on the basis of a previously obtained relation among the first set temperature Tw, the plurality of second set temperatures, and the measured value of the power consumption Pt of the cooling system  100 , and the plurality of obtained second set temperatures, such that the power consumption Pt of the cooling system  100  becomes the minimum. By virtue of such a configuration, since the relation among the first set temperature Tw, the plurality of second set temperatures, and the measured value of the power consumption Pt of the cooling system  100  is obtained beforehand, it is possible to reduce the power consumption Pt of the entire cooling system  100  while reducing the processing load of the setting section  70 . 
     Fifth Embodiment 
     A fifth embodiment will now be described. In the following description, the difference between the fifth embodiment and the fourth embodiment will be mainly described. Notably, portions identical with those of the first and fourth embodiments are denoted by the same reference numerals, and their description will not be repeated. 
     The setting section  70  may be configured to learn, through operation of the cooling system  100 , a predetermined relation among the first set temperature Tw, the plurality of second set temperatures, and the measured value of the power consumption Pt of the cooling system  100 . In such a case, the setting section  70  determines a direction in which the first set temperature Tw must be changed, for example, so as to decrease the measured value of the power consumption Pt, and changes the first set temperature Tw in that direction so as to decrease the measured value of the power consumption Pt. 
     The setting section  70  successively obtains the first set temperature Tw, the plurality of second set temperatures, and the measured value of the power consumption Pt of the cooling system  100 ; learns a predetermined relation among the first set temperature Tw, the plurality of second set temperatures, and the measured value of the power consumption Pt of the cooling system  100 ; and variably sets the first set temperature Tw on the basis of the learned predetermined relation and the plurality of obtained second set temperatures such that the power consumption Pt of the cooling system  100  becomes the minimum. By virtue of such a configuration, the setting section  70  can learn the predetermined relation through operation of the cooling system  100 . Therefore, as the learning progresses, the accuracy of the predetermined relation increases, whereby the power consumption Pt of the entire cooling system  100  can be reduced further. 
     Sixth Embodiment 
     A sixth embodiment will now be described. In the following description, the difference between the sixth embodiment and the fourth embodiment will be mainly described. Notably, portions identical with those of the first and fourth embodiments are denoted by the same reference numerals, and their description will not be repeated. 
     In the present embodiment as well, as shown by broken lines at the right end of  FIG.  1   , the cooling system  100  includes several hundred or more chillers in addition to the chillers  40 A to  40 C. Each chiller does not include the bypass flow passage  57  and the on-off valve  58 . Namely, each chiller does not perform free cooling. Also, the high-capacity refrigerator  10  does not include the bypass flow passage  27  and the on-off valve  28 . Namely, the high-capacity refrigerator  10  does not perform free cooling. 
     The power consumption Po of the high-capacity refrigerator  10  changes in accordance with the first set temperature Tw. The relation between the first set temperature Tw and the measured value of the power consumption Po of the high-capacity refrigerator  10  can be obtained beforehand through, for example, an experiment. Since the power consumption Po of the high-capacity refrigerator  10  also changes depending on the atmospheric temperature and thus the temperature of the fifth heat transfer medium supplied from the cooling tower  30  to the high-capacity refrigerator  10 . Therefore, for each atmospheric temperature, the relation between the first set temperature Tw and the measured value of the power consumption Po of the high-capacity refrigerator  10  is obtained beforehand through, for example, an experiment. Also, the power consumptions of the chillers change depending on the combination of the first set temperature Tw and the second set temperatures. The relation among the first set temperature Tw, the second set temperatures, and the measured values of the power consumptions of the chillers can be obtained beforehand through, for example, an experiment. 
     In view of this, the setting section  70  variably sets the first set temperature Tw, on the basis of a previously obtained first relation between the first set temperature Tw and the measured value of the power consumption Po of the high-capacity refrigerator  10 , a previously obtained second relation among the first set temperature Tw, the second set temperatures, and the measured values of the power consumptions of the chillers, and the plurality of obtained second set temperatures, such that the sum of the power consumption Po of the high-capacity refrigerator  10  and the power consumptions of the plurality of chillers becomes the minimum. By virtue of such a configuration, the power consumption Po of the high-capacity refrigerator  10  and the power consumptions of the chillers can be calculated more accurately, whereby the power consumption Pt of the entire cooling system  100  can be reduced further. 
     Notably, in the case where a change in the power consumption Po of the high-capacity refrigerator  10  with the atmospheric temperature is small, the atmospheric temperature can be considered to be constant. 
     Seventh Embodiment 
     A seventh embodiment will now be described. In the following description, the difference between the seventh embodiment and the sixth embodiment will be mainly described. Notably, portions identical with those of the first and sixth embodiments are denoted by the same reference numerals, and their description will not be repeated. 
     The setting section  70  may be configured to learn the above-described first relation between the first set temperature Tw and the power consumption Po of the high-capacity refrigerator  10  through operation of the cooling system  100 . Also, the setting section  70  may be configured to learn the above-described second relation among the first set temperature Tw, the second set temperatures, and the measured values of the power consumptions of the chillers through operation of the cooling system  100 . In such a case, the setting section  70  determines a direction in which the first set temperature Tw must be changed, for example, so as to decrease the sum of the measured value of the power consumption Po and the measured values of the power consumptions of the plurality of chillers, and changes the first set temperature Tw in that direction so as to decrease the sum of the measured value of the power consumption Po and the measured values of the power consumptions of the plurality of chillers. 
     The setting section  70  successively obtains the first set temperature Tw and the measured value of the power consumption Po of the high-capacity refrigerator  10 ; learns the first relation between the first set temperature Tw and the measured value of the power consumption Po; successively obtains the first set temperature Tw, the second set temperatures, and the measured values of the power consumptions of the chillers; learns the second relation among the first set temperature Tw, the second set temperatures, and the measured values of the power consumptions of the chillers; and variably sets the first set temperature Tw on the basis of the learned first and second relations and the plurality of obtained second set temperatures such that the power consumption Pt of the cooling system  100  becomes the minimum. By virtue of such a configuration, the setting section  70  can learn the first and second relations through operation of the cooling system  100 . Therefore, as the learning progresses, the accuracies of the first and second relations increase, whereby the power consumption Pt of the entire cooling system  100  can be reduced further. 
     Also, the above-described embodiments may be modified as follows. Portions identical with those of the above-described embodiments are denoted by the same reference numerals, and their description will not be repeated. 
     In the high-capacity refrigerator  10 , the bypass flow passage  27  and the on-off valve  28  may be omitted. In such a case, the high-capacity refrigerator  10  does not perform free cooling. 
     The control targets may be electrodes of semiconductor manufacturing apparatuses, substrate holding portions, etc. of other manufacturing apparatuses and processing apparatuses, or heat exchangers (heat exchange sections) which exchange heat with heat transfer mediums supplied to the electrodes, the substrate holding portions, etc. 
     Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims.