Method for monitoring a fuel cell system, and fuel cell system

Abstract

The invention relates to a method (10a; 10b; 10c) for monitoring a fuel cell system (12a; 12b; 12c) which converts at least one fuel into an exhaust gas which is discharged from the fuel cell system (12a; 12b; 12c), wherein in at least one method step, an amount of fuel in the exhaust gas is characterised. According to the invention, in order to characterise the amount of fuel, an oxygen parameter of the exhaust gas is determined at a first determination point in at least one first determination step (14a; 14b; 14c), and a further value of the oxygen parameter is determined at a further determination point in at least one further determination step (16a; 16b; 16c).

Description

[0001] R.414654

[0002] - 1 -

[0003] Description

[0004] Methods for monitoring a fuel cell system and fuel cell system

[0005] State of the art

[0006] A method for monitoring a fuel cell system, which converts at least one fuel into an exhaust gas that is discharged from the fuel cell system, has already been proposed, wherein in at least one process step a quantity of fuel in the exhaust gas is characterized.

[0007] Furthermore, a control method for a fuel cell system is known from DE 10 2021 212685 A1, which is based on an energy / heat balance. By applying the control method, fluctuations in the composition of the fuel used are compensated for by adjusting the fuel volume flow and / or an air volume flow. This allows the fuel cell system to be operated advantageously and efficiently regardless of the fuel composition.

[0008] Disclosure of the invention

[0009] The invention relates to a method for monitoring a fuel cell system which converts at least one fuel into an exhaust gas which is discharged from the fuel cell system, wherein in at least one process step of the method a quantity of fuel in the exhaust gas is characterized. R.414654

[0010] - 2 -

[0011] It is proposed that, to characterize the fuel quantity, an oxygen parameter of the exhaust gas is determined at a first measurement point in at least a first step of the process, and that a further value of the oxygen parameter is determined at a further measurement point in at least a further measurement step. The fuel cell system preferably comprises at least one fuel cell unit for an electrochemical conversion of the fuel and an oxygen-containing fluid. The at least one fuel cell unit preferably comprises at least one fuel cell, preferably a plurality of identically designed fuel cells, which can be arranged in one or more stacks. The at least one fuel cell preferably comprises at least one fuel electrode, at least one oxygen electrode, and at least one electrolyte arranged between them.The electrolyte is preferably oxygen-conducting, alternatively proton-conducting. Fuel cells of the same fuel cell unit are preferably connected electrically in series and fluidically in parallel. The fuel cell system comprises at least one fuel supply, which is fluidically connected to a fluid inlet of the at least one fuel electrode. Preferably, during operation of the fuel cell system, the at least one fuel electrode is supplied with the at least one fuel via the fuel supply. A hydrogen-containing, ammonia-containing, and / or hydrocarbon-containing fluid, in particular natural gas or biogas, is preferably used as fuel. The fuel cell system comprises at least one oxygen supply, which is fluidically connected to a fluid inlet of the at least one oxygen electrode.Preferably, during operation of the fuel cell system, the at least one oxygen electrode is supplied with the at least one oxygen-containing fluid, in particular ambient air or an industrial gas with a defined oxygen content, via the oxygen supply. The fuel cell system comprises at least one fuel exhaust gas discharge, which is fluidically connected to a fluid outlet of the at least one fuel electrode. The fuel cell system comprises at least one oxygen exhaust gas discharge, which is fluidically connected to a fluid outlet of the at least one oxygen electrode. Particularly preferably, the oxygen exhaust gas discharge and the fuel exhaust gas discharge are integrated into one another and include in particular R.414654.

[0012] - 3 - a common exhaust line through which exhaust gas from at least one oxygen electrode and exhaust gas from at least one fuel electrode are discharged together. Alternatively, the oxygen exhaust gas discharge and the fuel exhaust gas discharge each comprise a fluidically separate exhaust line. In the following, it is assumed that a common exhaust line exists for both the oxygen exhaust gas discharge and the fuel exhaust gas discharge. The fluid in the common exhaust line is referred to simply as exhaust gas. If a distinction between exhaust gas from the oxygen electrode or exhaust gas from the fuel electrode is necessary, this will be explicitly stated. Transferring the method according to the invention with separate handling of the exhaust gases from the electrodes is easily possible. The exhaust line preferably carries the exhaust gas out of the fuel cell system.Preferably, the exhaust gas line terminates in a chimney of the fuel cell system for the release of exhaust gas to the environment. Alternatively, the exhaust gas line terminates at a fluid interface for connection to an external disposal system, such as a central disposal line for multiple fuel cell systems, a supply line for a downstream CCh capture and storage plant, or the like.

[0013] The method preferably controls the amount of fuel in the exhaust gas to remain below a threshold value. The amount of fuel in the exhaust gas preferably describes the total amount of all remaining oxidizable components of the exhaust gas. These can be atoms and / or molecules of the originally supplied fuel, such as hydrogen, methane, or the like, and / or partially oxidized intermediate products that have formed within the fuel cell system, such as carbon monoxide in particular. In the simplest case, the threshold value is equal to the detection limit of the amount of fuel in the exhaust gas. Preferably, the threshold value is greater than the detection limit of the amount of fuel. The threshold value is preferably less than the lower explosive limit of the fuel in the exhaust gas.The fuel cell system preferably comprises a control unit that controls the fuel supply, the air supply and / or an electrical unit connected to the fuel cell unit, in particular in the course of a control method as is known from the prior art, for example from DE 10 2021 212685 A1. If the fuel quantity in the R.414654 exceeds

[0014] - 4 -

[0015] When the exhaust gas exceeds the threshold value, the control unit preferably shuts off at least the fuel supply, and preferably the entire fuel cell system. A "control unit" is understood to be, in particular, a unit with at least one control electronics unit. A "control electronics unit" is understood to be, in particular, a unit with a processor unit, a memory unit, and an operating program stored in the memory unit.

[0016] The phrase "characterizing the fuel quantity" means, in particular, determining a quantity that correlates with the fuel quantity, especially positively or negatively, in order to detect whether the threshold has been exceeded. This correlated quantity can be the fuel quantity itself or a quantity different from the fuel quantity, so that the absolute value of the fuel quantity does not need to be explicitly determined during the process. It is particularly preferred that, in at least one process step, the fuel quantity is characterized via the oxygen parameter, especially without explicitly determining the fuel quantity. It is conceivable that, in further process steps, the fuel quantity is explicitly determined, especially as a function of the oxygen parameter.

