Method for detecting the fill level of a water separator and fuel cell device

Abstract

Method for detecting the fill level of a water separator (14) in a fuel cell device (1) with a fuel cell stack (2) to which fuel is supplied on the anode inlet side by means of an anode supply line (8) and which has a water separator (14) coupled into an anode exhaust line (9) with a water separator valve (15) on the anode outlet side, comprising the following steps: • Adjusting the fuel pressure in the anode circuit to a predetermined fuel pressure by repeatedly opening and closing a pressure regulating valve coupled into an anode supply line (8), • Recording at least one reference value for a time when the pressure control valve is open and at least one reference value for a time when the pressure control valve is closed, • Opening the water separator valve (15) and draining any liquid collected in the water separator (14), • Maintaining the fuel pressure in the anode circuit at a predetermined fuel pressure with the water separator valve (15) open, by repeatedly opening and closing the pressure regulating valve and repeatedly recording the values ​​for the on time and the values ​​for the off time and • Closing the water separator valve (15) if at least one of the other values ​​of the start time and / or the end time deviates from the previously recorded reference value of the start time and / or the end time by a predetermined amount.

Description

[0001] The invention relates to a method for detecting the fill level of a water separator in a fuel cell device comprising a fuel cell stack to which fuel is supplied at the anode input via an anode supply line and which has a water separator coupled to an anode exhaust line at the anode output, including a water separator valve. The invention further relates to a fuel cell device.

[0002] Fuel cell devices are used for the chemical reaction of a fuel with oxygen to produce water, thereby generating electrical energy. As a core component, fuel cells contain the so-called membrane electron unit (MEU), which consists of a proton-conducting membrane and an electrode—an anode and a cathode—located on either side of the membrane. Modern fuel cell systems feature a water separator on the anode side, equipped with a water level sensor to determine the water level within the separator. This water level sensor is an additional component and is susceptible to errors caused by the longitudinal and lateral dynamics of the vehicle. The water level sensor reading can be distorted, for example, when cornering, driving on inclines, or experiencing strong vibrations during off-road driving or when encountering potholes.This also means, among other things, that the point at which all liquids have been discharged from the water separator is not detected in time. This leads to additional fuel gas leaks, which can cause limit reactions in the fuel cell system and also increases fuel consumption.

[0003] DE 10 2009 039 445 A1 discloses a method for draining liquid from a water separator in an anode circuit without the need for a level sensor, using a flow factor of a drain valve in conjunction with a pressure difference between a pressure level in a gas stream and a discharge area.

[0004] DE 10 2009 048 247 A1 discloses a method for a fuel cell system in which a drain valve of a water separator is kept open if a mass flow of an oxidizing agent exceeds a predetermined value and a pressure in an anode-side area of ​​the drain valve is greater than or equal to a pressure in a cathode-side area of ​​the drain valve.

[0005] DE 10 2011 011 147 A1 discloses a method for detecting a phase transition from liquid to gas, in which a switching ratio of an injector coupled into the anode supply line is used.

[0006] It is therefore an object of the present invention to provide a method for improved detection of the fill level of a water separator and an improved fuel cell device.

[0007] The method for determining the fill level of a water separator in a fuel cell device includes, in particular, the following steps: - Setting the fuel pressure in the anode circuit to a predetermined fuel pressure by repeatedly opening and closing a pressure regulating valve coupled into an anode supply line, - Recording at least one reference value for a time when the pressure control valve is open and at least one reference value for a time when the pressure control valve is closed, - Opening the water separator valve and draining any liquid collected in the water separator, - Maintaining the fuel pressure in the anode circuit at a predetermined fuel pressure with the water separator valve open, by repeatedly opening and closing the pressure regulating valve and repeatedly recording the values ​​for the start-up time and the values ​​for the shutdown time and - Closing the water separator valve if at least one of the other values ​​of the start time and / or the end time deviates from the previously recorded reference value of the start time and / or the end time by a predetermined amount.

