WO2005099016A2 - Fuel cell system - Google Patents

Fuel cell system Download PDF

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Publication number
WO2005099016A2
WO2005099016A2 PCT/JP2005/006992 JP2005006992W WO2005099016A2 WO 2005099016 A2 WO2005099016 A2 WO 2005099016A2 JP 2005006992 W JP2005006992 W JP 2005006992W WO 2005099016 A2 WO2005099016 A2 WO 2005099016A2
Authority
WO
WIPO (PCT)
Prior art keywords
fuel cell
voltage
cell stack
cell system
internal load
Prior art date
Application number
PCT/JP2005/006992
Other languages
French (fr)
Other versions
WO2005099016A3 (en
Inventor
Takashi Fukuda
Original Assignee
Nissan Motor Co., Ltd.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nissan Motor Co., Ltd. filed Critical Nissan Motor Co., Ltd.
Publication of WO2005099016A2 publication Critical patent/WO2005099016A2/en
Publication of WO2005099016A3 publication Critical patent/WO2005099016A3/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
    • H01M8/04955—Shut-off or shut-down of fuel cells
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04223—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids during start-up or shut-down; Depolarisation or activation, e.g. purging; Means for short-circuiting defective fuel cells
    • H01M8/04228—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids during start-up or shut-down; Depolarisation or activation, e.g. purging; Means for short-circuiting defective fuel cells during shut-down
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04223—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids during start-up or shut-down; Depolarisation or activation, e.g. purging; Means for short-circuiting defective fuel cells
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/043—Processes for controlling fuel cells or fuel cell systems applied during specific periods
    • H01M8/04303—Processes for controlling fuel cells or fuel cell systems applied during specific periods applied during shut-down
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04313—Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
    • H01M8/04537—Electric variables
    • H01M8/04544—Voltage
    • H01M8/04559—Voltage of fuel cell stacks
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
    • H01M8/04746—Pressure; Flow
    • H01M8/04753—Pressure; Flow of fuel cell reactants
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04007—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids related to heat exchange
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
    • H01M8/04089—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants
    • H01M8/04119—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04313—Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
    • H01M8/0432—Temperature; Ambient temperature
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04313—Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
    • H01M8/04492—Humidity; Ambient humidity; Water content
    • H01M8/04507—Humidity; Ambient humidity; Water content of cathode reactants at the inlet or inside the fuel cell
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells

Definitions

  • the present invention relates to a fuel cell system having a fuel cell that receives a fuel gas containing hydrogen and an oxidant gas such as air and generates electric power.
  • the present invention relates to controlling a stop operation of the fuel cell system.
  • A- fuel cell is an energy converter that receives a fuel gas containing hydrogen and an oxidant gas such as air in a complex of electrolyte and electrode catalyst to cause an electrochemical reaction that converts chemical energy into electrical energy.
  • a solid polymer electrolyte fuel cell employing a solid polymer membrane electrolyte is inexpensive and compact, has a high output density, and therefore, is expected for application to a power source of a mobile object such as an automobile.
  • a stop operation is carried out to decrease the pressure of fluids (fuel gas and oxidant gas).
  • fluids fuel gas and oxidant gas
  • the fuel cell is in a high-potential, no-load state. If the fuel cell is left in this high-potential, no-load state, the fuel cell will lose the catalytic power thereof. To prevent this, the potential of the fuel cell must be decreased after the completion of the stop operation. Due to this, Japanese Laid-open Patent Application Publication No. Hei-6-333586 discloses a fuel cell stopping method that achieves a usual stop operation on a fuel cell and then conducts a voltage decreasing operation on the fuel cell.
  • the disclosure stops the supply of a fuel gas to the fuel cell. More precisely, the disclosure carries out a first step (voltage decreasing operation) that disconnects the fuel cell from an external load, continues the supply of a fuel gas, stops the supply of an oxidant gas, and applies an internal load to the fuel cell and a second step that stops the supply of the fuel gas when the voltage of the fuel cell drops to a predetermined value and turns off a switch of the internal load to disconnect the internal load from the fuel cell.
  • a first step voltage decreasing operation
  • Hei-6-333586 turns off the switch of the internal load when the voltage of the fuel cell decreases to a predetermined value, to disconnect the internal load from the fuel cell.
  • the internal load has a relatively low resistance value to rapidly decrease the voltage of: the fuel cell and shorten the voltage decreasing operation, there is a risk of again increasing the v ⁇ oltage of the fuel cell after the disconnection of the internal load.
  • the voltage of the fuel cell reincreases after the disconnection of the internal load is dependent on the conditions of the fuel cell and system, cell volta-ge variation, and the like at the time of stoppage.
  • Hei-6-333586 disconnects the internal load without paying attention to the conditions of the fuel cell and system, cell voltage variation, and tt ⁇ e like at the time of stoppage. Accordingly, it is difficult for the related art to surely prevent the voltage reincrease of the fuel cell after disconnecting the internal load. Namely, the related art is insufficient to prevent the deterioration of the catalytic power of the fuel cell.
  • the present invention has been devised to solve the above-mentioned problems of the related art, and an object of the present invention is to provide a fuel cell system capable of surely preventing an increase in the voltage of a fuel cell after the disconnection o>f an internal load and securing the catalytic power of the fuel cell.
  • An aspect of the present invention provides a fuel cell system that includes a fuel cell configured to receive a fuel gas and an oxidant gas, electrochemically react the fuel gas with oxidant gas to generate electric power, an internal load configured to be electrically connectable to the fuel cell and consume the electric power generated by the fuel cell, a switch configured to electrically connect and disconnect the internal load to and from the fuel cell, a voltage detector configured to detect a voltage of the fuel cell, a shutdown controller configured to, when the fuel cell system is stopped, connect the internal load to the fuel cell through the switch, ffcie shutdown controller configured to stop the supply of the oxidant gas to the fuel cell, continue the supply of the fuel gas to the fuel cell, and consume oxygen remaining in the fuel cell to decrease the voltage of the fuel cell, a threshold changer configured to change a threshold value according to a state of the fuel cell system at the time of stoppage of the fuel cell system, and a disconnection controller configured to electrically disconnect the internal load from the fuel cell through the switch when the voltage of the fuel cell detected by the voltage detector decrease
  • a fuel cell system that includes a fuel cell including a plurality of power generation cells stacked in multiple layers, configured to receive a fuel gas and an oxidant gas, electrochemically react the fuel gas and oxidant gas with each other, and generate power, an internal load configured to be electrically connected to the fuel cell and consume power generated by the fuel cell, a switch configured to electrically connect and disconnect the internal load to and from the fuel cell, a voltage detector configured to detect voltages of the power generation cells, a shutdown controller configured to, when the fuel cell system is stopped, connect the internal load to the fuel cell through the switch, stop the supply of the oxidant gas to the fuel cell, continue the supply of the fuel gas to the fuel cell, and consume oxygen remaj-oing in the fuel cell to decrease the voltage of the fuel cell, and a disconnection controller configured to electrically disconnect the internal load from the fuel cell through the switch when a voltage variation across the power generation cells decreases to or below a predetermined value.
  • Figure 1 is a view generally showing a fuel cell system according to a fLrst embodiment of the present invention.
  • Figure 2 shows a characteristic curve indicating a relationship between operating load and operating pressure of the fuel cell stack 1.
  • Figure 3 explains a method of detecting a decrease in the average cell voltage of the fuel cell stack 1.
  • Figure 4 is a view showing an internal load circuit connected to the fuel cell stack 1 of the fuel cell system according to the first embodiment of the present invention.
  • Figure 5 is a functional block diagram showing functional blocks in the control uxiit 100, to perform the shutdown voltage decreasing operation.
  • Figure 6 shows a relationship between the temperature of the fuel cell stack 1 and timing to disconnect the internal load 41.
  • Figure 7 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the first embodiment is stopped.
  • Figure 8 shows a relationship between a moving average of operating load on the fuel cell stack 1 in a given period before stopping the system and a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • Figure 9 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the second embodiment is stopped.
  • Figure 10 shows a relationship between a moving average of operating pressure of the fuel cell stack 1 in a given period before stopping the system and a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • Figure 11 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the third embodiment is stopped.
  • Figure 12 shows a relationship between a moving average of conductivity of the coolant in a given period before stopping the system and a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • Figure 13 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the fourth embodiment is stopped.
  • Figure 14 shows a relationship between a difference between a moving average and a minimum of coolant conductivity and a threshold value to determine t-cming to disconnect the internal load 41 from the fuel cell stack 1.
  • Figure 15 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the fifth embodiment is stopped.
  • Figure 16 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the sixth embodiment is stopped.
  • FIG. 1 is a view generally showing a fuel cell system according to a first embodiment of the present invention.
  • This fuel cell system is used as, for example, a driving power source of a fuel-cell vehicle.
  • the system includes a fuel cell stack 1 to receive hydrogen and air and generate electric power.
  • the fuel cell stack 1 includes power generation cells. .
  • Each power generation cell has a fuel electrode to which a fuel gas, i.e., a hydrogen gas is supplied and an oxidant electrode to which an oxidant gas, i.e., air is supplied.
  • the fuel electrode and oxidant electrode are overlaid one upon another with an electrolyte being interposed between them.
  • the power generation cells are arranged in a multilayer structure to form the fuel cell stack 1.
  • the fuel cell stack 1 Based on an elecfrochemical reaction between hydrogen of the fuel gas and oxygen contained in the oxidant gas, the fuel cell stack 1 converts chemical energy into electrical energy.
  • hydrogen supplied to the fuel electrode causes a reaction to separate the same into hydrogen ions and electrons.
  • the hydrogen ions pass through the electrolyte and the electrons pass through an external circuit to generate electric power.
  • the hydrogen ions and electrons move to the oxidant electrode.
  • oxygen contained in the supplied air and the hydrogen ions moved through the electrolyte and the electrons react with each other to form water, which is discharged outside.
  • the electrolyte in the fuel cell stack 1 is, for example, a solid polymer electrolyte membrane, which has a high energy density, is producible at low cost, and is lightweight.
  • the solid polymer electrolyte membrane is, for example, a fluoride-resin-based ion-exchange membrane that transmits ions (protons) and functions as an ion conductive electrolyte when water-saturated.
  • the voltage of each power generation cell in the fuel cell stack 1 is monitored with a cell voltage monitor (voltage detector) 2, which provides information to a control unit 100 that generally controls operation of the fuel cell system.
  • the fuel cell stack 1 has a temperature sensor 3 to detect the temperature of the fuel cell stack 1. An output from the temperature sensor 3 is sent to the control unit 100.
  • the control unit 100 monitors the power generating conditions and temperature of the fuel cell stack 1.
  • a fuel gas i.e., hydrogen and an oxidant gas, i.e., air is supplied to the fuel electrode and oxidant electrode of each power generation cell.
  • the fuel cell system includes a hydrogen supply system and an air supply system.
  • the hydrogen supply system includes, for example, a hydrogen tank 4, a pressure control valve 5, a hydrogen supply passage 6, and an ejector 7.
  • the hydrogen tank 4 is a hydrogen supply source to supply hydrogen;
  • the pressure control valve 5 reduces the pressure of the hydrogen supplied from the hydrogen tank 4.
  • the pressure-reduced hydrogen is passed through the hydrogen supply passage 6 and ejector 7 to the fuel electrodes in the fuel cell stack 1.
  • Pressure at the fuel electrodes in the fuel cell stack 1 is detected by a pressure sensor 8.
  • the control unit 100 feeds the detected value from the pressure sensor 8 back to the pressure control valve 5, to thereby control the operation of the pressure control valve 5 so that the pressure at the fuel electrodes in the fuel cell stack 1 is kept at a desired value.
  • the supplied hydrogen is not entirely consumed in the fuel cell stack 1.
  • the remaining hydrogen (discharged from the fuel electrodes of the fuel cell stack 1) is mixed through the ejector 7 with hydrogen newly supplied from the hydrogen tank 4 through the hydrogen supply passage 6.
  • the mixed hydrogen is supplied to the fuel electrodes in the fuel cell stack 1.
  • a fuel electrode outlet side of the fuel cell stack 1 is provided with a hydrogen circulation passage 9 that circulates hydrogen discharged from the fuel electrodes of the fuel cell stack 1 to the ejector 7.
  • the ejector 7 uses the flow energy of hydrogen passing through the hydrogen supply passage 6, to circulate the hydrogen passing through the hydrogen circulation passage 9.
  • the fuel electrode outlet side of the fuel cell stack 1 is also connected to a hydrogen discharge passage 10, which is branched from the hydrogen circulation passage 9 and is used to discharge hydrogen from the fuel electrodes of the fuel cell stack 1 to the outside of the system.
  • a purge valve 11 On a downstream side from the location where the hydrogen discharge passage 10 is branched from the hydrogen circulation passage 9, there is arranged a purge valve 11.
  • the purge valve 11 has a function of switching the direction of hydrogen discharged from the fuel electrodes of the fuel cell stack 1.
  • the purge valve 11 is opened to purge hydrogen discharged from the fuel electrodes of the fuel cell stack 1 to the outside of the system through the hydrogen discharge passage 10.
  • impurities such as nitrogen may accumulate in the system. If the accumulation of impurities becomes excessive, a partial pressure of hydrogen will decrease to deteriorate the efficiency of the fuel cell stack 1 and will increase an average mass of circulated gas to deteriorate a hydrogen circulation flow rate at the ejector 7. In this case, the purge valve 11 is opened to purge hydrogen together with the impurities to the outside of the system through the hydrogen discharge passage 10.
  • the air supply system includes a compressor 12 and an air supply passage 13, to draw external air, pressurize the drawn air, and supply the pressurized air to the oxidant electrodes in the fuel cell stack 1.
  • the compressor 12 feeds air into the air supply passage 13 and into the oxidant electrodes of the fuel cell stack 1.
  • the air supply passage 13 has a filter 14 to trap micro-dust sulfur components, and oil from the compressor 12, so that clean air is supplied to the oxidant electrodes of the fuel cell stack 1.
  • An oxidant electrode outlet side of the fuel cell stack 1 is connected to an air discharge passage 15 to discharge air from the fuel cell stack 1. Oxygen and other components contained in air not consumed in the fuel cell stack 1 are discharged to the outside of the system through the air discharge passage 15.
  • the air discharge passage 15 has a pressure control valve 16 whose operation is controlled by the control unit 100 to maintain pressure at the oxidant electrodes in the fuel cell stack 1 at a desired value.
  • the air supply system includes a humidification unit for humidifying air to be supplied to the oxidant electrodes of the fuel cell stack 1.
  • the humidification unit includes a humidifier 17 arranged in the middle of the air supply passage 13, a water condenser 18 arranged upstream from the pressure control valve 16 in the air discharge passage 15 to reclaim water produced in the fuel cell stack 1, a water tank 19 to store the reclaimed water, a water circulation passage 20 to circulate and supply huntidifying water, a valve 21 arranged in the water circulation passage 20, and a pump 22.
  • the pump 22 of the hui-nidification unit 17 is controlled by the control unit 100 to pressurize and supply water from the water tank 19 to the humidifier 17 through the water circulation passage 20.
  • the humidifier 17 is, for example, a membrane humidifier that employs the pressurized water from the pump 22 to humidify air supplied to the oxidant electrodes of the fuel cell stack 1.
  • a humidified state of the air to be supplied to the oxidant electrodes of the fuel cell stack 1 is detected by a temperature and humidity sensor 23 arranged on the oxidant electrode inlet side of the fuel cell stack 1. The detected value from the temperature and humidity sensor 23 is sent to the control unit 100.
