FUEL CELL SYSTEM AND METHOD FOR OPERATING A FUEL CELL SYSTEM

Description

[0001] The invention relates to a fuel cell system with the features of the preamble of claim 1. Furthermore, the invention relates to a method for operating a fuel cell system. State of the art

[0002] Fuel cells, for example, several fuel cells connected to form a fuel cell stack in a fuel cell system, require a) a fuel, usually hydrogen, which is supplied to an anode of the fuel cell stack via an anode gas path, and b) oxygen, which is supplied to a cathode of the fuel cell stack via a cathode gas path, for energy generation. Air, typically drawn from the environment, usually serves as the oxygen source. Since the energy conversion process requires a certain air mass flow rate and pressure level, the air supplied to the cathode is first compressed using an air compressor located in the cathode gas path.

[0003] German patent DE102004022312A1 discloses a moisture exchange module for humidifying air supplied to the cathode. This protects the membrane from drying out and thus from damage or premature aging.

[0004] Fuel cell systems such as those shown in DE 102004022312 A1 have the disadvantage that the humidification device arranged in the cathode gas path requires a considerable amount of installation space and is also expensive to purchase. The present invention therefore aims to provide a fuel cell system that, in contrast, has a simpler design, in particular does not require a humidification device, thus saving installation space and costs.

[0005] To solve the problem, the fuel cell system with the features of claim 1 and the method with the features of claim 10 are proposed. Advantageous embodiments of the invention can be found in the respective dependent claims. Disclosure of the invention

[0006] The proposed fuel cell system comprises a fuel cell stack with a cathode to which air can be supplied as cathode gas via a cathode gas path. An air compressor is integrated into the cathode gas path. According to the invention, the cathode gas path branches downstream of the air compressor into a main path connectable to an inlet of the fuel cell stack and a secondary path connectable to an outlet of the fuel cell stack, wherein the main path and the secondary path can each be shut off individually or together by means of a shut-off device. Thus, compressed air can be selectively supplied to either the inlet or the outlet of the fuel cell stack. In addition, the air supply can be completely shut off when the system is switched off.

[0007] To ensure the cathode receives sufficient oxygen or air during normal operation of the fuel cell system, the air compressed by the air compressor is fed to the inlet of the fuel cell stack via the main path of the cathode gas path. The secondary path of the cathode gas path is closed by the shut-off device, and the air supplied to the cathode is routed through the fuel cell stack as usual. Alternatively, by closing the main path and opening the secondary path, the air compressed by the air compressor can be directed to the outlet of the fuel cell stack. The compressed air then enters the fuel cell stack via the outlet and exits via the inlet. This means that the fuel cell stack is traversed in the reverse direction.The air carries product water with it, so that the membranes of the fuel cell system can be heated with the help of the carried product water. In contrast, the fuel cell system is simpler in design, in particular it does not require a humidification device, so that space and costs can be saved.

[0008] To solve the problem, the fuel cell system with the features of claim 1 and the method with the features of claim 10 are proposed. Advantageous embodiments of the invention can be found in the respective dependent claims. Disclosure of the invention

[0009] The proposed fuel cell system comprises a fuel cell stack with a cathode to which air can be supplied as cathode gas via a cathode gas path. An air compressor is integrated into the cathode gas path. According to the invention, the cathode gas path branches downstream of the air compressor into a main path connectable to an inlet of the fuel cell stack and a secondary path connectable to an outlet of the fuel cell stack, wherein the main path and the secondary path can each be shut off individually or together by means of a shut-off device. Furthermore, the shut-off device comprises at least two additional movable shut-off elements by means of which the inlet and outlet of the fuel cell stack can be shut off. The air supply to the cathode of the fuel cell stack can also be shut off by means of these additional movable shut-off elements. They thus increase safety in the event of a shutdown.

[0010] Thus, compressed air can be selectively supplied to either the inlet or the outlet of the fuel cell stack. Furthermore, the air supply can be completely shut off when the system is shut down.

