Total polarization effect on the location of maximum temperature value in planar SOFC

The aim of the present study is the evaluation and the location of the maximum temperature values within the solid and porous components of a planar SOFC under the effect of total polarization: Ohmic, activation, concentration and the chemical reaction.The temperature field in SOFC components (interconnection, cathode, anode and electrolyte) is obtained by developing a mathematical model in FORTRAN language.The mathematical model predictions show the effect of the overpotentials on the thermal gradient and its locations in an SOFC with two geometries: i) anode or ii) electrolyte supported. The results are also discussed, following the SOFC low or high operating temperatures.     

Modeling of SOFC running on partially pre-reformed gas mixture

A two dimensional model is developed to study the transport and reaction processes in solid oxide fuel cells (SOFCs) fueled by partially pre-reformed gas mixture, considering the direct internal reforming (DIR) of methane and water gas shift (WGS) reaction in the porous anode of SOFC. Electrochemical oxidations of H₂ and CO fuels are both considered. The model consists of an electrochemical, a chemical model, and a computational fluid dynamics (CFD) model. Two chemical models are compared to examine their effects on SOFC modeling results. Different from the previous studies on hydrogen fueled SOFC, higher gas velocity is found to slightly decrease the performance of SOFC running on pre-reformed gas mixture, due to suppressed gas composition variation at a higher gas velocity. The current density distribution along the gas channels at an inlet temperature of 1173K is quite different from that at 1073K, as DIR reaction is facilitated at a higher temperature. It is also found that neglecting the electrochemical oxidation of CO can considerably underestimate the total current density of SOFC running on pre-reformed hydrocarbon fuels. An alternative method is proposed to numerically determine the open-circuit potential of SOFC running on hydrocarbon fuels. Electrochemical reactions are observed at open-circuit potentials.     

A CFD-based model of a planar SOFC for anode flow field design

This study presents a 3D CFD model of a planar SOFC with internal reforming for anode flow field design. The developed model reflects the influence of various factors on fuel cell performance including flow field design and kinetics of chemical and electrochemical reactions. The case study illustrates applications of the CFD model for planar SOFC with different anode flow field designs. Simulation results indicate the importance of the anode flow field design for planar SOFCs. The model is useful for optimization of fuel cell design and operating conditions.     

Analysis of reformate syngas mixture fed solid oxide fuel cell through experimental and 0-D thermodynamic modeling studies

In this study, the performance assessment of a solid oxide fuel cell (SOFC) fed with a reformate syngas mixture and having anode off-gas recirculation is done in terms of energy and exergy analyses. In this regard, a zero-dimensional (0-D) mathematical model for SOFCs is developed. This model is validated by the results of the in-house experimental studies. In addition, parametric studies are carried out to assess the effect of operating parameters on fuel cell performance. The results show that the proposed model is very agreeable with experimental studies. The maximum error found in the validated model is 6.8% at the operating temperature of 800 °C. In addition, it is shown that the anode off-gas recirculation ratio does not have a significant effect on the performance of the SOFC at low current densities. Furthermore, the exergy destruction rate of SOFC increases by 23.2% under the high current density condition (i = 1.4 A/cm²) when the fuel utilization ratio increases from 0.75 to 0.95.     

Numerical modelling of a cathode-supported solid oxide fuel cell (SOFC) in comparison with an electrolyte-supported model

Solid oxide fuel cell (SOFC) is a high-efficiency power generation system that is a large fuel option and converts chemical energy directly into electrical energy. In this study, the parameters affecting the performance of a cathode and an electrolyte supported SOFC are numerically investigated by developing a finite element method (FEM) based SOFC model involving conservation of mass, momentum, species and energy with ionic and electronic charge transfer. After the validation of the mathematical model, effects of support thickness, oxidant type, operating temperature and pressure on the performance of the cells are examined by using a FEM code Comsol. These effects of parameters are shown by the polarization and power curves. As a result of the study, it can be concluded that the performance of the SOFC increases with increasing the operating temperature and pressure values. Compared with the electrolyte-supported, the cathode-supported SOFC is shown to exhibit higher performance in terms of support thickness and operating parameters. The effect of support thickness is seen that the cell power is inversely proportional to the support layer thickness. The improved performance thus the higher hydrogen consumption can be attributed to the lack of N₂ and higher reaction rate when the pure oxygen is used as oxidant. In general it can be said that the cathode-supported SOFC always outperforms the electrolyte-supported SOFC.     

