Abstract:
This microgrid system ( 100 ) includes: a generator device ( 111 ) for outputting power; a plurality of sub-microgrids ( 110 ) that include utility customers ( 112 ) who consume power; a shared unit ( 120 ) connected to the plurality of sub-microgrids ( 110 ), and provided with a plurality of energy storage devices ( 121 ) for storing power; and a control device ( 130 ) that, when power outputted by the generator device ( 111 ) is to be stored in the energy storage devices ( 121 ), selects an energy storage device ( 121 ) as the storage destination for the power outputted by the generator device ( 111 ), in accordance with the respective status of the plurality of energy storage devices ( 121 ), and when power is to be supplied to utility customers ( 112 ), selects an energy storage device ( 121 ) as the supply source for supplying power to the utility customers ( 112 ), in accordance with the respective status of the plurality of energy storage devices ( 121 ).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a microgrid system and a control method for the microgrid system. 
       BACKGROUND ART 
       [0002]    In recent years, small-scale utility grid systems called microgrid systems have been gathering attention. In a microgrid system, generator devices, which are energy resources, and customers (such as factories, schools, houses, and shops), who consume power, are connected to each other by a utility grid, which is different from any commercial utility grid. 
         [0003]    A microgrid system controls power supply according to demand so as to make maximum use of the resources in the system, thereby attempting to provide stable and inexpensive power supply. For example, power consumed by the customers in a system is supplied as much as possible by the power generators in the system, and only insufficient power is bought from an ordinary commercial utility grid, whereby stable and inexpensive power supply is made possible. 
         [0004]    Examples of power generators often employed in microgrid systems are those using renewable energy, such as sunlight, wind power, and biomass energy. Power supply from such a power generator is unstable in some cases. 
         [0005]    To address this, PTL1 (Japanese Laid-open Patent No. 2012-055087) discloses a technique for controlling distribution of power from power supply sources to power consumption sources in a predetermined area on the basis of information such as environmental variation in power consumption by each of the power consumption sources and power supply by each of the power supply sources. With this technique, it is possible to provide stable power supply by controlling distribution of power from the power supply sources to the power consumption sources on the basis of information such as environmental variation. 
         [0006]    Further, PTL2 (Japanese Laid-open Patent No. 2013-110799) discloses a technique for providing an energy storage device, such as a storage battery, in a microgrid system. This technique attempts stable power supply by storing, in the storage battery, power output by a power generator (solar panel) in the microgrid system, and supplying the power stored in the storage battery to customers as needed. 
       CITATION LIST 
     Patent Literature 
       [0007]    [PTL1] Japanese Laid-open Patent No. 2012-055087 
         [0008]    [PTL2] Japanese Laid-open Patent No. 2013-110799 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0009]    As described above, power generators using renewable energy are often used in microgrid systems. Depending on the environment, this leads, in some cases, to a situation where power supply from a power supply source exceeds power consumption by power consumption sources, consequently producing excess power. 
         [0010]    In the technique disclosed in PTL1, a process to be performed when excess power is produced is not sufficiently taken into account, and therefore power output by the power supply sources is sometimes wasted. 
         [0011]    In the technique disclosed in PTL2, when excess power is generated, the excess power is stored in the energy storage device, thereby preventing power from being wasted. 
         [0012]      FIG. 1  is a graph illustrating charge and discharge characteristics of a typical storage battery.  FIG. 1  illustrates charge and discharge characteristics of a case in which the storage battery discharges only in the period from an initial time point (t 0 ) to a time point t 1  and is then charged.  FIG. 2  is a graph illustrating a relationship between a state of charge (SOC) and output voltage of the storage battery. 
         [0013]    As illustrated in  FIG. 1 , the output voltage of the storage battery gradually decreases from an initial voltage (4.2 V) after the start of the discharge and decreases abruptly when reaching a predetermined voltage (3.4 V in  FIG. 1 ). Moreover, as illustrated in  FIG. 2 , in a state where the output voltage of the storage battery is equal to or lower than the predetermined voltage (3.4 V), the SOC is equal to or lower than 20%. To increase the life of a storage battery, it is normally desirable to stop discharge of the storage battery and to charge the storage battery, when the SOC decreases to 20% or lower. 
         [0014]    According to the technique disclosed in PTL2, a power generator and an energy storage device are provided in a one-to-one relationship in the microgrid system, the energy storage device storing power output from the power generator. Due to this configuration, it is difficult for the technique disclosed in PTL2 to enable stable power supply when the SOC of the storage battery in the energy storage device decreases and consequently fluctuation of output voltage increases or power supply is stopped to charge the storage battery. Further, it is not possible to store excess power of the power generator after completion of charge of the storage battery, consequently wasting power output from the power generator in some cases. 
         [0015]    It is also conceivable to configure a storage battery with a storage capacity large enough to store sufficient power for power consumption by the customers in a microgrid system. However, the cost of a storage battery is typically not proportional to the storage capacity of the storage battery, and the cost per unit storage capacity increases as the storage capacity increases. Accordingly, using a storage battery having a large storage capacity as described above leads to an increase in cost. 
         [0016]    It is an object of the present invention to provide a microgrid system system capable of limiting an increase in cost, reducing waste of power, and achieving stable power supply, and a method of controlling the microgrid system. 
       Solution to Problem 
       [0017]    To achieve the above-mentioned object, a microgrid system according to the present invention includes: a plurality of sub-microgrids each of which comprises a power generator that outputs power and a customer that consumes power; a shared unit that is connected to each of the plurality of sub-microgrids and that includes a plurality of energy storage devices for storing power; and a control device that determines, when power output by any of the power generators is to be stored in the energy storage devices, an energy storage device to serve as a storage of the power output by the power generator, according to states of the plurality of respective energy storage devices, and that determines, when power is to be supplied to any of the customers, an energy storage device to serve as a supplier of the power to the customer, according to states of the plurality of respective energy storage devices. 
