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
  • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
  • H01M8/04186—Arrangements for control of reactant parameters, e.g. pressure or concentration of liquid-charged or electrolyte-charged reactants
  • H01M8/04194—Concentration measuring 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
  • H01M8/04313—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
  • H01M8/0438—Pressure; Ambient pressure; Flow
  • H01M8/04388—Pressure; Ambient pressure; Flow of anode reactants at the inlet or inside the fuel cell
• • 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
  • H01M8/0444—Concentration; Density
  • H01M8/04447—Concentration; Density of anode reactants at the inlet or inside the fuel cell
• • 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
  • H01M8/04313—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
  • H01M8/04604—Power, energy, capacity or load
• • 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
  • H01M8/04701—Temperature
  • H01M8/04731—Temperature of other components of a fuel cell or 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
  • H01M8/04298—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
  • H01M8/04858—Electric variables
  • H01M8/04865—Voltage
  • H01M8/0488—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
  • H01M8/04858—Electric variables
  • H01M8/04925—Power, energy, capacity or load
• • 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
  • H01M8/10—Fuel cells with solid electrolytes
  • H01M8/1009—Fuel cells with solid electrolytes with one of the reactants being liquid, solid or liquid-charged
  • H01M8/1011—Direct alcohol fuel cells [DAFC], e.g. direct methanol fuel cells [DMFC]
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M2250/00—Fuel cells for particular applications; Specific features of fuel cell system
  • H01M2250/30—Fuel cells in portable systems, e.g. mobile phone, laptop
• • 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
• • 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
  • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
  • Y02B90/00—Enabling technologies or technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02B90/10—Applications of fuel cells in buildings
• • 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 that controls a solid polymer fuel cell using a liquid as a fuel .
• Polymer electrolyte fuel cells are known as fuel cells sometimes called proton exchange membrane fuel cells and using a polymer membrane having ion conductivity as an electrolyte.
• Polymer electrolyte fuel cells include direct methanol fuel cells (DMFCs) .
• DMFCs direct methanol fuel cells
• Efforts have been made to develop direct methanol fuel cells serving as small power sources for portable devices because the direct methanol fuel cell requires no auxiliary device such as a vaporizer or a humidifier, methanol can be handled more easily than a gas fuel such as hydrogen, and the direct methanol fuel cell can be operated at low temperatures.
• the direct methanol fuel cell includes a membrane electrode assembly (MEA) , an anode (also referred to as a fuel electrode) , and a cathode (also referred to as an air electrode) .
• MEA membrane electrode assembly
• the direct methanol fuel cell is classified into an active type using an auxiliary device to supply a fuel to the anode and the cathode, and a passive type utilizing a natural force for fuel supply.
• the active DMFC enables an increase in cell output density, but requires auxiliary devices such as a pump which supplies a fuel to the anode and the cathode.
• the passive DMFC eliminates the need for auxiliary devices but offers a low cell density.
• methanol crossover also referred to as methanol crossleak; hereinafter referred to as methanol crossover
• the methanol crossover depends on the concentration of the fuel supplied to the anode in a power generation unit.
• efforts have been made to set the concentration of the fuel supplied to the power generation unit, to a predetermined range.
• JP-A 2005-32610 proposes a system that controls the flow rate of a fuel from a high-concentration fuel tank and a cathode water recovery unit to control the concentration of a mixed liquid.
• JP-A 2005-11633 proposes a system that controls the flow rate of the fuel from the high- concentration fuel tank and the amount of water recovered from the cathode side to make the concentration of the mixed liquid appropriate.
• JP-A H05-258760 proposes a system that controls the flow rate of the fuel from the high-concentration fuel tank and the amount of water fed from a water tank to control the amount and concentration of the mixed liquid.
• JP-A 2005-108713 proposes a system which feeds air discharged from the cathode into a concentration unit and which operates a concentration fan and a water recovery unit to vary the amount of water supplied to a mixture tank to control the concentration and amount of the mixed liquid.
• JP-A 2005-285828 discloses a technique of measuring a steady-state voltage output when current is applied from a fuel cell to an electric load, reducing the current from the fuel cell to the electric load substantially to zero and measuring a no- load voltage a given time later, and determining the methanol concentration of a fuel supplied to DMFC based on the measured steady-state voltage output and the no- load voltage.
• JP-A 2003-308867 disclose a method of inspecting DMFC which method determines acceptability of a fuel cell by observing a temporal change in the voltage of an electromotive unit caused by a change in current density in the electromotive unit.
• the conventional fuel cell systems include a concentration sensor that senses the concentration, the water recovery unit on the cathode side which adjusts the concentration or a fuel supply unit on the anode side, a water recovery circulating path that connects the cathode side and the anode side together, and the water tank that externally supplies water.
• the concentration sensor has a non- negligibly large occupation volume, increasing the size of the whole apparatus.
• JP-A 2005-285628 and JP-A 2003- 308867 propose the techniques of estimating the concentration without any concentration sensor.
• these sensing techniques have room for improvement; the techniques need to be stable over a long period of time, and the time required to obtain concentration information needs to be reduced.
• each of the conventional concentration sensors including a control circuit board, has a large volume; the conventional concentration sensors are disadvantageously too large to incorporate into a portable, compact device such as a cellular phone, a portable audio device, or a notebook personal computer.
• a portable, compact device such as a cellular phone, a portable audio device, or a notebook personal computer.
• An object of the present invention is to provide a fuel cell system including a fuel cell which uses a liquid as a fuel, the system allowing the fuel cell to be stably operated at high efficiency for a long time.
