Method for operating a fuel cell system, and control device

Abstract

The invention relates to a method for operating a fuel cell system (1), in which hydrogen from a tank (21) and recirculated hydrogen from a recirculation circuit (50) are fed as anode gas to at least one fuel cell (101) via a fuel line (20), and in which at least one fuel cell (101) is fed via a fuel line (20). The anode gas is removed from the recirculation circuit (50) by temporarily opening a purge valve (41), characterized in that the following steps are carried out:-opening the purge valve (41); detecting a pressure drop in the fuel line (20) upstream of the hydrogen metering valve (51); checking whether the valve characteristic curve of the purge valve (41) corresponds to the molar flow rate calculated according to the pressure drop. The invention further relates to a control device for carrying out the method or individual method steps.

Description

Field of the Invention

[0001] The present invention relates to a method for operating a fuel cell system. Furthermore, the present invention also relates to a control device which is configured to carry out the steps of the method. Background Art

[0002] A proton exchange membrane (PEM) fuel cell has a polymer electrolyte membrane which is arranged between an anode and a cathode. By means of the proton exchange membrane fuel cell, hydrogen gas supplied to the anode and oxygen gas supplied to the cathode in the form of air can be converted into electrical energy, heat and water. In order to increase the generated voltage, in practical applications, a plurality of fuel cells are combined into a fuel cell stack, also referred to as a "stack".

[0003] Through the diffusion process on the fuel cell, nitrogen gas reaches the anode side and enters the recirculation loop. Fresh fuel is an additional source of nitrogen gas, and this fresh fuel is not composed of 100% pure hydrogen gas. For a fuel cell, nitrogen gas is an inert gas, which reduces the cell voltage and thus the stack voltage, which in turn means an efficiency loss. Therefore, during a driving cycle, the anode gas is repeatedly discharged from the recirculation loop in order to reduce the nitrogen content. This discharge is achieved by means of a so-called purge valve.

[0004] According to the prior art, the supply of fresh hydrogen gas is carried out by means of hydrogen gas metering valves, which can be implemented as proportional valves. The adjustment strategy stipulates that the gas pressure in the anode path (measured at a defined position by means of a pressure sensor) is adjusted to a defined set pressure using this valve according to the system operating point. The reasons for supplementing fresh hydrogen gas may be: a) the consumption of hydrogen gas through electrochemical conversion; b) other losses of gas molecules in the anode chamber due to the long-term opening of the discharge valve when discharging the gas after completely draining the water and due to the opening of the purge valve.

[0005] Therefore, the present invention is directed to the following task: to describe a method for operating a fuel cell system which can reliably and at the same time inexpensively check the purge valve.

[0006] At the same time, the functional superiority of the medium pressure sensor in the fuel line can also be checked.

[0007] To solve this task, a method having the features of claim 1 is proposed. Advantageous developments of the present invention can be derived from the dependent claims. Furthermore, a control device for carrying out the method or the individual method steps is also described. Summary of the Invention

[0008] In the proposed method for operating a fuel cell system, hydrogen from a storage tank and recirculated hydrogen from a recirculation circuit are supplied as anode gas to at least one fuel cell via a fuel line. Since nitrogen, water, and small amounts of other gases accumulate in the anode gas of the recirculation circuit over time, the anode gas can be removed from the system by temporarily opening a purge valve.

[0009] The following steps are implemented herein:

[0010] - Open the purge valve;

[0011] - Detect a pressure drop in the fuel line upstream of the hydrogen metering valve;

[0012] - Check whether the valve characteristic curve of the purge valve matches the molar flow rate calculated based on the pressure drop.

[0013] When the purge valve is opened, the anode gas escapes from the recirculation circuit as the purge valve opens. Here, the set pressure in the recirculation circuit changes and drops below the desired set pressure. To maintain the set pressure, hydrogen must flow into the recirculation circuit through the hydrogen metering valve. To meet the increased demand for hydrogen, the opened hydrogen metering valve is further opened. This occurs suddenly and can be detected as a pressure change in the fuel line upstream of the hydrogen metering valve.

[0014] With the proposed method according to the invention, it is possible to determine, based on the pressure change in the fuel line, how much anode gas has left the recirculation circuit through the purge valve. This molar flow rate can be used to calibrate the valve characteristic curve of the purge valve. This is advantageous because, due to production reasons, the purge valve may have a dispersion of up to ±10% in the valve characteristic curve (relationship between volume flow rate and pressure). In addition, the valve characteristic curve drifts during the service life. Therefore, the purge valve can be simply calibrated by the proposed method, and thus the fuel cell system regulation can be calibrated. Overall, this results in less hydrogen consumption.

[0015] Advantageously, the signal of a pressure sensor in the fuel line is analyzed to detect a sudden change in the pressure in the fuel line, because this is a simple and accurate possibility for detecting a pressure change.

[0016] When the actuator current for controlling the hydrogen metering valve is analyzed to detect a sudden change in the pressure in the fuel line, an additional advantage is obtained because no additional costs are incurred due to the pressure sensor.

