The present application relates to a humidifier, preferably to a humidifier for process gases of fuel cells, as well as to a fuel cell system which comprises such a humidifier.

Fuel cells among others use gaseous process media, for example molecular hydrogen and/or air or oxygen for producing electricity.

Such fuel cells usually use a proton exchange membrane (PEM) . Such a PEM reaches to about 80 to 90 °C in service. It is important for the efficiency of the fuel cell as well as for the durability of the PEM, that in the environment of the PEM, the conditions with respect to temperature and humidity are kept rather stationary. It can be particularly disadvantageous for the durability and the efficiency of the fuel cell if the PEM runs dry. In order to deliberately adjust the degree of humidity of the process gases fed into the fuel cell, it is therefore common to humidify particular process gases before they are fed into a fuel cell. To this end, in known humidifiers, a water-permeable membrane is arranged between two flow plates, where the flow plate comprises channel structures. This water- permeable membrane separates a dry gas stream which needs to be humidified on the one side and a gas stream which needs to/can be dehumidified on the other side.

As the water-permeable membrane {at least for a minimum humidification) is essentially gas tight, the water content of the two gases in the humidifier almost balances out, while no mixture of the gases as such takes place.

It is however problematic that the known humidifiers are very expensive in their production and that the production and mounting tolerances need to be met very thoroughly in order to guarantee the desired degree of exchange of humidity and a clear separation of the gas streams . It is therefore the object of the present invention to provide for a humidifier and a fuel cell system with such humidifier, where the humidifier can be produced efficiently at large scale and where the humidifier due to a clever construction can be produced at low cost and be used without defects. This object is solved by a humidifier according to claim 1. This humidifier is particularly a humidifier for the humidification of process gases for fuel cells, which humidifier comprises:

- a first inlet for feeding dry gases as well as a first outlet for discharging of humidified gas as well as

- a second inlet for feeding humid gases as well as a second outlet for discharging of dehumidified gases,

- at least a first and a second flow plates as

well as a water transfer medium arranged between the first and the second flow plates which me- dium is essentially gas tight during service, where

- the first as well as the second flow plate each comprise channels for guiding gas and where on at least one flow plate at least in areas in the plane of the plane surface of the flow plate, the channel extend in such an undulating manner that the channels of the first as well as of the second flow plate at least in this areas do not engage with each other with positive fit.

As the channels of one flow plate at least in areas extend wave-shaped in such a manner that the channels of the first and the second flow plate at least in these areas do not engage with each other with posi- tive fit, it is always guaranteed that the lands between the channels overlap with sufficiently large areas and that the membrane located between the surfaces of the lands of the opposite flow plates are not shorn or stretched in an unnecessary manner. Such a shearing or stretching of the membrane - the water transfer medium - would not only lead to a destruction of the membrane - the water transfer medium - but also a deterioration of the efficiency of the humidifier. Such deterioration could for instance be caused by a local minimization of the effective channel cross section resulting from an engagement of the channel structures of the opposite flow plates with positive fit which could lead to a blocking or at least to an undesired increase of the gas flow resis- tance. Such negative effects are considerably reduced or even prevented by the invention, as the wave shape as such even when the plates are positioned only with reduced precision guarantees that the respective coverage of the surfaces of the lands is sufficient in order to on the one hand position the water transfer medium, the membrane and on the other hand to prevent all kinds of shearing.

In addition to the written description, this shall be emphasized by means of figures 4a and 4b. In the top views or views through the plates given in figures 4a and 4b, the two flow plates are shifted laterally, thus parallel to the flow direction, so that the upper plate extends beyond the edge of the lower plate on the right side and the lower plate extends beyond the upper plate on the left side. This shall only facilitate the explanation as the structures of the individual flow plates could otherwise not be recognized. Figures 4a and 4b also do not show the mem- brane for obviousness purposes. The cross sections are of schematic nature as the backsides of the flow plates are shown without structure.

On the left side of figure 4a, a section of a top view or a view through two flow plates arranged one on the other is shown. Here, the surfaces of the lands of the first flow plate are shown with a solid hatching that extends from the upper left to the lower right side . The surfaces of the lands of the opposite second flow plate are indicated with a dashed hatching that extends from the upper right to the lower left side. In a humidifier with extremely low production tolerances which is characterized by extremely high production cost, as it is given in the left part of figure 4a, an almost complete overlap of the surfaces of the lands is given.

