Developing fuel map to predict the effect of fuel composition on the maximum efficiency of solid oxide fuel cells

At any given cell operating condition, a fuel map can be developed to predict the effect of a fuel containing carbon, hydrogen, oxygen and inert gas atoms on the maximum cell efficiency (MCE) of solid oxide fuel cells (SOFCs). To create a fuel map, a thermodynamic model is developed to obtain the fuels that would yield identical MCE for SOFCs. These fuels make a continuous curve in the ternary coordinate system. A fuel map is established by developing continuous fuel curves for different MCEs at the same operating condition of a cell and representing them in the carbon-hydrogen-oxygen (C-H-O) ternary diagram. The graphical representation of fuel maps can be applied to predict the effect of the fuel composition and fuel processor on the MCE of SOFCs. As a general result, among the fuels that can be directly utilized in SOFCs, at the same temperature and pressure, the one located at the intersection of the H-C axis and the carbon deposition boundary (CDB) curve in the C-H-O ternary diagram provides the highest MCE. For any fuel that can be indirectly utilized in SOFCs, the steam reforming fuel processor always yields a higher MCE than auto-thermal reforming or partial oxidation fuel processors at the same anode inlet fuel temperature.     

Thermal stress and thermo-electrochemical analysis of a planar anode-supported solid oxide fuel cell: Effects of anode porosity

A 3D integrated numerical model is constructed to evaluate the thermal-fluid behavior and thermal stress characteristics of a planar anode-supported solid oxide fuel cell (SOFC). Effects of anode porosity on performance, temperature gradient and thermal stress are investigated. Using commercial Star-CD software with the es-sofc module, simulations are performed to obtain the current-voltage (I-V) characteristics of a fuel cell as a function of the anode porosity and the temperature distribution within the fuel cell under various operating conditions. The temperature field is then imported into the MARC finite element analysis (FEA) program to analyze thermal stresses induced within the cell. The numerical results are found to be in good agreement with the experimental data. It is shown that the maximum principal stress within the positive electrode-electrolyte-negative electrode (PEN) increases at a higher current and a higher temperature gradient. It is recommended that the temperature gradient should be limited to less than 10.6 °C mm⁻¹ to maintain the structural integrity of the PEN.     

Control-oriented modelling and analysis of a solid oxide fuel cell system

In this paper, a control-oriented model of a solid oxide fuel cell system is formulated and analyzed in detail. First, a lumped model based on first principle laws is formulated and tuned using experimental data coming from a real solid oxide fuel cell system test bench. The model calibration is carried out based on an optimization approach to minimize the error between the experimental data and the model one. To systematically analyze the system behavior, an equilibrium point analysis is formulated and developed. The analysis results show the maximum steady-state electrical power under each constant stack temperature. This will allow to appropriately select operation points during the system operation. Secondly, Lyapunov's theory is used to characterize the local stability of the equilibrium points. The results show that the equilibrium points are locally stable. Besides, comparison between the initial nonlinear model with the linearized model is performed to show the efficacy of the linearised model analysis. Finally, the frequency response of the linearized model is performed. This analysis provides key information about control system design in order to efficiently operate the solid oxide fuel cell system.     

Thermal stress modeling of anode supported micro-tubular solid oxide fuel cell

An anode-supported micro-tubular solid oxide fuel cell (SOFC) is analyzed by a two-dimensional axisymmetric numerical model, which is validated with the experimental I-V data. The temperature distribution generated by the thermo-electrochemical model is used to calculate the thermal stress field in the tubular SOFC. The results indicate that the current transport in the anode is the same at every investigated position. The stress of the micro-tubular cell occurs mainly because of the residual stress due to the mismatch between the coefficients of thermal expansion of the materials of the membrane electrode assembly. The micro-tubular cell can operate safely, but if there is an interfacial defect or a high enough tensile stress applied at the electrolyte, a failure can arise.     

