Method for operating a fuel cell system, controller

Abstract

The invention relates to a method for operating a fuel cell system (1) having a plurality of fuel cell stacks (100, 200), each having a cathode (110, 210) and an anode (120, 220), air being fed to the cathode (110, 210) via at least one air feed path (111, 211) and exhaust gas exiting from the fuel cell stacks (100, 200) being discharged via at least one exhaust gas path (112, 212), each anode (120, 220) being supplied with hydrogen via an anode circuit (121, 221). According to the invention, when the fuel cell system (1) is shut down, the exhaust gas of the first fuel cell stack (100) is introduced into the anode circuit (221) of the further fuel cell stack (200). By means of the introduced exhaust gas, the anodes (220) of the further fuel cell stack (200) are inert in a first stage of the shutdown process and dried in a second stage of the shutdown process. A controller for a fuel cell system (1) for performing the steps of the method according to the invention.

Description

Technical Field

[0001] The invention relates to a method for operating a fuel cell system having the features of the preamble of claim 1. The invention also relates to a control unit for a fuel cell system for carrying out the steps of the method.

[0002] A preferred area of ​​application is fuel cell vehicles, preferably fuel cell vehicles with start-stop operation. Background Art

[0003] Fuel cells are electrochemical energy converters. Hydrogen (H 2 ) and oxygen (O 2 ) can be used in particular as reaction gases. They are converted into electrical energy, water (H 2 O) and heat. The heart of a fuel cell is a membrane electrode assembly (MEA), which includes a membrane coated with catalytic materials on both sides to form electrodes. When a fuel cell is operating, hydrogen is supplied to one electrode (anode) and oxygen is supplied to the other electrode (cathode).

[0004] In order to increase the electrical power, a plurality of fuel cells are connected to form a fuel cell stack or stack in practice. In addition, a plurality of fuel cell stacks or fuel cell systems can be connected to form a so-called multi-stack system.

[0005] In the operation of a fuel cell system, the start-up and / or shutdown phases represent high loads, which can lead to degradation of the fuel cell. During startup, the main cause of this is the hydrogen-air front in the anode. When shutting down or shutting down, high voltages are generated because the anode is supplied with hydrogen and the cathode with oxygen, while no electrical load is drawn from the stack. This occurs in particular during long shutdown phases.

[0006] In order to counteract the degradation of the fuel cell during the start-up and / or shutdown phases, the oxygen present in the cathode can be consumed before the system is shut down by drawing current without additional air input. During this period, the anode continues to be supplied with hydrogen, so that the cell voltage is not critical. However, if air diffuses into the cathode, the cell voltage will increase and remain there for several hours, thereby causing destructive electrochemical reactions. Therefore, stop valves are usually provided on both the inlet and outlet sides, which should prevent air from entering the cathode when closed. However, since the stop valve is not completely sealed, its effectiveness is limited, especially during its service life. In addition, the stop valve is also accompanied by a considerable pressure loss.

[0007] In stationary applications, it is known to inertate the anode with nitrogen before shutting down in order to counteract undesired degradation. To this end, the nitrogen is kept in bottles. However, due to space restrictions, this is not possible in mobile applications. In addition, the nitrogen bottles must be refilled and maintained, which is disadvantageous in terms of cost.

[0008] Therefore, in an earlier application by the same applicant, for a multi-stack system, it has been suggested to use the exhaust gas exhausted from one fuel cell stack to inertate the anodes of another fuel cell stack. However, since the exhaust gas exhausted from the fuel cell stack is humid, due to containing liquid water and / or condensate, water accumulation may occur, which may hinder the gas supply at restart and thus lead to local hydrogen supply shortage. The accumulated water may also freeze and cause ice formation when the ambient temperature is below 0°C, which may result in an inability to restart. Summary of the invention

[0009] The object of the present invention is to counteract the degradation of fuel cells when shutting down a multi-stack fuel cell system without the above-mentioned problems occurring.

[0010] To solve this problem, a method is proposed having the features of claim 1. Advantageous embodiments can be found in the dependent claims. Furthermore, a control unit for a fuel cell system is specified for carrying out the steps of the method.

