Method for starting a fuel cell system

Abstract

A method for starting a fuel cell system is proposed, the fuel cell system comprising at least one fuel cell stack and a compressor assembly having a first compressor coupled to an electric motor and a second compressor coupled to a turbine arranged in a cathode path of the fuel cell system. The method may set a first start-up mode and a second start-up mode in which only the first compressor or both compressors are started, in particular in accordance with start-up conditions and operating conditions of the fuel cell stack.

Description

Technical Field

[0001] The invention relates to a method for starting a fuel cell system and a fuel cell system. Background Art

[0002] In vehicles in which at least a portion of the electric power required for the electric drive is provided by a fuel cell system, a compressor for compressing ambient air and supplying it to the fuel cell system is usually integrated. Many different compressor assemblies are known, which are constructed in a single-flow, dual-flow and single-stage or multi-stage manner. Here, thermofluidic machines are usually used as compressors. A two-stage compressor assembly includes, for example, a first compressor with an electric drive. A second compressor connected downstream or upstream can be driven by a turbine arranged in the cathode path of the fuel cell system and in particular in its exhaust path.

[0003] In some operating modes or operating conditions, a low cathode pressure in the range of the ambient pressure is required for the fuel cell, especially at low temperatures of the fuel cell, in order to discharge the produced water sufficiently. However, when both compressors are in operation, these requirements cannot be met due to operating limits such as pumping limits, stop limits, minimum and maximum permissible rotational speeds, axial forces / transverse forces on the compressor bearings, etc. If the pressure is too high in this case, too little reaction water is transported away, as a result of which water accumulates and after a certain period of time the fuel cell is filled up, so that the oxygen supply fails and the cell voltage and the supplied current are interrupted.

[0004] Since a compressor driven by a turbine requires sufficient enthalpy and therefore also a pressure at the turbine inlet in order to maintain its operation, for example, if the pressure is too low, the permissible lower speed limit may be reached. Summary of the invention

[0005] It is therefore an object of the present invention to specify an improved method for starting a fuel cell system in which low pressures can also be achieved if they are required or desired.

[0006] This object is achieved by a method having the features of independent claim 1. Advantageous embodiments and developments emerge from the dependent claims and the following description.

[0007] A method for starting a fuel cell system is proposed, wherein the fuel cell system includes at least one fuel cell stack and a compressor assembly, wherein the compressor assembly has a first compressor coupled to an electric motor and a second compressor coupled to a turbine arranged in a cathode path of the fuel cell system, and the method has the following steps: detecting a starting condition, wherein the starting condition is in the form of a required pressure level of at least one fuel cell stack and / or a temperature level of at least one fuel cell stack and / or a starting mode of at least one fuel cell stack; determining, based on the starting condition, whether the compressor assembly is started in a first starting mode in which the second compressor is started with a delayed start, or in a second starting mode in which the second compressor is started without a delayed start; and in the first starting mode, starting the first compressor and accelerating it until the idling speed of the first compressor is exceeded. and reaches a maximum predetermined first maximum starting speed; starts at least one fuel cell stack; operates at least one fuel cell stack at a maximum partial load; checks the temperature level of at least one fuel cell stack and / or the required pressure level of at least one fuel cell stack; and, after exceeding the corresponding threshold value, starts the second compressor and accelerates it until the idling speed of the second compressor is exceeded and a maximum predetermined second maximum starting speed is reached; and, once the first and second compressors are in a predetermined operating state, operates at least one fuel cell stack up to the set maximum load; in a second starting mode, starts the first compressor and the second compressor and correspondingly accelerates and starts at least one fuel cell stack; and, once the first and second compressors are in a predetermined operating state, operates at least one cell stack up to the set maximum load.

[0008] The method according to the invention enables the fuel cell system and in particular the compressor assembly to be started in such a way that operating modes which require a low system pressure in the cathode path of the fuel cell stack can be considered very simply. For example, this may require a frozen start at an external temperature below 0° C., a cold start at a temperature slightly above 0° C., a warm-up phase to a target operating temperature, operation of the fuel cell stack below the target operating temperature (e.g., cooling due to too low a load, e.g., downhill and at low ambient temperatures) or a drying process of the fuel cell stack when the vehicle is parked, a waste heat utilization function at low pressure. However, the compressor assembly can have an electrically operated compressor and a turbine operated compressor, as is common in the prior art. The compressor assembly can be started in such a way that when the electric compressor is started or operated, the low cathode pressure can be adjusted to a pressure close to the ambient pressure by delaying the start of the turbine operated compressor.

