Diagnostic method for a solid oxide fuel cell system

Abstract

The invention relates to a diagnostic method for a solid oxide fuel cell system (1) which has a fuel cell stack (2) with a plurality of solid oxide fuel cells (3), comprising the following measures: a) generating an electrical output voltage (U) through the fuel cell stack (2) such that an electrical current (I) flows from the fuel cell stack (3) to an electrical load (4) electrically connected to the fuel cell stack (2), b) interrupting the electrical connection of the fuel cell stack (2) to the electrical load (4) at a determined switch-off time (t1) such that the electrical current (I) from the fuel cell stack (3) to the load (4) is stopped, c) evaluating the time curve ((U(t)) of the electrical output voltage (U) for a predetermined (first) time interval (ΔT) as of the switch-off time (t1).

Description

[0001] R. 413096

[0002] - 1 -

[0003] Diagnostic procedure for a solid oxide fuel cell system

[0004] The present invention relates to a diagnostic method for a solid oxide fuel cell system and a diagnostic arrangement with such a solid oxide fuel cell system.

[0005] Besides efficient power delivery, reliable, long-term stable operation is crucial for the successful commercialization of a solid oxide fuel cell (SOFC) system. The robustness of SOFC operation is primarily determined by the long-term stability of the SOFC fuel cell stack or the individual solid oxide fuel cells within the stack, as well as by the individual components of the fuel cell stack, such as the SOFC cells, electrical contact resistors, and the interconnect between the individual fuel cells.

[0006] However, in-situ monitoring of the individual fuel cell stack components within the system is not possible or only possible with great difficulty, since the SOFC fuel cell stack is implemented as a closed unit within the SOFC system.

[0007] Acting like a "black box," the SOFC fuel cell stack, during operation, delivers only an electrical output voltage generated at the fuel cell stack for a given electrical load current. This voltage depends on selected system variables such as gas supply and temperature control, as well as on the condition of the individual components of the fuel cell stack. R. 413096

[0008] - 2 -

[0009] However, in order to improve the quality and aging behavior of the SOFC-

[0010] To monitor the fuel cell stack throughout its entire lifespan and to be able to take early countermeasures if problems are diagnosed during this monitoring, continuous monitoring of the fuel cell stack or its components is desired with as little impact as possible on the ongoing operation of the fuel cell system.

[0011] It is therefore an object of the present invention to demonstrate new approaches in the development of diagnostic methods for solid oxide fuel cell systems. In particular, an improved method is to be created which enables simple monitoring of the fuel cell stack as well as the diagnosis of faults during the ongoing operation of the fuel cell system.

[0012] This problem is solved by the subject matter of the independent patent claims. Preferred embodiments are the subject matter of the dependent claims.

[0013] The basic idea of ​​the invention is therefore to evaluate the electrical output voltage generated by the fuel cell stack during operation from the point in time when the electrical current generated by the fuel cell stack and supplied to an electrical consumer is switched off.

[0014] This takes advantage of the fact that electrochemical reaction processes and transport processes in the individual fuel cells of the fuel cell stack have a significant influence on the time course of the decay of the electrical output voltage immediately after switching off the fuel cell system, especially immediately after an electrical separation of the electrical consumer from the fuel cell stack.

[0015] In particular, it can be exploited that different processes in fuel cells have different characteristic time constants, meaning they proceed at different speeds. R. 413096

[0016] - 3 -

[0017] The fastest processes in electrochemical converters—that is, those with the shortest time constant—are charge transport processes, followed by electrochemical reactions, and then by mass transport. In the electrical voltage signal, the fast processes follow the excitation directly, while the slow processes decay. In the frequency spectrum, this means that fast processes have a high frequency, and slow processes a low frequency.

[0018] By analyzing the temporal profile of the electrical voltage signal, conclusions can be drawn about the condition of the fuel cells in the fuel cell stack, and thus the condition of the fuel cell stack can be diagnosed. For this purpose, the measured temporal profile of the electrical output voltage can be compared with stored reference values.

[0019] A particular advantage of the proposed solution is that the diagnostic procedure can be performed during a mandatory shutdown of the fuel cell system, which occurs at a specific point in its operation. This ensures that the actual active operation of the fuel cell system is not disrupted.

[0020] To record the temporal profile of the electrical output voltage provided by the fuel cell stack when the fuel cell system is switched off, a simple electrical voltage sensor can be used. This sensor only needs to be designed to determine the temporal profile of the electrical output voltage with sufficiently high temporal resolution. The provision of special hardware for performing the diagnostic procedure is not required.