[0011] To ensure the cathode receives sufficient oxygen or air during normal operation of the fuel cell system, the air compressed by the air compressor is fed to the inlet of the fuel cell stack via the main path of the cathode gas path. The secondary path of the cathode gas path is closed by the shut-off device, and the air supplied to the cathode is routed through the fuel cell stack as usual. Alternatively, by closing the main path and opening the secondary path, the air compressed by the air compressor can be directed to the outlet of the fuel cell stack. The compressed air then enters the fuel cell stack via the outlet and exits again via the inlet. This means that the fuel cell stack is traversed in the reverse direction. The air carries product water with it, which is used to humidify the membranes of the fuel cells within the fuel cell stack.A humidification device in the cathode gas path can therefore be omitted.

[0012] Switching between the main and secondary paths can be controlled, for example, based on time and / or the water content of the cathode exhaust gas. Furthermore, operating modes are possible in which both the main and secondary paths remain open, preventing the air compressor from working against the shut-off device. The flow direction through the fuel cell stack can be determined by the open flow cross-section. Additionally, in the event of shutdown, both the main and secondary paths can be closed using the shut-off device, ensuring that no more air is supplied to the cathode. This eliminates the need for the normally required shut-off valves, resulting in further savings in terms of installation space and costs.

[0013] In a further development of the invention, it is proposed that the main path and / or the secondary path of the cathode gas path can be connected to a cathode exhaust path by means of the blocking device. The previously compressed air can be easily fed to the outlet of the fuel cell stack via the connection of the secondary path to the cathode exhaust path. For this purpose, the secondary path opens into the cathode exhaust path. When the flow direction through the fuel cell stack is reversed, the air exiting the fuel cell stack via its inlet can be introduced into the cathode exhaust path via the connection of the main path to the cathode exhaust path. Therefore, preferably, the main path and the secondary path are always connected to the cathode exhaust path simultaneously when the fuel cell stack is subjected to reverse flow for membrane humidification. That is, always when the main path of the cathode gas path is blocked by means of the blocking device.

[0014] Furthermore, it is proposed that the main path of the cathode gas pathway be connectable to a secondary path of the cathode exhaust pathway, and vice versa. This ensures that different flow paths are available for the compressed air introduced into the cathode exhaust pathway via the secondary path and for the air introduced into the cathode exhaust pathway via the main path, as the flow directions are opposite.

[0015] According to a preferred embodiment of the invention, both the cathode gas path and the cathode exhaust path each have a main and a secondary path. This means that both the cathode gas path and the cathode exhaust path branch out. This results in a tree-like structure of the flow paths. Which flow path is usable at any given time can be controlled or regulated by means of the shut-off device.

[0016] The blocking device preferably comprises movable blocking elements for shutting off the main path and the secondary paths of the cathode gas path. This means that the blocking device includes at least two movable blocking elements. Further movable blocking elements are preferably arranged in the main path and the secondary path of the cathode gas path. In this case, the blocking device comprises at least four movable blocking elements. The movable blocking elements can, for example, be in the form of flaps. In this way, the blocking device can be implemented particularly cost-effectively.

[0017] According to an advantageous embodiment of the locking device, at least two locking elements are arranged to rotate about a common axis of rotation. The locking elements can thus be actuated together. To enable one locking element to be moved to an open position and simultaneously the other locking element to a locked position, it is further proposed that the angular position of the locking elements be offset by an angle α. The angle α can, for example, be 90°.

[0018] If the movable locking elements are arranged not only in the main and secondary paths of the cathode gas path, but also in the main and secondary paths of the cathode exhaust gas path, they are arranged analogously, that is, rotatably about a common axis of rotation and, furthermore, preferably offset from each other in their angular position by an angle α. The axis of rotation can, moreover, be the same axis of rotation about which the locking elements for blocking the main and secondary paths of the cathode gas path are rotatably arranged. This allows for a particularly compact arrangement of the locking elements of the blocking device and is therefore particularly space-saving. Furthermore, all locking elements can be controlled or actuated simultaneously.

[0019] During normal operation of the fuel cell system, the two main paths—that is, the main path of the cathode gas path and the main path of the cathode exhaust gas path—are preferably open, and the two secondary paths are each blocked by a movable locking element of the blocking device. To reverse the flow direction in the fuel cell stack, the blocking device is actuated so that the two main paths are now blocked and the two secondary paths are open. For this purpose, the two locking elements located in the secondary paths are moved from the blocked position to the open position. The two locking elements located in the main paths are moved from the open position to the blocked position.

[0020] Preferably, the locking elements have freewheels. These freewheels ensure that, in the event of shutdown, the open locking elements can be moved into a locked position without opening the locking elements already in the locked position. This means that, in order to completely shut off the air supply in the event of shutdown, all locking elements can be moved into a locked position simultaneously.