[0017] The oxygen parameter is preferably a parameter that correlates with the oxygen content in the exhaust gas. The oxygen parameter can, for example, be an oxygen concentration or an air-fuel ratio of the exhaust gas, or an equivalent quantity that describes the oxygen fraction of the exhaust gas. The oxygen parameter can be detected by means of at least one hardware sensor of the fuel cell system arranged in / on the exhaust pipe, for example, an optical oxygen sensor, an oxygen resistance probe, a lambda probe, or the like, or it can be calculated by the control unit from other system parameters available, particularly within the framework of the control procedure.Depending on the actual measurement principle used by the hardware sensor, a measured value of the hardware sensor that correlates with the oxygen content can also be used directly as an oxygen parameter, particularly without explicitly determining the oxygen content. For example, when using a wide-area sensor (R.414654).

[0018] - 5 - A broadband lambda sensor, used as a hardware sensor, allows the electrical pumping current of a pump cell of the broadband lambda sensor to be used directly as an oxygen parameter, particularly without specifically determining the air-fuel ratio. An oxygen parameter that exhibits cross-sensitivity to another component of the exhaust gas, for example hydrogen, is particularly preferred. The first measurement point is preferably located in the exhaust gas line. The second measurement point is preferably located downstream of the first measurement point in the exhaust gas line. The measurement point and / or the second measurement point can be measuring points of a hardware sensor or evaluation points of a mathematical model. The control unit particularly preferentially compares the values ​​of the oxygen parameter to characterize the amount of fuel in the exhaust gas.

[0019] The term "intended" should be understood to mean specifically programmed, designed, and / or equipped. The fact that an object is intended for a specific function should be understood to mean, in particular, that the object fulfills and / or executes this specific function in at least one application and / or operating state.

[0020] The design according to the invention enables advantageously cost-effective monitoring of the exhaust gas. In particular, expensive methods for directly determining the composition of the exhaust gas and the amount of fuel contained therein can be dispensed with. Furthermore, leaks between the monitoring points can be advantageously and unambiguously identified.

[0021] It is further proposed that, to verify the determination steps, a cross-comparison be carried out using the values ​​of the exhaust gas oxygen parameter determined in the first determination step and in the subsequent determination step. Preferably, the oxygen parameter is determined in at least two independent ways. For example, by means of two hardware sensors of the fuel cell system, by means of one hardware sensor and a calculation from other system parameters, or by two independent calculations from other system parameters. Preferably, the control unit evaluates at regular intervals, in particular at least once. R.414654

[0022] - 6 - per day, a reaction of the investigation step and the subsequent investigation step to a change in at least one system parameter in order to evaluate the functionality of the investigation steps. Preferably, the control unit controls the fuel supply and / or the air supply to perform the cross-comparison, for example, to change a volume flow rate of the fuel and / or the oxygen-containing fluid. Additionally or alternatively, the control unit controls the electrical unit to perform the cross-comparison, for example, to change an electrical current generated by the fuel cell unit. If the cross-comparison yields a negative result, the control unit preferably shuts down the fuel cell system or puts the fuel cell system into restricted safety operation until the next maintenance.The design according to the invention advantageously minimizes the risk of a false-negative detection of the fuel quantity.

[0023] It is further proposed that, for the purpose of performing the cross-comparison, at least one system parameter of the fuel cell system is actively changed only if no change in this system parameter has been detected within a predetermined period. System parameters can fluctuate due to external influences, such as ambient temperature, atmospheric pressure, or the like, particularly when using ambient air as the oxygen-containing fluid. Preferably, the control unit registers externally induced fluctuations in the system parameters in order to perform the cross-comparison. If no externally induced fluctuation in the system parameters occurs within a predetermined repetition rate of the cross-comparison, the control unit preferably actively changes at least one of the system parameters, as described by way of example in the last section.The design according to the invention allows the cross-comparison to be carried out advantageously in a resource-efficient manner. Furthermore, an additional deflection of the fuel cell system from an equilibrium position can be avoided, and an operating point of the fuel cell system can be advantageously kept stable.

[0024] It is further proposed that in at least one, in particular the first and / or at least one further, of the investigative steps of procedure R.414654

[0025] - 7 - the oxygen parameter is detected by means of a hardware sensor. The hardware sensor can be, for example, an optical oxygen sensor, an oxygen resistance probe, a lambda probe, or the like. For example, the first determination step is carried out using the hardware sensor. The at least one further determination step is preferably carried out with another hardware sensor located at a distance from the hardware sensor or by means of a calculation from other system parameters. The hardware sensor and the other hardware sensor can be identical in construction, have different designs, or be based on different measurement principles. In a particularly preferred embodiment, the hardware sensor and / or the other hardware sensor is designed as a broadband lambda probe. Preferably, the hardware sensor and / or the other hardware sensor is arranged in the exhaust pipe.For example, the additional hardware sensor is arranged downstream of the other hardware sensor with respect to the exhaust gas. The design according to the invention allows the oxygen parameter to be advantageously measured directly and cost-effectively. Furthermore, a safety standard, for example according to EN 14459, can be implemented advantageously and easily. In particular, the method can be implemented advantageously and easily with double-fault safety.

[0026] It is further proposed that in at least one, in particular the first and / or at least one further, step of the method for determining the oxygen parameter, the oxygen parameter is calculated from acquired control parameters of the fuel cell system. Control parameters are preferably system parameters of the fuel cell system and / or quantities derived therefrom, which are used by the control unit, particularly within the framework of the control procedure. The control parameters can be acquired by sensors of the fuel cell system and / or provided by local control elements of individual components of the fuel cell system.For example, the control or regulating unit uses as control or regulating parameters a flow parameter of the fuel in the fuel supply, a flow parameter of the oxygen-containing fluid in the oxygen supply, a quantity correlated with the composition of the fuel in the fuel supply, an electrical current provided by the fuel cell unit, an efficiency of the R.414654.

[0027] - 8 -

[0028] Fuel cell unit and / or the like for determining the oxygen parameter in the exhaust gas. In at least one embodiment of the method, the oxygen parameter is detected by the hardware sensor in the first determination step and calculated in the subsequent determination step, or vice versa. In at least one embodiment, the oxygen parameter is detected by the hardware sensors in both the first and subsequent determination steps and calculated in an additional determination step of the method. In at least one embodiment of the method, the oxygen parameter is calculated in both the first and subsequent determination steps. The inventive design allows the method to be optionally further validated by an additional determination step or to be implemented with advantageously few components.

[0029] It is further proposed that the fuel composition be determined to calculate the oxygen parameter. Preferably, the fuel cell system comprises at least one fluid analysis unit, in particular a gas analysis unit. The fluid analysis unit can be integrated into the fuel supply or positioned upstream of it. The fluid analysis unit includes, for example, sensors for measuring the fuel's speed of sound, its heat capacity, its viscosity, its infrared spectrum, or the like, and / or an input unit for inputting information provided by a fuel supplier. Preferably, the control unit determines the fuel composition based on measured values ​​from the fluid analysis unit.In particular, correlations of the measured values ​​with the stoichiometric coefficients for, for example, hydrogen, oxygen, carbon and / or nitrogen and / or with the electron transfer number are stored in a memory of the control unit in order to characterize the fuel composition. The design according to the invention allows the fuel composition to be determined advantageously accurately and the oxygen parameter to be determined advantageously reliably.

