Method for operating a multi-stack system, and control device

Abstract

The invention relates to a method for operating a multi-stack system (1) comprising a plurality of stacks (2) each having a cathode (2.1) and an anode (2.2), wherein: the cathodes (2.1) are fed with air by means of a shared air system (3) or one dedicated air system each, and the anodes (2.2) are fed with hydrogen by means of a shared anode circuit (4) or one dedicated anode circuit each; and the waste heat from the stacks (2) is dissipated by means of a shared cooling system (5) having a cooling circuit (6) and having a coolant pump (7) for conveying a coolant, said coolant pump being integrated into the cooling circuit (6), and is released to the surroundings by means of a radiator (8) integrated into the cooling circuit (6). According to the invention, the stacks (2) are dried before the multi-stack system (1) is shut down, wherein: a coolant temperature of 60°C to 70°C is set in a first phase of the drying process and the coolant temperature is lowered in a second phase of the drying process; and the transition from the first phase to the second phase takes place only when predefined drying conditions have been established in all stacks (2). The invention also relates to a control device for a multi-stack system (1) for carrying out steps of a method according to the invention.

Description

[0001] Description

[0002] title

[0003] Method for operating a multi-stack system, control unit

[0004] The invention relates to a method for operating a multi-stack system having the features of the preamble of claim 1. Furthermore, the invention relates to a control device for a multi-stack system for executing steps of the method.

[0005] The preferred area of ​​application is fuel cell vehicles with a multi-stack system for generating drive energy.

[0006] State of the art

[0007] Fuel cells are electrochemical energy converters. Hydrogen (H2) and oxygen (O2) can be used as reaction gases. These are converted into electrical energy, water (H2O), and heat using a fuel cell. The core of a fuel cell is a membrane-electrode assembly (MEA), which comprises a membrane coated on both sides with a catalytic material to form electrodes. During operation, the fuel cell supplies hydrogen to one electrode, the anode, and oxygen to the other, the cathode.

[0008] To increase electrical power, a large number of fuel cells are connected to form a fuel cell stack. Furthermore, several fuel cell stacks can be interconnected to form so-called multi-stack systems.

[0009] The heat generated during operation of a stack is dissipated via a cooling circuit and released into the environment via a main cooler. Regardless of the number of stacks, a cooling circuit with a coolant pump is usually available for this purpose. A directional control valve can be integrated into the cooling circuit to bypass the main cooler. Any additional water generated can be separated using a water separator and collected in a separate container, which is then emptied periodically.

[0010] Since the accumulating water cannot be completely removed, there is a risk of icing impairing the system during a cold or frozen start. To prevent this, the system is typically dried before shutdown, with air supplied to the cathode side and hydrogen to the anode side. Since water vapor that is not removed during drying can condense and then freeze after shutdown, the gas temperature is typically lowered during drying to promote condensation. Lowering the gas temperature can be achieved by lowering the coolant temperature.

[0011] In a multi-stack system with one cooling circuit and one coolant pump, individual stack control of the coolant temperature is not possible, making effective drying of the stacks difficult to implement.

[0012] The present invention therefore addresses the problem of optimizing the drying of stacks required when a multi-stack system is shut down. This should ultimately improve the freeze-start capability of the multi-stack system.

[0013] To achieve this objective, the method having the features of claim 1 is proposed. Advantageous embodiments are set forth in the subclaims. Furthermore, a control unit for a multi-stack system for executing steps of the method is specified.

[0014] Disclosure of the invention

[0015] A method is proposed for operating a multi-stack system comprising several stacks, each with a cathode and an anode. The cathodes are supplied with air via a common or separate air system, and the anodes are supplied with hydrogen via a common or separate anode circuit. The waste heat from the stacks is dissipated via a common cooling system with a cooling circuit and a coolant pump integrated into the cooling circuit for pumping a coolant, and is released to the environment via a radiator integrated into the cooling circuit.According to the invention, the stacks are dried before the multi-stack system is shut down, wherein in a first drying phase a coolant temperature of 60°C to 70°C is set and in a second drying phase the coolant temperature is reduced and wherein the transition from the first phase to the second phase only takes place when predefined drying conditions are met by all stacks.

