Fuel cell system

Abstract

comprising fuel cell system (1): a fuel cell unit (20a, 20b) comprising a first and a second fuel cell (20a, 20b) which supplies electrical power to a load device (50); a first and a second supply system (10a, 10b) which are set up to control a first and a second flow velocity (Qa, Qb) of cathode gas, which is supplied to the first and second fuel cell (20a, 20b) respectively; a switching device (36a, 36b) capable of switching the fuel cell unit (20a, 20b) and the load device (50) between an electrically connected state in which the fuel cell unit (20a, 20b) is electrically connected to the load device (50) and an electrically disconnected state in which the fuel cell unit (20a, 20b) is electrically disconnected from the load device (50); a switching control unit (60) which is configured to switch the fuel cell unit (20a, 20b) and the load device (50) into the electrically disconnected state when a required power (P) of the fuel cell unit (20a, 20b) is less than or equal to a threshold value (P1); an open-circuit voltage reference unit (60) configured to maintain a first open-circuit voltage (Va) of the first fuel cell (20a) and a second open-circuit voltage (Vb) of the second fuel cell (20b) in the electrically disconnected state; and a supply system control unit (60) configured to increase and decrease the first and second open-circuit voltages (Va, Vb) such that they converge within a first and second target range, respectively, by controlling the first and second supply systems (10a, 10b) such that the first and second flow velocities (Qa, Qb) are increased and decreased, where a lower limit (VLa) of the first target range is greater than a lower limit (VLb) of the second target range.

Description

Field of invention

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

[0002] If the power demand of a fuel cell is less than or equal to a threshold value, the fuel cell is electrically disconnected from the load devices. If the fuel cell's open-circuit voltage is too high in this state, the fuel cell's cathode catalyst could be eluted. Therefore, the open-circuit voltage is prevented from becoming too high by supplying a sufficient amount of hydrogen to the fuel cell and by controlling the cathode gas flow rate to be lower than usual, thus reducing the amount of oxygen remaining inside the fuel cell. However, if the power demand increases while the amount of oxygen remaining in the fuel cell is too low, the actual power response deteriorates.For this reason, the flow rate of the cathode gas is increased and decreased in such a way that the open-circuit voltage converges within a target range (see, for example, the Japanese unapproved publication JP 2016 - 96 086 A).

[0003] In a fuel cell system that includes such fuel cells, it is conceivable that the open-circuit voltages of the fuel cells are low when the required power increases. In this case, the response of the actual power to the required power may deteriorate. Summary of the invention

[0004] It is therefore an object of the present invention to create a fuel cell system in which the response to a required power is improved.

[0005] The above problem is solved by a fuel cell system comprising: a fuel cell unit comprising a first and a second fuel cell which supplies electrical power to a load device; a first and a second supply system which are configured to control a first and a second flow velocity of cathode gas which is supplied to the first and second fuel cells respectively; a switching device which is capable of switching the fuel cell unit and the load device between an electrically connected state in which the fuel cell unit is electrically connected to the load device and an electrically disconnected state in which the fuel cell unit is electrically disconnected from the load device;a switching control unit configured to switch the fuel cell unit and the load device to the electrically isolated state when a required power of the fuel cell unit is less than or equal to a threshold value; an open-circuit voltage reference unit configured to maintain a first open-circuit voltage of the first fuel cell and a second open-circuit voltage of the second fuel cell in the electrically isolated state; and a supply system control unit configured to increase and decrease the first and second open-circuit voltages such that they converge within a first and a second setpoint range, respectively, by controlling the first and second supply systems such that the first and second flow velocities are increased and decreased, wherein a lower limit of the first setpoint range is greater than a lower limit of the second setpoint range.

[0006] The lower limit of the first target range is higher than the lower limit of the second target range, thus ensuring a period during which the first open-circuit voltage is higher than the second open-circuit voltage. This prevents both the first and second open-circuit voltages from being in low states, thereby improving the response to a required power output.

[0007] One value in the first or second target range can be larger than that of the other value in the first or second target range.

[0008] The first and second target ranges can overlap at least partially.

[0009] An upper limit of the first target range can be identical to an upper limit of the second target range.

[0010] An upper limit of the first target range can be greater than an upper limit of the second target range.

[0011] Furthermore, the above problem is solved by a fuel cell system comprising: a fuel cell unit comprising a first and a second fuel cell which supplies electrical power to a load device; a first and a second supply system which are configured to control a first and a second flow velocity of cathode gas which is supplied to the first and second fuel cells respectively; a switching device which is capable of switching the fuel cell unit and the load device between an electrically connected state in which the fuel cell unit is electrically connected to the load device and an electrically disconnected state in which the fuel cell unit is electrically disconnected from the load device;a switching control unit configured to switch the fuel cell unit and the load device into the electrically isolated state when a required power of the fuel cell unit is less than or equal to a threshold value; an open-circuit voltage reference unit configured to maintain a first open-circuit voltage of the first fuel cell and a second open-circuit voltage of the second fuel cell in the electrically isolated state;and a feed system control unit configured to increase and decrease the first and second open-circuit voltages such that they each converge within a first and a second setpoint range by controlling the first and second feed systems such that the first and second flow velocities are increased and decreased, wherein a lower limit of the first setpoint range is identical to a lower limit of the second setpoint range and an upper limit of the first setpoint range is greater than an upper limit of the second setpoint range. Effects of the invention

[0012] According to the invention, it is possible to create a fuel cell system in which the response to a required power output is improved. Brief description of the drawing Fig. Figure 1 is a configuration view of a fuel cell system installed in a vehicle; Fig. Figure 2 is a timing diagram that illustrates an example of open-circuit voltage control; Fig. Figure 3 is an explanatory view of a magnitude relationship between upper limits and lower limits; Fig. Figure 4 is a flowchart that illustrates an example of open-circuit voltage control; Fig. Figure 5 is a flowchart that illustrates an example of the open-circuit voltage control of a first fuel cell; Fig. Figure 6 is a flowchart that illustrates an example of the open-circuit voltage control of a second fuel cell; and Fig. Figures 7A to 7D are explanatory views of magnitude relationships between upper and lower limits in variations of open-circuit voltage control. Detailed description: Fuel cell system configuration

