Performance of solid oxide fuel cell with chemical looping gasification products as fuel

Chemical looping gasification (CLG) can achieve the utilization of solid fuels for syngas production. The CLG system integrated with solid oxide fuel cell (SOFC) is a promising energy conversion way. In this work, an integration system of CLG and SOFC is evaluated via the implementation of a multi-field coupling modelling, where the products from the CLG are directly transported into the SOFC as the fuel and the coke deposition effect on the cell performance is evaluated. The results reveal that SOFC temperature using pure hydrogen as fuel has an increase of around 4 K compared to that with gas mixture as fuel owing to the inhibition of carbon deposition. It is found that the arrangement of anode and cathode in the countercurrent mode can promote the overall uniformity of current density compared to that in the cocurrent flow. Moreover, the impact of operating parameter of the CLG system on the SOFC performance is also examined. The results demonstrate that the increase of fuel reactor (FR) temperature and H₂O/C molar ratio in the CLG system is beneficial to the inhibition of carbon deposition and the enhancement of the SOFC performance.     

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.     

Evaluation of system configurations for solid oxide fuel cell-based micro-combined heat and power generators in residential applications

The evaluation of solid oxide fuel cell (SOFC) combined heat and power (CHP) system configurations for application in residential dwellings is explored through modeling and simulation of cell-stacks including the balance-of-plant equipment. Five different SOFC system designs are evaluated in terms of their energetic performance and suitability for meeting residential thermal-to-electric ratios. Effective system concepts and key performance parameters are identified. The SOFC stack performance is based on anode-supported planar geometry. A cell model is scaled-up to predict voltage–current performance characteristics when served with either hydrogen or methane fuel gas sources. System comparisons for both fuel types are made in terms of first and second law efficiencies. The results indicate that maximum efficiency is achieved when cathode and anode gas recirculation is used along with internal reforming of methane. System electric efficiencies of 40% HHV (45% LHV) and combined heat and power efficiencies of 79% (88% LHV) are described. The amount of heat loss from small-scale SOFC systems is included in the analyses and can have an adverse impact on CHP efficiency. Performance comparisons of hydrogen-fueled versus methane-fueled SOFC systems are also given. The comparisons indicate that hydrogen-based SOFC systems do not offer efficiency performance advantages over methane-fueled SOFC systems. Sensitivity of this result to fuel cell operating parameter selection demonstrates that the magnitude of the efficiency advantage of methane-fueled SOFC systems over hydrogen-fueled ones can be as high as 6%.     

The influence of flow direction variation on the performance of a single cell for an anode-substrate flat-panel solid oxide fuel cell

This study was performed for a computational investigation of a single cell for an anode-substrate flat-panel solid oxide fuel cell (SOFC) to scrutinize the performance related to thermodynamic potential and overpotentials according to three other flow configurations: parallel flow, countercurrent flow, and perpendicular flow. To understand the performance differences based on the typical three flow configurations, the contour plots of temperature, species, and current density were simulated, and the trends and the portions of the diverse overpotentials were analyzed. The calculated results demonstrated that the parallel flow configuration had a tendency to deliver the highest performance and the lowest overpotentials of the three configurations because the temperature and H₂ concentration in the parallel flow configuration were changed countercurrently along the anode flow direction. These overpotentials were complemented by interacting with the more uniform current density and the total impedance induced by the opposite directional change for the temperature and H₂ concentration.In designing the anode-substrate flat-panel SOFC, the uniformity of flow rate in each channel, which affects significantly to both performance and lifetime of the cell, has been checked. From this numerical analysis result, the design performance of single cell was satisfactorily verified by obtaining negligible flow deviation in each channel of the designed separator deviation, which was less than 3% of the average velocity.     

Single solid oxide fuel cell modeling and optimization

In this paper a dynamic model of a single solid oxide fuel cell (SOFC) is developed using a volume element methodology. It consists of a set of algebraic and ordinary differential equations derived from physical laws (e.g., the first law of thermodynamics, Fick's law, and Fourier's law), which allow for the prediction of the temperature and pressure spatial distribution inside the single SOFC, as functions of geometric and operating parameters. The thermodynamic model is coupled with an electrochemical model that is capable of determining the voltage, current, and power output. Based on the simulation results, the internal configuration (structure of the positive electrode-electrolyte-negative electrode assembly) and the operating conditions (air stoichiometric ratio and fuel utilization factor), as well as their impact on the performance of the single SOFC are discussed. Optimal geometric and operating parameters are obtained so that electrical power of the single SOFC at the nominal operating point is maximized. The method used is general and the fundamental optimization results are sharp, showing up to a 357% single SOFC performance variation within the studied parameters’ range, therefore these findings show the potential to use the model as a tool for future SOFC design, simulation and optimization.     

