Method for operating a fuel cell system and controller for the method

Abstract

The invention relates to a method for operating a fuel cell system (1), in particular a PEM fuel cell system, wherein hydrogen-containing anode gas is supplied to at least one fuel cell (2) via an anode gas path (3) and anode gas flowing out of the fuel cell (2) is conducted back via a recirculation path (4), wherein, in order to reduce the nitrogen proportion contained in the anode gas, a flushing 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 by means of at least one sensor (6) and used as an adjustment variable when adjusting the flushing of the recirculation path (4). The invention also relates to a controller (10) for carrying out the method according to the invention.

Description

TECHNICAL FIELD

[0001] The invention relates to a method for operating a fuel cell system, in particular a PEM fuel cell system. Furthermore, the invention relates to a controller which is able to carry out the method according to the invention. BACKGROUND

[0002] By means of a fuel cell system of the type described above, chemical energy can be converted into electrical energy using hydrogen and oxygen. To this end, the fuel cell system comprises at least one fuel cell which has an anode, a cathode and an electrolyte arranged between the anode and the cathode. If a polymer membrane (PEM = Proton Exchange Membrane) is used as electrolyte, this relates to a PEM fuel cell or a PEM fuel cell system.

[0003] The electrical energy obtained by means of the fuel cell system can be used as drive energy, for example for driving a vehicle. In this case, the required hydrogen is carried in suitable tanks on board the vehicle. Furthermore, the required oxygen is taken from the ambient air.

[0004] During the operation of the fuel cell system, the fuel cell ages, in particular due to thinning of the membrane and / or due to a reduction in the effectiveness of the platinum covering. This leads to an increase in the amount of nitrogen which diffuses from the cathode side to the anode side during the service life of the fuel cell. As a result, the ratio of nitrogen to hydrogen in the anode region changes, more precisely at the expense of the hydrogen content. As a result, the efficiency of the fuel cell system decreases.

[0005] In order to counteract this, in particular in order to minimize the nitrogen content in the anode gas, the anode region is periodically flushed. However, in the process, not only nitrogen, but also hydrogen is removed from the anode region, which increases the consumption of hydrogen. If the fuel cell system is used to drive a vehicle, this is accompanied by a reduction in the driving distance of the vehicle. SUMMARY

[0006] It is therefore an object of the present invention to reduce the hydrogen consumption in the operation of a fuel cell system.

[0007] In order to solve this task, a method having the features of claim 1 is provided. Advantageous embodiments can be derived from the dependent claims. Furthermore, a controller having the features of claim 8 is provided.

[0008] In the proposed method for operating a fuel cell system, in particular a PEM fuel cell system, hydrogen-containing anode gas is supplied to at least one fuel cell via an anode gas path, and anode gas flowing out of the fuel cell is conducted back via a recirculation path, wherein, in order to reduce the nitrogen proportion contained in the anode gas, a flushing valve arranged in the recirculation path is opened and the recirculation path is flushed. According to the application, the hydrogen content of the anode gas is determined by means of at least one sensor and is used as an adjustment variable when adjusting the flushing of the recirculation path.

[0009] The flushing of the recirculation path is therefore carried out in a regulated manner. By means of this regulation it is ensured that the recirculation path is flushed only when required. This is the case when the hydrogen content of the anode gas is below a predefined limit value. Only then is the flushing process introduced. In this way the number of flushing processes can be reduced. As a result, the hydrogen loss that accompanies the flushing is reduced, so that the hydrogen consumption is reduced and the driving distance of a vehicle driven by means of the fuel cell system is increased.

[0010] It is furthermore ensured that there is a minimum content of hydrogen gas in the anode gas, since the hydrogen content is monitored to flush in time.

[0011] In order to determine or monitor the hydrogen content of the anode gas, a hydrogen sensor is preferably used. Hydrogen sensors are already known from the prior art. See, for example, the publications DE 10 2005 058 830 A1 and DE 10 2005 058 832 A1, which are prior applications of the same applicant. The sensors described therein can also be used in the method described herein.

[0012] The hydrogen content of the anode gas is preferably determined by means of a sensor which is arranged in the recirculation path or in a flushing path which can be connected to the recirculation path via the flushing valve. That is to say, the hydrogen content of the anode gas on the outlet side of the at least one fuel cell is determined. Since the anode gas flowing out is supplied to the anode gas path again by means of the recirculation path and the hydrogen content of the anode gas in the anode gas path is known upstream of the introduction of the anode gas from the recirculation path, the hydrogen content of the anode gas downstream of the introduction into the anode gas path of the recirculation path can be determined by means of these variables. This hydrogen content is dimensioned sufficiently in order to ensure the efficiency of the at least one fuel cell.