• One aspect of the present invention provides a fuel cell system comprising: a fuel tank in which a liquid fuel is stored; a power generation unit including a membrane electrode assembly with an electrolyte membrane and an anode and a cathode arranged opposite each other across the electrolyte membrane, the fuel being supplied to the anode to react with air supplied to the cathode so that the power generation unit generates a power; a load adjustment unit which selectively connects a first load and a second load to the power generation unit; and a control unit configured to control a fuel supply from the fuel tank to the power generation unit, which includes a sensing unit sensing a voltage output from the power generation unit, a processing unit outputting a load adjustment signal to the load adjustment unit, and a database unit with a concentration conversion database and a flow rate control database, wherein the processing unit starts a control operation with reference to the flow rate control database in a state in which the first load is connected to the power generation unit, and outputs the load adjustment signal to the load adjustment unit to switch
• a fuel cell system comprising: a fuel tank in which a liquid fuel is stored; a power generation unit including a membrane electrode assembly with an electrolyte membrane and an anode and a cathode arranged opposite each other across the electrolyte membrane, the fuel being supplied to the anode to react with air supplied to the cathode so that the power generating unit generates a power; a load adjustment unit which selectively connects a first load and a second load to the power generation unit; and a control unit configured to control a fuel supply from the fuel tank to the power generation unit, which includes a sensing unit sensing a voltage output from the power generation unit, a processing unit outputting a load adjustment signal to the load adjustment unit, and a database unit with a concentration conversion database and a flow rate control database, with the first load connected to the power generation unit, the processing unit starting a control operation with reference to the flow rate control database, wherein the processing unit starts a control operation with reference to the flow rate control database in
• FIG. 1 is a block diagram schematically showing a fuel cell system according to a first embodiment of the present invention
• FIG. 2 is a cross sectional view schematically showing a stack structure in a power generation unit shown in FIG. 1 and a cross sectional view schematically showing a structure of each cell;
• FIG. 3 is a graph showing a voltage characteristic in the stack structure when a load adjustment unit shown in FIG. 1 is so switched as to produce a stepwise change of a load current supplied from the stack structure;
• FIG. 4 is a graph showing a measurement of a relationship between a methanol crossover current and a voltage difference ⁇ , which is produced from a crossover voltage;
• FIG. 5 is a flowchart showing a fuel supply control process executed by a control unit shown in FIG. 1;
• FIG. 6 is a flowchart showing a load variation response control loop in the control unit shown in FIG. 1;
• FIG. 7 is a graph illustrating a method of determining the voltage difference ⁇ V utilizing a minimum voltage value and a maximum voltage value in the system shown in FIG. 1;
• FIG. 8 is a graph showing a method of controlling a fuel supply amount based on the absolute value of the minimum voltage value in the system shown in FIG. 1;
• FIG. 9 is a block diagram schematically showing a fuel cell system according to a third embodiment of the present invention.
• FIG. 10 is a flowchart illustrating an operation in an open circuit voltage control loop in the systems shown in FIGS. 1 and 9;
• FIG. 11 is a block diagram schematically showing a fuel cell system according to a fifth embodiment of the present invention.
• FIG. 12 is a flowchart showing a fuel supply control process executed by a control unit shown in FIG. 11;
• FIG. 13 is a flowchart showing a process of controlling a fuel concentration in the fuel cell system shown in FIG. 11;
• FIG. 14A is a graph showing a relationship, in Example 1, between the supply fuel concentration and the voltage difference ⁇ V between the minimum voltage Vl and a steady-state voltage V2 obtained a given time later, both values being shown in FIG. 3, and a graph showing a relationship, in Example 2, between the supply fuel concentration and the voltage difference ⁇ V between the minimum voltage Vl and the maximum voltage V3, both values being shown in FIG. 7; and
• FIG. 14B is a graph showing a relationship, in Example 3, between a first minimum voltage value shown in FIG. 8 and the supply fuel concentration.
• FIG. 1 shows a configuration of a fuel cell system 1 according to a first embodiment of the present invention.
• the fuel cell system 1 compries a power generation unit 7 including a stack structure 10 as described with reference to FIG. 2 to generate power, a fuel tank 2 in which a fuel of a relatively high concentration containing such as a high-concentration methanol or a mixed solution of a methanol fuel and a small amount of water (a water solution of methanol) , and auxiliary devices 3 that support power generation in the power generation unit 7.
• the auxiliary devices 3 are composed of a mixture tank 5 in which a mixed solution of methanol and water having an optimum concentration for reaction in the power generation unit 7, a fuel supply unit 4 that feeds methanol or a mixed solution of methanol and water from the fuel tank 2 to the mixture tank 5, a fuel circulation unit 6 that feeds the mixed solution of methanol and water in the mixture tank 5 to an anode in the power generation unit 7 to return a fraction of the solution not used by the power generation unit 7 to the mixture tank 5 again, a load adjustment unit 8 which imposes a load on the power generation unit 7 to detect current provided to the load and which then adjusts the load to control output power from the power generation unit 7, and a control unit 9 that controls appropriate units in the auxiliary devices 3.
• the control unit 9 is composed of a sensing and processing unit 9a including a sensing unit that senses required information such as an output detection signal dependent on the output power from the power generation unit 7, a temperature detection signal dependent on a temperature of the power generation unit 7, and detection signals from the auxiliary devices 3, and a processing unit that provides operational instructions to the fuel supply unit 4, the power generation unit 7, and the load adjustment unit 8 to control these auxiliary devices 3, and a database unit 9b in which operational information is already stored which is required to set the operational instructions provided to the appropriate units based on the detection information including the detection signals detected by the appropriate units.
• the power generation unit 7 may further include a temperature control unit that controls a reaction temperature as described below with reference to a fifth embodiment. Based on a control instruction from the control unit 9, the temperature control unit may control the reaction temperature in the stack structure 10.
• the power generation unit 7 and the auxiliary devices 3 are connected by a fluid supply system.
• the fuel tank 2 and the fuel supply unit 4 are connected by a supply line Ll.
• the fuel supply unit 4 and the mixture tank 5 are connected by a supply line L2.
• a fuel of a relatively high concentration from the fuel tank 2 is supplied to the fuel supply unit 4 via the supply line Ll.
• the fuel from the fuel supply unit 4 is replenished in the mixture tank 5 via the supply line L2 as required.
• the mixture tank 5 is connected to the fuel circulation unit 6 through a supply line L3.
• the fuel circulation unit 6 is connected to the power generation unit 7 through a supply line L4.
• the power generation unit 7 is connected to the mixture tank 5 through a supply line L5.
• the fuel circulation unit 6 is operated to feed the fuel stored in the mixture tank 5 to the fuel circulation unit 6 via the supply line L3.
