Method for determining the state of a fuel cell system

Abstract

The invention relates to a method (100) for diagnosing the state of a fuel cell system (200), the method (100) having the steps of: - starting (101) the fuel cell system (200) with closed anode shut-off valves (207) and closed cathode shut-off valves (209); - opening (103) the anode shut-off valves (207) while the cathode shut-off valves (209) remain closed; - introducing (105) fresh hydrogen into an anode chamber (203) of a fuel cell stack (201) of the fuel cell system (200) while a cathode chamber (205) of the fuel cell stack (201) is closed, - applying (107) an electric charging current to the fuel cell stack (201), the charging current having a reversed polarity relative to a subsequent normal operation such that a voltage applied to respective fuel cells of the fuel cell stack (201) increases; - determining (109) the state of the fuel cell system (200) on the basis of a voltage curve at at least one fuel cell of the fuel cell stack (201) and / or an anode pressure measured by means of at least one anode pressure sensor provided at the anode chamber (203), - opening (111) the cathode shut-off valves (209), and - outputting (113) the state by means of an output interface (215).

Description

[0001] R.415639

[0002] - 1 -

[0003] Description

[0004] title

[0005] Method for determining the state of a fuel cell system

[0006] The presented invention relates to a method for determining a state of a fuel cell system and a fuel cell system according to the attached claims.

[0007] State of the art

[0008] Diagnosing the condition of a fuel cell system allows, on the one hand, the user to be informed about this condition and, on the other hand, the fuel cell system to be adjusted or adapted to this condition in order to maximize, for example, its RUL (remaining useful life) and energy efficiency.

[0009] In this context, the acquisition of detailed information on the electrochemical active surface area (ECSA), membrane permeability and double-layer capacity is particularly relevant.

[0010] These quantities can be determined using the so-called Galvanostatic Charge Method (GCM) in a stationary flowing gas atmosphere, such as nitrogen on the cathode side and hydrogen on the anode side.

[0011] One variant (AGCM - Adapted Galvanostatic Charge Method) involves inerting not with flowing gases, but with a closed inert volume, so that closed anode and cathode shut-off valves are present and no mass exchange with the environment occurs. R.415639

[0012] - 2 -

[0013] The trapped, inert gas volume is processed using the system's own special procedure for shutting down a fuel cell system, known as "cathode oxygen depletion." After deactivating the fuel cell stack, a waiting period of 3–6 hours is required to allow the trapped gases to homogenize through diffusion processes both within the two half-cells and across the membrane between them. An equilibrated state is a prerequisite for initiating state determination or characterization based on a measurement that relies on the fuel cell stack being recharged from its equilibrated state at least once, but preferably several times, with one or more different charging currents.

[0014] Charging initiates various electrochemical processes that generate a characteristic voltage response, from which various stack characteristics are extracted, allowing conclusions to be drawn about the state of the fuel cell system.

[0015] After each charging process, an equilibrated starting state must be established again, which necessitates waiting times for equilibration before and during the measurement.

[0016] Furthermore, the fuel cell stack may be briefly energized while unattended. Additionally, a processing unit must be woken up for measurement, or it may not be switched off, which minimizes the energy efficiency of the fuel cell system.

[0017] Disclosure of the invention

[0018] Within the scope of the presented invention, a method for diagnosing the condition of a fuel cell system and a fuel cell system are presented. Further features and details of the invention will become apparent from the respective dependent claims, the description, and the drawings. Features and details related to R.415639 apply.

[0019] - 3 - methods according to the invention are described, of course also in connection with the fuel cell system according to the invention and vice versa, so that with regard to the disclosure of the individual aspects of the invention, mutual reference is always made or can be made.

[0020] The presented invention serves to provide a robust fuel cell system.

[0021] Thus, according to a first aspect of the presented invention, a method for diagnosing the condition of a fuel cell system is presented.