[0009] In parallel with the two start-up modes, the fuel cell system as a whole can be operated in a first or second operating state, which takes into account the first or second start-up mode. In the first operating state, the power of the at least one fuel cell stack can be reduced to partial load and, for example, after reaching a sufficient operating temperature, it can be transferred to a second operating state in which both compressors are operated and the at least one fuel cell stack can be operated up to maximum load.

[0010] This allows for better coordination of the operation between the at least one fuel cell stack and the compressor assembly. Water management in the fuel cell system is improved and flooding of the fuel cells is avoided. Overall, aging of the at least one fuel cell stack is avoided or reduced and the overall efficiency of the fuel cell system is improved.

[0011] If the start-up of the second compressor should fail, when the speed of the second compressor remains at zero or does not reach the idling speed, the system remains in a partial load operating state until an attempt is made to start the second compressor again when necessary.

[0012] In an advantageous embodiment, the method further comprises: opening a second compressor bypass arranged in parallel with the second compressor when the first compressor is started while the second compressor is stationary; and closing the second compressor bypass when the second compressor is started. A higher air flow can be generated in the compressor assembly by the second compressor bypass. As a result, the fuel cell system is less restricted during operation, since the mass flow of the compressor assembly can be directed to the greatest extent around the second compressor, which has a favorable influence not only on the pumping limit of the first compressor but also on the lateral forces or axial forces of the second compressor.

[0013] In an advantageous embodiment, the start-up condition is detected predictively or in real time by evaluating at least one physical variable detectable by a sensor and / or based on a simulation of the physical variable. As previously described, this can be the required pressure level or temperature of at least one fuel cell stack. The latter can be estimated or simulated, for example, by a mathematical model.

[0014] In an advantageous specific embodiment, the at least one physical variable comprises a refrigerant temperature of at least one fuel cell stack. The refrigerant temperature can be measured by a refrigerant temperature sensor and provides information about the temperature within the fuel cell stack.

[0015] In an advantageous embodiment, in a first starting mode, when the first compressor is accelerated, the second compressor is traversed by compressed air provided by the first compressor, wherein the second compressor is locked and / or wherein the rotational speed of the first compressor is limited to a predetermined partial load rotational speed. Thus, the lateral and axial forces exerted on the stationary second compressor due to the flow through it can be limited to avoid damage. The stationary second compressor constitutes an obstacle and causes pressure losses. Therefore, the pumping limit of the first compressor is reached more quickly than when the second compressor is actively running. The limitation of the partial load rotational speed can be achieved in a simple manner by: compared with the operation with the running second compressor, the operation of the first compressor is significantly limited. Alternatively to this, it is also possible to calculate the forces from the current thermodynamic data and thus calculate the limitation of the partial load rotational speed based on the model.

[0016] In an advantageous embodiment, before the second compressor is started and after the second compressor bypass is closed, the rotational speed of the first compressor is limited to a rotational speed that results in an enthalpy of the cathode exhaust gas flow that exceeds the enthalpy required for starting the second compressor by a maximum of 10%. Thus, the enthalpy for starting the second compressor is sufficient, but at the same time the axial forces and transverse forces on the second compressor are also limited.

[0017] In an advantageous embodiment, if the required pressure level cannot be reached during the simultaneous operation of the first compressor and the second compressor, but is always exceeded, the operation of the second compressor is interrupted. If the pressure level for sensible water management is below the minimum pressure of the compressor assembly with two compressors in operation, the second compressor is switched off again. In order to prevent repeated switching off and starting of the second compressor, the decision to switch off can be implemented with a hysteresis function. In addition, the decision can also be made to depend on the operating mode, for example, in order to dry the at least one fuel cell stack during the switch-off, a low pressure close to the ambient pressure is required in the at least one fuel cell stack.

[0018] In addition, the present invention relates to a fuel cell system, which has at least one fuel cell stack, a compressor assembly (the compressor assembly has at least one first compressor that can be operated electrically and a second compressor arranged downstream or upstream, coupled to a turbine arranged in a cathode path of the fuel cell system) and a control unit for controlling the fuel cell system, wherein the fuel cell system is constructed to implement a method for starting a fuel cell system according to any of the preceding claims with the aid of the control unit.

