Numerical study of a flat-tube high power density solid oxide fuel cell: Part I. Heat/mass transfer and fluid flow

The flat-tube high power density (HPD) solid oxide fuel cell (SOFC) is a new design developed by Siemens Westinghouse, based on their formerly developed tubular type SOFC. It has increased power density, but still maintains the beneficial feature of secure sealing of a tubular SOFC. In this paper, a three-dimensional numerical model to simulate the steady state heat/mass transfer and fluid flow of a flat-tube HPD-SOFC is developed. In the numerical computation, governing equations for continuity, momentum, mass, and energy conservation are solved simultaneously. The highly coupled temperature, concentration and flow fields of the air stream and the fuel stream inside and outside the different chambers of a flat-tube HPD-SOFC are investigated. The variation of the temperature, concentration and flow fields with the current output is studied. The heat/mass transfer and fluid flow modeling and results will be used to simulate the overall performance of a flat-tube HPD-SOFC, and to help optimize the design and operation of a SOFC stack in practical applications.     

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 physical properties and operating parameters on numerically developed flat-tube solid oxide fuel cell performance

Fuel cells are promising as a clean energy conversion device in the era of global warming threat. Solid oxide fuel cells stand out among other fuel cell types because they are especially feasible for high-temperature applications. Besides the operating parameters, which are frequently encountered problems in SOFCs, physical parameters also directly affect cell performance. For this reason, careful examination of the effects of the parameters while designing the SOFC will contribute to the increase of the maximum power to be provided from the cell in applications. In this study, a solid oxide fuel cell (SOFC) created in flat-tube geometry is numerically modeled. The effect of the electrode and electrolyte layers on the performance is investigated parametrically on the created geometry. In addition, the effect of temperature on cell power is investigated comparatively by making analyzes at different temperatures for each case. In the analyzes, performance values are investigated for electrode layer thicknesses of 0.75 mm, 0.5 mm, and 0.25 mm, and electrolyte layer thicknesses of 1.25 mm, 1 mm, and 0.75 mm, respectively. As a result of the study, the highest cell power is obtained at 0.25 mm of the anode layer thickness. The maximum predicted average cell power of the flat tubular solid oxide fuel cell (FT-SOFC) is approximately 68.2 mW/cm² for the operating temperature of 1273 K. In the study, the effect of electrolyte conductivity on the performance of the developed cell is also investigated. It is observed that the cell with a conductivity value of 100 S/m at 1073 K operating temperature has the best performance. In addition, in the last part of the study, the performance of SOFC under non-uniform operating temperature conditions is also examined and a comparison is made for this situation, which is frequently experienced in practice. The results show that small changes in fuel cell operating temperature affect the cell performance in positive and negative directions depending on the increasing and decreasing temperature values.     

Parameter optimization for tubular solid oxide fuel cell stack based on the dynamic model and an improved genetic algorithm

The tubular solid oxide fuel cell (SOFC) stack has important parameters that need to be identified and optimized for the control of high performance. In this paper, a simple SOFC electrochemical model which its parameters need to be optimized is introduced to implement stack control for high output power. A dynamic SOFC model is built based on three sub-models to provide a large numbers simulated data and different condition for optimization. Unlike the traditional parameter optimization method--simple genetic algorithm (SGA), an improved genetic algorithm (IGA) is introduced. The proposed method shows more accuracy and validity by comparing the different results using SGA and IGA methods, the simulated data, and experimental data. The models and IGA method are adapted to control processes.     

Effect of inlet flow maldistribution in the stacking direction on the performance of a solid oxide fuel cell stack

This study examines the performance of a ten-cell solid oxide fuel cell (SOFC) stack with a non-uniform flow rate in the stacking direction. The author develops a two-dimensional numerical method to solve the electrochemical, mass and energy equations one stack at a time. The energy equations couple the heat exchange between the interconnector and both the cell and the flowing gas of adjacent cells. Moreover, this paper considers two boundary conditions, adiabatic and constant temperature, on the top and bottom faces of the SOFC. The results show that the non-uniform inlet flow rate of the fuel dominates the current density distribution; it causes the cell voltage to vary by over 13% for both boundary conditions. In addition, the constant temperature condition in this study can produce 3% more power than with the adiabatic condition. On the other hand, the air dominates the temperature field of a SOFC, and the non-uniform inlet flow rate of the air produces a variation of 3% in the average cell temperature of the cells when the boundary condition is adiabatic. This non-uniform effect on the electrical performance of each stack is apparently larger than in the transverse direction, which has been examined in our previous research.     