[0021] As a result, a simple diagnostic procedure is created that can diagnose the condition of the SOFC fuel cells in the fuel cell stack easily and with high accuracy. R. 413096

[0022] - 4 -

[0023] Following the above inventive concept, a diagnostic method according to the invention for a solid oxide fuel cell system, which has at least one, preferably at least two, particularly preferably several, fuel cell stacks with several solid oxide fuel cells, comprises at least three measures a) to c):

[0024] In a first measure a), an electrical output voltage is generated by the fuel cell stack, so that an electric current flows from the fuel cell stack to an electrical consumer electrically connected to the fuel cell stack.

[0025] In a second measure (b), the electrical connection between the fuel cell stack and the electrical load is interrupted at a specific time. This can be achieved using an electrical or electronic switch located in the electrical conductor connecting the fuel cell stack to the electrical load. The switch can be moved to an open state, in which it interrupts the electrical conductor and thus the electrical connection between the fuel cell stack and the electrical load. This stops the electrical current flowing from the fuel cell stack to the load.

[0026] In a third step (c), the temporal profile of the electrical output voltage is evaluated for a predetermined (first) time interval starting from the switch-off time. This evaluation can include a comparison with stored reference values. In this way, conclusions can be drawn about the state of the fuel cells in the fuel cell stack.

[0027] In a preferred embodiment of the method according to the invention, the termination of current generation, i.e., the stopping of the electric current flow in measure b), occurs instantaneously or abruptly. In this embodiment, the electric current flow thus decreases essentially stepwise, ideally with an infinitely negative slope, to a zero value at the time of switch-off. R. 413096

[0028] - 5 - in particular an electrical or electronic switch, especially a power transistor, provided in the electrical connection between the fuel cell stack and the electrical consumer, is opened, thereby interrupting the electrical connection between the fuel cell stack and the consumer.

[0029] According to an advantageous embodiment of the method according to the invention, the evaluation according to measure c) can include a frequency analysis of the temporal profile of the output voltage over the predetermined time interval and / or the calculation of a frequency spectrum. For this purpose, a transformation of the temporal profile of the output voltage into a frequency domain can be performed.

[0030] In this advanced training, a frequency spectrum of the electrical output voltage can be calculated from its temporal profile. Before calculating the frequency spectrum, it can be useful to filter the output voltage's temporal profile, i.e., the time signal, particularly using a high-pass, low-pass, or band-pass filter, and alternatively or additionally, to limit it to a specific time interval. This allows for a particularly precise spectral analysis of the output voltage's temporal profile after it is switched off. Consequently, the charge transport processes, electrochemical reaction and transport processes, and mass transport processes occurring within the fuel cell stack can be determined with high accuracy, thus enabling a highly precise assessment of the fuel cell stack's current operating and wear state.The frequency spectrum can be determined particularly usefully using a Fourier transform or a model-based fitting method.

[0031] In another preferred embodiment, in measure a) the electric current from the fuel cell stack is converted to the electrical R via a DC-AC converter. 413096

[0032] - 6 -

[0033] Consumers are involved. In this embodiment, step b) interrupts the electrical current flow from the DC-AC converter. Modern fuel cell systems often already have such a DC-AC converter installed as standard equipment to convert the DC output voltage generated by the fuel cell stack into a more suitable AC voltage for further use. Therefore, a separate DC-AC converter is not required.

[0034] According to an advantageous further development of the preferred embodiment of the method according to the invention described above, the DC-AC converter can comprise at least one electrical and / or electronic switch, in particular a power transistor. In measure a), this switch is in a closed state, so that the electric current flows from the fuel cell stack via the DC-AC converter to the electrical load. According to this further development, in measure b), the switch is then switched to an open state, in which it interrupts the electrical connection between the fuel cell stack and the load, so that the electric current flow from the fuel cell stack to the load stops. With the aid of such a switch, the electric current from the fuel cell stack to the electrical load can be abruptly terminated, so that the electric current waveform drops to zero in the form of a falling step.Such a stepwise decrease in electric current to zero is accompanied by a curve in the electrical output voltage that allows for particularly precise conclusions about the condition of the electrodes of the solid oxide fuel cells. Consequently, the evaluation carried out in measure c) can lead to particularly accurate results.

