Patent Publication Number: US-2006014056-A1

Title: Reformer and fuel cell system having the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims priority to and the benefit of Korean Patent Application No. 10-2004-0047557, filed on Jun. 24, 2004 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference.  
     FIELD OF THE INVENTION  
      The present invention relates to a fuel cell system and more particularly to a fuel cell system having an improved reformer.  
     BACKGROUND OF THE INVENTION  
      In general, a fuel cell is a system for generating electric energy through an electrochemical reaction between oxygen and hydrogen contained in hydrocarbon materials such as methanol, ethanol, and natural gas.  
      Recently developed polymer electrolyte membrane fuel cells (hereinafter, referred to as PEMFCs) have been shown to exhibit excellent output characteristics, low operating temperatures, and fast starting and response characteristics. PEMFCs have a wide range of application including use as mobile power sources for vehicles, as distributed power sources for homes or buildings, and as small-sized power sources for electronic apparatuses.  
      A fuel cell system employing the PEMFC scheme basically requires a stack, a reformer, a fuel tank, and a fuel pump. The stack constitutes an electricity generation set having a plurality of unit cells. The fuel pump supplies fuel from the fuel tank to the reformer. The reformer reforms the fuel to generate hydrogen which is supplied to the stack.  
      The reformer generates hydrogen from the fuel containing hydrogen by a catalytic chemical reaction using thermal energy. Accordingly, the reformer may include a heat source for generating the thermal energy and a reforming reaction section for absorbing the thermal energy and generating hydrogen from the fuel.  
      In the reformer of a conventional fuel cell system, the heat source and the reforming reaction section are provided in separate vessels that are connected to one another through pipes. Each of the heat source and the reforming reaction section is generally formed as a single module with honeycombed passages parallel to the flow direction of fuel, where a catalyst layer for promoting a reaction is formed on the passages. Such modules can be manufactured by injection-molding a ceramic material and forming the catalyst layer on the surface of the passages. However, in a conventional reformer, since the passages through which the fuel passes are isolated from each other, the flow distribution of fuel is not uniform. In addition, since the diffusion rate of fuel in the catalyst layer is lowered, the whole reaction efficiency of the reformer is deteriorated. Furthermore, since the manufacturing process of the modules is complex, the manufacturing productivity is deteriorated.  
     SUMMARY OF THE INVENTION  
      In accordance with the present invention a reformer which can enhance reaction efficiency and thermal efficiency with a simple structure is provided, as well as a fuel cell system having the reformer.  
      According to one aspect of the present invention, a reformer of a fuel cell system is provided. The reformer includes a main body defining an inner space, a reformer inlet and a reformer outlet. Within the inner space is a reaction section which forms a passage for fuel. The reaction section includes a heat-generating element for generating thermal energy from externally applied energy and a catalyst layer formed on the surface of the heat-generating element.  
      The reformer may further comprise an electrical power supply for supplying electric current to the heat-generating element. The heat-generating element may be made of metal having good conductivity.  
      The heat-generating element may have a pleated or corrugated shape.  
      A support layer may be formed between the heat-generating element and the catalyst layer to support the catalyst layer.  
      An insulating layer for electrically insulating the heat-generating element and the main body may be formed on inner surfaces of the main body.  
      According to another embodiment of the present invention, a fuel cell system is provided with a reformer for generating hydrogen from fuel, at least one electricity generator for generating electric energy through an electrochemical reaction of hydrogen and oxygen, a fuel supply unit for supplying the fuel to the reformer, and an oxygen supply unit for supplying oxygen to the electricity generator. The reformer is as was described above  
      The fuel supply unit and the reformer inlet of the main body may be connected through a first supply line and the reformer outlet of the main body and the electricity generator may be connected through a second supply line.  
      The fuel supply unit may include a fuel tank for storing a fuel containing hydrogen and a fuel pump connected to the fuel tank to transfer the fuel to the reformer.  
      The oxygen supply unit may include an air pump for supplying air to the electricity generator, and the air pump and the electricity generator may be connected through a third supply line.  
      A plurality of the electricity generators may be stacked to form a stack.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
       FIG. 1  is a block diagram schematically illustrating a fuel cell system according to an embodiment of the present invention;  
       FIG. 2  is an exploded perspective view illustrating a stack shown in  FIG. 1 ;  
       FIG. 3  is a perspective view of a reformer of a fuel cell system according to an embodiment of the present invention; and  
       FIG. 4  is a cross-sectional view of the reformer shown in  FIG. 3 . 
    
    
     DETAILED DESCRIPTION  
      Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the attached drawings such that the present invention can be put into practice by those skilled in the art. However, the present invention is not limited to the exemplary embodiments, but may be embodied in various forms.  
       FIG. 1  is a block diagram schematically illustrating a fuel cell system according to an embodiment of the present invention.  