[0008] The method utilizes the effect that the volume to be compensated for by the pressure regulator in the case of an open water separator valve, in order to maintain the specified fuel pressure, differs depending on whether liquid water or anode gas is being separated. Once the water separator is empty after the water separator valve has been actuated, the on and off times of the pressure regulating valve change due to the different outflow volume (now gaseous). Thus, the gas discharge phase, i.e., the point at which the water separator no longer contains any liquid, and is therefore empty, can be detected very reliably and quickly. This eliminates the need for costly sensors, such as a water level sensor, or allows for the addition of such sensors to verify the water level sensor's reading using the method according to the invention.Precisely recording the exact moment when the liquid has been completely drained from the water separator results in fuel savings and thus a more efficient fuel cell device. Furthermore, a high fuel concentration in the exhaust gas, which can lead to limit reactions, can be avoided. The pressure regulating valve can also be designed as an injector coupled into the anode supply line, with a variable flow diameter. The on and off times are preferably recorded continuously. However, it is also possible to record the on and off times only at or after the water separator valve opens, so that the first on and off time values ​​recorded after the water separator valve opens serve as reference values.

[0009] To adjust the specified fuel pressure, i.e., the desired power output of the fuel cell device, as effectively as possible, the fuel pressure is adjusted by repeatedly opening and closing the pressure control valve in such a way that the fuel pressure curve exhibits a periodic or oscillating pattern. In this context, it is particularly preferred if the periodic or oscillating pattern corresponds to a sawtooth waveform.

[0010] The at least one reference value for the start time and the at least one reference value for the stop time are preferably acquired after the water separator valve opens. In an alternative embodiment, however, it is also possible to acquire the reference value for the start time and the at least one reference value for the stop time before the water separator valve opens.

[0011] To increase the accuracy of the reference values ​​and thus reduce the error susceptibility of the procedure, it is preferred if a plurality of reference values ​​of the start time and / or a plurality of reference values ​​of the end time are recorded, if a mean or median is calculated from at least a portion of the reference values ​​of the start time and / or the end time, if a mean or median of the recorded values ​​of the start times and / or end times is calculated, and if the water separator is closed precisely when the mean or median of the values ​​deviates from the mean or median of the reference values ​​by a predetermined amount.

[0012] Alternatively or additionally, it is possible to determine a reference period duration from at least one of the reference values ​​of the start time and one of the reference values ​​of the end time, and to close the water separator valve if the period duration determined from the values ​​of the start time and the end time deviates from at least the previously determined reference period duration by a predetermined amount.

[0013] The fuel cell device, comprising a fuel cell stack to which fuel is supplied on the anode side via an anode supply line, a pressure regulating valve coupled into the anode supply line for adjusting the fuel pressure, a water separator having a water separator valve coupled into an anode exhaust line on the anode outlet side, and a control unit, is characterized in particular by the fact that the control unit is configured to carry out the process.

[0014] The advantages relating to the process apply analogously to the fuel cell device.

[0015] It is particularly advantageous if the water separator also has a level sensor. This allows the results of the level sensor to be compared with the results from the method according to the invention in order to identify as precisely as possible the point at which the liquid outlet transitions into the gas outlet. In addition, the level sensor can be used to trigger the opening of the water separator valve at a predetermined fill level in the water separator.

[0016] In an alternative embodiment, it is preferred that the water separator is sensor-free. In this case, the level sensor can be completely replaced using a water model, thus eliminating the need for costly and component-intensive sensors in the water separator.

[0017] The features and combinations of features mentioned above in the description, as well as those subsequently mentioned in the figure description and / or shown in the figures alone, can be used not only in the combinations specified, but also in other combinations or on their own, without departing from the scope of the invention. Thus, embodiments that are not explicitly shown or explained in the figures, but which can be derived and generated from the explained embodiments by separate combinations of features, are also to be considered as encompassed and disclosed by the invention.

[0018] Further advantages, features, and details of the invention will become apparent from the claims, the following description of preferred embodiments, and the drawings. These show: Fig. 1 a schematic representation of a fuel cell device, and Fig. 2. Temporal progression of start and end times

[0019] In the Fig. Figure 1 schematically shows a fuel cell device 1 which has a fuel cell or a plurality of fuel cells combined to form a fuel cell stack 2.

