fuel cell system

Abstract

To provide a technique for making the difference in the amount of residual water in each fuel cell stack relatively small even when wastewater treatment for a plurality of fuel cell stacks is completed at the same time.SOLUTION: A fuel cell system includes a plurality of fuel cell stacks, a gas supply unit, an impedance detection unit, and a control device, and when the control device stops power generation of the plurality of fuel cell stacks, the control device is configured to supply gas to the plurality of fuel cell stacks, and be able to perform wastewater treatment to discharge water remaining in the plurality of fuel cell stacks. The wastewater treatment includes restricting the gas delivery flow rate for the fuel cell stack whose detected value has reached a predetermined value among the plurality of fuel cell stacks until the detected value reaches the predetermined value for all of the plurality of fuel cell stacks.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The technology disclosed in this specification relates to a fuel cell system. [Background technology]

[0002] In a fuel cell system, when power generation in a fuel cell stack is stopped, water generated by a chemical reaction remains in the fuel cell stack. If this residual water is left in the fuel cell stack, it may freeze in low-temperature environments such as below freezing, causing the fuel cell stack to malfunction.

[0003] In this regard, Patent Document 1 describes a fuel cell system that, when power generation by a fuel cell stack is stopped, sends oxidizing gas to the fuel cell stack and performs a drainage process to discharge water remaining in the fuel cell stack. At this time, the amount of water remaining in the fuel cell stack is determined based on the impedance of the fuel cell stack. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-180160 Summary of the Invention [Problem to be solved by the invention]

[0005] When multiple fuel cell stacks are electrically connected in series, it is preferable to simultaneously complete the drainage process for all fuel cell stacks in order to control their operation. However, the drainage process for each fuel cell stack is rarely completed at the same time, and there are often differences in the timing at which the drainage process is completed. In this case, if the discharge process is terminated when drainage is completed for some fuel cell stacks, residual water will remain in the remaining fuel cell stacks. Furthermore, if the drainage process is continued until drainage is completed for all fuel cell stacks, the drainage process will be performed more than necessary for fuel cell stacks that have already completed drainage.

[0006] In view of the above circumstances, this specification provides a technique for relatively reducing the difference in the amount of residual water in each fuel cell stack even when the drainage treatment of multiple fuel cell stacks is completed simultaneously. [Means for solving the problem]

[0007] The technology disclosed in this specification is embodied in a fuel cell system. In a first aspect thereof, the fuel cell system includes a plurality of fuel cell stacks electrically connected in series, a gas supply unit that supplies gas to the plurality of fuel cell stacks, an impedance detection unit that detects the impedance of the plurality of fuel cell stacks, and a control device that controls the operation of the plurality of fuel cell stacks and the gas supply unit and acquires the detected value by the impedance detection unit. The control device is configured to supply the gas to the plurality of fuel cell stacks and execute a drainage process to discharge water remaining in the plurality of fuel cell stacks when power generation of the plurality of fuel cell stacks is stopped. The drainage process includes limiting the gas output flow rate for a fuel cell stack among the plurality of fuel cell stacks whose detected value has reached a predetermined value until the detected value reaches a predetermined value for all of the plurality of fuel cell stacks.

[0008] In the above-described fuel cell system, the control device monitors the impedance of each fuel cell stack during the drainage process. The lower the residual water volume in the fuel cell stack, the higher the impedance detected for that fuel cell stack. Therefore, a fuel cell stack for which an impedance exceeding a predetermined value is detected is considered to have a relatively small amount of residual water, while a fuel cell stack for which an impedance below the predetermined value is detected is considered to have a relatively large amount of residual water. Therefore, in the drainage process of the present technology, the gas output flow rate is limited for fuel cell stacks whose impedance has reached a predetermined value until the impedances of all fuel cell stacks reach the predetermined value. With this configuration, the residual water volumes in all fuel cell stacks can be equalized during the drainage process. As a result, the drainage process can be resumed without limiting the gas output flow rate for each fuel cell stack, and the drainage processes of multiple fuel cell stacks can be completed simultaneously, thereby relatively reducing the difference in the residual water volumes in each fuel cell stack.

