Method for drying the cathode path of a fuel cell stack in a fuel cell system

Abstract

The invention relates to a method for drying the cathode path (2) of a fuel cell stack (1) in a fuel cell system, wherein the drying time is regulated according to the time profile of the stack voltage (VS) under constant operating conditions of the fuel cell stack (1) by the subsystems cooling system (7), hydrogen system (8), air system (13) and / or electrical system.

Description

The present invention relates to a method for drying the cathode path of a fuel cell stack in a fuel cell system.In fuel cell systems of the type of interest here, the oxidizing agent oxygen from the ambient air and the reducing agent and hydrogen serving as fuel are generally used in order to react to water in a fuel cell stack consisting of a plurality of fuel cells and thus to generate an electrical power by electrochemical conversion.Prior ArtIn particular in the case of mobile fuel cell systems, according to the generally known prior art, the start of the fuel cell system can be implemented both functionally under all worldwide relevant environmental conditions and at different long downtime periods and can be implemented with respect to applicable lifetime requirements.Thus, it is important, when a fuel cell system is started to freeze, to bring the fuel cell stack out of the freezing zone as quickly as possible, so that water which is produced does not freeze at critical points in the fuel cell stack. In the event of a defective freezing start, both the fuel cell stack can be irreversibly damaged and the fuel cell system can lose the startability. In vehicle applications, the vehicle together with the fuel cell system would first have to be brought into a sufficiently warm environment in order to enable starting.To prevent these technical problems, it is important how much water the fuel cell stack contains before the start or at the beginning of the start. This amount of water must be within a predetermined tolerance range so that the fuel cell stack can, on the one hand, still incorporate the water occurring during the start into its storable components; on the other hand, it also does not dry it so completely that no proton conductivity of the membrane is possible at all or even damage to the membrane occurs in the case of excessively dry states.For these reasons, the states and operating modes upstream of the start of the fuel cell system are relevant in order to already ensure preparatory measures for restarting, namely in particular the shutdown of the fuel cell system including the drying method according to the invention and functionalities in the standstill phase, such as a so-called parking purge.Heretofore, the drying of a cathode path of a fuel cell stack by means of an air compression system of the fuel cell system is carried out by so-called blowing out for a defined period of time.It is the object of the present invention to adapt the required drying time to the drying requirement and in particular to end it in a suitable time window so that the fuel cell stack is not shut down too wet, the membrane of the fuel cell stack does not become too dry and / or the shut-down of the fuel cell system does not last too long.It should be noted that a fuel cell stack that is too moist may cause subsequent problems during a start of freezing as a result of degradation, damage and / or malfunction of the fuel cells. A membrane which is too dry is subject to degradation by shrinkage, hydrogen diffusion from anode to cathode with undesired concentration increase in the cathode, unnecessary hydrogen consumption and, in the extreme case, damage. Stopping for too long a time may result in an inconveniently long time for the user.Disclosure of the InventionThe object is achieved by a method according to claim 1.According to the invention, the drying duration is adjusted according to the time profile of the stack voltage under constant operating conditions of the fuel cell stack by at least one of the subsystems air system, cooling system, hydrogen system and / or electrical system.In other words, the duration of the drying method is thus controlled on the basis of the voltage profile of the stack, with constant stack operating conditions preferably being set in by the subsystems mentioned. The trajectory of the voltage profile is preferably evaluated with respect to the first and second derivation, in particular the inflection point, and the percentage or absolute voltage change, and the respectively matching drying duration of the current drying process is thus controlled. Purely time-controlled drying of the cathode path can thereby be dispensed with. It should be noted that the method according to the invention can also be applied to single cell voltages or the measured impedance of fuel cells.The advantages of the method according to the invention consist in particular in the fact that the humidity of the membrane in the fuel cell stack can be kept within a predefined tolerance band, which improves the freezing startability. In addition, degradation up to damage to the fuel cell stack is avoided and the time for shutting down the fuel cell system only runs as long as necessary and thus time-optimized. Further measures which improve the invention are described in more detail below together with the description of a preferred exemplary embodiment of the invention on the basis of the figures.Exemplary EmbodimentsIt shows: FIG. 1 shows a schematic illustration of a fuel cell system, FIG. 2 shows a graphical representation of the time profile of different variables of the fuel cell system during a drying process, FIG. 3 shows a graphical representation of a U-I characteristic curve with different membrane humidities and exemplary operating point for the drying method, and FIG. 4 shows a graphical representation of a time profile of individual cell voltages of a fuel cell stack during the drying process.According to FIG. 1, a fuel cell system of the type of interest here known per se substantially consists of a fuel cell stack 1 comprising a plurality of fuel cells, with a conventional cathode path 2, cooling path 3 and anode path 4.The cathode path 2 is part of a cooling circuit with a motor-driven coolant pump 6, a heat exchanger 5 existing with the environment and a downstream three-way valve 5 afor switching between a small and a large coolant circuit of the cooling system 7.The anode path 4 of the fuel cell stack 1 is part of a hydrogen system 8, further comprising a motor-driven hydrogen recirculation blower 9, a subsequent ejector pump 10, to which hydrogen stored in a tank 12 is supplied via a hydrogen metering valve 11.The cathode path 2 is part of an air system 13, further comprising in particular a motor-controlled air compressor 14 for extracting air from the environment 15 via an air filter 16, which is led downstream via a heat exchanger 15 in order to pass cooled into the cathode path 2 of the fuel cell stack 1. In addition, conventional valves-and therefore not described in detail-are integrated for control purposes.The fuel cell system further comprises a likewise conventional electrical system, which is not shown in any further