JP2008226593A - Fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell system that has superior power controllability at the time of voltage stepping down treatment. <P>SOLUTION: The fuel cell system includes a feed back control means in which a power output voltage is stepped down to a target voltage by controlling the output of the fuel cell by a voltage feed-back control based on a voltage deviation that is obtained by adding a proportional term resulting from multiplying an proportional gain on a deviation between a target current and a measured current of the fuel cell to an integration term in which that deviation is time integrated and then multiplied with an integration gain. The integration term is zero-cleared by the feedback control means at a point A where the current characteristic against voltage stepping down of the fuel cell is changed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

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

  A fuel cell is a power generation system that directly converts energy released during an oxidation reaction into electrical energy by oxidizing fuel by an electrochemical process. Both sides of an electrolyte membrane for selectively transporting hydrogen ions Has a stack structure in which a plurality of membrane-electrode assemblies are sandwiched between a pair of electrodes made of a porous material. Among them, a solid polymer electrolyte fuel cell using a solid polymer membrane as an electrolyte is expected to be used as an in-vehicle power source because it is low-cost and easy to downsize and has a high output density. .

In general, this type of fuel cell has an optimum temperature range of 70 to 80 ° C. for power generation. However, in an environment such as a cold region, it may take a long time to reach the optimum temperature range after startup. Therefore, various warm-up systems are being studied. For example, Japanese Patent Application Laid-Open No. 2004-30979 discloses a technique for controlling the self-heating amount of a fuel cell by performing low-efficiency operation with lower power generation efficiency than normal operation, and warming up the fuel cell. ing. According to this method, since the fuel cell can be self-warmed, it is not necessary to mount a warm-up device, and the convenience is excellent.
JP 2004-30979 A

  In order to implement the low efficiency operation, it is necessary to set the output voltage of the fuel cell to a voltage lower than the voltage determined by the IV characteristic curve (current vs. voltage characteristic curve) while supplying power to the system. As a method for performing the voltage drop processing, for example, in a state where the supply of the oxidizing gas is stopped while supplying the fuel gas to the fuel cell, the difference between the required power of the system and the target current of the fuel cell determined from the current voltage and the current filter value is calculated. Based on the PI control (proportional integral control), a voltage deviation is calculated every calculation cycle, and feedback control is performed so that the output of the fuel cell matches the system required power.

  However, as the voltage of the fuel cell is lowered, there is a point where the current characteristic with respect to the voltage drop changes. Therefore, when the voltage drop is controlled by PI control, the power controllability is lowered due to the influence of the integral term indicating the steady-state deviation. Sometimes. This is because there is a point where the factor that lowers the output voltage of the fuel cell changes from a voltage drop due to discharge from the fuel cell capacity component (parasitic capacitance) to a voltage drop due to the generation of residual oxidizing gas inside the fuel cell. It is.

  Therefore, an object of the present invention is to propose a fuel cell system capable of solving such a problem and performing a voltage drop process of a fuel cell excellent in power controllability.

  In order to solve the above problems, a fuel cell system according to the present invention includes a fuel cell, a proportional term obtained by multiplying the deviation between the target current of the fuel cell and the measured current by a proportional gain, and integrating the deviation over time. Feedback control means for lowering the output voltage to the target voltage while controlling the output of the fuel cell by voltage feedback control based on the voltage deviation obtained by adding the integral term obtained by multiplying the integral gain. The feedback control means clears the integral term to zero at the point where the current characteristic with respect to the voltage drop of the fuel cell changes.

  Since the target current and the measured current match at the point where the current characteristic with respect to the voltage drop changes, the power controllability can be improved by clearing the integral term indicating the steady deviation between the target current and the measured current to zero.

  Here, the point at which the current characteristics change with respect to the voltage drop changes from a voltage drop due to discharge from a capacitive component formed parasitically inside the fuel cell to a voltage drop due to power generation of the oxidizing gas present inside the fuel cell. It is a point. At the point where the current characteristic with respect to the voltage drop changes, it has been found that the actual measured current of the fuel cell matches the target current, and that the time derivative of the output voltage of the fuel cell is below a predetermined threshold.

