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utilization of reactants is accomplished incrementally, and the low voltage associated with high utilization is restricted to stacks which produce only a portion of the total power. Manifolding problems can further limit the practical reactant utilization of conventional MCFC systems. Ideally, fuel and oxidant streams are distributed equally among individual cells in a stack. Today's manifolds, however, have not been able to achieve this, and cells are typically supplied with unequal reactant flows. This causes the composition of outlet reactant streams to be variable among the cells. At high utilizations, this variability leads to a significant reduction in stack voltage. Therefore, conventional systems avoid such high utilizations. However, when stacks are networked in series, reactant streams can be thoroughly mixed between cells. This reduces the variability in reactant composition and helps to minimize the stack voltage loss. Another study (7) maximized the efficiency of conventional and series-connected fuel cell systems by optimizing cell voltage and current density. The study found that the optimum fuel utilization in the series-connected system was higher than that in the conventional system. Most importantly, the higher fuel utilization and lower current density of the series-connected system combined to give more efficient performance than the conventional system. 8.5.6 DisadvantagesofMCFCNetworks For recycling to improve the performance of an MCFC network, it must provide benefits that outweigh its inherent disadvantages. If carbon dioxide is not separated from the anode-anode recycle, the concentration of carbon dioxide in the anode is increased. This reduces the Nernst potential. The Nernst potential is similarly reduced by the anode-cathode recycle if steam is not condensed out, since recycled steam dilutes reactant concentrations in the oxidant. In addition, part of the power generated by the network is consumed by the equipment necessary to circulate the recycle streams. Such circulation equipment, along with the additional ducting required by recycling, also increases the capital cost of the MCFC network. Given the same initial feed streams, the flowrate of reactants through stacks networked in series is much larger than the flowrate of reactants through stacks in a conventional system. Conventional fuel cell systems divide the initial feed streams among many stacks arranged in parallel. However, the initial feed streams in an MCFC network are not divided, but fed directly into the first of a series of many stacks. Perhaps the greatest disadvantage of MCFC networks is that this increased flowrate creates larger pressure drops. Another potential disadvantage of an MCFC network is the interdependence of the stacks in series. A problem with one stack could alter the performance of succeeding stacks. Furthermore, bypassing or isolating a problematic stack in a network could be a difficult control process. In the conventional parallel configuration, stack performance is not so interrelated. 8.5.7 ComparisonofPerformance Two ASPEN (Advanced System for Process Engineering, public version) simulations compare the performance of conventional and networked fuel cell systems having identical recycle schemes and steam bottoming cycles. Each simulated system was composed of three MCFC stacks operating at the same temperature and pressure. The Nernst potential of each MCFC in both systems was reduced by 0.3 volts due to activation, concentration and ohmic voltage 8-88PDF Image | Fuel Cell Handbook (Seventh Edition)
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