Anode-supported micro tubular SOFCs for advanced ceramic reactor system

In this study, micro tubular SOFCs under 1 mm diameter have been fabricated and investigated at 450–550 °C operating temperature with H₂ fuel. The performance of the 0.8 mm diameter tubular SOFC was 110–350 mW cm⁻² at 450–550 °C operating temperatures. To maximize the performance of the cell as well as to optimize the geometry of tubular cells, a current collecting method used in the experiment was examined. A model was proposed to estimate the loss of performance for single cell due to the current collecting method as functions of anode tube length and thickness. The results showed that the losses of performance were calculated to be 0.8, 2.0, and 4.6% at 450, 500, and 550 °C operating temperatures, respectively, for the 0.8 mm diameter tubular SOFC with the length of 1.2 cm.     

Comparison of different current collecting modes of anode supported micro-tubular SOFC through mathematical modeling

A two-dimensional model comprising fuel channel, anode, cathode and electrolyte layers for anode-supported micro-tubular solid oxide fuel cell (SOFC), in which momentum, mass and charge transport are considered, has been developed. By using the model, tubular cells operating under three different modes of current collection, including inlet current collector (IC), outlet current collector (OC) and both inlet and outlet collector (BC), are proposed and simulated. The transport phenomena inside the cell, including gas flow behavior, species concentration, overpotential, current density and current path, are analyzed and discussed. The results depict that the model can well simulate the diagonal current path in the anode. The current collecting efficiency as a function of tube length is obtained. Among the three proposed modes, the BC mode is the most effective mode for a micro-tubular SOFC, and the IC mode generates the largest current density variation at z-direction.     

Mechanistic modelling of a cathode-supported tubular solid oxide fuel cell

A two-dimensional mechanistic model of a tubular solid oxide fuel cell (SOFC) considering momentum, energy, mass and charge transport is developed. The model geometry of a single cell comprises an air-preheating tube, air channel, fuel channel, anode, cathode and electrolyte layers. The heat radiation between cell and air-preheating tube is also incorporated into the model. This allows the model to predict heat transfer between the cell and air-preheating tube accurately. The model is validated and shows good agreement with literature data. It is anticipated that this model can be used to help develop efficient fuel cell designs and set operating variables under practical conditions. The transport phenomena inside the cell, including gas flow behaviour, temperature, overpotential, current density and species concentration, are analysed and discussed in detail. Fuel and air velocities are found to vary along flow passages depending on the local temperature and species concentrations. This model demonstrates the importance of incorporating heat radiation into a tubular SOFC model. Furthermore, the model shows that the overall cell performance is limited by O₂ diffusion through the thick porous cathode and points to the development of new cathode materials and designs being important avenues to enhance cell performance.     

Numerical modelling and comparative analysis of direct ammonia fuelled protonic and oxygen- ion conducting tubular solid oxide fuel cell

The main emphasis of this work is developing a 3D numerical model and investigating the performance characteristic of a direct ammonia fuelled protonic-conducting tubular solid oxide fuel cell (NH₃-T–SOFC–H) in comparison with the corresponding hydrogen-fuelled one and direct ammonia feed oxygen -ion conducting tubular solid oxide fuel cell (NH₃-T–SOFC–O) under the same operating parameters and geometrical shape. The findings revealed that NH₃-T–SOFC–H has outstanding performance over T–SOFC–O counterparts at intermediate temperature (973 K) when operated under similar working conditions and geometrical designs. On the other hand the NH₃-T–SOFC–O is promising for higher operating temperatures. The outcomes of the study are also confirmed that the power performance of NH₃-AS-T–SOFC–O is better than the other supports of both electrolytes when the anode electrode is constructed at the outside portion of the tubular cell. Yet, the other remarkable result found in this study is that NH₃– CS- T–SOFC–O has outstanding performance compared to all supports of both electrolytes when the fuel electrode is built in the inner portion of the tube. In addition, the finding indicates that the power performance of ammonia-fuelled tubular cells is strongly influenced by the anode position, operating temperatures, and pressures in both electrolytes yet the effect of cell temperature is more influential in the protonic-conducting cell. It is also observed that the performance of ES-T-SOFC is lower than AS- and CS-T-SOFC in both electrolytes and anode positions.     

