Method for adjusting a fuel cell system

Abstract

The invention presented relates to a method (100) for adjusting a fuel cell system (200). The presented procedure (100) includes: - Initiating (101) a shutdown procedure to shut down a fuel cell stack (201) of the fuel cell system (200) by closing cathode shut-off valves (213) of the fuel cell system (200), - Adjusting (103) an electric current delivered by a fuel cell stack (201) of the fuel cell system (200) by means of a computing unit (211) such that a voltage applied to the fuel cell stack (201) approaches a predetermined switching value (305), - Short-circuiting (105) of the fuel cell stack (201) via a cut-off resistor (209) as soon as the voltage applied to the fuel cell stack (201) falls below the switching value (305), wherein, in the event that the voltage applied to the fuel cell stack (201) rises again after it has dropped to a reference value greater than the switching value (305), the fuel cell stack (201) is short-circuited via the shutdown resistor (209) and the switching value (305) is increased for a subsequent shutdown procedure.

Description

[0001] The presented invention relates to a method for adjusting a fuel cell system, a fuel cell system and a program product according to the appended claims. State of the art

[0002] To minimize wear and tear on PEM fuel cell systems, a special shutdown procedure known as a "bleeddown" is used. This involves closing shutoff valves at the cathode inlet and cathode outlet of a fuel cell stack in a respective fuel cell system, preventing the flow of fresh, oxygen-containing air.

[0003] The hydrogen present on the anode side completely converts any oxygen still trapped in the cathode volume, causing the voltage applied to the fuel cell stack to drop significantly due to the reduction in the oxygen partial pressure. In the target state of the shutdown procedure, no oxygen is present in the cathode volume. Disclosure of the invention

[0004] Within the scope of the invention presented, a method for adjusting a fuel cell system, a fuel cell system, and a 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 method according to the invention naturally also apply in connection with the fuel cell system according to the invention or the 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.

[0005] The invention presented serves in particular to provide a robust and seal-optimized fuel cell system.

[0006] Thus, according to a first aspect of the invention presented, a method for adjusting a fuel cell system is presented.

[0007] The presented method comprises initiating a shutdown procedure for switching off a fuel cell stack of the fuel cell system by closing cathode shut-off valves of the fuel cell system, adjusting an electrical current output by a fuel cell stack of the fuel cell system by means of a computing unit such that a voltage applied to the fuel cell stack approaches a predetermined switching value and short-circuiting the fuel cell stack via a shutdown resistor as soon as the voltage applied to the fuel cell stack falls below the switching value, wherein in the event that the voltage applied to the fuel cell stack rises again after having fallen to a reference value which is greater than the switching value, the fuel cell stack is short-circuited via the shutdown resistor and the switching value is increased for a subsequent shutdown procedure.

[0008] The presented method is based on monitoring the voltage applied to a fuel cell stack during a shutdown procedure. Since this voltage only exhibits a characteristic curve in which the voltage drops below the switchover value when the cathode subsystem is technically sound or airtight, if a measured voltage does not exhibit this characteristic curve, it can be assumed that the corresponding cathode subsystem is technically faulty.

[0009] According to the method presented, a deviation of a respective voltage curve from the characteristic curve is detected in particular by the fact that the voltage curve, after having dropped to a reference value that is greater than the switching value, rises again and accordingly does not drop to the switching value. In this case, the fuel cell stack is short-circuited via the shutdown resistor, and the switching value is increased for a subsequent shutdown procedure, i.e., a so-called "bleeddown."

[0010] By increasing the switching value, the fuel cell stack is short-circuited via the shutdown resistor at a voltage higher than originally intended, e.g., at the reference value. Accordingly, the reference value can also be increased proportionally to the switching value to allow for a further increase in the switching value, if necessary, during a subsequent shutdown procedure.

[0011] According to the invention, a deviation of a respective measured voltage curve from the characteristic curve or a defect, in particular a leakage of a respective cathode subsystem, causes an increase in the switching value. This means that a state of the cathode subsystem can be deduced from the magnitude of the switching value.

