Abstract:
A solid-oxide fuel cell stack assembly comprising a plurality of sub-stacks, preferably two sub-stacks each containing one-half the total number of fuel cells. Cathode air and fuel gas are passed through the first sub-stack, wherein they are partially reacted and also heated. The exhaust cathode air and the exhaust fuel gas from the first sub-stack are directed to the respective inlets of the second sub-stack, becoming the supply cathode air and fuel gas therefor. A first heat exchanger in the flow paths between the sub-stacks and a second heat exchanger ahead of the sub-stacks can help to balance the performance of the two stacks. The result of dividing the number of cells into a plurality of sub-stacks, wherein the exhaust of one sub-stack becomes the supply for the next sub-stack, is that fuel efficiency and utilization are improved, thermal stresses are reduced, and electrical power generation is increased.

Description:
This invention was made with Government support under DE-FC26-02NT41246 awarded by DOE. The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     The present invention relates to fuel cells; more particularly, to solid-oxide fuel cells; and most particularly, to arrangements for optimizing the operating conditions, longevity, efficiency, fuel utilization, and electrical output of a solid-oxide fuel cell stack. 
     BACKGROUND OF THE INVENTION 
     Fuel cells for 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). 
     In some applications, for example, as an auxiliary power unit (APU) for a transportation application or a stationary power unit (SPU) for a stationary application, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a catalytic liquid or gaseous hydrocarbon oxidizing reformer, also referred to herein as “fuel gas”. 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 hydrocarbon fuel, resulting ultimately in water and carbon dioxide. Both reactions are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 1000° C. 
     A complete fuel cell stack assembly includes a plurality of fuel cells, for example, 60 cells in the form of sub-assemblies, and a plurality of components known in the art as interconnects which electrically connect the individual fuel cell subassemblies in series electrically. Typically, the interconnects include a conductive foam or weave disposed in the fuel gas and air flow spaces adjacent the anodes and cathodes of the fuel cells. 
     In known prior art fuel cell stack assemblies, each subassembly contains porting which, when joined to the porting of adjacent subassemblies, creates a supply manifold and an exhaust manifold for both the cathode air to the plurality of cathodes and the fuel gas to the plurality of anodes. Thus, all of the cathodes are in parallel pneumatic flow and all of the anodes are in parallel pneumatic flow. The total air is divided among the plurality of cathodes such that each increment of air passes over only a single cathode and then is collected in the air exhaust manifold. Similarly, the total fuel gas entering the stack assembly is divided among the plurality of anodes such that each increment of fuel gas passes over only a single anode and is then collected in the fuel gas exhaust manifold. This flow scheme is sensitive to uneven flow distribution at low anode flow rates. Generally, only a portion of the fuel cell gas is consumed, or utilized, in the single pass through the stack. High fuel utilization is desirable for high system efficiency; however, stack power density decreases with increasing fuel utilization due to fuel gas concentration gradients in the SOFC stack. 
     Air entering a prior art SOFC stack assembly at ambient temperature must be pre-heated to accommodate and regulate the temperature of the SOFC stack; and to this end, it is known to pass the incoming air through a cathode air heat exchanger immediately ahead of the fuel cells using hot exhaust air as the heat source, thus increasing the thermal efficiency of the system (recuperation). Also, the fuel gas is typically formed in a hydrocarbon reformer and thus may exit the reformer at about 650° C., although both the fuel gas and the cathode air are desirably substantially hotter than 650° C. for optimum fuel consumption and electrical generation (stack efficiency). 
     Another area of concern in prior art fuel cell stack assemblies is the temperature rise through the stack, the hydrogen/oxygen reaction being highly exothermic. High temperature gradients produce high stresses within the stack and can reduce stack durability. Temperature gradients through the traditional stack may be reduced through increased cathode air massflow, but this results in reduced air utilization for the fuel cell system and a resulting loss of system efficiency. 
     What is needed in the art is a means for increasing the fuel efficiency of the fuel cell system and electrical output of an SOFC stack assembly. 
