Method for operating a fuel cell system

Abstract

Method for operating a fuel cell system (10) with: a fuel cell stack (20) with a plurality of unit cells (24) in which anode gas flow paths (105) are formed; an anode gas supply unit (50) which is configured to supply anode gas to the fuel cell stack (20) via an anode gas supply opening (25) of the fuel cell stack (20); a pressure sensor (52) arranged between the anode gas supply opening (25) and the anode gas supply unit (50) to measure an anode gas supply pressure; a current sensor (92) configured to measure the current of the fuel cell stack (20); and a controller (80) which is configured to control an anode gas supply quantity of the anode gas supply unit (50) and thereby control the anode gas supply pressure, wherein the controller (80) if a blockage foreseeability condition is met, which indicates that the occurrence of a blockage due to liquid water in the anode gas flow paths (105) of at least one unit cell (24) is foreseeable: • calculated an initial pressure to adjust a stoichiometric ratio of anode gas to a predetermined value, wherein the stoichiometric ratio is calculated from a current strength of the fuel cell stack (20) and the anode gas supply quantity, • compares the first pressure with a second pressure to clear a blockage in the anode gas flow paths (105) due to liquid water, • a higher value is determined from the first print and the second print than the selected print, and • controls the anode gas supply quantity so that the anode gas supply pressure becomes the selected pressure.

Description

BACKGROUND AREA

[0001] The present disclosure relates to a method for operating a fuel cell system comprising a fuel cell stack. STATE OF THE ART

[0002] Anode gas flow paths in unit cells of a fuel cell stack can become clogged by liquid water generated during power generation. A purging process is performed to prevent the anode gas flow paths from becoming clogged by liquid water when the fuel cell system is stopped. In a fuel cell system disclosed in JP 2014-197 481 A, a purging process using anode gas as the purge gas is performed to drain liquid water located in the anode exhaust gas outlet flow paths from the unit cells, thereby preventing the anode exhaust gas outlet flow paths from becoming clogged. Furthermore, with regard to the prior art, reference is made to US Pat. No. 9,147,896 B2 and JP 2015-109 137 A.

[0003] However, always performing the purging process using large amounts of anode gas when the fuel cell system is stopped would reduce fuel economy. Therefore, it is desirable to keep the supply amount of anode gas as small as possible. Furthermore, the inventor of the present disclosure has found that clogging due to liquid water in the anode gas flow paths can occur not only when the fuel cell system is stopped, but also during normal operation or low-load operation, causing the problem of unit cells experiencing anode gas starvation and deterioration. SUMMARY

[0004] The present disclosure, which has been made to solve the problems described above, can be implemented in the following aspects. Solution to the problem

[0005] (1) According to one aspect of the present disclosure, a method for operating a fuel cell system is provided. The fuel cell system includes: a fuel cell stack having a plurality of unit cells in which anode gas flow paths are formed; an anode gas supply unit configured to supply anode gas to the fuel cell stack via an anode gas supply port of the fuel cell stack; a pressure sensor disposed between the anode gas supply port and the anode gas supply unit for measuring an anode gas supply pressure; a current sensor configured to measure a current of the fuel cell stack; and a controller configured to control an anode gas supply amount of the anode gas supply unit, thereby controlling the anode gas supply pressure.The controller is configured to, when a clogging foreseeability condition is met, indicating that clogging due to liquid water in the anode gas flow paths of at least one unit cell is foreseeable: calculate a first pressure for setting a stoichiometric ratio of anode gas to a predetermined value, wherein the stoichiometric ratio is calculated from a current intensity of the fuel cell stack and the anode gas supply amount, then compare the first pressure with a second pressure for releasing clogging in the anode gas flow paths due to liquid water, determine a higher value of the first pressure and the second pressure as the selected pressure, and control the anode gas supply amount so that the anode gas supply pressure becomes the selected pressure.

[0006] According to the fuel cell system in this aspect, since the anode gas supply amount is controlled by using the higher of the first and second pressures, the anode gas supply is reduced compared to cases where a large amount of anode gas is supplied to ensure a pressure sufficiently higher than the previous pressures at all times. Furthermore, since the anode gas supply amount is adjusted after the clogging predictability condition is satisfied regardless of the operating state of the fuel cell system, liquid water in the anode gas flow paths of the unit cells is discharged from the unit cells without using large amounts of anode gas, so that clogging of the anode gas flow paths is resolved, thereby preventing deterioration of the unit cells.

