DE102021207749A1 - Method for controlling a drying process of a fuel cell system - Google Patents

Abstract

The invention relates to a method for controlling a drying process of a fuel cell system (100) with at least one stack (101), in particular when the fuel cell system (100) is switched off, preferably in preparation for a start, in particular a freeze start and/or cold start, of the fuel cell system (100 ), Having the following steps: - operating a cathode system (10) of the fuel cell system (100) to dry the fuel cell system (100), - pulsing and/or varying an air mass flow (dm/dt) through the stack (101), and/or- Pulsating and/or varying a cathode pressure (p) in the stack (101).

Description

The invention relates to a method for controlling a drying process of a fuel cell system with at least one stack, in particular when shutting down the fuel cell system, preferably in preparation for a start, in particular a freeze start and/or cold start, of the fuel cell system according to the independent method claim. In addition, the invention relates to a corresponding control unit and a corresponding computer program product.

State of the art

• - funktional zu realisieren und
• - dabei die Lebenszeitanforderungen an das System zu erreichen.

In vehicles, so-called fuel cell vehicles, in which the drive energy is supplied by one or more fuel cell systems, among other things, the oxidizing agent oxygen from the ambient air and the reducing agent or fuel hydrogen are used to convert water (or water vapor) in the fuel cell. to react and thus to supply electrical power through electrochemical conversion. The fuel cell systems usually include several fuel cells that are combined to form a stack. With mobile fuel cell systems, the challenge is starting the system under all globally relevant conditions and with different lengths of vehicle downtime:

• - to implement functionally and
• - while achieving the lifetime requirements for the system.

• - Trocknungsverfahren des Stacks beim Abstellen des Systems, und/oder
• - Purge-Vorgang im Stillstand des Systems.

In the case of freezing starts, the focus is, among other things, on getting the stack out of the freezing zone (temperature > 0°C) as quickly as possible so that the water that forms does not freeze at critical points in the stack. In the event of a faulty freeze start, both the stack can suffer massive irreversible damage and the system cannot be started. In the event of a failed freeze start, the vehicle would have to be taken to a "warm" area or towed. It is usually important how much water the stack contains before the start or at the beginning of the start. This amount of water should advantageously be within a tolerance band so that the stack can store the water that accumulates during the start in its storable components (e.g. membrane, gas diffusion layer, etc.) without getting blockages from freezing water. On the other hand, the stack should not be completely dried, so that proton conductivity of the membrane is not possible and the membrane is damaged by states that are too dry. States and operating modes before the start are therefore essential in order to ensure preparatory measures for the restart, such as e.g. e.g.:

• - Drying procedure of the stack when shutting down the system, and/or
• - Purge process when the system is at a standstill.

The drying of the cathode path can be carried out for a defined time, for example, using an air compression system (to "blow out" the stack).

Disclosure of Invention

The present invention provides: a method for controlling a drying process of a fuel cell system with at least one stack, in particular when shutting down the fuel cell system, preferably in preparation for a start, in particular a cold start or freeze start, of the fuel cell system with the features of the independent method claim. In addition, the invention provides a corresponding control unit and a corresponding computer program with the features of the independent claims. Features and details that are described in connection with the different embodiments and/or aspects of the invention also apply, of course, in connection with the other embodiments and/or aspects and vice versa, so that the disclosure of the individual embodiments and/or aspects is reversed is or can always be mutually referred to.

According to the first aspect, the present invention provides: a method for controlling a drying process of a fuel cell system with at least one stack, in particular when shutting down the fuel cell system, preferably in preparation for a start, in particular a freeze start and/or cold start, of the fuel cell system.

• - Betreiben eines Kathodensystems des Brennstoffzellensystems zum Trocknen des Brennstoffzellensystems,
• - Pulsieren und/oder Variieren eines Luftmassenstroms bzw. eines Luftvolumenstroms durch den Stack, und/oder
• - Pulsieren und/oder Variieren eines Kathodendrucks im Stack.

The procedure has the following steps:

• - Operating a cathode system of the fuel cell system to dry the fuel cell system,
• - Pulsating and/or varying an air mass flow or an air volume flow through the stack, and/or
• - Pulsating and/or varying a cathode pressure in the stack.

The steps of the method according to the invention can be carried out in the given order or in a modified order. The steps of the method according to the invention can simultaneously, at least partly at the same time and/or in succession.