Thermal field in SOFC fed by hydrogen: Inlet gases temperature effect

In the present work, the effect of the hydrogen and the air temperature values on the temperature distribution in a Planar Solid Oxide Fuel Cell is studied by the aid of a two-dimensional mathematical model. Two different configurations of the Solid Oxide Fuel Cells are examined: i) the Anode Supported Planar Solid Oxide Fuel Cell (ASP_SOFC) and ii) the Electrolyte Supported Planar Solid Oxide Fuel Cell (ESP_SOFC). In order to describe the temperature distribution within the SOFC, the coupling of the mass and energy transport phenomena along with the electrochemistry is required. The studied parameters are: a) the hydrogen and the air temperature values and b) the geometry configurations. The complex system of the governing equations is numerically solved with the finite differences method and the calculation of the temperature distribution within each domain of the SOFCs is calculated via the 2-D mathematical model processed by FORTRAN language. Finally, the mathematical model predictions for the temperature distribution under the influence of the studied parameters are thoroughly discussed.     

Mini CHP based on the electrochemical generator and impeded fluidized bed reactor for methane steam reforming

The paper presents a configuration of mini CHP with the methane reformer and planar solid oxide fuel cell (SOFC) stacks. This mini CHP may produce electricity and superheated steam as well as preheat air and methane for the reformer along with cathode air used in the SOFC stack as an oxidant. Moreover, the mathematical model for this power plant has been created. The thermochemical reactor with impeded fluidized bed for autothermal steam reforming of methane (reformer) considered as the basis for the synthesis gas (syngas) production to fuel SOFC stacks has been studied experimentally as well. A fraction of conversion products has been oxidized by the air fed to the upper region of the impeded fluidized bed in order to carry out the endothermic methane steam reforming in a 1:3 ratio as well as to preheat products of these reactions. Studies have shown that syngas containing 55% of hydrogen could be produced by this reactor. Basic dimensions of the reactor as well as flow rates of air, water and methane for the conversion of methane have been adjusted through mathematical modelling.The paper provides heat balances for the reformer, SOFC stack and waste heat boiler (WHB) intended for generating superheated water steam along with preheating air and methane for the reformer as well as the preheated cathode air. The balances have formed the basis for calculating the following values: the useful product fraction in the reformer; fraction of hydrogen oxidized at SOFC anode; gross electric efficiency; anode temperature; exothermic effect of syngas hydrogen oxidation by air oxygen; excess entropy along with the Gibbs free energy change at standard conditions; electromotive force (EMF) of the fuel cell; specific flow rate of the equivalent fuel for producing electric and heat energy. Calculations have shown that the temperature of hydrogen oxidation products at SOFC anode is 850 °C; gross electric efficiency is 61.0%; EMF of one fuel cell is 0.985 V; fraction of hydrogen oxidized at SOFC anode is 64.6%; specific flow rate of the equivalent fuel for producing electric energy is 0.16 kg of eq.f./(kW·h) while that for heat generation amounts to 44.7 kg of eq.f./(GJ). All specific parameters are in agreement with the results of other studies.     

A numerical investigation of modeling an SOFC electrode as two finite layers

Modeling an electrode in solid oxide fuel cells (SOFCs) as two finite layers is numerically investigated. A simulation is conducted using the developed mathematical model, wherein an SOFC electrode is considered as a porous composite structure of electron- and ion-conducting particles. Moreover, an electrochemical reaction is considered to occur throughout the electrode. In other words, an electrode is treated as a reaction zone layer having triple phase boundaries (TPBs) scattered throughout the electrode, consistent with the micro modeling approach of treating electrodes. The model takes into account the transport of multi-component mixture in an electrode together with electrochemical reaction, the transport of electrons and ions in the respective electron-conducting and ion-conducting particles of the electrode. It is found that both the dimensionless electronic and ionic current densities remain constant with respective values of one and zero for most part of the anode before started to vary towards the end of the anode. Further, from the distributions of dimensionless electronic and ionic current densities in the anode, it can be deduced that the part of the anode (electrode) where the value of dimensionless electronic current density is one, can be considered as an electron-conducting (ion insulator) layer, referred to as the anode (electrode) backing layer; the part of the anode (electrode), where there is a variation in the electronic and ionic current densities and electrochemical reaction rate is most effective, can be considered as a mixed-conducting layer, referred to as the reaction zone layer. Finally, a parametric study is conducted to investigate the effect of key operating and design conditions on the thickness of the reaction zone layer in an SOFC anode.     