[0013] Fig. Figure 1 is a configuration view of a fuel cell system 1 (hereinafter referred to simply as the system) installed in a vehicle. The system 1 comprises cathode gas supply systems 10a and 10b, fuel cells 20a and 20b (hereinafter referred to simply as FCs), power control systems 30a and 30b, batteries 40a and 40b (hereinafter referred to as BATs), an electric motor 50, and an electronic control unit (ECU) 60. The system 1 also includes an anode gas supply system (not shown) that supplies hydrogen gas as anode gas to the FCs 20a and 20b, and a cooling system (not shown) that circulates cooling water through the FCs 20a and 20b, thus cooling them.

[0014] The FCs 20a and 20b are fuel cells that use cathode gas and fuel gas to generate electrical power. Each FC 20a and 20b is formed by stacking solid polymer electrolyte unit cells. FCs 20a and 20b are identical fuel cells and have the same power rating, although they are not limited to this rating. FCs 20a and 20b are examples of a fuel cell unit and also represent a first and second fuel cell, respectively.

[0015] The cathode gas supply systems 10a and 10b supply air containing oxygen as cathode gas to the FCs 20a and 20b, respectively. Specifically, the cathode gas supply systems 10a and 10b each comprise supply pipes 11a and 11b, outlet pipes 12a and 12b, bypass pipes 13a and 13b, air compressors 14a and 14b, bypass valves 15a and 15b, charge air coolers 16a and 16b, and backpressure valves 17a and 17b.

[0016] The inlet pipes 11a and 11b are each connected to the cathode inlet collector pipes of the FCs 20a and 20b. The outlet pipes 12a and 12b are each connected to the cathode outlet collector pipes of the FCs 20a and 20b. The bypass pipe 13a is connected to the inlet pipe 11a and the outlet pipe 12a. Likewise, the bypass pipe 13b is connected to the inlet pipe 11b and the outlet pipe 12b. The bypass valve 15a is formed on a common section of the inlet pipe 11a and the bypass pipe 13a. Similarly, the bypass valve 15b is formed on a common section of the inlet pipe 11b and the bypass pipe 13b. The bypass valve 15a switches the connection state between the supply pipe 11a and the bypass pipe 13a. Similarly, the bypass valve 15b switches the connection state between the supply pipe 11b and the bypass pipe 13b.The air compressor 14a, the bypass valve 15a, and the charge air cooler 16a are arranged on the supply pipe 11a in this order, starting from the upstream side. The backpressure valve 17a is arranged on the outlet pipe 12a and on the upstream side of a common section of the outlet pipe 12a and the bypass pipe 13a. Similarly, the air compressor 14b, the bypass valve 15b, and the charge air cooler 16b are arranged on the supply pipe 11b in this order, starting from the upstream side. The backpressure valve 17b is arranged on the outlet pipe 12b and on the upstream side of a common section of the outlet pipe 12b and the bypass pipe 13b.

[0017] Air compressors 14a and 14b supply air containing oxygen as cathode gas to FCs 20a and 20b via supply pipes 11a and 11b, respectively. The cathode gas supplied to FCs 20a and 20b is discharged through outlet pipes 12a and 12b. Charge air coolers 16a and 16b cool the cathode gas supplied to FCs 20a and 20b. Backpressure valves 17a and 17b adjust the backpressure on the cathode side of FCs 20a and 20b.

[0018] The power control systems 30a and 30b comprise fuel cell DC-DC converters 32a and 32b (hereinafter referred to as FDCs), battery DC-DC converters 34a and 34b (hereinafter referred to as BDCs), switches 36a and 36b, and auxiliary power unit DC-DC converters 39a and 39b (hereinafter referred to as AINVs). Furthermore, the power control systems 30a and 30b share a motor inverter 38 (hereinafter referred to as MINV), which is connected to the electric motor 50. The FDCs 32a and 32b each adapt DC power from the FCs 20a and 20b and supply the DC power to the MINV 38. BDCs 34a and 34b each adapt the DC power from BATs 40a and 40b, and supply the DC power to the MINV 38. The electrical power generated by FCs 20a and 20b is stored in BATs 40a and 40b, respectively.The MINV 38 converts the input DC power into three-phase AC power and supplies the three-phase AC power to the electric motor 50. The electric motor 50 drives wheels 5 to propel the vehicle.

[0019] Switch 36a opens and closes in response to a command from ECU 60. When switch 36a opens, FC 20a and BAT 40a are electrically disconnected from MINV 38, and when switch 36a closes, FC 20a and BAT 40a are connected to MINV 38. Similarly, switch 36b opens and closes in response to a command from ECU 60. When switch 36b opens, FC 20b and BAT 40b are electrically disconnected from MINV 38, and when switch 36b closes, FC 20b and BAT 40b are connected to MINV 38. In the disconnected state, FCs 20a and 20b are electrically disconnected from load devices that include the electric motor 50.Switches 36a and 36b are examples of a switching device capable of switching FCs 20a and 20b and the load devices between the electrically connected state, in which FCs 20a and 20b are electrically connected to the load devices, and the electrically disconnected state, in which FCs 20a and 20b are electrically disconnected from the load devices. The load devices in this case comprise auxiliary units for FCs 20a and 20b and auxiliary units for the vehicle, in addition to the electric motor 50. The auxiliary units for FCs 20a and 20b include the air compressors 14a and 14b described above, the bypass valves 15a and 15b, the backpressure valves 17a and 17b, and injection devices to supply the anode gas, which is contained in the anode gas supply system, to FCs 20a and 20b, respectively. The auxiliary units for the vehicle include, for example, an air compressor, lighting devices, and warning lights.Even when FCs 20a and 20b and BATs 40a and 40b are disconnected from MINV 38 by switches 36a and 36b, the storage capacity of BATs 40a and 40b is increased by BDCs 34a and 34b, respectively, and is capable of being supplied to air compressors 14a and 14b, bypass valves 15a and 15b, and backpressure valves 17a and 17b via AINVs 39a and 39b. In the present embodiment, switches 36a and 36b are described as components that are separate from FDCs 32a and 32b, but switches formed within FDCs 32a and 32b, respectively, can switch between the connected and disconnected states.

