METHOD AND SYSTEM FOR IDENTIFYING A LEAK WITHIN A MEMBRANE OF A FUEL CELL

Abstract

A method for identifying a leak (4′) within a membrane (4) of a fuel cell (5) during operation of a motor vehicle, comprising the steps: reducing the power provided by the fuel cell (5) starting from an output power to a minimum value; determining measurement values of the current cell voltage of the fuel cell (5) whilst the reduced power at the minimum value is provided by the fuel cell (5); and assessing a state of the membrane (4) of the fuel cell (5) on the basis of the determined measurement values in order to identify a leak (4′). The power reduced by the fuel cell (5) whilst measurement values of the current cell voltage are determined is provided at the same level by at least one further energy source.

Claims

1. A method for detecting a leak (4′) within a membrane (4) of a fuel cell (5) during the operation of a motor vehicle, the method comprising the steps of: a) reducing the power that is provided by means of the fuel cell (5) starting from an output power to a minimal value; b) determining measurement values of the prevailing cell voltage of the fuel cell (5) while providing the power that has reduced to the minimal value by means of the fuel cell (5); c) assessing the state of the membrane (4) of the fuel cell (5) with the aid of the determined measurement values for detecting a leak (4); wherein the reduced power from the fuel cell (5) during the procedure of determining the measurement values of the prevailing cell voltage is compensated for in an equal amount by means of at least one further energy source.

2. The method as claimed in claim 1, wherein prior to the reduction of the provided power a test is performed with respect to the prevailing feasibility of the method, wherein the test includes performing a comparison of the duration of the operation of the fuel cell (5) under full load with a comparison value.

3. The method as claimed in claim 1, wherein the power that is provided by means of the fuel cell (5) reduces during the method—and is increased back to the same extent after the measurement values of the prevailing cell voltage have been determined, after and in dependence upon the performed assessment procedure, wherein the reduced power from the fuel cell (5) is compensated for by means of the at least one further energy source and this power that is provided by means of the at least one further energy source is reduced accordingly by the same amount after the power that is provided by the fuel cell (5) increases.

4. The method as claimed in claim 1, wherein the power provided by means of the fuel cell (5) is reduced prior to and/or during the procedure of determining measurement values of the prevailing cell voltage and in dependence upon the performed assessment procedure to a value of less than 2% of the maximal power.

5. The method as claimed in claim 1, wherein the power that is provided by means of the fuel cell (5) is reduced in steps prior to and/or during the procedure of determining measurement values of the prevailing cell voltage.

6. The method as claimed in claim 1, wherein the procedure of determining measurement values is performed within less than 10 seconds.

7. The method as claimed in claim 1, wherein the procedure of assessing a state for detecting a leak (4′) within the membrane (4) of a fuel cell (5) includes performing at least one comparison between the measured values and reference values, wherein the measured values originate from different sensors.

8. The method as claimed in claim 1, wherein in dependence upon the assessment a warning signal is emitted and/or the fuel cell (5) is switched into an emergency operating mode.

9. The method as claimed in claim 1, wherein the individual steps of the method are repeated periodically during the operation of the fuel cell (5).

10. The method as claimed in claim 1, wherein the method is used in a vehicle, in particular in a fuel cell vehicle.

11. A system for operating a motor vehicle, the system comprising: at least one control unit for reducing the power that is provided by means of a fuel cell (5) starting from an output power to a minimal value, at least one measuring unit for determining measurement values of the prevailing cell voltage of the fuel cell (5) while providing the reduced power to the minimal value by means of the fuel cell (5), at least one processing unit for assessing a state of the membrane (4) of the fuel cell (5) with the aid of the determined measurement values for detecting a leak (4′); wherein the system comprises at least one further energy source for compensating in an equal amount for the power that has reduced during the procedure of determining measurement values of the prevailing cell voltage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Further advantages, features and details of the invention are disclosed in the description below in which exemplary embodiments of the invention are described in detail with reference to the drawings. In this case, the features mentioned in the claims and in the description can be essential to the invention in each case individually alone or in any combination.

[0020] In the drawing:

[0021] FIG. 1a illustrates a lateral view of an intact fuel cell without a leak,

[0022] FIG. 1b illustrates a sectional view of a defective fuel cell having a leak that has occurred within the membrane of the fuel cell,

[0023] FIG. 2 illustrates the polarization curve of an intact PEM fuel cell and a defective PEM fuel cell having a leak that has occurred within the membrane,

[0024] FIG. 3 plots the progression of the cell voltage of an intact fuel cell and a defective fuel cell during the performance of the method in accordance with the invention,

[0025] FIG. 4 shows a flow diagram for illustrating the progress of the method in accordance with the invention for detecting a leak within the membrane of a fuel cell.

[0026] Identical reference numerals are used in the figures for the same technical features.