  • the control unit 100 controls the pump 22 so that air supplied to the oxidant electrodes may have a desired humidity for the operation of the fuel cell stack 1.
  • the conductivity of the hun-ddifying water sent from the pump 22 to the humidifier 17 is detected by a conductivity sensor 24 and is supplied to the control unit 100. Excessive water in the humidifier 17 is returned to the water tank 19 through a hurnidi-cying water reclaim passage 25.
  • the passage 25 includes a pressure sensor 26 and a pressure adjust valve 27.
  • the control unit 100 controls the pressure adjust valve 27 according to a detected value from the pressure sensor 26, to ma tain pressure between the pump 22 and the pressure adjust valve 27 substantially at a constant value.
  • the water tank 19 has a water level sensor 28.
  • the control unit 100 determines a shortage or an excess of water in the water tank 19 and informs an operator of the detected state.
  • the fuel cell system has a cooling mechanism to cool the fuel cell stack 1. With a sohd polymer electrolyte, for example, the fuel cell stack 1 has a relatively low operating temperature of about 80°C and may need to be cooled if overheated.
  • the cooling mechanism includes a coolant circulation passage 29 and a coolant pump 30 to circulate a coolant
  • the coolant is a mixture of, for example, water and ethylene glycol serving as an antifreeze and is circulated to cool and maintain the fuel cell stack 1 at an optimum temperature.
  • the coolant circulation passage 29 of the cooling mechanism includes a radiator 31.
  • the radiator 31 is provided with a radiator fan (not shown) controlled by the control unit 100, to adjust the temperature of the coolant and realize a desired radiator outlet temperature.
  • a bypass passage 32 In parallel with the radiator 31 , there is a bypass passage 32.
  • a branch of the bypass passage 32 is provided with a thermostat three-way valve 33, which is operated according to the temperature of the coolant to change the direction of a coolant flow.
  • a coolant temperature at which the three-way valve 33 is switched is, for example, 50°C.
  • the coolant circulation passage 29 to pass the temperature-adjusted coolant has a reservoir tank 34.
  • the reservoir tank 34 absorbs thermal expansion and contraction of the coolant and replenishes the coolant
  • the top of the reservoir tank 34 is open to the atmosphere to provide a reservoir function.
  • a coolant inlet of the fuel cell stack 1 has a temperature sensor 37, and a coolant outlet thereof has a temperature sensor 38.
  • the temperature sensor 37 detects the temperature of coolant before entering the fuel cell stack 1, and the temperature sensor 38 detects the temperature of coolant exiting the fuel cell stack 1.
  • the detected values from the temperature sensors 37 and 38 are sent to the control unit 100, which uses them with a detected value from the temperature sensor 3 on the fuel cell stack 1 to control the cooling of the fuel cell stack 1.
  • the control unit 100 is, for example, a microcomputer involving a CPU, ROM, RAM, and peripheral interfaces.
  • the control unit 100 reads detected values from an outside air temperature sensor (not shown), the cell voltage monitor 2, the temperature sensor 3, and the like, carries out dete ⁇ ninations and operations accordingly, and provides various control signals to control various parts of the fuel cell system.
  • the control unit 100 When the fuel cell system is stopped, the control unit 100 according to this embodiment conducts a voltage decreasing operation to decrease the voltage of the fuel cell stack 1. This voltage decreasing operation carried out when the fuel cell system is stopped is hereinafter referred to as the "shutdown voltage decreasing operation.” The details of the shutdown voltage decreasing operation will be explained later.
  • a normal operation of the fuel cell system according to the embodiment with the above-mentioned configuration will be explained in connection with an example that employs the fuel cell system as a driving power source of a fuel cell vehicle.
  • the driver of the vehicle operates an accelerator.
  • output power electric power
  • the pressure control valve 5 adjusts the pressure of hydrogen, and the pressure-adjusted hydrogen is supplied to the fuel electrodes of the fuel cell stack 1.
  • the compressor 12 supplies air
  • the humidifier 17 humidifies the air and supplies the humidified air to the oxidant electrodes of the fuel cell stack 1.
  • Figure 2 shows a characteristic curve indicating a relationship between operating load and operating pressure of the fuel cell stack 1.
  • an abscissa represents operating load and an ordinate operating pressure.
  • the first embodiment sets an operating pressure according to an operating load as shown in Fig. 2. Namely, a low operating pressure is set under a low operating load and a high operating pressure is set under a high operating load.
  • the temperature and humidity sensor 23 monitors the temperature and humidity of the oxidant electrode inlet of the fuel cell stack 1. If the humidity of air supplied to the oxidant electrodes of the fuel cell stack 1 is insufficient the operating pressure is increased to increase the pressure of the humidifier 17, to thereby resolve the shortage of humidity.
  • the coolant pump 30 adjusts the flow rate of the coolant according to the quantity of heat generated by the fuel cell stack 1 so that the coolant is passed through the fuel cell stack 1 at a proper flow rate. Namely, the flow rate is corrected according to the temperature of the fuel cell stack 1 detected by the temperature sensor 3 installed on the fuel cell stack 1, or according to the temperature of the fuel cell stack 1 estimated from coolant temperatures detected by the temperature sensors 37 and 38 of the cooling mechanism. If the temperature of the fuel cell stack 1 is very low, the thermostat three-way valve 33 is changed so that the coolant may pass through the bypass passage 32 without passing through the radiator 31.
  • the outlet temperature of the radiator 31 is controlled substantially at a fixed value by controlling the rotation speed of the radiator fan (not shown) according to the temperature of the fuel cell stack 1 detected by the temperature sensor 3 or by the temperature sensors 37 and 38.
  • the water condenser 18 in the air supply system condenses and reclaims water from air discharged from the fuel cell stack 1.
  • the reclaimed water is guided through the water circulation passage 20 into the water tank 19 and is stored therein as humidifying water.
  • hydrogen discharged from the fuel electrodes of the fuel cell stack 1 is circulated through the hydrogen circulation passage 9 and ejector 7.
  • a concentration of impurities such as nitrogen in the passage 9 may increase due to the transmission of hydrogen through the solid polymer electrolyte membranes of the fuel cell stack 1, or the passage 9 may be clogged with water to decrease a cell voltage.
  • the cell voltage monitor 2 monitors a cell voltage. If the voltage of a given cell decreases by a predeterudined value (for example, 0.2 N) from an average cell voltage, or if an average cell voltage decreases by a predetermined level (for example, 0.1
  • FIG. 3 shows voltage characteristic curves of the fuel cell stack 1 relative to operating load.
  • the voltage characteristic curves (I-N curves) of the fuel cell stack 1 relative to operating load are stored as table data in the control unit 100.
  • the table data is used to find a reference average cell voltage according to an operating load and a temperature of the fuel cell stack 1.
  • the reference average cell voltage is compared with an average cell voltage actually detected during operation, to provide an average cell voltage decrease.
  • a cell voltage decrease for a relatively long period may be learned and corrected to grasp an average cell voltage decrease due to gradual deterioration of the fuel cell stack 1.
  • the fuel cell system provides output in response to a driver's accelerator operation, and a vehicle driving motor (not shown) drives the vehicle. If the driver turns off a key, a system shutdown signal is supplied to the control unit 100. Then, the control unit 100 conducts the shutdown voltage decreasing operation to stop the system.
  • the shutdown voltage decreasing operation achieved when the fuel cell system according to the first embodiment is stopped will be explained.
  • FIG. 4 is a view showing an internal load circuit connected to the fuel cell stack 1 of the fuel cell system according to the first embodiment of the present invention.
  • the fuel cell system of the embodiment connects the internal load circuit of Fig. 4 to the fuel cell stack 1.
  • the internal load circuit includes an internal load 41 made of a solid resistor and a switch 42 to connect and disconnect the internal load 41 to and from the fuel cell stack 1. Once the switch 42 is turned on to connect the internal load 41 to the fuel cell stack 1, electric power generated in the fuel cell stack 1 is consumed by the internal load 41.
  • Figure 5 is a functional block diagram showing functional blocks in the control unit
  • the functional blocks prepared in the control unit 100 to achieve the shutdown voltage decreasing operation include a shutdown controller 101 configured to, when the fuel cell system is stopped, connect the internal load 41 with the fuel cell through the switch 42, the shutdown controller 101 configured to stop the supply of the oxidant gas to the fuel cell, continue the supply of the fuel gas to the fuel cell, and consume oxygen ren aining in the fuel cell to decrease the voltage of the fuel cell, a threshold changer 102 configured to change a threshold value according to a state of the fuel cell system at the time of stoppage of the fuel cell system; and a disconnection controller 103 configured to electrically disconnect the internal load 41 from the fuel cell through the switch 42 when the voltage of the fuel cell detected by the voltage detector decreases to or below the threshold value.
  • the shutdown controller 101 carries out shutdown processes in response to a system shutdown signal generated in response to, for example, a key-off operation of the driver.
  • the shutdown processes include stopping the operation of the compressor 12 in the air supply system, stopping the supply of air to the fuel cell stack 1, opening the purge valve 11 in the hydrogen supply system, continuing the supply of hydrogen to the fuel cell stack 1, and turning on the switch 42 of the internal load circuit to connect the internal load 41 to the fuel cell stack 1.
  • the threshold setter 102 sets a threshold value used to determine Inning to disconnect the internal load 41 from the fuel cell stack 1. Namely, the internal load 41 is disconnected from the fuel cell stack 1 when the voltage of the fuel cell stack 1 (the total of cell voltages of the fuel cell stack 1 in this embodiment) decreases to a threshold value.
  • the threshold setter 102 sets such a threshold value used to determine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • the threshold setter 102 may set the threshold value according to a state of the fuel cell system when the system is stopped (for example, the temperature of the fuel cell stack 1 at the time of system shutdown). If the fuel cell stack 1 is left under a high-potential, no-load state during the stoppage of the fuel cell system, the catalytic power of the system will deteriorate. To prevent this, the shutdown voltage decreasing operation is carried out when the system is stopped, to decrease the voltage of the fuel cell stack 1. If the voltage of the fuel cell stack 1 is rapidly decreased, however, the voltage of the fuel cell stack will reincrease after the internal load 41 is disconnected from the fuel cell stack 1 to terminate the shutdown voltage decreasing operation.
  • the voltage reincreasing phenomenon of the fuel cell stack 1 occurs depending on the conditions such as temperature of the fuel cell stack 1 when the system is stopped. Generally, the lower the temperature of the fuel cell stack 1 at the time of stoppage of the system, the more frequently the voltage reincreasing phenomenon of the fuel cell stack 1 occurs after the completion of the shutdown voltage decreasing operation. Namely, the lower the temperature of the fuel cell stack 1 at system shutdown, the lower the threshold value for determining timing to disconnect the internal load 41 from the fuel cell stack 1 must be set so that the voltage of the fuel cell stack 1 is sufficiently decreased before disconnecting the internal load 41 from the fuel cell stack 1.
  • Figure 6 shows a relationship between the temperature of the fuel cell stack 1 and timing to disconnect the internal load 41.
  • An abscissa indicates the temperature of the fuel cell stack 1 when the system is stopped, and an ordinate indicates a threshold voltage to determine the disconnection timing.
  • the threshold setter 102 may employ a table containing such a relationship between the temperature of the fuel cell stack 1 and timing to disconnect the internal load 41. According to an actual temperature of the fuel cell stack 1 detected when the system, is stopped, the threshold setter 102 refers to the table and sets a threshold value to deterrnine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • the internal load disconnection controller 103 turns off the switch 42 of the internal load circuit when the voltage of the fuel cell stack 1 (the total of cell voltages of the fuel cell stack 1) decreases to or below the threshold value set by the threshold setter 102, to disconnect the internal load 41 from the fuel cell stack 1.
  • the threshold setter 102 sets a threshold value according to the temperature of the fuel cell stack 1 when the system is stopped.
  • the internal load disconnection controller 103 disconnects the internal load 41 from the fuel cell stack 1, to surely prevent the voltage reincreasing phenomenon of the fuel cell stack 1 after the completion of the shutdown voltage decreasing operation and secure the catalytic power of the fuel cell stack 1.
  • FIG. 7 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the first embodiment is stopped.
  • the shutdown controller 101 of the control unit 100 determines whether or not a system shutdown signal has been generated in response to, for example, a driver's key-off operation. If the system shutdown signal is present step S 102 starts a shutdown process. More precisely, the shutdown controller 101 stops the compressor 12 in the air supply system to stop the supply of air to the fuel cell stack 1, opens the purge valve 11 in the hydrogen supply system, continues the supply of hydrogen to the fuel cell stack 1, and turns on the switch 42 of the internal load circuit to connect the internal load 41 to the fuel cell stack 1.
  • the threshold setter 102 of the control unit 100 reads the temperature of the fuel stack 1 at the system stoppage. According to the read temperature of the fuel cell stack 1, the threshold setter 102 sets, in step S104, a threshold value to deteimine timing to disconnect the internal load 41 fro the fuel cell stack 1.
  • the temperature of the fuel cell stack 1 at the system stoppage may be the temperature of the fuel cell stack 1 itself detected by the temperature sensor 3. Alternatively, it may be a coolant temperature of the cooling mechanism that corresponds to the temperature of the fuel cell stack 1.
  • a coolant temperature at the inlet side of the fuel cell stack 1 detected by the temperature sensor 37, or a coolant temperature at the outlet side of the fuel cell stack 1 detected by the temperature sensor 38 is employable.
  • the internal load disconnection controller 103 of the control unit 100 obtains the voltage of the fuel cell stack 1, e.g., the total voltage of the power generation cells in the fuel cell stack 1 detected by the cell voltage monitor 2.
  • the internal load disconnection controller 103 determines whether or not the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S 104.
  • the voltage of the fuel cell stack 1 may be an average cell voltage instead of the total voltage of the power generation cells.
  • step S104 If the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S104, the internal load disconnection controller 103 turns off, in step S107, the switch 42 of the internal load circuit to disconnect the internal load 41 from the fuel cell stack 1.
  • step S108 the supply of -hydrogen to the fuel cell stack 1 is stopped and the humidification unit and cooling mechanism are stopped to complete the shutdown voltage decreasing operation.
  • the fuel cell system according to the first embodiment conducts the shutdown voltage decreasing operation when the system is stopped, through the processes of setting, according to the temperature of the fuel cell stack 1 at the system stoppage, a threshold value to deteimine the timing of c-isconnecting the internal load 41 from the fuel cell stack 1, and when the voltage of the fuel cell stack 1 decreases to or below the threshold value, disconnecting the internal load 41 from the fuel cell stack 1.
  • a threshold value to deteimine the timing of c-isconnecting the internal load 41 from the fuel cell stack 1
  • the voltage of the fuel cell stack 1 decreases to or below the threshold value, disconnecting the internal load 41 from the fuel cell stack 1.
  • the fuel cell system of this embodiment employs the same basic structure as that of the first embodiment mentioned above.
  • the second embodiment disconnects the internal load 41 from the fuel cell stack 1 at timing determined from a threshold value that is variable according to a state of the fuel cell system when the system is stopped.
  • the second embodiment employs, to determine the threshold value, an average value of operating load of the fuel cell stack 1 in a given period before stopping the system. Unlike the first embodiment that disconnects the internal load 41 from the fuel cell stack 1 when the voltage of the fuel cell stack 1 decreases to or below the threshold value, the second embodiment disconnects the internal load 41 from the fuel cell stack 1 if the voltage of the fuel cell stack 1 decreases to or below the threshold value and if a voltage variation across the power generation cells of the fuel cell stack 1 is equal to or below a predeteimined value.
