Method for operating a fuel cell system, computer program product, control unit, and fuel cell system

Abstract

The invention relates to a method for operating a fuel cell system (100) which is designed with at least one fuel cell stack (101), wherein the at least one fuel cell stack (101) has a corresponding air system (10), comprising: detecting a start request of the fuel cell system (100), checking whether a start under freezing conditions is required; if so, then: limiting a start method under freezing conditions to closed-loop controller modes for the air system (10) without using freezing-critical valves for the closed-loop control operation, carrying out the start method under freezing conditions without using freezing-critical valves for the closed-loop control operation, checking whether freezing-critical valves are ready for use; if so, then: enabling freezing-critical valves for the closed-loop control operation as a function of their readiness for use, continuing the start method with enabled valves.

Description

[0001] Description

[0002] title

[0003] Method for operating a fuel cell system, computer program product, control unit and fuel cell system

[0004] The invention relates to a method for operating a fuel cell system. Furthermore, the invention relates to a corresponding computer program, a corresponding control unit, and a corresponding fuel cell system for carrying out this method.

[0005] State of the art

[0006] In vehicles (Fuel Cell Vehicle or FCV for short) where propulsion energy is supplied (among other things) by one (or more) fuel cell systems (Fuel Cell System or FCS for short), oxygen from the ambient air is usually used as the oxidizing agent and hydrogen as the reducing agent or fuel to react in a fuel cell stack to form water (or water vapor) and thus deliver electrical power through electrochemical conversion.

[0007] The challenge with mobile fuel cell systems lies in starting the system under all globally relevant conditions and with varying vehicle standstill times.

[0008] - to implement functionally and

[0009] - while still meeting lifetime requirements.

[0010] Critical cases regarding freezing water and resulting damage and / or functional limitations include, for example, A1 Freezing of water, e.g., condensed due to temperature drop or accumulated in the exhaust path before / during shutdown.

[0011] - during the standstill phase or

[0012] - in a standby phase

[0013] (e.g., when the vehicle is powered by battery and the fuel cell system is switched off, e.g., due to zero power demand, e.g., when driving downhill, cooling due to wind, etc.).

[0014] A2 When the FCS starts, the resulting reaction water can freeze.

[0015] Water that has already thawed during startup can refreeze in other parts of the system.

[0016] Functional limitations, e.g., due to an aborted or inadequate freeze start, can subsequently lead to damage / degradation of the fuel cell stack and / or other components, especially in an air system.

[0017] Functional limitations can be caused in particular by blocked valves (referred to as throttle valves in the air system), e.g. at lower temperatures (e.g. -1 ...-20 °C).

[0018] Also critical is the short-distance case (“getting bread”), where there is only a short operating time and the components – especially in the cathode exhaust path – are not yet sufficiently warmed up and can therefore freeze and become blocked when the engine is switched off and in the subsequent standstill phase.

[0019] State-of-the-art measures regarding the valves are:

[0020] - Icebreaking function on the valves / throttle bodies,

[0021] - Oscillation function on the valves / throttle valves during operation,

[0022] - Installation of heating systems.

[0023] The combination of possible faults during a freeze start is high, especially in air systems with multiple valves. Frozen sensors can also lead to a faulty freeze start. Addressing all these fault scenarios with separate control modes and performing the necessary switching requires significant software effort, and the robustness of the numerous separate control modes can vary.

[0024] Disclosure of the invention

[0025] The present invention provides a method for operating a fuel cell system with the features of the independent method claim. Furthermore, the invention provides a corresponding computer program, a corresponding control unit, and a corresponding fuel cell system with the features of the dependent claims. Features and details described in connection with the different embodiments and / or aspects of the invention naturally also apply in connection with the other embodiments and / or aspects, and vice versa, so that the disclosure relating to the individual embodiments and / or aspects always includes, or can include, reciprocal references.