Computer simulation of a biomass gasification-solid oxide fuel cell power system using Aspen Plus

The operation and performance of a SOFC (solid oxide fuel cell) stack on biomass syn-gas from a biomass gasification CHP (combined heat and power) plant is investigated. The objective of this work is to develop a model of a biomass-SOFC system capable of predicting performance under diverse operating conditions. The tubular SOFC technology is selected. The SOFC stack model, equilibrium type based on Gibbs free energy minimisation, is developed using Aspen Plus. The model performs heat and mass balances and considers ohmic, activation and concentration losses for the voltage calculation. The model is validated against data available in the literature for operation on natural gas. Operating parameters are varied; parameters such as fuel utilisation factor (U_f), current density (j) and STCR (steam to carbon ratio) have significant influence. The results indicate that there must be a trade-off between voltage, efficiency and power with respect to j and the stack should be operated at low STCR and high U_f. Operation on biomass syn-gas is compared to natural gas operation and as expected performance degrades. The realistic design operating conditions with regard to performance are identified. High efficiencies are predicted making these systems very attractive.     

Study of a renewable biomass fueled SOFC: The effect of catalysts

This study demonstrated the feasibility of a tubular solid oxide fuel cell (SOFC) operating on fuels gasified from a carbon-bed containing a renewable biomass. Coconut shell carbon was used as the renewable biomass for the gasification to power the SOFC at 800 °C. The study was particularly focused on the effect of catalysts on the efficiency of the gasification process, thus the performance of tubular SOFC powered by these gasified fuels. SOFCs with catalysts such as Fe₂O₃ and K₂CO₃ added coconut shell carbon showed higher power density compared to that without catalyst. The improved cell performance can be ascribed to a promoted carbon conversion rate by the catalysts, and consequently higher concentration of gaseous fuels.     

CFD model for tubular SOFC stack fed directly by biomass

The energy transition can also be promoted by the sustainable use of biomass. Residual biomass in the Mediterranean areas can be exploited to a greater extent through highly efficient fuel cell systems. The Direct Biomass-SOFC project is based on a direct coupling between biomass power supply and SOFC tubular cells. This research project stems from the need to cover the electricity demand, avoiding the use of non-renewable sources. It will be investigated the unused or little-used biomass sources that can be exploited from the Mediterranean area.To this purpose, analyses were conducted to model a SOFC tubular cell stack by investigating the optimal configuration. The basic objective is to design a SOFC tubular cell stack, fed by syngas to produce at least 200 W. Two configurations were chosen: a square and a circular arrangement. Another objective of the study is to choose the best temperature control system. It have been selected a pressurised water system and an air system. The results show that the best performance is guaranteed by a square arrangement with an air temperature control system. The circular configuration provides less power than the square configuration, being limited by the multiple series connection to the lowest current value. The maximum electrical power produced with the square configuration is 225 W.     

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.     

Numerical analysis of output characteristics of tubular SOFC with internal reformer

In the solid oxide fuel cell (SOFC) system, the internal reforming of raw fuel will act as an efficient cooling system. To realize this cooling system, a special design of the internal reformer is required to avoid the inhomogeneous temperature distribution caused by the strong endothermic reforming reaction at the entrance of the internal reformer. For this purpose, a tubular internal reformer with adjusted catalyst density can be inserted into the tubular SOFC stack. By arranging this, the raw fuel flows along the axis of the internal reformer to be moderately reformed and returns at the end of the internal reformer as a sufficiently reformed fuel.In this paper, the output characteristics of this configuration are simulated using mathematical models, in which one-dimensional temperature and molar distributions are computed along the flow direction. By properly mounting the catalyst density in the internal reformer, the temperature distribution of the cell stack becomes moderate, and the power generation efficiency and the exhaust gas temperature are higher. Effects of other operating conditions such as fuel recirculation, fuel inlet temperature, air recirculation and air inlet temperature are also examined under the condition where the maximum temperature of the stack is kept at 1300 K by adjusting the air flow rate. Under this condition, these operating conditions exert a considerable effect on the exhaust temperature but have a slight effect on the efficiency.     

