Method for operating a fuel cell system, control unit

Abstract

The invention relates to a method for operating a fuel cell system (1), comprising at least one fuel cell stack (2) with a cathode and an anode, and an air system (3) for supplying the at least one fuel cell stack (2) with air, wherein the air is supplied to the at least one fuel cell stack (2) via an air supply path (4) of the air system (3) and the exhaust air exiting the at least one fuel cell stack (2) is discharged via an exhaust air path (5) of the air system (3). According to the invention, the following steps are performed when the at least one fuel cell stack (2) is switched off and / or during a shutdown phase: - Creating a cooling trajectory based on measurement and / or model data, - Predicting the time of ice formation in the supply air path (4) and / or in the exhaust air path (5) using the cooling trajectory and - Active drying of the supply air path (4) and / or the exhaust air path (5) before the predicted time, provided that this is within the shutdown phase. Furthermore, the invention relates to a control unit for executing steps of a method according to the invention.

Description

[0001] The present 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 control device for carrying out steps of the method.

[0002] The preferred field of application of the invention is mobile fuel cell systems or fuel cell vehicles. State of the art

[0003] Hydrogen-based fuel cell systems are considered the mobility concept of the future because they emit only water as exhaust gas and enable fast refueling times. The electrochemical reaction in the fuel cells requires air and hydrogen, which are provided by subsystems. The waste heat generated during the electrochemical reaction is dissipated using a cooling circuit and released into the environment via a vehicle radiator.

[0004] In practice, a large number of fuel cells are stacked on top of each other and connected to form a fuel cell stack. To increase performance, a fuel cell system can also comprise more than one stack. In this case, it is called a multi-stack system.

[0005] During a cold and / or freezing start of the fuel cell system, the fuel cell stack must be brought to operating temperature or heated up. Rapid heating is intended to prevent water and / or ice accumulation, as this can complicate or even completely prevent the start-up process. The risk of icing is only eliminated once the coolant in the cooling circuit has been safely heated above 0°C. Otherwise, the coolant will create freezing conditions when pumped into the stack.

[0006] Heating a stack can occur externally or internally through the electrochemical reaction within the stack. In both cases, heating prolongs the start-up process. In addition, the ice tolerance of the individual cells and the system as a whole must be increased due to the constant cooling to temperatures below 0°C. So-called ice buffers are provided in the cells for this purpose. The system can have at least one electric heater for this purpose.

[0007] To ensure a successful cold and / or freeze start without long preheating times, the two gas supply subsystems must also be freed of water. Otherwise, the actuators contained therein can become blocked due to ice formation, possibly requiring the start to be aborted. Approaches to this are known from the state of the art, particularly for the anode-side subsystem, which serves the hydrogen supply. Blocking the hydrogen supply would result in serious damage to the cells.

[0008] The present invention addresses the problem of removing water from the cathode-side subsystem of a fuel cell system, which serves to supply air, and specifically drying it. Drying is particularly relevant during short-distance vehicle operation at low ambient temperatures, since the exhaust air on the stack's exhaust side hardly heats up at these times, resulting in significant amounts of condensate.

[0009] To achieve this objective, the method is provided with the features of claim 1. Advantageous developments of the invention are set forth in the subclaims. Furthermore, a control device for executing steps of the method is provided. Disclosure of the invention

[0010] A method is proposed for operating a fuel cell system comprising at least one fuel cell stack with a cathode and an anode, as well as an air system for supplying the at least one fuel cell stack with air. The air is supplied to the at least one fuel cell stack via an air supply path of the air system, and the exhaust air exiting the at least one fuel cell stack is discharged via an exhaust path of the air system. According to the invention, the following steps are carried out when shutting down the at least one fuel cell stack and / or during a shutdown phase: - Creating a cooling trajectory based on measurement and / or model data, - Predicting the time of ice formation in the supply air path and / or in the exhaust air path using the cooling trajectory and - Active drying of the supply air path and / or the extract air path before the predicted time, provided that this is within the shutdown phase.

[0011] The proposed method has the advantage that the supply air path and / or the exhaust air path can be actively dried when there is a risk of ice formation due to ambient conditions and / or system conditions. If this risk exists, the timing is predicted so that ice formation can be prevented by early active drying.

[0012] In this way, the proposed method helps prevent errors during cold and / or freezing starts, thus improving the cold or freezing start capability of the fuel cell system. In particular, restart capability is improved during short-distance journeys and in cold environments.