[0007] (2) According to another aspect, the fuel cell system may further include: a system switch configured to issue an instruction to turn on and stop the fuel cell system; and a cell monitor configured to detect a cell voltage of the plurality of unit cells. It may be judged that the clogging predictability condition is satisfied when an instruction to stop the fuel cell system has been issued from the system switch, or when the cell monitor has detected a negative voltage from the fuel cell system during operation.

[0008] When an instruction to stop the fuel cell system has been issued, or when the cell monitoring device has detected a negative voltage, it is highly likely that clogging due to liquid water has occurred in the anode gas flow paths of the unit cells. In such a case, according to the fuel cell system in this aspect, since the controller adjusts the anode gas supply amount, liquid water present in the anode gas flow paths of the unit cells is drained from the unit cells, thus resolving the clogging of the anode gas flow paths and preventing deterioration of the unit cells.

[0009] (3) According to another aspect, the predetermined value of the stoichiometric ratio of the anode gas may be within a range of 1.2 to 1.3.

[0010] According to the fuel cell system of this aspect, anode gas supply to the fuel cell stack is sufficiently ensured. Therefore, the unit cells are free from anode gas deficiency condition, thus preventing deterioration of the unit cells.

[0011] (4) According to another aspect, the anode gas supply unit may include: an injector configured to supply anode gas from an anode gas tank to the anode gas supply port; an anode gas return piping configured to return anode gas discharged from an anode gas discharge port of the fuel cell stack to the anode gas supply port; and an anode gas circulation pump disposed in the anode gas return piping. The controller may be configured to control a discharge flow rate of the injector and / or a rotation speed of the anode gas circulation pump such that the stoichiometric ratio of the anode gas assumes the predetermined value.

[0012] According to the fuel cell system in this aspect, the stoichiometric ratio of anode gas is regulated to a predetermined value. Therefore, the required amount of anode gas is supplied to the fuel cells, preventing deterioration of the unit cells.

[0013] (5) According to another aspect, the second pressure may be a value resulting from an addition of anode gas supply pressure that decreases during a stop mode operation, which is executed after an instruction to stop the fuel cell system is issued from the system switch and before the fuel cell system is stopped, and a pressure increase that allows discharge of liquid water in the anode gas flow paths.

[0014] According to the fuel cell system in this aspect, when the second pressure is set as the selected pressure, liquid water located in the anode gas flow paths is reliably discharged, so that deterioration of the unit cells is suppressed.

[0015] The present disclosure may also be implemented in various ways other than the aspects described above. For example, the disclosure may be implemented in such modes as a purge method for fuel cell stacks and a fuel cell vehicle. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an explanatory view showing a schematic configuration of a fuel cell system according to an embodiment of the present disclosure; Fig. 2 is an explanatory view showing a schematic configuration of an anode-side separator of a unit cell; Fig. 3 is a flowchart showing a liquid water blockage release control; Fig. 4 is a graph showing an example of time variations in anode gas circulation pump speed, anode gas supply pressure, fuel cell current, and cell voltage during execution of liquid water clogging release control; and Fig. 5 is another graph showing an example of time variations in anode gas circulation pump speed, anode gas supply pressure, fuel cell current, and cell voltage during execution of the liquid water clogging release control. DESCRIPTION OF THE EMBODIMENTS

[0016] Fig. 1 is an explanatory view showing a schematic configuration of a fuel cell system 10 according to an embodiment of the present disclosure. The fuel cell system 10 is mounted, for example, in a vehicle to output electricity serving as a power source of the vehicle upon request from a vehicle driver. The fuel cell system 10 includes a fuel cell stack 20, an anode gas supply / circulation system 50, a cathode gas supply / discharge system 30, a coolant circulation system 70, a controller 80, a DC / DC converter 90, a power control unit (hereinafter referred to as "PCU") 91, and a cell monitor 95.

[0017] In the fuel cell stack 20, an end plate 21, an insulating plate 22, a current collector plate 23, a plurality of unit cells 24, a current collector plate 23, an insulating plate 22, and an end plate 21 are stacked in the following order. Each unit cell 24 consists of a membrane electrode assembly (not shown) and two separators sandwiching the membrane electrode assembly on both its anode and cathode sides. The fuel cell stack 20 also includes an anode gas supply port 25 and an anode gas discharge port 26. An anode gas collection pipe 25d is connected to the anode gas supply port 25.

