Diagnostic method for diagnosing a state of a fuel-cell system, fuel cell and computer-program product

Abstract

The present invention relates to a diagnostic method (100) for diagnosing a state of a fuel-cell system (300), the diagnostic method (100) comprising: - closing (101) cathode shut-off valves of a cathode subsystem (303) of the fuel-cell system (300), - applying (103) an electric current from a constant-current source to a fuel-cell stack (301) of the fuel-cell system (300), - measuring (105) a pressure curve of a pressure applied in the cathode subsystem (303) within a predetermined time window, beginning with when the electric current from the constant-current source is applied to the fuel-cell stack (301), - determining (107), on the basis of the measured pressure curve, a characteristic value which quantifies a state of the fuel-cell system (300), - outputting (109) the characteristic value.

Description

[0001] Description

[0002] title

[0003] DIAGNOSTIC METHOD FOR DIAGNOSIS OF A CONDITION OF A FUEL CELL SYSTEM, FUEL CELL AND COMPUTER PROGRAM PRODUCT

[0004] The presented invention relates to a diagnostic method, a fuel cell system and a computer program product according to the appended claims.

[0005] State of the art

[0006] To diagnose the condition of a fuel cell system, such as the so-called "state of health" (SOH), voltage-controlled diagnostic methods such as cyclic voltammetry are generally used. In this method, each individual fuel cell in a fuel cell stack is subjected to a voltage triangular waveform several times to determine a characteristic value, particularly as a function of an active platinum surface, i.e., an electrochemically active area of ​​the respective fuel cell.

[0007] Disclosure of the invention

[0008] Within the scope of the invention presented, a diagnostic method, a fuel cell system, and a computer program product are presented. Further features and details of the invention emerge from the respective subclaims, the description, and the drawings. Features and details described in connection with the diagnostic method according to the invention naturally also apply in connection with the fuel cell system according to the invention or the computer program product according to the invention, and vice versa, so that with regard to the disclosure of the individual aspects of the invention, reference is always made to each other. The invention presented serves, in particular, to provide a possibility for determining the status of a fuel cell system.

[0009] Thus, according to a first aspect of the invention presented, a diagnostic method for diagnosing a state of a fuel cell system is presented.

[0010] The presented diagnostic method comprises closing cathode shut-off valves of a cathode subsystem of the fuel cell system, applying an electric current from a constant current source to a fuel cell stack of the fuel cell system, measuring a pressure profile of a pressure present in the cathode subsystem in a predetermined time window, starting with the application of the electric current from the constant current source to the fuel cell stack, determining a characteristic value that quantifies a state of the fuel cell system based on the measured pressure profile and outputting the characteristic value.

[0011] In the context of the invention presented, a constant current source is understood to mean a direct current source, in particular a single-quadrant current source.

[0012] In the context of the invention presented, a characteristic value is understood to be a value on a scale or a numerical value.

[0013] The presented diagnostic method is based on a current-controlled process. This means that a specified electric current is applied to a fuel cell stack or to the respective fuel cells of the fuel cell stack as an independent variable. The change in the pressure in the cathode subsystem of the fuel cell system that occurs in response to the application of the electric current is determined and evaluated as a dependent variable, i.e., used to determine a characteristic value. Accordingly, a characteristic value that quantifies the condition of the fuel cell or fuel cell system is determined based on the pressure curve during application of the electric current and during subsequent self-discharge.

[0014] The determined characteristic value is output, i.e. provided or stored, on an output unit, such as a display and / or a memory.

[0015] A pressure sensor can be used to measure the pressure in the cathode subsystem of the fuel cell system.

[0016] By applying an electric current from a constant current source to the fuel cell stack of the fuel cell system as provided according to the invention, electrochemical processes are initiated which cause an electrochemical pumping of hydrogen diffused to the cathode side from the cathode side to the anode side.

[0017] In this process, hydrogen is split into protons and electrons on a catalytic layer on the cathode side and recombined on the anode side. The protons pass through the membrane, while the electrons participate in the external circuit as part of the charging current.

[0018] Due to the redistribution of hydrogen molecules due to the application of electrical current from the constant current source to the fuel cell stack of the fuel cell system, the cathode side is "drained" and the anode side is "pumped full." Due to the closed cathode volume, the pressure on the cathode side therefore drops until all hydrogen molecules have been pumped to the anode side. After that, the pressure stabilizes.