[0012] It may be provided that, in the event that the switching value is increased to a value that is greater than a predetermined first diagnostic threshold value, additional fuel is metered into an anode subsystem of the fuel cell stack when the fuel cell system is started.

[0013] By injecting additional fuel—i.e., in addition to the fuel present in the anode subsystem—into the anode subsystem of a fuel cell stack during startup of a fuel cell system, any air-hydrogen front that may be present is driven more quickly through the anode volume. Accordingly, the fuel cells of the fuel cell stack are exposed for a shorter time to the damaging mechanisms that operate during the existence of this air-hydrogen front. Additional hydrogen injection thus shortens these so-called air-to-air starts and thus mitigates their damaging effects.

[0014] A first diagnostic threshold defines a condition of the cathode subsystem in which such dosing of additional fuel is appropriate, since leaks in the cathode subsystem already exist and promote air-to-air starts.

[0015] It may further be provided that, in the event that the switching value is increased to a value that is greater than a predetermined second diagnostic threshold, an error message is output indicating a possible leak in a cathode subsystem of the fuel cell stack.

[0016] A second diagnostic threshold, which is in particular greater than the first diagnostic threshold, defines a condition of the cathode subsystem in which a significant leak exists that cannot be completely compensated, for example, by supplying additional fuel during startup. Accordingly, an error message can be issued if the switchover value is increased to a value greater than the second diagnostic threshold. This error message can, for example, be displayed on a screen or stored in a memory, such as an error log.

[0017] It may further be provided that, in the event that the switching value is increased to a value that is greater than the second diagnostic threshold, an anode subsystem of the fuel cell stack is inerted during the shutdown procedure.

[0018] To protect a fuel cell system from damage caused by an air-to-air start, the anode subsystem can be inerted during a shutdown process.

[0019] It can further be provided that the first diagnostic threshold value and / or the second diagnostic threshold value is determined empirically on the basis of a test setup or is determined dynamically on the basis of a mathematical model of the fuel cell system or is determined on the basis of respective switching values ​​set in a plurality of fuel cell systems.

[0020] A variable to be set for the first diagnostic threshold value and / or the second diagnostic threshold value defines a state of the fuel cell system as technically OK or technically not OK, so that the respective variable can be determined, for example, empirically based on technically faulty fuel cell systems.

[0021] Alternatively, a mathematical model of a particular fuel cell system can be used to simulate its state and to select the respective variable in such a way that it separates a given state from a deviating or inadmissible state.

[0022] Alternatively, a representative value, such as three times the standard deviation of the switching values, can be determined for a diagnostic threshold based on the respective switching values ​​set in a large number of fuel cell systems.

[0023] It may further be provided that the error message includes an instruction for maintenance of the cathode subsystem, in particular valves of the cathode subsystem.

[0024] Since a significant leakage in the cathode subsystem can be assumed if the switching value is increased to a value greater than the second diagnostic threshold and an error message is issued, the error message can include an instruction to maintain the cathode subsystem, in particular valves of the cathode subsystem, so that a corresponding fuel cell system is protected from major damage caused by operation with, for example, defective cathode shut-off valves.

[0025] It can further be provided that in the event that the voltage applied to the fuel cell stack rises again after it has fallen to a reference value which is greater than the switching value, the computing unit is deactivated.

[0026] Deactivating the computing unit protects it from damage caused by short-circuiting the fuel cell stack.

[0027] According to a second aspect, the presented invention relates to a fuel cell system for converting energy.

[0028] The fuel cell system comprises a fuel cell stack with a cathode subsystem and an anode subsystem, a voltage sensor for detecting a voltage applied to the fuel cell stack, a cut-off resistor and a computing unit.

[0029] The computing unit is configured to carry out a possible embodiment of the presented method based on values ​​determined by the voltage sensor.