[0022] The presented method comprises starting the fuel cell system with the anode and cathode shut-off valves closed, opening the anode shut-off valves while the cathode shut-off valves remain closed, introducing fresh hydrogen into an anode compartment of a fuel cell stack of the fuel cell system while a cathode compartment of the fuel cell stack is sealed, applying an electrical charging current to the fuel cell stack with a polarity reversed relative to subsequent normal operation, such that the voltage applied to the respective fuel cells of the fuel cell stack increases, and determining the state of the fuel cell system based on a voltage profile at at least one fuel cell of the fuel cell stack and / or an anode pressure measured by means of at least one anode pressure sensor arranged on the anode compartment.the opening of the cathode shut-off valves and the output of the status via an output interface.

[0023] In the context of the presented invention, an output interface is understood to be an interface for providing information, such as a communication interface. To output the status, the output interface can include an output unit, such as a display (R.415639).

[0024] - 4 - and / or transfer the state to a memory or provide the state to a control function for adjusting the fuel cell system.

[0025] The presented method is performed during the start-up process of a fuel cell system and is based on a closed half-cell, namely a closed cathode subsystem or a closed cathode compartment. While the cathode compartment is closed, the other half-cell, namely the anode subsystem or the anode compartment, is flushed with fresh hydrogen. This configuration makes it possible to utilize a so-called anode flush, i.e., a purging of the anode compartment with fresh hydrogen, which is standard practice during a start-up process, to perform a diagnostic check of the fuel cell system, i.e., to determine the condition of the fuel cell system.

[0026] By using the anode flush to carry out the presented method, the state of the fuel cell system is determined without the need for additional hydrogen, making the presented method and fuel cell system particularly energy efficient.

[0027] Furthermore, waiting times for the fuel cell system to be ready for use are minimized, and it is prevented that a fuel cell system is left unattended under voltage.

[0028] The initial state of the presented process is a completely closed fuel cell stack in which both the anode and cathode shut-off valves are closed, thus preventing any mass exchange between the fuel cell stack and the environment. A coolant flow may optionally be active. In this state, the fuel cell stack, specifically the anode and cathode compartments, contains hydrogen, nitrogen, and gaseous water. Depending on how long ago normal operation lasted, the gases in the fuel cell stack are distributed more or less homogeneously within the half-cells and within each half-cell.

[0029] To start the fuel cell system, first the anode compartment, i.e., only the anode compartment, not the cathode compartment, is filled with fresh hydrogen from R.415639.

[0030] - 5 - a tank. Accordingly, no preconditioned gas is used for purging the anode compartment, i.e., the so-called anode flush. For this purpose, the anode shut-off valves, i.e., the inlet and outlet valves on the anode compartment, are opened, and any existing anode recirculation system is also purged. The cathode shut-off valves on the cathode compartment remain closed, so that no air, especially no oxygen from the environment, enters the cathode compartment. The aim of the anode flush is to achieve a sufficiently high or predetermined hydrogen content in the anode compartment. This can be achieved, for example, by adjusting the mass flow rate of fresh hydrogen or the pressure level of the anode inlet.

[0031] While the cathode volume is closed, air is bypassed around it in the air system. This serves to sufficiently dilute the hydrogen mass flow from the anode outlet, as the anode exhaust is introduced into the cathode exhaust downstream of the stack. Dilution of hydrogen is necessary to prevent an explosive mixture.

[0032] After the anode flush, the cathode shut-off valves are opened and the cathode compartment is circulated with fresh ambient air via an air system, such as a blower or compressor. As a result, oxygen and hydrogen are now present at both the cathode and anode electrodes, whereupon a voltage (OCV, open circuit voltage, approximately 0.9–1.1 V) is established.

[0033] It may be provided that a low load of 5 to 50 mA / cm² is used. 2is set in the direction of the regular current direction so that the voltage level is lowered below OCV to avoid damage to the catalyst.

[0034] The anode subsystem then switches to its normal operation, e.g. an anode inlet pressure control or a hydrogen concentration control, and the start-up is complete.

[0035] It may be provided that the determination of the state of the fuel cell system in a time range between a start of the R.415639

[0036] - 6 -

[0037] The process involves applying an electric current to the fuel cell stack and opening the cathode shut-off valves.

[0038] To diagnose or determine the state of the fuel cell system, a charging current is applied to the fuel cell stack during the anode flush. This current is polarized opposite to the regular current direction during normal operation of the fuel cell system. Accordingly, the fuel cell stack acts as a power sink during charging.