[0016] In the proposed process, drying is carried out in two phases, which differ in that a different coolant temperature is set in each phase. In the second phase, the coolant temperature is lower than the coolant temperature in the first phase to promote the condensation of water vapor. However, beforehand, all stacks must meet predefined drying conditions, i.e., be "dried." Only when all stacks meet these conditions does the second drying phase begin. This procedure ensures effective drying of all stacks, minimizing the risk of icing during the subsequent shutdown phase. Accordingly, the freeze-start capability of the multi-stack system increases.

[0017] The proposed method thus enables synchronized drying of several stacks in a multi-stack system with only one cooling circuit and only one coolant pump.

[0018] In the second drying phase, the coolant temperature is advantageously lowered to between 10°C and 20°C. A corresponding coolant temperature leads to gas temperatures in the stacks that favor the condensation of water vapor.

[0019] To adjust the coolant temperature, a directional control valve integrated into the cooling circuit is preferably operated, via which a radiator bypass is connected to the cooling circuit. By operating the directional control valve, the radiator bypass can be opened so that the coolant is bypassed past the radiator. This means that the heat absorbed by the coolant in the stacks cannot be dissipated to the environment via the radiator, so that the coolant temperature does not drop. By operating the directional control valve, the radiator bypass can also be closed so that the coolant is fed to the radiator and the heat absorbed in the stacks can be dissipated to the environment via the radiator. As a result, the coolant temperature also drops.

[0020] Therefore, it is preferable to open the radiator bypass via the directional control valve during the first drying phase and close the radiator bypass via the directional control valve during the second drying phase. Closing the radiator bypass then leads to a reduction in the coolant temperature during the second drying phase.

[0021] Furthermore, it is proposed that the coolant quantity in the cooling circuit be distributed among the multiple stacks via a directional control valve integrated into the cooling circuit. The directional control valve allows the coolant quantity supplied to each stack to be individually adjusted for each stack, thus further optimizing the synchronization of drying across all stacks.

[0022] Furthermore, a control unit for a multi-stack system is proposed, wherein the control unit is configured to execute steps of a method according to the invention. For example, the control unit can actuate the at least one directional control valve integrated into the cooling circuit, so that the radiator bypass is opened or closed and / or the coolant quantity is distributed among the individual stacks according to the control unit's instructions.

[0023] The invention and its advantages are explained in more detail below with reference to the accompanying drawings. These show:

[0024] Figure 1 is a schematic representation of a multi-stack system that can be operated according to a method according to the invention,

[0025] Figure 2 is a schematic representation of the multi-stack system of Figure 1 showing the coolant flow in a cooling circuit of the system during a first drying phase, Figure 3 is a schematic representation of the multi-stack system of Figure 1 showing the coolant flow in the cooling circuit during a second drying phase and

[0026] Figure 4 is a flow chart illustrating a preferred sequence of a method according to the invention.

[0027] Detailed description of the drawings

[0028] Figure 1 shows a multi-stack system 1 with two stacks 2. Each stack 2 has a cathode 2.1 and an anode 2.2.

[0029] The cathodes 2.1 are supplied with air via a common air system 3. The air is taken from the ambient air and fed via an air supply path 12, first to an air filter 13, and then to an air compressor 14. Downstream of the air compressor 14, a cooler 15 and / or a humidifier 16 can be integrated to condition the air before it enters a stack 2. Downstream of the humidifier 16, the air supply path 12 branches off to connect both stacks 2 to the air system 3. Each stack 2 can be separately separated from the air system 3 via shut-off valves 17. Furthermore, a stack bypass 18 with an integrated bypass valve 19 is provided to bypass the stacks 2. The air escaping from the cathodes 2.1 of the stacks 2 is discharged via an exhaust air path 20 of the air system 3.

[0030] The anodes 2.2 are each supplied with hydrogen via an anode circuit 4. Hydrogen escaping from the anodes 2.2 is recirculated via the anode circuit 4, with recirculation being achieved passively by means of a jet pump 21 and actively by means of a blower 22. Since recirculated hydrogen becomes enriched with nitrogen, the anode circuit 4 is purged from time to time by opening a valve 23, the so-called purge valve. Fresh hydrogen is added via a hydrogen metering valve 24, which also drives the jet pump 21. Furthermore, a water separator 25 with a container 26 is integrated into each anode circuit 4. Separating water is collected in the container 26. A valve 27, the so-called drain valve, is provided for emptying the container 26.Both the purge valve and the drain valve are connected to the exhaust air path 20 of the air system 3 via a connecting line 28, so that the gases discharged via these lines are mixed with the air in the exhaust air path 20 before being released into the environment. Furthermore, each anode circuit 4 can be connected to the supply air path 12 of the air system 3 via a further connecting line 29 and a valve 30 arranged therein.

