Method for Diagnosing at Least One Fuel Cell Stack of a Fuel Cell Device, Computer-Readable Storage Medium, and Fuel Cell Diagnostic System

Abstract

A method for diagnosing at least one fuel cell stack of a fuel cell device by way of a fuel cell diagnostic system includes: impressing a sinusoidal first and at least one sinusoidal second AC current into the fuel cell stack; recording a sinusoidal first and second voltage response of the fuel cell stack; evaluating the first voltage response and evaluating the second voltage response by way of an analytical algorithm for a differential impedance analysis; determining a first resistance, a second resistance and a capacitance of the fuel cell stack by specifying an equivalent circuit diagram for the fuel cell stack; and diagnosing the fuel cell stack on the basis of the determined first resistance, the determined second resistance and the determined capacitance, wherein the diagnosis is carried out in real time. A computer-readable storage medium and a fuel cell diagnostic system are also described.

Claims

1-10. (canceled)

11. A method for diagnosing at least one fuel cell stack of a fuel cell device via a fuel cell diagnostic system, the method comprising: applying a sinusoidal first alternating current at a first frequency and applying at least one sinusoidal second alternating current at a second frequency different from the first frequency to the fuel cell stack; detecting a sinusoidal first voltage response of the fuel cell stack at the first frequency and detecting a sinusoidal second voltage response of the fuel cell stack at the second frequency; evaluating a first AC voltage as a function of the first voltage response and evaluating a second AC voltage as a function of the second voltage response via an analytical algorithm for a differential impedance analysis of the fuel cell stack by way of an electronic computing unit of the fuel cell diagnostic system, wherein the detected sinusoidal voltage response of the fuel cell stack at various excitation frequencies is used to determine a capacitance; determining a first resistance, a second resistance, and the capacitance of the fuel cell stack as a function of the evaluation by specifying an equivalent circuit diagram for the fuel cell stack, wherein the sinusoidal voltage response of the fuel cell stack at the various excitation frequencies is used to determine the second resistance; and diagnosing the fuel cell stack as a function of the determined first resistance, the determined second resistance, and the determined capacitance, wherein the diagnosis is carried out in real time, wherein the diagnosis is carried out during driving operation of a motor vehicle.

12. The method according to claim 11, wherein a double-layer capacitance of the fuel cell stack is determined as the capacitance.

13. The method according to claim 11, wherein a fault is determined as a diagnosis by means of the electronic computing unit by evaluating the determined capacitance as a function of a present cell voltage of the fuel cell stack.

14. The method according to claim 11, wherein a cause of fault is determined as a diagnosis by means of the electronic computing unit by evaluating the determined capacitance as a function of the determined second resistance.

15. The method according to claim 11, wherein, in the equivalent circuit diagram, the second resistance is specified as connected in parallel to the capacitance and the first resistance is specified as connected in series thereto.

16. The method according to claim 11, wherein, in the equivalent circuit diagram, the first resistance is specified as the ohmic resistance of the fuel cell stack, the second resistance as the electrochemical resistance of the fuel cell stack, and the capacitance as the apparent capacitance of the fuel cell stack.

17. The method according to claim 11, wherein the AC voltages are applied at a frequency between 100 mHz and 1 kHz.

18. The method according to claim 16, wherein the frequency is between 100 Hz and 300 Hz.

19. The method according to claim 11, wherein an undersupply of hydrogen in the fuel cell stack is diagnosed upon a determination of a drop of the capacitance and a uniform second resistance.

20. The method according to claim 19, wherein a present moisture value of a polymer membrane of a fuel cell of the fuel cell stack is taken into consideration in the determination of the drop of the capacitance.

21. The method according to claim 11, wherein an undersupply of oxygen in the fuel cell stack is diagnosed upon a determination of a constant capacitance and a rising second resistance.

22. A computer-readable storage medium, on which program instructions are stored, which, upon execution by a microprocessor, cause it to carry out the method according to claim 11.

