Patent Publication Number: US-2007114005-A1

Title: Heat exchanger assembly for fuel cell and method of cooling outlet stream of fuel cell using the same

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
      This application claims the benefit of German Patent Application No. 102005056181.0, filed on 18 Nov. 2005 in the German Patent Office, and Korean Patent Application No. 10-2006-0108021, filed on 2 Nov. 2006, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a heat exchanger assembly and a method of cooling the outlet stream of a fuel cell using the heat exchanger assembly. In particular, the present invention relates to heat exchangers which are used as water condensers in fuel cell systems, especially in Direct Methanol Fuel Cell (DMFC) Systems, used for supplying power for mobile electronic devices.  
      2. Description of the Related Art  
      A fuel cell is an electrochemical device producing electricity from an external fuel supply of hydrogen and oxygen. Typical reactants used in a fuel cell are hydrogen on the anode side and oxygen on the cathode side. Fuel cells are often considered to be very attractive in modern applications for their high efficiency and ideally emission-free use. In principle, the only by-product of a hydrogen fuel cell is water vapor. There are several different types of fuel cells, each using a different chemistry. Fuel cells are usually classified by the type of electrolyte they use. Some types of fuel cells work well for use in stationary power generation plants. Others may be useful for small portable applications or for powering cars.  
      In a hydrogen/oxygen proton-exchange membrane (or “polymer electrolyte”) fuel cell (PEMFC), a proton-conducting polymer membrane separates anode and cathode sides. Each side has an electrode, typically carbon paper coated with a platinum catalyst. On the anode side, hydrogen diffuses to the anode catalyst where it dissociates into protons and electrons. The protons are conducted through the membrane to the cathode, but the electrons are forced to travel in an external circuit, supplying power, because the membrane is electronically insulating. On the cathode catalyst, oxygen molecules react with the electrons which have travelled through the external circuit and with the protons to form water. In this example, the only waste product is water vapor and/or liquid water.  
      Other fuels are natural gas, propane and methanol. Methanol is a liquid fuel easy to transport and distribute, so methanol may be a likely candidate to power portable devices. A Direct Methanol Fuel Cell (DMFC) relies upon the oxidation of methanol on a catalyst layer to form carbon dioxide. Water is consumed at the anode and is produced at the cathode. Protons (H + ) are transported across the proton exchange membrane to the cathode where they react with oxygen to produce water. Electrons are transported via an external circuit from anode to cathode providing power to external devices. DMFCs have the advantage that they do not require the use of a reformer to extract hydrogen from the fuel. This allows DMFCs to have a compact design so they can be used in, e.g., mobile telecommunication devices.  
      In detail, the DMFC is composed of an anode, a cathode and an electrolyte film sandwiched between the anode and the cathode. A methanol aqueous solution is employed as the fuel. A fuel supply is connected with the fuel cell to supply fuel to the anodes. An air supply supplies air to the cathodes. A heat exchanger is connected to a cathode exhaust for cooling an exhaust stream, condensing water from an exhaust gas, and discharging the water to be mixed with the fuel. The condensed water is re-circulated to the fuel supply unit and re-used. The fuel does not need to be diluted with water in advance, leading to a further reduction of the size of the fuel cell.  
      DMFC systems are disclosed in US 20040166389 and US 20040062964. The latter addresses the issue of condensing water in a heat exchanger of a DMFC system in order to separate it from the exhaust stream of the fuel cell and to re-circulate the water and mix it with the fuel.  
      However, such downsized fuel cells need efficient heat exchangers in order to prevent damage. Due to corrosion reasons stainless steel is most frequently used as material for fuel cell heat exchangers. According to the state-of-the-art, different types of heat exchangers are used for this purpose. In plate-type heat exchangers, a cathode outlet stream and a cooling air stream are fed over opposite faces of a stainless steel plate, exchanging heat through the plate. A certain number of plates are stacked on top of each other in order to increase the exchanging surface. This type of heat exchanger possesses the problem of difficult integration with a cooling fan, since the cooling air stream has an uneven geometrical distribution. Additional space for flow shaping is needed, leading to a bulky device.  
      Another type of heat exchangers used are of a tubular type. Here one tube is bent in a serpentine like manner. There are restrictions to the length of these tubes—and accordingly to their surface area—because in order for the stream to be cooled, a certain drop in pressure must not be surpassed. In order to increase the heat exchange rate with the cooling air, metal lamellae are inserted between the tubes to increase the exchange surface. Nevertheless, due to the poor heat conductivity of the heat exchanger material, mainly stainless steel, the performance of this type of device is poor.  