The same is obvious from the right part of figure 4a. However, there the channels do not have a linear but an undulating course. The overlap of the channels in the left and the right part of the figure is empha- sized by the cross sections.

Especially for an industrial production at large scale, it is however important that a functional humidifier can be produced even when the actual or achievable tolerances are less strict. With the conventional system, this is however not possible as is obvious from the left part of figure 4b. One can realize that the surfaces of the lands of the one flow plate (indicated with the solid hatching leading from the upper left to the lower right side) and the surfaces of the lands of the opposite flow plate (given by the dashed hatching ascending from the left to the right) due to the large tolerances are shifted against each other and therefore engage with each other which almost leads to a positive fit. As a consequence, a membrane - not shown here - arranged between the two plates is shorn in an undesired manner or even destroyed, as is demonstrated in section C-C, showing the interdigitating positioning between the first 2.1 and the second plate 2.2. Especially when the pressure is increased this can even lead to the situation that the channel cross sections used for the conduct of the process gases are blocked and therefore that the humidifier only operates at non admissible low level.

This can be excluded with the concept according to the invention shown in the right part of figure 4b. Even with a torsion of the plane surface or a shift of the first and the second plate relative to each other, the wave-shape of the channel structures provides for a sufficient overlap of the surfaces of the lands, so that a destruction of the membrane - the water transfer medium - can be minimized or completely prevented.

Sections D-D and E-E clearly show that the channel structures do only incompletely overlap with each other. The overlap that remains between the channel structures is however sufficient for a water transfer through the membrane. This membrane is not shown in this sectional view.

In order to provide a sufficient support of the water transfer medium (e.g. of a humidifier membrane), it should be guaranteed that the overlap of the opposite surfaces of the lands of the two flow plates, thus of the surfaces of the lands, which partly also touch the humidifier membrane in all installed situations is at least 10%, preferably at least 20%, in spite of positioning and production tolerances. The active area is formed by that area of a flow plate, in which the channels extend as well as by the areas between the channels, the lands. The active area thus represents the complete area of the water transfer medium which can potentially conduct water. Typical shapes of the active area are rectangular shapes. The active area can however also have the shape of higher polygons, e.g. of hexagons. One usually chooses the shape of the active area in such a manner that the area available for the water transfer is maximized.

It has to be ascertained that a shearing of the water transfer medium, thus of the membrane or other layers arranged between the flow plates by the channels is prevented. With a concurrent guidance of the gases on both sides of the water transfer medium, the danger of a shifted compression is very probable, especially with straight courses of the channels in both interacting flow plates, see figure 4b left side. As a consequence, the main load direction of the local compression of the water transfer medium deviates from the orthogonal direction of the membrane surface and therefore leads to a shearing effect on the water transfer medium. Therefore, according to the invention, the cannels in at least one of the flow plates are designed wave- shaped in order to prevent such negative consequences. The amplitude of the wave structure should be at least the channel width of the opposite flow plate. On the other hand, the amplitude should not be too large, neither. It is preferred that the amplitude is at the most three times the width of the channel of the opposite flow plate. With this, the space available can be used in a most efficient way and the pressure loss over the flow channels is limited. If both flow channels possess wave- shaped channels, the structures should either have different wave lengths or be shifted relative to each other in the direction of the extension of the channel by at least a quarter and by at the most three quarters of a wavelength, thus by about half a wave length. With this, it is guaranteed that independent from a shift of the two flow plates, the local com- pression of the water transfer medium by the flow plates is always given with a main load direction orthogonal to the water transfer medium and that a shearing is effectively prevented. This way, one generally achieves that the overlap of the surfaces of the lands arranged opposite each other is sufficient in all installation situations.

