Comparison of Finite Volume SOFC Models for the Simulation of a Planar Cell Geometry

Abstract

This paper discusses the development of a finite volume model for a planar solid oxide fuel cell. Two different levels of detail for the definition of the basic cell elements are considered, the first with the assumption of isothermal behavior for a finite volume, defined by a portion of the cell PEN structure with pertinent air and fuel channels, and the second with a more refined element subdivision, capable of simulating temperature differences at a smaller scale. The model applies a detailed electrochemical and thermal analysis to a planar SOFC of a defined geometry (with co‐flow, counter‐flow or cross‐flow configuration), material properties and input flows. Electrochemical modeling includes an evaluation of ohmic, activation and diffusion losses as well as a kinetic model of the hydrocarbon reactions involved. The model calculates internal profiles of temperature, flow composition, current density, and cell energy balances. Internal heat exchange coefficients are evaluated with a specific fluid‐dynamic analysis. After a preliminary calibration of the model, a comparison of the simulation results generated by the two models is presented and a parametric analysis to investigate the effects of different assumptions on a selection of key parameters (heat losses, air stoichiometric ratio and inlet temperatures) is carried out. The results show that the refined model developed here could significantly help in the design of efficient fuel cell stack projects and in the careful consideration of the influence of heat losses, air ratio and the endothermic reforming reaction on cell temperature distribution and global performances.

Modeling Elementary Heterogeneous Chemistry and Electrochemistry in Solid-Oxide Fuel Cells

“…The channel is taken to be very short and the flow rates are sufficiently high so that there is essentially no depletion. Thus, the gas space in the anode and cathode compartments behaves as Heterogeneous reforming in the anode.-In this example the anode fuel stream is a mixture of 66% H 2 , 22% CO, and 12% CH 4 . This mixture is essentially the gas-phase chemical equilibrium composition that results from an initial mixture of 60% CH 4 and 40% H 2 O at 800°C, which might be the result of some upstream reforming process.…”

“…The CO also reacts with steam, shifting toward more H 2 and CO 2 . The reforming and shifting behavior persists within the anode until the CH 4 and CO are largely consumed by about 3 cm along the channel.…”

“…[49][50][51][52] Specifically, the UBI-QEP method is used to evaluate kinetic parameters for the desorption of O 2 , CH 4 49 and nonoxidative methane decomposition and formation ͑reactions 27-34͒. 51 Steam and CO 2 formation rates ͑reactions 13-18, 21, and 22͒, HCO formation ͑re-actions 25 and 26͒, and oxidative decomposition and formation of CH 4 ͑reactions 35-42͒ are derived from a theoretical study of dry methane reforming.…”

“…54 Owing to a lack of data on Ni surfaces, sticking coefficients for oxygen and steam ͑reactions 3 and 7͒ are estimated from studies of methane partial oxidation on rhodium. 55 Sticking coefficients for CH 4 and CO 2 adsorption ͑reac-tions 5 and 9͒ are derived from experimental studies of methane reforming and oxidation over Ni-coated monoliths. 56 The data for CO adsorption/desorption ͑reactions 11 and 12͒ are taken from AlSarraf et al 57 Elementary reaction mechanisms can be applied more generally than global mechanisms that may be validated only for specific geometric configurations and operating conditions.…”

“…Experimental conditions consider partial oxidation and steam reforming of CH 4 , including water-gas-shift and methanation processes. 56 The CH 4 /O 2 ratio ranged from 1.5 to 2 and steam was incorporated up to a steam/carbon ratio of 4.…”