Fuel cell system

Abstract

The invention relates to a fuel cell system. A fuel cell system is provided with a plurality of power generation modules each including one or more fuel cell stacks connected in series between the plurality of power generation modules, and an auxiliary device for controlling a fuel gas and an oxidant gas supplied to the fuel cell stacks. The system is provided with: a plurality of independent control units for controlling, on the basis of a control command, each of the auxiliary devices of the corresponding power generation modules among the plurality of power generation modules; and a unified control unit that sets a current command so as to obtain an output corresponding to a requested output requested by the system within a range in which the voltage of each fuel cell stack of the plurality of power generation modules is not lower than a lower limit voltage, and transmits the current command to the plurality of independent control units.

Description

Technical Field

[0001] The present invention relates to a fuel cell system. Background Art

[0002] In the past, a fuel cell system of this type has been proposed that includes a plurality of fuel cell stacks having a plurality of fuel cells connected in series, and an adjustment circuit for adjusting the current value of each fuel cell stack in the series, and a compensating current smaller than the main part of the current flows through at least one adjustment circuit (for example, refer to patent document 1).

[0003] In addition, as a fuel cell system, a fuel cell system is proposed that includes: multiple power modules, each of which includes at least one fuel cell stack, multiple DC / DC converters, each of which has an input end and an output end and is electrically connected to the corresponding power modules among the multiple power modules at the input end, and a DC power bus connected in parallel to the output ends of the multiple DC / DC converters and capable of being electrically connected to a load (for example, refer to patent document 2).

[0004] Patent Document 1: Japanese Patent Application No. 2012-533146

[0005] Patent Document 2: Japanese Patent Application No. 2022-523187

[0006] The system described in Patent Document 1 requires a regulator circuit, which, because the compensation current flows into the regulator circuit, degrades efficiency due to losses and increases costs due to the increased number of components. Furthermore, the system described in Patent Document 2 requires a DC / DC converter for each power module, which degrades efficiency due to switching losses and increases costs due to the increased number of components. Summary of the Invention

[0007] A main object of the fuel cell system of the present invention is to provide a fuel cell system that can achieve high output with a simple structure.

[0008] In order to achieve the above-mentioned main object, the present invention adopts the following mechanism.

[0009] The fuel cell system of the present invention aims at the following aspects:

[0010] A fuel cell system comprising: a plurality of power generation modules, each of which includes one or more fuel cell stacks for generating power through a reaction between a fuel gas and an oxidant gas, and auxiliary equipment for controlling the supply of the fuel gas and the oxidant gas to the fuel cell stacks, wherein the fuel cell stacks are connected in series between the plurality of power generation modules, the fuel cell system comprising:

[0011] a plurality of independent control units, which respectively control auxiliary equipment of corresponding power generation modules among the plurality of power generation modules based on control instructions; and

[0012] The unified control unit sets a current instruction and sends it to the plurality of independent control units in such a manner that an output corresponding to a required output of the system is obtained within a range in which the voltage of each fuel cell stack of the plurality of power generation modules does not fall below a lower limit voltage.

[0013] In the fuel cell system of the present invention, each fuel cell stack is connected in series between multiple power generation modules. Within a range where the voltage of each fuel cell stack in each of the multiple power generation modules does not fall below a lower voltage limit, current commands are set and sent to independent control units that control each power generation module in a manner that produces an output corresponding to the system's required output. This allows for a fuel cell system capable of achieving high output with a simple structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a schematic diagram of the structure of the fuel cell system according to this embodiment.

[0015] Figure 2 It is a simplified structural diagram of multiple power generation units including power generation modules and auxiliary equipment.

[0016] Figure 3 This is a simplified structural diagram of the power generation module.

[0017] Figure 4 This is a flowchart showing an example of an operation mode switching processing procedure.

[0018] Figure 5 This is a flowchart showing an example of a power generation control program.

[0019] Figure 6 This is an explanatory diagram showing the relationship between the terminal voltage and the current density of a single cell.

[0020] Figure 7 This is a flowchart showing an example of a start-stop processing routine.

[0021] Figure 8 This is a flowchart showing a power generation control routine according to a modified example.

[0022] Figure 9 It is a simplified structural diagram of a fuel cell system according to a modified example. DETAILED DESCRIPTION

[0023] Modes for carrying out the present invention will be described with reference to the accompanying drawings.

[0024] Figure 1 is a simplified structural diagram of the fuel cell system 10 of this embodiment. Figure 2is a simplified structural diagram of a plurality of power generation units 11 including power generation modules 20 and auxiliary equipment 30, Figure 3 2 is a simplified structural diagram of the power generation module 20 .

[0025] like Figure 1 As shown, the fuel cell system 10 of the embodiment includes a plurality of power generation units 11 and a unified control device 100 that manages the plurality of power generation units 11 .

[0026] like Figure 1 As shown, each of the plurality of power generation units 11 includes a power generation module 20 including a fuel cell stack 21 , various auxiliary equipment 30 required for operating the fuel cell stack 21 , and a module control device 90 for controlling the various auxiliary equipment 30 .

[0027] like Figure 3 As shown, the power generation module 20 includes, in addition to the fuel cell stack 21, a burner 22, heat exchangers 23 and 24, and an evaporator 25, all housed in a thermally insulated module housing 29. Furthermore, a heater may be provided in the evaporator 25 to compensate for any heat shortage in the evaporator 25. Various auxiliary equipment 30 include a fuel supply system 40, an air supply system 50, a circulation system 60, a waste heat recovery system 70, and a water supply system 80.

[0028] The fuel cell stack 21 has a plurality of solid oxide type cells, each of which includes an electrolyte such as stabilized zirconia (e.g., YSZ), a fuel electrode of a composite of a catalyst metal such as Ni and stabilized zirconia arranged on one side of the electrolyte, and an air electrode such as LSCF arranged on the other side of the electrolyte. In this embodiment, each fuel cell stack 21 is a reversible solid oxide cell stack having a power generation mode (FC mode) in which power is generated by the reaction of hydrogen with oxygen contained in the air, and an electrolysis mode (EC mode) in which hydrogen is generated by electrolysis of high-temperature steam while power is supplied from the power source 1. In addition, a system power supply, a regenerative energy source such as a solar power generation device, a battery, etc. can be used as the power source 1. A temperature sensor 94 is provided near the fuel cell stack 21. The temperature sensor 94 detects a temperature related to the temperature of the fuel cell stack 21 (stack-related temperature Tst).

