Numerical analysis of coupled transport and reaction phenomena in an anode-supported flat-tube solid oxide fuel cell

Heat and mass transfer with electrochemical reaction in an anode-supported flat-tube solid oxide fuel cell (FT-SOFC) is studied by means of three-dimensional numerical simulation. The distributions of the reaction fields in the anode-supported FT-SOFC are found to be similar to those in the planar SOFC with co-flow arrangement. However, in comparison with the latter, the concentration and activation overpotentials of the former can be reduced by additional reactant diffusion through the porous rib of the fuel channel. Parametric survey reveals that, for a fixed activation overpotential model, the output voltage can be improved by increasing the pore size of anode, while the cross-sectional geometry has smaller effect on the cell performance. Based on the results of three-dimensional simulation, we also develop a simplified numerical model of anode-supported FT-SOFC, which takes into account the concentration gradients in the thick anode of complex cross-sectional geometry. The simplified model can sufficiently predict the output voltage as well as the distributions of temperature and current density with very low computational cost. Thus, it can be used as a powerful tool for surveying wide range of anode-supported FT-SOFC design parameters.     

The current density and temperature distributions of anode-supported flat-tube solid oxide fuel cells affected by various channel designs

The advantages of a flat-tube solid oxide fuel cell (FT-SOFC) include easy sealing, low stack volume and low resistance to current collection. Because the performance of the FT-SOFC is closely linked to the interior hydrogen channel design, this paper studied various channel designs using a numerical approach. Ordinary FT-SOFCs have many channels with small cross-sectional areas. Unfortunately, this design makes it possible for the sealing material to block the channel entrance. Furthermore, it is difficult for the hydrogen to be evenly distributed under this type of design. To overcome these problems, a new design was developed to reduce the number of channels and increase the cross-sectional area. Contrary to expectations, the numerical approach applied here revealed that new design resulted in poor hydrogen transport into the support region and an average current density of 2846.7 A/m², lower than that of the traditional design. Another design with a gradually increasing width from inlet to outlet was applied to increase the performance, maintain the mechanical strength and reduce the pressure drop of SOFCs. This design improved the average current density to 3135.4 A/m². However, the cell performance with this channel design decreased significantly when the channel inlet became too narrow or when the outlet became too wide.     

Understanding thermal and redox cycling behaviors of flat-tube solid oxide fuel cells

The performance stability of solid oxide fuel cells (SOFCs) under thermal and redox cycles is vital for large-scale applications. In this work, we investigated the effects of thermal and redox cycles on cell performances of flat-tube Ni/yttria-stabilized zirconia (Ni/YSZ) anode-supported SOFCs. Cell performance was considerably affected by the duration of oxidation during redox cycles and the heating rate during the thermal cycles. The cell tolerated 20 short-term redox cycles (5 min oxidation) without significant performance degradation. Besides, the cell exhibited superior stability during 8 thermal cycles with a slow heating rate (4 °C min⁻¹) to that with a fast heating rate (8 °C min⁻¹). These results reflected that the thick anode support (2.7 mm) offered strong resistance to the shocks caused by redox and thermal cycling. Moreover, the morphological changes of the Ni phase during the redox and thermal cycling were investigated using Ni-film anode cells. Agglomeration of Ni particles and dissociation between the Ni film and the YSZ substrate were confirmed after 5 redox cycles, whereas no significant changes in Ni film emerged after 8 thermal cycles.     

Multi-physics modeling of a symmetric flat-tube solid oxide fuel cell with internal methane steam reforming

Methane is regarded as one of the ideal fuels for solid oxide fuel cells (SOFCs) due to its huge reserves and transportation properties. In this study, a 3D numerical model coupling with chemical reaction, electrochemical reaction, mass transfer, charge transfer, and heat transfer is developed to understand the heat and mass transfer processes of methane steam direct internal reforming based on double-sided cathodes (DSC) SOFC. After the model verification, the parametric simulations are performed to study the effects of operating voltage, inlet temperature, and steam to carbon (S/C) ratio on the performance of a DSC. It is found that the non-uniform distribution of flow rate among channels results in the non-uniform distribution of each physical field. Increasing the inlet temperature significantly enhances the performance of DSC, however, when the temperature is above 1073 K, the concentration loss and the temperature gradient of DSC increase, which is not conducive to the long-term operation of the DSC. In addition, we revealed the effect of the S/C ratios on the heat and mass transfer process. This study provides an insight into the heat and mass transfer process of SOFC with a mixture of steam and methane and operating conditions for enhancing the performance.     

