Membranstapel

Abstract

The invention relates to a membrane stack (1) of a humidifier. The membrane stack (1) comprises several membrane layers (2), several first spacers (3a), and several second spacers (3b). The membrane layers (2) and the spacers (3a, 3b) are stacked alternately in a stacking direction (ST). The respective first spacer (3a) and the adjacent membrane layers (2) form a membrane unit (4). In the membrane unit (4), a membrane segment (12) is wound around the spacer (3a), with the ends (12a, 12b) of the membrane segment (12) overlapping in an overlapping area (13). The invention also relates to a humidifier with the membrane stack (1) and a housing.

Description

[0001] The invention relates to a membrane stack of a humidifier for a fuel cell system, in particular for a motor vehicle, according to the preamble of claim 1. The invention also relates to a humidifier with the membrane stack.

[0002] A humidifier can be used in a fuel cell system with a fuel cell. The humidifier can humidify dry air flowing to the fuel cell with moist air flowing from the fuel cell. The fuel cell is typically a stack, i.e. a stack of several fuel cell elements stacked on top of one another. The humidifier typically comprises a membrane stack through which dry air and moist air can flow in a cross-flow without mixing. The membrane stack typically contains several flat membranes and several spacers stacked alternately in a stacking direction. Channels on the moist air side and channels on the dry air side are formed between the membranes; these channels alternate in the stacking direction and are separated from each other by the membranes.The membranes are airtight and moisture-permeable, allowing dry air and moist air to transfer moisture across the membranes. Typically, the membranes and spacers in the membrane stack are bonded together using an adhesive. The adhesive also forms the dry air and moist air channels between the membranes. Because the number of membranes and spacers is high, the effort required to manufacture the membrane stack is correspondingly high.

[0003] DE 10 2007 029 596 A1 discloses a membrane stack in which a membrane piece is wrapped around a spacer on one side, thus forming two membranes from each membrane piece. Unfortunately, the effort required to bond the membranes to the spacer remains high.

[0004] The object of the invention is therefore to provide an improved or at least alternative embodiment for a membrane stack of a humidifier of the generic type, in which the described disadvantages are overcome. The object of the invention is also to provide a corresponding humidifier.

[0005] This object is achieved according to the invention by the subject matter of the independent claims. Advantageous embodiments are the subject matter of the dependent claims.

[0006] The present invention is based on the general idea of ​​providing a membrane stack with a special structure of the membrane layers, wherein the spacers are wrapped around the membrane to form a membrane layer, so that the ends of the membrane can be connected to one another by the wrapping.

[0007] The membrane stack of a humidifier according to the invention is provided or designed for a fuel cell system, in particular for a vehicle. The humidifier can be provided or designed, in particular, for humidifying dry air flowing to a fuel cell of the fuel cell system with moist air flowing from the fuel cell of the fuel cell system. The membrane stack has a plurality of flat membrane layers, wherein the respective membrane layers are stacked spaced from one another in a stacking direction. The membrane stack also has a plurality of first spacers and a plurality of second spacers, wherein the first spacers and the second spacers are arranged alternately between the membrane layers in the stacking direction.The membrane stack can be flowed through transversely to the stacking direction through the respective first spacers in a first flow direction and through the respective second spacers in a second flow direction oriented transversely to the first flow direction. The respective first spacer and the membrane layers adjacent to the respective first spacer form or constitute a coherent membrane unit. The membrane layers of the respective membrane unit are formed by a coherent membrane piece, wherein the membrane piece is wound on the first spacer in a winding direction encircling the first flow direction. In the respective membrane unit, the membrane piece is wound completely around the first spacer according to the invention, wherein a first end of the membrane piece and an opposite second end of the membrane piece overlap and lie one above the other in an overlap region.

[0008] The respective membrane layers or the respective membrane section can be airtight and permeable to moisture or water vapor, so that in the membrane stack the moist air and the dry air cannot flow through the membrane layers and yet can exchange moisture or water vapor with each other. The membrane stack can be cuboid-shaped and the membrane layers can be flat and rectangular. The respective first spacers can form first channels through which flow can occur between the respective adjacent membrane layers, and the respective second spacers can form second channels through which flow can occur between the respective adjacent membrane layers. Since the first spacers and the second spacers alternate in the stacking direction, the first channels and the second channels can also alternate in the stacking direction.The first channels can flow in the first flow direction and the second channels can flow in the second flow direction and therefore the membrane stack can flow through in cross-flow without mixing.