The fuel cell system according to the invention can preferably be used for mobile applications, for example in vehicles, in particular fuel-powered vehicles. The fuel cell system according to the invention can serve as the main energy supplier for a vehicle. At the same time, however, it is also conceivable that the fuel cell system according to the invention can supply energy for an auxiliary drive and/or auxiliary drive of a vehicle, for example a hybrid vehicle. The fuel cell system according to the invention can also be used for stationary applications, for example in generators.

The fuel cell system according to the invention can have one or more stacks, each with several stacked fuel cells and the associated functional systems, including: media systems (air or cathode system, fuel or anode system, cooling system) and an electrical system. The fuel cell system according to the invention can preferably comprise a number of modules in the form of individual stacks with a number of stacked fuel cells.

• - durch Pulsationen und/oder Variationen des Luftmassenstroms (bzw. des Luftvolumenstroms) im Stack,

und/oder

• - durch Pulsationen und/oder Variationen des Kathodendrucks im Stack, die Trocknung des Stacks:
• - direkt auf einer Kathodenseite, und
• - zumindest indirekt auch auf Anodenseite

und auch die Trocknung des Kathodensystems verbessert/optimiert erfolgen kann.The invention proposes that the drying process or operation:

• - due to pulsations and/or variations in the air mass flow (or the air volume flow) in the stack,

and or

• - by pulsations and/or variations of the cathode pressure in the stack, the drying of the stack:
• - directly on a cathode side, and
• - at least indirectly also on the anode side

and the drying of the cathode system can also be improved/optimized.

• - beim Abstellen,
• - in Stillstandphasen, und/oder
• - an unterschiedlichen Betriebsarten (Stack-Betrieb, Bypass-Betrieb) angewandt werden, ggf. mit unterschiedlichen Betriebsparametern.

The procedure can, for example

• - when parking,
• - in standstill phases, and/or
• - be applied to different operating modes (stack mode, bypass mode), possibly with different operating parameters.

The method can preferably also be combined, synchronized and/or coupled with the pressure difference monitoring and/or regulation with regard to the membrane (on the cathode side/on the anode side) and/or with anode drying.

The flow as well as the flow form in the cathode path of the stack experiences a high variance in terms of flow conditions, Reynolds numbers, pressure pulses, pressure differences, etc., so that different geometries within the stack are dehumidified and dried in an improved way.

• - eine verbesserte Entfeuchtung/Trocknung des Stacks beim Abstellen und/oder in den Stillstandphasen,
• - verbesserte Gefrierstarts,
• - Verminderung der Degradation des Stacks, und/oder
• - durch Variation und Bewegung der Drosselklappen im Abgaspfad des Kathodensystems während des Trocknens wird außerdem die Gefahr des Einfrierens der Aktoren/Drosselklappen beim Trocknen eliminiert.

The process can achieve several significant benefits:

• - improved dehumidification/drying of the stack when shutting down and/or in the standstill phases,
• - improved freeze starts,
• - reducing the degradation of the stack, and/or
• - By varying and moving the throttle valves in the exhaust gas path of the cathode system during drying, the risk of the actuators/throttle valves freezing during drying is also eliminated.

In the following, mass flow is mentioned (volume flow is possible in the same way).

• OP1 Trocknen eines Kathodenpfades und einer Abluftleitung des Kathodensystems, wobei insbesondere Absperrventile geöffnet sind, ein Bypassventil geschlossen ist und das Trocknen durch ein Regelventil und den Betrieb einer Gasfördermaschine bestimmt wird,
• OP2 Trocknen eines Bypasspfades und zumindest zum Teil einer Abluftleitung des Kathodensystems, wobei insbesondere Absperrventile geschlossen sind, ein Bypassventil geöffnet ist und das Trocknen durch ein Regelventil und den Betrieb einer Gasfördermaschine bestimmt wird,

und/oder

• OP3 Trocknen eines Kathodenpfades, eines Bypasspfades und einer Abluftleitung des Kathodensystems, wobei insbesondere Absperrventile geöffnet sind und das Trocknen durch Regelventil, das Bypassventil und den Betrieb einer Gasfördermaschine bestimmt wird.

