JP2017168299A - Flooding determination method in fuel battery system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method capable of determining local flooding in a cell plane without using a current detection plate.SOLUTION: The flooding determination method in fuel battery system includes: a step for measuring an impedance of a fuel battery 40 with an impedance measurement part 81; a step for estimating a moisture content of the fuel battery 40 with a moisture content estimation part; and a step for determining it that local flooding occurs in the fuel battery 40 if the measured impedance is not less than a first threshold value and the estimated moisture content is not less than a second threshold value.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a flooding determination method in a fuel cell system.

  It is known that the output of the fuel cell is affected by the internal state of the fuel cell, for example, the wetness of the electrolyte membrane. Specifically, when the amount of moisture in the fuel cell is small and the electrolyte membrane is dry (so-called dry-up), the internal resistance increases and the output voltage of the fuel cell decreases. On the other hand, if the internal water content of the fuel cell is excessive (so-called flooding), the fuel cell electrode is covered with water, so that the diffusion of oxygen and hydrogen as reactants is hindered and the output voltage decreases. To do.

  In order to operate the fuel cell with high efficiency, it is necessary to optimally manage the internal moisture content of the fuel cell. The amount of moisture inside the fuel cell has a correlation with the impedance of the fuel cell. At present, the impedance of the fuel cell is measured by the AC impedance method to indirectly grasp the moisture state inside the fuel cell. .

  In the method of measuring the impedance of a general fuel cell, only the impedance of the entire fuel cell stack in which a plurality of cells are stacked can be measured, and the local internal moisture content in the plane of the cell cannot be grasped. was there. Therefore, in order to solve such a problem, the following Patent Document 1 discloses a method of detecting a local internal moisture content in the cell plane using a current detection plate.

JP 2009-252706 A

  However, in the method of Patent Document 1, a current detection plate in which a conductive part is arranged for each point to be detected is necessary in order to grasp the local internal moisture content in the cell plane.

  The present invention has been made in view of such problems, and an object thereof is to provide a method capable of determining local flooding of a fuel cell without using a current detection plate.

  In order to solve the above problems, a flooding determination method in a fuel cell system according to the present invention is a flooding determination method in a fuel cell system having a fuel cell in which a plurality of cells are stacked, and impedance measurement of the impedance of the fuel cell is performed. The step of measuring by the unit, the step of estimating the water content of the fuel cell by the water content estimation unit, and the measured impedance is not less than a predetermined value and the estimated water content is not less than the predetermined value. Determining that local flooding has occurred in the fuel cell.

  In the determination method based only on the impedance, the wet state of the fuel cell can be determined as the total amount, but local flooding in the cell plane cannot be determined. Usually, the impedance increases according to the dry state of the fuel cell. However, even when local flooding occurs in the fuel cell, the impedance increases, and the two cannot be distinguished. In the present invention, determination is made by considering not only the measured impedance value but also the estimated water content. By looking at this water content, whether the increase in impedance is due to the dry state of the fuel cell or local flooding Can be determined. Moreover, in this invention, in order to determine such local flooding, it can determine without using the electric current detection board in which the some electroconductive part is arrange | positioned.

  ADVANTAGE OF THE INVENTION According to this invention, the method which can determine the local flooding of a fuel cell can be provided, without using an electric current detection board.

It is a figure which shows the principal part structure of a fuel cell system. It is an equivalent circuit diagram which shows the electrical property of a cell. It is the graph which displayed the alternating current impedance of the fuel cell on the complex plane. It is a flowchart which shows the local flooding determination process by a control part.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. It should be noted that the following description of the preferred embodiment is merely an example, and is not intended to limit the present invention, its application, or its use.

  FIG. 1 is a diagram showing a main configuration of the fuel cell system 100. In the following description, a fuel cell hybrid vehicle (FCHV) is assumed as an example of the vehicle. However, not only the vehicle but also various moving bodies (for example, ships, airplanes, robots, etc.), stationary power sources, The present invention can also be applied to a portable fuel cell system.

  The fuel cell 40 (hereinafter also referred to as FC) is a means for generating electric power from supplied reaction gas (fuel gas and oxidizing gas), and is of various types such as a solid polymer type, a phosphoric acid type, and a molten carbonate type. A fuel cell can be used. The fuel cell 40 has a stack structure (fuel cell stack) in which a plurality of single cells including MEAs and the like are stacked in series. The output voltage (hereinafter referred to as FC voltage) and output current (hereinafter referred to as FC current) at the actual operation point of the fuel cell 40 are detected by the voltage sensor 140 and the current sensor 150, respectively. A fuel gas such as hydrogen gas is supplied from the fuel gas supply source 10 to the fuel electrode (anode) of the fuel cell 40, while an oxidizing gas such as air is supplied from the oxidizing gas supply source 70 to the oxygen electrode (cathode). Supplied.