[0029] In the FC mode, hydrogen gas supplied from the fuel supply system 40 is introduced as fuel gas to the fuel electrode of the fuel cell stack 21 via the fuel gas supply pipe 21a, and air supplied from the air supply system 50 is introduced as oxidant gas to the air electrode of the fuel cell stack 21 via the oxidant gas supply pipe 21b. 2-), the oxide ions pass through the electrolyte and react with hydrogen in the fuel electrode to obtain electrical energy. In the fuel electrode of each single cell, the fuel electrode exhaust gas not used in the electrochemical reaction (power generation) is heat-exchanged with the fuel gas (hydrogen) supplied to the fuel electrode from the fuel supply system 40 in the heat exchanger 23, and then discharged to the outside of the module housing 29. In addition, the fuel electrode exhaust gas passes through the fuel electrode exhaust piping 62 and is supplied to the circulation system 60. After being cooled and the water vapor contained in the fuel electrode exhaust gas is removed by the condenser 61 provided in the circulation system 60, it passes through the fuel electrode exhaust piping 63 and is supplied to the burner 22. In addition, in the air electrode of each single cell, the air electrode exhaust gas not used in the electrochemical reaction (power generation) is directly supplied to the burner 22. The fuel electrode exhaust introduced into the burner 22 is a combustible gas containing hydrogen, which is mixed with the air electrode exhaust gas containing oxygen introduced into the burner 22. The mixed gas is burned in the burner 22, thereby maintaining the fuel cell stack 21 at an appropriate temperature by the combustion heat. Furthermore, the burner 22 generates combustion exhaust gas, which exchanges heat with air supplied to the air electrode from the air supply system 50 in the heat exchanger 24. The combustion exhaust gas then passes through the combustion exhaust gas piping 72 and is supplied to the waste heat recovery system 70. After the waste heat is recovered by the waste heat recovery system 70, the combustion exhaust gas is discharged to the outside air.

[0030] On the other hand, in the EC mode, water vapor and a small amount of hydrogen supplied from the water supply system 80 and the fuel supply system 40 are introduced as fuel gas to the fuel electrode of the fuel cell stack 21 via the fuel gas supply pipe 21a, and air supplied from the air supply system 50 is introduced as scavenging gas to the air electrode of the fuel cell stack 21 via the oxidant gas supply pipe 21b. When a predetermined voltage of electricity is supplied between the terminals of the fuel cell stack 21 (reversible solid oxide cell stack) by the power supply 1, the water vapor introduced into the fuel electrode is decomposed into hydrogen and oxygen ions (O ions) by electrolysis in the fuel electrode. 2-), oxygen ions permeate the electrolyte, generating oxygen in the air electrode. Furthermore, in this embodiment, since a small amount of hydrogen is also supplied to the fuel electrode along with water vapor, the fuel electrode is maintained in a reducing atmosphere, thereby suppressing oxidative degradation of the fuel electrode. The hydrogen generated by the fuel electrode is discharged as fuel electrode exhaust along with the water vapor that has not reacted with electrolysis. After heat exchange with water vapor supplied to the fuel electrode from the water supply system 80 in the heat exchanger 23, it is discharged outside the module housing 29. Furthermore, the fuel electrode exhaust containing hydrogen and water vapor passes through the fuel electrode exhaust piping 62 and is supplied to the circulation system 60. After being cooled and having its water vapor removed by the condenser 61 provided in the circulation system 60, it is stored in the hydrogen tank 2 via the manifold 3 and the on-off valve 4. Furthermore, the on-off valve 4 is closed in the FC mode and open in the EC mode. Furthermore, a portion of the fuel electrode exhaust (hydrogen) passing through the condenser 61 passes through the fuel electrode exhaust piping 63 and is supplied to the burner 22. On the other hand, the oxygen generated by the air electrode is directly supplied to the burner 22 as air electrode exhaust together with the air passing through the air electrode. The combustion heat generated by the combustion of the mixed gas of the fuel electrode exhaust and the air electrode exhaust in the burner 22 is transferred to the evaporator 25. The evaporator 25 evaporates the water (raw water) supplied from the water supply system 80 to generate water vapor and heats the generated water vapor. In addition, in the burner 22, combustion exhaust gas is generated. After the generated combustion exhaust gas exchanges heat with the air supplied to the air electrode from the air supply system 50 in the heat exchanger 24, it passes through the combustion exhaust gas piping 72 and is discharged to the outside air via the waste heat recovery system 70.

[0031] The fuel supply system 40 includes: a hydrogen supply pipe 31 connected to the hydrogen tank 2 at one end, a branch pipe 41 branching from the other end of the hydrogen supply pipe 31 to each power generation module 20, and a hydrogen blower 42 respectively provided on each branch pipe 41. By operating the hydrogen blower 42, the hydrogen in the hydrogen tank 2 is pressurized (supplied) to the power generation module 20. A hydrogen blower 42 is provided on each branch pipe 41, so each hydrogen blower 42 is independently controlled, thereby enabling the hydrogen supply amount to be controlled per power generation module 20. In addition, an on-off valve 32 (double valve), a negative pressure prevention valve (not shown), etc. are provided on the hydrogen supply pipe 31. In addition to the hydrogen blower 42, each branch pipe 41 is also provided with a zero regulator 43 (equalizing valve), a flow sensor 44, etc. The flow sensor 44 detects the flow rate (fuel flow rate Fg) per unit time of the hydrogen (fuel gas) flowing in the branch pipe 41. The hydrogen gas introduced into the power generation module 20 is heated by heat exchange with the fuel electrode exhaust gas in the heat exchanger 23 and then supplied to the fuel electrode of the fuel cell stack 21 .

[0032] The air supply system 50 includes an air supply pipe 51 connected to each power generation module 20, a filter 52 installed at the inlet of each air supply pipe 51, and an air blower 53 installed in each air supply pipe 51. The air blower 53 is activated to draw air from the filter 52 and pressure-feed (supply) the drawn air to the power generation module 20. Since each air blower 53 is installed in each air supply pipe 51, each air blower 53 can be independently controlled, thereby controlling the air supply rate for each power generation module 20. Furthermore, a flow rate sensor 54 is installed in each air supply pipe 51. The flow rate sensor 54 detects the flow rate per unit time (air flow rate Fa) of the air flowing through the air supply pipe 51. The air introduced into the power generation module 20 is heated by heat exchange with the combustion exhaust gas in the heat exchanger 24 before being supplied to the air electrode of the fuel cell stack 21.

[0033] The circulation system 60 includes a condenser 61 having independent heat exchange channels for each power generation module 20; a fuel electrode exhaust pipe 62 having one end connected to each power generation module 20 (on the fuel electrode side of the fuel cell stack 21) and the other end connected to the inlet of each heat exchange channel of the condenser 61; a fuel electrode exhaust pipe 63 having one end connected to the outlet of each heat exchange channel of the condenser 61 and the other end connected to each power generation module 20 (on the burner 22 side); a circulation pipe 64 connecting the condenser 61 to heat utilization equipment; and a circulation pump 65 provided in the circulation pipe 64. The fuel electrode exhaust discharged from the fuel electrode side of the fuel cell stack 21 is supplied to the burner 22 after water vapor contained in the fuel electrode exhaust is removed by the condenser 61 through heat exchange with the heat exchange medium circulating in the circulation pipe 64. Furthermore, the water obtained by condensing the water vapor in the condenser 61 is stored in a water tank 81. The water stored in the water tank 81 is used as raw water in the EC mode.