[0031] The multi-stack system 1 shown in Figure 1 also has a cooling system 5 with a cooling circuit 6, in which a coolant is circulated by means of a coolant pump 7. The coolant is passed through the stacks 2 to dissipate the heat generated during operation. The coolant is distributed among the stacks 2 via a directional control valve 11 integrated into the cooling circuit 6. The heat absorbed by the coolant in the stacks 2 can be dissipated to the environment via a radiator 8 integrated into the cooling circuit 6. A radiator bypass 10, connected via a directional control valve 9, is provided to bypass the radiator 8.

[0032] If the multi-stack system 1 is to be shut down, the stacks 2 are first dried. Drying occurs in two phases, which are explained below with reference to Figures 2 to 4. Figure 2 uses arrows to indicate the coolant flow in the cooling circuit 6 during the first drying phase. Figure 3, also using arrows, shows the coolant flow in the cooling circuit 6 during the second drying phase. The sequence of a method according to the invention is described by way of example with reference to Figure 4.

[0033] In step S1 of Figure 4, the first drying phase of the stacks 2 is initiated before the multi-stack system 1 is shut down. In step S2, the directional control valve 9 is first actuated so that the coolant is guided past the radiator 8 via the radiator bypass 10 (see arrows in Figure 2). In this way, the coolant temperature in the cooling circuit 6 is set to approximately 65°C. In step S3, the drying of the stacks 2 can then be carried out. In step S4, a check is carried out to determine whether the first stack 2 meets predefined drying conditions. If this is the case, a check is carried out in step S5 to determine whether the second stack 2 also meets the predefined drying conditions. Only when both checks result in a positive result does the transition from the first to the second drying phase take place. For this purpose, the directional control valve 9 is actuated in step S6, so that the radiator bypass 10 is closed.The coolant is then fed to radiator 8 (see arrows in Figure 3). The coolant can dissipate the heat absorbed in the stacks 2 to the environment via radiator 8, so that the coolant temperature drops, for example, to 15°C. This causes the water vapor contained in the gases to condense and can be removed from the system as liquid water in step S7. Once all liquid water has been removed from the system, the process can be terminated in step S8.

Claims

Claims 1. A method for operating a multi-stack system (1) comprising a plurality of stacks (2), each having a cathode (2.1) and an anode (2.2), wherein the cathodes (2.1) are supplied with air via a common or individual air system (3) and the anodes (2.2) hydrogen is supplied via a common or each own anode circuit (4), wherein the waste heat of the stacks (2) is dissipated via a common cooling system (5) with a cooling circuit (6) and a coolant pump (7) integrated into the cooling circuit (6) for conveying a coolant and is released to the environment via a radiator (8) integrated into the cooling circuit (6), characterized in that before the multi-stack system (1) is switched off, the stacks (2) are dried, wherein in a first drying phase a coolant temperature of 60°C to 70°C is set and in a second drying phase the coolant temperature is reduced and wherein the transition from the first phase to the second phase only takes place when previously defined drying conditions are met by all stacks (2).

2. Method according to claim 1, characterized in that in the second phase of drying the coolant temperature is reduced to 10°C to 20°C.

3. Method according to claim 1 or 2, characterized in that, in order to adjust the coolant temperature, a directional control valve (9) integrated into the cooling circuit (5) is actuated, via which a radiator bypass (10) is connected to the cooling circuit (5).

4. Method according to claim 3, characterized in that in the first phase of drying the radiator bypass (10) is opened via the directional control valve (9) and in the second phase of drying the radiator bypass (10) is closed via the directional control valve (9).

5. Method according to one of the preceding claims, characterized in that the coolant quantity in the cooling circuit (6) is distributed to the plurality of stacks (2) via a directional control valve (11) integrated in the cooling circuit (6).

6. Control device for a multi-stack system (1), wherein the control device is configured to carry out steps of a method according to one of the preceding claims.

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