Method and device for operating a fuel cell system with a stack

Abstract

The invention relates to a method for operating a fuel cell system (1) having a stack (2) to which anode gas with a fuel is supplied via a stack inlet (21) in an anode circuit (6) on one anode side, which reacts with an oxidizing agent in the stack (2), wherein anode exhaust gas is discharged via a stack outlet (22), which contains, among other things, a residue of fuel and at least one further gaseous component, wherein fuel contained in the anode exhaust gas is fed back to the stack (2) by recirculation. In order to make the operation of the fuel cell system (1) simpler and / or more cost-effective, the gas composition at the stack inlet (21), at the stack outlet (22) or in the stack (2) is determined by means of a differential pressure measurement across the stack (2).

Description

[0001] The invention relates to a method and a device for operating a fuel cell system with a stack to which anode gas with a fuel is supplied in an anode circuit on one anode side via a stack inlet, which reacts with an oxidizing agent in the stack, wherein anode exhaust gas is discharged via a stack outlet, which contains, among other things, a residue of fuel and at least one further gaseous component, wherein fuel contained in the anode exhaust gas is fed back to the stack by recirculation. State of the art

[0002] German patent application DE 10 2021 203 349 A1 discloses a method for operating a fuel cell system in which hydrogen is supplied to a fuel cell stack as anode gas and anode gas escaping from the fuel cell stack is recirculated, and in which water contained in the anode gas is separated with the aid of a water separator, collected in a container and removed from the fuel cell system by opening a drain valve, wherein the time required to completely empty the container is determined to control the drain valve, and the filling time is set depending on the determined emptying time, wherein the time at which emptying begins is assumed to be known and the time at which emptying ends is determined by evaluating the voltage of the fuel cell stack.German patent application DE 10 2022 200 298 A1 discloses a method for operating a fuel cell system in which hydrogen is supplied to an anode of a fuel cell stack via an anode circuit and recirculated, the recirculation being effected by means of a fan integrated into the anode circuit, the fan being driven by a turbine integrated into a hydrogen path connecting the anode circuit to a high-pressure hydrogen tank, so that hydrogen taken from the high-pressure tank drives the turbine. Disclosure of the invention

[0003] The object of the invention is to make the operation of a fuel cell system simpler and / or more cost-effective.

[0004] The object is achieved in a method for operating a fuel cell system with a stack to which anode gas with a fuel is supplied via a stack inlet in an anode circuit on one anode side, which fuel reacts with an oxidizing agent in the stack, wherein anode exhaust gas containing, among other things, a residue of fuel and at least one further gaseous component is discharged via a stack outlet, wherein fuel contained in the anode exhaust gas is fed back to the stack by recirculation, in that the gas composition at the stack inlet, at the stack outlet or in the stack is determined by means of a differential pressure measurement across the stack. The stack comprises several fuel cells. For this reason, the stack is also referred to in German as a fuel cell stack. The fuel is, for example, hydrogen.In addition to the fuel, in particular hydrogen, the anode gas contains at least one other gaseous component, such as nitrogen. The nitrogen is an inert gas that is used, among other things, to test the fuel cell system before commissioning. In addition to the nitrogen, the anode gas may also contain unavoidable impurities. The oxidizing agent is, for example, oxygen contained in air, which is preferably supplied to the stack in compressed form. The claimed fuel cell system is preferably a low-temperature fuel cell system with liquid product water. Recirculation in the anode circuit is preferably achieved with the aid of a gas delivery unit, such as a blower or a jet pump. A jet pump works according to the Venturi principle.The fuel cell system can include a jet pump and a blower. However, there are also systems that only include a jet pump. Such systems are also referred to as jet-pump-only systems. The claimed method provides a function for determining the fuel concentration, in particular the hydrogen concentration, in the anode circuit. This offers, among other advantages, that the otherwise required hydrogen sensor in the anode path can be omitted. With the claimed differential pressure measurement across the stack, the gas composition at the stack inlet can be determined, particularly in jet-pump-only systems, using simple pressure sensors and temperature sensors, but without hydrogen sensors.

