1,721,016 research outputs found
Multiscale modelling potentialities for solid oxide fuel cell performance and degradation analysis
No full textSolid oxide fuel cells are electrochemical devices that are able to directly convert the chemical energy of fed fuels to electricity as well as to provide heat through exhausted gases allowing a higher energy efficiency compared to tradition thermal engines. However, the state-of-the-art materials show a drastic performance drop after too few working hours because of irreversible microstructural changes. Here the main issue consists of improving cell durability by optimising its structure and operative conditions. Modelling can significantly support this target, permitting a better understanding of different phenomena and providing information that are difficult to directly measure. However, degradation simulation is a quite challenging task due to the complexity of the studied systems, where different phenomena overlap, as well as due to the numerous data requested on both electrochemical and microstructural features. Depending on the available cell information and the analysis detail level, a multiscale modelling approach is a promising solution for providing effective results with reduced computational efforts. Based on a macroscale characterization, for example, semi-empirical degradation functions can be directly derived from electrochemical impedance spectra and area-specific resistance variations without knowing anything on the microstructure in order to estimate global cell performance and durability through a lumped-parameter model. Whereas, when aiming at the identification of an aged element specific behaviour, detailed formulations have to be introduced for each mechanism following a microscale approach. In such cases, a local-level modelling is fundamental in view of uneven distributions of properties on the cell plane which influence locally the degradation process development and resulting performance
Performance modelling of intermediate temperature solid oxide cells applied as electrochemical air separation unit
No full textOxygen production is a highly energy-consuming process, above all at the required purity increase. The state-ofthe-art application consists of cryogenic distillation widely used for a high production scale, while the adsorption and polymeric membrane technologies are more convenient for low demands without reaching the performance of the first yet. Solid oxide cells are a promising alternative since the performance in terms of the energy demand and the purity degree is independent from the system capacity, making them suitable for several application fields. Nevertheless, the technology readiness level is still too low for commercial use, requiring further improvements on material performance and durability, cell design and process management. Performing a detailed multiscale feasibility analysis, the work discusses the use of planar stacked cells working at intermediate temperatures and atmospheric pressure. High-performing co-doped double perovskite electrodes allow for optimising the separation kinetics. At the air side, the molecular oxygen dissociates through an externally applied potential into ions that migrate inside a highly anionic conductive electrolyte and reconvert to O2 at the pure oxygen side
Operating principles, performance and technology readiness level of reversible solid oxide cells
No full textThe continuous increase of energy demand with the subsequent huge fossil fuel consumption is provoking dramatic environmental consequences. The main challenge of this century is to develop and promote alternative, more eco-friendly energy production routes. In this framework, Solid Oxide Cells (SOCs) are a quite attractive technology which could satisfy the users’ energy request working in reversible operation. Two operating modes are alternated: from “Gas to Power”, when SOCs work as fuel cells fed with hydrogen-rich mixture to provide both electricity and heat, to “Power to Gas”, when SOCs work as electrolysers and energy is supplied to produce hydro-gen. If solid oxide fuel cells are an already mature technology with several stationary and mobile applications, the use of solid oxide electrolyser cells and even more reversible cells are still under investigation due to their insufficient lifetime. Aiming at providing a better understanding of this new technological approach, the study presents a detailed description of cell operation in terms of electrochemical behaviour and possible degradation, highlighting which are the most commonly used performance indicators. A thermodynamic analysis of system efficiency is proposed, followed by a comparison with other available electrochemical devices in order to underline specific solid oxide cell advantages and limitations
2D simulation for CH4internal reforming-SOFCs: An approach to study performance degradation and optimization
No full textSolid oxide fuel cells (SOFCs) are a well-developed technology, mainly used for combined heat and power production. High operating temperatures and anodic Ni-based materials allow for direct reforming reactions of CH4 and other light hydrocarbons inside the cell. This feature favors a wider use of SOFCs that otherwise would be limited by the absence of a proper H2 distribution network. This also permits the simplification of plant design avoiding additional units for upstream syngas production. In this context, control and knowledge of how variables such as temperature and gas composition are distributed on the cell surface are important to ensure good long-lasting performance. The aim of this work is to present a 2D modeling tool able to simulate SOFC performance working with direct internal CH4 reforming. Initially thermodynamic and kinetic approaches are compared in order to tune the model assuming a biogas as feed. Thanks to the introduction of a matrix of coefficients to represent the local distribution of reforming active sites, the model considers degradation/poisoning phenomena. The same approach is also used to identify an optimized catalyst distribution that allows reducing critical working conditions in terms of temperature gradient, thus facilitating long-term applications
