Abstract:
A fuel cell system having an apparatus for gas drying that includes, but is not limited to at least one cooling element with at least one first surface and at least one detachment device. The cooling element is designed to be thermally connected to a heat sink and to come into contact with gas flowing past. The detachment device is movably held relative to the first surface and is designed to detach frozen water from the first surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of International Application No. PCT/EP2010/066034, filed Oct. 25, 2010, which application claims priority to German Application No. 10 2009 051 212.8, filed Oct. 29, 2009 and to U.S. Provisional Application No. 61/256,015, filed Oct. 29, 2009, which are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The technical field relates to a fuel cell system with an apparatus for drying exhaust gases of the fuel cell system, to a method for drying exhaust gases of a fuel cell system, and to an aircraft with at least one such fuel cell system. 
     BACKGROUND 
     For modern commercial aircraft, occasionally fuel cell systems are conceived or already used in order to handle various tasks. Apart from electricity generation, other tasks can also be carried out, for example rendering a fuel tank inert by introducing the exhaust gases of a fuel cell system. Because of the way a fuel cell operates, the exhaust gas usually contains water vapor. Generally-speaking, if humid gases are used for rendering a fuel tank inert, there is a problem in that fuels, in particular kerosene, are hygroscopic. Furthermore, a bacteria population can form in the tank, which bacteria population could influence sensors for acquiring the fill level of the tank so that acquisition becomes imprecise. Furthermore, within the fuel tank or the fuel itself, ice crystals could form that could result in damage to engine injection nozzles and fuel filters in cruising flight of the aircraft or during below-zero temperatures on the ground. There is thus a requirement for introducing dry gases into the fuel tank in order to be able to render the fuel tank inert. 
     DE 10 2005 054 885 A1 and US 2007/0111060 A1 disclose a safety system for reducing the danger of explosion of a fuel tank, in which system a protective-gas production device comprises a fuel cell system with a fuel cell, and provides a protective gas which during operation of the fuel cell system is produced by the fuel cell. 
     In prior art various methods and systems are known that are used for drying gases, in particular air. Thus it would, for example, be possible to carry out adsorption with hygroscopic media, for example silica gel. However, the water absorption capacity of a hygroscopic medium is finite, and consequently after use it would have to either be replaced or regenerated. In particular in an aircraft, replacement leads to pronounced weight problems, and constant emptying and refilling leads to increased maintenance effort. Furthermore, regeneration would be possible with a corresponding heat input, for example, with heated air. However, this would place in doubt the effectiveness of the fuel cell system, because thermal regeneration would require considerable expenditure of energy. If no regeneration is to be carried out, due to the above-mentioned saturation, exhaust gas drying is possible only for a limited period of time. Generally speaking, in such methods dew points, i.e., temperatures, are attained at which there is a state of equilibrium between condensing water and evaporating water, which dew points or temperatures reach far into the double-digit negative region. 
     A further method for drying air takes place by water transfer with a selective membrane, with the use of a partial pressure differential. To this effect a membrane would be used that separates a gas to be dried from a dry gas, where, due to a partial pressure differential, water is made to pass through the membrane. As an alternative to the dry gas it would also be possible to increase the static pressure on that membrane side on which the gas to be dried is located. The drying performance of this method is limited by the achievable partial pressure differential. Particularly low dew points of a membrane compressed-air dryer are only achieved with the use of quite a high operating pressure and the accompanying high compressor performance necessary. 
     A further, third, method from prior art for gas drying would take place by cooling the gas to below the dew point, for which purpose basically only a heat exchanger and a heat sink or a cooling medium are required. Following cooling, and for final separation of liquid water from gaseous residual gas, a drip catcher or the like could be used. However, this principle requires quite considerable cooling capacity because liquid product water is present, and the energy released during the phase transition needs to be discharged. The cold used to cool the gas can in part be recovered in a downstream recuperative heat exchanger. Basically, in this arrangement the attainable dew point is limited by the freezing point, because in the design currently in widespread use icing occurring within the heat exchanger can result in the blocking of gas ducts. 
     Correspondingly, it may be considered at least one object to provide a system for cooling the exhaust gas of a fuel cell system, which system for cooling reduces or entirely eliminates the above-mentioned disadvantages. In particular, it may be considered at least another object to provide a system for drying exhaust gas of a fuel cell system, which system for drying with the use of as little energy as possible makes it possible to dry the exhaust gas as effectively as possible without significantly increasing the complexity of the fuel cell system or its periphery, while at the same time minimizing the additional weight. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background. 
