JP5105222B2 - Fuel cell system - Google Patents

Description

  The present invention relates to a fuel cell system, and more particularly to cell voltage management of a fuel cell stack.

  In recent years, low-pollution vehicles have been developed as part of efforts to deal with environmental problems, and one of them is a fuel cell vehicle using a fuel cell stack as an on-vehicle power source. The fuel cell stack has a stack structure in which a plurality of cells are stacked in series, and each cell has an anode electrode disposed on one surface of the electrolyte membrane and a cathode electrode disposed on the other surface. A membrane-electrode assembly. By supplying fuel gas and oxidizing gas to the membrane-electrode assembly, an electrochemical reaction occurs, and chemical energy is converted into electrical energy. In particular, a solid polymer electrolyte fuel cell stack using a solid polymer membrane as an electrolyte is easy to downsize at low cost and has a high output density, so that it is expected to be used as an in-vehicle power source. Yes.

When operating a fuel cell system, it is necessary to detect a power generation abnormality caused by insufficient supply of reaction gas due to flooding or drying of the membrane-electrode assembly, and recover the cell state so that sufficient output can be taken out. . Japanese Patent Application Laid-Open No. 2004-165058 proposes a technique for monitoring cell voltage drop and executing cell voltage recovery processing while limiting output when the cell voltage falls below a certain lower limit voltage threshold value.
JP 2004-165058 A

  However, in a fuel cell stack formed by stacking a plurality of cells, the voltage drop factor varies depending on the cell placement location, so if cell voltage control is performed with the same lower limit voltage threshold for all cells If the cell voltage recovery process is performed more than necessary or the output is excessively limited, the operation efficiency may be reduced.

  Therefore, in view of such a problem, the present invention has an object to appropriately set a lower limit voltage threshold value and a lower limit voltage allowable time threshold value according to a cell arrangement location.

In order to solve the above problems, a fuel cell system according to the present invention includes a fuel cell stack in which a plurality of cells are stacked, and a cell voltage recovery process or an output current limit process for a cell group in which the cell voltage is lower than a lower limit voltage threshold and a cell voltage control means for the lower limit voltage threshold of the cell group arranged in a substantially central portion of the fuel cell stack, voltage lower than the lower limit voltage threshold of the cell group arranged on an end portion of the fuel cell stack It is. The cell voltage control means may perform a cell voltage recovery process or an output current limit process for a cell group in which the time during which the cell voltage is below the lower limit voltage threshold exceeds the lower limit voltage allowable time threshold.

  The cell group arranged at the cell stack end portion (the anode electrode side end portion and the cathode electrode side end portion) is within the cell, between the cells, and between the electrodes than the cell group arranged at the substantially central portion of the cell stack. The temperature difference is likely to occur, and the cell voltage is likely to decrease due to insufficient supply of reaction gas due to flooding or the like. In particular, at the time of cold start, since it is highly necessary to avoid damage to the cell due to insufficient fuel gas supply, it is necessary to strictly manage the cell voltage at the end of the cell stack.

  On the other hand, in the cell group disposed in the substantially central portion of the cell stack, such flooding is rarely caused, and the cell voltage reduction factor is mainly due to an increase in resistance due to drying of the electrolyte membrane. Since the damage to the cell due to the drying of the electrolyte membrane is smaller than the damage to the cell due to insufficient fuel gas supply, the cell voltage management of the cell group arranged at the substantially central part of the cell stack is arranged at the end of the cell stack. It can be made more gradual than the cell voltage management of the cell group.

  For the above reasons, the cell voltage management is performed by setting the lower limit voltage threshold of the cell group arranged at the substantially central portion of the cell stack to a lower voltage than the lower limit voltage threshold of the cell group arranged at the end of the cell stack. As a result, it is possible to avoid a decrease in operation efficiency caused by executing a cell voltage recovery process more than necessary or excessively limiting the output of the cell group arranged in the substantially central part of the cell stack.

