Fuel cell system with branch device

Abstract

A fuel cell system (2), in particular for providing electrical energy to drive an electric motor (5) in a motor vehicle (1), has at least one fuel cell (4); an air supply path (6) configured to supply air from the environment to the at least one fuel cell (4); an exhaust air path (8) configured to discharge an exhaust air stream (14) from the at least one fuel cell (4); an exhaust air recirculation path (10) configured to return a return air stream (19) branched off from the exhaust air stream (14) to the air supply path (6); and a branching device (12) configured to divert a partial air stream from the exhaust air stream (14) and return it as a return air stream (19) to the exhaust air recirculation path (10).The branching device (12) is designed to utilize at least one gravity effect to branch off the partial airflow from the exhaust airflow (14) in such a way that the partial airflow does not carry a significant proportion of liquid water.

Description

[0001] The invention relates to a fuel cell system, in particular a fuel cell system for providing electrical energy to drive at least one electric motor in a motor vehicle equipped with an exhaust air branching device. The invention also relates to a motor vehicle with such a fuel cell system and a method for operating a fuel cell system according to the invention. State of the art

[0002] In vehicles where electrical drive energy is supplied by at least one fuel cell system (FCS), the oxidant oxygen from the ambient air is typically used to react with hydrogen in a membrane-based fuel cell to form water, particularly water vapor, and to provide electrical energy through the associated electrochemical conversion. The water, or water vapor, is released into the environment.

[0003] A key aspect of fuel cell operation is water management within the fuel cell membrane. The membrane must be kept sufficiently moist to conduct protons.

[0004] A humidification concept used to keep the membrane in the fuel cell sufficiently moist involves recirculating a portion of the moist exhaust air flowing out of the fuel cell back into the fuel cell. The portion of the moist exhaust air recirculated back into the fuel cell should, if possible, not contain liquid water, e.g., in the form of water droplets. Furthermore, components in the supply air path, such as compressor impellers, should not be exposed to liquid water droplets. Disclosure of the invention

[0005] It is therefore an object of the invention to provide a fuel cell system with a branching device which makes it possible to branch off a partial air flow from a moist exhaust air flow flowing out of a fuel cell in such a way that the partial air flow does not contain a significant proportion of liquid water.

[0006] A fuel cell system according to the invention, which is provided in particular for providing electrical energy for driving at least one electric motor in a motor vehicle, comprises at least one fuel cell; an air supply path designed to supply oxygen-containing air from the environment to the at least one fuel cell; an exhaust air path designed to discharge an exhaust air flow from the at least one fuel cell; an exhaust air return path designed to return a partial air flow branched off from the exhaust air flow as a return air flow into the air supply path; and a branching device designed to branch off the return air flow from the exhaust air flow and guide it into the exhaust air return path.The branching device is designed to use at least one gravity effect to branch off the partial air flow with as little liquid water as possible from the exhaust air flow containing liquid water and gaseous fluid.

[0007] The invention also includes a method of operating a fuel cell system according to the invention, wherein the method comprises diverting a partial air flow from the exhaust air flow of the at least one fuel cell without a significant proportion of liquid water and returning it as a return air flow into the supply air path of the at least one fuel cell, so that it returns to the at least one fuel cell.

[0008] The branching device of a fuel cell system according to the invention makes it possible to branch off a partial air flow from the moist exhaust air flow of the at least one fuel cell with the aid of at least one gravity effect, without the partial air flow containing a significant proportion of liquid water, e.g. in the form of water droplets, so that no liquid water droplets are introduced into the supply air path.

[0009] Liquid water contained in the exhaust air stream remains in the exhaust air path and is released into the environment together with the remaining, non-diverted part of the exhaust air stream.

[0010] Preferably, water condensing in the exhaust air recirculation path is also recirculated from the exhaust air recirculation path into the exhaust air path and discharged into the environment together with the exhaust air flow.

[0011] In this way, a defined return air flow, which does not contain a significant amount of liquid water, can be provided for metering into the fuel cell's supply air flow. This enables improved control of the fuel cell system, particularly the water management of the fuel cell system, when operating with exhaust air recirculation.

[0012] Compliance with the requirements for the supply air supplied to at least one fuel cell can be improved.

