Abstract:
A filter medium includes a ceramic mass having a void formed by burning a contained carbon to be eliminated. The ceramic mass has an inflow side for a fluid to flow thereinto and an outflow side for the fluid to flow out therefrom. The inflow side and the outflow side separate from each other with a distance. The inflow side and the outflow side have holes depressed inward from the sides, respectively.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2006-330482 filed on Dec. 7, 2006; the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a filter medium for filtering a fluid flowing in a fluid path connected to a fuel cell and a filtration system utilizing the filter medium. 
         [0003]    Various devices employing energy sources instead of fossil fuels have been developed recently. Techniques for using fuel cells as driving sources have been developed increasingly. In the field of vehicles using internal combustion engines such as automobiles, vehicles using fuel cells instead of such internal combustion engines have been being developed. 
         [0004]    The fuel cell has fluid paths connected thereto such as circuits for feeding hydrogen which is a reducing material serving as a fuel and oxygen which is an oxidizing material and a cooling circuit for preventing increases in temperature on the fuel cell stack. Methods for filtering such fluids flowing in the fluid paths have been studied. 
         [0005]    The present applicant has disclosed an ion exchange filter for preventing ionization of pure water flowing in a pure water humidifying circuit and cooling water flowing in a cooling circuit in the fuel cell (Japanese Patent Application Laid-open No. 2005-166267). 
         [0006]    Meanwhile, as in Japanese Patent Application Laid-Open No. 2003-269131, for example, filters made of ceramic are provided in emission systems for automobile engines to remove particles from emissions. 
         [0007]    According to the fluid path connected to the fuel cell, an air feeding circuit is connected to a compressor while a circulation circuit in a cooling device to a pump for circulating cooling water. Such devices may produce impurities such as fine abrasion powders during their operations. The previous studies on the fuel cell do not recognize such impurities produced from the devices. Therefore, any measures have not been taken for removing the impurities. The fluid path connected to the fuel cell has relatively high temperatures. Filter paper and non-woven fabric commonly utilized for a filter medium have low heat-resistance, being not suitable for the filtration system of the fuel cell. 
         [0008]    When ceramic is employed for the filter medium, it is effective for systems to be filtered with high temperatures. 
         [0009]    However, ceramic is brittle. If ceramic is employed for the filter medium and the resulting ceramic filter medium is utilized in vibrating systems, ceramic powders may spill or the filter medium may be chipped. Extremely high purity is required in the fluid path of the fuel cell. Ceramic is difficult to be employed for the filter medium in fluid paths for fuel cells mounted in vibrating automobiles. 
         [0010]    To solve the problem, ceramic powders may be compacted using binders. However, fluids may be contaminated by dissolution of such binders. 
       SUMMARY OF THE INVENTION 
       [0011]    The invention is directed to a filter medium using a ceramic mass having excellent heat- and vibration-resistances without dissolution of a binder. 
         [0012]    The invention is directed to a filtration system that filters a fluid in a fluid path connected to a fuel cell by using the filter medium to maintain high purity. 
         [0013]    The first aspect of the invention provides a filter medium. The filter medium includes a ceramic mass having a void formed by burning a contained carbon to be eliminated. The ceramic mass has an inflow side for a fluid to flow thereinto and an outflow side for the fluid to flow out therefrom. The inflow side and the outflow side separate from each other with a distance. The inflow side and the outflow side have holes depressed inward from the sides, respectively. 
         [0014]    The second aspect of the invention provides a filter medium. The filter medium includes a ceramic mass having a void formed by burning a contained carbon to be eliminated. The ceramic mass has an inflow side for a fluid to flow thereinto and an outflow side for the fluid to flow out therefrom. The inflow side and the outflow side separate from each other with a distance. The ceramic mass has a surface having a circumferential surface between the inflow side and the outflow side. At least the circumferential surface is coated with a coating material. 
         [0015]    The third aspect of the invention provides a filter medium. The filter medium includes a ceramic mass having a void formed by burning a contained carbon to be eliminated. The ceramic mass has an inflow side for a fluid to flow thereinto and an outflow side for the fluid to flow out therefrom. The inflow side and the outflow side separate from each other with a distance. The ceramic mass has a surface having a circumferential surface between the inflow side and the outflow side. At least the circumferential surface is coated with a coating material. The inflow side and the outflow side have holes depressed inward from the sides, respectively. 
         [0016]    The ceramic mass may have an entire surface coated with a coating material. The coating material may have communication holes coinciding with the holes of the inflow side and the outflow side. The communication holes may extend through the coating material in a thickness direction of the coating material and communicate with outside of the ceramic mass. 