     What is further needed in the art is a means for decreasing thermal stresses within a stack assembly, thereby improving stack durability, without reducing system air utilization and decreasing system efficiency. 
     It is a principal object of the present invention to increase the electrical output of an SOFC multi-cell stack of a given size. 
     It is a further object of the present invention to increase the fuel efficiency of an SOFC stack module. 
     It is a still further object of the present invention to extend the working lifetime of an SOFC stack assembly. 
     It is a still further object of the present invention to reduce thermal stresses in the SOFC stack, without reducing system air utilization, or for a given thermal stress on the SOFC stack, increase system utilization. 
     SUMMARY OF THE INVENTION 
     Briefly described, a solid-oxide fuel cell stack assembly comprising a plurality of individual fuel cell sub-assemblies is divided into a plurality of sub-stacks, preferably two sub-stacks each containing one-half of the fuel cell sub-assemblies. Other divisions of the stack into sub-stacks and apportionments of the cells into sub-stacks are comprehended by the invention. Cathode air and fuel gas are passed conventionally through the first sub-stack, wherein they are partially consumed and also heated. The exhaust cathode air and the exhaust fuel gas from the first sub-stack are directed to the respective inlets of the second sub-stack, becoming the supply cathode air and fuel gas therefor. This allows a second pass of the fuel gas, thereby increasing fuel efficiency. A heat exchanger in the air and fuel flow paths between the sub-stacks can be used to adjust gas temperatures to help balance the performance of the two stacks. For example, heat can be stripped from the inter-stack air and fuel gas and, via a second heat exchanger disposed ahead of the first sub-stack, can further preheat the air and fuel gas entering the first sub-stack, thereby improving the efficiency of the first sub-stack and making the operating condition of the two sub-stacks more nearly alike. Alternatively, the stripped heat may be exhausted to the environment to bring the operating temperature of the second sub-stack closer to the temperature of the first sub-stack. 
     The two or more substacks connected by series flow of anode and cathode gasses can be connected electrically in series, parallel, or run electrically independent from one another with separate controls as, for example, separate electronics to control the current or voltage of each substack independently. 
     A sub-stack module in accordance with the invention comprises first and second sub-stacks arranged in flow series as just described with a first heat exchanger between the stacks and a second heat exchanger ahead of the first stack. A plurality of sub-stack modules may be ganged in flow parallel and connected in electrical series to provide any desired electrical voltage. There can also be connected electrically in parallel or be run electrically independent from one another. In such arrangements, it can be useful to provide an auxiliary cooling unit whose output is controllably divided as by a plenum and valving among the plurality of modules such that the thermal operating conditions of all sub-stacks and all modules are optimal. 
     The result of dividing the number of cells into a plurality of stacks, wherein the exhaust of one stack becomes the supply for the next stack, is that fuel efficiency is improved, thermal stresses are reduced, electrical power generation is increased, and configurational flexibility is enhanced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic drawing of a prior art multiple-cell fuel cell stack arrangement wherein the cathode air and anode gas flow to the multiple cells in series; 
         FIG. 1   a  is a schematic drawing of a prior art multiple-substack arrangement wherein the cathode air and anode gas flow to the multiple cells in each substack in series and to the multiple-substacks in parallel; 
         FIG. 2  is a schematic drawing of a first embodiment of a multiple-cell fuel cell stack arrangement in accordance with the invention, comprising a plurality of sub-stacks; 
         FIG. 3  is a schematic drawing of a second embodiment, showing a first heat exchanger disposed between the sub-stacks; 
         FIG. 4  is a schematic drawing of a third embodiment, showing a second heat exchanger disposed ahead of the first sub-stack; 
         FIG. 4   a  is a schematic drawing like that shown in  FIG. 4 , showing a closed cooling system for supplying the heat exchangers; 
         FIG. 5  is a schematic drawing of a fourth embodiment, showing provision of additional cathode cooling air to the cathode inlet of the second sub-stack; 
         FIG. 6  is a schematic drawing showing a fuel cell module having first and second sub-stacks and first and second heat exchangers, substantially as shown in  FIG. 4 ; 
         FIG. 7  is a schematic drawing like that shown in  FIG. 6 , showing a distributed cooling air system for providing cooling air to the first and second heat exchangers; and 
         FIG. 8  is a schematic drawing of a fuel cell assembly comprising three modules, such as is shown in  FIG. 7 , and a cooling air system for supplying regulated air flow to the heat exchangers. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a prior art solid-oxide fuel cell stack  10  contains a plurality of individual fuel cells (not visible in  FIG. 1 ) arranged in flow parallel and electrical series, as is well known in the fuel cell art.  FIG. 1   a  shows an alternate prior art multiple-substack arrangement having fuel cell sub-stacks  10 ′ wherein, like stack  10  in  FIG. 1 , each substack contains a plurality of fuel cells arranged in parallel flow. Substacks  10 ′ are arranged in parallel flow pneumatic flow relative to each other and may be arranged  10  electrically in series (as shown), in parallel, or controlled independently. 
