JP5359621B2 - Fuel cell system and control method thereof - Google Patents

Description

  The present invention relates to a fuel cell system and a control method thereof, and more particularly, to a fuel cell system that performs intermittent operation of a fuel cell and a control method thereof.

  2. Description of the Related Art Conventionally, as described in Patent Document 1, for example, a fuel cell system is known in which a fuel cell and a secondary battery are connected in parallel to an inverter connected to a motor that is a load. In such a fuel cell system, the required power required to drive the motor is supplied by the fuel cell and the oxidizing gas by supplying the fuel gas and the oxidizing gas to the fuel cell, or the generated power is used. The operation of the fuel cell is controlled by appropriately selecting whether to stop the supply and to output the fuel cell from the secondary battery in a power generation halt state. Hereinafter, such operation control of the fuel cell is referred to as “intermittent operation”, the operation state in the power generation suspension state is referred to as “power generation suspension mode”, and the operation state in the normal power generation state is referred to as “normal power generation mode”. .

  In the fuel cell system of Patent Document 1, when the required power is compared with a threshold and the required power is less than the threshold and the required power can be output from the secondary battery, the power generation suspension mode is selected. On the other hand, it is described that the normal power generation mode is selected when the required power is equal to or greater than the threshold value. And it is described that the said threshold value is adjusted according to the open end voltage (OCV) of a fuel cell.

  Patent Document 2 discloses a fuel cell system that includes a motor, an inverter, a fuel cell, and a battery, and performs intermittent operation of the fuel cell. In this fuel cell system, it is described that when the impurity concentration in the fuel off-gas on the anode side of the fuel cell becomes a predetermined value or more during the power generation stop mode, the power generation stop mode is switched to the normal power generation mode. Here, it is described that the impurity concentration in the fuel off-gas is calculated based on the detected values of the hydrogen concentration and the water vapor concentration obtained from the impurity concentration sensor.

  Furthermore, in the fuel cell system that is intermittently operated disclosed in Patent Document 3, if it is determined that the performance of the fuel cell is degraded when the fuel cell power generation suspension mode is continued, the fuel cell is changed from the power generation suspension mode to the normal power generation mode. It is described that it will migrate. In this fuel cell system, it is described that it is determined whether or not performance degradation of the fuel cell occurs based on the nitrogen concentration in the fuel off gas.

JP 2005-71797 A JP 2006-318964 A JP-A-2005-26054

  In the fuel cell system of Patent Document 1, it is necessary to use the fuel cell in an open-ended voltage state. However, in the fuel cell, when the open-ended voltage is increased, the catalyst used in the fuel cell is eluted and deteriorated. However, there is a problem that the performance of the fuel cell is lowered.

  Further, in the fuel cell systems of Patent Documents 2 and 3, it is necessary to provide a special detection means for detecting the impurity concentration in the fuel off-gas of the fuel cell, resulting in high costs.

  An object of the present invention is to improve the responsiveness and energy efficiency of a fuel cell that operates intermittently while suppressing catalyst deterioration of the fuel cell in a fuel cell system that operates the fuel cell intermittently.

A fuel cell system according to the present invention includes a fuel cell that generates power by receiving supply of fuel gas and oxidant gas, and a voltage converter in parallel with the fuel cell with respect to a load device to which generated power is supplied from the fuel cell. A fuel cell system comprising: a power storage device connected to the fuel cell; and a control device for controlling the gas supply to the fuel cell and controlling the operation of the voltage converter, wherein the control device generates power for the fuel cell. By controlling the gas supply to the fuel cell based on the comparison between the required power and the threshold value, intermittent operation control is performed to switch the operation state of the fuel cell between the normal power generation mode and the power generation suspension mode, and the fuel to the fuel cell is The terminal voltage of the fuel cell does not exceed the upper limit voltage lower than the open-circuit voltage during the power generation stop mode in which the supply of gas and oxidizing gas is completely or almost stopped Run the high-potential avoidance control to, and a current value flowing from the fuel cell to execute the control to reduce the threshold in accordance with decreases during the high-potential avoidance control has been has power dormant mode, having a control arrangement.