[0019] It is also pointed out once again that the topological order of the compressors is not important for the implementation of the solution according to the invention.

[0020] In a preferred embodiment, the fuel cell system also has a second compressor bypass arranged in parallel with the second compressor, wherein the control unit is configured to open the second compressor bypass when the first compressor is started in a first starting mode when the second compressor is stationary, and to close the second compressor bypass when the second compressor is started.

[0021] In an advantageous embodiment, the fuel cell system further comprises a turbine bypass arranged in parallel with the turbine, wherein the control unit is designed to open the turbine bypass when the first compressor is started in the first starting mode. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In the following, further measures to improve the present invention are described in more detail together with the description of preferred embodiments of the present invention according to the accompanying drawings. It shows:

[0023] Figure 1 A schematic illustration of a fuel cell system in an exemplary compressor sequence;

[0024] Figure 2 Schematic, block-based illustrations of aspects of the method;

[0025] Figure 3 Schematic, block-based illustration of further aspects of the method. DETAILED DESCRIPTION

[0026] Figure 1 A fuel cell system 2 is shown which is controlled by a control unit 3. The fuel cell system 2 has a compressor assembly 4 which comprises a first compressor 6 with an electric motor 8 and an inverter 10. The first compressor 6 is connected to the motor 8 via a first shaft 12, which is supplied with an operating voltage via the inverter 10. The inverter 10 can be controlled from the outside in order to adjust the speed of the motor 8 as required. Here, the first compressor 6 is implemented as a second compression stage, and the second compressor 14 is implemented as a first compression stage. This represents the first stage of the overall air compression.

[0027] The second compressor 14 and the turbine 16 driving the second compressor are coupled to each other via a second shaft 18. The second compressor 14 is located downstream of the first compressor 6 and provides compressed air to a supply path 20 of a fuel cell stack 22. Exhaust gas from the fuel cell stack 22 flows into an exhaust path 24 and from there into the turbine 16. The exhaust gas can then flow out to an environment 26 downstream of the turbine 16. The supply path 20 and the exhaust path 24 can be combined into a cathode path.

[0028] In addition, an intercooler 28 is provided downstream of the second compressor 14, which cools the compressed supply air before it enters the fuel cell stack 22. Downstream of the intercooler 28 is a first shut-off valve 30, which prevents the flow of supply air into the fuel cell stack 22 as required. Downstream of the fuel cell stack 22 is a second shut-off valve 32, which is used to prevent interruption of the connection of the fuel cell stack 22 to the exhaust path 24 as required. The fuel cell stack 22 can be bypassed by a fuel cell bypass 34. The fuel cell bypass 34 has a fuel cell bypass valve 36 that can be opened and closed as required.

[0029] A heat exchanger 38 may be provided between the fuel cell stack 22 and the second compressor 14, which transfers heat from the supply air path 20 to the exhaust air path 24. Further, an additional intercooler 40 may be arranged between the first compressor 6 and the second compressor 14, which cools the air flowing into the second compressor 4. An air filter 42 is arranged in front of the first compressor 6, which filters the air from the environment 26 and feeds it to the first compressor 6. The turbine 16 can be bypassed by a turbine bypass 44. The turbine bypass 44 has a turbine bypass valve 46, which can be opened and closed as required. In addition, an exhaust shut-off valve 48 is also provided downstream of the turbine 16. Another heat exchanger 50 may be arranged directly downstream of it, which absorbs part of the heat from the supply air path 20 directly downstream of the first compressor 6 and transfers it to the outflowing exhaust gas. A second compressor bypass 52 is arranged in parallel with the second compressor 14, and the second compressor bypass has a second compressor bypass valve 54. Thereby, the second compressor 14 can be bypassed by the compressed air from the first compressor 6 .

[0030] Under the control of the control unit 3, the fuel cell system 2 can execute a method for starting the fuel cell system 2, which method Figure 2 and 3 Explained in more detail in .

[0031] Figure 2 Some aspects of the method are shown in a schematic, block-based diagram. First, a start condition is detected 56 in order to then decide 58 whether the compressor assembly 4 is to be started in a first start mode or in a second start mode. If the temperature of the fuel cell stack 22 is too low, a cold start or a frozen start is set, a low pressure is predefined, and the first start mode with delayed start of the second compressor 14 is selected.