[0034] The circulation system 60 further includes a return pipe 66 that branches from the fuel electrode exhaust pipe 63 downstream of the condenser 61 and is connected between the hydrogen blower 42 and the zero regulator 43 in the branch pipe 41 of the fuel supply system 40, and a regulating valve 67 (a solenoid valve) disposed in the return pipe 66. By opening the regulating valve 67, a portion of the fuel electrode exhaust gas that has passed through the condenser 61 can be recirculated and supplied from the fuel supply system 40 to the power generation module 20. Furthermore, in this embodiment, by adjusting the opening of the regulating valve 67, the ratio of the recirculated amount of fuel electrode exhaust gas to the discharged amount from the fuel cell stack 21, i.e., the reflux rate Re, can be varied.

[0035] The waste heat recovery system 70 includes a heat exchanger 71 connected to each combustion exhaust gas pipe 72, a heat storage tank 73, a circulation pipe 74 connecting the heat exchanger 71 and the heat storage tank 73, and a circulation pump 75 provided in the circulation pipe 74. When the circulation pump 75 is operated, the heat (waste heat) of the combustion exhaust gas is recovered by heat exchange between the heat exchange fluid in the heat storage tank 73 and the combustion exhaust gas. Furthermore, the waste heat recovery system 70 includes a circulation pipe 76 connecting the heat storage tank 73 to heat utilization equipment such as a factory, and a circulation pump 77 provided in the circulation pipe 76. When the circulation pump 77 is operated, the heat recovered in the heat storage tank 73 can be supplied to the heat utilization equipment.

[0036] The water supply system 80 includes a water tank 81 for storing water (raw water), a water supply pipe 82 connected to the water tank 81 at one end, branch pipes 83 branching from the other end of the water supply pipe 82 to each power generation module 20, and a water pump 84 provided in each branch pipe 83. The water pumps 84 are operated to pump (supply) the raw water in the water tank 81 to the power generation module 20. Because each water pump 84 is provided in each branch pipe 83, each water pump 84 can be independently controlled, allowing the amount of raw water supplied to be controlled for each power generation module 20.

[0037] An evaporator 25 is connected between the branch pipe 83 of the water supply system 80 and the fuel gas supply pipe 21a. The raw water evaporates in the evaporator 25 and is converted into water vapor. As described above, the fuel gas supply pipe 21a is provided with a heat exchanger 23 for exchanging heat between the fuel gas flowing through the fuel gas supply pipe 21a and the fuel electrode exhaust gas discharged from the fuel cell stack 21. Furthermore, the oxidant gas supply pipe 21b is provided with a heat exchanger 24 for exchanging heat between the oxidant gas flowing through the oxidant gas supply pipe 21b and the combustion exhaust gas discharged from the burner 22.

[0038] The fuel cell stacks 21 included in each of the multiple power generation units 11 are connected in series with a single power conditioner 15. The direct current generated by each fuel cell stack 21 is converted by the power conditioner 15 and supplied to a load L. A voltage sensor 91 is provided between the output terminals of the fuel cell stack 21 of each power generation module 20 to detect the output voltage of the fuel cell stack 21. Furthermore, a voltage sensor 92 is provided between one terminal (fuel terminal) of the fuel cell stack 21 at one end of the series-connected fuel cell stacks 21 and the other terminal (air terminal) of the fuel cell stack 21 at the other end to detect the overall voltage (total voltage Vt) of each fuel cell stack 21. Furthermore, a current sensor 93 is provided on the power line connecting the fuel cell stacks 21 in series to detect the current flowing in the power line. In addition, one terminal (fuel terminal) of the fuel cell stack 21 arranged at one end of the fuel cell stack 21 connected in series is electrically connected to the power supply terminal on the negative side of the power supply 1 via the power regulator 15, and the other terminal (air terminal) of the fuel cell stack 21 arranged at the other end is electrically connected to the power supply terminal on the positive side of the power supply 1 via the power regulator 15.

[0039] The power conditioner 15 has a DC / DC converter and an inverter, which converts the direct current from each fuel cell stack 21 into alternating current at a voltage that can be interconnected with the system power supply (for example, AC200V) and outputs it. The power conditioner 15 is connected to a power supply substrate (not shown). The power supply substrate converts the power from each fuel cell stack 21 and the power supply 1 into direct current suitable for driving various auxiliary devices 30, the module control device 90, and the unified control device 100 and supplies them to them. In addition, in the auxiliary equipment room where the power conditioner 15, the power supply substrate, etc. are configured, a cooling fan and a ventilation fan (not shown) for cooling the power conditioner 15 and the power supply substrate are configured.

[0040] Although not shown, each module control device 90 is configured as a microprocessor centered around a CPU. In addition to the CPU, it also includes a ROM for storing processing programs, a RAM for temporarily storing data, input / output ports, and a communication port. Input signals such as the stack temperature Tst from a temperature sensor 94 located near the corresponding fuel cell stack 21, the voltage V from a voltage sensor 91 located between the output terminals of the corresponding fuel cell stack 21, the fuel flow rate Qg from a flow rate sensor 44 located in the corresponding branch pipe 41 of the fuel supply system 40, and the air flow rate Qa from a flow rate sensor 54 located in the corresponding air supply pipe 51 of the air supply system 50 are input to each module control device 90 via its input ports. Furthermore, each module control device 90 outputs control signals for the corresponding hydrogen blower 42 of the fuel supply system 40, the corresponding air blower 53 of the air supply system 50, the corresponding regulating valve 67 of the circulation system 60, and the corresponding water pump 84 of the water supply system 80 via its output ports.

[0041] The unified control device 100 is constructed as a microprocessor centered around a CPU 101. In addition to the CPU 101, it also includes a ROM 102 for storing processing programs, a RAM 103 for temporarily storing data, an EEPROM 104 as non-volatile memory, input / output ports, and a communication port. The total voltage Vt detected by the voltage sensor 92 and the current I from the current sensor 93 are input to the unified control device 100 via the input port. Furthermore, control signals for the on-off valves 4 and 32, the circulation pump 65 of the circulation system 60, and the circulation pumps 75 and 77 of the waste heat recovery system 70 are output from the unified control device 100 via the output port. Furthermore, the unified control device 100 is communicatively connected to the unified control device 100 via a communication bus 12, allowing for the exchange of control signals and data.

[0042] Next, the operation of the fuel cell system 10 of this embodiment configured as described above will be described. Figure 4 1 is a flowchart showing an example of an operation mode switching process executed by the unified control device 100. This process is repeatedly executed in units of a predetermined time.

