FUEL CELL SYSTEM

Abstract

Fuel cell system (100), comprising: a fuel cell (10); a voltage detector (310) configured to detect an output voltage of the fuel cell (10); a current detector (320) configured to detect an output current of the fuel cell (10), an AC signal supply unit (330) configured to supply an AC signal to electrodes at both ends of the fuel cell (10); a phase difference calculation unit (210) configured to calculate a phase difference between the detected alternating current and the detected alternating voltage based on a detected alternating voltage detected by the voltage detector (310) and a detected alternating current detected by the current detector (320); and an estimation unit (220) configured to estimate, in accordance with the phase difference, a distribution characteristic amount of electric power generation representing a distribution of electric power generation in a cell surface (11s) of the fuel cell (10) using a predetermined relationship between the distribution characteristic amount of electric power generation and the phase difference, wherein the distribution characteristic amount of electric power generation includes a value indicating a difference between a maximum value and a minimum value of a local current density in the cell surface (11s).

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The present disclosure relates to a fuel cell system. 2. Description of the related art

[0002] A fuel cell includes a membrane-electrode assembly formed with an electrolyte membrane and two electrode-catalyst layers sandwiching the electrolyte membrane. Electric power is generated on a surface-like power generation region of the membrane-electrode assembly. In this specification, such a power generation region is referred to as a "cell surface." If the cell surface has too discrepant an electric power generation distribution, efficient electric power generation is difficult to achieve. Accordingly, techniques for measuring the electric power generation distribution in the cell surface have been conventionally developed.

[0003] JP 2006-318 784 A discloses a fuel cell system including a measuring device for measuring a current density distribution in the cell surface of a fuel cell. The fuel cell system includes a current density sensor specifically designed to measure the current density distribution in the cell surface. The current density sensor measures the current density distribution. DE 10 2018 107 175 A1 discloses a fuel cell system comprising a fuel cell stack configured to include a plurality of cells and output a direct current output, a signal superposition unit configured to superimpose an alternating current signal, the current value of which is controlled, onto the output current, wherein the alternating current signal includes alternating current components having a first frequency and a second frequency higher than the first frequency, and a voltage measurement unit configured toto measure a voltage value of the output current to which the alternating current signal has been applied, a phase difference detection unit configured to detect a phase difference between a phase of the first frequency of the current value in the alternating current signal and a phase of the first frequency of the current value measured by the voltage measuring unit, a calculation unit configured to calculate a water amount in at least one of the plurality of cells using the alternating current signal, and a use determination unit configured to determine not to use a calculation result obtained by the calculation unit when at least one condition that an absolute value of the phase difference is greater than a first absolute value and a condition that the absolute value of the phase difference is smaller than a fourth absolute value are met, and to determine,to use the calculation result obtained by the calculation unit when the absolute value of the phase difference lies within a range between a second absolute value that is smaller than the first absolute value and a third absolute value that is greater than the fourth absolute value. Another fuel cell system is the subject of JP 2008 - 103 257 A. SUMMARY OF THE INVENTION

[0004] However, conventional techniques require a dedicated current density sensor to obtain the electrical power generation distribution across the cell surface. This complicates the fuel cell structure and increases costs. Under these circumstances, there is a need for techniques that can estimate the electrical power generation distribution across the cell surface without requiring a current density sensor.

[0005] For efficient power generation in a fuel cell, it is desirable to appropriately control the fuel cell's humidity. Since the amount of water in the fuel cell is estimated and the humidity is controlled according to the previously estimated amount of water, the estimation process or humidity control is complicated. Accordingly, there is a demand for techniques that can appropriately control the fuel cell's humidity using a simpler method than conventional methods.

[0006] The present invention is practicable according to the following aspects.

[0007] According to one aspect of the present invention, a fuel cell system is provided. The fuel cell system includes: a fuel cell; a voltage detector; a current detector; an AC signal supply unit; a phase difference calculation unit; and an estimation unit. The voltage detector is configured to detect an output voltage of the fuel cell. The current detector is configured to detect an output current of the fuel cell. The AC signal supply unit is configured to supply an AC signal to electrodes at both ends of the fuel cell. The phase difference calculation unit is configured to calculate a phase difference between the detected AC current and the detected AC voltage based on a detected AC voltage detected by the voltage detector and a detected AC current detected by the current detector.The estimation unit is configured to estimate, in accordance with the phase difference, an electric power generation distribution characteristic amount representing a distribution of electric power generation in a cell surface of the fuel cell, using a predetermined relationship between the electric power generation distribution characteristic amount and the phase difference. The electric power generation distribution characteristic amount includes a value indicating a difference between a maximum value and a minimum value of a local current density in the cell surface.

[0008] The inventor of this application found that there is a correlation between the electric power generation distribution magnitude, which indicates the electric power generation distribution in the cell surface of a fuel cell, and a phase difference between the detected alternating current and the detected alternating voltage. Specifically, the inventor of this application found that there is a predetermined relationship between a difference between the maximum and minimum values ​​of the local current density in the cell surface and the phase difference. Based on this relationship, the fuel cell system estimates an electric power generation distribution magnitude in accordance with the phase difference. Therefore, the fuel cell system can estimate the electric power generation distribution in the cell surface of the fuel cell without providing a current density sensor.

[0009] In the fuel cell system, the cell surface may have a surface-integrated cathode gas passage from an inlet to an outlet of the cell surface for a cathode gas flowing in the cell surface.When the surface-integrated cathode gas passage is divided into an upstream section on the inlet side and a downstream section on the outlet side, the electric power generation distribution severity amount may indicate whether the electric power generation distribution in the cell surface corresponds to a first electric power generation distribution in which a maximum value of the local current density exists in the upstream section and a minimum value of the local current density exists in the downstream section, or corresponds to a second electric power generation distribution in which the maximum value of the local current density exists in the downstream section and the minimum value of the local current density exists in the upstream section.

