The invention relates to a fuel cell system. In addition, the invention relates to a method for operating a fuel cell system.
Fuel cells, for example a plurality of fuel cells of a fuel cell system connected to form a fuel cell stack, require for energy generation a) a fuel, as a rule hydrogen, which is fed via an anode gas path to an anode of the fuel cell stack, and b) oxygen, which is fed via a cathode gas path to a cathode of the fuel cell stack. Air taken from the environment is usually used as the oxygen supplier. Since the energy conversion process requires a certain air mass flow and a certain pressure level, the air fed on the cathode side is compressed beforehand by means of an air compressor arranged in the cathode gas path.
DE102004022312A1 discloses a moisture exchange module for moistening air fed to the cathode. In this way, the membrane can be protected against drying out and thus against damage or premature aging.
Such fuel cell systems, as shown in DE 102004022312 A1, have the disadvantage that the moistening device arranged in the cathode gas path requires a not inconsiderable installation space and is moreover expensive to procure. The present invention is therefore based on the object of specifying a fuel cell system which is of simpler design than this, in particular does not require a moistening device, so that installation space and costs can be saved.
To achieve the object, a fuel cell system and a method are proposed.
The proposed fuel cell system comprises a fuel cell stack with a cathode which can be fed with air 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 that can be connected to an inlet of the fuel cell stack and into a secondary path that can be connected to an outlet of the fuel cell stack, wherein the main path and the secondary path can each be shut off individually or together with the aid of a shut off device. Compressed air can thus be optionally fed to the inlet or to the outlet of the fuel cell stack. In addition, the air supply can be completely shut off in the event of a shutdown.
In order to supply the cathode with sufficient oxygen or air during normal operation of the fuel cell system, the air compressed with the aid of 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 shut off with the aid of the shut off device, and the air fed to the cathode is routed as usual through the fuel cell stack. By shutting off the main path and opening the secondary path, the air compressed with the aid of the air compressor can alternatively be fed to the outlet of the fuel cell stack. The compressed air thus enters the fuel cell stack via the outlet and exits again via the inlet. This means that the fuel cell stack is flowed through in the reverse direction. In the process, the air entrains product water, so that with the aid of the entrained product water the membranes of the fuel cells of the fuel cell stack are moistened. A moistening device in the cathode gas path can thus be omitted.
The switching between the main path and the secondary path can be controlled or regulated, for example, on the basis of a time sequence and/or depending on the water load of the cathode exhaust gas. In addition, operating modes are possible in which both the main path and the secondary path are kept open so that the air compressor does not have to operate against the shut off device. The flow direction through the fuel cell stack can be predetermined via the respectively open flow cross-section. Furthermore, in the case of a shutdown, both the main path and the secondary path can be shut off by means of the shut off device such that it is ensured that air is no longer fed to the cathode. This in turn allows the normally provided shut off valves to be omitted, so that further potential savings in terms of installation space and costs result.
In a development of the invention, it is proposed that the main path and/or the secondary path of the cathode gas path be connectable to a cathode exhaust gas path by means of the shut off device. The previously compressed air can be fed to the outlet of the fuel cell stack in a simple manner via the connection of the secondary path to the cathode exhaust gas path. To this end, the secondary path opens into the cathode exhaust gas path. In the case of the reverse flow direction through the fuel cell stack, the air exiting via the inlet of the fuel cell stack can be introduced into the cathode exhaust gas path via the connection of the main path to the cathode exhaust gas path. Preferably, therefore, the main path and the secondary path are always connected to the cathode exhaust gas path at the same time when the flow through the fuel cell stack is in the reverse flow direction in order to moisten the membranes. In other words, always when the main path of the cathode gas path is shut off with the aid of the shut off device.
Furthermore, it is proposed that the main path of the cathode gas path be connectable to a secondary path of the cathode exhaust gas path, and the secondary path of the cathode gas path be connectable to a main path of the cathode exhaust gas path. This ensures that different flow paths are available for the compressed air introduced into the cathode exhaust gas path via the secondary path of the cathode gas path and for the air introduced into the cathode exhaust gas path via the main path of the cathode gas path. This is because the flow direction is opposite.
According to a preferred embodiment of the invention, the cathode gas path as well as the cathode exhaust gas path thus each have a main path and a secondary path. That is to say, not only the cathode gas path but also the cathode exhaust gas path branch. This results in a tree-like structure of the flow paths. Which flow path is usable in each case can be controlled or regulated with the aid of the shut off device.
For this purpose, the shut off device preferably has movable shut off elements for shutting off the main path and the secondary path of the cathode gas path. In other words, the shut off device comprises at least two movable shut off elements. Further movable shut off elements are preferably arranged in the main path and in the secondary path of the cathode exhaust gas path. In this case, the shut off device comprises at least four movable shut off elements. The movable shut off elements can, for example, be in the form of flaps. In this way, the shut off device can be realized in a particularly cost-effective manner.
According to an advantageous embodiment of the shut off device, at least two shut off elements are arranged rotatably about a common axis of rotation. The shut off elements can thus be actuated together. So that a shut off element can be transferred into an open position and at the same time the other shut off element can be transferred into a shut off position, it is further proposed that the angular position of the shut off elements be offset by an angle α. The angle α can, for example, be 90°.