[0021] The two additional movable locking elements can be designed analogously to the locking elements described above, for example, in the form of simple flaps. These can in turn be arranged to rotate about a common axis of rotation, so that they can be actuated together. However, their angular position is preferably not offset from each other, since the two additional movable locking elements are simultaneously either in the locked position or in the open position.

[0022] Preferably, the two additional movable locking elements can be controlled independently of the locking elements described above. With the help of these two additional movable locking elements, a bypass function can thus be implemented, eliminating the need for the bypass path and the bypass valve located therein. This further simplifies the fuel cell system.

[0023] The air compressor of the proposed fuel cell system has at least one compressor wheel, preferably mounted on a common shaft with a turbine wheel located in the cathode exhaust path. This allows the air compressor to be operated with exceptional energy efficiency, as energy is recovered via the turbine wheel. Furthermore, the proposed use of product water protects the turbine wheel from damage caused by droplet impact. This is because the product water is used to moisten the fuel cell membranes, resulting in significantly less, or even no, product water being discharged with the cathode exhaust.

[0024] To solve the aforementioned problem, a method for operating a fuel cell system comprising a fuel cell stack with a cathode is proposed. In this method, air, previously compressed by an air compressor, is supplied to the cathode via a cathode gas path during normal operation. According to the invention, the flow direction of the compressed air through the fuel cell stack is temporarily reversed for membrane humidification. This allows the product water to be utilized in a way that simultaneously eliminates the need for an additional humidification device in the cathode gas path. This reduces the space requirements and costs of the fuel cell system.

[0025] To reverse the flow direction through the fuel cell stack, a main path of the cathode gas path connected to an inlet of the fuel cell stack is preferably blocked, and a secondary path of the cathode gas path connected to an outlet of the fuel cell stack is opened, preferably by means of a blocking device. This means that the fuel cell system according to the invention described above is particularly suitable for carrying out the method according to the invention, since here the cathode gas path comprises a main and a secondary path as well as a blocking device for selectively blocking the two paths. If both the main path and the secondary path can be blocked by means of the blocking device, the air supply towards the cathode can be completely blocked by means of the blocking device in the event of shutdown. The blocking device can thus replace at least one shut-off valve, thereby further simplifying the fuel cell system.

[0026] A locking device with movable locking elements, for example in the form of flaps, is preferably used. This allows the locking device to be implemented in a space-saving and cost-effective manner. The movable locking elements are preferably arranged to rotate about at least one axis. By rotating the locking elements, the main and / or the secondary path of the cathode gas path can be blocked. A freewheel mechanism ensures that both the main and the secondary path are blocked simultaneously by the locking elements.

[0027] The invention is explained in more detail below with reference to the accompanying drawings. These show: Fig. 1 a schematic representation of a cathode region of a fuel cell system, Fig. 2 a schematic representation of a cathode area of ​​a fuel cell system according to the invention during normal operation, Fig. 3a schematic representation of the cathode area of ​​the fuel cell system of the Fig. 2 when the flow direction through the fuel cell stack is reversed, Fig. 4 a schematic representation of the cathode area of ​​the fuel cell system of the Fig. 1 in case of shutdown, Fig. 5 a schematic representation of a cathode area of ​​a second fuel cell system according to the invention. Detailed description of the drawings

[0028] Fig. 1Figure 1 shows a fuel cell system 1 comprising a fuel cell stack 2 with a cathode 3 and an anode 25. Air can be supplied to the cathode 3 via a cathode gas path 4. The air is drawn from the environment, passed through an air filter 17, and compressed by an electrically driven air compressor 5 located in the cathode gas path 4. Since the air heats up during compression, it is cooled before an inlet 6 of the fuel cell stack 2 by means of a cooling device 18 located downstream of the air compressor 5 in the cathode gas path 4. The previously compressed and cooled air is also humidified by means of a humidification device 19 located downstream of the cooling device 18. The humidification is intended to prevent the membranes of the fuel cells of the fuel cell stack 2 from drying out.Although water is produced during the energy conversion process in the fuel cells, this cannot prevent the membranes from drying out, as it is discharged from the fuel cell stack along with the depleted gases.

