Electromagnetic Device Optimization with Stochastic Methods

none1Device optimization using metaheuristic methods has been successfully applied to electromagnetic devices since their development in the early 1980s. Some recent examples of the application of metaheuristics in electromagnetic device design include, among others, genetic algorithms [Zaoui2007], evolution strategies [Coelho2007], Tabu search [Cogotti2000], artificial immune systems [Campelo2006], particle swarm optimization (PSO) [Ciuprina2002]. In this chapter the author summarizes some of his experiences in the use of two stochastic optimization techniques which are very suitable to typical electromagnetic devices and systems. First the algorithms are briefly introduced and then their application to typical challenging problems, including Polymer Exchange Membrane Fuel Cells (PEMFC), high- field-uniformity solenoids and Superconducting Magnetic Energy Storage (SMES) systems, is presented.noneP. AlottoAlotto, Piergiorgi

Development of graphene oxide/nafion polymeric membranes toward the improvement of direct methanol fuel cell membranes

Nowadays the direct methanol fuel cell (DMFC) technology attracts a lot of interest because it represents a low temperature, high efficiency power source that can be advantageously used for mobile and portable applications and thus, it can be a valid alternative to the use of lithium battery technology. The DMFC is an electrochemical system that produces energy by oxidizing the liquid fuel (a mixture of water and methanol) without auxiliary devices. The use of DMFC has several advantages: easy fuel storage, low cost of methanol, operation at low temperature and pressure, small system size and low weight. However, several issues hinder the spreading of this technology. Firstly, the fuel cell membrane is generally composed by Nafion, a perfluorosulfonic polymer. Such membrane is prone to allow the passage of methanol, the so called cross-over, that strongly reduces DMFC performance. Furthermore, high cost and limited operating temperature range represent a strong limit to the commercialization of that technology. Research activities are focused on developing new polymer electrolyte membrane (PEM) materials aiming at overcoming the crossover. Among several types of material, graphene oxide (GO) has been considered as a promising element that offers great results in terms of water uptake and mechanical properties. Several authors have reported that GO contributes to reduce methanol permeability because it acts as a barrier, due to its higher tortuosity, while proton conductivity shows an opposite trend. In addition, it is well known that temperature, methanol concentration as well as flow rate affect proton conductivity and methanol crossover and, consequently, DMFC performance. It is therefore necessary to investigate what are the best conditions to make better use of this innovative material. The objective of this study is to assess the potential of GO in improving DMFC performance. To this end, several composite Nafion/GO membranes with different GO loading were manufactured using casting method. The internship at the School of Chemical Engineering at the University of Birmingham, allowed me to learn the methodology to prepare and characterize polymeric membranes. The composite membranes demonstrated higher mechanical strength, enhanced water uptake but lower proton conductivity than recast Nafion. Once the optimum loading was estimated, the performance of the DMFC, in a passive configuration, was analysed through the analysis of the polarization and power curves. It was revealed that the DMFC performance was enhanced by increasing the temperature. The DMFC performance increases when using GO membranes when increasing methanol concentration and flow rate. However, it is necessary to use the appropriate range of methanol concentration and anode flow rate. Extending the anode flow rate and methanol concentration has a dual effect: increasing the flow of the reactant allows to obtain higher performance despite enhancing the methanol crossover and losses. At one point, the loss will be no longer counterbalanced, and performance starts decreasing. Comparing the results with those of recast Nafion, it was demonstrated that by utilising GO-Nafion composite membranes, an increase in the maximum power density, open circuit condition and operating range, at all operating conditions, can be achieved. So, the detriment of proton conductivity was counterbalanced by the reduction of fuel cell crossover

Rapid Prototyping of Microfluidic Devices:Realization of Magnetic Micropumps, Fuel Cells and Protein Preconcentrators