  • the phenomenon that the voltage of the fuel cell stack 1 reincreases after the shutdown voltage decreasing operation of disconnecting the internal load 41 from the fuel cell stack 1 is dependent not only on the temperature of the fuel cell stack 1 when the system is stopped but also on operating load on the fuel cell stack 1 in a given period before stopping the system.
  • the lower the operating load on the fuel cell stack 1 in a given period before stopping the system the more frequently the reincreasing phenomenon of the voltage of the fuel cell stack 1 occurs after the completion of the shutdown voltage decreasing operation.
  • Figure 8 shows a relationship between a moving average of operating load on the fuel cell stack 1 in a given period before stopping the system and a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • the threshold setter 102 of the control unit 100 in the fuel cell system may employ a table containing such a relationship between a moving average of operating load on the fuel cell stack 1 in a given period before stopping the system and a threshold value to determine liming to disconnect the internal load 41 from the fuel cell stack 1.
  • the threshold setter 102 refers to the table and sets a threshold value to deteimine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • the table representative of the graph of Fig. 8 may be prepared according to tests on actual fuel cell systems.
  • the shutdown voltage decreasing operation stops the supply of air to the fuel cell stack 1 and continues the supply of hydrogen, to the same, to consume oxygen remaining around the oxidant electrodes.
  • cell voltages of the power generation cells of the fuel cell stack 1 may vary from one to another. If the cell voltage variation is large, the voltage of the fuel cell stack 1 will increase after the coiripletion of the shutdown voltage decreasing operation.
  • the internal load disconnection controller 103 of the control unit 100 of this embodiment is configured to turn off the switch 42 of the internal load circuit to disconnect the internal load 41 from the fuel cell stack 1 if the voltage of the fuel cell stack 1 decreases to or below a threshold value set by the threshold setter 102 and if a voltage variation across the power generation cells of the fuel cell stack 1 becomes equal to or below a predetermined value.
  • Figure 9 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the second embodiment is stopped. In the fuel cell system according to the second embodiment several processes are carried out before starting the shutdown voltage decreasing operation. Namely, step S201 reads load (operating load) on the operating fuel cell stack 1 from time to time.
  • Step S202 computes a moving average of the read operating load and stores the computation result in a memory of the control unit 100.
  • a period for which a moving average of operating load is computed is, for example, five minutes from the present time to five minutes before. Alternatively, an optimum period may be set according to the characteristics of the fuel cell stack 1 and system.
  • the second embodiment computes a moving average of operating load in a given period (for example, a period of five minutes) before the stoppage of the system. Instead of the moving average, a maximum value of operating load in a given period may be employable. In this case, step S202 updates a maximum value of operating load and stores the updated maximum value in the memory of the control unit 100.
  • step S203 the shutdown controller 101 of the control unit 100 dete ⁇ riines whether or not a system shutdown signal has been generated in response to, for example, a driver's key-off operation. If the system shutdown signal is present step S204 starts a shutdown process. More precisely, the shutdown controller 101 stops the compressor 12 in the air supply system to stop the supply of air to the fuel cell stack 1, opens the purge valve 11 in the hydrogen supply system, continues the supply of hydrogen to the fuel cell stack 1, and turns on the switch
  • the threshold setter 102 of the control unit L00 reads the moving average of operating load stored in step S202 in the memory of the control unit 100. According to the read moving average of operating load, the threshold setter 102 sets, in step S206, a threshold value to deteimine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • step S207 the internal load disconnection controller 103 of the control unit 100 obtains the voltage of the fuel cell stack 1, e.g., an average voltage of the power generation cells (average cell voltage) in the fuel cell stack 1 detected by the cell voltage monitor 2.
  • step S208 the internal load disconnection controller 103 determines whether or not the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S206. If the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S206, step S209 computes a voltage variation across the power generation cells of the fuel cell stack 1. In this example, a difference obtained by subtracting a minimum voltage from a maximum voltage of the power generation cells detected by the cell voltage monitor 2 is used as a value representative of the cell voltage variation.
  • Step S210 deteimines whether or not the cell voltage variation found in step S209 is equal to or below a predetermined value. If the cell voltage variation is equal to or below the predete-mined value, step S211 turns off the switch 42 of the internal load circuit to disconnect the internal load 41 from the fuel cell stack 1. In step S212, the supply of hydrogen to the fuel cell stack 1 is stopped and the humidification unit and cooling mechanism are stopped to complete the shutdown voltage decreasing operation.
  • the fuel cell system conducts the shutdown voltage decreasing operation when the system is stopped, through the processes of setting, according to operating load on the fuel cell stack 1 in a given period before stopping the system, a threshold value to deteimine the timing of disconnecting the internal load 41 from the fuel cell stack 1, and when the voltage of the fuel cell stack 1 decreases to or below the threshold value and when a cell voltage variation in the fuel cell stack 1 becomes equal to or below a predetermined level, disconnecting the internal load 41 from the fuel cell stack 1.
  • a fuel cell system according to a third embodiment of the present invention will be explained.
  • the fuel cell system of this embodiment employs the same basic structure as that of the first embodiment mentioned above.
  • the third embodiment disconnects the internal load 41 from the fuel cell stack 1 at timing determined 1 from a threshold value that is variable according to a state of the fuel cell system when the system is stopped.
  • the third embodiment employs, as a state of tb-e fuel cell system used as a reference to set the threshold value, an average operating pressure of the fuel cell stack 1 in a given period before stopping the system.
  • the third embodiment disconnects the internal load 41 from the fuel cell stack 1 when the voltage of the fuel cell stack 1 decreases to or below the threshold value and when a voltage variation across the power generation cells of the fuel cell stack 1 is equal to or below a given value.
  • the phenomenon that the voltage of the fuel cell stack 1 reincreases after the shutdown voltage decreasing operation of disconnecting the internal load 41 from the fuel cell stack 1 is also dependent on the operating pressure of the fuel cell stack 1 in a given period before stopping the system. Generally, the lower the operating pressure of the fuel cell stack 1 in a given period before stopping the system, the more frequently the reincreasing phenomenon of the voltage of the fuel cell stack 1 occurs after the completion of the shutdown voltage decreasing operation.
  • Figure 10 shows a relationship between a moving average of operating pressure of the fuel cell stack 1 in a given period before stopping the system and a threshold value to deteimine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • the threshold setter 102 o-f the control unit 100 in the fuel cell system may employ a table containing such a relationship between a moving average of operating pressure of the fuel cell stack 1 in a given period before stopping the system and a threshold value to deteimine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • the threshold setter 102 refers to the table and sets a threshold value to deteimine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • the table representative of the graph of Fig. 10 may be prepared according to tests on actual fuel cell systems.
  • FIG 11 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the third embodiment is stopped.
  • step S301 reads a pressure (operating pressure) of the operating fuel cell stack 1, for example, an inlet pressure of the fuel cell stack 1 from time to time.
  • step S302 computes a moving average of the read operating pressure and stores the computation result in a memory of the control unit 100.
  • a period for which the moving average of operating pressure is computed is, for example, five minutes from the present time to five minutes before. Alternatively, an optimum period may be set according to the characteristics of the fuel cell stack 1 and system.
  • he operating pressure of the fuel cell stack 1 is an inlet pressure of the fuel cell stack 1. Any othe-r pressure is also acceptable.
  • the third embodiment computes a moving average of operating pressure in a given period (for example, a period of five minutes) before the stoppage of the system. Instead of the moving average, a maximum value of operating pressure in a given period may be employable.
  • step S302 updates a maximum value of operating pressure and stores the updated maximum value in the memory of the control unit 100.
  • the shutdown controller 101 of the control unit 100 deteimines whether or not a system shutdown signal has been generated in response to, for example, a driver's key-off operation. If the system shutdown signal is present step S304 starts a shutdown process. More precisely, the shutdown controller 101 stops the compressor 12 in the air supply system to stop the supply of air to the fuel cell stack 1, opens the purge valve 11 in the hydrogen supply system, continues the supply of hydrogen to the fuel cell stack 1, and turns on the switch
  • the threshold setter 102 of the control unit 100 reads the moving average of operating pressure stored in step S302 in the memory of the control unit 100. According to the read moving average of operating pressure, the threshold setter 102 sets, in step S306, a threshold value to deteimine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • step S307 the internal load disconnection controller 103 of the control unit 100 obtains the voltage of the fuel cell stack 1, e.g., an average voltage of the power generation cells (average cell voltage) in the fuel cell stack 1 detected by the cell voltage monitor 2.
  • step S308 the internal load disconnection controller 103 deteimines whether or not the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S306. If the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S306, step S309 computes a voltage variation across the power generation cells of the fuel cell stack 1.
  • Step S310 determines whether or not the cell voltage variation found in step S309 is equal to or below a predetermined level. If the cell voltage variation is equal to or below the level, step S311 turns off the switch 42 of the internal load circuit to disconnect the internal load 41 from the fuel cell stack 1.
  • step S312 the supply of hydrogen to the fuel cell stack 1 is stopped and the humidification unit and cooling mechanism are stopped to complete the shutdown voltage decreasing operation.
  • the fuel cell system according to the third embodiment conducts the shutdown voltage decreasing operation when the system is stopped, through the processes of setting, according to an operating pressure of the fuel cell stack 1 in a given period before stopping the system, a threshold value to determine the timing of disconnecting the internal load 41 from the fuel cell stack 1, and when the voltage of the fuel cell stack 1 decreases to or below the threshold value and when a cell voltage variation in the fuel cell stack 1 becomes equal to or below a predetermined level, disconnecting the internal load 41 from the fuel cell stack 1.
  • a fuel cell system according to a fourth embodiment of the present invention will be explained.
  • the fuel cell system of this embodiment employs the same basic structure as that of the first embodiment mentioned above. like the first embodiment the fourth embodiment disconnects the internal load 41 from the fuel cell stack 1 at timing determined from a threshold value that is variable according to a state of the fuel cell system when the system is stopped.
  • the fourth embodiment employs, as a state of the fuel cell system used as a reference to set the threshold value, an average conductivity of the coolant (supphed to the fuel cell stack 1 to adjust the temperature of the fuel cell stack 1) in a given period before stopping the system.
  • the fourth embodiment disconnects the internal load 41 from the fuel cell stack 1 when the voltage of the fuel cell stack 1 decreases to or below the threshold value and when a voltage variation across the power generation cells of the fuel cell stack 1 becomes equal to or below a given value.
  • the phenomenon that the voltage of the fuel cell stack 1 reincreases after the shutdown voltage decreasing operation of disconnecting the internal load 41 from the fuel cell stack 1 is also dependent on the conductivity of the coolant in a given period before stopping the system.
  • the lower the conductivity of the coolant in a given period before stopping the system the more frequently the reincreasing phenomenon of the voltage of the fuel cell stack 1 occurs after the completion of the shutdown voltage decreasing operation. Namely, the lower the conductivity of the coolant in a given period before stopping the system, the lower the threshold value for deteimining timing to disconnect the internal load 41 from the fuel cell stack
  • Figure 12 shows a relationship between a moving average of conductivity of the coolant in a given period before stopping the system and a threshold value to deteimine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • the threshold setter 102 of the control unit 100 in the fuel cell system according to the fourth embodiment may employ a table containing such a relationship between a moving average of conductivity of the coolant in a given period before stopping the system and a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • the threshold setter 102 refers to the table and sets a threshold value to deteimine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • the table representative of the graph of Fig. 12 may be prepared according to tests on actual fuel cell systems.
  • Figure 13 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the fourth embodiment is stopped. In the fuel cell system according to the fourth embodiment several processes are carried out before starting the shutdown voltage decreasing operation. Namely, step S401 reads a conductivity of the coolant in the operating fuel cell stack 1 from time to time.
  • Step S402 computes a moving average of the read coolant conductivity and stores the computation result in a memory of the control unit 100.
  • a period for which the moving average of coolant conductivity is computed is, for example, five minutes from the present time to five minutes before. Alternatively, an optimum period may be set according to the characteristics of the fuel cell stack 1 and system.
  • This embodiment detects a conductivity of the coolant in front of the fuel cell stack 1. If the fuel cell system is of an internal humidifying type that directly supplies humidifying water to the fuel cell stack 1, the conductivity of the huirddifying water may be detected instead of the coolant conductivity.
  • the forth embodiment computes a moving average of coolant conductivity in a given period (for example, a period of five minutes) before the stoppage of the system.
  • step S402 updates a maximum value of coolant conductivity and stores the updated maximum value in the memory of the control unit 100.
  • the shutdown controller 101 of the control unit 100 deteimines whether or not a system shutdown signal has been generated in response to, for example, a driver's key-off operation. If the system shutdown signal is present step S404 starts a shutdown process.
  • the shutdown controller 101 stops the compressor 12 in the air supply system to stop the supply of air to the fuel cell stack 1, opens the purge valve 11 in the hydrogen supply system, continues the supply of hydrogen to the fuel cell stack 1, and turns on the switch 42 of the internal load circuit to connect the internal load 41 to the fuel cell stack 1.
  • the threshold setter 102 of the control unit 100 reads the moving average of coolant conductivity stored in step S402 in the memory of the control unit 100.
  • the threshold setter 102 sets, in step S406, a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • the internal load disconnection controller 103 of the control unit 100 obtains the voltage of the fuel cell stack 1, e.g., an average voltage of the power generation cells (average cell voltage) in the fuel cell stack 1 detected by the cell voltage monitor 2.
  • the internal load disconnection controller 103 determines whether or not the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S406. If the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S406, step S409 computes a voltage variation across the power generation cells of the fuel cell stack 1.
  • Step S410 deteimines whether or not the cell voltage variation found in step S409 is equal to or below a predetermined value. If the cell voltage variation is equal to or below the predetermined value, step S411 turns off the switch 42 of the internal load circuit to disconnect the internal load 41 from the fuel cell stack 1.
  • the fuel cell system according to the fourth embodiment conducts the shutdown voltage decreasing operation when the system is stopped, through the processes of setting, according to the conductivity of a coolant in a given period before stopping the system, a threshold value to determine the timing of disconnecting the internal load 41 from the fuel cell stack 1, and when the voltage of the fuel cell stack 1 decreases to or below the threshold value and when a cell voltage variation in the fuel cell stack 1 becomes equal to or below a predetermined level, disconnecting the internal load 41 from the fuel cell stack 1.
  • a fuel cell system according to a fifth embodiment of the present invention will be explained. This embodiment is a modification of the fourth embodiment mentioned above.
  • the fourth embodiment refers to an average of coolant conductivity in a given period before stopping the system, and based on the average, sets a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • the fifth embodiment refers to a difference between an average of coolant conductivity in a given period before stopping the system and a rninimum of coolant conductivity from the start of the system, and based on the difference, sets a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • the conductivity of a coolant greatly varies depending on the configuration of a fuel cell system and the operating condition of a fuel cell stack, or depending on aging. It is possible, therefore, that an average of coolant conductivity in a given period before stopping the system is insufficient to set a proper threshold value to deteimine t-tming to disconnect the internal load 41 from the fuel cell stack 1. To cope with this problem, the fuel cell system of the fifth embodiment considers conductivity deterioration.
  • the fifth embodiment finds a difference between an average of coolant conductivity in a given period before stopping the system and a minimum coolant conductivity measured from the start of the system, and according to the difference, selects a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • Figure 14 shows a relationship between a difference between a moving average and a ininimum of coolant conductivity and a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • the threshold setter 102 of the control unit 100 in the fuel cell system according to the fifth embodiment may employ a table containing such a relationship between a difference between a moving average and a minimum of coolant conductivity and a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • the threshold setter 102 refers to the table and sets a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • the table representative of the graph of Fig. 14 may be prepared according to tests on actual fuel cell systems. Instead of a minimum coolant conductivity from the start of the system, a minimum moving average may be used in consideration of the stability of data.