[0032] Under the first starting condition, the shutoff valves 30 and 32 are closed, the cathode bypass 34 and the turbine bypass 44 as well as the second compressor bypass 52 are open, which can be referred to as throttling 60. The first compressor 6 starts 62 and accelerates to a predetermined partial load speed taking into account the limitation 64, which can be tolerated when the second compressor 14 is switched off. The transverse and axial forces exerted on the stationary second compressor 14 by the flow through it - as long as this is generally the case through the second compressor bypass 52 - can be limited by limiting the speed. In addition, the pumping limit of the first compressor 6 is also not reached.

[0033] The fuel cell stack 22 is started 66 and operated under restrictions, wherein the shutoff valves 30 and 32 are open and the cathode bypass 34 is closed. The maximum possible partial load mass flow is followed by a limited operation of the first compressor 6 at a partial load speed, resulting in the maximum possible partial electrical power of the fuel cell stack 22. For a system without a second compressor bypass 52, the maximum possible partial electrical power is smaller than for a system with a second compressor bypass 52. However, for relevant situations such as frozen start and cold start, this partial power is usually sufficient. In the first start mode, the set power is limited and at the same time the warm-up gradient is also limited.

[0034] Then, the state and operation of the second compressor 14 are continuously monitored 68, in order in particular to ensure that the second compressor 14 is not rotating or to take into account that it is rotating. During the rotation, the rotation speed of the first compressor 6 can be reduced if necessary.

[0035] In addition, the fuel cell stack temperature is continuously checked 70. Based on this, it can be decided 72 when the second compressor 14 is put into operation 74. This can also be done at a correspondingly higher required pressure level. Since the temperature in the preheating phase changes slowly and the preheating gradient can be detected or adjusted, the start of the second compressor 14 can also be predicted. If, for example, the temperature of the coolant at the outlet of the fuel cell stack 22 is greater than 50° C., the second compressor 14 can be started. Since the start can be carried out quickly, the switch is dynamically non-critical. Then, after the second compressor 14 has been successfully started, the fuel cell stack 22 can operate normally 76 until it is fully loaded according to the usual restrictions suitable for the fuel cell system 2.

[0036] If a second start mode is set, the first compressor 6 and the second compressor 14 are started simultaneously 78. The turbine bypass 44 is closed so that the second compressor 14 starts with the first compressor 6. If present, the second compressor bypass 52 is also closed. If the second compressor 14 is not to be started, for example due to a stuck shaft, a restricted operation can be carried out as in process step 62. On the other hand, conventional operation can be started 80.

[0037] exist Figure 3 , further aspects of the method are shown in . Based on the operating conditions 82, a decision 84 is made whether the second compressor 14 is to be shut down (for example, when the compressor assembly 4 cannot meet the low pressure requirements). Preferably, this is performed with a corresponding hysteresis. If an early stop of the second compressor 14 is desired, the compressor assembly 4 is transferred 86 to an idling operating state so that the second compressor 14 is not shut down at a high load point. In this case, the rotational speed of the second compressor 14 should only be not significantly higher than the idling speed. Then the shutdown 88 is performed. In order to shut down the compressor 14 and the turbine 16, the turbine bypass 44 is opened and, if present, the compressor bypass 52 is also opened. Then, the first compressor 6 and the fuel cell stack 22 are continued to be operated 90, wherein the rotational speed of the first compressor 6 is to be coordinated with the absence of the second compressor 14 or other restrictions 64.

[0038] To terminate operation of the fuel cell system 2, the power of the fuel cell stack 22 is reduced. If the air mass flow is no longer required, ie, for the fuel cell stack 22, for residual hydrogen or other dilution, the first compressor 6 is switched off 92.

[0039] If the deactivation of the second compressor 14 is not premature, the compressors 6 and 14 are deactivated 94 simultaneously.

[0040] These two methods can be combined. In the short start-stop phase, it may not be necessary to start the second compressor, so that the second compressor can also remain switched off. This also has advantages for reducing bearing wear.

[0041] In this part of the method, it is also useful to monitor 68 the operation of the second compressor 14 .

[0042] In addition, it should be pointed out again that, unlike Figure 1 Alternatively, the order of the compressor stages can also be selected in exactly the opposite manner.