• the fed fuel is fed to the power generation unit 7 via the supply line L4 by the fuel circulation unit.
• a mixed solution and carbon dioxide (reaction product) which are not consumed by the power generation unit 7 are separated from each other.
• the mixed solution is fed to a line L5, and the carbon dioxide (reaction product) is emitted to the outside of the power generation unit 7.
• the mixed solution fed to the line L5 is returned into the mixture tank 5.
• the power generation unit 7 and the auxiliary devices 3 are connected by a signal and current wiring system.
• the control unit 9 is connected to the fuel supply unit 4 via a signal line Sl.
• the power generation unit 7 is connected to the control unit 9 through a signal line S2.
• the load adjustment unit 8 is connected to the control unit 9 through a signal line S3.
• the flow rate of a fuel fed from the fuel supply unit 4 to the mixture tank 5 is measured.
• Flow rate information including the measured flow rate signal is transmitted to the control unit 9 via the signal line Sl.
• the control unit 9 transmits a flow rate control instruction (flow rate setting signal) specifying the supply flow rate, to the fuel supply unit 4 via the signal line Sl.
• the fuel from the fuel supply unit 4 is fed to the mixture tank 5 according to the flow rate control instruction.
• An output voltage from the stack structure 10 in the power generation unit 7 and a temperature detected in the stack structure 10 are transmitted to the control unit 9 via the signal line S2 as voltage information (voltage signal) and temperature information (temperature signal) , respectively.
• the power generation unit 7 is connected to an external power source (external battery; not shown in the drawings) and to the load adjustment unit 8 via a current line S4.
• the external power source external battery
• the load adjustment unit 8 applies a load on the power generation unit 7 via the current line S4.
• a value of load current detected by the load adjustment unit 8 is transmitted to the control unit 9 via the signal line S3 as load current information (load current signal) .
• a load control instruction set by the control unit 9 is provided to the load adjustment unit 8 via the signal line S3 by the control unit 9.
• the load adjustment unit 8 connects a load corresponding to the set load specified according to the load control instruction, to the power generation unit 7.
• a load current flowing through the set load is detected and transmitted to the control unit 9 as load current information.
• the power generation unit 7 includes the stack structure 10 such as one shown in FIG. 2. With reference to FIG. 2, the stack structure 10 will be described in detail. As shown in FIG. 2, in the stack structure 10, a plurality of unit cells 16 are stacked and electrically connected in series between an anode power collection plate 12 and a cathode power collection plate 14. The unit cells 16 stacked between the anode power collection plate 12 and the cathode power collection plate 14 are arranged and fixedly tightened between a pair of tightening plates 18A and 18B using fixtures 19A and 19B. The anode power collection plate 12 and the cathode power collection plate 14 are connected to the load adjustment unit 8. Thus, current generated by the stack structure 10 is collected by the cathode power collection plate 14 and supplied to the load adjustment unit 8.
• the unit cell 16 includes an electrolyte membrane electrode assembly (hereinafter also referred to as MEA) 20.
• An anode channel plate 22 is provided on one side of the electrolyte membrane electrode assembly 20.
• a cathode channel plate 24 is provided on the other side of the electrolyte membrane electrode assembly 20.
• the electrolyte membrane electrode assembly 20 is sandwiched between the anode channel plate 22 and the cathode channel plate 24.
• a structure is thus formed in which the electrolyte membrane electrode assembly 20 is closed by a gasket 26 connected to the anode channel plate 22 and the cathode channel plate 24.
• the anode channel plate and the cathode channel plate are insulated from each other by the gasket 26.
• the gasket 26 also prevents the fuel and air from leaking from MEA to the exterior.
• an anode catalyst layer is formed on one side of the electrolyte membrane.
• a cathode catalyst layer is formed on the other side of the electrolyte membrane.
• a channel is formed in the anode channel plate opposite an MEA anode side so that a water solution of methanol used as a fuel flows through the channel.
• the water solution of methanol is fed to MEA via the channel.
• a gas resulting from reaction in MEA is discharged via the channel in the anode channel plate.
• a channel is formed in the cathode channel plate opposite an MEA cathode side so that air flows through the channel. Air is fed to MEA via the channel, and water resulting from reaction in MEA and passing through MEA is discharged via the channel in the cathode channel plate.
• the membrane electrode assembly is formed by coating a catalyst layer on opposite surfaces of a polymer electrolyte membrane to form a catalyst layer and joining a gas diffusion layer to the outside of the catalyst layer so as to perform smooth power collection, fuel supply, and discharge of a reaction product.
• a catalyst layer made of Nafion (a trade mark of Dupont) manufacture by Dupont may be used as a polymer electrolyte membrane.
• a commercially available Pt-Ru catalyst, a commercially available Pt catalyst, and the like may be used as an anode catalyst (anode catalyst layer) and a cathode catalyst (cathode catalyst layer) .
• Commercially available carbon paper, carbon fibers, or carbon nonwoven cloth may be used as a gas diffusion layer.
• the diffusion layer may include a micro porous layer made up of carbon and a water-repellent material.
• the anode channel plate 22 and the cathode channel plate 24 are provided to allow discharge of the fuel and product to the MEA anode catalyst layer and to allow supply of air and discharge of the product to the MEA cathode catalyst layer, respectively.
• the anode channel plate 22 and the cathode channel plate 24 may be optionally shaped.
• a serpentine channel plate may be used as the anode channel plate 22.
• a breathing channel plate allowing air from atmosphere to be directly taken in may be used as the cathode channel plate 24.
• a water solution of methanol is supplied to the channel in the anode channel plate 22 via the channel L4.
• Air is supplied to the channel in the cathode channel plate 24 by using a breathing scheme of taking air from the atmosphere directly into the channel in the cathode channel plate 24 instead of feeding air to the channel in the cathode channel plate 24 via another channel connected to the channel in the anode channel plate 24.
• a breathing scheme of taking air from the atmosphere directly into the channel in the cathode channel plate 24 instead of feeding air to the channel in the cathode channel plate 24 via another channel connected to the channel in the anode channel plate 24.
