Modelling and Performance Evaluation of Solid Oxide Fuel Cell for Building Integrated Co‐ and Polygeneration

Models of fuel cell based combined heat and power systems, used in building energy performance simulation codes, are often based on simple black or grey box models. To model a specific device, input data from experiments are often required for calibration. This paper presents an approach for the theoretical derivation of such data. A generic solid oxide fuel cell (SOFC) system model is described that is specifically developed for the evaluation of building integrated co‐ or polygeneration. First, a detailed computational cell model is developed for a planar SOFC and validated with available numerical and experimental data for intermediate and high temperature SOFCs with internal reforming (IT‐DIR and HT‐DIR). Results of sensitivity analyses on fuel utilisation and air excess ratio are given. Second, the cell model is extended to the stack model, considering stack pressure losses and the radiative heat transfer effect from the stack to the air flow. Third, two system designs based on the IT‐DIR and HT‐DIR SOFCs are modelled. Electric and CHP efficiencies are given for the two systems, as well as performance characteristics, to be used in simulations of building integrated co‐ and polygeneration systems.     

Interconnect materials for next-generation solid oxide fuel cells

Highly-efficient solid oxide fuel cell (SOFC) systems are gaining increased attention for future energy conversion applications. Many planar SOFC stack designs utilise ferritic stainless steel (FSS) interconnect components. During operation, surface corrosion of FSS interconnects degrades stack operation by increasing electrical resistance and introducing other deleterious material interactions. To minimise these effects, various surface modifications and coatings are currently under investigation. Two of these methods under development for this application are: metal organic chemical vapour deposition (MO-CVD); and, large area filtered arc deposition (LAFAD). SOFC interconnect-relevant corrosion behaviour of an MO-CVD coating on Crofer 22 APU, AL453, Fe30Cr and Haynes230, and complex, amorphous LAFAD AlCrCoMnTiYO coatings on FSS 430 were investigated. Both of these surface modifications and coatings exhibit significantly improved corrosion protection as compared with uncoated FSS samples.     

Exergoeconomic Analysis of a Combined Ammonia Based Solid Oxide Fuel Cell System

An exergoeconomic study of an ammonia‐fed solid oxide fuel cell (SOFC) based combined system for transportation applications is presented in this paper. The relations between capital costs and thermodynamic losses for the system components are investigated. The exergoeconomic analysis includes the SOFC stack and system components, including the compressor, microturbine, pressure regulator, and heat exchangers. A parametric study is also conducted to investigate the system performance and costs of the components, depending on the operating temperature, exhaust temperature, and fuel utilization ratio. A parametric study is performed to show how the ratio of the thermodynamic loss rate to capital cost changes with operating parameters. For the devices and the overall system, some practical correlations are introduced to relate the capital cost and total exergy loss. The ratio of exergy consumption to capital cost is found to be strongly dependent on the current density and stack temperature, but less affected by the fuel utilization ratio.     

Pressurized Operation of a Planar Solid Oxide Cell Stack

Solid oxide cells (SOCs) can be operated either as fuel cells (SOFC) to convert fuels to electricity or as electrolyzers (SOEC) to convert electricity to fuels such as hydrogen or methane. Pressurized operation of SOCs provide several benefits on both cell and system level. If successfully matured, pressurized SOEC based electrolyzers can become more efficient both energy‐ and cost‐wise than PEM and Alkaline systems. Pressurization of SOFCs can significantly increase the cell power density and reduce the size of auxiliary components. In the present study, a SOC stack was successfully operated at pressures up to 25 bar. The pressure dependency of the measured current‐voltage (I–V) curves and impedance spectra on the SOC stack are analyzed and the relation between various system parameters and pressure is derived. With increasing pressure the open circuit voltage (OCV) and the reaction kinetics (electrode performance) increases for thermodynamic and kinetic reasons, respectively. Further, the summit frequency of the gas concentration impedance arc and the pressure difference across the stack and heat exchangers is seen to decrease with increasing pressure following a power‐law expression. Finally a durability test was conducted at 10 bar.     

A Degradation Model for Solid Oxide Fuel Cell Anodes Due to Impurities in Coal Syngas: Part II Estimation of Tolerance Limits

An engineering analysis based on calibrated numerical predictions was performed to estimate the minimum allowable impurity concentrations in coal syngas intended to be used in Solid Oxide Fuel Cells (SOFCs) operating for over 10,000 h. Arsine and phosphine, impurities that are known to have the most deleterious effects on the cell performance due to their affinity to have strong relations with the anode catalyst by formation of secondary phases, were investigated. Time to failure was taken as the operation time when 60% performance loss is incurred, estimated by the previously developed one‐dimensional degradation model. Limiting concentrations were determined for arsine and phosphine fuel contaminants for electrolyte and anode supported SOFCs. Predicted lifetimes for single cells can provide a basis for estimation of SOFC stack lifetimes operating on coal syngas. Extrapolation of results from the numerical simulations based on accelerated laboratory tests at relatively higher concentrations can provide guidance into predicting the cell failure at low impurity concentrations.     