Claims

1. A method for starting a fuel cell system (2), the fuel cell system having at least one fuel cell stack (22) and a compressor assembly (4), the compressor assembly having a first compressor (6) coupled to an electric motor (8) and a second compressor (14) coupled to a turbine (16) arranged in a cathode path of the fuel cell system (2), the method comprising the following steps: - detecting (56) a start-up condition in the form of a required pressure level of the at least one fuel cell stack (22) and / or a temperature level of the at least one fuel cell stack (22) and / or a start-up method of the at least one fuel cell stack, - based on the starting condition, deciding (58) whether the compressor assembly (4) is to be started in a first starting mode with delayed starting of the second compressor (14) or in a second starting mode with non-delayed starting of the second compressor (14), - in the first starting mode, starting (62) the first compressor (6) and accelerating until the idling speed of the first compressor (6) is exceeded and a predetermined first maximum starting speed is reached at most; Starting (66) the at least one fuel cell stack (22); operating the at least one fuel cell stack (22) at a maximum partial load; checking (70) the temperature level of the at least one fuel cell stack (22) and / or the required pressure level of the at least one fuel cell stack (22); and starting (74) the second compressor (14) and accelerating it after exceeding a corresponding threshold value until the idling speed of the second compressor (14) is exceeded and a second predetermined maximum starting speed is reached at the maximum; and, as soon as the first compressor and the second compressor (6, 14) are in a predetermined operating state, operating (76) the at least one fuel cell stack (22) up to a set maximum load, - in the second start-up mode, starting (78) the first compressor (6) and the second compressor (14) and correspondingly accelerating and starting the at least one fuel cell stack; And, once the first compressor and the second compressor (6, 14) are in a predetermined operating state, the at least one fuel cell stack (22) is operated (80) up to a set maximum load.

2. The method according to claim 1, The method further comprises: when the first compressor (6) is started in the first starting mode when the second compressor (14) is stationary, opening a second compressor bypass (52), the second compressor bypass being arranged in parallel with the second compressor (14), and, when the second compressor (14) is started, closing the second compressor bypass (52).

3. The method according to claim 1 or 2, in, The start condition is detected (56) predictively or in real time by evaluating at least one physical variable detectable by a sensor and / or by a simulation based on the physical variable.

4. The method according to claim 3, in, The at least one physical variable includes a coolant temperature of the at least one fuel cell stack (22).

5. The method according to any one of the preceding claims, in, In the first starting mode, during acceleration of the first compressor (6), compressed air provided by the first compressor flows through the second compressor (14) at least partially. Therein, the second compressor (14) is locked and / or the speed of the first compressor (6) is limited to a predefined partial load speed.

6. The method according to claim 2, in, Before starting the second compressor (14) and after closing the second compressor bypass (52), the first compressor (6) is brought to a rotational speed that results in an enthalpy of the cathode exhaust gas flow that ensures the enthalpy necessary for starting the second compressor (14), preferably exceeding 10%.

7. A method according to any one of the preceding claims, in, If, during the simultaneous operation of the first compressor (6) and the second compressor (14), the required pressure level cannot be achieved but is always exceeded, the operation of the second compressor (14) is interrupted.

8. A fuel cell system (2), comprising: - at least one fuel cell stack (22), a compressor assembly (4) having at least one first compressor (6) which can be operated electrically and a second compressor (14) which is arranged downstream or upstream, the compressor assembly being coupled to a turbine (16) which is arranged in the cathode path of the fuel cell system (2), and - a control unit (3) for controlling the fuel cell system (2), in, The fuel cell system (2) is designed to carry out a method for starting a fuel cell system (4) according to any one of the preceding claims by means of the control unit (3).

9. The fuel cell system (2) according to claim 8, The fuel cell system further comprises a second compressor bypass (52), which is arranged parallel to the second compressor (14). in, The control unit (3) is configured to open the second compressor bypass (52) when the first compressor (6) is started in a first starting mode when the second compressor (14) is stationary, and to close the second compressor bypass when the second compressor (14) is started, and also to open the second compressor bypass when the second compressor (14) is to be stopped due to the operation of both compressors.

10. The fuel cell system (2) according to claim 8 or 9, The fuel cell system further comprises a turbine bypass (44) which is arranged in parallel with the turbine (16). in, The control unit (3) is configured to open the turbine bypass (44) when the first compressor (6) is started in the first starting mode, and to close the turbine bypass when the second compressor (14) is to be started, and also to open the turbine bypass when the second compressor (14) is to be stopped during operation of both compressors.

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