      Referring to  FIG. 1 , the fuel cell system  100  depicted is a polymer electrode membrane fuel cell (PEMFC) scheme, where fuel is reformed to generate hydrogen, and the hydrogen generated is electrochemically reacted with oxygen to generate electric energy.  
      The fuel used for the fuel cell system  100  may include liquid or gas fuel containing hydrogen such as methanol, ethanol, or natural gas. However, for ease of discussion, the fuel described below is a liquid fuel.  
      The fuel cell system  100  may utilize pure oxygen stored in an additional storage device as oxygen for reacting with hydrogen or may utilize air containing oxygen. The latter is exemplified in the following description.  
      The fuel cell system  100  basically comprises a stack  10  for generating electric energy through an electrochemical reaction between hydrogen and oxygen, a reformer  20  for generating hydrogen from the fuel, a fuel supply unit  30  for supplying the fuel to the reformer  20 , and an oxygen supply unit  40  for supplying oxygen to the stack  10 .  
       FIG. 2  is an exploded perspective view illustrating the stack shown in  FIG. 1 .  
      The stack  10  is composed of an electricity generator set which is formed by sequentially stacking a plurality of electricity generators  11 .  
      Each electricity generator  11  includes separators  16  (also referred to as “bipolar plates” in the art) that are disposed in close contact with both surfaces of a membrane-electrode assembly (MEA)  12 .  
      The MEA  12  has a predetermined active area where hydrogen and oxygen electrochemically react with each other and has a structure that an anode electrode is formed on one surface, a cathode electrode is formed on the other surface, and an electrolyte membrane is interposed between both electrodes.  
      The anode electrode converts hydrogen into electrons and hydrogen ions through the oxidation of hydrogen. The cathode electrode generates heat of a predetermined temperature and moisture through the reduction of the hydrogen ions and oxygen. The electrolyte membrane performs an ion exchange function of moving the hydrogen ions generated from the anode electrode to the cathode electrode.  
      The separators  16  function as conductors for connecting the anode electrodes and the cathode electrodes in series to each other, and also supply the MEAs  12  with hydrogen and oxygen.  
      Pressing plates  13  and  13 ′ are provided on the outermost ends of the stack  10  for bringing a plurality of electricity generators  11  in close contact with each other. However, other designs are available and the stack  10  may be constructed such that the pressing plates  13  and  13 ′ are excluded and the separators  16  positioned at the outermost sides of a plurality of electricity generators  11  can perform the function of the pressing plates  13  and  13 ′. The pressing plates  13  and  13 ′ may have the inherent function of a separator  16 , in addition to the function of bringing a plurality of electricity generators  11  into close contact with each other.  
      A first inlet  13   a  for supplying hydrogen generated from the reformer  20  to the electricity generators  11  and a second inlet  13   b  for supplying air supplied from the oxygen supply unit  40  to the electricity generators  11  are formed in one pressing plate  13  of the pressing plates  13  and  13 ′. A first outlet  13   c  for discharging the remaining hydrogen not participating in the reaction of the electricity generators  11  and a second outlet  13   d  for discharging the non-reacted air containing moisture generated from the bonding reaction of hydrogen and oxygen in the electricity generators  11  are formed in the other pressing plate  13 ′.  
      In the present invention, the reformer  20  generates hydrogen from the fuel containing hydrogen through a chemical catalytic reaction using thermal energy. The structure of the reformer  20  is described in more detail later with reference to  FIGS. 3 and 4 .  
      The fuel supply unit  30  for supplying the fuel to the reformer  20  includes a fuel tank  31  for storing the liquid fuel and a fuel pump  33  for pumping the fuel from the fuel tank  31 . The reformer  20  and the fuel tank  31  are connected to each other through a tubular shaped first supply line  81 . The reformer  20  and the first inlet  13   a  of the stack  10  are connected to each other through a tubular shaped second supply line  82 .  
      The oxygen supply unit  40  includes an air pump  41  for feeding air with a predetermined pumping power to the stack  10 . The air pump  41  and the second inlet  13   b  of the stack  10  are connected to each other through a third supply line  83 .  
      Hereinafter, an example of the reformer  20  is described in detail with reference to the attached drawings.  
       FIG. 3  is a perspective view illustrating the reformer of a fuel cell system according to an embodiment of the present invention and  FIG. 4  is a cross-sectional view of the reformer shown in  FIG. 3 .  
      Referring to the figures, the reformer  20  according to the present invention includes a main body  21 , a reaction section  25  which is disposed inside the main body  21  and which generates hydrogen from the fuel containing hydrogen, and a power supply  29 , shown schematically as supplying power to the reaction section  25  to generate thermal energy.  
