Performance evaluation of different configurations of biogas-fuelled SOFC micro-CHP systems for residential applications

Three configurations of solid oxide fuel cell (SOFC) micro-combined heat and power (micro-CHP) systems are studied with a particular emphasis on the application for single-family detached dwellings. Biogas is considered to be the primary fuel for the systems studied. In each system, a different method is used for processing the biogas fuel to prevent carbon deposition over the anode of the cells used in the SOFC stack. The anode exit gas recirculation, steam reforming, and partial oxidation are the methods employed in systems I–III, respectively. The results predicted through computer simulation of these systems confirm that the net AC electrical efficiency of around 42.4%, 41.7% and 33.9% are attainable for systems I–III, respectively. Depending on the size, location and building type and design, all the systems studied are suitable to provide the domestic hot water and electric power demands for residential dwellings. The effect of the cell operating voltage at different fuel utilization ratios on the number of cells required for the SOFC stack to generate around 1 kW net AC electric power, the thermal-to-electric ratio (TER), the net AC electrical and CHP efficiencies, the biogas fuel consumption, and the excess air required for controlling the SOFC stack temperature is also studied through a detailed sensitivity analysis. The results point out that the cell design voltage is higher than the cell voltage at which the minimum number of cells is obtained for the SOFC stack.     

The financial viability of an SOFC cogeneration system in single-family dwellings

In the near future, fuel cell-based residential micro-CHP systems will compete with traditional methods of energy supply. A micro-CHP system may be considered viable if its incremental capital cost compared to its competitors equals to cumulated savings during a given period of time. A simplified model is developed in this study to estimate the operation of a residential solid oxide fuel cell (SOFC) system. A comparative assessment of the SOFC system vis-à-vis heating systems based on gas, oil and electricity is conducted using the simplified model for a single-family house located in Ottawa and Vancouver. The energy consumption of the house is estimated using the HOT2000 building simulation program. A financial analysis is carried out to evaluate the sensitivity of the maximum allowable capital cost with respect to system sizing, acceptable payback period, energy price and the electricity buyback strategy of an energy utility. Based on the financial analysis, small (1–2 kW_e) SOFC systems seem to be feasible in the considered case. The present study shows also that an SOFC system is especially an alternative to heating systems based on oil and electrical furnaces.     

Fused filament fabrication for anode supported SOFC development: Towards advanced,scalable and cost-competitive energetic systems

Solid Oxide Fuel Cells (SOFC) are electrochemical devices that efficiently convert hydrogen or other fuels into energy and heat and are considered a promising and environmentally friendly alternative to mitigate the effects of climate change. The difficulties found for the SOFC devices implementation into the market have triggered an enhanced need of research on the manufacturing strategy, since the cells and stack fabrication remain a complicated procedure. In this work, the capabilities of an additive manufacturing (AM) process, as it is Fused Filament Fabrication (FFF), applied to anode supported SOFC were explored for the first time, due to the strengths this technology offers in terms of materials versatility and cost-competitive process. Moreover, the ability of tuning the support porosity during the printing process was demonstrated, minimizing the pore former concentration required and avoiding reformulation of the feedstock to achieve higher porosity in the anode. In the cell fabrication, low-cost techniques for thin film deposition were employed. The preliminary geometry investigated represents a novel SOFC device that shows an electrochemical performance comparable to that found in the conventional SOFC fabrication and it opens the possibility of potential single-cell and stack designs in which the use of ceramic sealing could be avoided and the system durability increased.     

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.     

Design and eco-technoeconomic analyses of SOFC/GT hybrid systems accounting for long-term degradation effects