[0035] The temporal profile of the electrical output voltage can be measured particularly conveniently using an electrical voltage sensor, which can preferably be integrated into the DC-AC converter. This eliminates the need for a separate voltage sensor, resulting in cost advantages. Alternatively or additionally to this electrical voltage sensor R. 413096

[0036] - 7 - but an electrical voltage sensor designed separately for the DC-AC converter can also be provided.

[0037] The electrical voltage sensor can particularly preferably be configured for high-resolution temporal measurement of the electrical output voltage. If the frequency analysis of the temporal profile of the electrical output voltage is to be performed over a specific frequency range or bandwidth, the required temporal resolution and, consequently, the sampling frequency of the electrical voltage sensor can be determined using the Nyquist-Shannon theorem. Advantageously, the frequency range can be between 10 mHz and 1 MHz, and particularly preferably between 100 mHz and 100 kHz.

[0038] According to another advantageous embodiment, the generation process according to measure a) can include switching on the electric current at a specific switch-on time. In this embodiment, according to an additional measure d), the electrical output voltage is monitored for a predetermined time interval from the switch-on time. This embodiment also takes advantage of the fact that electrochemical reaction processes as well as charge and mass transport processes in the individual fuel cells of the fuel cell stack have a significant influence on the temporal profile of the electrical output voltage that builds up after switch-on or immediately after the electrical connection of the electrical load to the fuel cell stack.

[0039] The invention further relates to a diagnostic arrangement. The diagnostic arrangement according to the invention comprises a solid oxide fuel cell system for generating electrical energy, which includes at least one fuel cell stack comprising several solid oxide fuel cells for providing an electrical output voltage generated by the solid oxide fuel cells during operation. Preferably, the solid oxide fuel cell system comprises at least two, and more preferably several, such fuel cell stacks. Furthermore, the diagnostic arrangement according to the invention comprises R. 413096

[0040] - 8 - arrangement of an electrical load that can be electrically connected to or connected with the fuel cell stack and which, in a state electrically connected to the fuel cell stack, can be supplied with an electric current generated by the fuel cell stack. Furthermore, the diagnostic arrangement comprises a control device that is set up or programmed to carry out the method according to the invention described above. The advantages of the method according to the invention, explained above, are therefore transferred to the diagnostic arrangement according to the invention.

[0041] In a preferred embodiment of the diagnostic arrangement according to the invention, the electrical load is electrically connected to the fuel cell stack by means of a DC-AC converter. Such a DC-AC converter can be used, in particular, to determine the electrical output voltage if a corresponding electrical voltage sensor is installed in the DC-AC converter.

[0042] According to an advantageous embodiment of the diagnostic arrangement according to the invention, the DC-AC converter has an electrical input terminal electrically connected to the fuel cell stack and an electrical output terminal that can be connected to or is connected to the electrical load. The input terminal can be connected to the electrical output terminal via an electrical conductor path.In this further development, the DC-AC converter comprises at least one electrical and / or electronic switch, which is preferably arranged in the conductor path and which can be switched by means of the control device between a closed state, in which the electrical input terminal for supplying the electrical load with electrical current is connected to the electrical output terminal via the switch, and an open state, in which this connection is broken so that no electrical current can flow from the fuel cell stack to the electrical load. R. 413096.

[0043] - 9 -

[0044] Further important features and advantages of the invention will become apparent from the dependent claims, the drawings and the associated description of the figures based on the drawings.

[0045] It is understood that the features mentioned above and those to be explained below can be used not only in the combinations specified, but also in other combinations or on their own, without leaving the scope of the present invention.

[0046] Preferred embodiments of the invention are shown in the drawings and are explained in more detail in the following description, wherein identical reference numerals refer to identical or similar or functionally identical components.

[0047] They show, schematically:

[0048] Fig. 1 shows the structure of a diagnostic arrangement according to the invention,

[0049] Fig. 2 shows a flowchart illustrating the method according to the invention.

[0050] Fig. 3a shows the time course of the electric current from the fuel cell stack to the electrical consumer in the time range of the switch-off time,

[0051] Fig. 3b shows the time course of the electrical output voltage generated by the fuel cell stack in the time range of the switch-off time,

[0052] Fig. 4a shows an electrical frequency spectrum calculated from the time course of the electrical output voltage, R. 413096

[0053] - 10 -

[0054] Fig. 4b shows an impedance spectrum corresponding to the frequency spectrum of figure 4a,

[0055] Fig. 5a shows the time course of the electric current from the fuel cell stack to the electrical consumer in the time range of a switch-on time,

[0056] Fig. 5 shows the time course of the electrical output voltage generated by the fuel cell stack in the time range of the switch-on time.