[0013] Preferably, the ambient temperature, the coolant temperature and / or the air temperature in the supply air path and / or the exhaust air path is / are recorded by sensors and taken into account as measurement data when creating the cooling trajectory. The ambient temperature can be an indication for estimating the risk of ice formation. However, the temperature in the system also depends on other factors, such as the shutdown duration. If this is very short, the system cools down less, so that the risk of ice formation is significantly lower. Therefore, it is advantageous - alternatively or additionally - to record the air temperature in the supply air path and / or the exhaust air path. The temperature can be recorded in particular in the area of ​​a stack inlet and / or a stack outlet, in which valves and / or flaps are located, with the help of which the fuel cell stack can be separated from the air system.During shutdown, these valves and / or flaps are closed to prevent oxygen from entering the stack. If these valves and / or flaps freeze, they must first be thawed during startup, which prolongs the startup process. Furthermore, the valves and / or flaps can be damaged. If the coolant temperature is taken into account, it is preferably also measured in the area of ​​the stack inlet and / or stack outlet.

[0014] Furthermore, the measurement data is preferably updated during the shutdown phase by repeated measurements, and the cooling trajectory is corrected taking the updated measurement data into account if the repeated measurements yield different measurement data. This allows for a subsequent temperature change, such as a change in the ambient temperature, to be responded to. If, for example, the ambient temperature drops faster than expected, active drying of the supply air path and / or the exhaust air path can be brought forward to reliably prevent ice formation.

[0015] If not just one or more fuel cell stacks are shut down, but the entire fuel cell system, it may first need to be woken up to perform a new measurement and, if necessary, correct the cooling trajectory. During a longer shutdown phase, such a wake-up process can be triggered at regular intervals.

[0016] Furthermore, it is proposed that measurement data from previous shutdown phases be taken into account when creating the cooling trajectory. The process can therefore also be a learning process.

[0017] Furthermore, model data from thermal simulation calculations and / or cooling curves are preferably taken into account when creating the cooling trajectory. The model data can be stored in a control unit of the fuel cell system or made available to the control unit from a central server via a communication interface.

[0018] Furthermore, weather data is preferably taken into account when creating the cooling trajectory. The weather data is preferably made available via a communication interface, for example, for communication with a cloud.

[0019] The cooling trajectory to be created can therefore be based on current and / or predictive data. Furthermore, previous data from previous shutdown phases can be used, allowing the process to be developed as a learning process.

[0020] In a further development of the invention, it is proposed that a temperature threshold be defined, which, if exceeded, triggers active drying of the supply air path and / or the exhaust air path. The introduction of a temperature threshold increases the certainty that ice formation will not occur in the air system. Preferably, a temperature threshold is selected that lies in a range between 0°C and 5°C, more preferably between 0°C and 3°C. A temperature threshold above 0°C allows for the consideration of measurement inaccuracies and / or temperature distributions.

[0021] Furthermore, predefined temperature thresholds can be used to wake up the system, ensuring that temperature changes are reliably detected. These temperature thresholds are preferably in a range between 5°C and 30°C.

[0022] To actively dry the supply air path and / or the exhaust air path, the cathode of the at least one fuel cell stack is preferably purged with air, preferably by opening valves and / or flaps that separate the fuel cell stack from the air system. Drying by means of air purging replaces an electric heater and / or preheating strategies, thus saving components, reducing energy costs, and achieving fast start-up times.

[0023] The drying achieved with the help of air purging can be used, in particular, to re-dry the supply air path and / or the exhaust air path. This is the case if drying was already performed before shutting down at least one fuel cell stack. Re-drying then primarily serves to remove condensate that only accumulates during the shutdown phase. Condensate can collect, particularly on or in valves and / or flaps, freeze, and then block them during the next start-up. Re-drying prevents this.

[0024] Ideally, before opening the valves and / or flaps that separate the at least one fuel cell stack from the air system, the anode of the at least one fuel cell stack is purged with hydrogen. By supplying the anode with hydrogen while purging the cathode with air, the cell voltages rise to open-circuit voltage (OCV). During this process, no current is drawn, meaning the current terminals are preferably open. Due to the already lowered stack temperature, this is considered a low risk.

[0025] According to a preferred embodiment of the invention, the drying process is carried out in several, preferably two, stages, wherein in a first stage the valves and / or flaps separating the at least one fuel cell stack from the air system are opened and in a second stage the valves and / or flaps are closed again, while at the same time another valve or flap arranged in a stack bypass path is opened, so that the air is supplied via the stack bypass path to at least one component integrated into the exhaust air path, in particular at least one valve, for drying. The at least one valve can in particular be a downstream pressure control valve, a bypass valve of a bypass path for bypassing an exhaust air turbine integrated into the exhaust air path and / or a valve in a recirculation path for exhaust air recirculation.In the first stage, the valves and / or flaps located close to the stack can be dried, while in the second stage, the valves and / or flaps farther from the stack can be dried. By staggering the drying times or performing them one after the other, more effective drying can be achieved.

