Intermittent exhaust air return during fuel cell system operation

Abstract

The invention relates to a method for operating a fuel cell system (100), comprising the following components: at least one fuel cell stack (101), and a cathode system (10) for supplying an oxygen-containing reactant in the form of supply air (L1) to the at least one fuel cell stack (101), the cathode system (10) having the following components: at least one supply air tube (11), the invention relates to a fuel cell system comprising at least one fuel cell stack (101), at least one supply air line (11) for supplying supply air (L1) to the at least one fuel cell stack (101), and at least one exhaust air line (12) for discharging exhaust air (L2) from the at least one fuel cell stack (101), in which at least one supply air line (11) is provided a compression unit (KE) for compressing the supply air (L1), an air connection (LV) from the exhaust air (L2) to the supply air (L1) is provided between the at least one exhaust air line (12) and the at least one supply air line (11) of the cathode path (10), the air connection (LV) being used to provide an intermittent exhaust air return (EGR) into the supply air (L1) in at least one operating mode (M) of the fuel cell system (100).

Description

Technical Field

[0001] The invention relates to a method for operating a fuel cell system. The invention also relates to a computer program product, a control unit and a fuel cell system. Background Art

[0002] In a drive system with a fuel cell system, oxygen from the ambient air is usually used to react with hydrogen in the fuel cell to generate water or water vapor and thus obtain electrical energy. Ambient air is supplied to the fuel cell stack by means of a delivery system or a compression system. The compression system is implemented to provide a specific air mass flow and / or a specific pressure level. The extrusion or compression of the supply air is usually carried out by a thermal fluid machine (single-stage, multi-stage or multi-flow). Here, optionally, in order to compress the air, the energy recovery of the outflowing moist exhaust air can be achieved by means of a turbine (e.g., an electrically driven turbocharger or a turbocharger without an electric drive). Higher system pressures can be achieved by two-stage compression and energy recovery by means of a turbine (e.g., in order to achieve a high-performance fuel cell system).

[0003] In some systems, the feed air is humidified by means of a humidifier. The humidifier requires installation space and causes pressure losses. In other systems, humidification is provided inside the stack without a separate humidifier for the feed air. For this purpose, the membranes in the stack are designed to be thin, so that the membranes are moistened by means of the product water produced in the cathode path and the interaction with the anode path within the stack and its recirculation. Internal humidification has limitations in terms of the operating range, especially in the high load range or at increased stack temperatures. Summary of the invention

[0004] According to a first aspect, the present invention provides a method for operating a fuel cell system having the features of an independent method claim. Furthermore, according to a second aspect, the present invention provides a computer program product having the features of an independent product claim, according to a third aspect, a control unit having the features of an independent device claim, and according to a fourth aspect, a fuel cell system having the features of an independent system claim. Further features, advantages and details of the present invention are derived from the dependent claims, the description and the drawings. Naturally, the features and details described in connection with individual aspects of the present invention are also associated with other aspects of the present invention and vice versa, so that in the disclosure of the individual aspects of the present invention, reference is always made to one another or can be made to one another.

[0005] The present invention provides: a method for operating a fuel cell system, wherein the fuel cell system has the following components:

[0006] - at least one fuel cell stack; and

[0007] a cathode system for supplying a reactant containing oxygen in the form of feed air to at least one fuel cell stack,

[0008] Among them, the cathode system has the following components:

[0009] - (at least one) supply air pipe for providing supply air to at least one fuel cell stack,

[0010] - and (at least one) exhaust air duct for conducting exhaust air away from at least one fuel cell stack,

[0011] wherein a compression unit for compressing the supply air is arranged in the (at least one) supply air pipe,

[0012] wherein an air connection from exhaust air to supply air is provided between (at least one) exhaust air pipe and (at least one) supply air pipe of the respective cathode path (from one stack_i to the same stack_i, from a stack_i to another stack_k, from a plurality of stacks_i,j to one and / or a plurality of identical or different stacks_k,m, wherein i,j,k,m can be identical or different counters),

[0013] The air connection serves in this case to provide an intermittent return of exhaust air into the (corresponding) feed air in at least one operating mode of the fuel cell system.

