Method for operating a fuel cell system, and control device

Abstract

The invention relates to a method for operating a fuel cell system (1), comprising a fuel cell stack (2) having a cathode (2.1) and an anode (2.2), wherein hydrogen is supplied to the anode (2.2) via an anode circuit (3), which hydrogen is removed from a tank and is metered into the anode circuit (3) by means of a hydrogen metering valve (4), and wherein anode gas containing residual hydrogen and exiting the anode (2.2) is recirculated via the anode circuit (3). According to the invention, the hydrogen metering valve (4) is operated in a pulsed manner at least temporarily, in particular in partial load operation, such that pressure pulses promoting the water discharge are generated in the anode circuit (3) and in the anode (2.2). The invention also relates to a control device for carrying out the method or method steps.

Description

[0001] Description

[0002] Procedures for operating a

[0003] The invention relates to a method for operating a fuel cell system with the features of the preamble of claim 1. Furthermore, the invention relates to a control unit for carrying out the method or for carrying out method steps.

[0004] The preferred application area of ​​the invention is mobile fuel cell systems or fuel cell vehicles.

[0005] State of the art

[0006] Hydrogen-based fuel cells convert hydrogen and oxygen into electrical energy, heat, and water. The hydrogen is supplied to an anode, and the oxygen—in the form of ambient air—to a cathode of the fuel cell. The anode is supplied with hydrogen via an anode circuit of an anode subsystem, through which fresh hydrogen from a tank and recirculated anode gas exiting the fuel cell are supplied. This gas contains residual hydrogen, nitrogen, and water. The nitrogen migrates from the cathode to the anode side via diffusion. The water is product water, which is generated as a byproduct of the electrochemical reaction in the fuel cell.

[0007] To prevent hydrogen shortages, the anode circuit is purged periodically. The amount of hydrogen removed from the anode circuit during purging is replaced with fresh hydrogen from the tank. Liquid water contained in the anode gas is removed by a water separator integrated into the anode circuit. Effective removal of product water via the water separator requires that it be carried out of the anode by the anode gas. However, depending on the specific operating conditions, water removal may be insufficient, resulting in too much water remaining in the anode and thus impairing the performance of the fuel cell stack.

[0008] The present invention addresses the problem of providing an operating strategy for optimizing water discharge from the anode of a fuel cell stack. To solve this problem, the method with the features of claim 1 is proposed. Advantageous embodiments of the invention are described in the dependent claims. Furthermore, a control unit for a fuel cell system is described.

[0009] Disclosure of the invention

[0010] A method for operating a fuel cell system is proposed, comprising a fuel cell stack with a cathode and an anode, wherein hydrogen is supplied to the anode via an anode circuit, which is taken from a tank and metered into the anode circuit by means of a hydrogen metering valve, and wherein anode gas containing residual hydrogen escaping from the anode is recirculated via the anode circuit.

[0011] According to the invention, the hydrogen metering valve is operated in pulsed mode, at least temporarily, particularly during partial load operation, so that pressure pulses promoting water discharge are generated in the anode circuit and in the anode.

[0012] The pressure pulses generated by the pulsed operation of the hydrogen metering valve increase the flow velocity of the anode gas through the anode and thus the water discharge. At low flow rates, such as during partial load operation, these pulses are essential for water discharge. This is because water discharge correlates with the pressure drop across the anode. If this drop falls below a certain threshold or minimum differential pressure, no water is discharged. The pressure pulses allow a differential pressure to be set that is at least temporarily above the minimum differential pressure, thus ensuring that water is discharged.

[0013] In a further development of the invention, it is proposed that the pressure drop across the anode be measured and the water discharge be quantified based on this pressure drop. The discharged quantity can be compared to the produced quantity, which depends on the respective operating conditions and is therefore also quantifiable. If the comparison shows that too much water has apparently remained in the anode, the control of the hydrogen metering valve can be adjusted. In particular, it is possible to switch to pulsed operation. The pressure drop across the anode can optionally be measured using a differential pressure sensor or using two absolute pressure sensors. A first absolute pressure sensor measures the pressure at the anode inlet, and another measures the pressure at the anode outlet.