  Here, the term “power generation suspension” means that the power generation state is not normal, and does not indicate a state where power generation is not performed at all.

  In the fuel cell system according to the present invention, when the current value increases due to supply of fuel gas or oxidant gas during the power generation suspension mode, the control device increases the threshold value accordingly. Is preferred.

A control method of a fuel cell system according to the present invention includes a fuel cell that generates power by receiving supply of fuel gas and oxidant gas, and a voltage in parallel with the fuel cell with respect to a load device to which generated power is supplied from the fuel cell. A control method for a fuel cell system, comprising: a power storage device connected via a conversion device; and a control device for controlling gas supply to the fuel cell and controlling the operation of the voltage conversion device. By controlling the gas supply to the fuel cell based on the comparison between the power required for power generation and the threshold value, intermittent operation is performed to switch the operation state of the fuel cell between the normal power generation mode and the power generation halt mode, and the fuel gas to the fuel cell And the terminal voltage of the fuel cell does not exceed the upper limit voltage lower than the open-circuit voltage during the power generation stop mode in which the supply of the oxidizing gas is completely or almost stopped To perform high-potential avoidance control, in the high-potential avoidance control has been that power idle mode, the current value flowing from the fuel cell to reduce the threshold with decreasing.

  Further, in the control method of the fuel cell system according to the present invention, when the current value increases due to supply of fuel gas or oxidizing gas during the power generation suspension mode, the threshold value is increased accordingly. preferable.

In the fuel cell system and the control method thereof according to the present invention, the terminal voltage of the fuel cell has an upper limit voltage lower than the open-circuit voltage during the power generation stop mode in which the supply of fuel gas to the fuel cell is completely or substantially stopped. High potential avoidance control is performed so as not to exceed, and control is performed to decrease the threshold value as the current value flowing out from the fuel cell decreases during the power generation suspension mode in which the high potential avoidance control is performed.

  In the fuel cell in the power generation halt mode, power generation is continued due to the electrochemical reaction between the remaining fuel gas and oxidizing gas, and the inter-terminal voltage tends to rise to the open-circuit voltage. By performing the control, the terminal voltage does not reach the open end voltage, so that catalyst deterioration can be suppressed.

  In addition, in the fuel cell in the power generation suspension mode under high potential avoidance control, the fuel gas supply to the fuel cell is basically stopped, so that the weak power generation is continued and the fuel is consumed as described above. As a result, the concentration of fuel contained in the fuel gas in the fuel cell gradually decreases, while the concentration of impurities such as nitrogen and water vapor that permeate from the cathode through the electrolyte membrane increases. Along with this, the current value flowing out from the fuel cell tends to gradually decrease. In this way, when the supply of fuel gas or the like is resumed in a state where the impurity concentration is high and the gas quality is considerably lowered and the mode is shifted from the power generation suspension mode to the normal power generation mode, the gas quality of the fuel gas in the fuel cell is improved. It takes time to return to a state where a predetermined output can be output, and this time becomes longer as the gas quality deteriorates, resulting in poor responsiveness. On the other hand, in the present invention, the threshold value is changed according to the current value, for example, the threshold value is decreased as the current value decreases, thereby lowering the reference for transition from the power generation suspension mode to the normal power generation mode. By shifting to the normal power generation mode before the gas quality deteriorates considerably, the responsiveness of the fuel cell that operates intermittently can be improved.

  On the other hand, when the threshold value is constant, a situation in which the fuel cell shifts from the power generation suspension mode to the normal power generation mode in a state where the gas quality of the fuel gas in the fuel cell has not deteriorated so much can occur frequently. In this case, fuel gas is wasted and energy efficiency (or fuel consumption) is deteriorated. On the other hand, in the present invention, the threshold value is changed according to the current value. For example, when the current value increases due to fuel gas replenishment during the power generation suspension mode, the threshold value is increased to increase the threshold value. Thus, it is possible to suppress the situation where the fuel cell shifts from the power generation suspension mode to the normal power generation mode in a state where the gas quality of the fuel gas has not deteriorated so much, and to improve the energy efficiency of the fuel cell that operates intermittently.