[0014] A fuel cell system within the scope of the invention can have at least two or more fuel cell stacks (so-called fuel cell stacks or simply stacks), each of which has a plurality of stacked, repeating units in the form of fuel cells, for example PEM fuel cells.

[0015] Advantageously, the fuel cell system within the scope of the present invention can be used for mobile applications, for example in a motor vehicle, or for stationary applications, for example in a generator system.

[0016] Advantageously, the fuel cell system within the scope of the invention can have a control unit which is designed for regulating the cathode system, in particular for regulating the compression unit and preferably for regulating the intermittent exhaust air recirculation according to the method according to the invention.

[0017] The cathode system within the framework of the present invention may also be referred to as an air system.

[0018] The compression unit within the scope of the present invention can also be referred to as an air compression system. The compression unit within the scope of the present invention can have at least one compressor, wherein in particular at least one compressor can be driven by an electric motor (for example by means of an electric motor) and / or mechanically (preferably by means of a turbine or a turbocharger). In addition, the at least one compressor can be implemented in a single-stage, two-stage or dual-flow manner. In addition, the compression unit can have at least one turbine to support the at least one compressor.

[0019] Preferably, the air connection within the framework of the present invention can have a pipe between (at least one) exhaust air pipe and (at least one) supply air pipe (from one stack_i to the same stack_j, from stack_i to another stack_k, from multiple stacks_i, j to one and / or multiple identical or different stacks_k, m, where i, j, k, m can be identical or different counters) and a recirculation valve.

[0020] Exhaust air recirculation within the scope of the present invention may be abbreviated to EGR.

[0021] The core of the concept is to provide an intermittent operating strategy for exhaust air recirculation, which is advantageous, for example, in a continuous high load range of the system or also in other load ranges. The intermittent exhaust air recirculation can provide a variable control of operating parameters of the fuel cell system, in particular the cathode system. In addition, the intermittent exhaust air recirculation can optionally be combined with a corresponding control of the anode system and / or the cooling system.

[0022] By means of the intermittent exhaust air recirculation it can be ensured that at least one fuel cell stack:

[0023] - neither drying out (in the area of ​​the inlet to the stack cathode),

[0024] - without overflow (near or on the outlet region of the stack cathode),

[0025] - and can continuously output high load or maximum load.

[0026] In particular, the following (additional optional) parameters can be changed:

[0027] - cathode pressure,

[0028] - cathode mass flow,

[0029] - cathode outlet activity, and

[0030] - EGR recirculation rate (and cathode inlet humidity at the same time).

[0031] With the help of the present invention, several advantages can be achieved:

[0032] - temporarily humidifying the supply air at the input to the stack without power loss or degradation,

[0033] - temporarily improve the drainage of water from the outlet of the pile,

[0034] - temporary depletion of oxygen in the pile,

[0035] - Improve the service life of the heap,

[0036] - Reduced pile degradation (less dry-out, etc.)

[0037] -Cost savings in system design, etc.

[0038] For example, intermittent exhaust air recirculation can be advantageous for, in particular, continuous high-load operation (e.g. 50%-100%, in particular 75%-100% or even 80%-100% of the maximum power) and / or maximum-load operation of the fuel cell system. Continuous can mean that there is a long-term high-load or maximum-load demand on the fuel cell system, such that:

[0039] - no longer maintains humidification inside the pure pile, and / or

[0040] - This application case already leads to a cost-intensive design during development (enlargement of the stack or the air system).

[0041] Furthermore, intermittent exhaust air recirculation can be advantageous in the case of low loads, for example when the feed air mass flow is low, so that the discharge of waste water from the stack is insufficient and there is a risk of liquid water accumulating in the stack or the stack overflowing. If the mass flow of fresh air is increased, however, the oxygen in the cathode can also increase, which can no longer be done in a compatible manner with flow and water management. By means of intermittent exhaust air recirculation, the discharge of water can be achieved, preferably without simultaneously increasing the oxygen mass in the cathode path through the stack. Advantageously, the cathode pressure can be kept essentially constant.

