Method for operating a fuel cell system; fuel cell system

Abstract

The invention relates to a method for operating a fuel cell system (100) having at least one fuel cell stack (11) in which a cathode chamber (K) is arranged and having a cathode feed line (31) which opens into the cathode chamber (K) in the flow direction, having a recirculation line (34) which is connected to the cathode feed line (31) and in which a recirculation valve (35) is arranged, wherein, during a drying process of the fuel cell stack, the air humidity in the cathode chamber K is set by carrying out the following steps at least once: a. determining a current air humidity in the cathode feed line (31), b. comparing the current air humidity with at least one limit value which describes an air humidity, c. at least partially opening the recirculation valve (35) over a first time period if the current air humidity is less than or equal to the at least one limit value.

Description

[0001] Description

[0002] Method for operating a fuel cell system; fuel cell system

[0003] The invention relates to a method for operating a fuel cell system having the features of the preamble of independent claim 1. Furthermore, the invention relates to a fuel cell system having the features of the preamble of independent claim 10.

[0004] State of the art

[0005] It is known from the prior art that fuel cell systems exist that have an anode system and a cathode system. The anode system consists of an anode supply line that supplies fuel, in particular hydrogen, to a fuel cell stack, and a recirculation line that recirculates anode exhaust gas to the anode supply line using a conveying unit. Furthermore, the cathode system consists of a cathode supply line, in which a compressor can be arranged, and a cathode outlet line through which cathode exhaust gas is conveyed from the cathode system into the exhaust system.

[0006] Starting the fuel cell system at temperatures below 0°C can be challenging due to the freezing of product water. To prevent product water from freezing, a drying process is used to remove water from the fuel cell system by flowing air through the cathode system and / or hydrogen through the anode system.

[0007] From published patent application DE 10 2019 214 748 A1, a method is known in which a drying process for the cathode system and the anode system is taught during a shutdown procedure of a fuel cell system. Disclosure of the invention

[0008] The inventive method for operating a fuel cell system according to independent claim 1 and the fuel cell system with the features according to independent claim 10 have the advantage that, during a drying process, at least a portion of cathode exhaust gas, hereinafter referred to as recirculation, is added to the outside air supplied to a cathode chamber. The outside air has a lower humidity than the cathode exhaust gas. Outside air is understood to be the air outside the fuel cell system.

[0009] Advantageously, during the drying process of the fuel cell stack, the air humidity in the cathode chamber is adjusted by performing the following steps at least once: a. Determining a current air humidity in the cathode supply line b. Comparing the current air humidity with at least one limit value that describes an air humidity c. At least partially opening the recirculation valve over a first period of time if the current air humidity is less than or equal to the at least one limit value

[0010] The inventive method makes it possible to dry the cathode compartment without drying out a membrane. The membrane borders the cathode compartment and is proton-conductive when wet. During the drying process of the cathode compartment, water is also removed from the membrane, which can cause it to dry out unintentionally and reduce the proton conductivity necessary for subsequent operation.

[0011] The drying out of the membrane is most pronounced in the area where the outside air enters the cathode chamber, since the outside air is least saturated with water when it enters the cathode chamber and can absorb the most water.

[0012] By drying with outside air, to which at least a proportion of recirculated air has been added and which therefore has a humidity above a limit value, the cathode chamber, which has a very high humidity of approximately 100%, can be dried and at the same time the membrane is prevented from drying out during the drying process.

[0013] The method according to the invention can efficiently prevent the cathode chamber from being dried unevenly during the drying process due to excessively low humidity of the supplied outside air, since the incoming air first dries the area of ​​the cathode chamber that is closest to the cathode supply line, while in other areas there is still insufficient drying.

[0014] This also prevents the area in the cathode compartment closest to the cathode lead from being over-dried due to too low humidity of the supplied outside air during the drying process, which leads to increased mechanical stresses across the membrane as it contracts, which in turn can lead to an increase in contact resistance and material fatigue of the membrane and is disadvantageous for the operation of the fuel cell system.

[0015] Furthermore, the process according to the invention allows the mass flow supplied to the cathode chamber to be increased during the drying process, thus shortening the drying process. The mass flow supplied to the cathode chamber consists of outside air and can additionally contain a portion of recirculated air to ensure a suitable humidity level.

[0016] It is advantageous if a recirculation valve is at least partially closed when at least one limit value is exceeded. This ensures that the current humidity remains at a level at which the drying process can be carried out efficiently.

[0017] Advantageously, steps a., b., and c. are repeated after a second period of time. This allows for regular checks during the drying process to determine whether the current humidity is at a suitable level to ensure efficient and uniform drying. It is advantageous to have a number of threshold values, each of which is assigned a valve opening degree. This allows the drying process to proceed in stages, resulting in improved, even, and gentle drying of the cathode chamber.