[0030] It is further proposed that, for the calculation of the oxygen parameter, a fuel composition can be implicitly determined by control or regulation parameter R.414654

[0031] - 9 - of the fuel cell system is taken into account. When controlling the fuel cell system, in particular according to the control method of DE 10 2021 212685 A1, the amount of fuel supplied is preferably adjusted by the control unit by closing an energy / heat balance so that the fuel cell system is supplied with the energy that is actually required. Preferably, the control unit increases the amount of fuel supplied if the fuel has a relatively low energy density. Preferably, the control unit decreases the amount of fuel supplied if the fuel has a relatively high energy density.The control unit preferably determines whether the fuel's energy density is high or low based on the energy / heat balance, particularly based on temperature measurements and without needing to explicitly determine the fuel's composition. Preferably, the control unit, especially during fault-free operation, maintains a system-wide fuel utilization rate of the fuel cell system at a setpoint within the control accuracy of the control method, regardless of the fuel's composition. This setpoint can be variable depending on the age or degradation level of the fuel cell unit. System-wide fuel utilization is the ratio of fuel converted in the fuel cell system to the total fuel supplied to the fuel cell system.Depending on the design of the fuel cell system, the system-wide fuel utilization can differ from or be identical to the local fuel utilization of the fuel cell unit. Preferably, the control unit uses the system-wide fuel utilization to calculate the oxygen parameter, in particular by expressing the fuel composition through the system-wide fuel utilization. The control unit can use the target fuel utilization value or an actual value of the system-wide fuel utilization to calculate the oxygen parameter. The control unit preferably calculates the actual value using the electron transfer number, for which it determines an approximate value, in particular from the energy / heat balance, without needing to explicitly know the fuel composition, as is done, for example, in DE 10 2021 212685 A1 R.414654.

[0032] - 10 - is known, or which they determine depending on a measured value of the fluid analysis unit. The inventive design allows the oxygen parameter to be advantageously determined even without explicit knowledge of the fuel composition. In particular, the costly fluid analysis unit can be dispensed with. By using the actual value of the system-wide fuel consumption, the oxygen parameter can be advantageously determined accurately, since fluctuations in the fuel composition are thereby taken into account.

[0033] In a specific embodiment of the method, a threshold value, in particular the aforementioned oxygen threshold value, of the oxygen parameter for emergency shutdown of the fuel cell system is adjusted depending on the fuel composition. The control unit preferably adjusts the oxygen threshold value based on the energy density and / or electron transfer number of the fuel, for which it determines an approximate value, particularly from the energy / heat balance, without needing to explicitly know the fuel composition, as is known, for example, from DE 10 2021 212685 A1, or which it determines based on a measured value from the fluid analysis unit. Preferably, the control unit sets the oxygen threshold value based on a monotonically, and in particular strictly monotonically, increasing relationship to the energy density and / or electron transfer number.The oxygen threshold can be defined, for example, as a continuous function or a monotonic step function of the energy density and / or the electron transfer number by the control unit, or it can be read from a suitably designed table stored in the control unit's memory. Due to the specific design in which the oxygen threshold depends on the fuel composition, the control unit can advantageously distinguish between a decrease in the oxygen parameter due to fluctuations in fuel quality and due to faults and / or damage in the fuel cell system. In particular, the risk of a false-positive detection of a fault and / or damage to the fuel cell system due to fuel quality fluctuations can be advantageously minimized. Alternatively, the control unit R.414654.

[0034] - 11 - the oxygen threshold is set independently of the fuel composition.

[0035] It is further proposed that, in at least one step of the process, a leakage of the exhaust gas heat exchanger of the fuel cell system is inferred from the determined values ​​of the oxygen parameter. The exhaust gas line preferably forms a primary side of the exhaust gas heat exchanger. The fuel supply preferably forms a secondary side of the exhaust gas heat exchanger. The exhaust gas heat exchanger is preferably designed to transfer heat from the exhaust gas to the fuel in the fuel supply. If the exhaust gas heat exchanger is damaged, fuel from the fuel supply can enter the exhaust gas line directly, i.e., bypassing the fuel cell unit, thereby increasing the amount of fuel in the exhaust gas line. When using at least two hardware sensors, preferably at least one is arranged upstream of the exhaust gas and at least one is arranged downstream of the exhaust gas heat exchanger on / in the exhaust gas line.The control unit preferably infers damage to the exhaust gas heat exchanger from a deviation in the oxygen parameters detected by the hardware sensors. When using a single hardware sensor, this sensor can be positioned upstream or downstream of the exhaust gas heat exchanger. The control unit preferably infers damage to the exhaust gas heat exchanger from a deviation of the oxygen parameter detected by the hardware sensor from a calculated value. If only calculated oxygen parameter values ​​are used, or as additional confirmation, the control unit preferably infers damage to the heat exchanger when an oxygen threshold value is undershot. The design according to the invention allows for the advantageously reliable and cost-effective detection of damage to the exhaust gas heat exchanger.

[0036] It is further proposed that in at least one step of the process, a leak in the oxygen parameter, particularly in the oxygen supply of the fuel cell system already mentioned, is inferred. In the event of a leak in the oxygen supply, the oxygen-containing R.414654 escapes.

[0037] - 12 -

[0038] Fluid from the fuel cell system. In the event of an oxygen supply leak, the fuel cell unit receives less oxygen than specified by the control unit. Preferably, the control unit infers an oxygen supply leak from a deviation of the calculated value of the oxygen parameter from a value detected by at least one hardware sensor. In particular, if the calculated value is greater than the detected value of the oxygen parameter, the control unit infers an oxygen supply leak. The design according to the invention provides advantageously robust detection of damage to the oxygen supply.

[0039] Furthermore, a fuel cell system is proposed comprising at least one fuel cell, in particular one already mentioned, for the electrochemical conversion of a fuel to an exhaust gas, with at least one exhaust gas line, in particular one already mentioned, for the discharge of the exhaust gas, and with at least one control unit, in particular one already mentioned, for carrying out a method according to the invention. Preferably, the fuel cell system includes at least one hardware sensor for detecting the oxygen parameter in the exhaust gas line and / or at least one calculation routine for determining the oxygen parameter in the exhaust gas line is stored in the memory of the control unit. The fuel cell system preferably comprises the fuel cell unit, the oxygen supply, the fuel supply, the oxygen exhaust gas discharge, and the fuel exhaust gas discharge.

