Method and control device for operating a fuel cell system

Abstract

The invention relates to a method for operating a fuel cell system, in which hydrogen from a tank and recycled hydrogen are supplied as anode gas to at least one fuel cell via an anode circuit (1), and water (6) contained in the anode gas is separated using a water separator (2) integrated in the anode circuit (1), collected in a container (3) and removed from the system by temporarily opening a drain valve (4). According to the invention, in order to detect a full container (3), the following steps are carried out: opening a purge valve (5) arranged in the container (3), detecting the time point when the opening cross section of a hydrogen metering valve integrated in the anode circuit (1) changes suddenly in order to maintain a target pressure in the anode circuit (1), comparing this time point with the opening time point of the purge valve (5). The invention further relates to a control device for carrying out the method or the individual method steps.

Description

[Technical field]

[0001] The present invention relates to a method for operating a fuel cell system, in particular a polymer electrolyte membrane (PEM) fuel cell system, and further to a controller arranged to carry out the steps of the method. [Background technology]

[0002] A PEM fuel cell has a polymer electrolyte membrane disposed between an anode and a cathode. PEM fuel cells can be used to convert hydrogen, which is fed to the anode, and oxygen, which is fed to the cathode in the form of air, into electrical energy, heat, and water. In practical applications, multiple fuel cells are integrated into a fuel cell stack, also called a "stack," to increase the voltage generated.

[0003] The anode gas flowing out of the PEM fuel cell usually contains unused hydrogen and is therefore recirculated and fed back to the anode via the anode circuit. However, nitrogen diffusing from the cathode side towards the anode side accumulates in the recirculated anode gas over time. To ensure a sufficient supply of hydrogen to the fuel cell, the anode area must therefore be purged from time to time. For this purpose, a valve, the so-called purge valve, is opened and the anode gas is drained off via the purge valve. Furthermore, a hydrogen metering valve is opened to replace the drained amount with fresh hydrogen from the tank. In this way, not only is the supply of hydrogen to the fuel cell ensured, but at the same time the gas pressure in the anode circuit is kept at a predefined target pressure.

[0004] Furthermore, the water produced during the electrochemical reaction in the fuel cell must be removed from the anode gas. For this purpose, a water separator with a container in which the separated water is collected is integrated into the anode circuit. Depending on the filling level in the container, another valve, the so-called drain valve, is opened and the container is emptied. In order to know when the container should be emptied, the filling level in the container can be monitored by means of a filling level sensor. However, in mobile applications, the filling level sensor is subject to shaking and vibrations that can affect the measurement result, which makes the use of filling level sensors problematic. Furthermore, filling level sensors increase costs. Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention therefore addresses the problem of providing a method for operating a fuel cell system that allows reliable and at the same time inexpensive monitoring of the filling level in a container for collecting separated water without the use of a filling level sensor. [Means for solving the problem]

[0006] In order to achieve this object, a method is proposed having the features of claim 1. Advantageous developments of the invention can be read from the dependent claims. Furthermore, a control device for carrying out the method or individual method steps is provided.

[0007] In the proposed method of operating a fuel cell system, hydrogen from a tank and recycled hydrogen are supplied as anode gas to at least one fuel cell via an anode circuit. The method further comprises the steps of separating water contained in the anode gas using a water separator integrated in the anode circuit, collecting it in a container and removing it from the system by temporarily opening a drain valve. To detect a full container, according to the invention, the following steps are carried out: - opening a purge valve disposed in the vessel; - detecting the time when the opening cross section of a hydrogen metering valve installed in the anode circuit changes suddenly in order to maintain the target pressure in the anode circuit; - comparing this time with the opening time of the purge valve.

[0008] If the container is full or if the water level is above the connection point of the purge valve, when the purge valve opens, water will flow out first, instead of gas. In this case, the target pressure of the anode circuit remains almost unchanged, so that the opening cross section of the hydrogen metering valve is essentially constant. Only when gas and not water is being led out via the purge valve does the open hydrogen metering valve need to be opened further in order to maintain the target pressure. This is done abruptly, so that it can be regarded as a clear sign that gas has started to flow out. The time lag between the opening of the purge valve and the change in the opening cross section of the hydrogen metering valve is therefore recognized as a full container. The maximum filling height is then preset by the height position of the purge valve.

[0009] As long as only water and not gas is discharged from the vessel via the purge valve, there is no need to replenish the hydrogen, despite the smaller water column, since the discharge of water only has a small effect on the target pressure of the anode circuit.

[0010] In a further development of the invention, it is proposed that in order to detect the time when the opening cross section of the hydrogen metering valve changes suddenly, the actuator current for controlling the hydrogen metering valve is evaluated. The actuator current is proportional to the opening of the hydrogen metering valve or to the metered hydrogen volume flow rate. The actuator current is therefore a regulation variable for regulating the target pressure of the anode circuit.

[0011] To evaluate the actuator current, a control device of the fuel cell system, which is preferably used to control the hydrogen metering valve, is used. If the anode gas is drawn off immediately after the purge valve opens, the hydrogen metering valve must be controlled with a relatively high current in order to maintain the target pressure in the anode circuit. If water flows out first when the purge valve opens, the hydrogen metering valve must counter-regulate only slightly, if at all, in order to expand the volume of the anode circuit to reach the target pressure. The changed actuator current can be detected precisely and at a high measurement frequency by the control device. The evaluation is carried out in particular by means of a suitable algorithm, which can be stored in the control device.

