Method for diagnosing an operating state of a fuel cell system, method for controlling a fuel cell system and fuel cell system

Abstract

The invention relates to a method for diagnosing the operating state of a fuel cell system, comprising at least one fuel cell and a system for operating medium management, an operating model (M) of the fuel cell is created, the operating model comprises an electrochemical model (ecM) and a physical model (pM) coupled thereto, by means of which the electrical operating power (P) of the fuel cell is determined on the basis of the electrical operating current (J) and the thermodynamic operating parameters, and by means of which the time-dependent spatial distribution of the thermodynamic operating parameters is determined, in order to diagnose the operating state, at least the following steps are continuously carried out: detecting an electrical operating current (J) of the fuel cell and a state variable (psi) of an operating medium of the fuel cell, detecting a state variable of the operating medium on the input side and / or the output side of the fuel cell, and determining an electrical operating power of the fuel cell by means of an operating model on the basis of the electrical operating current and the detected state variable.

Description

Technical Field

[0001] The present invention relates to a method for diagnosing the operating state of a fuel cell system, a method for controlling a fuel cell system based thereon, and a fuel cell system comprising at least one fuel cell, a system for operating medium management and a control unit, the control unit being designed to execute the method according to the invention. Background Art

[0002] Fuel cell systems for driving vehicles are usually based on proton exchange membrane fuel cells (PEM fuel cells) using hydrogen as fuel. Such fuel cell systems include a PEM fuel cell stack and systems connected thereto for operating medium management, in particular for anode hydrogen supply, cathode air supply and cooling.

[0003] The electrochemical and physical processes that take place during the operation of a PEM fuel cell are very complex and the functional components, in particular the proton exchange membrane, are subject to various degradation and wear processes. In order to control the fuel cell system, efforts are made to understand the corresponding operating state as accurately as possible, for which purpose, in the prior art, for example, impedance spectroscopy is performed to diagnose the electrochemical processes in the fuel cell in real time. Summary of the invention

[0004] The subject matter of the invention is a method for diagnosing the operating state of a fuel cell system having at least one fuel cell and a system for managing an operating medium, wherein an operating model of the fuel cell is created, the operating model comprising an electrochemical model and a physical model coupled thereto, wherein

[0005] - the electrical operating power of the fuel cell can be determined from the electrical operating current and thermodynamic operating parameters using an electrochemical model, and

[0006] The time-dependent spatial distribution of the thermodynamic operating parameter can be determined with the aid of the physical model, and at least the following steps are continuously performed for the diagnosis of the operating state:

[0007] - detecting the electrical operating current of the fuel cell and the state variables of the operating medium of the fuel cell, wherein the state variables of the operating medium are detected at the input side and / or the output side of the fuel cell, and - determining the electrical operating power of the fuel cell based on the detected electrical operating current and the detected state variables of the operating medium with the aid of an operating model.

[0008] The idea of ​​the present invention is to use a comprehensive operating model for the continuous diagnosis of the electrical operating power of a fuel cell, wherein the operating model models the electrochemical process within the fuel cell with suitable spatial and temporal resolution, such as the kinetics of ion diffusion and electrochemical sub-reactions through the membrane, and models the relevant thermodynamic operating parameters with the help of a physical model. The relevant thermodynamic operating parameters include in particular the temperature of the cell membrane, the temperature of the operating medium, the pressure of the operating medium, the volume flow of the operating medium, the mass concentration of the operating medium and / or the humidity of the operating medium. Here, the electrochemical model and the physical model are coupled to each other. The values ​​of the thermodynamic operating parameters determined by the physical model form the input variables of the electrochemical model. For example, the concentration loss of reactants and / or the values ​​of the local waste heat determined by the efficiency of the cell reaction determined by the electrochemical model are transmitted to the physical model again. The electrical operating power of the fuel cell modeled based on the current actually flowing is output as a measure for its operating state. The operating power obtained in this way can be used in particular to control the fuel cell system. In addition, it is possible to draw conclusions about the instantaneous degree of degradation, for example by comparison with an ideal polarization curve, in particular a polarization curve determined experimentally for a fuel cell system that is as good as new, ie not worn out.

[0009] In the operation of the fuel cell system, in particular, modeling for diagnosing the electrical operating power is continuously performed, for which purpose the current drawn by the connected electrical consumer, in particular the engine of an electric vehicle, is continuously detected as an input variable for the operating model. As a further input variable, state variables of the operating medium of the fuel cell are continuously detected, wherein the operating medium of the fuel cell can be considered, for example, a fuel, in particular hydrogen, an oxidant, in particular fresh air taken in, an anode exhaust gas, a cathode exhaust gas and / or a cooling fluid. The detected state variables include in particular the temperature, pressure, volume flow and / or moisture content of the operating medium, wherein sensors arranged upstream and / or downstream of the fuel cell can be used for the detection.