  According to the present invention, the voltage controllability can be improved by clearing the integral term indicating the steady deviation between the target current and the measured current to zero at the point where the current characteristic with respect to the voltage drop changes.

Embodiments according to the present invention will be described below with reference to the drawings.
FIG. 1 shows a system configuration of a fuel cell system 10 according to the present embodiment.
The fuel cell system 10 functions as an in-vehicle power supply system mounted on a fuel cell vehicle. The fuel cell stack 20 generates electric power by receiving supply of reaction gas (fuel gas, oxidant gas), and air as oxidant gas. Gas supply system 30 for supplying the fuel cell stack 20 with hydrogen, fuel gas supply system 40 for supplying hydrogen gas as the fuel gas to the fuel cell stack 20, and power for controlling charge and discharge of power A system 50, a cooling system 60 for cooling the fuel cell stack 20, and a controller (ECU) 70 for controlling the entire system are provided.

  The fuel cell stack 20 is a solid polymer electrolyte cell stack formed by stacking a plurality of cells in series. 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. The fuel cell stack 20 as a whole undergoes an electromotive reaction of the formula (3).

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)

  A voltage sensor 71 for detecting the output voltage of the fuel cell stack 20 and a current sensor 72 for detecting the generated current are attached to the fuel cell stack 20.

  The oxidizing gas supply system 30 includes an oxidizing gas passage 34 through which oxidizing gas supplied to the cathode electrode of the fuel cell stack 20 flows, and an oxidizing off gas passage 36 through which oxidizing off gas discharged from the fuel cell stack 20 flows. . In the oxidizing gas passage 34, an air compressor 32 that takes in the oxidizing gas from the atmosphere via the filter 31, a humidifier 33 for humidifying the oxidizing gas supplied to the cathode electrode of the fuel cell stack 20, and an oxidizing gas supply A throttle valve 35 for adjusting the amount is provided. The oxidizing off gas passage 36 includes a back pressure adjusting valve 37 for adjusting the oxidizing gas supply pressure, and a humidifier 33 for exchanging moisture between the oxidizing gas (dry gas) and the oxidizing off gas (wet gas). Is provided.

  The fuel gas supply system 40 includes a fuel gas supply source 41, a fuel gas passage 45 through which fuel gas supplied from the fuel gas supply source 41 to the anode electrode of the fuel cell stack 20 flows, and fuel discharged from the fuel cell stack 20. A circulation passage 46 for returning the off gas to the fuel gas passage 45, a circulation pump 47 that pumps the fuel off gas in the circulation passage 46 to the fuel gas passage 43, and an exhaust drainage passage 48 that is branched and connected to the circulation passage 47. Have.

  The fuel gas supply source 41 is composed of, for example, a high-pressure hydrogen tank or a hydrogen storage alloy, and stores high-pressure (for example, 35 MPa to 70 MPa) hydrogen gas. When the shut-off valve 42 is opened, the fuel gas flows out from the fuel gas supply source 41 into the fuel gas passage 45. The fuel gas is decompressed to, for example, about 200 kPa by the regulator 43 and the injector 44 and supplied to the fuel cell stack 20.

  The fuel gas supply source 41 includes a reformer that generates a hydrogen-rich reformed gas from a hydrocarbon-based fuel, and a high-pressure gas tank that stores the reformed gas generated by the reformer in a high-pressure state. It may be configured.

  The regulator 43 is a device that regulates the upstream side pressure (primary pressure) to a preset secondary pressure, and includes, for example, a mechanical pressure reducing valve that reduces the primary pressure. The mechanical pressure reducing valve has a housing in which a back pressure chamber and a pressure adjusting chamber are formed with a diaphragm therebetween, and the primary pressure is reduced to a predetermined pressure in the pressure adjusting chamber by the back pressure in the back pressure chamber. It has a configuration for the next pressure.

  The injector 44 is an electromagnetically driven on-off valve capable of adjusting the gas flow rate and gas pressure by driving the valve body directly with a predetermined driving cycle with an electromagnetic driving force and separating the valve body from the valve seat. The injector 44 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 slidably accommodated and opens and closes the injection hole.