[0042] During normal operation, firstly, only humidification inside the stack can be carried out without intermittent exhaust air recirculation. If during normal operation a high ambient temperature prevails and a high stack temperature is generated, but the temperature difference with the environment is still sufficient to remove the waste heat from the stack, the stack must be operated with a high pressure pCath1 and a low combustion air ratio lambdaCathl (or Lambda; also abbreviated as λ, air ratio or air number, expressed stoichiometrically: mCathStack=lambda*mCathStoechiometrisch) in order to maintain humidification inside the stack. In this case, the input area / starting area of ​​the stack cathode is in danger of drying out (drying out of the membrane) after a certain time td1. After a certain time td2, the input area / starting area of ​​the stack cathode is in danger of partial overflow (drying out of the membrane) due to the high water production under high load conditions. Both effects lead to power losses and may also sometimes lead to degradation of the stack. Advantageously, intermittent exhaust air recirculation can be introduced after time tS10=min(td1, td2).

[0043] Preferably, the intermittent exhaust air recirculation can include humidification of the interior of the stack and additionally exhaust air recirculation.

[0044] When the time tS10 =min(td1 , td2 ) has expired, the intermittent exhaust air return can be initiated or triggered.

[0045] Advantageously, the intermittent return of exhaust air does not lead to a reduction in power, at least not to a significant reduction in power.

[0046] For the intermittent return of exhaust air, the recirculation valve CVRezi is partially opened in order to introduce part of the exhaust air into the supply air.

[0047] In this case, the cathode pressure is reduced to pCath2 and the mass flow is increased to mCath2. The activity actCath2 at the cathode outlet is also temporarily increased.

[0048] That is, the air in the cathode removes more water and the humidification of the supply air is improved.

[0049] By increasing the mass flow in the stack, the water in the discharge / end region is conveyed away better, ie the risk of overflow is significantly reduced.

[0050] Advantageously, the increased mass flow does not increase the lambda in the cathode, since the exhaust air supplied is more oxygen-depleted than the air mass flow from the environment. If the air system additionally slightly increases the air supply, the lambda can preferably remain almost the same. This is possible even if the air system no longer has a speed reserve, since the air system can deliver more mass flow at the same speed due to the reduced system pressure (in the cathode). Alternatively or additionally to this, a speed reserve can be provided in the air system.

[0051] Due to the humidification by means of the air connection between exhaust air and supply air and the intermittent exhaust air return, in particular the inlet region of the stack can be significantly humidified, so that the risk of the membrane drying out can be significantly reduced.

[0052] The intermittent exhaust air recirculation can be performed for a specific time tS20. The time tS20 can be determined according to different effects (such as humidification at the cathode inlet tx1, flooding reduction tx2, possible permissible duration tx3 of lambda reduction), for example, tS20 = min (max (tx1, tx2), tx3).

[0053] The intermittent exhaust air return can have different control forms (different start and / or switch-off times, different gradients, etc.) The different control forms can be individually determined depending on the system and / or depending on the design and / or depending on the operating conditions and / or depending on the marginal conditions.

[0054] During the individual phases (intermittent exhaust air return start-up and shut-off), the relevant parameters can also be set constantly or variably.

[0055] Furthermore, the intermittent exhaust air recirculation may be carried out proactively, for example by trajectory planning and / or as a function of weather data, navigation data, a predicted power trajectory, or the like.

[0056] As mentioned above, intermittent exhaust air recirculation can advantageously be carried out during, in particular, continuous high-load operation of the fuel cell system and / or its maximum-load operation in order in particular to humidify the feed air, preferably at high ambient temperatures.

[0057] As already mentioned above, intermittent exhaust air recirculation can also be used to increase the mass flow through at least one fuel cell stack in order to particularly support the removal of water from the fuel cell stack, preferably without increasing the oxygen mass in at least one cell stack, preferably without changing the cathode pressure.

[0058] Advantageously, the intermittent exhaust air recirculation can also be used to deplete the mass flow through the at least one fuel cell stack with oxygen.

[0059] For simplicity, the intermittent return of exhaust air can be provided with an opening, in particular a partial opening, of the recirculation valve. In this way, a part of the exhaust air can be introduced into the supply air.