         [0018]    To achieve the above-mentioned object, a method of controlling a microgrid system is a control method of microgrid system having a power generator that outputs power, wherein 
         [0019]    the microgrid system is provided with a plurality of sub-microgrids each of which includes a power generator that outputs power and a customer that consumes power, and a shared unit that is connected to each of the plurality of sub-microgrids and that includes a plurality of energy storage devices for storing power, the method 
         [0020]    determines, when power output by any of the power generators is to be stored, an energy storage device to serve as a storage of the power output by the power generator, according to states of the plurality of respective energy storage devices; and 
         [0021]    determines, when power is to be supplied to any of the customers, an energy storage device to serve as a supplier of the power to the customer, according to states of the plurality of respective energy storage devices. 
       Advantageous Effect of Invention 
       [0022]    According to the present invention, it is possible to limit an increase in cost, to reduce waste of power, and to provide stable power supply. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0023]    [ FIG. 1 ]  FIG. 1  is a graph illustrating charge and discharge characteristics of a typical storage battery. 
           [0024]    [ FIG. 2 ]  FIG. 2  is a graph illustrating a relationship between a state of charge and output voltage of the typical storage battery. 
           [0025]    [ FIG. 3 ]  FIG. 3  is a diagram illustrating a configuration of main part of a microgrid system of a first exemplary embodiment of the present invention. 
           [0026]    [ FIG. 4 ]  FIG. 4  is a diagram illustrating a configuration of main part of a microgrid system of a second exemplary embodiment of the present invention. 
           [0027]    [ FIG. 5 ]  FIG. 5  is a flowchart presenting operations of the microgrid system illustrated in  FIG. 4  performed to store excess power of a DER  211 - 1  in an ESD. 
           [0028]    [ FIG. 6 ]  FIG. 6  is a flowchart presenting operations of the microgrid system illustrated in  FIG. 4  performed to store excess power of a DER  211 - 2  in an ESD. 
           [0029]    [ FIG. 7 ]  FIG. 7  is a flowchart presenting operations of the microgrid system illustrated in  FIG. 4  performed to supply power to a LOAD  212 - 1 . 
           [0030]    [ FIG. 8 ]  FIG. 8  is a flowchart presenting operations of the microgrid system illustrated in  FIG. 4  performed to supply power to a LOAD  212 - 2 . 
           [0031]    [ FIG. 9 ]  FIG. 9  is a diagram illustrating a configuration of main part of a microgrid system of a third exemplary embodiment of the present invention. 
           [0032]    [ FIG. 10 ]  FIG. 10  is a flowchart presenting operations of the microgrid system illustrated in  FIG. 9  performed to store excess power of a DER in an ESD. 
           [0033]    [ FIG. 11 ]  FIG. 11  is a flowchart presenting operations of the microgrid system illustrated in  FIG. 9  performed to supply power to a LOAD. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0034]    Exemplary embodiments for carrying out the present invention are described below with reference to the drawings. 
       First Exemplary Embodiment 
       [0035]      FIG. 3  is a diagram illustrating a configuration of main part of a microgrid system  100  of a first exemplary embodiment of the present invention. 
         [0036]    The microgrid system  100  illustrated in  FIG. 3  includes sub-microgrids  110  ( 110 - 1  and  110 - 2 ), a shared unit  120 , and a control device  130 . 
         [0037]    The sub-microgrid system  110 - 1  includes a power generator  111 - 1  and a customer  112 - 1 . 
         [0038]    The sub-microgrid system  110 - 2  includes a power generator  111 - 2  and a customer  112 - 2 . 
         [0039]    The supply unit  120  is connected to each of the sub-microgrid systems  110 - 1  and  110 - 2 , and includes a plurality of energy storage devices  121  ( 121 - 1  and  121 - 2 ). 
         [0040]    The power generators  111 - 1  and  111 - 2  output power. 
         [0041]    The energy storage devices  121 - 1  and  121 - 2  store power. 
         [0042]    The customers  112  consume power. 
         [0043]    The control device  130  controls the power generators  111  and the energy storage devices  121 - 1  and  121 - 2 . 
         [0044]    Next, operation of the microgrid system  100  is described. 
         [0045]    In a case of storing power output by one of the power generators  111  in the energy storage devices  121 , the control device  130  determines the energy storage device  121  to serve as the storage for storing the power output by the power generator  111 , according to the states of the respective energy storage devices  121 - 1  and  121 - 2 . Further, in a case of supplying power to one of the customers  112 , the control device  130  determines the energy storage device to serve as a supplier for supplying the power to the customer  112 , according to the states of the respective energy storage devices  121 - 1  and  121 - 2 . 
         [0046]    As described above, the microgrid system  100  of this exemplary embodiment includes the control device  130  that determines the energy storage device to serve as a storage of power output by each of the power generators  111  and the energy storage device to serve as a supplier of power to each of the customers  112 , according to the states of the energy storage devices  121 - 1  and  121 - 2 . 
         [0047]    By determining the energy device to serve as a storage of power output by each of the power generators  111 , according to the states of the energy storage devices  121 - 1  and  121 - 2 , it is possible to store the power in the energy device with enough capacity to store the power and to hence possibly reduce waste of power. 
         [0048]    Further, by determining the energy storage device  121  to serve as a supplier of power to each of the customers  112 , according to the states of the energy storage devices  121 - 1  and  121 - 2 , it is possible to prevent a situation where any of the energy storage devices  121  in a low SOC supplies power to the customer  112 . Thus, stable power supply to the customers  112  is enabled. In addition, it is possible to prevent a situation where any of the energy storage devices  121  in a low SOC continues to supply power, and to hence possibly increase the life of each of the energy storage devices  121 . 
         [0049]    Further, by providing the plurality of energy storage devices  121 , it is possible to reduce the cost per unit storage capacity and to hence limit an increase in cost, in comparison with a case of providing a single energy storage device having a large storage capacity. 
       Second Exemplary Embodiment 
       [0050]      FIG. 4  is a diagram illustrating a configuration of main part of a microgrid system  200  of a second exemplary embodiment of the present invention. 
         [0051]    The microgrid system  200  illustrated in  FIG. 4  includes sub-microgrids  210  ( 210 - 1  and  210 - 2 ), a shared unit  220 , and a control device  230 . 