[0039] The charging current triggers various electrochemical processes in the fuel cell stack, leading to a characteristic voltage increase in each fuel cell of the fuel cell stack.

[0040] The charging current can be approximately 5 - 50 mA / cm² 2It is advantageous to choose a relatively high charging current to accelerate the diagnosis and thus fit within the narrow time window of the anode flush. Higher charging currents are also easier for power electronics to set, since their DC-DC converters typically have a current setting range of 0–600 A, where 0 A cannot be set and the control accuracy at the lower end of the setting range is comparatively poor.

[0041] Rapid diagnostic execution also positively impacts the accuracy of state determination, as it minimizes parasitic processes such as electrical short circuits. This increases the accuracy in determining other parameters, such as ECSA, which are calculated using charge summation. Furthermore, rapid diagnostic execution justifies the assumption of constant fluidic boundary conditions during the diagnostic process.

[0042] In the presented method, the hydrogen content on the anode side increases from the start of the anode flush and can be considered constant after a certain time (final value: 100% hydrogen). The hydrogen partial pressure can then be directly determined from an anode pressure measured by a pressure sensor installed on the anode side (R.415639).

[0043] - 7 - can be. The pressure sensor can be installed at the anode inlet or outlet. It is also possible to determine an average pressure of both pressure sensors.

[0044] Various processes occur in the cathode compartment during the anode flush provided for according to the invention:

[0045] 1. Hydrogen is electrochemically pumped from the cathode compartment to the anode compartment.

[0046] 2. Nitrogen and gaseous water diffuse through the membrane (permeation) from the cathode compartment into the anode compartment due to the resulting partial pressure gradient.

[0047] 3. Also due to a partial pressure difference, hydrogen diffuses from the anode compartment into the cathode compartment. However, due to electrochemical processes, this hydrogen is pumped back into the anode compartment.

[0048] 4. Hydrogen in the areas of the cathode compartment that are spatially further away from the cathode electrodes of the fuel cells also begins to diffuse towards the cathode-side electrodes due to the concentration gradient that develops within the cathode compartment. From there, it is also electrochemically pumped into the anode compartment.

[0049] 5. The cathode pressure will decrease at a constant temperature because a total number of molecules leave the volume.

[0050] It may be provided that the state is determined by determining at least one parameter from the following list of parameters: double-layer capacitance, membrane permeability, short-circuit resistance, electrically active area.

[0051] The parameters double-layer capacitance, membrane permeability, short-circuit resistance, and electrochemically active area can be mathematically determined from operating parameters measured or determined during the anode flush, in particular a voltage measured at the respective fuel cells and an anode pressure measured in the anode compartment. These parameters can be directly used as state indicators (R.415639).

[0052] - 8 - output or assigned to a key figure or value that describes the state.

[0053] Alternatively or additionally, the respective parameters measured during the anode flush can be assigned to a state, i.e., a key figure or characteristic value, using a machine learner.

[0054] It may also be stipulated that before reaching the open circuit voltage of, for example, 0.8 V, a charging current of, for example, 0.066 - 0.23 A / cm² is applied. 2 applied to the fuel cell stack to prevent aging of the catalyst layers.

[0055] It may also be provided that the introduction of fresh hydrogen into the anode compartment for a predetermined period between

[0056] The tests are performed in 2 seconds and 30 seconds.

[0057] Depending on the spatial geometry of the fuel cell system and its system topology, different anode flush times may be specified for its start-up.

[0058] It may also be provided that the application of the charging current to the fuel cell stack and the determination of the state of the fuel cell system before opening the cathode shut-off valves are carried out several times.

[0059] By repeatedly performing the determination of the state of the fuel cell system, several measured values ​​are obtained and the state is determined in a particularly valid manner.