[0018] The anode gas supply / circulation system 50 functions as an anode gas supply unit for supplying anode gas to the fuel cell stack 20 via the anode gas supply port 25 of the fuel cell stack 20. The supply of the anode gas in the anode gas supply / circulation system 50 includes supplying from an anode gas storage container and supplying through an anode gas reflux. In the example of Fig. 1, hydrogen is used as the anode gas. The anode gas supply / circulation system 50 includes an anode gas tank 40, a shut-off valve 41, an anode gas supply piping 60, a regulator 51, an injector 54, an anode gas return piping 61, an anode gas circulation pump 55, a gas-liquid separator 56, a shut-off valve 57, and a drain / empty piping 58.

[0019] The anode gas tank 40 is a storage container for hydrogen gas at a high pressure of several tens of MPa. The anode gas supply piping 60 is connected between the anode gas tank 40 and one end CL of the anode gas header piping 25d to supply hydrogen discharged from the anode gas tank 40 to the fuel cell stack 20. As mentioned above, the shutoff valve 41, the regulator 51, and the injector 54 are arranged on the anode gas supply piping 60 in this order from the anode gas tank 40 side. The regulator 51 regulates the hydrogen pressure. The injector 54 supplies hydrogen discharged from the anode gas tank 40 to the anode gas supply port 25 of the fuel cell stack 20 via the anode gas header piping 25d.

[0020] The anode gas return pipe 61 is a pipe for returning anode gas discharged from the anode gas discharge port 26 of the fuel cell stack 20 to the anode gas supply port 25. The anode gas return pipe 61 has one end connected to the anode gas discharge port 26 of the fuel cell stack 20, and the other end connected to one end CL of the anode gas collection pipe 25d. That is, hydrogen carried in the anode gas supply pipe 60 and the hydrogen carried in the anode gas return pipe 61 accumulate in the anode gas collection pipe 25d and is supplied to the anode gas supply port 25 of the fuel cell stack 20.

[0021] In the anode gas return piping 61, the gas-liquid separator 56 and the anode gas circulation pump 55 are arranged in this order, as mentioned from a side closer to the anode gas discharge port 26 of the fuel cell stack 20. The gas-liquid separator 56 separates the gas and liquid discharged from the anode gas discharge port 26 of the fuel cell stack 20. Gas discharged from the anode gas discharge port 26 mainly consists of hydrogen that was not used for power generation, nitrogen that has permeated from the cathode side to the anode side of each unit cell 24, and steam generated by power generation. Liquid discharged from the anode gas discharge port 26 mainly consists of liquid water generated by power generation.The gas-liquid separator 56 separates nitrogen and liquid water from the gas and liquid, and discharges the separated nitrogen and liquid water to the outside via the shutoff valve 57 and the discharge / drain piping 58. Hydrogen and steam remaining in the gas-liquid separator 56 are returned to the anode gas return piping 61 as anode gas by the anode gas circulation pump 55. A pressure sensor 52 for measuring the anode gas supply pressure is arranged between the anode gas supply port 25 of the fuel cell stack 20 and the anode gas supply / circulation system 50, that is, on the anode gas collection piping 25d.

[0022] Controlled by a controller 80, the cathode gas supply / discharge system 30 supplies cathode gas and discharges cathode gas to and from the fuel cell stack 20. In the example of Fig. 1, air is used as the cathode gas. The cathode gas supply / discharge system 30 includes a cathode gas supply line 32, a compressor 31, a three-way valve 33, a bypass line 38, a pressure control valve 36, and a cathode gas discharge line 39.

[0023] The cathode gas supply piping 32 is connected to the fuel cell stack 20 to supply air drawn in from outside the fuel cell stack 20. As mentioned, an outside air temperature sensor 35, an air flow meter 34, a compressor 31, and a three-way valve 33 are arranged on the cathode gas supply piping 32, in this order, from the air inlet side. The outside air temperature sensor 35 measures the temperature of the air before it is drawn in. The air flow meter 34 measures the quantity of intake air. The compressor 31 compresses the intake air. The three-way valve 33 is connected to the bypass piping 38 to regulate the air flow rate flowing to the fuel cell stack 20 and the bypass piping 38. The bypass piping 38 is connected to the cathode gas discharge piping 39.