         [0052]    The sub-microgrid  210 - 1  includes a distributed energy resource (DER)  211 - 1  and a customer (LOAD)  212 - 1 . 
         [0053]    The sub-microgrid  210 - 2  includes a DER  211 - 2  and a LOAD  212 - 2 . 
         [0054]    The shared unit  220  is connected to each of the sub-microgrids  210 - 1  and  210 - 2 , and includes a plurality of energy storage devices (ESDs)  221  ( 221 - 1  and  221 - 2 ). 
         [0055]    The DERs  211 - 1  and  211 - 2  are power generators that convert renewable energy, e.g., sunlight, wind power, or biomass energy, to power and output the power. 
         [0056]    The ESDs  221 - 1  and  221 - 2  are energy storage devices that store power output by the DERs  211 - 1  and  211 - 2 . 
         [0057]    As described above, in the microgrid system  200 , the shared unit  220  includes the same number of ESDs  221  as that of the sub-microgrids  210 . In other words, in the microgrid system  200 , the ESDs  221  are provided so as to correspond to the respective sub-microgrids  210 . It is assumed below that the ESD  221 - 1  is provided so as to correspond to the sub-microgrid  210 - 1  (the DER  211 - 1  and the LOAD  212 - 1 ). It is also assumed that the ESD  221 - 2  is provided so as to correspond to the sub-microgrid  210 - 2  (the DER  211 - 2  and the LOAD  212 - 2 ). 
         [0058]    The LOAD  212 - 1  consumes power output by the DER  211 - 1  or power stored in the ESD  221 - 1  or  221 - 2 . 
         [0059]    The LOAD  212 - 2  consumes power output by the DER  211 - 2  or power stored in the ESD  221 - 1  or  221 - 2 . 
         [0060]    The control device  230  controls the DERs  211  and the ESDs  221 . 
         [0061]    Next, operation of the microgrid system  200  is described. 
         [0062]    First, operation performed when excess power of the DER  211 - 1  is to be stored in the ESDs  221  is described with reference to the flowchart presented in  FIG. 5 . 
         [0063]    The control device  230  monitors the state of power supply and demand in the system and causes, when determining that there is excess power in an output by the DER  211 - 1 , the ESDs  221  to store the excess power. 
         [0064]    First, the control device  230  determines whether or not the ESD  221 - 1  corresponding to the DER  211 - 1  is discharging (Step S 101 ). 
         [0065]    When determining that the ESD  221 - 1  is not discharging (Step S 101 : No), the control device  230  determines whether or not the SOC indicating the charge level of the ESD  221 - 1  is lower than 100% (Step S 102 ). 
         [0066]    When determining that the SOC of the ESD  221 - 1  is lower than 100% (Step S 102 : Yes), the control device  230  causes the ESD  221 - 1  to store the excess power of the DER  211 - 1  (i.e., charges the ESD  221 - 1 ) (Step S 103 ). 
         [0067]    Subsequently, the control device  230  determines whether or not the SOC of the ESD  221 - 1  is 100% after, for example, a predetermined time period has elapsed (Step S 104 ). 
         [0068]    When determining that the SOC of the ESD  221 - 1  is not 100% (Step S 104 : No), the control device  230  returns to the operation in Step S 103 . 
         [0069]    When determining that the SOC of the ESD  221 - 1  is 100% (Step S 104 : Yes), the control device  230  determines that the charging of the ESD  221 - 1  is completed and determines whether or not there is (still remains) excess power of the DER  211 - 1  (Step S 105 ). 
         [0070]    When determining that there is no excess power of the DER  211 - 1  (Step S 105 : No), the control device  230  terminates the process. 
         [0071]    When determining that there is excess power of the DER  211 - 1  (Step S 105 : Yes), the control device  230  determines whether or not the ESD  221 - 2  is discharging (Step S 106 ). 
         [0072]    When determining that the ESD  221 - 2  is discharging (Step S 106 : Yes), the control device  230  terminates the process. 
         [0073]    When determining that the ESD  221 - 2  is not discharging (Step  5106 : No), the control device  230  determines whether or not the SOC of the ESD  221 - 2  is lower than 100% (Step S 107 ). 
         [0074]    When determining that the SOC of the ESD  221 - 2  is not lower than 100%, i.e., determining that the SOC of the ESD  221 - 2  is 100% (Step S 107 : No), the control device  230  terminates the process. 
         [0075]    When determining that the SOC of the ESD  221 - 2  is lower than 100% (Step S 107 : Yes), the control device  230  causes the ESD  221 - 2  to store the excess power of the DER  211 - 1  (i.e., charges the ESD  221 - 2 ) (Step S 108 ). 
         [0076]    Subsequently, the control device  230  determines whether or not the SOC of the ESD  221 - 2  is 100% after, for example, a predetermined time period has elapsed (Step S 109 ). 
         [0077]    When determining that the SOC of the ESD  221 - 2  is not 100% (Step S 109 : No), the control device  230  returns to the operation in Step S 108 . 
         [0078]    When determining that the SOC of the ESD  221 - 2  is 100% (Step S 109 : Yes), the control device  230  determines that the charging of the ESD  221 - 2  is completed and determines whether or not there is (still remains) excess power of the DER  211 - 1  (Step S 110 ). 
         [0079]    When determining that there is no excess power of the DER  211 - 1  (Step S 110 : No), the control device  230  terminates the process. 
         [0080]    When determining that there is excess power of the DER  211 - 1  (Step S 110 : Yes), the control device  230  returns to the operation in Step S 101 . 
         [0081]    When determining that the ESD  221 - 1  is discharging (Step S 101 : Yes), the control device  230  advances to the operation in Step S 106 . When determining that the SOC of the ESD  221 - 1  is not lower than 100%, i.e., the SOC of the ESD  221 - 1  is 100% (Step S 102 : No), the control device  230  advances to the operation in Step S 106 . 
         [0082]    As described above, when excess power of the DER  211 - 1  is to be stored in the ESDs  221 , the control device  230  preferentially charges the ESD  221 - 1 , which corresponds to the DER  211 - 1 . The control device  230  charges the other ESD  221  when the ESD  221 - 1  is discharging, when the SOC of the ESD  221 - 1  is 100%, or when there is still excess power of the DER  211 - 1  after completion of charging of the ESD  221 - 1 . 