[0020] Each fuel cell comprises a membrane electrode assembly consisting of an anode and a cathode, as well as a proton-conducting membrane separating the anode from the cathode. The membrane is formed from an ionomer, preferably a sulfonated tetrafluoroethylene polymer (PTFE) or a perfluorinated sulfonic acid (PFSA) polymer. Alternatively, the membrane can be formed from a sulfonated hydrocarbon membrane.

[0021] A catalyst is additionally added to the anodes and / or the cathodes, wherein the membranes are preferably coated on their first side and / or on their second side with a catalyst layer made of a precious metal or of mixtures comprising precious metals such as platinum, palladium, ruthenium or the like, which serve as reaction accelerators in the reaction of the respective fuel cell.

[0022] Fuel (for example, hydrogen) is supplied to the anodes via anode compartments within the fuel cell stack 2. In a polymer electrolyte membrane fuel cell (PEM fuel cell), fuel or fuel molecules are split into protons and electrons at the anode. The membrane allows the protons (for example, H₂) to be released. + ) through, but is impermeable to electrons (e - The following reaction takes place at the anode: 2H₂→4H₂ + +4e -(Oxidation / Electron Release). While the protons pass through the membrane to the cathode, the electrons are conducted to the cathode or to an energy storage device via an external circuit. Cathode compartments within the fuel cell stack 2, also provided by the bipolar plate, can supply the cathodes with cathode gas (for example, oxygen or oxygen-containing air), so that the following reaction takes place on the cathode side: O₂ + 4H₂ + +4e - →2H2O (reduction / electron uptake).

[0023] Compressed air is supplied to the fuel cell stack 2 via a cathode fresh gas line 3 through a compressor 4. The fuel cell is also connected to a cathode exhaust line 6. On the anode side, hydrogen is supplied to the fuel cell stack 2 from a hydrogen tank 5 via an anode supply line 8 to provide the reactants required for the electrochemical reaction in the fuel cell. These gases are directed to and from the active surface of polar or bipolar plates via a media port. Flow channels, separated by ribs, are formed in the polar or bipolar plate for the further distribution of the gases to the membrane electrode arrangement, for their discharge from the fuel cell stack 2, and for the passage of a cooling medium.

[0024] A valve and / or also one in the Fig. The illustrated injector 13, which functions as a pressure regulating valve, can be suitable for achieving the desired partial pressure of fresh fuel within an anode circuit, which is created by the anode recirculation line 7. For this purpose, the pressure regulating valve, i.e., the injector 13, can be repeatedly opened and closed, thus generating a periodic or oscillating fuel pressure profile. With such an anode recirculation line 7, the fuel not consumed in the fuel cell stack 2 can be fed back to the anode chambers upstream of the fuel cell, so that the anode recirculation line 7 connects back to the anode supply line 8. Spent fuel is removed from the fuel cell via the anode exhaust line 9. Alternatively or additionally, a recirculation fan, also shown in the figure, can be used to effect the recirculation.The anode recirculation line 7 also includes a water separator 14 with a water separator valve 15. The water separator 14 can optionally be equipped with a level sensor or be sensorless. A control unit is configured to set a predetermined fuel pressure, depending on the required power output of the fuel cells. Due to the hysteresis pressure control provided by the control unit, with a maximum and minimum hysteresis, the fuel pressure curve exhibits a sawtooth pattern. The injector 13 is not limited to open or closed positions but can also assume any intermediate position between them. A pressure sensor is provided on the anode inlet side downstream of the injector 13 and on the anode outlet side upstream of the water separator 14 for setting the fuel pressure.

