METHOD FOR OPERATING A FUEL CELL SYSTEM AND CONTROL DEVICE

Abstract

The invention relates to a method for operating a fuel cell system (1), in particular a PEM fuel cell system, in which at least one fuel cell (2) is supplied with a hydrogen-containing anode gas via an anode gas path (3) and anode gas exiting the fuel cell (2) is returned via a recirculation path (4), wherein, in order to reduce a nitrogen content in the anode gas, a flush valve (5) arranged in the recirculation path (4) is opened and the recirculation path (4) is flushed. According to the invention, the actual composition of the anode gas is determined using at least one sensor (6) and the ageing status of the at least one fuel cell (2) is determined by comparing the determined actual composition with a target composition and/or an actual composition determined earlier. The invention also relates to a control device (7) for carrying out the method according to the invention.

Claims

1. A method for operating a fuel cell system (1), the method comprising: supplying at least one fuel cell (2) with a hydrogen-containing anode gas via an anode gas path (3), and returning anode gas exiting the fuel cell (2) via a recirculation path (4), wherein, in order to reduce a nitrogen content in the anode gas, a flushing valve (5) arranged in the recirculation path (4) is opened and the recirculation path (4) is flushed, wherein a composition of the anode gas is determined by means of at least one sensor (6) and the ageing state of the at least one fuel cell (2) is determined by comparing the determined actual composition with a nominal composition and/or an actual composition determined at an earlier time.

2. The method as claimed in claim 1, wherein a defined operating point is reached in order to determine the actual composition of the anode gas.

3. The method as claimed in claim 1, wherein the hydrogen content of the anode gas is determined by means of the sensor (6), the nitrogen content is determined from the hydrogen content, and the ageing state of the at least one fuel cell (2) is derived from the nitrogen content.

4. The method as claimed in claim 1, wherein the actual composition of the anode gas present in the recirculation path (4) is determined by means of the sensor (6).

5. The method as claimed in claim 1, wherein the actual composition of the anode gas discharged via the flushing valve (4) is determined by means of the sensor (6).

6. The method as claimed in claim 1, wherein the ageing state of the at least one fuel cell (2) is determined at regular temporal intervals.

7. The method as claimed in claim 1, wherein the ageing state of the flushing valve (5) and/or the sensor (6) is taken into account and, to determine the ageing state of the flushing valve (5) and/or the sensor (6), the hydrogen content of the anode gas discharged via the flushing valve (5) in a defined state A is compared with the hydrogen content in a defined state B.

8. The method as claimed in claim 7, wherein state A is achieved in load-free state of the fuel cell system (1) by lowering the pressure on the cathode side relative to the anode side.

9. The method as claimed in claim 7, wherein state B is achieved in normal operation of the fuel cell system (1) by reaching a defined load point.

10. A control device (7) which is configured to carry out a method as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The invention is described in more detail below with reference to the appended drawing. This shows a schematic illustration of a fuel cell system according to the invention.

DETAILED DESCRIPTION

[0024] The fuel cell system 1 illustrated schematically in the drawing serves to drive a vehicle. It constitutes merely one possible exemplary embodiment of a fuel cell system for performance of the method according to the invention.

[0025] The fuel cell system comprises at least one fuel cell 2 or several fuel cells 2 in a stacked arrangement. Air as a cathode gas is supplied to a cathode 8 of the fuel cell system 1 via a cathode gas path 10, while hydrogen as an anode gas is supplied to an anode 9 of the fuel cell system 1 via an anode gas path 3.

[0026] The air supplied to the cathode 8 is taken from the environment. Firstly, the air is compressed by means of a compressor 12 arranged in the cathode gas path 10. After compression, the air is cooled again via a cooling device 17 (also arranged in the cathode gas path 10) and is additionally moistened by means of a downstream moistening device 18. The compressor 12, the cooling device 17 and/or the moistening device 18 are optional. Cathode outlet gas exiting from the at least one fuel cell 2 is discharged via a cathode outlet gas path 11. The cathode outlet gas is supplied to an outlet gas turbine 13, which is arranged in the cathode outlet gas path 11 and supports an electric motor 14 for driving the compressor 12 arranged in the cathode gas path 10. The outlet gas turbine 13 is also optional. The cathode gas path 10 and the cathode outlet gas path 11 in the present case can be connected together via a bypass path 15 depending on the switch position of a bypass valve 16.

[0027] The hydrogen serving as an anode gas is stored in tanks 19 and supplied to the anode 9 by means of a suction jet pump 20 arranged in the anode gas path 3. Anode gas which exits from the at least one fuel cell 2 again is returned to the anode gas path 3 via a recirculation path 4 so it is not lost to the system. For this, a recirculation fan 22 is arranged in the recirculation path 4, but this is not however absolutely necessary.

[0028] Since the anode gas becomes enriched over time with nitrogen which diffuses from the cathode region to the anode region, from time to time the recirculation path 4 must be flushed. For this, a flushing valve 5 is arranged in the recirculation path 4 upstream of the recirculation fan 22. Liquid water contained in the anode gas can firstly be removed by means of a water separator 21 arranged in the recirculation path 4 upstream of the flushing valve 5.

[0029] The quantity of nitrogen which diffuses from the cathode region to the anode region is dependent on the ageing state of the at least one fuel cell 2, so that over time, i.e. as the age of the fuel cell 2 increases, the nitrogen proportion of the anode gas also rises, to the detriment of the hydrogen content. As a result, the efficiency of the fuel cell 2 falls.

[0030] To prevent this, according to the invention, the ageing state of the fuel cell 2 is determined from the composition of the anode gas. When necessary, the fuel cell 2 can be exchanged. Also, the recirculation path 4 is flushed depending on the ageing state of the at least one fuel cell 2. This means that flushing takes place not—as usual—at model-based temporal intervals, but as required. This extends the flushing intervals and increases the efficiency of the system.

[0031] The ageing state of the at least one fuel cell 2 is determined by comparing the actual composition, i.e. the currently determined anode gas composition, with a nominal composition. If the comparison shows a change in composition, from this an ageing or the ageing state of the at least one fuel cell 2 can be concluded. To determine the actual composition, a defined operating point is reached so that comparability is guaranteed. Then the actual composition is determined by means of the sensor 6, which in the present case is arranged in the cathode outlet gas path 11. Comparison of the actual composition with the nominal composition is carried out by means of a control device 7 in which the nominal composition is stored as a reference.

[0032] As an alternative to the arrangement of the sensor 6 illustrated in the figure, the sensor (sensor 6′) may also be placed directly in the recirculation path 4.