DE102021202053B4 - Diagnosis of the moisture condition of a PEM fuel cell stack - Google Patents

Abstract

Method for determining a moisture state of a membrane (103) of a fuel cell system (100), the method comprising
Determining an optimal pressure loss (201) between an inlet pressure (P1) of air at a fluid inlet (101) of the fuel cell system (102) and an outlet pressure (P2) at a fluid outlet (102) of the fuel cell system (100) as a function of an inlet volume flow (VL) of air at the fluid inlet (101) at an optimal moisture state of the membrane (103) of the fuel cell system (100),
Determining a dry pressure loss (202) between the inlet pressure (P1) of air at the fluid inlet (101) and an outlet pressure (P2) at the fluid outlet (102) as a function of the inlet volume flow (VL) for a dried-out membrane (103) of the fuel cell system (100),
Measuring a pressure loss between the inlet pressure (P1) of air at the fluid inlet (101) and the outlet pressure (P2) at the fluid outlet (102) as a function of the inlet volume flow (VL) of air at the fluid inlet (101),
Forming a first reference value (R1) consisting of a difference between the measured pressure loss and the dry pressure loss (202),
Forming a second reference value (R2) consisting of a difference between the optimal pressure loss (201) and the dry pressure loss (202),
Forming a quotient from the first reference value (R1) and the second reference value (R2),
where the quotient is indicative of a moisture state of the membrane (103).

Description

Technical area

The present invention relates to a method for determining a moisture state of a membrane of a fuel cell system.

Background of the invention

Energy generation systems with no or reduced production of harmful exhaust gases such as carbon monoxide will play an increasingly important role in the future. One such energy generation system is the fuel cell, which generates energy based on the chemical reaction between hydrogen and atmospheric oxygen and releases water as a product. In fuel cell systems, hydrogen is usually supplied to an anode side, where electrons are released and hydrogen protons diffuse through a corresponding membrane (e.g., a polymer electrolyte membrane) towards a cathode side of a fuel cell system to react with atmospheric oxygen. On the cathode side, the hydrogen protons react with atmospheric oxygen to form water, which is then removed as a product on the cathode side.

The efficiency of a fuel cell system depends in particular on the humidity level or optimal humidification of the membrane. If the membrane is too dry, efficiency decreases due to an increase in electrical resistance in the membrane. If the membrane is too humid, too much water condenses. The resulting water droplets flood the channels of a bipolar plate of the fuel cell system on the cathode side. This impedes mass transport and reduces the efficiency of the fuel cell system by limiting the chemical reaction.

From the JP 2009-259758 A A fuel cell system is known that operates under at least one of a non-humidified state and a high-temperature state. In the fuel cell system, a fuel gas channel and an oxidant gas channel are arranged such that the flow direction of the fuel gas is opposite to that of the oxidant gas in a fuel cell. Furthermore, the fuel cell system has a means for detecting the humidity near the inlet of the oxidant gas channel and a means for controlling the fuel gas to increase the humidity near the inlet of the oxidant gas channel when the humidity detecting means determines that the humidity near the inlet of the oxidant channel is too low. The humidity is increased by either increasing the flow rate of the fuel gas or reducing the pressure of the fuel gas.

From the WO 2008/142 564 A1 A control device for a fuel cell system is known. The control device has measuring means for measuring a pressure loss of fuel gas in a fuel cell stack and determining means for determining that an electrolyte membrane in the fuel cell stack is dry when a decrease in the pressure loss of the fuel gas is measured.

From the JP 2010/033975 A A fuel cell system capable of performing high-precision dry/wet control of an electrolyte film during purging is known. The fuel cell system includes: a fuel cell having an electrolyte film sandwiched by electrodes, a purge device for supplying purge gas to the fuel cell, and a control device for controlling the dry/wet state of the electrolyte film by controlling an electric current generated by the fuel cell during purging by the purge device.

Description of the invention

It is an object of the present invention to improve the performance of a fuel cell system depending on a moisture state of a membrane of the fuel cell system.

This object is achieved by a method for determining a moisture state of a membrane of a fuel cell system and by a fuel cell system according to the independent claims.

According to a first aspect of the present invention, a method for determining a moisture content of a membrane of a fuel cell system is described. According to the method, an optimal pressure loss between an inlet pressure of air at a fluid inlet of the fuel cell system and an outlet pressure at a fluid outlet of the fuel cell system is determined as a function of an inlet volume flow of air at the fluid inlet at an optimal moisture content of the membrane of the fuel cell system.

Furthermore, a dry pressure loss between the inlet pressure of air at the fluid inlet and an outlet pressure at the fluid outlet is determined depending on the inlet volume flow when the membrane of the fuel cell system is dried out.

An (actual) pressure loss between the inlet pressure of air at the fluid inlet and the outlet pressure at the fluid outlet is measured as a function of the inlet volume flow of air at the fluid inlet.

According to the method, a first reference value R1 is then calculated, consisting of a difference between the measured pressure loss and the dry pressure loss, and a second reference value R2 is calculated, consisting of a difference between the optimal pressure loss and the dry pressure loss. A quotient R1/R2 is calculated from the first reference value R1 and the second reference value R2, with the quotient being indicative of the membrane's moisture content.

According to a further aspect of the present invention, a fuel cell system is described which has a membrane separating a cathode side and an anode side. Furthermore, the fuel cell system has a fluid inlet through which air can be supplied to the cathode side at an inlet pressure, and a fluid outlet through which a fluid can be discharged from the cathode side as a product at an outlet pressure. The fuel cell system further has a control unit configured to carry out the above-described inventive method.