[0030] Accordingly, the computing unit can be configured to: - close the cathode subsystem's cathode shut-off valves to initiate a shutdown procedure to shut down the fuel cell stack, - to adjust an electric current delivered by the fuel cell stack in such a way that a voltage applied to the fuel cell stack approaches a predetermined switching value, - to short-circuit the fuel cell stack via a shutdown resistor as soon as the voltage applied to the fuel cell stack falls below the switching value, wherein the computing unit is configured to short-circuit the fuel cell stack via the shutdown resistor and to increase the switching value for a subsequent shutdown procedure if the voltage applied to the fuel cell stack rises again after it has fallen to a reference value that is greater than the switching value.

[0031] Advantages that have been described in detail for the method for adjusting a fuel cell system according to the first aspect of the invention apply equally to the fuel cell system for converting energy according to the second aspect of the invention.

[0032] According to a third aspect, the presented invention relates to a program product comprising program instructions which, when executed on a computing unit, configure the computing unit to carry out a possible embodiment of the presented method.

[0033] 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.

[0034] They show: Fig. 1 a possible design of the presented procedure, Fig. 2 a possible design of the presented fuel cell system, Fig. 3 a comparison of voltage curves in different states of the fuel cell system according to Fig. 2.

[0035] In Fig. 1 shows a method 100 for adjusting a fuel cell system.

[0036] The method 100 includes an initiation step 101 in which a shutdown procedure for shutting down a fuel cell stack of the fuel cell system is initiated by closing cathode shutoff valves of the fuel cell system.

[0037] Furthermore, the method 100 comprises an adjustment step 103 in which an electrical current delivered by a fuel cell stack of the fuel cell system is adjusted by means of a computing unit such that a voltage applied to the fuel cell stack approaches a predetermined switching value,

[0038] Furthermore, the method 100 comprises a first short-circuiting step 105, in which the fuel cell stack is short-circuited via a switch-off resistor as soon as the voltage applied to the fuel cell stack falls below the switching value.

[0039] Furthermore, the method 100 comprises a determination step 107 in which it is determined whether the voltage applied to the fuel cell stack increases again after it has fallen to a reference value that is greater than the switching value, and in the event that the determination step 107 shows that the voltage applied to the fuel cell stack increases again after it has fallen to a reference value that is greater than the switching value, a second short-circuiting step 109 in which the fuel cell stack is short-circuited via the shutdown resistor, and an adaptation step 111 in which the switching value is increased for a subsequent shutdown procedure.

[0040] In Fig. 2 shows a fuel cell system 200 for converting energy.

[0041] The fuel cell system 200 comprises a fuel cell stack 201 with a cathode subsystem 203 and an anode subsystem 205, a voltage sensor 207 for detecting a voltage applied to the fuel cell stack 201, a cut-off resistor 209, and a computing unit 211.

[0042] The computing unit 211 executes a program with program code means that configure the computing unit 211 to close cathode shutoff valves 213 of the cathode subsystem 203 to initiate a shutdown procedure for shutting down the fuel cell stack 201, to adjust an electrical current output by the fuel cell stack 201 such that a voltage applied to the fuel cell stack 201 approaches a predetermined switching value, and to short-circuit the fuel cell stack 201 via a shutdown resistor as soon as the voltage applied to the fuel cell stack 201 drops below the switching value.

[0043] Furthermore, the computing unit 211 is configured to short-circuit the fuel cell stack 201 via the shutdown resistor and to increase the switching value for a subsequent shutdown procedure in the event that the voltage applied to the fuel cell stack 201 rises again after it has fallen to a reference value that is greater than the switching value.

[0044] In Fig. 3 shows a diagram 300 which extends on its abscissa over time and on its ordinate over a voltage applied to a fuel cell stack.

[0045] A first curve 301 corresponds to a voltage drop during a shutdown procedure in a technically flawless fuel cell system.

[0046] A second curve 303 corresponds to a voltage drop during a shutdown procedure in a fuel cell system whose cathode subsystem has a leak.

[0047] At a time t_0, cathode shutoff valves are closed to perform a so-called "bleeddown." While the first waveform 301 continuously decreases, so that when the voltage reaches a switching value 305 at a time t_1, the fuel cell stack can be short-circuited in a second phase 307, the second waveform 303 only decreases in a first phase 309 and does not reach the switching value 305. Instead, the voltage of the second waveform 303 increases again due to oxygen diffusing through the leak into the cathode subsystem of the fuel cell stack. Accordingly, the switching value 305 must be increased to trigger the short circuit of the fuel cell stack.