[0020] The ECU 60 comprises a central processing unit (CPU), a read-only memory (ROM), and a random access memory (RAM). The ECU 60 is electrically connected to a pedal opening sensor 3, the air compressors 14a and 14b, the bypass valves 15a and 15b, the backpressure valves 17a and 17b, the FDCs 32a and 32b, the BDCs 34a and 34b, and the switches 36a and 36b.

[0021] The ECU 60 calculates the accelerator pedal opening degree of a driver's accelerator pedal, based on the reading from the accelerator pedal opening degree sensor 3. The ECU 60 calculates the amount of electrical power required to drive the electric motor 50, also based on this accelerator pedal opening degree. Finally, the ECU 60 calculates the total power required by FCs 20a and 20b, based on the power needed to drive the auxiliary components for FCs 20a and 20b, such as the air compressors 14a and 14b, the vehicle's auxiliary components such as the electric motor 50, and the storage power of the BATs 40a and 40b. The ECU 60 controls the FDCs 32a and 32b and the BDCs 34a and 34b to supply the MINV 38 with the amount of electrical power from the FCs 20a and 20b that corresponds to the required power of the entire FCs 20a and 20b.In the present specification, “required power” does not mean each required power of the FCs 20a and 20b, but rather the required power of all FCs 20a and 20b, that is, the required power of the fuel cell unit.

[0022] As described in detail later, the ECU 60 also performs an open-circuit voltage control. This control is implemented by a switching control unit, an open-circuit voltage reference unit, and a supply system control unit, which are functionally implemented by the CPU, ROM, and RAM. Open circuit voltage

[0023] For example, if the accelerator pedal opening decreases, the required power also decreases. If the required power is less than or equal to a threshold value described later, switches 36a and 36b electrically disconnect FCs 20a and 20b from the load devices, and the vehicle is then brought into an idle state. In this disconnected state, where FCs 20a and 20b are electrically disconnected from the load devices, they are in a state where power generation is temporarily halted. In such a state, any current in FCs 20a and 20b is zero, and any voltage in this state is referred to as the open-circuit voltage. If an increase in the accelerator pedal opening causes the required power in such an idle state to rise above the threshold value again, switches 36a and 36b electrically connect FCs 20a and 20b to the load devices.Therefore, the electrical power generated by the FCs 20a and 20b is supplied to the electric motor 50, and then the vehicle is in a driving state.

[0024] In the isolated state, where FCs 20a and 20b are electrically disconnected from the load devices, no oxygen or hydrogen is consumed during electrical power generation. However, hydrogen seeps through an electrolyte membrane from one anode side to the other, causing the hydrogen and oxygen to react and form water on the cathode side. This reduces the oxygen concentration on the cathode side.

[0025] A decrease in the oxygen concentration on the cathode side lowers the open-circuit voltage. The open-circuit voltage is preferably maintained at a high level even in the disconnected state, taking into account the response of the actual power output of FCs 20a and 20b when the required power increases after the open-circuit voltage has been lowered. This is because a high open-circuit voltage indicates a high oxygen concentration on the cathode side, and the electrical power output of FCs 20a and 20b increases accordingly when the required power rises under conditions of high oxygen concentration on the cathode side. However, the open-circuit voltage is too high, so the cathode catalyst could potentially be eluted, reducing the power performance of FCs 20a and 20b.Therefore, the open-circuit voltage control described above is implemented to maintain the open-circuit voltage within a predetermined target range. In this control, the flow rate of the cathode gas supplied to each of FCs 20a and 20b fluctuates, causing each open-circuit voltage to repeatedly rise and fall to converge within the target range. Furthermore, the target range described above is not one in which a problem immediately arises if the open-circuit voltage deviates from it. The target ranges for the open-circuit voltages of FCs 20a and 20b are examples of the first and second target ranges, respectively. Timing diagram of an open-circuit voltage control

[0026] Fig. Figure 2 is a timing diagram that illustrates an example of open-circuit voltage control. Fig. Figure 2 represents a curve of the required power P, the connected state between each of the FCs 20a and 20b and the load devices, a transition of the respective flow velocities Qa and Qb of the cathode gas supplied to the FCs 20a and 20b, and a transition of the respective voltages Va and Vb of the FCs 20a and 20b. Furthermore, the flow velocities Qa and Qb are controlled during the execution of the no-load voltage control, which is described below, by adjusting the opening degree of the bypass valves 15a and 15b, while the rotational speed of the air compressors 14a and 14b is kept constant, and the opening degree of the back-pressure valves 17a and 17b is kept constant.

[0027] For example, if the accelerator pedal opening gradually decreases while the vehicle is in a driving state, the required power gradually decreases from time t0. As the required power decreases, the flow velocities Qa and Qb decrease, so the power of FCs 20a and 20b decreases. Furthermore, as the power of FCs 20a and 20b decreases, the current of FCs 20a and 20b decreases, and the voltage Va and Vb increase.