  • Figure 15 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the fifth embodiment is stopped. In the fuel cell system according to the fifth embodiment several processes are carried out before starting the shutdown voltage decreasing operation. Namely, step S501 reads a conductivity of the coolant in the operating fuel cell stack 1 from time to time.
  • Step S502 stores a minimum of the read coolant conductivities in a memory of the control unit 100. Namely, if the coolant conductivity read in step S501 is smaller than the previously stored value, step S502 stores the read value in the memory, and if the previously stored value is smaller than the read value, keeps the previously stored value in the memory. In this way, a minimum coolant conductivity from the start of the system is always updated and stored.
  • Step S503 computes a moving average of the coolant conductivities read in step S501 and stores the computed result in the memory of the control unit 100. A period for which a moving average of coolant conductivity is computed is, for example, five minutes from the present time to five minutes before.
  • an optimum period may be set according to the characteristics of the fuel cell stack 1 and system.
  • This embodiment detects a coolant conductivity before the coolant is fed into the fuel cell stack 1. If the fuel cell system is of an internal humidifying type that directly supplies huimdi-fying water to the fuel cell stack 1, the conductivity of the hurm ' difying water may be detected instead of the conductivity of the coolant
  • the shutdown controller 101 of the control unit 100 determines whether or not a system shutdown signal has been generated in response to, for example, a driver's key-off operation. If the system shutdown signal is present step S505 starts a shutdown process.
  • the shutdown controller 101 stops the compressor 12 in the air supply system to stop the supply of air to the fuel cell stack 1, opens the purge valve 11 in the hydrogen supply system, continues the supply of hydrogen to the fuel cell stack 1, and turns on the switch 42 of the internal load circuit to connect the internal load 41 to the fuel cell stack 1.
  • the threshold setter 102 of the control unit 100 reads the minimum coolant conductivity from the start of the system stored in step S502 as well as the moving average of coolant conductivity stored in step S503.
  • step S507 the threshold setter 102 computes a difference between the read moving average of coolant conductivity and the read minimum coolant conductivity. This process can determine a coolant conductivity deterioration level based on a normal coolant conductivity. According to the difference computed in step S507 between the moving average of coolant conductivity and the minimum coolant conductivity, step S508 sets a threshold value to determine timing to disconnect the intemal load 41 from the fuel cell stack 1. Even if the coolant conductivity is relatively unstable in the system, or even if the coolant conductivity deteriorates due to aging, the fifth embodiment can set a proper threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1.
  • step S509 the internal load disconnection controller 103 of the control unit 100 obtains the voltage of the fuel cell stack 1, e.g., an average voltage of the power generation cells (average cell voltage) in the fuel cell stack 1 detected by the cell voltage monitor 2.
  • step S510 the intemal load disconnection controller 103 deteimines whether or not the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S508. If the voltage of the fuel cell stack 1 is equal to or below the threshold value, step S511 computes a voltage variation across the power generation cells of the fuel cell stack 1. In this example, a difference between a minimum voltage and a maximum voltage of the power generation cells detected by the cell voltage monitor 2 is used as a value representative of the cell voltage variation.
  • Step S512 determines whether or not the cell voltage variation found in step S511 is equal to or below a predete ⁇ nined value. If the cell voltage variation is equal to or below the predetermined value, step S513 turns off the switch 42 of the internal load circuit to disconnect the internal load 41 from the fuel cell stack 1. In step S514, the supply of hydrogen to the fuel cell stack 1 is stopped and the humidification unit and cooling mechanism are stopped to complete the shutdown voltage decreasing operation.
  • the fuel cell system conducts the shutdown voltage decreasing operation when the system is stopped, through the processes of setting, according to a coolant conductivity difference in a given period before stopping the system, a threshold value to dete ⁇ nine the timing of disconnecting the internal load 41 from the fuel cell stack 1, and when the voltage of the fuel cell stack 1 decreases to or below the threshold value and when a cell voltage variation in the fuel cell stack 1 becomes equal to or below a predeteimined level, disconnecting the internal load 41 from the fuel cell stack 1.
  • the fuel cell system of the fifth embodiment refers to a difference between an average of coolant conductivity in a given period before stopping the system and a minimum of coolant conductivity from the start of the system, and based on the difference, sets a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1. Even if the coolant conductivity in the system is relatively unstable, or even if the coolant conductivity deteriorates due to aging, the fifth embodiment can set a proper threshold value to dete ⁇ nine timing to disconnect the internal load 41 from the fuel cell stack 1. (Sixth embodiment) A fuel cell system according to a sixth embodiment of the present invention will be explained.
  • the fuel cell system of this embodiment conducts the shutdown voltage decreasing operation without comparing the voltage of the fuel cell stack 1 (a total voltage of the power generation cells or an average cell voltage) with a threshold value. Namely, the sixth embodiment disconnects the internal load 41 from the fuel cell stack 1 when a voltage variation across the power generation cells in the fuel cell stack 1 exceeds a predetermined value.
  • the shutdown voltage decreasing operation stops the supply of air to the fuel cell stack 1 and continues only the supply of hydrogen thereto to consume oxygen remaining around the oxidant electrodes of the fuel cell stack 1. At this time, voltages of the power generation cells in the fuel cell stack 1 may vary from one to another.
  • the sixth embodiment relies on, like the second to fifth embodiments, whether or not a voltage variation across the power generation cells in the fuel cell stack 1 is equal to or below a predetermined value when dete ⁇ nining the timing of disconnecting the internal load 41 from the fuel cell stack 1.
  • the second to fifth embodiments rely on not only the cell voltage variation but also whether or not the total voltage of the power generation cells in the fuel cell stack 1 or an average cell voltage of the fuel cell stack 1 decreases to a threshold value.
  • the sixth embodiment refers to only the cell voltage variation.
  • Step S601 deteimines whether or not a system shutdown signal has been generated in response to, for example, a driver's key-off operation. If the system shutdown signal is present step S602 starts a shutdown process.
  • the shutdown controller 101 stops the compressor 12 in the air supply system to stop the supply of air to the fuel cell stack 1, opens the purge valve 11 in the hydrogen supply system, continues the supply of hydrogen to the fuel cell stack 1, and turns on the switch 42 of the internal load circuit to connect the internal load 41 to the fuel cell stack 1.
  • Step S603 computes a voltage variation across the power generation cells of the fuel cell stack 1 according to voltages of the power generation cells detected by the cell voltage monitor 2.
  • a difference between a minimum voltage and a maximum voltage of the power generation cells detected by the cell voltage monitor 2 is used as a value representative of the cell voltage variation.
  • the present invention is not limited to this.
  • a statistical technique may be employed to find a standard deviation of all or some power generation cells and use the standard deviation as a value representative of the cell voltage variation.
  • Step S604 deteimines whether or not the cell voltage variation found in step S603 is equal to or below a predetermined value. If the cell voltage variation is equal to or below the predetermined value, step S605 turns off the switch 42 of the internal load circuit to disconnect the internal load 41 from the fuel cell stack 1.
  • step S606 the supply of hydrogen to the fuel cell stack 1 is stopped and the humidification unit and cooling mechanism are stopped to complete the shutdown voltage decreasing operation.
  • the fuel cell system according to the sixth embodiment conducts the shutdown voltage decreasing operation when the system is stopped.
  • the shutdown voltage decreasing operation of the sixth embodiment disconnects the internal load 41 from the fuel cell stack 1 when a voltage variation across the power generation cells in the fuel cell stack 1 becomes equal to or below a predetermined value. This effectively prevents the voltage reincreasing phenomenon of the fuel cell stack 1 after the completion of the shutdown voltage decreasing operation and secures the catalytic power of the fuel cell stack 1.
  • the fuel cell system of the sixth embodiment refers to only a cell voltage variation in the fuel cell stack 1 when deteimining the timing to disconnect the internal load 41 from the fuel cell stack 1. Namely, the sixth embodiment does not deteimine whether or not a total voltage of the power generation cells or an average cell voltage thereof decreases to a set value. For this, the sixth embodiment involves simplified processes and can reduce load on the control unit
  • the fuel cell system properly controls timing to disconnect an internal load from a fuel cell, to surely prevent the voltage reincreasing problem of the fuel cell after the disconnection of the internal load and secure the catalytic power of the fuel cell.
  • the entire contents of Japanese patent application P2004-111855 filed April 6 th , 2004 are hereby incorporated by reference.
  • the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof.
  • the present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

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Abstract

An aspect of the present invention provides a fuel cell system that includes a fuel cell configured to receive a fuel gas and an oxidant gas, electrochemically react the fuel gas with oxidant gas to generate electric power, an internal load configured to be electrically connectable to the fuel cell and consume the electric power generated by the fuel cell, a switch configured to electrically connect and disconnect the internal load with the fuel cell, a voltage detector configured to detect a voltage of the fuel cell, a shutdown controller configured to, when the fuel cell system is stopped, connect the internal load with the fuel cell through the switch, the shutdown controller configured to stop the supply of the oxidant gas to the fuel cell, continue the supply of the fuel gas to the fuel cell, and consume oxygen remaining in the fuel cell to decrease the voltage of the fuel cell, a threshold changer configured to change a threshold value according to a state of the fuel cell system at the time of stoppage of the fuel cell system, and a disconnection controller configured to electrically disconnect the internal load from the fuel cell through the switch when the voltage of the fuel cell detected by the voltage detector decreases to or below the threshold value.

Description

DESCRIPTION
FUEL CELL SYSTEM
TECHNICAL FIELD The present invention relates to a fuel cell system having a fuel cell that receives a fuel gas containing hydrogen and an oxidant gas such as air and generates electric power. In particular, the present invention relates to controlling a stop operation of the fuel cell system.
BACKGROUND ART To cope with recent environmental problems such as atmospheric pollution due to automobile exhaust gas and global warming due to carbon dioxide, fuel cell technology that realizes clean exhaust and high energy efficiency is drawing attention. A- fuel cell is an energy converter that receives a fuel gas containing hydrogen and an oxidant gas such as air in a complex of electrolyte and electrode catalyst to cause an electrochemical reaction that converts chemical energy into electrical energy. Among several types of fuel cells, a solid polymer electrolyte fuel cell employing a solid polymer membrane electrolyte is inexpensive and compact, has a high output density, and therefore, is expected for application to a power source of a mobile object such as an automobile. To shut down the fuel cell in a fuel cell system, a stop operation is carried out to decrease the pressure of fluids (fuel gas and oxidant gas). Just after the completion of such a stop operation, the fuel cell is in a high-potential, no-load state. If the fuel cell is left in this high-potential, no-load state, the fuel cell will lose the catalytic power thereof. To prevent this, the potential of the fuel cell must be decreased after the completion of the stop operation. Due to this, Japanese Laid-open Patent Application Publication No. Hei-6-333586 discloses a fuel cell stopping method that achieves a usual stop operation on a fuel cell and then conducts a voltage decreasing operation on the fuel cell. When the voltage of the fuel cell is dropped to a predetermined value, the disclosure stops the supply of a fuel gas to the fuel cell. More precisely, the disclosure carries out a first step (voltage decreasing operation) that disconnects the fuel cell from an external load, continues the supply of a fuel gas, stops the supply of an oxidant gas, and applies an internal load to the fuel cell and a second step that stops the supply of the fuel gas when the voltage of the fuel cell drops to a predetermined value and turns off a switch of the internal load to disconnect the internal load from the fuel cell.
DISCLOSURE OF INVENTION The method disclosed in the Japanese Laid-open Patent Application Pufehcation No.
Hei-6-333586 turns off the switch of the internal load when the voltage of the fuel cell decreases to a predetermined value, to disconnect the internal load from the fuel cell. At this time, if the internal load has a relatively low resistance value to rapidly decrease the voltage of: the fuel cell and shorten the voltage decreasing operation, there is a risk of again increasing the v^oltage of the fuel cell after the disconnection of the internal load. Whether or not the voltage of the fuel cell reincreases after the disconnection of the internal load is dependent on the conditions of the fuel cell and system, cell volta-ge variation, and the like at the time of stoppage. The method disclosed in the Japanese Laid-open Patent Application Publication No. Hei-6-333586 disconnects the internal load without paying attention to the conditions of the fuel cell and system, cell voltage variation, and ttαe like at the time of stoppage. Accordingly, it is difficult for the related art to surely prevent the voltage reincrease of the fuel cell after disconnecting the internal load. Namely, the related art is insufficient to prevent the deterioration of the catalytic power of the fuel cell. The present invention has been devised to solve the above-mentioned problems of the related art, and an object of the present invention is to provide a fuel cell system capable of surely preventing an increase in the voltage of a fuel cell after the disconnection o>f an internal load and securing the catalytic power of the fuel cell. An aspect of the present invention provides a fuel cell system that includes a fuel cell configured to receive a fuel gas and an oxidant gas, electrochemically react the fuel gas with oxidant gas to generate electric power, an internal load configured to be electrically connectable to the fuel cell and consume the electric power generated by the fuel cell, a switch configured to electrically connect and disconnect the internal load to and from the fuel cell, a voltage detector configured to detect a voltage of the fuel cell, a shutdown controller configured to, when the fuel cell system is stopped, connect the internal load to the fuel cell through the switch, ffcie shutdown controller configured to stop the supply of the oxidant gas to the fuel cell, continue the supply of the fuel gas to the fuel cell, and consume oxygen remaining in the fuel cell to decrease the voltage of the fuel cell, a threshold changer configured to change a threshold value according to a state of the fuel cell system at the time of stoppage of the fuel cell system, and a disconnection controller configured to electrically disconnect the internal load from the fuel cell through the switch when the voltage of the fuel cell detected by the voltage detector decreases to or below the threshold value. Another aspect of the present invention provides a fuel cell system that includes a fuel cell including a plurality of power generation cells stacked in multiple layers, configured to receive a fuel gas and an oxidant gas, electrochemically react the fuel gas and oxidant gas with each other, and generate power, an internal load configured to be electrically connected to the fuel cell and consume power generated by the fuel cell, a switch configured to electrically connect and disconnect the internal load to and from the fuel cell, a voltage detector configured to detect voltages of the power generation cells, a shutdown controller configured to, when the fuel cell system is stopped, connect the internal load to the fuel cell through the switch, stop the supply of the oxidant gas to the fuel cell, continue the supply of the fuel gas to the fuel cell, and consume oxygen remaj-oing in the fuel cell to decrease the voltage of the fuel cell, and a disconnection controller configured to electrically disconnect the internal load from the fuel cell through the switch when a voltage variation across the power generation cells decreases to or below a predetermined value.
BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a view generally showing a fuel cell system according to a fLrst embodiment of the present invention. Figure 2 shows a characteristic curve indicating a relationship between operating load and operating pressure of the fuel cell stack 1. Figure 3 explains a method of detecting a decrease in the average cell voltage of the fuel cell stack 1. Figure 4 is a view showing an internal load circuit connected to the fuel cell stack 1 of the fuel cell system according to the first embodiment of the present invention. Figure 5 is a functional block diagram showing functional blocks in the control uxiit 100, to perform the shutdown voltage decreasing operation. Figure 6 shows a relationship between the temperature of the fuel cell stack 1 and timing to disconnect the internal load 41. Figure 7 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the first embodiment is stopped. Figure 8 shows a relationship between a moving average of operating load on the fuel cell stack 1 in a given period before stopping the system and a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1. Figure 9 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the second embodiment is stopped. Figure 10 shows a relationship between a moving average of operating pressure of the fuel cell stack 1 in a given period before stopping the system and a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1. Figure 11 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the third embodiment is stopped. Figure 12 shows a relationship between a moving average of conductivity of the coolant in a given period before stopping the system and a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1. Figure 13 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the fourth embodiment is stopped. Figure 14 shows a relationship between a difference between a moving average and a minimum of coolant conductivity and a threshold value to determine t-cming to disconnect the internal load 41 from the fuel cell stack 1. Figure 15 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the fifth embodiment is stopped. Figure 16 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the sixth embodiment is stopped.