23. A fuel cell diagnostic system having an electronic computing unit and having a computer-readable storage medium, wherein the fuel cell diagnostic system is designed to carry out the method according to claim 11.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The technology disclosed here will be explained on the basis of the figures. In the figures:

[0027] FIG. 1 shows a schematic illustration of a motor vehicle having a fuel cell diagnostic system; and

[0028] FIG. 2 shows a schematic illustration of a flow chart of an embodiment of the method.

DETAILED DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 shows a schematic side view of a motor vehicle 10 having a fuel cell diagnostic system 12. The fuel cell diagnostic system 12 has at least one electronic computing unit 14 and a computer-readable storage medium 16. The motor vehicle 10 furthermore has a fuel cell device 18. The fuel cell device 18 in particular has a plurality of fuel cell stacks 20, in the present case, three fuel cell stacks 20. A respective fuel cell stack 20 can in turn have a plurality of fuel cells.

[0030] The technology presented in this disclosure relates to a method for diagnosing at least one of the fuel cell stacks 20 of the fuel cell device 18 by way of the fuel cell diagnostic system 12. A sinusoidal first AC voltage is applied at a first frequency and at least one sinusoidal second AC voltage is applied at a second frequency different from the first frequency to the fuel cell stack 20. A sinusoidal first voltage response of the fuel cell stack 20 at the first frequency and a sinusoidal second voltage response of the fuel cell stack 20 at the second frequency are detected. The first AC voltage is evaluated as a function of the first voltage response and the second AC voltage is evaluated as a function of the second voltage response by way of an analytical algorithm 22 for a differential impedance analysis of the fuel cell stack 20 by way of the electronic computing unit 14 of the fuel cell diagnostic system 12. A first resistance 24, a second resistance 26, and a capacitance 28 of the fuel cell stack 20 are determined as a function of the evaluation by specifying an equivalent circuit diagram 30 for the fuel cell stack 20. The fuel cell stack 20 is diagnosed as a function of the determined first resistance 24, the determined second resistance 26, and the determined capacitance 28.

[0031] In particular, occurring faults can result in impairment of the system performance, service life, and the fuel consumption/system efficiency. These faults can be partially intercepted via the operating strategy for the fuel cell device 18, but, for this purpose, it may be necessary to transfer data in real time which contain items of information about malfunctions of individual fuel cells. This is possible by way of the presented method.

[0032] In particular, FIG. 2 shows that, in the equivalent circuit diagram 30, the second resistance 26 is specified as connected in parallel to the capacitance 28 and the first resistance 24 is specified as connected in series thereto. Furthermore, FIG. 1 shows that the first resistance 24 is in particular specified as the ohmic resistance of the fuel cell stack 20, the second resistance 26 as the electrochemical resistance of the fuel cell stack 20, and the capacitance 28 as the apparent capacitance of the fuel cell stack 20. The capacitance 28 is in particular a double-layer capacitance of the fuel cell stack.

[0033] In particular, the method presented for the fuel cell stack 20 can also be applied in the case of a single fuel cell.

[0034] In the following description of the alternative exemplary embodiment illustrated in FIG. 2, identical reference signs are used for features which are identical and/or at least comparable in comparison to the first exemplary embodiment shown in FIG. 1 in their design and/or mode of action. If these are not explained in detail once again, their design and/or mode of action corresponds to the design and/or mode of action of the above-described features.

[0035] FIG. 2 shows a schematic view of a flow chart for carrying out the method. In particular, FIG. 2 shows that in a first step S1, a DC-DC converter installed in the fuel cell diagnostic system 12, which can also be referred to, for example, as a DC/DC converter, applies the sinusoidal AC voltage to the fuel cell stack 20. In a second step S2, the frequencies of the sinusoidal AC voltage are varied. In particular, for example, the applied AC voltages can be applied essentially between 100 mHz and 1 kHz, preferably between 100 Hz and 300 Hz. In other words, the first frequency can be applied between 100 mHz and 1 kHz and the second frequency can be applied between 100 mHz and 1 kHz. In particular, for example, it can be provided that at least five different frequencies are applied as the AC voltage to the fuel cell stack 20. In a third step S3, the detected sinusoidal voltage response of the fuel cell stack 20 at the various excitation frequencies is used to determine the capacitance 28, in particular the double-layer capacitance of the fuel cell stack 20.

