Method for operating a fuel cell system, control unit and fuel cell system

Abstract

The invention relates to a method for operating a fuel cell system (100) with at least one fuel cell stack (101) and at least one air system (10), wherein an exhaust gas recirculation path (EGR) is provided in the at least one air system (10) between a supply air path (11) to the at least one fuel cell stack (101) and an exhaust air path (12) from the at least one fuel cell stack (101), wherein a mixing point (MS) is arranged between the exhaust gas recirculation path (EGR) in the supply air path (11) downstream of an air filter (AF) and upstream of a, in particular second, air compression device (Comp), the method comprising: - Determining a current and a predictive content of liquid water in a mixed fluid at the inlet to the air compression device (Comp), - Carrying out at least one measure (M1, M2, M3, M4) to reduce the content of liquid water in a mixed fluid at the inlet to the air compression device (Comp) depending on the determination.

Description

[0001] The invention relates to a method for operating a fuel cell system. Furthermore, the invention relates to a computer program product, a control unit, and a fuel cell system. State of the art

[0002] In vehicles (FCV, English for "Fuel Cell Vehicle"), in which the drive energy is supplied by one or more (PEM) fuel cell systems (FCS, English for "Fuel Cell System"), the oxidizing agent oxygen from the ambient air is usually used to react with hydrogen in the fuel cell to form water (or water vapor) and thus to supply electrical power through electrochemical conversion.

[0003] A crucial aspect of PEM stack operation is water management in the membrane and thus also in the cathode path. The membrane must be sufficiently humidified to conduct protons. The risk of drying out is particularly high in the cathode inlet area. Therefore, various humidification concepts exist, including: - external humidification by membrane humidifier - internal stack humidification via anode-cathode interaction, possibly combined with other concepts (such as the following) - Water injection with water storage - Exhaust Gas Recirculation (EGR)

[0004] In exhaust gas recirculation or EGR, part of the moist stack exhaust gas is directed from the exhaust path into the supply air path (usually related to the same stack).

[0005] The exhaust gas recirculation or EGR can 1. by means of a pressure gradient between the extraction point in the exhaust gas path and the inflow point in the supply air path 2. or by means of additional fluid compression, which raises the exhaust gas to a higher pressure level.

[0006] The inflow point in the supply air path where fresh air and exhaust fluid are mixed can be referred to as a mixing point.

[0007] In the case of pressure difference-driven exhaust gas recirculation or EGR (Var.1), an EGR valve (preferably controllable and preferably tight-closing) is used according to SdT to regulate and / or prevent the EGR fluid flow. Disclosure of the invention

[0008] The present invention provides a method for operating a fuel cell system having the features of the independent method claim. Furthermore, the invention provides a corresponding computer program product, a corresponding control unit, and a corresponding fuel cell system having the features of the independent claims. Features and details described in connection with the various embodiments and / or aspects of the invention naturally also apply in connection with the other embodiments and / or aspects, and vice versa, so that reciprocal reference is or can always be made to the individual embodiments and / or aspects with regard to the disclosure.

[0009] The present invention provides, according to the first aspect: a method for operating a fuel cell system having at least one fuel cell stack and at least one air system.

[0010] In the at least one air system, an exhaust gas recirculation path is provided between an air supply path to the at least one fuel cell stack and an exhaust air path from the at least one fuel cell stack (or another fuel cell stack).

[0011] A mixing point between the exhaust gas recirculation path in the supply air path is arranged downstream of an air filter and upstream of a, in particular second, air compression device.

[0012] The procedure comprises the following steps: - Determining a current and a predictive content of liquid water in a mixed fluid at the inlet to or upstream of the air compression device, - Carrying out at least one measure to reduce the content of liquid water in a mixed fluid at the inlet to or upstream of the air compression device in dependence on the determining.

[0013] The steps of the method can be performed in the specified order or in a modified order. The steps of the method can be performed simultaneously, at least partially simultaneously, and / or sequentially.

[0014] The fuel cell system (or "system" for short) can preferably be used for mobile applications, for example, in vehicles, particularly fuel-powered vehicles, preferably in the commercial vehicle sector with high service life requirements. The fuel cell system can serve as the main energy supplier for a vehicle. Furthermore, the fuel cell system according to the invention can serve as an energy supply for a power take-off drive and / or an auxiliary drive of a vehicle, for example, a hybrid vehicle. However, the fuel cell system can also be used for stationary applications, for example, in generators.

