JP5099580B2 - Fuel cell system - Google Patents

Description

  The present invention relates to a fuel cell system that generates electricity by receiving supply of fuel gas and oxidizing gas.

  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.

In the battery reaction in the fuel cell system, moisture is generated, so that the effective electrode area of the cell decreases due to moisture condensation and the like, and there is a possibility that the diffusion of the reaction gas to the membrane-electrode assembly may be hindered. In such a situation, voltage distribution occurs between the cells, and the cell with the lowest voltage may reach 0 V or less and be overdischarged. When operating the fuel cell system, it detects such state deterioration as flooding or a shortage of reactive gas generated locally for some reason, and prevents damage to the fuel cell due to overcurrent in advance. It is necessary to restore the state of the fuel cell so that a sufficient output can be obtained. In particular, at low temperature starting such as below freezing, the cell temperature is low, so that the cell activity tends to become unstable, and the rated electromotive force may not be output. Japanese Patent Application Laid-Open No. 2004-165058 discloses a method for specifying a voltage drop factor when starting a fuel cell and performing a cell voltage recovery process.
JP 2004-165058 A

  However, in the method described in Japanese Patent Application Laid-Open No. 2004-165058, an output voltage (theoretical value) corresponding to the output current of the fuel cell is calculated using a current-voltage characteristic map corresponding to the temperature of the fuel cell prepared in advance. In addition, the output voltage (actually measured value) of the fuel cell is detected by a voltage sensor, and the logical value of the output voltage is compared with the actually measured value to determine whether or not the state of the fuel cell is normal. If so, a method of estimating whether the cause of the voltage drop is caused by temperature is adopted. Thus, since the voltage drop factor is specified through a plurality of processing steps, there is a disadvantage that it takes a long time for the cell voltage recovery processing.

  Therefore, an object of the present invention is to propose a fuel cell system capable of specifying a cell voltage decrease factor by a simple method.

In order to solve the above problems, a fuel cell system according to the present invention includes a fuel cell stack formed by stacking a plurality of cells, cell voltage detection means for detecting output voltages of the plurality of cells, and a fuel cell stack. a output current and voltage converting means for controlling the output voltage, surge sweep current of the fuel cell stack stepwise when below the one of the cell lower threshold voltage of the plurality of cells, the voltage converting means And a measuring means for measuring the output voltage drop rate of the fuel cell stack when the sweep current is increased stepwise, and the output voltage decrease rate of the fuel cell stack when the sweep current is increased stepwise is less than a predetermined threshold. And determining means for determining that the cause of the cell voltage decrease is an insufficient supply of oxidizing gas .

A fuel cell system according to another aspect of the present invention includes a fuel cell stack formed by stacking a plurality of cells, cell voltage detection means for detecting output voltages of the plurality of cells, output current of the fuel cell stack, and Voltage conversion means for controlling the output voltage, the voltage conversion means for rapidly increasing the sweep current of the fuel cell stack stepwise when any one of the plurality of cells falls below the lower threshold voltage, and the sweep current Measurement means for measuring the output voltage drop rate of the fuel cell stack when the current rapidly increases stepwise, and when the output voltage decrease rate of the fuel cell stack when the sweep current rapidly increases stepwise is greater than or equal to a predetermined threshold Determining means for determining that the cause of the cell voltage decrease is insufficient fuel gas supply.

  Here, when the output voltage reduction rate of the fuel cell stack when the sweep current of the fuel cell stack is increased stepwise is less than a predetermined threshold, the determination means is that the cell voltage reduction factor is insufficient oxidizing gas supply And when the output voltage drop rate of the fuel cell stack when the sweep current of the fuel cell stack suddenly increases stepwise is equal to or greater than a predetermined threshold, it is determined that the cell voltage drop factor is insufficient fuel gas supply. .

  According to the applicant's experiment, the rate of decrease in the output voltage of the fuel cell stack when the sweep current of the fuel cell stack is increased in a stepwise manner is higher than that in the case where the cause of cell voltage decrease is insufficient supply of oxidizing gas. It has been confirmed that the shortage is greater. Based on these experimental results, by comparing the output voltage drop rate of the fuel cell stack when the sweep current of the fuel cell stack is increased stepwise with a predetermined threshold, the cause of the cell voltage drop is the lack of oxidizing gas supply. It can be determined whether there is a fuel gas supply shortage or not.