Influence of fuel flow rate on the performance of micro tubular solid oxide fuel cell

Through mathematical analysis, the performance of micro tubular solid oxide fuel cell (SOFC) is evaluated successfully, which is in good agreement with the experimental data under different fuel flow. The results show that under the condition of high fuel utilization, a part of current path passing through the anode can increase by 16% compared with that under the condition of low fuel utilization, which can reduce the output performance of the cell. In the process of increasing fuel flow, the current gradually increases and reaches the platform value. When the length of cathode is long and the inner diameter of anode is small, the loss can be effectively reduced by changing the thickness of anode tube.     

A mathematical model of a tubular solid oxide fuel cell with specified combustion zone

A numerical model has been developed to simulate the effect of combustion zone geometry on the steady state and transient performance of a tubular solid oxide fuel cell (SOFC). The model consists of an electrochemical submodel and a thermal submodel. In the electrochemical model, a network circuit of a tubular SOFC was adopted to model the dynamics of Nernst potential, ohmic polarization, activation polarization, and concentration polarization. The thermal submodel simulated heat transfers by conduction, convention, and radiation between the cell and the air feed tube. The developed model was applied to simulate the performance of a tubular solid oxide fuel cell at various operating parameters, including distributions of circuits, temperature, and gas concentrations inside the fuel cell. The simulations predicted that increasing the length of the combustion zone would lead to an increase of the overall cell tube temperature and a shorter response time for transient performance. Enlarging the combustion zone, however, makes only a negligible contribution to electricity output properties, such as output voltage and power. These numerical results show that the developed model can reasonably simulate the performance properties of a tubular SOFC and is applicable to cell stack design.     

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.     

Numerical modelling of methane-powered micro-tubular, single-chamber solid oxide fuel cell

An experimentally validated, two-dimensional, axisymmetric, numerical model of micro-tubular, single-chamber solid oxide fuel cell (MT-SC-SOFC) has been developed. The model incorporates methane full combustion, steam reforming, dry reforming and water-gas shift reaction followed by electrochemical oxidation of produced hydrogen within the anode. On the cathode side, parasitic combustion of methane along with the electrochemical oxygen reduction is implemented. The results show that the poor performance of single-chamber SOFC as compared to the conventional (dual-chamber) SOFC (in case of micro-tubes) is due to the mass transport limitation on the anode side. The gas velocity inside the micro-tube is far too low when compared to the gas-chamber inlet velocity. The electronic current density is also non-uniform over the cell length, mainly due to the short length of the anode current collector located at the cell outlet. Furthermore, the higher temperature near the cell edges is due to the methane combustion (very close to the cell inlet) and current collection point (at the cell outlet). Both of these locations could be sensitive to the silver current collecting wire as silver may rupture due to cell overheating.     

Transport, sensitivity, and dimensional optimization studies of a tubular Solid Oxide Fuel Cell

In this paper, the impact of the design and operating parameters of a tubular Solid Oxide Fuel Cell (SOFC) is studied using a well-validated steady-state model. The profiles of species concentrations and pressure in the flow channels when the cell terminal voltage is changed are studied. In addition, both the axial and radial profiles of the species concentrations inside the electrodes are presented. The model is also used to study the effects of the parameters that can significantly influence the design criteria of an anode-supported tubular SOFC. The effects of the flowrate of H₂, inlet pressure, and the cell temperature on the power output from the cell are studied. Cell characteristic parameters such as the porosity of the electrodes, effective diffusivity of the species, and the rate of the electrochemical reactions are varied and their impact on the cell performance is observed. The influence of the cell design parameters such as the thickness of the electrodes and the electrolyte on the steady-state polarization curve are also studied. Finally, a dimensional study is presented. In the dimensional study, the radius of the anode flow channel, the length of the cell, and the annulus size are varied keeping the solid volume and the total cell volume constant. This study shows that it is possible to design guidelines for the optimum performance of the cell.     

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.     

