Method for operating a fuel cell system and control device therefor - Patents.com

Abstract

The present invention relates to a method for operating a fuel cell system (1), in particular a PEM fuel cell system, and further the present invention relates to a control device (10) for carrying out the method according to the invention. In the system, hydrogen-containing anode gas is supplied to at least one fuel cell (2) via an anode gas path (3), and the anode gas leaving the fuel cell (2) is fed back via a recirculation path (4), and a flushing valve (5) disposed in the recirculation path (4) is opened to flush the recirculation path (4) in order to reduce the proportion of nitrogen contained in the anode gas. According to the present invention, the hydrogen content of the anode gas is determined using at least one sensor (6) and is used as a control variable for controlling the flushing of the recirculation path (4).

Description

Technical Field

[0001] The present invention relates to a method of operating a fuel cell system, particularly a PEM fuel cell system. In addition, the present invention relates to a control device that enables the execution of the method according to the present invention.

Background Art

[0002] Using a fuel cell system of the type listed above, 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 the cathode. When a polymer membrane (PEM = "proton exchange membrane") is used as the electrolyte, this is a PEM fuel cell or a PEM fuel cell system.

[0003] The electrical energy obtained using a fuel cell system can be used, for example, as driving energy for driving a vehicle. In this case, the required hydrogen is carried together in a suitable tank mounted on the vehicle. Further, the required oxygen is taken in from the ambient air.

[0004] During the operation of the fuel cell system, the fuel cell deteriorates over time, particularly due to the thinning of the membrane and / or the decrease in the effectiveness of the platinum coating. This results in an increase in the amount of nitrogen diffusing from the cathode side to the anode side over the service life of the fuel cell. Along with this, the ratio of nitrogen to hydrogen in the anode region also changes, that is, the hydrogen content is sacrificed. Accordingly, the efficiency of the fuel cell system decreases.

[0005] To address this, the anode region is periodically flushed, particularly to minimize the nitrogen content in the anode gas. However, flushing removes not only nitrogen but also hydrogen from the anode region, leading to increased hydrogen consumption. When fuel cell systems are used to power vehicles, this results in a shorter driving range. [Overview of the project] [Problems that the invention aims to solve]

[0006] Therefore, the fundamental problem of the present invention is to reduce hydrogen consumption when a fuel cell system is in operation. [Means for solving the problem]

[0007] To solve this problem, a method having the constituent elements of claim 1 is described. Preferred embodiments can be read from the dependent claims. Furthermore, a control device having the constituent elements of claim 8 is described.

[0008] In a 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 leaving the fuel cell is fed back via a recirculation path. To reduce the nitrogen content in the anode gas, a flushing valve located in the recirculation path is opened to flush the recirculation path. According to the present invention, the hydrogen content of the anode gas is determined using at least one sensor and used as a control amount when controlling the flushing of the recirculation path.

[0009] Accordingly, flushing of the recirculation path is performed under control. The control ensures that the recirculation path is flushed only when necessary. This occurs when the hydrogen content of the anode gas falls below a predetermined limit. Only then is the flushing process initiated. In this way, the number of flushing processes can be reduced. As a result, hydrogen loss associated with flushing is also reduced, thereby decreasing hydrogen consumption and extending the driving range of vehicles powered by fuel cell systems.

[0010] Furthermore, the presence of a minimum concentration of hydrogen in the anode gas is guaranteed because the hydrogen content is monitored, and flushing is performed as needed.

[0011] A hydrogen sensor is preferably used to determine or monitor the hydrogen content of the anode gas. Hydrogen sensors are already known from the prior art. For example, see German Patent Application Publication No. 102005058830 and German Patent Application Publication No. 102005058832, which are prior applications of the same applicant. The sensors described therein can also be applied to the method described here.

[0012] The hydrogen content of the anode gas is preferably determined using a sensor located in the recirculation path or in a flushing path that can be connected to the recirculation path via a flushing valve. That is, the hydrogen content of the anode gas is determined at the outlet side of at least one fuel cell. The anode gas that exits is supplied back into the anode gas path via the recirculation path, and since the hydrogen content of the anode gas in the anode gas path upstream of the introduction of anode gas from the recirculation path is known, the hydrogen content of the anode gas downstream of the introduction of the recirculation path to the anode gas path can be determined by referring to these amounts. The hydrogen content is set so as to be sufficient to guarantee the efficiency of at least one fuel cell.