[0025] Since several fuel cells are combined in the fuel cell stack 2, a sufficiently large quantity of cathode gas must be supplied. Therefore, compressor 4 provides a large mass flow of cathode gas or fresh gas flow, whereby the temperature of the cathode gas increases significantly as a result of compression. The conditioning of the cathode gas or fresh gas flow, i.e., its adjustment with respect to the desired temperature and humidity in the fuel cell stack 2, takes place in a humidifier downstream of compressor 4. This humidifier saturates the membranes of the fuel cells with moisture to increase their efficiency, as this promotes proton transport.

[0026] The inventive method for detecting the fill level of a water separator 14 in a fuel cell device 1 comprises, in particular, the following steps: First, the fuel pressure in the anode circuit is set to a predetermined fuel pressure by repeatedly opening and closing the pressure regulating valve coupled into the anode supply line 8, in this case the injector 13. As already described above, the fuel pressure is set such that the fuel pressure curve has a periodic or oscillating profile, in particular a sawtooth profile.

[0027] At least one reference value is recorded for a time 16, during which the pressure control valve / injector 13 is open, and at least one reference value is recorded for a time 17, during which the pressure control valve / injector 13 is closed. The water separator valve 15 opens, for example, when an optional level sensor in the water separator 14 detects a limit level, or when a control unit, based on a water model, initiates the opening of the water separator valve 15. The liquid collected in the water separator 14 is drained, i.e., purged. By repeatedly opening and closing the pressure control valve / injector 13, the fuel pressure in the anode circuit is maintained at a predetermined fuel pressure while the water separator valve 15 is open. Values ​​for the time 16 and the time 17 are recorded repeatedly or continuously.The water separator valve 15 is closed exactly when at least one of the values ​​of the input time 16 and / or the output time 17 deviates from the previously recorded reference value of the input time 16 and / or the output time 17 by a predetermined amount.

[0028] The method utilizes the effect that the volume to be equalized differs when the water separator valve is open, depending on whether liquid water or anode gas is being separated. This is exemplified by the Fig.Figure 2 shows the course of the on times 16 and off times 17 before and after the opening of the water separator valve 15. The solid line 18 shows the opening and closing of the water separator valve 15. The curve 19 formed by circles shows the fuel concentration in the exhaust gas, and the curve 20 formed by crosses shows the fuel concentration at the anode. From time 21, when the water flow changes to a gas / fuel flow, i.e., from time 21 when the water separator 14 is empty, the off times 17 of the pressure regulating valve / injector 13 shorten, and the on times 16 lengthen as the fuel flow changes from water to gas in the pressure regulating valve. Depending on the specified fuel pressure, the on times 16 of the pressure regulating valve / injector 13 may also lengthen, and the off times 17 may shorten.The closing of the water separator valve 15 according to the invention upon detection of time 21 thus reduces fuel consumption and fuel concentration in the anode exhaust gas.

[0029] The reference value for the start time 16 and the reference value for the end time 17 can be acquired both after and before the water separator valve 15 opens. In particular, the acquisition of the reference values ​​and the values ​​for the start time 16 and the end time 17 is continuous. Thus, in one embodiment, it is also possible to begin the repeated or continuous acquisition of the start time 16 and the end time 17 only after or upon opening of the water separator valve 15. The first or one of the first values ​​for the start time 16 or the end time 17 after opening the water separator valve 15 then serve as the reference values ​​for the start time 16 and the end time 17.

[0030] To determine the time 21 of the transition from water discharge to gas discharge more precisely, it is advantageous to record a number of reference values ​​for time 16 and / or time 17. A mean or median is calculated from a portion of the reference values ​​for time 16 and / or time 17. Conversely, a mean or median is calculated from the recorded values ​​for time 16 or time 17, i.e., the values ​​recorded after the water separator valve 15 has opened. If the mean or median of these values ​​deviates from the mean or median of the reference values ​​by a predetermined amount, the water separator valve 15 is closed.In this context, it is particularly advantageous if 3 to 5 consecutive reference values ​​are formed to create a mean or median, and if 3 to 5 consecutive recorded values ​​of the entry times 16 or exit times 17 are formed to create a mean or median, and the mean or median of the reference values ​​is compared with the mean or median of the values.

