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) with at least one fuel cell stack (101), wherein the fuel cell system (100) comprises an air system (10) for supplying the at least one fuel cell stack (101) with an oxygen-containing reactant, wherein the air system (10) comprises a first air compressor (11) and a second air compressor (12), wherein the method serves to provide a multi-target operating strategy for operating the fuel cell system (100) by means of load sharing between the first air compressor (11) and the second air compressor (12). Furthermore, the invention relates to a computer program product, a control unit (ECU) and a fuel cell system (100).

Description

[0001] 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. State of the art

[0002] In vehicles (Fuel Cell Vehicle or FCV for short) where propulsion energy is supplied (among other things) by one (or more) fuel cell system(s) (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.

[0003] Ambient air is supplied to a fuel cell stack via an air system comprising an air supply system and / or an air compression system. The air supply system provides a corresponding (variable) air mass flow and pressure level.

[0004] The air conveying system typically includes at least one thermal turbomachine (or in other words, an air compressor) which is driven by an electric motor (English: "Electric Driven Air Compressor" or EAC for short).

[0005] Optionally, in addition to air compression (single-stage or multi-stage), energy recovery from outgoing moist air can be implemented using a turbine.

[0006] To obtain higher system pressures, e.g. for high-performance FCS systems, two-stage compression and energy recuperation via turbine (EACT) can be used.

[0007] Some systems may have two electrically driven air compressors (EACs), with at least one of the two air compressors optionally also being turbine-driven. Disclosure of the invention

[0008] 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.

[0009] According to the first aspect, the present invention provides: a method for operating a fuel cell system (hereinafter referred to simply as the system) with at least one (or more) fuel cell stack(s) (hereinafter referred to simply as the stack).

[0010] The fuel cell system includes an air system for supplying at least one fuel cell stack with an oxygen-containing reactant.

[0011] The air system preferably has a first (especially electrically driven) air compressor and a second (especially electrically driven) air compressor.

[0012] The method was developed to provide a multi-target operating strategy for operating the fuel cell system by means of load sharing between the first air compressor and the second air compressor.

[0013] The procedure comprises the following steps: (110) Determining a reference variable for controlling the load distribution between the first air compressor and the second air compressor, (120) Determining at least one balancing mode with at least one target variable for operating the fuel cell system, in particular mapping the target variable as a function of the reference variable, (130) Performing a load distribution between the first air compressor and the second air compressor according to a specified value of the reference variable for a desired value and / or a desired range of values ​​of the at least one target variable.

[0014] 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.

[0015] Preferably, the fuel cell system can comprise several modules in the form of individual stacks and the associated functional systems.

[0016] The air system can have two air compressors without turbine assistance.

[0017] The air system can have two air compressors, at least one of which can be turbine-driven.

[0018] Multiple stacks can be supplied by a common air system.

[0019] A common air system can have two air compressors without turbine assistance.

[0020] A common air system can also have two air compressors, at least one or both of which can be turbine-driven.

[0021] The fuel cell system can be used for mobile applications, e.g. in vehicles (FCV, short for "Fuel Cell Vehicle"), where the drive 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.

[0022] The fuel cell system can also be used for stationary applications, e.g. in generators.

[0023] The invention recognizes that the air system must cover a large operating range under various conditions, such as: Operating range 1. Idle, low load, partial load, medium load, high load, maximum load, Operating area 2. Ambient pressures, altitude travel, Operating area 3. Ambient temperatures, e.g., driving in hot climates, freezing starts, cold starts, etc., and ambient humidity, Operating area 4. Supply of one or more stacks, Operating area 5. Operation with / without exhaust gas recirculation (EGR) etc.

[0024] There are several operational objectives for the air system, such as: Operational objective 1. Meet stack requirements (pressure, mass flow, temperature, humidity, etc.), Operational objective 2: Meet the dilution requirements of purge / drain, Operational objective 3: Minimal electrical power consumption (optimization of consumption), Operational objective 4. Enable functionalities (e.g. EGR functionality), Operational objective 5. Ensure component protection (e.g., adhere to permissible temperature limits), etc.

