Device and method for operating a fuel cell system

Abstract

Device for operating a fuel cell system (1) with at least one fuel cell (2) which has a cathode compartment (3), with a supply line (6) leading to the cathode compartment (3) for supplying oxidizing agent, and a discharge line (7) leading away from the cathode compartment (3), in which a first shut-off element (12) is arranged, wherein a second shut-off element (13) is arranged in the supply line (6), wherein the second shut-off element (13) is arranged upstream of a compressor (8) connected to the supply line (6), and wherein the first shut-off element (12) is arranged downstream of an expander (11) connected to the discharge line (7).

Description

[0001] The invention relates to a device for operating a fuel cell system with at least one fuel cell, which has a cathode compartment, a supply line leading to the cathode compartment for supplying oxidizing agent, and a discharge line leading away from the cathode compartment in which a first shut-off element is arranged. The invention further relates to a method for operating a fuel cell system.

[0002] One of the main degradation mechanisms for the fuel cell stack is caused by high cell potentials. A threshold voltage of approximately 850 mV is considered to be the limit for this. This degradation effect is intensified by the increased air stoichiometry at low electrical power levels, which leads to a significant reduction in the inlet humidity at the fuel cell stack.

[0003] German patent application DE 10 2004 057 140 A1 discloses a method for operating a fuel cell stack in which age-related degradation is prevented by diluting the oxidizing agent, at least temporarily, so that the voltage delivered by each individual cell remains below a critical voltage value. Furthermore, the cathode gas flowing from the cathode compartment is recirculated. Additionally, a valve is arranged in the discharge line leading away from the cathode compartment, through which the dilution of the oxidizing agent can be regulated.

[0004] By recirculating the cathode exhaust gas, high cell voltages caused by oxygen depletion can also be avoided. Furthermore, this can increase the inlet humidity at the fuel cell stack and also enable the recirculation of moist exhaust gas.

[0005] Another major degradation mechanism for the fuel cell stack is caused by the presence of gas mixtures of oxygen and hydrogen in the fuel cell's anode compartment, particularly during fuel cell start-up. During system downtime, the oxygen diffuses across the membrane into the anode compartment, while the hydrogen is introduced during start-up.

[0006] EP 1 665 427 B1 discloses a hydrogen passivation shutdown system for a fuel cell power plant. An anode flow path is in fluid communication with an anode catalyst to direct hydrogen fuel so that it flows alongside the anode catalyst, and a cathode flow path is in fluid communication with a cathode catalyst to direct an oxidizer so that it flows alongside the cathode catalyst of a fuel cell. Hydrogen fuel can be transferred between the anode and cathode flow paths. A hydrogen storage system is attached to the anode flow path to absorb and store hydrogen during fuel cell operation and to release the hydrogen into the fuel cell each time it is shut down.

[0007] Further devices and methods for operating fuel cell systems to avoid degradation are described in US 2007 / 0128474 A1, US 2007 / 0087233 A1, US 4 820 594 A, DE 60 2004 000 440 T2 and US 2003 / 0219636 A1.

[0008] The object of the present invention is to provide a device and a method for operating a fuel cell system in which the aforementioned degradation mechanisms can be at least substantially reduced.

[0009] This problem is solved by a device having the features of claim 1 and a method having the features of claim 7.

[0010] An inventive device for operating a fuel cell system with at least one fuel cell having a cathode compartment comprises a supply line leading to the cathode compartment for supplying oxidizing agent to the cathode compartment. Furthermore, the device comprises a discharge line leading away from the cathode compartment of the fuel cell, in which a first shut-off element is arranged. A second shut-off element is arranged in the supply line. This design of the device makes it possible to at least reduce the aforementioned degradation mechanisms, since the entire system can be shut off from the environment as needed.