[0026] According to the first aspect, the present invention provides: a method for operating a fuel cell system (or hereinafter referred to as system), comprising at least one fuel cell stack (or hereinafter referred to as stack), wherein the at least one fuel cell stack comprises a corresponding air system, comprising:

[0027] (4) Detect a start request of the fuel cell system, (8) Check if a freeze start is required, if so: (20) Restrict a freeze start procedure to control modes for the air system without the use of freeze-critical valves for control, wherein at least one valve can change its status as a non-freeze-critical valve to a freeze-critical valve and / or vice versa during the freeze start procedure [This is the case when the anode remains filled with hydrogen and the cathode is / remains inerted upon shutdown (this is the so-called usual case).

[0028] In systems where the anode can and is inerted during shutdown, the cathode path can also remain open, i.e., the shut-off valve (CVout) from the cathode path of the stack can remain open or partially open and does not need to be moved during a freeze start until freezing is no longer critical.

[0029] (24) Performing the freeze-start procedure without using freeze-critical valves for control and, if necessary, a warm-up procedure,

[0030] (30) Check the operational readiness of freeze-critical valves; if so, then:

[0031] (34) Releasing freeze-critical valves for control depending on their operational readiness,

[0032] (36) Continue the starting procedure with the valves released.

[0033] The fuel cell system (or system for short) can have several fuel cell stacks (or stacks for short), each with several stacked fuel cells and the associated functional systems, including: media systems (air or cathode system, fuel or anode system, cooling system) and an electrical system.

[0034] Preferably, the fuel cell system can comprise several modules in the form of individual stacks and the associated functional systems.

[0035] Multiple stacks can comprise a common air or cathode system.

[0036] The fuel cell system can be used for mobile applications, e.g., in vehicles (FCV, short for "Fuel Cell Vehicle"), where the propulsion energy is supplied, among other things, by a (PEM) fuel cell system (FCS, short for "Fuel Cell System") with one or more fuel cell stacks. The fuel cell system can also be used for stationary applications, e.g., in generators.

[0037] The idea can be used for single-stage air compression systems or for multi-stage air compression systems.

[0038] The proposed method advantageously serves the following purposes:

[0039] - to avoid freeze-start problems in or due to the air system,

[0040] - to prevent damage / degradation of the fuel cell stack and / or other components, especially in an air system, due to failed or suboptimal freeze starts,

[0041] - to avoid freeze-start problems caused by couplings between the air system and other subsystems, e.g., anode system, coolant system,

[0042] - to ensure robust freeze starts.

[0043] This is made possible by ensuring robustness, functionality and controllability in the air system:

[0044] 1. by freezing the system without using freezing-critical valves (control and / or regulation operation dependent on sensors)

[0045] 2. possibly by temperature-dependent release of freezing-critical valves and switching of the control modes of the air system

[0046] 3. If applicable, a defined freeze-start-capable setting of the valves when the system is shut down, stopped and / or wake-up operation.

[0047] The procedure recognizes that:

[0048] - Freeze starts should preferably be performed at / near ambient pressure,

[0049] - the newer valves / throttle valves maintain their set position (even when de-energized).

[0050] The proposed method offers several significant advantages:

[0051] - Avoidance of freeze-start problems in or through the air system,

[0052] - to prevent damage / degradation of the fuel cell stack and / or other components, especially in an air system, - to ensure system functionality during freeze starts,

[0053] - robust operating strategies,

[0054] - Reduction of the software effort required for freeze starts, responsiveness to errors, etc.,

[0055] - Reduction and / or avoidance of electric heating for valves / throttle valves,

[0056] - Cost reduction (design),

[0057] - Energy reduction (operation) etc.

[0058] Critical valves / throttle valves for freeze start are referred to below as freeze-critical or freeze-start critical valves.

[0059] Advantageously, it is possible to determine, depending on the topology and also the design, which valves may be critical to freezing or freezing start and which may not.

[0060] Examples of valves that are critical to freezing or freezing start include:

[0061] D In the exhaust path:

[0062] - Pressure regulating valve,

[0063] - Turbine bypass valves,

[0064] - shut-off valve near the stack, possibly with control function.

[0065] D In the bypass path:

[0066] - Stack bypass valve.

[0067] D In the EGR path:

[0068] - EGR valve.

[0069] D In the supply air path:

[0070] - shut-off valve near the stack, possibly also with control function,

[0071] - Humidifier bypass valve.

