Abstract:
A fuel cell system comprises at least one fuel cell having an air inlet and an exhaust gas outlet, an air supply device connectable to the air inlet and an exhaust gas extracting device connectable to the exhaust gas outlet. Further, a water extraction device has at least two drying units, wherein the water extraction device is adapted for selectively providing a fluid connection from the air supply device to the air inlet of the fuel cell be means of one of the at least two drying units and from the exhaust gas extracting device to the exhaust gas outlet by means of another one of the at least two drying units. Thereby, an optimal drying process is conducted that does not influence a dynamic power generation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to the European Patent Application No. 12 172 241.7, filed Jun. 15, 2012 and to U.S. Provisional Patent Application No. 61/660,276, filed Jun. 15, 2012, which are each incorporated herein by reference in their entirety. 
       TECHNICAL FIELD 
       [0002]    The technical field is related to a fuel cell system and a method for operating a fuel cell system. 
       BACKGROUND 
       [0003]    The operation of a fuel cell system with the supply of air as an oxidant and an appropriate fuel gas requires the constant adaption of air flow to the respective power demand of the fuel cell. Generally, air is supplied to the fuel cell by means of a compressing device at an air inlet or alternatively by means of an extracting device, which extracting device sucks off air from the exhaust gas outlet. 
         [0004]    DE 10 2011 004 318 A1 proposes to combine an active air supply by a compressing device at an air inlet and an extracting device connected to an exhaust gas outlet. This improves the ability of conducting rapid changes in power demands, as the air flow may rapidly be adapted, without air being blocked by increasing flow resistances or by a distinct drop in pressure. 
         [0005]    It is furthermore known to use oxygen depleted air in the form of fuel cell exhaust gas for inerting a space in a vehicle, e.g. an aircraft fuel tank. In order to prevent a supply of water to the respective inerted space, it is necessary to considerably remove the water vapor from the exhaust gas. To achieve this, different methods are known. For example, gas separation may be conducted by means of a membrane that is permeable for gas but substantially impermeable for water vapor, wherein a certain partial pressure is necessary for a successful drying process requiring a compressing means for the exhaust gas to be dried. Furthermore, it is known to use a sorption means-based drying process, e.g. rotatable sorption wheels, containing a hygroscopic material that absorbs water vapor from exhaust gas. 
         [0006]    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 
       [0007]    The use of a membrane-based drying process or a sorption means-based drying process may be optimized for a better performance when an active air supply by means of an air compressing unit and an extracting means are used simultaneously. Furthermore to use or produce by products that are necessary for regenerating sorption based drying means shall be prevented. 
         [0008]    Accordingly, the present disclosure provides a fuel cell system that has an improved ability for rapid power demand changes and at the same time clearly having an improved ability to dry exhaust air from the exhaust gas outlet. 
         [0009]    In the following description of the present disclosure the drying units are also referred to as first drying unit, second drying unit and third drying unit or the first one of the drying units, a second one of the drying units and a third one of the drying units. The numbering of the drying units clarifies a timely or spatial relationship between the drying units of the water extraction device. It is clear that the water extraction device may comprise further drying units, such as a fourth drying unit, a fifth drying unit and so on. 
         [0010]    A fuel cell system according to the various embodiments of the present disclosure comprises at least one fuel cell having an air inlet and an exhaust gas outlet, an air supply device connectable to the air inlet and an air extracting means connectable to the exhaust gas outlet. The fuel cell system furthermore comprises a water extraction device, the water extraction device having at least two drying units, wherein the water extraction device is adapted for selectively providing a fluid connection from the air supply device to the fuel cell air inlet through a first one of the at least two drying units and from the air extracting means to the exhaust gas outlet through a second one of the at least two drying units. 
         [0011]    The fuel cell system may thereby use a combination of one or two independent air conveying means, i.e. a means for providing an overpressure in the region of the air supply or air inlet, (exemplarily in a cathode region of the fuel cell) and an underpressure means in the region of the exhaust gas extraction or the exhaust gas outlet, (exemplarily in an anode region of the fuel cell). The air supply device may thereby produce a flow of air in the direction of the air inlet. Therefore, the air supply device may be an air compression device. Alternatively, the air supply device may furthermore be an opening that allows air to be sucked into an air supply line. The extraction device produces suction at the exhaust gas outlet of the fuel cell. This combination may improve the air supply through the whole fuel cell system and all peripheral components attached thereto. Possibly including all air ducts, filters, air inlet openings, exhaust gas pipes etc. The air supply is thereby substantially independent from any ambient pressure such that, for example, in the use of aircraft, the actual flight state does not influence the operation of the fuel cell even if it is installed in an unpressurized part of an aircraft fuselage. Thereby, a rapid change of power demand is clearly improved as stated in DE 10 2011 004 318 A1. 