DETAILED DESCRIPTION

[0027] FIG. 1a illustrates a lateral view of an intact fuel cell 5 without a leak. In order to provide a greater amount of energy in a fuel cell system, a multiplicity of fuel cells 5 is combined in one fuel cell stack. For the sake of simplicity, only one fuel cell 5 is illustrated here. The fuel cell 5 comprises an anode 6 and a cathode 8 that are separated from one another by means of the membrane 4. Both the anode 6 and also the cathode 8 are electrically connected to the membrane 4. During the operation, the anode gas in the present case hydrogen 1 flows around the anode 6. In addition to hydrogen 1, the gas that is present at the anode also comprises individual nitrogen molecules 2 that can be present in the gas in particular in the case of hydrogen 1 that is recovered from the exhaust air. During the operation, the cathode gas flows around the cathode 8, in the present case the cathode gas is fresh air containing oxygen and comprising a nitrogen portion 2 and an oxygen portion 3. The membrane 4 is formed in the present case as a proton exchange membrane that is permeable for protons but as far as possible impermeable for the reactants of the fuel cell reaction, hydrogen 1 and oxygen 3. During the operation of the fuel cell 5, the fuel in this case hydrogen 1 oxidizes at the anode 6 in a catalytic manner while discharging electrons to form protons. The protons pass through the proton exchange membrane into the cathode chamber that is filled with oxygen-containing gas. The electrons are discharged from the fuel cell 5 and flow to the cathode 8 by way of an electrical connection that is not illustrated here. By means of absorbing the electrons, the oxidation agent in this case oxygen 1 is reduced at the cathode 8 to anions that react directly with the protons to form water. The described reaction creates a voltage that can be measured between the anode and the cathode and that depends in particular upon the reactants, the quality of the cell, the temperature and the cell load. The theoretically achievable cell voltage in the case of a hydrogen/oxygen fuel cell amounts to 1.23 V at a temperature of 25° C. However, generally only cell voltages of approximately 1 V are achieved owing to the reduced degrees of implementation as a result of impure reactants, wear experienced by the cells and in particular during the operation of the cells.

[0028] FIG. 1b illustrates a sectional view of a defective fuel cell 5 having a leak 4′ that has occurred within the proton exchange membrane 5. In this case, as a result of the leak, the reactants such as portions of the hydrogen 1 present in the anode chamber can penetrate the cathode chamber and react there directly with oxygen 3 to form water. The hydrogen 1 that is converted in this manner does not consequently contribute to the cell voltage with the result that in dependence upon the magnitude of the leak 4′ smaller cell voltages are measured and less energy delivered. In addition, in the case of larger leaks 4′, a direct reaction of this type can be also be problematic for reasons of safety owing to their high reaction energy.

[0029] FIG. 2 illustrates the polarization curve of an intact and a defective PEM fuel cell 12 having a leak 4′ that has occurred within the membrane 4 of the cell 5. If a fuel cell 5 is loaded with a current, then the cell voltage of the loaded cell 5 drops owing to activation losses and ohmic losses as the load increases. A varying load produces in this case a characteristic continuous current-voltage progression, the so-called current-voltage curve or polarization curve of a fuel cell 5. It becomes clear with the aid of this characteristic curve progression why the method in accordance with the invention should be performed to detect a leak within a membrane 4 of a fuel cell 5 with the aid of a voltage drop owing to the measuring sensitivity at the best in the fully unloaded state of a fuel cell 5 in the case of an open circuit voltage 14 (OVC). In this state, the voltage differences V.sub.a are the greatest between an intact system 10 and a defective system 12 having a leak 4′ that has occurred within the membrane 4. In contrast, if the measuring procedure is performed when the fuel cell 5 is subjected to a high load, then it is only possible with great difficulty to detect a leak 4′ with the aid of the difference of the cell voltage V.sub.b.

[0030] FIG. 3 illustrates the progression of the cell voltage of an intact and a defective fuel cell 5 during the performance of the method in accordance with the invention. In this case, the progression can be divided into three sections a, b, c. In this case, section a represents the progression of the cell voltage prior to a reduction of the power that is provided by the fuel cell 5 in the case of a current 30a provided by the fuel cell 5, whereas section b represents the progression of the cell voltage after a reduction of the power provided by the fuel cell 5 to a minimal value at the provided current 30b and illustrates the preferred measurement section for obtaining measurement values 40. Finally, the section c represents the progression of the cell voltage after performing the procedure of obtaining the measurement value, wherein the power provided by the fuel cell 5 is increased back to the originally provided current 30c. The curve 32 illustrates here the progression of the current provided by the at least one further energy source. The curve 34 illustrates the voltage progression of an intact fuel cell 5, whereas the curve 36 represents the voltage progression of a defective fuel cell 5 having a leak 4′ that has occurred within the membrane 4.

