Method for operating a fuel cell system. Fuel cell system

Abstract

The invention relates to a method for operating a fuel cell system (1) with at least two fuel cell stacks (2) which are supplied with compressed air via separate supply air paths (3), in each of which at least one air compressor unit (4) with at least one shaft-rotor unit (7) supported by gas bearings (5, 6) is integrated, wherein the gas bearings (5, 6) are tempered, in particular cooled, by at least two parallel connected shaft-rotor units (7), which branch off from a first supply air path (3) into an air path (8), are supplied via the air path (8) to the gas bearings (5, 6) to be tempered, in particular to be cooled, and are then introduced into a further supply air path (3) downstream of a shaft-rotor unit (7) integrated into the further supply air path (3). The invention further relates to a fuel cell system (1) that is suitable for carrying out the method or can be operated according to the method.

Description

[0001] The invention relates to a method for operating a fuel cell system with multiple fuel cell stacks. This means that the fuel cell system is designed as a multi-stack system. Furthermore, the invention relates to a fuel cell system that is suitable for carrying out the method or operable according to the method.

[0002] The preferred field of application of the invention is mobile fuel cell systems or vehicles in which two or more fuel cell stacks are used to generate drive energy. State of the art

[0003] In vehicles that use fuel cells to generate propulsion energy, the oxidizing agent oxygen from the ambient air is typically used to react with hydrogen in the fuel cells to form water or water vapor, thereby generating electrical power through electrochemical conversion. To provide a variable air mass flow and a suitable pressure level, the ambient air is fed to the fuel cells using an air conveying and compression system. High-performance air conveying and compression systems are known from the state of the art for this purpose. These systems are typically designed in multiple stages, thus enabling higher system pressures than single-stage air compression systems.

[0004] In a multi-stage air conveying and compression system, several air compressor units are typically connected in series. Thermal turbomachines, particularly radial compressors, are typically used as air compressor units, driven by electric motors. For energy recovery, the air compressor unit can have a turbine, to which the exhaust air or exhaust air exiting the fuel cells is fed.

[0005] If multiple fuel cell stacks are operated at high pressure in a fuel cell system ("multi-stack system"), the stack efficiency and the system efficiency can be increased. However, this generally requires multi-stage air compression and at least one-stage energy recuperation using a turbine.

[0006] The present invention is concerned with the task of improving air compression in a multi-stack system with regard to energy and system efficiency.

[0007] To achieve this object, the method having the features of claim 1 and the fuel cell system having the features of claim 8 are proposed. Advantageous developments of the invention can be found in the respective subclaims. Disclosure of the invention

[0008] A method is proposed for operating a fuel cell system with at least two fuel cell stacks, which are supplied with compressed air via separate supply air paths, each of which integrates at least one air compressor unit with at least one shaft-rotor unit supported by gas bearings. The gas bearings of at least two shaft-rotor units connected in parallel are tempered, in particular cooled, with compressed air, which is branched from a first supply air path into an air path, supplied via the air path to the gas bearings to be tempered, in particular to be cooled, and then introduced into a further supply air path downstream of a shaft-rotor unit integrated into the further supply air path.

[0009] In the proposed method, the air diverted from an air supply path for temperature control, in particular cooling, of gas storage devices is not simply discharged into the environment, but is made available to the system again by being fed into another air supply path. Since the air is fed downstream of a shaft-rotor unit of an air compressor unit integrated into the air supply path and thus into a pressurized area of ​​the fuel cell system, only a small proportion of the energy previously used to compress the air is lost. By introducing the air into the pressurized area, the energy required to compress the air can be reduced. Accordingly, system efficiency increases, while system costs and hydrogen consumption decrease. This is because the electrical energy required for compression is usually generated by the fuel cell stacks of the fuel cell system.

[0010] The proposed method also has the advantage that, during a cold or freezing start and / or during start / stop operation, the gas bearings connected via the air path can be used for mutual temperature control, particularly heating. Accordingly, the degradation of the gas bearings during a cold or freezing start and / or during start / stop operation can be reduced.