      Other tube-type heat exchangers use multiple parallel tubes, which are connected through registers attached to the ends of the tubes, directing the flow from one tube to the adjacent one. This type of heat exchanger is disadvantageous because the registers consume considerable space which cannot be used for heat exchangers. In addition, the assembly of the tubes with the registers is costly.  
     SUMMARY OF THE INVENTION  
      The present invention provides a heat exchanger assembly and a method of cooling an outlet stream of a fuel cell using a heat exchanger assembly increasing the efficiency of the heat exchange in fuel cell systems. The present invention also provides a heat exchanger for a fuel cell system and a method of cooling an outlet stream of a fuel cell, especially for a Direct Methanol Fuel Cell (DMFC) system, which provide a minimized volume for a given heat exchange capacity and a low pressure drop for both the cathode stream and the cooling air.  
      According to an aspect of the present invention, there is provided a heat exchanging assembly for fuel cell systems comprising a heat exchanger and a ventilation unit for generating a stream of cooling air through the heat exchanger, the ventilation unit comprising a circular ventilation means and a housing, the heat exchanger having a width extending in the y-direction, a depth extending in the x-direction, a height extending in the z-direction, a front and a rear, bounded by a plane, extending in the y-z plane, and two sides, bounded by planes, and extending in the x-z plane, wherein the heat exchanger comprises: an inlet manifold with an inlet opening, extending in the x-direction, an outlet manifold with an outlet opening, extending in the x-direction, being spaced apart from the inlet manifold, and a plurality of hollow heat exchanging elements, to allow a flow of a medium contained therein from the inlet manifold to the outlet manifold, the heat exchanging elements extending from the inlet to the outlet in the y-z plane in a serpentine manner, being arranged parallel to each other in the x-direction and spaced-apart to provide empty space between the heat exchanging elements, and comprising first sections extending in the z-direction, and second sections connecting successive first sections, wherein the ventilation unit is arranged parallel to the sides of the heat exchanger extending in the x-z plane, the cooling air flows through the free-space between the heat exchanging elements in an opposite direction to the flow of the medium within the heat exchanging elements, and the diameter of the ventilation means has a value corresponding to at least 66% of the smaller value of either the depth or the height of the heat exchanger.  
      The diameter of the ventilation means may have a value corresponding to at least 80%, preferable of 90%, even more preferable of 95% of the smaller value of either the depth or the height of the heat exchanger.  
      The cross section of each of the first sections may have a main axis which is parallel to the flow of cooling air, wherein the width of the cross section along the main axis is greater than the width of the cross section along a second axis perpendicular to the flow of cooling air.  
      The cross section of the first sections may have an oval shape, the main axis of the oval being arranged parallel to the flow of cooling air.  
      The outlet manifold may be realized as a water separator for separating condensed water from the medium flowing through the heat exchanger, the outlet manifold being in direct contact to the outlet ends of the heat exchanging elements.  
      The ventilation unit may be arranged on a downstream side of the heat exchanger and comprises a fan or blower for blowing air from the downstream side to an upstream side of the exchanger, wherein the downstream side is defined as the direction in which the medium flows within the heat exchanging elements.  
      The ventilation unit may be arranged on an upstream side of the heat exchanger and comprises a fan or blower for sucking air from a downstream side to an upstream side of the exchanger, wherein the upstream side is defined as the direction in which the medium flows within the heat exchanging elements.  
      The heat exchanging elements may have a tubular structure.  
      The second sections of the heat exchanging elements may have a u-shaped shaped tubular structure.  
      The u-shaped tubular structure may consist of straight sections arranged at right angles to each other.  
      The second sections of the heat exchanging elements may be provided with a straight tubular structure.  
      The connections between the first sections and the second sections of the heat exchanging elements may be at right angles or substantially right angles.  
      The middle axis of the tube sections and intersect each other at right angles.  
      The cross section of the second sections of the heat exchanging elements may have an oval shape.  
      The ventilation means may comprise a fan or a blower.  
      The fuel cell system may be a direct methanol fuel cell (DMFC) system.  
      The inlet of the heat exchanger may be connected to a cathode outlet of a fuel stack of a fuel cell system.  