[0010] In the fuel cell system, the electric power generation distribution magnitude amount estimated in accordance with the phase difference indicates whether the electric power generation distribution corresponds to the first electric power generation distribution or the second electric power generation distribution.

[0011] Accordingly, it is possible to estimate the distribution of electric power generation in the cell surface of the fuel cell without providing a current density sensor.

[0012] In the fuel cell system, the electric power generation distribution characteristic amount may indicate whether the electric power generation distribution in the cell surface corresponds to the first electric power generation distribution or the second electric power generation distribution with a positive or negative sign of the electric power generation distribution characteristic amount, respectively.

[0013] The fuel cell system distinguishes whether the electrical power generation distribution corresponds to the first electrical power generation distribution or the second electrical power generation distribution with a positive or negative sign of the electrical power generation distribution magnitude. Therefore, the fuel cell system can easily estimate an electrical power generation distribution based on the electrical power generation distribution magnitude.

[0014] The fuel cell system may further include an operating condition control unit configured to, when the electric power generation distribution amount estimated based on the phase difference is outside a predetermined allowable range, change an operating condition of the fuel cell system such that the electric power generation distribution amount falls within the allowable range.

[0015] In the fuel cell system, when the distribution characteristic amount of electric power generation, which represents a difference between the maximum value and the minimum value of the local current density in the cell surface, leaves an allowable range, the fuel cell system can be controlled under an appropriate operating condition such that the distribution characteristic amount of electric power generation falls within the allowable range.

[0016] In the fuel cell system, the estimation unit may estimate, in accordance with the phase difference, whether a humid state of the fuel cell is a first state in which a humidity of the fuel cell is lower than a predetermined humidity range, a second state in which the humidity of the fuel cell is within the humidity range, or a third state in which the humidity of the fuel cell is higher than the humidity range.

[0017] Since the fuel cell system can estimate whether the fuel cell is in one of three states related to the humidity of the fuel cell system according to a phase difference, appropriate control can be performed according to the estimation.

[0018] The fuel cell system may further include an operating condition control unit configured to, when it is estimated that the fuel cell is in the first state or the third state, change an operating condition of the fuel cell system such that the fuel cell is placed in the second state.

[0019] In the fuel cell system, when the fuel cell is in the first state in which the humidity of the fuel cell is low or is in the third state in which the humidity is high, the fuel cell system may be appropriately controlled to be in the second state.

[0020] According to another aspect of the present invention, a fuel cell system is provided. The fuel cell system includes: a fuel cell; a voltage detector; a current detector; an AC signal supply unit; a phase difference calculation unit; and an operating condition control unit. The voltage detector is configured to detect an output voltage of the fuel cell. The current detector is configured to detect an output current of the fuel cell. The AC signal supply unit is configured to supply an AC signal to electrodes at both ends of the fuel cell. The phase difference calculation unit is configured to calculate a phase difference between the detected AC current and the detected AC voltage based on a detected AC voltage detected by the voltage detector and a detected AC current detected by the current detector.The operating condition control unit is configured to control a humid state of the fuel cell by changing an operating condition of the fuel cell system in accordance with the phase difference. When the phase difference is a second value greater than a first value, the operating condition control unit sets the operating condition of the fuel cell system to an operating condition in which a humidity of the fuel cell is lower than a humidity of the fuel cell when the phase difference is the first value.

[0021] In the fuel cell system, the humidity of the fuel cell is controlled according to the phase difference. Therefore, the humidity of the fuel cell can be appropriately controlled using a simpler process than the conventional one.

[0022] The present invention can be implemented in various aspects. For example, the present invention can be implemented in the aspect other than the fuel cell system, such as a controller of the fuel cell system, a control method, or the like. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Features, advantages, and technical and economic significance of exemplary embodiments of the invention are described below with reference to the accompanying drawings, in which like reference numerals designate like elements and in which: Fig. 1 is an explanatory view showing the configuration of a fuel cell system of an embodiment; Fig. 2 is an explanatory view showing an apparatus configuration concerning estimation of electric power generation distribution in a cell surface; Fig. 3 is a timing chart showing a relationship between a detected alternating current and a detected alternating voltage; Fig. 4 is a diagrammatic view showing an example of a first electric power generation distribution in a cell surface; Fig. 5 is a diagrammatic view showing an example of a second electric power generation distribution in the cell surface; Fig. 6 is a diagrammatic view showing a relationship between a phase difference of current and voltage and a distribution amount of electric power generation; and Fig. 7 is a flowchart showing a method for measuring the phase difference and switching operation modes. DETAILED DESCRIPTION OF EMBODIMENTS

[0024] Fig. 1 is an explanatory view showing the configuration of a fuel cell system 100 according to an embodiment. The fuel cell system 100 is mounted on a vehicle 110, for example. The fuel cell system 100 outputs electric power in response to a driver's request, which serves as a driving power source of the vehicle 110.

[0025] The fuel cell system 100 includes a fuel cell stack 10, a controller 20, a cathode gas supply unit 30, an anode gas supply unit 50, a DC-DC converter 80, a power control unit (hereinafter referred to as "PCU") 81, a load 82, and a secondary battery 83. The controller 20 is configured as one or more ECUs including a CPU, a memory, and an interface. The controller 20 performs control of the fuel cell system 100 by executing computer programs stored in the memory. However, some or all of the functions of the controller 20 can be performed by hardware circuitry.

