Abstract:
A method and apparatus to increase fuel cell reliability and maintainability is disclosed. The apparatus includes a recuperating loop consisting of a spiral tube that surrounds a stack or a combination of several stacks. If the fuel cell stacks are externally manifolded, the recuperating loop may also surround the external manifold. The spent hot gases from the stack directly flows over the recuperating loop to transfer heat to a coolant flowing through the loop providing heat exchange by convection and radiation. The spent hot gases may be manifolded and may not flow over the recuperating loop. In this case, the heat exchange is by radiation between the hot fuel cell stack(s) and the recuperating loop.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a fuel cell system which uses the waste heat from hot exhaust coming out of fuel cell stacks to increase the overall fuel cell efficiency. More particularly, the present invention focuses on a method and apparatus to simplify the heat exchange to recuperate the high temperature gas at the outlet of a fuel cell without the complexity of an external recuperator or heat exchanger. The current invention may also eliminate a recuperator from a fuel cell system while maintaining the heat recuperation efficiency to increase the system reliability and maintainability. In effect, the current invention replaces complicated and costly heat exchanger or multiple heat exchangers and associated complex piping and ducting. 
     Fuel cells electrochemically react fuels with oxidants to generate electricity. The key components in a fuel cell include a cathode material, an electrolyte material, and an anode material. The electrolyte is a non-porous material sandwiched between the cathode and anode materials. The fuel and oxidant fluids are typically gases and are continuously passed through separate passageways. A fuel gas can be hydrogen, a short chain hydrocarbon, or a gas containing desired chemical species in some form. An oxidant may be an oxygen containing gas, or quite commonly air. Reactant gases, fuels and oxidants, are typically pre-heated before being fed to electrolyte. Electrochemical conversion occurs at or near the three-phase boundaries of each electrode (cathode and anode) and the electrolyte. The fuel is electrochemically reacted with the oxidant to produce a DC electrical output. The anode or fuel electrode enhances the rate at which electrochemical reactions occur on the fuel side. The cathode or oxidant electrode functions similarly on the oxidant side. 
     One of the common constructions of fuel cells is a solid oxide fuel cell (SOFC) that uses solid electrolytes for power generation. Solid electrolytes are either ion conducting ceramic or polymer membranes. In the former instance, the electrolyte is typically made of a ceramic, such as dense yttria-stabilized zirconia (YSZ) ceramic, that is a nonconductor of electrons, which ensures that the electrons must pass through the external circuit to do useful work. With such an electrolyte, the anode is oftentimes made of nickel/YSZ cermet and the cathode is oftentimes made of doped lanthanum manganite. 
     SOFCs of various construction geometries have been designed. These include the tubular, segmented cells in series, and planar geometries. These various constructions are described in “Ceramic Fuel Cells” by N. Q. Minh, Journal of the American Ceramic Society, 76, p. 563, 1993. 
     Sometimes, a planar construction resembles a cross-flow heat exchanger in a cubic configuration. The planar cross flow fuel cell is built from alternating flat single cell membranes (which are trilayer anode/electrolyte/cathode structures) and bipolar plates (which conduct current from cell to cell and provide channels for gas flow into a cubic structure or stack). The bipolar plates are oftentimes made of suitable metallic materials. The cross-flow stack is manifolded externally on four faces for fuel and oxidant gas management. 
     A radial or co-flow design is another popular design in fuel cell construction. An annular or circular shaped anode and cathode sandwich an electrolyte therebetween. Annular or circular shaped separator plates sandwich the combination of anode, cathode, and electrolyte. A fuel manifold and an oxidant manifold respectively direct fuel and oxidant to a central portion of the stack so that the fuel and oxidant can flow radially outward from the manifolds. 
     Regardless of the particular fuel cell configuration, the electrochemical reaction between the fuel and oxidant produces electric energy, spent fuel and oxidant exhaust. Quite often, the exhaust gas from a fuel cell is the original reactant gas which has been depleted of the particular migrating species in ionic form as a result of the electrochemical reaction. This conversion of fuel and oxidant to electricity in a fuel cell also produces heat, particularly at high current/power densities, which is removed to maintain the fuel cell at an efficient operating temperature. 
     Conventional thermal management in a fuel cell forces a cooling medium, either a liquid or gaseous coolant stream, through the fuel cell assembly. Coolants, such as water or air, are used for fuel cell heat exchange depending on the operating temperature of the fuel cell. The coolant enters a fuel cell at a temperature near the operating temperature of the fuel cell. When the coolant passes through the fuel cell, the waste energy from the electrochemical reaction in the fuel cell is carried away by the heat capacity of the coolant. The volume flow of the coolant is closely related to the temperature rise of the cooling medium that is determined by the constraints associated with thermal stress of the components in the fuel cell such as ceramic cells in an SOFC. The heat exchange system typically incorporates mechanical components to facilitate the heat transfer to the cooling medium. Flow channels are routinely employed to keep coolant, fuel and oxidant in their separate passage. In many cases, the cooling medium is also a reactant, e.g., air in an SOFC. The effectiveness of heat exchange is, thus, dependent on the available heat transfer surfaces and efficiency of radiation and convection. 