Effects of methane processing strategy on fuel composition,electrical and thermal efficiency of solid oxide fuel cell

Natural gas is a cheap and abundant fuel for solid oxide fuel cell (SOFC), generally integrating the SOFC system with methane pre-treating system for improving the stability of SOFC. In this paper, the accurate effects of methane processing strategy on fuel composition, electrical efficiency and thermal efficiency of SOFC are investigated based on the thermodynamic equilibrium. Steam reforming of methane is an endothermic process and can produce 3 mol of H₂ and 1 mol of CO from 1 mol of methane, and thus the electrical efficiency of SOFC is high at the same O/C ratio and equivalent fuel utilization, whereas the thermal efficiency is low. On the contrary, partial oxidation of methane is an exothermal process and only produces 2 mol of H₂ and 1 mol of CO from 1 mol of methane, and thus the electrical efficiency of SOFC is low at the same O/C ratio and equivalent fuel utilization, whereas the thermal efficiency is high. When the O/C ratio is 1.5, the electrical efficiency of SOFC is 55.3% for steam reforming of methane, while 32.7% for partial oxidation of methane. High electrical efficiency of SOFC can be achieved and carbon deposition can be depressed by selecting suitable O/C ratio from methane pretreatment according to the accurate calculation and analysis of effects of different methane processing strategies on the electrical efficiency and thermal efficiency of SOFC.     

Analysis of a solid oxide fuel cell and a molten carbonate fuel cell integrated system with different configurations

A solid oxide fuel cell with internal reforming operation is run at partial fuel utilization; thus, the remaining fuel can be further used for producing additional power. In addition, the exhaust gas of a solid oxide fuel cell still contains carbon dioxide, which is the primary greenhouse gas, and identifying a way to utilize this carbon dioxide is important. Integrating the solid oxide fuel cell with the molten carbonate fuel cell is a potential solution for carbon dioxide utilization. In this study, the performance of the integrated fuel cell system is analyzed. The solid oxide fuel cell is the main power generator, and the molten carbonate fuel cell is regarded as a carbon dioxide concentrator that produces electricity as a by-product. Modeling of the solid oxide fuel cell and the molten carbonate fuel cell is based on one-dimensional mass balance, considering all cell voltage losses. Primary operating conditions of the integrated fuel cell system that affect the system efficiencies in terms of power generation and carbon dioxide utilization are studied, and the optimal operating parameters are identified based on these criteria. Various configurations of the integrated fuel cell system are proposed and compared to determine the suitable design of the integrated fuel cell system.     

Performance analysis and temperature gradient of solid oxide fuel cell stacks operated with bio-oil sorption-enhanced steam reforming

A high temperature gradient within a solid oxide fuel cell (SOFC) stack is considered a major challenge in SOFC operations. This study investigates the effects of the key parameters on SOFC system efficiency and temperature gradient within a SOFC stack. A 40-cell SOFC stack integrated with a bio-oil sorption-enhanced steam reformer is simulated using MATLAB and DETCHEM. When the air-to-fuel ratio and steam-to-fuel ratio increase, the stack average temperature and temperature gradient decrease. However, a decrease in the stack temperature steadily reduces the system efficiency owing to the tradeoff between the stack performance and thermal balance between heat recovered and consumed by the system. With an increase in the bio-oil flow rate, the system efficiency decreases because of the lower resident time for the electrochemical reaction. This is not, however, beneficial to the maximum temperature gradient. To minimize the temperature gradient of the SOFC stack, a decrease in the bio-oil flow rate is the most effective way. The maximum temperature gradient can be reduced to 14.6 K cm⁻¹ with the stack and system efficiency of 76.58 and 65.18%, respectively, when the SOFC system is operated at an air-to-fuel ratio of 8, steam-to-fuel ratio of 6, and bio-oil flow rate of 0.0041 mol s⁻¹.     