      According to another aspect of the present invention, there is provided a method of cooling the outlet stream of a fuel cell using a heat exchanger assembly, the method comprising: guiding the outlet stream of a fuel cell into the inlet of a heat exchanger; guiding the stream from the inlet ends of the heat exchanging elements of the heat exchanger to the outlet ends of the heat exchanger elements, which leads to a cooling effect of the stream; and increasing the cooling effect of the guided stream by providing a flow of cooling air around the heat exchanging elements from the downstream side to the upstream side of the heat exchanger.  
      The method may further comprise: condensing water within the heat exchanger.  
      The method may further comprise: separating the condensed water and the air stream in a water separator being in direct contact with the outlet ends of the heat exchanging elements of the heat exchanger.  
      The method may further comprise: increasing the cooling effect within the heat exchanger by providing the first sections with a cross section whose width in a main axis direction parallel to the flow of cooling air is a greater length than its width in a second axis direction perpendicular to the flow of cooling air. 
    
    
     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 schematic diagram of a fuel cell supply system employing a heat exchanging assembly according to an embodiment of the present invention;  
       FIG. 2  is a schematic diagram of the heat exchanging assembly according to an embodiment of the present invention;  
       FIG. 3  is a schematic side view illustrating a heat exchanger illustrated in  FIG. 2 ;  
       FIG. 4  is a schematic diagram of a heat exchanging assembly according to another embodiment of the present invention; and  
       FIG. 5  is a schematic diagram of a heat exchanging assembly according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention will now be described more fully with reference to the accompanying drawings in which exemplary embodiments of the invention are shown.  
       FIG. 1  is a schematic diagram of a fuel cell supply system employing a heat exchanging assembly according to an embodiment of the present invention. Referring to  FIG. 1 , the fuel cell supply system is realized as a Direct Methanol Fuel Cell (DMFC) system. A fuel cell stack  10  has an air inlet  11  and an air outlet  13 . An air pump or fan  12  supplies reaction air to a stack cathode through the air inlet  11 . An anode cycle for diluted fuel consisting of a CO 2  separator  20  mounted downstream from a stack fuel outlet  16  removes CO 2  from a reaction stream and vents it to the outside through a venting opening  21 . In a mixer  22  the fuel stream is mixed with pure fuel from a fuel tank  30 . A fuel pump  23  feeds the diluted fuel back to the fuel inlet  15  of the stack.  
      A heat exchanger  50  is mounted in the outlet stream of a fuel cell cathode. A ventilation unit  55 , e.g. a fan, is used to cool the heat exchanger, leading to cooling of the outlet stream and condensation of water. This two phase flow exits at the outlet  52  of the heat exchanger  50 . The ventilation unit  55  and the heat exchanger  50  form the heat exchanging assembly according to the current embodiment of the present invention. Downstream of the heat exchanger  50 , a water separator  60  is mounted in order to separate liquid water from the air stream. The separated water is fed back to the anode cycle of the fuel cell system by a condensate pump  70 , and the residual air is vented through an outlet  61  to the outside.  
       FIG. 2  is a schematic diagram of the heat exchanging assembly according to an embodiment of the present invention.  FIG. 2  provides a coordinate system for illustrative purposes of the current embodiment. The surface of the page corresponds to a y-z plane, the x-direction points away from a viewer into the page.  
      The heat exchanging assembly for fuel cell systems of the present invention comprises a heat exchanger  50  and a ventilation unit  55 . The heat exchanger  50  comprises an inlet manifold  102  with an inlet opening  101 , an outlet manifold  104  with an outlet opening  103 , and a plurality of heat exchanging elements  105  disposed between the inlet opening  101  of the inlet manifold  102  and the outlet opening  103  of the outlet manifold  104  and connecting the inlet manifold  102  and the outlet manifold  104 . To maximize the efficiency of heat exchange in the heat exchanger  50 , cooling air generated by the ventilation unit  55  flows in the opposite direction to the flow of a medium within the heat exchanging elements  105 . The present invention will now be described in detail.  
      The ventilation unit  55  is used to generate a stream of cooling air through the heat exchanger  50  and comprises a circular ventilation means  56  and a housing  57 . The housing  57  can be rectangular or square. The ventilation means  56  is arranged in the housing  57  of the ventilation unit  55  and can be a fan or a blower.  