[0072] Valves in the exhaust path can be most critical due to the humid air (and sometimes air containing droplets). Valves can also experience problems due to condensation effects if the stack was shut down in a very humid, warm state and then cools down to low temperatures.

[0073] One measure for critical valves could be to equip them with heaters.

[0074] This is particularly relevant when a valve must move during freeze-start, for example, a close-coupled shut-off valve (CVout) (under start conditions with an inerted cathode path but no inerted anode path). Therefore, such a valve (CVout) will preferably be equipped with a heater. If such a valve (CVout) is unheated, its icebreaking function must be sufficient to ensure it functions properly.

[0075] Valves with a heating element do not necessarily have to be designated as freezing-critical or freezing-start-critical valves.

[0076] A stack-adjacent shut-off valve (CVout) can refreeze after opening, so it could again become a freezing-critical or freezing-start-critical valve during operation if, for example, it has not been used for control purposes for a period of time.

[0077] Furthermore, to check (8) whether a freeze start is required, at least one of the following actions can be performed:

[0078] (6) Determining starting conditions and / or a suitable starting procedure.

[0079] In this way, starting conditions, e.g., comprehensive internal system conditions such as the temperature in the air system, and / or external conditions such as the ambient temperature, can be checked. This allows for the targeted selection of a suitable starting procedure, such as a freeze start, cold start, warm start, normal operation, etc. Furthermore, if the check (8) to determine whether a freeze start is required yields a "no," at least one of the following actions can be performed:

[0080] (10) Release all necessary valves,

[0081] (14) Carry out the starting procedure with all required valves.

[0082] In other words, operation can proceed as usual (e.g., cold start, warm start, normal operation, etc.) if a freeze start is not required.

[0083] Advantageously, checking (30) the operational readiness of freeze-critical valves can be carried out until the operational readiness of all freeze-critical valves is verified. In this way, a safe transition to cold start, warm start, normal operation, etc., can be ensured.

[0084] Furthermore, it is conceivable that when checking (30) the operational readiness of freeze-critical valves:

[0085] - a temperature threshold, e.g. of an exhaust gas,

[0086] - a warm-up time for valves critical to freezing, and / or

[0087] - an energy input threshold, e.g. through exhaust gas, is checked / will be checked.

[0088] A temperature threshold could be, for example, 15 °C. If, for example, the exhaust gas temperature exceeds 15 °C for 2 minutes, then no valves in the exhaust gas path can be blocked by ice.

[0089] A threshold can also affect the warm-up times of the valves, which can affect the integrated enthalpy of the exhaust gas mass flow.

[0090] Combining threshold values, e.g. with regard to energy input, can also be provided for.

[0091] Threshold values ​​can generally be determined individually for each valve. When determining threshold values, individual installation positions, warm-up conditions, etc., can be taken into account, which can differ for different valves. Furthermore, different valves can be individually enabled.

[0092] Advantageously, to check (30) the operational readiness of freeze-critical valves, at least one of the following actions can be performed:

[0093] (26) Inspection and / or diagnosis of valves critical to freezing.

[0094] In this way, the freezing-critical valves can be specifically checked for their operational readiness. Furthermore, various diagnostic options are possible.

[0095] Furthermore, it may be provided that the checking (30) of the operational readiness of freeze-critical valves is carried out individually for different valves. It may also be provided that the release (34) of freeze-critical valves is carried out individually, in a cascaded manner, and / or in a specific sequence, e.g., depending on their criticality for the freezing start.

[0096] Furthermore, it is conceivable that during the execution (24) of the freeze-start procedure, valves may change their status from non-freeze-critical to freeze-critical valves and / or vice versa. For example, some valves must be moved during the freeze-start. At the beginning of the freeze-start, the risk of freezing may not yet be present or may not be high. After opening at the beginning of the freeze-start, water / moisture may accumulate in the system, causing these valves to refreeze. In this case, the valves can be reclassified as freeze-critical valves. Alternatively or additionally, for valves equipped with heaters, the heaters can be (re)activated to defrost these valves.