[0019] In the hypothetical case that all hydrogen molecules were immediately available at the catalytic layer, a course of the cathode pressure would decrease strictly monotonically and linearly to a point where a hydrogen concentration close to zero is reached.

[0020] Experiments have surprisingly shown that a pressure curve measured in a real fuel cell system after applying an electric current from a constant current source to the fuel cell stack is not strictly linear, but rather transitions from a linear reduction phase to a diffusion phase, in which the pressure decreases more slowly than the preceding linear reduction phase. This slowing of the pressure decrease is caused by a lack of hydrogen molecules at the catalytic layer, since not all hydrogen molecules are immediately available at the catalytic layer; rather, some of the hydrogen molecules must first diffuse to the catalytic layer.

[0021] The presented invention is based on the assumption that the time required for hydrogen molecules to diffuse to the catalytic layer varies with the state of the gas diffusion layer of a particular fuel cell. Accordingly, the pressure curve in the diffusion phase, in which the pressure decreases more slowly relative to the preceding linear decrease phase, becomes increasingly flatter with increasing age or increasing compaction of the gas diffusion layer. It follows that a change in the pressure curve over time or in relation to a reference value can be used to determine the state of the fuel cell system, and this state can be quantified using a characteristic value.

[0022] For example, the characteristic value can be determined based on a gradient, in particular a derivative of the pressure curve in the region of the pressure curve in which the pressure decreases more slowly relative to a preceding linear reduction phase, i.e. in the diffusion phase.

[0023] Optionally, a characteristic value can be assigned to each determined gradient value in the diffusion phase using an assignment scheme. For example, the characteristic value can correspond to a color scheme, particularly a traffic light scheme, that color-codes the classes "good," "wear," and "defect."

[0024] It can be provided that the characteristic value is determined on the basis of an effective diffusion coefficient of hydrogen from the cathode subsystem, wherein the effective diffusion coefficient of hydrogen from the cathode subsystem is determined on the basis of a gradient in a region of the pressure curve in which the pressure decreases more slowly relative to a preceding linear reduction phase.

[0025] A diffusion coefficient that describes the diffusion of hydrogen in the cathode volume can be calculated, for example, using Fick's first law or another mass transport model combined with the ideal gas equation.

[0026] It may further be provided that the characteristic value is determined based on a point in time at which the pressure begins to decrease more slowly relative to a preceding linear reduction phase.

[0027] Alternatively or in addition to the evaluation of the pressure curve in the diffusion phase, a point in time at which the pressure begins to decrease more slowly relative to a preceding linear reduction phase, i.e. the pressure curve makes a "kink" or changes its gradient, can be used to determine the characteristic value.

[0028] The point in time at which the pressure begins to decrease more slowly relative to a preceding linear reduction phase varies analogously to the gradient of the course in the diffusion phase with the state of the fuel cell system, in particular the compaction of the gas diffusion layer.

[0029] It can further be provided that the characteristic value is determined repeatedly and a change in the characteristic value over time is determined, wherein, if the change in the characteristic value over time is greater than a predetermined diagnostic threshold value, a warning message is output on an output unit.

[0030] By evaluating various parameters determined over a specified period of time, particularly across various operating cycles of a fuel cell system, a progression of the fuel cell system's condition can be determined. Accordingly, if a critical parameter or a critical change in the parameter occurs, for example, due to a particularly large difference between two consecutive operating cycles, it can be concluded that the fuel cell system is in a critical condition, and a warning message can be issued indicating the critical condition.

[0031] It can further be provided that, if the characteristic value is determined at a fuel cell stack temperature of less than or equal to 0°C, the characteristic value is compared with a reference value which corresponds to a characteristic value determined at a fuel cell stack temperature of 0°C, wherein, in the event that a deviation between the characteristic value and the reference value is below a predetermined icing threshold value, the fuel cell system is prepared for a freeze start.

[0032] Tests have also surprisingly shown that the pressure profile temporarily changes when the fuel cell stack ices over, resulting in a pressure profile similar to that of a compacted gas diffusion layer. Accordingly, if the characteristic value is determined at a fuel cell stack temperature of less than or equal to 0°C, icing of the fuel cell stack can be diagnosed based on the characteristic value by comparing the characteristic value with a reference value corresponding to a characteristic value determined at a fuel cell stack temperature of 0°C.

