Abstract:
A moisture exchanger ( 10 ) for transferring moisture between two gases, including a plurality of hollow fiber membranes ( 12 ). The moisture exchanger ( 10 ) includes at least one partition ( 34 ) between the hollow fiber membranes ( 12 ) and in that the plurality of hollow fiber membranes ( 12 ) is subdivided, at least in a section ( 36 ) of the length thereof, into zones ( 38 ) that are connected in parallel.

Description:
[0001]    The present invention relates to a moisture exchanger for transferring moisture between two gases, including a multitude of hollow fiber membranes. The present invention further relates to a fuel cell system including the same. 
       BACKGROUND 
       [0002]    Fuel cells utilize the chemical reaction of a fuel with oxygen yielding water to generate electrical energy. For this purpose, fuel cells include the so-called membrane electrode assembly (MEA) as a core component, which is a combination of a proton-conducting membrane and a respective electrode situated on either side of the membrane (anode and cathode). In addition, gas diffusion layers (GDL) may be situated on either side of the membrane electrode assembly on the sides of the electrodes facing away from the membrane. In general, the fuel cell is formed by a plurality of MEAs arranged in a stack, whose electric power outputs are added up. During operation of the fuel cell, the fuel, in particular hydrogen (H 2 ), or a hydrogen-containing gas mixture, is supplied to the anode, where an electrochemical oxidation from H 2  to H +  takes place, giving off electrons. A (waterbound or anhydrous) transport of the protons H +  from the anode area to the cathode area takes place via the electrolyte or the membrane which separates the reaction chambers from each other in a gas-tight manner and electrically insulates them. The electrons provided at the anode are fed to the cathode via an electrical line. The cathode is supplied with oxygen, or an oxygen-containing gas mixture, so that a reduction from O 2  to O 2−  takes place, taking up the electrons. At the same time, these oxygen anions react in the cathode area with the protons transported via the membrane, forming water. Due to the direct conversion of chemical into electrical energy, fuel cells achieve a higher efficiency at low process temperatures compared to other energy converters by circumventing the Carnot factor. 
         [0003]    The most advanced fuel cell technology at present is based on polymer electrolyte membranes (PEM), in which the membrane itself is made up of a polymer electrolyte. Acid-modified polymers, in particular perfluorinated polymers, are frequently used for this purpose. The most widely common representative of this class of polymer electrolytes is a membrane made of a sulfonated polytetrafluoroethylene copolymer (trade name: Nafion; copolymer of tetrafluoroethylene and a sulfonyl acid fluoride derivative of a perfluoroalkyl vinyl ether). The electrolytic conduction takes place via hydrated protons, which is why the presence of water is a requirement for the proton conductivity, and it is necessary to moisten the operating gases during operation of the PEM fuel cell. Due to the need for water, the maximum operating temperature of these fuel cells is limited to below 100° C. at normal pressure. Distinguishing it from high-temperature polymer electrolyte membrane fuel cells (HT PEM fuel cells), whose electrolyte conductivity is based on an electrolyte bound to a polymer backbone of the polymer electrolyte membrane with the aid of an electrostatic coordinative bond (for example, phosphoric acid-doped polybenzimidazole (PBI) membrane) and which are operated at temperatures of 160° C., this fuel cell type is also referred to as a low-temperature polymer electrolyte membrane fuel cell (LT PEM fuel cell). 
         [0004]    DE 10 2004 022 310 B4 describes a moisture exchanger module including hollow fiber membranes situated in a bundle. At the center of the bundle, a perforated tube extends through the bundle in the axial direction, which is closed in the center of its axial expansion by a blocking element. The cavities of the bundle provided between the hollow fiber membranes are also closed by a blocking element in the center of the axial expansion of the bundle. During operation, the moist exhaust gas of the fuel cell flows through the individual hollow fiber membranes. The air to be moistened supplied to the fuel cell flows into the tube via one of its ends, leaves the tube upstream from the blocking element via the perforation, and flows through the cavities of the bundle to the outside, in part radially and in part axially. The air to be moistened thereafter flows around the blocking element in the bundle, and again flows in part radially, in part axially through the cavities on the other side of the blocking element to the perforation of the tube. In this way, a mixture of a cross-flow and a counter-flow moisture exchanger is implemented. 
