Method for operating a fuel cell system, 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 hydrogen-containing anode gas is supplied to at least one fuel cell (2) by means of an anode gas path (3) and anode gas discharged from the fuel cell (2) is recirculated by means of a recirculation path (4), wherein, in order to reduce the nitrogen proportion contained in the anode gas, a purge valve (5) arranged in the recirculation path (4) is opened and the recirculation path (4) is purged. According to the invention, the actual 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 target composition and / or with an actual composition determined at an earlier point in time. The invention also relates to a control device (7) for carrying out the method according to the invention.

Description

Technical Field

[0001] This invention relates to a method for operating a fuel cell system, particularly a PEM fuel cell system. The invention also relates to a control device that implements the method according to the invention. Background Technology

[0002] Using the aforementioned type of fuel cell system, chemical energy can be converted into electrical energy using hydrogen and oxygen. For this purpose, the fuel cell system includes at least one fuel cell having an anode, a cathode, and an electrolyte disposed between the anode and cathode. If a polymer membrane (PEM = "Proton Exchange Membran") is used as the electrolyte, then a PEM fuel cell or PEM fuel cell system is involved.

[0003] The electrical energy generated by a fuel cell system can be used as driving energy, for example, to propel a vehicle. In this case, the required hydrogen is carried in a suitable tank within the vehicle. The necessary oxygen is obtained from ambient air.

[0004] During the operation of a fuel cell system, the fuel cell ages, particularly due to membrane thinning and / or a decrease in the effectiveness of the platinum coating. This leads to an increase in the amount of nitrogen diffusing from the cathode side to the anode side over the lifetime of the fuel cell. Consequently, the nitrogen-to-hydrogen ratio in the anode region changes, specifically at the expense of hydrogen content. Consequently, the efficiency of the fuel cell system decreases.

[0005] This invention is based on the objective of improving efficiency in the operation of fuel cell systems.

[0006] A method having the features of claim 1 is described to solve this task. Advantageous embodiments are known from the dependent claims. Furthermore, a control device having the features of claim 10 is described. Summary of the Invention

[0007] In the proposed method for operating a fuel cell system, particularly a PEM fuel cell system, hydrogen-containing anode gas is supplied to at least one fuel cell via an anode gas path, and the anode gas discharged from the fuel cell is recirculated via a recirculation path. To reduce the nitrogen content in the anode gas, a flushing valve arranged in the recirculation path is opened, and the recirculation path is flushed. According to the invention, the actual composition of the anode gas is determined using at least one sensor, and the aging state of the at least one fuel cell is determined by comparing the determined actual composition with a target composition and / or the actual composition determined at an earlier time.

[0008] This invention utilizes the previously mentioned relationship between fuel cell aging and the accompanying nitrogen enrichment in the anode gas. If the composition of the anode gas is known, the aging state of the fuel cell can be deduced in this respect. If the aging state is determined to be severe, making efficient system operation no longer possible, replacement can be performed. In this way, system efficiency is improved again.

[0009] Another advantage of the proposed method is that, given knowledge of the actual composition of the anode gas, the recirculation path can be flushed as needed. Subsequently, the number of flushing processes is reduced, or the flushing intervals become longer. Correspondingly, hydrogen loss associated with each flush is also reduced. Knowing the actual composition of the anode gas allows for simultaneous optimization of the flushing process, leading to further efficiency improvements.

[0010] To determine the actual composition of the anode gas, a predefined operating point is preferred. This ensures that the determined values ​​are comparable. The operating point to be started can be defined, for example, by system performance, pressure level, and / or gas mass flow rate in the anode and / or cathode regions.

[0011] The actual components thus determined can be compared with the target components and / or actual components determined at an earlier time using the same method according to the invention. Therefore, the actual components determined according to the method are preferably stored in a data storage device and made available for analysis and processing during the operation of the fuel cell system. The same applies to the target components, which should be considered for comparison with the determined actual components.