[0028] For example, when the accelerator pedal opening reaches zero, the required power P is less than or equal to a threshold value P1 at time t1. The threshold value P1 is a preset value and serves as a threshold to determine whether the required power P can be considered essentially zero. In other words, the threshold value P1 is a threshold to determine whether there should be no power generation request to FCs 20a and 20b. Furthermore, if the required power P is greater than the threshold value P1, the flow velocities Qa and Qb are controlled such that they are each greater than a target flow velocity QH. Moreover, the threshold value P1 is not, for example, restricted to essentially zero and can represent a low power requirement sufficient to propel the vehicle and operate the auxiliary equipment, which can be adequately handled by BATs 40a and 40b alone.

[0029] At time t2, when the required power P is essentially constant, and after a predetermined time in minutes has elapsed from time t1, when the required power P is less than or equal to the threshold value P1, the flow velocities Qa and Qb are each controlled such that they reach the target flow velocity QL. The target flow velocity QL is less than the target flow velocity QH. The flow velocities Qa and Qb are each controlled so that they lie between the target flow velocity QH and the target flow velocity QL by controlling the opening degree of the bypass valves 15a and 15b such that the flow velocities of cathode gas flowing through the bypass tubes 13a and 13b increase.If the flow velocities Qa and Qb are each controlled to achieve the target flow velocity QL, the IU characteristic of FCs 20a and 20b deteriorates compared to an IU characteristic during normal power generation. Therefore, the voltages Va and Vb begin to decrease from time t2 onwards.

[0030] At time t3, after a predetermined time has elapsed since time t2, at which point the voltages Va and Vb begin to decrease, FCs 20a and 20b are disconnected from the load devices, and the open-circuit voltage control begins. Thereafter, the voltages Va and Vb rise immediately, and the current of FCs 20a and 20b (not shown) reaches zero. In the state where FCs 20a and 20b are disconnected from the load devices, the voltages Va and Vb correspond to their respective open-circuit voltages. Furthermore, the disconnection time is adjusted such that the voltages Va and Vb, which rise immediately after FCs 20a and 20b are disconnected from the load devices, are each less than or equal to the upper limits VHa and VHb; this will be discussed in detail later. The upper limits VHa and VHb are preset.

[0031] The voltages Va and Vb decrease from time t3 onwards due to the leakage described above. This means that the target flow velocity QL is set to a low flow velocity so that the open-circuit voltage does not increase, even when considering leakage. This is because the rate of oxygen concentration decrease due to leakage varies depending on the operating environment and the operating time of the fuel cell, and it is difficult to determine the decrease rate in advance.

[0032] When the voltage Va of the FC 20a reaches a lower limit VLa at time t4, the flow velocity Qa of the cathode gas to the FC 20a is controlled to increase to the target flow velocity QH, causing the voltage Va to begin rising. The target flow velocity QH is set to such a rate that the oxygen concentration increases even when a decrease in oxygen concentration due to seepage is taken into account. The lower limit VLa, which will be described in detail later, is preset.

[0033] If the voltage Va at time t5 is greater than or equal to the upper limit VHa, the flow velocity Qa is controlled in such a way that it drops again to the target flow velocity QL, and then the voltage Va begins to drop.

[0034] When the voltage Vb reaches a lower limit VLb at time t6, the flow velocity Qb is controlled to increase to the target flow velocity QH, and then the voltage Vb begins to increase. The lower limit VLb is preset. When the voltage Va reaches the lower limit VLa at time t7, the flow velocity Qa is controlled to increase to the target flow velocity QH, and then the voltage Va begins to increase. When the voltage Va reaches the upper limit VHa at time t8, the flow velocity Qa is controlled to decrease to the target flow velocity QL, and then the voltage Va begins to decrease. When the voltage Vb reaches the upper limit VHb at time t8, the flow velocity Qb is likewise controlled to increase to the target flow velocity QL, and then the voltage Vb begins to decrease.

[0035] As described above, the upper limits VHa and VHb are the upper limits of the target ranges for the open-circuit voltage, and the lower limits VLa and VLb are the lower limits of the same. Therefore, the flow velocities Qa and Qb are controlled to rise and fall in such a way that the voltages Va and Vb repeatedly rise and fall to converge within the target range. The upper limits VHa and VHb are voltage values ​​suitable for preventing the elution of the cathode catalyst, as described above, and can be within the range of, for example, 0.75 V to 0.9 V, and in particular, 0.85 V.The lower limits VLa and VLb are voltage values ​​suitable to ensure good response to the required power and can be within a range of, for example, 0.5 V to 0.8 V, and in particular may be 0.77 V of the lower limit VLa and 0.7 V of the lower limit VLb.

[0036] When the required power P begins to increase at time t9, the required power P is greater than the threshold P1 at time t10, and FCs 20a and 20b are connected to the load devices. Thereafter, the voltages Va and Vb immediately decrease, and the current of FCs 20a and 20b (not shown) immediately increases. By controlling the flow velocities Qa and Qb such that they are each greater than the target flow velocity QH at time t11, the voltages Va and Vb decrease, and the power of FCs 20a and 20b (not shown) increases. In the present embodiment, the flow rates Qa and Qb increase at time t11 after FCs 20a and 20b were connected to the load devices at time t10, but this is not limited to this. The FCs 20a and 20b can be connected to the load devices after the flow velocities Qa and Qb have increased.The flow velocities Qa and Qb can increase and the FCs 20a and 20b can be connected to the load devices simultaneously.

[0037] Fig. Figure 3 is an explanatory view of a magnitude relationship between the upper limits VHa and VHb and the lower limits VLa and VLb described above. As in Fig. As shown in Figure 3, the lower limit VLa is greater than the lower limit VLb. Therefore, the voltage Va is kept in a high state so that it is not less than the lower limit VLa, ensuring a period during which the voltage Va is higher than the voltage Vb. Accordingly, both voltages Va and Vb are prevented from becoming low. For example, if the required power P increases and exceeds the threshold P1, and both voltages Va and Vb are low, the response of the actual power of FCs 20a and 20b to the required power P could deteriorate and affect the driving capability, since both voltages Va and Vb would be low. In the present embodiment, the occurrence of the above problem is prevented.