  An exhaust / drain valve 49 is disposed in the exhaust / drain passage 48. The exhaust / drain valve 49 is operated according to a command from the controller 70 to discharge the fuel off-gas and impurities including impurities in the circulation passage 46 to the outside. By opening the exhaust drain valve 49, the concentration of impurities in the fuel off-gas in the circulation passage 46 is lowered, and the hydrogen concentration in the fuel off-gas circulating in the circulation system can be increased.

  The fuel off gas discharged through the exhaust drain valve 49 is mixed with the oxidizing off gas flowing through the oxidizing off gas passage 34 and diluted by a diluter (not shown). The circulation pump 47 circulates and supplies the fuel off-gas in the circulation system to the fuel cell stack 20 by driving the motor.

  The power system 50 includes a DC / DC converter 51, a battery 52, a traction inverter 53, a traction motor 54, and auxiliary machinery 55. The DC / DC converter 51 boosts the DC voltage supplied from the battery 52 and outputs it to the traction inverter 53, and the DC power generated by the fuel cell stack 20, or the regenerative power collected by the traction motor 54 by regenerative braking. And a function of charging the battery 52 by stepping down the voltage. The charge / discharge of the battery 52 is controlled by these functions of the DC / DC converter 51. Further, the operation point (output voltage, output current) of the fuel cell stack 20 is controlled by voltage conversion control by the DC / DC converter 51.

  The battery 52 functions as a surplus power storage source, a regenerative energy storage source during regenerative braking, and an energy buffer during load fluctuations associated with acceleration or deceleration of the fuel cell vehicle. As the battery 52, 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 53 is, for example, a PWM inverter driven by a pulse width modulation method, and converts a DC voltage output from the fuel cell stack 20 or the battery 52 into a three-phase AC voltage in accordance with a control command from the controller 70. The rotational torque of the traction motor 54 is controlled. The traction motor 54 is a three-phase AC motor, for example, and constitutes a power source of the fuel cell vehicle.

  Auxiliary machines 55 are motors (for example, power sources such as pumps) arranged in each part in the fuel cell system 10, inverters for driving these motors, and various on-vehicle auxiliary machines. (For example, an air compressor, an injector, a cooling water circulation pump, a radiator, etc.) is a general term.

  The cooling system 60 includes refrigerant passages 61, 62, 63, and 64 for flowing the refrigerant circulating in the fuel cell stack 20, a circulation pump 65 for pumping the refrigerant, and heat exchange between the refrigerant and the outside air. A radiator 66, a three-way valve 67 for switching the refrigerant circulation path, and a temperature sensor 74 for detecting the refrigerant temperature are provided. During normal operation after the warm-up operation is completed, the refrigerant flowing out of the fuel cell stack 20 flows through the refrigerant passages 61 and 64 and is cooled by the radiator 66, and then flows through the refrigerant passage 63 and flows into the fuel cell stack 20 again. Thus, the three-way valve 67 is controlled to open and close. On the other hand, during the warm-up operation immediately after system startup, the three-way valve 67 is controlled to open and close so that the refrigerant flowing out of the fuel cell stack 20 flows through the refrigerant passages 61, 62, 63 and again into the fuel cell stack 20.

  The controller 70 is a computer system including a CPU, a ROM, a RAM, an input / output interface, and the like, and each part of the fuel cell system 10 (the oxidizing gas supply system 30, the fuel gas supply system 40, the power system 50, and the cooling system 60). It functions as a control means for controlling. For example, when the controller 70 receives the start signal IG output from the ignition switch, the controller 70 starts the operation of the fuel cell system 10, and the accelerator opening signal ACC output from the accelerator sensor or the vehicle speed signal output from the vehicle speed sensor. The required power of the entire system is obtained based on VC or the like.

  The required power of the entire system is the total value of the vehicle running power and the auxiliary machine power. Auxiliary power is the power consumed by in-vehicle accessories (humidifiers, air compressors, hydrogen pumps, cooling water circulation pumps, etc.), and equipment required for vehicle travel (transmissions, wheel control devices, steering devices, and suspensions) Power consumed by devices, etc., and power consumed by devices (air conditioners, lighting fixtures, audio, etc.) disposed in the passenger space.

