Method for drying a fuel cell system

Abstract

The invention relates to a method (100) for drying a fuel cell system (200), said method (100) comprising: - opening (101) at least one purge valve (211) of the fuel cell system (200), - determining (103) an anode pressure in an anode subsystem (203) of the fuel cell system (200), - opening (105) a hydrogen metering valve (207) for metering hydrogen into the anode subsystem (203) in the event that the anode pressure is below a predefined lower threshold value (119), - reducing (107) an amount of hydrogen metered into the anode subsystem (203) by the hydrogen metering valve (207) in the event that the anode pressure is above a predefined upper threshold value (121), wherein during opening (105) of the hydrogen metering valve (207), the hydrogen metering valve (207) is completely opened, and wherein the opening (105) of the hydrogen metering valve (207) and the reducing (107) of the amount of hydrogen metered into the anode subsystem (203) by the hydrogen metering valve (207) are repeated until a predefined termination condition is met.

Description

[0001] R. 414588

[0002] - 1 -

[0003] Description

[0004] title

[0005] Method for drying a fuel cell system

[0006] The presented invention relates to a method for drying a fuel cell system and a fuel cell system according to the attached claims.

[0007] State of the art

[0008] PEM fuel cell systems comprise an anode subsystem into which fuel, primarily hydrogen, is injected, and a cathode subsystem into which air is introduced. The reaction of hydrogen with oxygen in the air produces water. This typically occurs primarily in the cathode subsystem and to a lesser extent in the anode subsystem. The air is generally passed through the cathode subsystem and, after circulating through a fuel cell stack, discharged via an exhaust line. In the anode subsystem, the medium is recirculated from the anode outlet back to the anode inlet via a recirculation line, with fresh hydrogen being injected.

[0009] The main reason for the closed design of the anode subsystem is to ensure the most complete possible conversion of the supplied fuel.

[0010] To remove product water accumulation and diffused nitrogen from the anode subsystem, water separators as well as purge and drain valves are used, which allow the hydrogen concentration in the anode subsystem to be adjusted and the collected product water to be separated. R. 414588

[0011] - 2 -

[0012] During normal operation of a fuel cell system, reliable removal of product water from the system is essential for a long service life. This prevents localized deficiencies and the associated damage processes.

[0013] However, excessive drying of the membrane should be avoided, as this also leads to accelerated aging processes.

[0014] A special situation arises during the operation of a fuel cell system when it is shut down and subsequently cold-started, particularly under freezing conditions. Without sufficient water removal from the fuel cell system, ice can form in the anode subsystem. This ice can disrupt the hydrogen supply, leading to a hydrogen shortage or preventing the necessary water or gas removal. Both can damage and ultimately cause the fuel cell system to fail.

[0015] To ensure that a fuel cell system can be started safely even under freezing conditions, heaters are sometimes used to thaw critical areas, which, however, costs a lot of time and energy.

[0016] Furthermore, drying phases are known to occur when shutting down a fuel cell system, in which, for example, the anode subsystem is purged with dry, fresh hydrogen to drive out water and remove it from the anode subsystem by evaporation. Since only a relatively small gas flow occurs via the open purge valve, liquid water is only removed to a limited extent, but the membrane is dried relatively thoroughly over a large area. Therefore, it can still happen that water accumulations remain in the fuel cell system and freeze. R. 414588

[0017] - 3 -

[0018] Disclosure of the invention

[0019] Within the scope of the presented invention, a method for drying a fuel cell system and a fuel cell system are introduced. Further features and details of the invention will become apparent from the respective dependent claims, the description, and the drawings. Features and details described in connection with the method according to the invention also apply in connection with the fuel cell system according to the invention, and vice versa, so that the disclosure regarding the individual aspects of the invention is always, or can always be, mutually referenced.

[0020] The invention presented here serves in particular to provide a means of drying a fuel cell system.

[0021] Thus, according to a first aspect of the presented invention, a method for drying a fuel cell system is presented.

[0022] The presented method comprises opening at least one purge valve of the fuel cell system, determining an anode pressure in an anode subsystem of the fuel cell system, opening a hydrogen metering valve to introduce hydrogen into the anode subsystem if the anode pressure is less than a predetermined lower threshold, and reducing the amount of hydrogen injected into the anode subsystem by the hydrogen metering valve if the anode pressure is greater than a predetermined upper threshold, wherein the hydrogen metering valve is fully opened upon opening, and wherein the opening of the hydrogen metering valve and the reduction of the amount of hydrogen injected into the anode subsystem by the hydrogen metering valve are repeated until a predetermined termination condition is met. R. 414588

[0023] - 4 -

[0024] In the context of the presented invention, a flushing valve is to be understood as a so-called "purge valve", a so-called "drain valve" or a combination of these valves to form a so-called "purge / drain valve".