Furthermore, in a method it can be provided that, in order to dry the fuel cell system, the cathode system is operated in different operating modes, in particular comprising:

• OP1 Drying of a cathode path and an exhaust air line of the cathode system, with shut-off valves in particular being open, a bypass valve being closed and the drying being determined by a control valve and the operation of a gas extraction machine,
• OP2 drying of a bypass path and at least part of an exhaust air line of the cathode system, with shut-off valves in particular being closed, a bypass valve being open and the drying being determined by a control valve and the operation of a gas extraction machine,

and or

• OP3 Drying of a cathode path, a bypass path and an exhaust air line of the cathode system, shut-off valves in particular being open and drying being determined by the control valve, the bypass valve and the operation of a gas extraction machine.

The method proposed here and the operating modes can be implemented with the appropriate actuators depending on the topology of the air system. In this way, different operating modes can be set in the cathode system and used in an advantageous manner for drying the fuel cell system.

Advantageously, the cathode system can be operated sequentially, in parallel and/or alternately in different operating modes. In this way, the flow/flow pattern in the cathode path of the stack can be flexibly varied in terms of flow conditions, Reynolds numbers, pressure pulses, pressure differences, etc., so that the different geometries within the stack are dehumidified and dried with improved effectiveness and efficiency.

According to a further advantage, the cathode system can be operated in different operating modes with respectively defined periods of time and/or defined thresholds, such as for transported mass flows. In this way, a simple control of the method and in particular a simple switching between the different operating modes can be made possible.

Provision can be made for a bypass valve and/or a gas pumping machine to be controlled variably in order to pulsate and/or vary the air mass flow through the stack, it being possible in particular for shut-off valves to be opened. With the help of the bypass valve, the flow through the stack and through the bypass can be variably divided. In this way, pulsations and/or variations in the air mass flow through the stack can be made possible in a simple manner.

Provision can be made for a control valve and/or a gas pumping machine to be controlled variably in order to pulsate and/or vary the cathode pressure in the stack, it being possible in particular for shut-off valves to be opened. A bypass valve can remain closed. By controlling the control valve, the pressure in the cathode system can be adjusted in a simple manner. In this way, pulsations and/or variations in the pressure in the stack can be made possible in a simple manner.

Advantageously, for pulsing and/or varying the air mass flow through the stack and for pulsing and/or varying the cathode pressure in the stack, a control valve, a bypass valve and/or a gas pumping machine can be controlled variably, with shut-off valves in particular being able to be opened. In other words, in this case, a number of actuators in the cathode system can be variably controlled in order to provide a variety of flows/flow patterns in the cathode path of the stack. The control valve can be arranged both in the main path of the exhaust gas path and in a bypass path of a turbine in the exhaust gas path.

Preferably, pressure differences between an anode side and a cathode side of the stack can be monitored and/or taken into account when carrying out the method, in particular through pressure pulsations and/or pressure variations. In this way, the pressure differences across the membrane can be taken into account.

As a precaution and to avoid large pressure differences, pressure pulsations and/or pressure variations can be limited in an amplitude when carrying out the method.

Furthermore, in addition to indirect drying of the anode, direct anode drying can optionally be carried out with a purge and/or flush process (i.e. drain and/or purge or purge-drain process) and also optionally with a metering in of fresh fuel be provided.

Cathode drying and anode drying can advantageously be combined and/or synchronized when carrying out the method.

• - Verbrauchen von Brennstoff, bspw. durch ein Purge- und/oder Drain-Vorgang, wobei vorzugsweise ein Luftmassenstrom auf Einhalten einer Minimalgrenze überwacht wird,

und/oder

• - Zudosieren von Brennstoff, bspw. aus einem Tank in einem Anodensystem

It can be particularly advantageous if pressure differences between an anode side and a cathode side of the stack are compensated for when carrying out the method, in particular by:

• - consumption of fuel, e.g. by a purge and/or drain process, with an air mass flow preferably being monitored for compliance with a minimum limit,

and or

• - Metering of fuel, e.g. from a tank in an anode system

By consuming fuel, the pressure on the anode side can be lowered. The pressure on the anode side can be increased by metering in fuel.

Furthermore, it can be advantageous for the method to be carried out when the fuel cell system is at a standstill and/or in a passive operating mode. In this way, the start of the system can be prepared in an advantageous manner.

In addition, it can be advantageous that a wake-up function of the fuel cell system is carried out to carry out the method, in particular by an internal and/or external control unit, and/or that the method is used as a preparatory function for starting the fuel cell system and/or in /is carried out during a shutdown of the fuel cell system.

