Higher Total Reactant Utilization

In an improved design, called an MCFC network, reactant streams are ducted such that they are fed and recycled among multiple MCFC stacks in series. Figure 9-19b illustrates how the reactant streams in a fuel cell network flow in series from stack to stack. By networking fuel cell stacks, increased efficiency, improved thermal balance, and higher total reactant utilizations can be achieved. Networking also allows reactant streams to be conditioned at different stages of utilization. Between stacks, heat can be removed, streams can be mixed, and additional streams can be injected. 

Reactant Utilization and Gas Composition Reactant utilization and gas composition have major impacts on fuel cell efficiency. It is apparent from the Nemst equations in Table 2-2 that fuel and oxidant gases containing higher partial pressures of electrochemical reactants produce a higher cell voltage. Utilization (U) refers to the fraction of the total fuel or oxidant introduced into a fuel cell that reacts electrochemically. In low-temperature fuel cells, determining the fuel utilization is relatively straightforward when H2 is the fuel, because it is the only reactant involved in the electrochemical reaction," i.e. 

In PEM fuel cells, uniformity of the current density across the entire active area is critical for optimizing the fuel cell performance. A non-uniform current density in the fuel cell can drastically affect different parameters of the fuel cell, such as reduced reactant and catalyst utilization along the active area, decrease in total efficiency and lifetime, and durability failure modes. Thus, determination of the current density distribution information is vital for designing PEM fuel cells that achieve higher performance and longer life.130 A number of methods for measuring current distribution in PEM fuel cells have been demonstrated the following sections discuss some of these methods in further detail. 

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