[0038] Furthermore, the difference between the upper limit VHb and the lower limit VLb is greater than the difference between the upper limit VHa and the lower limit VLa. Therefore, during the period in which the open-circuit voltage control is performed, the number of switching cycles between increasing and decreasing the flow velocity Qb is less than the number of switching cycles between increasing and decreasing the flow velocity Qa. Accordingly, the frequency with which the opening degree of bypass valve 15b is changed to switch the flow velocity Qb is less than the frequency with which the opening degree of bypass valve 15a is changed to switch the flow velocity Qa. This prevents the deterioration of the service life of bypass valve 15b.

[0039] Furthermore, the target voltage range Va, from the upper limit VHa to the lower limit VLa, partially overlaps the target voltage range Vb, from the upper limit VHb to the lower limit VLb. This prevents a large deviation in output power between FCs 20a and 20b. Flowchart of an open-circuit voltage control

[0040] Fig. Figure 4 is a flowchart illustrating an example of the idle voltage control. First, the ECU 60 determines whether the required power P is less than or equal to the threshold P1 (step S1). As described above, the ECU 60 calculates the required power P based on the power needed to drive the electric motor 50 and the auxiliary components. For example, if the accelerator pedal opening is not zero and the vehicle is in a driving state, "no" is determined in step S1, and the current control process is terminated.

[0041] For example, if the accelerator pedal opening degree is zero, "yes" is determined in step S1, and the ECU 60 controls the flow velocities Qa and Qb to the target flow velocity QL as described above at time t2 (step S3). Accordingly, the oxygen concentration in the FCs 20a and 20b decreases, and the voltages Va and Vb also decrease.

[0042] Next, the ECU 60 receives the IU characteristic of FC 20a (step S5a) and determines whether the IU characteristic of FC 20a deteriorates sufficiently (step S7a). Then, the ECU 60 receives the IU characteristic of FC 20b (step S5b) and determines whether the IU characteristic of FC 20b deteriorates sufficiently (step S7b). That is, the target flow velocity QL is a flow velocity at which the IU characteristic deteriorates due to the low oxygen concentration on the cathode side of the fuel cell in the state in which the fuel cell is connected to the load devices. The IU characteristic is obtained based on the respective current and voltage values ​​of FCs 20a and 20b after the flow velocities Qa and Qb are controlled to the target flow velocity QL. It is assumed that the IU characteristic deteriorates as the current and voltage values ​​decrease.This means that if the current and voltage values ​​are less than or equal to their respective predefined values, it is determined that the IU characteristic has deteriorated sufficiently. If step S7a is determined to be "no", step S5a is executed again. Similarly, if step S7b is determined to be "no", step S5b is executed again.

[0043] If "yes" is determined in step S7a, the ECU 60 disconnects FC 20a from the load devices as at time t3 described above (step S9a). Likewise, if "yes" is determined in step S7b, the ECU 60 disconnects FC 20b from the load devices as at time t3 (step S9b). The processes of steps S9a and S9b are an example of processes performed by the switching control unit, which is configured to switch FCs 20a and 20b and the load devices to the disconnected state when the required power P is less than or equal to the threshold P1. As shown at time t3, the voltages Va and Vb increase by such a measure that they do not immediately exceed the upper limits VHa and VHb, respectively, after disconnection. This is because the separation described above is carried out after it has been determined in steps S7a and S7b that the IU characteristics have deteriorated sufficiently.This means that in step S7a, "the IU characteristic deteriorates sufficiently" means that the IU characteristic deteriorates by such a degree that the voltage that rises immediately after disconnection does not exceed the upper limit VHa. In step S7b, "the IU characteristic deteriorates sufficiently" means that the IU characteristic deteriorates by such a degree that the voltage that rises immediately after disconnection does not exceed the upper limit VHb.

[0044] Next, the ECU 60 executes the open-circuit voltage control of FC 20a (step S10a) and the open-circuit voltage control of FC 20b (step S10b). Additionally, the processes of steps S5a and S5b above, and subsequent processes, are executed in parallel. Flowchart of the open-circuit voltage control of the FC 20a

[0045] Fig. Figure 5 is a flowchart illustrating an example of the open-circuit voltage control of FC 20a. First, the ECU 60 controls the flow velocity Qa to achieve the target flow velocity QL (step S11a). If the flow velocity Qa was already controlled to the target flow velocity QL in step S3 described above, this state is maintained in step S11a. Next, the ECU 60 determines whether the required power P is less than or equal to the threshold value P1 (step S13a). If "no" is determined in step S13a, the ECU 60 performs feedback control to return FCs 20a and 20b to their normal power-generating state (step S100). The feedback control is described later.

[0046] If step S13a is determined to be "yes", the ECU 60 receives the voltage Va (step S15a) and determines whether the voltage Va is less than or equal to the lower limit VLa (step S17a). If step S17a is determined to be "no", the ECU 60 repeats the process from step S11a and subsequent processes. That is, as long as step S13a is determined to be "yes" and step S17a to be "no", the flow velocity Qa is controlled to the target flow velocity QL. For example, the flow velocity Qa is controlled to the target flow velocity QL after time t2 as described above. Since step S13a is executed repeatedly as long as step S17a is determined to be "no", the feedback control is performed even if the required power increases while the flow velocity Qa is controlled to the target flow velocity QL.

[0047] If step S17a determines "yes", for example as shown at time t7, the ECU 60 controls the flow velocity Qa to the target flow velocity QH (step S21a). Next, the ECU 60 determines whether the required power P is less than or equal to the threshold value P1 (step S23a). Similarly, if step S23a determines "no", the feedback control is executed (step S100).