[0017] In addition, it is proposed to pre-filter and / or time-average the signal on which the analysis of the actuator current is based. In this way, the accuracy of the analysis can be improved.

[0018] When analyzing and processing the pressure, the bounce time (Entprellzeit) of the purge valve can be considered. This means that a certain time offset between controlling and opening the purge valve affects (miteinflieβt) the analysis and processing. In this way, the accuracy of the analysis and processing can be further increased.

[0019] To ensure that only hydrogen from the storage tank remains in the recirculation loop, it is advantageous to wait until the pressure drop in the fuel line has reached a steady value (Plateauwert). In this way, the gas composition in the recirculation loop can be inferred, which has been determined in advance through system tests based on system operation.

[0020] For a fuel cell system having at least two fuel cell stacks, each of the at least two fuel cell stacks has a fuel line, a recirculation loop respectively, and a purge valve respectively. Advantageously, these purge valves are operated decoupled from each other such that only one purge valve can be opened at a time. In this way, the pressure drop can be clearly assigned to one purge valve.

[0021] Furthermore, a control device is proposed, which is configured to carry out the steps of the method according to the invention. In particular, the actuator current required to control and analyze the hydrogen metering valve can be detected and processed by means of this control device. To analyze the actuator current, corresponding algorithms are preferably stored in the control device. Description of the Drawings

[0022] The present invention and its advantages will be explained in more detail below based on the attached drawings.

[0023] Figure 1 A schematic diagram showing the topology of a fuel cell system according to a first embodiment, and

[0024] Figure 2 A schematic diagram showing the topology of a fuel cell system according to a second embodiment. Detailed Description of the Invention

[0025] In Figure 1 a schematic topology of a fuel cell system 1 according to a first embodiment of the present invention is shown, which fuel cell system has at least one fuel cell stack 101.

[0026] The fuel cell stack 101 has a cathode side 105 and an anode side 103. Hydrogen is supplied to the anode side 103 through a fuel line 20. The fuel line 20 is located between a pressure regulating valve 22 and a hydrogen metering valve 51.

[0027] Upstream of the pressure regulating valve 22, there is a high-pressure storage tank 21, which is connected to the pressure regulating valve 22 through an additional pipeline. Other components can be arranged in an additional pipeline and the fuel pipeline 20 to supply fuel to the anode side 103 of the fuel cell stack 101 as needed. With the help of the pressure regulating valve 22, so much hydrogen flows into the fuel pipeline 20 that the pressure in the fuel pipeline 20 is at an as-constant-as-possible value.

[0028] The excess fuel, as well as a certain amount of water and nitrogen that diffuse through the cell membrane to the anode side 103, are sent back to the recirculation loop 50 and mixed with the injected (zudosierten) fuel from the fuel pipeline 20.

[0029] To drive the flow in the recirculation loop 50, various components can be installed, such as a blower 52 or an ejector pump. A hydrogen metering valve 51 is arranged at the transition between the fuel pipeline 20 and the recirculation loop 50.

[0030] The hydrogen metering valve 51 ensures the supply of fresh hydrogen to the recirculation loop 50. The hydrogen metering valve 51 can be implemented as a proportional valve. The regulation strategy in the fuel cell system 1 stipulates that, according to the system operating point, the gas pressure in the recirculation loop 50 is adjusted to a defined set pressure using the hydrogen metering valve 51. The reason for supplementing fresh hydrogen may be the consumption of hydrogen due to the electrochemical conversion in the fuel cell stack 101 or other losses of gas molecules from the recirculation loop 50, such as due to opening the discharge valve 45 or the purge valve 41.

[0031] If the operating point changes, a larger or smaller amount of hydrogen will be transported from the fuel pipeline 20 to the recirculation loop 50 through the hydrogen metering valve 51 in a short time. Since the pressure regulating valve 22 responds more slowly in its behavior than the hydrogen metering valve 51, the pressure in the fuel pipeline 20 may fluctuate in a short time.

[0032] A water separator 2 is integrated in the recirculation loop 50 to separate water from the anode gas located in the recirculation loop 50. The water separator 2 has a container for collecting the separated water. To empty this container, the water separator 2 is connected to a discharge pipeline 46 through a discharge valve 45. The discharge pipeline 46 usually guides the excess water to an exhaust pipeline, which is connected to the surrounding environment.

[0033] A pressure sensor 25 is arranged in the fuel pipeline 20. The pressure sensor 25 is arranged upstream of the hydrogen metering valve 51 and measures the pressure between the pressure regulator 22 and the hydrogen metering valve 51 in the fuel pipeline 20.

[0034] According to the method provisions of the present invention, after opening the purge valve 41 and during or once the purge valve 41 is opened, the pressure in the fuel line 20 upstream of the hydrogen metering valve 51 is detected.

[0035] Based on the pressure drop in the fuel line 20 caused by opening the purge valve 41, the molar flow rate flowing out of the recirculation loop 50 through the purge valve 41 can be calculated.