Electrochemical study of a planar solid oxide fuel cell: Role of support structures

This paper presents a performance analysis of a planar solid oxide fuel cell (SOFC) with different support structures, i.e., electrode (anode and cathode) and electrolyte supports. An electrochemical model, taking into account structural and operational parameters and gas diffusion at the electrodes, is used to analyze the characteristics of the planar SOFC. Simulation results demonstrate that under cell operation at an intermediate temperature (1073 K), an anode-supported SOFC is superior to an electrolyte- and cathode-supported SOFC. Analysis of individual cell voltage loss indicates that ohmic loss dominates the performance of an electrolyte-supported SOFC whereas activation and ohmic overpotentials constitute the major loss in an electrode-supported counterpart. Sensitivity analyses of the anode-supported SOFC show that decreasing the electrolyte and anode thickness can improve cell performance. A decrease in operating temperature causes the cell to operate at a lower range of current density due to an increase in ohmic and activation overpotentials. Further, increasing the operating pressure and degree of pre-reforming reduces the concentration overpotential and thereby enhances cell performance.     

Thermochemical model and experimental validation of a tubular SOFC cell comprised in a 1 kWel stack designed for μCHP applications

Being aware of the needs for clean highly efficient micro combined heat and power (μCHP) systems for single and multifamily households, the Italian Ministry of Industry launched in 2009 the EFESO Project aiming to develop and operate four SOFC prototypes. An imperative part of the project foresaw computational modeling to optimize operating conditions of the power modules and pinpoint potential drawbacks in its design. This article deals with a 3-dimensional thermochemical model of a single SOFC tubular geometry cell comprised in a 1kW_el stack operating under similar conditions to the characterized power module. An analysis is presented on the effects of current density distribution, temperature distribution in the cell and pressure drop in the air and fuel channels, being these the most critical variables when operating the SOFC-powered μCHP system. This model will serve as a platform to generate a model of the whole stack which will be further validated by means of experimental activities.     

Electrochemical model for performance analysis of a tubular SOFC

Tubular solid oxide fuel cells (SOFC) are promising candidates for future energy conversion systems and expected to be applied widely for small‐scale distributed generation to large‐scale central station power plants because of their high electrical efficiency and high temperature exhaust gas utilization. This study presents an electrochemical model to determine the performance characteristics of tubular solid oxide fuel cell. Activation, ohmic and concentration polarizations are regarded as the major sources of irreversibility. The Butler–Volmer equation, Fick's law and Ohm's law are used to determine the polarization terms. Performance curves are simulated for single cell voltage and power under variable current density and validated with published experimental data for given operating conditions. All the variations of tubular SOFC's operational conditions such as operating pressure and temperature in the electrochemical processes is taken into consideration. The contribution of each polarization term to voltage losses is analysed with local characteristics such as pore size, electrolyte thickness and activation energy for evaluating the relative changes. Cell performance represented by cell voltage, power, efficiency and heat generation are analysed at its complete operating range, aiming at finding the set of optimal operating conditions maximizing the overall cell performance. As a conclusion from this study, the developed model is a simple and effective tool to analyse tubular SOFC in obtaining insight information about cell performance characteristics under different conditions. Copyright © 2006 John Wiley & Sons, Ltd.     

Numerical study of thermoelectric characteristics of a planar solid oxide fuel cell with direct internal reforming of methane

A three-dimensional mathematical thermo-fluid model coupling the electrochemical kinetics with fluid dynamics was developed to simulate the heat and mass transfer in planar anode-supported solid oxide fuel cell (SOFC). The internal reforming reactions and electrochemical reactions of carbon monoxide and hydrogen in the porous anode layer were analyzed. The temperature, species mole fraction, current density, overpotential loss and other performance parameters of the single cell unit were obtained by a commercial CFD code (Fluent) and external sub-routine. Results show that the current density produced by electrochemical reactions of carbon monoxide cannot be ignored, the cathode overpotential loss is the biggest one among the three overpotential losses, and that the proper decrease of the operating voltage leads to the increase of the current density, PEN structure temperature, fuel utilization factor, fuel efficiency and power output of the SOFC.     

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.     