[0013] Because the return air stream does not contain liquid water, components in the supply air path, such as compressor impellers, heat exchangers and valves, can be protected from droplet impact, flooding and other negative effects that liquid water in the supply air stream can have.

[0014] By means of a branching device according to the invention and a method according to the invention, the aging of the at least one fuel cell and other components of the fuel cell system can be slowed down, so that the fuel cell system has a longer service life.

[0015] According to the invention, the flow guidance of the exhaust air flow, gravity, inertial forces, centrifugal forces, flow guide surfaces, changes in the flow cross-sectional areas, drip edges, swirl flows, backflows, and / or similar effects are used to prevent a significant amount of liquid water from entering the exhaust air return path, so that essentially only moist gaseous fluid is present in the exhaust air return path and water condensed in the exhaust air return path is returned to the exhaust gas path.

[0016] The described effects can be applied individually. Several of the described effects can also be combined.

[0017] In one embodiment, the branching device is particularly designed to utilize at least one inertial effect in addition to gravity to divert the return air flow from the exhaust air flow without a significant proportion of liquid water. This allows the efficiency of water separation in the branching device to be further improved.

[0018] In one embodiment, the branching device is designed to change a flow direction of the exhaust air flow in order to separate liquid water, which is contained in the exhaust air flow, e.g. in the form of water droplets, from the exhaust air flow by the inertia of the liquid water and to prevent the liquid water from entering the exhaust air recirculation path.

[0019] The exhaust air path can, for example, be designed with a curve, wherein the exhaust air return path branches off from the exhaust air path in the region of the curve.

[0020] In one embodiment, a swirling region is additionally formed in the region of the curve of the exhaust air path. The swirling region can, in particular, comprise a region of the exhaust air path that has an expanded cross-section.

[0021] In one embodiment, the branching device is designed to generate a swirl or swirl flow in the exhaust air stream.

[0022] In one embodiment, the branching device is designed such that the exhaust air flow flows over a drip edge in order to separate liquid water contained in the exhaust air flow at the drip edge.

[0023] In one embodiment, the branching device is designed to increase the flow velocity of the exhaust air stream in the exhaust air path. For this purpose, a constriction, in particular a nozzle, can be provided in the exhaust air path through which the exhaust air stream flows.

[0024] In one embodiment, the branching device comprises an exhaust air path descending in the flow direction of the exhaust air flow and an exhaust air return path rising in the flow direction of the return air flow, which is in flow connection with the exhaust air path.

[0025] This prevents a significant amount of liquid water contained in the exhaust air stream from entering the exhaust air return path. Furthermore, water condensing in the exhaust air return path can flow back into the exhaust air path by gravity through the exhaust air return path, from where it can be discharged into the environment along with the remaining exhaust air stream in the exhaust air path.

[0026] In one embodiment, the exhaust air path has a gradient of at least 5%, in particular a gradient of at least 10%, preferably a gradient of at least 15% relative to the horizontal in the flow direction of the exhaust air stream when the branching device is installed in a motor vehicle. This ensures that the exhaust air path has a sufficient gradient, even in an inclined vehicle, in particular when traveling uphill or downhill, so that liquid water can flow out of the exhaust air path driven by gravity.

[0027] In one embodiment, the exhaust air return path has a smaller flow cross-section than the exhaust air path. This allows the required installation space to be reduced.

[0028] In one embodiment, the flow cross-section of the exhaust air path is expanded in an area surrounding the nozzle. By expanding the flow cross-section of the exhaust air path in the area of ​​the nozzle, a flow-reduced space can be created around the nozzle. Liquid water contained in the exhaust air stream can be efficiently separated in the flow-reduced space to prevent a significant amount of the liquid water from entering the exhaust air recirculation path. The exhaust air recirculation path can preferably be branched off from the flow-reduced space or from a return flow area.

[0029] In one embodiment, at least one flow guide surface is formed in the exhaust air path. The at least one flow guide surface can be arranged, in particular, upstream of a flow inlet of the exhaust air return path. A flow guide surface can also be used to create a flow-calmed space and / or a backflow area in the exhaust air path, in particular downstream of the flow guide surface, in which liquid water contained in the exhaust air stream can be efficiently separated to prevent a significant amount of the liquid water from entering the exhaust air return path.