The fuel cell system initially has an anode side and a cathode side, between which an electrical load or consumer is coupled, forming an electrical circuit. A hydrogen supply, for example a hydrogen tank, is coupled to the anode side. The membrane, through which hydrogen ions can diffuse towards the cathode side, is coupled between the anode and cathode sides. The released electrons are conducted to the cathode side via the electrical circuit. On the cathode side is the fluid inlet, through which air usually flows in. The oxygen in the air is ionized at the cathode and forms water with the hydrogen protons. The produced water is discharged via the fluid outlet on the cathode side. Typically, a larger amount of air flows in than the oxygen required for the reaction with hydrogen. The lambda value is therefore greater than 1. Accordingly, air also flows out at the fluid outlet. The produced water is absorbed by the air and flowed out through the flood outlet.

The incoming air can be controlled, for example, via an air compressor at the fluid inlet. Furthermore, the outlet pressure can be controlled, for example, via a controlled exhaust backpressure throttle valve. Furthermore, the fuel cell system is cooled. For this purpose, coolant channels are formed, through which a coolant flows.

The membrane is, in particular, a proton exchange membrane (PEMFC) and serves as an ion exchange membrane. The membrane can, for example, be coated on both sides with a catalytically active electrode, e.g., with a mixture of carbon (soot) and a catalyst, often platinum or a mixture of platinum and ruthenium (PtRu electrodes), platinum and nickel (PtNi electrodes), or platinum and cobalt (PtCo electrodes). Hydrogen molecules dissociate on the anode side and are oxidized to two protons each, releasing two electrons. These protons diffuse through the membrane. On the cathode side, oxygen is reduced by the electrons, which previously performed electrical work in an external circuit. Together with the protons transported through the membrane, water is produced. To utilize the electrical work, the anode and cathode are connected to the electrical load.

Furthermore, it has been found that the transport of protons through the membrane, as well as the ohmic resistance, are strongly correlated with the membrane's moisture content. If the membrane is dry, the proton flow is reduced and the ohmic resistance increases. If the moisture content is too high or the membrane is flooded, the proton flow is also disrupted and the ohmic resistance increases. The method according to the invention determines the membrane's moisture content accordingly, enabling a diagnosis of the condition of the fuel cell system.

It has been shown that, on the one hand, an optimal membrane moisture content is indicative of maximum fuel cell performance. Furthermore, it has been shown that a pressure difference between the inlet pressure of air at a fluid inlet and the outlet pressure of a fuel cell product (e.g., humid air or water) is indicative of fuel cell performance, so that the moisture content of the fuel cell membrane can be determined from this.

In order to determine the moisture content of a membrane in a fuel cell system, a so-called optimal pressure loss between an inlet pressure of air and an outlet pressure at a fluid outlet on the cathode side is first determined. The optimal pressure loss Loss is the pressure loss between the inlet pressure and the outlet pressure on the cathode side at which the maximum power of the fuel cell is achieved for a specific inlet air flow rate. The determination of the maximum power of the fuel cell as a function of a specific inlet air flow rate can, for example, be carried out offline on a test bench and saved in a table or corresponding setpoint file. Alternatively, the optimal pressure loss can be measured online, i.e., during operation of a fuel cell, as described in detail below.

Accordingly, a dry pressure drop between the inlet pressure and the outlet pressure on the cathode side is determined when the membrane is completely dry. The dry pressure drop is determined by completely drying out the flow channels and the membrane of the fuel cell, for example by increasing the temperature of the fuel cell until the membrane can be closed. In this state, a reference pressure drop characteristic curve, i.e. a dry pressure drop as a function of the inlet volume flow, is determined. The dry pressure drop can, for example, be determined on a test bench and saved in a table or corresponding setpoint file. Alternatively, the dry pressure drop can be measured online, i.e. during operation of a fuel cell, as described in detail below.

During fuel cell operation, a pressure loss between the inlet air pressure and the outlet pressure at the fluid outlet is also measured at a specific measured input flow rate. This specific actual pressure loss is then used to calculate the first reference value R1, which represents the difference between the measured pressure loss and the dry pressure loss at a specific input flow rate. The second reference value R2 is calculated by calculating the difference between the optimal pressure loss and the dry pressure loss at the specific input flow rate.

1. a.) R1/R2 =1 die Membran eine optimale Feuchte von 100% aufweist.
2. b.) R1/R2 > 1 ein Feuchtegrad von > 100% und somit eine Flutung der Membran vorliegt, und
3. c.) R1/R2 = 0 für 0% Feuchte und für eine komplett ausgetrocknete Membran bzw. Brennstoffzelle steht.

By setting R1/R2, we obtain the quotient, which allows us to determine whether the membrane is currently in an over-humidified or over-dry state (i.e., based on the measured pressure drop). Alternatively, the quotient can also be quantified as a percentage in the form of a fictitious relative humidity of the membrane as a moisture content in %. For example, this applies:

1. a.) R1/R2 =1 the membrane has an optimal humidity of 100%.
2. b.) R1/R2 > 1 a humidity level of > 100% and thus flooding of the membrane is present, and
3. c.) R1/R2 = 0 stands for 0% humidity and for a completely dried out membrane or fuel cell.

The present invention thus provides a quick and easy way to determine the moisture content of a membrane and thus the performance of the fuel cell. Only a few parameters, such as inlet pressure, outlet pressure, and air flow rate, are required, which are obtained from existing sensors of a fuel cell system. Accordingly, existing fuel cell systems can also be retrofitted with the analysis method described.

According to a further exemplary embodiment of the method, an upper threshold and a lower threshold are determined for the quotient. If the upper threshold is exceeded, flooding of the membrane is defined, and if the lower threshold is undershot, drying of the membrane is defined. In other words, a percentage range of the quotient can be determined in which an optimal or permissible operating state exists depending on the membrane moisture content. For example, the upper threshold can be selected at 110% and the lower threshold at 70%. If the quotient exceeds the value 1.1 or 110%, for example, flooding of the fuel cell is defined, so that drying processes are initiated. If the quotient falls below the value 0.7 or 70%, for example, drying of the fuel cell is defined, so that flooding processes or humidification measures of the fuel cell are carried out accordingly.