[0060] Furthermore, the intermittent exhaust air recirculation can provide for a variable control of at least one operating parameter of the fuel cell system, in particular of the cathode system, from the following parameters, for example:

[0061] - recirculation rate from exhaust air to supply air,

[0062] - cathode mass flow,

[0063] - cathode pressure, and / or

[0064] - Cathodic activity.

[0065] Furthermore, the intermittent exhaust air recirculation can provide for a periodic or aperiodic and / or symmetrical or asymmetrical control of at least one operating parameter of the cathode system. In this way, a flexible functionality can be provided.

[0066] Furthermore, it is conceivable that an intermittent exhaust air return may be introduced if:

[0067] - insufficient humidification of the interior of at least one fuel cell stack,

[0068] - the inlet region / starting region of at least one fuel cell stack has dried out or is in danger of drying out proactively, in particular after a certain time, and / or

[0069] - an outlet region / end region of at least one fuel cell stack may flood or may flood proactively, in particular after a certain time;

[0070] - when a high load requirement is present on at least one fuel cell stack for a longer period of time,

[0071] In particular, the stack voltage and / or the cell voltage and / or the impedance are measured in order to identify the state of the fuel cell stack and / or potential dangers, drying out, flooding, etc. in the fuel cell stack.

[0072] An at least temporary oxygen depletion is required in at least one fuel cell stack.

[0073] According to a further advantage, the intermittent exhaust air return can be carried out repeatedly and / or regularly and / or event-controlled and / or periodically. In this way, a flexible and extended functionality can be provided.

[0074] It may also be advantageous to implement intermittent exhaust air recirculation in a forward-looking manner, in particular as a function of weather data and / or navigation data. In this way, the requirements of the system (for example from the perspective of water management) can be taken into account particularly gently and particularly specifically for the system.

[0075] Furthermore, the present invention provides: a computer program product comprising instructions which, when implemented by a computer, cause the computer to perform the following method, which can be operated as described above. With the aid of the computer program product according to the invention, the same advantages as described above in connection with the method according to the invention can be achieved. Reference is hereby made in full to these advantages.

[0076] The invention further provides a control unit having a computing unit and a memory unit in which a code is stored which, when executed at least partially by the computing unit, executes the following method, which can be operated as described above. The same advantages as described above in conjunction with the method according to the invention can be achieved by means of the control unit according to the invention. Reference is hereby made in full to these advantages.

[0077] In addition, the present invention provides: a fuel cell system, wherein the fuel cell system has the following components:

[0078] - at least one fuel cell stack; and

[0079] - a cathode system for providing an oxygen-containing reactant in the form of feed air to at least one fuel cell stack;

[0080] The cathode system has the following components:

[0081] - (at least) one supply air pipe for providing supply air to at least one fuel cell stack,

[0082] - and (at least one) exhaust air duct for conducting exhaust air away from at least one fuel cell stack,

[0083] wherein a compression unit for compressing the supply air is arranged in the (at least one) supply air pipe,

[0084] wherein an air connection from exhaust air to supply air (from one stack_i to the same stack_i, from said stack_i to other stacks_k, from a plurality of stacks_i, j to one and / or a plurality of identical or different stacks_k, m, wherein i, j, k, m can be identical or different counters) is provided between (at least one) exhaust air pipe and (at least one) supply air pipe,

[0085] The air connection is designed in order to provide an intermittent return of exhaust air into the feed air in at least one operating mode of the fuel cell system.

[0086] Furthermore, it may be advantageous if a control unit is provided in the fuel cell system, which control unit is designed to carry out a method which can be operated as described above.

[0087] The same advantages as those described above in conjunction with the method according to the invention can be achieved with the fuel cell system according to the invention. Reference is made here in full to these advantages. BRIEF DESCRIPTION OF THE DRAWINGS

[0088] The present invention and its developments and advantages are explained in more detail below with reference to the accompanying drawings, which schematically show:

[0089] Figure 1 Schematic representation of a fuel cell system,

[0090] Figure 2 Schematic representation of a fuel cell system,

[0091] Figure 3 A schematic diagram of intermittent exhaust air recirculation, and

[0092] Figure 4 Schematic representation of intermittent exhaust air recirculation. DETAILED DESCRIPTION

[0093] Figure 1 and 2 A fuel cell system 100 within the meaning of the present invention is shown in each case.