  Then, the controller 70 determines the distribution of the output power of each of the fuel cell stack 20 and the battery 52, calculates the power generation command value, and oxidizes so that the power generation amount of the fuel cell stack 20 matches the target power. The gas supply system 30 and the fuel gas supply system 40 are controlled. Further, the controller 70 controls the operating point (output voltage, output current) of the fuel cell stack 20 by controlling the DC / DC converter 51 and adjusting the output voltage of the fuel cell stack 20. The controller 70 outputs, for example, each U-phase, V-phase, and W-phase AC voltage command value to the traction inverter 53 as a switching command so as to obtain a target torque according to the accelerator opening, and the traction motor 54. The output torque and rotational speed of the motor are controlled.

FIG. 2 is an exploded perspective view of the cells 21 constituting the fuel cell stack 20.
The cell 21 includes an electrolyte membrane 22, an anode electrode 23, a cathode electrode 24, and separators 26 and 27. The anode electrode 23 and the cathode electrode 24 are diffusion electrodes having a sandwich structure with the electrolyte membrane 22 sandwiched from both sides. Separators 26 and 27 made of a gas-impermeable conductive member form fuel gas and oxidizing gas flow paths between the anode electrode 23 and the cathode electrode 24 while sandwiching the sandwich structure from both sides. . The separator 26 is formed with a rib 26a having a concave cross section. When the anode electrode 23 comes into contact with the rib 26a, the opening of the rib 26a is closed and a fuel gas flow path is formed. The separator 27 is formed with a rib 27a having a concave cross section. When the cathode electrode 24 comes into contact with the rib 27a, the opening of the rib 27a is closed and an oxidizing gas flow path is formed.

  The anode electrode 23 is composed mainly of a carbon powder supporting a platinum-based metal catalyst (Pt, Pt—Fe, Pt—Cr, Pt—Ni, Pt—Ru, etc.), a catalyst layer 23 a in contact with the electrolyte membrane 22, It has a gas diffusion layer 23b formed on the surface of the catalyst layer 23a and having both air permeability and electronic conductivity. Similarly, the cathode electrode 24 has a catalyst layer 24a and a gas diffusion layer 24b. More specifically, the catalyst layers 23a and 24a are made by dispersing carbon powder carrying platinum or an alloy made of platinum and another metal in a suitable organic solvent, adding an appropriate amount of an electrolyte solution, and forming a paste, thereby forming the electrolyte membrane 22 Screen printed on top. The gas diffusion layers 23b and 24b are formed of carbon cloth, carbon paper, or carbon felt woven with carbon fiber yarns. The electrolyte membrane 22 is a proton conductive ion exchange membrane formed of a solid polymer material, for example, a fluororesin, and exhibits good electrical conductivity in a wet state. A membrane-electrode assembly 25 is formed by the electrolyte membrane 22, the anode electrode 23, and the cathode electrode 24.

  At the interface between the electrolyte membrane 22 and the catalyst layer 23a and at the interface between the electrolyte membrane 22 and the catalyst layer 24a, electrons and hydrogen ions involved in the electrochemical reaction shown in the above formulas (1) and (2) gather. As a result, an electric double layer is formed. Since the voltage generated by the electrons and hydrogen ions collected in the electric double layer is consumed as an energy source for activating each of hydrogen gas and oxygen gas in the ground state, it is generally referred to as activation overvoltage. . It is known that the electric double layer formed at the interface functions as an electric energy storage source, and its dynamic electric characteristics are equivalent to a capacitor.

  When the generated current is rapidly increased or decreased, the ohmic voltage drop caused by the ohmic resistance of the electrolyte membrane 22 follows the change in the generated current with good responsiveness, but the activation overvoltage generated in the electric double layer is It cannot follow the change in current with good responsiveness, and slowly settles to an equilibrium state over a period of time. The reason why such a difference occurs is that the electrical characteristics of the electrolyte membrane 22 can be modeled as a resistance element, whereas the electrical characteristics of the electric double layer can be modeled as a capacitor.