[0009] In a second aspect, in the first aspect, the wastewater treatment may further include correcting the gas discharge flow rate in accordance with the temperature of the plurality of fuel cell stacks. The amount of wastewater discharged from a fuel cell stack varies depending on the saturated water vapor amount, which depends on the temperature of the fuel cell stack, as well as the gas discharge flow rate supplied to the fuel cell stack. Therefore, by correcting the gas discharge flow rate to each fuel cell stack in accordance with the temperature of each fuel cell stack, the difference in the amount of residual water in each fuel cell stack can be eliminated in a relatively short time.

[0010] In a third aspect, in the first or second aspect, the gas may be an oxidizing gas supplied to the cathode side of a plurality of fuel cell stacks. In a fuel cell stack, water is produced mainly by a chemical reaction of gas on the cathode side. Therefore, by performing a wastewater treatment using the oxidizing gas supplied to the cathode side of the fuel cell stack, the remaining water in the fuel cell stack can be efficiently discharged to the outside.

[0011] In a fourth aspect, in any one of the first to third aspects, the gas supply unit may include, for each of the plurality of fuel cell stacks, an oxidizing gas supply path connected to the fuel cell stack's supply port and equipped with a compressor, an oxidizing gas discharge path connected to the fuel cell stack's discharge port, and a shunt path connecting the oxidizing gas supply path and the oxidizing gas discharge path to each other and equipped with a shunt valve. In this case, limiting the oxidizing gas output flow rate during wastewater treatment may include opening the shunt valve to divert the oxidizing gas from the shunt path to the oxidizing gas discharge path. With this configuration, the output flow rate of the oxidizing gas supplied to the fuel cell stack can be adjusted without precisely controlling the operation of the compressor.

[0012] Additionally or alternatively, limiting the flow rate of the oxidizing gas in the wastewater treatment may include limiting the flow rate of the oxidizing gas sent by the compressor. With this configuration, the amount of oxidizing gas supplied to the fuel cell unit can be adjusted regardless of whether or not the above-mentioned branch path is provided. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a circuit diagram including a fuel cell system 10 according to an embodiment. [Figure 2] 1 is a diagram schematically showing the configuration of a fuel cell system 10 according to an embodiment. [Figure 3] 3 is a flow diagram illustrating the drainage treatment executed by the control device 70. In Fig. 3, Imp(A) means the impedance of the first fuel cell stack 22A, and Imp(B) means the impedance of the second fuel cell stack 22B. [Figure 4]4(A) is a diagram showing an example of the impedance of the first fuel cell stack 22A, the air discharge flow rate, and the openings of the inlet valve 36, the flow dividing valve 40, and the outlet valve 44 for the first fuel cell unit 20A. FIG. 4(B) is a diagram showing an example of the impedance of the second fuel cell stack 22B, the air discharge flow rate, and the openings of the inlet valve 36, the flow dividing valve 40, and the outlet valve 44 for the second fuel cell unit 20B. [Figure 5] FIG. 4 is a diagram for explaining the relationship between the amount of residual water in the fuel cell stack 22 and impedance. [Figure 6] 6A and 6B are diagrams illustrating an example in which the air discharge flow rate during wastewater treatment is corrected in accordance with the temperatures of the fuel cell stacks 22A and 22B. Fig. 6A corresponds to Fig. 3A, and Fig. 6B corresponds to Fig. 3B. In Fig. 6B, the air discharge flow rate of Fig. 3B is corrected. DETAILED DESCRIPTION OF THE INVENTION

[0014] A fuel cell system 10 of this embodiment will be described with reference to the drawings. The fuel cell system 10 of this embodiment is mainly installed in large fuel cell vehicles (e.g., automobiles, buses, trucks, and trains) and stationary fuel cell devices. Note that the fuel cell system 10 may also be installed in various mobile bodies other than vehicles (e.g., ships and airplanes).