[0094] A fuel cell system 100 within the meaning of the present invention has the following elements:

[0095] - at least one fuel cell stack 101, and

[0096] a cathode system 10 for supplying a reactant containing oxygen in the form of feed air L1 to at least one fuel cell stack 101,

[0097] The cathode system 10 comprises the following components:

[0098] - (at least one) supply air pipe 11 for providing supply air L1 to at least one fuel cell stack 101,

[0099] and (at least one) exhaust air pipe 12 for conducting exhaust air L2 out of the at least one fuel cell stack 101,

[0100] In this case, a compression unit KE for compressing the supply air L1 is arranged in (at least one) supply air pipe 11.

[0101] wherein an air connection LV is provided between (at least one) exhaust air pipe 12 and (at least one) supply air pipe 11 from exhaust air L2 to supply air L1 (from one stack_i to the same stack_i, from stack_i to another stack_k, from multiple stacks_i,j to one and / or multiple identical or different stacks_k,m, wherein i,j,k,m can be identical or different counters),

[0102] The air connection LV is used, in this case, to provide an intermittent exhaust air recirculation EGR into the feed air L1 in at least one operating mode M of the fuel cell system 100 .

[0103] The fuel cell system 100 can be used for mobile applications, such as in a motor vehicle, or for stationary applications, such as in a generator system. The fuel cell system 100 according to the invention can have at least one or more fuel cell stacks 101 (so-called fuel cell stacks or simply stacks), which each have a plurality of stacked repeating units in the form of a plurality of fuel cells 101, such as PEM fuel cells.

[0104] Therefore, the fuel cell system 100 includes a cathode system 10 having an inlet air pipe 11 to the stack 101 and an exhaust air pipe 12 from the stack 101. An air filter AF is usually arranged at the inlet of the inlet air pipe 11 to filter harmful chemicals and particles or prevent them from entering the system 100.

[0105] The compression unit KE in the cathode system 10 serves to draw air from the environment U and to provide it in the form of feed air L1 to the stack 101. After passing through the stack 101, exhaust air L2 from the stack 101 is discharged to the environment U again.

[0106] The compression unit KE may include at least one electrically operated compressor V (see Figure 1 Right and Figure 2 left) and / or at least one compressor V driven mechanically, in particular by means of a turbine T (see Figure 1 Left and Figure 2 on the right) and / or at least one (not shown for reasons of simplicity only) compressor in the air supply line 11 which is driven by an electric motor and mechanically, in particular by means of a turbine.

[0107] Furthermore, the compression unit KE can have at least one turbine T in the exhaust air line 12 , which is arranged in operative connection with at least one compressor V in the supply air line 11 in order to assist the at least one compressor V in compressing the supply air L1 .

[0108] like Figure 1 and 2 Specifiably, at least one or more feed air coolers IC may be provided downstream of the compression unit KE. Optionally, a heat exchanger and / or a humidifier may be provided within the cathode system 10 , which are not shown only for reasons of simplicity.

[0109] Shut-off valves (not shown for reasons of simplicity only) may be provided before and after the fuel cell stack or stack 101 .

[0110] Furthermore, a valve CVexh may be provided in the exhaust air pipe 12 as a pressure regulator.

[0111] A bypass line 13 having a bypass valve ByCath may be provided between the supply air line 11 and the exhaust air line 12 .

[0112] Preferably, the air connection LV within the scope of the invention can have a pipe between the exhaust air pipe 12 and the supply air pipe 11 and a recirculation valve CVRezi.

[0113] Exhaust air recirculation within the scope of the present invention may be abbreviated to EGR.

[0114] The present invention proposes an intermittent operating strategy for the exhaust air recirculation, which can be advantageous, for example, in a continuous high load range of the fuel cell system 100 but also in other load ranges.

[0115] The intermittent exhaust air recirculation can provide for variable control of different operating parameters of the fuel cell system 100, in particular of the cathode system 10. Furthermore, the intermittent exhaust air recirculation can optionally be combined with a corresponding control of the anode system and / or the cooling system.