[0011] Preferably, both shut-off elements are closed in a specific operating state of the fuel cell system. Preferably, both shut-off elements are closed after the fuel cell system is switched off. In particular, the two shut-off elements and their operation in the aforementioned specific operating state of the fuel cell system enable the oxidizer present in the fuel cell system or in a subsystem to be consumed when the fuel cell system is switched off, and the system or subsystem to be hermetically sealed.

[0012] This can be reliably achieved by placing the two shut-off elements virtually at the system input and the system output.

[0013] A shut-off element can be, for example, a valve, a flap, or any other throttling element.

[0014] The second shut-off element is located upstream of a compressor connected to the supply line. The first shut-off element is located downstream of an expander connected to the discharge line.

[0015] Preferably, the device comprises a recirculation device for returning the cathode gas, wherein the recirculation device has a return line extending between the supply line and the discharge line, in which a flow resistance element is arranged. This recirculation device returns a portion of the cathode gas to the cathode compartment. This also contributes positively to reducing undesirable cell potentials, thereby increasing the service life of the fuel cell system. Furthermore, this return of the cathode gas can increase the inlet humidity at the fuel cell inlet.

[0016] Preferably, the return line branches off from the discharge line upstream of the first shut-off element and opens into the supply line downstream of the second shut-off element.

[0017] It can also be provided that the return line branches off directly from the discharge line at a point where a first shut-off element designed as a 3-way element is arranged. Preferably, the 3-way element is designed as a 3-way valve.

[0018] Preferably, a flow resistance element designed as a recirculation valve is arranged in the return line.

[0019] The second shut-off element, located in the supply line, is preferably designed as an oxidant control valve and system inlet valve. The first shut-off element, located in the discharge line, can be designed as a system outlet valve and, if a recirculation device is present, additionally as a recirculation valve.

[0020] However, it can also be provided that the first shut-off element is designed as a system outlet flap and exhaust gas control valve, and that a recirculation valve is arranged separately from this first shut-off element in the return line of the return device.

[0021] In a method according to the invention for operating a fuel cell system with at least one fuel cell, an oxidizing agent can be supplied to the cathode compartment of the fuel cell via a supply line. A cathode gas can be discharged from the cathode compartment via a discharge line, wherein the supply of oxidizing agent to the cathode compartment can be metered by a second shut-off element arranged in the supply line, and the supply line can be shut off by the second shut-off element. Depending on a specific operating state of the fuel cell system, the supply line is closed by the second shut-off element and the discharge line by a first shut-off element arranged in the discharge line. This approach can at least significantly reduce the aforementioned main degradation mechanisms, thereby increasing the service life of the fuel cell system.

[0022] The second shut-off element is arranged upstream of a compressor connected to the supply line, and the first shut-off element is arranged downstream of an expander connected to the discharge line.

[0023] Preferably, the two shut-off elements are closed when the fuel cell system is switched off, before the remaining portion of the oxidizing agent in the fuel cell system is consumed and the fuel cell system is sealed gas-tight.

[0024] Exemplary embodiments of the invention are explained in more detail below with reference to schematic drawings. These show: Fig. 1 a first embodiment of a device according to the invention; Fig. 2 a second embodiment of a device according to the invention; Fig. 3 a third embodiment of a device according to the invention; and Fig. 4 a fourth embodiment of a device according to the invention.

[0025] In the figures, identical or functionally equivalent elements are given the same reference symbols.

[0026] In Fig. Figure 1 shows a schematic representation of a fuel cell system 1, in which only the components necessary for understanding the invention are shown.

[0027] The fuel cell system 1 comprises a fuel cell 2 with a cathode compartment 3 and an anode compartment 4, which are separated by a membrane 5. In the exemplary embodiment, the fuel cell 2 is designed as a PEM fuel cell, although this specific design is not to be understood as limiting the invention.

[0028] Preferably, the fuel cell system 1 comprises a fuel cell stack with a plurality of fuel cells 2.

[0029] Furthermore, in the exemplary embodiment, the fuel cell system 1 is designed as a mobile fuel cell system, which can in particular be arranged in a vehicle.