[0014] Preferably, when quantifying water discharge based on the pressure drop across the anode, at least one additional measured and / or modeled parameter is considered. This measure improves the correlation between the pressure drop across the anode and the water discharge, thus increasing the accuracy of the water discharge quantification. The at least one additional measured and / or modeled parameter to be considered could be, for example, the hydrogen and / or nitrogen concentration in the anode gas and / or the temperature. The hydrogen and / or nitrogen concentration can be measured using a gas concentration sensor. The temperature can be measured with a temperature sensor.

[0015] Furthermore, it is proposed that in pulsed operation, the duration and / or frequency of the hydrogen metering valve activation be varied. These parameters allow for targeted control of the pressure pulses generated in the anode circuit or the anode itself, in order to optimize water discharge. For example, if the water discharge is too low, it can be increased by lengthening the period.

[0016] The pulsed operation of the hydrogen metering valve may be limited, particularly by the stress capacity of the membranes separating the anode from the cathode in the fuel cells of the fuel cell stack. If the differential pressure across the membrane exceeds a permissible maximum value, it can be damaged.

[0017] As a further development measure, it is therefore proposed that the pressure difference between the anode inlet and the cathode inlet and / or between the anode outlet and the cathode outlet be determined and taken into account when controlling the hydrogen metering valve. In particular, the determined value can be compared with a maximum permissible value and – if necessary – the control of the hydrogen metering valve in pulsed operation can be varied to prevent exceeding the maximum permissible value. For example, the period during the control of the hydrogen metering valve can be shortened so that pressure peaks are reduced and the differential pressure remains below the maximum permissible value.

[0018] Ensuring sufficient water discharge and maintaining a maximum permissible differential pressure across the cell membranes can require conflicting measures, leading to a conflict of objectives that cannot be resolved solely by adjusting the control of the hydrogen dosing valve. In cases of competing requirements, a change in the operating point or load may be necessary, with the decision preferably being made by a higher-level control unit.

[0019] The pressure difference between the anode inlet and the cathode inlet and / or between the anode outlet and the cathode outlet can be determined using at least one differential pressure sensor. Alternatively, two or more absolute pressure sensors can be used, depending on whether the pressure is to be measured only at the respective inlet or outlet, or at the anode inlet and outlet as well as at the cathode inlet and outlet.

[0020] According to a preferred embodiment of the invention, a hydrogen metering valve designed as a proportional valve is used. In this proportional design, the pressure at the anode inlet can be modulated. The pressure profile can thus be varied; for example, a sine profile can be converted into a sawtooth profile. Due to inertia, a pressure change at the anode inlet results in a pressure change at the anode outlet only after a time delay. This time delay, in turn, results in different differential pressures across the anode, depending on the respective pressure profile. These differential pressures simultaneously serve as an indicator of the volume flow through the anode. By modeling the pressure profile and thus the pressure difference across the anode, it is possible to simulate different time-dependent pressure differential profiles for identical, time-averaged pressure differences.to generate volumetric flows. This results – depending on the profile shape – in different times during which the pressure difference exceeds the threshold required for water discharge. Simultaneously, the absolute magnitude of the pressure difference influences the instantaneous water discharge to a certain extent. This leads to the sinusoidal profile discharging water over a longer period than, for example, the sawtooth profile. From an integral perspective, therefore, a clever modeling of the pressure profile can result in different, time-averaged water discharges.

[0021] Modeling the pressure profile, and thus the pressure difference across the anode, also influences the time during which there is no effective flow in the anode. These periods are among the phases in which the pressure difference across the anode is zero. Without effective flow, however, the risk of local hydrogen depletion due to hydrogen reaction increases. By modeling the pressure profile, these phases can be shortened, thereby minimizing the risk of local hydrogen depletion.

[0022] Possible profile shapes include, in addition to the sinusoidal and sawtooth profiles, other profile shapes such as a triangular profile, a PWM profile, or different combinations of the aforementioned profile shapes (so-called "wavefolding").