• the fuel circulation unit ⁇ is operated to supply a water solution of methanol of a predetermined concentration stored in the mixture tank 5, to the stack structure 10. Air is taken directly into the channel in the cathode channel plate 24 by means of breathing.
• the fuel permeates through the channel through which the fuel flows, to the anode catalyst layer.
• the cathode side of the cathode channel plate 24 is exposed to outside air, which permeates through the cathode catalyst layer.
• the load adjustment unit 8 is operated to activate the load connected to the stack structure 10. Then, the following reaction occurs in the anode catalyst layer, that is, on the anode side of MEA.
• the electrochemical reaction between the methanol and water generates carbon dioxide, hydrogen ions, and electrons.
• the following reaction occurs in the cathode catalyst layer, that is, on the cathode side of the membrane electrode assembly.
• the resulting water is absorbed to the cathode catalyst layer on the cathode side and emitted through the supplied air.
• Protons (H+) generated by the anode catalyst flow through the anode catalyst membrane and the polymer electrolyte membrane to the cathode catalyst layer. Electrons (e-) flow through the load adjustment unit 8.
• the carbon dioxide (C02) generated by the anode catalyst layer is emitted to the exterior of the stack structure 10 via the channel in the anode channel plate 22.
• gas-liquid separation mechanism is provided in the mixture tank 5, the supply line L5, or the stack structure 10. No reacted water solution of methanol is returned through the supply line L5 to the mixture tank 5 again, where the solution is mixed with the fuel fed from ' the fuel tank 2 via fuel supply unit4.
• the water (H2O) generated in the cathode catalyst layer is partly reversely diffused to the anode catalyst layer side through the polymer electrolyte membrane. The remaining fraction of the water is emitted to the atmosphere.
• the fuel cell system senses information resulting from crossover voltage to control the fuel concentration of the fuel supplied to the stack structure 10 so as to set power generation efficiency within a predetermined range.
• the voltage values need to be obtained with each of operation conditions such as temperature, load, fuel concentration, and fuel supply amount set to the same value.
• Sensing information includes a response characteristic of a voltage (hereinafter referred to as a stack voltage) output from the stack structure 10 when a load imposed on the stack structure 10 is varied as described above, such as a difference between a minimum voltage and a maximum voltage both output from the stack structure 10 as a result of the load variation, a difference between the minimum voltage and a steady-state voltage with a substantially equal value obtained after the minimum voltage, or the minimum voltage output from the stack structure 10 as a result of a variation in load.
• These voltage differences or the minimum voltage are measured to estimate the fuel concentration. Based on the estimated fuel concentration, the fuel concentration of the fuel supplied to the stack structure 10 is set back to the appropriate value or maintained at the appropriate value.
• protons migrate, and methanol and water pass through the polymer electrolyte membrane to migrate to the cathode catalyst layer side and the anode catalyst layer side, respectively.
• the transmitted methanol causes a crossover voltage in the cathode catalyst layer.
• the voltage output what is called the stack voltage
• the stack voltage the voltage output from the stack structure 10 lowers to reduce the power generation efficiency.
• fuel shortage occurs in an area in which fuel supply shortage is likely to occur. This similarly reduces the power generation efficiency.
• the crossover voltage needs to be sensed so that the amount of fuel supplied to the stack structure 10 can be controlled so as to set the power generation efficiency within the predetermined range.
• the response characteristic of the stack voltage such as one shown in FIG. 3 is utilized as information resulting from the crossover voltage as described below.
• FIG. 3 shows the response characteristic of the stack voltage observed when the load adjustment unit 8 switches the load to vary a load current I led out of the stack structure 10 from a load current Il to a load current 12 so that the change appears like a step.
• the stack voltage V thereafter reaches the maximum voltage value (Vraax) and then converges gradually to a substantially constant value (steady-state voltage V2) .
• the area A (A is the area of catalyst layer of each cell in stack 10) of the stack structure 10 has a given value.
• the current density i (il, i2) may be used, which is obtained by dividing the load current I (II, 12) by the area A.
• the load current I (II, 12) is used in the description below.
• One of the first and second loads includes a load with a zero load value.
• the other has a predetermined value .
• the point in time Tl is defined as a timing when the stack voltage exhibits the first minimum voltage Vl immediately after the load variation.
• the point in time T2 is defined as a point in time when after exhibiting the first minimum voltage Vl and then reaching the first maximum voltage Vmax, the stack voltage converges to the steady-state voltage V2.
• the point in time T2 can be optionally set provided that the point in time T2 is later than the point in time of first minimum voltage (minimum voltage Vl) and the point in time of the first maximum voltage value (Vmax) .
• the point in time T2 can normally be set between 10 to 60 seconds after the load variation.
• A denotes a given value.
• FIG. 4 is a graph of results of measurement of a relationship between the methanol crossover current I and the voltage difference ⁇ V, resulting from the crossover voltage.
• the axis of ordinate in FIG. 4 indicates the methanol cross over current density
• the fuel cell system 1 is uniquely set, the methanol cross over current and methanol cross over the current density can be uniquely converted into each other by multiplication by the given value A as described above.
• a methanol crossover current density corresponds to the methanol crossover current.
• the measured crossover current density in FIG. 4 is measured using a technique disclosed in a known document (J. Electrochem. Soc. 153, A543 (2006)). The results of measurement in FIG.
• FIG. 5 shows details of an operation performed by the control unit 9 to control the load adjustment unit 8 and the fuel supply unit 4.
• a concentration-flow rate control database (containing a control program) is stored in the database unit 9b to allow the control operation shown in FIG.
• the sensing and processing unit 9a references a voltage difference-concentration database (voltage difference- concentration conversion table) in the database unit 9b based on the output voltage detection signal, the temperature detection signal, and the like input to the sensing and processing unit 9a via the signal line S2, to perform the control operation described below.
• the operation performed by the sensing and processing unit 9b will be described below.
• the processing unit 9a starts the control operation by referencing the flow rate control database at a predetermined timing. Then, as shown in step S02, the sensing and processing unit 9a in the control unit 9 provides a load variation instruction to the load adjustment unit 8 via the signal line S3. Upon receiving the instruction, the load adjustment unit 8 executes a load variation process as shown in step S03 to change the load connected to the power generation unit 7. The sensing and processing unit 9a continues to monitor the voltage output from the power generation unit 7 via the signal line S2.