[0015] The fuel cell system can comprise multiple fuel cell stacks (or stacks for short), each with multiple 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. Preferably, the fuel cell system can comprise multiple modules in the form of individual stacks with multiple stacked fuel cells.

[0016] The exhaust air may also be referred to as exhaust gas or exhaust fluid. The exhaust gas recirculation path may also be referred to as an exhaust gas recirculation line or exhaust air recirculation line. The air compression device may also be referred to as an air compressor or compressor.

[0017] The invention recognizes that the exhaust fluid is moist and may also contain liquid water (water droplets). Condensation can also occur in the exhaust gas recirculation line, which is where water droplets can form.

[0018] At the mixing point, these water droplets can enter the supply air path. Further downstream of the mixing point, the water droplets can be carried to the air compression device. If water droplets hit the compressor impeller, significant degradation can occur. Furthermore, the gas bearings of the rotor-shaft system can be damaged. Other components, such as sensors, can also be damaged or experience functional limitations due to liquid water.

[0019] The process is designed to ensure component protection of the air compression device downstream of the mixing point when using exhaust gas recirculation, and to extend the service life of the air compression device. Furthermore, the process can enable component protection for other components and reduce functional limitations caused by liquid water.

[0020] The method can be advantageously applied to two-stage compressor systems. Furthermore, the method can be applied to single-stage compressor systems.

[0021] Advantageously, the method may comprise the following measures to reduce the content of liquid water in a mixed fluid at the inlet to or upstream of the air compression device depending on the determination: M1: Limiting an EGR rate or an opening cross-section of an EGR valve so that the humidity at the compressor inlet (downstream of the mixing point) falls below 100% (i.e., no liquid water is present).

[0022] This measure can be supported (and / or at least partially replaced) by further measures for better mixing, e.g. installation of a mixer to evaporate possible droplets, and / or further process engineering measures M2 and / or M3 and / or M4, which are described below.

[0023] M2: Load sharing (so-called balancing) between two air compressors ( Fig. ), so that the humidity of the compressor supply air (the mixed air after the mixing point) is significantly reduced or falls below 100%.

[0024] To reduce the humidity at the compressor inlet, the first compressor can be operated at a higher load and the second compressor at a lower load.

[0025] Due to the higher temperature level of the fresh air after a first compression stage, possible liquid water from the EGR fluid can be better absorbed (evaporated / vaporized).

[0026] The load distribution can also be adjusted accordingly when conditions (both stack operating conditions and environmental conditions) change.

[0027] If, for example, the risk of excessive humidity at the second compressor inlet decreases, the first compressor can be operated at a lower load.

[0028] If, for example, the risk of excessive humidity at the second compressor inlet increases, the first compressor can be operated at a higher load.

[0029] M3: Cooling of the fresh air, e.g. by an intercooler, can be reduced so that a higher fresh air temperature is available at the mixing point.

[0030] M4: Increasing the fluid flow through the stack bypass path, whereby the stack exhaust fluid is mixed with stack bypass air with a lower moisture content and then the EGR fluid extraction downstream contains a lower proportion of liquid water and the humidity is reduced.

[0031] The advantages of the process are the protection of components of the air compression device, in particular the compressor impeller and the gas bearings located downstream of the mixing point, as well as the increase in service life.

[0032] Advantageously, the at least one measure may comprise a first measure according to which a limitation for an exhaust gas recirculation fluid, preferably for a cross section of the exhaust gas recirculation path, is carried out, in particular adaptively.

[0033] According to a further advantage, the at least one measure can comprise a second measure according to which a balancing, preferably for power distribution, is carried out between a first air compression device and a second air compression device, in particular adaptively.

[0034] According to a further advantage, the at least one measure can comprise a third measure according to which cooling of a supply air upstream of the air compression device is reduced, in particular adaptively.