[0010] In an advantageous embodiment of the method according to the invention, the electrochemical model is based on an electrical equivalent circuit diagram, and / or the physical model is based on a thermal equivalent circuit diagram and / or a hydraulic equivalent circuit diagram. In particular, all models are based on a suitable equivalent circuit diagram. The electrical equivalent circuit diagram is a diagram of the electrochemical working mode of a fuel cell by means of decomposition into equivalent individual electrical components, wherein the equivalent circuit diagram simulates the behavior of the fuel cell as accurately as possible. In particular, linear and nonlinear resistors and capacitors are provided as passive electrical poles for modeling. Similar to the electrical equivalent circuit diagram, a thermal equivalent circuit diagram or a hydraulic equivalent circuit diagram can be established to model heat transfer and fluid mechanical phenomena in the transport of the operating medium. Here, node rules and grid rules known from power grid analysis can be advantageously used for calculations. For example, a potential difference corresponds to a temperature difference or a pressure difference, an electric current corresponds to a heat flow or a volume flow, and a resistance corresponds to a thermal resistance or a flow resistance, wherein the thermal resistance is particularly composed of components of heat conduction and convection.

[0011] In particular, the operating model has a discretization into interconnected segments, wherein each segment includes an electrochemical submodel and a physical submodel coupled thereto. Each segment represents a spatial section of the fuel cell being modeled and can be segmented in a manner optimized for the respective application and the available computing power.

[0012] In an operating mode for a fuel cell system having a fuel cell stack including a plurality of fuel cells electrically connected in series, the equivalent circuit diagrams of the electrochemical model and the physical model have a corresponding plurality of circuit planes connected one above the other.

[0013] Furthermore, the present invention relates to a method for controlling a fuel cell system comprising at least one fuel cell and a system for managing an operating medium, wherein a method for diagnosing an operating state of the fuel cell system according to one of the above-described embodiments is performed and wherein management of the operating medium is controlled as a function of the diagnosed operating state of the fuel cell system.

[0014] The invention further relates to a fuel cell system having at least one fuel cell, a system for operating medium management and a control unit which is designed to carry out the diagnostic method according to the invention and / or the control method according to the invention. The control unit is at least indirectly connected for communication with all components of the fuel cell system required for carrying out the method and in particular also has a storage and calculation unit for storing an operating model and continuously determines the electrical operating power of the fuel cell based on the detected electrical operating current of the operating medium and the detected state variables with the aid of the operating model.

[0015] The invention further relates to a computer program product having a program code stored on a computer-readable medium for executing the method according to the invention according to one of the above-described embodiments. The computer program product can be stored in particular on a control unit of the fuel cell system according to the invention and executed by the control unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Embodiments of the present invention are described in detail with reference to the following drawings.

[0017] Figure 1 a schematic diagram showing the electrochemical model and the physical model; and

[0018] Figure 2 A schematic diagram of an operating model according to the present invention is shown. DETAILED DESCRIPTION

[0019] Figure 1 A schematic diagram of a segment S as part of an operating model according to the present invention is shown, wherein the segment S includes an electrochemical submodel ecM and a physical submodel pM coupled thereto. The index tuple [ijk] represents the position of the segment S in the three-dimensional discretized operating model. The electrochemical submodel ecM is based on an electrical equivalent circuit diagram, which models activation losses and concentration losses as an example. Activation losses are generated by voltage losses when charges are kinetically inhibited from passing from the electronic conduction phase to the ion conduction phase, i.e., when overcoming the electrochemical double layer at the interface between the battery membrane and the anode or cathode. Activation losses limit the electrical operating power of the fuel cell at low current density. In the present case, the corresponding equivalent circuit diagram includes a double-layer capacitor connected in parallel with an ohmic resistor. In the diffusion-controlled region of the polarization curve, i.e., at high current density, concentration losses occur due to the high conversion rate of the reactants, which leads to a sharp drop in electrical operating power. In the present case, concentration losses are modeled by a series of Warburg impedances. The specific values ​​of the diodes in the illustrated equivalent circuit diagram of the electrochemical submodel ecM are related to thermodynamic operating parameters Σ. The thermodynamic operating parameters are provided by the physical submodel pM coupled to the electrochemical submodel ecM.

[0020] In the present embodiment, the physical submodel is based on a thermal equivalent circuit diagram thM and a hydraulic equivalent circuit diagram hydM. The thermal equivalent circuit diagram thM includes two resistors connected in series, which represent the contributions of heat conduction and convection to the heat transfer resistance of the segment S. The hydraulic equivalent circuit diagram hydM takes into account the flow resistance for the fuel on the anode side and the oxidant on the cathode side. The flow resistance is given, for example, by the geometry of the media channels of the bipolar plates and the structure of the diffusion layer on the electrodes.