[0060] It may also be provided that, during the introduction of fresh hydrogen into the anode compartment, air is conveyed through an air system and routed via a bypass line past the fuel cell stack into an exhaust tract. R.415639

[0061] - 9 -

[0062] Hydrogen exits the anode compartment into the exhaust pipe via a purge / drain line. This creates a gas mixture with an increasing hydrogen content over time. To prevent the formation of an explosive mixture in the exhaust tract of the fuel cell system or a corresponding vehicle, an air system can be activated during the described process, particularly during the anode flush. This means that an air compression device, such as a compressor, begins to rotate, and a volume of air flows through the system. However, this air flow does not pass through the cathode compartment of the fuel cell stack, but instead bypasses the fuel cell stack via a bypass line.

[0063] It may also be provided that, after the charging current is applied to the fuel cell stack and before the cathode shut-off valves are opened, an electric current is drawn with a polarity relatively similar to that provided for in normal operation.

[0064] To discharge the fuel cell stack, a discharge current can be applied to the fuel cell stack after the charging current, but in the opposite direction to the charging direction, i.e., in the direction or polarity of normal operation. This makes an additional data point available for each charging data point.

[0065] It may also be provided that the charging current and the anode pressure are adjusted in such a way that a voltage curve measured at at least one fuel cell of the fuel cell stack crosses a predetermined voltage level three times.

[0066] To determine the state of the fuel cell system, the parameters double-layer capacitance (CDL), membrane permeability (kN2), and short-circuit resistance (R) can be used. sc and electrochemically active area ECSA, also measured in "roughness factor", via which hydrogen desorption / adsorption is determined according to an electrochemical equivalent circuit. R.415639

[0067] - 10 -

[0068] The voltage profile can be evaluated by exploiting the fact that at a certain voltage level uev, usually between 0.4 and 0.5 V cell voltage, no desorption / adsorption processes take place and only the first three parameters mentioned (CDL, kn2, and R) are relevant. sc ) remain in the system of equations.

[0069] Accordingly, based on the equation for charge conservation, a formula (1) is obtained as follows:

[0070] The following applies: cell voltage (u), anode pressure (p) A~), charging current (i). Parameter (p) is a proportionality constant. To solve equation 1 with three unknowns, it must be set up three times with different parameters at three different positions but at the same voltage level uev. This yields the following system of equations, which has a unique solution if the individual equations are linearly independent.

[0071] With the two degrees of freedom for controlling the fuel cell system during the anode flush, i.e., the adjustment of current and anode pressure, the voltage level (uev) can be crossed three times, thus enabling a linearly independent and well-conditioned parameterization of the above system of equations. This can be achieved, for example, if the anode pressure p AThe current rises / falls monotonically, and the current does not rise monotonically at points 1, 2, and 3. The voltage level (uev) can be the same at each point and between 0.4 V and 0.5 V to prevent desorption / adsorption. R.415639

[0072] - 11 -

[0073] Accordingly, it can be provided that the anode pressure is regulated in such a way that it rises or falls monotonically and the charging current is reduced at least once and then rises again.

[0074] The remaining parameter Roughness Factor can be determined using the obtained parameters by reinserting the charge conservation equation (set up at each time t) and integrating the desorption current between the voltage limits lev and uev.

[0075] After the voltage level has been crossed three times, the cathode compartment can be opened and air can flow through it, which, after establishing a current flow in the normal operating direction, directly switches to the normal operating mode.

[0076] In particular, it may be provided that the parameters double-layer capacitance, membrane permeability, short-circuit resistance and electrically active area are determined using an electrochemical equivalent circuit diagram.

[0077] According to a second aspect, the presented invention relates to a fuel cell system for converting energy.

[0078] The fuel cell system comprises a fuel cell stack, which includes an anode compartment and a cathode compartment; controllable anode shut-off valves for opening or closing the anode compartment; controllable cathode shut-off valves for opening or closing the cathode compartment; a hydrogen metering unit for metering hydrogen into the anode compartment; a current / voltage source, such as a DC / DC converter connected to an HV battery, for applying a charging current to the fuel cell stack; and a computing unit, wherein the computing unit is configured to control the anode shut-off valves, the cathode shut-off valves, the hydrogen metering unit, and the current / voltage source to carry out a possible embodiment of the presented method. R.415639

[0079] - 12 -

[0080] It may be provided that the fuel cell system includes a bypass line that connects an air system for conveying air to an exhaust tract of the fuel cell system, with the bypass line directing the air past the fuel cell stack.