[0005] A preferred embodiment of the method is characterized in that, during the differential pressure measurement, pressure differences between a first pressure at the stack inlet and a second pressure at the stack outlet are measured and recorded as stack pressure data. The pressure measurement at the stack inlet and at the stack outlet can be carried out using relatively simple and inexpensive pressure sensors. The pressure sensors are connected to a control unit for control purposes. When evaluating the differential pressure measurements, the relationship that the differential pressure across the stack increases when the concentration of the other gaseous component, in particular the nitrogen concentration, in the anode circuit increases is advantageously exploited. The continuously recorded stack pressure data can thus be used to draw conclusions about the nitrogen concentration and, advantageously, also about the hydrogen concentration in the anode circuit.

[0006] A further preferred embodiment of the method is characterized in that at least one additional measurement is carried out during the differential pressure measurement in order to acquire additional data, from which, together with the stack pressure data, a multidimensional data model is formed, with the aid of which a current gas composition in the anode circuit is determined. In this way, the gas composition, in particular the hydrogen concentration, in the stack can advantageously be deduced without the use of a hydrogen sensor. The term additional measurement encompasses an actual additional measurement. However, the term additional measurement also encompasses the incorporation of additional information that is taken into account in the claimed method. Such additional information can, for example, include a position of a bypass valve relative to a gas injector.

[0007] A further preferred embodiment of the method is characterized in that the fuel cell system is measured at various operating pressures as part of the additional measurement. For example, an absolute pressure at a point in the stack, i.e., at the stack inlet or stack outlet, is used as the operating pressure. Alternatively or additionally, a pressure upstream of a gas injector of a jet pump can be used as the operating pressure. The operating pressures can also be advantageously measured using relatively simple and inexpensive pressure sensors.

[0008] A further preferred embodiment of the method is characterized in that the fuel cell system is measured at different anode gas compositions as part of the additional measurement. The different anode gas compositions can be realized, for example, by appropriately controlling valves in the fuel cell system. For example, the supply of fresh fuel, in particular hydrogen, can be adjusted by controlling a gas supply valve. The size of the supplied fuel mass flow also affects the recirculation in the anode circuit. Furthermore, the anode gas composition can be specifically changed by controlling a bypass valve, which can be used to bypass the jet pump.The activation of a purge valve, through which impurities, primarily nitrogen, are removed from the anode circuit, also has a significant impact on the anode gas composition. As part of a plausibility check, for example, control measures can be taken to compensate for the discharged anode exhaust gas by opening the gas supply valves accordingly.

[0009] A further preferred embodiment of the method is characterized in that the fuel cell system is measured at various valve positions during the additional measurement. These are advantageously valve positions of valves through which anode gas is supplied or discharged. Of course, bypass valve positions of bypasses must also be taken into account, which can be used to bypass a component, such as a jet pump, in the fuel cell system. The fuel cell system can comprise only one gas injector. However, the fuel cell system can also comprise two or more gas injectors.

[0010] Another preferred embodiment of the method is characterized in that the fuel cell system is measured at different temperatures as part of the additional measurement. These temperatures are preferably gas temperatures measured on the anode side in or on the stack. However, temperatures measured on the coolant side can also be used particularly advantageously here, because the stack temperature is primarily influenced and defined by the temperature of a cooling system on one coolant side of the stack. These temperatures can advantageously be measured using simple and inexpensive temperature sensors.

[0011] A further preferred embodiment of the method is characterized in that the fuel cell system is measured at various stack currents as part of the additional measurement. The stack currents are continuously recorded on an electrical connection side of the stack during operation of the fuel cell system. This effectively improves the quality of the claimed method in determining the gas composition at the stack inlet without additional costs.