Accordingly, according to an exemplary embodiment of the method, the moisture state of the membrane can be implemented based on the formed quotient.

According to an exemplary embodiment, the adjustment of the moisture state of the membrane is implemented by adjusting the inlet pressure at the fluid inlet. For example, a compressor can be arranged at the fluid inlet of the fuel cell, which compressor can be controlled in particular by the control unit. If the inlet pressure increases, the measured pressure difference between the fluid inlet and the fluid outlet point also increases. This in turn leads to a too dry state of the Membrane to approximate the optimal pressure loss or the optimal moisture state of the membrane. This can be explained, for example, by the fact that less moisture is transported away at a higher pressure loss.

According to another exemplary embodiment, the moisture content of the membrane is controlled by adjusting the volume flow. For example, with an increased volume flow, more water in the air can be absorbed and thus more effectively removed. This leads to a drying out of the membrane. Thus, the air lambda value on the cathode side is increased, in particular, because more air or more atmospheric oxygen is supplied than is required for the reaction with the hydrogen protons.

According to another exemplary embodiment, the moisture content of the membrane is adjusted by adjusting the temperature of the fuel cell system. If the fuel cell is operated at higher temperatures, the membrane and, accordingly, the fuel cell system will dry out further. If the fuel cell is operated at cooler temperatures, the membrane and, accordingly, the fuel cell system will be humidified or operated at a higher humidity. Accordingly, the moisture content of the membrane can be adjusted by regulating the cooling fluid.

According to a further exemplary embodiment, the temperature of the fuel cell system is adjusted by adjusting the temperature of a cooling medium and/or controlling a coolant supply of the cooling medium that cools the fuel cell system. The fuel cell system can, for example, have an integrated cooling system, wherein a cooling fluid, for example a cooling gas or a cooling liquid, is guided through corresponding cooling channels through the fuel cell system. The temperature of the fuel cell system can, for example, be controlled by the volume flow of the cooling fluid through the fuel cell in order to regulate heat transport. Furthermore, the temperature of the cooling fluid itself can be adjusted in order to regulate heat transport accordingly. A correspondingly warmer cooling fluid cools the fuel cell less, so that it operates at a higher temperature. Accordingly, a reduced volume flow of the cooling fluid contributes to hotter operation of the fuel cell.

According to another exemplary embodiment, the determination of an optimal pressure drop is determined by first operating the fuel cell system from a minimum to a maximum current and measuring the resulting cell voltage of the fuel cell system. The control parameters of the fuel cell system are adjusted or controlled until a constant cell voltage is established. When the constant cell voltage is established, a fuel cell power is determined, whereby the achievement of maximum fuel cell power is indicative of the optimal moisture content of the membrane. As explained above, the optimal or maximum fuel cell power can be achieved when the membrane is at an optimal moisture content. The optimal moisture content of the membrane, in turn, depends on the control parameters of the fuel cell. For example, a certain temperature of the fuel cell system leads to a certain moisture content of the membrane. If maximum power is achieved at this certain moisture content of the membrane, this is the optimal moisture content of the membrane and can be recorded or stored for further determination depending on the adjusted air volume flow at the fluid inlet on the cathode side.

The determination of the optimal moisture state of the membrane can, for example, be carried out offline on a test bench or online during the operation of a fuel cell.

According to an exemplary embodiment, the at least one control parameter is selected from the group consisting of the inlet pressure, the outlet pressure, an air lambda on the cathode side and the temperature of the fuel cell system.

The described method for determining the moisture content of a membrane of a fuel cell system, as well as the fuel cell system itself, can be used, for example, for stationary energy generation. Furthermore, the method and the fuel cell system can be used in mobile systems, such as a hydrogen-powered automobile, watercraft, or aircraft.

With the method according to the invention, a small number of common sensors (e.g., in an automobile) in the cathode circuit of the fuel cell system are sufficient to diagnose the moisture content of the membrane or the fuel cell system. These sensors monitor both the mass air flow (MAF) and the pressure at the inlet (P1) and outlet (P2) of the fuel cell system, as well as the exhaust gas temperature (T2) at the fluid outlet. The diagnostic result cannot be distorted by dynamic load changes in the fuel cell. By pre-off line measurement to determine the pressure losses in the cathode circuit of the optimally humidified membrane or fuel cell system, there is no need for additional cell voltage monitoring in the subsequent fuel cell system during operation. Furthermore, simple calibration means that the diagnostic function can be individually applied to a wide variety of different fuel cell systems, e.g. in the automotive sector. The diagnostic function does not lead to incorrect diagnosis in the case of non-humidity-related causes of a drop in fuel cell efficiency, such as too little hydrogen on the anode side or highly dynamic load changes. This precise distinction between cases is important to diagnose because, for example, drying out and flooding of the membrane should be treated with opposing measures in the fuel cell system control system in order to avoid permanent damage to the fuel cell.

In addition to improving fuel cell efficiency, the drying and flooding of the membrane are fully reversible within a time frame of several seconds. This applies if the humidity level is controlled back to an optimal value. If this does not happen, degradation of the fuel cell system, i.e., permanent damage to the membrane and thus reduced fuel cell efficiency, can be expected. The system and method according to the invention prevent this by actively monitoring and controlling the moisture content of the membrane during operation of the fuel cell system.

The diagnostic function according to the invention is characterized by simple calibration, which allows it to be applied to a wide variety of different fuel cell systems for mobile and stationary applications. The offline method for determining pressure losses via pressure sensors in the cathode circuit allows for the detection of a reduction in fuel cell efficiency due to drying out and flooding in the fuel cell system without the need to install additional cell voltage monitoring in the fuel cell system.

The method may in particular represent a computer-implemented control method which contains instructions for controlling a computer system, in particular the control unit, in order to coordinate the operation of the fuel cell system or the method in a suitable manner in order to achieve the effects associated with the method according to the invention.