  The fuel gas supply source 10 includes, for example, a hydrogen tank, various valves, and the like, and controls the amount of fuel gas supplied to the fuel cell 40 by adjusting the valve opening, the ON / OFF time, and the like. The oxidizing gas supply source 70 includes, for example, an air compressor, a motor that drives the air compressor, an inverter, and the like, and adjusts the amount of oxidizing gas supplied to the fuel cell 40 by adjusting the rotational speed of the motor.

  The battery 60 is a chargeable / dischargeable secondary battery, and is composed of, for example, a nickel metal hydride battery. Of course, instead of the battery 60, a chargeable / dischargeable capacitor (for example, a capacitor) other than the secondary battery may be provided. The battery 60 and the fuel cell 40 are connected in parallel to an inverter 110 for a traction motor, and a DC / DC converter 130 is provided between the battery 60 and the inverter 110.

  The inverter 110 is, for example, a pulse width modulation type PWM inverter, which converts DC power output from the fuel cell 40 or the battery 60 into three-phase AC power in accordance with a control command given from the control unit 80, and a traction motor ( Hereinafter, it is simply supplied to the motor 115. The motor 115 is a motor for driving the wheels 116 </ b> L and 116 </ b> R (load), and the rotation speed of the motor is controlled by the inverter 110.

  The DC / DC converter 130 is a full-bridge converter configured by, for example, four power transistors and a dedicated drive circuit (all not shown). The DC / DC converter 130 has a function of boosting or stepping down a DC voltage input from the fuel cell 40 or the like and outputting it to the battery 60 side. Further, charging / discharging of the battery 60 is realized by the function of the DC / DC converter 130.

  An auxiliary machine 120 such as a vehicle auxiliary machine or an FC auxiliary machine is connected between the battery 60 and the DC / DC converter 130. The battery 60 is a power source for these auxiliary machines 120. The vehicle auxiliary equipment refers to various electric power devices (lighting equipment, air conditioning equipment, hydraulic pump, etc.) used during vehicle operation, and the FC auxiliary equipment is used to operate the fuel cell 40. It refers to various power devices (pumps for supplying fuel gas and oxidizing gas, etc.).

  The control unit 80 includes a CPU, a ROM, a RAM, and the like, and includes a voltage sensor 140 that detects an FC voltage, a current sensor 150 that detects an FC current, a temperature sensor 50 that detects the temperature of the fuel cell 40, and a charge state of the battery 60. Each part of the system is centrally controlled on the basis of sensor signals input from an SOC sensor for detecting the accelerator and an accelerator pedal sensor for detecting the opening of the accelerator pedal. In addition, the control unit 80 includes a water content estimation unit (not shown). The impedance measuring unit 81 can measure the impedance by applying an alternating current to the fuel cell 40 based on each sensor signal. The water content estimation unit can estimate the water content of the fuel cell 40 based on each sensor signal. The control unit 80 uses the measurement result from the impedance measurement unit 81 and the estimation result from the water content estimation unit for local flooding determination processing in the cell plane. Details of the local flooding determination process will be described later.

FIG. 2 is an equivalent circuit diagram showing the electrical characteristics of the cells constituting the fuel cell.
The equivalent circuit of the cell has a circuit configuration in which R1 is connected in series to a parallel connection circuit of R2 and C. Here, R1 corresponds to the electrical resistance of the electrolyte membrane, and R2 corresponds to the resistance-converted activation overvoltage and diffusion overvoltage. C corresponds to the electric double layer capacity formed at the interface between the anode electrode and the electrolyte membrane and at the interface between the cathode electrode and the electrolyte membrane. When a sine wave current having a predetermined frequency is applied to this equivalent circuit, the voltage response is delayed with respect to the change in current.

  FIG. 3 is a graph showing the AC impedance of the fuel cell on a complex plane. The horizontal axis indicates the real part of the AC impedance, and the vertical axis indicates the imaginary part of the AC impedance. ω is the angular frequency of the sinusoidal current.

  When a sine wave signal from high frequency to low frequency is applied to the equivalent circuit shown in FIG. 2, a graph as shown in FIG. 3 is obtained. When the frequency of the sine wave signal is infinitely large (ω = ∞), the AC impedance is R1. The AC impedance when the frequency of the sine wave signal is very small (ω = 0) is R1 + R2. The AC impedance obtained when the frequency of the sine wave signal is changed between a high frequency and a low frequency draws a semicircle as shown in FIG.

  As described above, by using the AC impedance method, it is possible to separately measure R1 and R2 in the equivalent circuit of the fuel cell.

  Next, the flooding determination method in this embodiment will be described. FIG. 4 is a flowchart showing local flooding determination processing of the fuel cell.