         [0017]    The fourth aspect of the invention provides a filtration system installed in a fluid path connected to a fuel cell for filtrating a fluid circulating through the fluid path by a filtration device. The filtration device has a ceramic mass having a void formed by burning a contained carbon to be eliminated. The ceramic mass has an inflow side for a fluid to flow thereinto and an outflow side for the fluid to flow out therefrom. The inflow side and the outflow side separate from each other with a distance. The inflow side and the outflow side have holes depressed inward from the sides, respectively. The filtration device is located downstream of a flow driver allowing the fluid to circulate through the fluid path. 
         [0018]    The fifth aspect of the invention provides a filtration system installed in a fluid path connected to a fuel cell for filtrating a fluid circulating through the fluid path by a filtration device. The filtration device has a ceramic mass having a void formed by burning a contained carbon to be eliminated. The ceramic mass has an inflow side for a fluid to flow thereinto and an outflow side for the fluid to flow out therefrom. The inflow side and the outflow side separate from each other with a distance. The ceramic mass has a surface having a circumferential surface between the inflow side and the outflow side. At least the circumferential surface is coated with a coating material. The filtration device is located downstream of a flow driver allowing the fluid to circulate through the fluid path. 
         [0019]    The sixth aspect of the invention provides a filtration system installed in a fluid path connected to a fuel cell for filtrating a fluid circulating through the fluid path by a filtration device. The filtration device has a ceramic mass having a void formed by burning a contained carbon to be eliminated. The ceramic mass has an inflow side for a fluid to flow thereinto and an outflow side for the fluid to flow out therefrom. The inflow side and the outflow side separate from each other with a distance. The ceramic mass has a surface having a circumferential surface between the inflow side and the outflow side. At least the circumferential surface is coated with a coating material. The inflow side and the outflow side have holes depressed inward from the sides, respectively. The filtration device is located downstream of a flow driver allowing the fluid to circulate through the fluid path. 
         [0020]    The ceramic mass may have an entire surface coated with a coating material. The coating material may have communication holes coinciding with the holes of the inflow side and the outflow side. The communication holes may extend through the coating material in a thickness direction of the coating material and communicate with outside of the ceramic mass. 
     
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         [0021]      FIG. 1  is a schematic diagram of a circuit system for a fuel cell in a filtration system according to an embodiment of the invention; 
           [0022]      FIG. 2  is a vertical cross-sectional view of a schematic internal structure of the filter illustrated in  FIG. 1 ; 
           [0023]      FIG. 3  is a perspective view of the ceramic filter illustrated in  FIG. 2 ; 
           [0024]      FIG. 4  is a vertical cross-sectional view of a schematic structure of the ceramic filter illustrated in  FIG. 3 ; 
           [0025]      FIG. 5  is a vertical cross-sectional view of a schematic structure of a ceramic filter different from that of  FIG. 4 ; 
           [0026]      FIG. 6  is a vertical cross-sectional view of a schematic structure of a ceramic filter different from that of  FIGS. 3 to 5 ; and 
           [0027]      FIG. 7  is a vertical cross-sectional view of a schematic structure of a ceramic filter different from that of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    Embodiments of the invention will be described below with reference to the accompanying drawings 
         [0029]      FIG. 1  is a schematic view of a fuel cell system that utilizes a ceramic filter  20  which is a filter medium of the invention and a filtration system. A fuel cell stack  1  is connected to a hydrogen feeding circuit  2  for feeding hydrogen to the fuel cell stack  1 , an aspiration circuit  3  for feeding air from the atmosphere, and a cooling circuit  4  for cooling the fuel cell stack  1 . 
         [0030]    The hydrogen feeding circuit  2  has a hydrogen tank  5  that stores hydrogen serving as a fuel for the fuel cell. The hydrogen feeding circuit  2  also has a pipe connecting the hydrogen tank  5  to the fuel cell stack  1 . The hydrogen tank  5  contains a high pressure hydrogen filled therein. The filled hydrogen is reduced in pressure by a reducing valve to be fed to the fuel cell stack  1 . 
         [0031]    The aspiration circuit  3  has a pipe connecting the atmosphere to the fuel cell stack  1 . The pipe has a compressor  6  serving as a flow driver that pressurizes air for allowing the pressurized air to flow to the fuel cell stack  1 . In the aspiration circuit  3 , the filter  10  is connected to the downstream side of the compressor  6 . The filter  10  filters impurities contained in the circuit. 