Study of species, temperature distributions and the solid oxide fuel cells performance in a 2-D model

A two-dimensional mathematical model for a planar SOFC (solid oxide fuel cell) is constructed. The distribution of the chemical species, the temperature and the performance (power) and the current density were calculated using a single-unit model with double channels of co-flow pattern. The finite volume method was employed for the calculation. The method was based on the fundamental conservation laws of continuity, momentum, energy and mass. The equations are implemented in FORTRAN language. The effects of several heat sources and flow rates on the calculated results were also investigated. The reference SOFC polarization curve has been calculated by imposing a uniform temperature of 800 K, a pressure equal to 1 atm; H₂ and O₂ molar fractions equal to 0.97 and 0.21 respectively. Results of temperature, chemical species distributions, performance and efficiency under several heat sources are shown and discussed. At a current density of about 23500 A/m², the power densities under all sources and chemical sources reached their maximums of 12965 W/m² and 16209 W/m² (i.e. 25% lower) respectively. However the temperature increment in the anode is analyzed toward all sources and chemical reaction. The temperature maximum values for each heat source type reached 1005.81 K and 984.69 K respectively.     

Three‐dimensional nonisothermal modeling of solid oxide fuel cell coupling electrochemical kinetics and species transport

A three‐dimensional (3D) nonisothermal model is developed and applied for anode‐supported planar solid oxide fuel cell (SOFC). The mass and momentum, species, ion, electric, and heat transport equations are solved simultaneously by implementing the electrochemical kinetics and electrochemical reaction as volumetric source terms. The interconnect land limits the O₂ transport under the land and lowers the local current density under the land. The effects of interconnect land width and cathode substrate thickness on SOFC cell performance are quantified in this study. Cathode stoichiometry is found to have a large effect on the SOFC cell temperature distribution. Under low‐cathode stoichiometry, significant temperature gradients are seen in the SOFC cell. Higher‐cathode stoichiometry is beneficial for lower temperature and more uniform current density distribution in SOFC cell. Co‐flow and counter‐flow arrangements are investigated and discussed with the model. Counter‐flow arrangement is found to induce a high temperature and high current density region near the H₂ inlet. On the other hand, co‐flow arrangement leads high temperature and high current density to occur relatively downstream, a slightly lower maximum temperature on cell and considerably more uniform current density distribution. A 67.2‐cm² SOFC cell is simulated considering the side cooling effect. The side cooling effectively lowers the cell temperature, at the same time, causes temperature, current density, and fuel utilization nonuniformity in the across multichannel direction. Because of the strong coupling of the in‐plane current density distribution and temperature distribution, limiting the locally high temperature and temperature gradient is critical for achieving a more uniform current density distribution in anode‐supported planar SOFC.     

Micro modeling of solid oxide fuel cell anode based on stochastic reconstruction

A novel modeling scheme of SOFC anode based on the stochastic reconstruction technique and the Lattice Boltzmann Method (LBM) is proposed and applied to the performance assessment and also to the optimization of anode microstructures. A cross-sectional microscopy image is exploited to obtain a two-dimensional phase map (i.e., Ni, YSZ and pore), of which two-point correlation functions are used to reconstruct a three-dimensional model microstructure. Then, the polarization resistance of the reconstructed anode is obtained by the LBM simulation. The predicted anodic polarization resistance for a given microstructure and its sintering temperature dependence are in good agreement with the literature data. Three-dimensional distributions of potential and current can be obtained, while and the effect of working temperature is discussed. The proposed method is considered as a promising tool for designing SOFC anodes.     

Numerical investigation of the chemical and electrochemical characteristics of planar solid oxide fuel cell with direct internal reforming

A fully three-dimensional mathematical model of a planar solid oxide fuel cell (SOFC) with complete direct internal steam reforming was constructed to investigate the chemical and electrochemical characteristics of the porous-electrode-supported (PES)-SOFC developed by the Central Research Institute of Electric Power Industry of Japan. The effective kinetic models developed over the Ni/YSZ anode takes into account the heat transfer and species diffusion limitations in this porous anode. The models were used to simulate the methane steam reforming processes at the co- and counter-flow patterns. The results show that the flow patterns of gas and air have certain effects on cell performance. The cell at the counter-flow has a higher output voltage and output power density at the same operating conditions. At the counter-flow, however, a high hotspot temperature is observed in the anode with a non-fixed position, even when the air inlet flow rate is increased. This is disadvantageous to the cell. Both cell voltage and power density decrease with increased air flow rate.     

