Method for optimizing the purging of a fuel cell system after a standstill period

Abstract

A fuel cell system (100) is proposed. The fuel cell system (100) comprises at least one fuel cell stack (102), wherein the fuel cell stack (102) has a plurality of fuel cells, and a control unit (124). The control unit (124) is configured to detect a standstill period of the fuel cell stack (102), to determine a time duration for setting a predetermined hydrogen concentration prior to switching on the fuel cell stack (102) based on the detected standstill period, and to set the predetermined hydrogen concentration prior to switching on the fuel cell stack (102) based on the determined time duration.

Description

[0001] R.416591

[0002] - 1 -

[0003] Description

[0004] title

[0005] Fuel cell system

[0006] State of the art

[0007] Hydrogen-based fuel cell systems are considered a mobility concept of the future, as they emit only water as exhaust gas and enable rapid refueling. Fuel cell systems require oxygen, typically from the ambient air in vehicles, and hydrogen for the chemical reaction. The only reaction product is water, which is released in varying proportions as a gas and a liquid. The water is produced at the catalyst layer on one side of the cathode and transported by a gas diffusion layer (GDL) towards the respective gas flow channels.

[0008] Polymer electrolyte membrane (PEM) fuel cell systems convert hydrogen into electrical energy using oxygen, generating waste heat and water as byproducts. A PEM fuel cell consists of an anode supplied with hydrogen, a cathode supplied with air, and the polymer electrolyte membrane positioned between them, where the air and oxygen are converted into electricity, water, and heat. In practical applications, several such fuel cells are stacked to increase the electrically generated voltage.

[0009] Systemically, an approach has become established for supplying the PEM anode with hydrogen, in which the still hydrogen-rich anode exhaust gas is fed back to the anode inlet along with fresh hydrogen via gas supply units. This is called recirculation. A measure of recirculation is the ratio of hydrogen supplied to the stack to hydrogen consumed by the electrochemical reaction. This ratio is called lambda. R.416591

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[0011] Lambda, along with hydrogen concentration, is an essential parameter for the fuel cell stack. A sufficiently high lambda value ensures that the catalyst in the stack is adequately supplied with hydrogen across the entire flow range. This is because nitrogen also reaches the anode side through diffusion processes. Nitrogen is an inert gas for the electrochemical reaction taking place in the fuel cell. Due to recirculation, nitrogen accumulates in the anode, reducing the amount of hydrogen that can be supplied to it. Consequently, the lambda value decreases, which can lead to a reduction in cell voltage. If the cell is no longer adequately supplied with hydrogen, this can damage the cells.

[0012] Despite the advantages of existing fuel cell systems, there is still room for improvement. Especially after a prolonged period of inactivity, nitrogen accumulates on the anode, necessitating a first flush to establish a suitable hydrogen concentration before starting the fuel cell system. This process is slow, delaying the start-up and thus reducing the system's availability. Furthermore, purge air must be supplied from the cathode system to dilute any hydrogen. Currently, the flush time is fixed, resulting in unnecessary hydrogen being expelled.

[0013] Disclosure of the invention

[0014] Within the scope of the present invention, a fuel cell system, a method for operating a fuel cell system, a motor vehicle, and a computer program product are therefore proposed, which largely avoid the disadvantages of known fuel cell systems, methods for operating a fuel cell system, motor vehicles, and computer programs, and which, in particular, determine the time required to reach a target hydrogen concentration necessary for starting the fuel cell system after the system has been switched off, especially after only a brief period of time. R.416591

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[0016] A fuel cell system according to the invention comprises at least one fuel cell stack, wherein the fuel cell stack has several fuel cells, and a control unit. The control unit is configured to

[0017] - to record the downtime of the fuel cell stack,

[0018] - to determine a time period for setting a predetermined hydrogen concentration before switching on the fuel cell stack based on the recorded downtime, and

[0019] - to set the predetermined hydrogen concentration before switching on the fuel cell stack based on the determined time duration.