It shall be mentioned that "to extend wave- shaped" shall mean that at least two channels (or lands} arranged one next to the other in a top view on the plane of the plate show an undulating shape, which means that in their course at least one wave crest follows a wave trough. This is different from coincidental wave shapes, which may occur in known flow plates, especially in their edge regions, and which aim on the sealing or compression of these plates. It is preferred that at least two channels area arranged one adjacent the other with each of these channels comprising at least five, preferably at least more than ten consecutive waves, each of them with a wave crest and a wave trough. It is most preferred if the complete channel structure of at least one flow plate consists of channels with such an undulating arrangement. One should however mention that it can also be sufficient for a realization of the inventive idea that the wave-shaped courses are only given in areas.

A vector decomposition of the gas flow in a channel in the plane of the plate results in a steady progression of a main flow direction which is overlapped by a flow direction which directs orthogonally to this main flow direction and which points alternately to both directions . This alternating movement does not necessarily need to extend along the whole length of the channel. A further advantageous embodiment of the invention provides that the first and/or second flow plate comprises passage holes for the conduct of media in the direction of the stack, with these passage holes being preferably arranged at the periphery of the plate and having a circular, oval or oblong shape. Due to this, it is possible that pairs of the flow plates can be stacked on each other in an arbitrary manner, in order to achieve a sufficient humidification.

In an advantageous embodiment of the invention, the first and/or second flow plate comprises sealing structures or one or several sealing coatings. It is for instance possible that for the sealing, elas- tomeric layers or elastomeric profiles are moulded, printed and/or applied to the surface. It is also possible to integrally form an individual bead - be it a full bead or a half bead - in a flow plate. This is particularly useful for the production at a large industrial scale, as in a flow plate, which is produced from metal, the sealing structures formed as beads can be stamped simultaneously with the stamping of the channel structures. It is also possible to combine these integral beads additionally with elastomeric sealing structures. The bead extends preferably in a circumferential way in the edge region of the area structured with channels . It is also possible that the first and/or second flow plate comprise connecting structures, which can be designed as hooks, zip looks or clips. Here, an interlocking connection of the flow plates can be achieved for instance by providing recesses in the material itself or by folding over of particular sections of the material. The plates can also be con- nected to each other by means of a circumferential tongue and groove connection. Multiple local connections comparable to a push button are possible as well. It is also possible that the flow plates are adhesively connected to each other. Advantageous variants are designed in such a way that metallic flow plates are welded to each other point- or line- shaped, especially by laser welding. With polymer- based flow plates, it is preferred that they are con- nected to each other by melting the polymeric material. These connections can take place in the actual plate area, especially in their peripheral areas, but also in lug-shaped projections. Furthermore, additionally or alternatively, connecting coatings are possible, thus adhesives which are located between the flow plates. In the same way, brazing connections or the like are possible as well.

It is however also possible that the flow plate con- prises a frame- shaped basic plate, where the channel structures are inserted as inlays. Here, the laid- in channel structure and the surrounding frame can consist in the same material or also in different materials. It is for instance possible to produce the frame from a polymeric material and the inlay channel structures from a metallic material. With the laid- in channel structures various embodiments are possible. These channel structures can be designed as woven material, as knitted material, as expanded skeleton, as lamellae, as dimpled foils, as wave structures or as stampings in plastically deformed materials (for instance, metallic plates, polymeric foams or metallic foams) . The channel structures can additionally comprise swirling structures in order to equilibrate hu- midity concentrations and/or the flow of the process gases . A further advantageous embodiment provides that the flow plates at least in areas are produced from thermoplastic, elastomeric or thermoset polymers and/or are at least in parts produced from corrosion-stable metallic materials, especially from stainless steel. Among the elastomers, a large variety of materials are possible, both elastomers which can be worked with by moulding and thermoplastic elastomers. In a further advantageous embodiment, the wetting properties of the flow plates are modified by plasma or reactive plasma treatment and/or in such a way that the flow plates have hydrophilic or hydrophobic coatings or microstructures . By a deliberate adaptation of the respective properties, it is possible to avoid accumulations of humidity or to achieve a balanced distribution of the humidity to the gas to be humidified. It is especially advantageous to coat the surfaces of the flow plates arranged on opposite sides of a membrane differently so that the transport of humidity is deliberately influenced.