  As the predetermined threshold value, it is preferable to use the minimum value of the output voltage drop rate of the fuel cell stack when the sweep current of the fuel cell stack is increased stepwise in a state where the fuel gas supply is insufficient.

  According to the present invention, the cell voltage decrease factor can be determined by measuring the rate of decrease in the output voltage of the fuel cell stack when the sweep current of the fuel cell stack is increased in a stepped manner. The voltage drop factor can be specified.

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.

  A cell monitor 21 as a cell voltage detecting means is attached to the fuel cell stack 20. The cell monitor 21 detects the output voltage of each cell 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 opened according to a command from the controller 70, thereby discharging the fuel off-gas and moisture containing impurities in the circulation flow path 36 to the outside. As a result, the concentration of impurities in the hydrogen off-gas in the circulation channel 36 decreases, and the hydrogen concentration in the fuel off-gas that is circulated increases.

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

  The oxidizing gas piping system 40 has an oxidizing gas supply channel 44 through which oxidizing gas supplied to the cathode electrode of the fuel cell stack 20 flows, and a discharge channel 45 through which oxidizing off-gas discharged from the fuel cell stack 20 flows. is doing. The oxidizing gas supply flow path 44 is provided with an air compressor 42 that takes in the oxidizing gas through the filter 41 and a humidifier 43 that humidifies the oxidizing gas fed by the air compressor 42. The discharge 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 exchanges moisture between a high-humidity oxidizing off gas (wet gas) containing a large amount of moisture generated by the battery reaction and a low-humidity oxidizing gas (dry gas) taken from the atmosphere. Then, the oxidizing gas supplied to the fuel cell stack 20 is humidified.

  The power system 60 includes a DC / DC converter 61, a battery 62, a traction inverter 63, a traction motor 64, a current sensor 65, and a voltage sensor 66. When the fuel cell vehicle is powered by the traction motor 64, the DC / DC converter 61 boosts the output voltage of the battery 62 and supplies DC power to the traction inverter 63, while the fuel cell vehicle regeneratively brakes by the traction motor 64. When recharging, the battery 62 is charged by reducing the regenerated DC voltage. The DC / DC converter 61 also has a function of reducing the output voltage of the fuel cell stack 20 and charging the battery 62 in order to store surplus generated power of the fuel cell stack 20.

  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 air 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. Voltage and output current). Further, the controller 70 outputs the AC voltage command values of the U-phase, V-phase, and W-phase to the traction inverter 63 as switching commands, for example, so as to obtain the target vehicle speed according to the accelerator opening, The output torque and the rotation speed of the motor 64 are controlled.

Next, a method for determining the cell voltage drop factor will be outlined with reference to FIGS. Here, FIG. 2 is a graph showing a change in the output voltage V _SEL of the fuel cell stack 20 when the sweep current I _SEL of the fuel cell stack 20 is increased stepwise. FIG. 3 is a graph showing the IV characteristics (current-voltage characteristics) of the fuel cell stack 20.

The controller 70 monitors the detection value of the cell monitor 21 at regular intervals. When the controller 70 detects that the cell voltage of a part of the cells constituting the fuel cell stack 20 has fallen below a predetermined lower threshold voltage during startup or operation of the fuel cell stack 20, the DC / DC converter 61 is controlled, and the sweep current I _SEL of the fuel cell stack 20 is rapidly increased from I ₀ to I ₁ in a stepwise manner at time t ₀ . At this time, if the cell voltage decrease factor is insufficient supply of the reaction gas, the output voltage V _SEL of the fuel cell stack 20 rapidly decreases from V ₀ at the time t ₀ as shown by the solid line in FIG. However, it has been confirmed by experiments of the present applicant that it starts to rise and eventually settles at V ₂ around time t ₂ .

When the power generation state of the fuel cell stack 20 is normal (when sufficient reaction gas is supplied to the fuel cell stack 20 and the cell voltage does not decrease), the fuel cell stack is at the timing of time t _0. When the sweep current I _{SEL of} 20 is suddenly increased from I ₀ to I ₁ in steps, the output voltage V _NOMAL of the fuel cell stack 20 is changed from V ₀ at the time t ₀ as shown by the one-dot chain line in FIG. slowly decrease, to settle into V ₁ in the vicinity of the time t ₁ in time has been confirmed by the applicant of the experiment.