[0020] The rationale for measuring the standby time is that hydrogen diffuses from the anode to the cathode, and after the system is switched off, any remaining oxygen reacts with hydrogen to form water. A minimum hydrogen concentration must be reached for the subsequent start-up. By measuring the standby time, it is possible to determine how much hydrogen has diffused to the cathode and is therefore missing at the anode for a start-up. The invention makes it possible to determine the time required to reach the target hydrogen concentration necessary for starting the fuel cell system after a (brief) shutdown. The goal is to minimize hydrogen discharge and thus keep the flushing time as short as possible, while still achieving the minimum hydrogen concentration required for starting.

[0021] The control unit can be configured to determine the time required to reach the predetermined hydrogen concentration before switching on the fuel cell stack, using at least one characteristic map. To determine the flush time, a characteristic map can be used that is populated by measurements or simulation and outputs a flushing time based on a standstill period.

[0022] The control unit can be configured to determine the time required to reach the predetermined hydrogen concentration before switching on the fuel cell stack using at least one diffusion model. Alternatively, the flushing time can be calculated in the control unit using the standby time and a diffusion model. R.416591

[0023] - 4 -

[0024] The diffusion model can consider the standby time and at least one other parameter selected from the following group: humidity in the fuel cell stack, operating point before shutdown, anode pressure, cathode pressure, ambient temperature, and ambient pressure. Another implementation is to model the diffusion processes across the membrane and the reaction using a physical model. In this case, the standby time is the most important parameter, but the parameters mentioned above can be expanded upon.

[0025] The control unit can be configured to set the predetermined hydrogen concentration by adjusting a predetermined anode pressure. This allows for reliable setting of the minimum hydrogen concentration required for starting.

[0026] The control unit can be configured to set the predetermined anode pressure by injecting hydrogen into an anode chamber of the fuel cell stack. This is achieved by setting a desired anode pressure through hydrogen injection (pressure increase).

[0027] The fuel cell system may also include a drain valve and / or a purge valve. The control unit can be configured to set the predetermined hydrogen concentration by releasing anode gas through the drain valve and / or purge valve. The time required to release the anode gas via the purge and drain valves is calculated. Thus, a pressure increase is only permissible up to a defined limit, meaning that the desired hydrogen concentration cannot usually be achieved by increasing the pressure alone. This is accomplished, however, by setting a desired anode pressure by injecting hydrogen (pressure increase) and then further increasing the hydrogen concentration by purging the anode gas through a purge and drain valve. R.416591

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[0029] Furthermore, a method for operating a fuel cell system is proposed, wherein the fuel cell system comprises at least one fuel cell stack, and the fuel cell stack comprises multiple fuel cells. The method includes the following steps:

[0030] - Recording the downtime of the fuel cell stack,

[0031] - Determining a time period for setting a predetermined hydrogen concentration before switching on the fuel cell stack based on the recorded downtime, and

[0032] - Setting the predetermined hydrogen concentration before switching on the fuel cell stack based on the determined time duration.

[0033] The rationale for measuring the standby time is that hydrogen diffuses from the anode to the cathode, and after the system is switched off, any remaining oxygen reacts with hydrogen to form water. A minimum hydrogen concentration must be reached for the subsequent start-up. By measuring the standby time, it is possible to determine how much hydrogen has diffused to the cathode and is therefore missing at the anode for a start-up. The invention makes it possible to determine the time required to reach the target hydrogen concentration necessary for starting the fuel cell system after a (brief) shutdown. The goal is to minimize hydrogen discharge and thus keep the flushing time as short as possible, while still achieving the minimum hydrogen concentration required for starting.

[0034] The method can further include determining the time required to reach the predetermined hydrogen concentration before switching on the fuel cell stack, using at least one characteristic curve. To determine the flush time, a characteristic curve can be used that is populated by measurements or simulation and outputs a flushing time based on a standstill period.

[0035] The procedure can further include determining the time required to reach the predetermined hydrogen concentration before switching on the fuel cell stack using at least one diffusion model. Another possibility is to calculate the flushing time using the standby time and a diffusion model in the control unit. R.416591

[0036] - 6 -

[0037] The diffusion model can consider the standby time and at least one other parameter selected from the following group: humidity in the fuel cell stack, operating point before shutdown, anode pressure, cathode pressure, ambient temperature, and ambient pressure. Another implementation is to model the diffusion processes across the membrane and the reaction using a physical model. In this case, the standby time is the most important parameter, but the parameters mentioned above can be expanded upon.

