Abstract:
A heat exchanger for a solid-oxide fuel cell assembly. A plurality of parallel tubes conveys fuel cell stack exhaust gas from a first manifold means to a second manifold means. The tubes are highly corrugated to increase the wall area and decrease the wall thickness. The tubes are disposed in a jacket through which is passed incoming air to be heated. The tubes may be linear between two manifolds, or they may be curved such that the first and second manifold functions are accommodated within a single component.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of application Ser. No. 10/375,834, which was filed on Feb. 25, 2003. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to solid-oxide fuel cells (SOFCs); more particularly, to heat exchangers for heating incoming combustion air in an SOFC assembly; and most particularly, to an improved heat exchanger for increasing heat exchange efficiency and reducing heat exchanger manufacturing cost and complexity.  
       BACKGROUND OF THE INVENTION  
       [0003]     Fuel cells combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid oxide electrolyte layer through which oxygen anions migrate; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs).  
         [0004]     In some applications, for example, as an auxiliary power unit (APU) for an automotive vehicle, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a catalytic gasoline oxidizing reformer. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the liquid hydrocarbon, resulting ultimately in water and carbon dioxide. Both reactions are exothermic, and both are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 100° C.  
         [0005]     Air enters an SOFC fuel cell at ambient temperature and desirably is preheated before being sent to the fuel cell stacks. A convenient and economical way to heat the air is by abstracting heat via a heat exchanger from the fuel cell exhaust which exits the fuel cell combustor at about 950° C. In the prior art, a typical heat exchanger employed for this purpose is of a well known plate-and-frame design wherein a plurality of heat-exchange modules is assembled as a stack. A plurality of alternating hot and cold gas flow spaces are separated by heat transfer plates. A typical prior art heat exchanger for use in an SOFC may comprise more than 100 individual plates and frames and can require more than 200 feet of brazing to seal the edges of all the modules, and is thus complicated and expensive to fabricate.  
         [0006]     What is needed is an efficient heat exchanger for an SOFC wherein the number of components and fabrication costs are significantly reduced.  
         [0007]     It is a principal object of the present invention to reduce the cost and complexity of an SOFC heat exchanger.  
       SUMMARY OF THE INVENTION  
       [0008]     Briefly described, a heat exchanger for a solid-oxide fuel cell assembly includes a plurality of parallel tubes for conveying a first gas, preferably a hot gas, from a first manifold means to a second manifold means. The only brazing required is to attach each tube to each manifold. Preferably, the tubes are highly corrugated in bellows-like form to increase the wall area and decrease the wall thickness. The tubes are disposed in a jacket through which is passed a second gas, preferably a cool gas. The tubes may be linear between two manifolds, or they may be curved such that the first and second manifold functions are accommodated within a single component.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     These and other features and advantages of the invention will be more fully understood and appreciated from the following description of certain exemplary embodiments of the invention taken together with the accompanying drawings, in which:  
         [0010]      FIG. 1  is an isometric view from the front, partially exploded, of a prior art plate-and-frame heat exchanger;  
         [0011]      FIG. 2  is an exploded isometric view from the front of a first embodiment of a heat exchanger in accordance with the invention;  
         [0012]      FIG. 3  is an exploded isometric view from the front of a second embodiment of a heat exchanger in accordance with the invention; and  
         [0013]      FIG. 4  is an exploded isometric view from the front of a third embodiment of a heat exchanger in accordance with the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     Referring to  FIG. 1 , a prior art heat exchanger  10  comprises a plurality of alternating hot and cold fluid flow spaces formed by alternating rectangular plates  12  and frames  14 . In an SOFC assembly, the hot fluid is hot exhaust gas from the fuel cell stack and the cold fluid is combustion air entering the assembly. Each plate and frame has perforated extensions  16  at all four edges such that, when assembled into a solid stack  18 , the perforations define inlet and exhaust manifolds  20 , 22  for a first fluid  23  flowing across the plates in a first direction  24 , and inlet and exhaust manifolds  26 , 28  for a second fluid  29  flowing across opposite sides of the plates in a second and orthogonal direction  30 . The extensions of first frames  14   a  are open on their inner edges to permit access of first fluid  23  from manifold  20  to first sides of plates  12 , and the extensions of second frames  14   b  are open on their inner edges to permit access of second fluid  29  to the opposite sides of plates  12 . It will be seen that the sequence of plates and frames  12 - 14   a - 12 - 14   b  represents a modular repeat, and that the full stack is simply a plurality of such modular repeats, the number of modules being as desired for a particular heat exchange requirement. In prior art SOFC heat exchanger  10 , the number of modules is typically about 25, requiring 100 or more components. After the entire stack of plates and frames is assembled, the edges of all plates and frames are sealed as by brazing to prevent fluid leakage from the heat exchanger.  
         [0015]     Referring to  FIG. 2 , a first embodiment  110  of an SOFC heat exchanger in accordance with the invention includes a plurality of parallel metal tubes  112  having ends  113  (one set of ends not visible). Lower end plate  114  is provided with a plurality of openings  116  arranged in a pattern, each opening being surrounded by a lip  118  for receiving a first end (not visible) of a tube  112 . Upper end plate  120  is similarly provided with openings  122  and lips (not visible). The exemplary pattern of openings and tubes is four staggered rows of five tubes each. Obviously, other patterns are possible within the scope of the invention. The tubes are attached to the end plate lips as by brazing.  