Temperature Measurement and Distribution Inside Planar SOFC Stacks

In this work, a kind of thin K‐type thermocouple and self‐developed CAS‐I sealant were used to assembly solid oxide fuel cell (SOFC) stacks and temperatures of unit cells inside a planar SOFC stack were measured. The open circuit voltage testing of the stack and characterization of the interface between sealant and components suggested excellent sealing effect by applying the developed method. The effect of discharging direct‐current on temperature and temperature distribution inside the designed SOFC stack was investigated. The results showed that the discharging current had a great impact and the gas flow rate had a slight impact on the temperatures of unit cells. Temperature distribution of unit cells inside the stack was much non‐uniform and there is a significant temperature difference between various components of the stack and heating environment. The relationship between temperatures and cell performance showed that the worse the cell performance, the higher the cell surface temperature. When the stack was discharged at a constant current and the temperature of cell surface was over 950 °C, the higher the temperature, the more drop the corresponding voltage.     

Modelling the impact of creep on the probability of failure of a solid oxide fuel cell stack

In solid oxide fuel cell (SOFC) technology a major challenge lies in balancing thermal stresses from an inevitable thermal field. The cells are known to creep, changing over time the stress field. The main objective of this study was to assess the influence of creep on the failure probability of an SOFC stack. A finite element analysis on a single repeating unit of the stack was performed, in which the influence of the mechanical interactions, the temperature-dependent mechanical properties and creep of the SOFC materials are considered. Moreover, stresses from the thermo-mechanical simulation of sintering of the cells have been obtained and were implemented into the model of the single repeating unit. The significance of the relaxation of the stresses by creep in the cell components and its influence on the probability of cell survival was investigated. Finally, the influence of cell size on the failure probability was investigated.     

国外SOFC研究机构及研发状况

在简单介绍固体氧化物燃料电池(solid oxide fuel cells,SOFCs)堆结构的基础上,重点详细介绍几个世界级具有堆发电能力的机构及其研发状况,指出研发机构与大型企业集团联合开发、锁定目标、重点突破是目前各发达国家研究的特点。     

Operation viability and performance of solid oxide fuel cell fuelled by different feeds

The performances of solid oxide fuel cells (SOFCs) fed by different types of feed, i.e. biogas, biogas-reformed feed, methane-reformed feed and pure hydrogen, are simulated in this work. Maximum temperature gradient and maximum cell temperature are regarded as indicators for operation viability investigation whereas power density and electrical efficiency are considered as performance indicators. The change in operating parameters, i.e. excess air, fuel feed rate and operating voltage, affects both the performance and operation viability of SOFC, and therefore, these operating parameters should be carefully selected to obtain best possible power density and reasonable temperature and temperature gradient. Pure hydrogen feed offers the highest SOFC performance among the other feeds. Extremely high excess air is required for SOFC fed by biogas to become operation viable and, in addition, its power density is much lower than those of SOFCs fed by the other feeds. Methane-reformed feed offers higher power density than biogas-reformed feed since H₂ concentration of the former one is higher.     

Modeling of Direct H2S Fueled Solid Oxide Fuel Cells with Reforming Chemical Kinetics

A direct H₂S fueled SOFC model is developed based on Ni‐YSZ/YSZ/YSZ‐LSM button cell test stand. The model considers the detailed reforming chemical processes of H₂S and multi‐physics transport processes in the fuel cell and fuel supply tubes. The model is validated using experimental data. Extensive simulations are performed to study the complicated interactions between multi‐physics transport processes and chemical/electrochemical reactions. The results elucidate the fundamental mechanisms of direct H₂S fueled SOFCs. It is found that suitably increasing the H₂O content in the supplied H₂S fuel can improve SOFC electrochemical performance; high operating temperature may facilitate the reforming of H₂S and improve the electrochemical performance. The sulfur poisoning effect may be mitigated by increasing the H₂O content in the fuel, increasing the operating temperature, decreasing the flow rate, and/or making the cell work at low voltage (or high current) conditions.     