      The heat exchanger  50  is a three-dimensional structure and has a width  113  extending in the y-direction, a depth  114  extending in the x-direction, and a height  115  extending in the z-direction as illustrated in  FIG. 2 . The heat exchanger  50  has a front and a rear, which are both bounded by a plane and extend in the y-z plane. The two side planes of the heat exchanger  50  extend in the x-y plane. The heat exchanger  50  comprises the inlet manifold  102  with the inlet opening  101 , which extends in the x-direction, and the outlet manifold  104  with the outlet opening  103 , which extends in the x-direction, wherein the outlet manifold  104  is spaced apart from the inlet manifold  102 .  
      The heat exchanger  50  further comprises the plurality of heat exchanging elements  105 . The heat exchanging elements have a hollow structure. The hollow structure allows the medium contained in the heat exchanging elements  105  to flow from the inlet manifold  102  to the outlet manifold  104 . The heat exchanging elements  105  extend from the inlet  101  to the outlet  103  in the y-z plane in a serpentine manner. The elements  105  are arranged parallel to each other, stacked in the x-direction, and spaced-apart to generate a free space between the heat exchanging elements  105 . The heat exchanging elements  105  may be self-supportive. Thus, support plates or other support structures are not necessary. This ensures a formation in which empty space between adjacent heat exchanging elements  105  is maximized. The empty space is necessary to allow a sufficient flow of cooling air generated by the ventilation unit  55  through the heat exchanger  50 . The heat exchanging elements  105  comprise first sections  106 , extending in the z-direction, and second sections  107 , connecting successive first sections  106 . The first sections  106  in  FIG. 2  are placed upright. Each of the heat exchanging elements  105  comprises a plurality of the first sections  106 , which are arranged to be spaced-apart from each other. Each pair of adjacent first sections  106  is connected by the second sections  107 , which essentially extend in the y-direction, thereby forming the serpentine structure of the heat exchanging elements  105 . This structure allows provides efficient heat exchange due to the large surface for heat exchange.  
      The heat exchanger  50  exchanges heat of a medium flowing through its heat exchanging elements  105 . In the present invention the exhaust gas of the fuel cell  10  enters the inlet opening  101  of the heat exchanger  50 . The medium is distributed into the heat exchanging elements  105  connected to the manifold  102  via the inlet manifold  102 . The medium flows from the inlet manifold  102  to the outlet manifold  104  and then to the outlet opening  103 , through the heat exchanging elements  105 . In  FIG. 2 , the net flow of the medium is in the y-direction, i.e. from right to left. Within each heat exchanging element  105  the medium flows upward in the z-direction through the first sections  106 , then to the left in a y-direction through the first second section  107 , then downward through the second first section  106 , then again to the left through the second sections  107 , then upward again through the third first section  106 , etc., thereby establishing a net flow of the medium in the y-direction.  
      In order to increase the heat exchange of the heat exchanger  50 , a ventilation unit  55  is arranged parallel to a side of the heat exchanger  50  extending in the x-z plane. The ventilation unit  55  generates a flow of cooling air through the heat exchanger  50 . The cooling air flows through the empty space between the heat exchanging elements  105  in the opposite direction to the flow of the medium within the heat exchanging elements  105 . Thus, the ventilation unit  55  is provided in a counterflow arrangement respective to the flow of the medium within the heat exchanger  50 . In the above structure, the medium within the heat exchanger  50  flows in the opposite direction to the flow of the cooling air generated by the ventilation unit  55 , thereby remarkably increasing the efficiency of heat exchange in the heat exchanger  50 .  
      In particular, in order to ensure an efficient cooling effect, the diameter of the ventilation means  56  has a value corresponding to at least 66% of the smaller value of either the depth  114  or the height  115  of the heat exchanger  50 . Preferably, the diameter of the ventilation means  56  has a value corresponding to at least 80%, more preferably of 90%, even more preferably of 95% of the smaller value of either the depth  114  or the height  115  of the heat exchanger  50 . The diameter preferably does not exceed 150% of the larger value of either the depth  114  or the height  115 , more preferably 120%, even more preferably 100%.  
      In the current embodiment as illustrated in  FIG. 2 , the ventilation unit  55  is realized as a fan or blower being located on the downstream side with respect to the flow of the medium within the heat exchanger  50 . The upstream side is defined as the side of the heat exchanger  50  connected to the inlet  101 . Correspondingly, the downstream side is the side of the heat exchanger  50  connected with the outlet  103 . The ventilation unit  55  forms a cooling air stream from the downstream side to the upstream side of the heat exchanger  50  as shown in  FIG. 2 . In order to save space, the ventilation unit  55  is arranged directly adjacent to the downstream side of the heat exchanger  50 .  