     To achieve higher voltages for specific applications, the individual electrochemical cells are connected together in series to form a stack. To achieve higher currents, individual cells are connected in parallel. Electrical connection between cells is achieved by the use of an electrical interconnect between the cathode and anode of adjacent cells. The electrical interconnect oftentimes also provides for passageways which allow oxidant fluid to flow past the cathode and fuel fluid to flow past the anode, while keeping these fluids separated. Also typically included in the stack are ducts or manifolding to conduct the fuel and oxidant into and out of the stack. 
     In a traditional fuel cell system design, hot exhaust gas from the fuel cell is fed to the heat exchanger, and preheated reactant gas is received from the heat exchanger through insulated piping. For high temperature fuel cells such as SOFCs and molten carbonate fuel cells, costly alloy materials are often used for such piping. The heat exchanger and piping also require considerable installation and maintenance expense, particularly in a multi-stack fuel cell. An effective heat exchange system in a multi-stack fuel cell requires some piping surfaces at each individual stack to maintain an optimal operating temperature for each and overall fuel stack efficiency. One of the problems encountered by a fuel cell design in this aspect can be illustrated by the following example: 
     When a circulating coolant is used to circulate through the passageways in the fuel assembly, a pump may be required to circulate the coolant. Furthermore, connecting tubes, an expansion tank, radiator, thermal and/or other controls may be necessary to properly complete the heat exchange system. In addition to the added expense and the complexity of integrating the circulating coolant system with the multi-stack fuel assembly, other issues are the need of electricity to operate the pump, the pump is subject to mechanical failure, coolant may become contaminated or ionized resulting in electric short circuits or shunts in the fuel assembly, and the coolant may leak into the fuel cell reaction areas and/or freeze-up. This is a major challenge to a fuel cell thermal designer from the standpoint of weight, cost, structure integrity, maintenance and reliability. 
     Many have attempted to provide a simpler solution to the heat exchange requirement in a fuel cell assembly. Of particular interest are the following references: 
     U.S. Pat. No. 3,595,699 focuses on an active cooling method to maintain fuel cell temperature by electrically monitoring the current when it is increased. This allows the control system to react when fuel cell temperatures rise, and by how much before the temperature rises. However, the patent does not cover preheating or thermal integration, and is certainly not related to the overall system efficiency. 
     U.S. Pat. No. 5,338,622 claims a gas permeable membrane to transfer heat from the fuel cell exhaust to a cooling medium, which is not a reactant of the fuel cell, predominantly by radiation to a separate medium, or by a direct contact with the stack. The use of a gas permeable membrane and a non-reactant heat transfer medium adds complexity to the heat exchange system. A related patent, U.S. Pat. No. 5,462,817, further claims radiative heat transfer from a hot stack (not its exhaust) to a closed loop heat exchanger containing a separate cooling medium that is not a reactant for the fuel cell nor is a product. The cold and hot streams flow in two separate passages (FIG.  5 ). This decouples flows between the heat exchange fluid or medium and the reactants and products. The oxidant of the fuel is not preheated by the heat exchanger. Energy is needed to circulate the coolants and a separate heating mechanism must be provided to preheat the oxidant. 
     U.S. Pat. No. 5,340,664 provides for reactant preheat by means of a thermally integrated heat exchange system. The patent claims the use of heat from multiple fuel cell stacks. The heat is exchanged in a double thermal enclosure with incoming air and optionally provides the heat for endothermic reformer which is also enclosed in the thermal enclosure. The heating loops are embedded in the enclosure walls, and heat transfer is mainly by radiation. The heating passages are embedded in the enclosure wall and located between two insulation layers. Similar to U.S. Pat. No. 5,462,817, previously described, the patent requires one hot and one cold flow passage to complete the heat exchange process. The patent further specifies the use of a screw culvert heat exchanger (FIGS. 2 and 3) and plate passages (FIGS.  6  and  7 ). The claimed system may be thermally efficient, however, the mechanical structures needed for implementation add complexity, cost and weight to the fuel cells. 
     Other patents simplify the structural requirements for efficient exhaust heat re-uses by mixing fuel cell air and fuel exhaust with fresh colder air and fuel respectively, then recycle back into the cell (U.S. Pat. No. 6,136,462). No heat exchanger is needed for the claimed process. The system mixes heat as well as flow from two separate sources making the flow management quite complex. In yet another simplification attempt, a heat-pipe concept is used for heat exchange where heat transfer takes place through evaporation of an enclosed liquid (U.S. Pat. No. 6,146,779). The presence of a liquid coolant that facilitates heat transfer, again, adds a different complexity to the mechanical structure of a fuel cell. 