[0026] The fuel cell stack 10 has a stack structure formed from a stack of a plurality of single cells. Although not shown, each of the single cells 11 includes a membrane-electrode assembly having an electrolyte membrane and an anode electrode catalyst layer and a cathode electrode catalyst layer disposed on both sides of the electrolyte membrane. Each of the single cells 11 also includes a set of separators for supporting the membrane-electrode assembly. The electrolyte membrane is, for example, a solid polymer membrane with proton conductivity. The single cell 11 generates electric power upon receiving anode gas and cathode gas supplied as reactant gas. In the present embodiment, hydrogen gas is used as the anode gas, and air is used as the cathode gas. Note that other types of anode gas and cathode gas are also usable.

[0027] The cathode gas supply unit 30 includes a cathode gas line 31, an atmospheric pressure sensor 37, an air flow meter 32, a compressor 33, a first on-off valve 34, a flow dividing valve 36, a cathode exhaust line 41, and a first regulator 42.

[0028] The atmospheric pressure sensor 37 is provided in the cathode gas line 31 to measure an inlet pressure of the cathode gas line 31. The air flow meter 32 is provided in the cathode gas line 31 to measure the flow rate of intake air. The compressor 33 is connected to the fuel cell stack 10 through the cathode gas line 31. Under the control of the controller 20, the compressor 33 compresses the cathode gas sucked in from the outside and supplies the compressed cathode gas to the fuel cell stack 10.

[0029] The first on-off valve 34 is provided between the compressor 33 and the fuel cell stack 10. The first on-off valve 34 is opened and closed under the control of the controller 20. The flow dividing valve 36 is provided between the compressor 33 and the cathode exhaust line 41 to divide the amount of air flowing into the fuel cell stack 10 and the cathode exhaust line 41 under the control of the controller 20.

[0030] The cathode exhaust line 41 discharges the cathode exhaust gas discharged from the fuel cell stack 10 to the exterior of the fuel cell system 100. The first regulator 42 is a backpressure control valve that regulates the pressure at a cathode gas outlet of the fuel cell stack 10.

[0031] The anode gas supply unit 50 includes an anode gas line 51, an anode gas tank 52, a second on-off valve 53, a second regulator 54, injectors 55, a pressure sensor 56, an anode exhaust gas line 61, a gas-liquid separator 62, an outlet valve 63, a circulation line 64, and an anode gas pump 65.

[0032] The anode gas line 51 is a supply passage for supplying anode gas to the fuel cell stack 10. The anode gas tank 52 is connected to an anode gas inlet manifold of the fuel cell stack 10 through the anode gas line 51 to supply anode gas to the fuel cell stack 10. The second on-off valve 53, the second regulator 54, and the injectors 55 are provided in this order from an upstream side in the anode gas line 51. The second on-off valve 53 is opened and closed by the control of the controller 20. The second regulator 54 regulates the pressure of the anode gas on the upstream side of the injectors 55. The pressure sensor 56 is provided on the downstream side of the injectors 55 in the anode gas line 51. The pressure sensor 56 measures a pressure value downstream of the injectors 55.

[0033] The injectors 55 are on-off valves that are electromagnetically controlled in accordance with a drive period or valve opening time set by the controller 20. The injectors 55 regulate the supply amount of anode gas supplied to the fuel cell stack 10. In the present embodiment, the injectors 55 are provided in the anode gas line 51. The controller 20 controls the amount of anode gas supplied to the fuel cell stack 10 by controlling the drive period or valve opening time of the injectors 55 such that a measured value of the pressure sensor 56 does not become lower than a target pressure value. The target pressure value is determined in accordance with the electric power demanded by the fuel cell stack 10.

[0034] The anode exhaust line 61 connects an anode exhaust outlet of the fuel cell stack 10 and the cathode exhaust line 41. The anode exhaust line 61 is a discharge passage for discharging anode exhaust from the fuel cell stack 10. The anode exhaust includes anode gas or nitrogen gas that is not used in a power generation reaction.

[0035] The gas-liquid separator 62 is provided in the anode exhaust line 61. The gas-liquid separator 62 separates water as impurities from the anode gas discharged from the fuel cell stack 10 and stores the separated water.

[0036] The exhaust valve 63 is an on-off valve provided in the anode exhaust line 61. The exhaust valve 63 is provided vertically below the gas-liquid separator 62. The exhaust valve 63 is opened and closed by the controller 20. When the exhaust valve 63 is opened, the water in the gas-liquid separator 62 and then the anode exhaust gas are discharged from the exhaust valve 63. The water and the anode exhaust gas discharged from the exhaust valve 63 are discharged to the outside through the cathode exhaust line 41, which is connected to the anode exhaust line 61.

[0037] The circulation line 64 connects a line section on the downstream side of the injectors 55 in the anode gas line 51 and the gas-liquid separator 62. The circulation line 64 is equipped with the anode gas pump 65. The anode gas pump 65, which is controlled by the controller 20, pumps the anode exhaust gas, which is separated from water by the gas-liquid separator 62, to the anode gas line 51. In the fuel cell system 100, the anode exhaust gas containing the anode gas is circulated and recirculated to the fuel cell stack 10 to increase the utilization efficiency of the anode gas.

[0038] The DC-DC converter 80 increases the voltage output from the fuel cell stack 10 and supplies the voltage to the PCU 81. The PCU 81 includes an inverter and supplies electrical power to the load 82, such as a traction motor for driving a wheel, through the inverter. The PCU 81 also controls an output current of the fuel cell stack 10 based on a command from the controller 20.