Numerical investigation of flow/heat transfer and structural stress in a planar solid oxide fuel cell

The present work investigates the effects of the temperature and thermal stress distributions in a planar solid oxide fuel cell (SOFC) unit cell. A computational fluid dynamic (CFD) analysis of a planar anode-supported SOFC that considers electrochemical reactions is performed, and the thermal stresses are calculated. The static friction coefficients are assumed to range from 0.05 to 0.3, and conservatively, a perfectly bonded condition is assumed. The results show that the electrolyte is the weakest component and has the maximum stress because the electrolyte is the thinnest and the Young modulus is the highest. Thus, the contact between the anode electrode and the electrolyte, and between the cathode electrode and the electrolyte, would be the perfectly bonded condition. As a result, this research showed that the stresses induced by constraint forces with various contact conditions were dominant for the structural stability in a SOFC. Therefore, static friction coefficients on operative high temperature conditions are important to predict the structural integrity in a SOFC, and they will be investigated in future works in order to improve the structural stability in a stack design as well as in a SOFC.     

Development of a high power density 2.5 kW class solid oxide fuel cell stack

We have developed a 2.5 kW class solid oxide fuel cell stack. It is constructed by combining 70 power generation units, each of which is composed of an anode-supported planar cell and separators. The power generation unit for the 2.5 kW class stack were designed so that the height of the unit were scaled down by 2/3 of that for our conventional 1.5 kW class stack. The power generation unit for the 2.5 kW class stack provided the same output as the unit used for the conventional 1.5 kW class stack, which means that power density per unit volume of the 2.5 kW class stack was 50% greater than that of the conventional 1.5 kW class stack.     

Optimization of interconnect flow channels width in a planar solid oxide fuel cell

Finite element analysis is an effective method to investigate the uniformity of species distribution in solid oxide fuel cells. This paper presents a 3-D model with coupled mass transport, electron transfer and electrochemical reaction. Based on a working anode-supported SOFC with gas channels and porous electrodes, the model is validated by measured IV curves. The simulated results deviate no more than 3% from the measured data and indicate significant dependency of current density distribution on the gas composition distribution. Meanwhile, the simulated results also indicate that the ridges of the interconnect have negative effects on electrochemical reaction due to the limitation of mass transport. In further study, cases of various channel widths are calculated and analyzed, finding that the optimal width ratios for electrochemical in cathode and anode are 0.5.     

A numerical study on the heat and mass transfer characteristics of metal-supported solid oxide fuel cells

The heat and mass transfer characteristics of solid oxide fuel cells (SOFCs) need to be considered when designing SOFCs because they heavily influence the performance and durability of the cells. The physical property models, the governing equations (mass, momentum, energy and species balance equations) and the electrochemical reaction models were calculated simultaneously in a 3-dimensional SOFC simulation. The current density-voltage (I-V) curves measured experimentally from a single SOFC were compared with the simulation data for code validation purposes. The error between the experimental data and the numerical results was less than 5% at operating temperatures from 700 °C to 850 °C. The current density and the mass transfer rate of an anode-supported SOFC were compared with those of a metal-supported SOFC. The metal-supported SOFC had a 17% lower average current density than the anode-supported SOFC because of the bonding layer, but it showed better thermal stability than the anode-supported SOFC because of its more uniform current density distribution. The current density, temperature and pressure drop of the metal-supported SOFC were investigated for several channel designs. A high current density was observed near the hydrogen inlet and at the intersection of the hydrogen and air channels. However, there was a low current density under the rib and at the cell edge because of an insufficient reactant diffusion flux. When the proper channel design was applied to the metal-supported SOFC, the average current density was increased by 45%.     

