Method for operating a fuel cell system, and fuel cell system

Abstract

The invention relates to a method for operating a fuel cell system (100) comprising at least one fuel cell stack (11) and a cathode system (300), wherein the cathode system (300) is equipped with a cathode supply line (31), a cathode outlet line (32), and a cathode recirculation line (34) in order to conduct cathode exhaust gas out of the cathode outlet line (32) and into the cathode supply line (31). The cathode system is also equipped with a cathode path (K) which is connected to the cathode supply line (31) and to the cathode outlet line (32) and which is provided within the fuel cell stack (11) so that a minimum flow rate of a gas mixture is provided for the cathode path (K) in order to prevent and / or remove a buildup of liquid water in the cathode path (K).

Description

[0001] Description

[0002] title

[0003] Method for operating a fuel cell system; fuel cell system

[0004] 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.

[0005] State of the art

[0006] 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 to a fuel cell stack, and an anode recirculation line that recirculates anode exhaust gas to the anode supply line via a feed unit.

[0007] The cathode system consists of a cathode supply line, which can house a compressor and supply air to the fuel cell stack. Furthermore, the cathode system consists of a cathode outlet line through which cathode exhaust gas is transported from the cathode system into the exhaust system.

[0008] Disclosure of the invention

[0009] The inventive method for operating a fuel cell system with the features according to independent claim 1 has the advantage that accumulations of liquid water in a cathode path can be reliably prevented. The cathode path is located in the fuel cell stack and is arranged between the cathode supply line and the cathode outlet line. To reliably prevent accumulations of liquid water, a minimum flow rate of a gas mixture for the cathode path is determined.

[0010] If the current flow rate of the gas mixture in the cathode path corresponds to the minimum flow rate, this advantageously ensures that the fuel cell system has a sufficient flow rate of gas mixture during operation. This prevents accumulation of liquid water in the cathode path because the flow rate of gas mixture in the cathode path is sufficient to remove the water from the cathode path.

[0011] It is advantageous if the current flow rate of the gas mixture in the cathode path is specifically adjusted from a cathode reactant and / or a portion of the cathode exhaust gas. This can improve the efficiency of the fuel cell system. The efficiency of the fuel cell system is also improved if the delivery rate of a first delivery unit, which is located in the cathode supply line and delivers air into the cathode path, is reduced. This reduces the power requirement of the first delivery unit, which improves the efficiency of the fuel cell system.

[0012] It is advantageous to determine the current flow rate of the cathode reactant and compare the minimum flow rate of the gas mixture with the current flow rate of the cathode reactant. This allows you to determine whether the current flow rate of the cathode reactant already covers the minimum flow rate of the gas mixture.

[0013] If the current flow of the cathode reactant corresponds to the minimum flow, a means for interrupting the flow in the cathode recirculation line is advantageously activated. The means for interrupting the flow in the cathode recirculation line can reduce or interrupt the flow so that no flow passes through the cathode recirculation line. This allows the method according to the invention to be carried out more efficiently, since the flow is adjusted efficiently.

[0014] It is advantageous if the current flow of the cathode reactant is determined and the minimum flow of the gas mixture is compared with the flow of the cathode reactant and if the minimum flow is higher than the current flow of the cathode reactant, additional cathode exhaust gas is fed into the cathode path.

[0015] It is advantageous if the current flow of the cathode reactant in the cathode path depends on the power requirements of the fuel cell system. This allows a stoichiometric supply of the cathode reactant to be maintained in the fuel cell system.

[0016] If the current flow of the cathode reactant in the cathode path K is adapted to the power requirement of the fuel cell system 100, the required amount of cathode reactant can be supplied to the fuel cell system 100 efficiently and in a targeted manner.

[0017] It is advantageous to add a portion of the cathode exhaust gas to the gas mixture to adjust the current flow rate of the gas mixture in the cathode path to the minimum flow rate. This allows a minimum flow rate to be set in the cathode path while simultaneously supplying an optimal amount of cathode reactant to the fuel cell system.

[0018] The proportion of cathode exhaust gas in the gas mixture can advantageously be adjusted via a switching position of a first valve arranged in the cathode recirculation line and / or a second valve arranged in the cathode recirculation line. This represents an efficient way to adjust the proportion of cathode exhaust gas in the gas mixture.

[0019] By using a first valve and a second valve, it can be avoided that, in the event of a high pressure gradient within the cathode system, which occurs in particular at a high load point of the fuel cell system, cathode exhaust gas flows through the first valve when the first valve has a closed switching position.

[0020] The proportion of cathode exhaust gas in the gas mixture can be advantageously adjusted via the speed of a recirculation conveyor unit located in the cathode recirculation line. The use of a recirculation conveyor unit, which is a component already present in the fuel cell system, ensures a cost-effective and space-saving design.

