Method for operating a fuel cell system, and fuel cell system

Abstract

The invention relates to a method (100) for operating a fuel cell system (200), said method (100) comprising: operating (101) the fuel cell system (200) in a normal operation with open cathode isolation valves (207) and a predefined anode operating pressure as well as a predefined cathode operating pressure, providing electrical current via a fuel cell stack (201) of the fuel cell system (200); lowering (103) the anode operating pressure below a predefined anode threshold and the cathode operating pressure below a predefined cathode threshold, in response to a control command to switch the fuel cell system (200) into a standby operation; initiating (105) a passive bleed down, in which the cathode isolation valves (207) are closed and the provision of electrical current via the fuel cell stack (201) is ended; introducing (107) hydrogen into the anode chamber (203), in response to a control command for a restarting of the fuel cell system (200) from the standby operation; and opening (109) the cathode isolation valves (207). The invention also relates to a fuel cell system (200).

Description

[0001] R.415918

[0002] - 1 -

[0003] Description

[0004] title

[0005] Methods for operating a fuel cell system and fuel cell system

[0006] The present invention relates to a method for operating a fuel cell system and to a fuel cell system.

[0007] State of the art

[0008] Fuel cell systems are increasingly used in various applications, particularly in vehicles. A key aspect of operating fuel cell systems is efficiency and lifespan. To optimize these, it is desirable to put the system into standby mode when full power is not required. However, it is crucial that the system can be quickly restarted when full power is needed.

[0009] Conventional methods for switching off and restarting fuel cell systems are often time-consuming and can lead to undesirable effects such as the accumulation of hydrogen in the cathode compartment, which can delay the restart.

[0010] Disclosure of the invention

[0011] According to a first aspect of the invention, a method for operating a fuel cell system is provided. The method comprises operating the fuel cell system in normal operation with open cathode shut-off valves and a predetermined anode operating pressure and a predetermined cathode operating pressure, using R.415918.

[0012] - 2 - electric current through the fuel cell stack, the reduction of the anode operating pressure below a predetermined anode clamp value and the cathode operating pressure below a predetermined cathode threshold value, in response to a control command to switch the fuel cell system into standby mode, the initiation of passive bleeddown by closing the cathode shut-off valves and terminating the power supply, the reintroduction of hydrogen into the anode compartment, in response to a control command to restart the fuel cell system from standby mode and the opening of the cathode shut-off valves upon restart.

[0013] The presented method offers several advantages. In particular, it enables a rapid restart within a maximum of one second, and often significantly less than one second.

[0014] Furthermore, the presented method prevents an accumulation of hydrogen in the cathode compartment during the standby operation of the fuel cell system.

[0015] Furthermore, the presented method optimizes the gas concentration for an efficient restart of the fuel cell system.

[0016] The presented method can be applied in multi-stack systems, i.e., in fuel cell systems with multiple fuel cell stacks, where individual fuel cell stacks can be switched off.

[0017] Furthermore, the presented method enables a controlled transition to standby mode. This minimizes energy consumption during standby operation. In particular, the method prepares the fuel cell system for a rapid restart.

[0018] At the heart of the process is a controlled transition from normal operation to standby mode and back. During normal operation, the system operates with open cathode shut-off valves and predefined operating pressures in the anode and cathode compartments. During the transition to standby mode, these pressures are deliberately reduced below predefined threshold values ​​R.415918.

[0019] - 3 - reduced. This minimizes energy consumption during standby operation and simultaneously prepares the fuel cell system for a quick restart.

[0020] A key element of the presented invention is the initiation of a passive bleeddown. This involves closing the cathode shut-off valves and terminating the power supply by the fuel cell stack. Prior to this passive bleeddown, a defined electrical current can be drawn to precisely adjust the oxygen concentration in the cathode compartment. This prevents undesired reactions during standby operation and optimizes the gas concentrations for subsequent restart. Furthermore, it prevents the fuel cell stack voltage from rising above 0.85V per cell during standby operation, which would lead to fuel cell degradation.

[0021] It can also be provided that the cathode shut-off valves open at a speed that is at least 20% higher than the speed at which the cathode shut-off valves open when the fuel cell system is started without the application of the starting voltage. This means that the cathode shut-off valves are opened at an increased, and in particular the highest possible, speed during restart. This enables a very fast restart of the fuel cell system. Accordingly, the time during which the fuel cell system is not operating at full capacity can be minimized.

[0022] It may further be provided that, prior to the cessation of electrical current supply by the fuel cell stack and after the closure of the cathode shut-off valves, i.e., prior to passive bleeddown, an electrical current is drawn from the fuel cell stack, with the electrical current being drawn until a termination criterion is met in order to establish an oxygen concentration in the cathode compartment. This means that an electrical current is drawn prior to passive bleeddown to establish the oxygen concentration. This optimizes the gas concentrations for the standby operation of the fuel cell system. This further prevents undesired reactions during standby operation, R.415918.