[0040] The oxygen supply preferably comprises at least one supply line for guiding the oxygen-containing fluid from an external source to the fuel cell unit and at least one fluid control element arranged on the supply line for adjusting a flow parameter, in particular a volume flow rate, a mass flow rate, and / or a volumetric flow rate, of the oxygen-containing fluid through the fuel cell unit. The fluid control element can be designed as a fluid conveying device, in particular as a fan, blower, compressor, or pump, or as a control valve. Preferably, the oxygen supply includes at least one preheater for temperature-controlling the oxygen-containing fluid. The preheater can be, for example, an electric heater or R.414654.

[0041] - 13 - designed as the secondary side of another exhaust gas heat exchanger of the exhaust gas line.

[0042] The fuel supply preferably comprises at least one supply line for conveying the fuel from an external source to the fuel cell unit and at least one fluid control element arranged on the supply line for adjusting a flow parameter, in particular a volumetric flow rate, a mass flow rate, and / or a volumetric flow rate, of the oxygen-containing fluid through the fuel cell unit. The fluid control element can be designed as a fluid conveying device, in particular as a fan, blower, compressor, or pump, or as a control valve. Depending on the fuel used, the fuel supply may, for example, include a reformer for reforming the fuel. The fuel cell system can be configured with or without exhaust gas recirculation, which feeds the exhaust gases back into the fuel supply.In a configuration with exhaust gas recirculation, the exhaust gas recirculation and / or the fuel supply comprises a recirculation fluid conveyor, in particular a fan, blower, compressor, for recirculating the fuel-side exhaust gas into the fuel and preferably at least one recuperator for protecting the recirculation fluid conveyor from the temperature of the recirculated exhaust gas.

[0043] The fuel cell unit preferably comprises at least 100 fuel cells, and in particular at least several hundred fuel cells. The fuel cells are, for example, designed as solid oxide fuel cells, molten carbonate fuel cells, phosphoric acid fuel cells, alkaline fuel cells, proton exchange membrane fuel cells, or the like. The oxygen exhaust gas discharge and / or the fuel exhaust gas discharge preferably includes at least one afterburner for the thermal conversion of fuel residues exiting the fuel cell unit. Preferably, the common exhaust gas line is connected to an outlet of the afterburner. Preferably, the fuel cell system includes the exhaust gas heat exchanger, which is preferably arranged downstream of the afterburner on the exhaust gas line. Within the fuel supply, the exhaust gas heat exchanger is preferably located upstream of the reformer and downstream of the R.414654

[0044] - 14 -

[0045] Fluid actuating element, in particular downstream of an exhaust gas recirculation outlet.

[0046] The fuel cell system preferably comprises the electrical unit for handling the current supplied by the fuel cell unit. The control unit is preferably connected, via signaling, at least to the fluid actuator of the fuel supply, the fluid actuator of the oxygen supply, and the electrical unit. Particularly preferably, the fuel cell system includes a sensor unit, which is connected to the control unit via signaling and is designed for controlling the fuel cell system. In a particularly advantageous embodiment, the sensor unit comprises sensors that enable the control unit to perform the control procedure according to DE 10 2021 212685 A1.

[0047] The design according to the invention makes it possible to provide a fuel cell system that can be operated advantageously and safely.

[0048] It is further proposed that the fuel cell system comprises at least one, in particular the aforementioned, first hardware sensor, which is arranged at the first measurement point in the exhaust gas line for detecting the oxygen parameter, and at least one, in particular the aforementioned, further hardware sensor, which is arranged downstream of the first hardware sensor at the further measurement point in the exhaust gas line for detecting a further value of the oxygen parameter. The hardware sensors are preferably arranged on different sides of the exhaust gas heat exchanger in / on the exhaust gas line, particularly to be able to infer a leakage of the exhaust gas heat exchanger in the event of deviating measured values. The design according to the invention allows an advantageously high protection class according to EN 14459 to be achieved in an advantageously simple manner.

[0049] It is further proposed that the fuel cell system includes at least one sensor unit, in particular the one already mentioned, for recording control or regulation parameters of the fuel cell system, which together with the control or regulation unit includes at least one soft sensor for determining the R.414654

[0050] - 15 -

[0051] The oxygen parameter of the exhaust gas is measured by the soft sensor, which can evaluate the oxygen parameter at the first measurement point, at the second measurement point, or at an additional measurement point. Preferably, the sensor unit comprises at least one flow meter for measuring at least one flow parameter of the oxygen-containing fluid in the oxygen supply and / or for measuring at least one flow parameter of the fuel in the fuel supply. The flow meter can be integrated into the corresponding fluid control element, for example, as part of a local control loop for stabilizing an operating point of the fluid control element, or it can be arranged downstream of the fluid control element. Preferably, the sensor unit also includes at least one ammeter for measuring an electrical current supplied by the fuel cell unit.The design according to the invention allows the exhaust gas to be reliably monitored with advantageously few additional hardware sensors in the exhaust pipe. In particular, the exhaust gas can also be reliably monitored without hardware sensors in the exhaust pipe.

[0052] Furthermore, it is proposed that the fuel cell system comprises at least one, in particular exactly one, hardware sensor, especially the first hardware sensor or the further hardware sensor, in the exhaust gas line for detecting the oxygen parameter, and at least one, in particular the aforementioned, sensor unit for detecting control parameters of the fuel cell system, which together with the control unit forms at least one soft sensor for determining a further value of the oxygen parameter. Preferably, a measuring point of the hardware sensor and an evaluation point of the software sensor are spaced apart from each other, for example, on different sides of the exhaust gas heat exchanger with respect to the exhaust gas line. The design according to the invention enables advantageously reliable monitoring of the exhaust gas.In particular, a beneficially wide range of information can be provided at advantageously low costs, which makes it possible to identify any errors and / or damage that occur.

[0053] The inventive method and / or the inventive fuel cell system are not / should not be limited to the application described above and R.414654

[0054] - 16 -

[0055] The embodiment may be limited. In particular, the inventive method and / or the inventive fuel cell system may, to achieve a functionality described herein, have a different number of individual elements, components, units, and process steps than that specified herein. Furthermore, values ​​within the specified limits of the value ranges stated in this disclosure shall also be considered disclosed and freely usable.

[0056] Drawings

[0057] Further advantages become apparent from the following description of the drawings. The drawings illustrate three exemplary embodiments of the invention. The drawing, the description, and the claims contain numerous features in combination. A person skilled in the art will expediently consider the features individually and combine them into meaningful further combinations.

[0058] They show:

[0059] Fig. 1 shows a schematic representation of a fuel cell system according to the invention with two hardware sensors,

[0060] Fig. 2 shows a schematic flowchart of a method according to the invention, which can be carried out with the fuel cell system from Figure 1.