[0116] By means of the intermittent exhaust air recirculation it can be ensured that at least one fuel cell stack 101:

[0117] – neither drying out (in the area of ​​the inlet to the stack cathode),

[0118] – No overflow (near or at the outlet area of ​​the stack cathode)

[0119] - and can continuously output high loads or maximum loads.

[0120] In particular, the following (additional optional) parameters can be changed:

[0121] - cathode pressure pCath,

[0122] - cathode mass flow mCath,

[0123] - cathode outlet activity aCath, and

[0124] - EGR recirculation rate (and therefore cathode inlet humidity).

[0125] By means of the present invention, several advantages can be achieved:

[0126] - humidifying the supply air L1 at the inlet in the stack 101 (at least temporarily) without power loss or derating,

[0127] - improving (at least temporarily) the drainage of water from the outlet of the stack 101,

[0128] - (at least temporary) oxygen depletion in the stack 101,

[0129] - Improve the life of the stack 101,

[0130] - Reduced heap degradation (less drying out, etc.)

[0131] - Cost savings in the design of the system 100, etc.

[0132] like Figure 3 Specifically, intermittent exhaust air recirculation can be advantageous for, in particular, continuous high-load operation (e.g., 50%-100%, in particular 75%-100% or even 80%-100% of the maximum power) and / or maximum-load operation of the fuel cell system 100. Continuous can mean that there is a long-term high load or maximum load requirement on the fuel cell system 100, such that:

[0133] - no longer maintains humidification inside the pure earth pile, and / or

[0134] Such applications would already lead to a cost-intensive design during development (enlargement of the stack 101 or of the cathode system 10 ).

[0135] like Figure 4Specifically, an intermittent exhaust air recirculation can be advantageous in the case of low loads, for example when the feed air mass flow mCath is so low that the removal of water from the stack 101 is insufficient and there is a risk that liquid water accumulates in the stack 101 or that the stack 101 overflows. If the mass flow of fresh air is increased, however, the oxygen in the cathode can increase, which can no longer be done in a compatible manner with flow and water management. By means of intermittent exhaust air recirculation, the removal of water from the stack 101 can be achieved, preferably without simultaneously increasing the oxygen mass in the cathode path via the stack. In this case, the cathode pressure pStack can advantageously be kept essentially constant, such as Figure 4 Schematically indicated.

[0136] In normal operation, only humidification of the interior of the stack can be carried out without intermittent exhaust air recirculation. If high ambient temperatures prevail in normal operation and high stack temperatures occur, but the temperature difference with the environment is still sufficient to sufficiently dissipate the waste heat of the stack, the stack must be operated with a high pressure pCath1 and a low combustion air ratio lambdaCath1 (or Lambda; also called λ, air ratio or air number, expressed stoichiometrically: mCathStack=lambda*mCathStoechiometrisch) in order to maintain humidification of the interior of the stack.

[0137] In this case, the input / starting area of ​​the stack cathode is in danger of drying out after a certain time td1.

[0138] After a certain time td2 , the output region / end region of the stack cathode is again in danger of partial flooding due to the high water production under high load.

[0139] Both of the above-mentioned effects lead to power losses and also partially to degradation of the stack 101. Advantageously, the intermittent exhaust air recirculation can be started after a time tS10 = min(td1, td2).

[0140] Preferably, the intermittent exhaust air recirculation can include humidification of the interior of the stack and additionally exhaust air recirculation.

[0141] Then, when the time tS10 =min(td1, td2) expires, an intermittent exhaust air recirculation can be introduced or triggered.

[0142] Advantageously, the intermittent return of exhaust air does not lead to a reduction in power, at least not to a significant reduction in power.

[0143] For intermittent exhaust air recirculation, the recirculation valve CVRezi can be opened into a partial position in order to introduce part of the exhaust air into the supply air.

[0144] like Figure 3 In this case, the cathode pressure pStack can be reduced to pCath2 and the mass flow mCath can be increased to mCath2. The activity at the cathode outlet actCath2 is also temporarily increased. That is, the air in the cathode discharges more water and improves the humidification of the supply air L1. By increasing the mass flow mCath in the stack 101, the water in the output / end region is better transported out, that is, the risk of overflow is significantly reduced.