The control unit comprises, in particular, a processor and is coupled to the relevant pressure sensors, in particular at the fluid inlet and outlet, as well as to the temperature sensors, in particular at the fluid outlet. Furthermore, the control unit is coupled to the cooling system and its volume flow sensor and temperature sensor of the cooling fluid. Furthermore, the control unit is coupled to the control systems of the fuel cell system in order to adjust the control parameters. Accordingly, the control unit can be coupled to a compressor for the inlet air at the fluid inlet and to a controllable exhaust backpressure throttle valve at the fluid outlet. Furthermore, the control unit can be coupled to a control valve and to a heating unit for the cooling fluid.

It should be noted that the embodiments described here represent only a limited selection of possible variants of the invention. It is thus possible to combine the features of individual embodiments in a suitable manner, so that a multitude of different embodiments can be considered as obviously disclosed to those skilled in the art with the variants explicitly described here.

Short description of the drawing

• 1 zeigt eine schematische Darstellung eines Brennstoffzellensystems gemäß einer Ausführungsform der vorliegenden Erfindung.
• 2 zeigt ein Diagramm, in welchem der Druckverlust zwischen Fluideingang zu Fluidausgang bei einem bestimmten Luftvolumenstrom dargestellt wird, gemäß einer beispielhaften Ausführungsform der vorliegenden Erfindung.
• 3 zeigt ein Diagramm des Luftmassenstroms MAF über die Zeit t.
• 4 zeigt ein Diagramm der Ausgangstemperatur T2 über die Zeit t.
• 5 zeigt ein Diagramm des Eingangsdrucks P1 über die Zeit t.
• 6 zeigt ein Diagramm des Ausgangsdrucks P2 über die Zeit t.
• 7 zeigt ein Diagramm des Stromstärkenverlaufs über die Zeit t.
• 8 zeigt ein Diagramm des Spannungsverlaufs der mittleren Zellspannung über die Zeit t.
• 9 zeigt ein Diagramm des Verlaufs des Luft-Volumenstroms über die Zeit t.
• 10 zeigt ein Diagramm einer gemessenen Druckdifferenz über die Zeit t.
• 11 zeigt ein Diagramm des Verlaufs einer bestimmten Trocken-Druckdifferenz über die Zeit t.
• 12 zeigt ein Diagramm des Verlaufs des ersten Referenzwerts R1 über die Zeit t.
• 13 zeigt ein Diagramm des Verlaufs des zweiten Referenzwerts R2 über die Zeit t.
• 14 zeigt ein Diagramm des Verlaufs des Quotienten R1/R2 über die Zeit t.
• 15 zeigt ein Diagramm des Verlaufs eines prozentualen Feuchtegrads über die Zeit t.
• 16 zeigt ein Diagramm des Verlaufs einer binären Diagnosefunktion bei Austrocknung über die Zeit t.
• 17 zeigt ein Diagramm des Verlaufs einer binären Diagnosefunktion bei Flutung über die Zeit t.

For further explanation and better understanding of the present invention, embodiments are described in more detail below with reference to the attached figure.

• 1 shows a schematic representation of a fuel cell system according to an embodiment of the present invention.
• 2 shows a diagram in which the pressure loss between fluid inlet and fluid outlet is represented at a certain air volume flow, according to an exemplary embodiment of the present invention.
• 3 shows a diagram of the air mass flow MAF over time t.
• 4 shows a diagram of the initial temperature T2 over time t.
• 5 shows a diagram of the inlet pressure P1 over time t.
• 6 shows a diagram of the output pressure P2 over time t.
• 7 shows a diagram of the current intensity over time t.
• 8 shows a diagram of the voltage curve of the average cell voltage over time t.
• 9 shows a diagram of the air volume flow over time t.
• 10 shows a diagram of a measured pressure difference over time t.
• 11 shows a diagram of the course of a certain dry pressure difference over time t.
• 12 shows a diagram of the course of the first reference value R1 over time t.
• 13 shows a diagram of the course of the second reference value R2 over time t.
• 14 shows a diagram of the course of the quotient R1/R2 over time t.
• 15 shows a diagram of the course of a percentage humidity level over time t.
• 16 shows a diagram of the course of a binary diagnostic function during dehydration over time t.
• 17 shows a diagram of the course of a binary diagnostic function during flooding over time t.

Detailed description of exemplary embodiments

Identical or similar components are provided with the same reference numerals in the figure. The representation in the figure is schematic and not to scale.

1 shows a schematic representation of a fuel cell system 100 according to an embodiment of the present invention. The fuel cell system 100 has at least one fuel cell 111 with an anode side and a cathode side, between which an electrical load or consumer 104 is coupled and an electrical circuit I, U is formed. A hydrogen supply, for example a hydrogen tank 115, is coupled to the anode side. A membrane 103 is coupled between the anode side and the cathode side, through which hydrogen ions can diffuse towards the cathode side. The released electrons are conducted to the cathode side via the electrical circuit. On the cathode side, there is initially a fluid inlet 101, through which air usually flows in. The oxygen in the air is ionized at the cathode and forms water with the hydrogen protons. The produced water is discharged via the fluid outlet 102 on the cathode side. Typically, a larger amount of air flows in than the amount of atmospheric oxygen required for the reaction with hydrogen. The lambda value is therefore greater than 1. Accordingly, air also flows out through the fluid outlet. The produced water is absorbed by the air and expelled through the flood outlet.

The incoming air can be controlled, for example, via an air compressor 112 at the fluid inlet 101. Accordingly, for example, an air mass flow MAF and an air volume flow VL can be set at the fluid inlet 101. Furthermore, the outlet pressure P2 can be controlled, for example, via a controlled exhaust backpressure throttle valve 113. However, the air mass flow MAF and/or the air volume flow VL can also be controlled accordingly. Furthermore, the fuel cell system is cooled. For this purpose, coolant channels are formed, through which a coolant flows. The coolant has, for example, the temperature Tk and is controlled by the coolant circuit 114.