[0035] According to a further advantage, the at least one measure can comprise a fourth measure, according to which a fluid flow via a stack bypass path (13) is increased. In other words, the at least one measure can comprise a fourth measure, according to which the content of liquid water in the EGR fluid is reduced by admixing drier fluid from the stack bypass path to the stack exhaust gas upstream of the removal of the EGR fluid from the exhaust path, thereby reducing the humidity and water content in the exhaust path.

[0036] According to one embodiment, it is conceivable that the first measure can serve as a primary measure, since the exhaust gas recirculation can be used to control the exhaust gas recirculation.

[0037] It is conceivable that, if a first measure has been initiated and if the content of liquid water in a mixed fluid at the inlet to the air compression device is too high and / or the first measure is too restrictive, a second measure and / or a third measure and / or a fourth measure is / will be initiated in support of and / or in replacement of the first measure.

[0038] According to one embodiment, it is conceivable that the third measure can be used according to availability, and / or that the third measure can be initiated in support of and / or as a replacement for the second measure.

[0039] According to one embodiment, it is conceivable that the fourth measure can be used depending on the operating point and / or that the fourth measure can be initiated in support of and / or as a replacement for the second or another measure.

[0040] In principle, it is conceivable that the at least one measure may comprise several measures that are carried out successively, at least partially overlapping and / or simultaneously.

[0041] Advantageously, when determining the current and predictive content of liquid water in a mixed fluid at the inlet to the air compression device, at least one of the following parameters in the operation of the fuel cell system can be taken into account, in particular adaptively: - an operating parameter in a cathode path through the at least one fuel cell stack, comprising: - Pressure, - temperature, - mass flow, - Mole current, - Humidity, and / or - Content of liquid water, - Environmental parameters, including: - Pressure, - temperature, - Humidity, and / or - Content of liquid water, - System parameters in the supply air path, the exhaust air path and / or the stack bypass path, including: - Pressure, - temperature, - mass flow, - Mole current, - Humidity, and / or - Content of liquid water.

[0042] In this way, a reliable determination of the current and predictive content of liquid water in a mixed fluid at the inlet to the air compression device can be enabled.

[0043] In principle, it is conceivable that the process steps are carried out successively, at least partially overlapping and / or simultaneously.

[0044] Furthermore, it is conceivable that the procedure can be carried out repeatedly.

[0045] Furthermore, it is conceivable that the process can be carried out if exhaust gas recirculation is required.

[0046] According to a further aspect, the invention provides a computer program product comprising instructions that, when the computer program product is executed by a computer, such as the processing unit of the control unit, cause the computer to perform the method, which can proceed as described above. The computer program product can achieve the same advantages as described above in connection with the method according to the invention. These advantages are incorporated herein by reference.

[0047] A corresponding control unit provides a further aspect of the invention. A computer program in the form of code can be stored in a memory unit of the control unit. When the code is executed by a computing unit of the control unit, the program performs a method that can proceed as described above. The control unit can achieve the same advantages as those described above in connection with the method according to the invention. These advantages are incorporated herein by reference in their entirety.

[0048] A corresponding fuel cell system with a corresponding control unit provides a further aspect of the invention. Using the fuel cell system, the same advantages described above in connection with the method according to the invention can be achieved. These advantages are incorporated herein by reference. Preferred embodiments:

[0049] The invention and its further developments, as well as their advantages, are explained in more detail below with reference to the accompanying drawings. They show schematically: Fig. 1 an example system topology, Fig. 2 another example system topology, Fig. 3 another example system topology, Fig. 4 another example system topology, and Fig. 5 shows an example of a proposed procedure.

[0050] In the different figures, identical parts of the invention are always provided with the same reference numerals, which is why they are usually only described once.

[0051] The Fig. 1 to 5 serve to explain a method which was developed for operating a fuel cell system 100 with at least one fuel cell stack 101 and at least one air system 10.

[0052] In the at least one air system 10, an exhaust gas recirculation path EGR is provided between an air supply path 11 to the at least one fuel cell stack 101 and an exhaust air path 12 from the at least one fuel cell stack 101 (or another fuel cell stack).

[0053] A mixing point MS between the exhaust gas recirculation path EGR in the supply air path 11 is arranged downstream of an air filter AF and upstream of a, in particular second, air compression device Comp.