[0057] Figure 1 shows a rough schematic representation of the structure of a diagnostic arrangement 10 according to the invention. The diagnostic arrangement 10 comprises a solid oxide fuel cell system 1 for generating electrical energy, which in turn has a fuel cell stack 2 with several solid oxide fuel cells 3 for providing an electrical output voltage U generated by the solid oxide fuel cells 3 during operation.

[0058] Furthermore, the diagnostic arrangement 1 includes an electrical consumer 4 connected to the fuel cell stack 2, which in a state electrically connected to the fuel cell stack 3 is supplied with an electric current I generated by the fuel cell stack 2 and thus with electrical energy.

[0059] The electrical consumer 4 is electrically connected to the fuel cell stack 3 by means of a DC-AC converter 12, which can be part of the solid oxide fuel cell system 1.

[0060] In the example scenario, the DC-AC converter 12 has an electrical input terminal 13 electrically connected to the fuel cell stack 3 and an R. 413096

[0061] - 11 - electrically connected to the electrical load 4 electrical output terminal 14. The DC-AC converter 12 can convert an electrical DC voltage supplied at the electrical input terminal 13 into an electrical AC voltage supplied at the electrical output terminal 1.

[0062] Furthermore, the DC-AC converter 12 includes an electronic switch 15, which is switchable between a closed state, in which the electrical input terminal 13 for supplying the electrical consumer 4 with the electrical current I is connected to the electrical output terminal 1 via the switch 15, and an open state, in which this connection is removed, so that no electrical current I can flow from the fuel cell stack 2 to the electrical consumer 4.

[0063] Furthermore, the diagnostic arrangement 10 comprises a control / regulation device 11, which is set up and programmed to carry out the method according to the invention. For this purpose, the electronic switch 15 can be controlled by means of the control / regulation device 11 and can be switched between the closed and the open state by the control / regulation device 11.

[0064] The inventive method is explained below by way of example using the flowchart of Figure 2.

[0065] According to a first measure a), an electrical output voltage U is generated by the fuel cell stack 2, so that an electric current I flows from the fuel cell stack 2 to an electrical consumer 4 electrically connected to the fuel cell stack 2.

[0066] During the execution of measure a), the electronic switch 15 of the DC-AC converter 12 is in the closed state (not shown), so that R. 413096

[0067] - 12 - the fuel cell stack 2 is electrically connected to the electrical consumer 4 and consequently the electric current I can flow from the fuel cell stack 2 to the electrical consumer 4.

[0068] According to a second measure b), the electrical connection between the fuel cell stack 2 and the electrical load 4 is interrupted at a switch-off time t1. For this purpose, the electronic switch 15 can be switched from the closed to the open state by the control unit 11 (see Figure 1). This stops the electrical current I from the fuel cell stack 2 to the electrical load 4. In this case, the electrical current flow is therefore stopped by means of the DC-AC converter 12.

[0069] To illustrate this, Figure 3a shows the time course l(t) of the electric current I from the fuel cell stack 2 to the electrical load 4. Accordingly, the cessation of power generation, i.e., the termination of the electric current flow in measure b), occurs instantaneously, i.e., abruptly. Looking at the time course l(t) of the electric current I, it can be seen that at the switch-off time t1, the electric current I decreases stepwise from a nominal value Io assumed before the switch-off time t1 (i.e., t < t1) to a zero value and remains at zero, i.e., I(t) = 0 for t > t1.

[0070] In a third step c), the time course U(t) of the electrical output voltage U is determined and evaluated for a predetermined time interval AT from the switch-off time t1.

[0071] For clarification, Figure 3b, corresponding to Figure 3a, shows the time course U(t) of the electrical output voltage U from the fuel cell stack 1 to the electrical load 4. R. 413096

[0072] - 13 -

[0073] As can be seen in Figure 3b, the electrical output voltage U can rise to an increased value Ui, the so-called open-circuit voltage, compared to a nominal value Uo assumed before the switch-off time t1. The course of this increase in U(t) is characteristic of the instantaneous electrochemical state of the solid oxide fuel cells 3 of the fuel cell stack 2.

[0074] In the example scenario, the evaluation according to measure c) includes a frequency analysis of the time course U(t) of the electrical output voltage U(t) over the predetermined time interval AT. For this purpose, a frequency spectrum U(f), exemplified in Figure 4a, can be calculated from the time course U(t) of the electrical output voltage using a Fourier transform. From this spectrum, the frequency-dependent impedance Z(f) of the fuel cell stack 2 can then be determined.