[0011] A method for operating a fuel cell system having a plurality of fuel cell stacks, each of which has a cathode and an anode, is proposed, wherein air is fed to the cathode via at least one gas supply path and exhaust gas discharged from the fuel cell stack is discharged via at least one exhaust gas path, wherein the anodes can be supplied with hydrogen via an anode circuit. According to the invention, when the fuel cell system is shut down, the exhaust gas of a first fuel cell stack is introduced into the anode circuit of another fuel cell stack, and the anodes of the other fuel cell stacks are inerted in a first phase of the shutdown process by means of the introduced exhaust gas and dried in a second phase of the shutdown process.

[0012] The shutdown process thus comprises at least two phases, a first phase for inerting and a second phase for drying. Since inerting is followed by drying, the largely oxygen-free but moist exhaust gas of the first fuel cell stack can be used to inertate the anodes of the further fuel cell stacks. The moisture is then removed by the subsequent drying. This means that the risk of the above-mentioned harmful accumulation of water is significantly reduced.

[0013] Since the exhaust gas of the first fuel cell stack is used to inertize the anodes of the further fuel cell stacks, it is not necessary to maintain an additional inert gas (eg nitrogen), thereby eliminating the need to carry and refill at least one gas cylinder.

[0014] In the proposed method, the first fuel cell stack is preferably operated in a lean-burn mode in a first phase of the shutdown process. In lean-burn mode, the fuel cell stack is operated in a substoichiometric manner, i.e., 空气 <1, thereby minimizing the oxygen content in the exhaust gas from the fuel cell stack. Lean operation thus helps to generate inert gas, which can then be used to inertize the anodes of other fuel cell stacks.

[0015] Furthermore, in the first phase, the air supply to the other fuel cell stack is interrupted, preferably by switching off the air delivery and air compression system integrated into the air supply path and / or by closing at least one valve, in particular a shut-off valve. This measure ensures that no further oxygen is supplied to the cathode of the fuel cell stack to be inerted.

[0016] As a further measure, it is proposed to close the pressure regulator integrated into the anode circuit of the other fuel cell stack in the first phase, that is, to interrupt the supply of hydrogen into the anode circuit so that it can be filled with the exhaust gas of the first fuel cell stack.

[0017] Furthermore, in the first phase, the shut-off valve arranged in the connecting line is opened to introduce the exhaust gas of the first fuel cell stack into the anode circuit of the other fuel cell stack. Only when the shut-off valve is opened does the exhaust gas flow from the exhaust gas path of the first fuel cell stack into the anode circuit of the other fuel cell stack. The shut-off valve is preferably opened after the hydrogen supply to the anode circuit is interrupted, thereby ensuring that only the exhaust gas or inert gas reaches the anode circuit.

[0018] Furthermore, it is proposed that in a first phase, a purge valve and / or a drain valve integrated into the anode circuit of the other fuel cell stack is opened. Via the opened purge valve and / or drain valve, the anode gas still present in the anode can be displaced with the introduction of exhaust gas, so that the anode circuit is filled with exhaust gas or inert gas.

[0019] During the transition from the first phase to the second phase, the lean operation of the first fuel cell stack is preferably terminated and normal operation is adopted, since the oxygen content of the gas used for drying has no effect on the drying of the anode in the second phase.

[0020] In the second phase, a bypass path is opened around the first fuel cell stack, preferably by opening a bypass valve. That is, since the bypass path connects the gas supply path to the exhaust path, air is not the exhaust gas but air is fed to the further fuel cell stack. Air, instead of inert gas, is therefore used to dry the anodes in the second phase of the shutdown process. If the air was previously compressed by means of an air delivery and air compression system integrated into the gas supply path, the air is strongly heated beforehand, so that the water absorption capacity is enhanced.

[0021] In the second phase of the shutdown process, in addition to the anode, the cathode of the other fuel cell stack can also be dried. In this case, the drying of the cathode can be carried out independently of the drying of the anode. Because preferably, in order to dry the cathode of the other fuel cell stack, the air supply interrupted in the first phase is established again, so that air is fed to the cathode again via the air supply path. Thus, "own" air is used to dry the cathode.

