Patent Publication Number: US-2007104992-A1

Title: Solid oxide fuel cell stack of modularized design

Description:
FIELD OF THE INVENTION  
      The present invention relates to a modularized solid oxide fuel cell stack, and more particularly, to a fuel cell stack comprising a plurality of fuel cell cassettes, each being substantially a planar solid oxide fuel cell arranged inside a detachable cassette, which can facilitate the maintenance and replacement problems troubling conventional fuel cell stack and thus reduce the cost of maintaining the same.  
     BACKGROUND OF THE INVENTION  
      Fuel Cells have emerged as one of the most promising technologies for the power source of the future since it has the property of low pollution and high efficiency of energy transformation. Fuel cells can be categorized into proton exchange membrane fuel cell (PEMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), Molten Carbonate Fuel Cell (MCFC), and solid oxide fuel cell (SOFC) according to the types of eletrolyte used thereby, wherein the PEMFC, the AFC, and the PAFC is operating at a low temperature range, the MCFC is operating at an intermediate temperature range, and the SOFC is operating at a high temperature range. In addition to the abovementioned fuel cells, there are direct methanol fuel cell (DMFC) and metal-air fuel cell, and so on. Among the different types of fuel cells, the high temperature solid oxide fuel cell (SOFC) is particularly interesting due to the following factors: (1) The highest degree of efficiency; and (2) The arising heat at high temperatures can be furtherly used in many different ways. Due to the mentioned advantages, the SOFC is being studied and developed for its application in the fuel cell technology.  
      Conventionally, an SOFC is constructed with two porous electrodes which sandwich an electrolyte. In an SOFC, fuel, e.g. methane, and oxidant, e.g. air, are preheated to a temperature close to the operating temperature of the SOFC, i.e. between 600° C.˜1000° C., and then being fed into the SOFC. When an oxygen molecule contacts the cathode/electrolyte interface as the air flows along the cathode (which is therefore also called the “air electrode”), it catalytically acquires four electrons from the cathode and splits into two oxygen ions. The oxygen ions diffuse into the electrolyte material and migrate to the other side of the cell where they encounter the anode (also called the “fuel electrode”). The oxygen ions encounter the fuel at the anode/electrolyte interface and react catalytically, giving off water, carbon dioxide, heat, and—most importantly—electrons. The electrons transport through the anode to the external circuit and back to the cathode, providing a source of useful electrical energy in an external circuit. Furthermore, the exhaust air with temperature higher than 700° C. and residual fuel, both being discharged at the exit of the SOFC, can be recycled for other usages.  
      Two possible design configurations for SOFCs have emerged: a planar design and a tubular design. In the tabular SOFC, components are assembled in the form of a hollow tube so that the tabular SOFC can keep good airtight even when subjecting to a high-temperature ambient, but is suffered by the problems of low power density and high internal impedance. On the other hand, the planar SOFC can provide good power density and preferred efficiency, but it is troubled by the difficulty of keeping airtight. The key factors of a planar SOFC include: a membrane electrode assembly (MEA), being composed of an anode, a cathode and an electrolyte; a manifold plate, for guiding fuel and air; and other relating parts, capable operating while being subjected to high temperature. Since the voltage output of a single fuel cell is far to low for many applications, it frequently becomes necessary to connect multiple fuel cells in series, parallel or series/parallel configuration as those disclosed in U.S. Pat. No. 6,296,962 B1, U.S. Pat. No. 6,649,296 B1, and U.S. Pat. application Ser. No. 2005/0089371 A1. However, gas-tight connections must be incorporated in the fuel cell stack to allow for a safe and efficient flow of reaction gases. Typically, a group of individual fuel cells are welded, soldered or otherwise bonded together into a single unitary stack by the use of glass ceramics capable of enduring 700° C.˜1000° C. operating temperature. Accordingly, if one cell must be removed and replaced, such as for testing or maintenance, the remaining cells are destroyed in the process. This leads to significant losses in time and money.  
     SUMMARY OF THE INVENTION  
      In view of the disadvantages of prior art, the primary object of the present invention is to provide a modularized solid oxide fuel cell stack comprising a plurality of fuel cell cassettes, each being substantially a planar solid oxide fuel cell arranged inside a detachable cassette, by which the maintenance and replacement problems caused by the use of glass ceramics for enabling the airtight of the fuel cell stack can be solved and thus the cost of maintaining the same can be reduced.  