[0048] If step S23a is determined to be "yes", the ECU 60 receives the voltage Va (step S25a) and determines whether the voltage Va is greater than or equal to the upper limit VHa (step S27a). If step S27a is determined to be "no", the ECU 60 repeats the process from step S21a. As long as step S23a is determined to be "yes" and step S27a to be "no", the flow velocity Qa is controlled to the target flow velocity QH. That is, as illustrated in the time span from time t7 to time t8 described above, the flow velocity Qa is controlled to the target flow velocity QH. Because step S23a is executed repeatedly as long as step S27a is determined to be "no", it is possible to handle the case where the required power increases while the flow velocity Qa is controlled to the target flow velocity QH.

[0049] If "yes" is determined in step S27a, the ECU 60 re-executes the process from step S11a and the subsequent processes, and the flow velocity Qa is controlled to the target flow velocity QL, for example, as shown at time t8. As described above, the voltage Va is controlled to repeatedly rise and fall between the upper limit VHa and the lower limit VLa by alternately controlling the flow velocity Qa to the target flow velocity QH and the target flow velocity QL. The processes of steps S11a, S17a, S21a, and S27a are an example of a process executed by the feed system control unit, which is configured to increase and decrease the open-circuit voltage of FC 20a so that it converges within an initial target range by controlling the bypass valve 15a such that the flow velocity Qa increases and decreases.

[0050] Furthermore, the process from step S13a can be executed before step S11a or between steps S15a and S17a. Similarly, the process from step S23a can be executed before step S21a or between steps S25a and S27a. Flowchart of the open-circuit voltage control of the FC 20b

[0051] Fig. Figure 6 is a flowchart illustrating an example of open-circuit voltage control for the FC 20b. First, the ECU 60 controls the flow velocity Qb to achieve the target flow velocity QL (step S11b). If the flow velocity Qb was already controlled to the target flow velocity QL in step S3 described above, this condition continues in step S11b. Next, the ECU 60 determines whether the required power P is less than or equal to the threshold value P1 (step S13b). If "no" is determined in step S13b, the ECU 60 executes the feedback control (step S100).

[0052] If "yes" is determined in step S13b, the ECU 60 receives the voltage Vb (step S15b) and determines whether the voltage Vb is less than or equal to the lower limit VLb (step S17b). If "no" is determined in step S17b, the ECU 60 repeats the process from step S11b and the subsequent processes. That is, as long as "yes" is determined in step S13b and "no" is determined in step S17b, the flow velocity Qb is controlled to the target flow velocity QL. For example, the flow velocity Qb is controlled to the target flow velocity QL after time t2 as described above. Because the process from step S13b is executed repeatedly as long as "no" is determined in step S17b, the feedback control is performed even if the required power increases while the flow velocity Qb is controlled to the target flow velocity QL.

[0053] If "yes" is determined in step S17b, the ECU 60 controls the flow velocity Qb to the target flow velocity QH, as shown, for example, at time t6 (step S21b). Next, the ECU 60 determines whether the required power P is less than or equal to the threshold value P1 (step S23b). Similarly, if "no" is determined in step S23b, the feedback control is executed (step S100).

[0054] If "yes" is determined in step S23b, the ECU 60 receives the voltage Vb (step S25b) and determines whether the voltage Vb is greater than or equal to the upper limit VHb (step S27b). If "no" is determined in step S27b, the ECU 60 repeats the process from step S21b. As long as "yes" is determined in step S23b and "no" is determined in step S27b, the flow velocity Qb is controlled to the target flow velocity QH. That is, as described above, the flow velocity Qb is controlled to the target flow velocity QH during the time interval from time t6 to time t8. Since the process from step S23b is executed repeatedly as long as "no" is determined in step S27b, the feedback control is executed even if the required power increases while the flow velocity Qb is controlled to the target flow velocity QH.

[0055] If "yes" is determined in step S27b, the ECU 60 re-executes the process from step S11b and the subsequent processes, and the flow velocity Qb is controlled to the target flow velocity QL, for example, as shown at time t8. As described above, the flow velocity Qb is alternately controlled to the target flow velocity QH and the target flow velocity QL, so that the voltage Vb is controlled to repeatedly rise and fall between the upper limit VHb and the lower limit VLb. The processes of steps S11b, S17b, S21b, and S27b are an example of a process executed by the feed system control unit, which is configured to increase and decrease the open-circuit voltage of FC 20b so that it converges within a second target range by controlling the bypass valve 15b such that the flow velocity Qb rises and falls.

[0056] Furthermore, the process from step S13b can be executed before step S11b or between steps S15b and S17b. Similarly, the process from step S23b can be executed before step S21b or between steps S25b and S27b.

[0057] As described above, each of the open-circuit voltage control settings for FCs 20a and 20b is essentially the same, and the upper limits VHa and VHb are also the same value. The difference lies in the fact that the lower limit VLa is greater than the lower limit VLb. By setting the upper and lower limits in this way, it is possible to prevent a deterioration in the service life of the supply system, which adjusts the flow rate of the cathode gas supplied to FC 20b, while ensuring the required power response. This prevents an increase in the manufacturing costs of the program that performs the open-circuit voltage control of FCs 20a and 20b and prevents an increase in the process load on the ECU 60. Return control

[0058] When the required power P is greater than or equal to the threshold P1 in the state where FCs 20a and 20b are disconnected from the load devices, the ECU 60 controls switches 36a and 36b to electrically connect FCs 20a and 20b and the load devices, as shown at time t10. This causes the voltages Va and Vb to drop immediately. In the connected state, the flow velocities Qa and Qb are controlled to increase in response to an increase in the required power P, as shown at time t11. Therefore, the power of FCs 20a and 20b also increases to supply the required power P. Even if the feedback control is executed in the open-circuit voltage control of one of FCs 20a and 20b, both FCs 20a and 20b return to the normal power-generating state. First modification of an open-circuit voltage control

[0059] Next, a first modification of the open-circuit voltage control is described. Fig. Figure 7A illustrates the relationship between the upper limits VHa and VHb and the lower limits VLa and VLb in the first variation of the open-circuit voltage control. In this first variation, the upper limit VHa is greater than the upper limit VHb, the lower limit VLa is greater than the lower limit VLb, and the upper limit VHb is less than the upper limit VHa and greater than the lower limit VLa. Because the lower limit VLa is greater than the lower limit VLb, a period during which the voltage Va is higher than the voltage Vb is ensured, thus improving the response to an increase in power demand.