Theoretical analysis of reversible solid oxide fuel cell based on proton-conducting electrolyte

A single reversible solid oxide fuel cell (RSOFC) can accomplish two functions: (1) as a solid oxide steam electrolyzer (SOSE) for hydrogen production and (2) as a solid oxide fuel cell (SOFC) for power generation. An electrochemical model was developed to study the performance of an RSOFC based on a proton-conducting electrolyte (RSOFC-H). In both SOSE and SOFC modes, the hydrogen electrode-supported configuration was identified as the most favorable design to achieve high energy conversion efficiency of RSOFC-H. For comparison, in a previous study on conventional RSOFC based on an oxygen ion-conducting electrolyte (RSOFC-O), the hydrogen electrode-supported configuration was found to be favorable in the SOFC mode but such configuration would cause high concentration overpotential in the SOSE mode. Thus, the oxygen electrode-supported configuration was desirable for RSOFC-O operating in the SOSE mode. The results obtained in this study show that RSOFC-H has a natural advantage over RSOFC-O in terms of structural design. The modeling study signifies the difference between RSOFC-H and RSOFC-O and can serve as a useful tool for further design optimization.     

The calibration of a model for simulating the thermal and electrical performance of a 2.8 kWAC solid-oxide fuel cell micro-cogeneration device

The concurrent production of heat and electricity within residential buildings using solid-oxide fuel cell (SOFC) micro-cogeneration devices has the potential to reduce primary energy consumption, greenhouse gas emissions, and air pollutants. A realistic assessment of this emerging technology requires the accurate simulation of the thermal and electrical production of SOFC micro-cogeneration devices concurrent with the simulation of the building, its occupants, and coupled plant components. The calibration of such a model using empirical data gathered from experiments conducted with a 2.8 kW_AC SOFC micro-cogeneration device is demonstrated. The experimental configuration, types of instrumentation employed, and the operating scenarios examined are treated. The propagation of measurement uncertainty into the derived quantities that are necessary for model calibration are demonstrated by focusing upon the SOFC micro-cogeneration system’s gas-to-water heat exchanger. The calibration coefficients necessary to accurately simulate the thermal and electrical performance of this prototype device are presented and the types of analyses enabled to study the potential of the technology are demonstrated.     

Artificial intelligence based structural optimization of solid oxide fuel cell with three-dimensional reticulated trapezoidal flow field

Three-dimensional Reticulated trapezoidal flow field (RTFF) is promising in improving the performance and durability of the solid oxide fuel cell (SOFC). However, the structural complexity makes it challenging for the geometry configuration of the splitter and mixer. To this end, an intelligent optimization framework is proposed by coupling artificial neural network (ANN) and non-dominated sorting genetic algorithm-II (NSGA-II), in order to maximize the net power density and oxygen uniformity simultaneously. The ANN prediction model is trained to obtain the computationally efficient surrogate model of the computational fluid dynamics (CFD) numerical simulation. NSGA-II is used for the multi-objective optimization of the RTFF structural parameters. The results illustrate that the prediction model is of high prediction precision and generalization capability. In comparison to SOFC with conventional parallel flow fields (CPFF), the degree of the performance improvement of SOFC with optimized RTFF depends on the working condition, i.e., fuel and air flow rates and operating temperatures. The SOFC with the optimal RTFF achieves a higher molar concentration of oxygen and a more uniform distribution of oxygen and current density than the CPFF SOFC. The proposed optimization framework provides an efficient design method for the development of the next-generation SOFC flow field.     

Three-dimensional computational fluid dynamics modeling of anode-supported planar SOFC

Modeling plays a very important role in the development of fuel cells and fuel cell systems. The aim of this work is to investigate the electrochemical processes of a Solid Oxide Fuel Cell (SOFC) and to evaluate the performance of the proposed SOFC design. For this aim a three-dimensional Computational Fluid Dynamics (CFD) model has been developed for an anode-supported planar SOFC with corrugated bipolar plates serving as gas channels and current collector. The conservation of mass, momentum, energy and species is solved by using the commercial CFD code FLUENT in the developed model. The add-on FLUENT SOFC module is implemented for modeling the electrochemical reactions, loss mechanisms and related electric parameters throughout the cell. The distributions of temperature, flow velocity, pressure and gaseous (fuel and air) concentrations through the cell structure and gas channels is investigated. The relevant fuel cell variables such as the potential and current distribution over the cell and fuel utilization are calculated and studied. The modeling results indicate that, for the proposed SOFC design, reasonably uniform distributions of current density over the active cell area can be achieved. The geometry of the cathode gas channel has a substantial effect on the oxygen distribution and thus the overall cell performance. Methods for arriving at improved cell designs are discussed.     