[0145] Advantageously, λ is not increased by the increased mass flow mCath, since the exhaust air L2 supplied is more oxygen-depleted than the air mass flow from the environment U. If, for example, the cathode system 10 additionally slightly increases the amount of air supplied, λ can preferably remain almost constant. It can even be the case that the cathode system 10 no longer has a speed reserve, since, due to the reduced system pressure pCath, the cathode system 10 can deliver a greater mass flow mCath at the same speed. Alternatively or additionally to this, a speed reserve can be provided in the cathode system 10.

[0146] By moistening the feed air L1 by means of intermittent exhaust air recirculation, in particular the inlet region of the stack 101 can be significantly humidified, so that the risk of drying out of the membranes in the stack 101 can be significantly reduced.

[0147] The intermittent exhaust air recirculation can be performed for a specific time tS20. The time tS20 can be determined based on different effects (eg humidification at the cathode inlet tx1, flooding reduction tx2, possibly allowed lambda reduction duration tx3), for example tS20 = min (max (tx1, tx2), tx3).

[0148] The intermittent exhaust air return can have different control forms (different start and / or switch-off times, different gradients, etc.) The different control forms can be individually determined depending on the system and / or depending on the design and / or depending on the operating conditions and / or depending on the marginal conditions.

[0149] Important parameters during the individual phases (intermittent exhaust air return start-up and shut-off) can also be set constantly or variably.

[0150] Furthermore, the intermittent exhaust air recirculation may be carried out proactively, for example by trajectory planning and / or as a function of weather data, navigation data, power trajectory or the like.

[0151] Advantageously, intermittent exhaust air recirculation EGR may be introduced when:

[0152] - the internal humidification of at least one fuel cell stack 101 is insufficient,

[0153] - the inlet region / starting region of at least one fuel cell stack 101 has dried out or is in danger of drying out proactively, in particular after a certain time dt1 , and / or

[0154] at least one outlet region / end region of the fuel cell stack 101 is flooded or is flooded proactively, in particular after a certain time dt2,

[0155] - in the event of a prolonged high load demand on at least one fuel cell stack 101, in which in particular the stack voltage and / or the individual cell voltage and / or the impedance are measured in order to detect the state of the fuel cell stack (101) and / or potential dangers, drying out, flooding, etc. in the fuel cell stack (101),

[0156] An at least temporary oxygen depletion is required in at least one fuel cell stack 101 .

[0157] According to a further advantage, the intermittent exhaust air return can be carried out repeatedly and / or regularly and / or event-controlled and / or periodically. In this way, a flexible and expanded functionality can be provided.

[0158] The above description of the drawings illustrates the invention only within the framework of examples. Naturally, individual features of the embodiments can be combined freely with one another, as long as this is technically reasonable, without leaving the framework of the invention.

Claims

1. A method for operating a fuel cell system (100), in, The fuel cell system (100) comprises the following components: - at least one fuel cell stack (101); and a cathode system (10) for supplying a reactant containing oxygen in the form of feed air (L1) to the at least one fuel cell stack (101), The cathode system (10) comprises the following components: - at least one supply air pipe (11) for providing supply air (L1) to the at least one fuel cell stack (101), - and at least one exhaust air pipe (12) for conducting exhaust air (L1) out of the at least one fuel cell stack (101), Wherein, a compression unit (KE) for compressing the supply air (L1) is arranged in the at least one supply air pipe (11), wherein an air connection (LV) from exhaust air (L2) to supply air (L1) is provided between the at least one exhaust air pipe (12) and the at least one supply air pipe (11), The air connection (LV) is used to provide an intermittent exhaust air recirculation (EGR) into the supply air (L1) in at least one operating mode (M) of the fuel cell system (100).

2. The method according to claim 1, It is characterized in that The intermittent exhaust air recirculation (EGR) is carried out during, in particular, continuous high-load operation and / or maximum-load operation of the fuel cell system (100) in order to humidify the feed air (L1), in particular, preferably at high ambient temperatures.