With the growing importance of miniaturized energy applications and the development of micro Total Analysis Systems (μTAS), we have realized microfluidic devices, namely, magnetic micropumps, microfluidic fuel cells and membrane-based protein preconcentrators, all having high application potential in future. The choice of rapid prototyping microfabrication technologies and the selection of affordable materials are important aspects, when thinking of commercialization. Thus, we have employed powder blasting, polymer molding and assembly technologies during devices fabrication throughout the thesis. The first type of microfluidic device that we present is a poly(methyl methacrylate) (PMMA) ball-valve micropump with two different designs of the electromagnetic actuator, as optimized by the finite element method. The integration of a permanent magnet in a flexible polydimethylsiloxane (PDMS) membrane, which is clamped into PMMA structure, is proposed for providing a large stroke of the pumping membrane, making the micropump bubble-tolerant and self-priming Focusing on low power consumption for μTAS integration, another type of magnetic micropump with active valves is realized. It consists of a microfluidic chamber structure in glass that is assembled with a PDMS sheet, which comprises two valving membranes and a central actuation membrane, having each an integrated permanent magnet that is peristaltically actuated by a rotating arc-shaped permanent magnets assembly. A lumped circuit model is developed to predict and describe the frequency-dependent flow rate behavior for this type of pump. Powder blasting and PDMS molding rapid prototyping technologies are employed for realization of these two types of micropumps. Fuel cells with fluid delivery and removal options, having chemical reaction sites and electrode structures that can be realized in a microfluidic format, have high potential for applications. Therefore, microfluidic direct methanol fuel cells with embedded ion- permselective medium are studied and such type of fuel cell is realized by integrating a narrow Nafion strip in a molded elastomeric structure. A mechanical clamping assembly technology enables leakage-free operation and stable performance. The characterization reveals its output power density, using H2O2-based oxidant, is among the high-performance direct methanol fuel cells in microscale. Re-using the technology of the fuel cell chip, with its particular ion-permselective Nafion membrane and assembly method, we also have developed a protein preconcentrator with high purification performance. Our device can preconcentrate negatively charged biomolecules located at the anodic compartment side of the Nafion strip within only a few minutes with a high preconcentration factor. Moreover, a complex microfluidic finite element model is proposed to study and understand the physics of the preconcentration effect. Finally, we conclude the thesis with an outlook on future developments based on our work of the project and on the assembly technologies for microfluidic device integration

Fuel Cell Renewable Hybrid Power Systems

Climate change is becoming visible today, and so this book—through including innovative solutions and experimental research as well as state-of-the-art studies in challenging areas related to sustainable energy development based on hybrid energy systems that combine renewable energy systems with fuel cells—represents a useful resource for researchers in these fields. In this context, hydrogen fuel cell technology is one of the alternative solutions for the development of future clean energy systems. As this book presents the latest solutions, readers working in research areas related to the above are invited to read it

Passive Gas-Liquid Separation Using Hydrophobic Porous Polymer Membranes: A Study on the Effect of Operating Pressure on Membrane Area Requirement

The use of hydrophobic porous polymer membranes to vent unwanted gas bubbles from liquid streams is becoming increasingly more common in portable applications such as direct methanol fuel cells (DMFCs) and micro-fluidic cooling of electronic circuits. In order for these portable systems to keep up with the ever increasing demand of the mobile user, it is essential that auxiliary components, like gas-liquid separators (GLS), continue to decrease in weight and size. While there has been significant progress made in the field of membrane-based gas-liquid separation, the ability to miniaturize such devices has not been thoroughly addressed in the available literature. Thus, it was the purpose of this work to shed light on the scope of GLS miniaturization by examining how the amount porous membrane required to completely separate gas bubbles from a liquid stream varies with operating pressure. Two membrane characterization experiments were also employed to determine the permeability, k, and liquid entry pressure (LEP) of the membrane, which provided satisfying results. These parameters were then implemented into a mathematical model for predicting the theoretical membrane area required for a specified two-phase flow, and the results were compared to experimental values. It was shown that the drastically different surface properties of the wetted materials within the GLS device, namely polytetrafluoroethylene (PTFE) and acrylic, caused the actual membrane area requirement to be higher than the theoretical predictions by a constant amount. By analyzing the individual effects of gas and liquid flow, it was also shown that the membrane area requirement increased significantly when the liquid velocity exceeded an amount necessary to cause the flow regime to transition from wedging/slug flow to wavy/semi-annular flow

