Thermodynamic assessment of solid oxide fuel cell system integrated with bioethanol purification unit

A solid oxide fuel cell system integrated with a distillation column (SOFC–DIS) has been proposed in this article. The integrated SOFC system consists of a distillation column, an EtOH/H₂O heater, an air heater, an anode preheater, a reformer, an SOFC stack and an afterburner. Bioethanol with 5 mol% ethanol was purified in a distillation column to obtain a desired concentration necessary for SOFC operation. The SOFC stack was operated under isothermal conditions. The heat generated from the stack and the afterburner was supplied to the reformer and three heaters. The net remaining heat from the SOFC system (Q_SOFC,Net) was then provided to the reboiler of the distillation column. The effects of fuel utilization and operating voltage on the net energy (Q_Net), which equals Q_SOFC,Net minus the distillation energy (Q_D), were examined. It was found that the system could become more energy sufficient when operating at lower fuel utilization or lower voltage but at the expense of less electricity produced. Moreover, it was found that there were some operating conditions, which yielded Q_Net of zero. At this point, the integrated system provides the maximum electrical power without requiring an additional heat source. The effects of ethanol concentration and ethanol recovery on the electrical performance at zero Q_Net for different fuel utilizations were investigated. With the appropriate operating conditions (e.g. C_EtOH = 41%, U_f = 80% and EtOH recovery = 80%), the overall electrical efficiency and power density are 33.3% (LHV) and 0.32 W cm⁻², respectively.     

Performance improvement of bioethanol-fuelled solid oxide fuel cell system by using pervaporation

This work proposes an improvement in performance with respect to the electrical efficiency of a bioethanol-fuelled Solid Oxide Fuel Cell (SOFC) system by replacing a conventional distillation column by a pervaporation unit in the bioethanol purification process. The simulation study indicates that the membrane separation factor has a significant influence on the electrical power and heat energy required to generate a feed of 25 mol% ethanol in water to the reformer. The values of overall electrical efficiency of the SOFC systems with a distillation column and with a pervaporation unit are compared under the thermally self-sufficient condition (Q_net = 0) which offers their maximum electrical efficiency. At the base case, the SOFC system with a pervaporation unit provides an electrical efficiency of 42% compared with 34% achieved from the system with a distillation unit, indicating a significant improvement by using a pervaporation unit. An increase in ethanol recovery can further improve the overall electrical efficiency. The study also reveals that further improvement of the membrane selectivity can slightly enhance the overall efficiency of the SOFC system. Finally, an economic analysis of a bioethanol-fuelled SOFC system with pervaporation is suggested as the basis for further development.     

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.     

Thermodynamic modeling of an integrated biomass gasification and solid oxide fuel cell system

An integrated process of biomass gasification and solid oxide fuel cells (SOFC) is investigated using energy and exergy analyses. The performance of the system is assessed by calculating several parameters such as electrical efficiency, combined heat and power efficiency, power to heat ratio, exergy destruction ratio, and exergy efficiency. A performance comparison of power systems for different gasification agents is given by thermodynamic analysis. Exergy analysis is applied to investigate exergy destruction in components in the power systems. When using oxygen-enriched air as gasification agent, the gasifier reactor causes the greatest exergy destruction. About 29% of the chemical energy of the biomass is converted into net electric power, while about 17% of it is used to for producing hot water for district heating purposes. The total exergy efficiency of combined heat and power is 29%. For the case in which steam as the gasification agent, the highest exergy destruction lies in the air preheater due to the great temperature difference between the hot and cold side. The net electrical efficiency is about 40%. The exergy combined heat and power efficiency is above 36%, which is higher than that when air or oxygen-enriched air as gasification agent.     

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.     

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.     

Design for segmented-in-series solid oxide fuel cell through mathematical modeling

The segmented-in-series solid oxide fuel cell comprising fuel channel, anode, cathode and electrolyte layers has been evaluated by developing a two-dimensional model, in which the equations have been solved numerically through finite element methods. The results indicate that the voltage of each membrane electrode assembly (MEA) exhibits a parabola-like curve and is higher than the appointed voltage of unit cell (0.7 V). From fuel inlet to outlet, the voltage of each MEA deceases due to the decreasing local H₂ concentration. When both the interconnector and electrolyte gap lengths are fixed, the cell module with 5 mm long anode gives the maximal power density for the SS-SOFC. Higher power densities can be achieved through increasing the cathode thickness.     

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.     

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%.     