[0025] The invention addresses a method for ensuring the multiple operating objectives for systems with two electric motor-driven air compressors for supplying one or more stacks.

[0026] The method can be advantageously used to adapt a load distribution (which can also be referred to as balancing) between: - a first air compressor (with / without turbine) and / or - a second air compressor (with / without turbine) to ensure multiple operational objectives at - variable environmental conditions, - different operating conditions, - Operation with / without EGR and switching (EGR enable / EGR disable) etc.

[0027] The idea utilizes the additional degree of freedom provided by the different load distribution through the two electrically driven air compressors between: - Compaction stage 1 (compaction ratio pi1) and / or - Compression level 2 (compression ratio pi2) to achieve several different operating objectives (a multi-objective operating strategy) in parallel / simultaneously / sequentially and to adapt them depending on the situation.

[0028] Optionally, a switching mode can be provided for cases where multiple operating goals cannot be achieved in parallel / simultaneously.

[0029] The idea utilizes the insight that the two electrically driven air compressors create a degree of freedom for varying the load distribution between the first compression stage (compression ratio pi1) and the second compression stage (compression ratio pi2). The overall compression ratio of the two-stage air compression can be expressed as follows: piGes=pi1*pi2.

[0030] Different load distribution allows for the addressing of various operational goals.

[0031] These can be performed adaptively depending on the boundary conditions, the system operating modes, the requested stack operating points, the aging states, existing limitations, errors, etc.

[0032] The procedure can take multiple objectives into account and determine a load distribution that is advantageous for multiple objectives.

[0033] Several advantages can be achieved using this method: - An operating strategy that can achieve / serve multiple goals through balancing, - H2 consumption optimization, - Adaptation of load distribution / balancing depending on operating modes, boundary conditions, limitations, aging states, etc. - Switching load sharing with / without EGR use and ensuring EGR release, etc.

[0034] Furthermore, it may be provided that the reference value is determined as a function of the compression ratio of the first air compressor and / or the compression ratio of the second air compressor. It may also be provided that the reference value is determined as a function of the combined compression ratio of the first and second air compressors.

[0035] For example, the reference variable for controlling the load distribution can be expressed as follows: ratBal=(pi1−1) / (piGes−1), with total compression ratio of the compression piGes=pi1*pi2.

[0036] This size has advantages because the optimum electrical power consumption is approximately independent or not very sensitive to mass flow (through the compressors) and the pressure level to be achieved.

[0037] Alternatively, other load distribution ratios can be used (e.g., electrical power ratio Pel1 / PelGes).

[0038] The individual target variables, such as temperatures, pressure differences, electrical power, humidity, activities, heat dissipation, mass flows, lambda, gradients of the variables, etc., can be stored as characteristic curves / maps / models depending on this factor / reference variable, so that simplified control unit-compatible or real-time capable control / regulation is possible.

[0039] The process can also be model-based (e.g., using MPC = model predictive control). For this purpose, the operational objectives can be formulated as a cost function or a performance function.

[0040] Furthermore, an operating point-dependent, real-time optimization can be performed.

[0041] The at least one balancing mode can include: (B1) Temperature-controlled balancing.

[0042] The at least one target variable can include at least one of the following fluid temperatures: - a fluid temperature at the inlet and / or outlet of a cathode path of the at least one fuel cell stack and / or - at least one inlet temperature at the inlet to or outlet from at least one component of the air system, such as a turbine, the first compressor, the second compressor and / or a sensor.

[0043] Advantageously, temperature-controlled balancing can be used to maintain desired values ​​of at least one target parameter.

[0044] Temperature-controlled balancing can directly influence the fluid temperatures in the cathode path.

[0045] In principle, other fluids can also be affected by couplings to the supply air path, cathode path and / or exhaust air path.

[0046] Indirectly, the coolant in a heat exchanger (intercooler) and / or the anode fluid in the anode path can also be affected - provided that a heat transfer between the air path and the anode path is installed.