The method can also be carried out at least in part, including initiation, adjustment and/or monitoring, by a control unit of the fuel cell system.

A corresponding control unit provides a further aspect of the invention. A computer program in the form of a code can be stored in a memory unit of the control unit, which, when the code is executed by an arithmetic unit of the control unit, carries out a method which can run as described above. The same advantages that were described above in connection with the method according to the invention can be achieved with the aid of the control unit. Reference is made in full to these advantages here.

The control unit can control the actuators in the functional systems of the fuel cell system in order to carry out the method accordingly.

In addition, the control unit can be in a communication link with an external processing unit in order to completely or partially outsource some method steps and/or calculations to the external processing unit.

According to a further aspect, the invention provides a computer program product, comprising instructions that are used when the computer program product is executed by a computer, such as a computer. B. the computing unit of the control unit, cause the computer to carry out the method, which can run as described above. With the aid of the computer program product, the same advantages can be achieved that were described above in connection with the method according to the invention and/or the control unit according to the invention. Reference is made in full to these advantages here.

Preferred embodiments:

• 1 ein beispielhaftes Brennstoffzellensystem im Sinne der Erfindung,
• 2 ein beispielhaftes Verdichterkennfeld, in welches ein erfindungsgemäßes Verfahren beispielhaft und ausschnittsweise hinsichtlich Betriebspunktänderungen eingezeichnet ist,
• 3 ein weiteres beispielhaftes Verdichterkennfeld, in welches ein erfindungsgemäßes Verfahren beispielhaft und ausschnittsweise hinsichtlich Betriebspunktänderungen eingezeichnet ist, und
• 4 ein weiteres beispielhaftes Verdichterkennfeld, in welches ein erfindungsgemäßes Verfahren beispielhaft und ausschnittsweise hinsichtlich Betriebspunktänderungen eingezeichnet ist.

The invention and its developments as well as its advantages are explained in more detail below with reference to drawings. They each show schematically:

• 1 an exemplary fuel cell system within the meaning of the invention,
• 2 an exemplary compressor map, in which a method according to the invention is drawn in as an example and in part with regard to changes in the operating point,
• 3 a further exemplary compressor map, in which a method according to the invention is drawn in as an example and in part with regard to changes in the operating point, and
• 4 another exemplary compressor map, in which a method according to the invention is drawn in as an example and in part with regard to changes in the operating point.

In the different figures, the same parts of the invention are always provided with the same reference numerals, which is why these i. i.e. R. only be described once.

the 1 FIG. 1 shows an exemplary fuel cell system 100 within the scope of the invention. The fuel cell system 100 usually includes a number of fuel cells that are combined to form a stack 101 . A cathode path K, an anode path A and a coolant path KM are routed through the stack. The fuel cell system 100 can also have multiple stacks 101 .

In addition, the fuel cell system 100 comprises at least four functional systems 10, 20, 30, 40, including: a cathode system 10 to supply a cathode compartment or the cathode path K of the stack 101 with an oxygen-containing gas mixture, an anode system 20 to supply an anode compartment or the to supply anode path A of stack 101 with a fuel-containing gas mixture or an anode fluid H2, a cooling system 30 to temper stack 101, and an electrical system 40, simply indicated for reasons of simplicity, to dissipate the generated electrical power from stack 101.

The fuel cell system 100 thus comprises a cathode system 10 with an air supply line 11 to the stack 101 and an exhaust air line 12 from the stack 101. An air filter AF is usually arranged at the inlet of the air supply line 11 in order to filter harmful chemical substances and particles or their entry into the system 100 to prevent.

The AirComp gas pumping machine in the cathode system 10 can be designed in the form of a compressor in order to draw in the air from the environment U and provide it to the stack 101 in the form of supply air. After going through the stack 101, an exhaust air is discharged from the system 100 back to the environment U.

Like it the 1 indicates, at least one supply air cooler IC and optionally a humidifier (not shown) can be provided downstream of the compressor.

Shutoff valves SV1, SV2 can be provided before and after the fuel cell stack or stack 101. In addition, a valve CVexh can be provided as a pressure regulator in the exhaust air line 12 .

In the supply air line 11 and / or in the exhaust air line 12, several sensors can be provided, such as. B. humidity sensors, temperature sensors, pressure sensors, mass and / or volume sensors, etc. All sensors are in the 1 only not shown for reasons of simplicity. A fuel sensor H2Sens is shown at the end of the exhaust line 12 .