[0033] In the event that a deviation between the characteristic value and the reference value is below a predetermined icing threshold, the fuel cell system is prepared for a freeze start, for example by limiting a coolant flow through the fuel cell system during start-up in order to quickly defrost the formed ice or by limiting a current provided by the fuel cell stack during freeze start in order to prevent damage to the fuel cell system caused by the ice.

[0034] In this case, the fuel cell stack temperature can be determined based on at least one of the following measures: coolant temperature, outside temperature, or model temperature calculated based on a cooling model of the fuel cell stack. If the fuel cell stack temperature is determined based on the coolant temperature, a coolant pump can be activated for a predetermined period of time prior to a time at which the coolant temperature is measured.

[0035] By activating the coolant pump before a time at which the coolant temperature is measured, false measurements caused by local hot spots in the coolant flow can be prevented.

[0036] It may further be provided that the diagnostic procedure is carried out after the fuel cell system has been shut down.

[0037] Since the cathode shutoff valves are closed to perform the presented diagnostic procedure, a phase following a shutdown of the fuel cell system is particularly suitable for performing the diagnostic procedure, so that the fuel cell system is not impaired during operation by the diagnostic procedure. In particular, the diagnostic procedure can be performed immediately after an anode post-drying, a so-called "parking purge," during which hydrogen is removed from the anode subsystem, thus creating a favorable equilibrium for the diffusion of hydrogen from the cathode subsystem into the anode subsystem.

[0038] According to a second aspect, the presented invention relates to a fuel cell system for converting energy, wherein the fuel cell system comprises a computing unit configured to carry out a possible embodiment of the presented diagnostic method.

[0039] In the context of the present invention, a computing unit is understood to mean a computer, a processor, a control unit, or any other programmable circuit. In particular, the computing unit can be a control unit of the fuel cell system. According to a third aspect, the present invention relates to a computer program product with program code means that, when the computer program product is executed on a computer, configure the computer to carry out a possible embodiment of the present diagnostic method.

[0040] The presented computer program product can be, for example, a file for downloading from a server or a data storage device such as a CD-ROM or a USB stick.

[0041] Further advantages, features, and details of the invention will become apparent from the following description, which describes exemplary embodiments of the invention in detail with reference to the drawings. The features mentioned in the claims and in the description may be essential to the invention individually or in any combination.

[0042] They show schematically:

[0043] Figure 1 shows a possible embodiment of the presented diagnostic procedure,

[0044] Figure 2 shows a detailed representation of the diagnostic procedure according to Figure 1, and

[0045] Figure 3 shows a possible design of the presented vehicle.

[0046] Fig. 1 shows a diagnostic method 100 for diagnosing a condition of a fuel cell system.

[0047] The diagnostic method 100 comprises a shut-off step 101 in which cathode shut-off valves of a cathode subsystem of the fuel cell system are closed or shut off, an application step 103 in which a fuel cell stack of the fuel cell system is supplied with electrical current from a constant current source, a measuring step 105 in which a pressure profile of a pressure present in the cathode subsystem is measured in a predetermined time window, starting with the application of the electrical current from the constant current source to the fuel cell stack, a determination step 107 in which a characteristic value that quantifies a state of the fuel cell system is determined based on the measured pressure profile and an output step 109 in which the characteristic value is output, for example, on an output unit or in a memory.

[0048] Fig. 2 shows a diagram 200 which spans on its abscissa over time and on its ordinate over a pressure applied in a cathode subsystem of a fuel cell system during a sequence of the diagnostic method 100.

[0049] A first curve 201 corresponds to an ideal curve in which all hydrogen molecules are immediately available on a catalytic layer of the cathode subsystem. Accordingly, curve 201 decreases linearly from a starting value p_0 to a lower limit p_lim at the beginning of the current supply to the fuel cell stack at time t0.

[0050] Since in the first curve 201 all hydrogen molecules are immediately available, a time ta at which a hydrogen concentration at the catalytic layer is close to 0 coincides with the lower limit p_lim.