     SUMMARY 
     A fuel cell system is provided having an apparatus for drying of exhaust gas of the fuel cell system. The apparatus for drying of exhaust gas of the fuel cell system comprises at least one cooling element with at least one first surface and at least one detachment device. 
     The cooling element is designed to be thermally connected to a heat sink and to come into contact with gas flowing past the first surface. In concrete terms this means that a cooling element of any shape can be connected in any desired manner with a heat sink in order to be cooled. In this arrangement connection with the heat sink can be carried out in completely different ways. For example, a cooling circuit could be provided that conducts a cooling medium as a heat sink through the cooling element so that heat from the cooling element is dissipated to the cooling medium. At the same time the cooling element could also be designed to establish a mechanical connection with a Peltier element or the like as a heat sink so that, as a result of contact with a cold side of a Peltier element, heat dissipation from the cooling element to the Peltier element, and thus cooling, can take place. Furthermore, it could also be possible to link a cold fluid as a heat sink from any source in any desirable manner with the cooling element so that, as a result of the low temperature of the fluid, cooling of the cooling element takes place. In this arrangement, for example, particularly cold ambient air from the surroundings of an aircraft in cruising flight could be considered, which air can be used either directly or by way of a heat exchanger implemented in the form of an outer-skin cooler. Likewise, the use of liquid hydrogen from a cryogenic tank could be considered as a heat sink, which hydrogen is used as fuel for the fuel cell. In order to operate a fuel cell it is necessary anyway to convert the hydrogen from its liquid form to a gaseous form so that a heat input could be advantageous. 
     Such a cooling element provides at least an advantage in that the content of water or water vapor of a gas flowing past the first surface of the cooling element freezes and collects on the first surface. With an adequately cold temperature of the cooling element below the freezing point and adequate impingement of the first surface with the gas to be dried, adequate drying of the gas is possible. 
     The above-mentioned detachment device is movably held relative to the first surface of the cooling element and is designed to detach water that has frozen onto the first surface, and consequently no excessive deposit of ice occurs. This embodiment or other embodiments is not limited to a particular type of detachment device; instead, here too any imaginable detachment devices can be considered. Mechanical detachment elements can be implemented with scraping elements, scraping edges or the like, engage the first surface and mechanically detach ice from the first surface. 
     The use of a mechanical detachment device also provides at least an advantage in that no saturation effects occur. Furthermore, no special materials need to be fed to the apparatus, which materials would allow the detachment of ice or drying of the gas. Furthermore, a compact design can be anticipated. 
     In another embodiment of the apparatus, the detachment device comprises an edge that is designed to scrape off ice from the first surface of the cooling element. The edge is thus preferably to be designed in such a manner that its shape corresponds to the shape of the first surface. For example, if the first surface is a planar straight surface, a planar and straight-line edge could be used to scrape ice from the first surface. In this manner, the quantity of ice that has accumulated on the first surface is always limited. Consequently, continuous adequate heat dissipation for the icing of water vapor of the gas is possible. 
     According to another embodiment of the apparatus, the cooling element is a hollow body. The first surface is an inside surface of the cooling element. In this manner, in particular, the introduction and the passing-through of gas is simplified because the cooling element by its hollow shape could practically represent an air line. By thermally connecting the cooling element with a heat sink, ice collects on the inside surface of the cooling element. This ice can be scraped off continuously, step-by-step or in an alternating manner. 
     The design of the cooling element in concrete terms provides a body that at least in some sections is of a hollow-cylindrical shape, because this variant is particularly easy to manufacture, and can thus reduce the costs for producing the apparatus to a low level. 
     With the use of a hollow cooling element, particularly with a design that at least in some sections comprises a hollow-cylindrical shape, in another embodiment of the apparatus it makes sense to use a spindle-shaped detachment device whose outer spindle edges are in contact with the inside surface of the cooling element. The spindle-shaped detachment device is preferably to be rotatably held on an axis that corresponds to the axis of extension of the cooling element. This concentric symmetric design makes possible uniform scraping-off on the entire inside surface. By means of continuous rotation of this spindle-shaped detachment device, which could, for example, comprise a helical scraping edge, ice is continuously scraped off the inside surface of the cooling element so that depending on the pitch and the number of helical turns of the detachment device the ice is removed immediately after it has collected on the inside of the cooling element. 