  Further, the cell voltage management is performed by setting the lower limit voltage allowable time threshold of the cell group arranged in the substantially central portion of the cell stack to be longer than the lower limit voltage allowable time threshold of the cell group arranged at the end of the cell stack. As a result, it is possible to avoid a decrease in operation efficiency caused by executing a cell voltage recovery process more than necessary or excessively limiting the output of a cell group arranged in the substantially central portion of the cell stack.

  According to the present invention, the cell voltage management of the cell group disposed in the substantially central portion of the cell stack is made more gradual than the cell voltage management of the cell group disposed in the end portion of the cell stack. It is possible to avoid a reduction in operating efficiency due to executing cell voltage recovery processing more than necessary or excessively limiting the output of the cell group arranged in the cell group.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 shows a system configuration of a fuel cell system 10 that functions as an in-vehicle power supply system for a fuel cell vehicle.

  The fuel cell system 10 includes a fuel cell stack 20 that generates power upon receiving supply of reaction gases (oxidation gas and fuel gas), a fuel gas piping system 30 that supplies hydrogen gas as fuel gas to the fuel cell stack 20, an oxidation An oxidizing gas piping system 40 that supplies air as gas to the fuel cell stack 20, a power system 60 that controls charging / discharging of power, and a controller 70 that controls the entire system are provided.

  The fuel cell stack 20 is, for example, a solid polymer electrolyte cell stack formed by stacking a large number of cells in series. The cell has a cathode electrode on one surface of an electrolyte membrane made of an ion exchange membrane, an anode electrode on the other surface, and a pair of separators so as to sandwich the cathode electrode and the anode electrode from both sides. Yes. The fuel gas is supplied to the fuel gas flow path of one separator, and the oxidizing gas is supplied to the oxidizing gas flow path of the other separator, and the fuel cell stack 20 generates power by this gas supply.

  In the fuel cell stack 20, the oxidation reaction of the formula (1) occurs at the anode electrode, and the reduction reaction of the equation (2) occurs at the cathode electrode. In the fuel cell stack 20 as a whole, the electromotive reaction of the formula (3) occurs.

H ₂ → 2H ⁺ + 2e ⁻ (1)
(1/2) O ₂ + 2H ⁺ + 2e ⁻ → H ₂ O (2)
H ₂ + (1/2) O ₂ → H ₂ O (3)

  The cell monitor 80 is a cell voltage detection means for detecting each voltage of a plurality of cells constituting the fuel cell stack 20.

  The fuel gas piping system 30 includes a fuel gas supply source 31, a fuel gas supply passage 35 through which fuel gas (hydrogen gas) supplied from the fuel gas supply source 31 to the anode electrode of the fuel cell stack 20 flows, and a fuel cell stack. A circulation passage 36 for recirculating the fuel off-gas (hydrogen off-gas) discharged from the fuel gas supply passage 35 to the fuel gas supply passage 35, and a circulation pump 37 for pressure-feeding the fuel off-gas in the circulation passage 36 to the fuel gas supply passage 35. And an exhaust passage 39 branched and connected to the circulation passage 36.

  The fuel gas supply source 31 is composed of, for example, a high-pressure hydrogen tank or a hydrogen storage alloy, and stores, for example, 35 MPa or 70 MPa of hydrogen gas. When the shut-off valve 32 is opened, hydrogen gas flows out from the fuel gas supply source 31 into the fuel gas supply channel 35. The hydrogen gas is decompressed to, for example, about 200 kPa by the regulator 33 and the injector 34 and supplied to the fuel cell stack 20.

  The fuel gas supply source 31 includes a reformer that generates hydrogen-rich reformed gas from hydrocarbon fuel, a high-pressure gas tank that stores the reformed gas generated by the reformer in a high-pressure state, and You may comprise.

  The injector 34 is an electromagnetically driven on-off valve capable of adjusting the gas flow rate and the gas pressure by driving the valve body directly with a predetermined driving cycle with an electromagnetic driving force and separating it from the valve seat. The injector 34 includes a valve seat having an injection hole for injecting gaseous fuel such as fuel gas, a nozzle body for supplying and guiding the gaseous fuel to the injection hole, and an axial direction (gas flow direction) with respect to the nozzle body. And a valve body that is movably accommodated and opens and closes the injection hole.

