Method for operating a fuel cell system. Fuel cell system

Abstract

The invention relates to a method for operating a fuel cell system (1) with several fuel cell stacks (2) which are supplied with air via an air system (3), wherein the air is supplied via a supply air path (4) of the air system (3), which branches into several individual supply air paths (5) to connect the individual fuel cell stacks (2), and wherein the air is compressed before entering a fuel cell stack (2) by means of at least one air compressor unit (6, 7). According to the invention, the air supplied to the fuel cell stacks (2) is selectively compressed either via a switchable air compressor unit (6) integrated into the supply air path (4) and / or individually for each stack, wherein air compressor units (7) arranged in the individual supply air paths (5) are used for stack-specific air compression. The invention further relates to a fuel cell system (1) with several fuel cell stacks (2) that is suitable for carrying out the method or can be operated according to the method.

Description

[0001] The invention relates to a method for operating a fuel cell system having the features of the preamble of claim 1. Furthermore, the invention relates to a fuel cell system suitable for carrying out the method or operable according to the method. The fuel cell system is designed as a multi-stack system.

[0002] The preferred field of application of the invention is mobile fuel cell systems or vehicles in which fuel cells combined to form fuel cell stacks, so-called “stacks”, are used to generate drive energy. State of the art

[0003] In vehicles that use fuel cells to generate propulsion energy, the oxidizing agent oxygen from the ambient air is typically used to react with hydrogen in the fuel cells to form water or water vapor, thereby generating electrical power through electrochemical conversion. To provide a variable air mass flow and a suitable pressure level, the ambient air is fed to the fuel cells using an air conveying and compression system. High-performance air conveying and compression systems are known from the state of the art for this purpose. These systems are typically designed in multiple stages, thus enabling higher system pressures than single-stage air compression systems.

[0004] In a multi-stage air conveying and compression system, several air compressor units are typically connected in series. Thermal turbomachines, particularly radial compressors, are typically used as air compressor units, driven by electric motors. For energy recovery, the air compressor unit can have a turbine, to which the exhaust air or exhaust air exiting the fuel cells is fed.

[0005] If multiple fuel cell stacks are operated at high pressure in a fuel cell system ("multi-stack system"), the stack efficiency and the system efficiency can be increased. However, this generally requires multi-stage air compression and at least one-stage energy recuperation using a turbine.

[0006] The present invention addresses the task of optimizing air compression in a multi-stack system with regard to energy efficiency and thus hydrogen consumption. Furthermore, degradation is to be minimized and reliability increased.

[0007] To achieve this object, the method having the features of claim 1 and the fuel cell system having the features of claim 7 are proposed. Advantageous developments of the invention can be found in the respective subclaims. Disclosure of the invention

[0008] A method is proposed for operating a fuel cell system with multiple fuel cell stacks supplied with air via an air system. The air is supplied via an air supply path of the air system, which branches into several individual air supply paths to connect the individual fuel cell stacks. Before entering a fuel cell stack, the air is compressed using at least one air compressor unit. According to the invention, the air supplied to the fuel cell stacks is compressed either via an air compressor unit integrated into the supply air path that can be switched on and off and / or individually for each stack, with air compressor units arranged in the individual air supply paths being used for stack-individual air compression.

[0009] The process requires a fuel cell system that has a centrally located air compressor unit and several decentrally located air compressor units, with the centrally located air compressor unit being switchable on and off. - only the centrally located air compressor unit, - the centrally arranged air compressor unit together with at least one decentrally arranged air compressor unit or - At least one decentralized air compressor unit is used for air compression without a centrally located air compressor unit. Air compression can therefore be carried out in one or more stages.

[0010] For single-stage air compression, the air compressor unit integrated into the supply air path or centrally located is shut down. To switch from single-stage air compression back to multi-stage air compression, the air compressor unit integrated into the supply air path or centrally located is switched back on. This enables a variety of different operating modes and thus optimized operation in terms of system efficiency and hydrogen consumption. Another variant for single-stage air compression is the operation of the centrally located air compressor unit with the decentralized or stack-individual air compressor units switched off.