[0026] Furthermore, it is proposed that the residual heat of a coolant from a cooling system connected to the at least one fuel cell stack be used to actively dry the supply air path and / or the exhaust air path. The coolant can, for example, be passed through a cooler integrated into the supply air path, thereby heating the air in the supply air path. Sufficient residual heat is present when the coolant temperature is above ambient temperature and / or above 0°C.

[0027] The proposed method can be used in fuel cell systems with a single stack or with multiple stacks (multi-stack systems). In the latter case, the method can be applied when a single stack is shut down and transferred to standby mode for an extended period. This is because the stack can then cool down at low ambient temperatures. This is the case, for example, when only partial load is requested for the entire multi-stack system for a very long time, and therefore a stack is not needed, so it is transferred to standby mode.

[0028] Furthermore, a control unit for a fuel cell system is proposed. The control unit is configured to carry out steps of a method according to the invention. In particular, a cooling trajectory can be created with the aid of the control unit based on measurement and / or model data. For this purpose, the control unit can use measurement and / or model data that are stored and / or saved in a data memory of the control unit. Alternatively or additionally, the control unit can use data that is made available via a communication interface from an external data memory or server. Based on the created cooling trajectory, the drying process can then be initiated during a shutdown phase by controlling the necessary valves and / or flaps.

[0029] Preferred embodiments of the invention are described below with reference to the accompanying drawings, which show: Fig. 1 a schematic representation of a fuel cell system that can be operated according to a method according to the invention, Fig. 2 a flowchart illustrating a preferred sequence of a method according to the invention, Fig. 3 a schematic representation of another fuel cell system that can be operated according to a method according to the invention, and Fig. 4 a flowchart illustrating a further preferred sequence of a method according to the invention. Detailed description of the drawings

[0030] The Fig. 1 shows an example of a fuel cell system 1 that can be operated according to a method according to the invention. The illustrated fuel cell system 1 comprises a fuel cell stack 2 and an air system 3 for supplying the fuel cell stack 2 with air. The air is supplied via an air supply path 4 and discharged via an exhaust air path 5. The required air is taken from the environment and then discharged back into it. Since the electrochemical reaction in the fuel cells of the fuel cell stack 2 requires a certain air mass flow and a certain pressure level, the air taken from the environment is compressed using an air compressor 16. This heats up the air so strongly that it is then cooled down using a downstream cooler 17. The cooler 17 is connected to a cooling system 9 for this purpose.

[0031] The fuel cell stack 2 can be separated from the air system 3 by closing valves 6. This is particularly the case when the fuel cell system 1 is shut down. Instead of the valves 6, flaps 7 can also be provided (see reference symbols in parentheses).

[0032] To bypass the fuel cell stack 2, a stack bypass path 12 with a valve 10 integrated therein is provided, which connects the supply air path 4 with the exhaust air path 5. Instead of the valve 10, a flap 11 can also be provided (see reference numerals in parentheses). The illustrated fuel cell system 1 also has an anode subsystem 8, via which the fuel cell stack 2 is supplied with hydrogen.

[0033] Should this be in the Fig. 1 are operated according to a method according to the invention, the following can be done with reference to the Fig. 2 steps explained must be carried out.

[0034] In step S10, the shutdown of the fuel cell system 1 is initiated. To do this, a check is first carried out in step S11 to determine whether the temperature has fallen below a predetermined threshold. This can be 0°C, for example, or just above it. If this is not the case (“-”), the fuel cell system 1 can be shut down in step S12 because there is no risk of ice formation. If, however, the temperature falls below the predetermined threshold (“+”), temperature measurement data is taken in step S13, on the basis of which a cooling trajectory is then created in step S14. Based on the cooling trajectory, the time at which water present in the system freezes is then predicted in step S15. If the time is far in the future, temperature measurement data can be taken again in step S16 and compared with the earlier data or the created cooling trajectory. If necessary.The cooling trajectory must be corrected based on the current data. In step S17, a time point is defined that lies before the predicted time point at which water present in the system freezes. Upon reaching this time point, the supply air path 4 and the exhaust air path 5 are dried in step S18. The drying duration depends on the system and can be calibrated individually for each system in advance. The process is terminated in step S19.

[0035] In the Fig. In the fuel cell system 1 shown in Figure 1, the valve 6 integrated into the exhaust air path 5 is designed as a combination valve, which can also be used to regulate the pressure in the exhaust air path 5. This is not always the case.