• the sensing and processing unit 9a sets the determined voltage difference ⁇ V to be a voltage parameter (voltage difference information) , and references the voltage difference-concentration conversion database in the database unit 9b as shown in step S04. Then, as shown in step S05, the sensing and processing unit 9a determines the methanol crossover voltage or current from the parameter of the voltage difference ⁇ V to estimate the fuel concentration of the fuel currently supplied to the power generation unit 7.
• step S05 where the fuel concentration is estimated from the voltage difference ⁇ V, the correlation between the voltage difference ⁇ V and the fuel concentration is varied under the effect environmental factors such as the temperature of the power generation unit 7 and a load value set for the load variation.
• the database unit 9b in the control unit 9 preferably includes, as a database, required correction information for the environmental factors which includes power generation unit temperature dependence data and load current value dependence data in order to allow a concentration D to be estimated from the voltage difference ⁇ V.
• the load adjustment unit 8 preferably includes a current sensor that detects current (load current) supplied to the selected load by the power generation unit 7.
• the power generation unit 7 preferably includes a temperature sensor to measure the temperature of the stack structure 10.
• a current signal from the current sensor and a temperature signal from the temperature sensor are input to the control unit 9 via the respective lines (not shown in the drawings) .
• the control unit 9 preferably references the power generation unit temperature dependence data and load current value dependence data in the database unit 9b based on the input temperature and current signals to derive a temperature correction coefficient and a current correction coefficient .
• the control unit 9 then uses the temperature and current correction coefficients to correct a concentration value in the voltage difference-concentration conversion table.
• the sensing and processing unit 9a estimates the concentration based on the database unit 9b. Then, as shown in step S07, the sensing and processing unit 9a transmits a specified flow rate control instruction to the fuel supply unit 4 via the signal line Sl according to the concentration information and a relationship between the concentration and the fuel flow rate stored in the database. Upon receiving the instruction, the fuel supply unit 4 performs control such that the fuel supply amount is set equal to a predetermined flow rate as shown in step S08. As a result, the concentration of the fuel supplied to the power generation unit 7 is estimated to enable control such that the concentration is set within the predetermined range.
• the method described above with reference to FIG. 5 estimates the concentration from the voltage difference ⁇ V to control the fuel supply amount based on the concentration value determined by the estimation.
• the crossover amount in the power generation unit can be controlled to within the predetermined range without the need to estimate the concentration.
• the voltage difference ⁇ V is sensed as information related directly to the possible crossover, and based on this value, the fuel supply amount is controlled so as to set the crossover within the predetermined range. This eliminates the need for step S05, where the concentration is estimated from the voltage difference ⁇ V, and for the correction information required to estimate the concentration.
• the technique of estimating the concentration from the voltage difference ⁇ V will be described below.
• the larger current value results in a larger voltage difference ⁇ V.
• the current value 12 is a rated current value required to obtain a predetermined power generation amount
• the current value Il can be set for no load (an open circuit voltage) or a very light load.
• the magnitude correlation between the current values Il and 12 may be inversed.
• an increase in the concentration D of the water solution of methanol stored in the mixture tank 5 increases the crossover voltage and thus the voltage difference ⁇ V.
• a decrease in the concentration D of the water solution reduces the crossover voltage and thus the voltage difference ⁇ V.
• the fuel supply unit 4 is operated so as to set the voltage difference ⁇ V within a predetermined range to control the flow rate of the supplied fuel from the fuel tank. This enables the possible crossover voltage to be set within a predetermined range, allowing the power generation efficiency to be controlled.
• control flow in which the flow rate of the supplied fuel from the fuel tank 2 is controlled so as to set the power generation efficiency within the predetermined range.
• the control flow will be described with reference to FIG. 6 showing details of a flowchart of load variation response control performed in the system.
• the system flow chart is prerecorded in the database unit 9b in the control unit 9, and the auxiliary devices are operated and controlled based on conditions in the flowchart.
• step SIl control is started so as to set the power generation efficiency within the predetermined range.
• a timer (not shown in the drawings) is set.
• step S12 the load current flowing through the load is set to II.
• step S13 the control unit 9 checks whether or not a time interval Tlim preset while the system is in operation has elapsed. If a measured time in the timer is shorter than the time interval Tlim, the process returns to step S12 again to wait for the time interval Tlim to elapse. Once the time interval Tlim elapses, the load adjustment unit 8 is operated to switch from the first load to the second load, and a load variation in the stack structure 10 is measured, as shown in step S14.
• a program in the sensing and processing unit 9a is set such that the load variation occurs periodically at constant time intervals Tlim while the fuel cell is in operation.
• the concentration of the fuel supplied to the power generation unit 7 is regularly estimated and controlled. The periodic estimation and control of the fuel concentration enables the system to operate at high power generation efficiency over a long time.
• the predetermined current value Il is set to flow to the load, which is maintained for a given time until the voltage time is stabilized.
• the current value is thereafter changed to 12.
• a response value from the stack structure 10 is monitored.
• the voltage difference ⁇ V is measured as shown in step S15, and the database unit 9b is referenced as shown in step S16.
• step S17 the voltage difference ⁇ V is compared with the predetermined range stored in the database unit 9b.
• step S17 if the voltage difference ⁇ V is within the predetermined range, since the crossover voltage is within the predetermined range, the control unit 9 determines the concentration of the fuel supplied to the power generation unit 7 is within the predetermined range.
• the control unit 9 uses the conversion table for the voltage difference ⁇ V and the concentration stored in the database unit 9b to determine whether or not the voltage difference ⁇ V is within the predetermined range.
• the condition for the fuel supply flow rate QO is also stored in the database unit 9b.
• step SIl the load is varied again as shown in step S14.
• step S15 the voltage difference ⁇ V is measured.
• the load variation may prevent the voltage V2 from being stabilized even after the elapse of the constant time interval Tlim.
• the temperature of the stack structure 10 is measured as a temperature signal.