Ionic Conductive Membranes for Fuel Cells

This book, titled “Ionic Conductive Membranes for Fuel Cells”, from the journal Membranes, discusses the state of the art and future developments in the field of polymer electrolyte membranes for fuel cells, an efficient and clean system for converting fuel into energy

Multiphysics modeling of liquid-feed direct methanol fuel cells and characterization of diffusive transport properties of gas diffusion layers

Mención Internacional en el título de doctorPolymer Electrolyte Membrane (PEM) fuel cells are leading candidates to replace today’s fossil-based energy economy, providing efficient and clean electric energy generation for the 21st century. The study of PEM fuel cells represent a multidisciplinary and dynamic field in which mechanical, chemical, and electrical engineering, as well as material design, converge and collaborate with each other, making research on this topic a continuous multiphysics challenge. Numerical modeling plays a crucial role for the analysis of the complex mass, charge, and heat transport phenomena that take place at the micrometric scales of the porous layers that make up the Membrane Electrode Assembly (MEA) of PEM fuel cells, and constitutes an essential tool for the optimization of fuel cell performance. The most promising PEM fuel cell technologies are hydrogen Polymer Electrolyte Membrane Fuel Cells (PEMFCs), and liquid-feed Direct Methanol Fuel Cells (DMFCs). Although major interest has nowadays shifted to high-performance PEMFCs for the automotive industry, liquid-feed DMFCs are attractive power sources for portable electronic devices due to the higher energy density and ease of handling and storage of liquid methanol. The aim of this thesis is to contribute to the understanding of both technologies using multiphysics and multiscale modeling techniques. A multiphysics macroscopic model of a liquid-feed DMFC that accounts for the effects of the inhomogeneous assembly compression of the Gas Diffusion Layer (GDL) is first presented. Then, the effective gas diffusive properties of GDLs under dry and partially water-saturated conditions are characterized by combining the Lattice Boltzmann Method (LBM) with X-ray Computed Tomography (XCT) images of carbon-paper GDLs. The achievement of these two objectives is divided into three tasks: 1. A Finite Element Method (FEM) model is developed to simulate the inhomogeneous assembly process of the GDL associated with the repetitive rib/channel pattern of the Bipolar Plate (BPP). The model fully accounts for the nonlinear orthotropic mechanical properties of carbon-paper GDLs, thereby providing a more realistic characterization compared to isotropic models extensively used in the literature. The proposed model, conveniently validated against previous experimental data, enables the calculation of the GDL porosity distribution, GDL intrusion into the channel, and the contact pressure profiles at the interfaces of the GDL with the BPP and the catalyst-coated membrane. This analysis constitutes a necessary first step towards the development of multiphysics Computational Fluid Dynamics (CFD) models of either PEMFCs or DMFCs aiming to explore the effects of assembly compression. For a given GDL compression ratio, the results show that a combination of channel width, GDL thickness, and shear modulus dominates the transmission of stresses from the rib to the unloaded region below the channel, whereas an accurate description of the nonlinear through-plane Young’s modulus is needed to properly capture the GDL compressive response under the rib. 2. A multiphysics multiphase isothermal CFD model of a liquid-feed DMFC is then developed and presented. The model is progressively sophisticated in three steps: i. The first studies are conducted on a 2D/1D across-the-channel model that includes a 2D two-phase description for the anode GDL and a local 1D single-phase description for the remaining components of the MEA, i.e., catalyst layers, membrane, and cathode GDL. The model incorporates the effect of non-uniform mass and charge transport properties of the anode GDL induced by the cell assembly process simulated with the FEM model (Task 1). The effective mass and charge transport properties of the GDL are correlated as a function of porosity using experimental data from anisotropic carbon paper. ii. The above 2D/1D across-the-channel