[0047] Additionally, the temperature-based balancing can also be mapped / matched to a waste heat balancing. This means that if waste heat is dissipated after each compressor, the waste heat distribution can be changed.

[0048] The at least one balancing mode can include: (B2) Consumption-based balancing, e.g. without exhaust gas recirculation.

[0049] The at least one target variable can include the total electrical power (PelGes) of the first air compressor and the second air compressor.

[0050] Advantageously, consumption-based balancing can serve to reduce and / or minimize the total electrical power (PelGes) of the first air compressor and the second air compressor.

[0051] The at least one balancing mode can include: (B3) Pressure differential-determined balancing, e.g. for enabling exhaust gas recirculation.

[0052] The at least one target variable can include a pressure difference across an EGR pathway.

[0053] Advantageously, pressure differential-determined balancing can serve to provide a sufficient pressure gradient for a directed flow to provide exhaust gas recirculation.

[0054] The at least one balancing mode can include: (B4) Consumption-based balancing, e.g. with exhaust gas recirculation.

[0055] The at least one target variable can include the total electrical power PelGes of the first air compressor and the second air compressor.

[0056] Advantageously, consumption-based balancing can serve to reduce and / or minimize the total electrical power PelGes of the first air compressor and the second air compressor.

[0057] The at least one balancing mode can include: (B5) Map-limiting balancing for component protection.

[0058] Advantageously, map-limited balancing can be used to comply with pumping limits, stalling limits, speed limits and / or bearing force limits.

[0059] The at least one balancing mode can include: (B6) Temperature-limiting balancing for component protection.

[0060] Advantageously, temperature-limiting balancing can serve to protect components by ensuring compliance with temperature limits.

[0061] Furthermore, several target variables can be taken into account when carrying out the procedure, especially simultaneously.

[0062] Furthermore, multiple operational objectives can be considered when carrying out the procedure, particularly with a specific weighting and / or prioritization of corresponding target variables. An example of superimposing multiple operational objectives for balancing (step 120) could include the operational objectives "power / consumption" and "temperature limit - component protection." If, for example, both operational objectives cannot be achieved exactly, then "power / consumption" can be determined as the "softer" operational objective compared to "component protection." In this case, the total electrical power can be increased to comply with temperature limits for component protection.

[0063] Furthermore, the process allows switching between multiple balancing modes to accommodate different target variables, particularly sequential ones. This can be advantageous when multiple operational objectives require differing balancing parameters.

[0064] 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.

[0065] 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.

[0066] 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. Preferred embodiments:

[0067] The invention, its further developments, and its advantages are explained in more detail below with reference to the drawings. Each drawing schematically shows: Fig. 1 to 4 exemplary designs of a fuel cell system, Fig. 5 an exemplary setup of a fuel cell system, Fig. 6. An exemplary sequence of a procedure and Fig. 7 to 10 exemplary balancing modes.

[0068] In the different figures, identical parts of the invention are always provided with the same reference numerals, which is why they are usually only described once.

[0069] The Fig. Items 1 to 10 serve to explain the invention idea, which proposes: a method for operating a fuel cell system 100 (hereinafter referred to as system 100) with at least one (or more) fuel cell stack(s) 101 (hereinafter referred to as stack(s) 101).

[0070] As it is Fig. As illustrated in Figures 1 to 5, the fuel cell system 100 has an air system 10 (which can also be referred to as a cathode system) for supplying the at least one fuel cell stack 101 with an oxygen-containing reactant.

[0071] As it is Fig. As illustrated in Figures 1 to 5, the air system 10 has an air conveying system which includes a first (preferably electrically operated) air compressor 11 and a second (preferably electrically operated) air compressor 12.

[0072] The air compressors 11, 12 represent thermal turbomachines and can be driven by electric motors (eng. “Electric Driven Air Compressor” or EAC for short).