3. The method according to claim 1 or 2, It is characterized in that The intermittent exhaust air recirculation (EGR) is used to increase the mass flow (mCath) through the at least one fuel cell stack (101) in order to particularly support the removal of water from the fuel cell stack (101). Preferably, the oxygen quality in the at least one fuel cell stack (101) is not increased, Preferably, the cathode pressure (pCath) is not changed, And / or, the intermittent exhaust air recirculation (EGR) is used to deplete the mass flow (mCath) passing through the at least one fuel cell stack (101) with oxygen.

4. The method according to any one of the preceding claims, It is characterized in that The intermittent exhaust air recirculation (EGR) provides for an opening, in particular a partial opening, of a recirculation valve (CVRezi).

5. The method according to any one of the preceding claims, It is characterized in that The intermittent exhaust air recirculation (EGR) implements a variable control of at least one operating parameter (BP) of the fuel cell system (100), in particular of the cathode system (10), from among the following parameters: - a recirculation rate from said exhaust air (L2) to said supply air (L1), - cathode mass flow, - cathode pressure, and / or - Cathodic activity.

6. The method according to the preceding claims, It is characterized in that The intermittent exhaust air recirculation (EGR) provides for a periodic or aperiodic and / or symmetrical or asymmetrical control of at least one operating parameter (BP) of the cathode system (10).

7. A method according to any one of the preceding claims, It is characterized in that The intermittent exhaust air recirculation (EGR) is introduced when: - the interior of the at least one fuel cell stack (101) is insufficiently humidified, - an inlet region / starting region of the at least one fuel cell stack (101) dries out or is in danger of drying out prospectively, in particular after a certain time (dt1), and / or - an outlet region / end region of the at least one fuel cell stack (101) overflows or overflows proactively, in particular after a certain time (dt2), - in the event of a high load requirement being present on the at least one fuel cell stack (101) for a longer period of time, In particular, the stack voltage and / or the cell voltage and / or the impedance are measured to identify the state of the fuel cell stack (101) and / or potential risks, drying out, overflow, etc. in the fuel cell stack (101). - requiring at least temporary oxygen depletion in the at least one fuel cell stack (101).

8. The method according to any one of the preceding claims, It is characterized in that The intermittent exhaust air return is performed repeatedly and / or regularly and / or event-controlled and / or periodically, And / or the intermittent exhaust air return is carried out proactively, in particular as a function of weather data and / or navigation data and / or as a function of a predicted power trajectory.

9. A computer program product comprising instructions which, when executed by a computer, cause the computer to perform the method according to any one of the preceding claims.

10. A control unit (200) having a computing unit and a memory unit, in which a code is stored which, when at least partially executed by the computing unit, carries out the method according to any of the preceding claims.

11. A fuel cell system (100), in, The fuel cell system (100) comprises the following components: - at least one fuel cell stack (101) and - a cathode system (10) for supplying a reactant containing oxygen in the form of an inlet air (L1) to the at least one fuel cell stack (101), The cathode system (10) comprises the following components: - an air supply pipe (11) for providing supply air (L1) to the at least one fuel cell stack (101), - and an exhaust air pipe (12) for conducting exhaust air (L2) from the at least one fuel cell stack (101), wherein a compression unit (KE) is arranged in the (at least one) supply air pipe (11) for compressing the supply air (L1), wherein an air connection (LV) from the exhaust air (L2) to the supply air (L1) is arranged between the at least one exhaust air pipe (12) and the at least one supply air pipe (11), The air connection (LV) is designed to provide an intermittent exhaust air recirculation (EGR) into the feed air (L1) in at least one operating mode (M) of the fuel cell system (100).

12. The fuel cell system (100) according to the preceding claim, It is characterized in that A control unit (200) according to claim 10 is provided to execute the method according to any one of the above claims, and / or, the compression unit (KE) has at least one compressor (VI, V2), In particular, at least one compressor (V1) is driven electromotorally and / or mechanically, for example by means of a turbine (T). In this case, preferably at least one compressor (V2) is driven by an electric motor, In this case, preferably at least one compressor (V2) is designed in a single-stage, two-stage or dual-flow manner, And / or, the compression unit (KE) has at least one turbine (T) to support at least one compressor (V).

Images