  An exhaust passage 39 is connected to the circulation passage 36 via an exhaust valve 38. The exhaust valve 38 is operated according to a command from the controller 70, and thereby discharges the fuel off-gas and impurities including impurities in the circulation flow path 36 to the outside. By opening the exhaust valve 38, the concentration of impurities in the hydrogen off-gas in the circulation passage 36 decreases, and the concentration of hydrogen in the fuel off-gas circulated increases.

  The diluter 50 receives the fuel off-gas discharged through the exhaust valve 38 and the exhaust passage 39 and the oxidizing off-gas flowing through the exhaust passage 45 to dilute the fuel off-gas. The diluted exhaust gas of the fuel off-gas is silenced by a muffler (silencer) 51, flows through the tail pipe 52, and is exhausted outside the vehicle.

  The oxidizing gas piping system 40 has an oxidizing gas supply passage 44 through which oxidizing gas supplied to the cathode electrode of the fuel cell stack 20 flows, and an exhaust passage 45 through which oxidizing off gas discharged from the fuel cell stack 20 flows. is doing. The oxidizing gas supply channel 44 is provided with a compressor 42 that takes in the oxidizing gas via the filter 41 and a humidifier 43 that humidifies the oxidizing gas fed by the compressor 42. The exhaust passage 45 is provided with a back pressure adjustment valve 46 for adjusting the oxidizing gas supply pressure and a humidifier 43.

  The humidifier 43 accommodates a hollow fiber membrane bundle composed of a large number of hollow fiber membranes. Inside the hollow fiber membrane, a highly humid oxidizing off gas (wet gas) containing a large amount of water generated by the battery reaction flows, while on the outside of the hollow fiber membrane is a low wet oxidizing gas taken from the atmosphere. (Dry gas) flows. Oxidizing gas can be humidified by performing water exchange between the oxidizing gas and the oxidizing off gas across the hollow fiber membrane.

  The power system 60 includes a DC / DC converter 61, a battery 62, a traction inverter 63, and a traction motor 64. The DC / DC converter 61 is a DC voltage converter, which boosts the DC voltage from the battery 62 and outputs it to the traction inverter 63, and steps down the DC voltage from the fuel cell stack 20 or the traction motor 64. And a function of charging the battery 62. The charge / discharge of the battery 62 is controlled by these functions of the DC / DC converter 61. Further, the operation point (output voltage, output current) of the fuel cell stack 20 is controlled by the voltage conversion control by the DC / DC converter 61.

  The battery 62 is a power storage device capable of storing and discharging electric power, and functions as a regenerative energy storage source at the time of brake regeneration and an energy buffer at the time of load fluctuation accompanying acceleration or deceleration of the fuel cell vehicle. As the battery 62, for example, a secondary battery such as a nickel / cadmium storage battery, a nickel / hydrogen storage battery, or a lithium secondary battery is suitable.

  The traction inverter 63 converts a direct current into a three-phase alternating current and supplies it to the traction motor 64. The traction motor 64 is a three-phase AC motor, for example, and constitutes a power source of the fuel cell vehicle.

  The controller 70 is a computer system including a CPU, a ROM, a RAM, and an input / output interface, and controls each part of the fuel cell system 10. For example, when the controller 70 receives an activation signal output from an ignition switch (not shown), the controller 70 starts operation of the fuel cell system 10 and an accelerator opening signal output from an accelerator sensor (not shown), Based on a vehicle speed signal output from a vehicle speed sensor (not shown), the required power of the entire system is obtained. The required power of the entire system is the total value of the vehicle running power and the auxiliary machine power. Auxiliary power includes, for example, power consumed by in-vehicle auxiliary equipment (humidifiers, air compressors, hydrogen pumps, cooling water circulation pumps, etc.), and devices required for vehicle travel (transmissions, wheel control devices, steering devices) , And suspension devices), and power consumed by devices (such as air conditioners, lighting fixtures, and audio devices) disposed in the passenger space.