[0036] As exemplified in the Fig. As shown in Figure 3, the fuel cell system 1 can also have a component 13 integrated into the exhaust air path 5, which is designed as a pressure control valve. Since this is arranged away from the stack, it can be kept less effectively frost-free if the air is guided through the entire air system 3 including the fuel cell stack 2 for drying. The same applies to further components 14, 15, which are integrated in the form of further valves into the air system 3 of the Fig. 3 are integrated. Component 14 is a valve in a bypass path 18 for bypassing an exhaust air turbine 19 integrated in the exhaust air path 5. Component 15 is a valve that is integrated in a recirculation path 20 for exhaust air recirculation. In order to prevent these components 13, 14, 15 or valves from freezing as effectively as possible, the drying can be carried out in two stages. In a first stage, the valves 6 or the flaps 7, which are normally closed during the shutdown phase, are opened so that the air is guided through the fuel cell stack 2. In the process, the valves 6 or the flaps 7 are dried. In the second stage, the valves 6 or the flaps 7 are then closed again and the valve 10 in the stack bypass path 12 is opened. The air is now fed past the fuel cell stack 2 to the components 13, 14, 15 in order to dry them.

[0037] As exemplified in the Fig. As shown in Figure 4, the method according to the invention can be applied both during a system shutdown and when shutting down an individual fuel cell stack of a multi-stack system. If the former is the case, the system shutdown is initiated in step S20.1. If the latter is the case, the fuel cell stack 2 is transferred to standby mode in step S20.2. Both methods have in common that a check is first carried out in step S21 to determine whether a predetermined temperature threshold is undershot, so that the implementation of the method according to the invention would be triggered. If this is the case ("+"), the method according to the invention is initiated in step S22. If this is not the case ("-"), no further measures are required and the method is terminated in step S23.

Claims

[1] Method for operating a fuel cell system (1), comprising at least one fuel cell stack (2) with a cathode and an anode and an air system (3) for supplying the at least one fuel cell stack (2) with air, wherein the air is supplied to the at least one fuel cell stack (2) via an air supply path (4) of the air system (3) and the exhaust air emerging from the at least one fuel cell stack (2) is discharged via an exhaust air path (5) of the air system (3), characterized by that when switching off the at least one fuel cell stack (2) and / or during a shutdown phase, the following steps are carried out: - Creating a cooling trajectory based on measurement and / or model data, - Predicting the time of ice formation in the supply air path (4) and / or in the exhaust air path (5) using the cooling trajectory and - Active drying of the supply air path (4) and / or the exhaust air path (5) before the predicted time, provided that this is within the shutdown phase. [2] Method according to claim 1, characterized by that the ambient temperature, the coolant temperature and / or the air temperature in the supply air path (4) and / or in the exhaust air path (5) are recorded by sensors and taken into account as measured data when creating the cooling trajectory. [3] Method according to claim 1 or 2, characterized by that the measurement data are updated during the shutdown phase by re-measuring and the cooling trajectory is corrected taking into account the updated measurement data. [4] Method according to one of the preceding claims, characterized by that measurement data from previous shutdown phases are taken into account when creating the cooling trajectory. [5] Method according to one of the preceding claims, characterized bythat model data from thermal simulation calculations and / or cooling curves are taken into account when creating the cooling trajectory. [6] Method according to one of the preceding claims, characterized by that weather data is taken into account when creating the cooling trajectory, which is preferably made available via a communication interface, for example for communication with a cloud. [7] Method according to one of the preceding claims, characterized by that a temperature threshold is defined which is preferably in a range between 0°C and 5°C, further preferably between 0°C and 3°C, which, when undershot, triggers active drying of the supply air path (4) and / or the exhaust air path (5). [8] Method according to one of the preceding claims, characterized byin that, for the active drying of the supply air path (4) and / or the exhaust air path (5), the cathode of the at least one fuel cell stack (2) is flushed with air, preferably by opening valves (6) and / or flaps (7) which separate the fuel cell stack (2) from the air system (3). [9] Method according to claim 8, characterized by that before opening the valves (6) and / or flaps (7), the anode of the at least one fuel cell stack (2) is flushed with hydrogen. [10] Method claim 8 or 9, characterized byin that the drying process is carried out in several, preferably two, stages, wherein in a first stage the valves (6) and / or flaps (7) separating the at least one fuel cell stack (2) from the air system (3) are opened and in a second stage the valves (6) and / or flaps (7) are closed again, while at the same time a further valve (10) or a further flap (11) arranged in a stack bypass path (12) is opened, so that the air is supplied via the stack bypass path (12) to at least one component (13, 14, 15) integrated into the exhaust air path (5), in particular at least one valve, for drying. [11] Method according to one of the preceding claims, characterized by that the residual heat of a coolant of a cooling system (9) connected to the at least one fuel cell stack (2) is used for actively drying the supply air path (4) and / or the exhaust air path (5). [12] Control unit for a fuel cell system (1), wherein the control unit is configured to carry out steps of a method according to one of the preceding claims.

Images