DE102013014959A1 - Method for operating a fuel cell system - Google Patents

Abstract

The invention relates to a method for operating a fuel cell system (5) having at least one fuel cell stack (6) constructed from individual cells, which is supplied via a flow compressor (15) on the cathode side with air as the oxygen supplier, and which via a line provided with a system bypass valve (20) (19) between the supply air side after the flow compressor (15) and the exhaust side has. The invention is characterized in that the system bypass valve (20) is controlled or regulated as a function of the average voltage per individual cell of the fuel cell stack (6) so that a predetermined limit value of the average voltage is not exceeded.

Description

The invention relates to a method for operating a fuel cell system according to the closer defined in the preamble of claim 1. The invention also relates to the use of such a method.

Fuel cell systems, in particular fuel cell systems for providing electric drive power for vehicles, are known from the general state of the art. Frequently, flow machines or flow compressors are used in such fuel cell systems for supplying air to supply the fuel cell on the cathode side with air as an oxygen supplier. The flow compressors can be coupled in a preferred manner with a turbine, as for example in the DE 101 20 947 A1 is described. In addition to the freewheel described there with an upstream second electric motor driven flow compressor, it is also conceivable and possible to combine the turbine, the flow compressor and the electric machine into one unit. One then speaks in the general state of the art of a so-called electric turbocharger or ETC (Electric Turbo Charger). In such structures, it is, as well as in the DE 101 20 947 A1 is shown, it is common practice to provide a line connection between the supply side after the flow compressor and the exhaust side, if present in front of the turbine. This so-called system bypass is provided with a system bypass valve or a Umblaseventil, as referred to in the aforementioned DE-document. This is typically to prevent the flow compressor from exceeding its surge limit.

Now, in particular in vehicle applications, the flow compressor is not completely shut down and braked to zero speed, for example when the fuel cell system is idling. Rather, the flow compressor is operated at a minimum speed in order to sufficiently dynamically provide the required, for example, full amount of air available again when needed. However, this creates the problem that further air is conveyed to the at least one fuel cell stack of the fuel cell system, so that it comes to a corresponding voltage build-up in the individual cells of the fuel cell stack. This is especially true when the fuel cell system on the anode side has an anode circuit in which always at least a residual amount of hydrogen over a longer period exists, and so can not be reduced immediately, for example, when the fuel cell system changes to idle.

As long as there is a sufficiently high load on the fuel cell, ie as long as sufficient power is drawn from the fuel cell or a sufficiently high current is drawn, this is not critical. However, if sufficient power is no longer taken from the fuel cell, for example because it is not needed and because a potential battery in the fuel cell system has already reached its maximum charge, then a correspondingly high voltage potential is produced at the individual cells in the region of the individual cells of the fuel cell stack. This voltage potential can easily become so high that sustained damage to the individual cells of the fuel cell stack occurs. In this context, one speaks of a degradation. Due to the high voltage potential across the individual cells, damage to the catalytic materials or the like may occur, for example. This is known per se from the general state of the art as a problem in fuel cells, in particular in PEM fuel cells.

The object of the present invention is now to provide a method for operating a fuel cell system in the construction specified in the preamble of claim 1, which efficiently counteracts this problem of degradation.

According to the invention this object is achieved by a method having the features in the characterizing part of claim 1. Advantageous embodiments and further developments emerge from the subclaims dependent thereon. In addition, a particularly preferred use is specified in claim 5.

The method according to the invention controls the system bypass valve in the connection between the supply air side downstream of the flow compressor and the exhaust side as a function of the average voltage per individual cell of the fuel cell stack. In this case, the control or regulation is carried out so that the system bypass valve is always opened or closed so far that a predetermined limit value of the average voltage is not exceeded. In particular, in idle mode, if no larger current is removed from the fuel cell stack, and if the flow compressor continues to run at its minimum speed, it comes through the continuing air supply to the cathode side to correspondingly high voltage potentials in the fuel cell stack, with the above-mentioned problem. In the method according to the invention, the system bypass valve or blow-off valve, which is typically present anyway, is now controlled in a voltage-dependent manner. As a result, the amount of air necessarily conveyed due to the minimum speed of the flow compressor can at least be increased partially through the system bypass and system bypass valve. Thus reaches on the cathode side of the fuel cell stack no or only a much smaller amount of air. As a result, the average voltage per single cell is limited accordingly, so that the fuel cell damaging mechanisms can be safely and efficiently counteracted.