     A different approach to recycle exhaust heat is disclosed in U.S. Pat. No. 6,194,092 by coupling a hydrogen storage tank and a heat exchanger which transfers heat from the stack by convection and radiation in an enclosure. The hot air in the enclosure is forced into the storage tank by the aid of fans. Storage tank, fan and hydrogen are required for the complete system. 
     A Japanese Patent (JP 60035469), again, uses a separate fluid medium to transfer heat. This medium is not a reactant, as many previously discussed patents. The coolant loop is between some of the cells in a fuel cell stack. Also, the coolant is manifolded to individual cells. The design of such a system is complex and costly. 
     From the preceding, it becomes apparent that there is a need for a cost effective solution to recuperate waste heat from fuel cell stacks to thermally enhance the performance of fuel cell stacks while improving reliability and maintenance of the fuel cell without the added cost, structure and design complexity. A simple method and apparatus that eliminates the use of separate fluid for heat transfer and employs different heat transfer mechanisms as appropriate is needed to broaden the use of fuel cell stacks in various applications. A method to recuperate the exhaust heat from fuel cell stack without the use of a heat exchanger system or additional fluid is needed. An apparatus, which is flexible to work with single or multiple fuel cell stacks using the same design principle for various fuel cell geometry or arrangement while adaptive to the different structural requirement, is also needed for a fuel cell application. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus and method to exchange heat from the hot exhaust of fuel cell stacks and use it to preheat the incoming cooler air. The apparatus and method disclosed in the present invention overcomes many of the problems discussed in the prior art. 
     In one aspect of the present invention, a very simple heat exchanger is used for heat transfer between incoming air, which may be used as a fuel cell reactant, and exhaust heat through a combination of radiation and convection. The apparatus operates in a single passage spiral loop. The heated gases are freely blown over the heating loop. The spiral loop operates outside the fuel cell stack to provide an effective means of heat transfer with structural and design simplicity. Fins and dimples can be further added to the spiral loop to enhance the heat exchange mechanism. The simple design allows the spiral loop to be used in conjunction with a single fuel cell stack or multiple fuel cell stacks. 
     In another aspect of the present invention, the hot gases may be manifolded and directed out of the fuel cell stack(s) without flowing over the spiral loop. In this case, the heat exchange is by radiation between the hot fuel cell stack(s) and the cold oxidant flowing in the spiral loop. 
     In another aspect of the present invention, a method of increasing fuel cell reliability and maintainability is disclosed by integrating the recuperating loop into the fuel cell stack(s), simplifying heat exchange by reducing or eliminating the need for external recuperation, surrounding a stack or combination of stacks with a cylindrical spiral tube, directly flowing hot gas over the spiral tube, providing heating to the cathode flow in the tube, and feeding the pre-heated flow to the inlet of the fuel cell stack as a reactant of the fuel cell assembly. In this embodiment, the design complexity associated with a recuperator or heat exchanger at the outlet from the stack is avoided. The requirement for an external cooling medium, which is often a non-reactant fluid, pump, and re-circulation loop for heat exchange, and the maintenance and reliability associated with these components, have been eliminated. Thus, the reliability of the fuel cell assembly may be increased due to the structural and design simplicity. 
     In another aspect of the present invention, a spiral loop apparatus for the heat exchange at fuel cell stacks is comprised of a tube in rectangular or circular shape, a tube support such as an anchored or creep structure or simple tube supports at the inlet and/or outlet of fuel cell stack(s), and surface augmentation such as dimpling and/or fins if necessary. For high-temperature fuel cells, the tube may be made of any high temperature metal or thermally conductive composites being placed at an appropriate distance from the fuel cell stacks to best facilitate the heat transfer through radiation and convection. The loop structure is an integrated part of the fuel cell structure and the fluid flow in the tube is one of the reactants of the fuel cell electrochemical combustion ingredients. 
     In yet another aspect of the present invention, a single heat exchange loop is provided as an integral component of the fuel cell stacks. The loop is integrated with the structure of the fuel cell. The spiral loop is used to carry away the fuel cell waste heat which flows in the form of hot gases, with the coolant being a reactant of the fuel cell flowing directly to the inlet of the fuel cell. The loop is designed so that it can work with a fuel stack, either with or without manifolds. In the case of having the loop wrapping around a manifold, the loop is designed to be around the fuel cell stacks, not penetrating or embedded in the fuel cell stacks. The waste heat directly heats the tube wall which, in turn, heats the cooling fluid inside the tube. The heated fluid in the tube feeds into the fuel cell inlet as the preheated oxidant of the fuel cell stacks, eliminating the need of a separate heater structure to preheat the fuel cell reactants and the reliability thereof. Since the tube goes around the stack, not penetrating or embedding in the fuel cell, there is no sealing requirement or structural integration issues in the design aspect of the fuel cell. 