[0023] - 4 - in particular an increased voltage of, for example, more than 0.85V per cell may occur.

[0024] It may further be provided that the termination criterion is determined by integrating the amount of electrical current supplied by the fuel cell stack over a duration for which electrical current is drawn from the fuel cell stack before the supply of electrical current ceases and after the cathode shut-off valves close, and by determining, using the Faraday equation, the amount of oxygen reacted during this duration, wherein the amount of hydrogen introduced into the anode compartment and the amount of oxygen introduced into the cathode compartment are adjusted such that, after the cathode shut-off valves close and after the supply of electrical current by the fuel cell stack ceases, the hydrogen concentration in the anode compartment is lower than a predetermined hydrogen concentration threshold.

[0025] The termination criterion is determined by integrating the current and calculating the amount of oxygen reacted.

[0026] Such a method allows for precise control of gas concentrations and optimizes energy consumption during the transition to standby mode.

[0027] It may also be provided that the termination criterion includes a termination time chosen such that, after the cathode shut-off valves have been closed and after the supply of electrical current by the fuel cell stack has ceased, the oxygen concentration in the anode space is less than a predetermined oxygen concentration threshold.

[0028] For example, the termination point can be chosen so that just a minimum amount of hydrogen remains in the anode compartment. This prevents the presence of oxygen in the anode compartment. This prevents R.415918.

[0029] - 5 -

[0030] Degradation by oxygen in the anode compartment. This can therefore increase the lifespan of the fuel cell system.

[0031] It may still be provided that the calculation of gas quantities is carried out according to the following formulas:

[0032] IT air ~ P athode ' V cathode / R ' T athode where n is an amount of substance, p is a pressure, V is a volume, R is the gas constant, T is a temperature, x is a concentration, and n ö(eed a reacted amount of oxygen.

[0033] There should be slightly more hydrogen in the fuel cell stack than is needed to react oxygen to form water. Accordingly: nH2

[0034] 2 > n 02

[0035] This results in:

[0036] A function executable on a control unit for calculating the number of oxygen particles released during bleeding down is thus:

[0037] In this context: tO = cathode space closed or cathode space is closed (or somewhere in between), and t1 = end of the bleeding down.

[0038] The gas quantities in the anode and cathode are calculated based on measured or estimated pressures. This has the advantage of precise control. R.415918

[0039] - 6 - the gas composition is enabled. The restart of the fuel cell system can be optimized by precise knowledge of the gas quantities.

[0040] Preferably, the anode threshold and / or the cathode threshold may be set between 450 mbar and 550 mbar, particularly between 490 mbar and 510 mbar, preferably at 500 mbar, absolute. Having the pressure thresholds in these specific ranges has the advantage of optimizing energy consumption during standby. Furthermore, this allows for a rapid pressure build-up when restarting the fuel cell system.

[0041] It can further be provided that the pressure in the anode compartment is increased during restart to a range between 1450 mbar and 1550 mbar, in particular to a range between 1490 mbar and 1510 mbar, preferably to 1500 mbar, so that a hydrogen concentration of at least 66% is achieved. Increasing the pressure in the anode compartment to a specific range during restart of the fuel cell system has the advantage of ensuring a sufficient hydrogen concentration for startup. Furthermore, it enables a rapid power build-up.

[0042] It can also be implemented that anode recirculation is switched off during or shortly before the reduction of the anode operating pressure to prevent new hydrogen from being fed into the fuel cell stack during passive bleeddown. This prevents unwanted hydrogen accumulation during bleeddown. Furthermore, the gas balance for the standby operation of the fuel cell system can be optimized.

[0043] It may also be provided that, during normal operation of the fuel cell system, the anode purge and drain valves are closed and no hydrogen is supplied, while electricity generation continues. This can reduce hydrogen consumption. R.415918

[0044] - 7 -

[0045] Such a step optimally prepares the fuel cell system for standby operation.

[0046] It can also be provided that the restart takes place within 1 second. This minimizes the reaction time of the fuel cell system and increases the efficiency and usability of the fuel cell system.

[0047] It can also be envisaged that the method is applied in a standby mode of a fuel cell system with a plurality of fuel cell stacks, in which at least one fuel cell stack of the plurality of fuel cell stacks is switched off while at least one other fuel cell stack remains in operation. This allows for flexible power adjustment of the fuel cell system. Furthermore, this can optimize the overall efficiency of the fuel cell system.

[0048] In general, determining the correct time to terminate this current draw in this process can be done as follows: The current is integrated, and the amount of reacted oxygen is calculated using a Faraday equation. This allows for extremely precise control of the gas concentrations and optimizes energy consumption during the transition to standby mode. The termination time is chosen so that after the bleeddown, enough hydrogen is present in the cathode compartment to react any oxygen present there, as this could otherwise diffuse into the anode compartment. This prevents degradation of the fuel cell system and increases its lifespan.