[0031] As an alternative to directly comparing the start times 16 and / or the end times 17, a reference period can be determined from at least one start time 16 and at least one subsequent end time 17. Furthermore, after opening the water separator valve 15, a period can be determined continuously or at least repeatedly from the recorded values ​​of the start time 16 and the end time 17. The water separator valve 15 is then closed when the period deviates from at least the previously determined reference period by the specified amount. REFERENCE MARK LIST: 1 Fuel cell device 2 fuel cell stacks 3 Cathode fresh gas line 4 compressors 5 hydrogen tank 6 Cathode exhaust line 7 Anode recirculation line 8 Anode supply line 9 Anode exhaust line 10 Fresh gas supply line 11 Cathode exhaust line 12 Cathode recirculation line 13 Injector 14 water separators 15 Water separator valve 16 time 17 Time Out 18 Actuation of water separator valve 19 Fuel Concentration Exhaust Gas 20 Fuel concentration anode 21 Time of transition from water passage to gas passage

Claims

[1] Method for detecting the fill level of a water separator (14) in a fuel cell device (1) with a fuel cell stack (2) to which fuel is supplied on the anode inlet side by means of an anode supply line (8) and which has a water separator (14) coupled into an anode exhaust line (9) with a water separator valve (15) on the anode outlet side, comprising the following steps: • Adjusting the fuel pressure in the anode circuit to a predetermined fuel pressure by repeatedly opening and closing a pressure regulating valve coupled into an anode supply line (8), • Recording at least one reference value for a time when the pressure control valve is open and at least one reference value for a time when the pressure control valve is closed, • Opening the water separator valve (15) and draining any liquid collected in the water separator (14), • Maintaining the fuel pressure in the anode circuit at a predetermined fuel pressure with the water separator valve (15) open, by repeatedly opening and closing the pressure regulating valve and repeatedly recording the values ​​for the on time and the values ​​for the off time and • Closing the water separator valve (15) if at least one of the other values ​​of the start time and / or the end time deviates from the previously recorded reference value of the start time and / or the end time by a predetermined amount. [2] Method according to claim 1, characterized by , that the adjustment of the fuel pressure to the specified fuel pressure is carried out by repeatedly opening and closing the pressure regulating valve in such a way that a fuel pressure curve has a periodic or oscillating profile. [3] Method according to claim 2, characterized by , that the periodic or oscillating pattern corresponds to a sawtooth pattern. [4] Method according to any one of claims 1 to 3, characterized by , that the reference value of the start time and the reference value of the stop time are recorded after the water separator valve (15) is opened. [5] Method according to any one of claims 1 to 3, characterized by , that the reference value of the start time and the reference value of the stop time are recorded before the water separator valve (15) is opened. [6] Method according to any one of claims 1 to 5, characterized by , that a plurality of reference values ​​of the start time and / or a plurality of reference values ​​of the end time are recorded, that a mean or median is formed from at least a part of the reference values ​​of the start time and / or the end time, that a mean or median of the recorded values ​​of the start times and / or end times is formed, and that the water separator is closed when the mean or median of the values ​​deviates from the mean or median of the reference values ​​by a predetermined amount. [7] Method according to any one of claims 1 to 6, characterized by , that a reference period duration is determined from at least one of the reference values ​​of the start time and one of the reference values ​​of the time out, and that the water separator valve (15) is closed when the period duration determined from the values ​​of the start time and the time out deviates from at least the previously determined reference period duration by a predetermined amount. [8] Fuel cell device (1) comprising a fuel cell stack (2) to which fuel is supplied on the anode side via an anode supply line (8), a pressure control valve coupled into the anode supply line (8) for adjusting the fuel pressure, a water separator (14) having a water separator valve (15) coupled into an anode outlet line (9) on the anode outlet side and a control unit configured to carry out the method according to any one of claims 1 to 7. [9] Fuel cell device according to claim 8, characterized by that the water separator (14) has a level sensor. [10] Fuel cell device according to claim 8, characterized by , that the water separator (14) is formed without sensors.

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