[0009] In the respective membrane unit, the two membrane layers are formed by a continuous membrane piece. For this purpose, the membrane piece can be expediently flexible. The membrane piece is completely wound around the first spacer. In other words, the continuous membrane piece forms two membrane layers that rest against the first spacer on both sides. In the overlapping region, the two ends of the membrane piece also lie on top of one another. In other words, the first / second end of the membrane piece is arranged between the second / first end of the membrane piece and the first spacer in the stacking direction. The length of the membrane piece defined in the first flow direction can expediently be identical to the length of the first spacer defined in the first flow direction, so that the first spacer is open transversely to the first flow direction and can therefore be flowed through.The total width of the membrane piece defined in the winding direction can be expediently greater than twice the width of the first spacer defined transversely to the first flow direction and transversely to the stacking direction in order to be able to wrap around the first spacer and form the overlay region.

[0010] The overlapping region can be arranged on the first spacer, in particular at the edge. The overlapping region can be arranged on the first spacer in the stacking direction between the first spacer and the second spacer. In other words, the overlapping region can be arranged within the membrane stack. By overlapping the two ends of the membrane piece in the overlapping region, the two ends of the membrane piece are better protected from the moisture contained in the humid air compared to open ends of the membranes in a conventional membrane stack. Firstly, the first / second end of the membrane piece lies between the second / first end of the membrane piece and the first spacer and is not directly exposed to the moisture contained in the humid air.Secondly, the second / first end is not directly exposed to the humid air flow and is therefore better protected from the moisture contained in the humid air. Overall, this minimizes or at least reduces the risk of delamination of the multilayer membrane layers or the multilayer membrane section.

[0011] In the respective membrane unit, the first spacer can be formed by an elongated first edge strip and an elongated second edge strip, and a flow element through which a flow can pass. The two edge strips can be aligned in the first flow direction, with the flow element arranged between them. The overlapping region of the membrane piece can then be arranged on the first edge strip and aligned transversely to the stacking direction. The two edge strips and the flow element can be integrally connected to one another, formed from a single piece of material, or inseparable or non-detachable from one another. The first spacer can, for example, be formed from plastic using an injection molding process.The flow element through which the flow can pass can, for example, have a wave structure oriented in the first flow direction and / or a rib structure through which the flow can pass in the first flow direction and / or a different structure. The respective edge strips can be elongated and almost cuboid-shaped.

[0012] The height of the two edge strips, defined in the stacking direction, can specify the height of a first channel formed between the membrane layers and through which flow passes. The width of a first channel formed between the membrane layers and through which flow passes, defined transversely to the stacking direction and transversely to the first flow direction, can be specified by the width of the flow element defined transversely to the stacking direction and transversely to the first flow direction. The length of a first channel formed between the membrane layers and through which flow passes, defined in the first flow direction, can be specified by the length of the flow element or the first spacer or the respective edge strip defined in the first flow direction.

[0013] In the respective membrane unit, the first end of the membrane piece can be integrally connected in the overlap region to the second end and / or to the first edge strip of the first spacer. In particular, the first end can be glued and / or welded, in particular ultrasonically welded or laser welded, to the second end of the membrane piece and / or to the first edge strip of the first spacer. In the respective membrane unit, at least one of the membrane layers can be integrally connected to the second edge strip of the first spacer. In particular, the respective membrane layer can be glued and / or welded, in particular ultrasonically welded or laser welded, to the second edge strip of the first spacer. In this case, an adhesive seam oriented in the first flow direction and / or a weld seam oriented in the first flow direction can be formed on the respective membrane unit.The respective adhesive seam and / or weld seam can additionally protect the two ends of the membrane piece from delamination.

[0014] In the respective membrane unit, the width of the overlapping region of the membrane piece, defined transversely to the stacking direction and transversely to the first flow direction, can be equal to or greater than 50% and / or equal to or less than 75% of the width of the first edge strip of the first spacer, defined transversely to the stacking direction and transversely to the first flow direction. In other words, the overlapping region can be arranged entirely on the first edge strip, transversely to the stacking direction and transversely to the first flow direction.

[0015] The overlay region may be arranged on the first edge strip in the stacking direction between the first edge strip and the second spacer.