Control strategy for a solid oxide fuel cell and gas turbine hybrid system

This paper presents a multi-loop control strategy for a SOFC/GT hybrid system. A detailed dynamic model of the system is presented and its part-load performance is studied. The control objectives are discussed, with the main issue being a fairly constant fuel cell temperature under all conditions. Based on the system configuration and part-load performance, input and output variables of the control system are detected. Control cycles are introduced and their design is discussed. The responses of the resulting system on load changes, external disturbances as well as malfunction and degradation incidents are investigated. The system is stable under all incidents. An error in fuel flow measurement or assumed fuel quality provokes a steady-state fuel cell temperature offset. For a degraded system, it may be advisable to readjust the control system to the new characteristics.     

Exergy analysis of a solid oxide fuel cell micropowerplant

In this paper, an analytical model of a micro solid oxide fuel cell (SOFC) system fed by butane is introduced and analyzed in order to optimize its exergetic efficiency. The micro SOFC system is equipped with a partial oxidation (POX) reformer, a vaporizer, two pre-heaters, and a post-combustor. A one-dimensional (1D) polarization model of the SOFC is used to examine the effects of concentration overpotentials, activation overpotentials, and ohmic resistances on cell performance. This 1D polarization model is extended in this study to a two-dimensional (2D) fuel cell model considering convective mass and heat transport along the fuel cell channel and from the fuel cell to the environment. The influence of significant operational parameters on the exergetic efficiency of the micro SOFC system is discussed. The present study shows the importance of an exergy analysis of the fuel cell as part of an entire thermodynamic system (transportable micropowerplant) generating electric power.     

Application of adaptive neuro-fuzzy inference system techniques and artificial neural networks to predict solid oxide fuel cell performance in residential microgeneration installation

This study applies adaptive neuro-fuzzy inference system (ANFIS) techniques and artificial neural network (ANN) to predict solid oxide fuel cell (SOFC) performance while supplying both heat and power to a residence. A microgeneration 5 kW_el SOFC system was installed at the Canadian Centre for Housing Technology (CCHT), integrated with existing mechanical systems and connected in parallel to the grid. SOFC performance data were collected during the winter heating season and used for training of both ANN and ANFIS models. The ANN model was built on back propagation algorithm as for ANFIS model a combination of least squares method and back propagation gradient decent method were developed and applied. Both models were trained with experimental data and used to predict selective SOFC performance parameters such as fuel cell stack current, stack voltage, etc.     

连接体结构对固体氧化物燃料电池性能影响的数值分析

固体氧化物燃料电池（SOFC）中连接体结构对电池性能有重要影响。为探究连接体结构对固体氧化物燃料电池性能的影响，建立了传统直通、圆柱形、矩形和凹形4种不同连接体结构SOFC的三维数值模型，并对其流体流动、组分传递、电化学反应和固体流体传热的多物理场耦合过程进行了数值模拟。结果表明，在一定条件下，圆柱形、矩形和凹形连接体结构有利于电池中气体的传输，使电池的电流密度和输出功率均有所提升，其中凹形连接体结构的提升效果最明显，圆柱形、矩形连接体结构的次之。不同孔隙率下圆柱形、矩形和凹形连接体结构均优于传统直通连接体结构，在阴极孔隙率较小时其优势更加明显。     

Analysis of a SOFC energy generation system fuelled with biomass reformate

Biomass reformation is an interesting path for hydrogen production and its use for efficient energy generation. The main target is the fully exploitation of the potential of renewable fuels. To this aim, the coupling a biomass reformer together with a high temperature solid oxide fuel cell (SOFC) stack shows some advantages for the similar operating temperature of the two processes and the internal reforming capability of the SOFC. The latter further allows less stringent composition requirements of the feed gas from a gasifier and internal cooling of the SOFC.In this work, a complete model of a SOFC coupled with a biomass gasifier is used to identify the main effects of the operating conditions on the fuel cell performance.The gasification process has been simulated by an equilibrium model able to compute the reformate composition under different operating conditions, whereas a 3D fluid dynamics simulation (FLUENT) coupled with an external model for the electrochemical reactions has been used to predict the fuel cell performance in terms of electrical response and mass-energy fluxes.A 14 kW integrated SOFC-gasifier system has been analysed with this model to address the response of a planar SOFC as a function of the gasifier operating conditions.     

The modeling of a standalone solid-oxide fuel cell auxiliary power unit

In this research, a Simulink model of a standalone vehicular solid-oxide fuel cell (SOFC) auxiliary power unit (APU) is developed. The SOFC APU model consists of three major components: a controller model; a power electronics system model; and an SOFC plant model, including an SOFC stack module, two heat exchanger modules, and a combustor module. This paper discusses the development of the nonlinear dynamic models for the SOFC stacks, the heat exchangers and the combustors. When coupling with a controller model and a power electronic circuit model, the developed SOFC plant model is able to model the thermal dynamics and the electrochemical dynamics inside the SOFC APU components, as well as the transient responses to the electric loading changes. It has been shown that having such a model for the SOFC APU will help design engineers to adjust design parameters to optimize the performance. The modeling results of the SOFC APU heat-up stage and the output voltage response to a sudden load change are presented in this paper. The fuel flow regulation based on fuel utilization is also briefly discussed.     

