Quasi-3-D Dynamic Model of an Internally Reforming Planar Solid Oxide Fuel Cell for Hydrogen Co-Production

A simplified quasi-3-dimensional model of a solid oxide fuel cell (SOFC) is developed to investigate the dynamics of internal reformation in an SOFC. The dynamic model solves dynamic equations that govern relevant physical and chemical processes in a simplified geometric representation of a planar SOFC. This makes the model complex enough to resolve major performance characteristics and simple enough to be used in dynamic analyses and controls development at the system level. The model solves dynamic mass, momentum and energy conservation equations to provide local temperature, species concentrations, and current density distributions. These distributions are resolved in two dimensions across the cell, but each 2-D distribution resolves 5 separate control volumes through the nodal unit cell: the PEN; anode and cathode gas compartments; and interconnects. Internal reforming chemical kinetic expressions are included in the model formulation. Simulations show that extent of internal reformation impacts the dynamic temperature difference across the cell. Steady state maximum temperature differential across the cell can be reduced to about 100 K with 100% internal reformation and a cross-flow configuration. A full hydrogen co-production system was then modeled by integrating the SOFC model with heat exchangers, combustor, blower, and hydrogen collector. For conditions of a constant cathode exhaust temperature of 1273 K and lower fuel utilization (60%–70%), the dominant thermal influence on the cell temperature was cooling by the endothermic reformation reactions. But at higher fuel utilization conditions, the dominant thermal influence was the convective cooling of the cathode gases. System simulations showed no tradeoff between power and H₂ production if the cathode exhaust temperature is held constant at 1273 K. High power and high H₂ production conditions were found to be synergistic: high hydrogen production leads to high electrochemical efficiency and lower air flow rate leading to fewer parasitic losses. Dynamic SOFC responses to manipulation of fuel flow rate within the range of fuel utilization between 60 and 85% indicate that the system can be adequately controlled to produce various amounts of hydrogen and electricity.