A bypass line 13 with a ByCath bypass valve can be provided between the supply air line 11 and the exhaust air line 12 . The bypass line 13 can advantageously be used to control the mass flow in the cathode system 10 and/or to dilute the exhaust air, which may contain hydrogen, from the fuel cell stack or stack 101 . The method can also be transferred to other air system topologies, which may include modified or expanded actuators (e.g. multi-stage compression, turbine in the exhaust gas path, other or additional control valves).

The anode system 20 has several components. The components used to supply fuel include a fuel tank 21 and at least one metering valve HGI. The dosing valve HGI can also have a shut-off function. If the metering valve HGI does not have a shut-off function, a separate shut-off valve can be provided at the entrance to the anode chamber or anode path A.

Other components in the anode system 20 are a jet pump JP and a recirculation pump HRB. In addition, a purge valve PV and/or a drain valve DV and/or a combined purge/drain valve PDV can be provided in the anode system 20 .

With the help of 2 until 4 describes a method within the meaning of the invention, which can be used to control a drying process of a fuel cell system 100, in particular when shutting down the fuel cell system 100, preferably in preparation for a start, in particular a freeze start and/or cold start, of the fuel cell system 100.

• - Betreiben eines Kathodensystems 10 des Brennstoffzellensystems 100 zum Trocknen des Brennstoffzellensystems 100,
• - Pulsieren und/oder Variieren eines Luftmassenstroms dm/dt bzw. eines Luftvolumenstroms durch den Stack 101, und/oder
• - Pulsieren und/oder Variieren eines Kathodendrucks p im Stack 101.

The procedure has the following steps,

• - Operating a cathode system 10 of the fuel cell system 100 to dry the fuel cell system 100,
• - Pulsating and/or varying an air mass flow dm/dt or an air volume flow through the stack 101, and/or
• - Pulsating and/or varying a cathode pressure p in the stack 101.

• - durch Pulsationen und/oder Variationen des Luftmassenstroms dm/dt bzw. des Luftvolumenstroms im Stack 101,

und/oder

• - durch Pulsationen und/oder Variationen des Kathodendrucks p im Stack 101,

die Trocknung des Stacks 101:

• - direkt auf einer Kathodenseite K, und
• - zumindest indirekt auch auf Anodenseite A

und auch die Trocknung des Kathodensystems 10 verbessert/optimiert erfolgen kann.The invention proposes that the drying process or the drying operation:

• - due to pulsations and/or variations in the air mass flow dm/dt or the air volume flow in the stack 101,

and or

• - by pulsations and/or variations of the cathode pressure p in the stack 101,

the drying of the stack 101:

• - directly on a cathode side K, and
• - at least indirectly also on the anode side A

and the drying of the cathode system 10 can also be improved/optimized.

• - beim Abstellen,
• - in Stillstandphasen, und/oder
• - an unterschiedlichen Betriebsarten (Stack-Betrieb, Bypass-Betrieb) angewandt werden, ggf. mit unterschiedlichen Betriebsparametern.

The procedure can, for example

• - when parking,
• - in standstill phases, and/or
• - be applied to different operating modes (stack mode, bypass mode), possibly with different operating parameters.

The method can preferably also be combined, synchronized and/or coupled with the pressure difference monitoring and/or regulation with regard to the membrane (on the cathode side/on the anode side) and/or with anode drying.

The flow and the flow form in the cathode path K of the stack 101 thus experience a high variance with regard to flow states, Reynolds numbers, pressure pulses, pressure differences, etc., so that different geometries within the stack 101 are dehumidified and dried in an improved manner.

The method can be used to improve dehumidification/drying of the stack 101 when the fuel cell system 100 is switched off and/or during the standstill phases. In addition, improved freezing starts can be made possible with the aid of the method. So again rum reduction of the degradation of the stack 101 can be ensured. In addition, by varying and moving the throttle valves such. B. the control valve CVex, in the exhaust line 12 during drying also eliminates the risk of the actuators / throttle valves freezing during drying.

Due to the drying of the cathode path K in the stack, the anode path A is also dried, at least in part, since the water transport can take place via the membrane from the anode to the cathode. In addition, the method can advantageously be combined with the method in the anode path A, such as. B. drying by means of recirculation, drying by means of fresh fuel H2 and/or a purge/drain process.