[0013] Alternatively or additionally, it is proposed that the hydrogen content of the anode gas is determined by means of a sensor which is arranged in the cathode offgas path, which can be connected to the recirculation path via the flushing valve, directly or indirectly through the flushing path. As a rule, the cathode offgas of at least one fuel cell is conducted through the cathode offgas path, wherein the offgas, which is primarily air, which is reduced in its oxygen content by the reactions in the fuel cell, and water are output again to the surroundings. However, since the cathode offgas can also contain further components, for example hydrogen, the components of the cathode offgas are monitored by means of at least one sensor, for example by means of a hydrogen sensor, before the cathode offgas is output to the environment. In this case, it is possible to use an already existing sensor to determine the hydrogen content of the anode gas, so that an additional sensor for carrying out the method according to the application can be dispensed with if necessary. Only the connection of the recirculation path to the cathode offgas path via the flushing valve, if not already present, more precisely directly or indirectly through the flushing path, has to be established.

[0014] In the case of the use of a sensor arranged in the cathode offgas path, the sensor is loaded with a gas mixture consisting of cathode offgas and anode gas from the recirculation path. In order to determine the hydrogen content of the anode gas, the proportion of the anode gas in the gas mixture is determined in advance.

[0015] The proportion of the anode gas in the gas mixture in the cathode offgas path can be determined in various ways.

[0016] For example, in order to determine the proportion of the anode gas in the gas mixture, the molar mass of the anode gas can be derived from the pressure difference between the pressures in the recirculation path and the cathode offgas path, the flow cross section of the cathode offgas path and the temperature in the cathode offgas path.

[0017] Preferably, in addition to the determination of the proportion of the anode gas in the gas mixture, the proportion of the cathode offgas in the gas mixture is also determined. Here, 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 via the cathode gas path. The molar mass of the cathode gas in the cathode gas path in turn gives a conclusion about the quantity of cathode offgas conducted through the cathode offgas path. Accordingly, the proportion of the cathode offgas in the gas mixture can thus be determined.

[0018] Advantageously, the hydrogen content of the anode gas is determined at regular time intervals. That is to say, the hydrogen content is monitored such that a reaction can be made in good time when the hydrogen content falls below a predefined limit value. In particular, the recirculation path can be flushed in order to remove the nitrogen contained in the anode gas from the system and thus to keep the hydrogen content at a determined level. If the predefined limit value is not undershot, the next flushing process can be postponed, which contributes to reducing the hydrogen consumption, since hydrogen is always also expelled from the system along with the nitrogen.

[0019] The method according to the application is preferably carried out by means of a controller, whereby a high degree of automation is achieved which does not require external intervention.

[0020] It is therefore also proposed within the scope of the application that a controller is provided which is set up to carry out the method according to the application. The controller can in particular be connected to the at least one sensor for ascertaining the hydrogen content of the anode gas in a data transfer manner, whereby the sensor passes its data to the controller. Furthermore preferably, the controller is connected to the flushing valve by means of a control line, whereby (depending on the evaluation of the sensor data) the flushing valve is opened in order to introduce a flushing process. BRIEF DESCRIPTION OF DRAWINGS

[0021] The application is explained in more detail below with the aid of the drawings. The Figure 1 A schematic diagram of a fuel cell system according to the application is shown. DETAILED DESCRIPTION

[0022] The fuel cell system 1, which is shown schematically in the drawing, is used to drive a vehicle. The fuel cell system comprises at least one fuel cell 2 or a plurality of fuel cells 2 which are arranged in a stack. Air is supplied as cathode gas to the cathodes 11 of the fuel cell system 1 via a cathode gas path 9, while hydrogen gas is supplied as anode gas to the anodes 12 of the fuel cell system 1 via an anode gas path 3.

[0023] The air supplied to the cathodes 11 is taken from the surroundings. The air is first compressed by means of a compressor 13 which is arranged in the cathode gas path 9. After compression, the air is cooled down again here by means of a cooling device 18 which is also arranged in the cathode gas path 9, and is additionally humidified by means of a downstream humidifying device 19. However, the cooling and / or humidification of the air is not mandatorily necessary, whereby the provision of the cooling device 18 and / or the humidifying device 19 is only optional.

[0024] The cathode off-gas flowing from the at least one fuel cell 2 is conducted away via a cathode off-gas path 8. Here, the cathode off-gas can be supplied, as in the example shown, to an off-gas turbine 14 arranged in the cathode off-gas path 8, which supports an electric motor 15 for driving a compressor 13 arranged in the cathode off-gas path 9. Furthermore, according to the example shown, the cathode gas path 9 and the cathode off-gas path 8 can be connected via a bypass path 16 depending on the switching state of a bypass valve 17.