[0081] Advantages described in detail in relation to the method for diagnosing the condition of a fuel cell system according to the first aspect of the invention apply equally to the fuel cell system for converting energy according to the second aspect of the invention and vice versa.

[0082] Further advantages, features, and details of the invention will become apparent from the following description, in which exemplary embodiments of the invention are described in detail with reference to the drawings. The features mentioned in the claims and in the description can each be essential to the invention individually or in any combination.

[0083] They each show schematically:

[0084] Figure 1 shows a possible embodiment of the presented method,

[0085] Figure 2 shows a detailed representation of the process according to Fig. 1 and

[0086] Figure 3 shows a possible embodiment of the presented fuel cell system.

[0087] Fig. 1 shows a method 100 for diagnosing the condition of a fuel cell system 200.

[0088] The procedure 100 includes a start step 101 in which the fuel cell system 200 is started, e.g. in response to a start command provided by a user, with the anode shut-off valves and cathode shut-off valves closed.

[0089] Furthermore, the method 100 comprises an opening step 103 in which the anode shut-off valves 207 are opened while the cathode shut-off valves 209 remain closed, an initiation step 105, R.415639

[0090] - 13 - in which fresh hydrogen is introduced into an anode compartment 203 of a fuel cell stack 201 of the fuel cell system 200, while a cathode compartment 205 of the fuel cell stack 201 is closed and a charging step 107 in which an electrical charging current is applied to the fuel cell stack 201, the charging current being applied with a polarity reversed relative to a subsequent normal operation, so that a voltage applied to each fuel cell of the fuel cell stack 201 increases.

[0091] Furthermore, the method 100 comprises a determination step 109, in which a state of the fuel cell system 200 is determined on the basis of a voltage profile on at least one fuel cell of the fuel cell stack 201 and / or an anode pressure measured by means of at least one anode pressure sensor arranged on the anode space 203, an opening step 111, in which the cathode shut-off valves 209 are opened and an output step 113, in which the state is output via an output interface 215.

[0092] Optionally, the procedure 100 includes a setting step 115 in which the fuel cell system is set according to the output state, for example by selecting a power provided by the fuel cell stack 201, in particular a maximum power, based on the state, in particular by using an assignment scheme.

[0093] Figure 2 shows a first diagram 120 and a second diagram 130, which show processes occurring in parallel over time.

[0094] The first diagram 120 spans time on its abscissa and a cell voltage of a fuel cell of the fuel cell stack 201 of the fuel cell system 200 on its ordinate.

[0095] The second diagram 130 spans time on its abscissa and a current and anode pressure at the fuel cell stack 201 of the fuel cell system 200 on its ordinate. R.415639

[0096] - 14 -

[0097] A curve 121, shown in the first diagram 120, arises under the conditions shown in the second diagram 130. The curve 121 intersects a voltage value Uev three times at positions 123, 125, and 127. Accordingly, the current and anode pressure values ​​corresponding to positions 123, 125, and 127, represented by curves 133 and 131 in the second diagram 130, can be used to solve equation 1 with three unknowns.

[0098] Figure 3 shows a fuel cell system 200 for converting energy.

[0099] The fuel cell system 200 comprises a fuel cell stack 201, which includes an anode compartment 203 and a cathode compartment 205, as well as controllable anode shut-off valves 207 for opening or closing the anode compartment 203, controllable cathode shut-off valves 209 for opening or closing the cathode compartment 205, a hydrogen metering unit 211 for metering hydrogen into the anode compartment 203, a current / voltage source 213, such as a DC / DC converter for applying a charging current to the fuel cell stack 201, an output interface 215, and a computing unit 217, wherein the computing unit 217 is configured to control the anode shut-off valves 207, the cathode shut-off valves 209, the hydrogen metering unit 211, the voltage source 213, and the to control output interface 215 in order to carry out the procedure 100 shown in Fig. 1.