[0075] Figure 4b shows an impedance spectrum corresponding to the frequency spectrum of Figure 4a, which is also known to those skilled in the art as the “Nyquist diagram”.

[0076] As can be seen from the representation of figures 4a and 4b, different spectral ranges S1, S2, S3 can be characteristic of different physical and electrochemical processes at the electrodes of the solid oxide fuel cells 3.

[0077] The spectral range labeled S1 in Figures 4a and 4b is, for example, an ohmic range characterized by charge transport processes; range S2 is influenced by electrochemical processes at the electrodes; and range S3 by mass transport at the electrodes. The impedance Z(f) in the respective spectral ranges S1, S2, and S3 thus allows conclusions to be drawn about the state of the solid oxide fuel cells 3. For this purpose, the determined frequency spectrum, including ranges S1, S2, and / or S3, can be compared with reference values ​​(not shown) stored in the control unit 11. R. 413096

[0078] - 14 -

[0079] The impedance spectrum shown in Figure 4b represents the negative imaginary part of the impedance Z (-lm(Z) or -Z") over the real part (Re(Z) or Z'). Along the curve K shown in Figure 4b lie all the measured frequency points; that is, each excited and measured frequency f results in an impedance point IP(Z', Z'), and the entirety of all impedance points IP yields the characteristic curve K.

[0080] If excitation is applied at a very high frequency, for example several hundred kHz, the excitation occurs so quickly that only charge transport S1 can actually be stimulated; all slower processes cannot follow the excitation. Therefore, only the S1 region is measured. If excitation is applied at a significantly lower frequency, for example 0.1 Hz, all the processes mentioned above, S1, S2, and S3, can respond and can be measured accordingly.

[0081] The time course U(t) of the electrical output voltage U can be measured and determined using an electrical voltage sensor 16 (see Figure 1), which in the example shown is part of the DC-AC converter 12. The voltage sensor 16 can be connected to the control unit 11 for data transmission and thus transmit data representing the measured time course U(t) of the electrical output voltage U(t) to the control unit 11, which can then process and evaluate this sensor data. Alternatively, the further processing and evaluation can also take place in an external data cloud. To calculate the frequency spectrum U(f) from the time course of the output voltage U(t), the electrical voltage sensor 16 must be designed for high-resolution temporal measurement of the electrical output voltage U. If the frequency spectrum U(f) is to be calculated over a specific frequency range, orIf the measurement is to be performed over a specific bandwidth B, the temporal resolution and, consequently, a sampling frequency f of the voltage sensor 16 can be determined by the Nyquist-Shannon theorem, which is well known to those skilled in the art. R. 413096.

[0082] - 15 -

[0083] Figures 5a and 5b show a representation corresponding to Figures 3a and 3b, respectively, of a variant of the method according to the invention. Accordingly, generating the electric current I according to measure a) also includes switching on the electric current l(t) at a specific switching-on time tO. For this purpose, the electrical switch 15 can be switched from the open to the closed state by the control device 11, so that the previously non-existent electrical connection between the fuel cell stack 2 and the electrical load 4 is established.

[0084] In this variant, an additional step d) (not shown in the flowchart of Figure 2 for clarity) evaluates the electrical output voltage for a predetermined time interval AT from the switch-on time to, analogous to step c). Step d) differs from step c) only in that the evaluation takes place from the switch-on time to, rather than from the switch-off time ti. Otherwise, the explanations above regarding step c) also apply, mutatis mutandis, to step d).