The fuel cell system 100 further comprises a control unit 110. The control unit 110 is coupled to the relevant pressure sensors, in particular at the fluid inlet 101 and the fluid outlet 102, as well as to the temperature sensors, in particular at the fluid outlet 102. Furthermore, the control unit 110 is coupled to the cooling system 114 and its volume flow sensor and temperature sensor of the cooling fluid. Furthermore, the control unit 110 is coupled to the control systems of the fuel cell system 100 in order to adjust the control parameters. Accordingly, the control unit 110 can be coupled to the compressor 112 of the inlet air at the fluid inlet 101 and to the controllable exhaust backpressure throttle valve 113 at the fluid outlet 102. Furthermore, the control unit 110 can be coupled to a control valve and to a heating unit of the cooling system 114. Furthermore, a sensor system is provided to determine the air mass flow MAF, the pressure P1 at the fluid inlet 101 and the pressure P2 at the fluid outlet 102 and the exhaust gas temperature T2.

2 shows a diagram illustrating the pressure loss P1-P2 between fluid inlet 101 and fluid outlet 102 at a specific air volume flow VL, according to an exemplary embodiment of the present invention. The method according to the invention can be explained in particular with reference to the diagram shown.

First, an optimal pressure loss or a pressure difference 201 between an inlet pressure P1 of air at a fluid inlet 101 of the fuel cell system 100 and an outlet pressure P2 at a fluid outlet 102 of the fuel cell system 100 is determined as a function of an inlet volume flow VL of air at the fluid inlet 101 at an optimal moisture state of the membrane 103 of the fuel cell system 100. Furthermore, a dry pressure loss or the pressure difference 202 at dry Membrane 103 between the inlet pressure P1 of air at the fluid inlet 101 and an outlet pressure P2 at the fluid outlet 102 as a function of the inlet volume flow VL with a dried-out membrane 103 of the fuel cell system 100.

An (actual) pressure loss 203 between the inlet pressure P1 of air at the fluid inlet 101 and the outlet pressure P2 at the fluid outlet 102 is measured as a function of the inlet volume flow VL of air at the fluid inlet 101.

According to the method, a first reference value R1 consisting of a difference between the measured pressure loss 203 and the dry pressure loss 202, and a second reference value R2 consisting of a difference between the optimal pressure loss 201 and the dry pressure loss 202 are then formed. A quotient R1/R2 is formed from the first reference value R1 and the second reference value R2, wherein the quotient is indicative of a moisture state of the membrane 103.

Accordingly, a dry pressure loss 202 between the inlet pressure P1 and the outlet pressure P2 on the cathode side is determined when the membrane 103 is completely dried out. The dry pressure loss 202 is determined by completely drying out the flow channels and the membrane of the fuel cell 100, for example, by increasing the temperature of the fuel cell 100 until the membrane 103 can be closed to a completely dry state. In this state, a reference pressure loss curve, i.e., a dry pressure loss 202 as a function of the inlet volume flow VL, is determined. The dry pressure loss 202 can, for example, be determined on a test bench and saved in a table or corresponding setpoint file. Alternatively, the dry pressure loss 202 can be measured during operation of a fuel cell 100.

Through a direct offline reference measurement (i.e., in advance) of the pressure loss 202 across the fuel cell system 100 in the dry state, the pressure loss characteristic curve of the dry cathode path with a dried-out membrane 103 is recorded. This curve describes the pressure loss 202 across the fuel cell system with completely dry flow channels and thus a dried-out membrane 103. In general, the pressure loss P1-P2 depends, for example, on water droplets from condensation in the flow path, in particular a bipolar plate of the fuel cell system 100. If there are no water droplets, then the pressure loss is minimal. In this case, it can be concluded that the membrane 103 is also dry.

To determine the optimal humidified membrane, the pressure loss characteristic curve 201 is recorded through reference measurements at different electrical load variations of the current I of the fuel cell system 100. The offline measured reference pressure loss characteristic curve 201 is stored, for example, in the control unit 110. Reference pressure loss characteristic curves can be further determined as a function of an air lambda, pressure in the cathode circuit, and fuel cell system temperature for each stack of the fuel cell system 100 and for each current I.

The “offline” reference preliminary measurement takes place, for example, on a fuel cell test bench with a fuel cell system 100 representative of the application with cell voltage measurement.

To record the reference pressure loss characteristic curve 202 for the case of dry flow channels and corresponding dried-out membrane 103, the pressure loss 202 across the fuel cell system 100 is measured when the fuel cell is not electrically active or at a low constant current I.

For the case of the reference pressure loss characteristic curve 201 with an optimally humidified membrane 103, a fuel cell current intensity variation from minimum to maximum current intensity I is performed. The boundary conditions of the fuel cell system 100 (air lambda, cathode pressure, temperature of the fuel cell system/stack 100) are adjusted such that the cell voltages U of the fuel cell system 100 are in a stable operating state and at maximum fuel cell efficiency or maximum power output. This operating state represents the optimal humidification state of the fuel cell 100 or its membrane 103 and allows the measurement of the corresponding pressure loss characteristic curve 201 for the optimally humidified membrane 103.

Furthermore, the pressure losses can be measured during ongoing fuel cell operation (online) if cell voltage measurements are available in the fuel cell system.

The pressure loss characteristic curve 202 for the case of dry flow channels, for example a bipolar plate of the fuel cell system 100 and correspondingly dried-out membrane 103, is measured during operation of the fuel cell system 100, in a state when no consumer is connected. is closed and, accordingly, with the fuel cell electrically deactivated or with a low constant current, a variation of the air volume flow VL from a minimum to a maximum value is carried out via the air compressor 112 of the fuel cell system 100. The value pairs of air volume flow VL and measured differential pressure 202 are stored in a table. For example, this measurement can be carried out during a deceleration phase of an electric vehicle in which the fuel cell system 100 is installed. The deceleration phase is characterized in that the fuel cell system 100 provides no or hardly any electrical energy.