Multi-objective optimal droop control of solid oxide fuel cell based integrated energy system

Solid oxide fuel cell (SOFC) based integrated energy system (IES) is promising in the future low-carbon power generation market, due to the high efficiency and flexibility. However, it is challenging for the dynamic control design in dealing with the conflicting objectives in terms of fast power tracking and overall efficiency during the transient process of load response. To this end, this paper develops a multi-objective optimal droop control strategy for the real-time power dispatch of the IES. Firstly, a nonlinear implicit dynamic model consisting of SOFC, lithium-ion battery, photovoltaic array and DC-DC converter is developed. Then, a multi-objective optimization is formulated to balance the power tracking performance and transient efficiency. Non-dominated sorting genetic algorithm-II (NSGA-II) is adopted to search the optimal parameters for droop controller. Simulation results demonstrates that the electricity loss of the proposed method can be reduced by 96.26% with a slight compromise in power tracking performance.     

Design and performance analysis of a de-coupled solid oxide fuel cell gas turbine hybrid

Hybrid solid oxide fuel cells (SOFC) cycles of varying complexity are widely studied for their potential efficiency, carbon recovery and co-production of chemicals. This study introduces an alternative de-coupled fuel cell-gas turbine hybrid arrangement that retains the high efficiency thermal integration of a topping cycle without the high temperature heat exchanger of a bottoming cycle. The system utilizes a solid-state oxygen transport membrane to divert 30%–50% of the oxygen from the turbine working fluid to the intermediate temperature SOFC. Thermodynamic modeling delineates design trade-offs and identifies a flexible operating regime with peak fuel-to-electric efficiency of 75%. Co-production of electricity and high purity hydrogen result in net energy conversion efficiencies greater than 80%. The potential to retrofit existing turbine systems, particularly micro-turbines and stand-by ‘peaker’ plants, with minimal impact to compressor stability or transient response is a promising pathway to hybrid fuel cell/turbine development that does not require turbomachinery modification.     

Effects of heat sources on the performance of a planar solid oxide fuel cell

This paper presents an analysis of the effects of heat sources on performance of a planar anode-supported solid oxide fuel cell (SOFC). Heat sources in SOFCs include ohmic heat losses, heat released by chemical and electrochemical processes and radiation. We take into account the first three types of heat source here while neglecting the last type as it is supposed to be negligibly small. The cell is working under conditions of direct internal reforming of methane and with co-flow configuration. The composite electrodes are discretized allowing the heat source associated with the electrochemical processes to be implemented in a layer of finite thickness. Two cases are investigated, one where the electrochemical heat source is implemented on the anode side (base case) and another where it is implemented on the cathode side.     

Multi-loop control strategy of a solid oxide fuel cell and micro gas turbine hybrid system

Solid oxide fuel cell and micro gas turbine (SOFC/MGT) hybrid system is a promising distributed power technology. In order to ensure the system safe operation as well as long lifetime of the fuel cell, an effective control manner is expected to regulate the temperature and fuel utilization at the desired level, and track the desired power output. Thus, a multi-loop control strategy for the hybrid system is investigated in this paper. A mathematical model for the SOFC/MGT hybrid system is built firstly. Based on the mathematical model, control cycles are introduced and their design is discussed. Part load operation condition is employed to investigate the control strategies for the system. The dynamic modeling and control implementation are realized in the MATLAB/SIMULINK environment, and the simulation results show that it is feasible to build the multi-loop control methods for the SOFC/MGT hybrid system with regard to load disturbances.     

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.     

Conceptual design of a novel ammonia-fuelled portable solid oxide fuel cell system

A novel portable electric power generation system, fuelled by ammonia, is introduced and its performance is evaluated. In this system, a solid oxide fuel cell (SOFC) stack that consists of anode-supported planar cells with Ni-YSZ anode, YSZ electrolyte and YSZ-LSM cathode is used to generate electric power. The small size, simplicity, and high electrical efficiency are the main advantages of this environmentally friendly system. The results predicted through computer simulation of this system confirm that the first-law efficiency of 41.1% with the system operating voltage of 25.6 V is attainable for a 100 W portable system, operated at the cell voltage of 0.73 V and fuel utilization ratio of 80%. In these operating conditions, an ammonia cylinder with a capacity of 0.8 l is sufficient to sustain full-load operation of the portable system for 9 h and 34 min. The effect of the cell operating voltage at different fuel utilization ratios on the number of cells required in the SOFC stack, the first- and second-law efficiencies, the system operating voltage, the excess air, the heat transfer from the SOFC stack, and the duration of operation of the portable system with a cylinder of ammonia fuel, are also studied through a detailed sensitivity analysis. Overall, the ammonia-fuelled SOFC system introduced in this paper exhibits an appropriate performance for portable power generation applications.     