[0060] Furthermore, since the upper limit VHb is lower than the upper limit VHa, the cathode catalyst of the FC 20b is further prevented from dissolving, and the performance of the FC 20b is further prevented from deteriorating. Such a configuration is suitable in a case where a deterioration in the performance of the FC 20b must be particularly prevented, for example, in a system where the FC 20b delivers more power to the auxiliary units than the FC 20a, or in a case where the rated power of the FC 20b is higher than that of the FC 20a. In the first modification, it is thus possible to prevent the deterioration in the performance of the FC 20b with a higher priority than with the FC 20a, while ensuring a responsiveness to an increase in the required power.

[0061] In the first modification, as in the embodiment described above, the difference between the upper limit VHb and the lower limit VLb is greater than the difference between the upper limit VHa and the lower limit VLa, thus preventing the deterioration of the service life of the bypass valve 15b. This makes it possible to prevent the deterioration of the service life of the bypass valve 15b, which is the auxiliary component of the FC 20b, while simultaneously preventing a deterioration in the performance of the FC 20b and ensuring the performance of the FC 20b over a long period.

[0062] In the first variation, the upper limit VHa is, for example, 0.84 V, the upper limit VHb is 0.82 V, the lower limit VLa is 0.77 V and the lower limit VLb is 0.7 V. Second variation of an open-circuit voltage control

[0063] Fig. Figure 7B is an illustrative view of the relationship between the upper limits VHa and VHb and the lower limits VLa and VLb in a second variation of the open-circuit voltage control. In this second variation, the upper limit VHb is greater than the upper limit VHa, the lower limits VLa and VLb are identical, and the upper limit VHa is less than the upper limit VHb and greater than the lower limit VLb. Because the upper limit VHb is greater than the upper limit VHa, a period during which the voltage Vb is higher than the voltage Va is ensured, thus improving the response to an increase in required power. Furthermore, because the upper limit VHa is lower than the upper limit VHb, the dissolution of the FC 20a's cathode catalyst is further suppressed, preventing the degradation of the FC 20a's performance.Furthermore, in the second modification, the difference between the upper limit VHb and the lower limit VLb is also greater than the difference between the upper limit VHa and the lower limit VLa, thus preventing the deterioration of the service life of the bypass valve 15b.

[0064] Furthermore, in the second modification, unlike the embodiment described above and the first modification, as well as the third and fourth modifications, which will be described later, the target range of the open-circuit voltage of the FC 20b corresponds to the first target range, and the target range of the open-circuit voltage of the FC 20a corresponds to the second target range. Accordingly, the upper limit VHb corresponds to the upper limit of the first target range, and the upper limit VHa corresponds to the upper limit of the second target range. In the second modification, for example, the upper limit VHa is 0.8 V, the upper limit VHb is 0.84 V, and each of the lower limits VLa and VLb is 0.75 V. Third variation of an open-circuit voltage control

[0065] Fig. Figure 7C illustrates the relationship between the upper limits VHa and VHb and the lower limits VLa and VLb in the third variation of the open-circuit voltage control. In this third variation, the upper limit VHb is greater than the upper limit VHa, and the lower limit VLb is less than the lower limit VLa. Therefore, since the difference between the upper limit VHb and the lower limit VLb is greater than the difference between the upper limit VHa and the lower limit VLa, the deterioration of the bypass valve 15b's lifetime is prevented. Furthermore, since the upper limit VHa is lower than the upper limit VHb, the dissolution of the FC 20a's cathode catalyst is prevented, thus preventing the deterioration of the FC 20a's performance.

[0066] In the third variation, the upper limit VHa is, for example, 0.8 V, the upper limit VHb is 0.84 V, the lower limit VLa is 0.75 V and the lower limit VLb is 0.7 V. Fourth variation of an open-circuit voltage control

[0067] Fig. Figure 7D is an illustrative view of the relationship between the upper limits VHa and VHb and the lower limits VLa and VLb in the fourth variation of the open-circuit voltage control. In the fourth variation, the upper limit VHb is greater than the upper limit VHa, the lower limit VLb is greater than the lower limit VLa, the upper limit VHa is less than the upper limit VHb and greater than the lower limit VLb, and the lower limit VLb is less than the upper limit VHa and greater than the lower limit VLa. Because the upper limit VHb is greater than the upper limit VHa and because the lower limit VLb is greater than the lower limit VLa, a period of time is ensured during which the voltage Vb is higher than the voltage Va, thus improving the response to an increase in required power.Since the upper limit VHa is lower than the upper limit VHb, the dissolution of the cathode catalyst of FC 20a is also prevented. Furthermore, since the difference between the upper limit VHb and the lower limit VLb is greater than the difference between the upper limit VHa and the lower limit VLa, the deterioration of the service life of the bypass valve 15b is further prevented.

[0068] In the fourth variation, the upper limit VHa is, for example, 0.8 V, the upper limit VHb is 0.84 V, the lower limit VLa is 0.75 V and the lower limit VLb is 0.77 V.

[0069] In the first to fourth variations, the target voltages Va and Vb also partially overlap, which prevents a difference in performance between FCs 20a and 20b due to the implementation of the open-circuit voltage control.

[0070] In the embodiment and variations described above, the target flow velocity QL can be zero.