[0030] In one embodiment, an exhaust air recirculation valve is provided in the exhaust air recirculation path, which allows the return air flow flowing through the exhaust air recirculation path to be regulated. The exhaust air recirculation valve can, in particular, be designed to selectively allow or block the return air flow through the exhaust air recirculation path.

[0031] The exhaust air recirculation valve can be a switching valve that can only be switched between an open state, in which it allows the return air flow, and a closed state, in which it blocks the return air flow. Alternatively, the exhaust air recirculation valve can also be a variable valve that allows the return air flow to be regulated variably in several stages or continuously.

[0032] In one embodiment, a fan or compressor is provided in the exhaust air return path, enabling the return air flow flowing through the exhaust air return path to be driven. The fan or compressor can be adjustable in multiple stages or continuously.

[0033] The invention also includes a motor vehicle having one or more electric motors, which is equipped with a fuel cell system according to the invention in order to provide electrical energy for the at least one electric motor. Short description of the characters

[0034] Embodiments of the invention are described below with reference to the accompanying figures.

[0035] It shows: Fig. 1 is a schematic view of a motor vehicle equipped with a fuel cell system according to the invention; Fig. 2 a schematic sectional view of a branching device according to the invention according to a first embodiment; Fig. 3 a schematic sectional view of a branching device according to the invention according to a second embodiment; Fig. 4 a schematic sectional view of a branching device according to the invention according to a third embodiment; Fig. 5 is a schematic sectional view of a branching device according to the invention according to a fourth embodiment; Fig. 6 is a schematic sectional view of a branching device according to the invention according to a fifth embodiment; Fig. 7 is a schematic sectional view of a branching device according to the invention according to a sixth embodiment; Fig. 8 is a schematic sectional view of a branching device according to the invention according to a seventh embodiment; Fig. 9 is a schematic sectional view of a branching device according to the invention according to an eighth embodiment; and Fig. 10 is a schematic sectional view of a branching device according to the invention according to a ninth embodiment. Character description

[0036] Fig. 1 shows a schematic view of a motor vehicle 1 with a fuel cell system 2 which is designed according to an embodiment of the invention.

[0037] The motor vehicle 1 has four wheels 3 and an electric motor 5, which is provided for driving at least two wheels 3 of the motor vehicle 1. The electric motor 5 can also be provided for driving all four wheels 3 of the motor vehicle 1. In an alternative embodiment, which is not explicitly shown in the figures, an electric motor 5 can be provided on at least one of the wheels 3, in particular on each of the wheels 3, of the motor vehicle 1.

[0038] The electric motor 5 is supplied with electrical energy via a motor control 7, which is provided by the fuel cell system 2.

[0039] The fuel cell system 2 comprises at least one fuel cell 4; an air supply path 6, which is designed to supply oxygen-containing air from the environment to the at least one fuel cell 4, in particular to a cathode of the at least one fuel cell 4, and an exhaust air path 8, which is designed to discharge an exhaust air stream from the at least one fuel cell 4, in particular from the cathode of the at least one fuel cell 4, and to discharge it into the environment.

[0040] The fuel cell system 2 may also have at least one fuel cell stack comprising a plurality of fuel cells 4.

[0041] In the supply air path 6, in the Fig. 1 Components not shown may be provided for filtering and / or compressing the supplied air.

[0042] In the exhaust air path 8, in the Fig. 1 Components not shown, e.g. valves and / or turbines, may be provided.

[0043] The fuel cell system 2 also comprises an exhaust air return path 10, which is designed to return a return air flow 19, which is branched off from the exhaust air flow from the exhaust air path 8, into the supply air path 6.

[0044] A fuel cell system 2, which may comprise multiple fuel cell stacks, may also comprise multiple exhaust air recirculation paths 10. The exhaust air recirculation paths 10 may be coupled across stacks.

[0045] In the exhaust air path 8, a branching device 12 is provided, which is designed to branch off a partial flow as return air flow 19 without a significant proportion of liquid water from the exhaust air flow 14 and to guide it into the exhaust air return path 10.

[0046] The transfer of the return air flow 19 from the exhaust air path 8 into the supply air path 6 can be passive, ie solely due to a pressure difference between the exhaust air path 8 and the supply air path 6.