         [0012]    According to the various exemplary embodiments of the present disclosure, for the purpose of improving the characteristics of the exhaust gas that is intended for use as an inerting gas, an improved water extraction device is included in the fuel cell system. As stated above the water extraction device may comprise at least two drying units. In order to allow for certain advantages in one example, the drying units can be designed as regenerable drying units. The said drying units may thereby comprise a hygroscopic material that is adapted to absorb or bind water vapor contents in the exhaust gas and furthermore is adapted to later release the absorbed water vapor as a supply of heat again. 
         [0013]    One of the at least two drying units is connectable between the air inlet of the fuel cell system and the air supply device. This means that firstly air, e.g. compressed air, flows through this particular one of the drying units and reaches the air inlet of the fuel cell system later. During the flow of compressed air through this drying unit, water bound in the hygroscopic material may be released into the air flow if it comprises a temperature clearly above the exhaust gas temperature of the fuel cell as explained in detail below. This particular one drying unit is thereby regenerated which means that the water content is successively reduced for preparing this drying unit to bind additional water vapor afterwards. 
         [0014]    The air that leaves this particular drying unit reaches the fuel cell and thereby acts as the oxidant for the electrochemical fuel cell process. Through the release of water from the drying unit, the temperature of the air slightly decreases and the humidity increases. It is advantageous to use such humidified air as an oxidant for the fuel cell if the fuel cell were realized as a PEM fuel cell, as the membrane shall always maintain a predetermined humidity level. 
         [0015]    The other one of the at least two drying units is later subjected to exhaust gas from the fuel cell. Thereby, water particles from the exhaust gas are absorbed in the hygroscopic material of the drying unit. Exhaust gas that leaves this other drying unit may then have a relatively low humidity that clearly improves the use of this exhaust gas for inerting purposes. 
         [0016]    One aspect of the present disclosure lies in that the water extraction device is adapted for selectively connecting different drying units either to the air supply side of the fuel cell or to the exhaust side. Thereby, each of the at least two drying units may either be regenerated during the air supply or may absorb water from the exhaust side. For assuring that at least one of the drying units is capable of drying exhaust gas the selective switching may be conducted in predetermined intervals. Furthermore, the selective switching may also be conducted in conjunction with a humidity sensor in the hydroscopic material and or, on a physical simulation model that is capable of simulating the situation process of the hydroscopic material, e.g. based on measured situation characteristics of the hydroscopic material depending on the humidity of the air and its temperature. 
         [0017]    In an exemplary embodiment of the present disclosure for heating air a heating unit is connected to the air supply device. The heating unit may thereby be positioned either upstream, i.e. at an inlet, or downstream, i.e. at an outlet, of the air supply device. In one example, the air supply device can be positioned upstream of the heating unit. 
         [0018]    In one of various exemplary embodiments a condensation device is arranged between the exhaust gas outlet of the fuel cell and the water extraction device. Thereby, approximately 90% of the water vapor content in the exhaust gas may already be extracted before reaching any drying unit. Such a condensation device may be operated for example through the use of a large surface, such as inside a heat exchanger, wherein the surface may dissipate heat to its surroundings. This condensation device thereby provides a pre-drying process which pre-drying process reduces the necessary capacity for water absorption in the drying units such that reliability may be improved as well as a desired switchover interval being prolonged. Furthermore, the necessary dimensions of the water absorbing surface area, of the hygroscopic material, may be reduced such that a distinct advantage in the reduction of installation space and weight is achieved. If necessary, if a sufficient air flow is not present, e.g. during the ground operation of an aircraft that comprises a fuel cell system according to the present disclosure, the condensation device may be equipped with an additional ventilating means for improving the heat dissipation and thereby the condensation. 
         [0019]    In one embodiment the water extraction device has a supply line for supplying compressed air, an extraction line for extracting exhaust gas, an oxidant line routing compressed air to the inlet of the fuel cell and an exhaust line receiving exhaust gas directly from the fuel cell, wherein each of the at least two drying units comprises a first port and a second port, wherein the first ports of each drying unit is connectable to the supply line through a corresponding supply valve, and connectable to the extraction line through a corresponding extraction valve, and wherein the second ports of each drying unit is connectable to the oxidant line through a corresponding oxidant valve and connectable to the exhaust line through a corresponding exhaust valve. Thereby, each drying unit may be connected to four different valves and thus the direction of flow and the relative humidity through all drying units can be changed from one extremum to the other. Thereby, a rather compact design of the water extraction device may be accomplished, as all drying units may be arranged parallel to each other and enclosed in one housing or being held in the same frame as the switching process allows the alteration of the actual flow path inside the water extraction device. 