Heat and mass transfer and fluid flow in cryo-adsorptive hydrogen storage system

Compared to room temperature adsorption, cryo-adsorptive hydrogen storage capacity has been greatly improved, and has become the central issue of the hydrogen storage research. Accurate simulation and optimization for cryo-adsorptive hydrogen storage has important guidance and application value to the experimental research, and the finite element software Comsol Multiphysics™ and system analysis software Matlab/Simulink™ can be used to simulate the cryo-adsorptive hydrogen storage. However, the computational fluid dynamics (CFD) software Fluent™ can provide more information on the heat and mass transfer and the fluid flow than above softwares. Based on the mass, momentum and energy conservation equations, this paper uses the modified Dubinin–Astakhov (D–A) adsorption isotherm model, linear driving force (LDF) model and dynamic thermal boundary condition which are implemented by means of CFD software Fluent to simulate the hydrogen adsorption processes of charging and dormancy in the case of liquid nitrogen cooling. We study the variations of temperature and pressure during the processes of charging and dormancy. The results show that the experimental data is in good agreement with the simulation results. We also analyze the effect of variable specific heat and anisotropic thermal conductivity on the heat and mass transfer and the fluid flow in cryo-adsorptive hydrogen storage system.     

Mechanisms of performance degradation induced by thermal cycling in solid oxide fuel cell stacks with flat-tube anode-supported cells based on double-sided cathodes

In this work, the degradation in output power of a stack with flat-tube anode-supported cells based on double-sided cathodes and its mechanism are studied. After 102 thermal cycles, the OCV keeps about 1.1 V and remains stable, showing that the one-cell stack exhibits a good sealing performance. During the first 100 thermal cycles, when the temperature ranges from 750 to 200 °C with a heating/cooling rate of 3 °Cmin⁻¹, the stack degradation mainly occurs during the first 34 thermal cycles, and the degradation rate is ~0.89%/cycle. During the 101th and 102nd thermal cycles, an additional loading force is applied on the cathode side of the stack at room temperature, and the results shows that the output power at 750 °C increases and finally exceeds the initial output. As a result, the primary cause for degradation induced by thermal cycling is believed to originate from the weak interface between the cathode and the interconnect, resulting in an increase in ohmic resistance. The stack degradation can therefore be recovered by a secondary loading force on the cathode side.     

Computational fluid dynamics modeling of a solid oxide electrolyzer cell for hydrogen production

A 2D computational fluid dynamics (CFD) model was developed to study the performance of a planar solid oxide electrolyzer cell (SOEC) for hydrogen production. The governing equations for mass continuity, momentum conservation, energy conservation and species conservation were discretized with the finite volume method (FVM). The coupling of velocity and pressure was treated with the SIMPLEC (Semi-Implicit Method for Pressure Linked Equations – Consistent) algorithm. Simulations were performed to investigate the effects of operating/structural parameters on heat/mass transfer and the electric characteristics of a planar SOEC. It is found that the gas velocity at the cathode increases significantly along the main flow channel, as the increase in H₂ molar fraction decreases the density and viscosity of the gas mixture at the cathode. It is also found that increasing the inlet gas velocity can enhance the SOEC performance. Another important finding is that the electrode porosity has small effect on SOEC performance. The results of this paper provide better understanding on the coupled heat/mass transfer and electrochemical reaction phenomena in an SOEC. The model developed can serve as a useful tool for SOEC design optimization.     