[0022] In order to establish the air supply to the cathode again, the air delivery and air compression system that was previously shut off in the first phase is switched on again, and / or the previously closed valve, in particular the shut-off valve, is opened again. The prerequisite for opening the shut-off valve is the presence of such a valve, since a check valve can also be provided instead of the shut-off valve. Since valves, either shut-off valves or check valves, are usually provided in the air supply path and the exhaust path, at least two valves are opened in the case of a shut-off valve.

[0023] Once the anodes of the other fuel cell stacks are dried, the connection of the exhaust gas path of the first fuel cell stack to the anode circuit of the other fuel cell stack can be interrupted again in the second phase, that is, the shut-off valve previously opened in the first phase and arranged in the connecting line connecting the exhaust gas path to the anode circuit is closed again.

[0024] Once the cathode of the other fuel cell stack is dried, the air supply to the cathode of the other fuel cell stack can be interrupted again in the second phase. That is, the air supply and air compression system are turned off. If shut-off valves are provided in the air supply path and the exhaust path, these shut-off valves are closed.

[0025] With the anode and cathode dry, the shutdown process is complete and the fuel cell system can be completely shut down.

[0026] Furthermore, a control unit for a fuel cell system is proposed, which is configured to carry out the steps of the method according to the invention. The method can thus be automated. Furthermore, a smooth transition from inerting in a first phase of the shutdown process to drying in a second phase can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The preferred embodiment of the present invention is explained in more detail below with reference to the accompanying drawings. It shows:

[0028] Figure 1 Schematic representation of a multi-stack fuel cell system which can be operated according to the method according to the invention,

[0029] Figure 2 According to the process of the method of the present invention,

[0030] Figure 3 Figure 1Schematic representation of a fuel cell system with an illustration of the flow direction of the exhaust gas in the first stage, and

[0031] Figure 4 Figure 1 Schematic illustration of a fuel cell system with an illustration of the flow direction of the air in the second phase of the shutdown process. DETAILED DESCRIPTION

[0032] Figure 1 A fuel cell system 1 according to the invention is shown, which has a first fuel cell stack 100 and a second fuel cell stack 200 .

[0033] The first fuel cell stack 100 has a cathode 110 and an anode 120. Air is supplied to the cathode 110 as an oxygen supplier via an air supply path 111. The air is taken from the environment and input into an air delivery and air compression system 113 via an air filter 114 in order to provide a certain air mass flow and a certain pressure level. Since the air becomes hot here, it is cooled by means of a heat exchanger 115 integrated into the air supply path 111 and further humidified downstream by means of a humidifier 116. The air then reaches the cathode 110 of the fuel cell stack 100 via a first valve 117 (designed as a non-return valve in the present case).

[0034] The exhaust gas of the fuel cell stack 100 is discharged via an exhaust gas path 112, in which a further valve 117 in the form of a non-return valve is arranged. Downstream of the valve 117, a humidifier 116 is integrated into the exhaust gas path 112, so that the moist exhaust gas can be used to humidify the input air. Downstream of the humidifier 116, the exhaust gas is fed to a turbine 131, followed by a pressure regulator 130. With the aid of the turbine, part of the energy used for compression can be recovered, since the air delivery and air compression system 113 can be driven with the aid of the turbine 131. In order to bypass the fuel cell stack 100, the air supply path 111 and the exhaust gas path 112 can be connected via a bypass path 118 with an integrated bypass valve 119.

[0035] The anode 120 is supplied with fresh anode gas or hydrogen and recirculated anode gas via the anode loop 121. The recirculation is achieved passively by means of a jet pump 124 and actively by means of a fan 123. Since the recirculated anode gas is enriched with nitrogen over time, which diffuses from the cathode side to the anode side, a purge valve 122 is provided in the anode loop 121. By opening the purge valve 122, the nitrogen-containing anode gas is discharged from the anode loop 121 and replaced by fresh anode gas via an opened hydrogen metering valve (not shown). Since the recirculated anode gas is also rich in water, a water separator 126 having a container 127 is integrated into the anode loop 121. By opening the discharge valve 128, the container 127 can be emptied from time to time.

[0036] The heat generated during operation of the fuel cell stack 100 is dissipated by means of the cooling circuit 129 .