[0031] In a first section a during an operation of the fuel cell 5 with a current 30a provided by the cell, there can be hardly any difference between the cell voltage of the intact fuel cell 34a and the cell voltage of a defective fuel cell 36a (cf. FIG. 2 voltage difference V.sub.b). However, after the reduction in accordance with the invention of the fuel cell current from the operating state 30a to a minimal value 30b, it is possible with the aid of the cell voltage to significantly differentiate between the intact system and the defective system. The cell voltage of the intact cell 34b increases to the value which the cell achieves in the unloaded state, in other words in the case of the open circuit voltage (cf. FIG. 2). In contrast, the cell voltage of the defective cell 36b drops because the leak 4′ causes an exchange between the reactants that leads to a direct reaction between hydrogen molecules 1 and oxygen molecules 3 with the result that the hydrogen 1 that is converted in this manner to form water does not amount to the cell potential. It is possible in this phase that represents the preferred measurement section and preferably lasts 2 to 5 seconds for a leak 4′ within the membrane 4 of a fuel cell 5 to be detected in a very sensitive manner.

[0032] In order to also be able to operate a fuel cell system during this phase 40, in which the power provided by the fuel cell 5 or the provided current is reduced to a minimal value 30b, the reduced power is compensated for by means of increasing the current 32a of at least one further energy source to a current 32b with the result that a constant power is made available to the relevant drive during the entire progression.

[0033] Subsequently, after measurement values have been determined for assessing a state with respect to a leak within the membrane 4 of a fuel cell 5, the current provided by the fuel cell 5 is increased back to a value 30c while the current provided by the at least one other energy source is reduced to the same extent back to a value 32c. In reaction thereto, the detected cell voltage of the intact cell drops back, whereas the cell voltage of the defective cell increases with the result that as in the first section a it is only possible with great difficulty to differentiate between the intact cell and the defects cell with the aid cell voltage.

[0034] FIG. 4 shows a flow diagram for illustrating the progression of the method in accordance with the invention for detecting a leak within the membrane of a fuel cell 5. The method comprises the steps 20 to 28.

[0035] During the operation of a fuel cell system, first of all in an optional step 20 a test is performed with respect to the prevailing feasibility of the method. This optional method step is performed first and foremost in order to be able to ensure that preferably during the entire performance of the method in accordance with the invention the reduced power from the fuel cell 5 can also be compensated for by the at least one other energy source. A test of this type can in this case comprise in particular a test of the prevailing charge state of the at least one further energy source. Furthermore, the test can preferably include a comparison of the currently available charge capacity with a predicted energy consumption during the performance of the method in accordance with the invention.

[0036] After this optional step for performing the test with respect to the prevailing feasibility of the method in accordance with the invention, in step 22 the power provided by the fuel cell 5 is reduced starting from an output power to a minimal value and this power is simultaneously compensated by means of at least one other energy source. The minimal value of the power provided by the fuel cell 5 is in the ideal case a value of 0 Watt, in other words the value that can be measured if the fuel cell 5 is not providing any power. However, it is likewise possible that the minimal value is higher, for example multiple kilowatt or the like. A reduction of the power to the minimal value can occur in this case in steps or also continuously. The at least one further energy source is preferably an electrical, in particular electrochemical, energy source.

[0037] After the reduction of the power provided by the fuel cell 5 to the minimal value and the simultaneous compensation by means of at least one further energy source, measurement values of the prevailing cell voltage of the fuel cell 5 are determined in step 24 of the method in accordance with the invention. In this case, it is possible to select preferably freely and in a variable manner both the time interval for determining the measurement values and also the rate at which the measurement values are determined. The measurement values are preferably determined in this case in a small time frame, for example within 2 to 5 seconds.

[0038] After the measurement values of the prevailing cell voltage of the fuel cell 5 have been determined, a state of the membrane of the fuel cell 5 is finally assessed within the scope of the method in accordance with the invention in step 26 with the aid of the determined measurement values for detecting a leak. In this case, the procedure of assessing a state of this type currently includes performing at least one comparison between the measured values and reference values, wherein the measured values preferably originate from different sensors and are in particular averaged and/or weighted prior to being compared with reference values. The reference value is in this case currently preferably a variably determinable value, with the aid of which it is possible to assess a state with respect to detecting a leak 4′ within the membrane 4 of a fuel cell 5. In the simplest case, the reference value represents the value of the cell voltage that is theoretically ideally to be expected in the case of the correspondingly provided current. However, the reference values are in particular dependent upon the system and/or dependent upon the prevailing environmental conditions, with the result that the reference values should preferably be accordingly adjusted prior to a meaningful comparison.

[0039] Finally, after the assessment procedure has been performed, in a subsequent step 28 at least insofar as a leak 4′ has not been detected the power provided by the fuel cell 5 is increased while correspondingly reducing the power provided by the at least one other energy source. In the event that a leak 4′ has been detected within the membrane 4 of the relevant fuel cell 5 within the scope of the assessment procedure, then warning signals can be generated alternatively or cumulatively and/or the fuel cell 5 can be switched into an emergency operating mode or not be switched back on.