[0054] The exhaust gas recirculation or EGR can 1. by means of a pressure gradient between the extraction point in the exhaust gas path and the inflow point in the supply air path (cf. Fig. 1, Fig. 3 and Fig. 4) 2. or by means of additional fluid compression, which raises the exhaust gas to a higher pressure level (cf. Fig. 2).

[0055] In the pressure difference driven exhaust gas recirculation or EGR (Var.1), an EGR valve EGRV (preferably controllable and preferably tight-closing) is used to regulate and / or prevent the EGR fluid flow.

[0056] The procedure comprises the following steps: - Determining a current and a predictive content of liquid water in a mixed fluid at the inlet to or upstream of the air compression device Comp, - Carrying out at least one measure M1, M2, M3, M4 to reduce the content of liquid water in a mixed fluid at the inlet to the air compression device Comp depending on the determination.

[0057] The steps of the method can be performed in the specified order or in a modified order. The steps of the method can be performed simultaneously, at least partially simultaneously, and / or sequentially.

[0058] The exhaust air is usually humid and may still contain liquid water (water droplets). Condensation can also occur in the exhaust gas recirculation (EGR) line, which is where water droplets can form.

[0059] At the mixing point MS, these water droplets can enter the supply air path 11. Further downstream of the mixing point MS, the water droplets can be carried to the air compression device Comp. If water droplets impact a compressor impeller, significant degradation can occur. If liquid water enters the gas bearing path of the air compression device Comp, bearing damage and / or significant degradation can occur.

[0060] The process is used to protect components of the air compression device (Comp) downstream of the mixing point (MS) when using exhaust gas recirculation (EGR) and to increase the service life of the air compression device (Comp). Additionally, other components can be protected from liquid water and functionality can be ensured (e.g., sensors).

[0061] The process can be advantageously used in two-stage compression systems (cf. Fig. 4). Furthermore, the method can be applied to single-stage compressor systems (see Fig. 3).

[0062] Advantageously, the method may comprise the following measures (M1 and / or M2 and / or M3 and / or M4) to reduce the content of liquid water in a mixed fluid at the inlet to the air compression device Comp depending on the determination: M1: Limiting an EGR rate or an opening cross-section of an EGR valve so that the humidity at the compressor inlet (downstream of the mixing point) falls below 100% (i.e., no liquid water is present).

[0063] This measure M1 can be supported (and / or at least partially replaced) by further measures for better mixing, e.g. installation of a mixer to evaporate possible droplets, and / or further process engineering measures M2 and / or M3 and / or M4, which are described below.

[0064] M2: Load sharing (so-called balancing) between two air compressors ( Fig. ), so that the humidity of the compressor supply air (the mixed air after the mixing point) is significantly reduced or falls below 100%.

[0065] To reduce the humidity at the compressor inlet, the first air compression device Comp can be operated at a higher load and the second air compression device Comp at a lower load.

[0066] Due to the higher temperature level of the fresh air after the first air compression device Comp, possible liquid water from the EGR fluid can be better absorbed (evaporated / vaporized).

[0067] The load distribution can also be adjusted accordingly when conditions (both stack operating conditions and environmental conditions) change.

[0068] If, for example, the risk of excessive humidity at the inlet to the second air compression device Comp decreases, the first air compression device Comp can be operated at a lower load.

[0069] If, for example, the risk of excessive humidity at the inlet to the second air compression device Comp increases, the first air compression device Comp can be operated at a higher load.

[0070] M3: Cooling of the fresh air, e.g. by an intercooler, if present, can be reduced so that a higher fresh air temperature is available at the mixing point MS.

[0071] M4: Increasing the fluid flow through the stack bypass path, whereby the stack exhaust fluid is mixed with stack bypass air with a lower moisture content and then the EGR fluid extraction downstream contains a lower proportion of liquid water and the humidity is reduced.

[0072] The Fig. 5 shows a possible sequence of a method according to the present disclosure.

[0073] The method relates to the operation of the system 100 with exhaust gas recirculation or EGR, so that the method can be started when the exhaust gas recirculation or EGR is enabled by the system controller (step 105).

[0074] In step 110, the system can be operated with the EGR enabled.