• the control unit 9 controls a cooling fan (not shown in the drawings) so as to set the temperature within a predetermined range, to control the amount of air from the cooling fan and thus the temperature of the stack structure 10.
• the system according to the embodiment of the present invention measures the voltage difference ⁇ V, which correlates with the crossover voltage in the power generation unit 7, and control the fuel supply amount according to the information on the measured voltage difference ⁇ V.
• the control of the fuel supply amount based on the measurement of the voltage difference information is more accurate than control performed by a technique of using a concentration sensor to sense the fuel concentration in the mixture tank 5 and estimating the crossover voltage based on the sensed concentration condition to control the fuel supply.
• the amount of fuel introduced into the stack structure 10 may disadvantageously be varied by temporal degradation of the membrane electrode assembly or the like.
• the concentration of the fuel supplied to the power generation unit 7 needs to be varied over time depending on a variation in the fuel introduction amount, in order to allow the operation to be continued with the power generation efficiency maintained within the predetermined range.
• the system according to the embodiment of the present invention eliminates the need to estimate the crossover voltage based on the information on the fuel concentration in the mixture tank 5 but uses the crossover voltage value directly as measurement information. This facilitates dealing with the temporal variation.
• the system according to the embodiment of the present invention also eliminates the need for special components such as the concentration sensor. This enables a reduction in the size and costs of the system.
• the system according to the embodiment of the present invention determines the voltage difference ⁇ V, which is used as sensing information, based on the response obtained in an unsteady state in which the system is subjected to disturbance.
• This method can decrease a required measurement time compared to the case in which the steady-state voltage value is used.
• the voltage difference ⁇ V obtained immediately after the load variation is determined and detected by calculating the difference between the minimum voltage Vl and the voltage value (steady-state voltage value) V2 obtained the given time later.
• the power generation stack voltage exhibits the maximum voltage V3 (maximum point voltage value) .
• the voltage difference ⁇ V between the minimum voltage value (minimum point voltage Vl) and the maximum voltage value (maximum point voltage V3) is determined by V3-V1.
• the fuel supply amount is controlled based on the voltage difference information.
• the control method is the same as that described in Embodiment 1 and will not be described below. This technique further reduces the time required to determine the voltage difference ⁇ V, allowing the control to follow a variation quickly.
• the stack structure 10 is subjected to the load variation, and at the same time, the stack voltage starts to be monitored and recorded.
• the control unit 9 then internally determines the minimum voltage Vl and the maximum voltage V3 from the recorded voltage value to obtain the voltage difference ⁇ V.
• the control unit 9 estimates the fuel concentration condition of the fuel supplied to the power generation unit 7.
• the steady- state voltage V2 which is obtained after the load variation, that is, an increase in load, is unlikely to be affected by the concentration variation because of the insignificant adverse effect of the methanol crossover.
• the voltage difference ⁇ V is determined to depend almost on the minimum voltage Vl and replaced with ⁇ V (voltage difference) oc - Vl (minimum voltage value) .
• the detailed flow in the flowchart in FIG. 7 is similar to than that in FIG. 5 and will thus not be described below.
• This method avoids measuring the steady-state voltage V2 after the load has been varied. This enables a reduction in the time required for the load variation process, allowing controllability of the system to be improved.
• the method also enables a reduction in the number of sampled voltage data, allowing a reduction in a process load on the control unit 9 and an increase in processing speed.
• the method determines the steady-state voltage V2 to be constant regardless of the conditions. Thus, if the system is subjected to a significant concentration variation or is significantly affected by the external factors such as the temperature, an error is likely to occur. Thus, the method is desirably used in a system for which the possible adverse effects of the external factors can be predetermined.
• FIG. 9 shows a configuration of a fuel cell system according to the third embodiment. In FIG. 9, the same units and positions as those shown in FIG.
• the system 1 shown in FIG. 9 is composed of the stack structure 10, which includes the electrodes, the fuel tank 2, in which the fuel or the mixed solution of the fuel and a small amount of water is contained, and the auxiliary devices 3, which support the power generation unit 7.
• the auxiliary devices 3 are composed of the fuel supply unit 4, which feeds the methanol or the mixed solution of the methanol and water from the fuel tank 2 to the stack structure 10, the load adjustment unit 8, which senses the current value of the power generation unit 7 to lead the load out of the power generation unit 7, and the control unit 9.
• the control unit 9 is composed of the sensing and processing unit 9a, which provides the operational instructions to the power generation unit 7 and the load adjustment unit 8 to control these auxiliary devices 3, and the database unit 9b, in which the operational information is already stored which is required to set the operational instructions provided to the appropriate units based on the detection information including the detection signals detected by the appropriate units.
• the fuel tank 2 and the fuel supply unit 4 are connected by the supply line Ll.
• the fuel supply unit 4 and the stack structure 10 are connected by a supply line L6.
• the fuel supply unit 4 is operated to feed the fuel in the fuel tank 2 into the anode channel plate 22 of the stack structure 10. Air is similarly fed into the cathode channel plate 24 of the stack structure 10.
• the load adjustment unit 8 is operated.
• the stack structure 10 then starts generating power.
• the power generation unit 7 includes a gas-liquid separation unit (not shown in the drawings) in addition to the stack structure 10 to discharge a gas resulting from reaction to the exterior.
• the fuel in the fuel tank 2 is fed directly to the stack structure 10.
• the system shown in FIG. 9 does not include the mixture tank 5, which mixes the fuel in the mixture tank 5 with no reacted water solution of methanol from the stack structure 10 and to which the fuel from the fuel tank 2 is fed so that the concentration of the fuel is adjusted to the predetermined value, or a circulation mechanism (for example, the fuel circulation unit 6 in FIG. 1) which feeds the fuel from the mixture tank 5 to the power generation unit 7 to circulate the no reacted fuel to the mixture tank 5 again. Consequently, the system shown in FIG. 9 is simplified.
• a large amount of fuel fed from the fuel tank 2 to the stack structure 10 increases the crossover voltage in the stack structure 10.
• a small amount of supplied water solution of methanol reduces the crossover voltage in the stack structure 10. If the crossover voltage decreases below a predetermined value, fuel shortage occurs to disable power generation.