[0061] Fig. 3 shows a schematic representation of an alternative embodiment of the fuel cell system according to the invention with a hardware sensor and a soft sensor.

[0062] Fig. 4 shows a schematic flowchart of a method according to the invention, which can be carried out with the fuel cell system from Figure 3.

[0063] Fig. 5 shows a schematic representation of a fuel cell system according to the invention without a hardware sensor and

[0064] Fig. 6 shows a schematic flowchart of a method according to the invention, which can be carried out with the fuel cell system from Figure 5. R.414654

[0065] - 17 -

[0066] Description of the exemplary implementations

[0067] Figure 1 shows a fuel cell system 12a. The fuel cell system 12a comprises at least one fuel cell, in particular a solid oxide fuel cell, for the electrochemical conversion of a fuel with the addition of oxygen to produce an exhaust gas. The fuel cell comprises, in particular, a fuel electrode, especially an anode, and an oxygen electrode, in particular a cathode. Preferably, the fuel cell system 12a comprises a plurality of fuel cells combined to form a fuel cell unit. The fuel cell system 12a includes a conversion unit 34a.Depending on the fuel to be converted and / or the type of fuel cell, the conversion unit 34a may be identical to the fuel cell unit or may include the fuel cell unit as well as additional conversion components, such as a reformer for reforming the fuel before it is transferred to the fuel cell unit and / or an afterburner for thermal conversion of fuel residues exiting the fuel cell unit.

[0068] The fuel cell system 12a preferably comprises a fuel supply 42a for the supply of fuel, in particular natural gas or biogas, to the fuel cell unit. The fuel cell system 12a preferably comprises an oxygen supply 20a for the supply of an oxygen-containing fluid, in particular ambient air, to the fuel cell unit. The fuel cell system 12a comprises at least one exhaust gas line 22a for the discharge of exhaust gas from the fuel cell system 12a. The exhaust gas line 22a terminates, for example, in a chimney, for the discharge of exhaust gas to the environment. The exhaust gas line 22a is preferably designed as a common exhaust gas line 22a, which is particularly intended to discharge both fuel-side exhaust gas and oxygen-side exhaust gas from the fuel cell unit. The abbreviated term "exhaust gas" refers to the fluid located in the exhaust gas line 22a.The exhaust gas in the exhaust pipe 22a can be, in particular, the same as the fuel-side exhaust gas, the oxygen-side exhaust gas, a mixture of the fuel-side exhaust gas and the oxygen-side exhaust gas, or a mixture of R.414654.

[0069] - 18 - in particular by the afterburner, further processed product of the fuel-side exhaust gas and / or the oxygen-side exhaust gas.

[0070] The fuel supply 42a preferably comprises a fluid control element 38a, in particular a blower or a compressor, for adjusting the supply rate of fresh fuel to the fuel cell unit. The fuel cell system 12a preferably comprises at least one exhaust gas heat exchanger 18a, the primary side of which is arranged in the exhaust gas line 22a and the secondary side of which is arranged in the fuel supply 42a. The secondary side of the exhaust gas heat exchanger 18a is preferably arranged downstream of the fluid control element 38a and upstream of the conversion unit 34a with respect to the fuel. The fuel supply 42a can be designed as a single-pass system, i.e., without exhaust gas recirculation, or the fuel cell system 12a can have exhaust gas recirculation 44a. The exhaust gas recirculation 44a is preferably designed to feed the fuel-side exhaust gas of the fuel cell unit back into the fuel supply 42a.In an embodiment with exhaust gas recirculation 44a, the fuel supply 42a comprises a recirculation linkage unit 36a. The recirculation linkage unit 36a preferably includes an outlet of the exhaust gas recirculation 44a into a fuel line of the fuel supply 42a and at least one fluid pump, in particular a blower or a compressor, for adjusting a recirculation rate of the exhaust gas recirculation 44a. Preferably, the recirculation linkage unit 36a additionally includes a recuperator coupling the exhaust gas recirculation 44a and the fuel line to protect the fluid pump from excessively high temperatures of the recirculated fuel-side exhaust gas. The recirculation linkage unit 36a is preferably arranged upstream of the exhaust gas heat exchanger 18a and downstream of the fluid control element 38a of the fuel supply 42a with respect to the fuel.The oxygen supply 20a comprises at least one fluid control element 40a, in particular a blower or a compressor, for adjusting a supply rate of oxygen-containing fluid to the fuel cell unit.

[0071] The fuel cell system 12a comprises at least one control unit 24a. The control unit 24a is intended for regulating the fuel cell system 12a. The control unit 24a is, in particular, R.414654

[0072] - 19 - furthermore, for carrying out a method 10a, which is explained in more detail in Figure 2. The fuel cell system 12a comprises at least one sensor unit for detecting control parameters of the fuel cell system 12a. The sensor unit preferably comprises at least one flow meter for detecting a flow parameter, in particular a volume flow and / or mass flow, of the fuel set by the fluid control element 38a of the fuel supply 42a. The sensor unit preferably comprises at least one flow meter for detecting a flow parameter, in particular a volume flow and / or mass flow, of the oxygen-containing fluid set by the fluid control element 40a of the oxygen supply 20a. The sensor unit preferably comprises at least one ammeter for detecting an electric current provided by the fuel cell unit.The sensor unit preferably comprises several temperature sensors (not shown here), in particular for detecting the fuel flowing into the fuel cell system 12a, the oxygen-containing fluid flowing into the fuel cell system 12a, and the exhaust gas flowing out of the fuel cell system 12a.

[0073] The fuel cell system 12a comprises at least one first hardware sensor 26a, which is arranged in the exhaust gas line 22a for detecting an oxygen parameter of the exhaust gas. The fuel cell system 12a comprises at least one further hardware sensor 28a, which is arranged downstream of the first hardware sensor 26a in the exhaust gas line 22a for detecting another value of the oxygen parameter. The first hardware sensor 26a is preferably arranged upstream of the exhaust gas heat exchanger 18a with respect to the exhaust gas. The further hardware sensor 28a is preferably arranged downstream of the exhaust gas heat exchanger 18a. The first hardware sensor 26a and / or the further hardware sensor 28a are, for example, configured as lambda sensors, in particular as broadband lambda sensors, for detecting the oxygen parameter as the air-fuel ratio.Oxygen parameters that can be used include, in particular, the respective electrical pumping current of the broadband lambda sensors or the combustion air ratios determined from them. R.414654.