[0073] As it is Fig. As indicated in points 1 to 5, energy recovery from outgoing, moist air can optionally be implemented using a turbine (English: "Electric Driven Air Turbine Compressor" or EACT for short).

[0074] Some systems 100 can have two electrically driven air compressors 11, 12 (or EAC for short), with at least one of the two air compressors optionally being turbine-driven (EACT).

[0075] The method was developed to provide a multi-target operating strategy for operating the fuel cell system 100 by means of load sharing between the first air compressor 11 and the second air compressor 12.

[0076] As it is Fig. As illustrated in section 6, the procedure has the following procedural step: 110 Determining a reference variable ratBal for controlling the load distribution between the first air compressor 11 and the second air compressor 12.

[0077] In the operation of the air system 10 including the air compression system, the load distribution (also called balancing) between the first air compressor 11 and the second air compressor 12 represents a variable degree of freedom.

[0078] In order to achieve the necessary or requested total compression ratio piGes=pi1*pi2, it is possible to derive different pairs of values ​​for the requirements pi1 and pi2.

[0079] This degree of freedom can be used advantageously to provide a multi-target operating strategy for operating the fuel cell system 100.

[0080] As it is Fig. As illustrated in section 6, the procedure has the following procedural step: 120 Determining at least one balancing mode B1, B2, B3, B4, B5, B6 with at least one target variable p, dm / dt, T, x, dp, P, Q for operating the fuel cell system 100.

[0081] The target variables p, dm / dt, T, x, dp, P, Q can be represented as a function of the reference variable ratBal (e.g. using characteristic curves / maps / models).

[0082] Depending on operating modes, operating conditions (including stack requirements, dilution requirements, environmental conditions, and component limitations), the following can be derived: ◯ which operational objectives the load balancing currently needs to take into account, ◯ how the load distribution / balancing should be carried out and ◯ in which area load sharing is currently possible.

[0083] Performing load splitting can be done both in real time and predictively.

[0084] As it is Fig. As illustrated in section 6, the procedure has the following procedural step: 130 Performing a load distribution between the first air compressor 11 and the second air compressor 12 according to a specific value of the reference variable ratBal for a desired value and / or a desired range of values ​​of at least one target variable p, dm / dt, T, x, dp, P, Q.

[0085] The invention recognizes that the air system 10 must cover a large operating range under various conditions, such as: Operating range 1. Idle, low load, partial load, medium load, high load, maximum load, Operating area 2. Ambient pressures, altitude travel, Operating area 3. Ambient temperatures, e.g., driving in hot climates, freezing starts, cold starts, etc., and ambient humidity, Operating area 4. Supply of one or more stacks, Operating area 5. Operation with / without exhaust gas recirculation (EGR) etc.

[0086] There are several operational objectives for air system 10, such as: Operational objective 1. Meet stack requirements (pressure p, mass flow rate dm / dt, temperature T, humidity x, etc.), Operational objective 2: Meet the dilution requirements of purge / drain, Operational objective 3: Minimal electrical power consumption (optimization of consumption), Operational objective 4. Enable functionalities (e.g. EGR functionality), Operational objective 5. Ensure component protection (e.g., adhere to permissible temperature limits, speeds, forces, etc.) etc.

[0087] The method can therefore be advantageously used to adapt a load distribution (so-called balancing) between: - the first air compressor 11 (with / without turbine) and / or - the second air compressor 12 (with / without turbine) to ensure several operating objectives at - variable environmental conditions, - different operating conditions, - Operation with / without EGR and switching (EGR enable / EGR disable) etc.

[0088] The idea utilizes the additional degree of freedom of the different load distribution provided by the two electrically driven air compressors 11, 12 between: - Compaction stage 1 (compaction ratio pi1) and / or - Compression level 2 (compression ratio pi2) to achieve several different operating objectives (a multi-objective operating strategy) in parallel / simultaneously / sequentially and to adapt them depending on the situation.

[0089] Optionally, a switching mode 145 can be provided in the event that multiple operating goals cannot be achieved in parallel / simultaneously.