[0016] In the respective membrane unit, the shorter end of the membrane piece can be arranged above the longer end of the membrane piece in the overlapping region of the membrane piece. In other words, the first end of the membrane piece wrapped around the first edge strip of the first spacer can lie above the second end of the membrane piece wrapped around the second edge strip of the first spacer. By arranging the ends of the membrane piece in this way, the longer second end and thus the entire membrane layer are held on one side by the first edge strip and on the other side by the shorter first end, and thus on both sides, thereby stabilizing the membrane layers and the respective membrane unit.

[0017] For each membrane unit, the height of the respective membrane unit, defined in the stacking direction, at the first edge strip of the first spacer and at the second edge strip of the first spacer can be the same or identical. For this purpose, the height of the first edge strip of the first spacer, defined in the stacking direction, can be reduced by at least the thickness of the membrane piece compared to the height of the second edge strip of the first spacer, defined in the stacking direction. As a result, an increase in the height of the membrane unit at the first edge strip of the first spacer can be compensated for by the ends of the membrane piece overlapping in the overlapping region. If the ends of the membrane piece are welded to the first edge strip, the height of the first edge strip can be reduced by exactly the thickness of the membrane piece.If the ends of the membrane piece are bonded to the first edge strip, the height of the first edge strip can be reduced by the thickness of the membrane piece plus the thickness of the adhesive layers. By reducing the height of the first edge strip, the height of the additional layer of the membrane piece in the overlap area can be compensated, so that the respective membrane units cannot cause a height shift or asymmetry of the membrane stack when stacking the membrane stack.

[0018] In the respective membrane unit, the respective edge strip of the first spacer can have an external rounded portion. The rounded portion can be rounded in the winding direction and extend along the entire edge strip in the first flow direction. The rounded portion can be formed, in particular, on a side of the respective edge strip facing away from the flow element. The rounded portion can, in particular, be shaped such that the membrane piece can be wound around the first edge strip and the second edge strip. The rounded portion can, in particular, prevent damage to the membrane piece at corners of the respective edge strip.

[0019] In the membrane stack, the respective second spacers can be shaped identically to the respective first spacers. Alternatively, the respective first spacers and the respective second spacers can differ in shape and / or size and / or material. For example, the respective second spacer can be formed by a flow element through which flow can pass and two elongated adhesive strips aligned in the second flow direction. The adhesive strips can have several connected or separate adhesive sections. The adhesive strips can be arranged on the edges of the respective membrane layers adjacent to the second spacer and can bond the two adjacent membrane layers to the flow element. A second channel through which flow can pass in the second flow direction can thus be formed between the two adhesive strips.

[0020] The invention also relates to a humidifier for a fuel cell system having a fuel cell for humidifying dry air flowing to the fuel cell with moist air flowing from the fuel cell. The humidifier comprises the above-described membrane stack and a housing. The membrane stack is accommodated in the housing and sealed to the housing such that the dry air and the moist air flow through the housing and the membrane stack without mixing. Furthermore, the membrane stack is aligned in the housing such that the dry air flows through the membrane stack in the first flow direction through the respective membrane units, and the moist air flows through the membrane stack in the second flow direction. Due to this alignment of the membrane stack in the humidifier, the dry air flows through the first spacers and the moist air flows through the second spacers.Since the pressure prevailing in the humid air is lower than the pressure prevailing in the dry air, the overlay area of ​​the membrane piece of the respective membrane unit - which is adjacent to the second spacer and open due to the structure of the membrane stack - can be protected.

[0021] Further important features and advantages of the invention emerge from the subclaims, from the drawings and from the associated description of the figures based on the drawings.

[0022] It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

[0023] Preferred embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description, wherein the same reference numerals refer to the same or similar or functionally identical components.

[0024] They show, schematically Fig. 1 a partial view of a membrane stack according to the invention; Fig. 2 a partial side view of the membrane stack according to the invention; Fig. 3 to 5 views of a membrane unit of the membrane stack according to the invention; Fig. 6 and Fig. 7 partial side views of the membrane unit of the membrane stack according to the invention; Fig. 8 a partial view of the membrane unit of the membrane stack according to the invention; Fig. 9 a partial view of a spacer of the membrane unit of the membrane stack according to the invention; Fig. 10 a partial view of a membrane piece of the membrane unit of the membrane stack according to the invention; Fig. 11 is a partially exploded view of the membrane stack according to the invention; Fig. 12 a partial exploded view of the membrane stack according to the invention in a first embodiment; Fig. 13 a partial exploded view of the membrane stack according to the invention in a second embodiment.