[0074] - 20 -

[0075] Figure 2 shows a flowchart of the process 10a for monitoring the fuel cell system 12a. During normal operation 48a of the process 10a, the fuel cell system 12a converts the fuel into the exhaust gas and discharges this from the fuel cell system 12a. During normal operation 48a, the control unit 24a regulates the fuel cell system 12a, preferably according to a control method such as that known, for example, from DE 10 2021 212685 A1.

[0076] To monitor the fuel cell system 12a, the control unit 24a characterizes the amount of fuel in the exhaust gas in at least one step of process 10a, in particular to assess whether the fuel cell system 12a is operating correctly and / or whether there is a risk of explosion. In the event of faulty operation and / or damage to the fuel cell system 12, for example, unreacted fuel may enter the exhaust gas line 22a and / or alter the ratio of reaction products to oxygen in the exhaust gas. To characterize the amount of fuel, the control unit 24a determines the oxygen parameter of the exhaust gas in at least a first determination step 14a of process 10a.

[0077] In the first determination step 14a, the first hardware sensor 26a detects the oxygen parameter. Preferably, the method 10a comprises a further determination step 16a in which the further hardware sensor 28a detects another value of the oxygen parameter. The method 10a preferably includes a comparison 46a in which the control unit 24a compares the oxygen parameter and the further value of the oxygen parameter. Preferably, during the comparison 46a, the control unit 24a performs filtering and / or debouncing of received sensor signals from the hardware sensors 26a and 28a. If the values ​​of the oxygen parameter deviate from each other by more than a predetermined tolerance, the control unit 24a preferably performs an emergency shutdown 32a of at least the fuel supply 42a, preferably of the entire fuel cell system 12a.In the course of the emergency shutdown 32a, the control or regulating unit 24a informs an operator of the fuel cell system 12a and / or a maintenance R.414654.

[0078] - 21 - preferably indicates that the fault consists of a leak in the exhaust gas heat exchanger 18a. If the oxygen parameter values ​​are within the specified tolerance, the control unit 24a preferably continues normal operation 48a. The control unit 24a preferably adjusts the specified tolerance depending on a predictive reliability or comparison reliability, particularly during an operating point change of the fuel cell system 12a and / or outside of normal operation 48a, especially during startup, shutdown, maintenance operation, purging, or the like of the fuel cell system 12a. The determination steps 14a, 16a, and the comparison 46a are preferably carried out continuously in parallel with normal operation 48a, particularly with a repetition rate of at least 15 mHz, preferably at least 1 Hz, and particularly at least 15 Hz.

[0079] Preferably, the control unit 24a extends the comparison 46a to a cross-comparison of the hardware sensors 26a and 28a at regular intervals, preferably at least once daily. Preferably, the control unit 24a checks in this cross-comparison whether the hardware sensors 26a and 28a react differently to a deviation of the fuel cell system 12a from the current operating point. If the hardware sensors 26a and 28a react differently, the control unit 24a executes the emergency shutdown 32a and preferably informs the operator and / or maintenance service that one of the hardware sensors 26a or 28a is not functioning correctly. If the hardware sensors 26a and 28a react identically, the control unit 24a continues normal operation 48a.In a deflection step 52a of the method 10a, the control unit 24a preferably actively adjusts at least one system parameter of the fuel cell system 12a in order to deflect the fuel cell system 12a from its current operating point. Preferably, in the deflection step 52a, the control unit 24a adjusts the fuel supply rate and / or the oxygen-containing fluid supply rate by means of the corresponding fluid control elements 38a, 40a. Alternatively or additionally, in the deflection step 52a, the control unit 24a changes the electric current flowing through the fuel cell unit. R.414654.

[0080] - 22 -

[0081] Method 10a preferably comprises a fluctuation recording 50a. To perform the cross-comparison, the control unit 24a executes the deflection step 52a only if no change in system parameters has been detected within a predetermined period. Preferably, the control unit 24a logs the system parameters of the fuel cell system 12a detected by the sensor unit and the oxygen parameter values ​​detected by the hardware sensors 26a and 28a. The predetermined period corresponds, in particular, to at least the maximum time interval between two cross-comparisons. Preferably, the control unit 24a evaluates a correlation between a fluctuation in the system parameters and a change in the oxygen parameters in order to perform the cross-comparison.If no sufficiently large fluctuation occurs in the recorded system parameters within the specified period to correlate with the recorded oxygen parameter values, the control unit 24a executes deflection step 52a. If the control unit 24a detects a correlation between a fluctuation in the system parameters and a change in the oxygen parameters, it evaluates this correlation as a cross-comparison and does not execute deflection step 52a. The control unit 24a can perform the cross-comparison after the maximum time interval between two cross-comparisons has elapsed and / or use a fluctuation in the recorded system parameters as a trigger to perform a cross-comparison.

[0082] Method 10a can include an additional determination step 30a. In the additional determination step 30a, an additional value of the oxygen parameter is calculated by the control unit 24a from the acquired control parameters of the fuel cell system 12a. For this purpose, the sensor unit, together with the control unit 24a, forms at least one soft sensor for determining the oxygen parameter of the exhaust gas. Preferably, the control unit 24a uses the additional value of the oxygen parameter in the comparison 46a or cross-comparison to determine a deviation of the calculated value from the acquired values ​​and, depending on this, to trigger the emergency shutdown 32a or to maintain normal operation 48a. If the calculated value is greater than the acquired values ​​of the oxygen parameter, the control unit 24a informs the operator R.414654.

[0083] - 23 - and / or the maintenance service preferably informs that the oxygen supply 20a has a leak. If one of the detected or calculated values ​​deviates from the others, the control unit 24a preferably informs the operator or maintenance service which value deviates. If only one of the detected or calculated values ​​deviates in at least three determination steps 14a, 16a, 30a, the control unit 24a preferably maintains normal operation 48a or restricted safety operation. Preferably, the control unit 24a filters and / or debounces sensor signals from the sensor unit, from which it calculates the oxygen parameter. The additional determination step 30a can be performed the same number of times, more often, or less often than determination step 14a and the further determination step 16a.For example, the additional investigation step 30a is executed each time one of the other investigation steps 14a or 16a is executed. For example, the additional investigation step 30a is only executed to confirm one of the other investigation steps 14a or 16a if they differ from each other.