         [0005]    DE 10 2008 028 832 A1 describes a humidifier including hollow fibers, which are situated in layers and fixed by a connecting means, which is situated in multiple tracks on the layers. These tracks are oriented transversely to the hollow fibers. As a result of the tracks, a maze-like flow guidance is achieved. 
         [0006]    According to  FIG. 1 , furthermore a moisture exchanger  10  having a plurality of hollow fibers  12  is known, which is designed according to a cross-flow principle. During operation of moisture exchanger  10 , a gas  14  to be dehumidified flows into open ends  13  of hollow fibers  12  and flows through hollow fibers  12 . From opposing open ends  13  of the same hollow fibers  12 , dehumidified gas  16  flows out of hollow fibers  12 . A gas  18  to be humidified flows into an inlet manifold  20  and is distributed in inlet manifold  20  across the length and width of hollow fibers  12 , from where gas  18  to be humidified flows around the outer surfaces of hollow fibers  12  up to an outlet manifold  22 . Humidified gas  24  thereafter leaves moisture exchanger  10  via outlet manifold  22 . 
       SUMMARY OF THE INVENTION 
       [0007]    It is an object of the present invention to provide a moisture exchanger having an increased efficiency. 
         [0008]    According to the present invention, a moisture exchanger for transferring moisture between two gases, including a multitude of hollow fiber membranes, is provided. As a characterizing feature, it is provided that the moisture exchanger includes at least one partition, which is situated between the hollow fiber membranes and divides the multitude of hollow fiber membranes, at least in a subarea of their longitudinal extension, into parallel-connected areas. 
         [0009]    By the at least one partition being situated between the hollow fiber membranes, and the multitude of hollow fiber membranes being divided into parallel-connected areas at least in a subarea of their longitudinal extension, a main flow direction of a flow occurring during operation on the outer surfaces of the hollow fiber membranes is essentially defined in this subarea by the at least one partition. In this way, a moisture exchanger which may be operated according to the counter-flow principle may be created in a simple manner. Analogously to heat exchangers, the counter-flow principle is understood to mean that a main flow direction of a flow through cavities in the interiors of the hollow fiber membranes is oriented counter to a main flow direction of a flow on the outer surfaces of the hollow fiber membranes. The counter-flow principle increases the efficiency of the moisture exchanger, whereby the same may have a more compact design. 
         [0010]    The flows through the cavities in the interiors of the hollow fiber membranes and on the outer surfaces of the hollow fiber membranes are typically gas flows, one of the gas flows having a higher concentration of water (water vapor) than the other. The hollow fiber membranes are water-permeable membranes. A hollow fiber membrane may thus be understood to mean a cylindrical fiber which includes continuous channels in the cross section, connecting the inner cavity of the hollow fiber membrane to the outer surface of the hollow fiber membrane. 
         [0011]    The multitude of hollow fiber membranes are preferably situated in the same direction. This means that hollow fiber membranes may be arranged as bundles or as stacks. In other words, the hollow fiber membranes may be arranged as a fiber packet. The arrangement in the same direction results in advantageous flow conditions in the cavities within the hollow fiber membranes and in the cavities between the hollow fiber membranes. 
         [0012]    In one preferred embodiment of the present invention, it is provided that the at least one partition separates the parallel-connected areas in the subarea from each other in a gas-tight manner. In this way, any cross-flow between the parallel-connected areas is prevented, and a pure counter-flow moisture exchanger is implemented in the subarea. 
         [0013]    In one preferred embodiment of the present invention, it is provided that the moisture exchanger includes an inlet, which is situated in particular laterally on the multitude of hollow fiber membranes, and/or an outlet, which is situated in particular laterally on the multitude of hollow fiber membranes, for supplying a gas to the outer surfaces of the hollow fiber membranes. In this way, the inlet and/or the outlet are fluidically connected to the outer surfaces of the hollow fiber membranes. By situating the inlet and/or the outlet laterally on the multitude of hollow fiber membranes, it is possible to implement compact outer dimensions of the moisture exchanger, while also reducing the material expenditure. 
         [0014]    Preferably, it is provided that the parallel-connected areas are situated in a row next to each other, the lateral inlet and/or the lateral outlet preferably being situated on one end of this row. This configuration ensures that, e.g., proceeding from the inlet, the flow around the outer surfaces of the hollow fiber membranes consecutively reaches the parallel-connected areas. With the aid of the at least one partition, in this way a particularly effective counter-flow moisture exchanger is implemented, in which, with the aid of the partition, additionally dead zones in the flow around the outer surfaces may be prevented, or at least be reduced, particularly easily. 
