Method for diagnosing performance of membrane humidifier of fuel cell system, method for controlling fuel cell system, and fuel cell system

Abstract

The invention relates to a method for diagnosing the performance of a membrane humidifier of a fuel cell system, the membrane humidifier being configured to humidify a fresh air stream by means of a water content in a cathode exhaust stream of the fuel cell system, creating a physical model of the membrane humidifier with physical variables in an initial state; creating a degradation model by means of which a change in the physical variable is determined as a function of the thermodynamic operating parameters of the fuel cell system; creating a water transport model by means of which a water mass flow through the membrane of the membrane humidifier is determined on the basis of the physical variables and the thermodynamic operating parameters; in order to diagnose, at least the following steps are carried out continuously: detecting a thermodynamic operating parameter of the fuel cell system; updating the physical variable on the basis of the detected thermodynamic operating parameter; and determining a water mass flow through the membrane humidifier by means of a water transport model on the basis of the thermodynamic operating parameters and the updated physical variables such that the water mass flow is provided as a measure of the performance of the membrane humidifier at the respective time.

Description

Technical Field

[0001] The invention relates to a method for diagnosing the effectiveness of a membrane humidifier of a fuel cell system, a method for controlling a fuel cell system based on the method, and a fuel cell system having a membrane humidifier and a control unit which is designed to carry out the method according to the invention. Background Art

[0002] Fuel cell systems for driving vehicles are generally 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 a plurality of subsystems for anode hydrogen supply, cathode air supply and cooling connected to the PEM fuel cell stack.

[0003] The subsystem for cathode air supply supplies pressurized air to the cathode of the PEM fuel cell stack, and the pressurized air is extracted from the working environment as fresh air. Special humidification of the included polymer membrane is essential for optimizing the power and service life of the PEM fuel cell. In order to adjust the water balance of the polymer membrane, a membrane humidifier is integrated into the subsystem of cathode air supply in the prior art, and the membrane humidifier is configured to humidify the pressurized air by means of the water content in the cathode exhaust of the fuel cell system. In the membrane humidifier, the wet cathode exhaust flow from the PEM fuel cell stack and the dry fresh air flow are contacted via a semipermeable membrane to achieve a material transfer of water from the exhaust flow for humidifying the pressurized air flow. Therefore, the membrane humidifier includes a dry side for the pressurized air flow to be humidified and a wet side for the wet exhaust flow from the cathode of the PEM fuel cell stack, wherein the dry side and the wet side are generally formed by a channel system, and the channel system is in contact with each other on the half side via a semipermeable membrane in the middle. A porous diffusion layer can also be provided between the channel system and the membrane to achieve as comprehensive and uniform water transmission as possible via the membrane.

[0004] The efficiency of such membrane humidifiers deteriorates during the operating period, for example due to clogging of the membrane or its operation-induced wear, for example due to swelling, scaling, crack formation or erosion due to particles contained in the fluid flow. Summary of the invention

[0005] The invention relates to a method for diagnosing the effectiveness of a membrane humidifier of a fuel cell system, wherein the membrane humidifier is configured to humidify a fresh air flow drawn in as charge air by means of the water content in a cathode exhaust gas flow of the fuel cell system, wherein

[0006] - creating a physical model of the membrane humidifier with physical variables in an initial state,

[0007] - creating a degradation model, with the aid of which the change in the physical variables in the physical model of the membrane humidifier can be determined as a function of the thermodynamic operating parameters of the fuel cell system,

[0008] - creating a water transport model, with the help of which the water mass flow through the membrane of the membrane humidifier can be determined based on the physical variables in the physical model of the membrane humidifier and the thermodynamic operating parameters, and

[0009] - For diagnosis, continuously perform at least the following steps:

[0010] i. Detect the thermodynamic operating parameters of the fuel cell system,

[0011] ii. updating the physical variables in the physical model of the membrane humidifier based on the detected thermodynamic operating parameters by means of the degradation model, and

[0012] iii. Determining the water mass flow through the membrane humidifier using a water transport model based on the detected thermodynamic operating parameters and the updated physical variables, so that the water mass flow is provided as a measure for the effectiveness of the membrane humidifier at the respective time.

[0013] The idea of ​​the invention is to record the operating history of the membrane humidifier based on continuously detected thermodynamic operating parameters and at the same time to determine and update the associated degradation of the membrane humidifier by calculation, so that the determination of the water mass flow transmitted from the wet side to the dry side takes into account the respective current degradation degree of the membrane humidifier at any time. In addition to determining the water mass flow as a central characteristic variable for the efficiency of the membrane humidifier, the physical variables determined according to the invention in the respective degradation state of the membrane humidifier can also be provided and used to form other suitable characteristic variables.