[0024] The cathode gas discharge piping 39 has an upstream end portion connected to the fuel cell stack 20, and intermediate portions connected to the bypass piping 38 and the drain / purge piping 58 of the anode gas supply / circulation system 50, respectively. The cathode gas discharge piping 39 discharges cathode off-gas discharged from the fuel cell stack 20, air branched to the bypass piping 38, and nitrogen plus liquid water discharged from the drain / purge piping 58 to the outside. The pressure regulating valve 36 is also placed on the cathode gas discharge piping 39. The pressure regulating valve 36 is located at a position closer to the fuel cell stack 20 than the connection point between the cathode gas discharge pipe 39 and the bypass pipe 38. The pressure regulating valve 36 regulates a pressure of air supplied to the fuel cell stack 20.

[0025] Controlled by the controller 80, the coolant circulation system 70 cools the fuel cell stack 20. The coolant circulation system 70 includes a refrigerant supply piping 74, a refrigerant discharge piping 73, a radiator 71, a bypass piping 77, a three-way valve 75, and a refrigerant pump 72. The refrigerant is supplied using, for example, water, ethylene glycol or other cold-resistant water, air, or the like. The refrigerant pump 72 is placed on the refrigerant supply piping 74 to supply the refrigerant to the fuel cell stack 20. The three-way valve 75 regulates a flow rate of the refrigerant flowing to the radiator 71 and the bypass piping 77.

[0026] The cell monitoring device 95 has a function of detecting a cell voltage of the plurality of unit cells 24. In the example of Fig. 1, the cell monitoring device 95, controlled by the controller 80, detects a total of two cell voltages per channel on a channel basis, with each two unit cells 24 being bundled on a channel. The number of unit cells 24 per channel may be one or else three or more. In the case where the number of unit cells 24 per channel is N, where N is an integer of 2 or more, assuming that cell voltages of (N-1) unit cells 24 out of N unit cells 24 are each an average cell voltage, a cell voltage of the remaining unit cell 24 can be estimated. The average cell voltage is a value determined by dividing a voltage in the fuel cell stack 20 measured by a stack voltage sensor 93 by the number of unit cells 24.A cell voltage estimated in this way is also equal to a "cell voltage detected by the cell monitor 95." Since only one of the N unit cells 24 can have a negative voltage, it is generally practical to assume that the (N-1) unit cells 24 correspond to the average cell voltage.

[0027] The DC-DC converter 90 increases a voltage output from the fuel cell stack 20 and, under the control of the controller 80, supplies the resulting voltage to the PCU 91. The PCU 91, which includes an inverter, supplies power to a load under the control of the controller 80. Under the control of the controller 80, the PCU 91 also regulates a current of the fuel cell stack 20. A current sensor 92 for measuring the current of the fuel cell stack 20 is placed between the fuel cell stack 20 and the DC-DC converter 90.

[0028] The controller 80 is configured as a computer, specifically an ECU (electronic control unit), including a CPU, a RAM, and a non-volatile memory. The controller 80 outputs signals to control the turning on and off of individual devices in the fuel cell system 10 in response to instructions (ON and OFF operations) from a system switch 200. The system switch 200 corresponds, for example, to an ignition switch or a power switch of vehicles. Also, the controller 80, receiving a power generation request, controls individual parts of the fuel cell system 10 to excite the fuel cell stack 20 to generate electricity. The controller 80 controls the anode gas supply pressure by controlling the anode gas supply amount of the anode gas supply / circulation system 50, that is, by controlling the gas supply amount of the injector 54 and the anode gas circulation pump 55.

[0029] Fig. 2 is an explanatory view showing a schematic configuration of an anode-side separator 100 of a unit cell 24 in the fuel cell stack 20 ( Fig. 1) seen from the side of the membrane electrode assembly. In Fig. 2, an X direction runs along a horizontal direction, a Z direction runs along a vertically upward direction, and a Y direction runs along a stacking direction of the unit cells 24. An anode gas inlet manifold 110, a coolant outlet manifold 160, and a cathode gas inlet manifold 130 are positioned so that they can be placed from top to bottom in one longitudinal end edge portion of the separator 100. In contrast, a cathode gas outlet manifold 140, a coolant inlet manifold 150, and an anode gas outlet manifold 120 are positioned so that they can be placed from top to bottom in the other end edge portion of the separator 100. The anode gas inlet manifold opening 110 and the anode gas outlet manifold opening 120 are respectively connected to the anode gas supply opening 25 and the anode gas discharge opening 26 of the Fig. 1 shown fuel cell stack 20.

