Method for operating a fuel cell system

Abstract

Method for operating a fuel cell system (100) with at least one fuel cell stack (11) and a cathode system (300) with a cathode supply line (31), a cathode outlet line (32) and a recirculation line (34), wherein a valve (35) is arranged within the recirculation line (34), wherein it is determined whether freezing of water at the valve (35) is to be expected within a defined period of time and if freezing of water at the valve (35) is to be expected within a defined period of time, the valve (35) is set to an open switching position.

Description

The invention relates to a method for operating a fuel cell system having the features of the preamble of independent claim 1.Prior ArtIt is known from the prior art that fuel cell systems exist which have an anode system and a cathode system. The anode system consists of an anode feed line which feeds fuel, in particular hydrogen, to a fuel cell stack, and a recirculation line which returns anode off-gas into the anode feed line by means of a conveying unit. Furthermore, the cathode system consists of a cathode feed line, in which a compressor can be arranged, and a cathode outlet line, by which cathode exhaust gas is conveyed from the cathode system into the exhaust line.Starting the fuel cell system at temperatures below 0° C. may be a challenge due to product water freezing. In order to prevent product water from freezing, air flows through the cathode system and / or hydrogen flows through the anode system in a drying process, so that water is discharged from the fuel cell system.Disclosure of the InventionThe method according to the invention for operating a fuel cell system having the features according to independent claim 1 has the advantage that damage to a valve which is arranged in a recirculation line of a cathode system can be avoided in a reliable manner.The method according to the invention makes it possible to dispense with a heating element in order to transfer the frozen water at the valve into a liquid state and to avoid damage to the valve.With the aid of the method according to the invention, the drying method can be shortened when the fuel cell system is shut down, since water accumulations are possible in the region of the valve which is arranged within a recirculation line.Damage to the valve can be avoided by determining whether freezing of water at the valve is to be expected within a defined time period and, if freezing of water is to be expected at the valve within a defined time period, the valve is set in an open switching position. Since the valve is not closed when frozen water is present on the valve, damage to the material due to a force exertion during opening is avoided.It is advantageous if, during the transition between two operating states, it is determined whether freezing of water at the valve is to be expected. An operating state is a state that may occur during a vehicle operating period, specifically an active state of the fuel cell system in which electric power generation is occurring and an inactive state in which electric power generation is not occurring. During the transition between two operating states, a change in the temperature in the fuel cell system can occur, so that water deposits can occur in the fuel cell system and water collects on the valve. By carrying out the method according to the invention, it is possible to prevent damage from frozen water on the valve despite water deposition on the valve.When shutting down the fuel cell system, it is advantageous to determine whether freezing of water at the valve is to be expected. The shutdown of the fuel cell system represents a transition between an active state and an inactive state of the fuel cell system. When the fuel cell system is in an inactive state, it is possible that the ambient temperature of the fuel cell stack decreases and water may thereby freeze at the valve, because the fuel cell system no longer generates waste heat. The method according to the invention can be used to prevent damage to the valve due to frozen water in a targeted and efficient manner.When starting the fuel cell system, it is advantageous to determine whether freezing of water at the valve is to be expected. The start of the fuel cell system represents a transition between an inactive state and an active state of the fuel cell system. When the fuel cell system is started, an inhomogeneous temperature distribution is present within the fuel cell system, such that individual regions are already heated and water is present in a liquid or gaseous state. If the water in liquid or gaseous state reaches the valve and the valve is not yet sufficiently heated, the water may freeze again. The method according to the invention can be used to prevent damage to the valve due to frozen water in the case of the start of the fuel cell system in a targeted and efficient manner.It is advantageous if it is determined during an operation stop whether freezing of water at the valve is to be expected. An operation stop of the fuel cell system is an inactive operating state. During the stop of operation, the ambient temperature of the fuel cell system may change, so that the method according to the invention can be carried out more efficiently and in a targeted manner if it is determined during the stop of operation whether freezing of water at the valve is to be expected.During the stop of operation, it is advantageously determined by cyclic or periodic repetitions whether freezing of water at the valve is to be expected. When the fuel cell system is stopped in operation, it is possible for the latter to take place over a relatively long period of time and for temperature changes in the ambient temperature to result as a result. If it is determined in the context of cyclical or periodic repetitions whether freezing of water at the valve is to be expected, the method can contribute efficiently to minimizing damage to the valve even during an operation stop.It is advantageous if it is determined whether freezing of water at the valve is to be expected by determining a temperature T and comparing it with a limit temperature TGrenzand if the temperature T is less than the limit temperature TGrenz, freezing of water at the valve is to be expected. By comparing the determined temperature T with the limit temperature TGrenz, it is possible to approximate, with little effort, in a method step whether freezing of water at the valve is to be expected.The temperature T is advantageously determined by means of a temperature sensor which is arranged in the recirculation line. By measuring the temperature T with a temperature sensor, an accurate determination of the temperature T can be made possible and provided for carrying out the method according to the invention.Advantageously, the temperature sensor is arranged on the valve. As a result, the temperature sensor can determine the temperature T for the valve very accurately.It is advantageous if the temperature T is determined by modelling the expected temperature profile at the valve. The modelling can be supported by cloud-based data, such as, for example, a temperature in the environment of a vehicle in which the fuel cell system is arranged and / or an expected temperature profile in the environment of the vehicle in which the fuel cell system is arranged. As a result, a temperature sensor can be dispensed with and a compact and cost-effective construction of the fuel cell system can be ensured.DESCRIPTION OF THE DRAWINGSThe fuel cell system and the method according to the invention are explained in more detail below with reference to drawings with preferred exemplary embodiments.The following are shown: FIG. 1 shows a schematic topology