[0011] Preferably, the pressure level in the first supply air path is set higher than in the second supply air path, so that a pressure gradient exists in the air path connecting the two supply air paths. The pressure level in the two supply air paths can then be used to adjust the air mass flow supplied to the gas storage via the air path. In this case, no separate control is required.

[0012] Furthermore, it is proposed that, to reverse the flow direction in the air path, the pressure level in the first supply air path be set lower than in the second supply air path. This reverses the pressure gradient in the air path, and the air required to cool the gas bearings is diverted from the "second" supply air path and fed into the "first" supply air path.

[0013] To interrupt the temperature control, especially the cooling, of the gas bearings, the pressure level in the first supply air path can be set to the same level as in the subsequent supply air path. Since there is no pressure gradient in the air path in this case, no air is diverted from any of the supply air paths. Since the gas bearings are not temperature controlled or cooled in this case, this method is particularly suitable for idle operation and / or for operation at low loads and / or at low ambient temperatures.

[0014] In a further development of the invention, it is proposed that a higher-level system controller be used to coordinate the air requirements of the fuel cell stacks with the air requirements of the gas storage. With the help of the higher-level system controller, the pressure level required to meet the requirements can then be adjusted in the supply air paths. When coordinating the air requirements, the degrees of freedom existing in a multi-stack system can be utilized. These include degrees of freedom created, for example, by power splitting between the individual fuel cell stacks and / or between the fuel cell stacks as energy converters and at least one battery as energy storage.

[0015] Advantageously, the air diverted into the air path for temperature control, in particular cooling, of the gas bearings is cooled by at least one cooling device before being supplied to the gas bearings of at least one shaft-rotor unit. Since compressed and thus heated air is diverted into the air path, the prior cooling of the air can further optimize the cooling of the gas bearings. Ideally, an existing cooling device is used to cool the diverted air, thus limiting the installation space required. The existing cooling device can, for example, be a coolant-cooled cooling jacket of a shaft-rotor unit, through which the air path is then routed.

[0016] Alternatively or additionally, it is proposed that the air diverted into the air path be cooled by at least one cooling device between the gas bearings of two shaft-rotor units. This means that the air in the air path is intercooled. Intercooling maintains a high cooling effect of the air. Intercooling can also be achieved using a coolant-cooled cooling jacket of a shaft-rotor unit, so that intercooling can be implemented with a largely space-neutral effect.

[0017] Furthermore, a fuel cell system with at least two fuel cell stacks is proposed, which can be supplied with air via separate supply air paths. At least one air compressor unit, each with at least one shaft-rotor unit supported by gas bearings, is integrated into the supply air paths. For temperature control, in particular cooling, of the gas bearings of at least two shaft-rotor units connected in parallel, an air path is provided that extends from a pressurized area of ​​a first supply air path across the gas bearings to be temperature controlled, in particular cooled, to a pressurized area of ​​another supply air path.

[0018] The proposed fuel cell system is particularly suitable for carrying out the method according to the invention or can be operated according to this method, so that the same advantages can be achieved. In particular, the energy required to temperature-regulate the gas bearings with compressed air can be reduced, since the air diverted for this purpose from a pressurized area of ​​an air supply path is subsequently fed back into a pressurized area of ​​another air supply path. This increases system efficiency and reduces hydrogen consumption. Furthermore, the gas bearings of at least two shaft-rotor units connected in parallel can be temperature-regulated mutually, which proves particularly advantageous during a cold or freezing start or in start / stop operation. In this case, the air supplied to the gas bearings is used for heating.In the majority of cases, however, the air supplied to the gas bearings via the air path is used to cool the gas bearings.

[0019] In a further development of the invention, it is therefore proposed that the air path for cooling the diverted air be routed through a cooling device. The prior cooling of the air improves the cooling effect. The cooling device can, for example, be a coolant-cooled cooling jacket of a shaft-rotor unit, which is usually present, so that no additional cooling device is required. This helps save installation space and costs. The air path is simply routed through the coolant-cooled cooling jacket of the shaft-rotor unit.