[0012] In the proposed method, the hydrogen content of the anode gas is preferably determined using a sensor. The nitrogen content can be determined based on the hydrogen content, and the aging state of at least one fuel cell can be deduced from the nitrogen content. Therefore, in principle, determining only the hydrogen content is sufficient to determine the actual composition. Thus, the sensor used in the method is preferably a hydrogen sensor. Such sensors are well known from the prior art. For example, see publications DE102005058830A1 and DE102005058832A1, which are earlier applications by the same applicant.

[0013] Furthermore, it is proposed to use sensors to determine the actual composition of the anode gas present in the recirculation path. That is, sensors arranged in the recirculation path are used to determine the actual composition of the anode gas.

[0014] Alternatively or supplementarily, the actual composition of the anode gas discharged through the flushing valve can be determined using sensors. In this case, multiple sensors or a single sensor are arranged downstream of the flushing valve and thus outside the anode region. The sensor can be arranged, for example, in the cathode exhaust gas path through which the flushing volume is removed from the system along with the cathode exhaust gas. Since a sensor for determining gas composition is typically already present in the cathode exhaust gas path, this sensor can be used to determine the hydrogen content if necessary, so that the method can be performed without additional sensing devices.

[0015] Advantageously, the aging state of at least one fuel cell can be determined at regular time intervals. In this way, it is possible to react early to changes in the actual composition of the anode gas or to the aging of at least one fuel cell.

[0016] An extended embodiment of the invention proposes that the aging condition of the flushing valve and / or sensors be considered when monitoring the aging condition of at least one fuel cell. This improves the accuracy in determining the aging condition of at least one fuel cell.

[0017] To determine the aging condition of the flushing valve and / or sensor, it is preferable to compare the hydrogen content of the anode gas discharged through the flushing valve under defined condition A with the hydrogen content under defined condition B. That is, the operating condition changes, or two different operating points are started and compared. The change in gas composition can then be attributed to the change in function of the components used to determine the hydrogen content.

[0018] Preferably, state A is achieved by reducing the pressure on the cathode side relative to the anode side under no-load conditions of the fuel cell system. This prevents nitrogen from diffusing from the cathode side to the anode side. Additionally, a flushing valve can be opened to flush the anode area.

[0019] Preferably, state B is achieved by starting a defined load point during normal operation of the fuel cell system.

[0020] Preferably, the hydrogen content in states A and B is determined at the time of system commissioning and stored as a reference. Then, the two operating points can be remeasured at a later time.

[0021] Since state A can be set independently of the aging state of at least one fuel cell, the amount of nitrogen diffused from the cathode side to the anode side through the membrane can be inferred by comparing measurements performed in the two operating states.

[0022] Furthermore, a control device is proposed, configured to implement the aforementioned method according to the invention. The control device enables the analysis and processing of sensor data from the sensors used in the method. Furthermore, the results and / or references of the analysis can be stored in the control device, enabling the comparisons required for performing the method. In particular, the target component and / or earlier actual components can be stored in the control device. For this purpose, the control device preferably includes a data storage device. Simultaneously, the control device can have an analysis and processing unit to perform the necessary analysis and processing of the sensor data. Furthermore, the control device can be connected to a flushing valve via a control line to operate or open the flushing valve based on the results of the analysis and processing, thereby flushing the recirculation path. Attached Figure Description

[0023] The invention will now be described in more detail with the aid of the accompanying drawings, which show schematic illustrations of a fuel cell system according to the invention. Detailed Implementation

[0024] The fuel cell system 1 schematically shown in the accompanying drawings is used to drive a vehicle. It only shows one possible embodiment of a fuel cell system for performing the method according to the invention.

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

[0026] The air supplied to the cathode 8 comes from the surrounding environment. Prior to this, the air is compressed by a compressor 12 arranged in the cathode gas path 10. After compression, the air is cooled again by a cooling device 17, also arranged in the cathode gas path 10, and additionally humidified by a downstream humidifier 18. The compressor 12, cooling device 17, and / or humidifier 18 are optional. Cathode exhaust gas from at least one fuel cell 2 is discharged through the cathode exhaust gas path 11. Here, the cathode exhaust gas is supplied to an exhaust gas turbine 13 arranged in the cathode exhaust gas path 11, which assists an electric motor 14 to drive the compressor 12 arranged in the cathode gas path 10. The exhaust gas turbine 13 is also optional. Currently, the cathode gas path 10 and the cathode exhaust gas path 11 can be connected via a bypass path 15 depending on the on / off position of the bypass valve 16.