of a fuel cell system, and FIG. 2 shows a flow chart of the method according to the invention.FIG. 1 shows a schematic topology of a fuel cell system 100 having at least one fuel cell stack 11 and a cathode system 300. Furthermore, the fuel cell system 100 includes a highly schematically illustrated anode system 200 and a cooling circuit, not illustrated.The anode system 200 supplies an anode space of the fuel cell stack 11 with a fuel or anode fluid, in particular hydrogen (H 2) as reactants. The anode system includes an anode supply line and a recirculation line. Anode exhaust gas is recirculated from an anode space, which is arranged in the fuel cell stack 11, to the anode feed line via the recirculation line. Between the anode feed line and the recirculation line, a jet pump with metering valve is arranged.The cathode system 300 supplies oxygen (O 2) as a reactant to a cathode space arranged in the fuel cell stack 11. Oxygen forms a constituent of air. By supplying air into the fuel cell system 100, the oxygen is made available to the fuel cell system as a reactant.In the cathode system 300, a cathode supply line 31, a cathode outlet line 32, a recirculation line 34 and a cathode path are arranged.The cathode feed line 31 opens into the fuel cell stack 11. oxygen is supplied to the fuel cell stack 11 via the cathode feed line 31.Gases, such as cathode exhaust gas and / or fluids, such as liquid water, are discharged from the cathode system 300 via the cathode outlet line 32.The recirculation line 34 is arranged between the cathode feed line 31 and the cathode outlet line 32 and connects these. As a result, cathode off-gas can flow from the cathode outlet line 34 into the cathode feed line 31.A compressor 33 is located inside the cathode feed line 31, and the compressor 33 is arranged downstream of the recirculation line 34 in the flow direction.The compressor 33 conveys air into the fuel cell stack 11. An increase in the output of the compressor 33 has the effect that an increased volume flow of air is supplied to the fuel cell stack 11 via the cathode feed line 31. By reducing the power of the compressor 33, a reduced volumetric flow of air is supplied to the fuel cell stack 11.A valve 35 and a temperature sensor 36 are arranged within the recirculation line 34. In an alternative embodiment, a recirculation conveying unit can additionally be arranged in the recirculation line 34. The recirculation conveyor may assist in the flow of cathode exhaust gas from the cathode outlet conduit 32 into the cathode supply conduit 31.With the aid of the valve 35, the flow rate of the cathode off-gas through the recirculation line 34 can be adjusted. When the valve 35 has a closed switching position, no cathode exhaust gas can flow through the recirculation line 34. When the valve 35 is proportionally opened, cathode exhaust gas flows proportionally through the recirculation line 34.The temperature sensor 36 can determine the temperature T in the recirculation line and / or at the valve 35 and provide the measured value for carrying out the method according to the invention. The temperature sensor 36 is arranged within the recirculation line 34 before or after the valve 35. In an alternative embodiment, the temperature sensor 36 may be disposed on the valve 35.A control unit 500 is provided in order to control or regulate all control and control processes in the fuel cell system 100. This also includes the processing of at least one measured value for carrying out the method according to the invention.In an alternative embodiment, more than one fuel cell stack 11 can also be arranged in the fuel cell system 100, without the execution of the method according to the invention being restricted thereby.FIG. 2 shows an exemplary embodiment of the method according to the invention.The method according to the invention makes it possible to avoid damage to the valve 35 by frozen water in a reliable manner. For this purpose, the valve 35 is placed in an open switching position, so that an opening of the valve 35 in the icy state or when water has frozen at the valve 35 and the associated material damage, in particular elastomeric material components, can be avoided.In a step S 100, the method is started. The method is preferably started when a transition between two operating states occurs, in particular during a shutdown or a startup of the fuel cell system 100. Furthermore, the method can be started during an operation stop and repeated cyclically or periodically during the operation stop.An operating state is a state that may occur during a vehicle operation period, specifically, an active state of the fuel cell system 100 in which electric power generation is occurring and an inactive state in which electric power generation is not occurring.In a step S 200, it is determined whether freezing of water within a defined time period is to be expected. The defined time interval is a fixed time indication, such as one hour or three hours. It is likewise possible to define the time period as a variable time indication, so that, for example, a time indication is defined in a stored table as a function of meteorological data.Whether freezing of water at the valve 35 is to be expected can be determined by determining a temperature T and comparing it with a limit temperature Tthreshold.The temperature T may be measured via a temperature sensor 36 on the valve 35 or near the valve 35.The temperature T can be determined by modelling the expected temperature profile at the valve 35. In this case, the lowest approximated temperature that is to be expected within the defined time period corresponds to the temperature T.The limit temperature TGrenzmay correspond to a temperature at which water is frozen. For determining the limit temperature TGrenz, the ambient pressure can optionally be taken into account as well, so that the limit temperature can be adapted to the ambient pressure via a stored table.It is also possible to set the limit temperature TGrenz to a temperature that is near the temperature at which water is frozen.If the temperature T is lower than the limit temperature Tthreshold, freezing of water at the valve 35 is expected.When freezing of water in the defined period is expected, a step S300 is executed. In step 300, the valve 35 is placed in an open switching position.If no freezing of water is expected in the defined period of time, a step S400 is executed.In step S 400, the method according to the invention is ended. Step S 400 is executed either after step S 200 or after step S 300.The method according to the invention can be carried out in a fuel cell system 100 having a plurality of fuel cell stacks 11 in parallel or sequentially.The method may further be performed at least in part by the controller 500 of the fuel cell system 100. A computer program in the form of a code can be stored in a memory unit of the control unit 500, which computer program, when the code is executed by a computing unit of the control unit 500, carries out a method which can run as described above. With the aid of the control unit 500, the same advantages can be achieved as have been described above in connection with the method according to the invention. These advantages are referred to in the present case in their entirety.The control unit 500 may be in communication with the sensors of the fuel cell system 100 to monitor the sensor values.The control unit 500 can actuate the actuators in the fuel cell system 100 in order to carry out the method accordingly.In addition, the control unit 500 can be in a communication connection with an external computing unit in order to transfer some method steps and / or calculations wholly or partly to the external computing unit.