[0051] A second curve 203 corresponds to a delivery state of a fuel cell system in which not all hydrogen molecules are immediately available at the catalytic layer, so that the curve 203, after a linear reduction phase 205, transitions into a diffusion phase 207 in which the pressure decreases more slowly relative to a preceding linear reduction phase 205.

[0052] Since the diffusion of hydrogen molecules to the catalytic layer depends on the state of the fuel cell system, in particular the compaction of the gas diffusion layer of the respective fuel cells of the fuel cell system, the gradient of the second curve 203 in the diffusion phase 207 is characteristic of the state of the fuel cell system. In the second curve 203, a transition from the linear lowering phase 205 to the diffusion phase 207 occurs at a time h, which is accordingly also characteristic of the state of the fuel cell system.

[0053] A third curve 209 corresponds to an aged state of a fuel cell system in which a short linear lowering phase 211 transitions into a long and flat diffusion phase 213.

[0054] In the second curve 203, a transition from the linear reduction phase 211 to the diffusion phase 213 takes place at a time t2, which is accordingly also characteristic of the state of the fuel cell system.

[0055] Accordingly, based on the gradient in a respective diffusion phase 207, 213 and / or the time h or t2, a characteristic value can be determined which describes a state of the respective fuel cell system, in particular its state of health.

[0056] Fig. 3 shows a fuel cell system 300. The fuel cell system comprises a fuel cell stack 301 with a cathode subsystem 303 and an anode subsystem 305, as well as a computing unit 307 configured to perform the diagnostic method 100 according to Fig. 1.

Claims

Claims 1 . Diagnostic method (100) for diagnosing a condition of a fuel cell system (300), the diagnostic method (100) comprising: - closing (101) cathode shut-off valves of a cathode subsystem (303) of the fuel cell system (300), - applying (103) an electric current from a constant current source to a fuel cell stack (301) of the fuel cell system (300), - measuring (105) a pressure profile of a pressure present in the cathode subsystem (303) in a predetermined time window, starting with the application of the electric current from the constant current source to the fuel cell stack (301), - determining (107) a characteristic value that quantifies a state of the fuel cell system (300) on the basis of the measured pressure curve, - Output (109) of the characteristic value.

2. Diagnostic method (100) according to claim 1, characterized in that the characteristic value is determined on the basis of an effective diffusion coefficient of hydrogen from the cathode subsystem (303), wherein the effective diffusion coefficient of hydrogen from the cathode subsystem (303) is determined on the basis of a gradient in a region of the pressure curve in which the pressure decreases more slowly relative to a preceding linear reduction phase (205, 211).

3. Diagnostic method (100) according to claim 1, characterized in that that the characteristic value is determined based on a point in time (h, t2) at which the pressure begins to decrease more slowly relative to a preceding linear reduction phase.

4. Diagnostic method (100) according to one of the preceding claims, characterized in that the characteristic value is determined repeatedly and a change in the characteristic value over time is determined, wherein, if the change in the characteristic value over time is greater than a predetermined diagnostic threshold value, a warning message is output on an output unit.

5. Diagnostic method (100) according to one of the preceding claims, characterized in that, if the characteristic value is determined at a fuel cell stack temperature less than or equal to 0°C, the characteristic value is compared with a reference value which corresponds to a characteristic value determined at 0°C fuel cell stack temperature, wherein in the event that a deviation between the characteristic value and the reference value is below a predetermined icing threshold value, the fuel cell system (300) is prepared for a freeze start.

6. Diagnostic method (100) according to claim 5, characterized in that the fuel cell stack temperature is determined based on at least one of the following measures: coolant temperature, outside temperature, model temperature calculated based on a cooling model of the fuel cell stack.

7. Diagnostic method (100) according to claim 6, characterized in that in the case that the fuel cell stack temperature is determined based on the coolant temperature, before a time at which the coolant temperature is measured, a coolant pump is activated for a specified period of time.

8. Diagnostic method (100) according to one of the preceding claims, characterized in that the diagnostic method (100) is carried out after a shutdown process of the fuel cell system (300).

9. A fuel cell system (300) for converting energy, wherein the fuel cell system (300) comprises a computing unit (307) configured to perform a diagnostic method (100) according to any one of claims 1 to 8.

10. Computer program product with program code means which, when the computer program product is executed on a computer, Configure a computer to perform a diagnostic method (100) according to any one of claims 1 to 8.

Images