Operating mode OP1 (see, for example, Figures 2 and 3):

At OP1 the shut-off valves SV1, SV2 are open, the bypass valve ByCath is closed and the operating point(s) for the air drying process is/are determined by the control valve CVexh and the operation of the gas extraction unit AirComp.

Operating mode OP2 (see e.g. Figure 2, left):

At OP2 the isolation valves SV1, SV2 are closed, the ByCath bypass valve is open and the operating point(s) for the air drying process is/are determined by the control valve CVexh and the operation of the gas extraction unit AirComp.

Operating mode OP3 (see, for example, Figures 2 and 4):

At OP3 the shut-off valves SV1, SV2 are open and the operating point(s) for the air drying process is/are determined by the control valve CVexh, the bypass valve ByCath and the operation of the gas extraction unit AirComp.

• - Sequentiell: z. B. erst OP1, dann OP2 oder umgekehrt,
• - Parallel: OP3 und/oder
• - Alternierend bzw. kombiniert: z.B. OP1, dann OP3, dann OP2 o.Ä. durchgeführt werden.

The operating modes can advantageously:

• - Sequential: e.g. B. first OP1, then OP2 or vice versa,
• - Parallel: OP3 and/or
• - Alternating or combined: eg OP1, then OP3, then OP2 or similar.

The operating modes can advantageously be carried out with respectively defined time durations (Time(OPx)>Limit) or other thresholds, such as integrated mass flows (mAirintegral(OPx)>Limit).

In combinations with anode drying with purge or flush (drain or purge or purge drain valve is open), a minimum mass flow dm/dt in the exhaust air line 12 of the cathode system 10 at the point at which the purge gas is introduced is advantageous in order to meet the dilution condition to comply with In this case, the method presented here can receive an additional requirement or limitation for the air mass flows (mfExhaust = mfStackAbgas + mfBypass > mfMin_Dilute).

• Hinsichtlich Massenstrom-Pulsationen sind die Aktoren ByCath und AirComp im Fokus bzw. vorzugsweise zu verwenden (vgl. 2).

Actuators CVex, ByCath and pulsating variables such as the compressor speed:

• With regard to mass flow pulsations, the focus is on the ByCath and AirComp actuators or they should preferably be used (cf. 2 ).

With regard to pressure pulsations, the focus is on the CVexh and AirComp actuators or they should preferably be used (cf. 3 ).

If both the mass flow dm/dt and the pressure p are pulsating, all actuators can also be controlled in parallel (cf. 4 ).

the 2 , 3 and 4 show, based on the compressor map of the AirComp gas pumping machine, which combines the relevant variables mass flow dm/dt, speed and pressure ratio p2/p1 (via the AirComp gas pumping machine, with the compressor end pressure p2 being indirectly indicative of the cathode pressure p in the stack 101), different applications of the procedure.

• - Regelventil CVexh verändert die Drosselcharakteristik des Kathodensystems 10 und
• - Bypassventil ByCath verändert die Massenstrom-Aufteilung zwischen der Bypassleitung 13 und dem Kathodenpfad K durch den Stack 101.

This is superimposed by:

• - Control valve CVexh changes the throttling characteristics of the cathode system 10 and
• - ByCath bypass valve changes the mass flow distribution between the bypass line 13 and the cathode path K through the stack 101.

The throttle flaps or the actuators CVexh and AirComp and the compressor speed can be adjusted or varied very quickly, so that pulsations of corresponding sizes with high dynamics are possible.

In the 2 , 3 and 4 In each case, the approximately resulting area of the cathode path K of the stack 101 is drawn in with arrows. The cathode inlet pressure p is slightly below the compressor end pressure p2, for example due to pressure losses in the cathode system 10, for example via a heat exchanger IC.

the 2 shows an operating point B of the AirComp gas pumping machine at a constant maintained speed speed_9 and at a set throttle valve position of the control valve CVexh. The branching of the mass flow dm/dt through the stack 101 and through the bypass line 13 can now be set by means of the bypass valve ByCath. Depending on the oscillation of the positions of the bypass valve ByCath (from ByPass=closed in OP1 to ByPass=open in OP2), smaller or larger mass flow pulsations in the stack 101 can be generated. The mass flow pulsations provide possible applications for the OP3 operating mode. Pressure pulsations could be added by additional variation of positions of the control valve CVexh.