    
    
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description, and claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a diagrammatic perspective view of a fuel cell stack system incorporating a heating loop according to an embodiment of the present invention; 
     FIG. 1 b  is a diagrammatic perspective view of a fuel cell stack that may be employed in the system of FIG. 1 a;    
     FIG. 1 c  is a cross sectional view of FIG. 1 b  showing the flow of reactants in a fuel cell stack according to an embodiment of the present invention; 
     FIG. 2 is a diagram of an isometric view of a fuel cell stack with an external manifold and a heating loop according to an embodiment of the present invention; 
     FIG. 3 is a diagram of a side view of two radial flow fuel cell stacks connected together showing an internal manifold feeding two stacks and a heating loop according to an embodiment of the present invention; 
     FIG. 4 is a diagram of an isometric view of a fuel cell stack incorporating a recuperator loop with the fuel cell reactant inlet and outlet ends shown according to an embodiment of the present invention; 
     FIG. 5 is a diagram of an isometric view of a fuel cell stack incorporating a recuperator loop including external manifold with the fuel cell reactant inlet and outlet ends shown according to an embodiment of the present invention; and 
     FIG. 6 is a diagram of an isometric view of two fuel cell stack connected together through external manifolds incorporating a recuperator loop with the fuel cell reactant inlet and outlet ends shown according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Referring to FIG. 1 a , an internally manifolded fuel cell system  10  is shown in an embodiment including a ceramic/metal solid oxide fuel cell stack  12  and a fully integrated heating loop  14 . Although a circular configuration is depicted, other configurations are within the scope of the present invention. The present invention contemplates that the fuel cell stack  12  can be used in the context of at least solid oxide fuel cells and molten carbonate fuel cells well known in the art. The fuel cell stack  12  may comprise an electrolyte/electrode sandwich and an interconnect plate as known in the art. Fuel and oxidant can be fed through either a bottom end  16  or a top end  18  of the fuel cell stack  12 . An electrochemical reaction produces thermal energy in the form of exhaust heat and spent gases, releasing the heat and spent gases radially out at an outer surface  20 . The fuel cell system  10  is thermally integrated by including a heating loop  14  that conforms to or matches the configuration of the outer surface  20  of the fuel cell stack  12 . The heating loop  14  is designed as to be close enough to the outer surface  20  of the fuel cell stack  12  to effectively transfer the heat produced in the electrochemical processes within the fuel cell stack  12 . 
     In the process of releasing the heat within the fuel cell system  10 , the temperature of the fuel cell stack  12  may increase. The increased temperature can adversely affect the efficiency of a fuel cell operation. Thus, it is commonly known to those skilled in the arts to design a mechanism for heat removal that keeps the fuel cell within a desirable temperature range during operation. To facilitate the heat removal process according to one of the embodiments of the present invention, a heating loop  14  may be provided to absorb the heat released from the fuel cell stack  12 . Heating loop  14  provides a flow channel for cooling fluid that travels from a first end of the channel to a second end. In one of the embodiments of the present invention, the cooling fluid may be one of the reactants necessary for the fuel cell stack  12  to initiate the electrochemical reaction, i.e., the fuel or oxidant. The oxidant may be any medium that is oxygen rich to provide the burning or combustion. The choice of oxidants normally includes air. For illustration purposes, air is used in the following discussion as the cooling fluid or agent that flows through the heating loop  14 . But, it should be obvious to those skilled in the art that the oxidant and the coolant can include other choices normally used in a typical fuel cell system  10 . It should also be obvious that the coolant agent and the oxidant may be different even though the present invention describes the cooling agent and oxidant to be the same for thermal integration. 
     Referring to FIGS. 1 b  and  1   c , a fuel cell stack  12  that may incorporate the present invention includes a plurality of fuel cells  13  and interconnects  15 ,  24  well known in the art. Accordingly, the fuel cell  13  and interconnects  15 ,  24  may be in the shapes of thin disks stacked in an axial direction of an embedded manifold  11 . The axial direction of the fuel cell stack  12  refers to the direction of reactant flow. The fuel cell  13  may be sandwiched between a first interconnect  15  and a second interconnect  24  in the axial direction of the fuel cell stack  10 . The embedded manifold  11  within the fuel cell stack  12  can be located close to the center of the stack  12 , connecting a plurality of alternating fuel cells and interconnects. In this embodiment of the present invention, the embedded manifold  11  may provide the mechanical support to balance the fuel cell  13  and the interconnects  15 ,  24  along the centers. In addition, the embedded manifold  11  may provide flow communication to the fuel cell  13  and interconnects  15 ,  24 . The fuel cell  13  may have two surfaces—an anode surface  22  and a cathode surface  23 . The two surfaces may be separated by the thickness of the cell, one surface on the opposite side of the other. 