[0038] The process can further include setting the predetermined hydrogen concentration by adjusting a predetermined anode pressure and / or venting or draining anode gas. This allows for reliable adjustment of the minimum hydrogen concentration required for startup.

[0039] The process can further include setting the predetermined anode pressure by injecting hydrogen into an anode chamber of the fuel cell stack. This is achieved by setting a desired anode pressure through hydrogen injection (pressure increase) and then further increasing the hydrogen concentration by purging the anode gas via a purge and drain valve.

[0040] Furthermore, a motor vehicle is proposed that incorporates such a fuel cell system.

[0041] Finally, a computer program product is proposed with program code means which, when the computer program product is executed on a computer, configure the computer to perform a method according to one of the embodiments described above or below.

[0042] The proposed computer program product could be, for example, a file for download from a server or a data carrier such as a CD-ROM or a USB flash drive. R.416591

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[0044] The advantages which have been described in detail with regard to the fuel cell system and operating methods for operating a fuel cell system according to the invention apply equally to the vehicle according to the invention and to the computer program product according to the invention.

[0045] Within the scope of the present invention, a fuel cell system can be understood to be a system comprising at least one fuel cell stack, an anode path comprising an anode, an anode gas supply line and an anode gas return line, a cathode path comprising a cathode, a cathode gas supply line and a cathode gas return line, a sensor unit for acquiring data to determine a state and / or a load change of the fuel cell system, a processing unit for determining a state and / or a load change of the fuel cell system, and a control unit. The control unit is configured to regulate the operation of the fuel cell system or the fuel cell stack. A fuel cell stack comprises at least two fuel cells, preferably at least 10 fuel cells, and even more preferably at least 100 fuel cells.The term fuel cell stack is used synonymously with fuel cell stack within the scope of the present invention. A fuel cell consists of electrodes between which an electrolyte (ion conductor) is located. The electrodes are the aforementioned anode and cathode. A liquid, such as alkalis or acids, or molten alkali carbonate can be used as the electrolyte. In high-temperature fuel cells, a solid is used as the electrolyte, such as ion-conducting ceramic or a polymer, which then forms a solid electrolyte. Membranes are also used. These are semipermeable membranes that are permeable only to one type of ion, e.g., protons. A membrane can also separate two different liquid electrolytes from each other. The energy is supplied by a reaction of oxygen with the fuel. This is often hydrogen, but organic compounds such as methane or methanol are also used.Both reactants are continuously supplied via the electrodes. The fuel cell system can further comprise a housing in which the at least one fuel cell stack is accommodated. The fuel cell system can further comprise a control unit for activating a device for targeted adjustment of an R.416591.

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[0047] The system includes water loading of a membrane of the fuel cell system and a device for targeted adjustment of a water loading of a membrane of the fuel cell system.

[0048] The fuel cell system has several subsystems, such as the anode subsystem or hydrogen subsystem, which includes the anode path and one or more hydrogen tanks, the cathode subsystem, which includes the cathode path, an air compressor and optionally a humidifier, the electrical subsystem, which includes electrical components such as electrical connections, battery, energy storage, and the cooling system, which includes coolant, coolant pump and fan.

[0049] In the context of the present invention, a downtime can be understood as a period during which the fuel cell system or even just the fuel cell stack is not operated, but is switched off.

[0050] Brief description of the drawings

[0051] Further optional details and features of the invention will become apparent from the following description of preferred embodiments, which are shown schematically in the figures.

[0052] They show:

[0053] Figure 1 shows a schematic representation of a fuel cell system according to an embodiment of the present invention; and

[0054] Figure 2 shows an exemplary representation of the temporal concentration profiles of different gases at an anode when the fuel cell system is at standstill.

[0055] Embodiments of the invention R.416591

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[0057] Figure 1 shows a schematic representation of a fuel cell system 100 according to an embodiment of the present invention. The fuel cell system 100 can, by way of example, be arranged in a vehicle. The vehicle can be a passenger car or a truck, although other types of vehicles are conceivable in principle.

[0058] The fuel cell system 100 comprises at least one fuel cell stack 102. The fuel cell stack 102 includes several fuel cells, of which only one is shown for illustrative purposes. The fuel cell system 100 further comprises an anode path 104, which includes an anode 106, an anode gas supply line 108, and an anode gas return line 110.