[0020] For multi-stage air compression, at least two air compressor units are preferably integrated into the first supply air path and / or the second supply air path. Higher system pressures can be achieved with the help of multi-stage air compression.

[0021] Furthermore, in multi-stage air compression, the gas bearings of the shaft-rotor units of the same compression stage are preferably connected via an air path in both the first and the further supply air paths. This means that the gas bearings of the shaft-rotor units of the first compression stage and the gas bearings of the shaft-rotor units of the further compression stage are each connected via an air path. In two-stage air compression in the first and further supply air paths, two air paths are therefore preferably provided for tempering, in particular cooling, the gas bearings.

[0022] Preferred embodiments of the invention are explained in more detail below with reference to the accompanying drawings. These show: Fig. 1 a schematic representation of a first fuel cell system according to the invention, Fig. 2 a schematic representation of two air compressor units for a fuel cell system according to the invention, Fig. 3 a schematic representation of another fuel cell system according to the invention and Fig. 4 a) to f) each show a schematic representation of a shaft-rotor unit for an air compressor unit of a fuel cell system according to the invention. Detailed description of the drawings

[0023] The Fig. Figure 1 shows, by way of example, a first preferred embodiment of a fuel cell system 1 according to the invention with several, in this case two, fuel cell stacks 2. Each fuel cell stack 2 is supplied with air via a separate supply air path 3. The air is taken from the environment 12 and fed via an air filter 13 to an air compressor unit 4, which comprises at least one shaft-rotor unit 7 mounted on gas bearings 5, 6 (for example, analogous to Fig. 2). The air or exhaust air exiting the fuel cell stacks 2 is each discharged back to the environment 12 via an exhaust air path 11. For energy recovery, a turbine 10 can be integrated into each of the exhaust air paths 11, which is coupled to the shaft-rotor unit 7 of the air compressor unit 4 integrated into the respective supply air path 3. Alternatively or additionally, a pressure control valve (not shown) can be integrated into each of the exhaust air paths 11. A stack bypass 14 with an integrated bypass valve 15 is provided to bypass the fuel cell stacks 2.

[0024] Each fuel cell stack 2 of the illustrated fuel cell system 1 is connected to a cooling system 20 for cooling. The hydrogen supply to each fuel cell stack is provided via an anode subsystem 21, with the hydrogen being stored in a hydrogen tank 22.

[0025] The two supply air paths 3 are connected via an air path 8, which branches off from the supply air path 3 on the pressure side of the integrated air compressor unit 4 or flows into the supply air path 3. In this case, the branch or the outlet is located in the respective air compressor unit 4. This is not mandatory. For example, the branch or the outlet can also be arranged outside the respective air compressor unit 4 (for example, analogous to the Fig. 2). The air path 8 passes over the gas bearings 5, 6 of the shaft-rotor units 7 of the two air compressor units 4, allowing them to be tempered, particularly cooled, using compressed air. The flow direction and the air mass flow in the air path 8 depend on the pressure gradient in the air path 8. This can be adjusted via the pressure level in the two supply air paths 3.

[0026] The Fig. 2 shows, by way of example, two air compressor units 4 that are integrated into separate supply air paths 3. Each air compressor unit 4 comprises a shaft-rotor unit 7 that is mounted on gas bearings 5, 6, with the gas bearings 5 ​​being designed as radial bearings and the gas bearings 6 as axial bearings. The shaft-rotor units 7 are each driven by an electric motor 19 and a turbine 10 that is integrated into the respective exhaust air path 11. Thus, two impellers 23 are each connected to a shaft 24. An air path 8 leading over all gas bearings 5, 6 connects the pressurized areas of the two supply air paths 3, so that compressed air can be supplied to the gas bearings 5, 6 for temperature control, in particular cooling. To cool the air in the air path 8, at least one cooling device 9 is provided, which can be, for example, a coolant-cooled cooling jacket of a shaft-rotor unit 7.In this case, the air diverted into the air path 8 is first guided through the cooling jacket and then through the gas bearings 5, 6. Intermediate cooling can also be achieved with the aid of at least one additional cooling device 9, which is arranged between the gas bearings 5, 6 of the two parallel-connected shaft-rotor units 7.