[0039] The secondary battery 83 is connected to the fuel cell stack 10 through the PCU 81 and the DC-DC converter 80. The secondary battery 83 stores the electric power generated by the fuel cell stack 10. The secondary battery 83, together with the fuel cell stack 10, also functions as an electric power supply source in the fuel cell system 100. The electric power of the secondary battery 83 is supplied to the load 82, the compressor 33, the anode gas pump 65, and various valves. A lithium-ion battery, a nickel-hydrogen battery, or the like can be used as the secondary battery 83.

[0040] The controller 20 controls each unit of the fuel cell system 100 in accordance with an electric power request and controls an output of the fuel cell stack 10 or the secondary battery 83. The electric power request includes an external electric power generation request and an internal electric power generation request. The external electric power generation request is made by the driver or the like of the vehicle 110 on which the fuel cell system 100 is mounted. The internal electric power generation request is made to supply electric power to the auxiliary machines of the fuel cell system 100.

[0041] Fig. 2 is an explanatory view showing a device configuration related to estimating an electric power generation distribution in the cell surface. Here, a cell surface 11s of the single cell 11 is shown. The fuel cell stack 10 has a plurality of manifolds formed to pass through the single cells 11. The manifolds include an anode gas supply manifold 12in, an anode gas discharge manifold 12out, a cathode gas supply manifold 14in, and a cathode gas discharge manifold 14out. Anode gas AG supplied by the anode gas supply unit 50 is distributed to each of the single cells 11 through the anode gas supply manifold 12in and discharged through the anode gas discharge manifold 12out. The cathode gas CG supplied by the cathode gas supply unit 30 is distributed to each of the individual cells 11 through the cathode gas supply manifold 14in and discharged through the cathode gas discharge manifold 14out.Other distributors are designed to supply and discharge a coolant.

[0042] Each of the individual cells 11 has a surface-integrated anode gas passage AP and a surface-integrated cathode gas passage CP extending in the cell surface 11s. In this example, the surface-integrated anode gas passage AP has a plurality of linear sections extending linearly in a longitudinal direction and curved sections provided between the adjacent linear sections. Overall, the surface-integrated anode gas passage AP has a meandering upward shape. The surface-integrated cathode gas passage CP has a linear shape extending downward from an upper end to a lower end of the cell surface 11s. Therefore, the flow of the anode gas AG and the flow of the cathode gas CG in the cell surface 11s are formed as orthogonal flows that are orthogonal to each other.It should be noted that the surface-integrated cathode gas passage CP and the surface-integrated anode gas passage AP may be formed in shapes other than those disclosed. For example, they may be formed as opposing streams. Both in the case of orthogonal streams as in . Fig. 2 and the case of opposite currents, an inlet of the surface-integrated cathode gas passage CP is present near an outlet of the surface-integrated anode gas passage AP, and an outlet of the surface-integrated cathode gas passage CP is present near an inlet of the surface-integrated anode gas passage AP. Note that the inlet of the surface-integrated cathode gas passage CP is an inlet of the cell surface 11s, and the outlet of the surface-integrated cathode gas passage CP is an outlet of the cell surface 11s.

[0043] The fuel cell stack 10 is connected to a voltage detector 310 that detects an output voltage of the fuel cell stack 10, and a current detector 320 that detects an output current of the fuel cell stack 10. The electrodes at both ends of the fuel cell stack 10 are connected to an AC signal supply unit 330 that supplies an AC signal. The AC signal is an input signal for measuring the impedance of the fuel cell stack 10 using an AC impedance method. Although both an AC current and an AC voltage can be used as the AC signal, the AC current is used in the present embodiment.

[0044] A detected AC voltage Vd detected by the voltage detector 310 and a detected AC current Id detected by the current detector 320 are input to a phase difference calculation unit 210. The phase difference calculation unit 210 has a function of calculating a phase difference Δθ between the detected AC current Id and the detected AC voltage Vd. The calculation can be performed, for example, by fast Fourier transform processing. The phase difference Δθ is input from the phase difference calculation unit 210 to an estimation unit 220. The estimation unit 220 has a function of estimating, in accordance with the phase difference Δθ, an electric power generation distribution magnitude amount representing an electric power generation distribution in the cell surface 11s of the fuel cell stack 10. The estimation function will be described later.

[0045] In the example of Fig. 2, the voltage detector 310 measures the voltage of the entire fuel cell stack 10. Instead of the entire fuel cell stack 10, the voltage detector 310 may measure the voltage of one or more individual cells 11 that form some of the components of the fuel cell stack 10. In this document, the term "fuel cell" is understood in a broader sense to refer to both the entire fuel cell stack 10 and one or more individual cells 11 that form some of the components of the fuel cell stack 10.

[0046] The system of Fig. 2 further includes an operating condition control unit 230. The operating condition control unit 230 controls an operating condition of the fuel cell system 100 in accordance with the phase difference Δθ calculated in the phase difference calculation unit 210. Specific examples of the control will be described later. The phase difference calculation unit 210, the estimation unit 220, and the operating condition control unit 230 may be configured to operate in the manner shown in Fig. 1 shown controller 20 are included.

[0047] Fig. Figure 3 is a timing chart showing a relationship between the detected AC current Id and the detected AC voltage Vd. Here, Fig. 3 shows the detected alternating current Id and the detected alternating voltage Vd, which are detected by superimposing an alternating current component ΔI on a direct current component Iconst, which is an output current of the fuel cell 10, by the alternating current signal supply unit 330. The alternating current component ΔI is a sine wave with a frequency f and an angular frequency ω (= 2πf). Waveforms other than the sine wave, such as a triangular wave, a rectangular wave, and a pulse wave, can also be used as the waveform of the alternating current signal superimposed by the alternating current signal supply unit 330.