Modeling of a planar solid oxide fuel cell based on proton‐conducting electrolyte

A 2D computational fluid dynamics (CFD) model is developed to study the performance of an advanced planar solid oxide fuel cell based on proton conducting electrolyte (SOFC‐H). The governing equations are solved with the finite volume method (FVM). Simulations are conducted to understand the transport phenomena and electrochemical reaction involved in SOFC‐H operation as well as the effects of operating/structural parameters on SOFC‐H performance. In an SOFC based on oxygen ion conducting electrolyte (SOFC‐O), mass is transferred from the cathode side to the anode side. While in an SOFC‐H, mass is transferred from the anode to the cathode, which causes different velocity fields of the fuel and oxidant gas channels and influences the distributions of temperature and gas composition in the cell. It is also found that increasing the inlet gas velocity leads to an increase in the local current density and a slight decrease in the SOFC‐H temperature due to stronger cooling effect of the gas species at a higher velocity. Another finding is that the electrode structure does not significantly affect the heat and mass transfer in an SOFC‐H at typical operating voltages. Copyright © 2009 John Wiley & Sons, Ltd.     

Effects of operating and design parameters on the performance of a solid oxide fuel cell–gas turbine system

We present a steady‐state thermodynamic model of a hybrid solid oxide fuel cell (SOFC)–gas turbine (GT) cycle developed using a commercial process simulation software, AspenPlus?. The hybrid cycle model incorporates a zero‐dimensional macro‐level SOFC model. A parametric study was carried out using the developed model to study the effects of system pressure, SOFC operating temperature, turbine inlet temperature, steam‐to‐carbon ratio, SOFC fuel utilization factor, and GT isentropic efficiency on the specific work output and efficiency of a generic hybrid cycle with and without anode recirculation. The results show that system pressure and SOFC operating temperature increase the cycle efficiency regardless of the presence of anode recirculation. On the other hand, the specific work decreases with operating temperature. Overall, the model can successfully capture the complex performance trends observed in hybrid cycles. Copyright © 2010 John Wiley & Sons, Ltd.     

Performance of tubular Solid Oxide Fuel Cell at reduced temperature and cathode porosity

The effect of decreasing the inlet temperature and the cathode porosity of tubular Solid Oxide Fuel Cell (SOFC) with one air channel and one fuel channel is investigated using Computational Fluid Dynamics (CFD) approach. The CFD model was developed using Fluent SOFC to simulate the electrochemical effects. The cathode and the anode of the cell were resolved in the model and the convection and conduction heat transfer modes were included. The results of the CFD model are presented at inlet temperatures of 700 °C, 600 °C and 500 °C and with cathode porosity of 30%, 20% and 10%. It was found that the Fluent-based SOFC model is an effective tool for analyzing the complex and highly interactive three-dimensional electrical, thermal, and fluid flow fields associated with the SOFCs. It is found that the SOFC can operate in the intermediate temperature range and with low porosity cathodes more efficient than at high temperatures given that the transport properties of the cathode, anode and the electrolyte can be kept the same.     

Performance of an anode-supported solid oxide fuel cell with direct-internal reforming of ethanol

A theoretical study of a solid oxide fuel cell (SOFC) fed by ethanol is presented in this study. The previous studies mostly investigated the performance of ethanol-fuelled fuel cells based on a thermodynamic analysis and neglected the presence of actual losses encountered in a real SOFC operation. Therefore, the real performance of an anode-supported SOFC with direct-internal reforming operation is investigated here using a one-dimensional isothermal model coupled with a detailed electrochemical model for computing ohmic, activation, and concentration overpotentials. Effects of design and operating parameters, i.e., anode thickness, temperature, pressure, and degree of ethanol pre-reforming, on fuel cell performance are analyzed. The simulation results show that when SOFC is operated at the standard conditions (V = 0.65 V, T = 1023 K, and P = 1 atm), the average power density of 0.51 W cm⁻² is obtained and the activation overpotentials represent a major loss in the fuel cell, followed by the ohmic and concentration losses. An increase in the thickness of anode decreases fuel cell efficiency due to increased anode concentration overpotential. The performance of the anode-supported SOFC fuelled by ethanol can be improved by either increasing temperature, pressure, degree of pre-reforming of ethanol, and steam to ethanol molar ratio or decreasing the anode thickness and fuel flow rate at inlet. It is suggested that the anode thickness and operating conditions should be carefully determined to optimize fuel cell efficiency and fuel utilization.     