[0047] Optionally, a fan or compressor 13 can be provided in the exhaust air return path 10, which supports the transfer of the return air flow 19 from the exhaust air path 8 into the supply air path 6.

[0048] An exhaust air return valve 15 can also be provided in the exhaust air return path 10, which makes it possible to selectively release or block the transfer of the return air flow 19 from the exhaust air path 8 into the supply air path 6.

[0049] The exhaust air recirculation valve 15 can be a switching valve that can only be switched between an open state, in which it releases the return air flow 19, and a closed state, in which it blocks the return air flow 19. Alternatively, the exhaust air recirculation valve 15 can also be a variable valve that allows the return air flow 19 to be variably regulated in several stages or continuously.

[0050] In addition to the exhaust air return path 10, a bypass path 24 can also be provided, which makes it possible to guide the supply air flow from the supply air path 6 partially or completely past the at least one fuel cell 4 into the exhaust air path 8.

[0051] Fig. 2 shows a schematic sectional view of a first embodiment of a branching device 12 according to the invention.

[0052] The Fig. The branching device 12 shown in Fig. 2 comprises a section of the exhaust air path 8. In this embodiment, the exhaust air path 8 in the branching device 12 is aligned parallel to the gravitational field, so that a weight force F G which is aligned parallel to the flow direction of the exhaust air stream 14.

[0053] In a region of the branching device 12, a nozzle-shaped constriction 16 is formed in the exhaust air path 8. The flow velocity of the exhaust air stream 14 flowing through the nozzle-shaped constriction 16 is increased.

[0054] Upstream of the outlet of the nozzle-shaped constriction 16, a branch channel 18 is formed in a flow-calmed zone 17 outside the nozzle-shaped constriction 16. The branch channel 18 is part of the exhaust air return path 10 or is fluidly connected to the exhaust air return path 10.

[0055] The branch duct 18 may have a smaller cross-section than the exhaust air path 8.

[0056] The exhaust air path 8 can, for example, have a diameter d0 in the range of 45 mm to 90 mm.

[0057] The nozzle-shaped constriction 16 can, for example, have a diameter d1 in the range of 50% to 90% of the diameter d0 of the exhaust air path 8 at its narrowest point.

[0058] The branch channel 18 can, for example, have a diameter d2 in the range from 18 mm to 40 mm.

[0059] These specifications refer to mass flows in the exhaust path in the range of 80-140 g / s. For smaller or larger mass flows, the above-mentioned diameters d0, d1, and d2 must be scaled accordingly.

[0060] When the exhaust air recirculation valve 15 is fully or partially opened, a portion of the exhaust air flow 14 flowing through the exhaust air path 8 is branched off from the exhaust air flow 14 after the outlet of the nozzle-shaped constriction 16 and flows against the weight force F G into the branch duct 18 and through the exhaust air return path 10 into the supply air path 6. The branched part of the exhaust air flow 14 is returned as return air flow 19 into the at least one fuel cell 4 through the supply air path 6.

[0061] By aligning the exhaust air path 8 in the flow direction “downwards”, ie parallel to the weight force F g , and by aligning the branch channel 18 “upwards”, ie against the weight force F g , it can be prevented that a significant amount of liquid water contained in the exhaust air stream 14 flowing through the exhaust air path 8 enters the exhaust air return path 10 and is returned through the supply air path 6 together with the return air stream 19 into the at least one fuel cell 4.

[0062] Fig. 3 shows a variant of the Fig. 2, in which the exhaust air path 8 is widened around the nozzle-shaped constriction 16. As a result, the flow-calmed zone 17, which is formed around the nozzle-shaped constriction 16 and from which the branch duct 18 branches off, can be larger than in the Fig. 2. This results in the flow in the flow-calmed zone 17 being able to be further calmed, and liquid water contained in the exhaust air stream 14 flowing through the exhaust air path 8 being able to be separated even more effectively from the partial flow passing through the branch duct 18 into the exhaust air return path 10.

[0063] Another variant of the Fig. 2 shown branching device 12 are in the Fig. 4 and Fig. 5 shown.

[0064] In these variants, the exhaust air path 8 is not vertical, ie not parallel to the gravitational force F G , aligned.