A novel design and performance of cone-shaped tubular anode-supported segmented-in-series solid oxide fuel cell stack

A novel design of cone-shaped tubular segmented-in-series solid oxide fuel cell (SOFC) stack is presented in this paper. The cone-shaped tubular anode substrates are fabricated by slip casting technique and the yttria-stabilized zirconia (YSZ) electrolyte films are deposited onto the anode tubes by dip coating method. After sintering at 1400 °C for 4 h, a dense and crack-free YSZ film with a thickness of about 7 μm is successfully obtained. The single cell, NiO-YSZ/YSZ (7 μm)/LSM-YSZ, provides a maximum power density of 1.78 W cm⁻² at 800 °C, using moist hydrogen (75 ml min⁻¹) as fuel and ambient air as oxidant.A two-cell-stack based on the above-mentioned cone-shaped tubular anode-supported SOFC is fabricated. Its typical operating characteristics are investigated, particularly with respect to the thermal cycling test. The results show that the two-cell-stack has good thermo-mechanical properties and that the developed segmented-in-series SOFC stack is highly promising for portable applications.     

Monolithic flat tubular types of solid oxide fuel cells with integrated electrode and gas channels

A new monolithic solid oxide fuel cell (SOFC) design stacked with flatten tubes of unit cells without using metallic interconnector plate is introduced and evaluated in this study. The anode support is manufactured in a flat tubular shape with fuel channel inside and air gas channel on the cathode surface. This design allows all-ceramic stack to provide flow channels and electrical connection between unit cells without needing metal plates. This structure not only greatly reduces the production cost of SOFC stack, but also fundamentally avoids chromium poisoning originated from a metal plate, thereby improving stack stability. The fuel channel was created in the extrusion process by using the outlet shape of mold. The air channel was created by grinding the surface of pre-sintered support. The anode functional layer and electrolyte were dip-coated on the support. The cathode layer and ceramic interconnector were then spray coated. The maximum power density and total resistance of unit cell with an active area of 30 cm² at 800 °C were 498 mW/cm² and 0.67 Ωcm², respectively. A 5-cell stack was assembled with ceramic components only without metal plates. Its maximum power output at 750 °C was 46 W with degradation rate of 0.69%/kh during severe operation condition for more than 1000 h, proving that such all-ceramic stack is a strong candidate as novel SOFC stack design.     

A novel system for the production of pure hydrogen from natural gas based on solid oxide fuel cell–solid oxide electrolyzer

In this paper, a novel process for the production of pure hydrogen from natural gas based on the integration of solid oxide fuel cells (SOFCs) and solid oxide electrolyzer cells (SOECs) is presented. In this configuration, the SOFC is fed by natural gas and provides electricity and heat to the SOEC, which carries out the separation of steam into hydrogen and oxygen. Depending on the system layout considered, the oxygen available at the SOEC anode outlet can be either mixed with the SOFC cathode stream in order to improve the SOFC performance or regarded as a co-product. Two configurations of the cell stack are studied. The first consists of a stack with the same number of SOFCs and SOECs working at the same current density. In this case, since in typical operating conditions the voltage delivered by the SOFC is lower than the one required by the SOEC, the required additional power is supplied by means of an electric grid connection. In the second case, the electricity balance is compensated by providing additional SOFCs to the stack, which are fed by a supplementary natural gas feed. Simulations carried out with Aspen Plus show that pure hydrogen can be produced with a natural gas to hydrogen LHV-efficiency that is about twice the value of a typical water electrolyzer and comparable to that of medium-scale reformers.     

Innovative design to improve the power density of a solid oxide fuel cell

This paper explores the possibility of improving the power density of a solid oxide fuel cell (SOFC). A three-dimensional computational model (CFD-ACE package), with the relevant sub-models was used for the study. The performance of the SOFC was examined with a thin wall, which splits the inlet section and runs up to half the length of the flow channels. The results obtained with this (thin-walled) geometry were consistently better than those obtained with plain geometry (without the thin wall). The polarization characteristics of the thin-walled geometry indicated that the maximum power density obtained was 1.18 W cm⁻² at an efficiency of around 60%. The corresponding values of maximum power density and the efficiency at which it was obtained for a plain geometry were 0.88 W cm⁻² and 50%, respectively. The enhanced performance of the thin-walled geometry was attributed to a better distribution of the reactants along the length of the SOFC. Studies were also conducted to verify the performance of the thin-walled geometry over a wide range of inlet mass flow rates. They revealed a superior performance of the thin-walled geometry compared to the plain geometry. At lower inlet mass flow rates, the difference between the two in performance was small, but at higher inlet mass flow rates the difference in performance was significant.     

