MCFC Network

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. 

Figure 9-21 shows an MCFC network. The arrangement of stacks in series, as well as a unique recycle scheme, allows an MCFC network to meet all the requirements of an MCFC power system, while achieving high efficiency. 

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 Nemst potential. The Nemst 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. 

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. 

Stacks in series approach reversibility. MCFC stack networks produce more power than conventional configurations because they more closely approximate a reversible process. To illustrate this fact, consider Figure 9-20, which compares the maximum power that could be generated by three different MCFC systems having identical feed stream compositions. ... 

When MCFC stacks are networked in series, reactant streams can be conditioned between the stacks — at different stages of utilization. The composition of reactant streams can be optimized between stacks by injecting a reactant stream (see Figure 9-19) or by mixing the existing reactant streams. 

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 Nemst potential of each MCFC in both systems was reduced by 0.3 volts due to activation, concentration and ohmic voltage... 

Computer simulations have demonstrated that a combined cycle system with MCFC stacks networked in series is significantly more efficient than an identical system with MCFC stacks configured in parallel. 

Molten carbonate fuel cells are interesting candidates for stationary power generation networks, similar to the SOFC concept. The technology of the SOFC seems to be further developed and superior to MCFCs, shadowing the advantages that MCFCs can provide. Once SOFCs become ripe for commercialization, MCFCs will probably not be able to compete. 

Fossil-fuel, nuclear, and hydroelectric power plants all require a transmission network to transport electridty from where it is generated to where it is used. MCFCs, with their smaller size and portability, can be placed much closer to where the power is needed, redudng the need for transmission Unes. With this in mind, explain why MCFCs may have a greater advantage for providing power in developing countries versus in developed countries. 

---