[0030] Furthermore, the fuel cell system 1 includes a feed branch with a feed line 6, through which oxidizing agent can be supplied to the cathode compartment 3. Oxygen and, in particular, air can be used as the oxidizing agent.

[0031] The supply line 6 leads into the cathode chamber 3, from which a discharge line 7 leads away. Cathode gas is discharged via this discharge line 7.

[0032] A compressor 8 is arranged in the supply line 6 and is driven by a motor 9 via a shaft 10. An expander 11, for example a turbine, is also mounted on the shaft 10 and is connected to the discharge line 7. This expander is also driven by the motor 9 and the shaft 10.

[0033] A first shut-off element 12 is arranged downstream in the discharge line 7 and thus in the direction of flow of the cathode gas in the discharge line 7 after the expander 11.

[0034] A second shut-off element 13 is arranged in the supply line 6, which in the embodiment shown is arranged upstream and thus in the flow direction of the oxidizing agent in the supply line 6 upstream of the compressor 8.

[0035] The two shut-off elements 12 and 13 are intended to seal off the fuel cell system 1 to the outside.

[0036] In the Fig. In the embodiment shown in Figure 1, the fuel cell system 1 further comprises a recirculation device 14, via which the cathode gas formed in the cathode chamber 3 can be returned to the cathode chamber 3 via the discharge line 7, a return line 15 associated with the recirculation device 14 and the supply line 6.

[0037] The return line 15 extends between the discharge line 7 and the supply line 6, branching off from the discharge line 7 at branch 16 and thus downstream of the expander 11. The return line 15 joins the supply line 6 at the junction 17, which is located between the second shut-off element 13 and the compressor 8.

[0038] In the embodiment shown, the second shut-off element 13 is designed as an oxidant control valve and as a system inlet flap.

[0039] The first shut-off element 12 is designed as a three-way valve and is specifically intended as a recirculation valve and system outlet flap. The return line 15 thus branches off directly from the second shut-off element 13.

[0040] In Fig. 2 shows a further embodiment in which, in contrast to the embodiment according to Fig. 1. The first shut-off element 12 is not designed as a three-way valve. Instead, it is designed as an exhaust gas control valve and system outlet flap. A separately designed flow resistance element 18, which in this embodiment is a recirculation valve, is arranged in the return line 15. In this embodiment, the return line 15 branches off from the discharge line 7 upstream of the first shut-off element 12 and downstream of the expander 11.

[0041] In Fig. Figure 3 shows a further embodiment in which, in contrast to the embodiment according to Fig. 1. The first shut-off element 12 is arranged between the expander 11 and the cathode chamber 3 in the discharge line 7. Otherwise, the shut-off element 12 is analogous to the shut-off element in Fig. 1 designed.

[0042] In Fig. Figure 4 shows a fourth embodiment in which the first shut-off element 12 is analogous to the shut-off element 12 in Fig. 2 is formed. Furthermore, the return line 15 branches off from the discharge line 7 at the branch 16 between the expander 11 and the cathode chamber 3. In the return line 15, a flow resistance element 18 is again formed according to the embodiment in Fig. 2 arranged.

[0043] Through the in the Fig. In the embodiments of a device for operating a fuel cell system shown in Figures 1 to 4, the reduction of the aforementioned main degradation mechanisms can be achieved. The high cell potentials caused by these main degradation mechanisms can thereby be at least substantially reduced. For this purpose, the two shut-off elements 12 and 13 are provided, located at the system inlet and outlet, respectively. These shut-off elements are closed when the fuel cell system 1 is switched off, before the residual oxygen in the fuel cell system 1 is consumed and the fuel cell system 1 is sealed gas-tight.

[0044] According to the explanations in Fig. 1 and Fig. 2 For cathode recirculation, a portion of the cathode gas is routed from the outlet of the expander 11 to the inlet of the compressor 8. The first shut-off element 12 regulates the amount of recirculated cathode gas, and in combination with the second shut-off element 13, which regulates the amount of oxidizing agent supplied, a corresponding application is provided.