Development of an In Situ Surface Deformation and Temperature Measurement Technique for a Solid Oxide Fuel Cell Button Cell

A novel experimental technique is developed to measure the in situ surface deformation and temperature of a solid oxide fuel cell (SOFC) anode surface along with the cell electrochemical performance. The experimental setup consists of a NexTech Probostat^™ SOFC button cell test apparatus integrated with a Sagnac interferometric optical method and an infrared sensor for in situ surface deformation and temperature measurements, respectively. The button cell is fed with hydrogen or simulated coal syngas under SOFC operating conditions. The surface deformation is measured over time to estimate the anode structural degradation. The cell surface transient temperature is also monitored with different applied current densities under hydrogen and simulated coal syngas. The experimental results are useful to validate and develop SOFC structural durability and electrochemical models.     

Behavior of Metallic Components During 4,000 h Operation of an SOFC Stack with Carbon Containing Fuel Gas

In the present study the behavior of the ferritic interconnect steel Crofer 22 APU and the nickel contacting material during exposure in anode gas of a solid oxide fuel cell (SOFC) stack was investigated. The stack had been operating for 4,000 h and was ten times subjected to thermal cycling within this time period. A temporary high carbon activity in the fuel gas combined with temperature changes resulted in local disintegration of the nickel mesh. Additionally, nickel diffusion from the nickel mesh into the steel resulted in the formation of an austenitic zone. Diffusion of steel constituents into the nickel mesh lead to the formation of Cr,Mn‐oxides in the latter. Presence of the nickel/steel contact allows transport of carbon from the gas into the steel, resulting in local internal carburization of the steel. In areas which were not in direct contact with the nickel mesh, the Crofer 22 APU interconnect formed a protective surface oxide scale and no indications for carbon uptake were found. Mechanisms for the experimentally observed effects, including the local disintegration of the nickel mesh, are presented.     

New Fabrication Technique for Series‐Connected Stack With Micro Tubular SOFCs

Solid oxide fuel cell (SOFC) is highly efficient and is a promising candidate for future power systems. Among the many types of SOFCs which have been reported, the micro tubular design offers improved thermal robustness, with the possibility of rapid start‐up/shut‐down. In this study, a new stack structure for anode‐supported micro tubular SOFCs was developed in which porous MgO matrices were used to position the micro tubular cell elements. This arrangement allowed for electrical interconnection of each cell in a series, using a silver paste and a connecting LSCF paste for the anode and the cathode, respectively, in the MgO support structure. With this technique, the bundle size could be easily increased towards the kW class module design.     

SOFC Thermal Transients: Modeling by Application of Experimental System Identification Techniques

Solid oxide fuel cell (SOFC) is one of the most promising technologies for future power generation. In order to make this technology marketable, many issues as cost reduction, durability, and operational management have to be overcome. Therefore, the understanding of thermodynamic and electrochemical mechanisms, that govern the SOFC behavior in steady‐state and in transient operation, becomes fundamental. In this context, the modeling of fuel cell (FC) thermal transient is of great interest because it can predict the temperature time variation, useful to the dimensioning of auxiliary devices and to avoid unwanted operational states affecting cell durability. In the present study, a 0‐D model of SOFC thermal transients was developed by applying system identification techniques, starting from experimental tests carried out on a stack made up of four single cells. Moreover, it was successfully validated in reference to further experimental data. The model allows to evaluate, in term of dynamic response, the effect of the main operating parameters on FC temperature. As further result, some control/regulation considerations useful to limit thermal stresses were proposed.     

Modeling and Optimization of Anode‐Supported Solid Oxide Fuel Cells on Cell Parameters via Artificial Neural Network and Genetic Algorithm

An artificial neural network (ANN) and a genetic algorithm (GA) are employed to model and optimize cell parameters to improve the performance of singular, intermediate‐temperature, solid oxide fuel cells (IT‐SOFCs). The ANN model uses a feed‐forward neural network with an error back‐propagation algorithm. The ANN is trained using experimental data as a black‐box without using physical models. The developed model is able to predict the performance of the SOFC. An optimization algorithm is utilized to select the optimal SOFC parameters. The optimal values of four cell parameters (anode support thickness, anode support porosity, electrolyte thickness, and functional layer cathode thickness) are determined by using the GA under different conditions. The results show that these optimum cell parameters deliver the highest maximum power density under different constraints on the anode support thickness, porosity, and electrolyte thickness.     