[0030] A plurality of strip-like anode gas flow paths 105 are formed in a central portion of the separator 100. Each anode gas flow path 105 has an anode gas inlet portion 111 communicating with the anode gas inlet collection port 110 and an anode gas outlet portion 121 communicating with the anode gas outlet collection port 120. In the example of Fig. 2, the anode gas flow paths 105 are winding flow paths formed from a plurality of strip-shaped partial flow paths 105p arranged at equal intervals and winding. Also, the anode gas inlet part 111 and the anode gas outlet part 121 are comb-shaped.

[0031] Hydrogen supplied to the anode gas inlet manifold 110 passes through the anode gas inlet part 111 and flows into the partial flow paths 105p. The hydrogen that has flowed into the partial flow paths 105p passes through the anode gas outlet part 121 while winding and flows to the anode gas outlet manifold 120. At this point, there is a possibility that if the hydrogen supplied to the anode gas inlet manifold 110 contains liquid water generated by power generation or condensed water that accumulates in the anode gas circulation pump 55 ( Fig. 1) or the like, the anode gas flow paths 105, in particular the anode gas inlet part 111, are blocked by such liquid water.

[0032] Fig. 3 is a flowchart showing a control for solving clogging by liquid water in the fuel cell system 10. This control process is normally and repeatedly executed during the operation of the fuel cell system 10. At step S210, the controller 80 decides whether a clogging predictability condition is satisfied, indicating that clogging due to liquid water in the anode gas flow paths 105 ( Fig. 2) of the separator 100 of the unit cells 24 is foreseeable. It is judged that the clogging foreseeability condition is met when an instruction to stop the fuel cell system 10 is issued from the system switch 200, or when a negative voltage is detected by the cell monitor 95 during the operation of the fuel cell system 10. In the case where the instruction to stop the fuel cell system 10 has been issued, turning off the system switch 200 causes the rotational speed of the anode gas circulation pump 55 to decrease to reduce noise and vibration. Consequently, the anode gas flow rate decreases, so that liquid water becomes more likely to be present within the unit cells 24, and therefore clogging of the anode gas flow paths 105 due to liquid water is likely to occur.Even in the case where the cell monitor 95 detects a negative voltage during the operation of the fuel cell system 10, there is a high probability that clogging has occurred due to liquid water in the anode gas flow paths 105, thus blocking the supply of the anode gas to the unit cells 24. Unlike the above cases, it can be judged that the clogging predictability condition is also satisfied in cases where, for example, the fuel cell system 10 stops and restarts and performs normal operation after performing a warm-up operation to raise the temperature of the fuel cell stack 20, or where the fuel cell system 10 performs normal operation at an outside air temperature below freezing.That is, since condensed water is very likely to occur in the anode gas circulation pump 55 due to the temperature difference between the anode gas and the anode gas circulation pump 55, in these two cases, it is very likely that the liquid water may possibly flow into the anode gas flow paths 105 of the unit cells 24 and cause clogging there.

[0033] If it is determined at step S210 that the clogging predictability condition is not met, the controller 80 executes step S210 again. Meanwhile, if it is determined that the clogging predictability condition is met, the process flow proceeds to step S220.

[0034] At step S220, the controller 80 determines whether an instruction to stop the fuel cell system 10 has been issued from the system switch 200. If it is determined that the instruction to stop the fuel cell system 10 has been issued, the process flow proceeds to step S230. At step S230, the controller 80 starts a stop mode operation before stopping the fuel cell system 10. The term "stop mode operation" refers to an operation performed before the operation of individual systems of the fuel cell system 10 is completely stopped. In the stop mode operation, for example, the compressor 31 of the cathode gas supply / discharge system 30 is first stopped, and the speed of the anode gas circulation pump 55 is reduced. Also, a small amount of current is drained from the fuel cell stack 20 to prevent the voltage of the unit cells 24 from becoming an open-circuit voltage (OCV).On the other hand, if it is judged at step S220 that no instruction to stop the fuel cell system 10 has been issued, the process flow proceeds to step S240, skipping step S230.