[0097] Firstly, it may be provided that if, during the check (30) of the operational readiness of freeze-critical valves, all or some freeze-critical valves are verified as not operational, then the execution (24) of the freeze-start procedure is continued without the use of the affected freeze-critical valves. Secondly, it may be provided that, if, during the check (30) of the operational readiness of freeze-critical valves, all freeze-critical valves are verified as operational, then at least one of the following actions is carried out:

[0098] (14) Continue the start-up procedure with all required valves or (40) Carry out normal operation with all required valves.

[0099] A particular advantage of the invention is that, when or after the fuel cell system is switched off (50), e.g., by a wake-up function, the valves critical to freezing are moved into freeze-start-compatible positions. This ensures that a subsequent freeze-start procedure finds the valves already in the appropriate position for a freeze start. Furthermore, this reduces the risk of damage to the valves.

[0100] Furthermore, it may be provided that when performing (24) the freeze start procedure without the use of freeze-critical valves, at least one of the following actions is carried out:

[0101] (21) Testing and / or diagnosing sensors, especially in the air system,

[0102] (23) Use of sensors for regulation (23a) and / or control (23b) depending on the verification (21),

[0103] (24a) Adapting the starting procedure without using freeze-critical valves or non-functional sensors.

[0104] This allows the current operational readiness of sensors to be taken into account in the process.

[0105] According to a further aspect, the invention provides a computer program product comprising instructions which, when executed by a computer, such as the processing unit of a control unit, cause the computer to carry out the method, which can proceed as described above. The same advantages described above in connection with the method according to the invention can be achieved with the computer program product. These advantages are fully referenced herein.

[0106] A corresponding control unit provides a further aspect of the invention. A computer program in the form of code can be stored in a memory unit of the control unit. When the code is executed by a processing unit of the control unit, this program performs a procedure that can proceed as described above. The same advantages described above in connection with the method according to the invention can be achieved with the help of the control unit. These advantages are fully referenced herein.

[0107] A corresponding fuel cell system also constitutes an aspect of the invention, wherein the fuel cell stack includes a corresponding control unit. The same advantages described above in connection with the method according to the invention can be achieved using the fuel cell system. These advantages are fully referenced herein.

[0108] Preferred embodiments:

[0109] The invention, its further developments, and its advantages are explained in more detail below with reference to the drawings. Each drawing schematically shows:

[0110] Figure 1 shows an exemplary system topology,

[0111] Figure 2 shows an exemplary system topology and

[0112] Figure 3 shows an example of a possible procedure.

[0113] In the different figures, identical parts of the invention are always provided with the same reference numerals, which is why they are generally described only once. Figures 1 to 3 serve to explain the invention, which proposes a method for operating a fuel cell system 100 (or, in short, system), which can be implemented with at least one or more fuel cell stack(s) 101 (or, in short, stack).

[0114] As shown in Fig. 1, a fuel cell stack 101 can have a corresponding air system 10, wherein the air system 10 can have an air intake path 11 and an exhaust path 12.

[0115] As indicated in Fig. 2, two fuel cell stacks 101 (see Fig. 2) can have a corresponding air system 10, wherein the air system 10 can have an air intake path 11 and an exhaust path 12.

[0116] Figure 3 serves to illustrate the proposed method, comprising:

[0117] (4) Recording a start request of the fuel cell system 100,

[0118] (8) Check if a freeze start is required; if so, then:

[0119] (20) Restricting a freeze-start procedure to control modes for the air system 10 without the use of freeze-critical valves kV for control, wherein at least one valve CVout must first be moved for the freeze-start and can change its status from a non-freeze-critical valve to a freeze-critical valve and / or vice versa during the freeze-start procedure

[0120] [This is the case when the anode remains filled with hydrogen and the cathode is / remains inerted when switched off (this is the so-called usual case).

[0121] In systems where the anode can and is inerted during shutdown, the cathode path can also remain open, i.e., the shut-off valve (CVout) from the cathode path of the stack can remain open or partially open and does not need to be moved during a freeze start until freezing is no longer critical.