[0014] In particular, the detected thermodynamic operating parameters of the fuel cell system include the pressure, temperature, mass flow and humidity of the fresh air and cathode exhaust gas drawn in through the dry side or wet side of the membrane humidifier. The operating parameters are continuously detected, for example, by means of a sensor device suitable for the purpose. Furthermore, during the detection, a distinction is advantageously made between continuous parameter changes, such as pressure, temperature or humidity fluctuations, and induced parameter changes, such as start-stop cycles of the fuel cell system.

[0015] In the physical model of the membrane humidifier, in particular the detailed configuration of the membrane is mapped, which represents the main wear component of the membrane humidifier. The membrane can have a plate-like or tubular configuration, for example. The physical variables provided by the model in a spatially resolved manner include in particular the membrane thickness, the membrane water content and the diffusion coefficient for water in the membrane. The values ​​of the physical variables are first generated for the initial state of the membrane humidifier, wherein the initial state can in particular correspond to the unworn, fully efficient state of the membrane.

[0016] The degradation model obtains the detected thermodynamic operating parameters as input variables and models the degradation effects on the membrane humidifier resulting therefrom, so that the values ​​of the physical variables in the physical model of the membrane humidifier change corresponding to the degree of degradation. With each detection cycle, the degradation model updates the physical variables based on the values ​​of the physical variables from the previous cycle and the currently detected thermodynamic operating parameters.

[0017] For example, the degradation model for the membrane considers at least one of the following degradation mechanisms: crack formation, crack growth, pinhole formation, fatigue, corrosion, erosion, contamination, swelling, or scrubbing.

[0018] Here, the degradation mechanism under consideration is advantageously modeled by means of a semi-empirical method. It is based on the corresponding physical degradation relationship, in which the determined variables are used, for example, as empirically determined approximations or expressed in parameterized form, in which the parameters are defined, for example, by experimentally determined values ​​or determined by regression of a training data set. In the chemical degradation process, for example, a relationship based on the Arrhenius formula (Arrhenius-Gleichung) is used, and the parameters involved are calibrated according to literature data. For example, the membrane is chemically degraded due to corrosive free radical erosion, and the degradation model models the free radical formation in order to quantify the corrosion of the membrane. In the mechanical degradation phenomenon, for example, mechanical stress is calculated and methods in material fatigue are used. For example, crack growth caused by repeated expansion and contraction of the membrane during the temperature and humidity cycle can be modeled by means of a method based on the crack growth law according to Paris. The result of each process of the degradation model is an update of the physical variables of the physical model of the membrane humidifier, that is, the values ​​of the physical variables are continuously matched to the corresponding current degradation state of the membrane humidifier according to the load set of the operating history.

[0019] The values ​​of the physical variables of the physical model of the membrane humidifier corresponding to the respective current degradation state form the input variables for the water transport model. In this case, for example, the physical variables can first be converted into expedient parameters of the water transport model. A particular advantage of the method according to the invention is that the degradation model is separated from the water transport model in terms of calculation, so that in principle different water transport models can be used for specific embodiments of the method.

[0020] When determining the water mass flow through the membrane humidifier with the aid of a water transport model, the water transport within the membrane is advantageously considered in three spatial dimensions, i.e., discretization is performed in two "in-plane" dimensions and in the "cross-plane" direction. The method allows detailed consideration of the changes in physical variables that are important for the water transport in the local plane due to degradation processes, i.e., with a suitable local resolution.

[0021] In an advantageous embodiment, the water transport model models the adsorption and desorption rates of water at the membrane surface by means of empirical methods. The adsorption and desorption rates are treated separately from each other and can have different values. In particular, these ratios are related to the local water content of the membrane.

[0022] In particular, the determination of the water mass flow in each cycle by means of the water transport model does not require any method iterations, so that the computing resources required for the execution of the method can be kept low, or a practical real-time diagnosis of the efficiency of the membrane humidifier is possible. This iteration-free method is implemented by using a previously calculated and stored characteristic curve family.

[0023] In a further advantageous embodiment, the diagnosis further comprises performing the following step: iv. comparing the determined water mass flow with a reference value in order to obtain a measure for the quality of the physical model and / or the degradation model and / or the water transport model.

[0024] Carrying out the additional diagnostic step enables particularly effective and robust detection of defects in the membrane humidifier and plausibility checking of the model used, thereby enabling an extension of the service life of the membrane humidifier and of the entire fuel cell system.

[0025] The present invention also relates to a method for controlling a fuel cell system having a membrane humidifier, wherein the method for diagnosing the performance of a membrane humidifier according to the present invention according to any of the above-mentioned embodiments is performed, and wherein the thermodynamic operating parameters of the fuel cell system are controlled according to the diagnosed performance of the membrane humidifier. By knowing the degradation mechanism of the membrane humidifier and the corresponding current degradation state, the operation of the fuel cell system can be controlled so that unfavorable operating states are avoided in a targeted manner. This helps to extend the service life of the membrane humidifier and contributes to higher efficiency of the operation of the fuel cell system.