If the movable shut off 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 will be arranged analogously, that is to say rotatably about a common axis of rotation and further preferably offset from one another in their angular position by an angle α. Furthermore, the axis of rotation can be the same axis of rotation about which the shut off elements are arranged rotatably for shutting off the main and secondary paths of the cathode gas path. This enables a particularly compact arrangement of the shut off elements of the shut off device and therefore particularly moderate requirements as regards installation space. Furthermore, all shut off elements can be controlled or actuated simultaneously.
In normal operation of the fuel cell system, the two main paths, i.e. the main path of the cathode gas path as well as the main path of the cathode exhaust gas path, are preferably open and the two secondary paths are in each case shut off by a movable shut off element of the shut off device. To reverse the flow direction in the fuel cell stack, the shut off device is actuated such that the two main paths are now shut off and the two secondary paths are open. For this purpose, the two shut off elements arranged in the secondary paths are moved from the shut off position into the open position. The two shut off elements arranged in the main paths are moved from the open position into the shut off position.
The shut off elements preferably have freewheels. The freewheels ensure that, in the event of a shutdown, the respectively open shut off elements can be transferred into a shut off position without the shut off elements already in the shut off position being opened. This means that for the complete shutting off of the air supply in the event of a shutdown, all shut off elements can be brought simultaneously into a shut off position.
Furthermore, it is proposed that the shut off device comprises at least two further movable shut off elements by means of which the inlet of the fuel cell stack and the outlet of the fuel cell stack can be shut off. The air supply to the cathode of the fuel cell stack can likewise be shut off with the aid of the further movable shut off elements. They thus increase safety in the event of a shutdown.
The two further movable shut off elements can be formed analogously to the previously described shut off elements, for example in the form of simple flaps. These can in turn be arranged rotatably about a common axis of rotation so that they can be actuated together. However, their angular position is preferably not mutually offset, since the two further movable shut off elements will be at the same time either in the shut off position or in the open position.
Preferably, the two further movable shut off elements are controllable independently of the previously described shut off elements. By means of the two further movable shut off elements, a bypass function can thus be realized at the same time, which enable the elimination of the bypass path and the bypass valve arranged therein. In this way, the fuel cell system can be further simplified.
The air compressor of the proposed fuel cell system has at least one compressor wheel, which is preferably arranged on a common shaft with a turbine wheel arranged in the cathode exhaust gas path. The air compressor can thus be operated particularly energy-efficiently since energy is recovered with the aid of the turbine wheel. As a result of the proposed use of product water, the turbine wheel is also protected against damage caused by droplet impact. This is because the product water is used to moisten the membranes of the fuel cells, so that significantly less or even no product water is discharged with the cathode exhaust gas.
In order to achieve the object mentioned at the outset, a method for operating a fuel cell system comprising a fuel cell stack with a cathode is also proposed. In the method, air which has been previously compressed by means of an air compressor is fed to the cathode in normal operation via a cathode gas path. According to the invention, the flow direction through the fuel cell stack of the air compressed by means of the air compressor is temporarily reversed for membrane moistening. In this way, the product water can be fed to a utilization which at the same time makes an additional moistening device in the cathode gas path unnecessary. In this way, the installation space requirement and the costs of the fuel cell system can be reduced.
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 shut off with the aid of a shut off device, and a secondary path of the cathode gas path connected to an outlet of the fuel cell stack is opened. This means that in particular the previously described fuel cell system according to the invention is suitable for carrying out the method according to the invention, since here the cathode gas path comprises a main path and a secondary path and also a shut off device for selectively shutting off the two paths. If both the main path and the secondary path can be shut off with the aid of the shut off device, in the event of a shutdown the air supply in the direction of the cathode can be completely shut off with the aid of the shut off device. The shut off device can thus replace at least one shut off valve so that the fuel cell system is further simplified.
A shut off device with movable shut off elements, for example in the form of flaps, is preferably used. The shut off device can thus be implemented in a space-saving manner and at the same time cost-effectively. The movable shut off elements are preferably arranged rotatably about at least one axis of rotation. The main path and/or the secondary path of the cathode gas path can thus be shut off by a rotational movement of the shut off elements. By means of a freewheel, it can be ensured that both the main path and the secondary path are shut off by the shut off elements, and this simultaneously.
The invention is explained in more detail below with reference to the accompanying drawings.
In the Drawings:
The cathode exhaust gas is introduced into a cathode exhaust gas path 9 via an outlet 7 of the fuel cell stack 2 and fed to an exhaust gas turbine arranged 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 into which the cathode exhaust gas flows thus supports the electromotive drive of the air compressor 5. Since the cathode exhaust gas entrains 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 in the cathode exhaust gas path 9 upstream of the exhaust gas turbine.
In the fuel cell system 1 shown in
The invention is explained below by way of example with reference to
The highly simplified schematic representation of
Movable shut off elements 10 of a shut off device 8 are arranged in the main paths 4.1, 9.1 and in the secondary paths 4.2, 9.2, wherein in the present case, they are simple flaps which are arranged rotatably about a common axis of rotation 11. The angular position of the shut off elements 10 of the main paths 4.1, 9.1 is in each case offset by an angle α to the angular position of the shut off 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 shut off.
In
Number | Date | Country | Kind |
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10 2020 213 266.6 | Oct 2020 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2021/077137 | 10/1/2021 | WO |