In a further advantageous embodiment of the invention, the main flow direction of the channel structure of the first flow plate is arranged essentially parallel to the main flow direction of the second flow plate. This means that the resulting main flow direction of a dehumidified gas on the one side of the water transfer medium runs for instance parallel and in the same direction to the corresponding humidified gas on the other side of the water transfer medium. It is however also possible that the flow directions are parallel but with opposite directions, thus with counter flow. It is further also possible that these flow directions are in principal orthogonal to each other, so that a cross flow results. It shall also be mentioned that the directions mentioned in this sections, parallel or orthogonal, respectively, shall not be meant in a mathematically exact meaning, but that they include deviations of up to 2°, preferably up to 0.5° from the ideal angles

<0°/l80° or 90°/270°) .

In a further advantageous embodiment, the first and second flow plates are different from each other in at least one of material, channel shape, channel size and channel direction. It is however also possible that the first and second flow plate are formed identically. As far as the definition of these terms is concerned, especially with respect to the definition of the respective wave parameters, it is referred to figures 3d and 3e. It shall be emphasized that these figures shall not be understood in an exemplary way in the section describing the figures but that these definitions constitute a definition for the complete invention.

The water transfer medium can be designed in different ways. In a first embodiment, the water transfer medium is a membrane without reinforcement. This is possible with the invention given that the wave- shaped form according to the invention prevents a crushing / shearing of single sections of the membrane right from the beginning. An advantageous embodiment provides that a membrane without reinforcement is put on a support medium, e.g. a (graphite) fibre paper, a (graphite) fibre (non woven) fabric, a cloth, a fibre woven fabric or a fleece. Fibre weavings from natural and/or poly- meric fibres are possible, which are loosely laid onto the flow plate or which are bonded to the flow plate for an increase of the support function.

In a further advantageous embodiment, the water transfer medium is given as a reinforced membrane and/or this membrane is bonded to the first and/or second flow plates in areas so that a sufficient support effect for bridging the channels is achieved. This can be realized by a force- locking connection or in an adhesive way. It should be remarked that the bridging alone provides for the support effect, no additional support medium is required. In this context, the membrane is however reinforced.

In a further advantageous embodiment, the water transfer medium is a porous medium, e.g. a coated and/or impregnated woven material (Texapore ^® , Ven- turi ^® ) , as a laminate of membranes (Goretex ^® ) , as a membrane impregnated with ionomer, as a ionomeric membrane (Nafion ^® ) , as a hydrophilic non- ionic membrane or as a diaphragma.

The present invention also relates to a fuel cell system that comprises at least one humidifier according to the invention as well as at least one fuel cell connected to the humidifier. It is advantageous if a exhaust gas conduct from a cathode of the fuel cell is connected with the second inlet of the humidifier for the supply of humid gases. In this way, it is possible to recycle the humidity comprised in the relatively humid cathode exhaust gas, which is for instance obtained when water results from the reaction of molecular hydrogen and molecular oxygen. This water is then transferred to the humidification of the fresh process gases that are fed to the fuel cell. The connection here does not necessarily lead to a spatially separated part, it is also possible to connect the second inlet used for the supply of humid gases to the humidifier directly to the cathode outlet of the endplate of the fuel cell stack. To this end, the humidifier can be directly connected to the respective endplate or be integrated into the latter at least in sections.

A further advantageous embodiment provides that the first inlet for the supply of dry gases to the humidifier is connected to an unit for the supply of the cathode with fresh air, such as a compressor. This is especially recommended as the relative hot and dry air that is supplied by such a compressor needs to be humidified before it is fed to the fuel cell .

Further advantageous embodiments are described in the further claims.

The invention shall now be explained on the example of several drawings . It is shown in

Figure 1 a humidifier according to the

invention;

Figures 2a,

2b and 2c details of a humidifier module consisting of two flow plates and a water transfer medium arranged therebetween;

Figures 3a

to 3e details of the channel structures of the flow plates; and

Figures 4a and 4b exemplary arrangements of overlappings of various flow channel structures.