[0025] Fig. 1 shows a partial view of a membrane stack 1 of a humidifier according to the invention. The membrane stack 1 comprises a plurality of membrane layers 2, which are stacked at a distance from one another in a stacking direction ST. Furthermore, the membrane stack 1 comprises a plurality of first spacers 3a and a plurality of second spacers 3b, which are stacked alternately between the membrane layers 2 in the stacking direction ST. The membrane layers 2 and the spacers 3a and 3b are flat and oriented transversely to the stacking direction ST, so that the membrane stack 1 is cuboid-shaped. The membrane layers 2 of the membrane stack 1 are airtight and permeable to moisture and water vapor.

[0026] In the membrane stack 1, the respective first spacers 3a and the respective membrane layers 2 adjacent to the respective first spacer 3a each form a coherent membrane unit 4. The structure of the respective membrane unit 4 is described below with reference to Fig. 3 to Fig. 5. In the membrane stack 1, the membrane units 4 and the second spacers 3b are stacked alternately in the stacking direction ST. The respective membrane units 4 are glued to the second spacers 3b, as will be explained below with reference to Fig. 11 to Fig. 13. The membrane stack 1 thus forms a coherent, inseparable unit that can be installed in a humidifier.

[0027] Due to its structure, the membrane stack 1 has four side surfaces 5a, 5b and 6a, 6b aligned parallel to the stacking direction - see also Fig. 11 to Fig. 13 - and two end faces 7a and 7b oriented transversely to the stacking direction ST. The first spacers 3a are shaped such that they connect the opposing side faces 5a and 5b of the membrane stack 1 to one another in an air-conducting manner and are fluidically separated from the other opposing side faces 6a and 6b of the membrane stack 1. In other words, the first spacers 3a are shaped such that first channels through which flow is formed are formed between the membrane layers 2 adjacent to them. The second spacers 3b are shaped such that they connect the opposing side faces 6a and 6b of the membrane stack 1 to one another in an air-conducting manner and are fluidically separated from the other opposing side faces 5a and 5b of the membrane stack 1. In other words, the second spacers 3b are shaped such that second channels through which flow is formed between the membrane layers 2 adjacent to them.The end faces 7a and 7b are formed by the respective last membrane layers 2 in the stacking direction ST.

[0028] The first spacers 3a or the first channels can be flowed through in a first flow direction SRa and the second spacers 3b or the second channels can be flowed through in a second flow direction SRb. The first flow direction SRa and the second flow direction SRb are aligned perpendicular to one another and perpendicular to the stacking direction ST. If the membrane stack 1 is installed in a humidifier, dry air and moist air can flow through the membrane stack 1 in a cross-flow manner without mixing. Preferably, the dry air can flow through the first spacers 3a in the first flow direction SRa and the moist air can flow through the second spacers 3b in the second flow direction SRb. Since the membrane layers 2 are airtight and moisture-permeable, the dry air and the moist air can flow through the membrane stack 1 without mixing and exchange moisture with each other.

[0029] In the illustrated embodiment, the first spacers 3a and the second spacers 3b are shaped differently from each other. The first spacer 3a is integrally formed and has two edge strips 8a and 8b - see also Fig. 3 to Fig. 7 - and a flow element 9 arranged therebetween. The two edge strips 8a and 8b are aligned in the first flow direction SRa and delimit the first spacer element 3a to the outside. The flow element 9 has a wave structure 10 through which flow can pass in the first flow direction SRa. The structure of the first spacer 3a is described below with reference to Fig. 6 and Fig. 7. The second spacer 3b has the flow-through flow element 9 with the wave structure 10 and two adhesive strips 11, each with two adhesive sections, which bond the flow element 9 to the adjacent membrane layers 2 or to the adjacent membrane units 4. The flow element 9 and the adhesive strips 11 are aligned such that the second spacer 3b can be flowed through in the second flow direction SRb. In principle, the second spacer 3b can also be designed differently - see Fig. 13 - be built.

[0030] Fig. Figure 2 shows a partial side view of the membrane stack 1 according to the invention from the side surface 6a. Here, the structure of the second spacer 3b is particularly clearly visible.