FIG. 3 is an equivalent circuit diagram modeling the dynamic electrical characteristics of the fuel cell stack 20.
The fuel cell stack 20 has a circuit configuration in which an ideal fuel cell 28 and a capacitor 29 are connected in parallel. The ideal fuel cell 28 is a model of a virtual fuel cell that does not have the above-described CV characteristics, and behaves equivalent to a variable power source in terms of electrical characteristics. The capacitor 29 is obtained by modeling the electric behavior of the electric double layer formed at the interface as a capacitive element. The external load 56 is a model of the power system 50. The current flowing out of the ideal fuel cell 28 is Ifc, the output voltage of the ideal fuel cell 28 (output voltage of the fuel cell stack 20) is Vfc, the current flowing into the capacitor 29 is Ic, and the current flowing out of the fuel cell stack 20 to the external load 56 is Is. When the capacitance of the capacitor 29 is C and the time is t, the following equations (4) to (5) are established.

Ifc = Ic + Is (4)
Ic = C · ΔVfc / Δt (5)

  As shown in the equations (4) to (5), when the output voltage Vfc is boosted, the current Ic flowing into the capacitor 29 increases according to the amount of change ΔVfc / Δt per unit time. The current Is flowing out to the load 56 decreases. On the other hand, when the output voltage Vfc is stepped down, the current Ic flowing into the capacitor 29 decreases according to the change amount ΔVfc / Δt per unit time, so that the current Is flowing from the fuel cell stack 20 to the external load 56 increases.

  In the present embodiment, when the stack temperature when the fuel cell system 10 is activated is lower than a predetermined temperature (for example, 0 ° C.), the low-efficiency operation is performed while the vehicle is traveling, and the fuel cell stack 20 is warmed up. Low-efficiency operation refers to operation with low power generation efficiency by increasing power generation loss by controlling the amount of reactant gas supplied to the fuel cell stack 20 by setting the air stoichiometric ratio to less than 1.0. The air stoichiometric ratio is an oxygen surplus ratio and indicates how much surplus oxygen is supplied to oxygen necessary for reacting with hydrogen without excess or deficiency. When low efficiency operation is performed with the air stoichiometric ratio set low, the concentration overvoltage becomes larger than that during normal operation, so heat loss (power generation loss) in energy that can be extracted by the reaction between hydrogen and oxygen increases.

  In the low-efficiency operation while the vehicle is running, the flow rate of the oxidizing gas supplied to the fuel cell stack 20 while fixing the output voltage of the fuel cell stack 20 to a constant voltage value lower than the voltage value based on its IV characteristic. Is variably controlled according to the required power. Here, the reason for fixing the output voltage of the fuel cell stack 20 to a constant voltage value is that when the output voltage of the fuel cell stack 20 is changed, as shown in the equations (4) to (5), the fuel cell stack 20 This is because charging / discharging of the power from the capacitor 29 occurs due to the capacity characteristics of the capacitor, and the power supplied from the fuel cell stack 20 to the external load 56 becomes excessive or insufficient.

  The output voltage of the fuel cell stack 20 during the low-efficiency operation is set to a voltage value that can achieve a quick warm-up operation and obtain a minimum motor output necessary for vehicle travel. From the viewpoint of early warm-up, it is desirable to set the output voltage of the fuel cell stack 20 as low as possible. However, if it is too low, the motor output required for vehicle travel may not be obtained. It is desirable to set the voltage so as to obtain an appropriate motor output during vehicle travel while satisfying the performance.

  Thus, since the output voltage of the fuel cell stack 20 during the low efficiency operation is fixed to a constant voltage, the controller 70 variably controls the supply amount of the oxidizing gas to the fuel cell stack 20 to obtain the required power ( The power generation control according to the accelerator opening etc. is implemented. For example, when the load is high, the flow rate of the oxidizing gas to the fuel cell stack 20 is increased, and when the load is low, the flow rate of the oxidizing gas to the fuel cell stack 20 is decreased. However, the fuel gas supply to the fuel cell stack 20 is assumed to be maintained at a constant flow rate.