[0015] 1 and 2, the fuel cell system 10 includes a plurality of fuel cell units 20 and a control device 70 that controls the operation of the plurality of fuel cell units 20. The plurality of fuel cell units 20 includes a first fuel cell unit 20A and a second fuel cell unit 20B. The number of the plurality of fuel cell units 20 included in the fuel cell system 10 is not particularly limited. In other embodiments, the number of the plurality of fuel cell units 20 may be three or more.

[0016] 1 and 2, each fuel cell unit 20 includes a fuel cell stack 22, an oxidizing gas supply unit 30, a fuel gas supply unit 50, and a cell monitor 60. The fuel cell stack 22 of the first fuel cell unit 20A and the fuel cell stack 22 of the second fuel cell unit 20B are electrically connected in series. Hereinafter, the fuel cell stack 22 of the first fuel cell unit 20A will be simply referred to as the first fuel cell stack 22A, and the fuel cell stack 22 of the second fuel cell unit 20B will be simply referred to as the second fuel cell stack 22B.

[0017] Although not particularly limited, the two fuel cell stacks 22 connected in series are connected to a battery 104 via a DC-DC converter 102 and supply power to the battery 104. The battery 104 contains multiple secondary battery cells and is configured to be repeatedly chargeable using external power. The DC-DC converter 102 is a converter that boosts power and controls the power transferred between the two fuel cell stacks 22 and the battery 104. The two fuel cell stacks 22 are also connected to a load 108 via a PCU 106 and supply power to the load 108. The PCU 106 contains a DC-DC converter and / or an inverter and controls the power transferred between the two fuel cell stacks 22 and the load 108. Similarly, the battery 104 is also connected to the load 108 via the PCU 106 and supplies power to the load 108. As will be described in detail later, the operation of the two fuel cell stacks 22 is monitored and controlled by a control device 70.

[0018] As shown in Fig. 2, the fuel cell stack 22 has a structure in which a plurality of fuel cell units 24 are stacked. The fuel cell stack 22 generates electricity by causing a chemical reaction between an oxidizing gas and a fuel gas within the plurality of fuel cell units 24. In the fuel cell unit 20 of this embodiment, air is used as the oxidizing gas supplied to the cathode side, and hydrogen gas is used as the fuel gas supplied to the anode side. That is, in this embodiment, air is an example of an oxidizing gas, and hydrogen gas is an example of a fuel gas.

[0019] The specific configuration of the fuel cell 24 is not particularly limited. Although not shown, for example, each fuel cell 24 includes a membrane electrode and gas diffusion layer assembly (MEGA), an anode-side separator, a cathode-side separator, and a support frame. The membrane electrode gas diffusion layer assembly is configured by laminating an anode-side gas diffusion layer, an anode electrode, an electrolyte membrane, a cathode electrode, and a cathode-side gas diffusion layer in this order.

[0020] 2, the oxidizing gas supply unit 30 is a unit for supplying oxidizing gas (air) to the fuel cell stack 22. The oxidizing gas supply unit 30 includes a compressor 32, an oxidizing gas supply path 34, an inlet valve 36, a flow dividing path 38, a flow dividing valve 40, an oxidizing gas discharge path 42, and an outlet valve 44.

[0021] The compressor 32 is provided in an oxidizing gas supply path 34. The oxidizing gas supply path 34 is connected to the cathode supply port 26 of the fuel cell stack 22. The cathode supply port 26 of the fuel cell stack 22 is connected to each of the plurality of fuel cell units 24 within the fuel cell stack 22. The air compressed by the compressor 32 is supplied to the cathode supply port 26 of the fuel cell stack 22 through the oxidizing gas supply path 34. An inlet valve 36 is provided in the oxidizing gas supply path 34. Although not particularly limited, the oxidizing gas supply unit 30 may further include an intercooler that cools the air that has been compressed by the compressor 32 and has become hot.