[0029] The cathode exhaust gas is introduced via an outlet 7 of the fuel cell stack 2 into a cathode exhaust gas path 9 and fed to an exhaust gas turbine located in the cathode exhaust gas path 9. The exhaust gas turbine has a turbine wheel 16, which is arranged on a shaft 15 with a compressor wheel 14 of the air compressor 5. The exhaust gas turbine, which is subjected to the flow of cathode exhaust gas, thus assists the electric motor drive of the air compressor 5. Since the cathode exhaust gas carries product water and water droplets contained in the cathode exhaust gas can lead to damage to the exhaust gas turbine ("droplet impact"), a water separator 20 is arranged upstream of the exhaust gas turbine in the cathode exhaust gas path 9.

[0030] In the Fig. 1 In the fuel cell system 1 shown, the cathode gas path 4 and the cathode exhaust path 9 can each be shut off via a shut-off valve 21. Closing the shut-off valves 21 prevents air from continuing to be supplied to the cathode 3 of the fuel cell stack 2 when the system is shut down. A bypass path 23 with a bypass valve 22 located therein allows the cathode gas path 4 and the cathode exhaust path 9 to be briefly closed to bypass the fuel cell stack 2. A pressure regulator 24 is also located upstream of the exhaust turbine in the cathode exhaust path 9.

[0031] The invention is described below with reference to the Figures 2 to 5 An example will be provided.

[0032] The highly simplified schematic representation of the Fig. 2An air compressor 5 with a compressor wheel 14 is shown, which is arranged on a shaft 15 together with a turbine wheel 16 of an exhaust gas turbine. The compressor wheel 14 is arranged in a cathode gas path 4, through which compressed air can be supplied to an inlet 6 of a fuel cell stack (not shown) by means of the air compressor 5. The turbine wheel 16 is arranged in a cathode exhaust gas path 9 and is supplied with cathode exhaust gas, which leaves the fuel cell stack via an outlet 7. The cathode gas path 4 and the cathode exhaust gas path 9 branch off, so that both the cathode gas path 4 and the cathode exhaust gas path 9 each form a main path 4.1, 9.1 and a secondary path 4.2, 9.2. This results in a tree-like structure of the flow paths.

[0033] In the main paths 4.1, 9.1 and in the secondary paths 4.2, 9.2, movable locking elements 10 of a locking device 8 are arranged, wherein these are simple flaps that are rotatably arranged about a common axis of rotation 11. The angular position of the locking elements 10 of the main paths 4.1, 9.1 is each offset by an angle α from the angular position of the locking elements 10 of the secondary paths 4.2, 9.2, so that either the main paths 4.1, 9.1 or the secondary paths 4.2, 9.2 can be locked.

[0034] In the Fig. 2The position of the blocking elements 10 during normal operation of the fuel cell system 1 is shown. During normal operation, the main paths 4.1 and 9.1 are open, and the secondary paths 4.2 and 9.2 are closed. The air compressed by the air compressor 5 is thus fed to the inlet 6 via the main path 4.1 of the cathode gas path 4. The depleted air or cathode exhaust gas reaches the turbine wheel 16 via the outlet 7 and the main path 9.1 of the cathode exhaust path 9 (see the arrows indicating the flow direction).

[0035] In the Fig. 3The position of the locking elements 10 was changed by actuating the locking device 8. For this purpose, the locking elements 10, which are designed as flaps, were rotated (see lateral arrow in the direction of rotation). Now, the locking elements 10 each open the secondary paths 4.2 and 9.2, while the main paths 4.1 and 9.1 are closed (see the arrows indicating the flow direction). Since the secondary path 4.2 of the cathode gas path 4 leads into the main path 9.1 of the cathode exhaust path 9.1, the air compressed by the air compressor 5 is now supplied to the outlet 7 of the fuel cell stack. The fuel cell stack is thus subjected to reverse airflow. The air carries product water with it, so that membrane humidification is achieved using the product water. The air guided through the fuel cell stack to the membrane humidification exits the fuel cell stack via inlet 6 and can enter the secondary path 9 via the main path 4.1 of the cathode gas path 4.2 of the cathode exhaust path 9 are discharged.

[0036] The Fig. 4 A further position of the locking elements 10 of the locking device 8 for the shutdown case can be seen. In this position, all locking elements 10 assume the same angular position, so that all main paths 4.1, 9.1 and all secondary paths 4.2, 9.2 are blocked. The air supply is thus completely interrupted.