Furthermore, the pressure loss characteristic curve 201 for the optimally humidified membrane 103 can be determined online during operation of the fuel cell system 100. During operation of the fuel cell system, which is used, for example, in an electric vehicle, with constant fuel cell power, optimal humidification can be achieved by evaluating the individual cell voltage measurements over a time interval. If the evaluation of the individual cell voltage measurements fulfills the defined stability criterion (e.g., constant voltage U), the measured value pair of air volume flow VL and measured differential pressure 201 is determined and saved for the delivered fuel cell power. The measurement under the aforementioned conditions is repeated until the complete pressure loss characteristic curve for the case of an optimally humidified membrane 103 has been generated. The reference measurement can be performed once per driving cycle of a vehicle.

During operation of the fuel cell, a pressure loss 203 between the inlet pressure P1 of air and the outlet pressure P2 at the fluid outlet 102 is also measured at a specific measured input volume flow VL. This specific actual pressure loss 203 is then used to calculate the first reference value R1, which represents the difference between the measured pressure loss 203 and the dry pressure loss 202 at a specific input volume flow VL. The second reference value R2 is calculated by calculating the difference between the optimal pressure loss 201 and the dry pressure loss 202 at the specific input volume flow VL.

If one now sets R1/R2 one obtains a quotient with the help of which one can conclude whether the membrane 103 is currently in an over-humidified or too dry state (i.e. based on the measured pressure loss 203).

In 3 until 17 The determination of a moisture state of a membrane 103 or a fuel cell system 103 according to the method of the present invention is described by way of example.

Describe 3 until 8 Recordings of measurement signals over time t (in seconds) during operation of the fuel cell system 100 at a constant current I and mass air flow MAF. And increasing temperature of the coolant that cools the fuel cell system 100. In the example, the temperature of the coolant is increased in order to move from an initially flooded and moist state of the membrane 103 at a low temperature of the fuel cell system 100 and a correspondingly low temperature of the cooling fluid to an ultimately dried-out state of the membrane 103 at a high fuel cell temperature and a correspondingly high temperature of the cooling fluid.

The diagnostic function will be illustrated by way of example using the measurement shown on the fuel cell system 100. The known sensors and parameters from 1 and 2 over time in 3 until 8 shown. In 7 and 8 The measured current I and the average cell voltage U of the fuel cell system 100 are shown. In the measurement shown, the fuel cell temperature was gradually increased via the cooling circuit. The increase in the fuel cell coolant temperature leads to an increase in the exhaust gas temperature T2 at the outlet 102. The increase in the fuel cell coolant temperature and the gas temperature leads to a reduction in the relative humidity within the cathode and the membrane 103 of the fuel cell system 100. The reduction in humidity reduces the cell voltages and thus the efficiency of the fuel cell system 100.

3 shows a diagram of the air mass flow MAF over time t, whereby in the example shown a constant air mass flow MAF is specified at approximately 6 g/s (grams per second).

4 shows a diagram of the outlet temperature T2 over time t. The outlet temperature T2 is measured, for example, at the fluid outlet 102 on the cathode side. The outlet temperature T2 is adjusted, for example, by heating the cooling fluid of the fuel cell system 100 over time t or by reducing the volume flow of the cooling fluid.

5 shows a diagram of the inlet pressure P1 over time t. For example, up to a duration of 1500 seconds, the inlet pressure P1 is kept constant by means of the air compressor 112 at maintained at just above 1000 mbar. To regulate the moisture content of the fuel cell system 100, the inlet pressure P1 is increased to approximately 1600 mbar starting at 1500 seconds, as described below.

6 shows a diagram of the output pressure P2 over time t. For example, up to a period of 1500 seconds, the output pressure P2 is kept constant at 1000 mbar by means of the air compressor 112. To regulate the moisture content of the fuel cell system 100, as described below, the output pressure P2 is increased to approximately 1500 mbar starting at 1500 seconds. This can be done, for example, using the exhaust backpressure throttle valve 113.

7 shows a diagram of the current I over time t. During the example shown, the current I is kept constant at approximately 100 A.

8 shows a diagram of the voltage curve over time t. In the example shown, the voltage is measured over time t. For the set pressure difference between 0 and 1500 seconds P1-P2, the voltage reaches a maximum at approximately 750 seconds. After regulating the inlet pressure P1 and the outlet pressure P2 from 1500 seconds, a generally higher voltage U is established.

In 9 until 17 the determined or measured quantities of the diagnostic function according to the invention are given along time based on the parameters set above.

9 shows a diagram of the air flow rate VL over time t. The flow rate VL increases steadily from approximately 350 l/min until the inlet pressure P1 and outlet pressure P2 are regulated after 1500 seconds. After that, the flow rate decreases to approximately 200 l/min due to the pressure increase at a constant mass flow.

10 shows a diagram of a measured pressure difference P1-P2 (203) over time t. The measured pressure difference P1-P2 decreases slightly from approximately 145 mbar to approximately 130 mbar until the inlet pressure P1 and the outlet pressure P2 are regulated starting at 1500 seconds. This decrease can be caused, for example, by the drying out of the membrane 103 due to the increase in temperature T2 of the fuel cell system 100. After that, the measured pressure difference P1-P2 decreases to approximately 80 mbar due to the decrease in the air flow rate and then increases slightly.

11 shows a diagram of the course of a specific dry pressure difference P1-P2 (202) over time t. The specific pressure difference P1-P2 increases slightly, partly due to the increasing volume flow VL, from 100 mbar to approximately 110 mbar until the inlet pressure P1 and the outlet pressure P2 are regulated from 1500 seconds onwards. Thereafter, the specific pressure difference P1-P2 (202) decreases to approximately 60 mbar. The values for the specific dry pressure difference P1-P2 (202) for a dried-out membrane 103 over time t and with respect to the volume flow VL can be retrieved, for example, from a database.

