Power supply system and operation method of same

Abstract

A power supply system 100 according to the present disclosure comprises: a plurality of power conditioners 10 that convert DC power into AC power; and a plurality of fuel cell unit groups 20 that each include a plurality of fuel cell units 30 and are respectively connected to the plurality of power conditioners 10. The number of operating fuel cell units 30 required to generate required power is secured, and the number of operating fuel cell units 30 in each of the plurality of fuel cell unit groups 20 is set so that the total efficiency calculated from the power conversion efficiency of each of the plurality of power conditioners 10 becomes the maximum value or a value equal to or greater than a prescribed ratio of the maximum value.

Description

Power supply system and its operating method

[0001] The present disclosure relates to a power supply system and a method for operating the same.

[0002] Power supply systems such as fuel cell systems may have multiple power generation units that can operate independently of each other to meet a wide range of customer needs or to flexibly respond to load fluctuations.

[0003] When the output power of a power generating unit is DC power, DC power needs to be converted to AC power in order to supply power to an AC load. Patent Document 1 discloses a fuel cell stack connection control device including a plurality of fuel cell stacks and a power conditioning unit (PCS) connected to the plurality of fuel cell stacks.

[0004] JP 2015-128041 A

[0005] The power conversion efficiency of a power conditioner varies depending on the ratio of input power to the capacity of the power conditioner (load factor). When the relationship between load factor and power conversion efficiency is plotted on a graph, with the horizontal axis representing the load factor and the vertical axis representing the power conversion efficiency, the graph may show an upward convex curve.

[0006] Here, a large-scale power supply system requires multiple power conditioners. A group of power generation units is connected to each of the multiple power conditioners. One power generation unit group includes multiple power generation units. That is, the output power of the multiple power generation units is input to a common power conditioner. In such a configuration, in order to reduce power conversion loss in the power conditioner, it is important to appropriately select the power generation unit to be operated in accordance with load fluctuations. Reducing power conversion loss in the power conditioner improves the energy conversion efficiency of the power supply system, which in turn reduces the unit price of electricity.

[0007] An object of the present disclosure is to provide a technique for reducing power conversion losses and efficiently operating a power supply system.

[0008] The present disclosure provides a power supply system comprising: a plurality of power conditioners that convert DC power to AC power; and a plurality of fuel cell unit groups, each including a plurality of fuel cell units and connected to each of the plurality of power conditioners; wherein the number of operating fuel cell units in each of the plurality of fuel cell unit groups is set so that the number of operating fuel cell units required to generate the required power is ensured and the overall efficiency calculated from the power conversion efficiency of each of the plurality of power conditioners is the maximum value or a value equal to or greater than a predetermined percentage of that maximum value.

[0009] In another aspect, the present disclosure provides a method for operating a power supply system, the power supply system including a plurality of power conditioners that convert DC power to AC power, and a plurality of fuel cell unit groups each including a plurality of fuel cell units and connected to each of the plurality of power conditioners, the method including setting the number of operating fuel cell units in each of the plurality of fuel cell unit groups so that the number of operating fuel cell units required to generate the required power is ensured, and an overall efficiency calculated from the power conversion efficiency of each of the plurality of power conditioners is equal to or greater than the maximum value or a predetermined percentage of that maximum value.

[0010] According to the technology of the present disclosure, it is possible to reduce power conversion loss and operate the power supply system efficiently.

[0011] A diagram of a power supply system according to a first embodiment. A detailed diagram of a portion of the power supply system shown in FIG. 1. A graph showing an example of the relationship between the load factor and the power conversion efficiency in a power conditioner. A flowchart for determining a fuel cell unit to be operated. Another flowchart for determining a fuel cell unit to be operated.

[0012] Hereinafter, embodiments will be described in detail with reference to the drawings. However, unnecessary detailed description may be omitted. For example, detailed description of already well-known matters or redundant description of substantially the same configuration may be omitted.

[0013] The accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter recited in the claims.

[0014] First Embodiment Hereinafter, a first embodiment will be described with reference to FIGS. 1 and 2. FIG.