[0085] R. 413096

[0086] - 16 -

[0087] Reference symbol list

[0088] 1 Fuel cell system

[0089] 2 fuel cell stacks

[0090] 3 solid oxide db ren nstoff ze 11 en

[0091] 4 electrical consumers

[0092] 10 Diagnostic Order

[0093] 11 Control / regulating device

[0094] 12 DC-AC converters

[0095] 13 electrical input connection

[0096] 14 electrical output connection

[0097] 15 electronic switches

[0098] 16 electrical voltage sensor

[0099] Kt) electric current

[0100] U(t) electrical output voltage

[0101] AT predetermined time interval t1 switch-off time to switch-on time

[0102] U(f) frequency spectrum R. 413096

[0103] - 17 -

[0104] Z impedance

[0105] Z(f) frequency-dependent impedance

[0106] IP impedance points

[0107] K curve f sampling sampling frequency

[0108] 51 Spectral range

[0109] 52 Spectral range

[0110] 53 Spectral range

[0111] *****

Claims

R. 413096 - 18 - Patent claims 1. Diagnostic method for a solid oxide fuel cell system (1) comprising at least one, preferably at least two, particularly preferably several, fuel cell stacks (2) with several solid oxide fuel cells (3), comprising the following measures: a) Generating an electrical output voltage (U) through the fuel cell stack (2) such that an electric current is drawn from the fuel cell stack (2). a) (I(t)) to an electrical consumer (4) electrically connected to the fuel cell stack (2), b) interrupting the electrical connection of the fuel cell stack (2) with the electrical consumer (4) at a specific switch-off time (t1) so that the electric current (I) from the fuel cell stack (3) to the electrical consumer (4) is stopped, c) evaluating the time course (U(t)) of the electrical output voltage (U) for a predetermined (first) time interval (AT) from the switch-off time (t1).

2. Method according to claim 1, characterized in that the cessation of current generation in measure b) occurs instantaneously, so that the electric current (I) preferably decreases to a zero value in a substantially stepwise manner.

3. Method according to claim 1 or 2, characterized in that R. 413096 - 19 - the evaluation according to measure c) includes a frequency analysis of a time course (U(t)) of the electrical output voltage (U) over the predetermined time interval (AT) as well as the calculation of a frequency spectrum (U(f)) of the time course (U(t)).

4. Method according to one of claims 1 to 3, characterized in that the calculation of the frequency spectrum (U(f)) is carried out by a Fourier transform.

5. Method according to one of the preceding claims, characterized in that in the course of measure c), in particular from the frequency spectrum (U(f)), a frequency-dependent impedance (Z(f)) of the fuel cell stack (2) is determined.

6. Method according to one of the preceding claims, characterized in that in measure a) the electric current (I) is guided from the fuel cell stack (2) via a DC-AC converter (12) to the electrical consumer (4), in measure b) the electric current flow (I (t)) is interrupted by means of the DC-AC converter (12).

7. Method according to claim 6, characterized in that the DC-AC converter (12) comprises at least one electrical and / or electronic switch (15) which in measure a) is in a closed state, so that the electric current (I) flows from the fuel cell stack (2) via the DC-AC converter (12) to the electrical load (4), and the switch (15) is switched to an open state in measure b). R. 413096 - 20 - so that the switch (15) interrupts the electrical connection between the fuel cell stack (3) and the electrical consumer (4), thereby stopping the flow of electric current (I) from the fuel cell stack (2) to the electrical consumer (4).

8. Method according to one of the preceding claims, characterized in that the time course of the electrical output voltage (U(t)) is detected by means of an electrical voltage sensor (16) provided in the DC-AC converter (12).

9. Method according to one of the preceding claims, characterized in that the generation according to measure a) comprises switching on the electric current (I) at a specific switch-on time (tO), and in an additional measure d) the course of the electrical output voltage (U(t)) is evaluated for a predetermined (second) time interval (AT) from the switch-on time (tO).

10. Diagnostic arrangement (10), comprising a solid oxide fuel cell system (1) for generating electrical energy, comprising at least one, preferably at least two, fuel cell stack (2) each comprising several solid oxide fuel cells (2) for providing an electrical output voltage (U) generated by the solid oxide fuel cells (3) during operation, with an electrical load (4) that can be electrically connected to or is connected to the fuel cell stack (2) and which, in a state electrically connected to the fuel cell stack (2), can be supplied with an electric current (I) generated by the fuel cell stack (2), with a control / regulation device (11) for carrying out the R. 413096 - 21 - The method is set up / programmed according to one of the preceding claims.

11. Diagnostic arrangement according to claim 10, characterized in that the electrical consumer (4) is electrically connected to the fuel cell stack (2) by means of a DC-AC converter (12).

12. Diagnostic arrangement according to claim 11, characterized in that the DC-AC converter (12) has an electrical input terminal (13) electrically connected to the fuel cell stack (2) and an electrical output terminal (1) that can be connected to or is connected to the electrical load (5), the DC-AC converter (12) comprising at least one electrical and / or electronic switch (15) which can be switched by means of the control / regulation device (11) between a closed state in which the electrical input terminal (13) for supplying the electrical load (4) with the electric current (I) is connected to the electrical output terminal (1) via the switch (15), and an open state in which this connection is removed, so that no electric current (I) can flow from the fuel cell stack (2) to the electrical load (4). *****

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