[0012] A further preferred embodiment of the method is characterized in that the multidimensional data model is used in a control unit to control a purge function in the fuel cell system. During the purge function, anode exhaust gas, particularly nitrogen-contaminated anode exhaust gas, is discharged from the anode circuit of the fuel cell system via the purge valve. To calibrate the data model due to potential manufacturing-related variations in the hydraulic resistance of the stack, additional reference operating points are advantageously approached during operation. The purge valve is deliberately opened for a long time to temporarily maximize the fuel concentration in the anode circuit.

[0013] The invention further relates to a device configured to perform all steps of the method described above. The device comprises, for example, a control unit for, in, or with a fuel cell system described above. The invention also relates to a computer program configured to perform all steps of the method described above. The invention further relates to a machine-readable storage medium having such a computer program stored thereon.

[0014] Further advantages, features and details of the invention will become apparent from the following description, in which various embodiments are described in detail with reference to the drawing. Short description of the drawing

[0015] They show: Fig. 1 a schematic representation of an anode region of a first fuel cell system for carrying out the claimed method; Fig. 2 a schematic representation of an anode region of a second fuel cell system for carrying out the claimed method; Fig. 3 a schematic representation of a third fuel cell system for carrying out the claimed method; and Fig. 4 a similar representation as in Fig. 3 according to a variant with two gas injectors. Description of the embodiments

[0016] The Fig. 1 and Fig. 2 show known fuel cell systems 1 as disclosed in the German patent application DE 10 2021 203 349 A1 mentioned above. Fig. Figure 1 shows the anode region of the fuel cell system 1. This comprises an anode circuit 6 for supplying a fuel cell stack 2 with anode gas, in particular hydrogen.

[0017] Anode gas exiting the fuel cell stack 2 is recirculated. For this purpose, a jet pump 7 and a recirculation fan 8 are arranged in the anode circuit 6. Since recirculated anode gas becomes enriched with nitrogen and water over time, a water separator 3 with a container 4 for collecting the separated water is also integrated into the anode circuit 6.

[0018] Furthermore, a drain valve 5 and a purge valve 9 are provided for emptying the tank 4. When the drain valve 5 is open, water flows out of the tank 4. When the purge valve 9 is open, anode gas escapes to reduce the nitrogen concentration of the anode gas. The amount discharged via the purge valve 9 is replaced by fresh anode gas, which is introduced into the anode circuit 6 via the jet pump 7. The discharged amount is removed from the system via an exhaust line 10. The above also applies to a gas volume to be replaced by discharged water, which is released by the described draining process.

[0019] A variant of the fuel cell system of the Fig. 1 is the Fig. 2. Here, the water separator 3 and the tank 4 are integrated into the recirculation fan 8. The drain valve 5, which serves to empty the tank 4, is also located there. The purge valve 9 is integrated as a completely independent component in the anode circuit 6. Otherwise, the Fig. 2 shown fuel cell system to the Fig. 1.

[0020] The Fig. The fuel cell system 1 shown in Figure 3 is similar to the one shown in Fig. 1 and Fig. 2 shown fuel cell system 1. Therefore, to designate identical or similar parts in the Fig. 1 to 3 the same reference numerals 1 to 10 are used. In Fig. 3, additional reference numerals 11 to 23 are used, which are referred to below with reference to the Fig. 3 will be explained.

[0021] The Fig. The fuel cell system 1 shown in Figure 3 comprises, in a design as a jet pump only system, only a jet pump 7, but no recirculation blower 8, which in Fig. 3 is only indicated as an option by a dashed line. An arrow indicates Fig. 3 a hydrogen supply 18 from above via a gas injector 19 is indicated.

[0022] The fuel gas, in particular hydrogen gas, supplied via the gas injector 19 can either be used in the jet pump as a drive in the anode circuit or can be bypassed by the jet pump 7 via a bypass valve 20. Pressure sensor symbols in Fig. 3 indicates that a first pressure, also referred to as stack inlet pressure 11, is detected at a stack inlet 21 of stack 2. A second pressure, also referred to as stack outlet pressure 12, is detected at a stack outlet 22 of stack 2.