Claims

[1] Method (100) for adjusting a fuel cell system (200), the method (100) comprising: - initiating (101) a shutdown procedure for shutting down a fuel cell stack (201) of the fuel cell system (200) by closing cathode shut-off valves (213) of the fuel cell system (200), - adjusting (103) an electrical current output by a fuel cell stack (201) of the fuel cell system (200) by means of a computing unit (211) such that a voltage applied to the fuel cell stack (201) approaches a predetermined switching value (305), - short-circuiting (105) the fuel cell stack (201) via a shutdown resistor (209) as soon as the voltage applied to the fuel cell stack (201) falls below the switching value (305), wherein in the event that the voltage applied to the fuel cell stack (201) rises again after it has fallen to a reference value which is greater than the switching value (305), the fuel cell stack (201) is short-circuited via the shutdown resistor (209) and the switching value (305) is increased for a subsequent shutdown procedure. [2] Method (100) according to claim 1, characterized by that, in the event that the switching value (305) is increased to a value which is greater than a predetermined first diagnostic threshold value, additional fuel is metered into an anode subsystem (205) of the fuel cell stack (201) when the fuel cell system (200) is started. [3] Method (100) according to claim 1 or 2, characterized bythat in the event that the switching value (305) is increased to a value which is greater than a predetermined second diagnostic threshold value, an error message is output which indicates a possible leakage in a cathode subsystem (203) of the fuel cell stack (201). [4] Method (100) according to claim 3, characterized by in that, in the event that the switching value (305) is increased to a value which is greater than the second diagnostic threshold value, an anode subsystem (205) of the fuel cell stack (201) is inerted during the shutdown procedure. [5] Method (100) according to claim 3 or 4, characterized by that the first diagnostic threshold value and / or the second diagnostic threshold value is determined empirically on the basis of a test setup or is determined dynamically on the basis of a mathematical model of the fuel cell system (200) or is determined on the basis of respective switching values ​​set in a plurality of fuel cell systems (200). [6] Method (100) according to claims 3 to 5, characterized by that the error message comprises an instruction for maintenance of the cathode subsystem (203), in particular valves (213) of the cathode subsystem (203). [7] Method (100) according to one of the preceding claims, characterized by that in the event that the voltage applied to the fuel cell stack (201) rises again after it has fallen to a reference value which is greater than the switching value (305), the computing unit (211) is deactivated. [8] Fuel cell system (200) for converting energy, the fuel cell system (200) comprising: - a fuel cell stack (201) with a cathode subsystem (203) and an anode subsystem (205), - a voltage sensor (207) for detecting a voltage applied to the fuel cell stack (201), - a cut-off resistor (209), - a computing unit (211), wherein the computing unit (211) is configured to carry out a method (100) according to one of claims 1 to 7 based on values ​​determined by the voltage sensor (207). [9] Fuel cell system (200) according to claim 8, characterized by that the computing unit (211) is configured to: - to close cathode shut-off valves (213) of the cathode subsystem (203) to initiate a shutdown procedure for shutting down the fuel cell stack (201), - adjusting an electric current delivered by the fuel cell stack (201) such that a voltage applied to the fuel cell stack (201) approaches a predetermined switching value (305), - to short-circuit the fuel cell stack (201) via a shutdown resistor (209) as soon as the voltage applied to the fuel cell stack (201) falls below the switching value (305), wherein the computing unit (211) is configured to short-circuit the fuel cell stack (201) via the shutdown resistor (209) and to increase the switching value (305) for a subsequent shutdown procedure in the event that the voltage applied to the fuel cell stack (201) rises again after it has fallen to a reference value which is greater than the switching value (305). [10] A program product comprising program instructions which, when executed on a computing unit (211), configure the computing unit (211) to perform a method (100) according to any one of claims 1 to 7.

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