[0035] In step S240, the controller 80 calculates a first pressure for setting a stoichiometric ratio of anode gas to a predetermined value. The term "stoichiometric ratio" here refers to an actual anode gas flow rate relative to a theoretically required anode gas flow rate for power generation. The theoretically required anode gas flow rate for power generation, ie, an anode gas flow rate actually consumed by the fuel cell stack 20 at the time of step S240, can be calculated from a current value of the fuel cell stack 20 detected by the current sensor 92 ( Fig. 1) is measured. Also, the actual anode gas flow rate, ie, an anode gas flow rate supplied to the fuel cell stack 20 at the time of step S240, may be determined based on an anode gas discharge flow rate from the injector 54 ( Fig. 1) and the speed of the anode gas circulation pump 55 ( Fig. 1). The calculation of the actual anode gas flow rate can be carried out by taking into account the gaseous discharge volume and the liquid discharge volume of the gas-liquid separator 56 ( Fig. 1) in addition to the anode gas discharge flow rate through the injector 54 and the speed of the anode gas circulation pump 55.

[0036] The term "predetermined value" of the anode gas stoichiometric ratio refers to a value of the anode gas stoichiometric ratio that prevents insufficient anode gas supply to the fuel cell stack 20. A value within a range of 1.2 to 1.3 is preferably used for the predetermined value of the stoichiometric ratio. The predetermined value has been stored in the non-volatile memory in the controller 80. The predetermined value of the anode gas stoichiometric ratio may be greater than 1.3.

[0037] At step S250, the controller 80 compares the first pressure calculated in step S240 with a second pressure aimed at unblocking liquid water in the anode gas flow paths 105. In this case, since the fuel cell system 10 has executed step S230, the second pressure may be set to a value resulting from the addition of an anode gas supply pressure that decreases during the stop mode operation after the system switch 200 issues the instruction to stop the fuel cell system 10, and a pressure increase that allows water in the anode gas flow paths 105 to be discharged. During the stop mode operation, when the speed of the anode gas circulation pump 55 is reduced to reduce noise and vibration, the anode gas supply pressure decreases accordingly.The anode gas supply pressure resulting from its decrease takes a value between 160 kPa and 190 kPa, for example. The pressure increase that allows discharge of liquid water located in the anode gas flow paths 105 can be calculated from a cross-sectional area of ​​the anode gas flow paths 105, a predicted amount of liquid water, and the like. Preferably, the lower limit of the pressure increase is set to 33 kPa to reliably ensure discharge of liquid water. Also, to prevent an excessive increase in the anode gas discharge concentration upon restart of the fuel cell system 10, the upper limit of the pressure increase is preferably set to 60 kPa. The second pressure does not necessarily have to be calculated as the added value of the anode gas supply pressure and the pressure increase described above, but can be determined in advance experimentally or empirically.

[0038] At step S260, the controller 80 uses either the first pressure or the second pressure, whichever is higher, as the selected pressure. At step S270, the controller 80 regulates the anode gas supply amount of the anode gas supply / circulation system 50 so that the anode gas supply pressure becomes the selected pressure. In this way, it is possible to drain liquid water remaining in the anode gas flow paths 105 of the unit cells 24 from the unit cells 24, thereby enabling the liquid water clogging to be released.

[0039] At step S280, the controller 80 determines whether the adjustment of the anode gas supply amount in step S270 has ended. A termination condition in this case may be, for example, that when the stop mode operation of step S230 has been executed, the cell voltage of the unit cells 24 drops to a terminal voltage value or lower. This terminal voltage value is a value at which the cell voltage is low enough to judge that subsequently setting the current to zero will never cause deterioration of the unit cells 24. For example, a value of 0.80 V to 0.90 V is used for the terminal voltage value. If it is determined that the adjustment of the anode gas supply amount has not ended, the process flow returns to step S270.On the other hand, if it is determined that the anode gas supply amount adjustment has ended, the process flow proceeds to step S290, where the liquid water clogging release control is ended. If the stop mode operation was started at step S230, the stop mode operation is also ended, stopping the operation of all systems of the fuel cell system 10.