[0031] Fig. 3 to Fig. 5 show views of the membrane unit 4 of the membrane stack 1 according to the invention. The respective membrane unit 4 comprises - as already described above - the first spacer 3a and two membrane layers 2 adjacent to the spacer 3a. The membrane layers 2 of the respective membrane unit 4 are formed by a continuous membrane piece 12, wherein the membrane piece 12 is wound around the first spacer 3a in a winding direction WR surrounding the first flow direction SRa. The membrane piece 12 forms one membrane layer 2 when it rests on one side of the first spacer 3a and the other membrane layer 2 when it rests on the other side of the first spacer 3a. The membrane layers 2 are therefore part of the membrane piece 12 and are connected to one another. A shorter first end 12a of the membrane piece 12 lies over a second, longer end 12b of the membrane piece 12, so that an overlay region 13 is formed.The overlay region 13 is formed on the first edge strip 8a and lies in the stacking direction ST between the first spacer 3a and the adjacent second spacer 3b.

[0032] In Fig. 3, the membrane unit 4 is shown, wherein the shorter first end 12a and the longer second end 12b of the membrane piece 12 are detached from the first spacer 3a.

[0033] In Fig. 4, the membrane unit 4 is shown, wherein the longer second end 12b of the membrane piece 12 abuts the first spacer 3a and the shorter first end 12a is detached from the first spacer 3a.

[0034] In Fig. 5 shows the membrane unit 4, with the shorter first end 12a and the longer second end 12b of the membrane piece 12 abutting the first spacer 3a. The two ends 12a and 12b of the membrane piece 12 are welded to each other and to the first edge strip 8a of the first spacer 3a via a weld seam 14. Since the shorter first end 12a of the membrane piece 12 lies above the longer second end 12b of the membrane piece 12, the longer end 12b is held at the first end 12a and at the edge strip 8a, and thus on both sides.

[0035] Fig. 6 and Fig. 7 show partial side views of the membrane unit 4 of the membrane stack 1 according to the invention.

[0036] Referring to Fig. 6, the overlay region 13 is formed on the first edge strip 8a of the first spacer 3a. The width B_13 of the overlay region 13, defined transversely to the stacking direction ST and the first flow direction SRa, is smaller than the width B_8a of the first edge strip 8a, defined transversely to the stacking direction ST and the first flow direction SRa. The width B_13 of the overlay region 13 can be between 50% and 75% of the width B_8a of the first edge strip 8a.

[0037] Referring to Fig. 7, the height H_4 of the respective membrane unit 4 at the first edge strip 8a, defined in the stacking direction ST, is composed of three times the thickness D_12 of the membrane piece 12 and the height H_8a of the first edge strip 8a, defined in the stacking direction ST. The height H_4 of the respective membrane unit 4 at the second edge strip 8b, defined in the stacking direction ST, is composed of twice the thickness D_12 of the membrane piece 12 and the height H_8b of the first edge strip 8b, defined in the stacking direction ST. In order for the height H_4 of the respective membrane unit at the first edge strip 8a and at the second edge strip 8b to be identical, the height H_8a of the first edge strip 8a is reduced by one times the thickness D_12 of the membrane piece 12 compared to the height H_8b of the second edge strip 8b.It is understood that when bonding the two ends 12a and 12b to the edge strip 8a, the thickness of the adhesive layers can also be taken into account so that the height H_4 of the membrane unit 4 remains identical at both edge strips 8a and 8b.

[0038] Referring to Fig. 6 and Fig. 7, the first spacer 3a has rounded portions 15a and 15b on the edge strips 8a and 8b. The rounded portions 15a and 15b are formed on the outside of the edge strips 8a and 8b and facing away from the flow element 9. The rounded portions 15a and 15b enable the membrane piece 12 to be wrapped around the first spacer 3a and prevent damage to the membrane piece 12 at protruding corners of the edge strips 8a and 8a. The respective rounded portions 15a and 15b expediently run in the winding direction WR and extend in the first flow direction SRa along the entire edge strip 8a and 8b.

[0039] Fig. Figure 8 shows a partial view of the membrane unit of the membrane stack 1 according to the invention at the second edge strip 8b. In principle, the membrane piece 12 can also be glued or welded to the second edge strip 8b on one membrane layer 2 or on both membrane layers 2.