      To achieve the above object, the present invention provides a modularized solid oxide fuel cell stack comprising: at least a fuel cell cassette; an air tank, for providing air to the fuel cell stack while being used for receiving the fuel cell cassette; a fuel tank, for providing fuel to the fuel cell stack; and a set of conducting strips, connecting to the fuel cell cassette for transmitting electricity out of the fuel cell stack; wherein the fuel cell cassette further comprises a planar fuel cell and a case, being used for receiving the planar fuel cell. Preferably, the planar fuel cell is composed of two membrane electrode assembly (MEA), each having an anode electrode an a cathode electrode, and a nickel mesh with an extending bar, sandwiched between the two MEAs, whereas the anode electrode of one of the two MEAs is placed facing the anode electrode of another MEA.  
      In a preferred embodiment of the invention, the modularized solid oxide fuel cell stack comprise a plurality of serial-connected fuel cell cassettes; wherein the serial connection is achieved by connecting the extending bar of the nickel mesh of any one of the plural fuel cell cassette to the case of a neighbor fuel cell cassette. By serially connecting more than two fuel cell cassettes, the output voltage of the fuel cell stack can be increased.  
      Preferably, the case of each fuel cell cassette further has a plurality of manifolds arranged therein, for guiding air to flow and distribute uniformly along the cathodes of the corresponding planar fuel cell received therein.  
      Preferably, the number of the manifolds is dependent on the characters of the membrane electrode assembly of the planar fuel cell.  
      Preferably, the case is made of a non-precious metal such as stainless steel, or a high temperature resisting material such as Inconel 600/625, or a conductive material with thermal expansion coefficient similarly to that of the fuel cell.  
      In a preferred embodiment of the invention, the air tank further comprises: a main air chamber; at least a air duct, for guiding high temperature air to flow into the air tank; and a hollow air distribution chamber, being arranged at a position between the main air chamber and the air duct; wherein a plurality of air holes are arranged at a surface of the air distribution chamber facing toward the main air chamber.  
      In another preferred embodiment of the invention, the fuel tank further comprises a hollow fuel distribution chamber, having a plurality of fuel holes arranged on a surface thereof facing toward the fuel cell; and a fuel duct, for guiding high temperature fuel to flow into the hollow fuel distribution chamber.  
      Preferably, an air react channel is formed between the air tank and the cathode of fuel cell cassette, and a fuel reaction channel is formed between the fuel tank and the anode of the fuel cell cassette, whereas an air tight seal for isolating the air react channel from the fuel reaction channel while keeping both air tight.  
      Preferably, the air tight seal can be accomplished by sintered glass ceramics or mica spacers.  
      In a preferred aspect, the fuel tank further comprises a residual fuel chamber with a fuel exiting duct, the residual fuel chamber being connected to the fuel cell cassette; wherein the residual fuel is collected and accumulated by the residual fuel chamber to be guided out of the fuel tank by the fuel exiting duct. Moreover, the air tank further comprises an air exiting duct for guiding the reacted air of the fuel cell cassette out of the air tank.  
      In a preferred aspect, the air tank further comprises an after-burn chamber, for enabling residual fuel to burn therein.  
      Preferably, the residual fuel and the reacted air are guided to flow in the after-burn chamber to be burned therein.  
      Preferably, the after-burn chamber further comprises a porous ceramics arranged therein for enhancing the burning efficiency of the residual fuel and air while enhancing the homogeneity of temperature distribution.  
      Preferably, the conductive strips are made of a metal of high temperature resistance.  
      Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows an assembly of a modularized solid oxide fuel cell stack according to a preferred embodiment of the present invention.  
       FIG. 1A  shows an assembly of a modularized solid oxide fuel cell stack with two air ducts according to another preferred embodiment of the present invention.  
       FIG. 2  is an exploded view of a modularized solid oxide fuel cell stack according to the present invention.  
       FIG. 3A  to  FIG. 3C  are schematic diagrams depicting the sequential assembling of a fuel cell cassette of the present invention.  
       FIG. 4A  to  FIG. 4B  are schematic diagrams depicting the steps of sequentially assembling two fuel cell cassettes to an air tank according to the present invention.  
       FIG. 5  is a three dimensional view of an air tank according to a preferred embodiment of the invention.  