[0018] Advantageously, the degree of opening of the recirculation valve is determined using a function with the parameters current air humidity and / or the previous duration of the drying process. This allows for an optimized drying process. For example, at the beginning of the drying process, a small degree of opening of the recirculation valve, for example, 0% to 5%, may be desired in order to efficiently remove moisture from the cathode chamber. At a later point in time, the degree of opening of the recirculation valve can be set higher than at the beginning of the drying process in order to prevent the cathode chamber, particularly the area in the cathode chamber closest to the cathode supply line, from being overdried due to excessively low air humidity during the drying process.

[0019] It is advantageous if the current humidity is determined using a sensor. A sensor is an established component that enables efficient and accurate determination of a measured value, in particular the current humidity, and can make it available to the method according to the invention.

[0020] Advantageously, the sensor is designed as a virtual sensor. The virtual sensor calculates the air humidity from the parameters pressure in the cathode system, in particular in the cathode supply line, and / or temperature in the cathode system, in particular in the cathode supply line, and / or humidity of the cathode exhaust gas from the cathode chamber. This eliminates the need for additional sensors in the fuel cell system, ensuring a compact and cost-effective design of the fuel cell system.

[0021] It is advantageous if the current humidity is determined based on a model. This eliminates the need for additional sensors in the fuel cell system, ensuring a compact and cost-effective design of the fuel cell system. The current humidity is advantageously determined via an impedance measurement of the membrane adjacent to the cathode compartment K. With an impedance measurement, additional sensors in the fuel cell system are eliminated, ensuring a compact and cost-effective design of the fuel cell system. The total resistance of the fuel cell stack correlates with the humidity of a membrane of the fuel cell stack, which is located between the cathode compartment and an anode compartment arranged in the fuel cell stack.A decrease in the humidity of the supplied flow and the associated increasing drying of the cathode chamber are accompanied by a reduction in membrane humidity and thus a decrease in proton conductivity, so that the current humidity can be correlated via the total resistance of the fuel cell stack. The resistance of individual cells that are part of the fuel cell stack can also be used to determine the current humidity.

[0022] The fuel cell system for implementing the method according to the invention advantageously comprises at least one fuel cell stack in which a cathode chamber is arranged, and a cathode system with a cathode supply line that opens into the cathode chamber in the direction of flow, with a recirculation line that is connected to the cathode supply line and in which a recirculation valve is arranged, and with a control unit configured to implement the method according to one of the above claims. Thus, the existing control unit can be used to control or regulate the method according to the invention and requires no additional components.

[0023] Description of the drawings

[0024] The fuel cell system according to the invention and the method according to the invention are explained in more detail below with reference to drawings with preferred embodiments.

[0025] They show:

[0026] Fig. 1 shows a schematic topology of a fuel cell system and Fig. 2 shows a first embodiment of a method according to the invention and

[0027] Fig. 3 shows a second embodiment of a method according to the invention.

[0028] Figure 1 shows a schematic topology of the fuel cell system 100 with at least one fuel cell stack 11 and a cathode system 300. Furthermore, the fuel cell system 100 includes a highly schematically illustrated anode system 200 and a cooling circuit (not shown).

[0029] The anode system 200 supplies an anode chamber A of the fuel cell stack 11 with a fuel, in particular hydrogen (H2) as a reactant. The anode system 200 includes an anode supply line and a recirculation line. Anode exhaust gas from an anode chamber A, which is arranged in the fuel cell stack 11, is recirculated to the anode supply line via the recirculation line. A jet pump with a metering valve is arranged between the anode supply line and the recirculation line.

[0030] In the cathode system 300, a cathode supply line 31, a cathode outlet line 32 and a recirculation line 34 are arranged.

[0031] The cathode system 300 supplies a cathode chamber K of the fuel cell stack 11 with oxygen (O2) as a reactant. Oxygen is a component of air. By supplying air to the fuel cell system 100, oxygen is made available to the system as a reactant.

[0032] The cathode supply line 31 opens into the fuel cell stack 11 in the direction of flow. Oxygen is supplied to the fuel cell stack 11, in particular to the cathode chamber K, via the cathode supply line 31.

[0033] The cathode outlet line 32 is connected downstream to the fuel cell stack 11. Gases, such as cathode exhaust gas and / or fluids, such as liquid water, are discharged from the fuel cell stack 11, in particular the cathode chamber K, via the cathode outlet line 32. A compressor 33 and a sensor 37 are located within the cathode supply line 31. The compressor 33 is arranged downstream of the recirculation line 34 in the flow direction.