[0021] It is advantageous if the fuel cell system for implementing the method according to the invention has a second valve arranged in the cathode recirculation line. This allows the recirculation line to be completely fluidically shut off from the cathode supply line and the cathode outlet line. Furthermore, this prevents any leakage paths, for example, via bearing points of the recirculation conveyor unit, and improves the efficiency when operating the fuel cell system without recirculation of cathode exhaust gas.

[0022] The fuel cell system according to the invention can preferably be used for mobile applications, for example in vehicles, especially fuel-powered vehicles. The fuel cell system according to the invention can serve as the main energy supplier for a vehicle. At the same time, however, it is also conceivable that the fuel cell system according to the invention can be a secondary drive and / or auxiliary drive of a vehicle, for example, a hybrid vehicle. The fuel cell system according to the invention can also be used for stationary applications, for example, in generators.

[0023] The fuel cell system according to the invention can comprise one or more stacks, each with a plurality of stacked fuel cells and the associated functional systems, comprising: cathode system, anode system, cooling system and an electrical system.

[0024] Description of the drawings 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 according to a first embodiment and

[0027] Fig. 2 shows a schematic topology of a fuel cell system according to a second embodiment and

[0028] Fig. 3 is a schematic diagram of flow rates in the cathode path and

[0029] Fig. 4 is a flow chart of the method according to the invention.

[0030] Figure 1 shows a first schematic topology of a fuel cell system 100 with at least one fuel cell stack 11, an anode system 200 and a cathode system 300.

[0031] The anode system 200 supplies an anode chamber A of the fuel cell stack 11 with a fuel or anode fluid, in particular hydrogen (H2) as a reactant. The anode system includes an anode supply line 22 and an anode recirculation line 21. Anode exhaust gas from an anode chamber A, which is arranged in the fuel cell stack 11, is recirculated to the anode supply line 22 via the anode recirculation line 21. A jet pump with a metering valve 26 is arranged between the anode supply line 22 and the recirculation line 21.

[0032] The cathode system 300 supplies a cathode path K 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 it as a reactant. A cathode supply line 31, a cathode outlet line 32, a cathode recirculation line 34, and a cathode path K are arranged in the cathode system 300.

[0033] The cathode supply line 31 leads into the fuel cell stack 11. Oxygen is supplied to the fuel cell stack 11 via the cathode supply line 31.

[0034] Gases, such as cathode exhaust gas and / or fluids, such as liquid water, are discharged from the cathode system 300 via the cathode outlet line 32.

[0035] The cathode recirculation line 34 is arranged between and connects the cathode supply line 31 and the cathode outlet line 32. This allows cathode exhaust gas to flow from the cathode outlet line 34 into the cathode supply line 31.

[0036] The cathode path K, which is integrated in the fuel cell stack 11, is arranged between the cathode supply line 31 and the cathode outlet line 32. The cathode path K, the cathode supply line 31, and the cathode outlet line 32 are connected to one another in a flow-through manner.

[0037] A cathode compressor 33 is located within the cathode supply line 31. The cathode compressor 33 is arranged upstream of the cathode recirculation line 34 in the flow direction.

[0038] The cathode compressor 33 pumps air into the fuel cell stack 11. The cathode compressor 33 can be used to vary the air flow rate. Increasing the power of the cathode compressor 33 results in an increased air flow rate being supplied to the fuel cell stack 11 via the cathode supply line 31. Reducing the power of the cathode compressor 33 results in a reduced air flow rate being supplied to the fuel cell stack 11.

[0039] A first valve 35 and / or a recirculation conveying unit 36 ​​and / or a second valve 37 can be arranged within the cathode recirculation line 34. The first valve 35 and / or the second valve 37 can be used to adjust the current flow rate of the cathode exhaust gas through the cathode recirculation line. If the first valve 35 and / or the second valve 37 are in a blocked switching position, no cathode exhaust gas can flow through the cathode recirculation line 34.

[0040] This allows the cathode recirculation line 34 to be completely fluidically sealed off from the cathode supply line 31 and the cathode outlet line 32. Furthermore, this prevents any leakage paths, for example, via bearing points of the recirculation conveying unit 36, and improves the efficiency of operating the fuel cell system 100 without recirculation of cathode exhaust gas.

[0041] During a standstill of the fuel cell system 100, the first valve 35 and the second valve 37 are closed to prevent gas diffusion through any leakage paths. During a standstill of the fuel cell system 100, no energy is supplied by the fuel cell system 100.

[0042] Using the recirculation conveying unit 36, the current flow rate of the cathode exhaust gas flowing from the cathode recirculation line 34 into the cathode supply line 31 can be varied. Increasing the power of the recirculation conveying unit 36 ​​results in an increased volume flow of cathode exhaust gas being supplied to the cathode path K via the cathode recirculation line 34. Reducing the power of the recirculation conveying unit 36 ​​results in a reduced volume flow of cathode exhaust gas being supplied to the cathode path K.