[0065] In the examples shown in the Fig. 4 and Fig. 5, the exhaust air path 8 is rather inclined to gravitational force F G aligned so that it forms a more ( Fig. 4) or less ( Fig. 5) has a steep gradient. The exhaust air path 8 can, for example, have a gradient in the range of 3% to 16%, in particular a gradient in the range of 5% to 13%.

[0066] An exhaust air path 8 which has a slight gradient can under certain circumstances be better accommodated in a vehicle 1 than an exhaust air path 8 which is aligned vertically.

[0067] Fig. 6 shows a further embodiment of a branching device 12 according to the invention, in which a flow guide surface 20 is provided in the exhaust air path 8 instead of a nozzle-shaped constriction 16. The flow guide surface 20 can be arranged in particular upstream of the opening of the branch duct 18 into the exhaust air path 8. The flow guide surface 20 can, for example, be designed as a deflector plate arranged in the exhaust air path 8.

[0068] The downstream end of the flow guide surface 20 can in particular be designed as a drip edge 21, to which water, which is contained in the exhaust air flow 14, e.g. in the form of water droplets, is guided and drips off there without entering the branch channel 18.

[0069] By means of the flow guide surface 20, a flow-calmed zone 17 is created in an area located behind or downstream of the flow guide surface 20 along the flow direction of the exhaust air flow 14, from which zone a portion of the exhaust air flow 14 flowing through the exhaust air path 8 is branched off into the branch duct 18 and flows as a return flow through the branch duct 18 and the exhaust air return path 10 into the supply air path 6, as previously described.

[0070] Fig. 7 shows a variant of the Fig. 6 shown branching device 12, in which the exhaust air path 8 is not perpendicular, ie parallel to the gravitational force F G, but only arranged with a slight gradient.

[0071] In the examples shown in the Fig. 2 to 7, the exhaust air path 8 is straight in the area of ​​the branching device 12.

[0072] The Fig. 8 to 10 show embodiments of branching devices 12 according to the invention, in which the exhaust air path 8 is bent or curved in the region of the branching device 12.

[0073] The exhaust air path 8 can, for example, have a deflection angle / bend angle in the range of 80° to 160°.

[0074] A curvature of the exhaust air path 8 leads to a deflection of the exhaust air flow 14 in the exhaust air path 8. This deflection of the exhaust air flow 14 can result in water droplets contained in the exhaust air flow 14 being carried into an outer region of the curvature due to their inertia (“cyclone effect”) and thus not reaching the branch duct 18.

[0075] In the Fig. In the embodiment shown in Fig. 8, the exhaust air path 8 curves from a horizontal direction X, which is transverse to the direction of the gravitational force F G downwards in a vertical direction Y, parallel to the gravitational force F G is aligned.

[0076] The branch duct 18 opens into the curved area 11 of the exhaust air path 8 and extends from the exhaust air path 8 against the direction of the gravitational force F G up.

[0077] Fig. 9 shows a variant of the Fig. 8. In the embodiment shown in the Fig. In the embodiment shown in Fig. 9, a region 9 of the exhaust air path 8 arranged upstream of the curved region 11 is formed with a slight gradient.

[0078] Furthermore, an extension or bulge 22 of the exhaust air path 8 is formed in the area of ​​the curved section 11. The extension or bulge 22 of the exhaust air path 8 further reduces the flow velocity of the exhaust air stream 14 in the exhaust air path 8 in the curved section 11, and the cyclone effect is enhanced. As a result, water droplets contained in the exhaust air stream 14 in the exhaust air path 8 are separated even more effectively from the exhaust air stream 14, preventing them from entering the branch duct 18 with the partial air stream branched off as the return air stream 19.

[0079] Fig. 10 shows a further variant of a curved branching device 12. The exhaust air path 8 of the Fig. 10 is curved in a horizontal plane parallel to the plane of the drawing of the Fig. 10 and orthogonal gravitational force F G In other words, in the Fig. 10, the exhaust air path 8 extends both upstream and downstream of the curved region 11 substantially in an approximately horizontal direction with a slight gradient.

[0080] The branch duct 18 branches off from the exhaust air path 8 in the area of ​​the curved area 11 and extends against the gravitational force F G parallel to the gravitational force F G up.