Performance assessment and optimization of an integrated solid oxide fuel cell-gas turbine cogeneration system

The combined solid oxide fuel cells and gas turbine (SOFC/GT) system is known to be a potential alternative for distributed power generation. In this paper, a novel SOFC/GT based cogeneration system, which integrated a transcritical carbon dioxide cycle (TRCC) with a LNG cold energy utilization system is proposed. A mathematical (zero-dimensional) model is developed to analyze the co-generation system performance from the perspective of thermodynamic (energy and exergy) and economic costs. The main parameters of the system are chosen to analyze their effects on thermodynamic performance. The results show that the current system can achieve 64.40% thermal efficiency and 62.13% exergy efficiency under given conditions, and can further improve efficiency through parameter optimization. Finally, the multi-objective optimization program using NSGA-II (Non-dominated Sorting Genetic Algorithm II) is used to obtain the optimal value of the system design parameters. In the multi-objective analysis, the thermodynamic efficiency and economic cost of the system are considered as objective functions. The optimization results show that the final optimized design selected from the Pareto front can achieve 63.08% thermal efficiency and 61.10% exergy efficiency, respectively.     

Thermal impact performance study for the thermal management of ammonia-fueled single tubular solid oxide fuel cell

With the substantial improvement of the direct ammonia fuel cells performance, it has become the key to the further development of ammonia fuel cells to deeply understand the heat and mass transfer process inside the cell and to study the thermal impacts generation mechanism during cell operation. In this paper, a whole-cell model of single tubular direct ammonia cracking solid oxide fuel cell (SOFC) is established, and the generation mechanism of thermal impacts inside the cell is analysed in a data-driven method. The model includes the coupling of chemical-electrochemical reactions, local current, local temperature, mass flow and energy transfer inside the cell. It's identified from model simulations that the key to the thermal impact optimization of direct ammonia cracking SOFCs is to reduce the effect of the excessively fast and unbalanced ammonia cracking reaction on the cell. Both introducing the ammonia pre-reforming reaction and improving the activation energy of the ammonia cracking reaction can increase the overall average temperature of the cell and improve the temperature distribution. The 96% ammonia pre-reforming SOFCs can improve the extreme temperature difference in the anode from 37.71 K to 0.52 K at the operating temperature of 800 °C. Increasing activation energy of ammonia cracking reaction by 1.5 times can also make the ammonia cracking reaction rate distribution more uniform at the fuel channel, it can improve the extreme temperature difference in the anode to 4.49 K. This study can enrich the basic theory and research methods of thermal management of direct ammonia cracking SOFCs, and provide theoretical support for further improving cell performance.     

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.     

Thermal imaging of solid oxide fuel cell anode processes

A Si-charge-coupled device (CCD), camera-based, near-infrared imaging system is demonstrated on Ni/yttria-stabilized zirconia (YSZ) fragments and the anodes of working solid oxide fuel cells (SOFCs). NiO reduction to Ni by H₂ and carbon deposition lead to the fragment cooling by 5 ± 2 °C and 16 ± 1 °C, respectively. When air is flowed over the fragments, the temperature rises 24 ± 1 °C as carbon and Ni are oxidized. In an operational SOFC, the decrease in temperature with carbon deposition is only 4.0 ± 0.1 °C as the process is moderated by the presence of oxides and water. Electrochemical oxidation of carbon deposits results in a ΔT of +2.2 ± 0.2 °C, demonstrating that electrochemical oxidation is less vigorous than atmospheric oxidation. While the high temperatures of SOFCs are challenging in many respects, they facilitate thermal imaging because their emission overlaps the spectral response of inexpensive Si-CCD cameras. Using Si-CCD cameras has advantages in terms of cost, resolution, and convenience compared to mid-infrared thermal cameras. High spatial (0.1 mm) and temperature (0.1 °C) resolutions are achieved in this system. This approach provides a convenient and effective analytical technique for investigating the effects of anode chemistry in operating SOFCs.     