The process control can be designed as a control, so that the degree of opening of the system bypass valve depending on the detected average voltage is constantly adjusted. On the other hand, it is also conceivable that a pure control is realized, that is, depending on the measured voltage, a degree of opening of the system bypass valve matching this voltage is set. On a concrete feedback, as it is characteristic of the scheme, can then be omitted to simplify the structure. The control allows in contrast to the control of a higher accuracy, but is also associated with the corresponding higher cost.

In another very advantageous embodiment of the method according to the invention, it is now also provided that the average voltage per single cell is first kept below the predetermined limit by loading the fuel cell stack with an electrical load, after which the system by-pass valve is not activated until there is no or too little decrease in power Dependence of the average voltage per single cell of the fuel cell stack is controlled. In this particularly preferred embodiment of the method according to the invention, the fuel cell stack is first loaded with an electrical load in order to keep the average voltage per individual cell below a critical limit value. This has the decisive advantage that in such a situation, a higher efficiency of the fuel cell system can be achieved, as if extracted air is blown unused back to the environment. As long as a corresponding electric power is required, for example in a vehicle in secondary consumers, or as long as it can be usefully stored in a battery for the operation of the fuel cell system or a vehicle equipped with it, this is done. Only when a sufficient power decrease from the fuel cell stack is no longer possible, for example, because no higher power requirement and the battery has already reached a full state of charge, then only the voltage-dependent control of the system bypass valve is used to an otherwise inevitable occurrence of increased voltage potentials at the individual cells of the Prevent fuel cell stack.

As a result, the blowing off of the air conveyed at minimum speed of the flow compressor air is reduced to a minimum amount, so as to ensure the best possible efficiency of the fuel cell system in most operating situations.

Since during normal operation of the fuel cell system, for example, when this supplies the drive power required to drive a vehicle, it will typically always be possible to keep the voltage of the individual cells below the critical limit. In an advantageous development of the method according to the invention, it is therefore provided that the control according to the invention does not start until the fuel cell system is idling in order to save unnecessary expenditure with regard to the acquisition of measured values or the like.

In principle, it is possible that the mean voltage per single cell of the fuel cell stack is determined by a single cell voltage monitoring and a corresponding average value is formed. However, this is extremely expensive, since it should be possible to dispense with such a single cell voltage monitoring as far as possible. In a particularly favorable and advantageous development of the method according to the invention, it is accordingly provided that the average individual voltage per cell is determined from the voltage of the fuel cell stack and the number of cells installed therein.

In addition to the voltage or the average voltage, it is of course also possible to use the variables which are electrically related to the voltage, such as the current or the average current or the power or the average power as input variables. These can either be used directly or can be correspondingly converted by their relationship to the voltage and thus used in the sense of the invention.

As already mentioned, the flow compressor may be a flow compressor of a turbocharger, which is constructed, for example, in the manner described in the aforementioned prior art as an idler, or which is part of a so-called electric turbocharger. However, it would equally well be conceivable for the flow compressor to be used as the sole flow compressor, so that a turbine in the fuel cell system is completely dispensed with. In all applications, the method described provides a simple and efficient way to avoid the mechanisms of degradation and to allow a long life of the fuel cell stack.

The particularly preferred use of the method according to the invention is now its application in a fuel cell system which provides electrical power for driving an at least partially electrically driven vehicle. Such a fuel cell system for providing electrical drive power for a vehicle is normally exposed to the typically occurring requirements of a vehicle to the conventional driving operation. Vehicles are characterized, in particular when used for example in city traffic, the fact that they are often operated at idle, as they roll out, for example, stand at a traffic light or the like. All of these situations are potentially critical to the life of the fuel cell, as this can lead to the problems described in detail above. Since the inventive method with minimal effort is able to counteract these problems, it is particularly advantageous for use in such fuel cell systems, which are operated relatively dynamically, and which often have to be used in idle mode.

Further advantageous embodiments of the method according to the invention will become apparent from the embodiment, which is described below with reference to the figure.

The sole accompanying figure shows a vehicle having an exemplary fuel cell system disposed therein.