         [0032]    The cooling circuit  4  is a closed circulating circuit. The cooling circuit  4  has a pump  7  serving as a flow driver for circulating cooling water in the cooling circuit  4 . The cooling circuit  4  also has a radiator  8  for cooling the circulating cooling water. The cooling circuit  4  has the filter  10  on the downstream side of the pump  7  and the upstream side of the fuel cell stack  1 . The filter  10  filters impurities contained in the cooling water circulating in the circuit. In addition to ordinary water, the cooking water employs a mixed solution of ethylene glycol and water having an anti-freezing property in consideration of utilizing the fuel cell system in cold climates. 
         [0033]    When fluids flowing in the hydrogen feeding circuit  2 , aspiration circuit  3 , and cooling circuit  4  are filtered, the following points should be considered. 
         [0034]    The hydrogen feeding circuit  2  will be described first. The hydrogen feeding circuit  2  has the pipe connecting the hydrogen tank  5  to the fuel cell stack  1 . The pipe does not have any portions communicating with the atmosphere. Dust thus does not enter the hydrogen feeding circuit  2  from the atmosphere during its normal operation. Fine impurities may remain in the hydrogen tank  5 . Impurities may enter the hydrogen feeding circuit  2  when hydrogen is refilled in the hydrogen tank  5  or the hydrogen tank  5  is replaced. The hydrogen feeding circuit  2  feeds hydrogen which is a fuel for the fuel cell, removing impurities for feeding hydrogen with high purity. Impurities thus should be removed reliably from the hydrogen feeding circuit  2 . Specifically, impurities of larger than 1 μm should be removed reliably. Finer impurities by the 0.1 μm may be removed. A filter medium capable of filtering such impurities is selected and provided in the filter  10 . The hydrogen feeding circuit  2  is for feeding hydrogen, and filters made of metals may be broken due to hydrogen brittleness. When the filter  10  is provided in the hydrogen feeding circuit  2 , the filter medium is made of materials that do not cause the hydrogen brittleness. 
         [0035]    The aspiration circuit  3  aspirates air from the atmosphere. The atmosphere contains various impurities, and it cannot be employed as a fuel for the fuel cell unless the impurities are removed. An oxygen contained in the atmosphere is also employed as a fuel for the fuel cell, and impurities larger than 1 μm should be removed reliably. A filter medium capable of removing finer impurities by the 0.1 μm may be provided in the filter  10 . The temperature of the aspiration circuit  3  is increased to about 160° C. A filter medium made of heat-resistance materials withstanding high temperatures is employed. 
         [0036]    The cooling circuit  4  will be described next. The cooling circuit  4  is a closed circuit, and impurities do not enter the circuit  4  from the outside. Components of the pump  7  wear out according to the operation of the pump  7 , thus producing abrasion powder. The filter  10  removes such abrasion powder from the cooling water to feed the resulting filtered water to the fuel cell stack  1 . 
         [0037]    The temperature of the cooling water circulating in the cooling circuit  4  is increased to about 120° C. The filter medium used in the filter  10  employs a heat-resistant material. A cooling medium is liquid, and the filter medium is prevented from serving as the source of contamination due to ion dissolution from the filter medium. In the ion dissolution from the filter medium, if ions are dissolved in the cooling water, the cooling circuit  4  is additionally provided with an ion exchange filter for removing such ions. If the temperature of the cooling water in the cooling circuit is decreased and dissolved ions are precipitated again as crystals, the ion exchange filter cannot remove such crystals. Materials for the filter medium of the filter  10  provided in the cooling circuit  4  are selected in consideration of such matters. 
         [0038]    Requirements for the filter medium are as follows. Namely, the filter medium should remove fine impurities reliably and have the heat-resistance. Further, the filter medium should not cause the hydrogen brittleness and ion dissolution. In view of such requirements, a ceramic filter  20  to be described is employed as the filter medium. 
         [0039]      FIG. 2  illustrates a schematic structure of the filter  10  employed in the circuits. The filter  10  is mainly formed of a casing  11  serving as a framework and the ceramic filter  20  placed in the casing  11 . Holders  12  are provided in the casing  11 . The ceramic filter  20  is supported by the holders  12 . 
         [0040]      FIGS. 3 and 4  illustrate an example of the ceramic filter  20  employed in the filter  10 . The ceramic filter  20  is formed in a disk with a certain thickness and provided with parallel flat sides. The ceramic filter  20  is formed of a ceramic mass  21  serving as the core and a coating material  22  coated on the surface of the ceramic mass  21 . The ceramic mass  21  serving as the core has voids therein. Such voids are formed by heating the raw ceramic mass so as to burn contained carbon to be eliminated. The coating material  22  of a resin material coats the entire surface of the ceramic mass  21 . 