[0025] The presented method is based on the fact that the anode pressure is increased abruptly by very quickly opening the hydrogen metering valve until the predetermined upper threshold is reached. For this purpose, the anode pressure is monitored, e.g. by means of a pressure sensor and continuously compared with the upper threshold.

[0026] As soon as the anode pressure is greater than or equal to the upper threshold, the hydrogen metering valve is fully closed or at least closed to such an extent that a pressure drop occurs in the anode subsystem due to the discharge of medium from the anode subsystem via the open at least one purge valve.

[0027] The sudden increase in anode pressure due to the complete opening of the hydrogen metering valve after a pressure drop results in a temporarily increased mass flow through the anode subsystem compared to operation with a continuously flowing anode subsystem.

[0028] This increased mass flow rate results in a particularly high throughput of fresh, dry hydrogen and, consequently, a correspondingly high pressure drop in the anode when the hydrogen metering valve closes. As a result, water is discharged from the anode subsystem, collected in the water separator, and discharged through the open purge valve.

[0029] Furthermore, the rapid and complete opening of the hydrogen metering valve leads to a rapid increase in the anode pressure and a resulting slight rocking motion of the fuel cell membrane, i.e., a so-called "shaking effect", which causes water droplets to be loosened from the fuel cells and carried away.

[0030] Since the hydrogen metering valve closes at least partially after reaching the upper threshold, the anode pressure drops again through the open purge valve (at least one of which is open). This results in a pressure drop rate of R. 414588.

[0031] - 5 - with which the anode pressure drops, is essentially determined by a pressure difference between the anode pressure and a pressure in a drainage tract, as well as by the at least one purge valve, in particular its cross-section and its flow parameters.

[0032] If the anode pressure is less than or equal to the predefined lower threshold, another pressure pulse is triggered by opening the hydrogen metering valve. This cyclical generation of pressure pulses allows the liquid water to be moved out of the fuel cell system. Because only a few pressure pulses are required, the liquid water is removed from the fuel cell system quickly and efficiently. In contrast to a long, continuous flow with a low mass flow rate, this process does not cause, or significantly reduces, large-scale drying of the fuel cell membrane.

[0033] It may be provided that the termination condition includes a predetermined duration or a predetermined value of a measurement taken by a sensor of the fuel cell system, wherein the predetermined value is selected from the following list of measurements: amount of water in the anode subsystem, temperature in the anode subsystem.

[0034] By using a predetermined duration as a termination condition, the presented procedure is carried out within a fixed and correspondingly controlled time frame.

[0035] By using a predetermined value for a measured value as a termination condition, the presented method ends in a predetermined and correspondingly known state of the fuel cell system, so that the fuel cell system can be safely operated from this known state.

[0036] It may also be provided that when the hydrogen metering valve is opened, the hydrogen metering valve is opened at the highest possible speed R. 414588

[0037] - 6 -

[0038] Opening the hydrogen metering valve at maximum speed generates a particularly strong pressure surge, i.e., a particularly strong pulse, which propagates through the anode subsystem. For this purpose, the hydrogen metering valve can, for example, be driven with the highest possible, i.e., maximum permissible, voltage and opened to its maximum permissible opening angle or cross-sectional area.

[0039] It may also be provided that when reducing the amount of hydrogen metered into the anode subsystem by the hydrogen metering valve, the hydrogen metering valve is completely closed or closed to such an extent that there is a pressure drop in the anode pressure across the at least one open purge valve.

[0040] Completely closing the hydrogen metering valve creates a particularly large pressure difference and a correspondingly large pressure surge when the hydrogen metering valve is subsequently opened.

[0041] It may also be provided that, during repeated opening of the hydrogen metering valve and reduction of the amount of hydrogen metered into the anode subsystem by the hydrogen metering valve, an amount of air introduced into a cathode subsystem of the fuel cell system by a blower is varied in time synchronized with the opening of the hydrogen metering valve.

[0042] By varying the amount of air introduced into the cathode subsystem of the fuel cell system, the fuel cell membranes are subjected to a pressure surge generated by the air volume in addition to the pressure surge generated by opening the hydrogen metering valve, causing the membranes to vibrate particularly strongly and resulting in the rapid and extensive removal and discharge of water adhering to the membranes.