[0027] Hydrogen gas, used as the anode gas, is stored in tank 19 and supplied to anode 9 by means of a suction jet pump 20 arranged in anode gas path 3. Anode gas discharged again from at least one fuel cell 2 is recycled back to anode gas path 3 through recirculation path 4, so that the system does not lose the anode gas. For this purpose, a recirculation fan 22 is arranged in recirculation path 4, although the recirculation fan is not mandatory.

[0028] Because the anode gas becomes rich in nitrogen diffused from the cathode region to the anode region over time, the recirculation path 4 must be flushed periodically. For this purpose, a flushing valve 5 is arranged in the recirculation path 4 upstream of the recirculation fan 22. Prior to this, liquid water contained in the anode gas can be removed by means of a water separator 21 arranged upstream of the flushing valve 5 in the recirculation path 4.

[0029] The amount of nitrogen diffusing from the cathode region to the anode region depends on the aging condition of at least one fuel cell 2, such that the nitrogen content of the anode gas increases over time (i.e., as the service life of fuel cell 2 increases), specifically at the expense of hydrogen content. As a result, the efficiency of fuel cell 2 decreases.

[0030] To prevent this, according to the present invention, the aging state of the fuel cell 2 is determined based on the composition of the anode gas. The fuel cell 2 can be replaced when necessary. Furthermore, the recirculation path 4 is flushed according to the aging state of at least one fuel cell 2. That is, flushing is performed on demand, rather than at model-based time intervals as is typically the case. This extends the flushing interval and improves the system efficiency.

[0031] The aging state of at least one fuel cell 2 is determined by comparing the actual components (i.e., the currently determined anode gas components) with the target components. If the comparison indicates a change in composition, the aging or aging state of at least one fuel cell 2 can be inferred in this respect. To determine the actual components, a defined operating point is initiated to ensure comparability. The actual components are then determined using sensor 6 currently arranged in the cathode exhaust gas path 11. The comparison between the actual components and the target components is performed using control device 7, in which the target components are stored as a reference.

[0032] Alternatively, the sensor (sensor 6') can be placed directly in the recirculation path 4, instead of the arrangement of sensor 6 shown in the attached figure.

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 anode gas discharged from the fuel cell (2) is recycled via a recirculation path (4), wherein, To reduce the nitrogen content in the anode gas, the flushing valve (5) arranged in the recirculation path (4) is opened and the recirculation path (4) is flushed. The actual composition of the anode gas is determined using at least one sensor (6), and the aging state of the at least one fuel cell (2) is determined by comparing the determined actual composition with the target composition and / or the actual composition determined at an earlier time. In this process, the aging state of the flushing valve (5) and / or the sensor (6) is taken into consideration, and in order to determine the aging state of the flushing valve (5) and / or the sensor (6), the hydrogen content of the anode gas discharged through the flushing valve (5) under defined state A is compared with the hydrogen content under defined state B. In the no-load state of the fuel cell system (1), state A is achieved by reducing the pressure on the cathode side relative to the anode side. State B is achieved by starting a defined load point during normal operation of the fuel cell system (1).

2. The method according to claim 1, Its features are, To determine the actual composition of the anode gas, a defined operating point is initiated.

3. The method according to claim 1 or 2, Its features are, The hydrogen content of the anode gas is determined by the sensor (6), the nitrogen content is determined based on the hydrogen content, and the aging state of the at least one fuel cell (2) is derived from the nitrogen content.

4. The method according to claim 1 or 2, Its features are, The actual composition of the anode gas present in the recirculation path (4) is determined by means of the sensor (6).

5. The method according to claim 1 or 2, Its features are, The actual composition of the anode gas discharged through the flushing valve (5) is determined by the sensor (6).

6. The method according to claim 1 or 2, Its features are, The aging state of the at least one fuel cell (2) is determined at regular time intervals.

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

8. A control device (7) configured to implement the method according to any one of claims 1 to 7.

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