       FIG. 3  is a schematic side view illustrating the heat exchanger  50  illustrated in  FIG. 2 . Referring to  FIG. 3 , the height  115  of the heat exchanger  50  is defined as the distance between the top of a heat exchanger element  115  and the top of the outlet manifold  102 , i.e. the location where the bottom of the heat exchanger element  105  merges with the outlet manifold  102 . The depth  114  of the heat exchanger  50  is defined as the distance between the outer edge of the first heat exchanging element  105  and the outer edge of the last heat exchanging element  105  of the heat exchanger  50 .  
       FIG. 4  is a schematic diagram of a heat exchanging assembly according to another embodiment of the present invention. Referring to  FIG. 4 , a ventilation means  56  is realized as a radial fan or blower located on the upstream side of a heat exchanger  50 . The ventilation means  56  sucks in air to form a cooling air stream from the downstream side to the upstream side of the exhaust stream in the heat exchanging elements  105 , and preferably blows the air out in a perpendicular direction, i.e. in a z-direction.  
      The ventilation means  56  of the ventilation unit  55  can comprise a fan or blower, e.g. an axial fan or a radial blower or any other device which is able to produce a specially extended flow of air.  
      The stream of exhaust gas coming from a fuel stack  10  connected to the inlet  101  of the heat exchanger  50  flows in the positive y-direction, whereas the cooling air flows in the negative y-direction as illustrated in  FIG. 4 . An outlet opening  103  is connected to a water separator  60 . During the gas flow from the inlet  101  via a manifold  102  through the plurality of heat exchanging elements  105  to an outlet manifold  104  and an outlet  103 , the exhaust stream is cooled through the surface of the heat exchanging elements  105 . The counterflow cooling air stream from the fan or blower  55  enhances the cooling effect. The cooling air stream flows through the heat exchanger  50  and passes the at least one heat exchanging element  105  thereby providing a cooling effect through the surface of the at least one heat exchanging element  105 .  
      In order to reduce the flow resistance to the cooling air stream, the cross section of the tubes can have an oval shape  110  at least in the straight sections  106 , wherein the main axis of the oval shape  110  is parallel to the flow direction of the cooling air.  
      Both for manufacturing and aerodynamic reasons it is advantageous when the tubes in a U-turn section  107  also have an oval shape, wherein the main axis of an oval cross section  111  is perpendicular to both the cooling air flow and the straight sections  106  of the tubes.  
       FIG. 5  is a schematic diagram of a heat exchanging assembly according to another embodiment of the present invention. Referring to  FIG. 5 , in order to have shorter U-turn sections  107  the tubes are manufactured with essentially perpendicular angles, i.e. the angles between the respective middle axes of the tube sections are such that the respective tube sections are essentially rectangular. The u-shape is formed by two extended parallel straight sections  106  with an interposed short section  107  being essentially straight and arranged perpendicular between the two sections  106  so as to combine the sections  106 . However, in another embodiment the first and second sections  106  and  107  are both provided as straight tubular sections arranged perpendicular to each other. In one embodiment at least the straight sections  106  may have an oval cross section with the main axis of the oval  110  being parallel to the cooling air flow.  
      In order to save space the outlet ends of the tubes  112  are connected directly to a water separator  60 . The top of the water separator  60  is structured to allow all outlet ends  112  of the heat exchanging elements  112  to merge into the water separator  60 . The water separator  60  has a converging structure from its top to its bottom. The bottom of the water separator  60  includes a spout connected to a water feedback connection  62 , see  FIG. 1 .  
      The material of the tubes in the present invention may be stainless steel in the first instance, but also titanium or plastic. The tubular structure of the present invention may be self-supportive to maximise free-space between the parallel heat exchanging elements  105 .  
      As described above, according to a heat exchanger assembly and a method of cooling the outlet stream of a fuel cell using the heat exchanger assembly, which provide a minimized volume for a given heat exchange capacity, a low pressure drop for the cooling air due to a linear exterior profile of heat exchange elements and a low pressure drop for the cathode stream due to a plurality of the heat exchange elements which are parallel to each other and connected through a manifold, and provide excellent reciprocal communication with a cooling fan.  
      Although the invention has been described with reference to certain embodiments of the invention, the invention is not limited to these embodiments. In particular, the invention is not limited to DMFC fuel cell systems. It is also clear to one skilled in the art that the heat exchanging assembly can be rotated in space. Through modifications and variations of the embodiments, additional embodiments can be realized without departing from the scope of the invention.