Method for operating a fuel cell system, control device

Abstract

The invention relates to a method for operating a fuel cell system having a fuel cell stack and an anode subsystem for supplying an anode region of the fuel cell stack with a hydrogen-containing anode gas, wherein nitrogen-enriched anode gas exiting the fuel cell stack is passively recirculated by means of a jet pump via an anode circuit of the anode subsystem. According to the invention, the following steps are carried out in order to detect an increased nitrogen content of the anode gas: - generating pressure pulses in the anode circuit, which lead to negative pressure jumps, - measuring the voltage at the cell level and / or at the stack level during the negative pressure jumps, and - comparing the measured voltage with a predefined threshold value, wherein it is concluded that the nitrogen content is unduly high if the voltage falls below the threshold value. The invention also relates to a control device for a fuel cell system.

Description

[0001] Description

[0002] title

[0003] Method for operating a fuel cell system, control unit

[0004] 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 executing steps of the method.

[0005] The preferred application area of ​​the invention is mobile fuel cell systems or fuel cell vehicles.

[0006] State of the art

[0007] Hydrogen-based fuel cells convert hydrogen and oxygen into electrical energy, heat, and water. To increase electrical output, multiple fuel cells are stacked together to form a fuel cell stack. The hydrogen is supplied to an anode section of the stack, and the oxygen—in the form of ambient air—is supplied to a cathode section.

[0008] The anode section of a fuel cell stack is supplied with hydrogen via an anode subsystem. This subsystem comprises an anode circuit through which fresh hydrogen is supplied from a tank, and through which anode gas, still containing hydrogen, exits the stack and is recirculated. Over time, the recirculated anode gas becomes enriched with nitrogen, which diffuses from the cathode to the anode, and with water, which is product water generated as a byproduct of the electrochemical reaction in the fuel cells. To prevent a hydrogen shortage, the anode circuit is purged periodically. For this purpose, a valve, the so-called purge valve, is opened, through which anode gas is released. The released amount is replaced with fresh hydrogen from the tank. This is done by opening a hydrogen metering valve.Liquid water contained in the anode gas is separated from the anode gas by means of a water separator integrated into the anode circuit and collected in a container. To empty the container, a valve, the so-called drain valve, is opened. The functions of the purge valve and the drain valve can also be integrated into a single valve.

[0009] The recirculation of anode gas through the anode circuit is usually achieved passively using a jet pump and / or actively using a blower. When a blower is used, the nitrogen content of the anode gas can be determined by analyzing the blower's performance, since the density of the gas mixture increases with the nitrogen content, thus requiring more work to displace the gas. However, if recirculation is achieved solely passively, this is not possible, so other measures must be taken to determine the nitrogen and, consequently, the hydrogen content. Otherwise, there is a risk of hydrogen insufficient supply, as the anode circuit is not purged frequently enough. One option is the use of a hydrogen concentration sensor, but these are expensive and also very sensitive.

[0010] The present invention is therefore concerned with providing an alternative method for determining the nitrogen content of an anode gas in an anode circuit of a fuel cell system, which does not require a device for active recirculation of anode gas and is as simple and cost-effective to implement as possible.

[0011] To solve the 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 specified.

[0012] Disclosure of the invention

[0013] A method is proposed for operating a fuel cell system comprising a fuel cell stack and an anode subsystem for supplying an anode region of the fuel cell stack with a hydrogen-containing anode gas. In this method, nitrogen-enriched anode gas exiting the fuel cell stack is passively recirculated via an anode circuit of the anode subsystem using a jet pump. According to the invention, the following steps are performed to detect an increased nitrogen content in the anode gas:

[0014] Generating pressure pulses in the anode circuit that lead to negative pressure jumps,

[0015] Capturing the voltage at cell level and / or stack level during negative pressure jumps and

[0016] Comparing the measured voltage with a predefined threshold value, whereby an impermissibly high nitrogen content is inferred if the threshold value is undershot.

[0017] As a general rule, the voltage at both the cell and stack levels decreases with increasing nitrogen content of the anode gas. However, this effect is not very pronounced and therefore difficult to evaluate during operation. Only at very high nitrogen concentrations does significant noise occur, eventually leading to a voltage drop below a permissible minimum. This must be prevented through early prediction. The problem is that the cell and stack voltages often show no measurable fluctuations for a long time with increasing nitrogen content, but then extreme fluctuations occur very quickly, leading to a system shutdown.

[0018] In the proposed method, pressure pulses in the anode circuit briefly reduce the pressure ratio across the stack. This disruption of the operating state selectively and temporarily impairs the supply to the stack. Consequently, the system's sensitivity increases, and a higher nitrogen concentration leads more quickly to a measurable voltage drop. This deliberately induced, measurable voltage drop can then be used for the early detection of an elevated nitrogen content in the anode gas. Furthermore, measures can be initiated to prevent an impending hydrogen shortage. The proposed method requires neither a blower nor a hydrogen concentration sensor in the anode circuit. Therefore, the method can be implemented simply and cost-effectively.

[0019] In a further development of the invention, it is proposed that when the threshold value is undershot, the anode circuit is purged by opening a purge valve and / or the purge rate is increased. Both measures – individually or in combination – reduce the nitrogen content of the anode gas and thus counteract a hydrogen undersupply. As a result, stable operation of the stack is achieved without excessively high nitrogen concentrations.