       FIG. 6  is the back view of the air tank of  FIG. 5 .  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
      For your esteemed members of reviewing committee to further understand and recognize the fulfilled functions and structural characteristics of the invention, several preferable embodiments cooperating with detailed description are presented as the follows.  
      Please refer to  FIG. 1  and  FIG. 2 , which are respectively an assembly of a modularized solid oxide fuel cell stack of the present invention and a an exploded view of  FIG. 1 . As seen in  FIG. 1  and  FIG. 2 , a modularized solid oxide fuel cell stack is comprised of two fuel cell cassettes  50 ,  50   a , an air tank  60 , a fuel tank  70  and a set of conductive strips  80 . It is noted that the configuration of the fuel cell cassette  50  is the same as that of the fuel cell cassette  50   a , and thus the general configuration of fuel cell cassette can be represented by that of the fuel cell cassette  50  as seen in  FIG. 3A ˜ FIG. 3C , in that the fuel cell cassette  50  is comprised of a fuel cell  30  and a case  40 .  
      In  FIG. 3A ˜ FIG. 3C , the fuel cell  30  is a rectangle planar cell, being comprised of two membrane electrode assembly (MEA)  10  and a nickel mesh  20  sandwiched between the two MEAs. Each of the two MEAs  10  is a three-tier structure, comprising an anode  11 , a solid electrolyte  12  and a cathode  13 , and the nickel mesh  20  is composed of main body  21 , sandwiched between the two MEAs, and an extending bar  22 , extruding out of the sandwich structure of two MEAs  10  and the main body  21 . Moreover, a hole  23  is arranged on the extending bar  22 . The major function of the nickel mesh  30  is to enabling nature gas, hydrogen or any other common fuel used in the fuel cell stack to be distributed homogeneously on the surfaces of the anodes by using the porous nature of the nickel mesh  30 , and the same time without adversely affecting the conductive of the anode  11  cause by the generation of oxide layer thereon since it is still being enable to be subject to a redox ambient. In addition, the electrons are transport through the anodes  11  to the external circuit by the nickel mesh  20 . As seen in  FIG. 3B , the anode  11  of one of the two MEAs  10  is placed facing the anode  11  of another MEA  10  while sandwiching the nickel mesh  20  therebetween with the extending bar  22  extruding out of the sandwiched structure of two MEAs  10  and the main body  21 . The side of the sandwiched structure where the extending bar  22  is extruding is used as fuel inlet  31  while the side there of opposite to the fuel inlet  30  is used as the fuel outlet  32 , so that fuel can be fed into the sandwiched structure through the nickel mesh  20  of the fuel inlet  31  and flow out of the same through the nickel mesh  20  of the fuel outlet  32 . In order to prevent fuel to leakage from the two sides  33 ,  34  other than the fuel inlet  31  and the fuel outlet  32  of the sandwiched structure, the other two sides  33 ,  34  can be sealed by the use of sintered glass ceramics or mica spacers.  
      The case  40 , being an open-end hollow structure, has a connecting plate  45  arranged in front of the case, whereas the dimension of the connecting plate  45  is larger than that of the cross section of the case  40 . There is a recess  43  being arranged on the connecting plate  45 . Preferably, the case  40  is made of a non-precious metal such as stainless steel, or a high temperature resisting material such as Inconel 600/625, or a conductive material with thermal expansion coefficient similarly to that of the fuel cell. The space  41  formed inside the case  40  is used for receiving the fuel cell  30 . Furthermore, the case  40  further has a plurality of manifolds  42  arranged on the top and bottom thereof and channeling from the front of the case to the back thereof, for guiding air to flow and distribute uniformly along the cathodes of the corresponding planar fuel cell received therein. In order to achieve an optimum matching, the number of the manifolds  42  and the size of each manifold  42  are adjusted according to the characters of the corresponding membrane electrode assembly  10 . As seen in  FIG. 3B  and  FIG. 3C , as the fuel cell  30  is inserted into the case  40 , the extending bar  22  of the nickel mesh  20  will extrude out of the case  40  while the inner sides of the case  40  are in compact contact to the cathodes  13  of the fuel cell  30 , such that air can be guided to flow in the case  40  through the plural manifolds and to react with the cathodes  13 . In order to prevent the air from leakage and thus contact the anodes  11  of the fuel cell  30 , the gaps formed between the case  40  and the inserted fuel cell  30  at the fuel inlet  31  should be sealed. The airtight seal  44  can be achieved by sintering glass ceramics coated at the joint of the connecting plate  45  and the fuel cell  30 .  