[0026] In an advantageous embodiment of the method, the water mass flow and / or the physical variable is compared with a stored comparison value reflecting the state of the membrane humidifier at given thermodynamic operating parameters. In particular, the stored comparison value can be a value associated with a new and perfectly functioning membrane humidifier (“Beginning of Life”) or with a membrane humidifier that is about to be replaced (“End of Life”).

[0027] In this way, the remaining service life of the membrane humidifier can be efficiently determined and displayed to a user of the fuel cell system. In addition, a targeted adjustment of thermodynamic operating parameters is achieved in this way in order to achieve a predetermined service life of the membrane humidifier.

[0028] The invention further relates to a fuel cell system having a membrane humidifier, which is configured to humidify a fresh air flow drawn in as charge air by means of the water content in the cathode exhaust gas flow of the fuel cell system, 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 to all components of the fuel cell system required for carrying out the method for communication and furthermore has, in particular, a storage and calculation unit for storing a physical model of the membrane humidifier and for storing and continuously calculating a degradation model and a water transport model.

[0029] 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 any of the above-described embodiments. The computer program product can in particular be stored on a control unit of the fuel cell system according to the invention and executed by the control unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Embodiments of the present invention are described in detail according to the following drawings. The drawings show:

[0031] Figure 1 A schematic diagram showing a portion of a fuel cell system according to the invention, and

[0032] Figure 2 A schematic diagram shows the sequence of a first embodiment of the diagnostic method according to the present invention. DETAILED DESCRIPTION

[0033] Figure 1 A portion of a fuel cell system 10 according to the invention is shown in a schematic diagram. The PEM fuel cell stack 3 and the subsystem for cathode air supply are shown, and in particular the subsystem for anode hydrogen supply and cooling are not shown.

[0034] The subsystem for cathode air supply comprises, on the suction side, a compressor 4 and a charge air cooler 5, followed by a membrane humidifier 1 with a dry side 1a, and, on the exhaust side, a wet side 1b of said membrane humidifier and an expander 6. The membrane humidifier 1 is configured to humidify a fresh air flow drawn as charge air by means of the water content in the cathode exhaust flow, so that the fresh air flow can be conveyed through the dry side 1a and the exhaust flow can be conveyed through the wet side 1b, wherein the dry side 1a and the wet side 1b are in contact via a semipermeable membrane (not shown) in between.

[0035] The control unit 2 is connected for communication with all components of the subsystem for cathode air supply in a manner not shown and is designed to carry out the method according to the invention for diagnosing the functionality of the membrane humidifier 1 and for controlling the fuel cell system 10 .

[0036] Figure 2 A schematic diagram showing the flow of a first embodiment of a method for diagnosing the performance of a membrane humidifier according to the present invention. Based on the creation of a physical model 100 of a membrane humidifier, at time t 0 Generate physical variables Σ in the initial state 0 The physical variables Σ include, in particular, the membrane thickness, the membrane water content and the diffusion coefficient of water in the membrane. Furthermore, a degradation model 200 is created, with the aid of which the change in the physical variables Σ can be determined as a function of the thermodynamic operating parameter Ψ of the fuel cell system, and a water transport model 300 is created, with the aid of which the water mass flow Q through the membrane of the membrane humidifier can be determined based on the physical variables Σ and the thermodynamic operating parameter Ψ.

[0037] In order to diagnose the effectiveness of the membrane humidifier, during operation of the fuel cell system, the membrane humidifier is continuously monitored, ie at successive times t 1 to i Follow the steps shown.

[0038] This includes the continuous detection of thermodynamic operating parameters Ψ of the fuel cell system, in particular the pressure, temperature, mass flow and humidity of the fresh air drawn in and the cathode exhaust gas flowing through the dry side or wet side of the membrane humidifier.

[0039] The physical variables Σ are updated based on the corresponding operating parameters Ψ with the aid of the degradation model 200. For example, at time t 1 Detection of operating parameters 1 After that, the physical variables Σ in the initial state 0 The value is converted to the updated value Σ 1 , the updated value corresponds to the value of 0 to 1 The degradation state of the membrane humidifier for the set of loads acting within the operating interval of . The physical variable Σ is updated with each method cycle so that the degradation state based on the complete and detailed operating history is always mapped.

[0040] As a measure for the effectiveness of the membrane humidifier at the respective operating time t, the water mass flow Q through the membrane humidifier is determined by means of the water transport model 300 based on the respectively detected thermodynamic operating parameters Ψ and the updated physical variables Σ.