the 3 shows an example of a possible application for operating mode OP1. The conveyed mass flow dm/dt can be conducted through the stack 101 by the AirComp gas conveying machine. The drying process starts, for example, at operating point A. The mAirComp gas pumping machine has, for example, a speed of speed_8 and the throttle valve position is CVexh(A). The CVexh control valve is now operated in an oscillating manner. In addition, the operating point can be changed transiently from CVexh(A) to CVexh(H). For this purpose, the speed of the AirComp gas pumping machine can be lowered transiently from speed_8 to speed_6. In this example, the transient adjustment of the operating point is superimposed with imposed oscillations around the target operating point that has changed in each case. This essentially results in pressure pulsations, subordinate to mass flow pulsations (secondary effect, smaller) and also a coverage of a pressure and mass flow range during drying. If the speed is kept constant at speed_8, the result is a trajectory from A to K. The trajectories can also be reversed accordingly, e.g. from K to A or H to A.

In the 4 an area is shown in which the cathode path K of the stack 101 can be subjected to pulsations of mass flow dm/dt and pressure p by operating the above-mentioned actuators (mAirComp, ByCath and CVexh).

The invention thus pays tribute to the fact that the complex structure of the stack 101 can have very different geometries and structures or surfaces (e.g. distribution channels, inlet areas, deflections, gas diffusion layers, grid/mesh, etc.). The invention addresses this by varying the operating modes OP1, OP2, OP3 during drying in order to better dehumidify the different geometries.

If pressure pulsations are used, the requirements for the pressure differences across the membrane (cathode-membrane-anode) can advantageously be monitored and/or taken into account. For this purpose z. B. the pressure pulsations are limited in terms of amplitude. If a trajectory is used for the pressure level, as is the case e.g. B. in the 3 is shown from A to H or from H to A, possibly also without pressure pulsations but with mass flow pulsations, the anode pressure can be tracked accordingly. In the 3 From A to H, for example, fuel can be consumed and/or the pressure level can be lowered by means of a purge/drain process. Conversely, from H to A, the pressure level in the anode can be increased by metering in fuel.

• Vorzugsweise wird der Prozess von höherem Druckniveau bis auf niedrigeres Druckniveau mit überlagerten Massenstrompulsationen durchgeführt. Zu Beginn des Trocknungsprozesses müssen flüssige Tröpfchen aus dem Stack 101 herausbefördert werden, was durch eine höhere Druckdifferenz über den Stack 101 und damit einen höheren Kathodeneintrittsdruck p erreicht werden kann. Gegen Ende des Trocknungsprozesses ist das Druckniveau eher niedrig, weil niedrig bedruckte Luft mehr Feuchtigkeit aufnehmen kann und damit den Zielzustand besser erreichen kann. Der Anodendruck kann entsprechend durch Brennstoff-Verbrauch und Purge-/Drain-Vorgang und ggf. durch Nachdosierung entsprechend adaptiert werden.

An advantageous example can provide:

• The process is preferably carried out from a higher pressure level down to a lower pressure level with superimposed mass flow pulsations. At the beginning of the drying process, liquid droplets have to be conveyed out of the stack 101, which can be achieved by a higher pressure difference across the stack 101 and thus a higher cathode inlet pressure p. Towards the end of the drying process, the pressure level tends to be low, because air with low pressure can absorb more moisture and can therefore reach the target state better. The anode pressure can be adapted accordingly through fuel consumption and the purge/drain process and, if necessary, through additional dosing.

For example, the process in the 4 run from C to M, possibly with corresponding mass flow pulsations and optionally small pressure pulsations. Or also in the 3 from A to H if mass flow pulsations are superimposed and the pressure pulsations are small or not at all.

The method can advantageously also be used with different parameters (amplitude of the pulsations, trajectories of the operating point changes, starting point, end point for the operating point changes, mass flow pulsations and/or pressure pulsations, etc.).

A corresponding control unit 200, which is shown schematically in FIG 1 is indicated provides a further aspect of the invention. A computer program in the form of a code can be stored in a memory unit of the control unit 200, which, when the code is executed by an arithmetic unit of the control unit 200, carries out a method which can run as described above.

The control unit 200 can correspondingly control the actuators (CVex, ByPass, AirComp and possibly other actuators) in the functional systems 10, 20, 30, 40 of the fuel cell system 100 in order to carry out the method as described above.