BEST MODE FOR CARRYING OUT THE INVENTION Various embodiments of the present invention will be described with reference to the accompanying drawings. It is to be noted that same or similar reference numerals are applied to the same or similar parts and elements throughout the drawings, and the description of the same or similar parts and elements will be omitted or simplified.
(First embodiment) Figure 1 is a view generally showing a fuel cell system according to a first embodiment of the present invention. This fuel cell system is used as, for example, a driving power source of a fuel-cell vehicle. In Fig. 1, the system includes a fuel cell stack 1 to receive hydrogen and air and generate electric power. The fuel cell stack 1 includes power generation cells. . Each power generation cell has a fuel electrode to which a fuel gas, i.e., a hydrogen gas is supplied and an oxidant electrode to which an oxidant gas, i.e., air is supplied. The fuel electrode and oxidant electrode are overlaid one upon another with an electrolyte being interposed between them. The power generation cells are arranged in a multilayer structure to form the fuel cell stack 1. Based on an elecfrochemical reaction between hydrogen of the fuel gas and oxygen contained in the oxidant gas, the fuel cell stack 1 converts chemical energy into electrical energy. In each power generation cell of the fuel cell stack 1, hydrogen supplied to the fuel electrode causes a reaction to separate the same into hydrogen ions and electrons. The hydrogen ions pass through the electrolyte and the electrons pass through an external circuit to generate electric power. Then, the hydrogen ions and electrons move to the oxidant electrode. At the oxidant electrode, oxygen contained in the supplied air and the hydrogen ions moved through the electrolyte and the electrons react with each other to form water, which is discharged outside. The electrolyte in the fuel cell stack 1 is, for example, a solid polymer electrolyte membrane, which has a high energy density, is producible at low cost, and is lightweight. The solid polymer electrolyte membrane is, for example, a fluoride-resin-based ion-exchange membrane that transmits ions (protons) and functions as an ion conductive electrolyte when water-saturated. The voltage of each power generation cell in the fuel cell stack 1 is monitored with a cell voltage monitor (voltage detector) 2, which provides information to a control unit 100 that generally controls operation of the fuel cell system. The fuel cell stack 1 has a temperature sensor 3 to detect the temperature of the fuel cell stack 1. An output from the temperature sensor 3 is sent to the control unit 100. According to the information from the cell voltage monitor 2 and temperature sensor 3, the control unit 100 monitors the power generating conditions and temperature of the fuel cell stack 1. To generate power in the fuel cell stack 1, a fuel gas, i.e., hydrogen and an oxidant gas, i.e., air is supplied to the fuel electrode and oxidant electrode of each power generation cell. For this, the fuel cell system includes a hydrogen supply system and an air supply system. The hydrogen supply system includes, for example, a hydrogen tank 4, a pressure control valve 5, a hydrogen supply passage 6, and an ejector 7. The hydrogen tank 4 is a hydrogen supply source to supply hydrogen; The pressure control valve 5 reduces the pressure of the hydrogen supplied from the hydrogen tank 4. The pressure-reduced hydrogen is passed through the hydrogen supply passage 6 and ejector 7 to the fuel electrodes in the fuel cell stack 1. Pressure at the fuel electrodes in the fuel cell stack 1 is detected by a pressure sensor 8. The control unit 100 feeds the detected value from the pressure sensor 8 back to the pressure control valve 5, to thereby control the operation of the pressure control valve 5 so that the pressure at the fuel electrodes in the fuel cell stack 1 is kept at a desired value. The supplied hydrogen is not entirely consumed in the fuel cell stack 1. The remaining hydrogen (discharged from the fuel electrodes of the fuel cell stack 1) is mixed through the ejector 7 with hydrogen newly supplied from the hydrogen tank 4 through the hydrogen supply passage 6. The mixed hydrogen is supplied to the fuel electrodes in the fuel cell stack 1. To achieve this, a fuel electrode outlet side of the fuel cell stack 1 is provided with a hydrogen circulation passage 9 that circulates hydrogen discharged from the fuel electrodes of the fuel cell stack 1 to the ejector 7. The ejector 7 uses the flow energy of hydrogen passing through the hydrogen supply passage 6, to circulate the hydrogen passing through the hydrogen circulation passage 9. The fuel electrode outlet side of the fuel cell stack 1 is also connected to a hydrogen discharge passage 10, which is branched from the hydrogen circulation passage 9 and is used to discharge hydrogen from the fuel electrodes of the fuel cell stack 1 to the outside of the system. On a downstream side from the location where the hydrogen discharge passage 10 is branched from the hydrogen circulation passage 9, there is arranged a purge valve 11. The purge valve 11 has a function of switching the direction of hydrogen discharged from the fuel electrodes of the fuel cell stack 1. The purge valve 11 is opened to purge hydrogen discharged from the fuel electrodes of the fuel cell stack 1 to the outside of the system through the hydrogen discharge passage 10. When hydrogen is cyclically used as mentioned above, impurities such as nitrogen may accumulate in the system. If the accumulation of impurities becomes excessive, a partial pressure of hydrogen will decrease to deteriorate the efficiency of the fuel cell stack 1 and will increase an average mass of circulated gas to deteriorate a hydrogen circulation flow rate at the ejector 7. In this case, the purge valve 11 is opened to purge hydrogen together with the impurities to the outside of the system through the hydrogen discharge passage 10. The air supply system includes a compressor 12 and an air supply passage 13, to draw external air, pressurize the drawn air, and supply the pressurized air to the oxidant electrodes in the fuel cell stack 1. The compressor 12 feeds air into the air supply passage 13 and into the oxidant electrodes of the fuel cell stack 1. The air supply passage 13 has a filter 14 to trap micro-dust sulfur components, and oil from the compressor 12, so that clean air is supplied to the oxidant electrodes of the fuel cell stack 1. An oxidant electrode outlet side of the fuel cell stack 1 is connected to an air discharge passage 15 to discharge air from the fuel cell stack 1. Oxygen and other components contained in air not consumed in the fuel cell stack 1 are discharged to the outside of the system through the air discharge passage 15. The air discharge passage 15 has a pressure control valve 16 whose operation is controlled by the control unit 100 to maintain pressure at the oxidant electrodes in the fuel cell stack 1 at a desired value. The air supply system includes a humidification unit for humidifying air to be supplied to the oxidant electrodes of the fuel cell stack 1. The humidification unit includes a humidifier 17 arranged in the middle of the air supply passage 13, a water condenser 18 arranged upstream from the pressure control valve 16 in the air discharge passage 15 to reclaim water produced in the fuel cell stack 1, a water tank 19 to store the reclaimed water, a water circulation passage 20 to circulate and supply huntidifying water, a valve 21 arranged in the water circulation passage 20, and a pump 22. The pump 22 of the hui-nidification unit 17 is controlled by the control unit 100 to pressurize and supply water from the water tank 19 to the humidifier 17 through the water circulation passage 20. The humidifier 17 is, for example, a membrane humidifier that employs the pressurized water from the pump 22 to humidify air supplied to the oxidant electrodes of the fuel cell stack 1. A humidified state of the air to be supplied to the oxidant electrodes of the fuel cell stack 1 is detected by a temperature and humidity sensor 23 arranged on the oxidant electrode inlet side of the fuel cell stack 1. The detected value from the temperature and humidity sensor 23 is sent to the control unit 100. According to the detected value, the control unit 100 controls the pump 22 so that air supplied to the oxidant electrodes may have a desired humidity for the operation of the fuel cell stack 1. The conductivity of the hun-ddifying water sent from the pump 22 to the humidifier 17 is detected by a conductivity sensor 24 and is supplied to the control unit 100. Excessive water in the humidifier 17 is returned to the water tank 19 through a hurnidi-cying water reclaim passage 25. The passage 25 includes a pressure sensor 26 and a pressure adjust valve 27. The control unit 100 controls the pressure adjust valve 27 according to a detected value from the pressure sensor 26, to ma tain pressure between the pump 22 and the pressure adjust valve 27 substantially at a constant value. The water tank 19 has a water level sensor 28. According to a detected value from the water level sensor 28, the control unit 100 determines a shortage or an excess of water in the water tank 19 and informs an operator of the detected state. The fuel cell system according to the first embodiment has a cooling mechanism to cool the fuel cell stack 1. With a sohd polymer electrolyte, for example, the fuel cell stack 1 has a relatively low operating temperature of about 80°C and may need to be cooled if overheated. The cooling mechanism includes a coolant circulation passage 29 and a coolant pump 30 to circulate a coolant The coolant is a mixture of, for example, water and ethylene glycol serving as an antifreeze and is circulated to cool and maintain the fuel cell stack 1 at an optimum temperature. The coolant circulation passage 29 of the cooling mechanism includes a radiator 31. The radiator 31 is provided with a radiator fan (not shown) controlled by the control unit 100, to adjust the temperature of the coolant and realize a desired radiator outlet temperature. In parallel with the radiator 31 , there is a bypass passage 32. A branch of the bypass passage 32 is provided with a thermostat three-way valve 33, which is operated according to the temperature of the coolant to change the direction of a coolant flow. A coolant temperature at which the three-way valve 33 is switched is, for example, 50°C. The coolant circulation passage 29 to pass the temperature-adjusted coolant has a reservoir tank 34. The reservoir tank 34 absorbs thermal expansion and contraction of the coolant and replenishes the coolant The top of the reservoir tank 34 is open to the atmosphere to provide a reservoir function. In front of the reservoir tank 34, there is an ion filter 35 to remove foreign matter and ions. The temperature-adjusted coolant is passed through the ion filter 35 to decrease the conductivity thereof. The conductivity of the coolant is detected with a conductivity sensor 36 and is transferred to the control unit 100. A coolant inlet of the fuel cell stack 1 has a temperature sensor 37, and a coolant outlet thereof has a temperature sensor 38. The temperature sensor 37 detects the temperature of coolant before entering the fuel cell stack 1, and the temperature sensor 38 detects the temperature of coolant exiting the fuel cell stack 1. The detected values from the temperature sensors 37 and 38 are sent to the control unit 100, which uses them with a detected value from the temperature sensor 3 on the fuel cell stack 1 to control the cooling of the fuel cell stack 1. The control unit 100 is, for example, a microcomputer involving a CPU, ROM, RAM, and peripheral interfaces. The control unit 100 reads detected values from an outside air temperature sensor (not shown), the cell voltage monitor 2, the temperature sensor 3, and the like, carries out deteπninations and operations accordingly, and provides various control signals to control various parts of the fuel cell system. When the fuel cell system is stopped, the control unit 100 according to this embodiment conducts a voltage decreasing operation to decrease the voltage of the fuel cell stack 1. This voltage decreasing operation carried out when the fuel cell system is stopped is hereinafter referred to as the "shutdown voltage decreasing operation." The details of the shutdown voltage decreasing operation will be explained later. A normal operation of the fuel cell system according to the embodiment with the above-mentioned configuration will be explained in connection with an example that employs the fuel cell system as a driving power source of a fuel cell vehicle. In a normal operation of the fuel cell system, the driver of the vehicle operates an accelerator. According to an accelerator opening, output power (electric power) is determined to determine a hydrogen quantity and an air quantity. According to these quantities, the pressure control valve 5 adjusts the pressure of hydrogen, and the pressure-adjusted hydrogen is supplied to the fuel electrodes of the fuel cell stack 1. At the same time, the compressor 12 supplies air, and the humidifier 17 humidifies the air and supplies the humidified air to the oxidant electrodes of the fuel cell stack 1. Figure 2 shows a characteristic curve indicating a relationship between operating load and operating pressure of the fuel cell stack 1. In Fig. 2, an abscissa represents operating load and an ordinate operating pressure. The first embodiment sets an operating pressure according to an operating load as shown in Fig. 2. Namely, a low operating pressure is set under a low operating load and a high operating pressure is set under a high operating load. During operation of the fuel cell system, the temperature and humidity sensor 23 monitors the temperature and humidity of the oxidant electrode inlet of the fuel cell stack 1. If the humidity of air supplied to the oxidant electrodes of the fuel cell stack 1 is insufficient the operating pressure is increased to increase the pressure of the humidifier 17, to thereby resolve the shortage of humidity. The coolant pump 30 adjusts the flow rate of the coolant according to the quantity of heat generated by the fuel cell stack 1 so that the coolant is passed through the fuel cell stack 1 at a proper flow rate. Namely, the flow rate is corrected according to the temperature of the fuel cell stack 1 detected by the temperature sensor 3 installed on the fuel cell stack 1, or according to the temperature of the fuel cell stack 1 estimated from coolant temperatures detected by the temperature sensors 37 and 38 of the cooling mechanism. If the temperature of the fuel cell stack 1 is very low, the thermostat three-way valve 33 is changed so that the coolant may pass through the bypass passage 32 without passing through the radiator 31. The outlet temperature of the radiator 31 is controlled substantially at a fixed value by controlling the rotation speed of the radiator fan (not shown) according to the temperature of the fuel cell stack 1 detected by the temperature sensor 3 or by the temperature sensors 37 and 38. During the operation of the fuel cell system, the water condenser 18 in the air supply system condenses and reclaims water from air discharged from the fuel cell stack 1. The reclaimed water is guided through the water circulation passage 20 into the water tank 19 and is stored therein as humidifying water. In the hydrogen supply system, hydrogen discharged from the fuel electrodes of the fuel cell stack 1 is circulated through the hydrogen circulation passage 9 and ejector 7. During operation of the fuel cell system, a concentration of impurities such as nitrogen in the passage 9 may increase due to the transmission of hydrogen through the solid polymer electrolyte membranes of the fuel cell stack 1, or the passage 9 may be clogged with water to decrease a cell voltage. To cope with this, the cell voltage monitor 2 monitors a cell voltage. If the voltage of a given cell decreases by a predeteririined value (for example, 0.2 N) from an average cell voltage, or if an average cell voltage decreases by a predetermined level (for example, 0.1
V), the purge valve 11 is opened to discharge the impurities together with hydrogen from the hydrogen circulation passage 6 and fuel cell stack 1 , to thereby restore a specified cell voltage. Figure 3 explains a method of detecting a decrease in the average cell voltage of the fuel cell stack 1. More precisely, Fig. 3 shows voltage characteristic curves of the fuel cell stack 1 relative to operating load. To detect a decrease in the average cell voltage of the fuel cell stack 1, the voltage characteristic curves (I-N curves) of the fuel cell stack 1 relative to operating load are stored as table data in the control unit 100. The table data is used to find a reference average cell voltage according to an operating load and a temperature of the fuel cell stack 1. The reference average cell voltage is compared with an average cell voltage actually detected during operation, to provide an average cell voltage decrease. To consider deterioration of the fuel cell stack 1 in a long period, a cell voltage decrease for a relatively long period may be learned and corrected to grasp an average cell voltage decrease due to gradual deterioration of the fuel cell stack 1. In the normal operation of the fuel cell system mentioned above, the fuel cell system provides output in response to a driver's accelerator operation, and a vehicle driving motor (not shown) drives the vehicle. If the driver turns off a key, a system shutdown signal is supplied to the control unit 100. Then, the control unit 100 conducts the shutdown voltage decreasing operation to stop the system. The shutdown voltage decreasing operation achieved when the fuel cell system according to the first embodiment is stopped will be explained. Figure 4 is a view showing an internal load circuit connected to the fuel cell stack 1 of the fuel cell system according to the first embodiment of the present invention. To achieve the shutdown voltage decreasing operation, the fuel cell system of the embodiment connects the internal load circuit of Fig. 4 to the fuel cell stack 1. The internal load circuit includes an internal load 41 made of a solid resistor and a switch 42 to connect and disconnect the internal load 41 to and from the fuel cell stack 1. Once the switch 42 is turned on to connect the internal load 41 to the fuel cell stack 1, electric power generated in the fuel cell stack 1 is consumed by the internal load 41. Figure 5 is a functional block diagram showing functional blocks in the control unit
100, to perform the shutdown voltage decreasing operation. According to the embodiment the functional blocks prepared in the control unit 100 to achieve the shutdown voltage decreasing operation include a shutdown controller 101 configured to, when the fuel cell system is stopped, connect the internal load 41 with the fuel cell through the switch 42, the shutdown controller 101 configured to stop the supply of the oxidant gas to the fuel cell, continue the supply of the fuel gas to the fuel cell, and consume oxygen ren aining in the fuel cell to decrease the voltage of the fuel cell, a threshold changer 102 configured to change a threshold value according to a state of the fuel cell system at the time of stoppage of the fuel cell system; and a disconnection controller 103 configured to electrically disconnect the internal load 41 from the fuel cell through the switch 42 when the voltage of the fuel cell detected by the voltage detector decreases to or below the threshold value. The shutdown controller 101 carries out shutdown processes in response to a system shutdown signal generated in response to, for example, a key-off operation of the driver. The shutdown processes include stopping the operation of the compressor 12 in the air supply system, stopping the supply of air to the fuel cell stack 1, opening the purge valve 11 in the hydrogen supply system, continuing the supply of hydrogen to the fuel cell stack 1, and turning on the switch 42 of the internal load circuit to connect the internal load 41 to the fuel cell stack 1. The threshold setter 102 sets a threshold value used to determine Inning to disconnect the internal load 41 from the fuel cell stack 1. Namely, the internal load 41 is disconnected from the fuel cell stack 1 when the voltage of the fuel cell stack 1 (the total of cell voltages of the fuel cell stack 1 in this embodiment) decreases to a threshold value. The threshold setter 102 sets such a threshold value used to determine timing to disconnect the internal load 41 from the fuel cell stack 1. The threshold setter 102 may set the threshold value according to a state of the fuel cell system when the system is stopped (for example, the temperature of the fuel cell stack 1 at the time of system shutdown). If the fuel cell stack 1 is left under a high-potential, no-load state during the stoppage of the fuel cell system, the catalytic power of the system will deteriorate. To prevent this, the shutdown voltage decreasing operation is carried out when the system is stopped, to decrease the voltage of the fuel cell stack 1. If the voltage of the fuel cell stack 1 is rapidly decreased, however, the voltage of the fuel cell stack will reincrease after the internal load 41 is disconnected from the fuel cell stack 1 to terminate the shutdown voltage decreasing operation.