         [0016]     Metal tubes  112  preferably are axially corrugated as by hydro-forming into bellows form such that the surface area of each tube is substantially greater than the surface area of a non-corrugated tube having equal length and diameter. Preferably, the surface area is at least doubled. In addition, the bellows-forming process, which is well known in the art, causes thinning of the tube wall. As a result, the thermal conductance of heat exchanger  110  can be as much as 200% greater than that of prior art heat exchanger  10  of comparable size.  
         [0017]     Preferably, tubes  112  are formed of a nickel-based high temperature alloy, for example, Inconel 625.  
         [0018]     A base plate  124  has a planar upper surface  126  for mating against a planar lower surface  128  of lower end plate  114 . Surface  126  is relieved in three areas. One is a central well  130  defining an intermediate manifold for mating with the central two rows of ten openings  116 ; the other two are lateral wells  132   a , 132   b , each of which defines an intake and exhaust manifold, respectively, which mates with a respective lateral row of five openings  116 . Well  132   a  is provided with slots  134  extending through plate  124  for mating with a supply such as an intake manifold (not shown) of a first fluid  23 , preferably the hot exhaust gas from the fuel cell stack. Well  132   b  is provided with similar slots  136  for mating with a return pathway through an exhaust manifold (not shown) for first fluid  23 .  
         [0019]     A cover plate  138  has a planar lower surface  140  for mating against a planar upper surface  142  of upper end plate  120 . Surface  142  is relieved in two wells  144   a , 144   b , each of which defines a first and second crossover manifold, respectively. Each well contains two respective lateral rows of five openings  122 . Wells  144   a , 144   b  are separated by a median  146 .  
         [0020]     The result of this arrangement is an “M” shaped path for gas through five parallel tube assemblies. A first gas (fuel cell exhaust gas) at a first starting temperature enters through slots  134 , passes through openings  116  into the first staggered row of five tubes  112 , passes upwards through openings  122  into crossover manifold  144   a , passes downwards through openings  122  into the second staggered row of five tubes  112 , passes through openings  116  into central well  130 , passes upward through openings  116  into the third staggered row of five tubes  112 , passes upward through openings  112  into second crossover manifold  144   b , passes downward through openings  112  into the fourth staggered row of five tubes  112 , passes downward through openings  116  into lateral well or manifold  132   b , and passes out of heat exchanger  110  via slots  136 .  
         [0021]     Referring still to  FIG. 2 , tubes  112  and upper plate  120  are surrounded by a jacket  150  defining a jacketed space  152  between jacket  150  and the walls of tubes  112 . Jacket  150  is sealed to cover plate  138  and to lower end plate  114 . Lower end plate  114  is attached to base plate  124  as by bolts (not shown) through bores  154 . Lower end plate  114  and base plate  124  are provided with slots  158 , 160 . A second gas at a second starting temperature (air to be heated) enters through inlet slots  158 , passes into jacketed space  152 , passes around corrugated tubes  112  abstracting heat therefrom, and exits through exhaust slots  160 .  
         [0022]     Referring to  FIG. 3 , a second embodiment  210  is identical in gas flow path to embodiment  110  but is substantially simplified in construction. The “M” shaped flow path is clearly visible in five staggered M-tubes  212  having ends  213 . Each M-tube  212  preferably is corrugated along its four linear portions as shown. Upper plate  120  and cover  138  are eliminated, their functions being cast into a closed jacket  250  conformable with M-tubes  212 . Further, lower end plate  214  is simplified to have only ten openings  216  rather than twenty openings  116  as in embodiment  110 . The total brazing required between tubes and plates is reduced from forty joints to ten. When tubes  212  are one-half inch in diameter, the total length of brazing required is about 15 inches, as compared to 200 inches required for prior art exchanger  10  or 60 inches for first embodiment  110 . Preferably, bottom plate  214  is provided with a plurality of attached fins  280  disposed adjacent M-tubes  212  for improving air flow around the tubes. Base plate  224  is simplified to eliminate central well  130  from embodiment  110 .  
         [0023]     The “M” flow path indicated in first and second embodiments  110 , 210  can give rise to undesirably high back pressures because of the relatively long flow path. Referring to  FIG. 4 , a third embodiment  310  reduces the flow path by half, albeit at a cost of sixty joints as in first embodiment  110 . The basic flow arrangement provided by the twenty corrugated tubes  312  (shown for clarity via a cutout in jacket  350 ), having tubular inserts  313 , is ten parallel U-shaped flow paths instead of five M-shaped flow paths. Lower end plate  314  is substantially identical with plate  114  in  FIG. 2 . Base plate  324  is configured as essentially a frame having two openings  332   a , 332   b  which become inlet and exit chambers when plate  324  is disposed between a mounting manifold (not shown) and plate  314 . Upper end plate  320  is welded to tubes  312  as in  FIG. 2 , and a single upper manifold space  380  is provided by a cutout in a new spacer frame element  382  into which tubes  312  debouch. Cover plate  338  is similar to cover plate  138 . The flow path then is simply from inlet opening  332   a  upwards through the forward ten tubes  312  into manifold space  380 , then downwards through the rear ten tubes  312  into exhaust opening  332   b . Jacket  350  may be substantially identical with jacket  150 .  
         [0024]     A potential drawback of flowing a gas through corrugated tubing is stagnation of gas within the recesses of the corrugations. Referring still to  FIG. 4 , each tube  312  preferably is provided with an internal spiral turbulator  355  which is installed into the tube prior to brazing. (For purposes of clarity, each turbulator is shown partially removed from the respective tubes). Turbulator  355  is formed from sheet metal, preferably a high-temperature alloy, and twisted through an axial angle such as 180° about its axis. The turbulator induces a swirling flow of gas through the tube, promoting flushing of gas from the corrugation recesses.  
         [0025]     While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.