12 shows a diagram of the course of the first reference value R1 over time t. The reference value R1 reflects the pressure difference between the measured pressure loss 203 (see 10 ) and the dry pressure loss 202 (see 11 ) again. Until the inlet pressure P1 and the outlet pressure P2 are regulated from 1500 seconds, the reference value R1 drops from approximately 30 mbar to approximately 15 mbar.

13 shows a diagram of the course of the second reference value R2 over time t. The second reference value R2 consists of a difference between the optimal pressure loss 201 and the dry pressure loss 202. Until the regulation of the inlet pressure P1 and the outlet pressure P2 from 1500 seconds, the reference value R2 hardly decreases and remains constant at approximately 24 mbar.

14 shows a diagram of the course of the quotient R1/R2 over time t. The quotient R1/R2 is calculated from the first reference value R1 and the second reference value R2, whereby the quotient is indicative of the moisture content of the membrane. The quotient R1/R2 decreases from a value above 1 to a value of 0.6 at approximately 1500 seconds, at which time the inlet pressure P1 and the outlet pressure P2 are regulated. After the inlet pressure P1 and the outlet pressure P2 have been regulated, the quotient continues to decrease slightly due to the inertia of the system and the time-delayed re-wetting of the membrane 103 before settling at an ideal value of 1 at approximately 1750 seconds.

15 shows a diagram of the course of a percentage humidity level over time t.

In the following, the steps of the method according to the invention are described by way of example with reference to 9 until 17 In particular, the method according to the invention uses the parameters air mass flow MAF, inlet pressure P1, outlet pressure P2, outlet temperature T2 to determine the humidification state of the membrane 103 calculated, which is identified as the cause of the low cell voltages.

First, the required volume flow VL is measured from a mass flow signal of the MAF sensor (see 3 and 9 ). This can be estimated, for example, using the ideal gas equation with the help of the inlet pressure P1 and the temperature T2 at the fluid outlet 102 as an average value.

The measured pressure loss P1 - P2 ( 10 ) via the cathode path of the fuel cell system 100 is determined as the currently measured value ( 10 )

The pre-calibrated reference dry pressure loss curve P1 - P2 (fully dried, see 11 ) of the forced dried-out fuel cell system 100 is known in advance from the offline preliminary investigation for each calculated volume flow VL.

For each measuring point along the time t, the difference R1 is calculated as pressure loss between the currently measured values and the pressure loss when the membrane is dried out (see 12 ) calculated.

In addition, the difference R2 of the pressure loss curve between the dried-out and optimally humidified membrane or fuel cell system is calculated from the pre-calibrated data (see for example 2 ). To determine the pressure loss curve P1 - P2 (optimally humidified membrane), either the offline method can be used in advance or the online method, as described above. The advantage of the offline method is that cell voltage measurement is no longer necessary for diagnosis during fuel cell operation.

Now the quotient R1/R2 is formed (see 14 ), which can be used to determine whether the membrane is in an over-moistened or over-dry state.

1. a.) R1/R2 =1 die Membran weist eine optimale Feuchte von 100% auf.
2. b.) R1/R2 > 1 zeigt einen Feuchtegrad von > 100% und somit eine Flutung der Membran, und
3. c.) R1/R2 = 0 für 0% Feuchte und für eine komplett ausgetrocknete Membran bzw. Brennstoffzelle.

Alternatively, the quotient can also be quantified as a percentage in the form of a “fictitious relative humidity” of the membrane as a moisture level in % (see 15 ). This includes:

1. a.) R1/R2 =1 the membrane has an optimal humidity of 100%.
2. b.) R1/R2 > 1 indicates a humidity level of > 100% and thus flooding of the membrane, and
3. c.) R1/R2 = 0 for 0% humidity and for a completely dried out membrane or fuel cell.

16 shows a diagram of the course of a binary diagnostic function during dehydration over time t and 17 shows a diagram of the course of a binary diagnostic function during flooding over time t.

For example, an upper threshold and a lower threshold are determined for the quotient R1/R2. If the upper threshold is exceeded, flooding of the membrane 103 is defined, and if the lower threshold is undershot, drying of the membrane 103 is defined. In other words, a percentage range of the quotient can be determined in which an optimal or permissible operating state exists depending on the membrane humidity.

For example, the upper threshold of the quotient R1/R2 can be set to 110% and the lower threshold to 70%. If the quotient exceeds the value 1.1 or 110% (see 15 and 17 ), flooding of the fuel cell system 100 is defined, so that drying processes are initiated. The diagnosis (flooding) will therefore be =1 if the upper threshold value is exceeded. This will result in a lower efficiency of the fuel cell system 100 and thus lower cell voltage measurements, as can be seen from the cell voltage in 8 at the beginning. Another reason for the decreasing efficiency is the change in the electrochemistry due to initially lower operating temperatures in the fuel cell system 100. If excess moisture no longer occurs and the upper threshold is undershot, the diagnosis (flooding) = 0 (see 17 ).

Due to the increase in temperature of the fuel cell system 100 in the present example, the membrane 103 and flow plate channels of the air slowly dry out, so that after approximately 600 seconds, sufficient drying has been achieved and flooding is no longer detected.

If the quotient falls below the value of 0.7 or 70%, for example, the fuel cell is considered to be dry (see 15 and 16 ), so that flooding processes or humidification measures of the fuel cell should be carried out accordingly. In the present example, the state "drying out" is determined from approximately 1100 seconds. The diagnosis (drying out) is therefore = 1. Due to this, in addition to the changing electrochemistry, a lower efficiency of the fuel cell 100 will also result. If sub-humidity no longer occurs, the diagnosis "drying out" is = 0. From 1500 seconds onwards, in the present example, the inlet pressure P1 and the outlet pressure P2 are adjusted so that the cell voltage does not drop any further and the membrane is thus protected from permanent thermal damage. Although the inertia system after 1500 seconds the humidity level continues to decrease (see 15 ), but then the humidity level continues to rise. From about 1800 seconds onwards, the lower threshold of the humidity level is exceeded, so that no more drying out is detected (see 15 and 17 ). The humidity level ideally settles at a quotient of 1 or 100%.