[0090] The method can take multiple operational objectives into account and determine a load distribution that is advantageous for multiple operational objectives.

[0091] For example, the reference variable ratBal for controlling the load distribution can be determined as follows: ratBal=pi1−1 / piGes−1, with total compression ratio of the compression piGes=pi1*pi2.

[0092] This size has advantages, as the optimum electrical power consumption is approximately independent or not very sensitive to the mass flow through the compressors and the pressure level to be achieved.

[0093] Alternatively, other load distribution ratios can be used (e.g., electrical power ratio Pel1 / PelGes).

[0094] The individual target variables, such as temperatures, pressure differences, electrical power, humidity, activities, heat dissipation, mass flows, lambda, gradients of the variables, etc., can be stored as characteristic curves / maps / models depending on this factor / reference variable, so that simplified control unit-compatible or real-time capable control / regulation is possible.

[0095] The process can also be model-based (e.g., using MPC for "Model Predictive Control"). For this purpose, the operational objectives can be formulated as a cost function or a performance function.

[0096] Furthermore, an operating point-dependent, real-time optimization can be performed.

[0097] As it is Fig. As shown in section 6, at least one balancing mode can be included: B1, B2, B3, B4, B5, B6: B1 Temperature-controlled balancing.

[0098] The at least one target variable p, dm / dt, T, x, dp, P, Q can include at least one of the following fluid temperatures: - a fluid temperature at the inlet and / or outlet of a cathode path of at least one fuel cell stack 101 and / or - at least one inlet temperature at the inlet to or outlet from at least one component of the air system 10, such as a turbine, the first compressor, the second compressor and / or a sensor.

[0099] Advantageously, temperature-controlled balancing B1 can be used to maintain desired values ​​of at least one target variable p, dm / dt, T, x, dp, P, Q.

[0100] Temperature-controlled balancing can directly influence the fluid temperatures in the cathode path.

[0101] In principle, other fluids can also be affected by couplings to the supply air path, cathode path and / or exhaust air path.

[0102] Indirectly, the coolant in a heat exchanger (intercooler) or the anode fluid in the anode path can also be affected - provided that a heat transfer between the air path and the anode path is installed.

[0103] Additionally, the temperature-based balancing can also be mapped / matched to a waste heat balancing. This means that if waste heat is dissipated after compressors 11 and 12, the waste heat distribution can be changed.

[0104] It is conceivable that the fluid temperature at the inlet to the at least one fuel cell stack 101 can be given priority.

[0105] Air that is too hot must not be introduced into the at least one fuel cell stack 101.

[0106] The Fig. Figure 7 shows, by way of example, the behavior of temperatures T at different locations in the system 100 and the balancing ranges derived from them.

[0107] For example: C1 = Temperature at the outlet of the first air compressor 11, C2 = Temperature at the outlet of the second air compressor 12, T1 = Temperature at the inlet of a turbine and / or Stk in = input temperature into the stack cathode path.

[0108] Temperatures at other locations can also be taken into account (both in the fluid and on / in components).

[0109] From the Fig. 7. The following example 1 can be derived: In order to maintain a temperature T at T1 of approximately 135 °C limit, the balancing can have a value of ratBal > 0.4.

[0110] From the Fig. 7. The following example 2 can be derived: In order to maintain a temperature T at the stack inlet of approximately 100 °C, the balancing can have a value of ratBal > 0.53.

[0111] According to the Fig. 7. An upper limit Tmax can be specified for a temperature, for example, a temperature C1 at the outlet of the first air compressor 11 and / or a temperature C2 at the outlet of the second air compressor 12, e.g., a maximum of 180 °C. To comply with this, the balancing in the example has a range between ratBal < 0.75 (from C1) and ratBal > 0.38 (from C2).

[0112] As it is Fig. As shown in section 6, at least one balancing mode can be included: B1, B2, B3, B4, B5, B6: B2 Consumption-based balancing, e.g. without exhaust gas recirculation (EGR).