     Cathode air  12  enters stack  10 ,  10 ′ at cathode inlet  14  and, after flowing across all of the individual cathode surfaces in the stack, exits the stack as spent air  16  at cathode air outlet  18 . Fuel gas  20 , for example, hydrogen and carbon monoxide from a hydrocarbon reformer (not shown), enters stack  10 ,  10 ′ at anode inlet  22  and, after flowing across all of the individual anode surfaces in the stack, exits the stack as spent fuel gas  24  at anode outlet  26 . Typically, the cathode air is pre-heated before entering the stack, and the fuel gas is also preheated or relatively hot as delivered from the reformer. Both gases, as they flow through the stack, undergo substantial heating, which heat when discharged to the environment as shown in  FIG. 1  represents a significant thermodynamic loss and consequent fuel inefficiency. Further, as the gases are progressively heated within the stack, very substantial thermal stresses can be created which can lead to short working lifetimes of stress-sensitive components in the fuel cells. 
     Further, the air and fuel gases, being passed across the cells in parallel, pass over only a single cathode or single anode surface before being discharged. Because an individual fuel cell reaction is relatively inefficient, a significant amount of fuel remains in the “spent” anode exhaust  24 . 
     EXAMPLE 1 
     An SOFC stack configured in accordance with  FIG. 1  was operated at a current of 0.9 amps/cm 2 . With a temperature differential across the stack of 150° C. (inlet temperature 650° C., outlet temperature 800° C.), the stack produced 4810 watts of electricity at 0.74 volts/cell, fuel utilization was 43%, stack efficiency was 21.8%, and an additional 1136 watts of heat was released within the stack. 
     Referring to  FIG. 2 , in a first embodiment  100  in accordance with the invention, a fuel cell stack  110  having a plurality of individual fuel cell elements, for example, 60 cells as in prior art stack  10 , is divided into a plurality of sub-stacks, for example, two sub-stacks  111   a , 111   b  each containing 30 fuel cell elements, the two sub-stacks being connected in series electrically. Other numbers of sub-stacks are fully comprehended by the invention, and the numbers of fuel cells may or may not be distributed equally among the sub-stacks. 
     In accordance with the invention, the cathode exhaust  116   a  and anode exhaust  124   a  from first sub-stack  111   a  are collected and delivered to the respective inlets  114   b , 122   b  of sub-stack  111   b.  It will be seen that anode exhaust  124   a  has been passed over only 30 anode surfaces in sub-stack  111   a  and thus has a large remaining fuel content. Similarly, cathode air exhaust  116   a  has a large remaining oxygen content. Further, the mass flow of air and fuel gas through each cell is doubled with respect to stack  10 , which has the advantage of reducing the temperature gradient across each sub-stack by providing additional cooling, thus reducing thermal stresses in the cells, substacks, and stacks. 