[0043] When the operation mode switching process is executed, the CPU 101 of the unified control device 100 first determines the operation mode to be executed between the FC mode and the EC mode (step S100). The operation mode can be determined based on the power demand. For example, when the load L requires power, the FC mode can be selected, and when the load L does not require power, the EC mode can be selected. In addition, in the fuel cell system 10 that has signed a demand response contract, when the demand for power is reduced (DR), the FC mode can be selected, and when the demand for power is increased (DR), the EC mode can be selected.

[0044] Next, if the CPU 101 determines that the operating mode is FC mode (step S102), it connects the branch pipe 41 of the fuel supply system 40 to the fuel gas supply pipe 21a and closes the on-off valve 4, thereby switching the gas supply and exhaust paths (fuel gas, oxidant gas, fuel electrode exhaust, and air electrode exhaust) to and from the fuel cell stack 21 (reversible solid oxide cell stack) to FC mode (step S104). The CPU 101 then sends control instructions to the module control devices 90 to control the various auxiliary devices 30 (hydrogen blower 42, air blower 53) to supply hydrogen to the fuel electrode of the fuel cell stack 21 and air to the air electrode (step S106), thereby starting power generation (step S108), and terminating the present procedure.

[0045] On the other hand, if the CPU 101 determines that the operating mode is the EC mode (step S102), it connects the branch pipe 41 of the fuel supply system 40 and the branch pipe 83 of the water supply system to the fuel gas supply pipe 21a, and opens the on-off valve 4, etc., to switch the routing of the gases (fuel gas, oxidant gas, fuel electrode exhaust, and air electrode exhaust) supplied to and discharged from the fuel cell stack 21 (reversible solid oxide cell stack) to the EC mode (step S110). Furthermore, the CPU 101 sends control instructions to each module control unit 90 to control the various auxiliary equipment 30 (hydrogen blower 42, air blower 53, and water pump 84) so ​​that water vapor and a small amount of hydrogen are supplied to the fuel electrode of the fuel cell stack 21, and air is supplied to the air electrode (step S112). The necessary power from the power source 1 is supplied between the electrodes of each fuel cell stack 21, thereby starting the electrolysis operation (step S114), and the present routine ends.

[0046] Next, the power generation operation (FC mode) will be described in detail. Figure 5 1 is a flowchart showing an example of a power generation control program executed by the CPU 101 of the integrated control device 100. This program is executed when the operation is started in the FC mode.

[0047] When the power generation control program is executed, the CPU 101 first inputs data necessary for control, such as the required power generation output (required power generation) Preq required by the fuel cell system 10, the voltage V of the fuel cell stack 21 of each power generation module 20 from the voltage sensor 91, the total voltage Vt from the voltage sensor 92, and the current I from the current sensor 93 (step S200). The voltage V is the voltage input from the voltage sensor 91 to each module control device 90 via communication.

[0048] Next, the CPU 101 sets a target current Itag to be output from the fuel cell stack 21 of each power generation module 20, based on the required power generation output Preq and the voltage V of each power generation module 20, within a range where the voltage V does not fall below the respective lower limit voltage Vlim (S202). Specifically, the CPU 101 sets the maximum current as the upper limit current Imax, the value obtained by dividing the required power generation output Preq by the total voltage Vt as the required current Ireq, and the smaller of the required current Ireq and the upper limit current Imax as the target current Itag, within a range where the lowest voltage Vmin among the voltages V of the fuel cell stack 21 of each power generation module 20 does not fall below the lower limit voltage Vlim. The CPU 101 then transmits the set target current Itag as a current command to the control device 90 of each module 20 (step S204).

[0049] Each module control device 90 controls the supply of fuel gas (hydrogen) and air based on the received target current Itag. The fuel gas supply is controlled by setting a target fuel flow rate Fgtag based on the target current Itag so that the fuel utilization rate Ufsystem reaches the target fuel utilization rate Uftag, controlling the hydrogen blower 42 so that fuel gas is supplied at the set target fuel flow rate Fgtag, and controlling the regulating valve 67 so that the recirculation rate Re reaches the target recirculation rate Retag. Specifically, the hydrogen blower 42 is controlled by setting a fuel duty ratio through feedback calculation based on the difference between the set target fuel flow rate Fgtag and the fuel flow rate Fg detected by the flow sensor 44, and driving the motor of the hydrogen blower 42 at the set fuel duty ratio. Here, the fuel utilization rate Ufsystem is the ratio of the fuel gas used by the fuel cell stack 21 to the fuel gas supplied by the fuel supply system 40 (hydrogen blower 42). The air supply amount is controlled by setting a target air flow rate Fatag so that the air utilization rate Ua reaches the target air utilization rate Uatag, and controlling the air blower 53 so that air is supplied at the set target air flow rate Fatag. Specifically, the air blower 53 is controlled by setting an air duty ratio through feedback calculation based on the difference between the set target air flow rate Fatag and the air flow rate Fa detected by the flow sensor 54, and driving the motor that controls the air blower 53 at the set air duty ratio. Here, the air utilization rate Ua is the ratio of air used by the fuel cell stack 21 to air supplied by the air supply system 50 (air blower 53).

[0050] Next, the CPU 101 multiplies the current I input in step S200 by the total voltage Vt to calculate the actual power generation output P of the entire fuel cell system 10 (step S206), and determines whether the actual power generation output P is less than the required power generation output Preq (step S208). If the CPU 101 determines that the actual power generation output P is not less than the required power generation output Preq, it determines that the actual power generation output P is not insufficient and terminates this routine.

[0051] On the other hand, if the CPU 101 determines that the actual power generation output P is less than the required power generation output Preq, it determines that the actual power generation output P is insufficient and determines whether the target recirculation rate Retag is less than the predetermined upper limit Remax (step S210). If the CPU 101 determines that the target recirculation rate Retag is less than the upper limit Remax, it sets the value obtained by adding the specified value ΔRe to the current target recirculation rate Retag as the new target recirculation rate Retag, and sends the set target recirculation rate Retag to the corresponding module control device 90 (step S212). The voltage V of the fuel cell stack 21 can be expressed by the following formula (1). Here, in formula (1), "η" represents the efficiency per unit heat output, "-ΔH" represents the enthalpy difference before and after the reaction when hydrogen and oxygen react to produce water, "n" represents the number of moles of fuel gas, and "F" represents the Faraday constant. In addition, "Ufstack" represents the ratio of the fuel gas used by the fuel cell stack 21 to the fuel gas supplied to the fuel electrode of the fuel cell stack 21. In addition, in the fuel cell system 10 of the present embodiment, the fuel electrode exhaust gas discharged from the fuel cell stack 21 is refluxed to the fuel supply system 40, so the fuel utilization rate Ufstack is different from the fuel utilization rate Ufsystem. It can be seen from formula (1) that the voltage V of the fuel cell stack 21 increases as the fuel utilization rate Ufstack decreases. In addition, the fuel utilization rate Ufstack can be expressed by the following formula (2). It can be seen from formula (2) that when the fuel utilization rate Ufsystem is set to be constant, that is, when the supply amount of fuel gas is set to be constant, the fuel utilization rate Ufstack decreases as the recirculation rate Re increases. It can be seen that if formula (2) is substituted into formula (1), the voltage V of the fuel cell stack 21 increases as the recirculation rate Re increases. Therefore, even if the target current Itag is limited by the lower limit voltage Vlim, the voltage V of the corresponding fuel cell stack 21 can be increased by increasing the corrected target recirculation rate Retag, and the actual power generation output P can be increased. As a result, the voltage V of each fuel cell stack 21 can be made not lower than the lower limit voltage Vlim, and the output shortage for the required power generation output Preq can be eliminated. In addition, in the present embodiment, the increase correction of the target recirculation rate Retag is performed in the following manner, that is, until the actual power generation output P becomes greater than the required power generation output Pref or the target recirculation rate Retag reaches the upper limit value Remax, the target recirculation rate Retag is gradually increased by a specified value ΔRe. In addition, the increase correction of the target recirculation rate Retag can be performed uniformly for all of the multiple power generation modules 20, or it can be performed sequentially starting from the power generation module 20 with the lower voltage V among the multiple power generation modules 20. After increasing the corrected target recirculation rate Retag in this way, the CPU 101 determines whether it is required to stop the power generation operation (step S214).If the CPU 101 determines that the stop of the power generation operation is not requested, the process returns to step S200 . If the CPU 101 determines that the stop of the power generation operation is requested, the process ends this routine.