  Furthermore, in the present invention, the current flowing out from the fuel cell in the power generation suspension mode under high potential avoidance control is detected, and the threshold value, which is a reference standard for switching the operating state of the fuel cell, is changed accordingly. Since it is common to provide a current sensor or a voltage sensor to monitor the output state of the fuel cell, a nitrogen concentration sensor for detecting the concentration of impurities contained in the fuel gas in the fuel cell during the power generation halt mode, Without requiring a special sensor such as a water vapor concentration sensor, the threshold value can be changed appropriately in response to the gas quality deterioration of the fuel gas.

FIG. 1 is a schematic configuration diagram of a fuel cell system according to an embodiment of the present invention. FIG. 2 is a flowchart showing a processing procedure for intermittent operation control of the fuel cell. FIG. 3 shows the current flowing out of the fuel cell under high potential avoidance control during the power generation suspension mode, and the time required to output a predetermined power when the power generation suspension mode is shifted to the normal power generation mode. It is a graph which shows the relationship. FIG. 4 is a diagram showing how the threshold value for determining the shift to the normal power generation mode is reduced as the current flowing out from the fuel cell decreases during the power generation halt mode. FIG. 5 is a diagram illustrating a state in which the threshold for determining the transition to the normal power generation mode is also increased when the current flowing out of the fuel cell starts to increase during the power generation suspension mode.

  Embodiments according to the present invention will be described below in detail with reference to the accompanying drawings. In this description, specific shapes, materials, numerical values, directions, and the like are examples for facilitating the understanding of the present invention, and can be appropriately changed according to the application, purpose, specification, and the like.

  FIG. 1 is a schematic system configuration diagram showing an example in which a fuel cell system 10 according to an embodiment of the present application is used as an in-vehicle power supply system of a fuel cell vehicle. The fuel cell system 10 includes a fuel cell 12 that generates electric power by receiving supply of fuel and oxidizing gas, an air supply system 30 for supplying oxygen in the air as oxidizing gas to the fuel cell 12, and hydrogen as fuel. A hydrogen supply system 50 for supplying the fuel cell 12, a power system 70 that electrically connects the fuel cell 12 and a motor 78 that is a load device, and an ECU (Electronic Control) that is a control device that performs overall control of the entire system. Unit) 90.

  The fuel cell 12 is a solid polymer electrolyte membrane cell stack in which a large number of fuel cells are stacked in an electrically connected state. In the fuel cell 12, an oxidation reaction represented by H2 → 2H ++ 2e− occurs at the fuel electrode (anode electrode), and a reduction represented by (1/2) O2 + 2H ++ 2e− → H2O at the air electrode (cathode electrode). A reaction occurs. The entire fuel cell 12 undergoes an electrochemical reaction represented by H2 + (1/2) O2 → H2O.

The fuel cell 12 is electrically connected to the power system 70 via the positive electrode bus 13 and the negative electrode bus 14. The positive electrode bus 13 is provided with a current sensor 18 for detecting a current (hereinafter referred to as “FC current”) I _FC output from the fuel cell 12. A voltage sensor 16 is provided between the positive electrode bus 13 and the negative electrode bus 14 to detect a voltage between terminals of the fuel cell 12 (hereinafter referred to as “FC voltage” as appropriate) V _FC . The detection signals of these sensors 16 and 18 are transmitted to the ECU 90 and used for control of the fuel cell system 10.

  The air supply system 30 includes an air supply passage 32 through which air supplied to the air electrode of the fuel cell 12 flows, and an air discharge passage 34 through which air discharged from the fuel cell 12 flows. In the air supply passage 32, an air compressor 38 that takes in air from the atmosphere via an air filter 36, a humidifier 40 that appropriately humidifies air compressed and pressurized by the air compressor 38, and the fuel cell 12. A shutoff valve 42 for shutting off the air supply is provided. On the other hand, the air discharge passage 34 is provided with a shutoff valve 44 for shutting off the discharge of air from the fuel cell 12 and a pressure regulating valve 46 for adjusting the air supply pressure. The humidifier 40 collects the generated water discharged together with the air from the fuel cell 12 in the air discharge passage 34 when the humidified water 40 passes through the humidifier 40 and supplies the air supplied through the air supply passage 32. It is configured to be used for humidification.

