Method for operating a fuel cell device and fuel cell vehicle

Abstract

Method for operating a fuel cell device (1) comprising a fuel cell stack (2) having a plurality of fuel cells (3), a fuel tank (6) connected to the fuel cell stack (2) via an anode fresh gas line (7), and a recirculation line (10) having a recirculation fan (9) which opens into the anode fresh gas line (7) upstream of the fuel cell stack (2), wherein an iteration line (11) having a control valve (13) branches off downstream of the recirculation fan (9) via a branch (12) from the recirculation line (10) and opens into the recirculation line (10) upstream of the recirculation fan (9), comprising the steps: a) determining the quantity of liquid water at the branch (12),b) Determining the degree of opening of the control valve (13) to determine the proportion of recirculated anode gas passed through the iteration line (11) to achieve a predetermined or predeterminable maximum quantity of liquid water at the branch (12).

Description

The invention relates to a method for operating a fuel cell device having a fuel cell stack having a plurality of fuel cells, having a fuel tank which is connected to the fuel cell stack via an anode fresh gas line, and having a recirculation line which has a recirculation fan and opens into the anode fresh gas line upstream of the fuel cell stack, an iteration line which has a control valve branching off from the recirculation line downstream of the recirculation fan via a branch and opening into the recirculation line upstream of the recirculation fan, comprising the steps: a) determining the quantity of liquid water at the branch, b) determining the degree of opening of the control valve in order to define the proportion of recirculated anode gas conducted through the iteration line in order to achieve a predefined or predefinable maximum quantity of liquid water at the branch.The invention further relates to a fuel cell vehicle.Fuel cells are used for the chemical reaction of a fuel with oxygen to form water in order to generate electrical energy. For this purpose, fuel cells contain, as core component, the so-called membrane-electrode unit which is a composite of a proton-conducting membrane and in each case one electrode (anode and cathode) arranged on both sides of the membrane. During operation of a fuel cell device having a plurality of fuel cells combined to form a fuel cell stack, the fuel, in particular hydrogen (H 2) or a hydrogen-containing gas mixture, is fed to the anode in a superstoichiometric manner, where electrochemical oxidation of H 2 to H + takes place with emission of electrons. Via the membrane, which separates the reaction spaces from one another in a gas-tight manner and electrically isolates them, the protons H + are transported from the anode space into the cathode space. The electrons provided at the anode are supplied to the cathode via an electrical line. Oxygen or an oxygen-containing gas mixture is supplied to the cathode, so that a reduction from O 2 to O 2- takes place with the absorption of the electrons.Unused fuel is returned to the anode via a recirculation line using a jet pump or a recirculation fan. In the cathode compartment, the oxygen anions react with the protons transported across the membrane to form water. This water must be taken out of the fuel cell and the fuel cell stack until a moisture level is reached which is required for operating the fuel cell system.Fuel cell devices therefore require careful water management since, on the one hand, it is necessary to prevent too much water from being present in the fuel cell or in the fuel cell stack, which leads to blockage of the flow channels for the supply of the reactants. If, on the other hand, there is too little water in the fuel cell, the proton conductivity of the membrane is limited, so that sufficient moisture and water supply to the membrane must be ensured.In order to make sufficient oxygen available from the air for the plurality of fuel cells combined in a fuel cell stack, air with the oxygen contained therein is compressed by means of a compressor in the cathode circuit for supplying the cathode spaces of the fuel cell stack, so that relatively warm and dry compressed air is present, the moisture content of which is not sufficient for the membrane electrode unit to be used in the fuel cell stack. Therefore, a humidifier is used which, in the case of two gaseous media with a different moisture content, brings about a transfer of the moisture to the drier medium by the dry air provided by the compressor being guided past a humidifier membrane permeable to water vapor, the other side of which is swept with the moist exhaust air from the fuel cell stack.It is problematic if frost conditions are present at a start-up of the fuel cell device, i.e. conditions in which water freezes. This may result in the required flow channels for the reactant gases and the product water being blocked by ice, thus it is known to perform a drying procedure when shutting down the fuel cell device. At a start of the fuel cell device, it may nevertheless lead to problems, since liquid water can be introduced into the fuel cell stack via the recirculated fuel when the fuel cell device warms up until its operating temperature is reached. This leads to interruptions in the single cell voltage and a creeping degradation of the fuel cell stack, wherein a complete undersupply of fuel may also be present due to blocked flow channels, with the risk of a stack fire, which may lead to a defect of the fuel cell device and to its defect when used in a fuel cell vehicle.DE 10 2018 218 083 A1 describes a method for discharging liquid from a fuel cell device which has a recirculation line, wherein a partial stream containing the liquid is discharged downstream of a recirculation blower via a valve. Similarly, DE 10 2008 058 959 A1 discloses a method for operating a fuel cell system which has an anode recirculation device. Downstream of a recirculation blower, a discharge valve is provided, via which a partial stream for reducing liquid water is discharged as required, which is then not available for recirculation. US 2015 125 766 A1 discloses a method for controlling a fuel cell module with a hydrogen recirculation pump. On the basis of an operating signal of the hydrogen recirculation pump, a liquid water fraction within the fuel cell module is detected, wherein water removal takes place by means of a temperature increase.DE 