[0036] In a fourth step S4, the sinusoidal voltage response of the fuel cell stack 20 at the various excitation frequencies is used to determine the second resistance 26.

[0037] In a fifth step S5, the dependency of the capacitance 28 is determined as a function of a respective cell voltage of the fuel cell stack 20, wherein this dependence is an unambiguous criterion that a fault exists in the fuel cell stack 20. In other words, it is provided that, by evaluating the determined capacitance 28, in particular the determined double-layer capacitance, as a function of the present cell voltage of the fuel cell stack 20, a fault is determined as a diagnosis by way of the electronic computing unit 14.

[0038] Furthermore, in a sixth step S6, the dependency of the second resistance 26 to the capacitance 28 is determined, wherein a cause of fault can thus be determined. In other words, it can be provided that by evaluating the determined capacitance 28 and as a function of the determined second resistance 26, a cause of fault is determined as the diagnosis by way of the electronic computing unit 14.

[0039] In a seventh step S7, the faults in the fuel cell stack 20 are diagnosed and identified by the evaluation of the second resistance 26 and the capacitance 28. In particular, it can be provided for this purpose that the diagnosis is carried out in real time. Furthermore, it can be provided that the diagnosis is carried out during driving operation of the motor vehicle 10.

[0040] For example, it can be provided that an undersupply of hydrogen in the fuel cell stack 20 is diagnosed upon a determination of a drop of the capacitance 28 and a uniform second resistance 26 or an undersupply of oxygen in the fuel cell stack 20 is diagnosed upon a determination of a constant capacitance 28 and a rising second resistance 26.

[0041] For example, it can then be provided that, upon a diagnosed undersupply of hydrogen, hydrogen can accordingly be resupplied by way of a regulation. Alternatively, upon an undersupply of oxygen, a regulation can be carried out so that oxygen can be resupplied. It is thus made possible, for example, that the service life of the fuel cell device 18 can be lengthened. Furthermore, the performance of the fuel cell device 18 can be improved and the fuel consumption can be reduced.

[0042] In particular, the invention may use the fact that, in contrast to the prior art, an analytical evaluation of the parameters of the equivalent circuit diagram 30 is carried out. In particular, computing effort can be saved and the analysis can be carried out in reduced time by the analytical evaluation. In particular, the diagnosis of the fuel cell stack 20 can thus be carried out in real time by a predefined equation, which in particular corresponds to the algorithm. Therefore, parameter fitting does not take place, as is the case in the prior art. In particular, the invention furthermore may use the finding here that the capacitance 28 represents a double-layer capacitance, due to which the diagnosis can be carried out reliably. An operating condition determination of the fuel cell device 18 can thus be carried out in real time.

[0043] Furthermore, the invention may use the fact in particular that the electrical capacitance 28 of the fuel cell stack 20 is decisively dependent in normal operation of the fuel cell system on the voltage of the fuel cell stack 20. If the ratio of voltage to capacitance 28 is less than a certain limit, there is a malfunction of the fuel cell stack 20. This criterion can be detected and tracked synchronously for all cells of the fuel cell stack. The electrochemical resistances, as the second resistance 26 corresponds to in particular, and also the capacitance 28 behave differently depending on the fault of the fuel cell stack 20, for example, upon an undersupply of gases, excessively high/low water content of the fuel cells. The identification of the fault can thus be carried out by way of the combination of the second resistance 26 and the capacitance 28.

[0044] The term “essentially” in the context of the technology disclosed here comprises in each case the precise property or the precise value and also respective deviations unimportant for the function of the property/the value.

[0045] The preceding description of the present invention only serves for illustrative purposes and not for the purpose of restricting the invention. Various changes and modifications are possible in the context of the invention without leaving the scope of the invention and its equivalents.