[0122] (24) Perform the freeze-start procedure without using freeze-critical valves kV for control and, if necessary, a warm-up procedure, (30) Check the operational readiness of freeze-critical valves kV, if so, then:

[0123] (34) Release of freeze-critical valves kV for control depending on their operational readiness,

[0124] (36) Continue the starting procedure with the valves released kV.

[0125] The proposed method advantageously serves the following purposes:

[0126] - to avoid freeze-start problems in or through the air system 10

[0127] - to prevent damage / degradation of the fuel cell stack 101 and / or other components, especially in an air system 10, due to failed or suboptimal freeze starts,

[0128] - to avoid freeze-start problems caused by couplings between the air system and other subsystems, e.g., anode system, coolant system,

[0129] - to ensure robust freeze starts.

[0130] This is made possible by ensuring robustness, functionality and controllability in the air system 10:

[0131] 1. by freeze-starting system 100 without using freeze-critical valves

[0132] (Regulatory and / or control operation depending on sensors)

[0133] 2. possibly by temperature-dependent release of freezing-critical valves and switching of the control modes of the air system 10

[0134] 3. If applicable, defined freeze-start-capable setting of the valves during shutdown, stopping and / or wake-up operation 50 of the system 100

[0135] The procedure recognizes that:

[0136] - Freeze starts should preferably be performed at / near ambient pressure,

[0137] - the newer valves / throttle valves maintain their set position (even when de-energized).

[0138] The proposed method offers several significant advantages:

[0139] - Avoidance of freeze-start problems in or through the air system 10,

[0140] - to prevent damage / degradation of the fuel cell stack 101 and / or other components, especially in an air system 10, - to ensure system functionality during freeze starts,

[0141] - robust operating strategies,

[0142] - Reduction of the software effort required for freeze starts, responsiveness to errors, etc.

[0143] - Reduction and / or avoidance of electric heating for valves / throttle valves,

[0144] - Cost reduction (design),

[0145] - Energy reduction (operation) etc.

[0146] Critical valves / throttle valves for a freeze start are referred to below as freeze-critical or freeze-start critical valves kV.

[0147] Advantageously, it is possible to determine, depending on the topology and also the design, which valves may be critical to freezing or freezing start and which may not.

[0148] Exemplary topologies are shown in Figs. 1 and 2.

[0149] Examples of valves that are critical to freezing or freezing start include:

[0150] D In exhaust path 12:

[0151] - Pressure regulating valve CVexh,

[0152] - ByTurb turbine bypass valves,

[0153] - shut-off valve near the stack, possibly with control function CVout.

[0154] D In the bypass path:

[0155] - Stack bypass valve ByCath.

[0156] D In the EGR path:

[0157] - EGR valve.

[0158] D In supply air path 11 :

[0159] - stack-near shut-off valve, possibly also with control function, CVin,

[0160] - Humidifier bypass valve ByHum. Valves in exhaust path 12 can be the most critical due to the humid air (and partly droplet-laden air).

[0161] However, valves can also experience problems due to condensation effects if the stack was shut down in a very humid, warm condition and then cools down to low temperatures.

[0162] One measure for critical kV valves could be to equip them with heaters.

[0163] This is particularly relevant when a valve must move during a freeze start, e.g., a CVout shut-off valve located near the stack (under start conditions with an inerted cathode path but no inerted anode path). Therefore, such a CVout valve will preferably be equipped with a heater.

[0164] Valves with a heating element do not necessarily have to be designated as freezing-critical or freezing-start-critical valves kV.

[0165] A CVout shut-off valve located near the stack can freeze again after being opened, so it could become a freezing-critical or freezing-start-critical valve again during operation if, for example, it has not been used for control purposes for a period of time.

[0166] Furthermore, it may be provided that when performing (24) the freeze start procedure without the use of freeze-critical valves kV, at least one of the following actions is carried out:

[0167] (21) Inspection and / or diagnosis of sensors, in particular in the air system 10,

[0168] (23) Use of sensors for regulation 23a and / or for control (23b) depending on the verification 21 ,

[0169] (24a) Adapting the start-up procedure without using freeze-critical valves kV, without using non-functional sensors. This allows the current operational readiness of sensors to be taken into account in the procedure.

