Modeling Validation and Simulation of an Anode Supported SOFC Including Mass and Heat Transport, Fluid Flow and Chemical Reactions

Abstract

Fuel cells are electrochemical devices that directly transform chemical energy into electricity, which are promising for future energy systems, since they are energy efficient and, when hydrogen is used as fuel, there are no direct emissions of greenhouse gases. The cell performance depends strongly on the material characteristics, the operating conditions and the chemical reactions that occur inside the cell. The chemical-and electrochemical reaction rates depend on temperature, material structure, catalytic activity, degradation and the partial pressures for the different species components. There is a lack of information, within the open literature, concerning the fundamentals behind these reactions. Experimental as well as modeling studies are needed to reduce this gap.In this study experimental data collected from an intermediate temperature standard SOFC with H 2 /H 2 O in the fuel stream are used to validate a previously developed computational fluid dynamics model based on the finite element method. The developed model is based on the governing equations of heat and mass transport and fluid flow, which are solved together with kinetic expressions for internal reforming reactions of hydrocarbon fuels and electrochemistry. This model is further updated to describe the experimental environment concerning cell design. Discussion on available active area for electrochemical reactions and average ionic transport distance from the anodic-to the cathodic three-phase boundary (TPB) are presented. The fuel inlet mole fractions are changed for the validated model to simulate a H 2 /H 2 O mixture and 30 % pre-reformed natural gas. Keywords: SOFC, Modeling, Validation, Active Area, IonicTransport Distance, COMSOL Multiphysics INTRODUCTION AND PROBLEM STATEMENTFuel cells (FCs) are promising due to advantages of higher efficiency and lower emissions of SO X , NO X and CO 2 than conventional power generation [1]. The solid oxide fuel cell (SOFC) is a high temperature fuel cell, which operates at 600-1000 ºC [2]. This temperature allows SOFCs to operate with different types of fuels from both fossil and renewable sources. It opens a way for an easier transition from conventional power generation with hydrocarbon based fuels to fuel cells with possibility for different fuels, especially SOFCs. Due to the increasing global awareness of that energy usage affects the environments, the interest of renewable energy has increased. SOFCs are generally more tolerant to contaminants than other fuel cells and the possibility to internally (as well as externally) reform the fuel make them interesting for renewable energy resources [1].Numerical results of SOFC models are only approximations of real conditions and the validation is an important and necessary step in the development of reliable and accurate computational models. A range of validity can be established between the developed model and experimental

SOFC modeling considering electrochemical reactions at the active three phase boundaries

“…These findings are different compared to our previous predictions [3,6,8], where the electrochemical reactions are defined at the electrode/electrolyte interfaces only and the cathodic activation polarization dominates over the anodic one. This difference might be particular due to a larger active TPB AV e applied on the cathodic side 14 The activation polarizations along the flow direction (x-direction) within the anode and the cathode for the standard case are shown in Figure 6.…”

“…This is an extension and improvement compared to our previous models [3,8,25], where the ion transport in the YSZ within the anode and the cathode is neglected. The expressions for ionic and electronic conductivity are introduced in section 2.4.…”

“…This difference might be particular due to a larger active TPB AV e applied on the cathodic side 14 The activation polarizations along the flow direction (x-direction) within the anode and the cathode for the standard case are shown in Figure 6. There can be compared with the ones predicted by our previous model [8] in Figure 7, where the electrochemical reactions as well as the activation and concentration polarization are defined as interface conditions at the electrode/electrolyte interfaces. For both models, the cell voltage is defined to 0.7 V and a similar trend is found, i.e., the total polarization decreases along the flow direction (as the temperature increases) and the activation polarizations dominate, compared to the ohmic and the concentration polarizations.…”

“…Note that the ionic conductivity in the electrodes was not included in our previous work [3,6,8,25]. To the authors knowledge this is the first cell scale SOFC model, which considers the microstructure effects by including the tortuosity factors and material volume fractions for the ion and electron transport within the anode and the cathode.…”

“…The advances compared to our previous work [3,6,8] include the addition of the governing equations for the ion transport (in YSZ) and the electron transport (in Ni or LSM), i.e., the current density distributions are calculated from local conditions (2D) throughout the cell, instead of just defining an average value (0D) [3,25] or with a 1D approach at the electrode/electrolyte interface [7]. Further advances are made for the electrochemical reactions, which are defined as source terms instead of interface conditions (as in [3,7,25]) concerning mass, heat, momentum, ion and electron governing transport equations.…”