[0023] Furthermore, a control unit for a fuel cell system is proposed, wherein the control unit is configured to execute a method or steps of a method according to the invention. The control unit is specifically designed to actuate the hydrogen metering valve. To optimize water discharge from the anode, the hydrogen metering valve can be operated at least temporarily in pulsed mode using the control unit. If necessary, the duration and / or frequency of the actuation can also be varied. If the measured values ​​acquired by the various pressure sensors are provided to the control unit, it can quantify the water discharge. Depending on this, the control unit can then actuate the hydrogen metering valve.Preferably, the control unit takes into account further measured and / or modeled parameters, such as the hydrogen and / or nitrogen concentration in the anode gas and / or the temperature. The control unit can also monitor whether a maximum permissible differential pressure across the cell membranes of the fuel cell stack is maintained. If there is a risk of exceeding this pressure when the control of the hydrogen metering valve is modified to ensure sufficient water discharge, the control unit can select a different operating point to meet both requirements.

[0024] The invention and its advantages are described in more detail below with reference to the accompanying drawings. These show:

[0025] Fig. 1 shows a schematic representation of a first fuel cell system for carrying out a method according to the invention,

[0026] Fig. 2 shows a schematic representation of a second fuel cell system for carrying out a method according to the invention.

[0027] Fig. 3 shows a schematic representation of a third fuel cell system for carrying out a method according to the invention,

[0028] Fig. 4 shows a schematic representation of a fourth fuel cell system for carrying out a method according to the invention,

[0029] Fig. 5 a) the pressure p over time t at the anode inlet (A) and at the anode outlet (B) with a pressure profile with a sinusoidal profile, b) the resulting differential pressure Ap across the anode, and c) the corresponding water discharge.

[0030] Fig. 6 a) the pressure p over time t at the anode inlet (A) and at the anode outlet (B) with a pressure profile with a sawtooth shape, b) the resulting differential pressure Ap across the anode, and c) the corresponding water discharge.

[0031] Fig. 7 is a diagram illustrating the time course of the pressure loss across the anode during pulsed operation of the hydrogen metering valve.

[0032] Fig. 8 shows a diagram illustrating the time course of the pressure loss across the anode during pulsed operation of the hydrogen metering valve, including the threshold SW for water discharge, which is exceeded in this case.

[0033] Fig. 9 shows a diagram illustrating the time course of the pressure loss across the anode during pulsed operation of the hydrogen metering valve, including the threshold SW for water discharge, which is not exceeded in this case.

[0034] Fig. 10 shows a flowchart illustrating a preferred sequence of a method according to the invention and

[0035] Fig. 11 is a flowchart illustrating a preferred sequence of an extended method according to the invention.

[0036] Detailed description of the drawings

[0037] Figure 1 shows an example of a fuel cell system 1 with a fuel cell stack 2. The fuel cell stack 2 has an anode 2.2, which must be supplied with hydrogen during operation of the fuel cell system 1. This supply is provided via an anode circuit 3, through which fresh hydrogen from a tank 8 and a gas mixture exiting the anode 2.2, containing residual hydrogen, nitrogen, and water, are fed to the anode 2.2. The gas mixture exiting the anode 2.2 is passively recirculated via the anode circuit 3 by means of a jet pump 10. A hydrogen metering valve 4 is connected upstream of the jet pump 10, by means of which the fresh hydrogen taken from the tank 8 is metered into the anode circuit 3. At the same time, a motive jet is generated in the jet pump 10 by means of the hydrogen metering valve 4, which effects the passive recirculation.To enable the addition of fresh hydrogen independently of the operation of the jet pump 10, a bypass valve 11 is provided, by means of which a bypass path 12 bypassing the jet pump 10 can be opened. A pressure regulator 9 is also provided to control the pressure of the hydrogen taken from tank 8.

[0038] A water separator 13 is integrated into the anode circuit 3 to separate the liquid water carried by the recirculated gas from the gas stream. The liquid water is collected in a container of the water separator 13. When the container is full, it is emptied by opening a valve. The valve can be a simple drain valve or—as shown by way of example in Figure 1—a drain / purge valve 14. In this case, the valve can also be used to purge the anode circuit 3.