[0170] As shown in Fig. 3, when a start (4) of the system 100 is initiated, it is possible to determine which start conditions are present and which start procedure (6) is carried out.

[0171] As shown in Fig. 3, if there is no freeze start (trigger (8) = no), the freeze-critical valves kV (which may advantageously have been moved to a specific freeze-start-capable position during the last shutdown (60)) can be released (13) and moved to the "non-freeze start position" or directly switched to the control mode (14), such as cold start, warm start, normal operation, etc. The air system 10 can then use all actuators, including the freeze-critical valves kV, as degrees of freedom for control.

[0172] As shown in Fig. 3, when a freeze start is initiated by means of trigger (8), the freeze-critical valves kV are not released for use (20) or are only released when a clear “ok function” is reported by means of diagnostics (26).

[0173] As shown in Fig. 3, the freeze start procedure (24) is then carried out using only one or more air compressor unit(s) Comp and the enabled valves (non-freeze-critical valves and, if necessary, freeze-critical valves kV enabled via diagnostics (26)).

[0174] For example, it may be that only one stack bypass valve ByCath is enabled, so that the air system 10 for the freezing start is controlled by means of air compressor unit Comp and stack bypass valve ByCath.

[0175] As shown in Fig. 3, it can be provided that, if sensors are frozen and / or implausible, the air system control can switch from a regulated operation (23a) with limited actuator functionality to a controlled operation (23b) with limited actuator functionality and limited sensor functionality. As shown in Fig. 3, the freeze-critical valves kV are only released when the trigger (30) is fulfilled.

[0176] - The trigger (30) can, for example, include a temperature threshold. For instance, it can be used to check whether the exhaust gas temperature exceeds 15 °C for 2 minutes. This would ensure that no valves in exhaust path 12 are blocked by ice.

[0177] - The threshold values ​​can also refer to heating times of the valves, integrated enthalpy of the exhaust gas mass flow, or combinations thereof.

[0178] - The threshold values ​​can also be set individually for each valve (since the installation position and warm-up conditions can vary greatly between different valves).

[0179] - The release (34) can be cascaded.

[0180] - Release (34) can be achieved using diagnoses (26).

[0181] - The release (34) can also only take place when the freeze start procedure (24) has already completed the freeze start and is in the warm-up phase,

[0182] - If all freezing-critical valves kV are released, then the starting procedure can (possibly depending on the time) switch to normal operation or to the “standard warm-up” (e.g. for cold start above 0 °C) (from (36) to (40) or from (36) to (14)).

[0183] - If the sensors are OK / plausible again, the control can also be switched back to a regulation mode (from (23b) to (23a)).

[0184] If system 100 is switched off (e.g., by standby or by switching off (50) the vehicle), then a process step (60) can be performed. In this step, valves can be moved to a freeze-start-capable position (advantageously independent of when and which start occurs subsequently). Alternatively to switching off (50), the process step (60) can also be performed by a wake-up at a temperature threshold, e.g., below 4 °C and possibly a negative temperature gradient (i.e., a falling temperature).

[0185] The dashed lines in Fig. 3 may take some time.

[0186] In the example of system topology according to Fig. 1, the following valves can be determined / declared as freeze-critical valves kV:

[0187] - in particular pressure regulating valve CVexh and shut-off valve CVout at the output from the stack 101 (where CVout can only refreeze after being opened for the freeze start),

[0188] - secondly also humidifier bypass valve ByHum, stack bypass valve ByCath and shut-off valve CVin at the entrance to the Stack 101.

[0189] Positions of valves in process step (60):

[0190] - Shut-off valves CVin, CVout are closed (if an inerted cathode path without an inerted anode path is present during shutdown).

[0191] Special case: If a stack 101 is completely filled with air (anode and cathode previously inerted, special topology), then shut-off valves CVin, CVout can also be open during the standstill phase.

[0192] - Stack bypass valve ByCath in partial position,

[0193] - Pressure regulating valve CVexh open, partially open, preferably mostly open,

[0194] - Humidifier bypass valve ByHum open, partially open, preferably mostly open.