[0022] The oxidizing gas discharge path 42 is connected to the cathode-side discharge port 27 of the fuel cell stack 22. The cathode-side discharge port 27 of the fuel cell stack 22 is connected to each of the plurality of fuel cell units 24 within the fuel cell stack 22. After the air has passed through the plurality of fuel cell units 24, it undergoes a reaction within the fuel cell stack 22 and is then discharged from the fuel cell stack 22 to the oxidizing gas discharge path 42 through the cathode-side discharge port 27. An outlet valve 44 is provided in the oxidizing gas discharge path 42. The outlet valve 44 is also called a backpressure valve, and together with the flow dividing valve 40, adjusts the discharge flow rate of the oxidizing gas supplied to the fuel cell stack 22.

[0023] The diversion path 38 connects the oxidizing gas supply path 34 and the oxidizing gas discharge path 42 to each other. The diversion path 38 is provided with a diversion valve 40. Depending on the opening and closing of the diversion valve 40 and the outlet valve 44, some or all of the air flowing through the oxidizing gas supply path 34 is sent to the oxidizing gas discharge path 42 via the diversion path 38 without being supplied to the fuel cell stack 22. This adjusts the flow rate of the oxidizing gas supplied to the fuel cell stack 22. The inlet valve 36 is located downstream of the branch point of the oxidizing gas supply path 34 and the diversion path 38, and the outlet valve 44 is located upstream of the junction point of the oxidizing gas discharge path 42 and the diversion path 38.

[0024] As shown in FIG. 2, the fuel gas supply unit 50 is a unit for supplying fuel gas (hydrogen gas) to the fuel cell stack 22. The fuel gas supply unit 50 includes a fuel gas tank 52, a fuel gas supply path 54, a supply control valve 56, and a fuel gas discharge path 58. The fuel gas tank 52 stores hydrogen gas. The fuel gas tank 52 is connected to the anode-side supply port 28 of the fuel cell stack 22 via the fuel gas supply path 54. The cathode-side supply port 26 of the fuel cell stack 22 is connected to each of the multiple fuel cell units 24 within the fuel cell stack 22. The fuel gas supply path 54 is provided with a supply control valve 56. When the supply control valve 56 is opened, the hydrogen gas supplied from the fuel gas tank 52 passes through the fuel gas supply path 54 and is supplied to the anode-side supply port 28 of the fuel cell stack 22.

[0025] The anode-side outlet 29 of the fuel cell stack 22 is connected to a fuel gas discharge path 58. The anode-side outlet 29 of the fuel cell stack 22 is connected to each of the plurality of fuel cell units 24 within the fuel cell stack 22. The hydrogen gas that flows into the fuel cell stack 22 is discharged to the outside from the fuel gas discharge path 58 via the anode-side outlet 29. In this case, the gas discharged to the outside from the fuel cell stack 22 may contain unreacted hydrogen gas. Therefore, each fuel cell unit 20 may further be provided with a circulation path (not shown) so that the unreacted hydrogen gas can be circulated to the fuel cell stack 22.

[0026] 2, the cell monitor 60 is disposed within the fuel cell stack 22 and is electrically connected to the plurality of fuel cell units 24. The cell monitor 60 outputs a voltage signal for detecting the voltage of each fuel cell unit 24. As will be described in more detail below, the voltage signal output by the cell monitor 60 is input to the control device 70 and used in the process of calculating the impedance of the fuel cell unit 24.