[0052] [Number 1]

[0053]

[0054] If the CPU 101 determines in step S210 that the target recirculation rate Retag is greater than the upper limit Remax, it sets the value obtained by subtracting a predetermined value ΔUf (e.g., 1%) from the current target fuel utilization rate Uftag (in this embodiment, the target value of the fuel utilization rate Ufsystem) as the new target fuel utilization rate Uftag, and transmits the set target fuel utilization rate Uftag to the corresponding module control unit 90 (step S216). By reducing and correcting the target fuel utilization rate Uftag using equation (1), the voltage V of the corresponding fuel cell stack 21 can be increased, thereby increasing the actual power generation output P. As a result, the voltage V of each fuel cell stack 21 can be kept from falling below the lower limit voltage Vlim, and the output shortfall relative to the required power generation output Preq can be eliminated. In this embodiment, the target fuel utilization rate Uftag is corrected by gradually decreasing the target fuel utilization rate Uftag by the predetermined value ΔUf until the actual power generation output P reaches or exceeds the required power generation output Pref. The target fuel utilization rate Uftag may be corrected to decrease at once for all of the plurality of power generation modules 20 , or may be corrected sequentially starting with the power generation module 20 having the lowest voltage V among the plurality of power generation modules 20 .

[0055] Next, the CPU 101 receives the stack temperature Tst from the temperature sensor 94 (step S218) and determines whether the received stack temperature Tst is above a predetermined temperature Tref (step S220). The predetermined temperature Tref is defined as a temperature near the upper limit of the appropriate temperature range for the fuel cell stack 21. If the CPU 101 determines that the stack temperature Tst is not above the predetermined temperature Tref, the CPU 101 determines that the temperature of the fuel cell stack 21 is within the appropriate temperature range and proceeds to step S214. On the other hand, if the CPU 101 determines that the stack temperature Tst is above the predetermined temperature Tref, the CPU 101 determines that the temperature has reached the upper limit of the appropriate temperature range. To cool the fuel cell stack 21, the CPU 101 sets the value obtained by subtracting a predetermined value ΔUa (e.g., 1%) from the current target air utilization rate Uatag as the new target air utilization rate Uatag, transmits the value to the corresponding module control unit 90 (step S222), and then returns to step S218. As described above, if the fuel utilization rate Ufsystem is reduced, the fuel electrode exhaust gas increases, and the temperature of the fuel cell stack 21 is increased by the combustion of the fuel electrode exhaust gas in the burner 22. Therefore, if the stack-related temperature Tst becomes higher than the specified temperature Tref, the target air utilization rate Uatag is reduced, and the fuel cell stack 21 is cooled by air. In this embodiment, the target air utilization rate Uatag is gradually reduced by a specified value ΔUa before the stack-related temperature Tst is lower than the specified temperature Tref, thereby performing a reduction correction of the target air utilization rate Uatag. If the CPU 101 determines in step S220 that the stack-related temperature Tst is lower than the specified temperature Tref, it proceeds to step S214. Moreover, in step S214, the CPU 101 returns to S200 unless there is a request to stop the power generation operation. If there is a request to stop the power generation operation, this program ends.

[0056] Thus, in the fuel cell system 10 of this embodiment, by increasing the target recirculation rate Retag or decreasing the target fuel utilization rate Uftag, the voltage V of the fuel cell stack 21 is increased, and the actual power generation output P is increased without increasing the current swept from the fuel cell stack 21. Figure 6 As shown, the voltage drop of the single cell is caused by ohmic resistance, overvoltage accompanying the electrode reaction of the fuel electrode and the air electrode, and the contact resistance between the single cells caused by stacking. Moreover, as the current density increases, the ohmic resistance, overvoltage, etc. increase, and the voltage between the terminals of the single cell decreases. That is, as the current density increases, the voltage V of the fuel cell stack 21 decreases. The current density can be replaced by the oxidation reaction rate of the fuel electrode. If the current (target current Itag) swept through the fuel cell stack 21 is increased, the oxygen ions (O 2-) will increase, and the amount of H2O in the fuel electrode will increase. The increase in H2O will oxidize the fuel electrode material (Ni) through the following chemical reaction (Ni+H2O→NiO+H2), thereby degrading the fuel electrode, that is, degrading the fuel cell stack 21. In the fuel cell system 10 of this embodiment, by increasing the voltage V of the fuel cell stack 21, the increase in the current (current density) swept through the fuel cell stack 21 relative to the required power output Preq can be suppressed, thereby suppressing the degradation of the fuel cell stack 21 and extending its service life.

[0057] Next, the operation of the fuel cell system 10 during startup and shutdown in the FC mode will be described. Figure 7 This is a flowchart showing an example of a startup / stop processing routine executed by the CPU 101 of the integrated control device 100. This routine is repeatedly executed in units of a predetermined time.

[0058] When the start-stop processing routine is executed, CPU 101 first determines whether it is time for system startup (step S300). If the CPU 101 determines that it is time for system startup, it reads the target recirculation rate Retag, target fuel utilization rate Uftag, and target air utilization rate Uatag for each power generation module 20 stored in a specified area of ​​EEPROM 104 (step S302), transmits the read target values ​​to the module control device 90 of the corresponding power generation module 20 (step S304), and then proceeds to step S306. As described above, the module control device 90 uses the received target values ​​to control the supply of fuel gas and air. If the CPU 101 determines that it is not time for system startup, it proceeds to step S306.