[0113] The at least one target variable p, dm / dt, T, x, dp, P, Q can include a total electrical power PelGes of the first air compressor 11 and the second air compressor 12.

[0114] Advantageously, the consumption-based balancing B2 can be used to reduce and / or minimize the total electrical power (PelGes) of the first air compressor 11 and the second air compressor 12.

[0115] Fig. 8: shows a consumption-based balancing B2 (without EGR).

[0116] The target variable (y-axis) can be the total electrical drive power PelGes of the two air compressors 11, 12. Electrical power P in kW can be identified on the y-axis.

[0117] As it is Fig. As indicated in section 8, this target value can be minimized. This also allows for a reduction or optimization of H2 consumption.

[0118] It will be according to Fig. 8. The optimum is sought, but other operational objectives can also be taken into account, as the Fig. 10 suggests.

[0119] Furthermore, a range around the optimum can also be defined as a value range (see shading in the Fig. 8).

[0120] Fig. Figure 10 shows an example of an overlap of multiple balancing objectives (step 120). In this example, two operational objectives, "minimum performance" (balancing mode B2) and "temperature limit compliance" (balancing mode B1), cannot be exactly achieved.

[0121] The "softer" operating goal in this case might be "power / consumption" compared to "component protection." Therefore, the marked operating point can be selected for balancing. While the electrical power P is not optimal, the power increase is moderate because the optimum is quite "shallow."

[0122] Alternatively, switching mode 145 could also be used, due to thermal inertia.

[0123] As it is Fig. As shown in section 6, at least one balancing mode can be included: B1, B2, B3, B4, B5, B6: B3 Pressure differential-determined balancing, e.g. for enabling exhaust gas recirculation (EGR).

[0124] The at least one target variable p, dm / dt, T, x, dp, P, Q can include a pressure difference dp_EGR across an EGR path.

[0125] Advantageously, the pressure differential-determined balancing B3 can serve to provide a sufficient pressure gradient for a directed flow to provide exhaust gas recirculation (EGR).

[0126] Fig. Figure 9 shows a pressure difference-determined balancing B3.

[0127] The target variable (y-axis) can be the pressure difference dp between an exhaust air path at an EGR branch point and an intake air path at an EGR inlet point.

[0128] To enable EGR, a sufficient pressure differential dp_EGR > dpThreshold must be present for it to function. This allows for directed flow in the EGR path (from the exhaust gas path towards the intake air path).

[0129] From the Fig. 9. The following example 3 can be derived: If, for example, dpThreshold = 250 mbar, then the balancing can have a value of ratBal < 0.48 in order to trigger an EGR release.

[0130] As it is Fig. As indicated in 6, it can include at least one balancing mode: B1, B2, B3, B4, B5, B6: B4 Consumption-based balancing, e.g. with exhaust gas recirculation (EGR).

[0131] The at least one target variable p, dm / dt, T, x, dp, P, Q can include a total electrical power PelGes of the first air compressor 11 and the second air compressor 12.

[0132] Advantageously, the consumption-based balancing B4 can be used to reduce and / or minimize the total electrical power PelGes of the first air compressor 11 and the second air compressor 12.

[0133] If EGR is activated, the fuel consumption-optimal operating points shift compared to B2. Therefore, switching to a balancing operating strategy B4 with EGR and a correspondingly adapted operating range can be advantageous here.

[0134] As it is Fig. As indicated in 6, it can include at least one balancing mode: B1, B2, B3, B4, B5, B6: B5 Map-limiting balancing for component protection.

[0135] Advantageously, the map-limited balancing B5 can be used to comply with pumping limits, stalling limits, speed limits and / or bearing force limits.

[0136] If, for example, one of the two air compressors 11, 12 reaches the surge limit (or the choking limit), then balancing can preferably be used to ensure component protection with regard to compressor pumps.

[0137] Balancing can also be advantageously used with regard to achieving maximum rotational speeds for rotors / shafts / bearings and / or bearing forces / bearing limitations. The ratio ratBal can be adjusted so that both air compressors 11, 12 operate within a defined characteristic curve or within the defined limits.