Then, the catalytic power of the fuel cell stack 1 will deteriorate. The voltage reincreasing phenomenon of the fuel cell stack 1 occurs depending on the conditions such as temperature of the fuel cell stack 1 when the system is stopped. Generally, the lower the temperature of the fuel cell stack 1 at the time of stoppage of the system, the more frequently the voltage reincreasing phenomenon of the fuel cell stack 1 occurs after the completion of the shutdown voltage decreasing operation. Namely, the lower the temperature of the fuel cell stack 1 at system shutdown, the lower the threshold value for determining timing to disconnect the internal load 41 from the fuel cell stack 1 must be set so that the voltage of the fuel cell stack 1 is sufficiently decreased before disconnecting the internal load 41 from the fuel cell stack 1. This surely prevents the voltage reincreasing phenomenon of the fuel cell stack 1 after the completion of the shutdown voltage decreasing operation. Figure 6 shows a relationship between the temperature of the fuel cell stack 1 and timing to disconnect the internal load 41. An abscissa indicates the temperature of the fuel cell stack 1 when the system is stopped, and an ordinate indicates a threshold voltage to determine the disconnection timing. The threshold setter 102 may employ a table containing such a relationship between the temperature of the fuel cell stack 1 and timing to disconnect the internal load 41. According to an actual temperature of the fuel cell stack 1 detected when the system, is stopped, the threshold setter 102 refers to the table and sets a threshold value to deterrnine timing to disconnect the internal load 41 from the fuel cell stack 1. The table representative of the graph of Fig. 6 may be prepared according to tests on actual fuel cell systems. The internal load disconnection controller 103 turns off the switch 42 of the internal load circuit when the voltage of the fuel cell stack 1 (the total of cell voltages of the fuel cell stack 1) decreases to or below the threshold value set by the threshold setter 102, to disconnect the internal load 41 from the fuel cell stack 1. In this way, the threshold setter 102 sets a threshold value according to the temperature of the fuel cell stack 1 when the system is stopped. When the voltage of the fuel cell stack 1 decreases to or below the threshold value, the internal load disconnection controller 103 disconnects the internal load 41 from the fuel cell stack 1, to surely prevent the voltage reincreasing phenomenon of the fuel cell stack 1 after the completion of the shutdown voltage decreasing operation and secure the catalytic power of the fuel cell stack 1. Figure 7 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the first embodiment is stopped. In step S101, the shutdown controller 101 of the control unit 100 determines whether or not a system shutdown signal has been generated in response to, for example, a driver's key-off operation. If the system shutdown signal is present step S 102 starts a shutdown process. More precisely, the shutdown controller 101 stops the compressor 12 in the air supply system to stop the supply of air to the fuel cell stack 1, opens the purge valve 11 in the hydrogen supply system, continues the supply of hydrogen to the fuel cell stack 1, and turns on the switch 42 of the internal load circuit to connect the internal load 41 to the fuel cell stack 1. With this shutdown process, oxygen remaining around the oxidant electrodes of the fuel cell stack 1 is consumed by power generation reaction with the continuously supphed hydrogen, and the generated power is consumed by the internal load 41. In step S103, the threshold setter 102 of the control unit 100 reads the temperature of the fuel stack 1 at the system stoppage. According to the read temperature of the fuel cell stack 1, the threshold setter 102 sets, in step S104, a threshold value to deteimine timing to disconnect the internal load 41 fro the fuel cell stack 1. The temperature of the fuel cell stack 1 at the system stoppage may be the temperature of the fuel cell stack 1 itself detected by the temperature sensor 3. Alternatively, it may be a coolant temperature of the cooling mechanism that corresponds to the temperature of the fuel cell stack 1. For example, a coolant temperature at the inlet side of the fuel cell stack 1 detected by the temperature sensor 37, or a coolant temperature at the outlet side of the fuel cell stack 1 detected by the temperature sensor 38 is employable. In step S105, the internal load disconnection controller 103 of the control unit 100 obtains the voltage of the fuel cell stack 1, e.g., the total voltage of the power generation cells in the fuel cell stack 1 detected by the cell voltage monitor 2. In step S106, the internal load disconnection controller 103 determines whether or not the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S 104. The voltage of the fuel cell stack 1 may be an average cell voltage instead of the total voltage of the power generation cells. If the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S104, the internal load disconnection controller 103 turns off, in step S107, the switch 42 of the internal load circuit to disconnect the internal load 41 from the fuel cell stack 1. In step S108, the supply of -hydrogen to the fuel cell stack 1 is stopped and the humidification unit and cooling mechanism are stopped to complete the shutdown voltage decreasing operation. As explained above, the fuel cell system according to the first embodiment conducts the shutdown voltage decreasing operation when the system is stopped, through the processes of setting, according to the temperature of the fuel cell stack 1 at the system stoppage, a threshold value to deteimine the timing of c-isconnecting the internal load 41 from the fuel cell stack 1, and when the voltage of the fuel cell stack 1 decreases to or below the threshold value, disconnecting the internal load 41 from the fuel cell stack 1. This effectively prevents the voltage reincreasing phenomenon of the fuel cell stack 1 after the completion of the shutdown voltage decreasing operation and secures the catalytic power of the fuel cell stack 1. (Second embodiment) A fuel cell system according to a second embodiment of the present invention will be explained. The fuel cell system of this embodiment employs the same basic structure as that of the first embodiment mentioned above. Like the first embodiment the second embodiment disconnects the internal load 41 from the fuel cell stack 1 at timing determined from a threshold value that is variable according to a state of the fuel cell system when the system is stopped.
Unlike the first embodiment that employs the temperature of the fuel cell stack 1 as the state of the fuel cell system to deteimine the threshold value, the second embodiment employs, to determine the threshold value, an average value of operating load of the fuel cell stack 1 in a given period before stopping the system. Unlike the first embodiment that disconnects the internal load 41 from the fuel cell stack 1 when the voltage of the fuel cell stack 1 decreases to or below the threshold value, the second embodiment disconnects the internal load 41 from the fuel cell stack 1 if the voltage of the fuel cell stack 1 decreases to or below the threshold value and if a voltage variation across the power generation cells of the fuel cell stack 1 is equal to or below a predeteimined value. The phenomenon that the voltage of the fuel cell stack 1 reincreases after the shutdown voltage decreasing operation of disconnecting the internal load 41 from the fuel cell stack 1 is dependent not only on the temperature of the fuel cell stack 1 when the system is stopped but also on operating load on the fuel cell stack 1 in a given period before stopping the system. Generally, the lower the operating load on the fuel cell stack 1 in a given period before stopping the system, the more frequently the reincreasing phenomenon of the voltage of the fuel cell stack 1 occurs after the completion of the shutdown voltage decreasing operation. Namely, the lower the operating load on the fuel cell stack 1 in a given period before stopping the system, the lower the threshold value for deteiminirig timing to disconnect the intemal load 41 from the fuel cell stack 1 is set so that the voltage of the fuel cell stack 1 is sufficiently decreased before disconnecting the internal load 41 from the fuel cell stack 1. This surely prevents the voltage reincreasing phenomenon of the fuel cell stack 1 after the completion of the shutdown voltage decreasing operation. Figure 8 shows a relationship between a moving average of operating load on the fuel cell stack 1 in a given period before stopping the system and a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1. The threshold setter 102 of the control unit 100 in the fuel cell system according to the second embodiment may employ a table containing such a relationship between a moving average of operating load on the fuel cell stack 1 in a given period before stopping the system and a threshold value to determine liming to disconnect the internal load 41 from the fuel cell stack 1. According to an actual moving average of operating load on the fuel cell stack 1 detected in a given period before stopping the system, the threshold setter 102 refers to the table and sets a threshold value to deteimine timing to disconnect the internal load 41 from the fuel cell stack 1. The table representative of the graph of Fig. 8 may be prepared according to tests on actual fuel cell systems. As mentioned above, the shutdown voltage decreasing operation stops the supply of air to the fuel cell stack 1 and continues the supply of hydrogen, to the same, to consume oxygen remaining around the oxidant electrodes. At this time, cell voltages of the power generation cells of the fuel cell stack 1 may vary from one to another. If the cell voltage variation is large, the voltage of the fuel cell stack 1 will increase after the coiripletion of the shutdown voltage decreasing operation. To cope with this problem, the internal load disconnection controller 103 of the control unit 100 of this embodiment is configured to turn off the switch 42 of the internal load circuit to disconnect the internal load 41 from the fuel cell stack 1 if the voltage of the fuel cell stack 1 decreases to or below a threshold value set by the threshold setter 102 and if a voltage variation across the power generation cells of the fuel cell stack 1 becomes equal to or below a predetermined value. Figure 9 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the second embodiment is stopped. In the fuel cell system according to the second embodiment several processes are carried out before starting the shutdown voltage decreasing operation. Namely, step S201 reads load (operating load) on the operating fuel cell stack 1 from time to time. Step S202 computes a moving average of the read operating load and stores the computation result in a memory of the control unit 100. A period for which a moving average of operating load is computed is, for example, five minutes from the present time to five minutes before. Alternatively, an optimum period may be set according to the characteristics of the fuel cell stack 1 and system. The second embodiment computes a moving average of operating load in a given period (for example, a period of five minutes) before the stoppage of the system. Instead of the moving average, a maximum value of operating load in a given period may be employable. In this case, step S202 updates a maximum value of operating load and stores the updated maximum value in the memory of the control unit 100. In step S203, the shutdown controller 101 of the control unit 100 deteπriines whether or not a system shutdown signal has been generated in response to, for example, a driver's key-off operation. If the system shutdown signal is present step S204 starts a shutdown process. More precisely, the shutdown controller 101 stops the compressor 12 in the air supply system to stop the supply of air to the fuel cell stack 1, opens the purge valve 11 in the hydrogen supply system, continues the supply of hydrogen to the fuel cell stack 1, and turns on the switch
42 of the internal load circuit to connect the internal load 41 to the fuel cell stack 1. With this shutdown process, oxygen remaining around the oxidant electrodes of the fuel cell stack 1 is consumed by power generation reaction with the continuously supplied hydrogen, and the generated power is consumed by the internal load 41. In step S205, the threshold setter 102 of the control unit L00 reads the moving average of operating load stored in step S202 in the memory of the control unit 100. According to the read moving average of operating load, the threshold setter 102 sets, in step S206, a threshold value to deteimine timing to disconnect the internal load 41 from the fuel cell stack 1. In step S207, the internal load disconnection controller 103 of the control unit 100 obtains the voltage of the fuel cell stack 1, e.g., an average voltage of the power generation cells (average cell voltage) in the fuel cell stack 1 detected by the cell voltage monitor 2. In step S208, the internal load disconnection controller 103 determines whether or not the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S206. If the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S206, step S209 computes a voltage variation across the power generation cells of the fuel cell stack 1. In this example, a difference obtained by subtracting a minimum voltage from a maximum voltage of the power generation cells detected by the cell voltage monitor 2 is used as a value representative of the cell voltage variation. The present invention is not limited to this. For example, a statistical technique may be employed to find a standard deviation of all or some power generation cells and use the standard deviation as a value representative of the cell voltage variation. Step S210 deteimines whether or not the cell voltage variation found in step S209 is equal to or below a predetermined value. If the cell voltage variation is equal to or below the predete-mined value, step S211 turns off the switch 42 of the internal load circuit to disconnect the internal load 41 from the fuel cell stack 1. In step S212, the supply of hydrogen to the fuel cell stack 1 is stopped and the humidification unit and cooling mechanism are stopped to complete the shutdown voltage decreasing operation. As explained above, the fuel cell system according to the second embodiment conducts the shutdown voltage decreasing operation when the system is stopped, through the processes of setting, according to operating load on the fuel cell stack 1 in a given period before stopping the system, a threshold value to deteimine the timing of disconnecting the internal load 41 from the fuel cell stack 1, and when the voltage of the fuel cell stack 1 decreases to or below the threshold value and when a cell voltage variation in the fuel cell stack 1 becomes equal to or below a predetermined level, disconnecting the internal load 41 from the fuel cell stack 1. This effectively prevents the voltage reincreasing phenomenon of the fuel cell stack 1 after the completion of the shutdown voltage decreasing operation and secures the catalytic power of the fuel cell stack 1. (Third embodiment) A fuel cell system according to a third embodiment of the present invention will be explained. The fuel cell system of this embodiment employs the same basic structure as that of the first embodiment mentioned above. Like the first embodiment the third embodiment disconnects the internal load 41 from the fuel cell stack 1 at timing determined1 from a threshold value that is variable according to a state of the fuel cell system when the system is stopped. Unlike the first embodiment the third embodiment employs, as a state of tb-e fuel cell system used as a reference to set the threshold value, an average operating pressure of the fuel cell stack 1 in a given period before stopping the system. Like the second embodiment the third embodiment disconnects the internal load 41 from the fuel cell stack 1 when the voltage of the fuel cell stack 1 decreases to or below the threshold value and when a voltage variation across the power generation cells of the fuel cell stack 1 is equal to or below a given value. The phenomenon that the voltage of the fuel cell stack 1 reincreases after the shutdown voltage decreasing operation of disconnecting the internal load 41 from the fuel cell stack 1 is also dependent on the operating pressure of the fuel cell stack 1 in a given period before stopping the system. Generally, the lower the operating pressure of the fuel cell stack 1 in a given period before stopping the system, the more frequently the reincreasing phenomenon of the voltage of the fuel cell stack 1 occurs after the completion of the shutdown voltage decreasing operation. Namely, the lower the operating pressure of the fuel cell stack 1 in a given period before stopping the system, the lower the threshold value for deteπriining timing to disconnect the internal load 41 from the fuel cell stack 1 must be set so that the voltage of the fuel cell stack 1 is sufficiently decreased before disconnecting the internal load 41 from the fuel cell stack 1. This surely prevents the voltage reincreasing phenomenon of the fuel cell stack 1 after the completion of the shutdown voltage decreasing operation. Figure 10 shows a relationship between a moving average of operating pressure of the fuel cell stack 1 in a given period before stopping the system and a threshold value to deteimine timing to disconnect the internal load 41 from the fuel cell stack 1. The threshold setter 102 o-f the control unit 100 in the fuel cell system according to the third embodiment may employ a table containing such a relationship between a moving average of operating pressure of the fuel cell stack 1 in a given period before stopping the system and a threshold value to deteimine timing to disconnect the internal load 41 from the fuel cell stack 1. According to an actual moving average of operating pressure of the fuel cell stack 1 detected in a given period before stopping the system, the threshold setter 102 refers to the table and sets a threshold value to deteimine timing to disconnect the internal load 41 from the fuel cell stack 1. The table representative of the graph of Fig. 10 may be prepared according to tests on actual fuel cell systems. Figure 11 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the third embodiment is stopped. In the fuel cell system according to the third embodiment several processes are carried out before starting the shutdown voltage decreasing operation. Namely, step S301 reads a pressure (operating pressure) of the operating fuel cell stack 1, for example, an inlet pressure of the fuel cell stack 1 from time to time. Step S302 computes a moving average of the read operating pressure and stores the computation result in a memory of the control unit 100. A period for which the moving average of operating pressure is computed is, for example, five minutes from the present time to five minutes before. Alternatively, an optimum period may be set according to the characteristics of the fuel cell stack 1 and system. According to this embodiment he operating pressure of the fuel cell stack 1 is an inlet pressure of the fuel cell stack 1. Any othe-r pressure is also acceptable. The third embodiment computes a moving average of operating pressure in a given period (for example, a period of five minutes) before the stoppage of the system. Instead of the moving average, a maximum value of operating pressure in a given period may be employable. In this case, step S302 updates a maximum value of operating pressure and stores the updated maximum value in the memory of the control unit 100. In step S303, the shutdown controller 101 of the control unit 100 deteimines whether or not a system shutdown signal has been generated in response to, for example, a driver's key-off operation. If the system shutdown signal is present step S304 starts a shutdown process. More precisely, the shutdown controller 101 stops the compressor 12 in the air supply system to stop the supply of air to the fuel cell stack 1, opens the purge valve 11 in the hydrogen supply system, continues the supply of hydrogen to the fuel cell stack 1, and turns on the switch