[0048] As is well known, a frequency range used in the AC impedance method is divided into a low-frequency range in which the real part of an impedance is equal to the sum of a proton displacement resistance and a gas reaction resistance, and a high-frequency range in which the real part of the impedance is equal to the proton displacement resistance. In the present embodiment, it is preferable to set the frequency f of the AC signal to a value in the low-frequency range. Specifically, it is preferable to set the frequency f to a value of 1 Hz or more and 150 Hz or less. It is even more preferable to set the frequency f to a value of 1 Hz or more and below 100 Hz. It is most preferable to set the frequency f to a value of 20 Hz or more and 80 Hz or less.In the present embodiment, unlike the general AC impedance method, it is sufficient to use only a specific frequency and calculate a phase difference Δθ between the detected AC current Id and the detected AC voltage Vd for the specific frequency.

[0049] The phase difference calculation unit 210 calculates the phase difference Δθ between the detected alternating current Id and the detected alternating voltage Vd. As is well known, an equivalent circuit of the fuel cell is formed from a resistance component and a capacitive component. Accordingly, as shown in Fig. As shown in Figure 3, a phase relationship is generally obtained in which the phase of the detected alternating current Id leads the detected alternating voltage Vd. In this document, the phase difference Δθ is expressed with a negative value when the phase of the detected alternating current Id leads the detected alternating voltage Vd, whereas the phase difference Δθ is expressed with a positive value when the phase of the detected alternating current Id lags the detected alternating voltage Vd.

[0050] The inventor of this application has found that between the phase difference Δθ and the distribution of electric power generation in the cell surface 11s, a relationship is established as described below with reference to Fig. 4 to Fig. 6 described correlation exists.

[0051] Fig. Fig. 4 is a diagram showing an example of a first electric power generation distribution PD1 in the cell surface 11s. A horizontal axis of Fig. 4 represents a position from the inlet to the outlet of the surface-integrated cathode gas passage CP, and a vertical axis represents a local current density. The surface-integrated cathode gas passage CP can be divided into an upstream section IP on the inlet side and a downstream section OP on the outlet side of the surface-integrated cathode gas passage CP.

[0052] In Fig. 4 illustrates two distributions PD1a, PD1b as the first electric power generation distribution PD1. In a first electric power generation distribution PD1a, the local current density exhibits a maximum value Imax at the inlet of the surface-integrated cathode gas passage CP and then decreases approximately monotonically toward the outlet, where a minimum value Imin is exhibited. In the other first electric power generation distribution PD1b, the local current density exhibits the maximum value Imax at a position slightly downstream of the inlet of the surface-integrated cathode gas passage CP and then decreases approximately monotonically toward the outlet, where the minimum value Imin is exhibited. The first electric power generation distributions PD1a, PD1b are similar in that the maximum value Imax of the local current density is present in the upstream section IP and the minimum value Imin is present in the downstream section OP.Experiments conducted by the inventor of this application indicate that there is a high probability that the first electric power generation distribution PD1 exists when the single cells 11 are dry. The reason for this is as assumed below. That is, when the single cells 11 are dry, the electrolyte membrane acquires a reduced electron mobility. This leads to a decrease in the local current density at the inlet of the surface-integrated anode gas passage AP (located near the outlet of the surface-integrated cathode gas passage CP, as shown in FIG. Fig. 2). The difference between the maximum value Imax and the minimum value Imin of the local current density tends to increase with increasing dryness of the individual cells 11.

[0053] In the present embodiment, as an electric power generation distribution severity amount representing the first electric power generation distribution PD1, a value I* obtained by subtracting the maximum value Imax, which is an extreme value Iip of the local current density in the upstream section IP, from the minimum value Imin, which is an extreme value Iop of the local current density in the downstream section OP is used. Therefore, the electric power generation distribution severity amount I* for the first electric power generation distribution PD1 becomes a negative value. The electric power generation distribution severity amount I* represents a difference between the maximum value Imax and the minimum value Imin of the local current density in the cell surface 11s.As described above, the difference between the maximum value Imax and the minimum value Imin of the local current density tends to increase with increasing dryness of the individual cells 11. Consequently, the absolute value of the distribution magnitude of electric power generation I* also increases with increasing dryness of the individual cells 11.

[0054] Fig. 5 is a diagrammatic view showing an example of second electric power generation distributions PD2a, PD2b in the cell surface 11s. In one second electric power generation distribution PD2a, the local current density records a minimum value Imin at the inlet of the surface-integrated cathode gas passage CP and then increases approximately monotonically toward the outlet, where a maximum value Imax is recorded. In the other second electric power generation distribution PD2b, the local current density records a minimum value Imin at the inlet of the surface-integrated cathode gas passage CP, then increases approximately monotonically toward the outlet, and records a maximum value Imax at a position slightly upstream of the outlet.The second electric power generation distributions PD2a, PD2b are similar in that the maximum value Imax of the local current density is present in the downstream section OP, and the minimum value Imin is present in the upstream section IP. Experiments conducted by the inventor of this application indicate that there is a high probability of the second electric power generation distribution PD2 being established when the single cells 11 are too humid. The reason for this is as assumed below. That is, when the single cells 11 are too humid, there is a high probability of water blockage occurring near the inlet of the surface-integrated cathode gas passage CP. As a result, sufficient supply of cathode gas is hindered near the inlet of the surface-integrated cathode gas passage CP, resulting in a decrease in the local current density.