Optionally, the control unit 200 can be in a communication link with an external computing unit in order to completely or partially outsource some method steps and/or calculations to the external computing unit.

The above description of the figures describes the present invention exclusively within the framework of examples. It goes without saying that individual features of the embodiments can be freely combined with one another, insofar as this makes technical sense, without departing from the scope of the invention.

Claims (11)

Method for controlling a drying process of a fuel cell system (100) with at least one stack (101), in particular when shutting down the fuel cell system (100), preferably in preparation for a start, in particular a freeze start and/or cold start, of the fuel cell system (100), having the following Steps: - Operating a cathode system (10) of the fuel cell system (100) to dry the fuel cell system (100), - pulsing and/or varying an air mass flow (dm/dt) through the stack (101), and/or - Pulsating and/or varying a cathode pressure (p) in the stack (101). procedure after claim 1 , characterized in that for drying the fuel cell system (100), the cathode system (10) is operated in different operating modes (OP1, OP2, OP3), in particular comprising: OP1 drying a cathode path (K) and an exhaust air line (12) of the cathode system (10 ), whereby in particular shut-off valves (SV1, SV2) are opened, a bypass valve (ByCath) is closed and the drying is determined by a control valve (CVex) and the operation of a gas extraction machine (AirComp), OP2 drying of a bypass path (13) and at least for Part of an exhaust air line (12) of the cathode system (10), with shut-off valves (SV1, SV2) in particular being closed, a bypass valve (ByCath) being open and drying being determined by a control valve (CVex) and the operation of a gas extraction machine (AirComp), and/or OP3 drying of a cathode path (K), a bypass path (13) and an exhaust air line (12) of the cathode system (10), in particular shut-off valves (SV1, SV2) being opened net and the drying is determined by the control valve (CVex), the bypass valve (ByCath) and the operation of a gas extraction machine (AirComp). procedure after claim 2 , characterized in that the cathode system (10) is operated sequentially, in parallel and / or alternately in different operating modes (OP1, OP2, OP3), and / or that the cathode system (10) with respectively defined time periods and / or defined thresholds, such as eg for conveyed mass flows, is operated in different operating modes (OP1, OP2, OP3). Method according to one of the preceding claims, characterized in that for pulsing and / or varying the air mass flow (dm / dt) through the stack (101) a bypass valve (ByCath) and / or a gas pumping machine (AirComp) are variably controlled, in particular shut-off valves (SV1, SV2) are open. Method according to one of the preceding claims, characterized in that for pulsating and/or varying the cathode pressure (p) in the stack (101), a control valve (CVex) and/or a gas pumping machine (AirComp) are variably controlled, with shut-off valves (SV1, SV2) are open, and preferably a bypass valve (ByCath) can be closed. Method according to one of the preceding claims, characterized in that a control valve ( CVex), a bypass valve (ByCath) and/or a gas pumping machine (AirComp) can be variably controlled, with shut-off valves (SV1, SV2) in particular being open. Method according to one of the preceding claims, characterized in that when the method is carried out, in particular by pressure pulsations and/or pressure variations, pressure differences between an anode side and a cathode side of the stack are monitored and/or taken into account, and/or that when the method is carried out be limited in an amplitude by pressure pulsations and/or pressure variations. Method according to one of the preceding claims, characterized in that cathode drying and anode drying are combined and/or synchronized when the method is carried out, and/or that pressure differences between an anode side and a cathode side of the stack are compensated when the method is carried out, in particular by: - Consumption of fuel, e.g. by a purge and/or drain process, wherein an air mass flow is preferably monitored for compliance with a minimum limit (mfMin_Dilute), and/or - Metering of fuel, e.g. from a tank (21) in a anode system (20). Method according to one of the preceding claims, characterized in that the method is carried out when the fuel cell system (100) is at a standstill and/or in a passive operating mode, and/or that a wake-up function of the fuel cell system (100) is used to carry out the method. is carried out, in particular by an internal and/or external control unit (200), and/or that the method is carried out as a preparatory function for starting the fuel cell system (100) and/or when the fuel cell system (100) is switched off. Control unit (200), having a memory unit in which a code is stored, and an arithmetic unit, wherein when the code is executed by the arithmetic unit, the method according to one of the preceding claims is executed. Computer program product, comprising instructions which, when the computer program product is executed by a computer, cause the computer to perform the method according to any one of the preceding Claims 1 until 9 to perform.

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