平板式阳极支撑固体氧化物燃料电池传质传热仿真研究

利用流体力学计算软件F luen t建立平板式阳极支撑固体氧化物燃料电池(SOFC)的三维数学模型。在阳极与阴极多孔电极中使用尘气模型模拟气体质量传输并采用B rinkm an-Forschhe im er-D acy模型来模拟多孔电极中黏性与惯性效应对气体流动的影响。研究给出了燃料气与空气在同向流与反向流情况下组分浓度、电压与温度分布。结果显示在同向流情况下,电池的最大功率密度较大与温度分布较均匀合理。研究给出了多孔电极结构参数(孔隙率、曲折因子与孔径尺寸)对电池性能的影响。结果表明比较计算的极化性能与文献的实验数据两者较好的吻合。     

Numerical study of a flat-tube high power density solid oxide fuel cell: Part II: Cell performance and stack optimization

The flat-tube high power density (HPD) solid oxide fuel cell (SOFC) is a geometry based on a tubular type SOFC, and is being developed by Siemens Westinghouse and other international companies in Japan and Korea. It has increased power density, but still maintains the beneficial feature of secure sealing for a tubular SOFC. In this paper, the electric performance of a flat-tube HPD SOFC is studied. This paper also investigates the effects of the stack chamber number, stack shape, and other stack geometry features on the performance of the flat-tube HPD SOFC. The results show that the performance of a flat-tube HPD SOFC is better than a tubular SOFC with the same active cell surface, and that increasing the number of chambers number can improve the overall performance of a flat-tube HPD SOFC. The height of a flat-tube HPD SOFC and the thickness of the ribs do not have much effect on the performance of the cell as is expected. This study will help to design and optimize the flat-tube HPD SOFC.     

Study on heating performances of steam generator for solid oxide fuel cell using waste heat from solid particles

This paper reports the heating performances of steam generator for solid oxide fuel cell using waste heat from solid particles. The model of trapezoid-fin-tube heat exchanger was set up by using FLUENT 14.0. The model has been used to investigate the effects of fin tip width (2 mm–4 mm) and fin height (34 mm–46 mm). The fin surface temperature, the particle temperature, the fin total heat flux, the heat recovery efficiency and the heat transfer coefficient were studied. The heating performance of steam generator is improved when the trapezoid-fins are placed on heat transfer tubes, which is conducive to increase the power generation efficiency of solid oxide fuel cell. When the fin height increases from 34 mm to 46 mm, the average temperature of calcined petroleum coke decrease from 414 K to 376 K, the maximum temperature decrease from 498 K to 442 K, the average heat transfer coefficient of internal and external heat exchanger increase 12.4% and 12.7% respectively, the heat recovery efficiency increases 4.3%. When the fin tip width increases from 2 mm to 3 mm, the average temperature reduce 6.7 K and the maximum temperature decrease 7.3 K, the average heat transfer coefficient of internal and external heat exchanger increase 3.8% and 3.7% respectively, the heat recovery efficiency increases 0.88%.     

Degradation of cathode current-collecting materials for anode-supported flat-tube solid oxide fuel cell

Different types of cathode current-collecting material for anode-supported flat-tube solid oxide fuel cells are fabricated and their electrochemical properties are characterized. Current collection for the cathode is achieved by winding Ag wire and by painting different conductive pastes of Ag–Pd, Pt, La_0.6Sr_0.4CoO₃ (LSCo), and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) on the wire. Cell performance at the initial operation time is in the order of Pt > LSCo > LSCF > Ag–Pd. On the other hand, the performance degradation rate is in the order of LSCo < LSCF < Pt < Ag–Pd. LSCo paste as a cathode current-collector shows the most stable long-term performance of 0.8 V, 300 mA cm⁻² at 750 °C, even under a thermal cycle condition with heating and cooling rates of 150 °C h⁻¹. The performance degradation of the Ag–Pd and Pt pastes is caused by increased polarization resistance due to metal particle sintering. From these results, it is concluded that a cathode current-collector composed of wound silver wire with LSCo paste is useful for anode-supported flat-tube cells as it does not experience any significant degradation during a long operation time.     