[0043] A control unit 500 is provided to regulate and control all control and regulation processes in the fuel cell system 100.

[0044] This also includes the processing of at least one measurement signal for carrying out the method according to the invention.

[0045] In an alternative embodiment, more than one fuel cell stack 11 can be arranged in the fuel cell system 100, without limiting the implementation of the method according to the invention. The method according to the invention can be implemented in a fuel cell system 100 in multiple fuel cell stacks 11 in parallel or sequentially.

[0046] Figure 2 shows a second schematic topology of a fuel cell system 100 having at least one fuel cell stack 11, an anode system 200 and a cathode system 300. The fuel cell system 100 shown in Figure 2 corresponds to the fuel cell system 100 shown in Figure 1, except for the differences mentioned below.

[0047] The cathode recirculation line 34 opens into the cathode supply line 31 upstream of the cathode compressor 33. The valve 35 is arranged within the cathode recirculation line 34.

[0048] The cathode compressor 33 supports recirculation of the cathode exhaust gas from the cathode outlet line 32 via the cathode recirculation line 34 into the cathode supply line 31.

[0049] In an alternative embodiment, in the second embodiment, the recirculation conveying unit 36 ​​can be arranged in the recirculation line 34 to support the flow of the cathode exhaust gas from the cathode outlet line 32 into the cathode supply line 31.

[0050] Figure 3 shows a highly schematic diagram showing the flow rates in the cathode path K as a function of current. The x-axis shows the generated current in amperes. The y-axis represents the flow rate of the gas mixture in the cathode path K in m / s.

[0051] The curve shown is an example of a flow rate curve in the cathode system, which depends on the flow velocity of the gas mixture in the cathode path K and the generated current. The curve is divided by the dashed line into a stable region and an unstable region. A stable region exists when the flow velocity is sufficient to transport water away from the cathode path K, and thus the current flow rate of the gas mixture corresponds to or exceeds the minimum flow rate. An unstable region exists when the flow velocity is not sufficient to permanently transport water away from the cathode path K, so that accumulations of liquid water can occur (so-called "flooding"). In the unstable region, the current flow rate is below the minimum flow rate.

[0052] The accumulation of liquid water in the fuel cell system 100, particularly in the gas diffusion layers, can lead to a local undersupply of reactants. The gas diffusion layers are arranged between the fuel cells within the fuel cell stack 11. The gas diffusion layers serve to effectively and evenly distribute the reactants hydrogen and oxygen.

[0053] It is possible that a brief drop in the flow velocity of the gas mixture in the cathode path K, for example for a period of 10 s, does not yet lead to flooding.

[0054] With the aid of the method according to the invention, it can be ensured that the fuel cell system is operated in the stable range.

[0055] Figure 4 shows an embodiment of the method according to the invention.

[0056] The method begins in step S100. In step S200, the current flow rate through the cathode path K required to prevent accumulation of liquid water is determined. For this purpose, the minimum flow rate is determined. The minimum flow rate can be calculated based on the operating state of the fuel cell system or derived from a characteristic map.

[0057] In a step S300, the current flow of the cathode reactant into the cathode path K is determined. The current flow of the cathode reactant can be determined via the amount of air conveyed into the cathode supply line 31, for example, via the speed of the cathode compressor 33.

[0058] In step S400, a check is performed to determine whether the current flow rate of the gas mixture in the cathode path K corresponds to the minimum flow rate and is covered by the cathode reactant. For this purpose, the minimum flow rate is compared with the current flow rate of the cathode reactant.

[0059] If the current flow rate of cathode reactant in the cathode path K equals or exceeds the minimum flow rate, step S800 is executed. Step S800 represents the end of the method. The current flow rate of cathode reactant in the cathode path K depends on the power demand of the fuel cell system 100 and is adjusted depending on the power demand.

[0060] The current flow of the cathode reactant in the cathode path K can be adjusted via the cathode feed unit 36.

[0061] Additionally, it is possible to control a means for interrupting the flow in the cathode recirculation line 34 when the flow of cathode reactant equals or exceeds the minimum flow of gas mixture in the cathode path K. The means for interrupting the flow in the cathode recirculation line 34 can be the first valve 35 and / or the second valve 37 in a closed switching position.

[0062] If the current flow of cathode reactant does not correspond to the minimum flow of gas mixture in the cathode path K or is lower than the minimum flow, step S500 is executed. In step S500, the additional amount of gas mixture required to achieve a current flow in the cathode path K that corresponds to the minimum flow or is higher than the minimum flow is determined.

[0063] The current flow can be set to a minimum flow by adjusting the current flow from the cathode reactant and a portion of the cathode exhaust gas. For this purpose, step S600 is executed.