         [0020]    The individual valves may be connected to a control unit that takes care of an appropriate switching logic that prevents inadvertent connections of both ports to the same line, or other cases that might hinder normal operation. It may be useful that this control unit also comprises a logic that leads to a switchover in predetermined intervals. Furthermore, the control unit may be connected to humidity sensors that allow monitoring the saturation state of the hygroscopic material in the individual drying units for allowing an automated switchover in the absence of predetermined time intervals. Still further, the control unit may comprise a numerical simulation algorithm that allows the parallel simulation of the actual saturation state of the hygroscopic material in the drying units depending on temperature, humidity and flow rates of the individual gas flows through the drying units. 
         [0021]    In one exemplary embodiment the water extraction device comprises a total of three drying units. After drying one of the drying units by means of heated air it is advantageous to provide a third drying unit for allowing the drying unit that has just been dried to cool down before it is further subjected to humid air that is to be dried. Thereby, the water extraction capacity may be increased on demand, e.g. by connecting more than just one drying unit to the exhaust gas outlet of the fuel cell. At the same time, more than just one drying unit may be regenerated by subjecting the respective drying units to the compressed air flow. 
         [0022]    In one embodiment the water extraction device is adapted for selectively providing a fluid connection from the air compressing device to the air inlet of the fuel cell through a first one of the three drying units, a fluid connection from the exhaust gas extracting device to the exhaust gas outlet through a second one of the three drying units in a cyclic manner, wherein a remaining third one of the three drying units is in an idle state. Hence, a first drying unit is provided between the air compressing device and the air inlet of the fuel cell. A second drying unit is connected between the exhaust gas extracting device and the exhaust gas outlet. A remaining, third drying unit is not connected to any of these components and therefore is in an idle state. After reaching a saturation state in the first drying unit that requires switching from a first connection to a second connection the first one of the drying units is connected between the exhaust gas extracting device and the exhaust gas outlet. The second one of the three drying units is disconnected from the exhaust gas extracting device and the exhaust gas outlet and is thereby set to an idle state. The third one of the three drying units is connected to the air compressing device and the air inlet and will thereby conduct the drying function. Afterwards, another switchover is conducted. Thereby, all three drying units will be set to two different connections or into an idle state. This is conducted in a cyclic manner such that each of the drying units are substantially used for the same amount of time in all three different connections. 
         [0023]    Another exemplary embodiment comprises an inerting port that is connectable to an enclosed space. The inert gas port may be situated downstream of the air extracting means and may provide for inert gas. 
         [0024]    The present disclosure furthermore relates to an aircraft having a fuel cell system as described above as well as at least one fuel tank having an inert gas input, wherein the inert gas input is connectable to an inert gas port positioned downstream of the air extracting means of the fuel cell system. Thereby, a highly flexible fuel cell system provides electrical energy for electrical consumers inside the aircraft, wherein the fuel cell system itself dynamically responds to rapidly changing load requirements without interrupting the fuel cell process. 
         [0025]    According to various exemplary embodiments, a method for operating a fuel cell system is provided. The method according to the present disclosure basically comprises supplying air, leading heated supplied air to a first one of at least two drying units such that the respective drying unit is regenerated, leading humid air from the respective drying unit to the fuel cell air inlet, routing humid exhaust gas from the exhaust gas outlet to a second one of the at least two drying units for absorption of the water vapor contents of the exhaust gas and conveying the dried exhaust to an inert gas port. The air supply device may thereby be adapted for supplying compressed air, i.e. as an air compressing device. 
         [0026]    As explained above, the method may also comprise switching over the connection of one drying unit to either the air supply device and the air inlet of the fuel cell, and the other to an exhaust gas outlet and an air extraction device. 
         [0027]    Another exemplary embodiment of the method comprises establishing a first connection of one drying unit between the air supply device and the air inlet of the fuel cell via the second drying unit and establishing a second connection between an exhaust gas outlet of the fuel cell and an exhaust gas extraction device via the first regenerated drying unit. Thereby at least two drying units may be used for drying exhaust gas and may be regenerated after reaching a certain saturation state. 