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Comparison of different drag models in CFD-DEM simulations of spouted beds
No full textSpouted beds are commonly simulated through the Computational Fluid Dynamics – Discrete Element Method approach. The choice of the drag model is still a matter of debate, as they feature peculiar operative conditions. In this work, we simulated two spouted beds containing Geldart-D particles. We tested seven drag models: three are classic models, while four are developed through advanced computational techniques. The results indicate that the key variable is the ratio between the operative and the minimum spouting gas velocity (u/ums). At u = ums only the Gidaspow model can always predict fluidisation, but at low u/ums values the Beetstra model is the best compromise. For higher values, the Rong and Di Felice models behave better, while the others overestimate the particles' velocity. These results can be useful to identify the best performing model and show there is a need for more appropriate models for spouted beds
Simulation of the gasification of agricultural residues using coco simulator
No full textThe free simulator COCO has been applied for the study of the gasification of biomass. A Matlab unit operation was included in the model to enable the calculation of equilibrium heterogeneous reactions. The validity of the model has been confirmed by comparing its results with experimental data using sunflower husk as feedstock. Once validated, the model has been applied to the study of the gasification of pruning of apple trees. The influence of excess air and temperature on the efficiency of the process has been analysed. Overall, the model is found to operate stably and consistently over gasification of biomass reactions. The model represents a quick and effective tool to assess the effect of the modification of a variable and its impact on the overall gasification process in an efficient and costless way
Electrochemical Characterization and Modelling of Anode and Electrolyte Supported Solid Oxide Fuel Cells
No full textSolid Oxide Fuel Cells (SOFCs) have emerged as an attractive alternative for efficient cogeneration of electricity and heat with reduced emissions during operation. High working temperatures result in optimized kinetics and higher efficiencies in comparison to other fuel cell types. Among different designs, Anode Supported Cells (ASCs) and Electrolyte Supported Cells are currently the most promising configurations on a commercial scale. This work analyses these two designs with a focus on electrochemical features as the main performance marker. The study was carried out using both theoretical and experimental approaches on planar single cells. A detailed test campaign at different operating conditions in terms of temperature, fuel and oxidant composition was designed. Electrochemical Impedance Spectroscopy and current-voltage (I-V) measurements were used to identify the contributions of different cell components. The electrochemical kinetics derived from the individual resistance terms was implemented in a 2D simulation tool (SIMFC-SIMulation of Fuel Cells) to obtain the detailed global cell behaviour and to understand local occurring mechanisms on anodic and cathodic cell planes. The model was validated for an anode supported cell consisting of Ni-YSZ/YSZ/LSCF-CGO and an electrolyte supported cell consisting of Ni-CGO/YSZ/LSCF-CGO, showing the possibility to tune the parameters depending on analysed cells
Multiscale analysis of Ni-YSZ and Ni-CGO anode based SOFC degradation: From local microstructural variation to cell electrochemical performance
No full textNickel based fuel electrodes are widely used for commercial solid oxide fuel cells showing a high catalytic ac-tivity, despite of involving severe microstructural changes which reduce the system lifetime. Needing a detailed knowledge of such phenomena, the authors compare the behaviour of two state-of-the-art planar cells, Ni-YSZ based anode supported cell and Ni-CGO based electrolyte supported cell, working for 1000 hours under a gal-vanostatic operation with H2 rich feed. Following a multiscale approach, the system was analysed in terms of both global performance and local properties. Experimental observations through electrochemical character-ization and microstructural analysis laid the basis for developing a physics-based model able to predict the cell operation at reference and aged status. Indeed, the kinetics was expressed as a function of microstructural features and considers the time evolution of some parameters. Ni-based electrode was identified as the first source of degradation due to Ni instability resulting in a reduction of catalytic activity and conductivity, correlated mainly to Ni particle coarsening and migration respectively. Each degradation mechanism prevailed depending on the material structure (i.e., initial particle size and distribution) and imposed working conditions (i.e., temperature, load and gas composition)
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