[0040] Fig. 4 is a graph showing an example of time variations in a rotational speed of the anode gas circulation pump 55, anode gas supply pressure, current of the fuel cell stack 20, and cell voltage during execution of the liquid water clogging release control. Fig. Fig. 4 shows a case where the controller 80 at step S260 Fig. 3 sets the second pressure P2 as the selected pressure.

[0041] During a period from time t0 to time t1, the fuel cell system 10 performs normal operation. Here, the term "normal operation" implies a state in which the reaction gases are sufficiently supplied and the fuel cell system 10 is operating successfully.

[0042] At time t1, an instruction to stop the fuel cell system 10 is issued by the system switch 200. In this example, when an instruction to stop the fuel cell system 10 is issued, it is judged that the clogging predictability condition is satisfied (S210-S220, Fig. 3). Accordingly, the fuel cell system 10 performs the stop mode operation during a period from time t1 to time t2 (S230, Fig. 3) to divert a small current from the fuel cell stack 20. In this case, the speed of the anode gas circulation pump 55 is reduced to reduce noise and vibration. As the speed of the anode gas circulation pump 55 decreases, the anode gas supply amount drops to a level corresponding to the pressure P0, and a first pressure P1 is calculated to ensure the stoichiometric ratio of the anode gas (S240, Fig. 3). In the example from Fig. 4, since the first pressure P1 is lower than the second pressure P2 to solve the blockage due to liquid water, the anode gas supply pressure is adjusted to the second pressure P2 (S250-S270, Fig. 3). With this arrangement, liquid water remaining in the anode gas flow paths 105 of the unit cells 24 is discharged from the unit cells 24, so that clogging due to liquid water can be resolved. At time t1, the compressor 31 of the cathode gas supply / discharge system 30 is stopped, resulting in the cell voltage of the unit cells 24 gradually decreasing.

[0043] At time t2, the cell voltage of the unit cells 24 drops below the final voltage value Vth, where the liquid water blockage release control is terminated (S280-S290, Fig. 3). Starting at time t2, the anode gas supply / circulation system 50 of the fuel cell system 10 is completely stopped.

[0044] Fig. 5 is another graph showing an example of time variations in a rotational speed of the anode gas circulation pump 55, anode gas supply pressure, current of the fuel cell stack 20, and cell voltage during execution of the liquid water clogging release control. Fig. Fig. 5 shows a case where the controller 80 at step S260 Fig. 3 sets the first pressure P1 as the selected pressure. Fig. 5 shows an operating state of the fuel cell system 10 in which a vehicle with the fuel cell system 10 mounted therein performs low-speed travel, acceleration travel, and constant-speed travel in this order, assuming that the vehicle is in a traffic jam.

[0045] During a period from time t0 to time t1, the fuel cell system 10 performs low-load operation because the fuel cell vehicle travels at a low speed (e.g., 10 km / h). Even during low-load operation, it is preferable that the anode gas supply / circulation system 50 and the cathode gas supply / discharge system 30 are operated such that the stoichiometric ratios of the anode gas and the cathode gas each reach values ​​within a suitable range (e.g., a range of 1.2 to 1.3).

[0046] While the fuel cell system 10 is performing low-load operation, the anode gas and the cathode gas reach low flow rates in response to the low load, so that a large amount of liquid water remains in the unit cells 24 to clog the anode gas flow paths 105. Consequently, the unit cells 24 experience an anode gas shortage, so that a negative cell voltage is detected at time t1. At the same time, in the example of Fig. 5, the fuel cell vehicle performs an acceleration run, where, as the current of the fuel cell stack 20 increases, the speed of the anode gas circulation pump 55 increases, and the anode gas supply pressure also increases. During the period from time t1 to time t2, the fuel cell system 10 performs normal operation.

[0047] In the example from Fig. 5, if a negative cell voltage is detected during operation of the fuel cell system 10, it is decided that the clogging predictability condition is met (S210-S220, Fig. 3). Therefore, the first pressure P1 is calculated to ensure the stoichiometric ratio of the anode gas (S240, Fig. 3). In the example from Fig. 5, the anode gas supply pressure is regulated to the first pressure P1, since the first pressure P1 is greater than the second pressure P2 to solve a blockage due to liquid water (S250-S270, Fig. 3). The controller 80 adjusts either the discharge flow rate of the injector 54 and / or the speed of the anode gas circulation pump 55 so that the stoichiometric ratio of the anode gas assumes a predetermined value and adjusts the anode gas supply pressure to the first pressure P1. With this arrangement, liquid water remaining in the anode gas flow paths 105 of the unit cells 24 is discharged from the unit cells 24, thus relieving the clogging due to the liquid water.