[0011] For example, the air compressor unit integrated into the supply air path can be shut down if not all fuel cell stacks and / or all fuel cell stacks are to be operated at low pressure. For particularly low pressure requirements, only the air compressor unit integrated into the supply air path can be operated, for example: - in the start / stop mode of a vehicle or - in low-load operation of the fuel cell stack - in low-pressure operation of the fuel cell stack - when the fuel cell system starts freezing - in the drying mode of the fuel cell system - when the vehicle is stationary, if the air supplied via the air system is only required to dilute a purge quantity discharged from an anode subsystem during a purge process (“parking purge”).

[0012] Using only the air compressor unit integrated into the supply air path during a vehicle's start / stop operation also has the advantage that the air compressor units located in the individual supply air paths are subjected to less stress and thus age more slowly. Consequently, degradation of the air compressor units is minimized.

[0013] A further advantage is that greater reliability is achieved, since in the event of a failure of an air compressor unit arranged in an individual supply air path, the air supply can also be ensured centrally via the air compressor unit integrated in the supply air path.

[0014] Preferably, when the air compressor unit integrated into the supply air path is shut down, a bypass valve located in a compressor bypass path is opened to bypass the air compressor unit. With the bypass valve open and the central air compressor unit shut down, the air then flows past the central air compressor unit via the compressor bypass path. This bypass valve can also be used to provide surge protection when the central air compressor unit is operating, i.e., air can be recirculated in reverse flow from the outlet of the central air compressor unit to the intake area. This ensures surge protection when pressure requirements exceed the surge limit of the central air compressor unit.

[0015] Furthermore, it is proposed that the air compressor unit integrated into the supply air path and the air compressor units integrated into the individual supply air paths are each driven by an electric motor. The electric motor can be controlled specifically so that the air mass flows and pressures in the individual supply air paths can be adjusted individually for each stack. The air compressor units integrated into the individual supply air paths are preferably each additionally driven by a turbine. In this way, part of the energy used for compression can be recovered.

[0016] Preferably, the air compressed by the air compressor unit integrated into the supply air path is cooled. Cooling the compressed air is particularly advantageous when the air compression occurs in multiple stages, so that intermediate cooling is achieved through the cooling. This can increase efficiency, as air from a lower temperature level can be compressed more efficiently than from a higher temperature level. For cooling or intermediate cooling, at least one cooling device is preferably used, which is integrated into the supply air path and / or into an individual supply air path.

[0017] In a further development of the invention, it is proposed that at least one further system be supplied with air via the air system, for example, a ventilation system and / or a braking system that requires air, in particular compressed air. In this way, the one air system can cover additional air requirements in the vehicle. For this purpose, the at least one further system is preferably connected to the supply air path via another individual supply air path, similar to the fuel cell stacks.

[0018] The fuel cell system advantageously includes a higher-level system controller. Using the higher-level system controller, a target value for at least one operating variable, in particular for the air mass flow, pressure, and / or humidity, can be defined and implemented via the operating modes of the individual air compressor units. The target value for the at least one operating variable can be implemented and adjusted individually for each stack.

[0019] With the help of the higher-level system control, for example, an energy management system can be implemented to optimize the operating strategy. With the help of the energy management system, trajectories for air mass flows, pressures, and, if necessary, other operating variables can be calculated and implemented via the operating modes of the individual air compressor units. Excessively rapid toggling between the various operating modes should be avoided, for example, by considering start / stop processes. Preferably, energy-related and / or degradation-related aspects or goals should also be considered.

[0020] If an air compressor unit, a fuel cell stack, or another operationally relevant component fails, system operation can be adjusted accordingly using the higher-level system control. If not all operating modes are feasible, the selection of operating modes can be restricted to optimize the operating strategy. In some cases, the upper limit of the maximum possible power of the fuel cell stacks may need to be reduced. If the fuel cell system is currently operating at high power levels, power derating can also be applied.

[0021] The switchable air compressor unit in the supply air path enables additional functions. For example, the air compressor unit can be switched on to compensate for a drop in ambient air pressure during uphill travel or high-altitude operation. Furthermore, at high ambient temperatures ("hot-country travel"), the air compressor unit can be switched on to operate the fuel cell stacks at higher pressures to prevent the membranes from drying out. Furthermore, the air compressor unit can be switched on to at least partially compensate for a decrease in the efficiency of a fuel cell stack over its lifetime. This means that the central compressor unit can also be used increasingly adaptively over its lifetime.