detail here.In order to obtain the most optimum possible evaluation capability by evaluating the voltage trajectory, the operating conditions of the fuel cell stack 1 are adjusted as constantly as possible during the drying process or at least in a section of the drying process which is relevant for determining the time duration or the termination of the drying process.The cooling system 7 constantly adjusts the conditions in the cooling path 3, in particular a constant fuel cell stack inlet temperature and a constant temperature difference of the coolant between inlet and outlet temperature.In the hydrogen system 8, the so-called anode circuit, the conditions are constantly adjusted or kept constant so that they change as little as possible or as little as possible at the anode of the fuel cell stack.The air system 13 constantly adjusts the conditions at the cathode inlet, i.e. in particular a constant air mass flow, a constant pressure level in the cathode path 2 and a constant cathode inlet temperature. The air mass flow is usually carried out with a high excess of air, so that both the quantity of water generated by the drawn flow is transported away and the actual drying takes place. Lambda air values greater than 10 are customary for this purpose.The electrical subsystem draws a constant current to enable the voltage rating to be sufficiently accurate.The aforementioned subsystems of the fuel cell system can be triggered simultaneously or also differently with respect to the beginning of the drying method. Corresponding preparations or operating parameter settings can also already be carried out in preceding method steps.According to the invention, during the drying process, the stack voltage V S is monitored, recorded and the trajectory is evaluated in real time under the constant operating conditions of the fuel cell stack specified above.FIG. 2 shows a correspondingly real measurement over time of the stack voltage V S as a voltage drop ΔU during the drying process. The time course takes place horizontally as can be seen. In this exemplary embodiment, the time interval A to B is approximately 76 seconds, in which a voltage drop ΔU of approximately 12% results. The time C marks the end of the drying process. The operating conditions of the fuel cell stack are at least approximately constant. In particular, a constant profile of the electric current I of the electric system is illustrated here by way of example.In addition to the aforementioned percentage voltage drop in the relevant evaluation region, the absolute voltage drop, the gradient (first derivative) and the curvature (second derivative) are also monitored in particular. Typically, during the drying process under constant operating conditions, an inflection point occurs during the time profile of the stack voltage V S. At the beginning of the drying process, the membrane, the gas diffusion layer, the channels, i.e. the complete surfaces, are well supplied with moisture or wetted with water. Despite the removal of moisture or water, the proton conductivity of the membrane is still sufficient, so that the stack voltage V S falls only slowly. As drying proceeds, the moisture content of the membrane falls more significantly. The gradient becomes steeper. A concentration gradient resulting from this between cathode and anode results in a transport of water from the still moist anode to the dry cathode after a certain time. This prevents an even greater voltage dip. The anode is thereby dried. This effect and further drying time again result in a smaller stress gradient. The drying is carried out sufficiently. Further drying would dry the membrane too much, with the disadvantages described above.FIG. 3 illustrates different U-I characteristic curves for different moisture states of a membrane of an exemplary fuel cell and the drying process, which preferably takes place here on a constantly drawn current. The resistance of the membrane increases as the membrane becomes drier, resulting in a lowered stack voltage compared to the optimized operating state.Specifically shown are U-I characteristic curves c 1, c 2 and c 3 with different membrane humidities and exemplary operating point for the drying method at constant current Idry and stack voltage UdryStart at the beginning of the relevant drying section for the evaluation and the stack voltage UdryEnd at the end of the relevant drying section. UmaxSoll, ImaxSoll, IminSoll limit the operating characteristic diagram of the fuel cell system as boundary conditions.The described effects and the current trajectory evaluation allow criteria for the termination of the drying process to be defined and then used to terminate the drying process variably, i.e. without a constant drying time, and therefore appropriately. A specific criterion or a combination of the following termination criteria can be used here:ratioU > ratioULimggU=0, i.e. the inflection point is reacheddUBestrag>dUBestragLimAfter the evaluation is completed, including the criteria being reached, the drying process can either be terminated immediately or a further time can be postdried in order to compensate for any uncertainties from the evaluation and the three-dimensional distribution of the thermodynamic variables in the fuel cell stack.The drying process is ended and a subsequent system state is carried out, for example a so-called bleeddown, i.e. closing of the shut-off valves in the cathode path, in order to exhaust the enclosed cathode volume with respect to oxygen.The method according to the invention cannot be applied solely to the entire stack voltage V S. If a single cell voltage can be measured using a so-called CVM device, single cell voltages or cell packets can also be evaluated analogously to the method according to the invention. A combined application can also be carried out taking into account single cell voltages and total voltage. The individual voltages allow the drying state for regions of the fuel cell stack to be additionally taken into account and the duration of the drying process to be controlled even more specifically. However, this requires sufficient robustness of the sensor system.The method according to the invention can moreover be advantageously combined with further measures and monitoring functions. For example, an impedance measurement of the stack voltages can be carried out. It should also be noted that the method according to the invention can also be carried out under two or more constant operating conditions, that is to say a state with two different parameter sets, for example a first parameter set for a fuel cell stack temperature and a further parameter set for a fuel cell stack temperature, wherein a transient transition takes place therebetween even without evaluation of the voltage.

Claims

Method for drying the cathode path (2) of a fuel cell stack (1) in a fuel cell system, wherein the drying duration is adjusted according to the temporal profile of the stack voltage (V S) under constant operating conditions of the fuel cell stack (1) by the subsystems cooling system (7), hydrogen system (8), air system (13) and / or electrical system.

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