     Still referring to FIGS. 1 b  and  1   c , two flow channels may be formed inside the embedded manifold  11 . A first flow channel  19  may provide the passage for the first reactant flow  17  necessary for the electrochemical reaction to occur in the fuel cells. The first reactant flow in the present embodiment is the fuel flow  17  for the purposes of the following discussion. The fuel flow  17  can enter the fuel cell stack  12  from the top surface  18  through a first end  35  of the embedded manifold  11  (FIG. 1 b ). The fuel flow  17  travels along the axial direction of fuel cell stack  12  inside the first flow channel  19  and towards the anode surface  22  of the fuel cell  13 . A second reactant, air, can enter the fuel cell stack  12  from the bottom surface  16  through a second end  27  of the embedded manifold  11 . The airflow  21  travels in a second flow channel  28  inside the embedded manifold  11  along the axial direction of fuel cell stack  12  but in the opposite direction to the fuel flow  17  and towards the cathode surface  23  of fuel cell  13 . 
     The airflow  21  communicates with the fuel cell  13  through a first set of orifices  29  and allows the air to flow over the cathode surface  23  of fuel cell  13 . The air  21  that exits the orifices  29  flows through a set of annular flow distributors  31 . The fuel flow  17  similarly flows through a second set of orifices  30  and the annular flow distributor  31  and then over the anode surface  22 . An electrochemical reaction occurs at the fuel cell  13  when the air, acting as an oxidant, is present on the cathode surface  23  and the fuel is present on the anode surface  22 . A practical limit of 90% fuel utilization may be possible according to one of the embodiments. It is not desirable to attempt to react all of the fuel in the fuel cell stack  12 . Thus, the un-reacted fuel may be combusted in a post-stack combustion chamber (not shown). 
     The end products of the electrochemical reaction at the fuel cell  13  are depleted air  32 , depleted fuel  33 , and waste heat (FIGS. 1 b  and  1   c ). The waste heat causes the depleted fuel  33  and depleted air  32  to be hotter than the air flow  21  and fuel flow  17  entering through the orifices  29 ,  30 . The hotter depleted fuel  33  travels radially outward along a first set of annular gaps or channels  26  formed by the first interconnect  24 . Likewise, the hotter depleted air  32  travels radially outward along a second set of gaps or channels  25  formed by the second interconnect  15 . When the exhaust is not manifolded as shown in FIGS. 1 b  and  1   c , the depleted gases combust at the exterior edges of the fuel cells, a process that produces more waste heat. 
     In one embodiment for an integrated configuration of the heating loop  14  and fuel cell stack  12  (FIG. 1 a ), the fuel cell stack  12  essentially operates adiabatically. The temperatures of hotter depleted air and fuel  32 ,  33  continue to rise as the combustion at the fuel cell  13  continues. The hotter depleted air and fuel  32 ,  33  exit the fuel cell stack  12  radially flowing through the channels formed by the interconnects. Depleted hot gases flow over the heating loop  14 . Convection and radiation processes occur between the hot exhaust gases and the cooler surface of the heating loop  14 . The temperature difference between the depleted hot gases and an exterior wall of the heating loop  14  effectuates a heat transfer mechanism to capture the heat of the depleted gases. 
     The flowing fluid in the heating loop  14  facilitates the heat recuperation process. Depending on the flow rate and volume of the fluid inside the heating loop  14 , a steady state temperature distribution can be reached among the depleted gases  32  and  33 , the exterior wall of the heating loop  14 , and the fluid flowing inside the heating loop  14 . More specifically, the air enters the heating loop  14  at a lower temperature, typically at room temperature. The hotter depleted air  32  and fuel  33  transfer heat to the heating loop  14  and then to the fluid (e.g., air), increasing the fluid temperature. The fluid then exits the heating loop  14  at a higher temperature. In one embodiment of the present invention, this hotter fluid can flow directly into the second flow channel  28  inside the embedded manifold  11  to be used as a pre-heated oxidant  21  (i.e., when the fluid is air) for the fuel cell stack  12  (FIG. 1 a ). 
     Thus, the heating loop  14  and fuel cell stack  12  may be fully integrated by sharing a continuous flow of oxidant. Furthermore, the radially projected heat may be recovered by the heating loop  14  for pre-heating the oxidant flow  21  into the fuel cell stack  12 . This embodiment of the present invention effectively removes the need for a separate coolant fluid and an additional pre-heater for the oxidant flow  21 . Therefore, the present design provides simplicity. 