[0059] Next, CPU101 determines whether it is the time to stop the system (step S306). If CPU101 determines that it is not the time to stop the system, the program ends. On the other hand, if CPU101 determines that it is the time to stop the system, the target reflow rate Retag, target fuel utilization rate Uftag and target air utilization rate Uatag of each current power generation module 20 are stored in the specified area of ​​EEPROM104 (step S308), and the program ends. In this way, CPU101 stores the target reflow rate Retag, target fuel utilization rate Uftag and target air utilization rate Uatag in EEPROM104 when the system stops, and reads the target reflow rate Retag, target fuel utilization rate Uftag and target air utilization rate Uatag from EEPROM104 when the system starts and sends them to the corresponding module control device 90, so that each power generation module 20 can be properly operated in the case of stopping and starting the system.

[0060] In the fuel cell system 10 of the present embodiment described above, each fuel cell stack 21 is connected in series between a plurality of power generation modules 20. Within the range where the voltage V of each fuel cell stack 21 of the plurality of power generation modules 20 does not fall below the lower limit voltage Vlim, a target current Itag is set and transmitted to each module control device 90 so as to obtain an output corresponding to the required power generation output Preq required by the system. Thus, a fuel cell system 10 capable of obtaining a high output can be provided with a simple structure.

[0061] Furthermore, in the fuel cell system 10 of this embodiment, when the actual power generation output P is insufficient relative to the required power generation output Preq, the target fuel utilization rate Uftag is reduced. Reducing the target fuel utilization rate Uftag increases the voltage V of the corresponding fuel cell stack 21, thereby increasing the actual power generation output P. As a result, the voltage V of each fuel cell stack 21 can be kept above the lower limit voltage Vlim, and the output deficiency relative to the required power generation output Preq can be eliminated. Furthermore, by gradually reducing the target fuel utilization rate Uftag by a predetermined value ΔUf until the deficiency in the actual power generation output P relative to the required power generation output Preq is eliminated, the deficiency in the actual power generation output P relative to the required power generation output Preq can be eliminated without causing abrupt changes (pulsations) in the power generation state (current and voltage) of the fuel cell stack 21. Furthermore, in the fuel cell system 10 of this embodiment, when the stack-related temperature Tst is above the predetermined temperature Tref, the target air utilization rate Uatag in the corresponding fuel cell stack 21 is reduced. Reducing the fuel utilization rate Ufsystem increases the amount of exhaust fuel, and combustion of the exhaust fuel increases the temperature of the fuel cell stack 21. Therefore, by reducing the target air utilization rate Uatag, it is possible to cool the fuel cell stack 21 with air and maintain the fuel cell stack 21 within an appropriate temperature range.

[0062] Furthermore, in the fuel cell system 10 of this embodiment, when the actual power output P is insufficient relative to the required power output Preq, the target recirculation rate Retag is increased. Since the voltage V of the fuel cell stack 21 increases as the recirculation rate Re increases, even if the target current Itag is limited by the lower limit voltage Vlim, increasing the target recirculation rate Retag can increase the voltage V of the corresponding fuel cell stack 21, thereby increasing the actual power output P. As a result, the voltage V of each fuel cell stack 21 does not fall below the lower limit voltage Vlim, and the output deficiency relative to the required power output Preq can be eliminated. Furthermore, by gradually decreasing the target recirculation rate Retag by a predetermined value ΔRe until the actual power output deficiency relative to the required power output Preq is eliminated, the deficiency in the actual power output relative to the required power output Preq can be eliminated without causing abrupt changes (pulsations) in the power generation state (current, voltage) of the fuel cell stack 21.

[0063] Furthermore, in the fuel cell system 10 of this embodiment, the target recirculation rate Retag, the target fuel utilization rate Uftag, and the target air utilization rate Uatag are stored in the EEPROM 104 when the system is stopped. When the system is started, the target recirculation rate Retag, the target fuel utilization rate Uftag, and the target air utilization rate Uatag are read from the EEPROM 104 and transmitted to the corresponding module control device 90. Therefore, regardless of whether the system is stopped or started, the operation of each power generation module 20 can be properly performed.

[0064] In the fuel cell system 10 of this embodiment, the fuel cell stack 21 is configured as a reversible solid oxide cell stack, and has power generation mode (FC mode) and electrolysis mode (EC mode) as operating modes, thereby being able to cope with power supply and demand.

[0065] In the above embodiment, the circulation system 60 includes a return pipe 66 for returning the fuel exhaust gas to the branch pipe 41, and a regulating valve 67 provided in the return pipe 66. However, a hole may be provided instead of the regulating valve 67. In addition, the return pipe 66 may be omitted. In the above case, the reflux rate Re cannot be changed, so the CPU 101 of the unified control device 100 replaces Figure 5 The power generation control program can be executed by omitting steps S210 and S212. Figure 8 The power generation control program can be

[0066] In the above embodiment, in FC mode, the fuel supply system 40 supplies hydrogen as fuel gas to the fuel electrodes of the fuel cell stack 21. However, it is also possible to reform a raw fuel gas such as natural gas or LP gas into a fuel gas containing hydrogen and supply the fuel gas to the fuel electrodes of the fuel cell stack 21. In this case, the fuel supply system includes a gas pump that pressurizes the raw fuel gas to the branch pipe 41 and a desulfurizer that removes sulfur from the raw fuel gas. Furthermore, the module housing 29 only needs to include an evaporator that receives water (reforming water) stored in the water tank 81 and generates water vapor, and a reformer that uses the water vapor from the evaporator to reform the raw fuel gas into fuel gas.

[0067] In the above embodiment, the fuel cell system 10 includes the FC mode and the EC mode as the operation modes, but it is not necessary to include the EC mode. Figure 9 As shown in the modified example of the fuel cell system 10B, the hydrogen recovery system such as the hydrogen tank 2, the collecting pipe 3, and the opening and closing valve 4 is omitted; the water supply system 80 such as the water tank 81, the water supply pipe 82, the branch pipe 83, and the water pump 84 is omitted. Instead, it is sufficient to connect the hydrogen supply source to the hydrogen supply pipe 31.

[0068] In the above embodiment, each of the plurality of power generation modules 20 included in the fuel cell system 10 includes one fuel cell stack 21. However, all or part of the plurality of power generation modules 20 may include a plurality of fuel cell stacks 21 connected in series.

[0069] In the above embodiment, the module control device 90 and the unified control device 100 are constituted by separate control units, but may be constituted by a single control unit.

[0070] The following describes the correspondence between the main components of the embodiment and the main components of the invention described in the section "Means for Solving the Problem." In the embodiment, the fuel cell stack 21 corresponds to the "fuel cell stack," the power generation module 20 corresponds to the "power generation module," the various auxiliary equipment 30 including the fuel supply system 40, air supply system 50, and circulation system 60, etc., corresponds to the "auxiliary equipment," the module control device 90 corresponds to the "independent control unit," and the unified control device 100 corresponds to the "unified control unit." Furthermore, the EEPROM 104 corresponds to the "storage unit." Furthermore, the burner 22 corresponds to the "combustion unit," and the temperature sensor 94 corresponds to the "temperature sensor." Furthermore, the return pipe 66 corresponds to the "return path," and the regulating valve 67 corresponds to the "regulator."