[0013] As an alternative or addition, it is proposed that the hydrogen content of the anode gas be determined using a sensor placed in the cathode exhaust gas path, which can be connected to the recirculation path directly by a flushing valve or indirectly via a flushing path. Through the cathode exhaust gas path, the cathode exhaust gas of at least one fuel cell is typically discharged, and the exhaust gas—primarily air with reduced oxygen content due to reactions in the fuel cell, and water—is released back into the environment. However, since the cathode exhaust gas may also contain other components, such as hydrogen, the composition of the cathode exhaust gas is monitored using at least one sensor, for example, a hydrogen sensor, before the cathode exhaust gas is released into the environment. In such cases, existing sensors can be used to determine the hydrogen content of the anode gas, thereby potentially eliminating the need for additional sensors to implement the method of the present invention. It is only necessary to establish a connection between the cathode exhaust gas path and the recirculation path—directly, or indirectly via a flushing path—if one has not already been established, through a flushing valve.

[0014] When a sensor is used that is positioned in the cathode exhaust gas path, the sensor is loaded with a mixed gas consisting of cathode exhaust gas and anode gas originating from the recirculation path. In order to determine the hydrogen content of the anode gas, the proportion of anode gas in the mixed gas is predetermined.

[0015] The proportion of anode gas in the mixed gas of the cathode exhaust gas path can be determined by various methods.

[0016] For example, to determine the proportion of anode gas in the mixed gas, the molar mass of anode gas can be derived from the pressure difference between the pressures in the recirculation path and the cathode exhaust gas path, the flow cross-sectional area of ​​the cathode exhaust gas path, and the temperature of the cathode exhaust gas path.

[0017] In addition to determining the proportion of anode gas in the gas mixture, it is preferable to also determine the proportion of cathode exhaust gas in the gas mixture. In this case, it is preferable to derive the molar mass of cathode gas from the latest operating point and the regulated and / or measured mass of air supplied to at least one fuel cell via the cathode gas path. The molar mass of cathode gas in the cathode gas path provides information about the amount of cathode exhaust gas discharged via the cathode exhaust gas path. Accordingly, the proportion of cathode exhaust gas in the gas mixture can be determined from this.

[0018] It is preferable that the hydrogen content of the anode gas be determined at regular time intervals. That is, the hydrogen content is monitored, so that when the hydrogen content falls below a predetermined limit, action can be taken without delay. In particular, the recirculation path can be flushed to remove nitrogen contained in the anode gas from the system, thereby maintaining the hydrogen content at a specified level. If the hydrogen content is not below the predetermined limit, the next flushing process can be postponed, which helps to reduce hydrogen consumption, because hydrogen, along with nitrogen, is always discharged from the system.

[0019] The method according to the present invention is preferably carried out using a control device, thereby achieving a high degree of automation that does not require external intervention.

[0020] Therefore, within the framework of the present invention, a control device set up to carry out the method according to the present invention is also proposed. In particular, the control device may be connected in a data transmission manner to at least one sensor for determining the hydrogen content of the anode gas, thereby the sensor transferring its data to the control device. It is even more preferable that the control device is connected to a flushing valve via a control line, thereby opening the flushing valve to initiate the flushing process, depending on the evaluation of the sensor data.

[0021] Next, the present invention will be described in detail with reference to the attached drawings.

Brief Description of the Drawings

[0022] [Figure 1] A schematic diagram of a fuel cell system according to the present invention is shown.

Embodiments of the Invention

[0023] The fuel cell system 1 schematically shown in the drawings serves to drive a vehicle and includes at least one fuel cell 2 or a plurality of fuel cells 2 arranged in a stacked configuration. Air is supplied as cathode gas to the cathode 11 of the fuel cell system 1 via the cathode gas path 9, and hydrogen is supplied as anode gas to the anode 12 of the fuel cell system 1 via the anode gas path 3.

[0024] The air supplied to the cathode 11 is taken in from the surroundings. The air is pre-compressed using a compressor 13 arranged in the cathode gas path 9. Also, via a cooling device 18 arranged in the cathode gas path 9, in this example the air is cooled again after compression and additionally humidified using a downstream humidifying device 19. However, cooling and / or humidifying of the air is not absolutely necessary, and thus the installation of the cooling device 18 and / or the humidifying device 19 is merely optional.

[0025] The cathode exhaust gas coming out of at least one fuel cell 2 is discharged via the cathode exhaust gas path 8. At this time, the cathode exhaust gas - as in the illustrated example - can be supplied to an exhaust gas turbine 14 arranged in the cathode exhaust gas path 8, which assists an electric motor 15 for driving the compressor 13 arranged in the cathode gas path 9. Further, as in the illustrated example, the cathode gas path 9 and the cathode exhaust gas path 8 may be connectable via a bypass path 16 depending on the switching position of a bypass valve 17.