[0043] It may also be stipulated that there is a pressure range between 60 mbar and 100 mbar between the lower and upper threshold values. R. 414588

[0044] - 7 -

[0045] Tests have shown that with a pulse height or pressure surge between 60mbar and 100mbar, a maximum number of pulses is possible within a given time range, so that product water located in the anode subsystem is discharged as quickly and safely as possible.

[0046] It may also be provided that at least one flushing valve includes a purge valve and / or a drain valve.

[0047] Opening all purge valves, especially a purge drain valve, results in a particularly strong pressure surge.

[0048] It may also be provided that the procedure is executed when the ambient temperature of the fuel cell system is below a predetermined activation threshold.

[0049] Since a freeze start can be ruled out at an ambient temperature above an activation threshold of, for example, 5°C, the implementation of the presented method to prevent freezing damage can be dispensed with.

[0050] It may also be provided that the procedure is executed again if the relative humidity in a fuel cell stack of the fuel cell system exceeds a predetermined humidity threshold, or if the temperature of the fuel cell stack falls below a predetermined stack temperature threshold, or if a predetermined time has elapsed since a deactivation command to deactivate the fuel cell system.

[0051] By repeating the process, for example in addition to running it when shutting down a respective fuel cell system, the fuel cell system can also be dried during operation or adjusted to a predetermined state with respect to relative humidity in its fuel cell stack. R. 414588

[0052] - 8 -

[0053] According to a second aspect, the presented invention relates to a fuel cell system for converting energy. The advantages described in detail for the method of drying a fuel cell system according to the first aspect of the invention apply equally to the fuel cell system for converting energy according to the second aspect of the invention, and vice versa.

[0054] The presented fuel cell system comprises a fuel cell stack, an anode subsystem, a cathode subsystem, a hydrogen metering valve for metering hydrogen into the anode subsystem, a blower for introducing air into the cathode subsystem, at least one purge valve for draining medium from the anode subsystem, and a computing unit, wherein the computing unit is configured to control the fuel cell system in order to execute a possible embodiment of the presented method.

[0055] In the context of the presented invention, a computing unit is understood to be a computer, in particular a cloud computer, a processor, a control unit or any other programmable circuit.

[0056] The presented computing unit is specifically configured to control the hydrogen metering valve of the presented fuel cell system in order to open or close it.

[0057] Further advantages, features, and details of the invention will become apparent from the following description, in which exemplary embodiments of the invention are described in detail with reference to the drawings. The features mentioned in the claims and in the description can each be essential to the invention individually or in any combination.

[0058] They each show schematically:

[0059] Figure 1 shows a possible embodiment of the presented method,

[0060] Figure 2 shows a detailed representation of the process according to Fig. 1, and R. 414588

[0061] - 9 -

[0062] Figure 3 shows a possible embodiment of the presented fuel cell system.

[0063] Fig. 1 shows a method 100 for drying a fuel cell system 200.

[0064] The method 100 comprises a first opening step 101, in which a purge valve 211 of the fuel cell system 200 is opened, a determination step 103, in which an anode pressure in an anode subsystem 203 of the fuel cell system 200 is determined, in particular by means of a pressure sensor, a second opening step 105, in which a hydrogen metering valve 207 is opened to meter hydrogen into the anode subsystem 203 if the anode pressure is less than a predetermined lower threshold 119, and a reduction step 107, in which a quantity of hydrogen metered into the anode subsystem 203 by the hydrogen metering valve 207 is reduced if the anode pressure is greater than a predetermined upper threshold 121.

[0065] When the hydrogen metering valve 207 is opened, the hydrogen metering valve 207 is fully opened.

[0066] Furthermore, the second opening step 105 and the reduction step 107 are repeated until a predetermined termination condition is met.

[0067] Fig. 2 shows the process 100 in detail.

[0068] In release step 110, predefined release conditions, such as target times, status information, probabilities of a potential freeze start, and availability requirements, are checked. If the release conditions are met, the purge valve 101 is opened in opening step 191, and subsequently, in opening step 105, the hydrogen metering valve 207 is opened in a pulsating motion. R. 414588

[0069] - 10 -

[0070] To open the hydrogen metering valve 207, a function for controlling the movement of the hydrogen metering valve 207 compares an anode pressure determined at a first time point with the lower threshold value 119 in a first adjustment step 113. If the anode pressure determined at the first time point falls below the lower threshold value 119, the hydrogen metering valve 207 is fully opened.