     As mentioned above, the fluid flow rate in the heating loop  14  can regulate the removal of thermal energy in the fuel cell system  10 . The heat transfer between the heating loop  14  and the fuel cell stack  12  may rely upon thermal radiation and convection. The radiation and convection through the gap is capable of high thermal flux when the temperature rise in the fuel cell stack  12  is optimized. Smaller temperature rises require greater airflow, increasing compressor parasitic power requirements. Greater temperature rises result in more severe thermal stresses and either higher stack operating temperatures (which may result in material property issues and increased exhaust gas heat losses) or lower stack operating temperatures (with more difficulty in maintaining fuel cell efficiency). For example, a fuel cell stack, which produces 31.5 watts from the electrochemical reaction, may produce 22.5 watts of power and 9 watts of heat. A post combustion chamber may further produce 3.5 watts of heat. The combined heat of 12.5 watts, which comprise 9 watts of heat plus 3.5 watts from post combustion heat, may suffer a 5.0 watt surface loss in the process. Assuming the air enters the heating loop  14  at 25° C. and a 200° C. temperature rise is required in the fuel cell stack  12 , the air may exit the heating loop  14  at 600° C., enter the fuel cell stack  12  at the same temperature, and leave the fuel cell stack  12  at 800° C. if the airflow remains at 0.155 kilogram per hour. The heat pick up by the heating loop  14  is thus 9 watts. A steady state performance calculation, as accomplished for this example, may be performed to determine the desired flow rate for specific design cases. 
     Also, as indicated above, the heating loop  14  can be mechanically integrated into the fuel cell system  10  by connecting an exit end of heating loop  14  to an inlet end of the oxidant flow channel  28 . Furthermore, portions of the interconnects  15 ,  24  and/or a separator plate  34  can be extended structurally beyond the outer surface  20  of the fuel cell stack  12  to provide mechanical support to the heating loop  14 . Care must be taken to electrically isolate the loop from the cells and interconnects by using an insulator to provide mechanical structure. Accordingly, the heating loop  14  of the present invention can be completely integrated into the fuel cell system  10  in reactant flow communication, as opposed to being a decoupled element of the system. 
     The material selection of the heating loop  14  is based on stack operating temperature and heat removal from the fuel cell system  10 . Thus, the optimal selection of the heating loop materials can be materials with good convective characteristics such as a high temperature metal like titanium and niobium or a thermally conductive metal like nickel based alloy and cobalt for solid oxide fuel cell applications. 
     In yet another embodiment of the heating loop  14 , surface augmentation can be provided, such as fins, to enhance heat transfer by increasing the contact surfaces in thermal convection. Dimpling, typically in the form of dents to increase exposure surfaces, can be added to the heating loop  14  surfaces to further enhance the heat transfer process. Surface augmentation of heating loop  14  may also be provided for reactant fluid in the heating loop  14 . 
     The geometrical aspect of the heating loop  14  can affect the heat transfer effectiveness in the fuel cell system  10 . The heating loop  14  in FIG.  1   a  is a cylindrical spiral type with a circular cross section around the fuel cell stack  12 . It should be obvious to those skilled in the art, however, that the cross section of the heating loop passage can be triangular, rectangular, or any shape that can effectively remove waste heat from the fuel cell stack  12 . The cylindrical spiral shape loop  14  around the fuel cell stack  12  can also be wrapped more densely with a space between two consecutive loops approximately 2 mm in the axial direction when the amount of heat generated is greater. 
     FIG. 2 is an embodiment of an externally manifolded fuel cell stack system  40  that can comprise a circular fuel cell stack  42 , a heating loop  44 , and an external manifold  46 . In a manner well known in the art, the external manifold  46  can flow gases into fuel cell stack  42  similar to that described in FIGS. 1 a-c . Although various materials can be used to construct the manifold  46 , preferred materials include ceramics, glass ceramics, metallic alloys, oxidation resistant metallic alloys, metal-ceramic composites and intermetallics. One geometrical shape of the external manifold  46  can be a rectangular but other shapes could be used. The outer peripheries of the external manifold  46  may extend substantially to the outer peripheries or outer surface  48  of the fuel cell stack  42 . The external manifold  46  is preferably positioned in a tangential location where the external manifold  46  and the fuel cell stack  42  are structurally connected adjacent to each other. As shown in FIG. 2, the external manifold  46  can extend axially from a first or top side  50  of stack  42  and to a second or bottom side  52  of the fuel cell stack  42 . Formed within the external manifold  46  are two interior cavities (not shown) for reactant flows into fuel cell stack  42  that extend along the axial length of the manifold  46  similar to those described in FIGS. 1 a-c.    
     As in the embodiment shown in FIGS. 1 a-c , an electrochemical reaction produces heat in the form of exhaust heat and spent gases are released radially toward the outer surface  48  of the fuel cell stack  42 . During this process, the temperature of the fuel cell stack  42  rises. Hotter depleted fuel and air causes the temperature of the external manifold  46  to rise as well. The temperature difference between the hotter depleted air/fuel flow and the wall of heating loop  44  creates a temperature gradient across the gap between the fuel cell stack  42  and the heating loop  44 . Similarly, the temperature difference between an outer surface  54  of the manifold  46  and the wall of heating loop  44  can create a temperature gradient across the gap between the manifold  46  and the heating loop  44 . Like in the above embodiment, the temperature gradient can effectuate a heat transfer mechanism to remove the heat, primarily through radiation and convection, from the fuel cell stack  42  and manifold  46 . 