[0059] The anode path 104 further comprises a hydrogen metering valve 112 and, optionally, a jet pump 114, which are arranged in the anode gas supply line 108. A predetermined quantity of hydrogen can be supplied to the anode 106 via the hydrogen metering valve 112. This hydrogen can, for example, be injected at a predetermined pressure by means of the hydrogen metering valve 112. The jet pump 114 ensures, in particular, recirculation, i.e., flow. The anode path 104 further comprises a water separator 116, a drain valve 118, a purge valve 120, and an anode recirculation blower 122. The water separator 116, the drain valve 118, and the purge valve 120 are arranged in the anode gas return line 110, with the drain valve 118 being located upstream of the water separator 116 and the purge valve 120 being located downstream of the water separator 116.The anode recirculation blower 122 is located between the anode gas supply line 108 and the anode gas return line 110 and is connected to both. A systemic approach has been established for supplying the anode 106 with hydrogen, whereby the still hydrogen-rich anode exhaust gas is fed back to the anode inlet along with fresh hydrogen by means of the anode recirculation blower 122. The water separator removes water from the anode exhaust gas. After a purging process, the anode gas can be blown out or discharged via the purge valve 120 and drain valve 118. R.416591.

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[0061] The fuel cell system 100 further includes a cathode path (not shown in detail) comprising a cathode, a cathode gas supply line and a cathode gas discharge line.

[0062] The fuel cell system 100 further comprises at least one sensor unit (not shown) for acquiring operating parameters of the fuel cell stack 102. The sensor unit may also be configured to acquire data for determining the state and / or load change of the fuel cell system 100. The fuel cell system may also include a processing unit (not shown) for determining the state and / or load change of the fuel cell system 100.

[0063] The fuel cell system 100 also includes a control unit 124. The control unit is designed to regulate or control the operation of the fuel cell system 100 or the fuel cell stack 102.

[0064] The control unit 124 is configured to detect the standstill time of the fuel cell stack. This is because the decrease in hydrogen concentration depends significantly on the standstill time of the fuel cell system and, in particular, the polymer electrolyte membrane.

[0065] The control unit 124 is still configured to determine the time required to establish a predetermined hydrogen concentration before switching on the fuel cell stack, based on the recorded downtime. The remaining hydrogen concentration, as well as the time required for the first flush, can be determined from the downtime.

[0066] In the simplest implementation, the duration or flush time can be determined from a characteristic curve based on the standstill time. Another implementation could involve modeling the diffusion processes across the membrane and the reaction using a physical model. In this case, the standstill time would be the most important parameter, but it could be extended to include the following:

[0067] Humidity in fuel cell stack 102, operating point before shutdown, anode pressure, cathode pressure, ambient temperature, ambient R-416591

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[0069] Pressure. The diffusion processes can thus be validated by monitoring the pressure in the anode and the pressure in the cathode. The ambient temperature and pressure may cause the fuel cell stack 102 to cool down or influence the equilibrium state.

[0070] Figure 2 shows an exemplary representation of the concentration profiles of various gases at an anode 106 when the fuel cell system 100 is at rest. The x-axis represents time in hours. The y-axis represents the gas concentration in mol%. Curve 126 represents the concentration of nitrogen over time. Curve 128 represents the concentration of hydrogen over time. Curve 130 represents the concentration of oxygen over time. Curve 132 represents the concentration of water vapor over time. Based on curve 128, a characteristic map can thus be created within the scope of the present invention to determine the time required to establish a predetermined hydrogen concentration before switching on the fuel cell stack 102.

[0071] The reason for this is that hydrogen diffuses from the anode 106 to the cathode, and after the shutdown process, any remaining oxygen reacts with hydrogen to form water. Especially after a prolonged period of inactivity, nitrogen accumulates on the anode 106, so that before starting the fuel cell system 100, a suitable hydrogen concentration for starting must first be established by venting the anode gas.