Experimental Investigations and Modeling of Direct Internal Reforming of Biogases in Tubular Solid Oxide Fuel Cells

Biogas‐fed Solid Oxide Fuel Cell (SOFC) systems can be considered as interesting integrated systems in the framework of distributed power generation. In particular, bio‐methane and bio‐hydrogen produced from anaerobic digestion of organic wastes represent renewable carbon‐neutral fuels for high efficiency electrochemical generators. With such non‐conventional mixtures fed to the anode of the SOFC, the interest lies in understanding the multi‐physics phenomena there occurring and optimizing the geometric and operation parameters of the SOFC, while avoiding operating and fuel conditions that can lead to or accelerate degradation processes. In this study, an anode‐supported (Ni‐YSZ) tubular SOFC was considered; the tubular geometry enables a relatively easy separation of the air and fuel reactants and it allows one to evaluate the temperature field of the fuel gas inside the tube, which is strictly related to the electrochemical and heterogeneous chemical reactions occurring within the anode volume. The experiments have been designed to analyze the behavior of the cell under different load and fuel utilization (FU) conditions, providing efficiency maps for both fuels. The experimental results were used to validate a multi‐physics model of the tubular cell. The model showed to be in good agreement with the experimental data, and was used to study the sensitive of some selected geometrical parameters modification over the cell performances.     

Comparative Study of the Leak Characteristics of Two Ceramic/Glass Composite Seals for Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) have the potential to play a significant role in a future clean energy economy. However, SOFCs still face major obstacles before they can be commercialized, with efficient sealing being among the most prominent. The present research focuses on the comparative study of microstructure, crystal phase evolution, and leak rates, for two ceramic/glass seals used in an SOFC. The leak test apparatus is a controlled facility designed to incorporate different mechanical loading, stack configurations, and thermal cycles. Simultaneous leak testing with an acoustic emission (AE) sensor was also used to identify any micro‐damage in seals. A two‐level factorial design was applied to the first sealing composition to identify the main and the interactive factors for leak rates. MINITAB^® was also used to determine a linear regression‐based leak rate model. The second seal formulation employed a more stable glass which led to reduced leak rates. Additional factors in a two‐level factorial design were investigated for the second seal formulation. Based on multiple experiments with different stack components, it was determined that the number of interfaces is most critical for leak rate, showing that even in the presence of thermal cycling, leakage is an interfacial dominated phenomenon.     

Tape Casting of Anode Supports for Solid Oxide Fuel Cells at Forschungszentrum Jülich

This contribution describes the development of tape casting for solid oxide fuel cells (SOFCs) anode supports starting with the characterization of the powders and ending with manufacturing of cells for stack testing. After casting the support, full cells were prepared by screen printing and sintering of the functional layers. The results of single‐cell and stack tests of the novel SOFC will be discussed. The new cell showed excellent electrochemical performance in single‐cell tests with more than 1.5 A/cm² (800°C, 0.7 V). Furthermore, stack tests showed no significant difference from earlier standard cells when operated at 800°C with a current density of 0.5 A/cm².     

Recent Developments in Solid Oxide Fuel Cell Materials

Recent developments in solid oxide fuel cell (SOFC) materials and stacks have been reviewed mainly from the viewpoint of the materials chemistry associated with stack development. Firstly the general features of SOFCs are described to clarify the materials problems which need to be solved and the technology that needs to be developed. The main, characteristic features of conventional SOFC materials are described with the emphasis put on the materials problems associated with the respective materials. Then the materials problems associated with stack development are described for tubular stacks and for planar stacks with oxide interconnect and with metal interconnect. Finally, new materials, design and processing are discussed in relation to new applications of SOFCs. The emphasis is placed on the fact that these are new challenges and therefore a more fundamental strategy for materials and stack development should be constructed on the basis of the materials science and technology developed mainly for conventional materials.     

Anode-Supported Tubular Micro-Solid Oxide Fuel Cell

A tubular anode-supported "micro-solid oxide fuel cell" (μSOFC) has been developed for producing high volumetric power density (VPD) SOFC systems featuring rapid turn on/off capability. An electrophoretic deposition (EPD)-based, facile manufacturing process is being refined to produce the anode support, anode functional and electrolyte layers of a single cell. μSOFCs (diameter <5 mm) have two main potential advantages, a substantial increase in the electrolyte surface area per unit volume of a stack and also rapid start-up. As fuel cell power is directly proportional to the active electrolyte surface area, a μSOFC stack can substantially increase the VPD of an SOFC device. A decrease in tube diameter allows for a reduction in wall thickness without any degradation of a cell's mechanical properties. Owing to its thin wall, a μSOFC has an extremely high thermal shock resistance and low thermal mass. These two characteristics are fundamental in reducing start-up and turn-off time for the SOFC stack. Traditionally, SOFC has not been considered for portable applications due to its high thermal mass and low thermal shock resistance (start-up time in hours), but with μSOFCs' potential for rapid start-up, new possibilities for portable and transportable applications open up.