Figure 1 shows a humidifier 1, which comprises a plu- rality of stacked humidifier modules 8, which are layered between two endplates 9 and clamped together by the latter ones. Gases are supplied and discharged via the end plates. For clarity reason neither a compressor connected to the stack for the supply of dry process gas nor a fuel cell unit to which humidified process gas is supplied are shown. The humidifier 1 is thus a humidifier for the humidification of process gases for fuel cells, which humidifier comprises a first inlet for the supply of dried gas {arrow B) as well as a first outlet for the discharge of humidified gas {arrow C) and a second inlet for the supply of humid gases (arrow A) as well as a second outlet for the discharge of dehumidified gases (arrow D} , at least a first and a second flow plate as well as an essentially gas-tight water transfer medium arranged between the first and second flow plate, where the first and the second flow plates each comprise channels for the guidance of gases and where the channels of at least one of the flow plates extend at least in sections in an undulating manner in the plane of the flow plate, so that the channels of the first and the second flow plate at least in

this/these section (s) do not engage with each other with positive fit.

In the following, the properties of the flow plates, especially with respect to the undulating course of the channels guiding the process gases in the flow plates, are discussed considering figures 2a to 4b. The humidifier module 8 of figure 2a is shown in an exploded representation in figure 2b. Here, a first flow plate 2.1 is shown which comprises a channel structure 3. Details of the course of the channels will be described in the context of figures 3a to 3e.

Further, the module comprises a second flow plate 2.2. Between the first flow plate 2.1 and the second flow plate 2.2 a water transfer medium 6 is arranged which is embodied as a membrane. Supporting media, namely graphite fibre paper 10, rest on this membrane. The module is compressed and/or glued in such a manner that gas guided through the channels 3 cannot exit from the active are, which in figure 2b is given with reference number "3" on the first flow plate 2.1. In the example shown, the active area 3, thus the area defined by the channels and the lands between the channels, is an essentially rectangular area, the edge of which is marked with a dotted line in the figure. Figure 2b does not show any sealing elements which provide for gas-tightness, e.g. beads and/or elastomeric seals, in the area surrounding the active area.

In order to achieve an additional sealing, the hu- midifier module may also be glued in its border region. Such pre-mounted humidifier modules 8 can be assembled to stacks of arbitrary thickness. It is possible to cast their outer edges additionally with a resin for additional sealing purpose or to protect the humidifier from external dirt, contamination and mechanical impacts. Passage of the gases in the direction of the stack is achieved through the passage openings 7a and 7b, which alternately supply the plates with gas. This is also obvious from the course of the distribution areas 70a, 70b in figure 2-b. The individual flow plates and/or the humidifier modules can be glued to each other, welded, cast, be connected to one another through melting and/or via mechanical connecting structures such as hooks or clips, which are not shown in the figures. The flow plates given essentially are integral flow plates consisting of only one piece, into which the channel structure has been stamped, embossed or moulded. In the example given, the flow plates consist of a corrosion-free metallic material. As an alternative, frame-shaped plates are possible, too, into which the channel structure is integrated as an inlay. The surface of the plates in the area of the channel structure may be coated in a hydrophilic or hydrophobic manner and can comprise micro structures.

The backside of the second flow plate 2.2 in figure 2b is also provided with channel structures, which are however not visible. With stamped plates, the structure is preferably formed through the plate, so that a protrusion (a depression) on the front side is opposed to a depression {a protrusion) on the back side of the plate. With polymeric plates, the channels are preferable formed during moulding. Here it is possible to shape the channels on the opposite sides of a plate also in a non-complementary manner. With flow plates with channel structures on both of their surfaces, the flow plate 2.2, delimits the actual module. At the same time, it is also the border of the adjacent module, which is not completely shown here. Thus, in this case a module comprises only one flow plate or, to be more precise, neighbouring modules share a flow plate at their interfaces.

The main flow direction of the channels on the surface pointing away from the observer are parallel to the main flow direction on the visible surface of the flow plate 2.1, so that the present humidifier is a humidifier with cocurrent flow or counter flow, the plates 2.1. and 2.2 can actually be used for both of these flow arrangements. It is particularly simple to produce the present arrangement at a large industrial scale, as the individual flow plates are essentially identical. This is however not mandatory. The same is analogously true fort he water transfer medium, which can be realized in many ways, see for instance the introductory section of this description.