[0015] [1-1. Configuration] Fig. 1 is a configuration diagram of a power supply system according to a first embodiment. The power supply system 100 includes a plurality of power conditioners 10 and a plurality of fuel cell unit groups 20. The plurality of fuel cell unit groups 20 are connected to the plurality of power conditioners 10, respectively. In other words, there is a one-to-one correspondence between the power conditioners 10 and the fuel cell unit groups 20. DC power generated in the fuel cell unit groups 20 is supplied to the power conditioner 10. In the power conditioner 10, the DC power is converted into AC power and output. The AC power is supplied to a load 102.

[0016] The load 102 may be a factory, a hospital, a school, a commercial facility, an office building, or the like.

[0017] 2 is a detailed diagram of a portion of the power supply system 100. The power conditioner 10 includes, for example, a DC-DC converter 12, a DC-AC inverter 14, and a control unit 16. DC power generated by the fuel cell unit group 20 is input to the DC-DC converter 12 and converted into DC power of a desired voltage. The DC power output from the DC-DC converter 12 is input to the DC-AC inverter 14 and converted into AC power suitable for supplying power to the load 102. The control unit 16 controls the DC-DC converter 12 and the DC-AC inverter 14. The control unit 16 is, for example, a computer including a storage device in which programs required for control are stored and a processor that reads and executes the programs from the storage device.

[0018] The power conditioner 10 may be a device called a PCS (Power Conditioning Subsystem). The number of power conditioners 10 is determined according to the maximum output of the power supply system 100. For example, when the maximum output of the power supply system 100 is 1 MW and the capacity of the power conditioner 10 is 100 kW, ten power conditioners 10 are provided in the power supply system 100. The capacity of the power conditioner 10 is the maximum power that the power conditioner 10 can output.

[0019] The fuel cell unit group 20 includes a plurality of fuel cell units 30. The plurality of fuel cell units 30 are connected in parallel to the power conditioner 10 so that the power generated by the plurality of fuel cell units 30 is supplied to the common power conditioner 10. Each of the plurality of fuel cell units 30 includes auxiliary equipment such as a pump and a blower, and a control unit (not shown) that controls these. The rated output of the fuel cell unit 30 is, for example, 5 kW. For example, one fuel cell unit group 20 is made up of 20 fuel cell units 30.

[0020] The power supply system 100 further includes a control unit 40. The control unit 40 controls the power conditioner 10 and the fuel cell unit group 20. The control unit 40 is, for example, a computer including a storage device in which a program required for control is stored and a processor that reads and executes the program from the storage device. The control unit 40 may be provided near the power conditioner 10 and the fuel cell unit group 20, or may be provided in a location remote from the power conditioner 10 and the fuel cell unit group 20. The functions of the control unit 40 may be provided by a cloud server or an edge server.

[0021] In accordance with commands from the control unit 40, the control unit 16 of the power conditioner 10 controls the DC-DC converter 12 and the DC-AC inverter 14. In accordance with commands from the control unit 40, the control unit (not shown) of the fuel cell unit 30 controls auxiliary equipment such as a blower and a pump so that power is generated in the fuel cell unit 30.

[0022] The maximum output of the power supply system 100 can be freely designed by increasing or decreasing the number of power conditioners 10 and the number of fuel cell unit groups 20. Furthermore, the multiple power conditioners 10 can be operated independently of each other. The fuel cell units 30 that make up the fuel cell unit group 20 can also be operated independently of each other.

[0023] [1-2. Operation] The operation of the power supply system 100 configured as above will be described below. In the following calculation example, the following numerical examples are used.

[0024] Maximum output of power supply system 100: 1 MW Number of power conditioners 10: 10 units Capacity of power conditioner 10: 100 kW Number of fuel cell units 30: 200 units Rated output of fuel cell unit 30: 5 kW

[0025] The control unit 40 acquires information indicating the required power from an external source. The information indicating the required power may be information provided to the control unit 40 from a higher-level control unit, or may be information input from a terminal by an administrator of the power supply system 100. For example, assume that the required power is 500 kW. There are many combinations of power conditioners 10 and fuel cell units 30 that can generate 500 kW of power.