[0027] As shown in FIGS. 1 and 2 , the control device 70 is a computer device having a processor, memory, etc. The control device 70 is communicatively connected to the fuel cell stack 22, the compressor 32 of the oxidizing gas supply unit 30, the inlet valve 36, the flow divider valve 40, the outlet valve 44, and the supply control valve 56 of the fuel gas supply unit 50, and can control and monitor their operation. The control device 70 calculates the power required for the fuel cell stack 22 based on an external power requirement. Based on the calculated power requirement, the control device 70 controls the operation of the compressor 32, the inlet valve 36, the flow divider valve 40, the outlet valve 44, and the supply control valve 56. This controls the pressure of the air and hydrogen gas supplied to the fuel cell stack 22, and adjusts the output power from the fuel cell stack 22 to the required power. As described above, the control device 70 receives a voltage signal from the cell monitor 60. The control device 70 also monitors the output current of the fuel cell stack 22. The control device 70 can calculate the impedance of the fuel cell 24 based on the voltage signal from the cell monitor 60 and the output current of the fuel cell stack 22. Here, the cell monitor 60 and part of the control device 70 in this specification are examples of the impedance detection unit of the present technology. Note that the control device 70 may be configured as a single computer device, or may be configured as a combination of multiple computer devices.

[0028] The control device 70 is configured to be able to perform a water discharge process. The water discharge process is mainly performed when power generation by the plurality of fuel cell stacks 22 is stopped. In the water discharge process, gas (here, air) is supplied to the plurality of fuel cell stacks 22, and water remaining in the plurality of fuel cell stacks 22 is discharged.

[0029] 3-5, the water discharge process performed by the control device 70 will be described. When the water discharge process starts ("START" in FIG. 3), the control device 70 operates the compressor 32 of each fuel cell unit 20. The control device 70 also opens the inlet valve 36 and the outlet valve 44 and closes the diverter valve 40 (time 0 in FIG. 4). As an example, as shown in FIGS. 4(A) and 4(B), in each fuel cell unit 20A, 20B, the opening degree of the inlet valve 36 and the outlet valve 44 is 100%, and the opening degree of the diverter valve 40 is 0%. At this time, in each fuel cell unit 20, all of the air compressed by the compressor 32 and flowing into the oxidizing gas supply path 34 flows into the fuel cell stack 22. The air that has passed through the fuel cell stack 22 is then discharged to the outside via the oxidizing gas discharge path 42, along with any remaining water in the fuel cell stack 22. At this time, the air discharge flow rate GA (hereinafter sometimes referred to as the first discharge flow rate GA) sent to the first fuel cell stack 22A and the air discharge flow rate GB (hereinafter sometimes referred to as the second discharge flow rate GB) sent to the second fuel cell stack 22B become a predetermined standard flow rate N.

[0030] The control device 70 then determines whether the impedance of the first fuel cell stack 22A is equal to or greater than a predetermined value PV (step S10). The predetermined value PV may be determined experimentally or through simulation. As an example, the predetermined value PV may be the impedance at which the slope of the graph of FIG. 5 showing the relationship between the residual water volume in the fuel cell stack 22 and the impedance of the fuel cell stack 22 changes significantly. For example, the predetermined value PV may be the impedance at which the impedance of the fuel cell stack 22 begins to change relatively significantly with respect to the change in the residual water volume in the fuel cell stack 22. In other embodiments, the predetermined value PV may be determined taking into account the influence of factors other than the residual water volume in the fuel cell stack 22 on the impedance. Measuring the impedance requires power generation by the fuel cell stack 22. The power generated by this power generation is used to supply air to the fuel cell stack 22.

[0031] If step S10 returns NO, it is assumed that the amount of water remaining in the first fuel cell stack 22A is relatively large and the water discharge process is proceeding relatively slowly. Conversely, if step S10 returns YES, it is assumed that the amount of water remaining in the first fuel cell stack 22A is relatively small and the water discharge process is proceeding relatively quickly.

[0032] 3 is initiated, the amount of residual water in each fuel cell stack 22 is relatively large, and the impedance of the first fuel cell stack 22A is smaller than the predetermined value PV, i.e., the result in step S10 is NO. In this case, the control device 70 determines whether the impedance of the second fuel cell stack 22B is equal to or greater than the predetermined value PV (step S12). The predetermined value PV in step S12 is equal to the predetermined value PV in step S10. As described above, immediately after the water discharge process of FIG. 3 is initiated, the amount of residual water in each fuel cell stack 22 is relatively large, and therefore the impedance of the second fuel cell stack 22B is also smaller than the predetermined value PV, i.e., the result in step S12 is NO.