Claims

R.415639 - 15 - Claims 1. Method (100) for diagnosing a condition of a fuel cell system (200), wherein the method (100) comprises: Starting (101) the fuel cell system (200) with the anode shut-off valves (207) and cathode shut-off valves (209) closed, Opening (103) the anode shut-off valves (207) while the cathode shut-off valves (209) remain closed, Introducing (105) fresh hydrogen into an anode compartment (203) of a fuel cell stack (201) of the fuel cell system (200), while a cathode compartment (205) of the fuel cell stack (201) is closed, Imprinting (107) an electric charging current onto the fuel cell stack (201), with a polarity reversed relative to a subsequent normal operation, so that a voltage applied to each fuel cell of the fuel cell stack (201) increases, Determining (109) a state of the fuel cell system (200) based on a voltage profile at at least one fuel cell of the fuel cell stack (201) and / or an anode pressure measured by means of at least one anode pressure sensor arranged on the anode space (203), Opening (111) the cathode shut-off valves (209) and outputting (113) the state through an output interface (215). R.415639 - 16 - 2. Method (100) according to claim 1 , characterized in that the determination of the state of the fuel cell system (200) takes place in a time interval between the start of the imprinting (107) of the electric current onto the fuel cell stack (201) and the opening (111) of the cathode shut-off valves (209).

3. Method (100) according to claim 1 or 2, characterized in that the state is determined by determining at least one parameter from the following list of parameters: Double layer capacitance, membrane permeability, short-circuit resistance, electrochemically active area of ​​a catalyst.

4. Method (100) according to one of the preceding claims, characterized in that the charging current is between 5 A / cm² 2 and 50 mA / cm 2 lies.

5. Method (100) according to one of the preceding claims, characterized in that the introduction of fresh hydrogen into the anode compartment (203) is carried out for a predetermined period between 2 seconds and 30 seconds.

6. Method (100) according to one of the preceding claims, characterized in that the imprinting (107) of the charging current onto the fuel cell stack (201) and the determination of the state of the fuel cell system (200) before opening the cathode shut-off valves (209) is carried out several times. R.415639 - 17 - 7. Method (100) according to one of the preceding claims, characterized in that during the introduction of fresh hydrogen into the anode compartment (203), air is conveyed through an air system and directed through a bypass line past the fuel cell stack (201) into an exhaust gas tract.

8. Method (100) according to one of the preceding claims, characterized in that after imprinting (107) the charging current onto the fuel cell stack (201) and before opening (111) the cathode shut-off valves (209) an electric current with a relatively similar polarity as provided for in normal operation is drawn.

9. Method (100) according to one of claims 1 to 7, characterized in that the charging current and the anode pressure are adjusted such that a voltage profile measured at at least one fuel cell of the fuel cell stack (201) crosses a predetermined voltage level (Uev) at least three times.

10. Method (100) according to claim 9, characterized in that the anode pressure is adjusted such that it rises or falls monotonically and the charging current is reduced at least once and then rises again.

11. Method (100) according to claim 9 or 10, characterized in that the parameters double layer capacitance, membrane permeability, short-circuit resistance and electrochemically active area are determined using an electrochemical equivalent circuit diagram. R.415639 - 18 - 12. Fuel cell system (200) for converting energy, the fuel cell system (200) comprising: a fuel cell stack (201) comprising an anode compartment (203) and a cathode compartment (205), controllable anode shut-off valves (207) for opening or closing the anode compartment (203), controllable cathode shut-off valves (209) for opening or closing the cathode compartment (205), a hydrogen metering unit (211) for metering hydrogen into the anode compartment (203), a current / voltage source (213) for imposing a charging current on the fuel cell stack (201), an output interface (215) and a computing unit (217), the computing unit (217) being configured to anode shut-off valves (207), the cathode shut-off valves (209), the to control the hydrogen dosing unit (211), the current / voltage source (213) and the output interface (215) in order to carry out a method (100) according to one of claims 1 to 11.

13. Fuel cell system (200) according to claim 12, characterized in that the fuel cell system (200) comprises a bypass line which connects an air system for conveying air to an exhaust tract of the fuel cell system in an air-conducting manner, wherein the bypass line directs the air past the fuel cell stack (201).

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