     As in the above embodiment, the heating loop  44  can be mechanically integrated into the fuel cell system as described in the above embodiment. Alternatively, it can be secured by mechanical fasteners or bonded onto the external manifold  46  thus reducing the cost, size and design complexity. The material selection and configuration of the heating loop  44  can also be as in the above embodiment. Consequently, a reactant fluid can also flow through the heating loop  44 , as in the above embodiment. Further, the heating loop  44  may have surface augmentation as above. 
     FIG. 3 shows a parallel connection of externally manifolded stacks. In general, the embodiment of FIG. 3 is the same as that in FIG. 2, except that there is a plurality of stacks in the former. A first fuel cell stack  80  and a second fuel cell stack  90  are connected in parallel according to the embodiment of FIG.  3 . For purposes of illustration, the first and second fuel cell stacks  80 ,  90  are depicted as radial fuel cell stacks. For simplicity, only two stacks are shown in FIG.  3 . In general, any number of fuel cell stacks can be fit inside the heating loop  87 . 
     As in the embodiment of FIG. 2, the first radial fuel cell stack  80  can include a first external manifold  81  that flows gases into the stack  80 . The outer peripheries or surface of the first external manifold  81  may extend substantially to the outer surface  85  of the fuel cell stack  80 . The first external manifold  81  is preferably positioned in a tangential location or at the outer edge of the fuel cell stack  80  where the external manifold  81  and the fuel cell stack  80  may be structurally connected adjacent to each other similar to that described in the embodiment of FIG.  2 . In FIG. 3, the first external manifold  81  may extend from a first or top side  83  of fuel cell stack  80  and to a second or bottom side  84  of the fuel cell stack  80 . Formed within the first external manifold  81  can be two interior cavities (not shown) that extend along the axial length of the first manifold  81 . Both cavities flow reactant gases into the manifold  81 . 
     The second radial fuel cell stack  90  may be constructed and operate in a fashion similar to the first radial fuel cell stack  80 . However, different constructions can be used. Having a construction and function similar to the first radial stack  80 , the second radial stack  90  can include a second external manifold  91  similar to the first manifold  81 . Therefore, formed within the second external manifold  91  can be two interior cavities that extend along the longitudinal length of the second manifold  91  to flow the gases into the manifold  91 . 
     For the purposes of illustration, the second fuel cell stack  90  and its second external manifold  91 , as shown in FIG. 3, are mirror images of the first fuel cell stack  80  and its manifold  81 . The outward faces of the first manifold  81  and the second manifold  91  are positioned to oppose each other. A connection element  89  is disposed between first and second manifolds  81  and  91 . The connection element  89 , as shown in FIG. 3, can be a rectangular shaped element extending from the first manifold  81  radially outward towards the second manifold  91 , forming a combined/integral external manifold feeding both fuel cell stacks. The connection element  89  may cover the entire lengths of the first and second manifolds. The connection element  89  can be in other forms or shapes. The first and second fuel cell stacks  80 ,  90  can be positioned at any relative angle to each other while the connection element  89  is shaped to provide the required linkage, forming a multi-stack fuel cell assembly. 
     Formed within the connection element  89  can be two interior cavities that extend along the axial length of the element  89  (not shown). Both cavities flow gases into the element  89 . The cavities therein can be connected internally to the cavities inside the first and second manifolds  81 ,  91  to provide flow communication for the multi-stack fuel cell assembly. The gases are then distributed to the first and second fuel cell stacks  80 ,  90  through the flow communication within the first and the second manifolds  81 ,  91 . 
     As with the above embodiments, electrochemical reactions produce thermal energy in the form of exhaust heat and spent gases are released radially outward from the fuel cell stacks  80 ,  90 . In the process of releasing the heat radially, the temperatures of the fuel cell stacks  80 ,  90  rise. Similarly the temperatures at the external manifolds  81 ,  91  and the connection element  89  rise. The temperature difference between the fuel cell stacks  80 ,  90  and the wall of heating loop  87  creates temperature gradients across the gaps between the fuel cell stacks  80 ,  90  and the heating loop  87 . Similarly, the temperature differences between the outer surfaces of the manifolds  81  and  91 , together with the connection  89 , and the wall of heating loop  87  create temperature gradients across the gap (not shown) between the manifolds  81 ,  91 , as well as the interconnect  89 , and the heating loop  87 . The temperature difference between the depleted hot gases and the wall of heating loop  87  creates a temperature gradient across the gap between the fuel cell stacks  80  and  90 , and the heating loop  87 . The temperature gradients effectuate a heat transfer mechanism to remove the heat by convection and radiation from the fuel cell stacks, manifolds, and interconnect. 