         [0015]    Preferably, it is provided that the lateral inlet and the lateral outlet are situated on opposite ends of the row. The inlet and the outlet are thus also situated on opposite sides of the multitude of the hollow fiber membranes. Due to the parallel-connected areas, the flows thus cover paths of identical lengths, so that the parallel-connected areas are equal to each other in terms of the flow. 
         [0016]    According to one preferred embodiment of the present invention, it is provided that a flow-through cross-sectional area, proceeding from the lateral inlet and/or the lateral outlet, increasingly decreases toward a parallel-connected area situated the farthest away from the inlet and/or the outlet. Proceeding from the inlet and/or the outlet toward the parallel-connected area situated the farthest away, the values of the cross-sectional areas are thus (strictly) monotonically decreasing. These embodiments ensure that a flow-through cross-sectional area which connects the inlet and/or the outlet to the parallel-connected areas is appropriately adapted to the volume flow. In this way, a flow velocity between the inlet and/or the outlet and the parallel-connected areas may be kept preferably constant, whereby losses are reduced. 
         [0017]    In one further preferred embodiment of the present invention, it is provided that the lateral inlet and the lateral outlet are situated on opposite ends of the longitudinal extension of the multitude of hollow fiber membranes. In this way, the length of the hollow fiber membranes is used optimally for the moisture transfer. 
         [0018]    The at least one partition is preferably situated in a longitudinal extension direction of the hollow fiber membranes between the lateral inlet and the lateral outlet. This prevents flow deflections and thus minimizes a loss of pressure. 
         [0019]    Preferably, it is provided that the moisture exchanger includes a respective manifold between the inlet and/or the outlet and the hollow fiber membranes. In the direction of the lateral extension of the multitude of hollow fiber membranes, the manifold preferably has maximally the extension of the inlet and/or outlet. Furthermore, the extension of the manifold may also exceed the extension of the inlet and/or outlet in the direction of the longitudinal extension of the multitude of hollow fiber membranes by maximally 100%, in particular by maximally 50%, preferably by maximally 25%. Furthermore, the manifold preferably extends across the entire width of the multitude of hollow fiber membranes. In this way, it is possible to distribute the gas to be humidified across the entire width of the moisture exchanger, and to have this gas flow around the entire length of the hollow fiber membranes. 
         [0020]    Furthermore, a fuel cell system including a moisture exchanger according to the present invention is provided. Due to the high efficiency of the moisture exchanger, the fuel cell system is characterized particularly by its compact design. 
         [0021]    Preferably, it is provided that outer surfaces of the multitude of hollow fiber membranes of the moisture exchanger are fluidically connected to a cathode inlet and a cathode outlet of a fuel cell of the fuel cell system in such a way that the moisture exchanger is operable as a counter-flow moisture exchanger. In this way, during operation of the fuel cell, water which developed during the fuel cell reaction is withdrawn from the exhaust gas of the fuel cell with the aid of the moisture exchanger and supplied to a fresh air flow. In this way, sufficient humidification of a membrane of the fuel cell is ensured. By operating the moisture exchanger as a counter-flow moisture exchanger, an optimal efficiency is achieved. 
         [0022]    The various specific embodiments of the present invention described in the present application may advantageously be combined with each other, unless they are designed differently in the individual case. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The present invention is described hereafter in exemplary embodiments based on the accompanying drawings. 
           [0024]      FIG. 1  shows a moisture exchanger according to the prior art; 
           [0025]      FIG. 2  shows an interior configuration of a moisture exchanger according to one preferred embodiment of the present invention; 
           [0026]      FIG. 3  shows a moisture exchanger according to one preferred embodiment of the present invention; 
           [0027]      FIG. 4  shows a further moisture exchanger according to one preferred embodiment of the present invention; and 
           [0028]      FIG. 5  shows a fuel cell system according to one preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]      FIG. 1  was already addressed with respect to the prior art. 
         [0030]      FIG. 2  shows an interior configuration of a moisture exchanger  10  according to one preferred embodiment of the present invention. A multitude of hollow fiber membranes  12  may be molded, for example, arranged in a fiber packet in a housing frame  26 , which is part of a housing  29 . At each of their two ends, hollow fiber membranes  12  have an open end  13 , which extends through housing frame  26 . Furthermore, hollow fiber membranes  12  have outer surfaces  15 , which are connected with the aid of channels, which are not shown, to cavities in the interior of hollow fiber membranes  12  situated between open ends  13 . Through the channels, water (H 2 O) enters through hollow fiber membranes  12  on their outer surfaces  15  during operation of moisture exchanger  10 . The water stems from a gas  14  which is to be dehumidified and transferred to a gas  18  to be humidified. 