10 2019 001 602 A1 discloses an anode circuit having a hydrogen tank which is connected to a fuel cell stack via a supply line having a jet pump. A fan is arranged in series with the jet pump. A recirculation line is guided to the jet pump, wherein a bypass line is guided around the blower. A valve device is arranged in each of the bypass line and in the line with the blower, so that the anode circuit can be operated with the jet pump alone at low load and with the blower alone at higher load. A bypass line to the blower is also shown in DE 10 2020 105 476 A1.It is the object of the present invention to provide an improved method for operating a fuel cell device and an improved fuel cell vehicle.This object is achieved by a method having the features of claim 1 and by a fuel cell vehicle having the features of claim 5. Advantageous embodiments with expedient developments of the invention are specified in the dependent claims.Within the scope of the invention, a method for operating a fuel cell device is provided having a fuel cell stack having a plurality of fuel cells, having a fuel tank which is connected to the fuel cell stack via an anode fresh gas line, and having a recirculation line having a recirculation fan which opens into the anode fresh gas line upstream of the fuel cell stack, wherein an iteration line having a control valve branches off from the recirculation line downstream of the recirculation fan via a branch and opens into the recirculation line upstream of the recirculation fan, comprising the steps: a) determining the amount of liquid water at the branch, b) determining the degree of opening of the control valve in order to define the proportion of the recirculated anode gas conducted through the iteration line in order to achieve a predefined or predefinable maximum quantity of the liquid water at the branch.With this method, the duration of the use of the iteration line is optimized and the additional use of the recirculation blower is limited to the required amount.For this purpose, it is expedient if the determination of the quantity of the liquid water at the branch is effected via a measurement of the temperature and the relative humidity and, in particular, a minimum temperature is determined in this case as a function of the relative humidity and the temperature at the branch, which minimum temperature excludes a subsequent condensation in the anode fresh gas line and / or the fuel cell stack.Furthermore, the regulating valve can be at least partially closed in order to check the requirement for further use of the iteration line. In this method step, the relative humidity and the temperature for the control operation can be checked whether a return through the iteration line is really necessary.The fuel cell device is characterized in that recirculated anode gas can be conducted repeatedly through the recirculation blower and is thereby subjected to a temperature increase by renewed compression in the recirculation blower. An increase in the water absorption capacity and also an increase in the temperature of the gas supplied to the fuel cell stack via the anode fresh gas line are associated with the increased temperature of the recirculated anode gas, so that the condensation on colder line sections is reduced after a frost start. The presence of liquid water at the inlet of the fuel cell stack is avoided and a higher system robustness is thereby achieved with an improved durability, since an undersupply of fuel is excluded. It should also be noted that, as a result of the use of the iteration line for the further heating of the recirculated anode gas, the discharge thereof is unnecessary and thus improved fuel use is achieved.It is very particularly preferred if a control valve is arranged in the iteration line. This creates the possibility of carrying out the use of the iteration line as required, in particular by closing the control valve to shut off the iteration line when the relative humidity in the recirculated anode gas is low, for example after reaching the standard operating conditions after a frost start. The opening and closing of the control valve can be effected independently of the actuation of a purge valve arranged in the recirculation line.In order to be able to better assess the need to use the iteration line, a sensor for detecting the temperature and / or a sensor for detecting the relative humidity of the recirculated anode gas is arranged downstream of the recirculation blower and upstream of the branch of the iteration line.Alternatively or additionally, a computing unit can be provided for the model-based determination of the relative humidity and / or the temperature of the recirculated anode gas on the basis of operating parameters of the fuel cell stack. In this case, the given load point, the extent of degradation, the ambient temperature, the temperature of the fuel cell stack, the individual cell voltage of the fuel cells, and the operating duration can be used in particular as operating parameters.The aforementioned advantages and effects also apply to a fuel cell vehicle having such a fuel cell device, which is distinguished in particular by a reduction in the loss of recirculated anode gas and thus an improved utilization of the fuel and thus an increase in the range. For this purpose, a control device is provided which is configured to carry out one of the above methods.The features and combinations of features mentioned above in the description and the features and combinations of features mentioned below in the description of the figures and / or shown alone in the figures can be used not only in the respectively specified combination but also in other combinations or alone without departing from the scope of the invention. Therefore, embodiments are also to be considered included and disclosed by the invention, which are not explicitly shown or explained in the figures, but which emerge from the explained embodiments and can be generated by separate combinations of features.Further advantages, features and details of the invention are evident from the claims, the following description of preferred embodiments and on the basis of the drawings. The following are shown: FIG. 1 shows a schematic illustration of a fuel cell device with an iteration line, FIG. 2 shows a representation of the temperature at the inlet of the anode as a function of the current density, shown in dashed lines for the prior art without an iteration