[0055] As an electric power generation distribution magnitude amount representing the second electric power generation distribution PD2, a value I* obtained by subtracting the minimum value Imin, which is an extreme value Iip of the local current density in the upstream section IP, from the maximum value Imax, which is an extreme value Iop of the local current density in the downstream section OP, is used. Therefore, the electric power generation distribution magnitude amount I* for the second electric power generation distribution PD2 becomes a positive value. The electric power generation distribution magnitude amount I* also represents a difference between the maximum value Imax and the minimum value Imin of the local current density in the cell surface 11s.

[0056] As described above, in the Fig. 4, the distribution characteristic amount of electrical power generation I* becomes a negative value, whereas in the first distribution of electrical power generation PD1 shown in Fig. 5, the electric power generation distribution magnitude I* becomes a positive value. Conversely, when the electric power generation distribution magnitude I* is negative, the electric power generation distribution in the cell surface 11s can be considered the first electric power generation distribution PD1. When the electric power generation distribution magnitude I* is positive, the electric power generation distribution in the cell surface 11s can be considered the second electric power generation distribution PD2.

[0057] Fig. 6 is a diagram showing a relationship RL between the phase difference Δθ of the detected alternating current Id to the detected alternating voltage Vd and the distribution magnitude of electric power generation I*. Diagram points of Fig. 6 shows the results of experiments conducted using the current density sensor, which is not the subject of the claimed invention. It can be seen that there is a practically linear relationship RL between the phase difference Δθ and the electric power generation distribution magnitude I*. When the electric power generation distribution magnitude I* is negative, it can be assumed that the electric power generation distribution in the cell surface 11s is the one shown in Fig. 4 is the first distribution of electric power generation PD1. If the distribution magnitude of electric power generation I* is positive, it can be assumed that the distribution of electric power generation in the cell surface 11s is the one shown in Fig. 5 is the second distribution of electrical power generation PD2. As in Fig. 4, the absolute value of the electric power generation distribution magnitude I* for the first electric power generation distribution PD1a becomes larger as the dryness of the single cell 11 is higher.

[0058] The humid state of the fuel cell includes a first state H1 in which the fuel cell is too dry, a second state H2 in which the fuel cell is appropriately humid, and a third state H3 in which the fuel cell is too humid. The second state H2 is the state in a predetermined humidity range. The first state H1 is the state in which the humidity is lower than the humidity range of the second state H2. The third state H3 is the state in which the humidity is higher than the humidity range of the second state H2. Fig. In the example shown in Figure 6, the range in which the electric power generation distribution magnitude I* is less than a lower limit value ILOW corresponds to the first state H1 in which the fuel cell is too dry. The range in which the electric power generation distribution magnitude I* is from the lower limit value ILOW to an upper limit value IHIGH corresponds to the second state H2 in which the fuel cell is appropriately humid. The range in which the electric power generation distribution magnitude I* exceeds the upper limit value IHIGH corresponds to the third state H3 in which the fuel cell is too humid. In this case, the appropriate range of the electric power generation distribution magnitude I* is the range from the lower limit value ILOW to the upper limit value IHIGH. HIGH .

[0059] Since the predetermined relationship RL exists between the phase difference Δθ and the distribution magnitude of electric power generation I*, it can be estimated whether the wet state of the fuel cell is H1, H2, or H3 based on the value of the phase difference Δθ. In short, if the phase difference Δθ is within a reasonable range RR, it can be considered that the fuel cell is in the second state H2 in which the fuel cell is reasonably wet. If the phase difference Δθ is smaller than a lower limit value Δθ LOW of the appropriate range RR, it can be assumed that the fuel cell is in the first state H1, in which the fuel cell is too dry. If the phase difference Δθ is greater than an upper limit Δθ HIGHthe appropriate range RR, it can be considered that the fuel cell is in the third state H3 in which the fuel cell is too humid. Since the appropriate range of the phase difference Δθ and the electric power generation distribution expression amount I* depends on the structure of the single cell 11 or the like, the appropriate range is set experimentally or empirically for each model of the fuel cell stack 10 in advance. The appropriate range of the phase difference Δθ and the electric power generation distribution expression amount I* is also called the "allowable range."

[0060] Fig. Figure 7 is a flowchart showing a process for measuring the phase difference Δθ and switching operating modes. The process is executed by the controller 20 in regular repetition after the fuel cell system 100 starts and normal operation is initiated. When the process of Fig. 7 is started, the fuel cell system 100 is operated in a normal operation mode. The normal operation mode is different from a first operation mode executed in step S160 to reduce the humidity of the fuel cell and a second operation mode executed in step S170 to increase the humidity of the fuel cell.

[0061] In step S110, an AC impedance measurement is performed, and a phase difference Δθ between the detected AC current Id and the detected AC voltage Vd is calculated. Specifically, the Fig. The AC signal supply unit 330 shown in Figure 2 applies an AC signal to the electrodes at both ends of the fuel cell. Based on the detected AC voltage Vd detected by the voltage detector 310 and the detected AC current Id detected by the current detector 320 in response to the applied AC signal, the phase difference calculation unit 210 calculates a phase difference Δθ. The phase difference Δθ is supplied to the estimation unit 220.

[0062] In step S120, the estimation unit 220 determines whether the phase difference Δθ in the Fig. 6 shown reasonable range RR. The range RR is a range from the lower limit Δθ LOW up to the upper limit Δθ HIGHIf the phase difference Δθ is within the appropriate range RR, it can be considered that the fuel cell is in the second state H2 in which the fuel cell is appropriately humidified. Accordingly, in step S130, the normal operation mode is maintained and the process of Fig. 7 is terminated. If the phase difference Δθ is not within the appropriate range RR, it is determined in step S140 whether the phase difference Δθ is greater than the upper limit Δθ HIGH of the appropriate range RR.