         [0083]    With this configuration, it is possible to store excess power of the DER  211 - 1  in the ESDs  221  without wasting the excess power. 
         [0084]    Next, operation performed when excess power of the DER  211 - 2  is to be stored in the ESDs  221  is described with reference to the flowchart presented in  FIG. 6 . 
         [0085]    The control device  230  monitors the state of power supply and demand in the system and causes, when determining that there is excess power in an output by the DER  211 - 2 , the ESDs  221  to store the excess power. 
         [0086]    First, the control device  230  determines whether or not the ESD  221 - 2  corresponding to the DER  212 - 1  is discharging (Step S 201 ). 
         [0087]    When determining that the ESD  221 - 2  is not discharging (Step S 201 : No), the control device  230  determines whether or not the SOC indicating the charge level of the ESD  221 - 2  is lower than 100% (Step S 202 ). 
         [0088]    When determining that the SOC of the ESD  221 - 2  is lower than 100% (Step S 202 : Yes), the control device  230  causes the ESD  221 - 1  to store the excess power of the DER  211 - 2  (i.e., charges the ESD  221 - 1 ) (Step S 203 ). 
         [0089]    Subsequently, the control device  230  determines whether or not the SOC of the ESD  221 - 2  is 100% after, for example, a predetermined time period has elapsed (Step S 204 ). 
         [0090]    When determining that the SOC of the ESD  221 - 2  is not 100% (Step S 204 : No), the control device  230  returns to the operation in Step S 203 . 
         [0091]    When determining that the SOC of the ESD  221 - 2  is 100% (Step S 204 : Yes), the control device  230  determines that the charging of the ESD  221 - 2  is completed and determines whether or not there is (still remains) excess power of the DER  211 - 2  (Step S 205 ). 
         [0092]    When determining that there is no excess power of the DER  211 - 2  (Step S 205 : No), the control device  230  terminates the process. 
         [0093]    When determining that there is excess power of the DER  211 - 2  (Step S 205 : Yes), the control device  230  determines whether or not the ESD  221 - 1  is discharging (Step S 206 ). 
         [0094]    When determining that the ESD  221 - 1  is discharging (Step S 206 : Yes), the control device  230  terminates the process. 
         [0095]    When determining that the ESD  221 - 1  is not discharging (Step S 206 : No), the control device  230  determines whether or not the SOC of the ESD  221 - 1  is lower than 100% (Step S 207 ). 
         [0096]    When determining that the SOC of the ESD  221 - 1  is not lower than 100%, i.e., determining that the SOC of the ESD  221 - 1  is 100% (Step S 207 : No), the control device  230  terminates the process. 
         [0097]    When determining that the SOC of the ESD  221 - 1  is lower than 100% (Step S 207 : Yes), the control device  230  causes the ESD  221 - 1  to store the excess power of the DER  211 - 2  (i.e., charges the ESD  221 - 1 ) (Step S 208 ). 
         [0098]    Subsequently, the control device  230  determines whether or not the SOC of the ESD  221 - 1  is 100% after, for example, a predetermined time period has elapsed (Step S 209 ). 
         [0099]    When determining that the SOC of the ESD  221 - 1  is not 100% (Step S 209 : No), the control device  230  returns to the operation in Step S 208 . 
         [0100]    When determining that the SOC of the ESD  221 - 1  is 100% (Step S 209 : Yes), the control device  230  determines that the charging of the ESD  221 - 1  is completed and determines whether or not there is (still remains) excess power of the DER  211 - 2  (Step S 210 ). 
         [0101]    When determining that there is no excess power of the DER  211 - 2  (Step S 110 : No), the control device  230  terminates the process. 
         [0102]    When determining that there is excess power of the DER  211 - 2  (Step S 110 : Yes), the control device  230  returns to the operation in Step S 201 . 
         [0103]    When determining that the ESD  221 - 2  is discharging (Step S 101 : Yes), the control device  230  advances to the operation in Step S 206 . When determining that the SOC of the ESD  221 - 2  is not lower than 100%, i.e., the SOC of the ESD  221 - 2  is 100% (Step S 102 : No), the control device  230  advances to the operation in Step S 206 . 
         [0104]    As described above, when excess power of the DER  211 - 2  is to be stored in the ESDs  221 , the control device  230  preferentially charges the ESD  221 - 2 , which corresponds to the DER  211 - 2 . The control device  130  charges the other ESD  221  when the ESD  221 - 2  is discharging, when the SOC of the ESD  221 - 2  is 100%, or when there is still excess power of the DER  211 - 2  after completion of charging of the ESD  221 - 2 . 
         [0105]    With this configuration, it is possible to store excess power of the DER  211 - 2  in the ESDs  221  without wasting the excess power. 
         [0106]    Next, operation performed when power is supplied from the ESDs  221  to the LOAD  212 - 1  is described with reference to the flowchart presented in  FIG. 7 . 
         [0107]    When power is to be supplied from the ESDs  222  to the LOAD  212 - 1 , the control device  230 , first, determines whether or not the ESD  221 - 1 , which corresponds to the LOAD  212 - 1 , is discharging (Step S 301 ). 
         [0108]    When determining that the ESD  221 - 1  is not discharging (Step S 301 : No), the control device  230  determines whether or not the SOC of the ESD  221 - 1  is higher than a predetermined threshold, e.g., 20% (Step S 302 ). Note, as described above, that it is desirable to charge, when the SOC of any of the ESDs (storage batteries)  221  decreases to 20% or lower, the ESD  221  for the purpose of increasing the life of each of ESD  221 . The predetermined threshold is set at a value of the SOC of the ESD (storage battery)  211  in a state where the ESD  221  is desired to be charged. 
         [0109]    When determining that the SOC of the ESD  221 - 1  is higher than 20% (Step S 302 : Yes), the control device  230  causes the ESD  221 - 1  to supply power to the LOAD  212 - 1  (Step S 303 ). 