[0037] The Fig. 5 shows a further development of the fuel cell system 1 of the Figures 2 to 4Here, the shut-off device comprises eight additional shut-off elements 12 in the form of flaps, which are rotatably arranged about a common axis of rotation 13 at the same angular position. With the aid of these additional shut-off elements 12, the inlet 6 and the outlet 7 can be shut off, so that the air supply is reliably interrupted in the event of shutdown. Furthermore, the additional shut-off elements 12 can be closed, and the shut-off elements 10 in the main path 4.1 of the cathode gas path 4 and in the secondary path 9.2 of the cathode exhaust gas path 9 can be opened. In this way, the shut-off device 8 enables bypass operation (see the arrows indicating the flow direction) and is thus able to create a bypass path 23 with a bypass valve 22 arranged therein, analogous to the Fig. 1 to replace.

Claims

1. Fuel cell system (1), comprising a fuel cell stack (2) with a cathode (3), to which air can be supplied via a cathode gas path (4) as cathode gas, wherein an air compressor (5) is integrated into the cathode gas path (4), wherein the cathode gas path (4) branches downstream of the air compressor (5) into a main path (4.1) which can be connected to an inlet (6) of the fuel cell stack (2) and a secondary path (4.2) which can be connected to an outlet (7) of the fuel cell stack (2), wherein the main path (4.1) and the secondary path (4.2) can each be shut off individually or together with the aid of a blocking device (8), characterized in that the blocking device (8) comprises at least two further movable blocking elements (12), by means of which the inlet (6) of the fuel cell stack (2) and the outlet (7) of the fuel cell stack (2) can be shut off.

2. Fuel cell system (1) according to Claim 1, characterized in that the main path (4.1) and / or the secondary path (4.2) of the cathode gas path (4) can be connected to a cathode off-gas path (9) with the aid of the blocking device (8).

3. Fuel cell system (1) according to Claim 2, characterized in that the main path (4.1) of the cathode gas path (4) can be connected to a secondary path (9.1) of the cathode off-gas path (9) and the secondary path (4.2) of the cathode gas path (4) can be connected to a main path (9.1) of the cathode off-gas path (9).

4. Fuel cell system (1) according to any of the preceding claims, characterized in that the blocking device (8) has movable blocking elements (10), for example in the form of flaps, for shutting off the main path (4.1) and the secondary path (4.2) of the cathode gas path (4), wherein further movable blocking elements (10) are preferably arranged in the main path (9.1) and in the secondary path (9.2) of the cathode off-gas path (9).

5. Fuel cell system (1) according to Claim 4, characterized in that at least two blocking elements (10) are arranged rotatably about a common rotation axis (11), wherein the angular position of the blocking elements (10) is preferably offset through an angle (α), and wherein the angle (α) is further preferably 90°.

6. Fuel cell system (1) according to Claim 5, characterized in that the blocking elements (10) have freewheels, so that they can be moved to the same angular position, for example.

7. Fuel cell system (1) according to Claim 1, characterized in that the further blocking elements (12) are arranged rotatably about a common rotation axis (13), preferably in the same angular position.

8. Fuel cell system (1) according to any of the preceding claims, characterized in that the air compressor (5) has at least one compressor wheel (14), which is preferably arranged on a common shaft (15) with a turbine wheel (16) arranged in the cathode off-gas path (9).

9. Method for operating a fuel cell system (1) according to any of the preceding claims, comprising a fuel cell stack (2) with a cathode (3), to which air compressed with the aid of an air compressor (5) is supplied via a cathode gas path (4) during normal operation, wherein the flow direction of the air compressed with the aid of the air compressor (5) through the fuel cell stack (2) is temporarily reversed to moisten the membrane.

10. Method according to Claim 9, characterized in that a main path (4.1) of the cathode gas path (4) connected to an inlet (6) of the fuel cell stack (2) is blocked and a secondary path (4.2) of the cathode gas path (4) connected to an outlet (7) of the fuel cell stack (2) is opened with the aid of a blocking device (8) to reverse the flow direction.

11. Method according to Claim 9 or 10, characterized in that a blocking device (8) with movable blocking elements (10, 12), for example in the form of flaps, is used, which blocking elements are preferably arranged rotatably about at least one rotation axis (11, 13).