Evaluation of fuel diversity in Solid Oxide Fuel Cell system

Operability of Solid Oxide Fuel Cell (SOFC) on numerous fuels has been widely counted as a leading advantage in literature. In a designed system, however, switching from a fuel to another is not practically a straightforward task as this causes several system performance issues in both dynamic and steady-state modes. In order to demonstrate the system fuel diversity capabilities, these consequences must be well-evaluated by quantifying the characteristic measures for numerous fuel cases and also potential combinations. From this viewpoint, the numerical predictive models play a critical role. This paper aims to investigate the performance of a SOFC system fed by various fuels using a demonstrated system level model. Process configuration and streams results of a real-life SOFC system rig published in literature are used to validate the model. The presented model is capable not only of capturing the system performance measures but also the SOFC internal variable distributions, allowing the multiscale study of fuel switching scenarios. The fuel change impacts on the system are simulated by considering various fuel sources, i.e., natural gas, biogas, and syngas. Moreover, applications of simulated fuel mixtures are assessed. The modelling results show significant concerns about fuel switching in a system in terms of variation of efficiencies, stack internal temperature and current density homogeneity, and environmental issues. Moreover, the results reveal opportunities for multi-fuel design to address the operation and application requirements such as optimisation of the anode off-gas recycling rate and the thermal-to-electrical ratio as well as the system specific greenhouse gases, i.e., g-CO_x/Wh release.     

Direct liquid methanol-fueled solid oxide fuel cell

Anode coking problem of solid oxide fuel cell (SOFC) when using hydrocarbon fuels has been the major barrier for the practice and commercialization of well-developed high performance SOFC. In this work, based on fuels consideration, we chose liquid methanol as the candidate fuel for SOFC with the configuration of NiO/SDC–SDC–SSC/SDC. For comparison, traditional fuels, hydrogen and ammonia, were tested. With methanol as fuel, the maximum power densities were 698, 430 and 223 mW cm⁻² at 650, 600 and 550 °C, respectively, which were higher than that with ammonia and lower than that of hydrogen. The electrochemical properties of the cells with the three fuels were investigated by AC impedance spectroscopy. The long-term stability of the cell with methanol, methane and ethanol were also studied at a constant output voltage of 0.5 V.     

Numerical modeling and parametric analysis of solid oxide fuel cell button cell testing process

In laboratory studies of solid oxide fuel cell (SOFC), performance testing is commonly conducted upon button cells because of easy implementation and low cost. However, the comparison of SOFC performance testing results from different labs is difficult because of the different testing procedures and configurations used. In this paper, the SOFC button cell testing process is simulated. A 2‐D numerical model considering the electron/ion/gas transport and electrochemical reactions inside the porous electrodes is established, based on which the effects of different structural parameters and configurations on SOFC performance testing results are analyzed. Results show that the vertical distance (H) between the anode surface and the inlet of the anode gas channel is the most affecting structure parameter of the testing device, which can lead to up to 18% performance deviation and thus needs to be carefully controlled in SOFC button cell testing process. In addition, the current collection method and the configuration of gas tubes should be guaranteed to be the same for a reasonable and accurate comparison between different testing results. This work would be helpful for the standardization of SOFC button cell testing.     

Thermodynamic analysis of an SOFC–GT–ORC integrated power system with liquefied natural gas as heat sink

To recover the waste heat from solid oxide fuel cell (SOFC) and improve the overall electrical efficiency, a new integrated power system driven by SOFC is proposed to achieve the cascade energy utilization. This system integrates an SOFC–GT system with an organic Rankine cycle (ORC) using liquefied natural gas (LNG) as heat sink to recover the cryogenic energy of LNG. Based on the mathematical model, a parametric analysis is conducted to examine the effects of some key thermodynamic parameters on the system performance. The results indicate that the overall electrical efficiency of 67% can be easily achieved for the current system, which can be further improved with parametric optimization. An increase in fuel flow rate of SOFC can raise the net power output, but it has a negative effect on SOFC and overall electrical efficiency. The compressor pressure ratio contributes to an increase in SOFC and overall electrical efficiency, which are contrary to the effects of air flow rate and steam-to-carbon ratio. Under the given conditions, compared with the Kalina sub-system, the ORC sub-system produces 12.6% more power output by utilizing the cryogenic energy of LNG with simple configuration.