[0027] Another fuel cell system 1 according to the invention is the Fig. 3. Here, for multi-stage air compression, two air compressor units 4.1, 4.2 are integrated into each of the two supply air paths 3, each comprising a shaft-rotor unit 7 mounted on gas bearings 5, 6. The shaft-rotor unit 7 of the first air compressor unit 4.1 is driven by an electric motor 19. The shaft-rotor unit 7 of the second air compressor unit 4.2 is driven by an electric motor 19 and a turbine 10. The turbines 10 can each be bypassed via a turbine bypass 16 with an integrated bypass valve 17. To bypass the fuel cell stacks 2, a stack bypass 14 with an integrated bypass valve 15 is provided. Furthermore, the fuel cell stacks 2 can be separated from the respective supply and exhaust air paths 3, 11 by means of shut-off valves 18.

[0028] In the Fig. 3, downstream of each air compressor unit 4.1, 4.2, a cooling device 9 is integrated into the supply air path 3 for cooling or intercooling the multi-stage compressed air. The intercooling between the two compression stages is achieved in this case with the aid of a cooling device 9 in the form of a heat exchanger through which a coolant from a cooling circuit (not shown) flows. The cooling after the respective second compression stage is achieved with the aid of a cooling device 9 in the form of a gas-gas heat exchanger through which the air or exhaust air in the exhaust air path 11 flows. Other cooling devices 9 can also be used. Furthermore, the air compressor units 4.1, 4.2 integrated into the supply air paths 3 do not have to be designed identically for each compression stage.

[0029] Two air paths 8 are provided to cool the gas bearings 5, 6 of the shaft-rotor units 7 of the air compressor units 4.1, 4.2 integrated into the supply air paths 3. A first air path 8 connects the two supply air paths 3, each on the pressure side of the shaft-rotor unit 7 of the first compression stage. A second air path 8 connects the two supply air paths 3, each on the pressure side of the shaft-rotor unit 7 of the second compression stage. A separate air path 8 is therefore provided for each compression stage.

[0030] The air compressor units 4, 4.1, 4.2 used in a fuel cell system 1 according to the invention can be designed in a variety of ways. Fig. 4a) to 4f) show exemplary embodiments of various shaft-rotor units 7 for an air compressor unit 4, 4.1, 4.2.

[0031] Fig. 4a) shows an air compressor unit 4 with a shaft-rotor unit 7, which comprises an impeller 23 arranged on a shaft 24. The shaft-rotor unit 7 is driven by an electric motor 19 (analogous to the shaft-rotor unit 7 of the air compressor units 4.1 of the fuel cell system 1 of the Fig. 3). The gas bearings 5, 6 for supporting the shaft-rotor unit 7 are not shown.

[0032] Fig. Figure 4b) shows an air compressor unit 4 with a shaft-rotor unit 7 designed as a multi-flow unit. For this purpose, the shaft-rotor unit 7 has two impellers 23.1, 23.2 arranged on a shaft 24. The shaft-rotor unit 7 is driven by an electric motor 19.

[0033] Fig. 4c) shows an air compressor unit 4 with a shaft-rotor unit 7, which also comprises two impellers 23.1, 23.2 coupled via a shaft 24. However, the impellers 23.1, 23.2 are not connected in parallel, but in series. This means that they are subjected to air flow one after the other. This enables multi-stage air compression with intermediate cooling, provided a cooling device 9 is integrated into the supply air path 3.