  The hydrogen supply system 50 is discharged from the fuel cell 12, for example, a hydrogen supply source 52 including a high-pressure hydrogen tank, a hydrogen supply passage 54 through which hydrogen gas supplied from the hydrogen supply source 52 to the fuel electrode of the fuel cell 12 flows. A hydrogen discharge passage 56 through which hydrogen off-gas flows, a circulation passage 58 branched from the hydrogen discharge passage 56 and connected to the hydrogen supply passage 54, and a hydrogen off-gas discharged from the fuel cell 12 from the hydrogen discharge passage 56 to the circulation passage 58. And a circulation pump 60 for circulating supply to the hydrogen supply passage 54 via the.

  A hydrogen supply passage 54 connected from the hydrogen supply source 52 to the fuel cell 12 includes, from the upstream side in the hydrogen gas supply direction, a shutoff valve 61 that shuts off the outflow of hydrogen gas from the hydrogen supply source 52, and a hydrogen supply source 52. A pressure regulating valve 62 that moderately depressurizes the ejected hydrogen gas, an injector 63 that controls the amount of hydrogen supplied to the fuel cell 12, a shutoff valve 64 for shutting off the supply of hydrogen gas to the fuel cell 12, and the fuel cell 12. A pressure sensor 65 for detecting the pressure of the supplied hydrogen gas is installed. On the other hand, the hydrogen discharge passage 56 is opened in order from the upstream side in the hydrogen offgas discharge direction, a shutoff valve 66 for shutting off the hydrogen offgas discharge from the fuel cell 12, and when the hydrogen offgas is discharged out of the system. A hydrogen off-gas discharge shut-off valve 67 is installed.

  In the hydrogen supply system 50 of the present embodiment, the hydrogen stored in the hydrogen supply source 52 will be described as being supplied to the fuel cell 12. However, the present invention is not limited to this, and a hydrocarbon-based fuel such as natural gas is used. Hydrogen rich gas generated by reforming with water vapor may be supplied to the fuel cell 12.

  As the shutoff valves 42, 44, 61, 64, 66, and 67 included in the air supply system 30 and the hydrogen supply system 50, electromagnetic valves that open or close in response to a command from the ECU 90 are preferably used. . The pressure regulating valves 46 and 62 are devices for regulating the upstream primary pressure to a preset secondary pressure. For example, a mechanical pressure reducing valve for reducing the primary pressure is preferably used. Further, the injector 63 is preferably configured by an electromagnetic on-off valve or the like having a valve body that can be opened and closed by an electromagnetic driving force, and the flow rate of hydrogen gas that passes through the opening degree or valve opening time of the valve body is controlled. The hydrogen gas pressure can be adjusted.

  The power system 70 includes a DC / DC converter (voltage conversion device) 72, a battery (power storage device) 74, an inverter 76 and an AC motor (load device) 78. The inverter 76 is electrically connected to the fuel cell 12 via the positive electrode bus 13 and the negative electrode bus 14, and has a function of converting DC power supplied from the fuel cell 12 into AC power and applying it to the AC motor 78. On the contrary, when the AC motor 78 functions as a generator during regenerative braking, it has a function of converting AC power output from the AC motor 78 into a DC voltage. The inverter 76 has a known configuration that can be constituted by, for example, a plurality of power switching elements such as IGBTs and diodes, and the power switching elements are turned on / off by the ECU 90, thereby causing direct current. The voltage can be converted to a three-phase alternating voltage or vice versa.

  As the AC motor 78, a three-phase synchronous AC motor can be suitably used. The AC motor 78 is driven by applying a three-phase AC voltage converted by the inverter 76. The driving force of the AC motor 78 is transmitted to the wheels (both not shown) via the axle, thereby obtaining the traveling force of the vehicle.