This study compares two SOFC/GT (solid oxide fuel cell with gas turbine) hybrid systems to that of two standalone SOFC systems via eco-technoeconomic analyses that account for long-term degradation effects. Four cases were examined: 1) standalone SOFC plant without a steam bottoming cycle; 2) standalone SOFC plant with a steam bottoming cycle; 3) SOFC/GT hybrid plant without a steam bottoming cycle; and 4) SOFC/GT with a steam bottoming cycle. This study employed a real-time 1D SOFC model with an empirical degradation calculation integrated with steady-state balance-of-plant models. Simulations used Matlab Simulink R2017a, Aspen Plus V10, and Python 3.7.4 with a pseudo steady-state approach. The results showed that, with some trade-offs, the SOFC/GT hybrid plant with the steam bottoming cycle is the best option, with an overall efficiency of 44.6% _LHV, an LCOE (levelized cost of electricity) of $US 77/MWh, and a CCA (cost of CO₂ avoided) of -$US 49.3/tonneCO₂e. The sensitivity analysis also indicated that SOFC/GT hybrid plants were less sensitive to SOFC price compared to standalone SOFC plants. The sensitivity analysis indicated that using a larger gas turbine and replacing the SOFC stack less frequently was the better design choice for the SOFC/GT hybrid plant.     

Thermal stress analysis of a planar SOFC stack

The aim of this study is, by using finite element analysis (FEA), to characterize the thermal stress distribution in a planar solid oxide fuel cell (SOFC) stack during various stages. The temperature profiles generated by an integrated thermo-electrochemical model were applied to calculate the thermal stress distributions in a multiple-cell SOFC stack by using a three-dimensional (3D) FEA model. The constructed 3D FEA model consists of the complete components used in a practical SOFC stack, including positive electrode–electrolyte–negative electrode (PEN) assembly, interconnect, nickel mesh, and gas-tight glass-ceramic seals. Incorporation of the glass-ceramic sealant, which was never considered in previous studies, into the 3D FEA model would produce more realistic results in thermal stress analysis and enhance the reliability of predicting potential failure locations in an SOFC stack. The effects of stack support condition, viscous behavior of the glass-ceramic sealant, temperature gradient, and thermal expansion mismatch between components were characterized. Modeling results indicated that a change in the support condition at the bottom frame of the SOFC stack would not cause significant changes in thermal stress distribution. Thermal stress distribution did not differ significantly in each unit cell of the multiple-cell stack due to a comparable in-plane temperature profile. By considering the viscous characteristics of the glass-ceramic sealant at temperatures above the glass-transition temperature, relaxation of thermal stresses in the PEN was predicted. The thermal expansion behavior of the metallic interconnect/frame had a greater influence on the thermal stress distribution in the PEN than did that of the glass-ceramic sealant due to the domination of interconnect/frame in the volume of a planar SOFC assembly.     

Experimental study of SOFC system heat-up without safety gases

Premixed safety gas is conventionally used to keep the anode of a solid oxide fuel cell (SOFC) under reducing conditions during heat-up. This article presents the results of an experimental study to heat up a SOFC system and stack without the said premixed safety gases, i.e. by utilizing a natural gas pre-reformer and anode off-gas recycling (AOGR). Firstly, ex-situ experiments were conducted to investigate the operability of a pre-reformer during system heat-up. It was found that any oxygen fed to the reformer hinders the reforming reactions at low temperatures. Secondly, based on the ex-situ findings, series of heat-up cycles were conducted with a complete 10 kW system using AOGR and a planar SOFC stack. In these experiments it was found that the system heat-up is possible with fuel gas and steam only, without the need for premixed reducing safety gases. Use of the fuel gas instead of a premixed safety gas did not result in a significant performance loss in the SOFC stack. Therefore, such a heat-up strategy was developed for SOFC systems that reduces the need of premixed safety gas storage space and thus decreases the system cost.     

Numerical evaluation of a parallel fuel feeding SOFC–PEFC system using seal-less planar SOFC stack

We propose a system that combines a seal-less planar solid oxide fuel cell (SOFC) stack and polymer electrolyte fuel cell (PEFC) stack. In the proposed system, fuel for the SOFC (SOFC fuel) and fuel for the PEFC (PEFC fuel) are fed to each stack in parallel. The steam reformer for the PEFC fuel surrounds the seal-less planar SOFC stack. Combustion exhaust heat from the SOFC stack is used for reforming the PEFC fuel. We show that the electrical efficiency in the SOFC–PEFC system is 5% higher than that in a simple SOFC system using only a seal-less planar SOFC stack when the SOFC operation temperature is higher than 973 K.     

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.     