By the way, the IV characteristic of the fuel cell stack 20 has an output characteristic as shown in FIG. 3, and this IV characteristic depends on the operating state (for example, impedance, temperature, etc.) of the fuel cell stack 20. It is known to change. The controller 70 obtains the output voltage based on the IV characteristic and the output current in a certain operation state by holding the IV characteristic as map data in advance according to each operation state of the fuel cell stack 20. Can do. As shown in the IV characteristics, the output voltage corresponding to the output current I ₀ is V ₀ , and the output voltage corresponding to the output current I ₁ is V ₁ . This current-voltage correspondence is in agreement with the characteristic curve of _VNOMAL shown in FIG.

  According to the experiment by the present applicant, the rate of decrease in the output voltage of the fuel cell stack 20 when the sweep current of the fuel cell stack 20 is increased stepwise is higher than that in the case where the cell voltage decrease factor is insufficient oxidizing gas supply. It has been confirmed that the fuel gas supply shortage is greater. Furthermore, when the fuel gas supply is insufficient, the cell voltage falls below the zero potential and drops to a negative potential. However, when the oxidizing gas supply is insufficient, the cell voltage falls only to the zero potential.

If the minimum value of the output voltage drop rate of the fuel cell stack 20 when the oxidizing gas supply is insufficient is X _0, and the minimum value of the output voltage drop rate of the fuel cell stack 20 when the fuel gas supply is insufficient is X ₁ , the oxidation When the gas supply is insufficient, the equation (1) is established, and when the fuel gas supply is insufficient, the equation (2) is established.
X ₀ ≦ dV _SEL / dt <X ₁ (1)
X ₁ ≦ dV _SEL / dT (2)
Note that dV _SEL / dt indicates the output voltage drop rate of the fuel cell stack 20 when the sweep current of the fuel cell stack 20 is increased in a stepped manner. This is represented by V _SEL after time t ₀ in FIG. This corresponds to a downward slope of (voltage drop per unit time).

When the fuel gas supply is insufficient, the cell voltage falls below the zero potential and drops to a negative potential. However, when the oxidizing gas supply is insufficient, the cell voltage drops only to the zero potential. In consideration of the characteristics, the expression (3) is established when the oxidizing gas supply is insufficient, and the expression (4) is established when the fuel gas supply is insufficient.
X ₀ ≦ dV _SEL / dt <X ₁ and V _NOMAL −V _SEL ≦ Y (3)
X ₁ ≦ dV _SEL / dT and Y <V _NOMAL −V _SEL (4)
Y is the maximum value of the voltage difference between V _NOMAL and V _SEL when the oxidizing gas supply is insufficient.

  The controller 70 uses the combination of the formulas (1) and (2), or the combination of the formulas (3) and (4), so that the cause of the cell voltage drop is an insufficient supply of the oxidizing gas, or the fuel gas supply. Can determine if there is a shortage.

FIG. 4 is a flowchart showing the cell voltage drop factor determination process.
If the controller 70 detects that the cell voltages of some of the cells constituting the fuel cell stack 20 have fallen below a predetermined lower threshold voltage by reading the detection values of the cell monitor 21 at regular intervals (step 401; YES) By controlling the DC / DC converter 61, the sweep current I _SEL of the fuel cell stack 20 is rapidly increased in a stepped manner from I ₀ to I ₁ (step 402).

Then, the controller 70 reads the detection value of the voltage sensor 66 and measures the output voltage decrease rate dV _SEL / dt of the fuel cell stack 20 (step 403).

  The controller 70 determines whether or not the above equation (1) is established (step 404). Here, if the expression (1) is satisfied (step 404; YES), the controller 70 determines that the cell voltage lowering factor is insufficient supply of oxidizing gas (step 406).

  If the expression (1) is not satisfied (step 404; NO), the controller 70 determines whether or not the above expression (2) is satisfied (step 405). Here, if the expression (2) is satisfied (step 405; YES), the controller 70 determines that the cell voltage lowering factor is insufficient fuel gas supply (step 407).

  If equation (2) is not satisfied (step 404; NO), the controller 70 determines that the cell voltage decrease factor is a factor other than the shortage of reactant gas supply (step 408).