[0048] At time t2, the fuel cell vehicle terminates the acceleration run to continue traveling at a constant speed. In response, the current of the fuel cell stack 20 decreases, while the speed of the anode gas circulation pump 55 and the anode gas supply pressure also decrease. Depending on the termination condition, for example, the current of the fuel cell stack 20 becomes less than an acceleration current I1, the fuel cell system 10 terminates the liquid water blockage release control (S280-S290, Fig. 3). During a period from time t2 to time t3, the fuel cell system 10 performs normal operation.

[0049] As described above, according to one embodiment of the present disclosure, since the controller 80 of the fuel cell system 10 controls the anode gas supply amount by using the higher of the first and second pressures, the anode gas supply is reduced compared to cases where a large amount of anode gas is supplied to ensure a pressure sufficiently higher than the previous pressures at all times. Also, since the controller 80 controls the anode gas supply amount during a stop or low-load operation of the fuel cell system 10 so that the anode gas supply pressure becomes a selected pressure, liquid water remaining in the anode gas flow paths 105 of the unit cells 24 is discharged from the unit cells 24, so that clogging of the anode gas flow paths 105 is resolved, thereby preventing deterioration of the unit cells 24.

Claims

[1] Method for operating a fuel cell system (10) comprising: a fuel cell stack (20) having a plurality of unit cells (24) in which anode gas flow paths (105) are formed; an anode gas supply unit (50) configured to supply anode gas to the fuel cell stack (20) via an anode gas supply opening (25) of the fuel cell stack (20); a pressure sensor (52) arranged between the anode gas supply port (25) and the anode gas supply unit (50) for measuring an anode gas supply pressure; a current sensor (92) configured to measure a current intensity of the fuel cell stack (20); and a controller (80) configured to control an anode gas supply amount of the anode gas supply unit (50) and thereby control the anode gas supply pressure, wherein the controller (80), when a clogging foreseeability condition is met, indicating that clogging due to liquid water in the anode gas flow paths (105) of at least one unit cell (24) is foreseeable: • calculates a first pressure to adjust a stoichiometric ratio of anode gas to a predetermined value, wherein the stoichiometric ratio is calculated from a current intensity of the fuel cell stack (20) and the anode gas supply amount, • comparing the first pressure with a second pressure to release a blockage in the anode gas flow paths (105) due to liquid water, • determines a higher value of the first pressure and the second pressure as the selected pressure, and • controls the anode gas supply amount so that the anode gas supply pressure becomes the selected pressure. [2] A method for operating a fuel cell system (10) according to claim 1, wherein the fuel cell system further comprises: a system switch (200) configured to give an instruction to switch on and stop the fuel cell system (10); and a cell monitoring device (95) configured to detect a cell voltage of the plurality of unit cells (24), wherein it is decided that the clogging foreseeability condition is met when an instruction to stop the fuel cell system (10) has been issued by the system switch (200) or when the cell monitoring device (95) has detected a negative voltage during operation of the fuel cell system (10). [3] A method for operating a fuel cell system (10) according to claim 1 or 2, wherein the predetermined value of the stoichiometric ratio of the anode gas is within a range of 1.2 to 1.

3. [4] A method for operating a fuel cell system (10) according to any one of claims 1 to 3, wherein the anode gas supply unit (50) comprises: an injector (54) configured to supply anode gas from an anode gas tank to the anode gas supply port (25); an anode gas return pipe (61) configured to return anode gas discharged from an anode gas discharge port of the fuel cell stack (20) to the anode gas supply port (25); and an anode gas circulation pump (55) placed in the anode gas return pipe (61), wherein the controller (80) controls a discharge flow rate of the injector (54) and / or a speed of the anode gas circulation pump (55) such that the stoichiometric ratio of the anode gas assumes the predetermined value. [5] A method of operating a fuel cell system (10) according to claim 2, wherein the second pressure is a value resulting from an addition of anode gas supply pressure which decreases during a stop mode operation which is carried out after an instruction to stop the fuel cell system (10) is issued from the system switch (200) and before the fuel cell system (10) is stopped, and a pressure increase which allows discharge of liquid water in the anode gas flow paths (105).

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