[0022] Furthermore, a fuel cell system with multiple fuel cell stacks and an air system for supplying the fuel cell stacks with air is proposed. The air system comprises an air supply path that branches into several individual air supply paths for connecting the individual fuel cell stacks. The air system also comprises at least one air compressor unit for compressing the air supplied to the fuel cell stacks. According to the invention, a first air compressor unit that can be switched on and off is integrated into the air supply path, and a further air compressor unit is integrated into each of the individual air supply paths for stack-specific air compression.

[0023] The proposed fuel cell system is particularly suitable for implementing the method according to the invention described above, or can be operated according to this method. Thus, the same advantages can be achieved. In particular, system efficiency can be increased and hydrogen consumption reduced. Furthermore, degradation can be minimized and reliability increased.

[0024] According to a preferred embodiment of the invention, a compressor bypass path with an integrated bypass valve is provided for bypassing the first air compressor unit integrated into the supply air path. When the air compressor unit is shut down, the bypass valve in the compressor bypass path can be opened, allowing air to bypass the air compressor unit via the bypass path. Furthermore, when the air compressor unit is switched on, the bypass valve integrated into the compressor bypass path can be used to prevent compressor surge at certain operating points by allowing a portion of the compressed air to flow back via the bypass path. The bypass valve can be set to a partially open position.

[0025] Preferably, the first air compressor unit integrated into the supply air path is single- or multi-flow and is driven by an electric motor. In the single-flow design, the air compressor unit has an impeller arranged on a shaft. In the multi-flow, in particular double-flow, design, the air compressor unit has several impellers arranged on the shaft, which are subjected to parallel air flow.

[0026] Furthermore, it is proposed that the air compressor units integrated into the individual supply air paths be driven by an electric motor and a turbine. The turbines can be used to recover some of the energy used for compression.

[0027] The first air compressor unit integrated into the supply air path and the additional air compressor units can be designed the same or differently. For example, the first air compressor unit can be designed as an axial compressor, and the additional air compressor units can each be designed as radial compressors. Furthermore, all air compressor units can be designed as radial compressors.

[0028] Furthermore, the air system preferably comprises at least one cooling device, which is integrated downstream of the first air compressor unit into the supply air path and / or into at least one individual supply air path. Since the air heats up considerably during compression, the cooling device can be used to cool the air before it is fed to the fuel cell stacks. In the case of multi-stage air compression, the at least one cooling device can be used to achieve intermediate cooling and thus increase system efficiency. This is because air can be compressed more efficiently from a lower temperature level than from a higher temperature level.

[0029] The proposed fuel cell system preferably has a higher-level system controller for defining a target value for at least one operating variable, in particular for the air mass flow, pressure, and / or air humidity, and for implementing the target value via the operating modes of the individual air compressor units. The system controller thus enables optimization of the operating strategy, preferably across multiple system levels. Furthermore, pre-optimization of the air system operating modes is possible. Once the operating modes have been determined, they can be implemented and adjusted using the system controller.

[0030] The invention and its advantages are explained in more detail below with reference to the accompanying drawings. These show: Fig. 1 a schematic representation of a first fuel cell system with an air system for supplying several fuel cell stacks with air, Fig. 2 a schematic representation of a second fuel cell system with an air system for supplying several fuel cell stacks with air, Fig. 3 a schematic representation of a third fuel cell system with an air system for supplying several fuel cell stacks with air and Fig. 4 a block diagram illustrating a preferred sequence of a method according to the invention. Detailed description of the drawings

[0031] The Fig. Figure 1 shows, by way of example, a first fuel cell system 1 according to the invention with a plurality of fuel cell stacks 2. The fuel cell system 1 comprises an air system 3 for supplying the fuel cell stacks 2 with air taken from the environment 16. The air is supplied via an air supply path 4 of the air system 3, which branches into a plurality of individual air supply paths 5 for connecting the individual fuel cell stacks 2. A further system 14 is connected to the air supply path 4 via a further individual air supply path 5. This system may be, for example, a ventilation system or a braking system that requires air.