      Generally, a fuel cell can only provides voltage of 0.6˜0.9V. Therefore, in order to increase the output voltage of the modularized solid oxide fuel cell stack, it is preferred to connect a plurality of fuel cell in serial so as to form a fuel cell stack. Please refer to  FIG. 4A  and  FIG. 4B , which are schematic diagrams depicting the steps of sequentially assembling two fuel cell cassettes  50 ,  50   a  to an air tank  60  according to the present invention. In  FIG. 4A  and  FIG. 4B , as the two fuel cell cassette  50 ,  50   a  are stacked to be inserted into the air tank  60 , the flexibility of extending bars  22 ,  22   a  enabling the same to be bended downwardly so as to fix the hole  23  of the extending bar  22  to the recess  43   a  of the connecting plate  45   a  connected to the lower fuel cell cassette  50   a  by a screw  51 . By which, the electrons generated from anode II of the upper fuel cell cassette  50  is collected by the corresponding nickel mesh  20  to be transmitted to the connecting plate  45   a  of the lower fuel cell cassette  50   a , and then being transmitted to the case  40   a  thereof for being provided to the cathode  13   a  thereof. It is noted that the number of fuel cell cassette to be used in the modularized fuel cell stack of the invention is not limited by the two fuel cell cassettes  50 ,  50   a , which can be as many as required, and thus the size of any components of the modularized fuel cell stack of the invention can be adjusted accordingly.  
      The configuration of the air tank  60 , the fuel tank  70 , the conductive strips  80  and the two fuel cell cassettes  50 ,  50   a  is illustrated with reference to the diagrams shown in  FIG. 2  and  FIG. 4B . As seen in  FIG. 2 , the air tank  60  having a front panel  66  arranged in front thereof further comprises: a main air chamber  61  for receiving the two fuel cell cassettes  50 ,  50   a ; at least a air duct  62 , for guiding high temperature air generated from a heat exchanger to flow into the air tank  60 ; and a hollow air distribution chamber  63 , having a plurality of air holes  64  to be arranged at a surface of the air distribution chamber  63  facing toward the main air chamber  61 , being arranged at a position between the main air chamber  61  and the air duct  62 . wherein the flowing speed of the high temperature air is first being reduced by the operation of the air distribution chamber  63 , and then the high temperature air is enabled to flow into the main air chamber  61  homogeneously through the plural air holes  64 . Moreover, the air tank  60  further comprises an after-burn chamber  65 , being arranged at an end thereof away from the fuel tank  70 , for enabling residual fuel and air to burn therein. Since the electrochemical reaction proceeding in the fuel cell stack as the fuel and air flowing across the two fuel cell cassettes  50 ,  50   a  can cause the temperature of the fuel cell stack to be raised to the range between 700° C. to 900° C., a burning effect can be caused instantly as soon as the residual fuel, which is mostly composed of hydrogen, is mixed with air in the after-burn chamber  65 . In addition, the size of the air duct  62  and the amount of air duct can be adjusted to optimize the air to be homogeneously distributed. Please refer to  FIG. 1A , which shows an assembly of a modularized solid oxide fuel cell stack with two air ducts  62 ,  62   a  according to another preferred embodiment of the present invention. The air duct  62  is arranged at a side of the air tank  60  while another air duct  62   a  is arranged at the opposite side of the air tank  60 . It is noted that the means for connecting the air duct  62   a  to the air tank  60  is similar to that of air duct  62  and thus is not described further herein. The symmetrical disposition of air ducts  62 ,  62   a  on the air tank  60  can increase the homogeneity of high temperature air to be distributed in the main air chamber  61 .  
