Flow Distribution Analysis of Solid Oxide Fuel Cell Stack under Electric Load Conditions

Abstract

The flow distribution of reactants is one of the key contributors to the proper stack operation. Uneven local utilization of the fuel is limiting the maximum practical utilization and decreasing electric efficiency of the stack. Flow distribution in the planar SOFC stack under electric load conditions and under high fuel utilization conditions is a complex process, affected by reactant gas physical characteristics, flow field geometry, dimensional characteristics of the cell package, stack manifolds design, flow direction in the inlet and outlet manifolds (U-flow, Z-flow), reactants flow field configuration (co-, cross-, counterflow), performance characteristics of the cell package and fabrication tolerances. In addition, flow distribution and pressure distribution in the fuel cell flow field and in the stack are coupled. The resulting pressure drop is contributing to the parasitic losses of the overall process. However, minimization of pressure drop in the flow field is limited by the resulting increase in the flow maldistribution. Efforts to predict flow distribution of reactants in the fuel cell stack have been reported in literature using both analytical and computational fluid dynamics (CFD) methods. The electrochemical reactions under electric load conditions, chemical reactions in the gas phase and temperature distribution effects are often neglected due to computational complexity of the resulting problem. In this work, the effect of fuel type was evaluated for a wide range of fuel streams derived from the process calculations (NG, LPG, DME, diesel, methanol, ethanol, biomass etc.) using typical process configurations (steam reforming, fuel recycling loop) for the SOFC process design. Localized reactant flow conditions were derived from CFD calculations, accounting for both electrochemical reactions under load and chemical reactions in the gas phase. The results are also presented for the oxidant flow, which is several times higher then the corresponding fuel flow, in a typical range of oxidant utilizations. Systematic analysis of a range of effects and their relative importance on the flow distribution is presented. Operation of the stack under electric load conditions increases flow maldistribution of the fuel for the U-flow manifold configuration. The effect of the electric load on the oxidant flow is negligible for typical oxidant utilizations. The loss of reactants through the cell package sealing can increase flow maldistribution meaningfully. The CFD model calculations have been verified by the pressure drop measurements in the flow field.