[0034] Compressor 33 pumps air into fuel cell stack 11. Compressor 33 can be used to vary the air flow rate. Increasing the power of compressor 33 results in an increased air flow rate being supplied to fuel cell stack 11 via cathode supply line 31. Reducing the power of compressor 33 results in a reduced air flow rate being supplied to fuel cell stack 11.

[0035] The sensor 37 is preferably arranged between the compressor 33 and the cathode chamber K. The sensor 37 can determine a measured value, in particular a humidity, in the cathode supply line 31 and provide the measured value for carrying out the method according to the invention.

[0036] In an alternative embodiment, the sensor 37 is omitted and the air humidity is determined based on a model.

[0037] The recirculation line 34 is arranged between the cathode supply line 31 and the cathode outlet line 32 and connects them. This allows cathode exhaust gas to flow from the cathode outlet line 32 into the cathode supply line 31.

[0038] A recirculation valve 35 is arranged in the recirculation line 34. The recirculation valve 35 can be used to adjust the flow of the cathode exhaust gas through the recirculation line 34. If the recirculation valve 35 is open to 0%, no cathode exhaust gas can flow through the recirculation line 34. If the recirculation valve 35 is partially open, cathode exhaust gas flows partially through the recirculation line 34.

[0039] In an alternative embodiment, a recirculation conveying unit can additionally be arranged in the recirculation line 34. The recirculation conveying unit can support the flow of cathode exhaust gas from the cathode outlet line 32 into the cathode supply line 31. An outlet valve 36 is arranged in the cathode outlet line 32. With the aid of the outlet valve 36, the mass flow of cathode exhaust gas discharged from the cathode system 300 can be additionally adjusted. It is also possible to build up a pressure gradient within the cathode system 300 by at least partially closing the outlet valve 36, so that a larger mass flow of cathode exhaust gas flows into the recirculation line 34.

[0040] A control unit 500 is provided to regulate and control all control-related processes in the fuel cell system 100. This also includes the processing of a measured value for executing the method according to the invention.

[0041] In an alternative embodiment, more than one fuel cell stack 11 can be arranged in the fuel cell system 100, without the execution of the method according to the invention being restricted thereby.

[0042] Figure 2 shows a first embodiment of the method according to the invention.

[0043] The inventive method makes it possible to dry the cathode compartment without drying out a membrane. The membrane borders the cathode compartment and is proton-conductive when wet. During the drying process of the cathode compartment, water is also removed from the membrane, which can cause it to dry out unintentionally and reduce the proton conductivity necessary for subsequent operation.

[0044] By drying with outside air, to which at least a proportion of recirculated air has been added and which therefore has a humidity above a limit value, the cathode chamber, which has a very high humidity of approximately 100%, can be dried and at the same time the membrane is prevented from drying out during the drying process.

[0045] The method is started in step S100. The inventive

[0046] The method is initiated or executed during a drying process of the fuel cell system 100. In step S200, the current air humidity is determined. The current air humidity can be determined using a sensor 37, in particular a humidity sensor.

[0047] In a first alternative embodiment, the sensor 37 can be designed as a virtual sensor 37. The virtual sensor 37 can be formed from the parameters pressure in the cathode system 300, in particular in the cathode supply line 31, and / or temperature in the cathode system 300, in particular in the cathode supply line 31, and / or humidity of the cathode exhaust gas from the cathode chamber K.

[0048] In a second alternative embodiment, the current air humidity can be determined based on a model. With a model-based determination of the current air humidity, the current air humidity can be determined, for example, based on the air humidity of the outside air and the air humidity of the recirculated air, as well as the mixing ratio of outside air to recirculated air in the cathode supply line 31.

[0049] In a third alternative embodiment, the current air humidity can be determined by measuring the impedance of the membrane adjacent to the cathode compartment K. The total resistance of the fuel cell stack 11 correlates with the humidity of a membrane of the fuel cell stack 11, which is located between the cathode compartment and an anode compartment arranged in the fuel cell stack. A decreasing air humidity of the supplied flow rate and the associated increasing drying of the cathode compartment K are accompanied by a reduction in the membrane humidity and thus a decreasing proton conductivity, so that the current air humidity can be correlated via the total resistance of the fuel cell stack 11. The resistance of only individual cells that are part of the fuel cell stack can also be used to determine the current air humidity.

[0050] In step S300, the current air humidity is compared with at least one limit value that describes an air humidity. If the current air humidity is less than or equal to the at least one limit value, the recirculation valve 35 is opened at least partially for a first period of time in a step S500. If the current air humidity is greater than the at least one limit value, the recirculation valve 35 is closed at least partially for a first period of time in a step S400.