         [0031]      FIG. 3  shows a moisture exchanger  10  according to one preferred embodiment of the present invention. Housing frame  26  known from  FIG. 2  may be closed toward the top and the bottom by housing covers  28 , which includes [sic; include] an inlet  30  situated laterally on the multitude of hollow fiber membranes  12  and/or an outlet  32  situated laterally on the multitude of hollow fiber membranes  12  for supplying outer surfaces  15  of hollow fiber membranes  12 . Outer surfaces  15  of hollow fiber membranes  12  are thus situated in a housing  29  which is closed with the exception of inlet  30  and outlet  32  and which includes housing frame  26  and housing covers  28 . For a simplified representation, the housing is shown in a semi-transparent representation in  FIG. 3 . In their simplest embodiment, inlet  30  and outlet  32  are continuous openings in a wall of housing  29 . Furthermore, inlet  30  and/or outlet  32  may also be designed in such a way that gas  18  to be humidified is distributed across the entire width of the multitude of hollow fiber membranes  12  and/or humidified gas  24  is collected across the entire width of the multitude of hollow fiber membranes  12 . 
         [0032]      FIG. 3  shows partitions  34  according to the present invention, which are situated between hollow fiber membranes  12 . Partitions  34  divide the multitude of hollow fiber membranes  12 , at least in a subarea  36  of their longitudinal extension, into parallel-connected areas  38 . Furthermore, partitions  36  divide the multitude of hollow fiber membranes  12  on their entire width. In the example, partitions  34  extend from one side of housing  29  to an opposite side of housing  29  and have a gas-tight design. Parallel-connected areas  38  in subarea  36  are separated from each other in a gas-tight manner with the aid of partitions  34 . Partitions  34  may be molded into the housing, in particular housing frame  26 , which is formed of a synthetic resin, for example. 
         [0033]    It is apparent that parallel-connected areas  38  are situated in a row next to each other, lateral inlet  30  and lateral outlet  32  being situated on opposite ends of this row. Furthermore, it is apparent that lateral inlet  30  and lateral outlet  32  are situated on opposite ends of the longitudinal extension of the multitude of hollow fiber membranes  12 . Furthermore, partitions  34  are situated between lateral inlet  30  and lateral outlet  32 . 
         [0034]    Compared to a moisture exchanger having no partitions  34 , partitions  34  effectively reduce fluidic dead zones  40  (i.e., areas through which only poor flow or no flow at all occurs). 
         [0035]      FIG. 4  shows the interior configuration of a moisture exchanger  10  according to one preferred embodiment of the present invention in greater detail. Moisture exchanger  10  shown in  FIG. 4  differs from that in  FIG. 3  in that partitions  34  are situated offset from each other. The offset is designed in such a way that a cross-sectional area  39  allowing through-flow, proceeding from lateral inlet  30  toward a parallel-connected area  38  situated the farthest from inlet  30 , increasingly decreases. Furthermore, a cross-sectional area  39  allowing through-flow increasingly decreases proceeding from lateral outlet  32  toward a parallel-connected area  38  situated the farthest away from outlet  32 . This embodiment may further reduce dead zones  40  since partitions  34  which are closest to dead zones  40  may be brought closer to dead zones  40 . Hollow fiber membranes  15  shown in  FIGS. 2 and 4  may also have a loosely undulated profile. 
         [0036]      FIG. 5  shows a fuel cell system  50  according to one preferred embodiment of the present invention. Fuel cell system  50  includes a fuel cell  52 , which has a cathode side  54  and an anode side  56 . Outer surfaces  15  of hollow fiber membranes  12  are connected to a cathode inlet  58  of fuel cell  52 , and open ends  13  of hollow fiber membranes  12  are fluidically connected to a cathode outlet  60  of fuel cell  52 . Fuel cell system  52  may be used to supply an electric motor with power to drive a vehicle, which is not shown. 
         [0037]    The operating principle of moisture exchanger  10  and of fuel cell system  50  according to one preferred embodiment of the present invention shall be described in greater detail hereafter. 