[0071] Furthermore, the correspondence between the main components of the embodiment and the main components of the invention described in the "Solutions to Problems" column is intended to specifically illustrate an example of how the embodiment implements the invention described in the "Solutions to Problems" column and is therefore not limited to the components of the invention described in the "Solutions to Problems" column. In other words, the interpretation of the invention described in the "Solutions to Problems" column should not be based on the description in that column; the embodiment is merely a specific example of the invention described in the "Solutions to Problems" column.

[0072] [Summary of the present embodiment]

[0073] As described above, the gist of the fuel cell system (10) of the present invention is that it has a plurality of power generation modules (20), each of which includes one or more fuel cell stacks (21) for generating power by the reaction of fuel gas and oxidant gas, and auxiliary equipment (30) for controlling the fuel gas and oxidant gas supplied to the above-mentioned fuel cell stacks (21), each fuel cell stack (21) is connected in series between the above-mentioned plurality of power generation modules (20), and the fuel cell system (10) has: a plurality of independent control units (90), which respectively control the auxiliary equipment (30) of the corresponding power generation modules (20) in the above-mentioned plurality of power generation modules (20) based on control instructions; and a unified control unit (100), which sets current instructions in a manner to obtain an output corresponding to the required output required by the system (10) within a range where the voltage of each fuel cell stack (21) of the above-mentioned plurality of power generation modules (20) is not lower than a lower limit voltage, and sends them to the above-mentioned plurality of independent control units (90) (S202, S204).

[0074] In the fuel cell system of the present invention, each fuel cell stack is connected in series between multiple power generation modules. Within a range where the voltage of each fuel cell stack in each of the multiple power generation modules does not fall below a lower voltage limit, current commands are set and sent to independent control units that control each power generation module in a manner that produces an output corresponding to the system's required output. This allows for a fuel cell system capable of achieving high output with a simple structure.

[0075] In such a fuel cell system (10) of the present invention, the unified control unit (100) may also send a control instruction (S206, S208: yes, S210, S212, S216) to control the auxiliary equipment in a manner that increases the voltage of the fuel cell stack when the actual output is insufficient relative to the required output.

[0076] By increasing the voltage of the fuel cell stack through the control of auxiliary equipment, the actual output can be increased without lowering the lower limit voltage, and the required output can be met. In addition, the voltage drop of the single cell is caused by the overvoltage accompanied by ohmic resistance, electrode reaction, etc. Moreover, as the current density increases, the ohmic resistance, overvoltage, etc. increase, and the voltage between the terminals of the single cell decreases. That is, the voltage of the fuel cell stack decreases. The current density can be replaced by the oxidation reaction rate of the fuel electrode. If the current swept through the fuel cell stack is increased, the oxidation of the fuel electrode is aggravated, resulting in degradation of the fuel cell stack. In the fuel cell system of the present invention, by increasing the voltage of the fuel cell stack, the increase of the swept current relative to the required output can be suppressed, so the degradation of the fuel cell stack can be suppressed and the service life can be extended.

[0077] In the fuel cell system (10) of the present invention in which the voltage of the fuel cell stack is increased by controlling the auxiliary equipment, the above-mentioned multiple independent control units (90) can also control the corresponding auxiliary equipment (30) in such a manner that the utilization rate of the fuel gas in the respectively corresponding fuel cell stack (21) becomes the target utilization rate, and the above-mentioned unified control unit (100) can also reduce the target utilization rate of the above-mentioned fuel gas when the actual output is insufficient relative to the above-mentioned required output (S206, S208: yes, S216).

[0078] This is because the voltage of the fuel cell stack increases as the utilization rate of the fuel gas decreases.

[0079] In this case, the unified control unit (100) may gradually reduce the target utilization rate of the fuel gas until the deficiency of the actual output relative to the required output is resolved.

[0080] In this way, the shortfall of the actual output relative to the required output can be eliminated without causing a sudden change (pulsation) in the power generation state (current, voltage) of the fuel cell stack.

[0081] In this case, a storage unit (104) for storing the current target utilization rate of the above-mentioned fuel gas may also be provided, and the above-mentioned unified control unit (100) may also start the operation (S300, S302, S304, S306, S308) of the corresponding power generation module (20) with the target utilization rate of the above-mentioned fuel gas stored in the above-mentioned storage unit (104) after stopping the system (10) and when starting the system (10) next time.

[0082] In this way, each power generation module can be properly operated regardless of whether the power generation system is stopped or started.

[0083] In addition, in the above case, the above-mentioned multiple power generation modules (20) may also include: a combustion unit (22) that burns the exhaust gas discharged from the corresponding fuel cell stack (21), and a temperature sensor (94) that detects a temperature related to the temperature of the corresponding fuel cell stack (21). The above-mentioned multiple independent control units (90) may also control the corresponding auxiliary equipment (30) in such a manner that the utilization rate of the oxidant gas in the corresponding fuel cell stack (21) becomes a target utilization rate. The above-mentioned unified control unit (100) may also reduce the target utilization rate of the above-mentioned oxidant gas in the corresponding fuel cell stack (21) when the temperature detected by the above-mentioned temperature sensor (94) is above a specified temperature (S218, S220: yes, S222).

[0084] If the fuel gas utilization rate decreases, exhaust gas increases, and the temperature of the fuel cell stack rises due to the combustion of exhaust gas in the combustion unit. Therefore, by reducing the target utilization rate of the oxidant gas, the oxidant gas can be increased, and the oxidant gas can be used to suppress excessive temperature increases in the fuel cell stack.

[0085] In this case, a storage unit (104) for storing the current target utilization rate of the oxidant gas may also be provided, and the unified control unit (100) may also start the operation (S300, S302, S304, S306, S308) of the corresponding power generation module (20) with the target utilization rate of the oxidant gas stored in the storage unit (104) after stopping the system (10) and when starting the system (10) next time.

[0086] In this way, each power generation module can be properly operated regardless of whether the power generation system is stopped or started.

[0087] In addition, in the fuel cell system (10) of the present invention in which the voltage of the fuel cell stack is increased by controlling an auxiliary device, the above-mentioned multiple power generation modules (20) may also have: a reflux path (66) for refluxing the fuel exhaust gas discharged from the respectively corresponding fuel cell stack (21) to the fuel gas supply path (41), and an adjustment unit (67) provided in the above-mentioned reflux path (66) for adjusting the reflux rate of the above-mentioned fuel exhaust gas. The above-mentioned unified control unit (100) may also increase the above-mentioned reflux rate (S206, S208: yes, S212) when the actual output is insufficient relative to the above-mentioned required output.