[0072] The control unit 124 is further configured to set the predetermined hydrogen concentration before switching on the fuel cell stack, based on the determined time duration. Specifically, the control unit 124 is configured to set the predetermined hydrogen concentration before switching on the fuel cell stack 102 by setting a predetermined anode pressure. More precisely, the control unit 124 is configured to set the predetermined anode pressure by injecting hydrogen into an anode chamber of the fuel cell stack 102. R.416591

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[0074] For the subsequent start-up, a minimum hydrogen concentration must be achieved. This is accomplished by setting the desired anode pressure through hydrogen injection (pressure increase) and then further increasing the hydrogen concentration by purging the anode gas via the purge valve 120 and drain valve 118. During purging, the hydrogen-nitrogen mixture is discharged, which reduces the system's efficiency. The invention makes it possible to minimize hydrogen discharge and thus keep the flush time as short as possible, while still achieving the minimum hydrogen concentration.

Claims

R.416591 - 13 - Claims 1. Fuel cell system (100), comprising at least one fuel cell stack (102), wherein the fuel cell stack (102) comprises multiple fuel cells, and a control unit (124) wherein the control unit (124) is configured to detect a standstill time of the fuel cell stack (102), to determine a time period for setting a predetermined hydrogen concentration before switching on the fuel cell stack (102) based on the detected standstill time, and to set the predetermined hydrogen concentration before switching on the fuel cell stack (102) based on the determined time duration.

2. Fuel cell system (100) according to the preceding claim, wherein the control unit (124) is configured to determine the time required to set the predetermined hydrogen concentration before switching on the fuel cell stack (102) by means of at least one characteristic map.

3. Fuel cell system (100) according to one of the preceding claims, wherein the control unit (124) is configured to determine the time required to set the predetermined hydrogen concentration before switching on the fuel cell stack (102) using at least one diffusion model.

4. Fuel cell system (100) according to the preceding claim, wherein the diffusion model takes into account as parameters the standby time and at least one parameter selected from the group consisting of: humidity in the fuel cell stack, operating point before shutdown, pressure in the anode, pressure in the cathode, ambient temperature, ambient pressure. R.416591 - 14 - 5. Fuel cell system (100) according to one of the preceding claims, wherein the control unit (124) is configured to adjust the predetermined hydrogen concentration by adjusting a predetermined anode pressure.

6. Fuel cell system (100) according to the preceding claim, wherein the control unit (124) is configured to set the predetermined anode pressure by injecting hydrogen into an anode space of the fuel cell stack (102).

7. Fuel cell system (100) according to one of the preceding claims, further comprising a drain valve (118) and / or a purge valve (120), wherein the control unit (124) is configured to adjust the predetermined hydrogen concentration by releasing anode gas through actuation of the drain valve (118) and / or the purge valve (120).

8. Method for operating a fuel cell system (100), wherein the fuel cell system (100) comprises at least one fuel cell stack (102), wherein the fuel cell stack (102) comprises multiple fuel cells, comprising the steps: Determining the idle time of the fuel cell stack (102), determining the time required to set a predetermined hydrogen concentration before switching on the fuel cell stack (102) based on the determined idle time, and Setting the predetermined hydrogen concentration before switching on the fuel cell stack (102) based on the determined time duration.

9. Method according to the preceding claim, further comprising determining the time required to adjust the predetermined hydrogen concentration before switching on the fuel cell stack (102) using at least one characteristic map. R.416591 - 15 - 10. Method according to one of the two preceding claims, further comprising determining the time required to adjust the predetermined hydrogen concentration before switching on the fuel cell stack (102) using at least one diffusion model.

11. Method according to the preceding claim, wherein the diffusion model takes into account as parameters the standstill time and at least one parameter selected from the group consisting of: moisture in the fuel cell stack, operating point before shutdown, pressure in the anode, pressure in the cathode, ambient temperature, ambient pressure.

12. Method according to any one of claims 8 to 11, further comprising adjusting the predetermined hydrogen concentration by adjusting a predetermined anode pressure and / or by venting anode gas.

13. Method according to the preceding claim, further comprising adjusting the predetermined anode pressure by injecting hydrogen into an anode space of the fuel cell stack (102).

14. Vehicle comprising a fuel cell system (100) according to any one of claims 1 to 8.

15. Computer program product comprising program code means which, when the computer program product is executed on a computer, configure the computer to perform a method according to any one of claims 8 to 13.

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