  Then, the controller 70 determines the distribution of the output power of the fuel cell stack 20 and the battery 62, and the rotational speed of the compressor 42 and the valve opening of the injector 34 so that the power generation amount of the fuel cell stack 20 matches the target power. And adjusting the output voltage of the fuel cell stack 20 by controlling the DC / DC converter 61 and adjusting the output voltage of the fuel cell stack 20. , Output current). Further, the controller 70 outputs, for example, each U-phase, V-phase, and W-phase AC voltage command value to the traction inverter 63 as a switching command so that the target vehicle speed according to the accelerator opening is obtained, and the traction motor 64 output torque and rotation speed are controlled.

  The controller 70 monitors the presence or absence of power generation abnormality of the fuel cell stack 20 based on the cell voltage of each cell detected by the cell monitor 80, and determines that the power generation abnormality has occurred, executes a cell voltage recovery process, Or, output is limited. Details of the cell voltage monitoring will be described later.

FIG. 2 is a schematic diagram showing the structure of the fuel cell stack 20.
As shown in the figure, a pair of terminal plates 22A and 22B for power extraction are arranged at both ends of a cell stack formed by stacking N cells (N is an integer of 2 or more). Yes. The terminal plate 22A is an anode electrode, and the terminal plate 22B is a cathode electrode. The outer sides of the terminal plates 22A and 22B are sandwiched and fixed by a pair of end plates 24A and 24B via insulating plates 23A and 23B. Each cell 21 is assigned a cell number, and the cell number increases from the terminal plate 22A toward the terminal plate 22B. For example, the cell number of the cell 21 closest to the terminal plate 22A is 1, and the cell number of the cell 21 closest to the terminal plate 22B is N.

FIG. 3 is a graph showing the cell voltage distribution of the fuel cell stack 20.
In the figure, the horizontal axis indicates the cell number, and the vertical axis indicates the cell voltage. In the fuel cell stack 20, the heat is removed from the end plates 24A and 24B that are in contact with the outside air, so that a group of cells (hereinafter, described) is arranged at the cell stack end portions (the anode electrode side end portion and the cathode electrode side end portion). For the sake of convenience, the temperature in the cell, between the cells, and between the electrodes is higher than the cell group (hereinafter referred to as the B group for convenience of description) disposed in the substantially central portion of the cell stack. Differences are likely to occur. For this reason, in group A, the cell voltage tends to decrease as it approaches the cell stack end. This is because moisture is condensed at a low temperature in the cell, and flooding occurs from that point, which tends to cause a drop in cell voltage. In particular, in the fuel cell stack 20 that has been left for a long time after the battery operation is stopped, moisture moves from the cathode electrode to the anode electrode through the electrolyte membrane. May occur. Since power generation due to insufficient fuel gas supply has been found to cause damage to the cells 21, the group A requires strict cell voltage management.

  On the other hand, in the group B, the above-mentioned flooding is rare, and the cell voltage lowering factor is mainly due to the increase in resistance (decrease in proton conductivity) due to drying of the electrolyte membrane. In the group B, since the temperature in the cell, between the cells, and between the electrodes is small, all the cells have the same cell voltage. Since the damage to the cell 21 due to the drying of the electrolyte membrane is smaller than the damage to the cell 21 due to insufficient fuel gas supply, the cell voltage management of the B group can be made more gradual than the cell voltage management of the A group. Specifically, the lower limit voltage threshold Vth2 of the B group can be set to a voltage lower than the lower limit voltage threshold Vth1 of the A group. Further, the B group lower limit voltage allowable time threshold T2 can be set longer than the lower limit voltage allowable time threshold T1 of the A group. Here, the lower limit voltage threshold refers to the lowest voltage that can allow the cell voltage to decrease from the viewpoint of damage to the cell 21 due to power generation abnormality, and the lower limit voltage allowable time threshold refers to damage that the cell 21 receives due to power generation abnormality. From this point of view, it means the longest time in which the cell voltage can be allowed to fall below the lower limit voltage threshold.