(Step S110)
First, the impedance measuring unit 81 measures the impedance of the fuel cell, and determines whether or not the measured impedance is equal to or greater than a first threshold value. If the impedance is greater than or equal to the first threshold, the process proceeds to step S120, and if the impedance is less than the first threshold, the determination flow ends. Normally, the impedance increases according to the dry state of the fuel cell, and the impedance also increases when the fuel cell is locally flooded. A first threshold value is set for.

  In this step S110, the AC impedance measurement of the fuel cell 40 by the impedance measuring unit 81 is performed by the following procedure.

(1) The impedance measuring unit 81 applies a sine wave signal to the direct current of the fuel cell 40.
(2) The impedance measuring unit 81 samples the response voltage detected by the voltage sensor 140 and the response current detected by the current sensor 150 at a predetermined sampling rate, performs a fast Fourier transform process (FFT process), and responds. Dividing the voltage and response current into real and imaginary components respectively, dividing the FFT-processed response voltage by the FFT-processed response current to calculate the AC impedance real and imaginary components, and on the complex plane The distance r from the origin and the phase angle θ are calculated (see FIG. 3). By measuring the response voltage and response current of the fuel cell 40, the AC impedance of the fuel cell 40 can be calculated.

(Step S120)
In step S120 following step S110 (YES), the water content estimation unit (not shown) of the control unit 80 estimates the water content of the fuel cell 40, and whether or not the estimated water content is greater than or equal to the second threshold value. Determine. If it is equal to or greater than the second threshold, the controller 80 determines that the flooding is local, and proceeds to step S130 (drainage control). If it is less than the second threshold, it is determined to be in the dry state, and step S140 (wetting control). Proceed to Note that when the impedance is increased from the first threshold (step S110), the second threshold is the impedance increased due to the dry state of the fuel cell, or the impedance is increased due to the occurrence of local flooding of the fuel cell. It is set to an arbitrary value that can be determined.

Here, the calculation method of the water content of the fuel cell 40 in step S120 will be described. Based on the following equation (1), the water content can be calculated by water balance calculation theoretically derived based on the power generation state of the fuel cell.
Theoretical water balance (g / s) = amount of water generated by FC power generation-amount of gas carried away ... (1)
Amount of water generated by FC power generation (g / s) = total current / (2 × 96485) × number of cells × 18
Gas removal water volume (g / s) ... Calculated based on cathode outlet gas flow rate and saturated vapor pressure at cathode outlet pressure

(Step S130)
In step S130 following step S120 (YES), since it is determined that the flooding is local, drainage control is performed by the control unit 80. The drainage control is not particularly limited as long as the excessive moisture generated in the fuel cell 40 is controlled to be discharged to the outside of the fuel cell 40. For example, the gas pressure is pulsated and supplied. It is possible to take this method.

(Step S140)
In step S140 following step S120 (NO), since it is determined that the fuel cell 40 is in the dry state, the control unit 80 performs wetting control. Wetting control is not particularly limited as long as control is performed to increase the internal moisture content of the fuel cell. For example, the reaction gas (air) supplied to the cathode side of the fuel cell 40 is humidified. It is possible to adopt a method of performing supply control.

  The flooding determination method in the present embodiment described above is a flooding determination method in the fuel cell system 100 including the fuel cell 40 configured by stacking a plurality of cells, and the impedance of the fuel cell 40 is measured by the impedance measuring unit 81. The step of estimating the water content of the fuel cell 40 by the water content estimation unit, and the impedance measured by the impedance measurement unit 81 is greater than or equal to the first threshold and the water content estimation unit estimated by the water content estimation unit. Determining that local flooding has occurred in the fuel cell 40 when the amount of water is greater than or equal to the second threshold value. Based on these measured impedance values and estimated water content, drainage control is performed when local flooding is determined, and wetting control is performed when determined dry.

  The embodiments of the present invention have been described above with reference to specific examples. However, the present invention is not limited to these specific examples. In other words, those specific examples that have been appropriately modified by those skilled in the art are also included in the scope of the present invention as long as they have the characteristics of the present invention.

10: Fuel gas supply source 40: Fuel cell 50: Temperature sensor 60: Battery 70: Oxidation gas supply source 80: Control unit 100: Fuel cell system 110: Inverter 115: Motor 120: Auxiliary equipment 130: DC / DC converter 140 : Voltage sensor 150: Current sensor

Claims (1)

A flooding determination method in a fuel cell system having a fuel cell in which a plurality of cells are stacked,
Measuring the impedance of the fuel cell with an impedance measuring unit;
Estimating the water content of the fuel cell by a water content estimation unit;
Determining that local flooding has occurred in the fuel cell when the measured impedance is greater than or equal to a predetermined value and the estimated water content is greater than or equal to a predetermined value. Method for determining flooding in the system.