     EXAMPLE 2 
     An SOFC stack configured in accordance with  FIG. 2  was operated at a current of 0.9 amps/cm 2 . 
     Sub-stack  111   a  was operated with a temperature differential of 100° C. (650° C. inlet temperature, 750° C. outlet temperature) and produced 2359 watts of electricity at 0.72 volts/cell. Fuel utilization was 21.8%, stack efficiency was 10.7%, and only 177 watts of heat were rejected into the stack. Note that this is a much lower fuel utilization and stack efficiency than for prior art stack  10 . 
     Sub-stack  111   b  had an inlet temperature of 750° C. and an outlet temperature of 850° C. Sub-stack  111   b  produced 2589 watts of electricity at 0.79 volts/cell. Fuel utilization was 28% and the stack efficiency was 15%. The reaction was slightly endothermic, requiring 138 watts of energy from the stack. 
     Advantages of the novel stack configuration in accordance with the invention ( FIG. 2 ) vs. the prior art configuration ( FIG. 1 ):
         a) the novel configuration produced more total electric power, 4948 watts vs. 4810 watts.   b) the novel configuration ran with a substantially reduced temperature gradient across each sub-stack, thus improving stack durability and reducing thermal stresses in the structure.   c) the novel configuration ran with substantially improved net fuel utilization, 49.8% vs. 43%
 
Regarding the viability of comparisons, the heat exchange between the chemical reactions and the stack structure in all cases was of a magnitude and sign such that if everything were shifted to adiabatic, the comparisons would favor the novel configuration even more.
       

     Referring to  FIG. 3 , in a second embodiment  200 , a first heat exchanger  130  is installed in the flow paths of cathode exhaust  116   a  and anode exhaust  124   a.  Heat exchanger  130  may be a single three-way heat exchanger or two separate two-way heat exchangers, as is known in the heat exchanging art; for simplicity of presentation, heat exchanger  130  (and all other heat exchangers herein) is shown as a single unit. Tempered air or other coolant  132  is provided through a first side of exchanger  130  such that the exhaust streams of the first sub-stack are cooled with waste heat  134  moving to the environment or to be used productively elsewhere in the system. This arrangement can permit the two sub-stacks  111   a , 111   b  to operate at very similar temperatures, and to do so at the lower end of the viable temperature range. This is advantageous from a materials or durability standpoint although at some sacrifice in thermal and electrical efficiency. 
     Referring to  FIG. 4 , in a third embodiment  300  a second heat exchanger  140  is provided in the entrance streams of cathode air  12  and fuel gas  20 . Some or all of waste heat  134  from first heat exchanger  130  is diverted through second heat exchanger  140  to additionally preheat cathode air  12  and fuel gas  20  prior to entry into first sub-stack  111   a.  Exhaust  134   a  from second heat exchanger  140  may be released to the environment or diverted for use productively elsewhere in the system. 
     In Example  2 , clearly second sub-stack  111   b  performed better than first sub-stack  111   a,  largely because of higher operating temperature. First sub-stack  111   a  can be shown to operate better if the operating temperature is raised 100° C. (which also raises the inlet and exhaust temperatures 100° C.). Doing so changes the stack reaction from exothermic to slightly endothermic and improves stack efficiency. The net effect if run under adiabatic conditions would be a reduction in temperature gradient across sub-stack  111   a.    
     Thus, the efficiency of a series flow arrangement of sub-stacks  111   a , 111   b  can be improved if the sub-stacks are operated under similar thermal regimes, either by simply lowering the inlet temperature of the second sub-stack or by also raising the inlet temperature of the first sub-stack. A highly desirable effect of either approach is a reduction in stack thermal stress. 
     Referring to  FIG. 4   a,  the cooling system for heat exchangers  130 , 140  may readily be provided as a closed loop  150  including recirculation means such as a coolant pump or blower  152 . 