Evaluation of the redox stability of segmented-in-series solid oxide fuel cell stacks

The tolerance for the reduction and oxidation (redox) reactions of the segmented-in-series solid oxide fuel cells (SIS-SOFCs) has been investigated. In conventional anode-supported solid oxide fuel cells (SOFCs), the anode and the substrate are typically prepared from Ni-YSZ-based materials which exhibit a significant dimensional change because of the redox reaction and cannot retain their structure. The substrate of the SIS-SOFCs is prepared from Ni-doped MgO-based material, which has a high redox tolerance, and the SIS-SOFC exhibits a good performance after the redox cycles.The degradation rate is approximately 0.15% per cycle in a redox condition of start-and-stop operation without fuel supply. In the other redox condition (when the fuel supply is interrupted for 1.5 min), the voltage of the SIS-SOFCs remains almost constant. However, the voltage of SIS-SOFCs decreases with an increase in the reoxidation time of the interruption in the fuel supply. The high redox tolerance is attributed to the fact that the diffusion coefficients, mean free path, and existence of the Ni particles in the substrate can effectively deter the oxidation of the anode.     

Performance of sorption-enhanced chemical looping gasification system coupled with solid oxide fuel cell using exergy analysis

Coal gasification system integrated with solid oxide fuel cell (SOFC) provides a promising energy conversion way owing to its high efficiency. To get a deep insight into the energy performance of this system, a thermodynamic evaluation is implemented. Meanwhile, the technologies of chemical looping and CO₂ sorption are introduced into this integration system. It is found that the addition of oxygen carrier and sorbent into coal gasification system can promote the output power of the SOFC with a higher exergy destruction, where the exergy efficiency of most modules in the system can reach 80% except tar separation. The results also reveal that a suitable improvement of gasifying agent amount is beneficial to the energy performance of the system. When the H₂O/C molar ratio is increased to 3.0, the SOFC exergy efficiency of 97% can be achieved.     

Fabrication of hydrogen electrode supported cell for utilized regenerative solid oxide fuel cell application

A utilized regenerative solid oxide fuel cell (URSOFC) provides the dual function of performing energy storage and power generation, all in one unit. When functioning as an energy storage device, the URSOFC acts like a solid oxide electrolyzer cell (SOEC) in water electrolysis mode; whereby the electric energy is stored as (electrolyzied) hydrogen and oxygen gases. While hydrogen is useful as a transportation fuel and in other industrial applications, the URSOFC also acts as a solid oxide fuel cell (SOFC) in power generation mode to produce electricity when needed. The URSOFC would be a competitive technology in the upcoming hydrogen economy on the basis of its low cost, simple structure, and high efficiency. This paper reports on the design and manufacturing of its anode support cell using commercially available materials. Also reported are the resulting performance, both in electrolysis and fuel cell modes, as a function of its operating parameters such as temperature and current density. We found that the URSOFC performance improved with increasing temperature and its fuel cell mode had a better performance than its electrolysis mode due to a limited humidity inlet causing concentration polarization. In addition, there were great improvements in performance for both the SOFC and SOEC modes after the first test and could be attributed to an increase in porosity within the oxygen electrode, which was beneficial for the oxygen reaction.     

Entropy generation analysis in a monolithic-type solid oxide fuel cell (SOFC)

The aim of the paper is to investigate possible improvements in the geometry design of a monolithic solid oxide fuel cells (SOFCs) through analysis of the entropy generation terms. The different contributions to the local rate of entropy generation are calculated using a computational fluid dynamic (CFD) model of the fuel cell, accounting for energy transfer, fluid dynamics, current transfer, chemical reactions and electrochemistry. The fuel cell geometry is then modified to reduce the main sources of irreversibility and increase its efficiency.     

Basic properties of a liquid tin anode solid oxide fuel cell

An unconventional high temperature fuel cell system, the liquid tin anode solid oxide fuel cell (LTA-SOFC), is discussed. A thermodynamic analysis of a solid oxide fuel cell with a liquid metal anode is developed. Pertinent thermochemical and thermophysical properties of liquid tin in particular are detailed. An experimental setup for analysis of LTA-SOFC anode kinetics is described, and data for a planar cell under hydrogen indicated an effective oxygen diffusion coefficient of 5.3 × 10⁻⁵ cm² s⁻¹ at 800 °C and 8.9 × 10⁻⁵ cm² s⁻¹ at 900 °C. This value is similar to previously reported literature values for liquid tin. The oxygen conductivity through the tin, calculated from measured diffusion coefficients and theoretical oxygen solubility limits, is found to be on the same order of that of yttria-stabilized zirconia (YSZ), a traditional SOFC electrolyte material. As such, the ohmic loss due to oxygen transport through the tin layer must be considered in practical system cell design since the tin layer will usually be at least as thick as the electrolyte.