[0040] Fig. Figure 9 shows a partial view of the first spacer 3a of the membrane unit 4 of the membrane stack 1 according to the invention at the second edge strip 8b. Here, the rounded portion 15b on the second edge strip 8b is particularly clearly visible.

[0041] Fig. 10 shows a partial view of the membrane piece 12 of the membrane unit 1 of the membrane stack 1 according to the invention. Here, in particular, the overlay region 13 is clearly visible, which is formed on the first edge strip 8a - not shown here.

[0042] Fig. 11 shows a partial exploded view of the membrane stack 1 according to the invention. As can be seen particularly clearly here, the second spacers 3b are arranged between the membrane units 4 and glued to them.

[0043] Fig. Figure 12 shows a partially exploded view of the membrane stack according to the invention in a first embodiment. The first embodiment shown here corresponds to the one shown in Fig. 1 to Fig. 10. In the first embodiment, the second spacer 3b is formed by the flow element 9 with the wave structure 10 and two adhesive strips 11, which bond the flow element 9 to the membrane layers 2 of the respective adjacent membrane units 4. An adhesive is first applied to the membrane unit 4 and, after the flow element 9 has been arranged on the membrane unit 4, to the flow element 9. The next membrane unit 4 is then arranged on the flow element 9. Two adhesive applications are therefore necessary here. When the adhesive hardens, the adhesive strips 11 are formed on the flow element 9 and the flow element 9 is thereby bonded to the adjacent membrane layers 2 of the adjacent membrane units 4.

[0044] Fig.13 shows a partial exploded view of the membrane stack 1 according to the invention in a second embodiment. Here, the second spacer 3b is shaped differently from the first embodiment of the membrane stack 1 and has the flow element 9 with a grid structure 16 and two adhesive strips 11. The adhesive strips 11 bond the flow element 9 to the membrane layers 2 of the respective adjacent membrane units 4. An adhesive is applied to the flow element 9, and the flow element 9 is arranged on the membrane unit 4, and the next membrane unit 4 is arranged on the flow element 9. Thus, only one adhesive application is necessary here.

[0045] When the adhesive hardens, the adhesive strips 11 are formed on the flow element 9 and thereby the flow element 9 is bonded to the adjacent membrane layers 2 of the adjacent membrane units 4. List of reference symbols 1 membrane stack 2 membrane layers 3a / 3b spacers 4 Membrane unit 5a / 5b side surface 6a / 6b side surface 7a / 7b frontal surface 8a / 8b verge strip 9 Flow element 10 Wave structure 11 adhesive strips 12 membrane piece 12a / 12b End of the membrane piece 13 Overlay area 14 Weld seam 15a / 15b rounding 16 Lattice structure B_8a Width of the first edge strip B_13 Width of the overlay area D_12 Thickness of the membrane piece H_4 Height of the membrane unit H_8a / H_8b Height of the edge strip SRa First flow direction SRb Second flow direction ST stacking direction WR winding direction QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2007 029 596 A1

[0003]