[0081] The Fig. The embodiments of branching devices 12 shown in Figures 2 to 10 are only examples.

[0082] Further embodiments of branching devices 12 according to the invention, not explicitly shown in the figures, are also possible, which use gravity effects and / or inertia effects to branch off a partial air flow as return air flow 19 from the exhaust air flow 14, which does not contain a significant proportion of liquid water.

Claims

[1] Fuel cell system (2), in particular for providing electrical energy for driving at least one electric motor (5) in a motor vehicle (1), with at least one fuel cell (4); an air supply path (6) which is designed to supply air from the environment to the at least one fuel cell (4); an exhaust air path (8) which is designed to discharge an exhaust air stream (14) from the at least one fuel cell (4); an exhaust air return path (10) designed to return a return air flow (19) branched off from the exhaust air flow (14) to the supply air path (6); and a branching device (12) which is designed to branch off a partial air flow from the exhaust air flow (14) and to guide it as a return air flow (19) into the exhaust air return path (10); wherein the branching device (12) is designed to use at least one gravity effect to branch off the partial air flow (19) from the exhaust air flow (14) without a significant proportion of liquid water. [2] Fuel cell system (2) according to claim 1, wherein the branching device (12) is additionally designed to use at least one inertia effect to branch off the partial air flow (19) from the exhaust air flow (14) without a significant proportion of liquid water. [3] Fuel cell system (2) according to claim 1 or 2, wherein the branching device (12) is designed to change a flow direction of the partial air flow (19) in relation to the exhaust air flow (14) and / or to allow the exhaust air flow (14) to flow over a drip edge or flow guide surface (21). [4] Fuel cell system (2) according to claim 3, wherein the exhaust air path (8) is formed with at least one curved region (11) and wherein the exhaust air return path (10) branches off from the exhaust air path (8) in the region of the at least one curved region (11). [5] Fuel cell system (2) according to claim 4, wherein a swirling region (22) is formed in the region of the at least one curved region (11); wherein the swirling region (22) comprises, in particular, a region of the exhaust air path (8) which has an expanded cross-section. [6] Fuel cell system (2) according to one of the preceding claims, wherein the branching device (12) is designed to generate a swirling flow in the exhaust air flow (14) and / or to increase the flow velocity of the exhaust air flow (14). [7] Fuel cell system (2) according to claim 6, wherein a constriction of the flow cross-section, in particular a nozzle (16), is formed in the exhaust air path (8). [8] Fuel cell system (2) according to claim 7, wherein the flow cross-section of the exhaust air path (8) is extended by the nozzle (16) and / or wherein the branch of the return path is arranged in a flow-calmed region (17) or a return flow region of the exhaust air fluid. [9] Fuel cell system (2) according to one of the preceding claims, wherein the branching device (12) comprises an exhaust air path (8) descending in the flow direction of the exhaust air flow (14) and an exhaust air return path (10) rising in the flow direction of the return air flow (19). [10] Fuel cell system (2) according to claim 9, wherein the exhaust air path (8) in the flow direction of the exhaust air stream (14) has a gradient of at least 5%, in particular a gradient of at least 10%, preferably a gradient of at least 15%, relative to a horizontal. [11] Fuel cell system (2) according to one of the preceding claims, wherein the exhaust air return path (10) has a smaller flow cross-section than the exhaust air path (8). [12] Fuel cell system (2) according to one of the preceding claims, wherein at least one flow guide surface (20) is formed in the exhaust air path (8); wherein the at least one flow guide surface (20) is arranged in particular upstream of a flow inlet of the exhaust air recirculation path (10) in the exhaust air path (8). [13] Fuel cell system (2) according to one of the preceding claims, wherein an exhaust air recirculation valve (15) and / or a fan (13) is provided in the exhaust air recirculation path (10). [14] Motor vehicle (1) with a fuel cell system (2) according to one of the preceding claims. [15] Method of operating a fuel cell system (2) according to one of claims 1 to 13, wherein the method comprises branching off a partial air flow from an exhaust air flow (14) of the at least one fuel cell (4) without a significant proportion of liquid water and returning it as a return air flow (19) to the supply air path (6) of the at least one fuel cell (4), wherein the method in particular comprises regulating the return air flow (19) with the aid of an exhaust air recirculation valve (15).

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