     Similar to the above embodiments, thermal radiation and convection through the gaps is capable of high thermal flux depending on the flow rate and volume of the coolant through the heating loop  87 . A steady state performance calculation can be performed in the fuel cell system design phase to choose the proper temperature rises in the fuel cell stacks  80  and  90 , the manifolds  81  and  91 , the connection element  89 , and the optimal fluid flow rate in the heating loop  87 . The gap distances can vary at different external locations, but generally in the same ranges, as in the above embodiments. Likewise, the heating loop  87  is configured to follow the exterior contours of the stacks, manifolds, and interconnect element, and it may be constructed and operated as in the above embodiments. 
     In FIG. 4, an embodiment of an internally manifolded, thermally integrated rectangular fuel cell system  100  for a ceramic/metal solid oxide fuel cell stack  102  including a fully integrated recuperator loop  104  is shown. Fuel cell stack  102  can have internal flow cavities/channels (not shown) that are similar in construction and operation to the embodiment shown in FIGS. 1 a-c . However, in contrast to the embodiment of FIGS. 1 a-c , this embodiment has an overall rectangular configuration. Accordingly, from a top cross sectional view (not shown), the various components of the fuel cell stack  102  have a rectangular cross section. Furthermore, flow channels  106 ,  108 ,  110  and  112  are outside the fuel cell stack  102 . The channels  106 ,  108  can allow external connections between different fuel cell stacks and with the fuel and oxidant flows from the recuperator loop  104  (not shown). The flow channels  110  and  112  can allow the exhaust gases exit the stack  102  (not shown). In such configuration, the exhaust hot gases do not flow over the heating loop  104 , but they are manifolded (not shown) and exit the stack through flow channels  110  and  112 . 
     The internal manifold (not shown) can extend from a first or topside of the stack  102 , through the stack  102 , and out of a second or bottom side of the stack  102 . A reactant gas may flow into one of the open flow channels  106  and  108  while the other reactant flows through another of the channels. As with the above embodiments, and in the process of releasing heat, the temperature of the fuel cell stack  102  rises. The temperature difference between the fuel cell stack  102  and the wall of recuperator loop  104  creates a temperature gradient across the gap between the two structures. The temperature gradient effectuates a heat transfer mechanism to remove the heat, primarily through radiation, from the fuel cell stack  102 . The design, construction, and operation can be the same as in the above heating loops. 
     FIG. 5 is an externally manifolded, thermally integrated fuel cell system  140  including a ceramic/metal solid oxide fuel cell stack  122  and a fully integrated recuperator loop  124 . Thus, this embodiment is similar to that shown in FIG. 4, except that the latter is internally manifolded. Thus, the fuel cell stack  122  includes a pair of external manifolds  132  that flow gases into fuel cell stack  122 . One external manifold can each be positioned at opposite ends of the fuel cell stack  122 . In this embodiment, a reactant gas may flow into one of the externally exposed channels  126 ,  128  while the exhaust gas flows through another of the channels  126 ,  128 . While the channels and manifolds for a second reactant and exhaust are not shown, they can be attached to the two exposed sides of the stack. The recuperator loop  124  is designed, constructed, and operated in a fashion similar to the above embodiments of recuperator loops. 
     Referring to FIG. 6, two serially connected fuel cell stacks with external manifolds are depicted. This embodiment is similar to that shown in FIG. 5, except that multiple stacks  180  and  190  are provided among multiple manifolds  181 ,  189 , and  191 . As in the above embodiments with multiple stacks, the stacks  180  and  190  may have the same or a different design and shape. In either event, an external manifold  181  can be positioned at one end of the fuel cell stack  180 . A second external manifold  191  can be positioned at an end of the second fuel cell stack  190 . The open ends of the stacks  180  and  190  are so positioned that they face each other with a third manifold  189  therebetween. The third manifold  189  can extend from the first stack  180  towards the second stack  190  forming an integral manifold that feeds both fuel cell stacks and provides the necessary flow and electric connections. The first and second fuel cell stacks  180 ,  190  can be positioned at any relative location to each other while the manifold  189  provides the required mechanical and gas communication linkage. Thus, one reactant can enter the manifold  189  via an inlet  186  and flow through the fuel cells to exhaust manifolds  181  and  191 . The depleted first reactant is exhausted through channels  187  and  188 . The other reactant can be supplied to each stack by attaching separate manifolds (not shown) to the two exposed sides of each stack. As shown in FIG. 6, a recuperator loop  184  can be designed, constructed, and operated like the above recuperator loops. 
     Although the present invention has been described in considerable detail with reference to certain versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the versions contained