[0034] Fig. 4d) shows an air compressor unit 4 with a shaft-rotor unit 7, which comprises two impellers 23.1, 23.2 coupled via a shaft 24, wherein the second impeller 23.2, however, belongs to a turbine 10. The shaft-rotor unit 7 is accordingly driven by means of an electric motor 19 and by means of a turbine 10 (analogous to the shaft-rotor unit 7 of the air compressor units 4.2 of the fuel cell system 1 of the Fig. 3).

[0035] Fig. 4e) shows an air compressor unit 4 with a shaft-rotor unit 7, which comprises two impellers 23.2, 23.3 coupled via a shaft 24, wherein the impeller 23.2 belongs to a turbine 10 for driving the shaft-rotor unit 7. An electric motor 19 is not provided.

[0036] Fig. 4f) shows, by way of example, a shaft-rotor unit 7 with a shaft 24 and two impellers 23.1, 23.2 arranged thereon, which are two turbine wheels for purely generator operation.

Claims

[1] Method for operating a fuel cell system (1) with at least two fuel cell stacks (2) which are supplied with compressed air via separate supply air paths (3), into each of which at least one air compressor unit (4) with at least one shaft-rotor unit (7) mounted via gas bearings (5, 6) is integrated, wherein the gas bearings (5, 6) are tempered, in particular cooled, by at least two shaft-rotor units (7) connected in parallel with compressed air which is branched off from a first supply air path (3) into an air path (8), fed via the air path (8) to the gas bearings (5, 6) to be tempered, in particular to be cooled, and then introduced into a further supply air path (3) downstream of a shaft-rotor unit (7) integrated into the further supply air path (3). [2] Method according to claim 1, characterized bythat the pressure level in the first supply air path (3) is set higher than in the further supply air path (3), so that there is a pressure gradient in the air path (8) which connects the two supply air paths (3). [3] Method according to claim 1 or 2, characterized by that in order to reverse the flow direction in the air path (8), the pressure level in the first supply air path (3) is set lower than in the further supply air path (3). [4] Method according to one of the preceding claims, characterized by that in order to interrupt a temperature control, in particular cooling, of the gas bearings (5, 6), the pressure level in the first supply air path (3) is set equal to that in the further supply air path (3). [5] Method according to one of the preceding claims, characterized by that a higher-level system control is used to coordinate the air requirements of the fuel cell stacks (2) and the air requirements of the gas storage (5, 6). [6] Method according to one of the preceding claims, characterized by that the air branched off into the air path (8) is cooled by means of at least one cooling device (9) before it is fed to the gas bearings (5, 6) of at least one shaft-rotor unit (7). [7] Method according to one of the preceding claims, characterized by that the air branched off into the air path (8) is cooled by means of at least one cooling device (9) between the gas bearings of two shaft-rotor units (7). [8] Fuel cell system (1) with at least two fuel cell stacks (2) which can be supplied with air via separate supply air paths (3), wherein at least one air compressor unit (4) with at least one shaft-rotor unit (7) mounted via gas bearings (5, 6) is integrated in each of the supply air paths (3), and wherein an air path (8) is provided for tempering, in particular cooling, the gas bearings (5, 6) of at least two shaft-rotor units (7) connected in parallel, which air path extends from a pressurised region of a first supply air path (3) via the gas bearings (5, 6) to be tempered, in particular to be cooled, to a pressurised region of a further supply air path (3). [9] Fuel cell system (1) according to claim 8, characterized by that the air path (8) for cooling the branched air is guided via a cooling device (9), for example a coolant-cooled cooling jacket of a shaft-rotor unit (7). [10] Fuel cell system (1) according to claim 8 or 9, characterized by that for multi-stage air compression at least two air compressor units (4.1, 4.2) are integrated into the first supply air path (3) and / or into the further supply air path (3). [11] Fuel cell system (1) according to claim 10, characterized by that in a multi-stage air compression, both in the first and in the further supply air path (3), the gas bearings (5, 6) of the shaft-rotor units (7) of the same compression stage are connected via an air path (8).

Images