[0049] The restart process is characterized by its speed. Hydrogen is rapidly introduced into the anode compartment, with the pressure increased to a specific area to ensure a sufficient hydrogen concentration. Simultaneously, the cathode shut-off valves are opened at an increased rate. This enables the fuel cell system to restart within just one second. This is a crucial advantage for the responsiveness and efficiency of the fuel cell system. R.415918

[0050] - 8 -

[0051] The presented method can include specific measures for optimizing gas management. For example, anode recirculation can be switched off during the reduction of the anode operating pressure to prevent unwanted hydrogen accumulation. Furthermore, during normal operation, the anode purge and drain valves can be closed, preventing hydrogen from being supplied while electricity generation continues. This reduces hydrogen consumption and optimally prepares the fuel cell system for standby operation.

[0052] A particular advantage of the invention lies in its applicability to multi-stack systems. The presented method makes it possible to switch off individual fuel cell stacks in fuel cell systems with multiple fuel cell stacks while others remain in operation. This allows for flexible power adjustment and optimizes the overall efficiency of the fuel cell system.

[0053] According to a second aspect of the invention, a fuel cell system is presented. The fuel cell system comprises at least one fuel cell stack, which includes an anode compartment and a cathode compartment, cathode shut-off valves, a hydrogen metering unit, and a computing unit configured to control the cathode shut-off valves and the hydrogen metering unit in order to carry out a possible embodiment of the presented method.

[0054] In the context of the presented invention, a computing unit is to be understood as a computer, a processor, a control unit or any other programmable circuit.

[0055] The computing unit is preferably configured to restart the fuel cell system within 1 second. The fuel cell system preferably comprises a plurality of fuel cell stacks.

[0056] The presented fuel cell system enables the practical implementation of the presented method according to the first aspect of the invention. R.415918

[0057] - 9 -

[0058] The fuel cell system enables integration into various applications, especially vehicles.

[0059] The presented fuel cell system is designed to carry out the presented method. That is, the fuel cell system preferably comprises a number of fuel cell stacks with respective anode and cathode compartments, cathode shut-off valves, and a specially configured computing unit. This practical implementation of the method according to the invention enables integration into various applications, particularly in vehicles.

[0060] In summary, the invention offers a highly innovative method for operating a fuel cell system, characterized by a fast and efficient transition between normal operation and standby mode. Precise control of gas concentrations, optimized pressure control, and ultra-fast restart significantly improve the efficiency, performance, and lifespan of the fuel cell system.

[0061] 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.

[0062] They each show schematically:

[0063] Figure 1 shows a possible embodiment of the presented method and

[0064] Figure 2 shows a possible embodiment of the presented

[0065] Fuel cell system.

[0066] Figure 1 shows a method 100 for operating a fuel cell system 200, for example, as shown in Figure 2. R.415918

[0067] - 10 -

[0068] The method 100 comprises an operating step 101 in which the fuel cell system 200 is operated in normal operation, with open cathode shut-off valves 207 and a predetermined anode operating pressure as well as a predetermined cathode operating pressure, with electrical current provided by a fuel cell stack 201 of the fuel cell system 200.

[0069] Furthermore, the procedure 100 includes a reduction step 103 in which the anode operating pressure is reduced below a predetermined anode clamp value and the cathode operating pressure is reduced below a predetermined cathode threshold value in response to a control command to switch the fuel cell system 200 into standby operation.

[0070] Furthermore, the procedure 100 includes a first initiation step 105 in which a passive bleeddown is initiated by closing the cathode shut-off valves 207 and stopping the supply of electrical current through the fuel cell stack 201.

[0071] Furthermore, the procedure 100 includes a reintroduction step 107, in which hydrogen is again introduced into the anode compartment 203. The reintroduction step 107 occurs in response to a control command for a restart of the fuel cell system 200 from standby mode.

[0072] Furthermore, the procedure 100 includes an opening step 109 in which the cathode shut-off valves 207 are opened.

[0073] Figure 2 shows a fuel cell system 200 for converting energy.

[0074] The fuel cell system 200 comprises at least one fuel cell stack 201, which includes an anode compartment 203 and a cathode compartment 205, cathode shut-off valves 207, which are movable between a closed position in which they seal off the cathode compartment 205 fluid-tight and an open position in which they allow fluid to flow through the cathode compartment 205, an R.415918

[0075] - 11 -

[0076] Hydrogen dosing unit 209 and an optional voltage source 211, in particular a bidirectional voltage source, such as a battery, and a computing unit 213. The computing unit 213 is configured to control the cathode shut-off valves 207, which

[0077] to control the hydrogen dosing unit 209 and the optional voltage source 211 in order to carry out the procedure 100 according to Fig. 1.