[0075] In step 120, the current and future / predictive content of liquid water in a mixed fluid at the inlet to the air compression device Comp (downstream of the mixing point MS) is determined. The following parameters can be considered, particularly adaptively, during operation of the fuel cell system: - an operating parameter BP in a cathode path through the at least one fuel cell stack 101, comprising: - Pressure, - temperature, - mass flow, - Mole current, - Humidity, and / or - Content of liquid water, - Environmental parameters UP, including: - Pressure, - temperature, - Humidity, and / or - Content of liquid water, - System parameters SP in the supply air path 11, in the exhaust air path 12 and / or stack bypass path 13, comprising: - Pressure, - temperature, - mass flow, - Mole current, - Humidity, and / or - Content of liquid water.

[0076] In step 130, the first measure M1 can be initiated, according to which a limitation for an exhaust gas recirculation fluid, preferably for a cross section of the exhaust gas recirculation path EGR, is carried out, in particular adaptively.

[0077] From the current or predictive parameters, it can be determined how large the maximum opening cross-section of the EGR path (or max. mass flow or molar flow of EGR fluid) may be so that the mixed fluid in the compressor inlet is not too moist or there is no risk of drops occurring.

[0078] This variable limitation can be continuously reset and adjusted in the control panel.

[0079] With the first measure M1, the maximum possible mass flow or molar flow of the EGR fluid can be limited.

[0080] If this measure M1 is too restrictive / limiting, then the second measure M2 can be applied additionally or alternatively in two-stage air compression systems.

[0081] In step 140, an assessment of the risk of exceeding a certain threshold, e.g. for humidity / activity / water content or similar, in the mixing fluid at the inlet to the air compression device Comp downstream of the mixing point MS can be carried out.

[0082] From the steps and / or parameters listed above, the following can be determined: - whether / when the danger of drops on the compressor occurs, or, - whether / when the current limit from measure M1 will be reached.

[0083] If this is the case, a trigger can be set in step 141 to initiate the second measure M2 and / or the third measure M3 and / or the fourth measure M4.

[0084] In step 150, the second measure M2 can be initiated, according to which a balancing, preferably for power distribution, between a first air compression device Comp and a second air compression device Comp is carried out, in particular adaptively.

[0085] Measure M2 can thus provide that the limits for the load distribution at the first air compression device Comp and at the second air compression device Comp (“balancing”) are changed in such a way that the humidity in the inflow to the second air compression device Comp decreases or that the risk of droplet impact is prevented.

[0086] The first air compression device Comp can be operated at a higher load and the second air compression device Comp at a lower load, so that the same stack operating point can be represented, but the conditions upstream of the second air compression device Comp change, in particular in favor of component protection.

[0087] Consequently, it can be provided that the limitation for the EGR path (measure M1 can be adjusted) can then be adjusted accordingly, so that a higher EGR rate is enabled.

[0088] In step 160, the third measure M3 and / or fourth measure M4 can be initiated, according to which cooling of a supply air L1 upstream of the air compression device Comp is reduced, in particular adaptively, so that a higher temperature of the fresh air is available at the mixing point MS.

[0089] The third measure, M3, has a less dynamic impact than the second measure, M2, so this measure, M3, can also be implemented in a cascaded manner, i.e., more slowly. It can also be applied only when the second measure, M2, is not feasible or reaches its limits.

[0090] The fourth measure M4 can be implemented as a more dynamic measure, so that this measure can also be carried out before measure M3.

[0091] Examples (particularly in response to changes in environmental conditions) for the second measure M2 and / or third measure M3 and / or fourth measure M4 may be the following: If, for example, the ambient temperature drops (humidity at the inlet to the second air compression device Comp would increase at the same EGR rate), then the load of the first air compression device Comp can be increased and the load of the second air compression device Comp can be reduced and / or the cooling of the fresh air by the intercooler IC can be reduced and / or the fluid quantity via the stack bypass can be increased.

[0092] If, for example, the ambient humidity decreases (e.g. tunnel passage), more degrees of freedom arise (humidity would decrease at the inlet to the second air compression device Comp at the same EGR rate), then the load of the first air compression device Comp can be reduced.