         [0028]    The method may therefore further comprise switching the first connection via the first drying unit and the second connection via the second drying unit in an alternating way such that one of the drying units is being regenerated in the first connection while the other one of the drying units is absorbing the water vapor contents of the exhaust gas in the second connection. 
         [0029]    In one exemplary embodiment the fuel cell system comprises at least three drying units and the method further comprises drying a third one of the at least three drying units, wherein the third drying unit is being dried and idle while the first drying unit is being regenerated in the first connection and the second drying unit is absorbing the water vapor contents of the exhaust gas in the second connection. 
         [0030]    Another embodiment comprises switching the first connection via the dried third drying unit and the second connection via the saturated second drying unit and drying the first drying unit in a cyclic way such that one of the drying units is being regenerated in the first connection while another one of the drying units is absorbing the water vapor contents of the exhaust gas in the second connection and yet another one of the drying units is being dried. The regeneration process of the drying units may thereby be improved. 
         [0031]    In one embodiment the drying of one of the drying units comprises heating and/or blowing air, e.g. through the introduction of heated ambient air or bleed air. 
         [0032]    Further, the connection may be changed such that in predetermined intervals, one of the drying units is regenerated while the other drying unit is accomplishing the drying process. 
         [0033]    A person skilled in the art can gather other characteristics and advantages of the disclosure from the following description of exemplary embodiments that refers to the attached drawings, wherein the described exemplary embodiments should not be interpreted in a restrictive sense. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    The various embodiments will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
           [0035]      FIG. 1  shows a fuel cell system according to the various teachings of the present disclosure in a block-oriented schematic view. 
           [0036]      FIG. 2  shows an aircraft with a fuel tank and a fuel cell system. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    The following detailed description is merely exemplary in nature and is not intended to limit the present disclosure or the application and uses of the present disclosure. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description. 
         [0038]      FIG. 1  shows a fuel cell system  2  having a fuel cell  4 , a water extraction device  6 , an air supply device  8  and an air extracting means  10 . The air supply device  8  is exemplarily realized as an air compressing device. The fuel cell  4  may be designed as a PEM fuel cell that comprises a membrane and operates at medium temperatures in the range of e.g. about 60° C. to about 80° C. For the purpose of increasing the available electrical power and for adapting the electrical voltage to a required level the fuel cell  4  does not necessarily comprise just a single fuel cell but may in one example, be realized as a fuel cell stack with a plurality of single fuel cells arranged in a staggered manner and connected by means of parallel and/or serial connections. 
         [0039]    For the purpose of supplying a fuel gas, a fuel gas supply unit may be present, which fuel gas supply unit is not depicted. Primarily, the fuel cell  4  may need a supply of hydrogen. This may be conducted by means of a pure hydrogen supply but may also be based on hydrogen containing fuel gas as an output from a reforming apparatus that converts a fuel based on carbohydrates to a hydrogen-containing gas. The core of the present disclosure does not depend on the characteristics of the fuel gas. 
         [0040]    The fuel cell  4  is furthermore connected to a fuel cell cooling unit  12 , exemplarily depicted in a block-like schematic view. The cooling unit  12  may comprise a heat exchanger which is flown through by a coolant heated by the fuel cell  4  flows, which coolant transfers heat to air, e.g. air from the surroundings of the fuel cell system, fuel of a fuel tank inside a vehicle, e.g. an aircraft, a fluid that delivers heat to heat consuming devices, such as an anti-icing system in an aircraft, or any other means that is capable of sufficiently receiving heat. 
         [0041]    The water extraction device  6  comprises a first drying unit  14 , a second drying unit  16  and a third drying unit  18 . These drying units  14 ,  16  and  18  are based on a hygroscopic, water-absorbing material contained in an appropriate housing that allows a flow-through of air. By supplying heated air, water bound in the hygroscopic material may be released. By supplying humid air, the water vapor is absorbed by the hygroscopic material. The hygroscopic material will at some point be saturated such that it will not absorb any additional water. For enabling the absorption of additional water it is regenerated by means of supplying heated air. 