Claims

R.415918 - 12 - Claims 1. Method (100) for operating a fuel cell system (200), wherein the method (100) comprises: - Operating (101) the fuel cell system (200) in normal operation, with open cathode shut-off valves (207) and a predetermined anode operating pressure and a predetermined cathode operating pressure, providing electrical current through a fuel cell stack (201) of the fuel cell system (200), - Lowering (103) the anode operating pressure below a predetermined anode clamp value and the cathode operating pressure below a predetermined cathode threshold value, in response to a control command to switch the fuel cell system (200) into standby mode, - Initiating (105) a passive bleeddown by closing the cathode shut-off valves (207) and ceasing the supply of electrical current through the fuel cell stack (201), - Introducing (107) hydrogen into the anode compartment (203) in response to a control command for a restart of the fuel cell system (200) from standby mode, and - Opening (109) the cathode shut-off valves (207).

2. Method (100) according to claim 1 , characterized in that the cathode shut-off valves (207) are opened at a speed that is at least 20% higher than the speed at which the cathode shut-off valves are opened when the fuel cell system is started without the application of the start voltage. R.415918 - 13 - 3. Method (100) according to claim 1 or 2, characterized in that before the supply of electric current by the fuel cell stack (201) is stopped and after the cathode shut-off valves (207) are closed, an electric current is drawn from the fuel cell stack (201), wherein the electric current is drawn until a termination criterion is met in order to establish an oxygen concentration in the cathode space (205).

4. Method (100) according to claim 3, characterized in that the termination criterion is determined by integrating an amount of electric current supplied by the fuel cell stack (201) over a duration for which an electric current is drawn from the fuel cell stack (201) before the supply of electric current by the fuel cell stack (201) ceases and after the cathode shut-off valves (207) close, and by determining an amount of oxygen reacted during this duration using the Faraday equation, wherein an amount of hydrogen introduced into the anode compartment (203) and an amount of oxygen introduced into the cathode compartment (205) are adjusted such that, after the cathode shut-off valves (207) close and after the supply of electric current by the fuel cell stack (201) ceases, a hydrogen concentration is present in the anode compartment (203).which is smaller than a predetermined hydrogen concentration threshold. R.415918 - 14 - 5. Method (100) according to claim 3, characterized in that the termination criterion comprises a termination time which is selected such that after closing the cathode shut-off valves (207) and after the cessation of the supply of electrical current by the fuel cell stack (201) an oxygen concentration in the anode space (203) is less than a predetermined oxygen concentration threshold.

6. Method (100) according to one of claims 3 - 5, characterized in that the method (100) further comprises: - Measuring or estimating the respective pressures in the anode compartment (203) and the cathode compartment (205) and - Calculating gas quantities in the anode compartment (203) and the cathode compartment (205) based on the measured or estimated pressures and the respective volumes of gas channels in the fuel cell stack (201).

7. Method (100) according to claim 6, characterized in that the calculation of the gas quantities is carried out according to the following formulas: where n is an amount of substance, p is a pressure, V is a volume, R is the gas constant, T is a temperature, x is a concentration, and n ö(eed a reacted amount of oxygen.

8. Method (100) according to one of the preceding claims, characterized in that the anode threshold and / or the cathode threshold is absolute between 450 mbar and 550 mbar, in particular between 490 mbar and 510 mbar, preferably at 500 mbar.

9. Method (100) according to any one of the preceding claims, R.415918 - 15 - characterized in that an anode recirculation is switched off during the lowering of the anode operating pressure or shortly before.

10. Method (100) according to one of the preceding claims, characterized in that, during the operation of the fuel cell system (200) in normal operation, the anode purge and drain valves are closed and no hydrogen is supplied, while electricity continues to be generated.

11. Method (100) according to one of the preceding claims, characterized in that the restart takes place within 1 second.

12. Method (100) according to one of the preceding claims, characterized in that the method (100) is applied in a standby mode of a fuel cell system (200) with a plurality of fuel cell stacks (201), wherein at least one fuel cell stack (201) of the plurality of fuel cell stacks (201) is switched off while at least one other fuel cell stack (201) remains in operation.

13. Fuel cell system (200), comprising - at least one fuel cell stack (201) comprising an anode compartment (203) and a cathode compartment (205), - Cathode shut-off valves (207), - a hydrogen dosing unit (209), and - a computing unit (213) configured to control the cathode shut-off valves (207) and the hydrogen metering unit (209) to perform a method (100) according to any one of claims 1 - 12.

14. Fuel cell system (200) according to claim 13, characterized in that, R.415918 - 16 - that the computing unit (213) is configured to perform a restart within 1 second.

15. Fuel cell system (200) according to claim 13 or 14, characterized in that the fuel cell system (200) comprises a plurality of fuel cell stacks (201).

Images