Characterization of electrochemical reaction and thermo-fluid flow in metal-supported solid oxide fuel cell stacks with various manifold designs

The metal-supported solid oxide fuel cell (SOFC), in which a metal plate is bonded to a ceramic cell, was recently introduced as a new fuel cell design. Metal-supported SOFCs do not suffer from gas leakage, because the metal plates are welded to the metallic interconnects, which also provides high mechanical strength. However, the bonding layer existing between the interconnect and the ceramic cell increases mass transfer resistance, resulting in decreased performance. To better understand and control the mass transfer rate, the manifold structure of the fuel cell stack as well as the channel design in each single cell should be studied. Using a numerical approach, physical property models, governing equations and electrochemical reaction models were calculated simultaneously. The experimentally measured current density–voltage curves were compared with the simulation data to validate the code. Current densities, temperatures and pressure distributions resulting from various manifold designs were presented as numerical results. The parallel manifold design displayed an average current density of 2820.1 A/m² and a relatively uniform current density distribution. The serpentine design yielded the highest average current density among the studied manifold designs, but the maximum pressure was 32 times higher than with the parallel design. Moreover, the large temperature difference observed with the serpentine design may result in a thermal expansion problem. The expanding manifold design yielded an average current density of 2885.9 A/m² and a maximum pressure of 6350 Pa. The pressure distribution with this manifold design was clearly related to the manifold structure. The tapering manifold design is the opposite of the expanding manifold; with this design, the average current density and maximum pressure were slightly lower than the expanding manifold. The dual-flow hybrid manifold design combines two different manifold structures: a serpentine hydrogen manifold and a parallel air manifold. The dual-flow hybrid design yielded an average current density of 2905.4 A/m² and a maximum pressure of 750 Pa.     

Heat and mass transfer effects in a direct methanol fuel cell: A 1D model

Models are a fundamental tool for the design process of fuel cells and fuel cell systems. In this work, a steady-state, one-dimensional model accounting for coupled heat and mass transfer, along with the electrochemical reactions occurring in the DMFC, is presented. The model output is the temperature profile through the cell and the water balance and methanol crossover between the anode and the cathode. The model predicts the correct trends for the influence of current density and methanol feed concentration on both methanol and water crossover. The model estimates the net water transfer coefficient through the membrane, α, a very important parameter to describe water management in the DMFC. Suitable operating ranges can be set up for different MEA structures maintaining the crossover of methanol and water within acceptable levels. The model is rapidly implemented and is therefore suitable for inclusion in real-time system level DMFC calculations.     

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.     

Efficiency analyses of ethanol-fueled solid oxide fuel cell power system

In this study, the balance of plant (BOP) of an ethanol-fueled SOFC is analyzed using the GCTool software package developed by Argonne National Laboratory (ANL). The effects of the excess air ratio and fuel utilization on the electric and heat efficiencies of the SOFC are systematically examined for two reforming methods (steam reforming and auto-thermal reforming) and two flow sheets (BOP A and BOP B). In BOP A, the cathode off-gas is passed directly to the afterburner together with the unreacted fuel, and the hot flue gas exiting the burner is then used to provide the thermal energy required for the ethanol reforming process. In BOP B, the cathode off-gas is passed through a heat exchanger in order to heat the ethanol fuel prior to the reforming process, and is then flowed into the burner with the unreacted fuel. The results show that given an SOFC inlet temperature of 650 °C, a fuel utilization of 70.2% and excess air ratios of 4, 6 and 7, respectively, the overall system efficiency is equal to 74.9%, 72.3% and 71.0%. In general, the results presented in this study provide a useful starting point for the design and development of practical ethanol-fueled SOFC test systems.     

Performance analysis of solid oxide fuel cell using reformed fuel

Fuelling SOFC with reformed fuel can be beneficial due to it being cheaper compared to pure hydrogen. A biomass fuel can be easily modeled as a reformed fuel, as it can be converted into H₂ and CO using gasification or biodegradation, the main composition of product from a reformer. Hence in this study it is assumed that feed to the fuel cell contains only H₂ and CO. A closed parametric model is formulated. Performance is analyzed with changes in temperature, pressure and fuel ratio; considering the possible voltage losses, like ohmic, activation, mass transfer and fuel crossover. Performance curves consisting of operating voltage, fuel utilization, efficiency, power density and current density are developed for both pure hydrogen and mixture of CO and H₂. Variations of open circuit voltage with temperature, power density with current density, operating voltage with current density and maximum power density with fuel utilization are also evaluated.