[0063] If the phase difference Δθ is greater than the upper limit Δθ HIGHof the appropriate range RR, it can be considered that the fuel cell is in the third state H3 in which the fuel cell is too humid. Accordingly, the process proceeds from step S140 to step S150. In step S150, the operating condition control unit 230 operates the fuel cell system 100 in the first operating mode for reducing the humidity of the fuel cell. In the first operating mode, one or more of the following operations may be used. (C-1a) Increasing the amount of water discharged from the inside of the fuel cell to the outside by increasing the rotational speed of the compressor 33 and thereby increasing the supply flow rate of cathode gas to the fuel cell. (C-1b) Increasing the amount of water discharged from the inside of the fuel cell to the outside by changing a set value of the first regulator 42 and thereby reducing the cathode gas pressure in the fuel cell.(C-1c) If the cathode gas supply unit 30 is equipped with a humidifier, reduce the amount of humidification by the humidifier. In the first operation mode using these operations, one or more of the rotational speed of the compressor 33, the set value of the first regulator 42, and the amount of humidification by the humidifier correspond to the control amount controlled by the operating condition control unit 230, that is, the operating condition of the fuel cell system 100.

[0064] Similarly, for the anode gas, one or more of the following operations can be used to reduce the humidity of the fuel cell. (A-1a) Increasing the amount of water discharged from the inside of the fuel cell to the outside by increasing the rotational speed of the anode gas pump 65 and thereby increasing the supply flow rate of anode gas to the fuel cell. (A-1b) Increasing the amount of water discharged from the inside of the fuel cell to the outside by temporarily reducing the amount of anode gas supplied through the injectors 55 and thereby reducing the anode gas pressure in the fuel cell. (A-1c) If the anode gas supply unit 50 is equipped with a humidifier, reducing the amount of humidification by the humidifier.

[0065] If the phase difference Δθ in step S140 is larger than the upper limit Δθ HIGH of the appropriate range RR, the phase difference Δθ is smaller than the lower limit Δθ LOWof the appropriate range RR. In this case, it can be considered that the fuel cell is in the first state H1 in which the fuel cell is too dry. Accordingly, the process proceeds from step S140 to step S160. In step S160, the operating condition control unit 230 operates the fuel cell system 100 in the second operating mode to increase the humidity of the fuel cell. In the second operating mode, one or more of the following operations may be used. (C-2a) Reducing the amount of water discharged from the inside of the fuel cell to the outside by reducing the rotational speed of the compressor 33 and thereby reducing the supply flow rate of cathode gas to the fuel cell. (C-2b) Reducing the amount of water discharged from the inside of the fuel cell to the outside by changing the set value of the first regulator 42 and thereby increasing the cathode gas pressure in the fuel cell.(C-2c) When the cathode gas supply unit 30 is equipped with a humidifier, increase the humidification amount by the humidifier.

[0066] For the anode gas, one or more of the following operations can be used analogously to increase the humidity of the fuel cell. (A-2a) Reducing the amount of water discharged from the inside of the fuel cell to the outside by reducing the rotational speed of the anode gas pump 65 and thereby reducing the supply flow rate of anode gas to the fuel cell. (A-2b) Reducing the amount of water discharged from the inside of the fuel cell to the outside by temporarily increasing the amount of anode gas supplied through the injectors 55 and thereby increasing the anode gas pressure in the fuel cell. (A-2c) If the anode gas supply unit 50 is equipped with a humidifier, increasing the amount of humidification by the humidifier.

[0067] After step S150 or step S160, the process following step S110 is executed again. When the phase difference Δθ returns to the appropriate range RR after the process returns from step S150 or step S160 to step S110, the process proceeds to step S130, in which the operating condition control unit 230 switches the operating mode to the normal operating mode and terminates the process of Fig. 7.

[0068] The process described above of Fig. 7 illustrates the process for changing the operating condition of the fuel cell system 100 such that the fuel cell state is adjusted to be the second state H2, which is a suitable humid state, when the fuel cell state is estimated to be the first state H1 or the third state H3, which are not the suitable humid state. When such a process is performed, the fuel cell system 100 can be appropriately controlled in accordance with the phase angle Δθ to be in the second state H2 in which the fuel cell is appropriately humid.

[0069] The process of Fig. 7 can also be considered a process of changing the operating condition of the fuel cell system 100 such that the electric power generation distribution characteristic amount I* falls within a predetermined allowable range when the electric power generation distribution characteristic amount I* estimated based on the phase difference Δθ is outside the allowable range. When such a process is performed in the case where the electric power generation distribution characteristic amount I*, which is a difference between the maximum value Imax and the minimum value Imin of the local current density in the cell surface 11s, is outside the allowable range, the fuel cell system 100 can be controlled under an appropriate operating condition such that the electric power generation distribution characteristic amount I* falls within the allowable range.

[0070] In Fig. 6 and Fig. 7, the humid state of the fuel cell is divided into three states H1 to H3 according to the phase difference Δθ. However, the humid state of the fuel cell may be divided into two states or four or more states. In any of these cases, the operating condition control unit 230 may change the operating condition of the fuel cell system 100 according to the phase difference Δθ and perform control of the humid state of the fuel cell. In this case, the operating condition control unit 230 preferably performs control under the operating condition where, when the phase difference Δθ is large, the humidity of the fuel cell is lower than when the phase difference Δθ is small.The control may also be considered as the control under the operating condition where, when the phase difference Δθ is a second value larger than a first value, the humidity of the fuel cell is lower than when the phase difference Δθ is the first value. For example, the control corresponds to the control of step S150 in FIG. Fig. 7. With the controller, the wet state of the fuel cell can be appropriately controlled in accordance with the phase difference Δθ.