[0071] As in Fig. As shown in Figure 2, the open-circuit voltage is controlled by adjusting the flow velocities Qa and Qb using square wave control, but this is not limited to this. At least one of the flow velocities Qa and Qb can be adjusted to rise and fall linearly or in a curved pattern, or it can be adjusted using sinusoidal control.

[0072] The flow velocity Qa is controlled by adjusting the opening degree of the bypass valve 15a in the embodiment and the modification described above, but this is not the only possible method. The flow velocity Qa can be controlled by adjusting at least one rotational speed of the air compressor 14a, the opening degree of the bypass valve 15a, and / or the opening degree of the backpressure valve 17a. For example, if the flow velocity Qa is controlled from a state in which the flow velocity Qa is greater than the target flow velocity QL to the target flow velocity QL, only the rotational speed of the air compressor 14a can be reduced without changing the opening degrees of the bypass valve 15a and the backpressure valve 17a.Alternatively, only the opening degree of the backpressure valve 17a can be reduced, without changing the rotational speed of the air compressor 14a or the opening degree of the bypass valve 15a. Furthermore, if only the rotational speed of the air compressor 14a is adjusted to control the flow velocity Qa, the energy loss due to the air compressor 14a is reduced compared to if only the opening degree of the bypass valve 15a or the backpressure valve 17a is adjusted. The same applies to the flow velocity Qb.

[0073] The system 1 described above includes, but is not limited to, the two air compressors 14a and 14b. For example, instead of the two air compressors 14a and 14b, a single air compressor and pipes connecting the air compressor and both supply pipes 11a and 11b are provided, and the flow velocities Qa and Qb can be controlled by controlling the opening degrees of the bypass valves 15a and 15b and the backpressure valves 17a and 17b.

[0074] System 1, described above, comprises two fuel cells 20a and 20b, but it can include three or more. In this case, at least two fuel cell open-circuit voltages can be controlled as described above. It is also possible to prevent the open-circuit voltages of all fuel cells from being brought into low states.

[0075] The fuel cell system is installed in the vehicle, but this is not limited to cars. For example, a stationary fuel cell system can be used. The vehicle can be not only a car, but also a motorcycle, a rail vehicle, a ship, an aircraft, or the like. Furthermore, the vehicle can be a hybrid vehicle that uses an electric motor and an internal combustion engine together.

[0076] Although some embodiments of the present invention have been described in detail, the present invention is not limited to the specific embodiments, but can be modified or altered within the scope of the present invention as claimed.

Claims

[1] comprising a fuel cell system (1): a fuel cell unit (20a, 20b) comprising a first and a second fuel cell (20a, 20b) which supplies electrical power to a load device (50); a first and a second supply system (10a, 10b) which are set up to control a first and a second flow velocity (Qa, Qb) of cathode gas, which is supplied to the first and second fuel cell (20a, 20b) respectively; a switching device (36a, 36b) capable of switching the fuel cell unit (20a, 20b) and the load device (50) between an electrically connected state in which the fuel cell unit (20a, 20b) is electrically connected to the load device (50) and an electrically disconnected state in which the fuel cell unit (20a, 20b) is electrically disconnected from the load device (50); a switching control unit (60) which is configured to switch the fuel cell unit (20a, 20b) and the load device (50) into the electrically disconnected state when a required power (P) of the fuel cell unit (20a, 20b) is less than or equal to a threshold value (P1); an open-circuit voltage reference unit (60) configured to maintain a first open-circuit voltage (Va) of the first fuel cell (20a) and a second open-circuit voltage (Vb) of the second fuel cell (20b) in the electrically disconnected state; and a supply system control unit (60) configured to increase and decrease the first and second open-circuit voltages (Va, Vb) such that they converge within a first and second target range, respectively, by controlling the first and second supply systems (10a, 10b) such that the first and second flow velocities (Qa, Qb) are increased and decreased, where a lower limit (VLa) of the first target range is greater than a lower limit (VLb) of the second target range. [2] Fuel cell system according to claim 1, wherein one size of the first or second target range is larger than that of the other of the first or second target range. [3] Fuel cell system according to claim 1 or 2, wherein the first and second target ranges overlap at least partially. [4] Fuel cell system according to one of claims 1 to 3, wherein an upper limit (VHa) of the first target range is identical to an upper limit (VHb) of the second target range. [5] Fuel cell system of one of claims 1 to 3, wherein an upper limit (VHa) of the first target range is greater than an upper limit (VHb) of the second target range. [6] comprising a fuel cell system (1): a fuel cell unit (20a, 20b) comprising a first and a second fuel cell (20a, 20b) which supplies electrical power to a load device (50); a first and a second supply system (10a, 10b) which are set up to control a first and a second flow velocity (Qa, Qb) of cathode gas, which is supplied to the first and second fuel cell (20a, 20b) respectively; a switching device (36a, 36b) capable of switching the fuel cell unit (20a, 20b) and the load device (50) between an electrically connected state in which the fuel cell unit (20a, 20b) is electrically connected to the load device (50) and an electrically disconnected state in which the fuel cell unit (20a, 20b) is electrically disconnected from the load device (50); a switching control unit (60) which is configured to switch the fuel cell unit (20a, 20b) and the load device (50) into the electrically disconnected state when a required power (P) of the fuel cell unit (20a, 20b) is less than or equal to a threshold value (P1); an open-circuit voltage reference unit (60) configured to maintain a first open-circuit voltage (Va) of the first fuel cell (20a) and a second open-circuit voltage (Vb) of the second fuel cell (20b) in the electrically disconnected state; and a supply system control unit (60) configured to increase and decrease the first and second open-circuit voltages (Va, Vb) such that they converge within a first and second target range, respectively, by controlling the first and second supply systems (10a, 10b) such that the first and second flow velocities (Qa, Qb) are increased and decreased, where a lower limit (VLa) of the first target range is identical to a lower limit (VLb) of the second target range, and an upper limit (VHa) of the first target range is greater than an upper limit (VHb) of the second target range.

Images