[0195] Positions of valves at the beginning of the freeze start procedure (at least up to step (34)):

[0196] - Shut-off valves CVin and CVout are open,

[0197] - Stack bypass valve ByCath in partial position (as defined in process step (60)), pressure control valve CVexh open, partially open, preferably mostly open

[0198] (as defined in procedure step (60)),

[0199] Humidifier bypass valve ByHum open (as defined by 60)

[0200] At the beginning of the freeze-start process, the mass flow is initially regulated only using the air compressor unit Comp.

[0201] Only when it has been verified that further valves are operational or functioning (preferably via diagnostics (26)) or by warming / defrosting) are further valves enabled for control. Enabling can also be done on a valve-by-valve basis (e.g., cascaded). For example, the air system 10 can also control the freeze start with the Comp air compressor unit and the ByCath.

[0202] Position of the valves / throttle valves in normal operation (40) or transitional operation (36) or a non-freeze start procedure (14) (e.g. cold start / warm-up / warm start):

[0203] - Shut-off valves CVin, CVout are open or additionally / optionally in control mode (especially shut-off valve CVout at the output from stack 101),

[0204] - Stack bypass valve ByCath in control mode,

[0205] - Pressure control valve CVexh in control mode,

[0206] - Humidifier bypass valve ByHum in control mode.

[0207] In the example of system topology according to Fig. 2, the following valves can be determined / declared as freeze-critical valves kV:

[0208] - in particular turbine bypass valves ByT1 , ByT2 and shut-off valves CVoutl , CVout2 at the output from the stacks 101 (whereby the shut-off valves CVoutl , CVout2 can only refreeze after being opened),

[0209] - secondarily also shut-off valves CVinl , CVin2, Stack- Bypass valve ByCath, EGR valves EGR.

[0210] Positions of valves in process step (60): - Shut-off valves CVinl , CVin2, CVoutl , CVout2 are closed (when there is an inerted cathode path without an inerted anode path when shutting down)

[0211] Special case: If stacks 101 are completely filled with air (anode and cathode previously inerted, special topology), then shut-off valves CVinl , CVin2, CVoutl , CVout2 can also be open during the standstill phase.

[0212] - Stack bypass valve ByCath in partial position,

[0213] - Pressure regulating valve CVexh open, partially open, preferably mostly open,

[0214] - Turbine bypass valves ByT1, ByT2 open, partially open, preferably mostly open,

[0215] - EGR valve EGR minimally / slightly open (not completely closed, allowing some warm air to flow through, but this does not significantly affect the freezing start)

[0216] Positions of valves at the beginning of the freeze start procedure (at least up to step (34)):

[0217] - Shut-off valves CVinl, CVin2, CVoutl, CVout2 are open

[0218] - Stack bypass valve ByCath in partial position (as defined in procedure step (60))

[0219] - Pressure regulating valve CVexh open, partially open, preferably mostly open (as defined in process step (60)),

[0220] - Turbine bypass valves ByT1 , ByT2 open, partially open, preferably mostly open (as defined in process step (60)),

[0221] - EGR valve EGR minimally / slightly open (as defined in procedure step (60)).

[0222] In the case of a freezing start, the mass flow is initially regulated using only one and / or both air compressor units Compl , Comp2.

[0223] Only when it has been verified that further valves are operational or functioning (preferably via diagnostics (26)) or by warming / defrosting) are further valves enabled for control. Enabling can also be done individually for each valve (e.g., cascaded). For example, the air system 10 can also control the freeze start with the air compressor unit Comp and the ByCath. Position of valves / throttles in normal operation (40) or transitional operation (36) or a non-freeze start procedure (14) (e.g., cold start / warm-up / warm start):

[0224] - Shut-off valves CVinl , CVin2, CVoutl , CVout2 are open or additionally in control mode (especially CVout),

[0225] - Stack bypass valve ByCath in control mode,

[0226] - Pressure control valve CVexh in control mode,

[0227] - Turbine bypass valves ByT1 , ByT2 in control mode (with the aim of closing as much as possible to increase or even maximize energy utilization via the turbine(s) Turbl , Turb2),

[0228] - EGR valve: EGR in control mode (when EGR is requested) or closed (when EGR is not requested)

[0229] A corresponding computer program product, a corresponding control unit ECU and a corresponding fuel cell system 100 with a corresponding control unit ECU represent further aspects of the invention.