[0064] In a step S600, the first valve 35 and / or the second valve 37 is set to an open switching position, through which the required amount of cathode exhaust gas can be directed via the cathode recirculation line 34 into the cathode supply line 31. When the first valve 35 and / or second valve 37 is partially open, cathode exhaust gas can flow partially from the cathode outlet line 32 via the cathode recirculation line 34 into the cathode supply line 31.

[0065] In the first embodiment, the recirculation conveying unit 36 ​​supports the flow of the cathode exhaust gas into the cathode feed line 31. In the second embodiment, the cathode exhaust gas flows into the cathode feed line 31 due to the pressure gradient generated by the cathode compressor 33, wherein the recirculation conveying unit 36 ​​can optionally support the flow of the cathode exhaust gas.

[0066] In the first embodiment of the proposed fuel cell system 100, the recirculation conveying unit 36 ​​can additionally be activated and the current flow of the cathode exhaust gas from the cathode outlet line 32 into the cathode supply line 31 can be adjusted via the speed of the recirculation conveying unit 36.

[0067] In step S700, a check is made to determine whether an event triggering the end of the method has occurred. An event triggering the end of the method may be a stoppage of the fuel cell system. If no event triggering the end of the method has occurred, step S200 is executed again.

[0068] If an event triggering the termination of the method has occurred, step S800 is executed. In step S800, the method according to the invention is terminated.

[0069] In an alternative embodiment, it is also possible to carry out the method according to the invention with more than one fuel cell stack 11.

[0070] The method according to the invention can be carried out in parallel or sequentially in a fuel cell system 100 in a plurality of fuel cell stacks 11. Furthermore, the method can be carried out at least partially 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 computer program carries out 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.

[0071] The control unit 500 may be in communication with the sensors of the fuel cell system 100 to monitor the sensor values.

[0072] The control unit 500 can control the actuators in the fuel cell system 100 in order to carry out the method accordingly.

[0073] 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.

[0074] 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 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) and a cathode system (300), wherein a cathode feed line (31), a cathode outlet line (32) and a cathode recirculation line (34) for conducting cathode exhaust gas from the cathode outlet line (32) into the cathode feed line (31) are arranged in the cathode system (300), as well as a cathode path (K) which is connected to the cathode feed line (31) and the cathode outlet line (32) and is arranged within the fuel cell stack (11), characterized in that a minimum flow rate of a gas mixture is determined for the cathode path (K) in order to avoid and / or remove accumulations of liquid water in the cathode path (K). 2.) Method according to claim 1, characterized in that the current Flow of the gas mixture in the cathode path (K), which is adjusted from cathode reactant and / or a portion of the cathode exhaust gas, corresponds at least to the minimum flow. 3.) Method according to claim 1 and 2, characterized in that the current flow is determined and compared with the minimum flow and if the current flow of the cathode reactant is higher than the minimum flow, a means for interrupting the flow in the cathode recirculation line (34) is controlled. 4.) Method according to claim 1 and 2, characterized in that the current flow is determined and compared with the minimum flow and if the current flow is lower than the minimum flow, the cathode exhaust gas is additionally passed into the cathode path (K). 5.) Method according to claim 1 and 2, characterized in that the amount of cathode reactant supplied in the cathode path (K) depends on the power requirement of the fuel cell system (100). 6.) Method according to claim 5, characterized in that the supplied The amount of cathode reactant in the cathode path (K) is adjusted via a cathode feed unit (36) arranged in the cathode feed line (31). 7.) Method according to claim 2 and 4, characterized in that a proportion of the cathode exhaust gas is adjusted in the current flow in order to adjust the current flow of the gas mixture in the cathode path (K) at least to the minimum flow. 8.) Method according to claim 2 and 7, characterized in that the proportion of cathode exhaust gas in the current flow is adjusted via a switching position of a first valve (35) arranged in the cathode recirculation line (34) and / or a second valve (37) arranged in the cathode recirculation line (34). 9.) Method according to claim 2 and 8, characterized in that the proportion of cathode exhaust gas in the current flow is adjusted via the speed of a recirculation conveying unit (36) arranged in the cathode recirculation line (34). 10.) Fuel cell system (100) for carrying out the method according to one of the above claims, with at least one fuel cell stack (11) and one cathode system (300), wherein a cathode supply line (31), a cathode outlet line (32) and a cathode recirculation line (34) with a first valve (35) for conducting cathode exhaust gas from the cathode outlet line (32) into the cathode supply line (31) are arranged in the cathode system (300), as well as a cathode path (K) which is connected to the cathode supply line (31) and the cathode outlet line (32) and is arranged within the fuel cell stack (11), characterized in that a second valve (37) is arranged in the cathode recirculation line.

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