[0033] If the answers to steps S10 and S12 are NO, the control device 70 maintains the first delivery flow rate GA and the second delivery flow rate GB at the aforementioned standard flow rate N (step S14). That is, the control device 70 maintains the openings of the inlet valve 36 and outlet valve 44 at 100% in each fuel cell unit 20A, 20B, and maintains the opening of the flow dividing valve 40 at 0% (from time 0 to time T1 in FIG. 4). This allows the drainage process for each fuel cell stack 22A, 22B to proceed.

[0034] The control device 70 then determines whether or not the termination condition is met (step S16). The termination condition here includes that the impedance of the first fuel cell stack 22A and the impedance of the second fuel cell stack 22B are both equal to or greater than the termination value FV. The termination value LV is the impedance value that allows the end of the drainage process for the fuel cell stack 22, and is a value greater than a predetermined value PV. As an example, as shown in FIG. 5, the termination value LV can be the impedance value that corresponds to when the amount of residual water in the fuel cell stack 22 is substantially zero. If the determination in step S16 is NO, the control device 70 returns to the processing of step S10.

[0035] As the water discharge process progresses in each fuel cell stack 22A, 22B, differences in the progress of the water discharge processes may occur. For example, the water discharge process in the first fuel cell stack 22A may progress relatively quickly, while the water discharge process in the second fuel cell stack 22B may progress relatively slowly. In this case, only the impedance of the first fuel cell stack 22A will reach the predetermined value PV (YES in step S10 and NO in step S20).

[0036] If step S10 is YES and step S20 is NO, the control device 70 limits the first delivery flow rate GA to the first fuel cell stack 22A to a predetermined limit flow rate n (step S22). The limit flow rate n is a value smaller than the standard flow rate N, and the specific numerical value is not particularly limited. Meanwhile, the second delivery flow rate GB to the second fuel cell stack 22B is maintained at the standard flow rate N. As a result, the first delivery flow rate GA becomes smaller than the second delivery flow rate GB. The control device 70 can adjust the first delivery flow rate GA to the limit flow rate n by limiting the flow rate of air delivered by the compressor 32 in the first fuel cell unit 20A and / or changing the opening of the outlet valve 44 or the dividing valve 40.

[0037] As an example, the control device 70 in this embodiment increases the opening degree of the flow dividing valve 40 in the first fuel cell unit 20A to a predetermined opening degree (e.g., 80%) and decreases the opening degree of the outlet valve 44 to a predetermined limited opening degree (e.g., 20%) (time T1 in FIG. 4). On the other hand, in the second fuel cell unit 20B, the opening degrees of the inlet valve 36 and the outlet valve 44 are maintained at 100%, and the opening degree of the flow dividing valve 40 is maintained at 0%. As a result, in the first fuel cell unit 20A, a portion of the air compressed by the compressor 32 flows into the oxidizing gas discharge path 42 via the flow dividing path 38. In other words, the flow rate of air sent to the first fuel cell stack 22A is limited. In this way, in the first fuel cell unit 20A, the first delivery flow rate GA is limited by opening the flow dividing valve 40 to divert air from the flow dividing path 38 to the oxidizing gas discharge path 42. As a result, in the first fuel cell stack 22A, necessary power generation continues, but wastewater treatment does not substantially progress.

[0038] If the water discharge process in the first fuel cell stack 22A does not substantially progress while the water discharge process in the second fuel cell stack 22B progresses, the progress of the water discharge processes in the two fuel cell stacks 22A and 22B will eventually become equal. That is, when the impedance of the first fuel cell stack 22A reaches a predetermined value PV (YES in step S10), the impedance of the second fuel cell stack 22B also reaches a predetermined value PV (YES in step S20).