  Further, a battery 74 is connected to the positive bus 13 and the negative bus 14 via a DC / DC converter 72 in parallel with the fuel cell 12 with respect to an inverter 76 connected to an AC motor 78. The DC / DC converter 72 boosts the DC power supplied from the battery 74 and supplies it to the inverter 76, and reduces the regenerative power from the AC motor 78 and the generated power from the fuel cell 12 for charging the battery. This is a bidirectional converter having a step-down function, and has a known configuration that can be composed of, for example, a power switching element such as an IGBT, a diode, a reactor, or the like.

  Further, the DC / DC converter 72 can perform the boosting and step-down functions as described above by receiving the control signal from the ECU 90 and controlling the power switching element to be turned off / off. Further, the DC / DC converter 72 maintains the connection point between the positive electrode bus 13 and the negative electrode bus 14 at a predetermined potential during the power generation suspension mode of the fuel cell 12, so that the potential of the fuel cell 12 becomes OCV (open end voltage). It can be used for high potential avoidance control that suppresses rising to This high potential avoidance control will be described later.

  The battery 74 functions as a surplus power storage source, a regenerative energy storage source at the time of regenerative braking, and an energy buffer at the time of load fluctuation accompanying acceleration or deceleration of the fuel cell vehicle. As the battery 74, for example, a secondary battery such as a nickel metal hydride battery or a lithium secondary battery is preferably used. However, instead of the battery, a capacitor that can store electricity without an internal chemical reaction may be used as the power storage device. The battery 74 is provided with an SOC sensor (not shown) for detecting an SOC (State of charge). Specifically, the SOC sensor can be configured by a current sensor that detects a battery current, and the ECU 90 monitors the remaining capacity of the battery 74 by integrating the detected values of the current sensor. Accordingly, charge / discharge restriction can be applied to the battery 74. Further, a temperature sensor and a voltage sensor for detecting the temperature and voltage of the battery 74 may be provided, and detection signals from these sensors may be input to the ECU 90 and used for managing the state of the battery 74.

  The ECU 90 is a computer system including a CPU that executes various programs, a ROM that stores various programs in advance, a RAM that temporarily stores detection data, and an input / output interface that is an input / output unit for various signals. Each part of the battery system 10 is controlled. For example, when the ECU 90 receives an activation signal IG output from an ignition switch (not shown) by a user operation, the ECU 90 starts operation of the fuel cell system 10 and an accelerator opening signal ACC or a vehicle speed sensor output from the accelerator sensor. The required power of the entire system 10 is calculated based on the vehicle speed signal Svc output from the vehicle.

  Then, the ECU 90 determines the distribution of the output power of each of the fuel cell 12 and the battery 74 and controls the air supply system 30 and the hydrogen supply system 50 so that the generated power of the fuel cell 12 matches the target power. At the same time, the DC / DC converter 72 is controlled to adjust the output voltage of the fuel cell 12, thereby controlling the operation point (FC voltage, FC current) of the fuel cell 12. Further, the ECU 90 outputs, for example, each U-phase, V-phase, and W-phase AC voltage command value to the inverter 76 as a switching command so as to obtain a target torque corresponding to the accelerator opening, and the AC motor 78. The output torque and rotation speed of the motor are controlled.

Next, intermittent operation and high potential control of the fuel cell 12 in the fuel cell system 10 having the above-described configuration will be described with reference to FIGS. FIG. 2 is a flowchart showing a processing procedure of intermittent operation control of the fuel cell 12, and FIG. 3 shows a current I _FC flowing out from the fuel cell 12 under high potential avoidance control during the power generation suspension mode and a normal power generation mode from the power generation suspension mode. FIG. 4 is a graph showing the relationship between the time required to output a predetermined generated power when shifting to, and FIG. 4 shows the current I _FC flowing out of the fuel cell 12 during the power generation stop mode. It is a figure which shows a mode that the threshold value Pthr for determining transfer to normal electric power generation mode is made small.