[0023] The indicated pressure sensors for the stack inlet pressure 11 and the stack outlet pressure 12 are connected to a control unit 23 indicated only by a rectangle. Further pressure sensors are connected to this control unit 23. Fig. 3 sensors (not shown), in particular pressure sensors and temperature sensors, are connected for control purposes. A hydrogen sensor 17 is in Fig. 3 is arranged in the anode circuit 6. This hydrogen sensor 17 can advantageously be omitted if the claimed method described below is used.

[0024] Stack 2 is symbolically divided into a stack anode 13 and a stack cathode 14. A rectangle indicates a stack cooling system 15. Another rectangle indicates an electrical connection 16 on stack 2.

[0025] Fuel cell system 1 is a polymer electrolyte membrane (PEM) fuel cell system. In the fuel cell system 1, hydrogen is converted into electrical energy using oxygen, generating waste heat and water. Converting hydrogen means consuming or removing hydrogen molecules from the anode side.

[0026] A PEM fuel cell consists of an anode supplied with hydrogen, a cathode supplied with air, and a polymer electrolyte membrane sandwiched between them. In practical applications, several such individual fuel cells are stacked to increase the generated electrical voltage.

[0027] Within the fuel cell stack 2, called stack, there are supply channels that supply the individual fuel cells with hydrogen and air and transport away the depleted moist air and the depleted anode exhaust gas.

[0028] Nitrogen enters the anode compartment through diffusion processes. Another source of nitrogen is the fresh fuel, which is not 100 percent pure hydrogen. Nitrogen is an inert gas for the fuel cell, reducing the cell voltage and thus the stack voltage, which in turn leads to a loss of efficiency. Therefore, gas is repeatedly removed from the anode compartment during a driving cycle to reduce the nitrogen content. This removal occurs via purge valve 9.

[0029] A systemic approach to supplying the PEM anode with hydrogen has been established in which the still hydrogen-rich anode exhaust gas is fed back to the anode inlet together with fresh hydrogen using gas delivery units. This is called recirculation. Jet pumps and hybrid solutions consisting of a jet pump and a hydrogen blower are used as gas delivery units. For particularly cost-effective systems, the solution with only one jet pump is preferred.

[0030] The Fig. The fuel cell system 1 shown in Figure 3 is also referred to as a jet pump bypass system due to the presence of the bypass 20. In the Fig. In the fuel cell system 1 shown in Figure 3, low-load operating points are primarily driven by the jet pump 7, and high-load operating points are supported by the bypass 20. This allows the jet pump 7 to be more specifically designed for low-load operating points.

[0031] The Fig. 4 shown variant of the fuel cell system 1 from Fig. 3 comprises, in addition to the gas injector 19, a second gas injector 25. The jet pump 7 is supplied with hydrogen from the hydrogen supply 18 via the gas injector 19. The bypass valve 20 can be supplied directly with hydrogen from the hydrogen supply 18 via the second gas injector 25.

[0032] The supply of fresh hydrogen is provided, for example, via hydrogen dosing valves, which can be designed as proportional valves. One control strategy involves using the hydrogen dosing valve to regulate the gas pressure within the anode path, measured by a pressure sensor, at a defined position to a defined target pressure depending on the system operating point.

[0033] Reasons for adding fresh hydrogen can be, on the one hand, consumption of hydrogen through electrochemical conversion and, on the other hand, losses of gas molecules from the anode chamber due, for example, to the drain valve 5 being opened for too long when gas is discharged after the water separator 3 has been completely emptied of water, or due to the purge valve 9 being opened.

[0034] Conventional purge valves are normally closed valves equipped with a defined orifice as a flow resistance. This flow resistance allows the maximum discharged gas volume to be defined.

[0035] In conventional fuel cell systems, purging occurs in a time-controlled manner, depending on the system's operating point, or based on or after defined incidents or events during system operation. If purging occurs frequently, the nitrogen concentration is kept low, but this also wastes fuel and reduces system efficiency.