In the only figure is a vehicle 1 to recognize. This is via an example indicated electric drive motor 2 driven by an axis 3 powered wheels 4 of the vehicle 1 to be moved. The electrical power for the electric drive motor 2 supplies a fuel cell system 5 which is exemplary in the vehicle 1 is indicated. At its core, this fuel cell system exists 5 from a fuel cell stack 6 , which is constructed as a stack of single cells. In particular, it can be designed as a PEM fuel cell stack. This fuel cell stack 6 In this case, in each of the individual cells, an anode region and a cathode region as well as a membrane and typically a cooling region are included. By way of example, an anode space is shown in the illustration of the figure 7 and a cathode compartment 8th in the fuel cell stack 6 indicated. The anode compartment 7 is hydrogen from a compressed gas storage 9 via a pressure regulating and dosing unit 10 fed. Unused hydrogen reaches the anode compartment 7 via a recirculation line 11 and a recirculation conveyor 12 back and is mixed with fresh hydrogen to the anode compartment 7 fed again. The recirculation conveyor 12 is shown here by way of example as a hydrogen circulation blower. It could just as well be designed as a gas jet pump, or as a combination of a fan and a gas jet pump. In this so-called anode circuit around the anode compartment 7 Over time, water and nitrogen accumulate through the membranes of the fuel cell from the cathode compartment 8th in the anode compartment 7 diffused. To collect the water and this together with gas from the recirculation line 11 to be able to drain, is in the representation of the figure by way of example a water separator 13 to recognize which via a drain valve 14 features. This structure is known from the prior art.

The cathode side is supplied with air as an oxygen supplier. The air is, for example, via an air filter, not shown here by a flow compressor 15 sucked in and over a humidifier 16 to the cathode compartment 8th promoted. In addition, a charge air cooler would be in the flow direction downstream of the flow compressor 15 possible, but this is not absolutely necessary and is known from the general state of the art, so that its illustration has been omitted here. The exhaust air from the cathode compartment 8th , which is correspondingly moist, passes again as a moisture supplier through the humidifier 16 , wherein the exhaust side is separated from the supply air side by membranes. These membranes, which may be formed, for example, as flat membranes or hollow fiber membranes, allow water vapor to pass, so that the humidity of the exhaust air, the supply air to the cathode compartment 8th can moisten accordingly. After the humidifier 16 the exhaust air flows through a turbine 17 in which she is relaxed, in the environment. Other components, such as a catalytic burner in the flow direction in front of the turbine 17 , are known in principle and would also be conceivable in the structure shown here. Now it is like that in the area of the turbine 17 thermal energy and pressure energy from the exhaust air of the fuel cell stack 6 at least partially recovered. This energy is used to drive the flow compressor 15 at least partially. In addition, required power is provided by an electric machine 18 delivered, which together with the turbine 17 and the flow compressor 15 sitting on a common shaft. In the case of a power surplus in the area of the turbine 17 can over the electric machine 18 also electrical power can be generated. This construction as an electrical machine 18 , Turbine 17 and flow compressor 15 is known from the general state of the art and is also referred to as an electric turbocharger or ETC (Electric Turbo Charger).

In the fuel cell system shown here 5 is now also a leader 19 with a so-called system bypass valve 20 to recognize. The system bypass valve 20 will typically be closed in regular operation. In this situation, the pressure regulating and dosing unit 10 Hydrogen dosed into the anode circuit and according to the power requirement by the electric drive motor 2 of the vehicle 1 , which ultimately, for example, by an accelerator pedal position of a user of the vehicle 1 is specified, electric power is in the fuel cell stack 6 generated. Over indicated electrical lines 21 This power comes in the area of power electronics 22 , which prepares this accordingly and to the electric traction motor 2 passes. About the power electronics 22 Also, if necessary, the electric machine 18 powered by the electric turbocharger. Surplus power, or when braking the vehicle in the regenerative operation of the electric drive motor 2 resulting power, may be in an electrical energy storage device 23 , For example, a high-voltage battery, a plurality of high-power capacitors or a combination thereof, are stored.