[0088] This is because the voltage of the fuel cell stack increases as the utilization rate of the fuel gas decreases, and when the supply flow rate of the fuel gas is constant, the utilization rate of the fuel gas decreases as the recirculation rate increases.

[0089] In this case, the unified control unit (100) may gradually increase the reflux rate until the deficiency of the actual output relative to the required output is eliminated or the reflux rate reaches an upper limit value.

[0090] In this way, the shortfall of actual output relative to required output can be eliminated without causing a sudden change (pulsation) in the power generation state (current, voltage) of the fuel cell stack.

[0091] In this case, a storage unit (104) for storing the current reflux rate may also be provided, and the unified control unit (100) may also start the operation (S300, S302, S304, S306, S308) of the corresponding power generation module (20) with the reflux rate stored in the storage unit (104) after stopping the system (10) and when starting the system (10) next time.

[0092] In this way, each power generation module can be properly operated regardless of whether the power generation system is stopped or started.

[0093] In addition, in the fuel cell system (10) of the present invention, the unified control unit (100) can also set the upper limit current in such a manner that the lowest voltage among the voltages of the fuel cell stacks (21) connected in series between the multiple power generation modules (20) is not lower than the lower limit voltage, set the required current based on the required output and the overall voltage of the fuel cell stacks, and set the current instruction based on the smaller of the required current and the upper limit current.

[0094] In this way, the required output can be met without deteriorating any of the fuel cell stacks.

[0095] In addition, in the fuel cell system (10) of the present invention, the above-mentioned fuel cell stack (21) can also be a reversible solid oxide type cell stack that can perform a power generation action of generating electricity through the reaction of fuel gas and oxidant gas, and an electrolysis action of generating hydrogen through high-temperature water vapor electrolysis. The above-mentioned unified control unit (100) can also send control instructions to the corresponding independent control unit (90) in a manner of switching the above-mentioned power generation action and the above-mentioned electrolysis action.

[0096] In this way, the supply and demand of electricity can be met.

[0097] While the modes for carrying out the present invention have been described above using the embodiments, the present invention is not limited to the embodiments and can of course be carried out in various forms without departing from the spirit of the present invention.

[0098] Possibility of industrial application

[0099] The present invention can be applied to the fuel cell system manufacturing industry and the like.

[0100] Description of Reference Signs

[0101] 10…Fuel cell system, 20…Power generation module, 21…Fuel cell stack, 22…Burner (combustion unit), 30…Auxiliary equipment, 41…Branch pipe (supply path), 66…Return path, 67…Adjustment valve (adjustment unit), 90…Module control device (independent control unit), 94…Temperature sensor, 100…Unified control device (unified control unit), 104…EEPROM (storage unit).

Claims

1. A fuel cell system comprising: a plurality of power generation modules, each of which includes one or more fuel cell stacks for generating electricity through a reaction between a fuel gas and an oxidant gas, and auxiliary equipment for controlling the supply of the fuel gas and the oxidant gas to the fuel cell stacks, wherein the fuel cell stacks are connected in series between the plurality of power generation modules, the fuel cell system comprising: a plurality of independent control units, which respectively control auxiliary equipment of corresponding power generation modules among the plurality of power generation modules based on control instructions; and The unified control unit sets a current instruction and sends it to the plurality of independent control units in such a manner that an output corresponding to a required output of the system is obtained within a range in which the voltage of each fuel cell stack of the plurality of power generation modules does not fall below a lower limit voltage.

2. The fuel cell system according to claim 1, wherein: The unified control unit transmits a control command to control the auxiliary equipment so as to increase the voltage of the fuel cell stack when the actual output is insufficient with respect to the required output.

3. The fuel cell system according to claim 2, wherein: The plurality of independent control units control the corresponding auxiliary equipment in such a manner that the utilization rate of the fuel gas in the corresponding fuel cell stack reaches the target utilization rate. The unified control unit reduces the target utilization rate of the fuel gas when the actual output is insufficient with respect to the required output.

4. The fuel cell system according to claim 3, wherein: The unified control unit gradually reduces the target utilization rate of the fuel gas until a deficiency in the actual output relative to the required output is resolved.

5. The fuel cell system according to claim 4, comprising: a storage unit storing the current target utilization rate of the fuel gas; The unified control unit starts the operation of the corresponding power generation module at the target utilization rate of the fuel gas stored in the storage unit when the system is next started after the system is stopped.

6. The fuel cell system according to any one of claims 3 to 5, wherein: The plurality of power generation modules include a combustion unit for combusting exhaust gas discharged from the corresponding fuel cell stacks, and a temperature sensor for detecting a temperature correlated with the temperature of the corresponding fuel cell stacks. The plurality of independent control units control the corresponding auxiliary equipment so that the utilization rate of the oxidant gas in the corresponding fuel cell stack reaches the target utilization rate. The unified control unit reduces a target utilization rate of the oxidant gas in a corresponding fuel cell stack when the temperature detected by the temperature sensor is equal to or higher than a predetermined temperature.

7. The fuel cell system according to claim 6, comprising: a storage unit storing the current target utilization rate of the oxidant gas; The unified control unit starts the operation of the corresponding power generation module at the target utilization rate of the oxidant gas stored in the storage unit when the system is next started after the system is stopped.

8. The fuel cell system according to any one of claims 2 to 5, wherein: The plurality of power generation modules include: a recirculation path for returning the fuel exhaust gas discharged from the corresponding fuel cell stack to the fuel gas supply path; and a regulating unit provided in the recirculation path for regulating the recirculation rate of the fuel exhaust gas. The unified control unit increases the reflux rate when the actual output is insufficient with respect to the required output.

9. The fuel cell system according to claim 8, wherein: The unified control unit gradually increases the reflux rate until a deficiency in actual output relative to the required output is resolved or the reflux rate reaches an upper limit.

10. The fuel cell system according to claim 9, comprising: a storage unit for storing the current reflux rate; The unified control unit starts the operation of the corresponding power generation module at the next system startup after the system is stopped, using the reflux ratio stored in the storage unit.

11. The fuel cell system according to any one of claims 1 to 5, wherein: The above-mentioned unified control unit sets the upper limit current in such a way that the lowest voltage among the voltages of each fuel cell stack connected in series between the above-mentioned multiple power generation modules is not lower than the lower limit voltage, sets the required current based on the above-mentioned required output and the overall voltage of each of the above-mentioned fuel cell stacks, and sets the above-mentioned current instruction based on the smaller of the above-mentioned required current and the above-mentioned upper limit current.

12. The fuel cell system according to any one of claims 1 to 5, wherein: The fuel cell stack is a reversible solid oxide type cell stack capable of performing power generation operation by the reaction of fuel gas and oxidant gas, and electrolysis operation by electrolyzing high-temperature steam to generate hydrogen. The unified control unit sends a control instruction to the corresponding independent control unit in a manner of switching the power generation action and the electrolysis action.

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