[0021] Figure 2 A schematic diagram of an operating model M for a method according to the invention for diagnosing the operating state of a fuel cell system is shown, the operating model comprising an electrochemical model ecM and a physical model pM coupled thereto. The electrical operating power P of the fuel cell can be determined with the aid of the electrochemical model ecM from the electrical operating current J and the thermodynamic operating parameter Σ, and the time-dependent spatial distribution of the thermodynamic operating parameter Σ can be determined with the aid of the physical model pM. In the course of the method according to the invention, the operating current J and the state variable Ψ of the operating medium of the fuel cell are continuously detected during the operation of the fuel cell system to be monitored and are transmitted to the operating model M as input variables.

[0022] The operating model M models a fuel cell system having a fuel cell stack, wherein the fuel cell stack has a plurality of fuel cells, wherein the equivalent circuit diagrams of the electrochemical model ecM and the physical model pM have a corresponding plurality of circuit planes ecM-Z, pM-Z connected one above the other.

[0023] Within each circuit plane ecM-Z, pM-Z there are segments corresponding to the spatial discretization of the respectively modeled fuel cell. In this case, the columns of the equivalent circuit diagram correspond to the course of the operating medium channels on the cathode side for the supplied oxidant, usually humidified fresh air, which is also responsible for humidifying the cell membrane. Therefore, in the electrochemical model ecM, a uniform membrane resistance R is used for each row of the equivalent circuit diagram in a simplified manner. M , as this is largely determined by the water content of the cell membrane and the operating medium channels perpendicular to the cathode side are assumed to be uniform. The physical submodel pM[ijk] is coupled piecewise to each electrochemical submodel ecM[ijk] so that electrochemical and physical processes are modeled with the same spatial resolution.

Claims

1. A method for diagnosing an operating state of a fuel cell system, the fuel cell system having at least one fuel cell and a system for managing an operating medium, wherein an operating model (M) of the fuel cell is created, the operating model comprising an electrochemical model (ecM) and a physical model (pM) coupled thereto, wherein the electrical operating power (P) of the fuel cell can be determined from the electrical operating current (J) and from thermodynamic operating parameters (Σ) by means of an electrochemical model (ecM), and - the time-dependent spatial distribution of the thermodynamic operating parameters (Σ) can be determined with the aid of the physical model (pM), And in which at least the following steps are continuously performed for diagnosing the operating state: - detecting an electrical operating current (J) of the fuel cell and a state variable (Ψ) of an operating medium of the fuel cell, wherein the state variable (Ψ) of the operating medium is detected on the input side and / or on the output side of the fuel cell, and - determining the electrical operating power of the fuel cell based on the detected electrical operating current (J) of the operating medium and the detected state variable (Ψ) with the aid of the operating model (M).

2. A method according to claim 1, wherein the thermodynamic operating parameters (Σ) determined in a time-dependent spatial distribution with the aid of the physical model (pM) include the temperature of the proton exchange membrane of the fuel cell, the temperature of the operating medium, the pressure of the operating medium, the volume flow of the operating medium, the mole concentration of the operating medium and / or the humidity of the operating medium.

3. The method according to claim 1 or 2, wherein a fuel, in particular hydrogen, an oxidant, in particular aspirated fresh air, an anode exhaust gas, a cathode exhaust gas and / or a cooling fluid are considered as the working medium of the fuel cell.

4. The method according to claim 1, wherein the electrochemical model (ecM) is based on an electrical equivalent circuit diagram, and / or the physical model (pM) is based on a thermal equivalent circuit diagram (thM) and / or a hydraulic equivalent circuit diagram (hydM).

5. The method according to claim 4, wherein the operating model has: discretization into interconnected segments (S), wherein each segment (S) includes an electrochemical sub-model and a physical sub-model coupled thereto.

6. A method according to claim 4 or 5, wherein the fuel cell system comprises a fuel cell stack having a plurality of fuel cells, and wherein the equivalent circuit diagrams of the electrochemical model (ecM) and the physical model (pM) have corresponding multiple upper and lower connected circuit planes (ecM-Z, pM-Z).

7. A method for controlling a fuel cell system, the fuel cell system having at least one fuel cell and a system for managing an operating medium, wherein a method for diagnosing an operating state of the fuel cell system according to any one of the above claims is performed, and wherein management of the operating medium is controlled based on the diagnosed operating state of the fuel cell system. 8 . A fuel cell system comprising at least one fuel cell, a system for managing an operating medium and a control unit which is designed to carry out the method according to claim 1 and / or the method according to claim 7 . 9 . A computer program product, comprising a program code, the program code being stored on a computer-readable medium, the computer program product being configured to execute the method according to claim 1 and / or the method according to claim 7 .

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