FIG. 4 shows the cell voltage management table 90.
The cell voltage management table 90 stores a lower limit voltage threshold value and a lower limit voltage allowable time threshold value for each of the A group and the B group. The controller 70 holds a cell voltage management table 90 and functions as cell voltage control means for managing the cell voltage based on the cell voltage output from the cell monitor 80. Specifically, the controller 70 determines whether or not the cell voltage of each of the A group and the B group is below the lower limit voltage threshold, and if the cell voltage is lower than the lower limit voltage threshold, the time is the lower limit voltage allowable time. Check if the threshold is exceeded. If the time during which the cell voltage is below the lower limit voltage threshold exceeds the lower limit voltage allowable time threshold, the cell voltage recovery process is executed or the output current limit process is executed.

  In this way, cell voltage management is performed for the B group more than necessary by setting the lower limit voltage threshold Vth2 of the B group to a voltage lower than the lower limit voltage threshold Vth1 of the A group. It is possible to avoid a decrease in operating efficiency due to execution or excessive output limitation.

  Further, the cell voltage management is performed more than necessary for the B group by setting the lower limit voltage allowable time threshold T2 of the B group to be longer than the lower limit voltage allowable time threshold T1 of the A group and performing cell voltage management. It is possible to avoid a decrease in operating efficiency due to execution of processing or excessive output limitation.

  Further, according to the cell voltage management according to the present embodiment, the output limit (current limit) of the fuel cell stack 20 can be minimized, so that it is possible to expect an improvement in operation efficiency. In addition, since the cell voltage distribution shown in FIG. 3 can be seen not only at low temperature start but also during battery operation, voltage management according to this embodiment is effective not only during low temperature start but also during battery operation. It is.

  The examples and application examples described through the embodiments of the invention can be used in combination as appropriate according to the application, or can be used with modifications or improvements, and the present invention is limited to the description of the embodiment described above. It is not something. It will be apparent from the scope of the claims that the embodiments added with such combinations or changes or improvements can also be included in the technical scope of the present invention.

  For example, in the above-described embodiment, the usage form in which the fuel cell system 10 is used as the in-vehicle power supply system is illustrated, but the usage form of the fuel cell system 10 is not limited to this example. For example, the fuel cell system 10 may be mounted as a power source of a mobile body (robot, ship, aircraft, etc.) other than the fuel cell vehicle. Further, the fuel cell system 10 according to the present embodiment may be used as a power generation facility (stationary power generation system) such as a house or a building.

1 is a system configuration diagram of a fuel cell system according to an embodiment. It is a schematic diagram which shows the structure of a fuel cell stack. It is a graph which shows the cell voltage distribution of a fuel cell stack. It is explanatory drawing of a cell voltage management table.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Fuel cell system 20 ... Fuel cell stack 70 ... Controller 80 ... Cell monitor 90 ... Cell voltage management table

Claims (2)

A fuel cell stack formed by stacking a plurality of cells;
Cell voltage control means for performing cell voltage recovery processing or output current limiting processing of a cell group whose cell voltage is below the lower limit voltage threshold , and
The fuel cell system, wherein a lower limit voltage threshold value of a cell group disposed at a substantially central portion of the fuel cell stack is lower than a lower limit voltage threshold value of a cell group disposed at an end portion of the fuel cell stack. The fuel cell system according to claim 1,
The cell voltage control means performs a cell voltage recovery process or an output current limit process of a cell group in which the time during which the cell voltage is lower than the lower limit voltage threshold exceeds the lower limit voltage allowable time threshold,
A fuel cell in which a lower limit voltage allowable time threshold value of a cell group disposed at a substantially central portion of the fuel cell stack is longer than a lower limit voltage allowable time threshold value of a cell group disposed at an end of the fuel cell stack. system.

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