42 of the internal load circuit to connect the internal load 41 to the fuel cell stack 1. With this shutdown process, oxygen remaining around the oxidant electrodes of the fuel cell stack 1 is consumed by power generation reaction with the continuously supplied hydrogen, and the generated power is consumed by the internal load 41. In step S305, the threshold setter 102 of the control unit 100 reads the moving average of operating pressure stored in step S302 in the memory of the control unit 100. According to the read moving average of operating pressure, the threshold setter 102 sets, in step S306, a threshold value to deteimine timing to disconnect the internal load 41 from the fuel cell stack 1. In step S307, the internal load disconnection controller 103 of the control unit 100 obtains the voltage of the fuel cell stack 1, e.g., an average voltage of the power generation cells (average cell voltage) in the fuel cell stack 1 detected by the cell voltage monitor 2. In step S308, the internal load disconnection controller 103 deteimines whether or not the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S306. If the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S306, step S309 computes a voltage variation across the power generation cells of the fuel cell stack 1. In this example, a difference obtained by subtracting a m-b-ιimum voltage from a maximum voltage of the power generation cells detected by the cell voltage monitor 2 is used as a value representative of the cell voltage variation. This, however, does not limit the present invention. For example, a statistical technique may be employed to find a standard deviation of all or some power generation cells and use the standard deviation as a value representative of the cell voltage variation. Step S310 determines whether or not the cell voltage variation found in step S309 is equal to or below a predetermined level. If the cell voltage variation is equal to or below the
Figure imgf000023_0001
level, step S311 turns off the switch 42 of the internal load circuit to disconnect the internal load 41 from the fuel cell stack 1. In step S312, the supply of hydrogen to the fuel cell stack 1 is stopped and the humidification unit and cooling mechanism are stopped to complete the shutdown voltage decreasing operation. As explained above, the fuel cell system according to the third embodiment conducts the shutdown voltage decreasing operation when the system is stopped, through the processes of setting, according to an operating pressure of the fuel cell stack 1 in a given period before stopping the system, a threshold value to determine the timing of disconnecting the internal load 41 from the fuel cell stack 1, and when the voltage of the fuel cell stack 1 decreases to or below the threshold value and when a cell voltage variation in the fuel cell stack 1 becomes equal to or below a predetermined level, disconnecting the internal load 41 from the fuel cell stack 1. This effectively prevents the voltage reincreasing phenomenon of the fuel cell stack 1 after the completion of the shutdown voltage decreasing operation and secures the catalytic power of the fuel cell stack 1. (Fourth embodiment) A fuel cell system according to a fourth embodiment of the present invention will be explained. The fuel cell system of this embodiment employs the same basic structure as that of the first embodiment mentioned above. like the first embodiment the fourth embodiment disconnects the internal load 41 from the fuel cell stack 1 at timing determined from a threshold value that is variable according to a state of the fuel cell system when the system is stopped. Unlike the first embodiment the fourth embodiment employs, as a state of the fuel cell system used as a reference to set the threshold value, an average conductivity of the coolant (supphed to the fuel cell stack 1 to adjust the temperature of the fuel cell stack 1) in a given period before stopping the system. Like the second and third embodiments, the fourth embodiment disconnects the internal load 41 from the fuel cell stack 1 when the voltage of the fuel cell stack 1 decreases to or below the threshold value and when a voltage variation across the power generation cells of the fuel cell stack 1 becomes equal to or below a given value. The phenomenon that the voltage of the fuel cell stack 1 reincreases after the shutdown voltage decreasing operation of disconnecting the internal load 41 from the fuel cell stack 1 is also dependent on the conductivity of the coolant in a given period before stopping the system. Generally, the lower the conductivity of the coolant in a given period before stopping the system, the more frequently the reincreasing phenomenon of the voltage of the fuel cell stack 1 occurs after the completion of the shutdown voltage decreasing operation. Namely, the lower the conductivity of the coolant in a given period before stopping the system, the lower the threshold value for deteimining timing to disconnect the internal load 41 from the fuel cell stack
1 must be set so that the voltage of the fuel cell stack 1 is sufficiently decreased before disconnecting the internal load 41 from the fuel cell stack 1. This surely prevents the voltage reincreasing phenomenon of the fuel cell stack 1 after the completion of the shutdown voltage decreasing operation. Figure 12 shows a relationship between a moving average of conductivity of the coolant in a given period before stopping the system and a threshold value to deteimine timing to disconnect the internal load 41 from the fuel cell stack 1. The threshold setter 102 of the control unit 100 in the fuel cell system according to the fourth embodiment may employ a table containing such a relationship between a moving average of conductivity of the coolant in a given period before stopping the system and a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1. According to an actual moving average of conductivity of the coolant detected in a given period before stopping the system, the threshold setter 102 refers to the table and sets a threshold value to deteimine timing to disconnect the internal load 41 from the fuel cell stack 1. The table representative of the graph of Fig. 12 may be prepared according to tests on actual fuel cell systems. Figure 13 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the fourth embodiment is stopped. In the fuel cell system according to the fourth embodiment several processes are carried out before starting the shutdown voltage decreasing operation. Namely, step S401 reads a conductivity of the coolant in the operating fuel cell stack 1 from time to time. Step S402 computes a moving average of the read coolant conductivity and stores the computation result in a memory of the control unit 100. A period for which the moving average of coolant conductivity is computed is, for example, five minutes from the present time to five minutes before. Alternatively, an optimum period may be set according to the characteristics of the fuel cell stack 1 and system. This embodiment detects a conductivity of the coolant in front of the fuel cell stack 1. If the fuel cell system is of an internal humidifying type that directly supplies humidifying water to the fuel cell stack 1, the conductivity of the huirddifying water may be detected instead of the coolant conductivity. The forth embodiment computes a moving average of coolant conductivity in a given period (for example, a period of five minutes) before the stoppage of the system. Instead of a moving average, a maximum value of coolant conductivity in a given period may be employable. In this case, step S402 updates a maximum value of coolant conductivity and stores the updated maximum value in the memory of the control unit 100. In step S403, the shutdown controller 101 of the control unit 100 deteimines whether or not a system shutdown signal has been generated in response to, for example, a driver's key-off operation. If the system shutdown signal is present step S404 starts a shutdown process. More precisely, the shutdown controller 101 stops the compressor 12 in the air supply system to stop the supply of air to the fuel cell stack 1, opens the purge valve 11 in the hydrogen supply system, continues the supply of hydrogen to the fuel cell stack 1, and turns on the switch 42 of the internal load circuit to connect the internal load 41 to the fuel cell stack 1. With this shutdown process, oxygen remaining around the oxidant electrodes of the fuel cell stack 1 is consumed by power generation reaction with the continuously supplied hydrogen, and the generated power is consumed by the internal load 41. In step S405, the threshold setter 102 of the control unit 100 reads the moving average of coolant conductivity stored in step S402 in the memory of the control unit 100. According to the read moving average of coolant conductivity, the threshold setter 102 sets, in step S406, a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1. In step S407, the internal load disconnection controller 103 of the control unit 100 obtains the voltage of the fuel cell stack 1, e.g., an average voltage of the power generation cells (average cell voltage) in the fuel cell stack 1 detected by the cell voltage monitor 2. In step S408, the internal load disconnection controller 103 determines whether or not the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S406. If the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S406, step S409 computes a voltage variation across the power generation cells of the fuel cell stack 1. In this example, a difference obtained by subtracting a rninimum voltage from a maximum voltage of the power generation cells detected by the cell voltage monitor 2 is used as a value representative of the cell voltage variation. The present invention is not limited to this. For example, a statistical technique may be employed to find a standard deviation of all or some power generation cells and use the standard deviation as a value representative of the cell voltage variation. Step S410 deteimines whether or not the cell voltage variation found in step S409 is equal to or below a predetermined value. If the cell voltage variation is equal to or below the predetermined value, step S411 turns off the switch 42 of the internal load circuit to disconnect the internal load 41 from the fuel cell stack 1. _Ln step S412, the supply of hydrogen to the fuel cell stack 1 is stopped and the humidification unit and cooling mechanism are stopped to complete the shutdown voltage decreasing operation. As explained above, the fuel cell system according to the fourth embodiment conducts the shutdown voltage decreasing operation when the system is stopped, through the processes of setting, according to the conductivity of a coolant in a given period before stopping the system, a threshold value to determine the timing of disconnecting the internal load 41 from the fuel cell stack 1, and when the voltage of the fuel cell stack 1 decreases to or below the threshold value and when a cell voltage variation in the fuel cell stack 1 becomes equal to or below a predetermined level, disconnecting the internal load 41 from the fuel cell stack 1. This effectively prevents the voltage reincreasing phenomenon of the fuel cell stack 1 after the completion of the shutdown voltage decreasing operation and secures the catalytic power of the fuel cell stack 1. (Fifth embodiment) A fuel cell system according to a fifth embodiment of the present invention will be explained. This embodiment is a modification of the fourth embodiment mentioned above. The fourth embodiment refers to an average of coolant conductivity in a given period before stopping the system, and based on the average, sets a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1. The fifth embodiment refers to a difference between an average of coolant conductivity in a given period before stopping the system and a rninimum of coolant conductivity from the start of the system, and based on the difference, sets a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1. The conductivity of a coolant greatly varies depending on the configuration of a fuel cell system and the operating condition of a fuel cell stack, or depending on aging. It is possible, therefore, that an average of coolant conductivity in a given period before stopping the system is insufficient to set a proper threshold value to deteimine t-tming to disconnect the internal load 41 from the fuel cell stack 1. To cope with this problem, the fuel cell system of the fifth embodiment considers conductivity deterioration. For this, the fifth embodiment finds a difference between an average of coolant conductivity in a given period before stopping the system and a minimum coolant conductivity measured from the start of the system, and according to the difference, selects a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1. Figure 14 shows a relationship between a difference between a moving average and a ininimum of coolant conductivity and a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1. The threshold setter 102 of the control unit 100 in the fuel cell system according to the fifth embodiment may employ a table containing such a relationship between a difference between a moving average and a minimum of coolant conductivity and a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1. According to an actually obtained difference between a coolant-conductivity moving average and a minimum coolant conductivity, the threshold setter 102 refers to the table and sets a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1. The table representative of the graph of Fig. 14 may be prepared according to tests on actual fuel cell systems. Instead of a minimum coolant conductivity from the start of the system, a minimum moving average may be used in consideration of the stability of data. Figure 15 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the fifth embodiment is stopped. In the fuel cell system according to the fifth embodiment several processes are carried out before starting the shutdown voltage decreasing operation. Namely, step S501 reads a conductivity of the coolant in the operating fuel cell stack 1 from time to time. Step S502 stores a minimum of the read coolant conductivities in a memory of the control unit 100. Namely, if the coolant conductivity read in step S501 is smaller than the previously stored value, step S502 stores the read value in the memory, and if the previously stored value is smaller than the read value, keeps the previously stored value in the memory. In this way, a minimum coolant conductivity from the start of the system is always updated and stored. Step S503 computes a moving average of the coolant conductivities read in step S501 and stores the computed result in the memory of the control unit 100. A period for which a moving average of coolant conductivity is computed is, for example, five minutes from the present time to five minutes before. Alternatively, an optimum period may be set according to the characteristics of the fuel cell stack 1 and system. This embodiment detects a coolant conductivity before the coolant is fed into the fuel cell stack 1. If the fuel cell system is of an internal humidifying type that directly supplies huimdi-fying water to the fuel cell stack 1, the conductivity of the hurm'difying water may be detected instead of the conductivity of the coolant In step S5O4, the shutdown controller 101 of the control unit 100 determines whether or not a system shutdown signal has been generated in response to, for example, a driver's key-off operation. If the system shutdown signal is present step S505 starts a shutdown process. More precisely, the shutdown controller 101 stops the compressor 12 in the air supply system to stop the supply of air to the fuel cell stack 1, opens the purge valve 11 in the hydrogen supply system, continues the supply of hydrogen to the fuel cell stack 1, and turns on the switch 42 of the internal load circuit to connect the internal load 41 to the fuel cell stack 1. With this shutdown process, oxygen remaining around the oxidant electrodes of the fuel cell stack 1 is consumed by power generation reaction with the continuously supplied hydrogen, and the generated power is consumed by the internal load 41. In step S506, the threshold setter 102 of the control unit 100 reads the minimum coolant conductivity from the start of the system stored in step S502 as well as the moving average of coolant conductivity stored in step S503. In step S507, the threshold setter 102 computes a difference between the read moving average of coolant conductivity and the read minimum coolant conductivity. This process can determine a coolant conductivity deterioration level