[0138] As it is Fig. As indicated in 6, it can include at least one balancing mode: B1, B2, B3, B4, B5, B6: B6 Temperature-limiting balancing for component protection.

[0139] Advantageously, the temperature-limiting balancing for component protection B6 can be used to comply with temperature limits.

[0140] If temperatures within a component reach a limit, balancing can mitigate the situation. Examples include temperatures in the inverter or the electric motor. For instance, if the stator temperature (electric motor) of the first air compressor 11 reaches its limit, balancing allows the second air compressor 12 to operate at a higher load.

[0141] The balancing can be adjusted in step 130: Step 132: The first air compressor 11 can be operated at a higher load, the second air compressor 12 can be operated at a lower load. Substep 134: The second air compressor 12 can be operated at a higher load, the first air compressor 11 can be operated at a lower load. Substep 136: The balancing can be maintained.

[0142] In step 140, operation can continue with adapted balancing, as implemented in step 130, until the control unit ECU performs the next adjustment.

[0143] The calculation step size in the control unit can be relatively small, e.g. 10 ms. Consequently, the balancing can be adapted quickly to the current situation.

[0144] The relationships described above can be dependent on the operating point. Therefore, the balancing can be adjusted in real time or reactively. Furthermore, prediction can also take place, i.e., a calculation of the balancing for the next future states (e.g., MPC for "Model Predictive Control"). In step 145, a toggle mode can be provided.

[0145] If several operating objectives are mutually exclusive, a switching mode or toggle mode 145 can be implemented in some cases. This allows the balancing to be switched back and forth between different modes in order to achieve one operating objective for a period of time and another for a period of time. This can be particularly advantageous when inertia within the system 100 can be utilized.

[0146] Example 1: thermal inertia. Since temperatures do not change very quickly, a temperature limitation can be disregarded for a while until the temperature has risen accordingly.

[0147] Example 2: Humidification status. EGR can be switched off for a period of time if the humidification status is good, as the membrane in the stack has a certain water retention capacity / inertia. The use of switching mode 145 can preferably employ a predictive operating strategy.

[0148] Furthermore, when carrying out the procedure, several target variables p, dm / dt, T, x, dp, P, Q can be taken into account, in particular simultaneously or in advance.

[0149] 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.