         [0041]    The coating material  22  is selected in view of the adhesion to the ceramic mass  21  serving as the core and the following capability with respect to the thermally expanded ceramic mass  21 . Polypropylene, nylon, and fluorine are used for the coating material  22 . The coating material  22  is not limited to such a resin material and epoxy, silicon, and rubber adhesives are used. 
         [0042]    The ceramic filter  20  has a side in the thickness direction. This side serves as an inflow side  23  for a fluid to flow thereinto for filtering. The ceramic filter  20  has the other side opposing the one side. The other side serves as an outflow side  24  for the filtered fluid to flow out. The inflow side  23  and outflow side  24  have a plurality of holes  25  depressed internally from the surfaces, respectively. In the ceramic filter  20 , the holes  25  formed in the inflow side  23  coincide in position with the holes  25  formed in the outflow side  24 . The holes  25  formed in the inflow side  23  and outflow side  24  are serially arranged on the identical lines. The holes  25  are formed with a tool such as a drill with an outer diameter corresponding to the diameter of the holes  25  after the surface of the ceramic mass  21  is coated with the coating material  22 . The position of a communication hole  25   b  in the coating material  22  coincides with the position of a hole  25   a  in the ceramic mass  21 . 
         [0043]    The plurality of holes  25  may be formed in the inflow side  23  and outflow side  24  in advance when the raw ceramic mass is formed. The holes  25  are masked not so as to be closed by the coating material  22  when the surface of the ceramic mass  21  is coated with the coating material  22 . The masked portion functions as the communication hole  25   b  when removing the mask. 
         [0044]    According to the ceramic filter  20 , a fluid flows from the holes  25  on the inflow side  23  toward the inside of the ceramic filter  20 . On the inflow side  23 , the hole  25   a  of the ceramic mass  21  functions as an inflow portion for allowing the fluid to enter the ceramic mass  21 . The communication hole  25   b  of the coating material  22  functions as a communicating portion for allowing the fluid which has flown in the circuit to enter the hole  25   a  of the ceramic mass  21 . The fluid entering the ceramic mass  21  passes through the voids in the ceramic mass  21  to flow toward the holes  25  on the outflow side  24 . Impurities contained in the fluid are thus filtered. The filtered fluid passes through the holes  25  on the outflow side  24  to flow out from the outflow side  24 . The hole  25   a  formed in the ceramic mass  21  functions as an outflow portion on the outflow side  24 . The communication hole  25   b  of the coating material  22  functions as a communicating portion for allowing the fluid to flow from the hole  25   a  of the ceramic mass  21  to the circuit. 
         [0045]    According to the ceramic filter  20 , the voids in the ceramic mass  21  serving as the core are formed by heating the raw ceramic mass so as to burn carbon contained therein and eliminate the same. Unlike ceramic masses prepared by compacting ceramic particles using binders, the ceramic filter does not employ such binders. If the fluid flows in the ceramic filter  20 , dissolution of the binder does not occur. The ceramic filter  20  does not serve as the source of contamination. 
         [0046]    However, articles on the surface of the ceramic mass  21  prepared as described above may be spilled, or the ceramic mass  21  may be chipped. To prevent such disadvantages, the entire surface of the ceramic mass  21  is coated with the coating material  22 . If the ceramic mass  21  is coated with the coating material  22 , the coating material  22  prevents the fluid from entering and exiting from the ceramic mass  21 . The holes  25  on the inflow side  23  and outflow side  24  facilitate smooth inflow and outflow of the fluid. The holes  25  increase the area of contacting the fluid with the ceramic mass  21 , thereby improving the filtration efficiency. 
         [0047]    While the holes  25  are formed in the inflow side  23  and outflow side  24  so as to be arranged on the identical lines, they may be formed as illustrated in  FIG. 5 . 
         [0048]    According to a ceramic filter  20 A illustrated in  FIG. 5 , the position of the hole  25  formed on the inflow side  23  is displaced from the position of the hole  25  formed on the outflow side  24 . When observing the cross section of the ceramic filter  20 A, the holes  25  are arranged in a zigzag pattern in the diameter direction of the ceramic filter  20 . The holes  25  on the top side are positioned between the holes  25  on the bottom side. The holes  25  on the bottom side are positioned between the holes  25  on the top side. According to the ceramic filter  20 A illustrated in  FIG. 5 , the entire surface of the ceramic mass  21  serving as the core is coated with the coating material  22 . The holes  25  on the inflow side  23  and outflow side  24  are formed with a tool such as a drill through the coating material  22 . 
         [0049]    The ceramic filter prepared by coating the entire surface of the ceramic mass with the coating material has been described. A ceramic filter prepared by coating the ceramic mass partially with the coating material may be employed. 