[0036] Now, this molar flow rate obtained from the calculation based on the pressure drop in the fuel line 20 is compared with the molar flow rate obtained from the valve characteristic curve of the purge valve 41.

[0037] A valve characteristic curve is stored for the purge valve 41, which illustrates the flow rate or flow coefficient related to the valve lift. With the flow coefficient and the pressure loss existing at the valve respectively, the flow rate can then be determined.

[0038] The valve characteristic curve of the purge valve 41 or the resulting molar flow rate can be compared with the molar flow rate calculated based on the pressure drop in the fuel line 20. If there is a deviation, the purge valve 41 can be recalibrated, and thus the fuel cell system regulation can be recalibrated. For this purpose, a new valve characteristic curve is stored in the control device of the fuel cell system 1.

[0039] According to the first embodiment form, the signal of the pressure sensor 25 in the fuel line 20 can be analyzed to detect the pressure in the fuel line 20.

[0040] According to another embodiment form, the actuator current for controlling the hydrogen metering valve 51 can be analyzed to detect the change in the pressure in the fuel line 20. If the actuator current rises above the threshold, a large amount of fuel is delivered through the hydrogen metering valve 51 to the recirculation loop 50. Therefore, the pressure in the fuel line 20 between the pressure regulating valve 22 and the hydrogen metering valve 51 drops.

[0041] To analyze the actuator current, the control device of the fuel cell system 1 can be used, with which the hydrogen metering valve 51 is controlled.

[0042] Preferably, during the method according to the present invention, the operating conditions or the load should not change. However, if this occurs, the occurring load change is taken into account when analyzing the actuator current or the pressure.

[0043] In an alternative embodiment, one can wait to determine the molar flow rate until the pressure drop in the fuel line 20 has reached a steady value. This is the case when the anode gas, which is a mixture consisting of hydrogen, nitrogen, water, and other gases, has been completely emptied through the purge valve 41 and only hydrogen from the storage tank 21 remains in the recirculation loop 50.

[0044] Figure 2 Figure 4 shows a fuel cell system 1 according to a second embodiment, which has at least two fuel cell stacks 101. Each fuel cell stack has a fuel line 20, a recirculation loop 50, a purge valve 41, a hydrogen metering valve 51, a pressure regulating valve 22, and a pressure sensor 25 arranged in the fuel line 20, respectively. The other components of the fuel cell system 1 with at least two fuel cell stacks 101 also do not differ from the arrangement described in the first embodiment. A single hydrogen storage tank 21 or a tank system is connected to an additional line 20a, which has branches and is connected to the respective pressure regulating valve 22.

[0045] When checking whether the valve characteristic curve of the purge valve 41 corresponds to the molar flow rate calculated based on the pressure drop, the effects and pressure fluctuations caused by opening at least one additional purge valve 41 should be avoided. If the purge valves 41 are operated decoupled from each other such that only one purge valve 41 can be opened at a time, the purge valve 41 can be clearly assigned to the pressure sensor 25 of the corresponding fuel cell stack 101.

Claims

1. A method for operating a fuel cell system (1), in which hydrogen from a storage tank (21) and recirculated hydrogen from a recirculation circuit (50) are supplied as anode gas to at least one fuel cell (101) via a fuel line (20), and in which anode gas is removed from the recirculation circuit (50) by temporarily opening a purge valve (41). Characterized in that: The following steps are implemented: - Open the purge valve (41); - Detect a pressure drop in the fuel line (20) upstream of the hydrogen metering valve (51); - Check whether the valve characteristic curve of the purge valve (41) matches the molar flow rate calculated based on the pressure drop.

2. The method according to claim 1, Characterized in that: In order to detect the pressure in the fuel line (20), the signal of a pressure sensor (25) in the fuel line (20) is analyzed and processed.

3. The method according to claims 1 to 2, Characterized in that: In order to detect the pressure in the fuel line (20), the actuator current for controlling the hydrogen metering valve (51) is analyzed and processed.

4. The method according to claim 3, Characterized in that: In order to analyze and process the actuator current, a control device (27) of the fuel cell system (1) is used, and the hydrogen metering valve (51) is controlled by means of the control device.

5. The method according to any one of claims 2 to 4, Characterized in that: The detected pressure drop and the analyzed and processed actuator current are compared to check the function of the pressure sensor (25).

6. The method according to any one of the preceding claims, Characterized in that: In order to determine the molar flow rate, wait until the pressure drop in the fuel line (20) reaches a steady value.

7. The method according to any one of the preceding claims, wherein: The fuel cell system (1) has at least two fuel cell stacks (101), and the at least two fuel cell stacks each have a fuel line (20), a recirculation circuit (50) and each have a purge valve (41). Characterized in that the purge valves (41) are operated decoupled from each other such that only one purge valve (41) can be opened at a time.

8. The method according to any one of the preceding claims, Characterized in that: The molar flow rate calculated based on the pressure drop is used to calibrate the valve characteristic curve of the purge valve (41).

9. A control device, which is arranged to implement the steps of the method according to any one of the preceding claims.

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