Modelling and evaluation of building integrated SOFC systems

This study presents the final results of a series of modelling steps which are undertaken for the performance assessment of the building cogeneration and polygeneration systems using solid oxide fuel cell (SOFC). Based on earlier work, generic SOFC cell stack and system models were developed and employed to analyze different SOFC systems configurations for optimal efficiencies, this SOFC system model is used to derive performance input data for transient whole-building and energy system simulation tools which contain simpler SOFC system models. These steps are shortly summarized here. Then the final step, the evaluation of building integrated co- and polygeneration SOFC systems in terms of primary energy demand and CO₂ emissions, employing such tools, is presented here for a polygeneration system with typical heating and cooling loads, and electricity demand profiles, for different SOFC systems, including a comparison to current standard technologies.     

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.     

Design and Analysis of S-CO_2 Cycle and Radial Turbine for SOFC Vehicle Waste-Heat Recovery

Solid oxide fuel cell(SOFC) vehicles are considered to have broad prospects for development, and the high operating temperature of SOFC results in great potential for waste-heat recovery. Many concepts for utilizing waste heat of SOFC have been suggested and studied, and most of them directly couple an SOFC to a gas turbine, which require the SOFC to operate at an elevated pressure and make the system less flexible and thus harder to operate. In recent years, with the development of turbine and heat exchanger technology, the supercritical carbon dioxide(S-CO_2) power cycle has raised widespread attractions for the waste recovery. This study explores the potential of S-CO_2 Brayton cycle to realize waste-heat recovery for an SOFC vehicle. The SOFC can operate at atmospheric pressure, and the hybrid system is easier to operate than the directly coupled systems. In this paper, a simple recuperated S-CO_2 Brayton cycle is proposed and the key component, radial inflow turbine is designed and focused. The flow state of the designed turbine is analyzed in detail based on computational fluid dynamics(CFD) numerical simulation. Five cases with different impeller tip clearances are numerically simulated to study its influence on the turbine performance. In addition, off-design performance analysis of the radial inflow turbine is conducted considering the temperature fluctuation of SOFC in practical applications.     

Design and numerical analysis of a planar anode-supported SOFC stack

In the present study, numerical simulations are conducted to examine the flow characteristics and attributes of electrochemical reactions in the stack through three-dimensional analysis using finite volume approach prior to the fabrication of the SOFC stack. The stack flow uniformity index is employed to investigate the flow uniformity whereas in the case of electrochemical modeling, different mathematical models are adopted to predict the characteristics of activation and ohmic overpotentials that occur during electrochemical reactions in the cell. The normalized mass flow rate is found almost same in each cell with flow uniformity index of 0.999. The calculated voltage and power curves under different average current densities are compared with experimental results for the model validation. The changes in the voltage and power of the SOFC stack, current density, temperature, over potential and reactants distributions in relation to varying amounts of reactants flow are also examined. The current density distribution in each cell is observed to vary along the anode flow direction. The temperature difference in each cell is almost same along the flow direction of reactants, and the irreversible resistance showed an opposite trend with a temperature distribution in each cell.     

Design and analysis of SOFC stack with different types of external manifolds

In this study, a four-cell stack of anode-supported planar solid oxide fuel cells (SOFCs) was designed and simulated to investigate the flow/heat transport phenomena and the performance of the SOFC stack. This SOFC stack was designed based on the external manifold types with one side open toward the cathode inlet and components such as base station, manifold, end plate, press jig, and housing. To investigate the performance of the SOFC stack, a step-by-step heat and flow analysis was conducted. First, the separator, functioning as a current collector and a gas channel, was designed to have repeated convex shapes. As the boundary of the flow passage was periodic in both streamwise and transverse directions, only a small part of the flow channel was simulated. In the case of simple homogeneous porous media, the computational results for flow resistance could be expressed by following Darcy's Law. Subsequently, these calculation results from the separator flow analysis were used in the housing and stack analysis. Second, the flow of the cathode region in the housing of SOFC stack was analyzed to verify the flow uniformity in the cathode channel of the separators. Finally, a stack analysis was executed using the electrochemical reaction model to investigate the performance and transport phenomena of the stack. Owing to the uniformity in flow and temperature, each SOFC cell exhibited similar contours of reactant gases, temperature, and current density. In the case of two different fuel utilizations with different flow rates, the low fuel utilization performed slightly better than the high fuel utilization.     