[0230] The preceding explanation of the embodiments describes the present invention solely by way of examples. Naturally, individual features of the embodiments can be freely combined with one another, provided this is technically feasible, without departing from the scope of the present invention.

Claims

Claims 1. Method for operating a fuel cell system (100) comprising at least one fuel cell stack (101), wherein the at least one fuel cell stack (101) has a corresponding air system (10), comprising: (4) Capture a start request of the fuel cell system (100), (8) Check if a freeze start is required, if so, then: (20) Restricting a freeze-start procedure to control modes for the air system (10) without using freeze-critical valves (kV) for control, (24) Performing the freeze-start procedure without the use of freeze-critical valves (kV) for control, (30) Check the operational readiness of freeze-critical valves (kV), if so: (34) Releasing freeze-critical valves (kV) for control depending on their operational readiness, (36) Continue the starting procedure with the valves released (kV).

2. Method according to the preceding claim, wherein at least one action is performed to check (8) whether a freeze start is required: (6) Determining starting conditions and / or a suitable starting procedure.

3. Method according to any of the preceding claims, wherein if checking (8) whether a freeze start is required yields a no, then: (10) Release all necessary valves, (14) Carry out the starting procedure with all required valves.

4. Method according to one of the preceding claims, wherein the checking (30) of the operational readiness of freeze-critical valves (kV) is carried out until the operational readiness of all freeze-critical valves (kV) is verified.

5. Method according to one of the preceding claims, wherein when checking (30) the operational readiness of freeze-critical valves (kV): - a temperature threshold, e.g. of an exhaust gas, - a warm-up time of freeze-critical valves (kV) and / or - an energy input threshold, e.g. through exhaust gas, is checked / will be checked.

6. Method according to one of the preceding claims, wherein at least one action is performed to check (30) the operational readiness of freeze-critical valves (kV): (26) Inspection and / or diagnosis of freeze-critical valves (kV).

7. Method according to any of the preceding claims, wherein the checking (30) of the operational readiness of freeze-critical valves (kV) is carried out individually for different valves, and / or wherein the release (34) of freeze-critical valves (kV) is carried out individually, cascaded and / or in a specific sequence, and / or wherein, during the execution (24) of the freeze start procedure, valves can change their status from non-freeze-critical valves to freeze-critical valves (kV) and / or vice versa.

8. Method according to any of the preceding claims, wherein, when checking (30) the operational readiness of freeze-critical valves (kV), all or some freeze-critical valves (kV) are verified as not operational, then the execution (24) of the freeze-start procedure is continued without the use of affected freeze-critical valves (kV).

9. Method according to any of the preceding claims, wherein, when checking (30) the operational readiness of freeze-critical valves (kV) all freeze-critical valves (kV) are verified as operational, then at least one of the following actions is performed: (14) Continue the starting procedure with all required valves (kV) or (40) Performing normal operation with all required valves (kV).

10. Method according to one of the preceding claims, wherein, during or after a shutdown (50) of the fuel cell system (100), e.g. by a wake-up function, the freezing-critical valves (kV) are moved into freeze-start capable positions.

11. Method according to any of the preceding claims, wherein when performing (24) the freeze-start method without the use of freeze-critical valves (kV), at least one of the following actions is performed: (21) Checking and / or diagnosing sensors, especially in the air system (10), (23) Use of sensors for regulation (23a) and / or control (23b) depending on the verification (21), (24a) Adapting the starting procedure without the use of freeze-critical valves (kV) and without the use of non-functional sensors.

12. Computer program product comprising instructions which, when executed by a computer, cause the computer to perform a method according to any one of the preceding method claims 1 to 11.

13. Electronic control unit (ECU), comprising a processing unit and a storage unit in which a code is stored which, at least Partial execution by the computing unit performs a method according to one of the preceding method claims 1 to 11 14. Fuel cell system (100) comprising a control unit (ECU) according to the preceding claim.

Images