[0039] The separation rate of the water separator 13 depends, among other things, on whether the water generated in the anode 2.2 during operation of the fuel cell system 1 is carried away with the anode gas. Since the water discharge from the anode 2.2 correlates with the pressure drop across the anode 2.2, the water discharge can be quantified based on the pressure drop or the differential pressure Ap. The fuel cell system 1 shown in Figure 1 therefore has two pressure sensors, which are designed as absolute pressure sensors 6, 7. A first absolute pressure sensor 6 is located at the anode inlet and a second absolute pressure sensor 7 is located at the anode outlet. The differential pressure Ap can be determined from their measured values.

[0040] As an alternative to two absolute pressure sensors 6, 7, a single differential pressure sensor 5 can also be used. This embodiment is shown by way of example in Figure 2.

[0041] If the water discharge across anode 2.2 has been quantified based on the pressure loss or differential pressure Ap, it can be assessed, given the amount of water produced at the respective operating point, whether the water discharge is sufficient. If this is not the case, the hydrogen metering valve 4 can be operated in pulsed mode according to the invention. Pulsed operation generates pressure pulses that promote the water discharge from anode 2.2.

[0042] As can be seen in the diagram in Figure 5a), where curve A shows the pressure profile at the anode inlet and curve B the pressure profile at the anode outlet, the pulsed operation of the hydrogen metering valve 4, i.e., the timed opening and closing of the hydrogen metering valve 4, generates a sinusoidal pressure pulse, with curve B being time-shifted relative to curve A. This results in a differential pressure Ap, the profile of which is shown in Figure 5b). The water discharge corresponding to the differential pressure Ap is shown in Figure 5c), where line C indicates the time-averaged water discharge. If a hydrogen metering valve 4 designed as a proportional valve is used instead of a switching valve 4, the pressure profile can also be modeled. Thus, in addition to the sinusoidal pressure profile, other pressure profiles can also be generated, for example, a sawtooth profile.Analogous to Figures 5a) to 5c), Figure 6a) shows the pressure profiles at the anode inlet (curve A) and the anode outlet (curve B), Figure 6b) shows the resulting differential pressure Ap, and Figure 6c) shows the corresponding water discharge, including the time-averaged water discharge (line C). Comparing Figure 6c) to Figure 5c), the time-averaged water discharge is approximately identical, while the maximum differential pressure Ap in Figure 6b) is significantly higher than in Figure 5b). By modeling the pressure profile, higher differential pressures can therefore be achieved, which is particularly advantageous if it is assumed that the differential pressure will not reach the minimum differential pressure necessary for water discharge. This will be explained below with reference to Figures 7 to 9.

[0043] Figure 7 shows an example of the differential pressure Ap profile during pulsed sawtooth operation of a hydrogen metering valve 4, resulting from the fact that the operating phase a, in which the hydrogen metering valve 4 is actuated, is significantly shorter than the operating phase b, in which the hydrogen metering valve 4 is not actuated. The ratio a / b is also referred to as the "duty cycle".

[0044] Figure 8 shows the same curve as Figure 7, with the addition of a threshold value SW. The differential pressure Ap must exceed the threshold value SW for any water to be discharged from anode 2.2. The threshold value SW thus defines the minimum differential pressure required for water discharge. Accordingly, water is discharged in phase At1, while no water is discharged in phase At2. If the differential pressure Ap curve flattens out, as shown in Figure 9, no water is discharged at any time, since the differential pressure Ap remains below the threshold value SW. Modeling the pressure profile can therefore be used to achieve the minimum differential pressure required for water discharge. In particular, the following process steps, explained with reference to Figure 10, can be carried out. First, it is checked (step S1) whether the water discharge from anode 2, quantified based on the differential pressure Ap, is within the specified range.2 is sufficient, for example, if it lies within a predefined target area. If this is not the case. Four pressure pulses can be generated by pulsed operation of the hydrogen metering valve, which promote water discharge (step S2). Subsequently, it is checked whether the water discharge is now within the target range. If this is still not the case, In a further step S2, the control of the hydrogen valve 4 in pulsed operation can be varied to further increase the water discharge. For example, the duration and / or frequency of the control of the hydrogen metering valve 4 can be changed. If the retest in step S1 shows that the water discharge is sufficient (",,+"), the selected control of the hydrogen metering valve 4 can be retained.