According to the present embodiment, the cell voltage decrease factor is determined by measuring the output voltage decrease rate dV _SEL / dt of the fuel cell stack 20 when the sweep current I _SEL of the fuel cell stack 20 is increased in a stepped manner. As a result, the cell voltage reduction factor can be identified by simple processing.

In the present embodiment, the DC / DC converter 61 and the controller 70 function as sweep current control means for rapidly increasing the sweep current I _SEL of the fuel cell stack 20 from I ₀ to I ₁ in a stepwise manner. The voltage sensor 66 and the controller 70 function as measurement means for measuring the output voltage drop rate dV _SEL / dt of the fuel cell stack 20. The controller 70 functions as a determination unit for determining a cell voltage decrease factor based on the output voltage decrease rate dV _SEL / dt.

  It should be noted that the examples described through the embodiments of the invention can be used in appropriate combination 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 a thing. For example, in the flowchart shown in FIG. 4, the expression (3) may be applied instead of the expression (1), and the expression (4) may be applied instead of the expression (2).

  Further, if it is determined that the cause of the cell voltage drop is the shortage of fuel gas supply, the controller 70 performs processing for eliminating the shortage of fuel gas supply (for example, increasing the fuel gas supply pressure, increasing the fuel gas supply amount). Etc.) to attempt cell voltage recovery. As a process for solving the shortage of fuel gas supply, for example, by controlling the injector 34, the fuel gas supply pressure supplied to the fuel cell stack 20 is increased, or the rotation speed of the circulation pump 37 is controlled. Thus, processing such as increasing the flow rate of the fuel gas flowing into the fuel cell stack 20 is executed.

  On the other hand, if it is determined that the cause of the cell voltage drop is an insufficient oxidizing gas supply, the controller 70 executes a process for eliminating the insufficient oxidizing gas supply (for example, increasing the oxidizing gas supply amount, etc.) It is preferable to attempt voltage recovery. As a process for solving the shortage of the oxidizing gas supply, for example, a process of controlling the rotation speed of the air compressor 42 and increasing the supply amount of the oxidizing gas flowing into the fuel cell stack 20 is executed.

1 is a system configuration diagram of a fuel cell system according to an embodiment. It is a graph which shows the change of the cell voltage when the sweep electric current of a fuel cell stack is increased in steps. It is a graph which shows the IV characteristic of a fuel cell stack. It is a flowchart which shows the process for discriminating a cell voltage fall factor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Fuel cell system 21 ... Cell monitor 61 ... DC / DC converter 65 ... Voltage sensor 66 ... Current sensor 70 ... Controller

Claims (3)

A fuel cell stack formed by stacking a plurality of cells;
Cell voltage detection means for detecting the output voltage of each of the plurality of cells;
Voltage conversion means for controlling the output current and output voltage of the fuel cell stack, wherein the sweep current of the fuel cell stack is stepped when any one of the plurality of cells falls below a lower threshold voltage. Rapidly increasing voltage conversion means ;
Measuring means for measuring the output voltage drop rate of the fuel cell stack when the sweep current is increased stepwise; and
A determination unit that determines that the cell voltage decrease factor is an insufficient supply of oxidizing gas when the output voltage decrease rate of the fuel cell stack when the sweep current is rapidly increased stepwise is less than a predetermined threshold ;
A fuel cell system comprising:   A fuel cell stack formed by stacking a plurality of cells;
  Cell voltage detection means for detecting the output voltage of each of the plurality of cells;
  Voltage conversion means for controlling the output current and output voltage of the fuel cell stack, wherein the sweep current of the fuel cell stack is stepped when any one of the plurality of cells falls below a lower threshold voltage. Rapidly increasing voltage conversion means;
  Measuring means for measuring the output voltage drop rate of the fuel cell stack when the sweep current is increased stepwise; and
  A determination unit that determines that the cell voltage decrease factor is insufficient fuel gas supply when the output voltage decrease rate of the fuel cell stack when the sweep current is increased in a stepwise manner is a predetermined threshold value or more;
  Fuel cell system comprising The fuel cell system according to claim 1 or 2 , wherein
The fuel cell system, wherein the predetermined threshold is a minimum value of the output voltage drop rate of the fuel cell stack when the sweep current is increased stepwise in a state where fuel gas supply is insufficient.

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