[0071] The operating condition control unit 230 preferably also performs control under the operating condition in which, when the phase difference Δθ is small, the humidity of the fuel cell is higher than when the phase difference Δθ is large. The control may also be considered to be the control under the operating condition in which, when the phase difference Δθ is a third value smaller than the first value, the humidity of the fuel cell is higher than when the phase difference Δθ is the first value. The control corresponds, for example, to the control of step S160 in Fig. 7. With this control, the wet state of the fuel cell can also be appropriately controlled in accordance with the phase difference Δθ.

[0072] As described above, the present embodiment utilizes the predetermined relationship RL between the electric power generation distribution magnitude I*, which represents a difference between the maximum value Imax and the minimum value Imin of the local current density in the cell surface 11s, and the phase difference Δθ. Based on the relationship RL, the fuel cell system 100 in the present embodiment estimates the electric power generation distribution magnitude I* in accordance with the phase difference Δθ. Consequently, the fuel cell system 100 can estimate the electric power generation distribution in the cell surface 11s of the fuel cell without providing a current density sensor.

[0073] In the disclosed embodiment, the electric power generation distribution magnitude I* is a value obtained by subtracting an extreme value Iip of the local current density in the upstream section IP from an extreme value Iop of the local current density in the downstream section OP. However, it is also possible to use other electric power generation distribution magnitudes I*. The electric power generation distribution magnitudes I* preferably include a value representing a difference between the maximum value Imax and the minimum value Imin of the local current density in the cell surface 11s.In that case, a value that does not indicate whether the electric power generation distribution in the cell surface 11s is the electric power generation distribution PD1 or the electric power generation distribution PD2 may be used as the electric power generation distribution magnitude amount I*.

[0074] In the embodiment, the electric power generation distribution magnitude I* indicates whether the electric power generation distribution corresponds to the electric power generation distribution PD1 or the electric power generation distribution PD2, which are distinguished by the positive and negative signs of the electric power generation distribution magnitude I*, respectively. However, the electric power generation distributions PD1, PD2 may be distinguished by other methods. For example, to distinguish the first electric power generation distribution PD1 and the second electric power generation distribution PD2, an independent bit other than a bit representing the difference between the maximum value Imax and the minimum value Imin may be used instead of the positive and negative signs.It should be noted that distinguishing the distributions of electrical power generation using the positive and negative signs of the distribution magnitude of electrical power generation I* offers the advantage that the distribution of electrical power generation can be easily estimated based on the distribution magnitude of electrical power generation I*.

Claims

[1] Fuel cell system (100), comprising: a fuel cell (10); a voltage detector (310) configured to detect an output voltage of the fuel cell (10); a current detector (320) configured to detect an output current of the fuel cell (10), an AC signal supply unit (330) configured to supply an AC signal to electrodes at both ends of the fuel cell (10); a phase difference calculation unit (210) configured to calculate a phase difference between the detected alternating current and the detected alternating voltage based on a detected alternating voltage detected by the voltage detector (310) and a detected alternating current detected by the current detector (320); and an estimation unit (220) configured to estimate, in accordance with the phase difference, a distribution characteristic amount of electric power generation representing a distribution of electric power generation in a cell surface (11s) of the fuel cell (10) using a predetermined relationship between the distribution characteristic amount of electric power generation and the phase difference, wherein the distribution characteristic amount of electric power generation includes a value indicating a difference between a maximum value and a minimum value of a local current density in the cell surface (11s). [2] Fuel cell system (100) according to claim 1, wherein: the cell surface (11s) has a surface-integrated cathode gas passage (CP) from an inlet to an outlet of the cell surface (11s) for a cathode gas flowing in the cell surface (11s); and when the surface-integrated cathode gas passage (CP) is divided into an upstream section on an inlet side and a downstream section on an outlet side, the electric power generation distribution severity amount indicates whether the electric power generation distribution in the cell surface (11s) corresponds to a first electric power generation distribution (PD1) in which the maximum value of the local current density is present in the upstream section and the minimum value of the local current density is present in the downstream section, or corresponds to a second electric power generation distribution (PD2) in which the maximum value of the local current density is present in the downstream section and the minimum value of the local current density is present in the upstream section. [3] The fuel cell system (100) according to claim 2, wherein the electric power generation distribution amount indicates whether the electric power generation distribution in the cell surface (11s) corresponds to the first electric power generation distribution (PD1) or the second electric power generation distribution (PD2) with positive and negative signs of the electric power generation distribution amount. [4] The fuel cell system (100) according to any one of claims 1 to 3, further comprising an operating condition control unit (230) configured to, when the electric power generation distribution magnitude amount estimated based on the phase difference is outside a predetermined allowable range, change an operating condition of the fuel cell system (100) such that the electric power generation distribution magnitude amount falls within the allowable range. [5] The fuel cell system (100) according to any one of claims 1 to 3, wherein the estimation unit (220) estimates, in accordance with the phase difference, whether a humid state of the fuel cell (10) is a first state (H1) in which a humidity of the fuel cell (10) is lower than a predetermined humidity range, a second state (H2) in which the humidity of the fuel cell (10) is within the humidity range, or a third state (H3) in which the humidity of the fuel cell (10) is higher than the humidity range. [6] The fuel cell system (100) according to claim 5, further comprising an operating condition control unit (230) configured to, when it is estimated that the fuel cell (10) is in the first state (H1) or the third state (H3), change an operating condition of the fuel cell system (100) such that the fuel cell (10) is placed in the second state (H2).

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