[0025] The hydrogen used as anode gas is stored in a tank 20 and supplied to the anode 12 via an anode gas path 3, in which an aspirator jet pump 21 is arranged here. The anode gas flowing again from the at least one fuel cell 2 is conducted back into the anode gas path 3 via a recirculation path 4, so that the system does not lose said anode gas. To this end, a recirculation blower 23 can be arranged in the recirculation path 4, as shown.

[0026] Since the anode gas is enriched over time with nitrogen gas diffusing from the cathode region into the anode region, it is sometimes necessary to flush the recirculation path 4. To this end, a flushing valve 5 is arranged in the recirculation path 4 upstream of the recirculation blower 23. Liquid water contained in the anode gas is removed beforehand by means of a water separator 22 arranged in the recirculation path 4 upstream of the flushing valve 5.

[0027] The anode gas enriched with nitrogen gas conducted via the flushing valve 5 is introduced here via a flushing path 7 into the cathode off-gas path 8 and is discharged together with the cathode off-gas to the surroundings. Here, not only the nitrogen gas is removed from the recirculation path 4, but also the hydrogen gas. The composition of the gas mixture present in the cathode off-gas path 8 is monitored by means of a sensor 6, wherein the sensor 6 detects here the hydrogen content of the gas mixture. To this end, the sensor 6 is arranged in the cathode off-gas path 8 in a position in which the sensor 6 is loaded with cathode off-gas and anode gas from the recirculation path 4.

[0028] Thus, knowing the amount of anode gas of the gas mixture, the hydrogen content of the anode gas can be derived from the hydrogen content of the gas mixture. Knowing the hydrogen content of the anode gas, the flushing of the recirculation path 4 can be adjusted as required. In this way, the number of flushing processes can be minimized, thereby reducing the hydrogen gas consumption and increasing the driving distance of the vehicle.

[0029] Instead of the position shown, the sensor 6 can also be arranged in the recirculation path 4 or in the flushing path 7. Since these paths are loaded only with anode gas, it is not necessary to first determine the share of anode gas in the gas mixture in order to ascertain the hydrogen content. This simplifies the method. However, the sensor 6 that can already be present cannot be utilized.

[0030] Irrespective of the arrangement of the sensors 6, the fuel cell system 1 also comprises a controller 10, which is connected to the sensors 6 in a data transmission manner, so that the data transmitted by the sensors 6 can be evaluated by means of the controller 10. Furthermore, the controller 10 is preferably connected to the flushing valve 5 by a control line, so that the flushing valve 5 can be actuated or opened by means of said control line when it is evaluated that the hydrogen content of the anode gas is below a predefined limit value.

Claims

1. A method for operating a fuel cell system (1), wherein, Hydrogen-containing anode gas is supplied to at least one fuel cell (2) via an anode gas path (3), and the anode gas flowing out of the fuel cell (2) is returned via a recirculation path (4), wherein, in order to reduce the nitrogen content contained in the anode gas, a flushing valve (5) arranged in the recirculation path (4) is opened and the recirculation path (4) is flushed. The hydrogen content of the anode gas is determined by at least one sensor (6) and is used as a regulating parameter when adjusting the flushing of the recirculation path (4). The hydrogen content of the anode gas is determined by means of a sensor (6) arranged in the cathode exhaust gas path (8), which can be directly or indirectly connected to the recirculation path (4) via the flushing valve (5) and the flushing path (7). The sensor (6) is loaded with a gas mixture consisting of cathode exhaust gas and anode gas from the recirculation path (4), and the proportion of anode gas in the gas mixture is determined to obtain the hydrogen content of the anode gas. In order to determine the proportion of the anode gas in the gas mixture, the molar mass of the anode gas is derived from the pressure difference between the recirculation path (4) and the cathode exhaust gas path (8), the flow cross-section of the cathode exhaust gas path (8), and the temperature in the cathode exhaust gas path (8).

2. The method according to claim 1, Its features are, To determine the proportion of the anode gas in the gas mixture, the proportion of the cathode exhaust gas is determined.

3. The method according to claim 1 or 2, Its features are, The hydrogen content of the anode gas is determined at regular time intervals.

4. The method according to claim 1 or 2, Its features are, The fuel cell system (1) is a PEM fuel cell system.

5. The method according to claim 2, Its features are, The molar mass of the cathode gas is derived from the current operating point and the adjusted and / or measured air mass supplied to the at least one fuel cell (2) via the cathode gas path (9).

6. A controller (10) configured to implement the method according to any one of the preceding claims.

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