[0041] A second specific embodiment (not shown) of the diagnostic method according to the invention corresponds essentially to the first specific embodiment, so that only the differences are discussed below.

[0042] A second embodiment comprises carrying out a comparison of the determined water mass flow Q with a reference value, wherein the reference value is derived by a control unit from humidity values ​​measured upstream of the wet side 1b and downstream of the dry side 1a of the membrane humidifier, and the humidity values ​​are measured by means of a humidity sensor (not shown) in each case. In this case, the additional step of comparing with the reference value can be carried out at any time t i It can be carried out, or it can be carried out at appropriate intervals of a coarser mesh.

[0043] In an alternative embodiment, the reference value is derived from an additional or different mathematical model, which is designed to determine the water mass flow in an alternative way. In another alternative embodiment, the reference value is derived from checking or ascertaining the humidity present in the fuel cell stack 3, wherein the humidity in the fuel cell stack 3 causes a certain water transport to the wet side 1b of the membrane humidifier 1, and thus a certain water mass flow Q. The humidity in the fuel cell stack 1 is checked or ascertained by means of an "on-board" electrical impedance spectroscopy device.

Claims

1. A method for diagnosing the effectiveness of a membrane humidifier (1) of a fuel cell system (10), wherein the membrane humidifier (1) is configured to humidify a fresh air flow drawn in as charge air by means of the water content in a cathode exhaust gas flow of the fuel cell system (10), wherein - creating a physical model (100) of the membrane humidifier (1) with physical variables (Σ) in an initial state, - creating a degradation model (200), by means of which the change of the physical variable (Σ) in the physical model (100) of the membrane humidifier (1) can be determined as a function of the thermodynamic operating parameters (Ψ) of the fuel cell system (100), - creating a water transport model (300) by means of which the water mass flow (Q) through the membrane of the membrane humidifier (1) can be determined based on the physical variables (Σ) and the thermodynamic operating parameters (Ψ) in the physical model (100) of the membrane humidifier (1), and - For diagnosis, continuously perform at least the following steps: i. detecting a thermodynamic operating parameter (Ψ) of the fuel cell system (100), ii. updating the physical variables (Σ) in the physical model (100) of the membrane humidifier (1) based on the detected thermodynamic operating parameters (Ψ) by means of the degradation model (200), and iii. determining the water mass flow (Q) through the membrane humidifier (1) based on the detected thermodynamic operating parameters (Ψ) and the updated physical variables (Σ) with the aid of the water transport model (300), so that the water mass flow (Q) is provided as a measure of the effectiveness of the membrane humidifier (1) at the corresponding time (t).

2. The method of claim 1, wherein the degradation model (200) for the membrane takes into account at least one of the following degradation mechanisms: crack formation, crack growth, pinhole formation, fatigue, corrosion, erosion, contamination, swelling or washing.

3. The method according to claim 1 or 2, wherein the degradation model (200) models at least one of the degradation mechanisms under consideration by means of a semi-empirical method.

4. The method according to any of the preceding claims, wherein the water transport model (300) considers water transport within the membrane in three spatial dimensions.

5. The method according to any one of the preceding claims, wherein the water transport model (300) models the adsorption and desorption rates of water at the membrane surface by means of empirical methods.

6. A method according to any of the preceding claims, wherein the diagnosis further comprises the following steps: iv. comparing the determined water mass flow (Q) with a reference value in order to obtain a measure of the quality of the physical model (100) and / or the degradation model (200) and / or the water transport model (300).

7. A method for controlling a fuel cell system (10), wherein the fuel cell system (10) has a membrane humidifier (1), which is configured to humidify a fresh air flow drawn in as boost air by means of the water content in the cathode exhaust flow of the fuel cell system (10), and wherein a method for diagnosing the effectiveness of the membrane humidifier (1) according to any of the above claims is performed, and wherein a thermodynamic operating parameter (Ψ) of the fuel cell system (10) is controlled based on the diagnosed effectiveness of the membrane humidifier (1).

8. The method according to claim 7, wherein: Given given thermodynamic operating parameters (Ψ), the water mass flow (Q) and / or the physical variable (Σ) are compared with stored comparison values ​​which reflect the state of the membrane humidifier (1).

9. A fuel cell system (10), comprising: a membrane humidifier (1) and a control unit (2), wherein the membrane humidifier is configured to humidify a fresh air flow drawn in as boost air by means of the water content in the cathode exhaust flow of the fuel cell system (10), and the control unit is configured to execute the method according to any one of claims 1 to 6 and / or the method according to claims 7 to 8. 10 . A computer program product having a program code stored on a computer-readable medium for executing the method according to claim 1 and / or the method according to claim 7 .

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