Evaluation of cost reduction potential for 1 kW class SOFC stack production: Implications for SOFC technology scenario

Recently, a commercial version of a residential solid oxide fuel cell (SOFC) system with a flat tubular cell has been developed. However, the system cost still remains very high, which is a barrier to its widespread use. In this study, the potential for cost reductions in SOFC stack production was investigated in order to contribute to the viability of the widespread use of such residential SOFC systems in future. A cost analysis of 700 W SOFC stack production based on a process integration modeling was conducted. The present bottom–up approach enabled us to perform a sensitivity analysis with a variety of parameters in terms of cell design, the production process and cell performance. This allowed us to investigate the effects of these factors on the production cost, thereby revealing the quantitative impact of each technological improvement on the cost reduction potential. The present analysis also revealed innovation pathways which could result in technology scenarios where residential SOFC systems could reach a break-even point in comparison with the baseload electricity cost. The analysis of the cost reduction potential presented here provides a useful viewpoint for developing a research strategy for state-of-the-art SOFC technology.     

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%.     

Cathode-side electrical contact and contact materials for solid oxide fuel cell stacking: A review

In a planar solid oxide fuel cell (SOFC) stack, a number of individual cells are stacked together to increase the voltage and power output. At both the cathode– and anode–interconnect interfaces, electrical contact layers are applied between the interconnect and electrodes during cell fabrication process or stack assembly to increase the electrode-interconnect contact area and to compensate for dimensional tolerance variation of the contacting components, thus minimizing ohmic contact resistance throughout the stack. As such, electrical contact is an essential component in SOFC stacks. In this paper, we review the cathode-side electrical contact design and contact materials for application in SOFC stacks. Following an introduction of the function and working principles of electrical contact, the material requirements for cathode-side contact layer in SOFC stacks are outlined. The current materials for the cathode–interconnect contact are thoroughly reviewed, including noble metals, conductive ceramics (e.g. perovskites and spinels), composites, and other more complex structures. Several potential directions for cathode–interconnect contact material research and development are also highlighted.     

Thermodynamic model and parametric analysis of a tubular SOFC module

Solid oxide fuel cells (SOFCs) have been considered in the last years as one of the most promising technologies for very high-efficiency electric energy generation from natural gas, both with simple fuel cell plants and with integrated gas turbine-fuel cell systems. Among the SOFC technologies, tubular SOFC stacks with internal reforming have emerged as one of the most mature technology, with a serious potential for a future commercialization. In this paper, a thermodynamic model of a tubular SOFC stack, with natural gas feeding, internal reforming of hydrocarbons and internal air preheating is proposed. In the first section of the paper, the model is discussed in detail, analyzing its calculating equations and tracing its logical steps; the model is then calibrated on the available data for a recently demonstrated tubular SOFC prototype plant. In the second section of the paper, it is carried out a detailed parametric analysis of the stack working conditions, as a function of the main operating parameters. The discussion of the results of the thermodynamic and parametric analysis yields interesting considerations about partial load SOFC operation and load regulation, and about system design and integration with gas turbine cycles.     

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.     

Cold start dynamics and temperature sliding observer design of an automotive SOFC APU

This paper presents a dynamic model for studying the cold start dynamics and observer design of an auxiliary power unit (APU) for automotive applications. The APU is embedded with a solid oxide fuel cell (SOFC) stack which is a quiet and pollutant-free electric generator; however, it suffers from slow start problem from ambient conditions. The SOFC APU system equips with an after-burner to accelerate the start-up transient in this research. The combustion chamber burns the residual fuel (and air) left from the SOFC to raise the exhaust temperature to preheat the SOFC stack through an energy recovery unit. Since thermal effect is the dominant factor that influences the SOFC transient and steady performance, a nonlinear real-time sliding observer for stack temperature was implemented into the system dynamics to monitor the temperature variation for future controller design. The simulation results show that a 100 W APU system in this research takes about 2 min (in theory) for start-up without considering the thermal limitation of the cell fracture.