[0037] The exhaust gas path 112 of the fuel cell stack 100 is connected to the anode loop 221 of the other fuel cell stack 200 via a connecting line 2 with an integrated shut-off valve 3. Therefore, when the shut-off valve 3 is opened, the exhaust gas of the first fuel cell stack 100 can be introduced into the anode loop 221 of the other fuel cell stack 200. The anode gas rich in exhaust gas then reaches the anode 220 of the other fuel cell stack 200 via the anode loop 221.

[0038] For the sake of simplicity, the two fuel cell stacks 100, 200 are currently configured to be largely identical. However, this is not a prerequisite for being able to carry out the method according to the invention. Identical components are represented by the same reference numerals, wherein the components of the first fuel cell stack 100 are preceded by "1" and the components of the second fuel cell stack 200 are preceded by "2". For the description of the components of the other fuel cell stack 200, reference is made to the description of the components of the first fuel cell stack 100. Unlike the first fuel cell stack 100, the valve 217 of the second fuel cell stack 200 is not designed as a check valve, but as a controllable shut-off valve.

[0039] Figure 1 The fuel cell system 1 shown can be operated in the following manner when shut down: Figure 2 Run the method shown.

[0040] Figure 2 The method shown comprises two stages: a first stage is used to inertize the anode 220 , and a second stage is used to dry the anode 220 and the cathode 210 .

[0041] The first stage includes steps S1 to S8, for which the following Figure 2 Combined with Figure 3 Give a description.

[0042] In step S1, the inerting of the anode 220 of the other fuel cell stack 200 is started by exhausting the first fuel cell stack 100. To this end, in step S2, the air supply to the fuel cell stack 200 is first interrupted, that is, the air supply and air compression system 213 are turned off. In addition, in step S3, the valve 217 is closed, which is currently possible because they are designed as controllable shut-off valves. If a shut-off valve should not be provided but a check valve, step S3 is omitted. Subsequently, in step S4, the cathode 210 of the fuel cell stack 200 is depleted of oxygen. Then, in step S5, the first fuel cell stack 100 is only still operated in lean combustion mode, so that λ 空气<1. The first fuel cell stack 100 thus generates an exhaust gas or inert gas which is as oxygen-free as possible, which can be used to inertify the anode 220 of the other fuel cell stack 200 .

[0043] Then, in step S6, the anode side pressure regulator 225 of the other fuel cell system 200 is closed, thereby interrupting the hydrogen input. Then, in step S7, the shut-off valve 3 in the connecting pipeline 2 is opened, so that the exhaust gas of the first fuel cell stack 100 reaches the anode loop 221 of the other fuel cell stack 200 (see Figure 3 , arrows 300 and 301). In order to remove the anode gas still present in the anode loop 221, the purge valve 222 and / or the drain valve 228 are opened. The anode loop 221 is filled with exhaust gas or inert gas and is inertized in this way. Excess exhaust gas is discharged via the opened purge valve 222 and / or the opened drain valve 228 via the connecting line 232 into the exhaust path 212 of the other fuel cell system 200 (see Figure 3 , arrows 320 and 303).

[0044] In step S9, it is checked whether the anode 221 is inert. If it is ("+"), the second stage can be entered. This includes steps S10 to S13 and is used to dry the anode 221 of the other fuel cell system 200. At the same time, the cathode 210 can be dried in the second stage. For this purpose, steps S14 to S17 are performed. Figure 2 Combined with Figure 4 Describe the steps in phase two of the shutdown process.

[0045] In step S10, the lean-burn operation of the first fuel cell stack 100 is terminated and normal operation is adopted, so that λ 空气 ≥1. In addition, the bypass valve 119 is opened so that air flows from the air supply path 111 through the bypass path 118 into the exhaust path 112. Therefore, mainly air and no exhaust gas is input to the other fuel cell stack 200 via the connecting line 2 (see Figure 4 , arrow 400). The air reaches the anode circuit 221 via the connecting line 2 and enters the anode 220 from there to dry the anode (see Figure 4 , arrow 401). The humid air exhausted from the anode 220 is discharged via the open purge valve 222 and / or the open exhaust valve 228 (see Figure 4 , arrow 402).