[0026] FIG. 3 is a graph showing an example of the relationship between the load factor and the power conversion efficiency of the power conditioner 10. The horizontal axis represents the load factor. The load factor is the ratio of input power to the capacity of the power conditioner 10. The vertical axis represents the power conversion efficiency. As shown in FIG. 3, the power conversion efficiency of the power conditioner 10 varies depending on the load factor. The power conversion efficiency of the power conditioner 10 is a value calculated from the load factor of the power conditioner 10. When the load factor is low, the power conversion efficiency of the power conditioner 10 is relatively low. In the example shown in FIG. 3, the power conversion efficiency increases from a load factor of 0% to a load factor of 65%. The maximum power conversion efficiency is achieved near a load factor of 65%. The power conversion efficiency gradually decreases from a load factor of 65% to 100% (rated output). In other words, the graph shows an upward convex curve. Therefore, the more power conditioners 10 operating at a load factor that indicates a high power conversion efficiency, the more efficiently the power supply system 100 can be operated. Since the power conversion efficiency can be calculated from the load factor, the overall efficiency when a plurality of power conditioners 10 are operated can also be easily calculated.

[0027] (When each fuel cell unit 30 is operated at its rated output) Figure 4 is a flowchart for determining which fuel cell unit 30 should be operated. The control unit 40 executes the processes shown in Figure 4. In the example shown in Figure 4, the condition is that each of the multiple fuel cell units 30 is operated at its rated output (5 kW). In other words, the fuel cell unit 30 will not be operated at an output less than the rated output.

[0028] In step S1, the candidate number Nc of power conditioners 10 and the load factor at the candidate number Nc are calculated. The minimum value of the candidate number Nc of power conditioners 10 is an integer equal to or greater than D / Cp, which is obtained by dividing the required power D by the capacity Cp of the power conditioner 10. The maximum value of the candidate number Nc is the number of power conditioners 10 included in the power supply system 100. Therefore, when the required power D is 500 kW, the candidate number Nc of power conditioners 10 is 5, 6, 7, 8, 9, or 10.

[0029] The load factor for each candidate number Nc can be calculated using the following formula (1). The load factor for each candidate number Nc is the load factor when the power conditioner 10 is operated with that number Nc. For example, when the number Nc of candidate units is 5, the load factor is 100×500 / (5×100)=100(%). When the number Nc of candidate units is 6, 7, 8, 9, or 10, the load factors are 83.3%, 71.4%, 62.5%, 55.6%, and 50.0%, respectively.

[0030] (Load factor) = 100 x D / (Nc x Cp)... (1) D: Required power Nc: Number of candidates Cp: Capacity

[0031] In step S2, the power conversion efficiency when the power conditioner 10 is operated at each calculated load factor is calculated. That is, the power conversion efficiency at each load factor is calculated using a table that describes the relationship in the graph shown in FIG. 3. If the control unit 40 has a table that describes the relationship in the graph shown in FIG. 3, the control unit 40 refers to the table to identify the power conversion efficiency corresponding to the load factor. However, instead of the table, a formula for identifying the relationship in the graph shown in FIG. 3 may be used to calculate the power conversion efficiency from the load factor.

[0032] In step S3, a load factor E1 indicating the maximum power conversion efficiency and a load factor E2 indicating the second highest power conversion efficiency are extracted. According to the graph (table) shown in FIG. 3 , the power conversion efficiency is 96.5% when the load factor is 71.4%. Therefore, the load factor E1 indicating the maximum power conversion efficiency is 71.4%. The load factor E2 indicating the second highest power conversion efficiency is 62.5%. The power conversion efficiency at a load factor of 62.5% is 96.0%. The load factor E1 (71.4%) is the load factor when seven power conditioners 10 are operated. The load factor E2 (62.5%) is the load factor when eight power conditioners 10 are operated. The number of operating power conditioners 10 when the maximum power conversion efficiency is achieved is defined as the first number of operating units N1. The number of operating power conditioners 10 when the second highest power conversion efficiency is achieved is defined as the second number of operating units N2.

[0033] In step S4, the number of fuel cell units 30 required to satisfy the required power D is calculated, and the number of operating fuel cell units 30 in each of the power conditioners 10 with the first operating number N1 corresponding to the load factor E1 is calculated so that the number of fuel cell units 30 assigned to the power conditioner 10 is an integer.

[0034] The number of fuel cell units 30 required to satisfy the required power D of 500 kW is 100. The value obtained by dividing the required number of fuel cell units 30 by the first number N1 (=7) of power conditioners 10 in operation is 14.3. Therefore, the number of fuel cell units 30 in operation in five power conditioners 10 is 14, and the number of fuel cell units 30 in operation in two power conditioners 10 is 15.