A problem of the flow compressor 15 , both in a single flow compressor 15 as well as its construction as ETC together with the electric machine 18 and the turbine 17 , as shown here, lies in the fact that it must run at relatively fast speeds to a sufficient amount of air, for example, at medium or full load of the fuel cell system 5 provide. Will he be idling the fuel cell system 5 slowed down completely, it takes a very long time until it reaches the required speed again. This is typically unacceptable. In operation of the vehicle 1 therefore becomes the flow compressor 15 always, even when idling the fuel cell system 5 , operated at a predetermined minimum speed in the order of typically 5-20% of the rated speed to ensure a fast run-up when needed can. This operation of the flow compressor 15 with given minimum speed but now ensures that in the corresponding operating situations, air in the cathode compartment 8th of the fuel cell stack 6 although this is not really desirable. If a sufficient decrease in performance can be ensured, this can be achieved, for example, by reducing the performance of secondary consumers in the vehicle 1 or storing the power in the electrical energy storage 23 It can be prevented that the voltage potentials on the individual cells undesirably increase. If such a decrease in performance is not possible, for example because there is insufficient power requirement in the vehicle 1 exists and at the same time the energy storage 23 is fully charged, then it can lead to a very harmful increase in potential at the individual cells of the fuel cell stack 6 come. This leads to a significant damage to the individual cells and ultimately to a degradation and a reduction in the life of the fuel cell stack 6 ,

In the embodiment of the fuel cell system shown here 5 or of the vehicle 1 It is now planned that on the power electronics 22 which at least the voltage of the fuel cell stack 6 knows anyway, a mean voltage per single cell is determined. Ideally, this is done by dividing the total voltage of the fuel cell stack 6 by the number of installed single cells. Reaches this average voltage of the individual cells of the fuel cell stack 6 now a value critical to the degradation, then via a control electronics, which in the embodiment shown here also in the power electronics 22 integrated, a voltage-dependent control of the system bypass valve 20 , This is thus dependent on the average voltage of the individual cells of the fuel cell stack 6 controlled so that air in the bypass on the fuel cell stack 6 pass directly into the turbine 17 and thus into the area of the exhaust air is passed. Although this is the efficiency of the fuel cell system 5 deteriorates accordingly, since the air has been promoted with a corresponding effort. However, by this blowing off of the conveyed air to the environment depending on the voltage of the individual cells of the fuel cell stack via the system bypass valve 20 be achieved that the potentials at the single cells of the fuel cell stack 6 even if no power from the fuel cell stack 6 each is so small that they do not reach harmful in terms of degradation areas. The structure thus deteriorates the efficiency of the fuel cell system 5 minimal over its service life, but allows prevention of degradation and thus a very gentle handling of the fuel cell stack 6 , As a result, a significant increase in the life is achieved, which provides a significant additional advantage over the minimum loss of efficiency accepted in this regard.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 10120947 A1 [0002, 0002]

Claims (5)

Method for operating a fuel cell system ( 5 ) with at least one fuel cell stack constructed from individual cells ( 6 ), which via a flow compressor ( 15 ) is supplied with air as an oxygen supplier on the cathode side, and which is supplied via a system bypass valve ( 20 ) provided line ( 19 ) between the supply air side after the flow compressor ( 15 ) and the exhaust side, characterized in that the system bypass valve ( 20 ) as a function of the mean voltage per single cell of the fuel cell stack ( 6 ) is controlled so that a predetermined limit value of the mean voltage is not exceeded. Method according to claim 1, characterized in that first by loading the fuel cell stack ( 6 ) the average voltage per individual cell is kept below the predetermined limit value with an electrical load, after which the system bypass valve (only if there is no or too little decrease in electrical power) 20 ) as a function of the mean voltage per single cell of the fuel cell stack ( 6 ) is controlled so that the predetermined limit value of the average voltage is not exceeded. A method according to claim 1 or 2, characterized in that the control or regulation of the system bypass valve ( 20 ) is applied only in idle depending on the average voltage per single cell of the fuel cell stack. Method according to claim 1, 2 or 3, characterized in that the mean voltage per single cell of the fuel cell stack ( 6 ) from the total voltage of the fuel cell stack ( 6 ) and the number of single cells is determined. Use of the method according to one of claims 1 to 4, for operating a fuel cell system ( 5 ), which in a vehicle ( 1 ) is used to provide electrical drive power.

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