model is upgraded to account for the effect of electrical contact resistances at the GDL/BPP interface, the diffusive resistance of thin anode and cathode Microporous Layers (MPLs), and the effect of assembly compression on the 1D single-phase description of the cathode GDL. iii. The 2D/1D across-the-channel model is further improved by including a fully 2D two-phase description for the cathode GDL, instead of the 1D single-phase formulation adopted in the two previous steps. The model also accounts for the effect of electrical contact resistances between the GDL-MPL diffusion medium and the membrane, and includes hydrogen evolution kinetics at the anode to give a realistic description of the electrochemical processes that occur under oxygen-starved conditions. The proposed 2D/1D across-thechannel model is also extended to a 3D/1D model and combined with 1D two-phase models for the anode and cathode channels, leading to an advanced 3D/1D + 1D model that is successfully validated against previous experimental data. Among other conclusions, the results show that fully hydrophobic relationships widely used in the literature to model capillary transport of carbon dioxide in the anode GDL lead to unrealistic results when inhomogeneous GDL compression effects are taken into account. By contrast, more realistic results are obtained when GDLspecific capillary pressure data including the mixed-wettability characteristics of GDLs are considered. The results also show that, in agreement with previous experimental data, there is an optimum assembly compression level that maximizes the cell performance due to the trade-off between ohmic and mass transport losses; the optimal compression level being strongly dependent on BPP material and, more weakly, on the actual working conditions. Beyond the GDL compression, there is an optimum methanol concentration that maximizes the power output due to the trade-off between anode polarization losses and cathode mixed overpotential caused by methanol crossover. For a given methanol solution, DMFC performance is largely affected by the oxygen supply rate, operating temperature, and gas (liquid) saturation level at the anode (cathode) GDL/channel interface. 3. The effective diffusivity of GDLs is characterized under both dry and wet conditions by performing pore-scale LBM simulations on XCT images of carbon-paper GDLs undergoing water-invasion experiments. Under dry conditions, the results show a good agreement with previous experimental data reported for morphologically similar GDLs. Under wet conditions, it is shown that the spatial distribution of water across the porous medium has a major effect that is not accounted for by the average (or total) amount of water contained in the porous medium. Specifically, it is found that the existence of local bottleneck regions near the invasion face drastically limits the diffusive flux through the porous medium. This finding, traditionally ignored in previous studies, has an important repercussion for two-phase macroscopic continuum models. In a subsequent step in the investigation, it is shown that macroscopic models require effective properties determined under uniform porosity and saturation conditions to provide a physically-consistent macroscopic formulation. Constitutive relationships for the effective diffusivity suitable for use in macroscopic models are determined from a massive computational campaign (∼2,500 simulations) considering GDL representative subdomains with locally homogeneous porosity and saturation as a proxy for representative elementary volumes. Using arithmetic and harmonic upscaling rules, it is confirmed that the correlations determined on the local scale are able to recover the global data obtained on the inhomogeneous full GDL domain. Moreover, good agreement is found for the under-the-channel region when the local correlations are upscaled to previous global data from running fuel cells. The results indicate, however, that the blockage of local diffusive transport in the under-the-rib region is larger, presumably due to water condensation and interferences of water fingers with the rib walls. Both effects were not present in the X-ray tomography data, which only considered capillary invasion, and should be addressed in future work.Las pilas de combustible de membrana polimérica (PEM fuel cells) son candidatos líderes para reemplazar la