     The cooling element is enclosed by a further body, which on its inside forms a gap to the cooling element. Through such a gap a cooling medium could enter, which results in cooling of the cooling element. The temperature of the inflowing cooling medium or the like should comprise a value that is adequately lower than approximately 0° C. In this respect the embodiments are not limited to a particular type of cooling medium; instead, a number of different cooling media could be used. Both liquid and gaseous cooling media could be considered, where, in the use in an aircraft, ambient air from a ram air inlet or the like could also be suitable as a cooling medium, at least in cruise flight. 
     According to an embodiment of the apparatus, a hollow-shaped cooling element on at least on one end comprises an opening-out shape so that the introduction of the gas to be dried and/or the discharge of the detached ice are/is facilitated. The opening-out shape could, for example, be designed so as to be funnel-shaped or trumpet-shaped and could serve as a reservoir for ice or meltwater. 
     In an opening-out region of a hollow cooling element an aperture could be arranged through which the detached ice or the meltwater obtained from the outside in the frozen phase by the action of heat can be discharged. In this arrangement the action of heat can be implemented by the inflowing gas. In this arrangement the cooling element is preferably positioned in such a manner that mixing of the outflowing gas with the water to be discharged can be prevented. The apparatus could, for example, prevent this by horizontal support, because accumulated ice or accumulated meltwater could fall out or drip off perpendicularly to the direction of flow of the gas. 
     With the use of a spindle-shaped detachment device a drive device could be used that is arranged as far as possible outside axes of extension of the cooling element and of the detachment device so that the incident flow over the first surface of the cooling element is not impeded. For example, an electrical motor, optionally with a suitable gear arrangement, could be selected as a suitable drive. However, the embodiments are not limited to the use of an electric motor. Instead, in particular in an aircraft environment, a pneumatic or hydraulic drive device could also be considered. With the use of a planar first surface, linear guiding of a detachment device could be considered that uses a corresponding linear guide gear arrangement on the drive device. At the same time it would make sense to arrange a corresponding gear arrangement on the motor in order to reduce the rotational speed and to increase the torque of a spindle-shaped detachment device. 
     A method is also provided for gas drying, which method essentially comprises the following steps. In the first instance a gas stream that is to be dried is directed onto a first surface of a cooling element; with a thermal connection with a heat sink the cooling element is cooled in order to, in this process, freeze the water vapor or water content contained in the gas, so that the water vapor or water content accumulates on the first surface. At substantially the same time, the same time, subsequently or alternatingly, a detachment device on the first surface is moved so that the ice that has accumulated on the first surface is removed. Optionally, removed ice is collected in a reservoir, and, furthermore optionally, is melted by exposure to external heat. The ice collected in the reservoir can fall out of it or can be discharged from it; likewise the ice that has optionally been melted by exposure to heat can be channeled out. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further characteristics, advantages and application options are disclosed in the following description of the exemplary embodiments and of the figures. The described and/or illustrated characteristics per se and in any combination form the subject of the invention, even irrespective of their composition in the individual claims or their interrelationships. Furthermore, identical or similar components in the figures have the same reference characters. 
         FIG. 1  shows a diagrammatic view of a first exemplary embodiment of the apparatus; 
         FIG. 2  shows a diagrammatic view of a second exemplary embodiment of the apparatus; 
         FIG. 3   a  and  FIG. 3   b  show diagrammatic views of two cooling options for the apparatus according to an embodiment; 
         FIG. 4  shows a diagrammatic view of the method according to an embodiment; and 
         FIG. 5  shows an aircraft with at least one fuel cell and at least one apparatus according to an embodiment for drying the exhaust gas of the layer composition; 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit application and uses. Furthermore, there is no intention to be bound by any theory presented in the preceding background or summary or the following detailed description. 
       FIG. 1  shows a schematic diagram of the apparatus for drying exhaust gas of a fuel cell system. A cooling element  2  of any desired shape (for the sake of simplicity only shown in sections in the diagram) is cooled with a connection with a heat sink (not shown in the diagram). The temperature of the cooling element  2  is below approximately 0° C. so that freezing or sublimation of water vapor in a gas  4  to be dried can be achieved. 
     The cooling element  2  comprises a first surface  6 , along which the gas  4  to be dried flows. The gas  4  comprises a defined content of water or water vapor that is to be discharged. As a result of the gas  4  flowing along the first surface  6  of the cooling element  2 , the water freezes or sublimates and accumulates as a layer of ice on the first surface  6 . The accumulation of ice cannot be carried out indefinitely, and for this reason a detachment device  8  is used that is held so as to be movable relative to the cooling element  2 . For example, the detachment device  8  comprises a scraping edge  10  that is in contact with the first surface  6 . As a result of the scraping edge  10  moving along the first surface  6 , the ice is scraped off. With continuous movement of the detachment device  8  along the first surface  6 , the surface  6  can always remain free of ice so that an ideal cooling effect can always act on the water content of the gas  4 , and consequently continuous, ideal, dehumidification of the gas  4  can be carried out. 