• the load is varied while the system is in operation.
• the crossover voltage is then estimated from the information value of the output response value ⁇ V (voltage difference) obtained from the stack structure 10 when the load is varied.
• the fuel supply unit 4 is then operated so as to set the output response value within the predetermined range. This improves the power generation efficiency of the system.
• Embodiments 1 and 2 For the technique of varying the load on the stack structure 10 and detecting the output response value ⁇ V resulting from the load variation, the method described above in Embodiments 1 and 2 can be utilized.
• the control flow in which the voltage difference ⁇ V is detected so that the fuel supply unit 4 is operated so as to set the voltage difference value within the predetermined range the corresponding method described in Embodiment 1 can be utilized.
• the system varying the load, measuring the voltage difference ⁇ V (voltage variation value) , and controlling the voltage variation value ⁇ V to within the predetermined range is in principle operated under the condition that the response of the voltage variation value ⁇ V is obtained.
• the load cannot be imposed, resulting in no load variation response.
• a control method is adopted such that in an environment in which control conditions are outside an appropriate range for a load variation response, the system is operated according to an open circuit voltage control loop that controls the fuel supply amount based on an open circuit voltage and such that after the environment is changed such that the control conditions are within the appropriate range, the system is shifted to a load variation response control loop described with reference to FIG. 6. Consequently, the system can be stably controlled.
• the control method of operating the system according to the open circuit voltage control loop is performed (1) when the system in a low temperature environment is started, (2) if in the flowchart shown in FIG. 6, the voltage difference ⁇ V is outside the predetermined range and smaller than a predetermined value, that is, the fuel concentration is extremely low, preventing the load from being varied, or (3) if any particular cells in the stack are subjected to a decrease in output voltage which has at least a given value compared to the other cells.
• FIG. 10 shows a flowchart in which the system is operated according to the open circuit voltage control loop. The operation of the system will be described with reference to FIG. 10.
• the system flowchart is pre-stored in the database unit 9b of the control unit 9 as a program. Based on the program, the sensing and processing unit 9a operates and controls the auxiliary devices according to the conditions.
• step S21 the operation is started.
• step S22 the system determines, based on information from the stack structure 10 and the auxiliary devices 3, whether or not the control conditions for the system are within the appropriate range for the load variation response. If the control conditions for the system are within the appropriate range, then as shown in step S23, the system is shifted to the load variation response control loop described with reference to FIG. 6.
• step S22 if the control conditions for the system are outside the appropriate range, then as shown in step S24, the load adjustment unit 8 is operated to measure the open circuit voltage.
• the open circuit voltage means that the load is not connected to the power generation unit 7 and that a terminal connecting the load to the power generation unit 7 is composed of an open circuit and that the voltage of the open terminal making up the open circuit for the power generation unit is measured.
• step S25 the control unit 9 controls the fuel supply unit 4 so as to maintain the amount of fuel supplied by the fuel supply unit 4 at the predetermined flow rate QO as shown in step S26. If in step S25, the open circuit voltage is outside the predetermined range, and in step S27, the voltage value is larger than a predetermined value, then as shown in step S28, the control unit 9 controls the fuel supply unit 4 so as to set the amount of fuel supplied by the fuel supply unit 4 to Qup, which is greater than the value of the predetermined flow rate QO.
• step S29 the control unit 9 controls the fuel supply unit 4 so as to set the amount of fuel supplied by the fuel supply unit 4 to Qlow, which is smaller than the value of the predetermined flow rate QO.
• step S22 the system determines again whether or not the control conditions are within the appropriate range for the load variation response. If in step S22, the control conditions are within the appropriate range for the load variation response, the system is shifted to the load variation response control loop, described with reference to FIGS. 5 and 6 and shown as step S23. If in step S22, the control conditions are outside the appropriate range, the system is shifted again to the open circuit voltage measurement loop, shown in steps S24 to S29.
• the control method is applicable to a system according to a fifth embodiment shown in FIG. 11.
• FIG. 11 the same units and positions as those shown in FIG. 1 are denoted by the same reference numerals as those shown in FIG. 1 and will thus not be described.
• the system shown in FIG. 11 is composed of the power generation unit 7, including the stack structure 10 with the electrodes and a temperature control unit 30 that controls the temperature of the stack structure 10, the fuel tank 2, which contains the fuel or the mixed solution of the fuel and a small amount of water, and the auxiliary devices 3, which support the power generation unit 7.
• the auxiliary devices 3 are composed of an on-off valve 40 that is opened and closed to start or stop feeding of methanol or the mixed solution of methanol and water from the fuel tank 2 to the fuel circulation unit 6, the fuel circulation unit 6 that feeds the mixed solution of methanol and water in the mixture tank 5 to the anode channel plate 22 of the power generation unit 7 to circulate the fraction of the solution not used by the power generation unit 7 again, the load adjustment unit 8, which senses the output value from the stack structure to lead the load out of the stack structure 10, and the control unit 9.
• the control unit 9 is composed of the sensing and processing unit 9a, which senses the output from power generation unit 7, the temperature, and the required information from the auxiliary devices to provide the operational instructions, and the database unit 9b in which information indicating what operational instruction to be given in connection with the sensed information.
• the temperature control unit 30 includes, for example, a cooling unit such as a fan which supplies air to the stack structure 10 and a sensor that detects the temperature of the stack structure 10.
• the fuel tank 2 and the on-off valve 40 are connected by the supply line Ll.
• the on-off valve 40 and the fuel circulation unit 6 are connected by supply lines L8 and L7.
• the fuel circulation unit 6 and the stack structure 10 are connected by the supply line L4.
• the stack structure 10 and the supply line L7 are connected by the supply line L5.
• the system shown in FIG. 11 does not include the mixture tank 5 so that the no reacted water solution of methanol is returned from the power generation unit 7 directly to the supply line L7 via the line L5.
• an amount of fuel equal to the volume by which the fuel in the stack structure 10 decreases is fed from the fuel tank 2 so as to always maintain the volume of the anode fuel solution circulating through the anode circulation line constant (L7, L4, and L5) .