      Please refer to  FIG. 5  and  FIG. 6 , which are respectively a three dimensional view of an air tank and a back view thereof according to a preferred embodiment of the invention. The characteristic of the air tank  600  is that the air and residual fuel can be fed into an after-burn chamber separately. In order to separate the residual fuel from the air, a residual fuel chamber  670  is arranged at the back of the main air chamber  610 , whereas the grooves  671  arranged at a side of the residual fuel chamber  670  facing toward the main air chamber  610  is used to connected to the back end of the corresponding fuel cell cassette. For ensuring all the residual fuel to enter the residual fuel chamber completely through the grooves  671 , airtight mica spacers (not shown in the figures) are added to the joint between the grooves  671  and the corresponding fuel cell cassette. After the residual fuel is collected and accumulated in the residual fuel chamber  670 , it is being fed to the after-burn chamber through the fuel exiting duct  680  connecting to the residual fuel chamber  670 . Moreover, as air fed into the main air chamber  610  through the air duct  620  is reacted to the fuel cell cassettes, the reacted air is fed into the after-burn chamber by way of an air exiting duct  680 . By the configuration described above, the air and residual fuel are fed into an after-burn chamber separately. For either the air tank  60  of  FIG. 2  or the air tank  600  of FOG.  5 , the after-burn chamber attached at the back of any of the two air tanks  60 ,  600  can have a porous ceramics arranged therein for enhancing the burning efficiency of the residual fuel and the air while enhancing the homogeneity of temperature distribution.  
      As seen in  FIG. 2  and  FIG. 4B , there are two mica spacers  91 ,  92  attached to the front panel  66 . The mica spacer  91  is inset in the inner frame  661  of the mica spacer  91  for preventing air from leaking out of the air tank  60  through the gap formed at the joint of the front panel  66  and the fuel cell cassettes  50 ,  50   a  and also for preventing the leakage of electricity generated by the two fuel cell cassettes  50 ,  50   a . The mica spacer  92  is inset in another in another inner frame  662  of the front panel  66  for preventing the connecting plates  45 ,  45   a  from contacting to the inner frame  662  and thus preventing the leakage of electricity caused thereby. In addition, there is an isolating plate being placed between the connecting plate  45  and the connecting plate  45   a  for preventing short circuit caused by the contacting of the two plates  45 ,  45   a.    
      As seen in  FIG. 1  and  FIG. 2 , the fuel tank  70  is connected to the front panel  66  of the air tank  66  by the use of a back panel  74  thereof so as to seal the fuel cell cassettes  50 ,  50   a  inside the air tank  60 . Moreover, a fuel duct  71  is connected to the fuel tank  70  for guiding high temperature fuel to feed into a hollow fuel distribution chamber  72  formed inside the fuel tank  70 , whereas the hollow fuel distribution chamber  72  has a plurality of fuel holes  73  arranged on a side thereof facing toward the fuel cell cassettes  50 ,  50   a . Furthermore, there is a mica spacer  93  to be arranged at the joint of the fuel tank  70  and the air tank  60  so as to isolate the two and prevent leakage.  
      The function of the set of conductive strips  80  is to conduct current out of the fuel cell stack of the invention, which comprises an anode strip  81  and a cathode strip  82 . Both the anode strip  81  and the cathode strip  82  are made of a high temperature resisting material, preferably, similar to that of the cases  40 ,  40   a . The anode strip  81  can be a Y-type clip having an opening  811  arranged on top thereof. As seen in  FIG. 4B , by using the opening  811  to clip the extending bar  22   a  of the lower fuel cell cassette  50   a , the current generated from the anode  11   a  of the lower fuel cell cassette  50   a  can be conducted out of the fuel cell stack. On the other hand, the cathode strip  82  is fixed to the hole  43  of the connecting plate  45  of the upper fuel cell cassette  50  by a screw  821 . For preventing the leakage of fuel caused by the installation of the conducive strips  81 ,  82 , an addition mica space  94  is being arranged between the fuel tank  70  and the spacer  93  for sandwiching the conductive strips  81 ,  82  between the two mica spacers  93 ,  94 , as shown in  FIG. 1 .  
      By the configuration described above, a modularized solid oxide fuel cell stack can be constructed. Operationally, as high temperature fuel is fed into the stack through the fuel duct  71  while hot air is guided into the same through the air duct  62 , electricity can be output by the operation of the anode strip  81  and the cathode strip  82 . Thereafter, the reacted air and residual fuel can be guided to the after-burn chamber  65  disposed at the back of the air tank  60 , where the residual fuel is burning out for generating high temperature exhaust gas to be discharged and recycled. As the configuration of replaceable fuel cell cassettes in the fuel cell stack of the invention, accordingly, if one fuel cell cassette must be removed and replaced, such as for testing or maintenance, the remaining fuel cell cassettes will not destroyed in the process. This leads to significant saving in time and money.  
      While the preferred embodiment of the invention has been set forth for the purpose of disclosure, modifications of the disclosed embodiment of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.