line and shown with a solid line for a fuel cell device with an iteration line according to FIG. 1, and FIG. 3 shows a time-dependent illustration of the cell voltage with its collapse upon entry of liquid water into the fuel cell stack.FIG. 1 schematically shows a fuel cell device 1, wherein the latter comprises a plurality of fuel cells 3 combined in a fuel cell stack 2.Each of the fuel cells 3 comprises an anode, a cathode and a proton-conductive membrane separating the anode from the cathode. The membrane is formed from an ionomer, preferably a sulfonated polytetrafluoroethylene (PTFE) polymer or a perfluorinated sulfonic acid (PFSA) polymer. Alternatively, the membrane can also be formed as a sulfonated hydrocarbon membrane.Fuel, for example hydrogen, can be supplied to the anode from a fuel tank 6 via an anode fresh gas line 7. In a polymer electrolyte membrane fuel cell (PEM fuel cell), fuel or fuel molecules are split into protons and electrons at the anode. The PEM allows the protons to pass through, but is impermeable to the electrons. The reaction takes place at the anode, for example: 2H 2 →4H ++ 4 e -( oxidation / electron emission). As the protons pass through the PEM to the cathode, the electrons are conducted to the cathode or to an energy store via an external circuit.The cathode gas (for example oxygen or oxygen-containing air) can be supplied to the cathode via a cathode space, so that the following reaction takes place on the cathode side: O 2+ 4 H ++ 4 e -→2H 2 O (reduction / electron uptake).Due to the plurality of fuel cells 3, a sufficiently large amount of cathode gas must be provided, so that a large cathode gas mass flow or fresh gas flow is provided by a compressor 8, wherein the temperature of the cathode gas increases greatly as a result of the compression of the cathode gas. The conditioning of the cathode gas or of the fresh air gas stream, i.e. its adjustment with respect to the temperature and humidity desired in the fuel cell stack 2, takes place in a charge air cooler 5 downstream of the compressor 8 and a humidifier 4 downstream of the latter, which brings about moisture saturation of the membranes of the fuel cells 3 in order to increase their efficiency, since this promotes proton transport.The fuel is supplied to the fuel cell stack 2 in a superstoichiometric amount, which is therefore not completely consumed as a result. For improved use of the fuel, the latter is recirculated by means of a recirculation line 10 having a recirculation fan 9 and fed back into the anode fresh gas line 7 upstream of the fuel cell stack 2. However, for example, when frost start conditions are present, a large relative humidity may be present in the recirculated anode gas, associated with a relatively low temperature. This can lead to condensation on cold line sections and thus to an introduction of liquid water into the fuel cell stack 2.FIG. 3 shows the dip in cell voltage at 8 seconds associated with the introduction of liquid water.In order to avoid this, an iteration line 11 is provided as shown in FIG. 1, which branches off from the recirculation line 10 downstream of the recirculation blower 9 via a branch 12 and opens back into the recirculation line 10 upstream of the recirculation blower 9. This iteration line 11 can thus be used to guide the recirculated anode gas multiple times through the recirculation blower 9, wherein an increase in the temperature of the anode(ab) gas is achieved by the multiple compression. This temperature increase can be effected to such an extent that subsequently no condensation takes place on cold power sections and thus the introduction of liquid water into the fuel cell stack 2 is avoided. From FIG. 2, the temperature increase by 10 K (Kelvin) is evident by the effect of a repeated passage of the recirculated anode gas through the iteration line 11 and the recirculation blower 9.FIG. 1 also indicates that a control valve 13 is arranged in the iteration line 11, which control valve can be used to restrict or prevent the use of the iteration line 11 for cases in which there is no risk of liquid water being introduced into the fuel cell stack 2.For this purpose, downstream of the recirculation blower 9 and upstream of the branch 12 of the iteration line 11, a sensor for detecting the temperature and / or a sensor for detecting the relative humidity of the recirculated anode gas is arranged. Alternatively or additionally, a computing unit can be provided for the model-based determination of the relative humidity and / or the temperature of the recirculated anode gas on the basis of operating parameters of the fuel cell stack 2.A method for operating such a fuel cell device 1 comprises the steps: a) determining the quantity of liquid water at the branch 12, b) determining the degree of opening of the control valve 13 for defining the proportion of recirculated anode gas conducted through the iteration line 11 for achieving a predefined or predefinable maximum quantity of liquid water at the branch 12.In this case, the determination of the quantity of liquid water at the branch 12 can be effected via a measurement of the temperature and the relative humidity and, as a function of the relative humidity and the temperature at the branch 12, a minimum temperature can be determined which excludes a subsequent condensation in the anode fresh gas line 7 and / or the fuel cell stack 2. In this case, the regulating valve 13 can also be at least partially closed in order to check the need for further use of the iteration line 11.This fuel cell device 1 shows its advantages in particular when used in a fuel cell vehicle, since the latter can be exposed to frequently changing environmental conditions on account of its mobility and there is also the possibility of a frost start. In particular, during this frost start, a fuel undersupply of the fuel cell stack 2 is avoided, wherein, however, also a discharge of fuel from the recirculation line 10 at this point and for this purpose is unnecessary and thus an increased efficiency and range is achieved; only in the case of the actuation of a purge valve arranged in the recirculation line, a corresponding discharge of fuel can still take place.LIST OF REFERENCE CHARACTERS:1 Fuel cell device 2 Fuel cell stack 3 Fuel cell 4 Humidifier 5 Charge air cooler 6 Fuel tank 7 Anode fresh gas line 8 Compressor 9 Recirculation blower 10 Recirculation line 11 Iteration line 12 Branch 13 Control valve 14 Separator