         [0110]    Subsequently, the control device  230  determines whether or not more power needs to be supplied to the LOAD  212 - 1 , after, for example, a predetermined time period has elapsed (Step S 304 ). 
         [0111]    When determining that no more power needs to be supplied to the LOAD  212 - 1  (Step S 304 : No), the control device  230  terminates the process. 
         [0112]    When determining that more power needs to be supplied to the LOAD  212 - 1  (Step S 304 : Yes), the control device  230  determines whether or not the SOC of the ESD  221 - 1  is 20% (Step S 305 ). 
         [0113]    When determining that the SOC of the ESD  221 - 1  is not 20% (Step S 305 : No), the control device  230  returns to the operation in Step S 303 . 
         [0114]    When determining that the SOC of the ESD  221 - 1  is 20% (equal to or lower than 20%) (Step S 305 : Yes), the control device  230  determines whether or not the ESD  221 - 2  is discharging (Step S 306 ). 
         [0115]    When determining that the ESD  221 - 2  is discharging (Step S 306 : Yes), the ESD  221 - 2  is discharging power to the other LOAD  212 , and hence the control device  230  terminates the process. 
         [0116]    When determining that the ESD  221 - 2  is not discharging (Step S 306 : No), the control device  230  determines whether or not the SOC of the ESD  221 - 2  is higher than a predetermined threshold, e.g., 20% (Step S 307 ). 
         [0117]    When determining that the SOC of the ESD  221 - 2  is not higher than 20% (Step S 307 : No), the control device  230  terminates the process. 
         [0118]    When determining that the SOC of the ESD  221 - 2  is higher than 20% (Step S 307 : Yes), the control device  230  causes the ESD  221 - 2  to supply power to the LOAD  212 - 1  (Step S 308 ). 
         [0119]    Subsequently, the control device  230  determines whether or not more power needs to be supplied to the LOAD  212 - 1 , after, for example, a predetermined time period has elapsed (Step S 309 ). 
         [0120]    When determining that no more power needs to be supplied to the LOAD  212 - 1  (Step S 309 : No), the control device  230  terminates the process. 
         [0121]    When determining that more power needs to be supplied to the LOAD  212 - 1  (Step S 309 : Yes), the control device  230  determines whether or not the SOC of the ESD  221 - 2  is 20% (Step S 310 ). 
         [0122]    When determining that the SOC of the ESD  221 - 2  is not 20% (Step S 310 : No), the control device  230  returns to the operation in Step S 308 . 
         [0123]    When determining that the SOC of the ESD  221 - 2  is 20% (equal to or lower than 20%) (Step S 310 : Yes), the control device  230  returns to the operation in Step S 301 . 
         [0124]    Meanwhile, when determining that the ESD  221 - 1  is discharging (Step S 301 : Yes) or when determining that the SOC of the ESD  221 - 1  is not higher than 20% (Step S 302 : No), the control device  230  advances to the operation in Step S 306 . 
         [0125]    As described above, when the ESDs  221  are to supply power to the LOAD  212 - 1 , the control device  230  preferentially causes the ESD  221 - 1 , which corresponds to the LOAD  212 - 1 , to supply power. In addition, the control device  230  causes the other ESD  221 - 1  to supply power to the LOAD  212 - 1  when the ESD  221 - 1  is discharging or when the SOC of the ESD  221 - 1  is not higher than 20%, while causing the other ESD  221  to supply power when the SOC of the ESD  221 - 1  decreases to 20% or lower. 
         [0126]    With this configuration, it is possible to prevent a situation where any of the ESDs  221  in a low SOC supplies power to the LOAD  212 - 1  and to hence provide stable power supply to the LOAD  212 - 1 . In addition, it is possible to prevent a situation where any of the ESDs  221  in a low SOC continues to supply power and to hence increase the life of each of the ESDs  221 . 
         [0127]    Next, operation performed when power is supplied from the ESDs  221  to the LOAD  212 - 2  is described with reference to the flowchart presented in  FIG. 8 . 
         [0128]    When power is to be supplied from the ESDs  221  to the LOAD  212 - 2 , the control device  230 , first, determines whether or not the ESD  221 - 2 , which corresponds to the LOAD  212 - 2 , is discharging (Step S 401 ). 
         [0129]    When determining that the ESD  221 - 2  is not discharging (Step S 401 : No), the control device  230  determines whether or not the SOC of the ESD  221 - 2  is higher than a predetermined threshold, e.g., 20% (Step S 402 ). 
         [0130]    When determining that the SOC of the ESD  221 - 2  is equal to or higher than 20% (Step S 402 : Yes), the control device  230  causes the ESD  221 - 2  to supply power to the LOAD  212 - 2  (Step S 403 ). 
         [0131]    Subsequently, the control device  230  determines whether or not more power needs to be supplied to the LOAD  212 - 2 , after, for example, a predetermined time period has elapsed (Step S 404 ). 
         [0132]    When determining that no more power needs to be supplied to the LOAD  212 - 2  (Step S 404 : No), the control device  230  terminates the process. 
         [0133]    When determining that more power needs to be supplied to the LOAD  212 - 2  (Step S 404 : Yes), the control device  230  determines whether or not the SOC of the ESD  221 - 2  is 20% (Step S 305 ). 
         [0134]    When determining that the SOC of the ESD  221 - 2  is not 20% (Step S 405 : No), the control device  230  returns to the operation in Step S 403 . 
         [0135]    When determining that the SOC of the ESD  221 - 2  is 20% (equal to or lower than 20%) (Step S 405 : Yes), the control device  230  determines whether or not the ESD  221 - 1  is discharging (Step S 406 ). 
         [0136]    When determining that the ESD  221 - 1  is discharging (Step S 406 : Yes), the ESD  221 - 1  is discharging power to the other LOAD  212 , and hence the control device  230  terminates the process. 
         [0137]    When determining that the ESD  221 - 1  is not discharging (Step S 406 : No), the control device  230  determines whether or not the SOC of the ESD  221 - 1  is higher than a predetermined threshold, e.g., 20% (Step S 407 ). 