Claims

Method for operating a fuel cell system (100) having at least one fuel cell stack (11) and a cathode system (300) having a cathode feed line (31), a cathode outlet line (32) and a recirculation line (34), wherein a valve (35) is arranged within the recirculation line (34), characterized in that it is determined whether freezing of water at the valve (35) is to be expected within a defined time period and, if freezing of water is to be expected at the valve (35) within a defined time period, the valve (35) is set in an open switching position.Method according to Claim 1, characterized in that, during the transition between two operating states of the fuel cell system (100), it is determined whether freezing of water at the valve (35) is to be expected.Method according to Claim 2, characterized in that, when the fuel cell system (100) is shutting down, it is determined whether freezing of water at the valve (35) is to be expected.Method according to Claim 2, characterized in that, when the fuel cell system (100) is started, it is determined whether freezing of water at the valve (35) is to be expected.Method according to Claim 1, characterized in that it is determined during an operation stop of the fuel cell system (100) whether freezing of water at the valve (35) is to be expected.Method according to Claim 5, characterized in that during the stop of operation it is determined in cyclic or periodic repetitions whether freezing of water at the valve (35) is to be expected.Method according to claim 1, characterised in that it is determined whether freezing of water at the valve (35) is to be expected by determining a temperature T and comparing it with a limit temperature TGrenz and that if the temperature T is less than the limit temperature TGrenz freezing of water at the valve (35) is to be expected.Method according to Claim 7, characterized in that the temperature T is determined by means of a temperature sensor (36) which is arranged in the recirculation line (34).Method according to claim 8, characterised in that the temperature sensor (36) is arranged on the valve (35).Method according to claim 7, characterised in that the temperature T is determined by modelling the temperature profile to be expected taking place, in particular modelling by cloud-based data.

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