Claims

Method for operating a fuel cell device (1) having a fuel cell stack (2) having a plurality of fuel cells (3), having a fuel tank (6) which is connected to the fuel cell stack (2) via an anode fresh gas line (7), and having a recirculation line (10) which has a recirculation fan (9) and opens into the anode fresh gas line (7) upstream of the fuel cell stack (2), an iteration line (11) which has a control valve (13) branching off from the recirculation line (10) downstream of the recirculation fan (9) via a branch (12) and opening into the recirculation line (10) upstream of the recirculation fan (9), comprising the steps: a) determining the amount of liquid water at the branch (12), b) determining the degree of opening of the control valve (13) for defining the proportion of the recirculated anode gas conducted through the iteration line (11) for achieving a predetermined or predeterminable maximum quantity of the liquid water at the branch (12).Method according to claim 1, characterised in that the determination of the quantity of liquid water at the branch (12) takes place via a measurement of the temperature and the relative humidity.Method according to Claim 1 or 2, characterized in that a minimum temperature is determined as a function of the relative humidity and the temperature at the branch (12), which minimum temperature excludes subsequent condensation in the anode fresh gas line (7) and / or the fuel cell stack (2).Method according to one of Claims 1 to 3, characterized in that the regulating valve (13) is at least partially closed in order to check the requirement for further use of the iteration line (11).Fuel cell vehicle, characterized in that a control unit is provided which is set up to carry out a method according to one of Claims 1 to 4.

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