[0039] If the answers to steps S10 and S20 are YES, the control device 70 adjusts the first delivery flow rate GA and the second delivery flow rate GB to the standard flow rate N (step S24). That is, the control device 70 maintains the opening degrees of the inlet valve 36 and the outlet valve 44 at 100% in each fuel cell unit 20A, 20B, and maintains the opening degree of the flow dividing valve 40 at 0% (time T2 in FIG. 4). As a result, after time T2 in FIG. 4, the drainage process in each fuel cell stack 22A, 22B progresses equally.

[0040] As the water discharge process in each fuel cell stack 22A, 22B progresses further, the impedances of both fuel cell stacks 22A, 22B become equal to or greater than the end value LV (YES in step S16). At this time, the control device 70 ends the water discharge process shown in Figure 3 (time T3 in Figure 4).

[0041] On the other hand, the water discharge process of the first fuel cell stack 22A may proceed relatively slowly, while the water discharge process of the second fuel cell stack 22B may proceed relatively quickly. In this case, only the impedance of the second fuel cell stack 22B reaches the predetermined value PV (NO in step S10 and YES in step S12), and the process of step S18 is executed instead of the process of step S22. In step S18, the control device 70 limits the second delivery flow rate GB to the second fuel cell stack 22B to a predetermined limit flow rate n, and maintains the first delivery flow rate GA to the first fuel cell stack 22A at the standard flow rate N. This makes the second delivery flow rate GB smaller than the first delivery flow rate GA. As a result, the second fuel cell stack 22B continues to generate necessary power, but the water discharge process does not substantially proceed.

[0042] In the fuel cell system 10 described above, the control device 70 monitors the impedance of each fuel cell stack 22 during the water discharge process. The smaller the residual water volume in the fuel cell stack 22, the higher the impedance detected for that fuel cell stack 22. Therefore, a fuel cell stack 22 for which an impedance exceeding a predetermined value PV is detected is considered to have a relatively small amount of residual water therein, while a fuel cell stack 22 for which an impedance below the predetermined value PV is detected is considered to have a relatively large amount of residual water therein. Therefore, in the water discharge process shown in FIG. 3, the air discharge flow rate is limited for the first fuel cell stack 22A, whose impedance has reached the predetermined value, until the impedances of the two fuel cell stacks 22A and 22B reach the predetermined value PV (from time T1 to time T2 in FIG. 4). With this configuration, the residual water volumes in the two fuel cell stacks 22A and 22B can be made equal during the water discharge process. This allows the drainage process to be resumed without restricting the air discharge flow rate for each fuel cell stack 22A, 22B (time T2 in Figure 4), and the drainage process for the two fuel cell stacks 22A, 22B to be completed simultaneously (time T3 in the figure), thereby making it possible to relatively reduce the difference in the amount of residual water in each fuel cell stack.

[0043] In the above-described embodiment, the air delivery rates GA and GB, which are not limited in delivery rate in steps S14, S18, S22, and S24 of Fig. 3, are equal to the standard flow rate N. In this regard, in other embodiments, the air delivery rates GA and GB, which are not limited in delivery rate in steps S14, S18, S22, and S24 of Fig. 3, may be greater than or less than the standard flow rate N.

[0044] 3, when limiting the second delivery flow rate GB, the diverter valve 40 in the second fuel cell unit 20B is opened to divert air from the diverter path 38 to the oxidizing gas discharge path 42. In this regard, in other embodiments, the second delivery flow rate GB may be limited by limiting the air delivery flow rate by the compressor 32 of the second fuel cell unit 20B. With this configuration, the delivery flow rate of air supplied to the fuel cell stack 22 can be adjusted regardless of whether the diverter path 38 is present or not.