  The low efficiency operation is performed until the stack temperature rises to a predetermined temperature (for example, 0 ° C.), and when the stack temperature reaches the predetermined temperature, the operation is switched to the normal operation.

FIG. 4 shows the IV characteristics of the fuel cell stack 20.
During normal operation, the operating point (output current Ifc, output voltage Vfc) is set to any operating point (for example, OP2) on the IV characteristic curve (current vs. voltage characteristic curve) 200 in order to increase power generation efficiency. Is done. On the other hand, during low-efficiency operation, the power generation efficiency is reduced to increase heat loss, so the operation point is set to an operation point having a voltage value lower than that of the IV characteristic curve 200 (for example, OP1). In low-efficiency operation, the output voltage Vfc is fixed at V1, so that by controlling the flow rate of air supplied from the air compressor 32 to the fuel cell stack 20, the output current Ifc is adjusted and the operation load (for example, the accelerator is opened). Power generation control according to the degree).

  By the way, immediately after the system is started (when the ignition switch is turned on), the output voltage of the fuel cell stack 20 is equal to the open-circuit voltage OCV. Processing is performed, and the output voltage of the fuel cell stack 20 is stepped down to a desired voltage value (for example, V1).

Next, details of the voltage drop process will be described.
FIG. 5 shows a functional block diagram for calculating the voltage command value V_ref of the fuel cell stack 20 during the voltage drop process. Each of these functions is realized by the calculation function of the controller 70.

  The target current calculation function 301 is based on the target power P_ref of the fuel cell stack 20 calculated according to the required power from the external load 56 and the voltage filter value V_flt that is the output value of the voltage sensor 71. Target current I_reg is calculated. The subtractor 302 obtains a deviation ΔI between the current filter value I_flt that is the output value of the current sensor 72 and the target current I_reg calculated by the target current calculation function 301.

The PI control function 303 calculates a proportional feedback correction value (proportional term: P = K _P × ΔI) by multiplying the deviation ΔI by the proportional gain K _P and integrates it into the time integral value of deviation (∫ΔIdt). By multiplying the gain K _I , an integral feedback correction value (integral term: I = K _I × ∫ΔIdt) is calculated, and a feedback voltage correction value ΔV_ref obtained by adding these is calculated.

  The delay circuit 304 stores the voltage command value V_ref before one calculation cycle. The subtractor 305 calculates the voltage command value V_ref of the next calculation cycle by subtracting the feedback voltage correction value ΔV_ref from the voltage command value V_ref of the previous calculation cycle. The DC / DC converter 51 performs control so that the output voltage Vfc of the fuel cell stack 20 matches the voltage command value V_ref.

The controller 70 is composed by multiplying a proportional term P of the deviation ΔI between the target current I_reg and the current filter value I_flt of the fuel cell stack 20 formed by multiplying a proportional gain K _P, the more integral gain K _I integrates the deviation ΔI time It functions as a feedback control means for feedback-controlling the output voltage of the fuel cell stack 20 based on the feedback voltage correction value ΔV_ref obtained by adding the integral term I. The controller 70 calculates the voltage command value V_ref for each calculation cycle as described above until the output voltage Vfc of the fuel cell stack 20 matches the target voltage V1.

  During the voltage drop process, the supply of the oxidizing gas from the oxidizing gas supply system 30 to the fuel cell stack 20 is stopped while the fuel gas supply from the fuel gas supply system 40 to the fuel cell stack 20 is maintained.

FIG. 6 shows the voltage versus time characteristics of the fuel cell stack 20 during the voltage drop process.
A voltage drop curve 400 shows a time change of the voltage filter value V_flt at the time of the voltage drop process. The period from the time t0 to the time t1 and the period from the time t2 to the time t3 are mainly abrupt due to discharge from the capacitor 29. A voltage drop is shown, and a period from time t1 to time t2 shows a gradual voltage drop mainly due to power generation of the oxidizing gas remaining in the fuel cell stack 20. As shown in this graph, it can be understood that there is a point A where the current characteristic with respect to the voltage drop changes at time t1.