[0051] In a step S600, which can be executed after step 400 or step 500, a second time period is run through before step S200 is executed again, provided that no event triggering the end of the method has occurred. An event triggering the end of the method can, for example, represent the termination of the drying process.

[0052] In step S700, the method according to the invention is terminated. The method according to the invention can be terminated when a predefined number of iterations of the method according to the invention have been completed. The method according to the invention can be terminated when the drying process is complete.

[0053] Figure 3 shows a second embodiment of the method according to the invention. The second embodiment of the method according to the invention corresponds to the first embodiment of the method according to the invention, except for the differences mentioned below.

[0054] In step S300, the current humidity is compared with a number of threshold values, each threshold value being assigned an opening degree of the recirculation valve 35. Each threshold value is assigned a percentage opening degree, so that at 0% opening degree, the recirculation valve 35 is closed, and 100% opening degree corresponds to a fully open recirculation valve 35.

[0055] In an alternative embodiment, the degree of opening of the recirculation valve 35 is determined via a function with the parameters current air humidity and / or previous duration of the drying process.

[0056] In the second embodiment of the method according to the invention, step S400 is omitted.

[0057] The method according to the invention can furthermore be carried out at least in part by the control unit 500 of the fuel cell system 100. A computer program in the form of code can be stored in a memory unit of the control unit 500. When the code is executed by a computing unit of the control unit 500, the program performs a method that can proceed as described above. With the aid of the control unit 500, the same advantages can be achieved that were described above in connection with the method according to the invention. These advantages are incorporated herein by reference in their entirety.

[0058] The control unit 500 may be in communication with the sensors of the fuel cell system 100 to monitor the sensor values.

[0059] The control unit 500 can control the actuators in the fuel cell system 100 in order to carry out the method accordingly.

[0060] In addition, the control unit 500 can be in communication with an external computing unit in order to outsource some process steps and / or calculations in whole or in part to the external computing unit.

[0061] According to a further aspect, the invention provides a computer program product comprising instructions that, when executed by a computer, such as the processing unit of the control unit 500, cause the computer to perform the method, which can proceed as described above. Using the computer program product, the same advantages can be achieved that were described above in connection with the method according to the invention and / or the control unit 500 according to the invention. These advantages are incorporated herein by reference.

Claims

Claims 1.) Method for operating a fuel cell system (100) with at least one Fuel cell stack (11) in which a cathode chamber (K) is arranged and with a cathode supply line (31) which opens into the cathode chamber (K) in the flow direction, with a recirculation line (34) which is connected to the cathode supply line (31) and in which a recirculation valve (35) is arranged, characterized in that during a drying process of the fuel cell stack, the air humidity in the cathode chamber K is adjusted by carrying out the following steps at least once: a. Determining a current air humidity in the cathode supply line (31) b. Comparing the current air humidity with at least one limit value that describes an air humidity c. At least partially opening the recirculation valve (35) over a first period of time if the current air humidity is less than or equal to the at least one limit value 2.) Method according to claim 1, characterized in that when the at least one limit value is exceeded, the recirculation valve (35) is at least partially closed. 3.) Method according to claim 1, characterized in that steps a., b. and c. are repeated after a second period of time has elapsed. 4.) Method according to claim 1, characterized in that it comprises a number of limit values, each limit value being assigned a degree of opening of the recirculation valve (35). 5.) Method according to claim 1, characterized in that the degree of opening of the Recirculation valve (35) is determined via a function with the parameters current air humidity and / or previous duration of the drying process. 6.) Method according to claim 1, characterized in that the current Humidity is determined via a sensor (37). 7.) Method according to claim 6, characterized in that the sensor (37) is designed as a virtual sensor and is formed from the parameters pressure in the cathode system (300), in particular in the cathode feed line (31) and / or temperature in the cathode system (300), in particular in the cathode feed line (31), and / or the humidity of the cathode exhaust gas from the cathode chamber K. 8.) Method according to claim 1, characterized in that the current Humidity is determined based on a model. 9.) Method according to claim 1, characterized in that the current Humidity is determined via an impedance measurement of a membrane that borders the cathode chamber K. 10.) Fuel cell system (100) with at least one fuel cell stack (11) in which a cathode chamber K is arranged and a cathode system (300) with a cathode feed line (31) which opens into the cathode chamber K in the direction of flow, with a recirculation line (34) which is connected to the cathode feed line (31) and in which a recirculation valve (35) is arranged and a control unit (500) which is configured to carry out the method according to one of the above claims.

Images