[0084] Preferably, the control or regulating unit 24a calculates the oxygen parameter based on a model that assumes a general oxidation reaction, in particular a hydrocarbon of the form C x H y , molecular hydrogen, molecular nitrogen, molecular oxygen and carbon dioxide are involved. In particular, the control unit 24a calculates the oxygen parameter X according to

[0085] , _ C Q2 ' f _ 1 _

[0086] V bs ii K H Ko Kc + -4 — where c 02the concentration of molecular oxygen in the oxygen-containing fluid, where V sf is the volume flow rate of the oxygen-containing fluid, where V bs the volume flow rate of the fuel, where K c the stoichiometric carbon coefficient, K H the stoichiometric hydrogen coefficient and K o The stoichiometric oxygen coefficient of the fuel is [missing information]. The volume flows are preferably measured by the sensor unit or provided by the fluid control elements 38a, 40a. The oxygen concentration in the oxygen-containing fluid can be [missing information], for example when using ambient air as a constant c. 02 = 0.21 stored, or from a Sauer- R.414654

[0087] - 24 - The material sensor of the sensor unit can be detected. The stoichiometric coefficients can be determined by the control unit 24a, for example, using a fluid analysis unit of the fuel cell system 12a and based on stored correlations of measured values ​​from the fluid analysis unit with the stoichiometric coefficients, or can be obtained from a fuel supplier.

[0088] Preferably, the composition of the fuel is implicitly taken into account for the calculation of the oxygen parameter by means of control or regulation parameters of the fuel cell system 12a, in particular without explicitly recording or determining the exact composition. Preferably, in the control procedure of normal operation 48a, a system-wide fuel utilization FU is used. sysThe fuel cell system 12a uses the oxygen parameter X as a control variable, whereby fluctuations in fuel composition are compensated for via an energy / heat balance. Preferably, the control unit 24a calculates the oxygen parameter X according to:

[0089] , . 4 • FU sys • F

[0090] X - c 02 • n s f— —

[0091] 1 1 1 cell where h sf the mass flow of the oxygen-containing fluid, F the Faraday constant, / the electric current through the fuel cell unit and N ZelleThe number of fuel cells in the fuel cell unit is determined by the sensor unit. The mass flow is preferably measured by the sensor unit, in particular directly or in the form of an equivalent quantity, for example, the volume flow, or supplied by the fluid control element 40a of the oxygen supply 20a. The electrical current is preferably detected by the ammeter. In the simplest case, a setpoint for system-wide fuel consumption can be used for fuel utilization. Alternatively, an actual value for system-wide fuel consumption can be used. The control unit 24a determines the actual value for system-wide fuel consumption preferably according to:

[0092] 2 • V 02 _ FU sys = - — y V bs • K e _ where V 02 - the volume flow of molecular oxygen ions through the fuel cell unit and K eThe electron transfer number of the fuel is determined by the control unit 24a, preferably based on the electric current, to determine the volume flow rate of molecular oxygen ions through the fuel cell unit. An approximate value for the electron transfer number of R.414654

[0093] - 25 -

[0094] The control or regulating unit 24a preferably derives the fuel from the energy / heat balance, as described, for example, in DE 10 2021 212685 A1.

[0095] Figures 3 to 6 show further embodiments of the invention. The following descriptions and drawings are essentially limited to the differences between the embodiments, whereby with regard to identically designated components, in particular with regard to components with the same reference numerals, reference may also be made to the drawings and / or the description of the other embodiments, especially Figures 1 to 2. To distinguish the embodiments, the letter a is appended to the reference numerals of the embodiment in Figures 1 to 2. In the embodiments of Figures 3 to 6, the letter a is replaced by the letters b to c.

[0096] Figure 3 shows a fuel cell system 12b with at least one fuel cell for the electrochemical conversion of a fuel to an exhaust gas. The fuel cell system 12b includes at least one exhaust gas line 22b for discharging the exhaust gas. The fuel cell system 12b includes at least one control unit 24b for carrying out a process 10b, which is explained in more detail in Figure 4.

[0097] The fuel cell system 12b comprises at least one hardware sensor 26b for measuring the oxygen parameter in the exhaust gas line 22b. Exactly one hardware sensor 26b for measuring the oxygen parameter in the exhaust gas line 22b is sufficient to carry out the method 10b. In an advantageously component-free variant, the fuel cell system 12b comprises exactly one hardware sensor 26b. In an advantageously fail-safe variant, the fuel cell system 12b can have several redundant hardware sensors. The fuel cell system 12b comprises at least one sensor unit for measuring control parameters of the fuel cell system 12b, which, together with the control unit 24b, forms at least one soft sensor for determining a further value of the oxygen parameter. The hardware sensor 26b is shown here by way of example upstream of an exhaust gas heat transfer device.

[0098] - 26 - carriers 18b in the exhaust gas line 22b. An evaluation point of the soft sensor is preferably located at a detection point downstream of the exhaust gas heat exchanger 18b in the exhaust gas line 22b. Alternatively, the hardware sensor 26b is located downstream of the exhaust gas heat exchanger 18b and the evaluation point of the soft sensor is located upstream of the exhaust gas heat exchanger 18b.

[0099] Regarding further features of the fuel cell system 12b, reference is made to the illustrations in Figures 1 and 2.

[0100] Figure 4 shows a flowchart of process 10b. Process 10b is designed to monitor the fuel cell system 12b. The fuel cell system 12b converts at least one fuel into an exhaust gas, which is discharged from the fuel cell system 12b. The control unit 24b characterizes the amount of fuel in the exhaust gas in at least one process step. To characterize the amount of fuel, the control unit 24b determines an oxygen parameter of the exhaust gas in at least a first determination step 14b of process 10b.

[0101] In the first determination step 14b, the hardware sensor 26b records the oxygen parameter. The method 10b comprises at least one acquisition step 54b in which system parameters of the fuel cell system 12b are acquired by the sensor unit. The method 10b comprises at least one further determination step 16b in which the control unit 24b calculates the oxygen parameter, in particular using the method as explained for the additional determination step 30a from Figure 2. The method 10b comprises a comparison 46b in which preferably at least one calculated value and at least one measured value of the oxygen parameter are compared with each other in order to detect a faulty operation 48b and / or damage to the fuel cell system 12b.

[0102] For further features of method 10b, reference is made to the illustrations in Figures 1 and 2. R.414654

[0103] - 27 -

[0104] Figure 5 shows a fuel cell system 12c with at least one fuel cell for the electrochemical conversion of a fuel to an exhaust gas. The fuel cell system 12c includes at least one exhaust gas line 22c for discharging the exhaust gas. The fuel cell system 12c includes at least one control unit 24c for carrying out a method 10c, which is explained in more detail in Figure 6. The fuel cell system 12c does not include a hardware sensor for measuring the oxygen parameter in the exhaust gas line 22c. The fuel cell system 12c includes at least one sensor unit for measuring control parameters of the fuel cell system 12c, which, together with the control unit 24c, forms at least one soft sensor for determining a further value of the oxygen parameter.

[0105] Regarding further features of the fuel cell system 12c, reference is made to the illustrations in Figures 1 to 4.