[0045] There may be limits to the adjustment of the control of the hydrogen metering valve 4, one of which is determined by the load-bearing capacity of the cell membranes of the fuel cells in the fuel cell stack 2. This is because the relatively thin cell membranes separate the anode 2.2 from the cathode 2.1 of the fuel cell stack 2, which is supplied with air as an oxygen source during operation of the fuel cell system 1. Hydrogen and oxygen are then converted into electrical energy, heat, and water in the fuel cells. To prevent damage to the cell membranes, the differential pressure across the membranes must not exceed a certain value. Therefore, this differential pressure must be monitored.

[0046] As shown in Figure 3, the pressure at the anode inlet and the cathode inlet can be measured using two absolute pressure sensors 16, 17. From this, the differential pressure across the cell membranes can then be determined. Alternatively, as shown in Figure 4, a differential pressure sensor 15 can be used to replace the two absolute pressure sensors 16, 17.

[0047] Knowing the differential pressure across the cell membranes, the extended method according to the invention, illustrated in Figure 11, can be carried out. Method steps S1 to S3 correspond to those of the method explained with reference to Figure 10. Steps S4 and S5 are now added. In step S4, it is checked whether the differential pressure across the cell membranes is below a maximum permissible value, so that there is no risk of damage to the cell membranes. If the result of the check is negative, the process continues. The control of the hydrogen metering valve 4 must be adjusted (back to step S2). If the test result is positive (",,+"), the pulsed

[0048] Operation of the hydrogen metering valve 4 will continue with the current control parameters.

Claims

Claims 1. Method for operating a fuel cell system (1) comprising a fuel cell stack (2) with a cathode (2.1) and an anode (2.2), wherein hydrogen is supplied to the anode (2.2) via an anode circuit (3), which is taken from a tank and metered into the anode circuit (3) by means of a hydrogen metering valve (4), and wherein anode gas containing residual hydrogen exiting from the anode (2.2) is recirculated via the anode circuit (3), characterized in that the hydrogen metering valve (4) is operated in a pulsed manner at least temporarily, in particular during partial load operation, so that pressure pulses promoting water discharge are generated in the anode circuit (3) and in the anode (2.2).

2. Method according to claim 1, characterized in that the pressure loss across the anode (2.2) is measured and the water discharge is quantified on the basis of the pressure loss, wherein preferably the pressure loss across the anode (2.2) is measured using a differential pressure sensor (5) or using two absolute pressure sensors (6, 7).

3. Method according to claim 2, characterized in that, when quantifying the water discharge based on the pressure loss across the anode (2.2), at least one further measured and / or modeled quantity, for example the hydrogen and / or nitrogen concentration in the anode gas and / or the temperature, is taken into account.

4. Method according to one of the preceding claims, characterized in that in pulsed operation the duration and / or the frequency of the actuation of the hydrogen metering valve (4) is varied.

5. Method according to claim 4, characterized in that the difference between the pressure at the anode inlet and at the cathode inlet and / or at the anode outlet and at the cathode outlet is determined and taken into account when controlling the hydrogen metering valve (4).

6. Method according to one of the preceding claims, characterized in that a hydrogen metering valve (4) designed as a proportional valve is used.

7. Method according to claim 6, characterized in that the pressure profile is varied with the aid of the hydrogen metering valve (4) designed as a proportional valve, so that a profile shape different from a sinusoidal profile is generated, for example a sawtooth, a triangular and / or a PWM profile.

8. Control unit for a fuel cell system (1), wherein the control unit is configured to perform a method or steps of a method according to any of the preceding claims.

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