[0012] The drain valve can then be controlled depending on the evaluation of the actuator current, in particular by opening the drain valve if it is recognized that the container is full. Control of the drain valve is likewise preferably effected by means of a control device. In this way the method can be fully automated.

[0013] Furthermore, it is proposed that the signals on which the evaluation of the actuator current is based are pre-filtered and / or time-averaged, in this way increasing the accuracy of the evaluation.

[0014] When estimating the actuator current, the bounce time of the purge valve is preferably taken into account, which means that some time lag between the actuation and opening of the purge valve is taken into account in the estimation, in this way the accuracy of the estimation can be further increased.

[0015] Furthermore, load changes that occur when the purge valve is opened are preferably taken into account when evaluating the actuator current, since load changes can result in changes in the control of the hydrogen metering valve, which in turn changes the actuator current.

[0016] Furthermore, a control device is proposed which is arranged to carry out the steps of the method according to the invention. In particular, the actuator current required for controlling the hydrogen metering valve can be detected and evaluated by means of a control valve. If the evaluation reveals a time lag in the rise of the actuator current after the opening of the purge valve, this leads to the conclusion that the container is full. In this case, the control device can be used to control and open the drain valve in order to empty the container. For the evaluation of the actuator current, a corresponding algorithm is preferably stored in the control device. [Brief description of the drawings]

[0017] [Figure 1] FIG. 2 is a schematic diagram of a water separator incorporated in the anode circuit of a fuel cell system. [Diagram 2] 4 is a diagram showing a graph of the actuator current profile depending on the filling level in the vessel of the water separator; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The invention and its advantages will be explained in more detail below with reference to the accompanying drawings.

[0019] FIG. 1 exemplarily shows a water separator 2 integrated into an anode circuit 1. The water separator 2 comprises a container 3 for collecting water 6, which is separated by means of the water separator 2 from the anode gas of the anode circuit 1. To empty the container 3, a drain valve 4 is provided on the bottom side. The drain valve is opened depending on the filling level of the container 3. Furthermore, a purge valve 5 is provided on the side of the container 3. The purge valve allows the anode gas, which has accumulated nitrogen, to be removed from the anode circuit 1. This, however, presupposes that the filling level in the container 3 is not so high that the purge valve 5 is located below the water level. Otherwise, when the purge valve 5 is opened, the water 6 flows out and not the gas 7. The water 6 is removed only when the filling level is H max The filling level is preset by the height position of the purge valve 5 in the container 3.

[0020] When the purge valve 5 opens, the gas 7 flowing out of the container 3 is replaced by fresh hydrogen, which is metered into the anode circuit by means of a hydrogen metering valve (not shown). For this purpose, the hydrogen metering valve or its actuator (not shown) are correspondingly controlled by a control device (not shown). The actuator current therefore rises as the purge valve 5 opens. This relationship is shown exemplarily in FIG. 2, where the middle diagram b) shows the opening and closing of the purge valve 5 versus time t, and the lower diagram c) shows the associated actuator current profile. It can be seen therein that the rise in the actuator current when the purge valve 5 is last opened is clearly delayed in time (see arrow 8). This is because the container 3 fills with water over time, which causes the opening time t of the purge valve 5 to increase. 1 Fill level is H max This is because the value was above Max at time t 1 This means that when the purge valve 5 opens at time t , initially only water 6 flows out of the container 3. Only when the filling level is low enough that gas is being drawn out does the actuator current also increase. This occurs at time t 2 Therefore, t 1 and 2 From the time difference between this and the time when the drain valve 4 is opened, it can be seen that the container 3 is full and that the drain valve 4 must be opened to empty it. [Explanation of symbols]

[0021] 1 Anode circuit 2 Water separator 3 containers 4 Drain valve 5 Purge valve 6 water 7. Gas 8 Arrows

Claims

1. A method of operating a fuel cell system, in which hydrogen from a tank and recycled hydrogen are supplied as anode gas to at least one fuel cell via an anode circuit (1), and water (6) contained in said anode gas is separated using a water separator (2) integrated in said anode circuit (1), collected in a container (3) and removed from the system by temporarily opening a drain valve (4), In order to detect that the water level (6) is above the connection of a purge valve (5) arranged in the container (3), the following steps are carried out: - opening the purge valve (5), - detecting the time (t2) at which the opening cross section of a hydrogen metering valve installed in the anode circuit (1) changes suddenly in order to maintain the target pressure in the anode circuit (1); - comparing said time (t2) with the opening time (t1) of said purge valve (5).

2. 2. The method according to claim 1, characterized in that an actuator current for controlling the hydrogen metering valve is evaluated to detect the time point (t2) at which the opening cross section of the hydrogen metering valve changes suddenly.

3. 3. The method of claim 2, wherein a controller of the fuel cell system used to control the hydrogen metering valve is used to estimate the actuator current.

4. 4. A method according to claim 3, characterized in that, preferably with the aid of the control device, the drain valve is controlled in dependence on an evaluation of the actuator current.

5. 5. The method according to claim 2, wherein the signals on which the estimation of the actuator current is based are previously filtered and / or averaged in time.

6. 5. The method according to claim 2, further comprising taking into account the time lag between the control and the opening of the purge valve (5) when evaluating the actuator current.

7. 5. The method according to claim 2, wherein the load change occurring when the purge valve (5) is opened is taken into account when evaluating the actuator current, the load change including the internal pressure change in the flow path.

8. A control device, the control device being configured to carry out the steps of the method according to any one of claims 1 to 4.

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