Fabrication of micro-tubular solid oxide fuel cells with a single-grain-thick yttria stabilized zirconia electrolyte

This study discusses the fabrication and electrochemical performance of micro-tubular solid oxide fuel cells (SOFCs) with an electrolyte consisting a single-grain-thick yttria stabilized zirconia (YSZ) layer. It is found that a uniform coating of an electrolyte slurry and controlled shrinkage of the supported tube leads to a dense, crack-free, single-grain-thick (less than 1 μm) electrolyte on a porous anode tube. The SOFC has a power density of 0.39 W cm⁻² at an operating temperature as low as 600 °C, with YSZ and nickel/YSZ for the electrolyte and anode, respectively. An examination is made of the effect of hydrogen fuel flow rate and shown that a higher flow rate leads to better cell performance. Hence a YSZ cell can be used for low-temperature SOFC systems below 600 °C, simply by optimizing the cell structure and operating conditions.     

Modeling of gas transport through a tubular solid oxide fuel cell and the porous anode layer

A design model is a necessary tool to understand the gas transport phenomena that occurs in a tubular solid oxide fuel cell (SOFC). This paper describes a computational model, which studies the gas flow through an anode-supported tubular SOFC and the subsequent diffusion of gas through its porous anode. The model is a numerical solution for the gas flow through a plug flow reactor with a diffusion layer, which includes the activation, ohmic, and concentration polarizations. Gas diffusion is modeled using the dusty-gas equations which include Knudsen diffusion. Mercury intrusion porosimetry (MIP) is used to experimentally determine micro-structural parameters such as porosity, tortuosity and effective diffusion coefficients, which are used in the diffusion equations for the porous anode layer. It was found that diffusion in the anode plays a key role in the performance of a tubular SOFC. The concentration gradient of hydrogen and water results in a lower concentration of hydrogen and a higher concentration of water at the reactive triple phase boundary (TPB) than in the fuel stream which both lead to a lower cell voltage. The gas diffusion determines the limiting current density of the cell where a higher concentration drop of hydrogen results in a lower limiting current density. The model validates well with experimental data and is used to improve micro-tubular solid oxide fuel cell designs.     

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.     

Fabrication and characterization of micro tubular SOFCs for operation in the intermediate temperature

Tubular SOFC systems appear to be well-suited to accommodate repeated cycling under rapid changes in electrical load and in cell operating temperatures. Our goal is to develop innovative processing method to fabricate new micro tubular SOFCs with sub-millimeter diameter and its stack module which enable to generate high volumetric power density. In this study, micro tubular SOFCs under 1 mm diameter have been successfully fabricated and tested in the intermediate temperature region (550 °C or under). The cell consists of NiO–Gd doped ceria (GDC) as an anode (support tube), GDC as an electrolyte and (La, Sr)(Fe, Co)O₃ (LSCF)–GDC as a cathode. The single tubular cell with 0.8 mm diameter and 12 mm length generated over 70 mW at 550 °C with H₂ fuel, which indicates that the cell generated over 0.3 W cm⁻² at 550 °C.     

Fabrication and characterization of components for cube shaped micro tubular SOFC bundle

Solid oxide fuel cells (SOFCs) have the highest energy efficiency among various power generators. However, SOFCs generally have problems regarding to heat stress due to the heating cycle during cell operation, especially quick start-up/shut-down and the size of SOFC systems, which limit their application use. Micro tubular SOFCs are expected to be a solution to these problems because they are considered to be robust for repeated cycling under rapid changes in cell operating temperatures. If highly dense micro tubular SOFC stacks become available, it will accelerate development of SOFC systems, as well as increase a variety of applications. Our study aims to fabricate compact and high power SOFC bundles, which are composed of tubular SOFCs with the diameter of sub-millimeters. In this study, as the first stage of the development, processing technologies of tubular SOFCs and cube shaped cathode matrices were examined. Micro tubular SOFCs were fabricated using extrusion and co-firing techniques. The tubular SOFCs were then, arranged in the cathode matrices, which were piled up to be a cube shaped bundle. Each component of the cube shaped micro tubular SOFC bundle (cube) will be discussed in detail.