economía energética actual basada en combustibles fósiles, proporcionando una generación de energía eléctrica eficiente y limpia para el siglo XXI. El estudio de pilas PEM es un campo multidisplinar y dinámico en el que la ingeniería mecánica, química y eléctrica, así como el diseño de materiales, convergen y colaboran entre sí, convirtiendo la investigación en este campo en un desafío multifísico continuo. El modelado numérico juega un papel crucial en el estudio de los complejos fenómenos de transporte de masa, carga, y calor que tienen lugar en las escalas micrométricas de las capas porosas que constituyen el conjunto membrana-electrodo (Membrane Electrode Assembly, MEA) de las pilas PEM, y constituye una herramienta esencial para optimizar el rendimiento de éstas. Las pilas tipo PEM más prometedoras son las pilas de hidrógeno (Polymer Electrolyte Membrane Fuel Cells, PEMFCs), y las pilas de metanol directo alimentadas con soluciones acuosas (liquid-feed Direct Methanol Fuel Cells, DMFCs). Aunque los mayores intereses en la actualidad están centrados en el desarrollo de pilas PEMFC de alto rendimiento para la industria automovilística, las pilas DMFC de alimentación líquida son atractivas para dispositivos electrónicos portátiles debido a la mayor densidad energética y la facilidad de manipulación y almacenamiento del metanol líquido. El propósito de esta tesis es contribuir al conocimiento de ambas tecnologías empleando técnicas de modelado multifísicas y multiescala. En primer lugar, se presenta un modelo macroscópico multifísco de una pila DMFC de alimentación líquida que tiene en cuenta los efectos de la compresión no homogénea de ensamblaje sobre las capas difusoras de gas o transporte (Gas Diffusion layers, GDLs). A continuación, la difusividad efectiva de GDLs en condiciones secas y parcialmente saturadas de agua son caracterizadas combinando el método de lattice Boltzmann (Lattice Boltzmann Method, LBM) con imágenes de rayos X de tomografía computarizada (X-ray Computed Tomography, XCT) de GDLs de papel de carbono. La consecución de estos dos objetivos se divide en tres tareas: 1. El desarrollo de un modelo de elementos finitos (Finite Element Method, FEM) para simular el proceso de ensamblaje de la GDL asociado al patrón repetitivo costilla/canal del plato bipolar (Bipolar Plate, BPP). El modelo incorpora una caracterización detallada de las propiedades mecánicas ortótropas de GDLs de papel de carbono, proporcionando una caracterización más realista en comparación con modelos isótropos extensamente empleados en la literatura. El modelo propuesto, validado con datos experimentales previos, permite el cálculo de la distribución de porosidad en la GDL, la intrusión de la GDL en el canal, y los perfiles de presiones de contacto en las interfaces de la GDL con el plato bipolar y el conjunto membrana-capa catalizadora. Este análisis constituye un primer paso necesario para el desarrollo de un modelo de dinámica de fluidos computacional (Computational Fluid Dynamics, CFD), ya sea de una pila PEMFC o DMFC, con el propósito de explorar los efectos de la compresión de ensamblaje. Para un ratio de compresión de la GDL dado, los resultados muestran que el ancho del canal, el espesor de la GDL, y el módulo de cortadura dominan la transmisión de esfuerzos de la costilla a la región no solicitada bajo el canal, mientras que una descripción detallada del módulo elástico no lineal en la dirección del espesor es necesaria para capturar adecuadamente la respuesta a compresión de la GDL bajo la costilla. 2. El desarrollo de un modelo CFD multifísico, multifásico, e isotermo de una pila de metanol directo de alimentación líquida. El modelo es sofisticado progresivamente en tres pasos: i. Los primeros estudios se llevan a cabo con un modelo 2D/1D incorporando una sección transversal de la MEA que incluye una descripción bifásica 2D para la GDL del ánodo y una descripción local 1D monofásica para los restantes componentes de la MEA (capas catalíticas, membrana, y GDL del cátodo). El modelo incorpora el efecto de las propiedades de transporte de masa y carga no uniformes inducidas por el proceso de ensamblaje de la pila simulado con el modelo FEM (Tarea 1). Las propiedades efectivas de transporte de masa y carga de la GDL son correlacionadas en función de la porosidad empleando datos experimentales correspondientes a papel de carbono anisótropo. ii. El