         [0138]    When determining that the SOC of the ESD  221 - 1  is not higher than 20% (Step S 407 : No), the control device  230  terminates the process. 
         [0139]    When determining that the SOC of the ESD  221 - 1  is higher than 20% (Step S 407 : Yes), the control device  230  causes the ESD  221 - 1  to supply power to the LOAD  212 - 2  (Step S 408 ). 
         [0140]    Subsequently, the control device  230  determines whether or not more power needs to be supplied to the LOAD  212 - 2 , after, for example, a predetermined time period has elapsed (Step S 409 ). 
         [0141]    When determining that no more power needs to be supplied to the LOAD  212 - 2  (Step S 409 : No), the control device  230  terminates the process. 
         [0142]    When determining that more power needs to be supplied to the LOAD  212 - 2  (Step S 409 : Yes), the control device  230  determines whether or not the SOC of the ESD  221 - 1  is 20% (Step S 310 ). 
         [0143]    When determining that the SOC of the ESD  221 - 1  is not 20% (Step S 410 : No), the control device  230  returns to the operation in Step S 408 . 
         [0144]    When determining that the SOC of the ESD  221 - 1  is 20% (equal to or lower than 20%) (Step S 410 : Yes), the control device  230  returns to the operation in Step S 401 . 
         [0145]    Meanwhile, when determining that the ESD  221 - 2  is discharging (Step S 401 : Yes) or when determining that the SOC of the ESD  221 - 2  is not higher than 20% (Step S 402 : No), the control device  230  advances to the operation in Step S 406 . 
         [0146]    As described above, when the ESDs  221  are to supply power to the LOAD  212 - 2 , the control device  230  preferentially causes the ESD  221 - 2 , which corresponds to the LOAD  212 - 2 , to supply power. In addition, the control device  230  causes the ESD  221 - 2  to supply power to the LOAD  212 - 2  when the ESD  221 - 2  is discharging or when the SOC of the ESD  221 - 2  is not higher than 20%, while causing the other ESD  221  to supply power when the SOC of the ESD  221 - 2  decreases to 20% or lower. 
         [0147]    With this configuration, it is possible to prevent a situation where any of the ESDs  221  in a low SOC supplies power to the LOAD  212 - 2  and to hence enable stable power supply to the LOAD  212 - 2 . In addition, it is possible to prevent a situation where any of the ESDs  221  in a low SOC continues to supply power and to hence increase the life of each of the ESDs  221 . 
         [0148]    As described above, the microgrid system  200  of this exemplary embodiment includes the control device  230  that determines the ESD  221  to serve as a storage of power output by each of the DERs  211  and the ESD  221  to serve as a supplier of power to each of the LOADs  212 , according to the states of the ESDs  221 - 1  and  221 - 2 . 
         [0149]    When excess power of one of the DERs  211  is to be stored in the ESDs  221 , the control device  230  causes the ESD  221  corresponding to the DER  211  to store the power and causes, upon completion of the charging of the ESD  221 , the other ESD  221  to store the power. 
         [0150]    When power is supplied to one of the LOADs  212 , the control device  230  causes the ESD  221  corresponding to the LOAD  212  to supply power to the LOAD  212  and causes, when the SOC of the ESD  221  decreases to a predetermined threshold or lower, the other ESD  221  to supply power to the LOAD  212 . 
         [0151]    With this configuration, when charging of the ESD  221  corresponding to the DER  211  is completed, power is stored in the other ESD  221 , thus possibly reducing waste of power. 
         [0152]    When the SOC of the ESD  221  supplying power to one of the LOADs  212  decreases, power is supplied to the LOAD  212  by the other ESD  221 . This prevents a situation where the ESD  221  in a low SOC supplies power to the LOAD  212 , and enables stable power supply to the LOAD  212 . In addition, it is possible to prevent a situation where any of the ESDs  221  in a low SOC continues to supply power and to hence possibly increase the life of each of the ESDs  221 . 
         [0153]    Further, by providing the plurality of ESDs  221 , it is possible to reduce the cost per unit storage capacity and to hence limit an increase in cost, in comparison with a case of providing a single ESD having a large storage capacity. 
         [0154]    Note that, although this exemplary embodiment has been described by using the example in which the microgrid system  200  includes two sub-microgrid systems  210  and the shared unit  220  includes two ESDs  221 , the microgrid system  200  is not limited to this. The microgrid system  200  may include three or more sub-microgrid systems  210 , and the shared unit  220  may include the same number of ESDs  221  as that of the sub-microgrid systems  210 . 
       Third Exemplary Embodiment 
       [0155]      FIG. 9  is a diagram illustrating a configuration of main part of a microgrid system  300  of a third exemplary embodiment of the present invention. Note that, in  FIG. 9 , similar components to those in  FIG. 4  are denoted by the same reference signs as those in  FIG. 4 , and description thereof is omitted. 
         [0156]    The microgrid system  300  illustrated in  FIG. 9  includes n sub-microgrid systems  210  ( 210 - 1  to  210 -n), a shared unit  220 , which is connected to each of the sub-microgrid systems  210  and includes m ESDs  221  ( 221 - 1  to  221 -m), and a control device  310 . Note that each of n and m is an integer equal to or larger than two. 
         [0157]    The control device  310  controls the DER  211 - 1  to the DER  211 -n and the ESDs  221 - 1  to  221 -m. 
         [0158]    Next, operation of the microgrid system  300  is described. 
         [0159]    First, operation performed when excess power of any DER  211  (DER  211 -x) among the DER  211 - 1  to the DER  211 -n is to be stored in the ESDs  221  is described with reference to the flowchart presented in  FIG. 10 . 
         [0160]    The control device  310  monitors the state of power supply and demand in the system and causes, when determining that there is excess power in an output by the DER  211 -x, the ESDs  221  to store the excess power. 
         [0161]    First, the control device  310  selects the ESD  221  that is neither being charged nor discharging and that has the lowest SOC, from among the ESDs  221 - 1  to  221 -m (Step S 501 ). 
         [0162]    Subsequently, the control device  310  causes the selected ESD  221  to store the excess power of the DER  211 -x (i.e., charges the selected ESD  221 ) (Step S 502 ). 