Tuneable diode laser gas analyser for methane measurements on a large scale solid oxide fuel cell

A new in-line, real time gas analyser is described that uses tuneable diode laser spectroscopy (TDLS) for the measurement of methane in solid oxide fuel cells. The sensor has been tested on an operating solid oxide fuel cell (SOFC) in order to prove the fast response and accuracy of the technology as compared to a gas chromatograph. The advantages of using a TDLS system for process control in a large-scale, distributed power SOFC unit are described. In future work, the addition of new laser sources and wavelength modulation will allow the simultaneous measurement of methane, water vapour, carbon-dioxide and carbon-monoxide concentrations.     

Development of 1 kW class solid oxide fuel cell stack using anode-supported planar cells

We have developed a 1 kW class solid oxide fuel cell (SOFC) stack composed of 50 anode-supported planar 120-mm-diameter SOFCs. Intermediate plates, which exhibited negligible deformation under operating conditions, were placed in the stack to cancel out the cumulative error related to the position and angle of the stack parts. The stack provided an electrical conversion efficiency of 54% (based on the lower heating value (LHV) of the methane used as a fuel) and an output of 1120 W when the fuel utilization, current density, and operating temperature were 67%, 0.28 A cm⁻², and 1073 K, respectively. The stack operated stably for almost 700 h.     

Monte-Carlo simulation and performance optimization for the cathode microstructure in a solid oxide fuel cell

A 3D micro-scale model is developed to simulate the transport and electrochemical reaction in a composite cathode. This model takes into account the details of the specific cathode microstructure such as random pore structure, active TPB (three phase boundary) site distribution, particle size and composition and their interrelationship to the charge transfer and mass transport processes. Especially, the pore structure and mass diffusion were incorporated into this model. Influence of the microsturcture parameters on the performance was investigated by numerical simulations.     

Energy and exergy analysis of internal reforming solid oxide fuel cell–gas turbine hybrid system

The aim of this work is to analyze methane-fed internal reforming solid oxide fuel cell–gas turbine (IRSOFC—GT) power generation system based on the first and second law of thermodynamics. Exergy analysis is used to indicate the thermodynamic losses in each unit and to assess the work potentials of the streams of matter and of heat interactions. The system consists of a prereformer, a SOFC stack, a combustor, a turbine, a fuel compressor and air compressor, recuperators and a heat recovery steam generator (HRSG). A parametric study is also performed to evaluate the effect of various parameters such as fuel flow rate, air flow rate, temperature and pressure on system performance.     

Thermodynamic analysis of an integrated power generation system driven by solid oxide fuel cell

A new integrated power generation system driven by the solid oxide fuel cell (SOFC) is proposed to improve the conversion efficiency of conventional energy by using a Kalina cycle to recover the waste heat of exhaust from the SOFC-GT. The system using methane as main fuel consists an internal reforming SOFC, an after-burner, a gas turbine, preheaters, compressors and a Kalina cycle. The proposed system is simulated based on the developed mathematical models, and the overall system performance has been evaluated by the first and second law of thermodynamics. Exergy analysis is conducted to indicate the thermodynamic losses in each components. A parametric analysis is also carried out to examine the effects of some key thermodynamic parameters on the system performance. Results indicate that as compressor pressure ratio increases, SOFC electrical efficiency increases and there is an optimal compressor pressure ratio to reach the maximum overall electrical efficiency and exergy efficiency. It is also found that SOFC electrical efficiency, overall electrical efficiency and exergy efficiency can be improved by increasing air flow rate. Also, the largest exergy destruction occurs in the SOFC followed by the after-burner, the waste heat boiler, the gas turbine. The compressor pressure ratio and air flow rate have significant effects on the exergy destruction in some main components of system.     

Solid oxide fuel cell systems for distributed power generation and cogeneration

Solid oxide fuel cell (SOFC) is a promising technology for decentralized power generation and cogeneration. This technology has several advantages: the high electric efficiency, which can be theoretically improved through integration in power cycles; the low emissions; and the possibility of using a large variety of gaseous fuels.     