Claims

[1] Membrane stack (1) of a humidifier for a fuel cell system, in particular for a vehicle, - wherein the membrane stack (1) has a plurality of flat membrane layers (2) and the respective membrane layers (2) are stacked spaced apart from one another in a stacking direction (ST), - wherein the membrane stack (1) has a plurality of first spacers (3a) and a plurality of second spacers (3b), and the first spacers (3a) and the second spacers (3b) are arranged alternately between the membrane layers (2) in the stacking direction (ST), - wherein the membrane stack (1) can be flowed through transversely to the stacking direction (ST) through the respective first spacers (3a) in a first flow direction (SRa) and through the respective second spacers (3b) in a second flow direction (SRb) oriented transversely to the first flow direction (SRa), - wherein the respective first spacer (3a) and the membrane layers (2) adjacent to the respective first spacer (3a) form a coherent membrane unit (4), - wherein the membrane layers (2) of the respective membrane unit (4) are formed by a continuous membrane piece (12), wherein the membrane piece (12) is wound on the first spacer (3a) in a winding direction (WR) surrounding the first flow direction (SRa), characterized by in that in the respective membrane unit (4) the membrane piece (12) is completely wound around the first spacer (3a), wherein a first end (12a) of the membrane piece (12) and an opposite second end (12b) of the membrane piece (12) overlap and lie one above the other in an overlapping region (13). [2] Membrane stack (1) according to claim 1, characterized by , - that in the respective membrane unit (4) the first spacer (3a) is formed by an elongated first edge strip (8a) and an elongated second edge strip (8b) and a flow element (9) through which a flow can pass, - that the two edge strips (8a, 8b) are aligned in the first flow direction (SRa) and the flow element (9) is arranged therebetween, and - that the overlay region (13) of the membrane piece (12) is arranged on the first edge strip (8a) and is aligned transversely to the stacking direction (ST). [3] Membrane stack (1) according to claim 2, characterized by , - that in the respective membrane unit (4), the first end (12a) of the membrane piece (12) in the overlapping region (13) is materially connected, preferably glued and / or welded, to the second end (12b) of the membrane piece (12) and / or to the first edge strip (8a) of the first spacer (3a), and / or - that in the respective membrane unit (4) at least one of the membrane layers (2) is integrally connected, preferably glued and / or welded, to the second edge strip (8b) of the first spacer (3a). [4] Membrane stack (1) according to claim 2 or 3, characterized by that in the respective membrane unit (4) the width (B_13) of the overlapping region (13) of the membrane piece (12) defined transversely to the stacking direction (ST) and transversely to the first flow direction (SRa) is equal to or greater than 50% and / or equal to or less than 75% of the width (B_8a) of the first edge strip (8a) of the first spacer (3a) defined transversely to the stacking direction (ST) and transversely to the first flow direction (SRa). [5] Membrane stack (1) according to one of claims 2 to 4, characterized by , - that in the respective membrane unit (4) in the overlapping area (13) of the membrane piece (12) the shorter end (12a) of the membrane piece (12) is arranged above the longer end (12b) of the membrane piece (12), and / or - that in the respective membrane unit (4), the first end (12a) of the membrane piece (12) wrapped around the first edge strip (8a) of the first spacer (3a) lies above the second end (12b) of the membrane piece (12) wrapped around the second edge strip (8b) of the first spacer (3a). [6] Membrane stack (1) according to one of claims 2 to 5, characterized by , - that in the respective membrane unit (4), a height (H_8a) of the first edge strip (8a) of the first spacer (3a) defined in the stacking direction (ST) is reduced by at least the thickness (D_12) of the membrane piece (12) compared to a height (H_8b) of the second edge strip (8b) of the first spacer (3a) defined in the stacking direction (ST), and / or - that in the respective membrane unit (4), the height (H_4) of the respective membrane unit (4) defined in the stacking direction (ST) is the same at the first edge strip (8a) of the first spacer (3a) and at the second edge strip (8b) of the first spacer (3a). [7] Membrane stack (1) according to one of claims 2 to 6, characterized by , - that in the respective membrane unit (4), the respective edge strip (8a, 8b) of the first spacer (3a) has a rounded portion (15a, 15b) on the outside, wherein the rounded portion (15a, 15b) is rounded in the winding direction (WR) and extends along the entire edge strip (8a, 8b) in the first flow direction (SRa), and / or - that in the respective membrane unit (4) the first spacer (3a) is made of plastic and / or is formed by an injection molding process. [8] Membrane stack (1) according to one of the preceding claims, characterized by , - that the respective first spacers (3a) and the respective second spacers (3b) differ in shape and / or size and / or material, or - that the respective second spacers (3b) are shaped identically to the respective first spacers (3a). [9] Membrane stack (1) according to one of the preceding claims, characterized by , - that the respective second spacer (3b) is formed by a flow element (10, 16) through which a flow can flow and two elongated adhesive strips (11) aligned in the second flow direction (SRb), and - that the adhesive strips (11) are arranged on the edge side of the respective membrane layers (2) adjacent to the second spacer (3b) and bond the two adjacent membrane layers (2) to the flow element (10, 16) through which flow can take place. [10] Humidifier for a fuel cell system, in particular for a vehicle, with a fuel cell for humidifying a dry air flowing to the fuel cell with a moist air flowing from the fuel cell, - wherein the humidifier comprises the membrane stack (1) according to one of the preceding claims and a housing, - wherein the membrane stack (1) is accommodated in the housing and sealed to the housing in such a way that the dry air and the moist air flow through the housing and the membrane stack (1) without mixing, and - that the membrane stack (1) is aligned in the housing such that the dry air flows through the membrane stack (1) in the first flow direction (SRa) through the respective membrane units (4) and the moist air flows through the membrane stack (1) in the second flow direction (SRb).

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