     Referring to  FIG. 5 , in a fourth embodiment  400  a second stream  412  of cathode air  12  is mixed with cathode exhaust air  116 a from first sub-stack  111   a.  Stream  412  is preferably tempered and metered to result in a cathode air inlet stream to second sub-stack  111   b  sufficient to decrease the temperature gradient across sub-stack  111   b.  This embodiment does not require either of heat exchangers  130 , 140  and their associated cooling system(s), but its effect is limited to lowering the temperature of the second sub-stack. A potential disadvantage of this embodiment is that a thermal imbalance is created between the temperatures of the cathode air and the fuel gas entering the second sub-stack, which can create undesirable mechanical stresses therein. 
     Alternatively, other gasses could supplement or replace second stream  412  in this injection scheme. Most notably, stream  412  could be augmented with oxygen. This would be especially useful if multiple (more than 2) substacks were series flow connected in a substack module. Such gas tailoring or selection could impact the SOFC chemistry in downstream substacks as well as provide additional cooling as already noted. 
     Referring to  FIG. 6 , a fuel cell stack module  500  in accordance with the invention comprises a first sub-stack  511   a,  a second sub-stack  511   b,  a precooler heat exchanger  540  ahead of the first sub-stack, and an intercooler heat exchanger  530  between the sub-stacks, which arrangement is substantially as described above. In addition, module  500  includes a temperature sensor T 2  in the gas flowstream between precooler  540  and first sub-stack  511  a to monitor the temperature of the cathode air  512  entering the sub-stack, which may be equal to the temperature of fuel gas  520  if their respective temperatures have been previously equalized to a temperature T 1  during passage of the gases through opposite sides of an equalizer heat exchanger  502  outside module  500 . A first coolant  580  may be passed through precooler  540  to adjust entry temperature T 2  as desired. A temperature sensor T 3  monitors the temperature of the gas exhausts from first sub-stack  511   a,  and a temperature sensor T 4  monitors the entry temperature of gases into second sub-stack  511   b.  A second coolant  582  may be passed through intercooler  530  to adjust entry temperature T 4  as desired, which generally should be about the same as temperature T 2 . Thus, when the two sub-stacks contain the same number of fuel cells, the performance and efficiency of the two stacks should be about the same. Sensors T 2  and T 3  permit monitoring of the temperature change across first sub-stack  511   a,  and sensors T 4  and T 5  permit monitoring of the temperature change across second sub-stack  511   b.    
     In a currently preferred mode of operation, cathode air  12  is passed through a cathode air heat exchanger  590  in known fashion, which may be heated by exhaust gas at temperature T 5 , for example, and may be further tempered by addition of bypass air  592 . Fuel gas  20  is supplied as by a hydrocarbon reformer  594  in known fashion. The temperature and flow rate of first coolant  580  is adjusted to provide a predetermined temperature T 2  of cathode air and fuel gas at the entrance to first sub-stack  511   a.  The temperature and flow rate of second coolant  582  is adjusted to provide a predetermined temperature T 4  of cathode air and fuel gas at the entrance to second sub-stack  511   b.  First and second coolants  580 , 582  may or may not be identical in substance, temperature, or flow rate. Referring to  FIG. 7 , a coolant air plenum  596  may be used to supply both coolants  580 , 582  to module  500  from a common source at a common temperature, the individual flow rates of coolant being governed by a single proportional valve  598 . 
     Referring to  FIG. 8 , a plurality of fuel cell stack modules  500 , in this example three such modules, may be grouped together in an arrangement  600  having several legs of series sub-stacks connected in flow parallel to cathode air  12  and fuel gas  20 . Because the optimal operating temperatures of the various first sub-stacks  511   a  and second sub-stacks  511   b  may differ slightly, the plurality of coolant flows  580 , 582  may be supplied from a common plenum  596  and individually flow-controlled by a plurality of control valves  597 . 
     While the substacks in the various embodiments, in accordance with the invention, are shown connected electrically in series, it is understood that they could alternatively be connected electrically in parallel (controlling voltage or total current) or operated electrically independent (controlling voltage or current for each substack). 
     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.