[0045] Although not particularly limited, as shown in FIG. 6, the above-described water discharge process may further include correcting the air discharge flow rate in accordance with the temperature of the fuel cell stack 22. In the fuel cell stack 22, the rate of progress of the water discharge process varies depending on the temperature (more specifically, the saturated water vapor amount) within the fuel cell stack 22. That is, the higher the temperature of the fuel cell stack 22 and the greater the saturated water vapor amount, the faster the water discharge process progresses. For example, when the temperature of the second fuel cell stack 22B is lower than that of the first fuel cell stack 22A, the saturated water vapor amount of the second fuel cell stack 22B becomes smaller than the saturated water vapor amount of the first fuel cell stack 22A. In this case, the second discharge flow rate GB to the second fuel cell stack 22B may be set to a flow rate N+α, which is the sum of the standard flow rate N and the correction flow rate α, and may be greater than the first discharge flow rate GA to the first fuel cell stack 22A. In this way, by correcting the discharge flow rates GA and GB to each fuel cell stack 22 in accordance with the temperature of each fuel cell stack 22, the difference in the amount of residual water in each fuel cell stack 22 can be eliminated in a relatively short time.

[0046] Although several specific examples have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above. The technical elements described in this specification or drawings exhibit technical utility either alone or in combination. [Explanation of symbols]

[0047] 10: Fuel cell system 20A: First fuel cell unit 20B: Second fuel cell unit 22A: First fuel cell stack 22B: Second fuel cell stack 24: Fuel cell 30: Oxidizing gas supply unit 32: Compressor 34: Oxidizing gas supply route 36: Inlet valve 38: Diversion route 40: Flow dividing valve 42: Oxidizing gas emission route 44: Outlet valve 50: Fuel gas supply unit 52: Fuel gas tank 54: Fuel gas supply route 56: Supply control valve 58: Fuel gas exhaust route 60: Cell monitor 70: Control device 102: DC-DC converter 104: Battery 108: Load

Claims

1. a plurality of fuel cell stacks electrically connected in series; a gas supply unit for supplying gas to the plurality of fuel cell stacks; an impedance detection unit for detecting impedances of the plurality of fuel cell stacks; a control device that controls the operations of the plurality of fuel cell stacks and the gas supply unit and acquires the detected value by the impedance detection unit; Equipped with the control device is configured to supply the gas to the plurality of fuel cell stacks when power generation of the plurality of fuel cell stacks is stopped, and to perform a drainage process of draining water remaining in the plurality of fuel cell stacks, The wastewater treatment includes: adjusting the gas delivery flow rate to a predetermined standard flow rate for each of the plurality of fuel cell stacks until the detected value reaches a predetermined intermediate value; limiting the gas delivery flow rate of a fuel cell stack among the plurality of fuel cell stacks for which the detection value has reached the intermediate value so that the gas delivery flow rate is reduced to a predetermined limited flow rate that is not zero; When the detected value reaches the intermediate value for all of the plurality of fuel cell stacks, adjusting the gas delivery flow rate for each of the plurality of fuel cell stacks back to the standard flow rate; and terminating the water discharge treatment when the detected value reaches a predetermined end value that is higher than the intermediate value for all of the plurality of fuel cell stacks. Fuel cell system.

2. 2. The fuel cell system according to claim 1, wherein the wastewater treatment further includes correcting the standard flow rate for each of the plurality of fuel cell stacks in accordance with the temperature of the fuel cell stack.

3. 3. The fuel cell system according to claim 1, wherein the gas is an oxidizing gas supplied to the cathode sides of the plurality of fuel cell stacks.

4. The gas supply unit supplies the gas to each of the plurality of fuel cell stacks. an oxidizing gas supply path connected to a supply port of the fuel cell stack and equipped with a compressor; an oxidizing gas exhaust path connected to an exhaust port of the fuel cell stack; a branch path connecting the oxidizing gas supply path and the oxidizing gas discharge path to each other and having a branch valve; Equipped with 4. The fuel cell system according to claim 3, wherein limiting the flow rate of the oxidizing gas discharged during the wastewater treatment includes opening the diversion valve to divert the oxidizing gas from the diversion path to the oxidizing gas discharge path.

5. 4. The fuel cell system according to claim 3, wherein limiting the flow rate of the oxidizing gas delivered in the wastewater treatment includes limiting the flow rate of the oxidizing gas delivered by a compressor.

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