FIG. 7 shows the current vs. time characteristics of the fuel cell stack 20 during the voltage drop process.
In the figure, a current filter value 500 indicates a time change of the current filter value I_flt during the voltage drop process, and a target current 600 indicates a time change of the target current I_reg during the voltage drop process. Immediately after starting the voltage drop process, the deviation between the current filter value 500 and the target current 600 is large, and the current filter value 500 cannot easily follow the target current 600. The deviation between the two gradually decreases with time, and the current filter value 500 matches the target current 600 at time t1. Here, reference numeral 800 indicates a steady deviation obtained by time-integrating the deviation between the current filter value 500 and the target current 600 during the period from time t0 to time t1, and is equal to the integral term I at the time t1.

  When the current filter value 500 coincides with the target current 600, that is, at the point A where the current characteristic with respect to the voltage drop changes, the integral term I indicating the steady deviation is cleared to zero, thereby improving the current follow-up controllability after the time t1. be able to.

  In order to facilitate understanding of the present embodiment, reference numeral 700 indicates a time change of the current filter value I_flt when the integral term I is not cleared to zero when the current filter value 500 matches the target current 600. If the integral term I is not cleared to zero, current follow-up control is performed in a state where steady deviations are accumulated, and it can be understood that the current follow-up controllability is deteriorated.

  Note that the point A at which the current characteristic with respect to the voltage drop changes is by detecting whether or not the current filter value 500 matches the target current 600, or when the time derivative of the voltage filter value V_flt falls below a predetermined threshold. By detecting this, its presence can be detected. The integral term I may be cleared to zero when the point A at which the current characteristic with respect to the voltage drop changes is detected.

  Note that the voltage drop process is performed as a preparation for performing the warm-up operation by the low-efficiency operation described above, and can also be performed, for example, for a catalyst activation process. It is known that when the fuel cell stack 20 is continuously operated at an oxidation potential, an oxide film is formed on the surface of the platinum catalyst, and the overvoltage increases, so that the IV characteristic decreases. By reducing the output voltage of the fuel cell stack 20 to the reduction potential, the oxide film covering the platinum catalyst can be removed and the IV characteristics can be recovered.

  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. FIG. It is a disassembled perspective view of a cell. It is an equivalent circuit diagram of a fuel cell stack. It is explanatory drawing of the operating point of a fuel cell stack. It is a functional block diagram for calculating a voltage command value at the time of voltage drop processing. It is a graph which shows the voltage versus time characteristic of the fuel cell stack at the time of a voltage drop process. It is a graph which shows the electric current vs. time characteristic of the fuel cell stack at the time of a voltage drop process.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Fuel cell system 20 ... Fuel cell stack 30 ... Oxidation gas supply system 40 ... Fuel gas supply system 50 ... Electric power system 60 ... Cooling system 70 ... Controller

Claims (4)

A fuel cell;
Based on a voltage deviation obtained by adding a proportional term obtained by multiplying the deviation between the target current of the fuel cell and the measured current by a proportional gain and an integral term obtained by integrating the deviation with time and further multiplying the integral gain. Feedback control means for lowering the output voltage of the fuel cell to a target voltage by voltage feedback control, and
The feedback control means is a fuel cell system in which the integral term is cleared to zero at a point where a current characteristic with respect to a voltage drop of the fuel cell changes. The fuel cell system according to claim 1,
The point at which the current characteristics change with respect to the voltage drop changes from a voltage drop due to discharge from a capacitive component formed parasitically inside the fuel cell to a voltage drop due to power generation of the oxidizing gas present inside the fuel cell. The point is the fuel cell system. The fuel cell system according to claim 1,
The point at which the current characteristic with respect to the voltage drop changes is a fuel cell system in which the measured current of the fuel cell matches the target current. The fuel cell system according to claim 1,
The point at which the current characteristic with respect to the voltage drop changes is a point at which the time derivative of the output voltage of the fuel cell is equal to or less than a predetermined threshold value.

Images