Figure 2c illustrates a plate arrangement for a humidifier with cross flow of the gases, as is obvious from the essentially orthogonal arrangement of the channels in plates 2.1 and 2.2. Here, no distribution area is given, but the flow channels start immediately at the edge of the passage openings 7a and 7b, respectively. Again, passage openings 7a and 7b alternately supply the plates with gas so that gases with different humidity flow on both sides of the membrane 6.

From figures 2a to 2c, the channel structure according to the invention is not completely visible yet, it will be obvious from figures 3a ff. In this context, figure 3a shows a partial cross section (section A-A of figure 3c) of areas of the flow plates 2.1 and 2.2 as well as of a membrane 6, a water transfer medium, arranged between these two flow plates. The back side of the flow plates is shown in a non-structured manner, it can however both be non- structured or structured. In the channels of the flow plate 2.1, thus in the non-hatched hollow interspaces between the flow plate 2.1 and the water transfer me- dium 6, a first process gas is flowing. A different second process gas streams through the corresponding hollow spaces between the water transfer medium 6 and the flow plate 2.2. The water transfer medium 6 is essentially gas tight, but permeable for liquids. Due to this, the respective gradient of the water concen- trations in the gases on both sides of the water transfer medium is equilibrated at least to a large degree and a humidification of the drier gas is achieved. It is obvious that the surfaces of the lands essentially rest one on the other, only the water transfer medium 6 lays in between. With the strong overlap of the surfaces of the lands shown in figure 3a, the water transfer medium 6 is not sheared or pressed into the channel .

Figure 3b shows a section B-B of figure 3c in which the overlap of the surfaces of the lands in the contact area 4 is even larger.

Finally, figure 3c shows a top view onto the channels of a humidifier module which is cut in the plane C indicated in figure 3a. It shows that both the channels of the first flow plate 2.1 and of the second flow plate 2.2 have a wave-shaped course in their plane. As a consequence, the surfaces of the land 5 are not pressed into the channels 3 of the respective opposite flow plate, so that here always a sufficient overlap (in total more than 10% of the total area of the water transfer medium in the humidified area) is given.

In figure 3d, a complete wave period of the channel structure of the flow plate 2.1 is visible. The wave length here is designated with „λ", the amplitude A is measured from the centre line of the wave to the uppermost edge of the land, as shown in figure 3d.

Further characteristics in the context of the channel and the land are obvious in figure 3e. Here, the inclination of the wall of the channel is denominated with a, with a being on average between 2 and 30°. The depth of the channel, t, is measured from the uppermost point of the land 5 to the deepest point of the channel 3. The channel depth t is advantageously

0.2 to 2 mm, preferably 0.3 to 1.1 mm. The width of the land b is measured as the distance of the channel walls at the height t divided by 2. The width of the land b is advantageously between 0.3 and 2.5 mm, preferably between 0.7 and 1.5 mm. With the preferred embodiment where the channel structures are embossed metallic channel structures, one can also identify radii, namely the radius r between the channel base to the wall and the radius R between the upper sur- face of the land and the wall of the channel. Both radii r and R are advantageously between 0.05 and 0.25 mm, preferably between 0.1 and 0.15 mm. The actual channel width b' advantageously ranges between 0.5 and 3 mm, preferably between 0.5 and 1.5 mm.

One can derive from figures 3a to 3e that in the contact area of first and second flow plate (the contact is indirect due to the membrane) , the amplitude A of the wave shape of the channels of the first flow plate corresponds to at least the channel width of the second flow plate. In a preferred embodiment both the first and the second flow plates are provided with wave- shaped channels at least in sections, as is obvious from figure 3c. Here, the channels of the op- posite flow plates are set off by λ/4 to ¾ λ in the main extension direction of the channels. List of reference numbers

1 humidifier

2.1 first flow plate

2.2 second flow plate

3 channels

3a channel area

4 contact area

5 land

6 water transfer medium

7 passage

8 humidifier module

9 end plate

10 diffusion medium, e.g. graphite fibre paper 11 channel extension direction

70a 70b distribution area

λ wave length

t channel depth

b land width b' channel width at half height

a inclination of channel wall

r radius at the transition between the channel base and the channel wall

R radius at the transition between the upper side of the land and the channel wall