[0106] Figure 6 shows a flowchart of process 10c. Process 10c is designed to monitor the fuel cell system 12c. The fuel cell system 12c converts at least one fuel into an exhaust gas, which is discharged from the fuel cell system 12c. The control unit 24c characterizes the amount of fuel in the exhaust gas in at least one process step. To characterize the amount of fuel, the control unit 24c determines an oxygen parameter of the exhaust gas in at least a first determination step 14c of process 10c. Preferably, process 10c includes at least one further determination step 16c for characterizing the amount of fuel. Preferably, the control unit 24c determines values ​​of the oxygen parameter independently of each other in determination steps 14c and 16c.Method 10c preferably comprises, for each determination step 14c, 16c, a separate acquisition step 54c, 56c for the independent acquisition of system parameters of the fuel cell system 2c by means of the sensor unit. Preferably, method 10c includes a comparison 46c in which the control unit 24c mutually verifies the calculated values ​​of the oxygen parameter for a cross-comparison. Preferably, in the comparison 46c, the control unit 24c compares the calculated values ​​with a threshold value. Preferably, the control unit 24c performs an emergency shutdown 32c, R.414654.

[0107] - 28 - if at least one of the calculated values ​​falls below the threshold. Preferably, the control or regulating unit 24c continues normal operation 48c of the procedure 10c as long as the calculated values, at least the majority, in particular all, calculated values ​​are above the threshold.

[0108] Preferably, the control unit 24c adjusts the threshold value of the oxygen parameter for emergency shutdown 32c of the fuel cell system 12c depending on the fuel composition. Preferably, the control unit 24c adjusts the threshold value depending on the electron transfer number or energy density of the fuel, which it derives, in particular, from an energy / heat balance of the fuel cell system 12c. Preferably, the control unit 24c sets a lower threshold value for the oxygen parameter the lower the energy density or electron transfer number.

[0109] In a particularly precise variant of the method 10c, several fuel parameters characterizing the fuel composition are processed by the control unit 24c, in addition to the energy density and / or electron transfer number. For example, a flow meter in the sensor unit could be based on ultrasonic measurement, so that in addition to recording a fuel flow parameter, the fuel's speed of sound is also determined. The speed of sound is particularly sensitive to the hydrogen content of the fuel. Furthermore, the fuel's heat capacity, density, and / or optical properties, or similar parameters, could be recorded. The control unit 24c compares the recorded fuel parameters, for example, with its own threshold value, to decide whether to continue normal operation 48c or to initiate an emergency shutdown 32c.Alternatively, a single threshold value is used, which is determined by the control or regulation unit 24c, for example in a Gaussian process, or by means of a polynomial model and / or by means of a neural network, depending on the detected fuel parameters.

[0110] Regarding further features of method 10c, reference is made to the illustrations in Figures 1 to 4.

Claims

R.414654 - 29 - Claims 1. Method (10a; 10b; 10c) for monitoring a fuel cell system (12a; 12b; 12c) which converts at least one fuel into an exhaust gas which is discharged from the fuel cell system (12a; 12b; 12c), wherein in at least one method step a quantity of fuel in the exhaust gas is characterized, characterized in that, for characterizing the quantity of fuel, in at least a first determination step (14a; 14b; 14c) an oxygen parameter of the exhaust gas is determined at a first determination point and in at least a further determination step (16a; 16b; 16c) a further value of the oxygen parameter is determined at a further determination point.

2. Method (10a; 10b; 10c) according to claim 1 , characterized in that a cross-comparison is carried out to verify the determination steps by means of the values ​​of the oxygen parameter of the exhaust gas determined in the first determination step (14a; 14b; 14c) and in the further determination step (16a; 16b; 16c).

3. Method (10a; 10b; 10c) according to claim 2, characterized in that, for the purpose of carrying out the cross-comparison, at least one system parameter of the fuel cell system (12a; 12b; 12c) is only actively changed if no change of this system parameter has been detected in a given period of time.

4. Method (10a; 10b; 10c) according to one of the preceding claims, characterized in that in at least one, in particular the first and / or the at least one further, of the determination steps (14a, 16a; 14b) the oxygen parameter is detected by means of a hardware sensor (26a, 28a; 26b, 28b; 26c, 28c). R.414654 - 30 - 5. Method (10a; 10b; 10c) according to one of the preceding claims, characterized in that in at least one, in particular the first and / or the at least one further, of the determination steps (30a; 16b; 14c, 16c) the oxygen parameter is calculated from recorded control or regulation parameters of the fuel cell system (12a; 12b; 12c).

6. Method (10a; 10b; 10c) according to claim 5, characterized in that a composition of the fuel is recorded for the calculation of the oxygen parameter.

7. Method (10a; 10b; 10c) according to claim 5 or 6, characterized in that, for the calculation of the oxygen parameter, a composition of the fuel is implicitly taken into account by control or regulation parameters of the fuel cell system (12a; 12b; 12c).

8. Method (10a; 10b; 10c) according to one of the preceding claims, characterized in that in at least one method step, a leakage of an exhaust gas heat exchanger (18a; 18b; 18c) of the fuel cell system is detected from the determined values ​​of the oxygen parameter. (12a; 12b; 12c) is closed.

9. Method (10a; 10b; 10c) according to one of the preceding claims, characterized in that in at least one method step, a leakage of an oxygen supply (20a; 20b; 20c) of the fuel cell system (12a; 12b; 12c) is inferred from the oxygen parameter.

10. Fuel cell system (12a; 12b; 12c) comprising at least one fuel cell for an electrochemical conversion of a fuel to an exhaust gas, comprising at least one exhaust gas line (22a; 22b; 22c) for a discharge of the exhaust gas, and comprising at least one control or regulating unit (24a; 24b; 24c) for a carrying out of a method (10a; 10b; 10c) according to one of the preceding claims. R.414654 - 31 - 11. Fuel cell system (12a) according to claim 10, comprising at least one first hardware sensor (26a) arranged to detect the oxygen parameter in the exhaust line (22a), and comprising at least one further hardware sensor (28a) arranged to detect a further value of the oxygen parameter downstream of the first hardware sensor (26a) in the exhaust line (22a).

12. Fuel cell system (12a; 12b; 12c) according to claim 10 or 11, comprising at least one sensor unit for detecting control or regulation parameters of the fuel cell system (12a; 12b; 12c), which together with the control or regulation unit (24a; 24b; 24c) forms at least one soft sensor for determining the oxygen parameter of the exhaust gas.

13. Fuel cell system (12a; 12b) according to claim 10, comprising at least one, in particular exactly one, hardware sensor (26a; 26b) for detecting the oxygen parameter in the exhaust gas line (22a; 22b) and comprising at least one sensor unit for detecting control or regulation parameters of the fuel cell system (12a; 12b), which together with the control or regulation unit (24a; 24b) forms at least one soft sensor for determining a further value of the oxygen parameter.

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