         [0163]    Subsequently, the control device  310  determines whether or not the SOC of the selected ESD  221  is 100% after, for example, a predetermined time period has elapsed (Step S 503 ). 
         [0164]    When determining that the SOC of the selected ESD  221  is not 100% (Step S 503 : No), the control device  310  returns to the operation in Step S 502 . 
         [0165]    When determining that the SOC of the selected ESD  221  is 100% (Step S 503 : Yes), the control device  310  determines whether or not there is (still remains) excess power of the DER  211 -x (Step S 504 ). 
         [0166]    When determining that there is no excess power of the DER  211 -x (Step S 504 : No), the control device  310  terminates the process. 
         [0167]    When determining that there is excess power of the DER  211 -x (Step S 504 : Yes), the control device  310  returns to the operation in Step S 501 . 
         [0168]    As described above, when excess power of one of the DERs  211  is to be stored in the ESDs  221 , the control device  310  sequentially charges the ESDs  211  from the ESD  211  that is neither being charged nor discharging and that has the lower SOC, until there remains no excess power. With this configuration, it is possible to store excess power of the DER  211  in the ESDs  221  without wasting the excess power. 
         [0169]    Next, operation performed when power is to be supplied from the ESDs  221  to any LOAD  212  (LOAD  212 -y) among the LOADs  212 - 1  to  212 -m is described with reference to the flowchart presented in  FIG. 11 . 
         [0170]    When power is to be supplied from the ESDs  221  to the LOAD  212 -y, the control device  310 , first, selects the ESD  221  that is neither being charged nor discharging and that has the highest SOC, from among the ESDs  221 - 1  to  221 -m (Step S 601 ). 
         [0171]    Subsequently, the control device  310  causes the selected ESD  221  to supply power to the LOAD  212 -y (Step S 602 ). 
         [0172]    Subsequently, the control device  310  determines whether or not the SOC of the selected ESD  221  is equal to or lower than 20%, after, for example, a predetermined time period has elapsed (Step S 603 ). 
         [0173]    When determining that the SOC of the selected ESD  221  is not equal to or lower than 20% (Step S 603 : No), the control device  310  returns to the operation in Step S 602 . 
         [0174]    When determining that the SOC of the selected ESD  221  is equal to or lower than 20% (Step S 603 : Yes), the control device  310  determines whether or not more power needs to be supplied to the LOAD  212 -y (Step S 604 ). 
         [0175]    When determining that no more power needs to be supplied to the LOAD  212 -y (Step S 604 : No), the control device  310  terminates the process. 
         [0176]    When determining that more power needs to be supplied to the LOAD  212 -y (Step S 604 : Yes), the control device  310  returns to the operation in Step S 601 . 
         [0177]    As described above, when power is to be supplied from the ESDs  221  to one of the LOADs  212 , the control device  310  sequentially causes the ESDs  211  to supply power to the LOAD  212  from the ESDs  211  that is neither being charged nor discharging and has the higher SOC, until the SOC of the ESD  221  decreases to the predetermined threshold or lower. 
         [0178]    With this configuration, it is possible to prevent a situation where any of the ESDs  221  in a low SOC supplies power to each of the LOADs  212  and to hence provide stable power supply to the LOADs  212 . In addition, it is possible to prevent a situation where any of the ESDs  221  in a low SOC continues to supply power and to hence possibly increase the life of each of the ESDs  221 . 
         [0179]    As described above, the microgrid system  300  of this exemplary embodiment includes the control device  310  that determines the ESD  221  to serve as a storage of power output by each of the DERs  211 - 1  to  211 -n and the ESD  221  to serve as a supplier of power to each of the LOADs  212 , according to the states of the ESDs  221 - 1  to  221 -m. 
         [0180]    When excess power of one of the DERs  211  is to be stored in the ESDs  221 , the control device  310  sequentially causes the ESDs  221  to store the power from the ESD  221  having the lower SOC and causes, upon completion of the charging of the ESD  221 , one of the other ESDs  221  to store the power. 
         [0181]    Further, when power is to be supplied to one of the LOADs  212 , the control device  230  sequentially causes the ESDs  221  to supply power, starting with the ESD  221  having the higher SOC until the SOC of the ESD  221  decreases to the predetermined threshold or lower. 
         [0182]    With this configuration, when charging of the ESD  221  corresponding to the DER  211  is completed, power is stored in one of the other ESDs  221 , thus reducing waste of power. 
         [0183]    Further, when the SOC of the ESD  221  supplying power to each of the LOADs  212  decreases, one of the other ESDs  221  supplies power to the LOAD  212 . This prevents a situation where any of the ESDs  221  in a low SOC supplies power to the LOAD  212 , and enables stable power supply to the LOAD  212 . In addition, it is possible to prevent a situation where any of the ESDs  221  in a low SOC continues to supply power and to hence possibly increase the life of each of the ESDs  221 . 
         [0184]    Further, by providing the plurality of ESDs  221 , it is possible to reduce the cost per unit storage capacity and to hence limit an increase in cost, in comparison with a case of providing a single ESD having a large storage capacity. 
         [0185]    The method performed in the microgrid system according to the present invention may be implemented as a program to be executed by a computer. In addition, the program can be stored in a storage medium and can be provided to the outside via a network. 
         [0186]    Part or the whole of the above-described exemplary embodiments may be described as, but not limited to, the following supplementary notes. 
         [0187]    (Supplementary Note 1) 
         [0188]    A microgrid system including: 
         [0189]    a plurality of sub-microgrids each of which includes a power generator that outputs power and a customer that consumes power; 
         [0190]    a shared unit that is connected to each of the plurality of sub-microgrids and that includes a plurality of energy storage devices for storing power; and 
         [0191]    a control device that determines, when power output by any of the power generators is to be stored in the energy storage devices, an energy storage device to serve as a storage of the power output by the power generator, according to states of the plurality of respective energy storage devices, and that determines, when power is to be supplied to any of the customers, an energy storage device to serve as a supplier of the power to the customer, according to states of the plurality of respective energy storage devices. 