based on a normal coolant conductivity. According to the difference computed in step S507 between the moving average of coolant conductivity and the minimum coolant conductivity, step S508 sets a threshold value to determine timing to disconnect the intemal load 41 from the fuel cell stack 1. Even if the coolant conductivity is relatively unstable in the system, or even if the coolant conductivity deteriorates due to aging, the fifth embodiment can set a proper threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1. In step S509, the internal load disconnection controller 103 of the control unit 100 obtains the voltage of the fuel cell stack 1, e.g., an average voltage of the power generation cells (average cell voltage) in the fuel cell stack 1 detected by the cell voltage monitor 2. In step S510, the intemal load disconnection controller 103 deteimines whether or not the voltage of the fuel cell stack 1 is equal to or below the threshold value set in step S508. If the voltage of the fuel cell stack 1 is equal to or below the threshold value, step S511 computes a voltage variation across the power generation cells of the fuel cell stack 1. In this example, a difference between a minimum voltage and a maximum voltage of the power generation cells detected by the cell voltage monitor 2 is used as a value representative of the cell voltage variation. The present invention is not limited to this. For example, a statistical technique may be employed to find a standard deviation of all or some power generation cells and use the standard deviation as a value representative of the cell voltage variation. Step S512 determines whether or not the cell voltage variation found in step S511 is equal to or below a predeteπnined value. If the cell voltage variation is equal to or below the predetermined value, step S513 turns off the switch 42 of the internal load circuit to disconnect the internal load 41 from the fuel cell stack 1. In step S514, the supply of hydrogen to the fuel cell stack 1 is stopped and the humidification unit and cooling mechanism are stopped to complete the shutdown voltage decreasing operation. As explained above, the fuel cell system according to the fifth embodiment conducts the shutdown voltage decreasing operation when the system is stopped, through the processes of setting, according to a coolant conductivity difference in a given period before stopping the system, a threshold value to deteπnine the timing of disconnecting the internal load 41 from the fuel cell stack 1, and when the voltage of the fuel cell stack 1 decreases to or below the threshold value and when a cell voltage variation in the fuel cell stack 1 becomes equal to or below a predeteimined level, disconnecting the internal load 41 from the fuel cell stack 1. This effectively prevents the voltage reincreasing phenomenon of the fuel cell stack 1 after the completion of the shutdown voltage decreasing operation and secures the catalytic power of the fuel cell stack 1. In particular, the fuel cell system of the fifth embodiment refers to a difference between an average of coolant conductivity in a given period before stopping the system and a minimum of coolant conductivity from the start of the system, and based on the difference, sets a threshold value to determine timing to disconnect the internal load 41 from the fuel cell stack 1. Even if the coolant conductivity in the system is relatively unstable, or even if the coolant conductivity deteriorates due to aging, the fifth embodiment can set a proper threshold value to deteπnine timing to disconnect the internal load 41 from the fuel cell stack 1. (Sixth embodiment) A fuel cell system according to a sixth embodiment of the present invention will be explained. The fuel cell system of this embodiment conducts the shutdown voltage decreasing operation without comparing the voltage of the fuel cell stack 1 (a total voltage of the power generation cells or an average cell voltage) with a threshold value. Namely, the sixth embodiment disconnects the internal load 41 from the fuel cell stack 1 when a voltage variation across the power generation cells in the fuel cell stack 1 exceeds a predetermined value. As mentioned above, the shutdown voltage decreasing operation stops the supply of air to the fuel cell stack 1 and continues only the supply of hydrogen thereto to consume oxygen remaining around the oxidant electrodes of the fuel cell stack 1. At this time, voltages of the power generation cells in the fuel cell stack 1 may vary from one to another. If the cell voltage variation is large, the voltage reincreasing phenomenon of the fuel cell stack 1 after the completion of the shutdown voltage decreasing operation will easily occur. To cope with this problem, the sixth embodiment relies on, like the second to fifth embodiments, whether or not a voltage variation across the power generation cells in the fuel cell stack 1 is equal to or below a predetermined value when deteπnining the timing of disconnecting the internal load 41 from the fuel cell stack 1. The second to fifth embodiments rely on not only the cell voltage variation but also whether or not the total voltage of the power generation cells in the fuel cell stack 1 or an average cell voltage of the fuel cell stack 1 decreases to a threshold value. On the other hand, the sixth embodiment refers to only the cell voltage variation. This is because, when the cell voltage variation is equal to or below a predetermined value, it can be assumed that the total voltage of the power generation cells in the fuel cell stack 1 or an average cell voltage of the fuel cell stack 1 is sufficiently low. Figure 16 is a flowchart showing the shutdown voltage decreasing operation carried out when the fuel cell system of the sixth embodiment is stopped. Step S601 deteimines whether or not a system shutdown signal has been generated in response to, for example, a driver's key-off operation. If the system shutdown signal is present step S602 starts a shutdown process. More precisely, the shutdown controller 101 stops the compressor 12 in the air supply system to stop the supply of air to the fuel cell stack 1, opens the purge valve 11 in the hydrogen supply system, continues the supply of hydrogen to the fuel cell stack 1, and turns on the switch 42 of the internal load circuit to connect the internal load 41 to the fuel cell stack 1. With this shutdown process, oxygen remai-ning around the oxidant electrodes of the fuel cell stack 1 is consumed by power generation reaction with the continuously supphed hydrogen, and the generated power is consumed by the internal load 41. Step S603 computes a voltage variation across the power generation cells of the fuel cell stack 1 according to voltages of the power generation cells detected by the cell voltage monitor 2. According to this embodiment a difference between a minimum voltage and a maximum voltage of the power generation cells detected by the cell voltage monitor 2 is used as a value representative of the cell voltage variation. The present invention is not limited to this. For example, a statistical technique may be employed to find a standard deviation of all or some power generation cells and use the standard deviation as a value representative of the cell voltage variation. Step S604 deteimines whether or not the cell voltage variation found in step S603 is equal to or below a predetermined value. If the cell voltage variation is equal to or below the predetermined value, step S605 turns off the switch 42 of the internal load circuit to disconnect the internal load 41 from the fuel cell stack 1. In step S606, the supply of hydrogen to the fuel cell stack 1 is stopped and the humidification unit and cooling mechanism are stopped to complete the shutdown voltage decreasing operation. As explained above, the fuel cell system according to the sixth embodiment conducts the shutdown voltage decreasing operation when the system is stopped. The shutdown voltage decreasing operation of the sixth embodiment disconnects the internal load 41 from the fuel cell stack 1 when a voltage variation across the power generation cells in the fuel cell stack 1 becomes equal to or below a predetermined value. This effectively prevents the voltage reincreasing phenomenon of the fuel cell stack 1 after the completion of the shutdown voltage decreasing operation and secures the catalytic power of the fuel cell stack 1. The fuel cell system of the sixth embodiment refers to only a cell voltage variation in the fuel cell stack 1 when deteimining the timing to disconnect the internal load 41 from the fuel cell stack 1. Namely, the sixth embodiment does not deteimine whether or not a total voltage of the power generation cells or an average cell voltage thereof decreases to a set value. For this, the sixth embodiment involves simplified processes and can reduce load on the control unit
100. In summary, the fuel cell system according to the present invention properly controls timing to disconnect an internal load from a fuel cell, to surely prevent the voltage reincreasing problem of the fuel cell after the disconnection of the internal load and secure the catalytic power of the fuel cell. The entire contents of Japanese patent application P2004-111855 filed April 6th, 2004 are hereby incorporated by reference. The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
INDUSTRIAL APPLICABILITY The following are examples of applications in which the present invention can be applied: fuel cell automobiles, railroad vehicles capable of traveling through areas where electric power service is not available, and stationary fuel cell systems.

Claims

What is claimed is: 1. A fuel cell system comprising: a fuel cell configured to receive a fuel gas and an oxidant gas, electiOchemically react the fuel gas with oxidant gas to generate electric power; an internal load configured to be electrically connectable to the fuel cell and consume the electric power generated by the fuel cell; a switch configured to electrically connect and disconnect the internal load with the fuel cell; a voltage detector configured to detect a voltage of the fuel cell; a shutdown controller configured to, when the fuel cell system is stopped, connect the internal load with the fuel cell through the switch, the shutdown controller configured to stop the supply of the oxidant gas to the fuel cell, continue the supply of the fuel gas to the fuel cell, and consume oxygen remaining in the fuel cell to decrease the voltage of the fuel cell; a threshold changer configured to change a threshold value according to a state of the fuel cell system at the time of stoppage of the fuel cell system; and a disconnection controller configured to electrically disconnect the internal load from the fuel cell through the switch when the voltage of the fuel cell detected by the voltage detector decreases to or below the threshold value.
2. The fuel cell system of claim 1, wherein: the threshold changer changes the threshold value according to one selected from the group consisting of a temperature of the fuel cell when the fuel cell system is stopped, a temperature of a coolant at an inlet of the fuel cell, and a temperature of the coolant at an outlet of the fuel cell.
3. The fuel cell system of claim 1, wherein: the threshold changer changes the threshold value according to one selected from the group consisting of maximum and average values of operating load of the fuel cell measured in a pi^eteimined period before the stoppage of the fuel cell system.
4. The fuel cell system of claim 1 , wherein: the threshold changer changes the threshold value according to one selected from the group consisting of maximum and average values of operating pressure of the fuel cell measured in a predetermined period before the stoppage of the fuel cell system.
5. The fuel cell system of claim 1 , further comprising: a cooler configured to cool the fuel cell with the use of a coolant wherein: the threshold changer changes the threshold value according to a conductivity of the coolant supplied to the fuel cell.
6. The fuel cell system of claim 5, wherein: the threshold changer changes the threshold value according to one selected from the group consisting of maximurn and average values of conductivity of the coolant in a predetermined period before the stoppage of the fuel cell system.
7. The fuel cell system of claim 5, wherein: the threshold changer changes the threshold value according to a difference between an average value of conductivity of the coolant in a predetermined period before the stoppage of the fuel cell system and a minirnum value of conductivity of the coolant after the start of the fuel cell system.
8. The fuel cell system of claim 5, further comprising: a humidifier configured to hurddify the oxidant gas with hurmdifying water, wherein: the threshold changer changes the threshold value according to a conductivity of humic-ifying water of the oxidant gas supphed to the fuel cell.
9. The fuel cell system of claim 8, wherein: the threshold changer changes the threshold value according to one selected from the group consisting of maximum and average values of conductivity of the hun- difying water of the oxidant gas in a predetermined period before the stoppage of the fuel cell system.
10. The fuel cell system of claim 8, wherein: the threshold changer changes the threshold value according to a change between an average conductivity of the oxidant gas in a predetermined period before the stoppage of the fuel cell system and a minimum conductivity of the oxidant gas from the start of the fuel cell system.
11. The fuel cell system of claim 1 , wherein: the fuel cell comprises a plurality of power generation cells arranged in multiple layers; and the voltage detector detects one selected from the group consisting of a total voltage and an average voltage of the power generation cells.
12. The fuel cell system of claim 11 , wherein: the disconnection controller electrically disconnects the internal load from the fuel cell through the switch if a voltage of the fuel cell detected by the voltage detector is equal to or below the threshold value and if a voltage variation across the power generation cells is equal to or below a predetermined value.
13. The fuel cell system of claim 12, wherein: the disconnection controller electrically disconnects the internal load from the fuel cell through the switch if a voltage of the fuel cell detected by the voltage detector is equal to or below the threshold value and if a difference between maximum and minimum voltages among the power generation cells is equal to or below a predeteimined value.
14. The fuel cell system of claim 12, wherein: the disconnection controller electrically disconnects the internal load from the fuel cell through the switch if a voltage of the fuel cell detected by the voltage detector is equal to or below the threshold value and if a standard deviation of voltages of the power generation cells is equal to or below a predetermined value.
15. A fuel cell system comprising: a fuel cell including a plurality of power generation cells stacked in multiple layers, configured to receive a fuel gas and an oxidant gas, electrochemically react the fuel gas and oxidant gas with each other, and generate power, an internal load configured to be electrically connected to the fuel cell and consume power generated by the fuel cell; a switch con-figured to electrically connect and disconnect the internal load with the fuel cell; a voltage detector configured to detect voltages of the power generation cells; a shutdown controller configured to, when the fuel cell system is stopped, connect the internal load with the fuel cell through the switch, stop the supply of the oxidant gas to the fuel cell, continue the supply of the fuel gas to the fuel cell, and consume oxygen remaining in the fuel cell to decrease the voltage of the fuel cell; and a disconnection controller configured to electrically disconnect the internal load from the fuel cell through the switch when a voltage variation across the power generation cells decreases to or below a predetermined value.
16. The fuel cell system of claim 15, wherein: the disconnection controller electrically disconnects the internal load from the fuel cell through the switch if a difference between maximum and minimum voltages among the power generation cells is equal to or below a pi^etermined value.
17. The fuel cell system of claim 15, wherein: the disconnection controller electrically disconnects the internal load from the fuel cell through the switch if a standard deviation of voltages of the power generation cells is equal to or below a predetermined value.
18. A fuel cell system comprising: a fuel cell for receiving a fuel gas and an oxidant gas, electrochemically reacting the fuel gas with oxidant gas to generate electric power; an internal load for being electrically connectable to the fuel cell and consuming the electric power generated by the fuel cell; a switch for electrically connecting and disconnecting the internal load with the fuel cell; a voltage detector for detecting a voltage of the fuel cell; a shutdown controller for, when the fuel cell system is stopped, connecting the internal load with the fuel cell through the switch, stop the supply of the oxidant gas to the fuel cell, the shutdown controller for continuing the supply of the fuel gas to the fuel cell, and consuming oxygen remaining in the fuel cell to decrease the voltage of the fuel cell; a threshold changer for changing a threshold value according to a state of the fuel cell system at the time of stoppage of the fuel cell system; and a disconnection controller for electrically disconnecting the internal load from the fuel cell through the switch when the voltage of the fuel cell detected by the voltage detector decreases to or below the threshold value.
19. The fuel cell system of claim 18, wherein: the threshold changer changes the threshold value according to one selected from the group consisting of a temperature of the fuel cell when the fuel cell system is stopped, a temperature of a coolant at an inlet of the fuel cell, and a temperature of the coolant at an outlet of the fuel cell.
20. The fuel cell system of claim 18, wherein: the threshold changer changes the threshold value according to one selected from the group consisting of maximum and average values of operating load of the fuel cell measured in a predeteimined period before the stoppage of the fuel cell system.
PCT/JP2005/006992 2004-04-06 2005-04-05 Fuel cell system WO2005099016A2 (en)

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