       FIG. 2  shows a more concrete exemplary embodiment of the apparatus  11 . In this arrangement a cooling element  12  is designed as a hollow cylinder through which the gas  4  to be dried flows. With adequate cooling, the first surface  14 , designed as an inside surface of the cooling element, is covered by ice, and the gas  4  is dried as it flows through the cooling element  12 . To remove the ice layer on the first surface  14  a detachment device  16  is used that is rotatably held on an axis  18 , where the axis  18  corresponds to the axis of extension of the cooling element  12  and consequently is arranged concentrically to the aforesaid. The drive of the detachment device  16  is implemented by a diagrammatically shown drive device  31  which by way of a shaft  33  is connected with the detachment device  16 , where the shaft  33  extends over a greater height than does the cooling element  12 , and consequently the inflow of the gas  4  to be dried is made possible. 
     In this arrangement the detachment device  16  comprises a spiral-shaped or helical edge  20  that continuously scrapes along the first surface  14  of the cooling element  12  when the detachment device  16  is rotating. In this manner continuous detachment of ice from the first surface  14  is carried out. 
     Preferably, the detachment device  16  comprises a helical turn arrangement that is sufficiently coarse to allow easy flow of the gas  4  through the apparatus, while at the same time, however, ice detachment can remain assured. 
     In the embodiment shown, the hollow-cylindrically-shaped cooling element  12  is enclosed by a further cylindrically-shaped body  22  that defines a gap  24  to the cooling element  12 . A cooling medium could flow through this gap  24 , which cooling medium by passing along a second surface  26  of the cooling element  12  results in cooling as a result of which the water content of the gas  4  freezes on the first surface  14 . 
     A lower region  28  of the cooling element  12  comprises an outward-expanding shape which, for example as a reservoir, provides sufficient space for accumulated ice that has been detached from the first surface  14 . Optionally, a corresponding aperture  30  can be provided through which the ice, or ice in the form of meltwater, which ice has been melted by exposure to external heat, is discharged. The outward-formed region  28  of the cooling element  12  could comprise a cover  32  that in the extension of the axis  18  comprises a cutout  34  that allows unimpeded flowing out of the gas  4 . 
       FIG. 3   a  diagrammatically shows apparatus  11 , which apparatus is connected with a cryogenic hydrogen tank  35  filled with liquid hydrogen. Liquid hydrogen enters the gap  24 , cools the cooling element  12 , and is returned to the tank  35  or is conveyed for use in a fuel cell or the like. 
       FIG. 3   b  diagrammatically shows a heat exchanger  37  that is cooled by ambient air  39 . A separate cooling circuit  41  connects the heat exchanger  37  with the apparatus so that direct introduction of ambient air can be prevented. After use in the heat exchanger  37  the ambient air  39  can be removed. As an alternative, ambient air can also flow directly through the gap  24 . 
     Furthermore,  FIG. 4  shows the essential steps of the method according to an embodiment. In the first instance a first surface of a cooling element is subjected  36  to a gas stream to be dried; by way of a thermal connection with a heat sink the cooling element is cooled  38  in order to, in this process, freeze the water vapor or water content contained in the gas so that the ice accumulates on the first surface. At the same time, subsequently or alternatingly, a detachment device on the first surface is moved  40  so that the ice that has accumulated on the first surface is removed. Optionally, removed ice is collected in a reservoir, and, furthermore optionally, is melted  42  by exposure to external heat. The ice collected in the reservoir can fall out of it or can be discharged  44  from it; likewise the ice that has optionally been melted by exposure to heat can be channeled out. 
     Finally,  FIG. 5  shows an aircraft  46  comprising at least one fuel cell system  48  that feeds into fuel tanks  52  an exhaust gas containing water vapor through apparatus  50  in a dried state for rendering inert said fuel tanks  52 . 
     In addition, it should be pointed out that “comprising” does not exclude other elements or steps, and “a” or “one” does not exclude a plural number. Furthermore, it should be pointed out that characteristics or steps which have been described with reference to one of the above exemplary embodiments can also be used in combination with other characteristics or steps of other exemplary embodiments described above. Moreover, while at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.