Temperature field,H2 and H2O mass transfer in SOFC single cell: Electrode and electrolyte thickness effects

The temperature increment in electrodes and electrolyte of a fuel cell is mainly attributed to the chemical reaction and the irreversibilities. The aim of this work is to study the increasing temperature of a SOFC single cell under the influence of the electrode and electrolyte thicknesses for its type of heat source. The hydrogen and water field are also discussed according to anode thickness.     

Performance degradation of solid oxide fuel cells due to sulfur poisoning of the electrochemical reaction and internal reforming reaction

Hydrogen sulfide is known to degrade the solid oxide fuel cell (SOFC) performance by adsorbing on the nickel anode catalyst. In this research, the mechanism underlying such SOFC degradation was evaluated based on a theoretical mathematical modeling approach and the sulfur coverage was calculated from a Temkin-like isotherm which is related to both temperature and hydrogen sulfide (H₂S) concentration. The influences of the cell temperature, H₂S concentration and electrochemical performance on both the sulfur coverage and cell polarization are studied in detail. Two specific models were considered to identify whether sulfur poisoning has a larger impact on cell performance through its effect on the electrochemical reaction or on the internal reforming reaction. It was found that sulfur poisoning has different effects on the hydrogen oxidation reaction and internal reforming reaction, leading to competing changes in cell performance with temperature and H₂S concentration.     

Thermodynamic analysis and experimental study of electrode reactions and open circuit voltages for methane-fuelled SOFC

Natural gas is one of the most important fuels for solid oxide fuel cell (SOFC). The relationships among the reactions of methane over the nickel-based anode, fuel compositions, carbon deposition, electromotive force (EMF) and open circuit voltage (OCV) of SOFC are investigated in this work. With the increase of temperature, EMF and OCV of SOFC decrease gradually when the cell uses humidified hydrogen as fuel. Reactivity of methane increases gradually with the increase of temperature, which can affect the EMF and OCV of SOFC. When the humidified mixture of nitrogen and methane is used as the fuel, the EMF and OCV of SOFC increase gradually with the increase of temperature. EMF and OCV of SOFC with humidified mixture of hydrogen and methane (M_CH4: M_H2: M_H2O = 12.2: 85.3: 2.5) as fuel decrease gradually with the increase of temperature when the temperature is lower than 873 K, which is similar to that with humidified hydrogen as fuel. While when the temperature is higher than 923 K, the EMF and OCV of SOFC with humidified mixture of hydrogen and methane as fuel increase gradually with the increase of temperature, which is similar to that with humidified mixture of nitrogen and methane as fuel. OCV of SOFC is mainly affected by thermodynamic equilibriums for methane-fuelled SOFC when the anode activity is high enough, which is close to the EMF calculated according to the thermodynamic equilibriums. While with the increase of carbon deposition, the anode activity decreases apparently and the OCV of SOFC also decreases apparently, which shows that the OCV is affected by the anode activity for methane-fuelled SOFC when the anode activity is low.     

Optimization of single SOFC structural design for maximum power

This paper improves previously published models by the authors for a single solid oxide fuel cell (SOFC), and introduces a procedure to optimize its external configuration and operating conditions, so that the net power is maximized. The previous models are hereby improved to include: i) a constant offset overpotential in total potential drop; ii) heat generation associated with all the potential losses; iii) temperature-dependent thermo-physical properties of fuel and air, and iv) pumping power to maximize fuel cell performance. The thermodynamic model is derived from physical laws (e.g., the first law of thermodynamics, Fick's law, Fourier's law) to obtain the temperature and pressure spatial distribution in the SOFC. The electrochemical model is validated by direct comparison with experimental data from the Pacific Northwest National Laboratory (PNNL), and allows for the computation of the SOFC voltage, current, and power output. Based on the simulation results, the structural design, the active three phase boundaries regions at the electrodes and the fuel utilization factor, and their impact on the SOFC performance are discussed. Subjected to fixed total volume, the optimal geometric and operating parameters are pursued so that the net power of the SOFC is maximized through a 4-way-optimization procedure. The method used is general and the numerically obtained maxima are sharp, taking into account that up to a 631% single SOFC performance variation was observed within the studied parameters' range. The fixed volume constraint was then relaxed, and the effect of total volume variation on performance was investigated, delivering the general optimal parameters for the 4-way maximized SOFC net power output within the studied total dimensionless fuel cell volume range. These findings show the potential to use the model as a tool for future SOFC design, simulation and optimization.