Techno-economic and environmental assessment of integrating SOFC with a conventional steam and power system in a natural gas processing plant

Solid oxide fuel cell (SOFC) technology has been proven to be a highly efficient electrochemical device that directly converts chemical energy into electrical energy with a potential to increase system efficiencies and to significantly reduce emissions in oil and gas operations. Enabling an effective integration and smarter utilization of SOFC systems at different scales and point-of-use can lead to an overall system efficiency improvement. In this research paper, two case studies, including the steam and power system in a gas plant, are considered for comparative analysis purposes. The first case study, base case, is a traditional steam and power system in a natural gas processing plant and the proposed case is the combined steam and power system with SOFC unit. Techno-economic and environmental analyses are performed for both cases. To carry out a comparative analysis, the power output of both cases is fixed at 20 MW. The results of this study show that a reduction of almost 35% in the emissions is possible and the cost of electricity becomes about 25% less for the proposed case.     

天然气水蒸气重整与SOFC耦合应用

固体氧化物燃料电池（SOFC）系统具有高能源效率和使用可再生燃料的可能性,将在未来的可持续能源系统中发挥重要作用。过去几年燃料电池的发展很快,但在成本、稳定性和市场份额方面,该技术仍处于早期发展阶段。在以天然气为燃料的SOFC系统中,燃料的重整过程和燃料利用水平都可能影响系统运行的稳定性、热量和能量平衡,从而影响系统的使用寿命、输出功率和效率。因此,对燃料重整过程的设计与控制对有效的SOFC电池运行具有重要意义。对天然气在SOFC系统中的重整器配置方式（包括外重整和内重整）、重整参数和重整燃料利用方式进行了详细的综述分析,并对未来天然气SOFC系统的发展进行了展望。     

Influence of current densities in SOFC and PEFC stacks on a SOFC–PEFC combined system

A solid oxide fuel cell (SOFC)–polymer electrolyte fuel cell (PEFC) combined system was investigated by numerical simulation. Here, the effect of the current densities in the SOFC and the PEFC stacks on the system's performance is evaluated under a constant fuel utilization condition. It is shown that the SOFC–PEFC system has an optimal combination of current densities, for which the electrical efficiency is highest. The optimal combination exists because the cell voltage in one stack increases and that of the other stack decreases when the current densities are changed. It is clarified that there is an optimal size of the PEFC stack in the parallel-fuel-feeding-type SOFC–PEFC system from the viewpoint of efficiency, although a larger PEFC stack always leads to higher electrical efficiency in the series-fuel-feeding-type SOFC–PEFC system. The 40 kW-class PEFC stack is suitable for the 110 kW-class SOFC stack in the parallel-fuel-feeding type SOFC–PEFC system.     

Fault-tolerant control for steam fluctuation in SOFC system with reforming units

Faults of solid oxide fuel cell (SOFC) systems can affect the characteristics of the stack and inhibit SOFC system commercialization. It has been found that the temperature fluctuation of the burner caused by fluctuation of steam flow rate would greatly affect the temperature of SOFC system and even exceed the safe operation range. Firstly, this paper introduces a mathematical model for the process of steam and natural gas reforming in a real SOFC system. Secondly, the cause of the burner temperature fluctuation is analyzed, and the model to simulate this faulty situation is established. Then, the Bayesian regularization neural network is used for fault diagnosis and good test results are obtained. Finally, fuzzy fault-tolerant control strategy is designed for the thermal safety problem of SOFC system. The simulation results validate the effectiveness of the proposed fault-tolerant control strategy.     

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.     

基于容阻建模技术的SOFC电堆数值模拟

借助MATLAB软件，通过建模对固体氧化物燃料电池堆的运行进行仿真。为了计算的方便，模型采用了容阻建模技术。采用由质量守恒、能量守恒、动量守恒和电化学过程组成的模型，对SOFC电堆通道中的气体组成、温度分布、电压分布和电流密度分布进行了计算分析。这些分析结果可对电堆的设计提供参考。