[0035] In step S5, the number of fuel cell units 30 required to satisfy the required power D is calculated, and the number of operating fuel cell units 30 in each of the power conditioners 10 of the second operating number N2 corresponding to the load factor E2 is calculated so that the number of fuel cell units 30 assigned to the power conditioner 10 is an integer.

[0036] The value obtained by dividing the required number of fuel cell units 30 by the second number N2 (=8) of operating power conditioners 10 is 12.5. Therefore, the number of operating fuel cell units 30 in the four power conditioners 10 is 12, and the number of operating fuel cell units 30 in the four power conditioners 10 is 13.

[0037] In step S6, the overall efficiency when the number of operating power conditioners 10 is the first number N1 and the overall efficiency when the number of operating power conditioners 10 is the second number N2 are calculated.

[0038] The overall efficiency when the number of operating power conditioners 10 is the first operating number N1 can be calculated using the following method. When the number of operating fuel cell units 30 is 14, the load factor is 70%. When the number of operating fuel cell units 30 is 15, the load factor is 75%. When the power conversion efficiency at a load factor of 70% is E70 and the power conversion efficiency at a load factor of 75% is E75, the overall efficiency can be calculated using the formula ((E70 x 5) + (E75 x 2)) / 7. In other words, the overall efficiency is the average value (arithmetic mean value) of the power conversion efficiencies of the multiple power conditioners 10 to be operated. With this configuration, the overall efficiency can be easily calculated. The number of operating fuel cell units 30 can be determined using the overall efficiency.

[0039] The overall efficiency when the number of operating power conditioners 10 is the second operating number N2 can be calculated using the following method. When the number of operating fuel cell units 30 is 12, the load factor is 60%. When the number of operating fuel cell units 30 is 13, the load factor is 65%. When the power conversion efficiency at a load factor of 60% is E60 and the power conversion efficiency at a load factor of 65% is E65, the overall efficiency can be calculated using the formula ((E60 x 4) + (E65 x 4)) / 8.

[0040] In step S7, the overall efficiency when operating the first number N1 of power conditioners 10 is compared with the overall efficiency when operating the second number N2 of power conditioners 10, and the number of operating power conditioners 10 is set to the number of operating power conditioners 10 that shows the highest overall efficiency. Once the number of operating power conditioners 10 is determined, the number of operating fuel cell units 30 in each of the fuel cell unit groups 20 is also determined. For example, when it is selected to operate the first number N1 of power conditioners 10, the number of operating fuel cell units 30 in each of the fuel cell unit groups 20 is set so that each of the five power conditioners 10 is supplied with DC power from 14 fuel cell units 30, and each of the two power conditioners 10 is supplied with DC power from 15 fuel cell units 30.

[0041] There is no particular limitation on the method for selecting seven power conditioners 10 from ten power conditioners 10. Similarly, there is no particular limitation on the method for selecting fourteen or fifteen fuel cell units 30 from twenty fuel cell units 30. For example, the power conditioners 10 and fuel cell units 30 can be selected so that fuel cell units 30 with a shorter cumulative operating time are operated preferentially.

[0042] According to the method described with reference to Figure 4, the number of operating power conditioners 10 and the number of operating fuel cell units 30 assigned to each power conditioner 10 are determined so that the overall efficiency is at or near the maximum value within a selectable numerical range while ensuring the number of operating fuel cell units 30 necessary to generate the required power D. In other words, the number of operating power conditioners 10 and the input power to each power conditioner 10 are determined. Power conversion loss in the power conditioners 10 can be reduced, allowing the power supply system 100 to be operated efficiently. This leads to a reduction in the unit price of electricity.

[0043] Furthermore, even in cases where operation of the fuel cell unit 30 at an output less than the rated output is not permitted, it is possible to increase the power conversion efficiency of the power conditioner 10. Not operating the fuel cell unit 30 at an output less than the rated output is advantageous from the perspective of the lifespan of the fuel cell stack of the fuel cell unit 30.

[0044] The number of operating power conditioners 10 may be set to the number of operating power conditioners 10 that shows the second or third highest overall efficiency. In other words, the number of operating power conditioners 10, the fuel cell units 30 to be operated, and the number of operating fuel cell units 30 may be selected so that the overall efficiency is equal to or greater than a predetermined percentage of the maximum value. For example, if the ratio of the overall efficiency when operating eight power conditioners 10 to the overall efficiency when operating seven power conditioners 10 is 0.95 or greater, it may be selected to operate eight power conditioners 10. For example, operating eight power conditioners 10 may be advantageous for leveling the operating times of the power conditioners 10 and the fuel cell units 30.

