METHOD FOR OPERATING A FUEL CELL SYSTEM AND CONTROL DEVICE FOR SAME

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 hydrogen content of the anode gas is determined using at least one sensor (6) and used as a control variable when controlling the flushing of the recirculation path (4). The invention also relates to a control device (10) for carrying out the method according to the invention.

Claims

1. A method for operating a fuel cell system (1), the method comprising: supplying a hydrogen-containing anode gas to at least one fuel cell (2) via an anode gas path (3), and recycling anode gas exiting from the fuel cell (2) via a recirculation path (4), wherein a flush valve (5) arranged in the recirculation path (4) is opened and the recirculation path (4) is flushed for reduction of a proportion of nitrogen contained in the anode gas, wherein the hydrogen content of the anode gas is determined with the aid of at least one sensor (6) and is used as a control variable in controlling the flushing of the recirculation path (4).

2. The method as claimed in claim 1, wherein the hydrogen content of the anode gas is determined with the aid of a sensor (6) arranged in the recirculation path (4) or in a flush path (7) connectable to the recirculation path (4) via the flush valve (5).

3. The method as claimed in claim 1, wherein the hydrogen content of the anode gas is determined with the aid of a sensor (6) arranged in a cathode waste gas path (8) directly connectable to the recirculation path (4) via the flush valve (5) or indirectly connectable to the recirculation path (4) via the flush path (7).

4. The method as claimed in claim 3, wherein a gas mixture consisting of cathode waste gas and anode gas from the recirculation path (4) is admitted to the sensor (6), and the proportion of anode gas in the gas mixture is determined for determination of the hydrogen content of the anode gas.

5. The method as claimed in claim 4, wherein the molar mass of the anode gas is derived from the pressure difference between the pressures in the recirculation path (4) and in the cathode waste gas path (8), the flow cross section of the cathode waste gas path (8) and the temperature in the cathode waste gas path (8) for determination of the proportion of anode gas in the gas mixture.

6. The method as claimed in claim 4, wherein the proportion of cathode waste gas is determined for determination of the proportion of anode gas in the gas mixture, wherein the molar mass of the cathode gas is preferably derived from the current operating point and the set and/or measured air mass supplied to the at least one fuel cell (2) via a cathode gas path (9).

7. The method as claimed in claim 1, wherein the hydrogen content of the anode gas is determined at regular time intervals.

8. A control device (10) configured to execute a method as claimed in claim 1.

Description

DETAILED DESCRIPTION OF THE DRAWING

[0021] The fuel cell system 1 depicted schematically in the drawing is used to drive a vehicle. It comprises at least one fuel cell 2 or multiple fuel cells 2 in a stacked arrangement. Air is supplied as cathode gas to a cathode 11 of the fuel cell system 1 via a cathode gas path 9, whereas hydrogen is supplied as anode gas to an anode 12 of the fuel cell system 1 via an anode gas path 3.

DETAILED DESCRIPTION

[0022] The air supplied to the cathode 11 is taken from the environment. The air is compressed beforehand with the aid of a compressor 13 arranged in the cathode gas path 9. In the present case, the air is cooled again after compression by means of a cooler 18 likewise arranged in the cathode gas path 9 and is additionally humidified with the aid of a downstream humidifier 19. However, the cooling and/or humidification of the air is/are not absolutely necessary, and so the provision of a cooler 18 and/or a humidifier 19 is merely optional.

[0023] Cathode waste gas exiting from the at least one fuel cell 2 is discharged via a cathode waste gas path 8. In this connection, the cathode waste gas canas in the example depictedbe supplied to a waste gas turbine 14 which is arranged in the cathode waste gas path 8 and which supports an electric motor 15 for driving the compressor 13 arranged in the cathode gas path 9. Furthermore, in the example depicted, the cathode gas path 9 and the cathode waste gas path 8 can be connectable via a bypass path 16 depending on the switch position of a bypass valve 17.

[0024] The hydrogen serving as anode gas is stored in tanks 20 and is supplied to the anode 12 via an anode gas path 3 in which a suction jet pump 21 is arranged in the present case. Anode gas exiting back out of the at least one fuel cell 2 is recycled into the anode gas path 3 via a recirculation path 4, and so it is not lost from the system. To this end, the recirculation path 4 canas depictedhave a recirculation fan 23 arranged therein.

[0025] Since the anode gas is enriched over time with nitrogen diffusing from the cathode region into the anode region, the recirculation path 4 must be flushed from time to time. To this end, a flush valve 5 is arranged in the recirculation path 4 upstream of the recirculation fan 23. Liquid water contained in the anode gas is removed beforehand with the aid of a water separator 22 arranged upstream of the flush valve 5 in the recirculation path 4.

[0026] In the present case, nitrogen-enriched anode gas discharged via the flush valve 5 is introduced into the cathode waste gas path 8 via a flush path 7 and is released to the environment together with the cathode waste gas. This removes not only nitrogen but also hydrogen from the recirculation path 4. The composition of the gas mixture present in the cathode waste gas path 8 is monitored with the aid of a sensor 6, the sensor 6 measuring the hydrogen content of the gas mixture in the present case. To this end, the sensor 6 is arranged in a position in the cathode waste gas path 8 in which cathode waste gas and anode gas from the recirculation path 4 are admitted thereto. Knowing the amount of anode gas in the gas mixture, it is accordingly possible to derive the hydrogen content of the anode gas from the hydrogen content of the gas mixture. Knowing the hydrogen content of the anode gas, it is possible to control the flushing of the recirculation path 4 in line with what is required. In this way, the number of flushing operations can be reduced to a minimum, so that hydrogen consumption drops and the range of the vehicle increases.

[0027] As an alternative to the position depicted, the sensor 6 can also be arranged in the recirculation path 4 or in the flush path 7. Since only anode gas is admitted thereto, determination of the hydrogen content does not first require determination of the proportion of anode gas in a gas mixture. This simplifies the method. However, it is not possible to rely on a sensor 6 possibly already present.

[0028] Irrespective of the arrangement of the sensor 6, the fuel cell system 1 further comprises a control device 10 which is data-transmittingly connected to the sensor 6, so that the data transmitted by the sensor 6 can be evaluated with the aid of the control device 10. Further preferably, the control device 10 is connected to the flush valve 5 via a control line, so that it can activate or open the flush valve 5 if the evaluation reveals that the hydrogen content of the anode gas is falling short of a specified limit value.