  In the fuel cell system 10, the energy efficiency of the system 10 is improved by performing an intermittent operation in which the operation state of the fuel cell 12 is intermittently switched between the normal power generation mode and the power generation suspension mode according to the operation load. Yes. In the low load region where the power generation efficiency is low, the fuel cell system 10 is controlled to the operation state in the power generation halt mode by setting the power generation required power P * of the fuel cell 12 to zero. Necessary electric power is covered by electric power from the battery 74. On the other hand, in the high load region where the power generation efficiency is high, the power generation required power P * of the fuel cell 12 is calculated based on the accelerator opening ACC, the vehicle speed Svc, and the like to obtain the operation state in the normal power generation mode. The power necessary for system operation is covered only by the power generated by the fuel cell 12. However, when the fuel cell 12 is in the normal power generation mode, the vehicle required power may be satisfied by combining the power generated by the fuel cell 12 and the power from the battery 74.

  A procedure for intermittent operation control of the fuel cell 12 will be described with reference to the flowchart shown in FIG. This control flow is executed in the ECU 90 every predetermined time (for example, several ms) while the fuel cell system 10 is in operation.

  First, it is determined whether or not the power generation required power P * (kW) for the fuel cell 12 is smaller than a predetermined threshold value Pthr (kW) (step S10). The threshold value Pthr here is set in consideration of the power generation efficiency of the fuel cell 12 and the energy efficiency of the entire system.

  If it is determined in the above determination that the required power generation P * is smaller than the threshold value Pthr (YES in step S10), the operating state of the fuel cell 12 shifts from the normal power generation mode to the power generation halt mode, or the power generation halt mode Is continued (step S12). In the power generation halt mode, the shut-off valves 42, 44, 61, 64, 66 are closed, the air compressor 38 and the circulation pump 60 are deactivated, and the supply of air and hydrogen to the fuel cell 12 is stopped (see FIG. 1).

  On the other hand, if it is determined in the above determination that the power generation required power P * is equal to or greater than the threshold value Pthr (NO in step S10), the fuel cell 12 is supplied with air and hydrogen and is operated in the normal power generation mode (step S14). ).

When the fuel cell 12 is in the power generation halt mode, high potential avoidance control is also executed (step S12). In this high potential avoidance control, the operation of the DC / DC converter 72 is controlled by a control signal from the ECU 90 so that the inter-terminal voltage V _FC of the fuel cell 12 does not exceed the upper limit voltage set lower than the open-end voltage OCV. Maintained. This upper limit voltage is preferably a potential at which a catalyst contained in each fuel cell of the fuel cell 12, for example, a platinum catalyst, does not elute, and the voltage per fuel cell is about 90% of the maximum output voltage. It is preferable to set so as to be.

  Thus, by performing high potential avoidance control for the fuel cell 12 whose operation state is in the power generation halt mode, catalyst deterioration of the fuel cell 12 can be suppressed. However, such high potential avoidance control may be performed not only during the power generation suspension mode but also when the fuel cell 12 is operated in the normal power generation mode.

When the fuel cell 12 is in the power generation halt mode, the supply of air and hydrogen to the fuel cell 12 is completely stopped, but the oxygen and hydrogen remaining in the manifold in the fuel cell 12 and the flow path of the fuel cell Weak power generation continues due to the electrochemical reaction, and the inter-terminal voltage V _FC of the fuel cell 12 tends to increase. By performing the high potential avoidance control as described above, the inter-terminal voltage V _FC becomes higher than the open-circuit voltage. Is kept constant so as not to exceed the lower upper limit voltage. This is shown in FIG. In FIG. 4, the horizontal axis represents time, and the vertical axis represents FC voltage V _FC , FC current I _FC , threshold value Pthr for determining the shift to the normal power generation mode, power generation required power to the fuel cell 12 in order from the top, And the FC operation state is shown.

As shown in FIG. 4, when the operating state of the fuel cell 12 is in the power generation suspension mode, the FC voltage V _FC is kept constant at the upper limit voltage lower than the OCV by the high potential avoidance control. The FC current I _FC flows out from the fuel cell 12 because the weak power generation is continued in the battery 12. This FC current I _FC is detected by the current sensor 18.