         [0050]      FIG. 6  is a cross-sectional view of a ceramic filter  30 . A disk-shaped ceramic mass  31  with a certain thickness is employed for the ceramic filter  30 . The circumferential surface of the ceramic mass  31  is coated with a coating material  32 . 
         [0051]    According to the ceramic filter  30 , the ceramic mass  31  serving as the core has voids therein formed by heating the raw ceramic mass so as to burn contained carbon and eliminate the same. The ceramic mass  31  is provided with parallel flat sides. One side of the sides is an inflow side  33  that a fluid enters. The other side is an outflow side  34  having the fluid to exit therefrom. The inflow side  33  and outflow side  34  have a plurality of holes  35  depressed internally from the surfaces, respectively. According to the ceramic filter  30  illustrated in  FIG. 6 , the positions of the holes  35  on the inflow side  33  are the identical to those on the outflow side  34 . Namely, the holes  35  on the inflow side  33  and the outflow side  34  are serially arranged on the identical lines. The holes  35  may be formed using a tool such as a drill or may be formed in advance when the raw ceramic mass  31  is formed. 
         [0052]    The coating material  32  coats the circumferential surface of a portion between the inflow side  33  and outflow side  34  of the ceramic mass  31 . The edges serving as boundaries between the circumferential surface and the inflow side  33  and between the circumferential surface and the outflow side  34  are easily chipped. To prevent such chipping, the coating material  32  extends further than the peripheries of the inflow side  33  and outflow side  34 . Resin materials including polypropylene, nylon, and fluorine are employed for the coating material  32 . The coating material  32  is not limited to such resin materials, and epoxy, silicon, and rubber adhesives are employed. 
         [0053]    The ceramic filter  30  illustrated in  FIG. 6  is mounted in the casing  11  of the filter  10  with its circumferential surface being held. The inflow side  33  and outflow side  34  of the ceramic filter  30  are not coated with the coating material  32 , the area of contacting the fluid is increased. The contact area is further increased by the inner circumferential surfaces of holes  35  on the inflow side  33  and outflow side  34 . 
         [0054]    According to the ceramic filter  30  that the coating material  32  coats only the circumferential surface of the ceramic mass  31 , as illustrated in  FIG. 7 , the holes  35  on the inflow side  33  may be displaced from those on the outflow side  34 .  FIG. 7  is a vertical cross-sectional view of a ceramic filter  30 A. The holes  35  on the inflow side  33  and outflow side  34  are arranged in a zigzag pattern in the diameter direction of the ceramic filter  30 A. 
         [0055]    The ceramic filter having entire surface or circumferential surface coated with the coating material has been described by way of example. A ceramic filter whose surface is not coated with the coating material may be employed. When the ceramic filters described are employed for the filter  10  in the hydrogen feeding circuit  2 , the aspiration circuit  3 , and the cooling circuit  4 , impurities with small particle diameters are filtered reliably. The temperature of the air in the aspiration circuit  3  is increased to about 160° C. The temperature of the cooling water in the cooling circuit  4  is increased to about 120° C. The ceramic filter has high heat-resistance, and the filtration efficiency is not decreased due to heat. The ceramic filter medium is not attacked by hydrogen, namely, does not cause hydrogen brittleness in the hydrogen feeding circuit  2 . When the ceramic filter is provided in the cooling circuit  4 , impurities with small particle diameters are filtered. The binder is not used, and the ceramic filter does not serve as the source of contamination. 
         [0056]    While a case where the invention is applied to a circuit for a fuel cell using hydrogen as a fuel has been described above, the invention is not limited thereto and may also be applied to a circuit for a fuel cell using methane of a reducing material as a fuel. When hydrogen is used as a fuel, the invention may be applied to a circuit with a device for reforming natural gas or methanol to produce hydrogen. 
         [0057]    Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teachings. The scope of the invention is defined with reference to the following claims. 
         [0058]    According to the invention, if the filter medium having an excellent heat resistance is used in the fuel cell system having a fluid of high temperatures to be filtered, the filter medium does not deteriorate due to heat. The ceramic mass being a core formed as the invention allows fine impurities to be efficiently filtered. Without using a binder, impurities contained in the binder do not dissolves, and the ceramic filter itself does not serves as a source of contamination. 
         [0059]    Formation of holes on inflow side and outflow side of the filter medium enlarges contact area with a fluid, improving filtration efficiency. This enlargement of the contact area with the fluid due to the holes further widens efficient filtration portion if the filter medium is partially clogged. 
         [0060]    Coating of the ceramic mass with a coating material effectively prevents disadvantage of the ceramic mass such as partial coming out and chipping.