[0045] In the explanations according to Fig. 3 and Fig. 4 A portion of the cathode gas is directed from the inlet of the expander 11 to the inlet of the compressor 8.

[0046] In particular, in the explanations according to Fig. 1 and Fig. 2. An arrangement without the expander 11 can also be provided. If the fuel cell system 1 is switched off or shut down, after depletion of the oxidizer by cathode recirculation in the arrangement according to the descriptions in Fig. 1 and Fig. 3, the second shut-off element 13 closed and the first shut-off element 12 placed in the recirculation position, so that the fuel cell system 1 is shut down with low oxygen levels and the subsequent diffusion of oxygen is made difficult or prevented.

[0047] In the explanations according to Fig. 2 and Fig. 4 This is done by closing the two shut-off elements 12 and 13 and at least partially opening, if this has not already been done, the flow resistance element 18. Reference symbol list 1 Fuel cell system 2 Fuel cell 3 Cathode space 4 anode compartment 5 Membran 6 Supply line 7 Drain line 8 compressors 9 engine 10 wave 11 Expanders 12, 13 barrier elements 14 Recirculation device 15 Return line 16 Junction 17 Junction 18 Flow resistance element

Claims

[1] Device for operating a fuel cell system (1) with at least one fuel cell (2) which has a cathode compartment (3), with a supply line (6) leading to the cathode compartment (3) for supplying oxidizing agent, and a discharge line (7) leading away from the cathode compartment (3) in which a first shut-off element (12) is arranged, wherein a second shut-off element (13) is arranged in the supply line (6), wherein the second shut-off element (13) is arranged upstream of a compressor (8) connected to the supply line (6), and wherein the first shut-off element (12) is arranged downstream of an expander (11) connected to the discharge line (7). [2] Device according to claim 1, characterized by , that both shut-off elements (12, 13) are closed in a specific operating state of the fuel cell system (1). [3] Device according to claim 2, characterized by, that both shut-off elements (12, 13) are closed after the fuel cell system (1) is switched off. [4] Device according to any one of the preceding claims, characterized by a recirculation device (14) for returning the cathode gas, which has a return line (15) extending between the supply line (6) and the discharge line (7), in which a flow resistance element (18) is arranged. [5] Device according to claim 4, characterized by , that the return line (15) branches off upstream of the first shut-off element (12) from the discharge line (7) and flows downstream of the second shut-off element (13) into the supply line (6). [6] Device according to any one of the preceding claims 1 to 3, characterized bya recirculation device (14) for returning the cathode gas, which has a return line (15) extending between the supply line (6) and the discharge line (7), and at the branch (16) of the return line (15) from the discharge line (7) a first shut-off element (12) designed as a 3-way element, in particular a 3-way valve (13), is arranged. [7] Method for operating a fuel cell system (1) with at least one fuel cell (2), in which an oxidizing agent can be supplied to the cathode compartment (3) of the fuel cell (2) via a supply line (6), and a cathode gas can be discharged from the cathode compartment (3) via a discharge line (7), wherein the cathode gas discharge can be metered by a first shut-off element (12) arranged in the discharge line (6), wherein, depending on a specific operating state of the fuel cell system (1), the supply line (6) is closed by a second shut-off element (13) and the discharge line (7) is closed by the first shut-off element (12), wherein the second shut-off element (13) is arranged upstream of a compressor (8) connected to the supply line (6), and wherein the first shut-off element (12) is arranged downstream of an expander (11) connected to the discharge line (7). [8] Method according to claim 7, characterized by, that the two shut-off elements (12, 13) are closed when the fuel cell system (1) is switched off, before the remaining portion of the oxidizing agent in the fuel cell system (1) is consumed and the fuel cell system (1) is sealed gas-tight.

Images