Developing fuel map to predict the effect of fuel composition on the maximum voltage of solid oxide fuel cells

A thermodynamic model is developed to determine the fuels that would yield an identical maximum cell voltage (MCV) for solid oxide fuel cells (SOFCs) at a given operating condition. These fuels make a continuous curve in the ternary coordinate system. A fuel map is established by developing the continuous fuel curves for different MCVs at the same operating condition and representing them in the carbon-hydrogen-oxygen (C-H-O) ternary diagram. Using the fuel map, the effect of the composition of a fuel containing carbon, hydrogen, oxygen, and inert gas atoms on the MCV of SOFCs can be easily studied. In addition to the effect of the fuel composition, the graphical representation of fuel maps can be applied to study the effect of the fuel processors on the MCV of SOFCs. As a general result, among fuels that can be directly utilized in SOFCs, at the same temperature and pressure, the one located at the intersection of the H-C axis and the carbon deposition boundary (CDB) curve in the C-H-O ternary diagram, provides the highest MCV for SOFCs. The results also show that for the fuels that cannot be directly utilized in SOFC, the steam reforming fuel processor always yields a higher MCV than the autothermal reforming or the partial oxidation fuel processors at the same inlet fuel temperature.     

Effects of fuel processing methods on industrial scale biogas-fuelled solid oxide fuel cell system for operating in wastewater treatment plants

The performance of three solid oxide fuel cell (SOFC) systems, fuelled by biogas produced through anaerobic digestion (AD) process, for heat and electricity generation in wastewater treatment plants (WWTPs) is studied. Each system has a different fuel processing method to prevent carbon deposition over the anode catalyst under biogas fuelling. Anode gas recirculation (AGR), steam reforming (SR), and partial oxidation (POX) are the methods employed in systems I-III, respectively. A planar SOFC stack used in these systems is based on the anode-supported cells with Ni-YSZ anode, YSZ electrolyte and YSZ-LSM cathode, operated at 800 °C. A computer code has been developed for the simulation of the planar SOFC in cell, stack and system levels and applied for the performance prediction of the SOFC systems. The key operational parameters affecting the performance of the SOFC systems are identified. The effect of these parameters on the electrical and CHP efficiencies, the generated electricity and heat, the total exergy destruction, and the number of cells in SOFC stack of the systems are studied. The results show that among the SOFC systems investigated in this study, the AGR and SR fuel processor-based systems with electrical efficiency of 45.1% and 43%, respectively, are suitable to be applied in WWTPs. If the entire biogas produced in a WWTP is used in the AGR or SR fuel processor-based SOFC system, the electricity and heat required to operate the WWTP can be completely self-supplied and the extra electricity generated can be sold to the electrical grid.     

Electrochemical properties and thermal neutral state of solid oxide fuel cells with direct internal reforming of methane

Solid oxide fuel cells (SOFCs) with direct internal reforming (DIR) provide a promising method to realize clean and efficient utilization of hydrocarbon fuels. Thse endothermic reforming reactions occur simultaneously with exothermic electrochemical reactions at the anode, making thermal neutral state achievable inside a fuel cell, providing reference to the thermal management. In this study, a calculation model combining experimental data and thermodynamic results was established, validating the possibility of achieving thermal neutral state in DIR-SOFCs. In the process of modeling, the electrochemical and thermodynamic characteristics in direct internal steam and dry reforming were elaborately compared, contributing to a more scientific understanding of anode reaction mechanism. Detailed experimental investigation was carried out to determine the influence of H₂O/CO₂ on the electrochemical properties of DIR-SOFCs, based on which the optimum steam-carbon ratio (S/C) and CO₂ to CH₄ ratios were obtained. Besides, analysis of distribution of relaxation times (DRT) combined with elementary reactions in CH₄H₂O and CH₄CO₂ atmospheres were proposed to distinguish different physical and chemical processes within anodes. The results of this study can be conducive to a more precise understanding of reaction mechanism on SOFC anodes and meaningful for practical application of DIR-SOFCs.