         [0038]    During operation, fuel cell  52  is supplied via anode side  56  with a fuel, e.g., hydrogen. Fuel cell  52  is supplied with fresh air (reaction air) via cathode side  54 . To prevent a polymer electrolyte membrane (PEM) of fuel cell  52  from drying out, the supplied fresh air is humidified. This takes place with the aid of moisture exchanger  10 , which withdraws moisture from an exhaust gas flow of fuel cell  52 , i.e., gas  14  to be dehumidified, and supplies the moisture to the fresh air flow, i.e., gas  18  to be humidified. The moisture present in the exhaust gas flow stems from the fuel cell reaction and is recirculated to fuel cell  52  with the aid of moisture exchanger  10 . 
         [0039]    The transfer of the moisture takes place with the aid of moisture exchanger  10  in that gas  14  to be dehumidified flows in through open ends  13  on one end of hollow fiber membranes  12 , flows through hollow fiber membranes  12 , and leaves through open ends  13  at the other end of hollow fiber membranes  12  as dehumidified gas  16 . Within hollow fiber membranes  12 , the moisture condenses due to capillary condensation in the channels which connect outer surfaces  15  to the inner cavities of hollow fiber membranes  12 . Gas  18  to be humidified enters inlet  30  and flows around outer surfaces  15  of hollow fiber membranes  12 . During the flow around the outer surfaces, the moisture condensed in the channels evaporates and is entrained by the flow. At the same time, the condensed water seals the channels in a gas-tight manner. Humidified gas  24  leaves moisture exchanger  10  via outlet  32  and is subsequently supplied to fuel cell  52 . 
         [0040]    Moisture exchangers  10  according to  FIGS. 3 and 4  differ compared to the previously known moisture exchangers (e.g., according to  FIG. 1 ) in that now an operation according to the counter-flow principle is enabled with the aid of partitions  34  situated according to the present invention. For this purpose, a main flow direction of gas  18  to be humidified outside hollow fiber membranes  12  is oriented opposite a main flow direction of gas  14  to be dehumidified within hollow fiber membranes  12  in parallel-connected areas  38 . 
         [0041]    The flow guidance of the two gases  14 ,  18  has a crucial influence on a concentration difference of the water content of the two gases  14 ,  18  which is present across the membranes of hollow fiber membranes  12 . Compared to the previously implemented cross-flow guidance according to  FIG. 1 , the counter-flow guidance now offers the advantage that the concentration difference between the two gases  14 ,  18  is approximately constant regardless of the position in moisture exchanger  10 , while it is lower with increasing water exchange in the case of the cross-flow guidance. Moreover, moisture exchanger  10  according to  FIG. 1  has a main flow direction  42  outside hollow fiber membranes  12  which is averaged between the inlet and the outlet, which entails the drawback that a flow around relatively large subareas of outer surfaces  15  is not possible, and thereby relatively large dead zones  40  are created. 
         [0042]    With the aid of the at least one partition  34  according to the present invention, which serves as a subdivision of the multitude of hollow fiber membranes  12 , dead zones  40  (i.e., the unused portions of hollow fiber membranes  12 ) are reduced on the one hand, and the flow is oriented in such a way that a counter-flow guidance in moisture exchanger  10  arises on the other hand. 
       LIST OF REFERENCE NUMERALS 
       [0043]      10  moisture exchanger 
         [0044]      12  hollow fiber membrane 
         [0045]      13  open end of a hollow fiber membrane 
         [0046]      14  gas to be dehumidified 
         [0047]      15  outer surface of a hollow fiber membrane 
         [0048]      16  dehumidified gas 
         [0049]      18  gas to be humidified 
         [0050]      20  inlet manifold 
         [0051]      22  outlet manifold 
         [0052]      24  humidified gas 
         [0053]      26  housing frame 
         [0054]      28  housing cover 
         [0055]      29  housing 
         [0056]      30  inlet 
         [0057]      32  outlet 
         [0058]      34  partition 
         [0059]      36  subarea of the longitudinal extension of the multitude of hollow fiber membranes 
         [0060]      38  parallel-connected areas 
         [0061]      39  flow-through cross-sectional area 
         [0062]      40  dead zone 
         [0063]      42  real main flow direction 
         [0064]      50  fuel cell system 
         [0065]      52  fuel cell 
         [0066]      54  cathode side 
         [0067]      56  anode side 
         [0068]      58  cathode inlet 
         [0069]      60  cathode outlet