[0150] 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

[1] Method for operating a fuel cell system (100) comprising at least one fuel cell stack (101), wherein the fuel cell system (100) comprises an air system (10) for supplying the at least one fuel cell stack (101) with an oxygen-containing reactant, wherein the air system (10) comprises a first air compressor (11) and a second air compressor (12), wherein the method serves to provide a multi-target operating strategy for operating the fuel cell system (100) by means of load sharing between the first air compressor (11) and the second air compressor (12), demonstrating the procedure: (110) Determining a reference variable (ratBal) for controlling the load distribution between the first air compressor (11) and the second air compressor (12), (120) Determining at least one balancing mode (B1, B2, B3, B4, B5, B6) with at least one target variable (p, dm / dt, T, x, dp, P, Q) for operating the fuel cell system (100), wherein in particular the target variable (p, dm / dt, T, x, dp, P, Q) is mapped as a function of the reference variable (ratBal), (130) Performing a load distribution between the first air compressor (11) and the second air compressor (12) according to a specified value of the reference variable (ratBal) for a desired value and / or a desired range of values ​​of at least one target variable (p, dm / dt, T, x, dp, P, Q). [2] Method according to claim 1, wherein the reference quantity (ratBal) is determined as a function of a compression ratio (pi1) of the first air compressor (11) and / or a compression ratio (pi2) of the second air compressor (12) and / or wherein the reference quantity (ratBal) is determined as a function of a total compression ratio (piGes) of the first air compressor (11) and the second air compressor (12). [3] Method according to claim 1 or 2, wherein the at least one balancing mode (B1, B2, B3, B4, B5, B6) comprises at least one of the following modes: (B1) Temperature-determined balancing, wherein in particular the at least one target variable (p, dm / dt, T, x, dp, P, Q) includes at least one of the following fluid temperatures: - a fluid temperature at the inlet and / or outlet to a cathode path of the at least one fuel cell stack (101) and / or - at least one inlet temperature at the inlet to or outlet from at least one component of the air system (10), such as a turbine, the first compressor, the second compressor and / or a sensor, wherein the temperature-determined balancing (B1) preferably serves to maintain desired values ​​of the at least one target variable (p, dm / dt, T, x, dp, P, Q). [4] Method according to the preceding claim, wherein the at least one balancing mode (B1, B2, B3, B4, B5, B6) comprises at least one of the following modes: (B2) Consumption-based balancing, e.g. without a functionality such as exhaust gas recirculation (EGR), wherein in particular the at least one target variable (p, dm / dt, T, x, dp, P, Q) comprises a total electrical power (PelGes) of the first air compressor (11) and the second air compressor (12), wherein preferably the consumption-based balancing (B2) serves to reduce and / or minimize the total electrical power (PelGes) of the first air compressor (11) and the second air compressor (12). [5] Method according to the preceding claim, wherein the at least one balancing mode (B1, B2, B3, B4, B5, B6) comprises at least one of the following modes: (B3) Pressure differential-determined balancing, e.g. for enabling exhaust gas recirculation (EGR), wherein in particular the at least one target variable (p, dm / dt, T, x, dp, P, Q) includes a pressure differential (dp_EGR) across an EGR path, wherein preferably the pressure differential-determined balancing (B3) serves to provide a sufficient pressure gradient for directed flow to enable exhaust gas recirculation (EGR). [6] Method according to any of the preceding claims, wherein the at least one balancing mode (B1, B2, B3, B4, B5, B6) comprises at least one of the following modes: (B4) Consumption-based balancing, e.g. with a functionality such as exhaust gas recirculation (EGR), wherein in particular the at least one target variable (p, dm / dt, T, x, dp, P, Q) includes a total electrical power (PelGes) of the first air compressor (11) and the second air compressor (12), wherein preferably the consumption-based balancing (B4) serves to reduce and / or minimize the total electrical power (PelGes) of the first air compressor (11) and the second air compressor (12). [7] Method according to any of the preceding claims, wherein the at least one balancing mode (B1, B2, B3, B4, B5, B6) comprises at least one of the following modes: (B5) Map-limiting balancing for component protection, wherein preferably the map-limited balancing (B5) serves to comply with pumping limits, stalling limits, speed limits and / or bearing force limits. [8] Method according to any of the preceding claims, wherein the at least one balancing mode (B1, B2, B3, B4, B5, B6) comprises at least one of the following modes: (B6) Temperature-limiting balancing for component protection, wherein the temperature-limiting balancing for component protection (B6) preferably serves to comply with temperature limits. [9] Method according to any one of the preceding claims, wherein several target variables (p, dm / dt, T, x, dp, P, Q) can be taken into account when carrying out the procedure, in particular simultaneously, and / or wherein several operational objectives can be taken into account when carrying out the procedure, in particular under a certain weighting and / or prioritization of corresponding target variables (p, dm / dt, T, x, dp, P, Q), and / or wherein, when performing the procedure, it is possible to switch between several balancing modes (B1, B2, B3, B4, B5, B6) in order to take different target variables (p, dm / dt, T, x, dp, P, Q), in particular sequentially, into account, and / or where the procedure is carried out in real time or predictively. [10] Computer program product comprising instructions which, when executed by a computer, cause the computer to perform a method according to any of the preceding method claims. [11] Electronic control unit (ECU) comprising a computing unit and a storage unit in which a code is stored which, when at least partially executed by the computing unit, performs a method according to any one of the preceding method claims 1 to 9. [12] Fuel cell system (100) comprising a control unit (ECU) according to the preceding claim.

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