      The main body  21  defines an inner space with a reformer inlet  22  through which fuel is supplied to the inner space and a reformer outlet  23  through which the hydrogen is discharged from the main body  21 . According to this embodiment, the main body  21  comprises a rectangular parallelepiped housing structure with the reformer inlet  22  and the reformer outlet  23  at opposite ends. However, the main body  21  is not limited to the above-mentioned shape, but may be made in any one of a number of different shapes with a cylindrical shape being just one example.  
      The main body  21  can be made of a material having a heat-shielding property such as from ceramic, stainless steel, or aluminum.  
      The reformer inlet  22  of the main body  21  and the fuel tank  31  of the fuel supply unit  30  are connected to each other through the first supply line  81  described above. The reformer outlet  23  of the main body  21  and the electricity generators  11  of the stack  10  are connected to each other through the second supply line  82  described above.  
      The reaction section  25  generates hydrogen from the fuel through a catalytic reformation reaction using the thermal energy, and is disposed inside the main body  21  to form a plurality of passages  24  through which the fuel passes. The reaction section  25  includes a heat generating element  26  for generating the thermal energy from energy applied externally and a catalyst layer  28  which is formed on the surface of the heat-generating element  26  and which promotes the reforming reaction of the fuel.  
      A support layer  27  supporting the catalyst layer  28  may be formed between the heat-generating element  26  and the catalyst layer  28 . The support layer  27  serves as a carrier supporting the catalyst layer  28  and may be made of a material such as alumina (Al 2 O 3 ), silica (SiO 2 ), or titania (TiO 2 ).  
      The heat-generating element  26  may be formed by shaping a metal plate made of a metal such as stainless steel, aluminum, copper, nickel, iron, or the like in a pleated or corrugated arrangement. Accordingly, when viewed in cross section from a direction perpendicular to the longitudinal direction of the passages  24 , the heat-generating element  26  has a wavelike shape.  
      By forming an insulating layer  205  on inner surfaces of the main body  21 , the main body  21  and the heat-generating element  26  can be electrically insulated from each other even when both of them are made of conductive materials. However, in one embodiment, it is preferred that the heat-generating element  26  is spaced from the inner surface of the main body  21 .  
      The power supply  29  is connected in series to both ends of the heat-generating element  26  and supplies electric current to the heat-generating element  26 . The electrical resistance of the heat-generating element  26  generates the desired thermal energy.  
      In the present embodiment, although an electrical power supply  29  is exemplified as a source for supplying energy to the heat-generating element  26 , various other sources may be used for supplying other types of energy to the heat-generating element  26 .  
      In the reformer  20  according to the present embodiment, since the sectional shape of the heat-generating element  26  forming the passages  24  for fuel has a corrugated shape, the distribution of fuel through the passage  24  is uniform and the flow of fuel is turbulent, thereby enhancing the contact area of the fuel to the surface of the catalyst layer  28 . In addition, since the structure of the reaction section in which the reforming reaction occurs is simple, it is simple to manufacture and productivity may be enhanced.  
      Operation of the fuel cell system according to the present invention will now be described in detail.  
      First, a predetermined amount of power is applied to the heat-generating element  26  from the power supply  29 . Then, the heat-generating element  26  generates thermal energy due to its electrical resistance  
      The fuel pump  33  supplies the fuel stored in the fuel tank  31  to the inner space of the main body  21  through the first supply line  81  and the reformer inlet  22 . The fuel flows through the passages  24  formed by the heat-generating element  26  where it absorbs thermal energy. Hydrogen is generated from the fuel through the reforming reaction of the fuel as it passes the catalyst layer  28 .  
      Subsequently, hydrogen generated from the fuel is discharged through the reformer outlet  23  of the main body  21  and the hydrogen is supplied to the first inlet  13   a  of the stack  10  through the second supply line  82 . At the same time, the air pump  41  supplies air to the second inlet  13   b  of the stack  10  through the third supply line  83 .  
      In the stack  10 , the hydrogen is supplied to the anode electrode of the membrane-electrode assembly  12  through the separators  16 . Oxygen in the air is supplied to the cathode electrode of the membrane-electrode assembly  12  through the separators  16 .  
      The anode electrode divides hydrogen into protons (hydrogen ions) and electrons by the oxidation reaction. The protons move to the cathode electrode through the electrolyte membrane and the electrons move to the cathode electrode of the neighboring membrane-electrode assembly  12  through the separators  16  or another terminal portion (not shown), but not through the electrolyte membrane. Current is generated by the flow of electrodes and heat and water are generated incidentally.  
      Although the exemplary embodiments of the present invention have been described, the present invention is not limited to the embodiments, but may be modified in various forms without departing from the scope of the appended claims, the detailed description, and the accompanying drawings of the present invention. Therefore, it is natural that such modifications belong to the scope of the present invention.