[0036] Knowledge of the nitrogen concentration is important, among other things, to optimize the frequency of purging with regard to system efficiency, but also to minimize damage to the cells. Fig. The hydrogen sensor 17 shown in Figure 3 can be used in the anode circuit 6. However, the hydrogen sensor 17 always represents an intervention in the anode path, which is associated with mechanical interfaces. Mechanical interfaces can, in turn, cause leaks. Such hydrogen sensors 17 are expensive, have a short service life in automotive applications, and are rarely available.

[0037] By the claimed method, the gas composition at the stack inlet 21 can be determined by means of a differential pressure measurement across the stack 2 at the Fig. 3 shown jet pump only system in a simple manner. However, the differential pressure measurement claimed in the method can also be carried out in the Fig. 1 and Fig. 2 can be used advantageously.

[0038] In the differential pressure measurement, the pressure is measured before and after stack 2. When evaluating the differential pressure measurement, the relationship is exploited that the differential pressure across stack 2 increases when the nitrogen concentration in the anode circuit 6 increases.

[0039] Within the scope of the claimed method, the fuel cell system 1 is measured for various operating pressures, gas compositions, positions of the bypass valve 20, temperatures, and stack currents. A multidimensional data model is generated from the measured values.

[0040] What has been said in the previous paragraph applies both to the Fig. 3 shown fuel cell system 1 with only one gas injector 19 as well as for the Fig. 4 shown fuel cell system 1 with two gas injectors 19 and 25. Via the gas injector 19, Fig.4 the hydrogen is introduced into the jet pump 7. The gas injector 25 bypasses the jet pump 7.

[0041] In the data model, for example, the hydrogen concentration or the hydrogen content at the stack inlet 21 is plotted as a function of the differential pressure across the stack 2. The data model can then be used in the control unit 3 to control the purge function.

[0042] To calibrate the data model due to potential manufacturing-related variations in the hydraulic resistance of stack 2, additional reference operating points are approached during operation. For this purpose, the purge valve 9 is deliberately opened for a long time to temporarily maximize the hydrogen concentration in the anode circuit 6. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2021 203 349 A1 [0002, 0016] DE 10 2022 200 298 A1

[0002]

Claims

[1] Method for operating a fuel cell system (1) with a stack (2) to which anode gas with a fuel is supplied in an anode circuit (6) on one anode side via a stack inlet (21), which reacts with an oxidizing agent in the stack (2), wherein anode exhaust gas is discharged via a stack outlet (22), which contains, among other things, a residue of fuel and at least one further gaseous component, wherein fuel contained in the anode exhaust gas is fed back to the stack (2) by recirculation, characterized by that the gas composition at the stack inlet (21), at the stack outlet (22) or in the stack (2) is determined by means of a differential pressure measurement across the stack (2). [2] Method according to claim 1, characterized by that during the differential pressure measurement, pressure differences between a first pressure at the stack inlet (21) and a second pressure at the stack outlet (22) are measured and recorded as stack pressure data. [3] Method according to one of the preceding claims, characterized by that at least one additional measurement is carried out during the differential pressure measurement in order to collect additional data from which, together with the stack pressure data, a multi-dimensional data model is created with the help of which a current gas composition in the anode circuit is determined. [4] Method according to claim 3, characterized by that the fuel cell system (1) is measured at different operating pressures as part of the additional measurement. [5] Method according to claim 3 or 4, characterized by that the fuel cell system (1) is measured at different anode gas compositions as part of the additional measurement. [6] Method according to one of claims 3 to 5, characterized by that the fuel cell system (1) is measured at different valve positions as part of the additional measurement. [7] Method according to one of claims 3 to 6, characterized by that the fuel cell system (1) is measured at different temperatures as part of the additional measurement. [8] Method according to one of claims 3 to 7, characterized by that the fuel cell system (1) is measured at different stack currents as part of the additional measurement. [9] Method according to one of claims 3 to 8, characterized by that the multidimensional data model is used in a control unit (23) for controlling a purge function in the fuel cell system (1). [10] Device adapted to carry out all the steps of a method according to any one of the preceding claims.

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