[0032] An air filter 17 is integrated into the supply air path 4 to remove harmful particles and harmful chemical substances from the air taken from the environment 16. Furthermore, a first air compressor unit 6, which can be switched on and off, is integrated into the supply air path 4. A compressor bypass path 8 with an integrated bypass valve 9 is provided for switching on and off. When the bypass valve 9 is closed, the air compressor unit 6 is switched on. When the air compressor unit 6 is in operation, the bypass valve 9 can also be used as an anti-surge or overflow valve. If the air compressor unit is switched off and the stack-individual air compressors are in operation, the bypass valve 9 is in the open position, i.e. the air mass flow flows via the compressor bypass 8.

[0033] Further air compressor units 7 are provided in the individual supply air paths 5 for supplying air to the fuel cell stacks 2, so that when the first air compressor unit 6 is switched on, multi-stage air compression can be realized.

[0034] The air compressor units 6, 7 can be identical or different. Possible embodiments are described below with reference to Fig. 2 and Fig. 3 described.

[0035] The Fig. The fuel cell system 1 according to the invention, shown in Figure 2, has an air system 3 with an air supply path 4 and a plurality of individual air supply paths 5 for supplying air to a plurality of, in this case three, fuel cell stacks 2. A first air compressor unit 6 is integrated into the air supply path 4 and can be switched on and off via a compressor bypass path 8 with an integrated bypass valve 9. The first air compressor unit 6 is driven by an electric motor 10. Downstream of the air compressor unit 6, a cooling device 12 is arranged in the air supply path 4. With the help of the cooling device 12, intermediate cooling can be carried out before the compressed air is distributed to the individual air supply paths 5. A further air compressor unit 7 is integrated into each individual air supply path 5 and is driven by an electric motor 10 and a turbine 11.The turbine 11 is arranged in an exhaust air path 15, through which the air exiting the respective fuel cell stack 2 is discharged to the environment 16. Downstream of each of the additional air compressor units 7, a cooling device 13 in the form of a gas-to-gas heat exchanger is provided for cooling the air. Furthermore, stack bypass paths 18 with integrated bypass valves 19 are provided for bypassing the fuel cell stacks 2.

[0036] Fig. 3 shows a modification of the fuel cell system 1 of Fig. 2. In contrast to the embodiment of the Fig. 2, the first air compressor unit 6 is designed as a multi-flow or double-flow unit. For this purpose, it has two impellers 6.1, 6.2, which are coupled via a shaft 6.3. The two impellers 6.1, 6.2 are subjected to parallel air flow. The air compressor unit 6 is also driven by an electric motor 10. A compressor bypass path 8 with an integrated bypass valve 9 is provided for switching the air compressor unit 6 on and off.

[0037] If the air compressor unit 6 is in operation, the bypass valve 9 can also be operated as a pump protection or overflow valve in a partially open position for certain operating points with higher pressure levels, whereby the flow direction is then reversed compared to the air flow via the bypass 8 when the air compressor unit 6 is switched off and the air compressor units 7 are in operation.

[0038] The Fig. Figure 3 also shows an alternative cooling concept, shown in dashed lines. In the alternative cooling concept shown, cooling devices 13 are integrated into the individual supply air paths 5 upstream of the air compressor units 7. The cooling device 12 for intermediate cooling integrated into the supply air path 4 can thus be omitted. In addition, additional cooling devices 13 can be provided downstream of the air compressor units 7.

[0039] The Fig. The fuel cell systems 1 shown in Figures 1 to 3 are suitable for carrying out the method according to the invention or can be operated according to this method. For this purpose, the fuel cell systems 1 can each comprise a higher-level system controller 20. Such a controller is shown in Figures Fig. 1 to 3 not shown.

[0040] The method according to the invention is described below with reference to Fig. 4 described.

[0041] The Fig.4 shows a higher-level system controller 20, with the aid of which an energy management system 21 can be set up to optimize the power split L1, L2 in the fuel cell system 1 and / or in combination with a high-voltage battery. Furthermore, with the aid of the energy management system 21, the operating strategy can be optimized through degrees of freedom of various system levels E1, E2, E3. This can include trajectory planning for air mass flows and pressures in the air system 3. With the aid of the system controller 20, the requirements or requested trajectories can be defined in a first step S10. In a subsequent step S11, further conditions for optimizing the operation of the air compressor units 6, 7 can be determined.In a step S12, suitable operating modes for the operation of the air compressor units 6, 7 are then sought for the respective requirements and / or conditions and implemented in a step S13, wherein the implementation may include:. S13.1 Operation only of the air compressor unit 6 integrated in the supply air path 4 S13.2 Operation of the air compressor unit 6 integrated in the supply air path 4 in combination with at least one air compressor unit 7 integrated in a single supply air path 5 or S13.3 Operation of at least one air compressor unit 7 integrated in an individual supply air path 5 with air compressor unit 6 switched off.