[0071] In a second adjustment step 115, the function for controlling the movement of the hydrogen metering valve 207 compares an anode pressure determined at a second time point later than the first with the upper threshold value 121. If the anode pressure determined at the second time point exceeds the upper threshold value 121, the hydrogen metering valve 207 is at least partially closed, so that the anode pressure decreases.

[0072] In a terminating step 117, the flushing valve 211 is closed when a termination condition is reached, such as after a predetermined duration.

[0073] Optionally, 111 additional drying processes can be carried out in an extra step.

[0074] Figure 3 shows a fuel cell system 200 for converting energy.

[0075] The fuel cell system 200 comprises a fuel cell stack 201, an anode subsystem 203, a cathode subsystem 205, a hydrogen metering valve 207 for metering hydrogen into the anode subsystem 203, a blower 209 for introducing air into the cathode subsystem 205, at least one purge valve 211 for draining medium from the anode subsystem 203, and a computing unit 213, wherein the computing unit 213 is configured to control the fuel cell system 200 to carry out the method 100 according to Fig. 1.

Claims

R. 414588 - 11 - Claims 1. Method (100) for drying a fuel cell system (200), wherein the method (100) comprises: Opening (101) at least one purge valve (211) of the fuel cell system (200), Determining (103) an anode pressure in an anode subsystem (203) of the fuel cell system (200), Opening (105) of a hydrogen metering valve (207) to meter hydrogen into the anode subsystem (203) in the event that the anode pressure is less than a predetermined lower (119) threshold value, Reducing (107) the amount of hydrogen metered into the anode subsystem (203) by the hydrogen metering valve (207) in the event that the anode pressure is greater than a predetermined upper threshold (121), wherein when the hydrogen metering valve (207) is opened (105) the hydrogen metering valve (207) is fully opened, and wherein the opening (105) of the hydrogen metering valve (207) and the reduction (107) of the amount of hydrogen metered into the anode subsystem (203) by the hydrogen metering valve (207) is repeated until a predetermined termination condition is met.

2. Method (100) according to claim 1, characterized in that the termination condition comprises a predetermined duration or a predetermined value of a measured value measured by a sensor of the fuel cell system (200), wherein the predetermined value is selected from the following list of measured values: amount of water in the anode subsystem, temperature in the anode subsystem.

3. Method (100) according to claim 1 or 2, characterized in that, R. 414588 - 12 - that when opening (105) the hydrogen metering valve (207) the hydrogen metering valve (207) is opened at the greatest possible speed 4. Method (100) according to one of the preceding claims, characterized in that when reducing (107) the amount of hydrogen metered into the anode subsystem (203) by the hydrogen metering valve (207), the hydrogen metering valve (207) is completely closed or closed to such an extent that a pressure drop of the anode pressure occurs across the at least one open purge valve (211).

5. Method (100) according to one of the preceding claims, characterized in that during the repeated opening (105) of the hydrogen metering valve (207) and reduction (107) of the amount of hydrogen metered into the anode subsystem (203) by the hydrogen metering valve (207), an amount of air introduced into a cathode subsystem (205) of the fuel cell system (200) by a blower (209) is varied in a time synchronized manner with the opening (105) of the hydrogen metering valve (207).

6. Method (100) according to one of the preceding claims, characterized in that a pressure range between 60mbar and 100mbar lies between the lower threshold (119) and the upper threshold (121).

7. Method (100) according to one of the preceding claims, characterized in that the at least one purge valve (211) comprises a purge valve and / or a drain valve.

8. Method (100) according to one of the preceding claims, characterized in that R. 414588 - 13 - that the procedure (100) is executed when the ambient temperature of the fuel cell system (200) is below a predetermined activation threshold.

9. Method (100) according to one of the preceding claims, characterized in that the method (100) is carried out again when a relative humidity in a fuel cell stack (201) of the fuel cell system (200) is above a predetermined humidity threshold value or a temperature of the fuel cell stack (201) is below a predetermined stack temperature threshold value or a predetermined time has elapsed after a deactivation command to deactivate the fuel cell system (200).

10. Fuel cell system (200) for converting energy, wherein the fuel cell system (200) comprises: a fuel cell stack (201), an anode subsystem (203), a cathode subsystem (205), a hydrogen metering valve (207) for metering hydrogen into the anode subsystem (203), a blower (209) for introducing air into the cathode subsystem (205), at least one purge valve (211) for draining medium from the anode subsystem (203), a computing unit (213), wherein the computing unit (213) is configured to control the fuel cell system (200) to carry out a method (100) according to any one of claims 1 to 9.

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