         [0042]    All drying units  14 ,  16  and  18  comprise a first port  14   a,    16   a,    18   a  and a second port  14   b,    16   b  and  18   b.  Each of the first ports are connectable to a supply line  20  downstream of the air supply device  8 , and connectable to an extraction line  22  upstream of the extraction device  10  by way of supply valves  24 ,  26  and  28  and extraction valves  30 ,  32  and  34 . On the other side, each second port  14   b,    16   b  and  18   b  is connectable to an air inlet  36  of the fuel cell  4  by means of oxidant valves  38 ,  40 ,  42  and connectable to an exhaust gas outlet  44  of the fuel cell  4  through exhaust valves  46 ,  48  and  50 . With this arrangement it is possible to route compressed air from the air supply device  8  through any of the first, second and third drying units  14 ,  16  and  18  for supplying air to the air inlet  36  of the fuel cell  4 . At the same time, the exhaust gas arising at the exhaust gas outlet  44  of the fuel cell  4  may be routed through any of the first, second or third drying units  14 ,  16  and  18  for the purpose of supplying dried exhaust gas to the air extraction device  10 . 
         [0043]    The basic idea of this arrangement lies in that each of the first, second and third drying units  14 ,  16  and  18  may be selectively regenerated in a cyclic manner by the supply of air in order to always have at least one drying unit capable of absorbing water vapor from exhaust gas of the fuel cell  4 . Furthermore, by-products such as separate regenerating air flows, can be prevented. 
         [0044]    The addition of a heater  52  directly downstream of the air supply device  8  leads to an elevated temperature of the source of compressed air that has a rather low relative humidity for improving the regeneration process. For example, the heater  52  may heat the compressed air to a temperature of about 120° C. Through the absorption of water contained in the hygroscopic material of the respective drying unit to be regenerated, and through leading this humidified air to the air inlet port  36  of the fuel cell  4  the temperature of the compressed air decreases to a level capable of operating the fuel cell  4 . 
         [0045]    The individual drying units  14 ,  16  and  18  are used in a cyclic way to absorb water vapor of the exhaust gas of the fuel cell  4 . 
         [0046]    For example, the first drying unit  14  is connected between the air supply device  8  and the air inlet  36  of the fuel cell  4  and thereby operates in a regeneration state, where absorbed water is removed from the hygroscopic material. At the same time the second drying unit  16  is connected between the exhaust gas extracting device  10  and the exhaust gas outlet  44  of the fuel cell  4  and thereby operates in a drying state, where water vapor from the humid exhaust gas is absorbed by the hygroscopic material until a certain saturation state is reached. In the meantime, the third drying unit  18  is in an idle state and thereby is neither drying humid exhaust gas nor is regenerated. 
         [0047]    After reaching a predetermined saturation state in the second drying unit  16  the connections of the drying units  14 ,  16  and  18  are switched such that the first drying unit  14  is disconnected from the air supply device  8  and the air inlet  36  of the fuel cell  4  to reach an idle state; the second drying unit  16  is connected to air supply device  8  and the air inlet  36  of the fuel cell  4  to be operated in an regeneration state; the third drying unit  18  is now connected between the exhaust gas extracting device  10  and the exhaust gas outlet  44  for being operated in a drying state. 
         [0048]    After the third drying unit reaches a predetermined saturation state, the connections of the three drying units  14 ,  16  and  18  is switched once more such that they are operated in the respective remaining operation state. The switching over of the connections may be conducted in a cyclic manner such that in predetermined intervals the connections are changed over. 
         [0049]    A pre-drying means  54 , e.g. realized as a condensation device, may be located directly downstream of the exhaust gas port  44  of the fuel cell  4 . There, a distinct surface area, which surface area is thermally connected to a heat dissipation means  56 , leads to continuous condensation water vapor being present in the exhaust gas, which condensate is then conveyed to a condensation water tank  58 . By this measure, approximately 90% of the water vapor content in the exhaust gas may be removed prior to reaching the individual drying units. Thereby, total humidity can be drastically decreased. 
         [0050]    The valves  24 - 50  are exemplarily connected to a control unit  59  that controls the appropriate switching process through an individual controlling of the valves  24 - 50 . The control unit  59  may be connected to humidity sensors integrated into the drying units  14 ,  16  and  18  and may also comprise a switching logic for switching over at predetermined intervals of time or may also follow an internal numerical simulation. 
         [0051]      FIG. 2  shows an aircraft  60  having a fuel tank  62  and a fuel cell system  2 . According to  FIG. 1 , dried inert gas that is provided by the exhaust gas extraction device  10  is supplied to the tank  62  in order to provide an inert atmosphere. By the advantageous drying process conducted as explained above hardly any water vapor reaches the tank  62  and thus prevents the accumulation or absorption of water in the fuel and also prevents the generation of bacteria inside the tank. 
         [0052]    While at least one exemplary embodiment has been presented in the foregoing 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 of the present disclosure in any way. Rather, the foregoing 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 of the present disclosure as set forth in the appended claims and their legal equivalents.