In the fuel cell 12 in the power generation suspension mode under high potential avoidance control as described above, the hydrogen concentration in the fuel cell 12 gradually decreases as the weak power generation is continued and hydrogen is consumed as described above. Thus, the concentration of impurities such as nitrogen and water vapor that permeate from the cathode (air electrode) side of each fuel cell through the electrolyte membrane to the anode (fuel electrode) side increases. Accordingly, the current value of the FC current I _FC flowing out from the fuel cell 12 tends to gradually decrease. As described above, when the supply of hydrogen and air is resumed in the state where the impurity concentration on the anode side of the fuel cell 12 is increased and the gas quality is considerably lowered to shift from the power generation suspension mode to the normal power generation mode, the anode in the fuel cell 12 It takes time to return to a state where the gas quality on the side is improved and a predetermined generated power can be output, and this time becomes longer as the gas quality deteriorates and the responsiveness deteriorates. This is shown in the graph of FIG. As shown in the figure, the smaller the FC current I _FC during the power generation halt mode, the longer the time Tr required for the fuel cell 12 to be able to output the predetermined power generation, that is, the responsiveness of the fuel cell 12 is deteriorated. I know.

2 and 4 again, the ECU 90 executes a process of changing the threshold value Pthr serving as a reference for transition from the power generation suspension mode to the normal power generation mode in accordance with the detected FC current I _FC (step). S16). Specifically, a process of setting the threshold value Pthr to be smaller as the FC current I _FC decreases. The threshold Pthr in process is derived from the relationship of the FC current I _FC and threshold Pthr previously stored in the map or table format in the ROM.

The manner in which the threshold value Pthr is changed to be smaller as the FC current I _FC is decreased is shown in the second and third stages from the top in FIG. In this example, the FC current I _FC decreases linearly with time, and the threshold value Pthr also decreases linearly with the passage of time. Of course, how to change the threshold value Pthr It is not limited to. For example, it may be set such that the threshold Pthr also curvedly decreases as the FC current I _FC is curved manner reduced, or the threshold Pthr even when the FC current I _FC is curvedly decreases linearly It may be set to be smaller or vice versa.

Then, the generator required power P * for the fuel cell 12 is equal to or greater than or equal to the threshold value Pthr that is set smaller in accordance with the FC current I _FC as described above, is determined (step S18). Until the determination in step S16 is affirmative, steps S16 and S18 are repeatedly performed, and when the power generation request power P * becomes _{equal to} or greater than the FC current I _FC , the supply of air and hydrogen to the fuel cell 12 is resumed. The operating state of the fuel cell 12 shifts from the power generation halt mode to the normal power generation mode, and at the same time, the threshold value Pthr is returned to the initial setting value (step S20).

As described above, according to the fuel cell system 10 of the present embodiment, the threshold value Pthr is set to be smaller as the FC current I _FC decreases, thereby lowering the reference for transition from the power generation suspension mode to the normal power generation mode. It becomes possible to shift to the normal power generation mode before the gas quality on the anode side of the fuel cell deteriorates considerably, and as a result, the responsiveness of the fuel cell 12 that operates intermittently can be improved.

On the other hand, when the threshold value Pthr is kept constant at a relatively low value in order to ensure the responsiveness at the time of transition from the power generation suspension mode to the normal power generation mode in the fuel cell 12 that operates intermittently, the hydrogen in the fuel cell 12 A situation in which the fuel cell 12 transitions from the power generation suspension mode to the normal power generation mode can occur frequently in a state where the gas quality of the gas has not deteriorated so much, and in that case, waste of hydrogen gas becomes large and energy efficiency (or fuel efficiency) is poor. Become. In contrast, by the fuel cell system 10 of the present embodiment, the threshold value Pthr in accordance with the decrease in the FC current I _FC initial value of the threshold Pthr sure you set high to some extent small, the fuel cell 12 to the intermittent operation While improving the responsiveness, it is possible to improve the energy efficiency by suppressing the waste of hydrogen gas as described above.

Next, a modification of the above embodiment will be described with reference to FIG. FIG. 5 shows a state in which the threshold value Pthr for determining the transition to the normal power generation mode is also increased when the FC current I _FC starts to increase during the power generation halt mode. It is the same figure as the step.