Optionally, a dependency on the operating point of the fuel cell system 100 (e.g., current, pressure, air lambda) can also be assigned to the upper and lower threshold values. The example shown applies, for example, to a current of 100 A for the fuel cell system 100.

If drying out or flooding has been diagnosed and the optimum humidity level has been exceeded or undershot for a longer period of time, this information can be passed on to a downstream humidity controller or the control unit 110. This controller can then, for example, reduce the temperature or increase or decrease the pressure in the cathode circuit or the air lambda via the exhaust back pressure. In the example shown, the exhaust back pressure was increased by continuously closing the back pressure throttle valve 113, which in 6 which is evident at the end by an increase in P2. This allows the humidity management to be returned to the optimal range, as shown in 15 until 17 in a time range between 600 and 1100 seconds or from approximately 1800 seconds onwards in the calculated humidity level at the end. This stabilizes the cell voltage again ( 8 ).

Additionally, it should be noted that "comprising" does not exclude other elements or steps, and "a" or "an" does not exclude a plurality. Furthermore, it should be noted that features or steps described with reference to one of the above embodiments may also be used in combination with other features or steps of other embodiments described above. Reference signs in the claims are not to be considered as limitations.

Claims (10)

A method for determining a moisture content of a membrane (103) of a fuel cell system (100), comprising: determining an optimal pressure loss (201) between an inlet pressure (P1) of air at a fluid inlet (101) of the fuel cell system (102) and an outlet pressure (P2) at a fluid outlet (102) of the fuel cell system (100) as a function of an inlet volume flow (VL) of air at the fluid inlet (101) at an optimal moisture content of the membrane (103) of the fuel cell system (100); determining a dry pressure loss (202) between the inlet pressure (P1) of air at the fluid inlet (101) and an outlet pressure (P2) at the fluid outlet (102) as a function of the inlet volume flow (VL) when the membrane (103) of the fuel cell system (100) is dry; measuring a pressure loss between the inlet pressure (P1) of air at the fluid inlet (101) and the outlet pressure (P2) at the fluid outlet (102) as a function of the inlet volume flow (VL) of air at the fluid inlet (101), forming a first reference value (R1) consisting of a difference between the measured pressure loss and the dry pressure loss (202), forming a second reference value (R2) consisting of a difference between the optimal pressure loss (201) and the dry pressure loss (202), forming a quotient of the first reference value (R1) and the second reference value (R2), wherein the quotient is indicative of a moisture state of the membrane (103). Procedure according to Claim 1 , Determining an upper threshold value and a lower threshold value for the quotient, wherein flooding of the membrane (103) is defined when the upper threshold value is exceeded and drying out of the membrane (103) is defined when the lower threshold value is undershot. Procedure according to Claim 1 or 2 , further comprising adjusting the humidity state of the membrane (103) based on the formed quotient. Procedure according to Claim 3 , wherein the adjustment of the humidity state of the membrane (103) is implemented by adjusting the inlet pressure at the fluid inlet (101). Procedure according to Claim 3 or 4 , wherein the adjustment of the humidity state of the membrane (103) is implemented by means of an adjustment of the volume flow at the fluid inlet (101). Procedure according to one of the Claims 3 until 5 , wherein the adjustment of the moisture state of the membrane (103) is implemented by means of an adjustment of a temperature of the fuel cell system (100). Procedure according to Claim 6 , wherein the setting of the temperature of the fuel cell system (100) is carried out by means of a temperature setting of a cooling medium and/or a control of a cooling supply of the cooling medium which cools the fuel cell system (100). Procedure according to one of the Claims 1 until 7 , wherein determining an optimal pressure loss (201) comprises operating the fuel cell system (100) from a minimum to a maximum current intensity and measuring the resulting cell voltage of the fuel cell system (100), controlling at least one control parameter of the fuel cell system (100) until a constant cell voltage is established, measuring a fuel cell power output at the constant cell voltage, wherein reaching a maximum fuel cell power output is indicative of the optimal moisture state of the membrane. Procedure according to Claim 8 , wherein the at least one control parameter is selected from the group consisting of the inlet pressure (P1), the outlet pressure (P2), an air lambda on the cathode side and the temperature of the fuel cell system (100). A fuel cell system comprising: a membrane (100) separating a cathode side and an anode side, a fluid inlet (101) through which air can be supplied to the cathode side at an inlet pressure (P1), a fluid outlet (102) through which a fluid can be discharged from the cathode side as a product at an outlet pressure (P2), a control unit (110) configured to: determine an optimal pressure loss (201) between the inlet pressure (P1) of air and the outlet pressure (P2) of the fluid as a function of an inlet volume flow (VL) of air at the fluid inlet (101) at an optimal humidity level of the membrane (103) of the fuel cell system (100), and determine a dry pressure loss (202) between the inlet pressure (P1) of air at the fluid inlet (101) and the outlet pressure (P2) at the fluid outlet (102) as a function of the inlet volume flow (VL) for a dried-out membrane (103) of the fuel cell system (100), Measuring a pressure loss between the inlet pressure (P1) of air at the fluid inlet (101) and the outlet pressure (P2) at the fluid outlet (102) as a function of the inlet volume flow (VL) of air at the fluid inlet (101), Forming a first reference value (R1) consisting of a difference between the measured pressure loss and the dry pressure loss (202), Forming a second reference value (R2) consisting of a difference between the optimal pressure loss (201) and the dry pressure loss (202), Forming a quotient of the first reference value (R1) and the second reference value (R2), wherein the quotient is indicative of a moisture state of the membrane (103).

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