         [0192]    (Supplementary Note 2) 
         [0193]    The microgrid system according to Supplementary Note  1 , wherein 
         [0194]    the energy storage devices are provided so as to correspond to the plurality of respective sub-microgrids, and, 
         [0195]    when power of any of the power generators is to be stored, the control device causes the energy storage device corresponding to the sub-microgrid including the power generator, to store the power of the power generator, and causes, upon completion of charging of the energy storage device, a different one of the energy storage devices to store the power of the power generator. 
         [0196]    (Supplementary Note 3) 
         [0197]    The microgrid system according to Supplementary Note 2, wherein, when power of any of the power generators is to be stored, the control device causes the energy storage device corresponding to the sub-microgrid including the power generator, to store the power of the power generator when the energy storage device is not discharging, and causes, when the energy storage device is discharging, a different one of the energy storage devices to store the power of the power generator. 
         [0198]    (Supplementary Note 4) 
         [0199]    The microgrid system according to Supplementary Note 2 or 3, wherein, when power is to be supplied to any of the customers, the control device causes the energy storage device corresponding to the sub-microgrid including the customer, to supply power to the customer, and causes, when a charge level of the energy storage device decreases to a predetermined threshold or lower, a different one of the energy storage devices to supply power to the customer. 
         [0200]    (Supplementary Note 5) 
         [0201]    The microgrid system according to Supplementary Note 4, wherein, when power is to be supplied to any of the customers, the control device causes the energy storage device corresponding to the sub-microgrid including the customer, to supply power to the customer when the energy storage device is not discharging, and causes, when the energy storage device is discharging, a different one of the energy storage devices to supply power to the customer. 
         [0202]    (Supplementary Note 6) 
         [0203]    The microgrid system according to Supplementary Note 1, wherein, when power output by any of the power generators is to be stored in the energy storage devices, the control device sequentially causes the energy storage devices to store the power of the power generator, from an energy storage device that is neither being charged nor discharging and that has a lower charge level among the plurality of energy storage devices. 
         [0204]    (Supplementary Note 7) 
         [0205]    The microgrid system according to Supplementary Note 6, wherein, when power is to be supplied to any of the customers, the control device sequentially causes the energy storage devices to supply power to the customer, from an energy storage device that is neither being charged nor discharging and that has a higher charge level among the plurality of energy storage devices. 
         [0206]    (Supplementary Note 8) 
         [0207]    A method of controlling a microgrid system including power generators that output power, 
         [0208]    the microgrid system being provided with a plurality of sub-microgrids each of which includes a power generator that outputs power and a customer that consumes power, and a shared unit that is connected to each of the plurality of sub-microgrids and that includes a plurality of energy storage devices for storing power, 
         [0209]    the control method including: 
         [0210]    determining, when power output by any of the power generators is to be stored, an energy storage device to serve as a storage of the power output by the power generator, according to states of the plurality of respective energy storage devices; and 
         [0211]    determining, when power is to be supplied to any of the customers, an energy storage device to serve as a supplier of the power to the customer, according to states of the plurality of respective energy storage devices. 
         [0212]    (Supplementary Note 9) 
         [0213]    The control method according to Supplementary Note 8, wherein the energy storage devices are provided so as to correspond to the plurality of respective sub-microgrids, and, when power of any of the power generators is to be stored, the energy storage device corresponding to the sub-microgrid including the power generator is caused to store the power of the power generator, and upon completion of charging of the energy storage device, a different one of the energy storage devices is caused to store the power of the power generator. 
         [0214]    (Supplementary Note 10) 
         [0215]    The control method according to Supplementary Note 9, wherein, when power of any of the power generators is to be stored, the energy storage device corresponding to the sub-microgrid including the power generator is caused to store the power of the power generator when the energy storage device is not discharging, and when the energy storage device is discharging, a different one of the energy storage devices is caused to store the power of the power generator. 
         [0216]    (Supplementary Note 11) 
         [0217]    The control method according to Supplementary Note 9 or 10, wherein, when power is to be supplied to any of the customers, the energy storage device corresponding to the sub-microgrid including the customer is caused to supply power to the customer, and when a charge level of the energy storage device decreases to a predetermined threshold or lower, a different one of the energy storage devices is caused to supply power to the customer. 
         [0218]    (Supplementary Note 12) 
         [0219]    The control method according to Supplementary Note 11, wherein, when power is to be supplied to any of the customers, the energy storage device corresponding to the sub-microgrid including the customer is caused to supply power to the customer when the energy storage device is not discharging, and when the energy storage device is discharging, a different one of the energy storage devices is caused to supply power to the customer. 
         [0220]    (Supplementary Note 13) 
         [0221]    The control method according to Supplementary Note 8, wherein, when power output by any of the power generators is to be stored in the energy storage devices, the energy storage devices are sequentially caused to store the power of the power generator, from an energy storage device that is neither being charged nor discharging and that has a lower charge level among the plurality of energy storage devices. 
         [0222]    (Supplementary Note 14) 
         [0223]    The control method according to Supplementary Note 13, wherein, when power is to be supplied to any of the customers, the energy storage devices are sequentially caused to supply power to the customer, from an energy storage device that is neither being charged nor discharging and that has a higher charge level among the plurality of energy storage devices. 
         [0224]    The invention of the present application has been described above with reference to the exemplary embodiments. However, the invention of the present invention is not limited to the above-described exemplary embodiments. Various changes understood by those skilled in the art can be made to the configuration and details of the invention of the present application within the scope of the invention of the present application. 
         [0225]    This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2014-035061, filed on Feb. 26, 2014, the disclosure of which is incorporated herein in its entirety by reference. 
       REFERENCE SIGNS LIST 
       [0226]      100 ,  200  Microgrid system 
         [0227]      110 ,  210  Sub-microgrid 
         [0228]      111  Power generator 
         [0229]      112  Customer 
         [0230]      120 ,  220  Shared unit 
         [0231]      121  Energy storage device 
         [0232]      130 ,  230 ,  310  Control device 
         [0233]      211  DER 
         [0234]      212  LOAD 
         [0235]      221  ESD