[0046] In step S11, it is checked whether the anode 220 has dried. If so ("+"), in step S12, the shutoff valve 3 in the connecting line 2 and the purge valve 222 and / or the drain valve 228 (if open) are closed again. Subsequently, in step S13, the air mass flow via the bypass path 118 can be interrupted or at least reduced by closing the bypass valve 119 of the first fuel cell stack 100.

[0047] At the same time, steps S14 to S17 for drying the cathode 210 can be performed. For this purpose, in step S14, the valve 217 previously closed in the first stage is opened again. In step S15, the air delivery and air compression system 213 is switched on again, so that the air supply to the cathode 210 is no longer interrupted (see Figure 4 , arrow 500). The air mass flow generated by the air delivery and air compression system 213 ultimately leads to drying of the cathode 210. The humid air exhausted from the cathode 210 is removed via the exhaust path 212 (see Figure 4 , arrows 501 and 502).

[0048] In step S16, it is checked whether the positive electrode 210 is already dry. If so ("+"), the air input to the cathode 210 can be interrupted again in step S17. For this purpose, the air delivery and air compression system 213 is turned off. In addition, valve 217 is closed.

[0049] The shutdown process ends in step S18.

Claims

1. A method for operating a fuel cell system (1) having a plurality of fuel cell stacks (100, 200), each of which has a cathode (110, 210) and an anode (120, 220), wherein: Air is input to the cathode (110, 210) via at least one air supply path (111, 211), and exhaust gas exhausted from the fuel cell stack (100, 200) is exhausted via at least one exhaust path (112, 212), wherein the anode (120, 220) is supplied with hydrogen via an anode loop (121, 221), respectively. It is characterized in that when the fuel cell system (1) is shut down, the exhaust gas of the first fuel cell stack (100) is introduced into the anode loop (221) of another fuel cell stack (200), and with the help of the introduced exhaust gas, the anode (220) of the other fuel cell stack (200) is inerted in the first stage of the shutdown process and dried in the second stage of the shutdown process.

2. The method according to claim 1, It is characterized in that In the first stage, the first fuel cell stack (100) operates in a lean-burn operation mode.

3. The method according to claim 1 or 2, It is characterized in that In the first phase, the air supply to the further fuel cell stack (200) is interrupted by switching off an air delivery and air compression system (213) integrated into the air supply path (211) and / or by closing at least one valve (217), in particular a shut-off valve.

4. The method according to any one of the preceding claims, It is characterized in that In the first phase, a pressure regulator (225) integrated into the anode circuit (221) of the further fuel cell stack (200) is closed.

5. The method according to any one of the preceding claims, It is characterized in that In the first stage, a shutoff valve (3) arranged in a connecting line (2) is opened to introduce the exhaust gas of the first fuel cell stack (100) into the anode circuit (221) of the further fuel cell stack (200).

6. The method according to any one of the preceding claims, It is characterized in that In the first phase, a purge valve (222) and / or a drain valve (228) integrated into the anode circuit (221) of the further fuel cell stack (200) is opened.

7. The method according to any one of the preceding claims, It is characterized in that In the transition from the first stage to the second stage, the lean-burn operation of the first fuel cell stack (100) is terminated and normal operation is adopted.

8. The method according to any one of the preceding claims, It is characterized in that In the second stage, a bypass path (118) bypassing the first fuel cell stack (100) is opened by opening a bypass valve (119).

9. The method according to any one of the preceding claims, It is characterized in that In the second phase, in addition to the anode (220), the cathode (210) of the further fuel cell stack (200) is also dried.

10. The method according to claim 10, It is characterized in that In order to dry the cathode (210) of the further fuel cell stack (200), the air supply which was interrupted in the first phase is reestablished.

11. The method according to any one of the preceding claims, It is characterized in that In the second stage, once the anode (112) of the other fuel cell stack (200) is dried, the connection between the exhaust path (112) of the first fuel cell stack (100) and the anode loop (221) of the other fuel cell stack (200) is interrupted.

12. The method according to any one of the preceding claims, It is characterized in that In the second phase, once the cathode (210) is dry, the air supply to the cathode (210) of the further fuel cell stack (200) is interrupted again.

13. A controller for a fuel cell system (1), the controller being arranged to perform the steps of the method according to any one of the preceding claims.

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