When the fuel cell 12 is in the power generation halt mode, the ECU 90 temporarily opens the shut-off valves 61 and 64 based on the detection signal input from the pressure sensor 65 in order to suppress a decrease in the gas pressure on the anode side in the fuel cell 12. In some cases, hydrogen gas is replenished by opening the valve. In such a case, the hydrogen concentration in the fuel cell 12 is slightly increased and the gas quality is slightly improved. As a result, the power generation performance is improved and the FC current I _FC is temporarily increased. In this case, it is preferable to change to also increase the threshold Pthr in accordance with the temporary increase in the FC current I _FC. In this way, the threshold value Pthr can be changed appropriately in accordance with the gas quality on the anode side of the fuel cell 12, and the energy efficiency can be improved more effectively by suppressing the waste of hydrogen gas described above. . Here, air may be replenished to the cathode side of the fuel cell 12 during the power generation suspension mode, and at this time, the FC current I _FC may increase due to a slight increase in the oxygen concentration in the fuel cell 12. In such a case, it is preferable to deal with the above as well.

  In the above-described embodiment, an example in which the fuel cell system is applied as a power supply system for a vehicle has been described. However, the fuel cell system according to the present invention is not limited to a vehicle such as a ship, an airplane, or a robot. As may be applied.

  DESCRIPTION OF SYMBOLS 10 Fuel cell system, 12 Fuel cell, 13 Positive electrode bus, 14 Negative electrode bus, 16 Voltage sensor, 18 Current sensor, 30 Air supply system, 32 Air supply channel, 34 Air discharge channel, 36 Air filter, 38 Air compressor, 40 Humidification , 42, 44, 61, 64, 66, 67 shutoff valve, 46, 62 pressure regulating valve, 50 hydrogen supply system, 52 hydrogen supply source, 54 hydrogen supply passage, 56 hydrogen discharge passage, 58 circulation passage, 60 circulation pump, 63 injector, 65 pressure sensor, 70 power system, 72 DC / DC converter, 74 battery, 76 inverter, 78 AC motor, 90 ECU.

Claims (4)

A fuel cell that generates power upon receipt of fuel gas and oxidant gas; and a power storage device that is connected in parallel to the fuel cell via a voltage converter to a load device that is supplied with power generated by the fuel cell. A control device for controlling gas supply to the fuel cell and controlling the operation of the voltage converter,
The control device controls the operation of the fuel cell between the normal power generation mode and the power generation suspension mode by controlling the gas supply to the fuel cell based on a comparison between the power generation required power for the fuel cell and a threshold value. To prevent the fuel cell terminal voltage from exceeding the upper limit voltage, which is lower than the open-circuit voltage, during the power generation stop mode in which the supply of fuel gas and oxidizing gas to the fuel cell is completely or substantially stopped. A control structure for performing potential avoidance control, and executing control for reducing the threshold value as the current value flowing out of the fuel cell decreases during the power generation suspension mode in which the high potential avoidance control is performed. Fuel cell system. The fuel cell system according to claim 1, wherein
The control device, the when the current value is increased by that there was a supply of fuel gas or oxidizing gas into the power generating idle mode, the fuel cell system, characterized in combined size of the threshold Kusuru that it. A fuel cell that generates power by receiving supply of fuel gas and oxidant gas; and a power storage device that is connected in parallel to the fuel cell to a load device to which generated power is supplied from the fuel cell via a voltage converter, A control device for controlling the gas supply to the fuel cell and controlling the operation of the voltage converter, and a control method for a fuel cell system,
  Performing intermittent operation to switch the operation state of the fuel cell between the normal power generation mode and the power generation suspension mode by controlling the gas supply to the fuel cell based on the comparison between the power generation required power for the fuel cell and the threshold value,
  High potential avoidance control that prevents the terminal voltage of the fuel cell from exceeding the upper limit voltage lower than the open-circuit voltage during the power generation stop mode in which the supply of the fuel gas and the oxidizing gas to the fuel cell is completely or almost stopped. Done
  The threshold value is decreased as the current value flowing out of the fuel cell decreases during the power generation suspension mode in which the high potential avoidance control is performed.
A control method for a fuel cell system. In the control method of the fuel cell system according to claim 3,
  A control method for a fuel cell system, wherein when the current value increases due to replenishment of fuel gas or oxidizing gas during the power generation halt mode, the threshold value is increased accordingly.

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