• the on-off valve 40 remains open to feed the fuel to the supply line L7 so that the fuel flows from the supply line Ll to the supply line L7 via the supply line L8.
• the on-off valve 40 is closed for safety to block the feeding from the fuel tank 2 to the supply line L7 to prevent outflow of the fuel.
• an amount of fuel equal to the volume by which the fuel in the stack structure 10 decreases as a result of the reaction and crossover is fed from the fuel tank 2.
• the amounts of fuel and water consumed by the stack structure 10 are varied to control the concentration of the fuel supplied to the power generation unit 7.
• the temperature control unit 30 varies the temperature of the stack structure 10 to vary the crossover amounts of water and methanol.
• the water solution of methanol is consumed by the reaction and permeation (crossover) , and the volume of the water solution of methanol circulating through the anode circulation line correspondingly decreases.
• the fuel tank 2 is configured so as to exert a predetermined pressure on the on-off valve 40, if the volume of the water solution of methanol circulating through the anode circulation line decreases as a result of the consumption of the water solution of methanol in the power generation unit 7, an amount of fuel in the fuel tank 2 which is equal to the decrease is fed to the anode channel plate 22 side of the power generation unit 7 via the lines L8 and L7
• the volume of the water solution of methanol circulating through the anode circulation line is always maintained constant. However, if a balance between consumption of the fuel in the power generation unit 7 and feeding of the fuel from the high-concentration fuel tank is disrupted, the concentration of the water solution of methanol supplied to the power generation unit 7 varies. If the concentration is outside the predetermined range, the power generation efficiency decreases. Then, the fuel concentration is estimated by the method associated with the load variation and described with reference to FIG. 3, 7, or 8. According to a flowchart in FIG. 12, the temperature of the power generation unit 7 is controlled so as to set the fuel concentration within the predetermined range. The flowchart shown in FIG. 12 is the same as that shown in FIG.
• step SO ⁇ concentration-flow rate control database in step SO ⁇
• a concentration-temperature control database composed of a table indicating correlation between the concentration and the temperature of the power generation unit 7 and that the fuel supply control instruction in step S07 is replaced with a power generation unit temperature change instruction.
• the temperature control unit 30 Upon receiving the instruction, the temperature control unit 30 controls the temperature control unit 30 so as to set the temperature of the power generation unit 7 to a predetermined value as shown in step S08. As a result, the concentration of the fuel supplied to the power generation unit 7 is estimated, thus enabling control such that the temperature is set within the predetermined range.
• FIG. 13 shows a concentration control method in the fuel, cell system shown in FIG. 12.
• the fuel concentration is estimated from the output difference ⁇ V, that is, the magnitude of a methanol crossover current, obtained when the voltage is varied
• the system determines in step S32 whether or not the estimated concentration value is within the predetermined range. If the system determines in step S32 that the estimated concentration is within the predetermined range, the temperature control unit 30 is controlled so as to maintain the temperature of the power generation unit 7 as shown in step S34. If the system determines in step S32 that the estimated concentration is outside the predetermined range and that the concentration has decreased as shown in step S36, the temperature control unit 30 is controlled so as to increase the temperature of the power generation unit 7 as shown in step S38.
• ⁇ V that is, the magnitude of a methanol crossover current
• the fan is controlled to reduce the cooling performance of the power generation unit 7. If the system determines in step S36 that the concentration has not decreased, the temperature control unit 30 is controlled so as to reduce the temperature of the power generation unit 7 as shown in step S40. To reduce the temperature of the power generation unit 7, for example, the fan is controlled to increase the cooling performance of the power generation unit 7.
• a heater (not shown in the drawings) may be provided in the power generation unit 7 and activated to heat the power generation unit 7.
• the crossover amounts of methanol and water in the power generation unit 7 observed after the increase in the temperature of the power generation unit 7 are greater than those observed before the increase in temperature.
• the amount of supplied fuel from the high-concentration fuel tank 2 increases by the volume by which the fuel has been reduced by the crossover. Thus, the concentration of the fuel supplied to the power generation unit can be increased.
• a membrane electrode assembly was produced based on a known manufacturing method such as one disclosed in JP-A 2006-259503 (KOKAI) .
• the stack structure 10 into which the membrane electrode assembly was incorporated was prepared.
• a fuel circulation pump was used to supply a water solution of methanol controlled to a predetermined concentration, to the anode via the anode channel plate at a fuel flow rate of 0.5 cc/min. Air in the atmosphere was led into the cathode channel plate by a known method such as one disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2007-95581 by means of breathing.
• the temperature of the stack structure 10 was controlled by the cooling fan so as to be stabilized at a predetermined value.
• Power was generated by a constant-current operation with a rated load value of 1.0 A.
• the temperature of the stack structure 10 was measured by inserting a thermocouple into a central portion of the stack structure 10. This setup was used to examine a relationship between the concentration of the fuel supplied to the power generation unit 7 and the load variation response result.
• the value of the output voltage during the increase in load from 0.1 A to 1.0 A was monitored in a 10-Hz time series to measure the voltage difference ⁇ V. As described with reference to FIG. 3, the voltage difference ⁇ V between the minimum voltage Vl and the steady-state voltage V2 obtained a given time later was measured. A relationship between the voltage difference ⁇ V and the concentration of the supplied fuel was then measured. This example was defined as Example 1.
• a relationship between the voltage difference ⁇ V and the concentration of the supplied fuel was then measured.
• This example was defined as Example 2.
• Example 3 a relationship between the first minimum voltage value described with reference to FIG. 8 and the concentration of the supplied fuel was measured.
• FIGS. 14A and 14B show the results of measurements in the above-described examples.
• a correlation was established between the concentration of the fuel supplied to the power generation unit 7 and the voltage difference ⁇ V.
• the results of the measurements thus indicated the effect of sensing the concentration of the fuel supplied to the power generation unit 7, in the output response obtained when the load was varied.
• the fuel cell system according to the present invention is characterized in that the concentration is estimated based on the output response from the power generation unit.
• the present invention can thus provide a small, stable concentration management system eliminating the need for a concentration sensor. As a result, the fuel cell system is stably operated at high power generation efficiency over a long time.

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• 2008
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