[0042] In a step S14, the duration of operation of the air compressor units 6, 7 in the respective operating modes is determined depending on the system dynamics.

[0043] In a step S15, an error diagnosis or error handling can also be carried out with the help of the higher-level system control 20.

Claims

[1] Method for operating a fuel cell system (1) with a plurality of fuel cell stacks (2) which are supplied with air via an air system (3), wherein the air is supplied via an air supply path (4) of the air system (3), which branches into a plurality of individual air supply paths (5) for connecting the individual fuel cell stacks (2), and wherein the air is compressed by means of at least one air compressor unit (6, 7) before entering a fuel cell stack (2), characterized by that the air supplied to the fuel cell stacks (2) is compressed optionally via an air compressor unit (6) integrated into the supply air path (4) and which can be switched on and off and / or individually for each stack, wherein air compressor units (7) arranged in the individual supply air paths (5) are used for the stack-individual air compression. [2] Method according to claim 1, characterized bythat when the air compressor unit (6) which is integrated in the supply air path (4) is switched off, a bypass valve (9) arranged in a compressor bypass path (8) to bypass the air compressor unit (6) is opened. [3] Method according to claim 1 or 2, characterized by that the air compressor unit (6) integrated in the supply air path (4) and the air compressor units (7) integrated in the individual supply air paths (5) are each driven by means of an electric motor (10), wherein preferably the air compressor units (7) integrated in the individual supply air paths (5) are each additionally driven by means of a turbine (11). [4] Method according to one of the preceding claims, characterized bythat the air compressed by means of the air compressor unit (6) integrated into the supply air path (4) is cooled, wherein preferably at least one cooling device (12, 13) is used which is integrated into the supply air path (4) and / or into an individual supply air path (5). [5] Method according to one of the preceding claims, characterized by that at least one further system (14) is supplied with air via the air system (3), for example a ventilation system and / or a braking system which requires air, in particular compressed air. [6] Method according to one of the preceding claims, characterized by that with the aid of a higher-level system control (20) a target specification for at least one operating variable, in particular for the air mass flow, the pressure and / or the air humidity, is defined and implemented via the operating modes of the individual air compressor units (6, 7). [7] Fuel cell system (1) with a plurality of fuel cell stacks (2) and an air system (3) for supplying the fuel cell stacks (2) with air, wherein the air system (3) comprises an air supply path (4) which branches into a plurality of individual air supply paths (5) for connecting the individual fuel cell stacks (2), and wherein the air system (3) comprises at least one air compressor unit (6, 7) for compressing the air supplied to the fuel cell stacks (2), characterized by that a first air compressor unit (6) which can be switched on and off is integrated into the supply air path (4) and a further air compressor unit (7) is integrated into each of the individual supply air paths (5) for stack-individual air compression. [8] Fuel cell system (1) according to claim 7, characterized by that a compressor bypass path (8) with integrated bypass valve (9) is provided for bypassing the first air compressor unit (6) integrated into the supply air path (4). [9] Fuel cell system (1) according to claim 7 or 8, characterized by that the first air compressor unit (6) integrated into the supply air path (4) is designed as a single-flow or multi-flow unit and is driven by means of an electric motor (10). [10] Fuel cell system (1) according to one of claims 7 to 9, characterized by that the air compressor units (7) integrated in the individual supply air paths (5) are each driven by means of an electric motor (10) and a turbine (11). [11] Fuel cell system (1) according to one of claims 7 to 10, characterized by that the air system (3) comprises at least one cooling device (12, 13) which is integrated downstream of the first air compressor unit (6) into the supply air path (4) and / or into at least one individual supply air path (5). [12] Fuel cell system (1) according to one of claims 7 to 11, characterized bya higher-level system control (20) for defining a target specification for at least one operating variable, in particular for the air mass flow, the pressure and / or the air humidity, and for implementing the target specification via the operating modes of the individual air compressor units (6, 7).

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