[0045] If the required power D is less than a predetermined power, the number of operating power conditioners 10 may be automatically set to one. For example, if the required power D is 35 kW, seven fuel cell units 30 must be operated. The load factor when operating seven fuel cell units 30 connected to one power conditioner 10 is 35%. According to the graph shown in FIG. 3 , the power conversion efficiency at a load factor of 35% is 95%, which is equal to the power conversion efficiency at rated operation (i.e., a load factor of 100%). Using multiple power conditioners 10 to operate seven fuel cell units 30 would result in a significant loss of power conversion efficiency. Therefore, when such a required power D is calculated, the number of operating power conditioners 10 is set to one, and the fuel cell unit 30 to be operated is selected from the fuel cell unit group 20 connected to that power conditioner 10. The "predetermined power" may be a power corresponding to a load factor lower than the power conversion efficiency at a load factor of 100%.

[0046] The required power D may be a value that does not take into account power conversion loss, or may be the total output power required of the multiple fuel cell units 30. Even if 100 5 kW fuel cell units 30 are operated to supply 500 kW of power, the output power will be less than 500 kW because the power conversion loss in the power conditioner 10 is not zero. The required power D is set so that the output power of the power supply system 100 and the power consumption of the load 102 are balanced.

[0047] (When partial load operation of fuel cell unit 30 is permitted) Figure 5 is a flowchart for determining the fuel cell unit 30 to operate. The control unit 40 executes the processes shown in Figure 5. In the example shown in Figure 5, partial load operation is permitted for multiple fuel cell units 30. Partial load operation means that the fuel cell unit 30 is operated at an output less than the rated output.

[0048] In step ST1, the number Nc of candidate power conditioners 10 and the load factor at the number Nc of candidate power conditioners 10 are calculated. The process in step ST1 is the same as step S1 shown in FIG.

[0049] In step ST2, the power conversion efficiency when the power conditioner 10 is operated at each of the calculated load factors is calculated. The process of step ST2 is the same as step S2 shown in FIG.

[0050] In step ST3, a load factor E1 that indicates the maximum power conversion efficiency is extracted. The number of operating power conditioners 10 when the maximum power conversion efficiency is achieved is defined as a first number N1 of operating power conditioners.

[0051] In step ST4, the number of fuel cell units 30 required to satisfy the required power D is calculated, and the number of operating fuel cell units 30 in each of the power conditioners 10 of the first operating number N1 corresponding to the load factor E1 is calculated.

[0052] The number of fuel cell units 30 required to satisfy the required power D of 500 kW is 100. Dividing the required number of fuel cell units 30 by the first number N1 (=7) of operating power conditioners 10 yields 14.3 (≈100 / 7). Therefore, some of the fuel cell units 30 to be operated operate at partial load, while the remaining fuel cell units 30 to be operated operate at rated output. In this embodiment, for each of the seven power conditioners 10, 14 fuel cell units 30 are operated at rated load, and one fuel cell unit 30 is operated at partial load (5 kW × 0.3 = 1.5 kW). This allows the number of operating fuel cell units 30 and the power generation output of each fuel cell unit 30 to be set so that the overall efficiency calculated from the power conversion efficiency of each of the multiple power conditioners 10 is maximized.

[0053] 5 , the number of operating power conditioners 10, the number of operating fuel cell units 30 assigned to each power conditioner 10, and the power generation output of each fuel cell unit 30 are determined so as to maximize the overall efficiency while ensuring the number of operating fuel cell units 30 required to generate the required power D. In other words, the number of operating power conditioners 10 and the input power to each power conditioner 10 are determined. Power conversion loss in the power conditioners 10 can be reduced, allowing the power supply system 100 to be operated efficiently. This leads to a reduction in the unit price of electricity.

[0054] Note that, also in the method described with reference to FIG. 5, the number of operating power conditioners 10 may be set to the number of operating power conditioners that exhibits the second or third highest overall efficiency.