[0026] Hydrogen, which serves as anode gas, is stored in the tank 20 and is supplied to the anode 12 via the anode gas path 3 in which the suction jet pump 21 is arranged in this example. The anode gas coming out of at least one fuel cell 2 again is fed back to the anode gas path 3 via the recirculation path 4, so that it is not lost from the system. For this purpose, a recirculation fan 23 may be arranged in the recirculation path 4 - as shown in the figure.

[0027] In the anode gas, since the concentration of nitrogen diffusing from the cathode region to the anode region increases over time, the recirculation path 4 has to be flushed occasionally. For this purpose, a flushing valve 5 is arranged upstream of the recirculation fan 23 in the recirculation path 4. The liquid water contained in the anode gas is removed beforehand using the water separator 22 arranged in the recirculation path 4 upstream of the flushing valve 5.

[0028] The anode gas with a high nitrogen concentration discharged through the flushing valve 5 is introduced into the cathode exhaust gas path 8 via the flushing path 7 and discharged to the surroundings together with the cathode exhaust gas in this example. At that time, not only nitrogen but also hydrogen is removed from the recirculation path 4. The composition of the mixed gas in the cathode exhaust gas path 8 is monitored using the sensor 6. In this example, the sensor 6 detects the hydrogen content rate of the mixed gas. For this purpose, the sensor 6 is arranged in the cathode exhaust gas path 8 at a position loaded by the cathode exhaust gas and the anode gas derived from the recirculation path 4. Accordingly, based on the knowledge of the amount of the anode gas in the mixed gas, the hydrogen content rate of the anode gas can be derived from the hydrogen content rate of the mixed gas. Based on the knowledge of the hydrogen content rate of the anode gas, the flushing of the recirculation path 4 can be controlled as needed. In this way, the number of times of the flushing process can be minimized, thereby reducing the hydrogen consumption and increasing the driving distance of the vehicle.

[0029] Alternatively, the sensor 6 can be placed in the recirculation path 4 or the flushing path 7, instead of the location shown in the diagram. Since these are loaded only by the anode gas, it is not necessary to first determine the proportion of anode gas in the mixed gas in order to determine the hydrogen content. This simplifies the method. However, in some cases, it may not be possible to use an already installed sensor 6.

[0030] Regardless of the arrangement of the sensor 6, the fuel cell system 1 further includes a control device 10 connected to the sensor 6 via a data transmission method, thereby enabling the data transmitted from the sensor 6 to be evaluated using the control device 10. It is even more preferable that the control device 10 is connected to the flushing valve 5 via a control line, thereby enabling the flushing valve 5 to be controlled or opened when an evaluation indicates that the hydrogen content of the anode gas falls below a predetermined limit value. [Explanation of symbols]

[0031] 1. Fuel cell system 2 fuel cell 3. Anode gas pathway 4. Recirculation pathway 5. Flushing valve 6 sensors 7. Flushing path 8 Cathode Exhaust Gas Paths 9. Cathode gas pathway 10 Control device

Claims

1. A method of 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 line (3), the anode gas leaving said fuel cell (2) is fed back via a recirculation line (4), and a flushing valve (5) arranged in said recirculation line (4) is opened to flush said recirculation line (4) in order to reduce the proportion of nitrogen contained in the anode gas, A method characterized in that the hydrogen content of the anode gas is determined using at least one sensor (6) and is used as a control variable when controlling the flushing of said recirculation path (4).

2. 2. The method according to claim 1, characterized in that the hydrogen content of the anode gas is determined by means of a sensor (6) arranged in the recirculation line (4) or in a flushing line (7) connectable to the recirculation line (4) via the flushing valve (5).

3. 3. The method according to claim 1, wherein the hydrogen content of the anode gas is determined by means of a sensor (6) arranged in a cathode exhaust gas line (8) that is connectable to the recirculation line (4) directly by the flushing valve (5) or indirectly via the flushing line (7).

4. 4. The method according to claim 3, characterized in that the sensor (6) is loaded with a gas mixture consisting of cathode exhaust gas and anode gas coming from the recirculation path (4) and the proportion of anode gas in the gas mixture is determined in order to determine the hydrogen content of the anode gas.

5. 5. The method according to 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 the cathode exhaust gas path (8), the flow cross-sectional area of ​​the cathode exhaust gas path (8), and the temperature of the cathode exhaust gas path (8) in order to determine the proportion of the anode gas in the mixed gas.

6. 6. The method according to claim 4 or 5, characterized in that the proportion of cathode exhaust gas is determined to determine the proportion of anode gas in the gas mixture, and preferably the molar mass of the cathode gas is derived from the latest operating point and the regulated and / or measured air mass supplied to the at least one fuel cell (2) via the cathode gas path (9).

7. 7. A method according to claim 1, wherein the hydrogen content of the anode gas is determined at regular time intervals.

8. A control device (10) set up to carry out the method according to any one of claims 1 to 7.

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