[0020] Furthermore, it is proposed that the amplitude of the pressure pulses generated in the anode circuit be varied. The sensitivity of the system can be adjusted via the amplitude of the pressure pulses so that a significant and therefore measurable voltage drop occurs.

[0021] Advantageously, pressure pulses with a rectangular shape are generated in the anode circuit. This means that the pressure in the anode circuit alternates between two values, resulting in distinct pressure jumps. An increased nitrogen content in the anode gas can thus be detected even more reliably. However, other shapes are also possible, such as sinusoidal, trapezoidal, or sawtooth shapes, which lead to a significant pressure drop within a short time.

[0022] The pressure values ​​generated during the pressure pulses preferably fluctuate around a mean target pressure in the anode circuit. This means that the pressure pulses are superimposed on the anode target pressure. During the negative pressure jumps, the voltage is then measured at the cell level and / or at the stack level. If the measurement shows a significant voltage drop, this indicates an elevated or critical nitrogen concentration.

[0023] The nitrogen content of the anode gas can be inferred from both the cell voltages and the stack voltage during the evaluation. Evaluating the cell voltages requires cell voltage monitoring. The stack voltage is recorded regularly. Preferably, when recording the voltage at the cell level, an average is calculated from the individual cell voltage values ​​and compared to a predefined threshold value.

[0024] When measuring the voltage at the stack level, the voltage signal is preferably low-pass filtered. At the stack level, the voltage signal can be evaluated for a drop below a threshold value (e.g., 99% of the slightly low-pass filtered voltage signal) during the pressure pulse. In this range, for example, a nitrogen concentration > 35% is reached. By increasing the activation of the purge valve, the nitrogen concentration can be reduced so that subsequent pressure pulses should no longer result in a voltage drop below the threshold value.

[0025] Advantageously, the pressure pulses are generated at regular intervals in the anode circuit. The nitrogen content of the anode gas can thus be continuously monitored. This ensures that an elevated critical nitrogen concentration is detected early and prevents a hydrogen shortage.

[0026] Since the proposed method does not require a blower, it is particularly suitable for detecting elevated nitrogen content in anode gas that is recirculated passively. Therefore, it is further proposed that the recirculation of anode gas through the anode circuit be achieved passively, preferably using a jet pump. This saves space and costs.

[0027] Furthermore, a control unit for a fuel cell system is proposed, wherein the control unit is configured to execute steps of a method according to the invention. In particular, the control unit can be used to evaluate the voltage detected at the cell level and / or at the stack level. For this purpose, a threshold value can be stored in the control unit. If the threshold value is undershot, a purge process can be triggered and / or the purge rate increased via the control unit. The invention and its advantages are described in more detail below with reference to the accompanying drawing. This drawing shows, in several diagrams, the voltage profiles at the cell level and at the stack level during pressure pulses for the detection of an increased nitrogen content in the anode gas.

[0028] Detailed description of the drawing

[0029] According to the inventive method, pressure pulses are generated in the anode circuit of the anode subsystem to detect an increased nitrogen content of the anode gas (see diagram a)). The pressure pulses lead to pressure jumps around a mean setpoint of the anode pressure. This means that negative pressure jumps are also generated. These lead to a deliberately induced, brief disturbance of the operating state and thus to a brief deterioration of the hydrogen supply. In this way, clearly recognizable voltage dips are generated at the cell level (see diagram c)) as well as at the

[0030] Stack level (see diagram d)) is created, which suggests an increased nitrogen content (see diagram b)).

Claims

Claims 1. A method for operating a fuel cell system with a fuel cell stack and an anode subsystem for supplying an anode area of ​​the fuel cell stack with a hydrogen-containing anode gas, wherein nitrogen-enriched anode gas exiting the fuel cell stack is passively recirculated via an anode circuit of the anode subsystem by means of a jet pump, characterized in that the following steps are carried out to detect an increased nitrogen content of the anode gas. Generating pressure pulses in the anode circuit that lead to negative pressure jumps, Capturing the voltage at cell level and / or stack level during negative pressure jumps and Comparing the measured voltage with a predefined threshold value, whereby an impermissibly high nitrogen content is inferred if the threshold value is undershot.

2. Method according to claim 1, characterized in that, when the threshold value is undershot, the anode circuit is purged by opening a purge valve and / or the purge rate is increased.

3. Method according to claim 1 or 2, characterized in that the amplitude of the pressure pulses generated in the anode circuit is varied.

4. Method according to one of the preceding claims, characterized in that pressure pulses in rectangular, sine, trapezoidal or sawtooth shape are generated in the anode circuit.

5. Method according to any one of the preceding claims, characterized in that, when measuring the voltage at cell level, an average value is formed from the individual cell voltage values ​​and the average value is compared with the predefined threshold value.

6. Method according to one of the preceding claims, characterized in that when the voltage is detected at the stack level, the voltage signal is low-pass filtered.

7. Method according to one of the preceding claims, characterized in that the pressure pulses are generated in the anode circuit at regular time intervals.

8. Method according to one of the preceding claims, characterized in that the recirculation of anode gas via the anode circuit is effected exclusively passively.

9. Control unit for a fuel cell system, wherein the control unit is configured to perform steps of a method according to any of the preceding claims.

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