[0055] 4, it is possible to search for a combination in which the overall efficiency calculated from the power conversion efficiencies of the multiple power conditioners 10 is the maximum value or a value equal to or greater than a predetermined percentage of the maximum value with a very small number of calculations. However, the overall efficiency may be calculated for all combinations of power conditioners 10 and fuel cell units 30 that satisfy the required power. The power conditioners 10 and fuel cell units 30 may be operated with the combination that shows the highest overall efficiency.

[0056] [1-3. Effects, etc.] As described above, according to this embodiment, the number of operating fuel cell units 30 required to generate the required power D is ensured. The number of operating fuel cell units 30 in each of the multiple fuel cell unit groups 20 is set so that the overall efficiency calculated from the power conversion efficiencies of the multiple power conditioners 10 is the maximum value or a value equal to or greater than a predetermined percentage of that maximum value. With this configuration, power conversion loss in the power conditioners 10 can be reduced, allowing the power supply system 100 to be operated efficiently. This leads to a reduction in the unit price of electricity.

[0057] In this embodiment, the overall efficiency may be the average value of the power conversion efficiencies of the multiple power conditioners 10, and the number of operating fuel cell units 30 may be set so that the average value is the maximum value or a predetermined percentage of the maximum value under conditions where each of the multiple fuel cell units 30 is operated at its rated output. With this configuration, the overall efficiency can be easily calculated. The number of operating fuel cell units 30 can be determined using the overall efficiency.

[0058] Furthermore, in this embodiment, the number of operating fuel cell units 30 may be set so that the overall efficiency is the maximum value or a predetermined percentage of that maximum value or more under the condition that at least some of the plurality of fuel cell units 30 are capable of partial load operation and the remaining plurality of fuel cell units 30 are operated at rated output. According to this configuration, the number of operating fuel cell units 30 and the power generation output of each of the fuel cell units 30 can be set so that the overall efficiency calculated from the power conversion efficiencies of the plurality of power conditioners 10 is the maximum value or a predetermined percentage of that maximum value or more.

[0059] Furthermore, in the present embodiment, the power conversion efficiency of each of the multiple power conditioners 10 may be a value calculated from the load factor of each of the multiple power conditioners 10, and the load factor may be the ratio of the input power to the power conditioner 10 to the capacity of the power conditioner 10. Because the power conversion efficiency can be calculated from the load factor, the overall efficiency when the multiple power conditioners 10 are operated can also be easily calculated.

[0060] The technique of the present disclosure is useful for a power supply system, and is particularly useful for a power supply system in which the power demand of a load changes.

Claims

1. A power supply system comprising: a plurality of power conditioners that convert DC power to AC power; and a plurality of fuel cell unit groups, each of which includes a plurality of fuel cell units and is connected to each of the plurality of power conditioners; wherein the number of operating fuel cell units in each of the plurality of fuel cell unit groups is set so that the number of operating fuel cell units required to generate the required power is ensured and an overall efficiency calculated from the power conversion efficiency of each of the plurality of power conditioners is equal to or greater than the maximum value or a predetermined percentage of that maximum value.

2. The power supply system of claim 1, wherein the overall efficiency is an average value of the power conversion efficiency of each of the plurality of power conditioners, and the number of operating fuel cell units is set so that the average value is equal to or greater than the maximum value or a predetermined percentage of the maximum value under conditions in which each of the plurality of fuel cell units is operated at rated output.

3. The power supply system of claim 1, wherein the number of operating fuel cell units is set so that, under conditions where at least a portion of the plurality of fuel cell units are capable of partial load operation and the remainder of the plurality of fuel cell units are operated at rated output, the overall efficiency is equal to or greater than the maximum value or a predetermined percentage of that maximum value.

4. The power supply system according to claim 1, wherein the power conversion efficiency of each of the plurality of power conditioners is a value calculated from a load factor of each of the plurality of power conditioners, and the load factor is a ratio of input power to the power conditioner to a capacity of the power conditioner.

5. A method for operating a power supply system, the power supply system comprising a plurality of power conditioners that convert DC power to AC power, and a plurality of fuel cell unit groups each including a plurality of fuel cell units and connected to each of the plurality of power conditioners, the method including setting the number of operating fuel cell units in each of the plurality of fuel cell unit groups so that the number of operating fuel cell units required to generate the required power is ensured, and an overall efficiency calculated from the power conversion efficiency of each of the plurality of power conditioners is equal to or greater than the maximum value or a predetermined percentage of that maximum value.

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