[0093] If the stack operating point is changed so that the water content of the exhaust gas fluid and thus of the EGR fluid increases, the load of the first air compression device Comp can be increased as a component protection measure and / or the cooling of the fresh air by the intercooler IC can be reduced and / or the fluid quantity via the stack bypass can be increased.

[0094] A corresponding computer program product, a corresponding control unit ecu and a corresponding fuel cell system 100 with a corresponding control unit ecu represent further aspects of the invention.

[0095] The above explanation of the embodiments describes the present invention exclusively within the scope of examples. Of course, individual features of the embodiments can be freely combined with one another, provided they are technically feasible, without departing from the scope of the present invention.

Claims

[1] Method for operating a fuel cell system (100) with at least one fuel cell stack (101) and at least one air system (10), wherein in the at least one air system (10) an exhaust gas recirculation path (EGR) is provided between an air supply path (11) to the at least one fuel cell stack (101) and an exhaust air path (12) from the at least one fuel cell stack (101), wherein a mixing point (MS) is arranged between the exhaust gas recirculation path (EGR) in the air supply path (11) downstream of an air filter (AF) and upstream of a, in particular second, air compression device (Comp), the method comprising: - Determining a current and a predictive content of liquid water in a mixed fluid at the inlet to and / or upstream of the air compression device (Comp), - Carrying out at least one measure (M1, M2, M3, M4) to reduce the content of liquid water in a mixed fluid at the inlet to the air compression device (Comp) depending on the determination. [2] Method according to claim 1, wherein the at least one measure (M1, M2, M3, M4) comprises a first measure (M1) according to which a limitation for an exhaust gas recirculation fluid, preferably for a cross section of the exhaust gas recirculation path (EGR), in particular adaptively, is carried out. [3] Method according to claim 1 or 2, wherein the at least one measure (M1, M2, M3, M4) comprises a second measure (M2) according to which a balancing, preferably for power distribution, between a first air compression device (Comp) and a second air compression device (Comp), in particular adaptively, is carried out. [4] Method according to one of the preceding claims, wherein the at least one measure (M1, M2, M3, M4) comprises a third measure (M3) according to which a cooling of a supply air (L1) upstream of the air compression device (Comp) is reduced, in particular adaptively. [5] Method according to one of the preceding claims, wherein the at least one measure (M1, M2, M3, M4) comprises a fourth measure (M4) according to which a fluid flow is increased via a stack bypass path (13). [6] Method according to one of the preceding claims, wherein, when a first measure (M1) has been initiated and when the content of a liquid water in a mixed fluid at the inlet to the air compression device (Comp) is too high and / or the first measure (M1) is too restrictive, a second measure (M2) and / or a third measure (M3) and / or a fourth measure (M4) is / are initiated in support of and / or in replacement of the first measure (M1). [7] Method according to one of the preceding claims, wherein the at least one measure (M1, M2, M3, M4) comprises a plurality of measures which are carried out successively, at least partially overlapping and / or simultaneously. [8] Method according to one of the preceding claims, wherein, when determining the current and the predictive content of liquid water in a mixed fluid at the inlet to the air compression device (Comp), at least one of the following parameters is taken into account during operation of the fuel cell system (100), in particular adaptively: - an operating parameter (BP) in a cathode path through the at least one fuel cell stack (101), comprising: - Pressure, - temperature, - mass flow, - Mole current, - Humidity, and / or - Content of liquid water, - Environmental parameters (UP), including: - Pressure, - temperature, - Humidity, and / or - Content of liquid water, - System parameters (SP) in the supply air path (11), in the exhaust air path (12) and / or stack bypass path (13), comprising: - Pressure, - temperature, - mass flow, - Mole current, - Humidity, and / or - Content of liquid water. [9] Method according to one of the preceding claims, wherein the method steps are carried out successively, at least partially overlapping and / or simultaneously, and / or wherein the method is carried out repeatedly, and / or wherein the method is carried out when exhaust gas recirculation is required. [10] Computer program product comprising instructions which, when the computer program product is executed by a computer, cause the computer to carry out a method according to one of the preceding claims. [11] Control unit (ecu), comprising 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 a method according to one of the preceding claims. [12] Fuel cell system (100) comprising a control unit (ecu) according to the preceding claim.

Images