modelo 2D/1D anterior es mejorado incorporando el efecto de las resistencias eléctricas de contacto en la interfaz GDL/BPP, la resistencia difusiva de finas capas microporosas (Microporous Layers, MPLs) en el ánodo y el cátodo, y el efecto de la compresión de ensamblaje en la descripción monofásica 1D de la GDL del cátodo. iii. El modelo 2D/1D es mejorado en mayor grado incluyendo una descripción completamente 2D de la GDL del cátodo, en lugar de la formulación monofásica 1D adoptada en los dos pasos previos. El modelo también tiene en cuenta el efecto de las resistencias eléctricas de contacto entre el medio difusor GDL-MPL y la membrana, e incluye la cinética de evolución de hidrógeno en el ánodo proporcionando una descripción realista de los procesos electroquímicos que ocurren en condiciones de escasez de oxígeno. El modelo 2D/1D propuesto también es extendido a un modelo 3D/1D y combinado con modelos bifásicos 1D para los canales del ánodo y el cátodo, dando lugar a un modelo 3D/1D + 1D que es validado satisfactoriamente con datos experimentales previos presentados en la literatura. Entre otras conclusiones, los resultados muestran que relaciones completamente hidrófobas ampliamente empleadas en la literatura para modelar el transporte capilar de dióxido de carbono en la GDL del ánodo conducen a resultados irreales cuando el efecto de la compresión no homogénea de la GDL es tenido en cuenta. Por el contrario, resultados más realistas son obtenidos al considerar datos de presión capilar específicos de GDLs que reflejan sus características de mojabilidad mixta. Asimismo, en acuerdo con resultados experimentales previos, los resultados muestran que existe un nivel de compresión de ensamblaje óptimo que maximiza las prestaciones de la pila debido a un balance entre pérdidas de transporte másicas y óhmicas; el nivel de compresión óptimo depende fuertemente del material del plato bipolar y, más débilmente, de las condiciones de operación. Más allá del nivel de compresión de la GDL, existe una concentración de metanol óptima que maximiza la potencia de la pila debido a un balance entre las pérdidas de polarización del ánodo y el sobrepotencial mixto del cátodo causado por el cruce de metanol líquido a través de la membrana. Para una concentración de metanol dada, el rendimiento de la pila se ve afectado en gran medida por la tasa de suministro de oxígeno, la temperatura de operación, y el nivel de saturación gaseoso (líquido) en la interfaz GDL/canal del ánodo (cátodo). 3. La difusividad efectiva de GDLs es caracterizada tanto en condiciones secas como parcialmente saturadas realizando simulaciones de LBM a la escala del poro en imágenes XCT de GDLs de papel de carbono tomadas durante experimentos de invasión de agua. En condiciones secas, los resultados muestran un buen acuerdo con resultados experimentales previos de GDLs con una morfología similar. En condiciones parcialmente saturadas, se muestra que la distribución espacial de agua a través del medio poroso posee un efecto importante que no es tenido en cuenta tan sólo considerando la cantidad de agua media (o total) contenida en el medio poroso. Específicamente, se ha comprobado que la existencia de cuellos de botella locales en las cercanías de la cara de invasión limita fuertemente el flujo difusivo a través del medio poroso. Este hallazgo, tradicionalmente ignorado en estudios previos, tiene una repercusión importante sobre modelos macroscópicos bifásicos. En un siguiente paso en la investigación, se muestra que modelos macroscópicos requieren propiedades efectivas determinadas en condiciones de porosidad y saturación uniformes para proporcionar una formulación macroscópica físicamente consistente. Relaciones constitutivas para la difusividad efectiva adecuadas para su uso en modelos macroscópicos son determinadas en base a los resultados de una campaña computacional masiva (∼2500 simulaciones) considerando subdominios de GDL representativos con distribuciones de porosidad y saturación localmente homogéneas. Esto permite cumplir parcialmente los requisitos para que los subdominios sean volúmenes elementarios representativos (Representative Elementary Volumes, REVs). La capacidad de las correlaciones determinadas en la escala local para recuperar los resultados globales obtenidos en los dominios de GDL no uniformes es confirmada usando reglas de escalamiento aritmético y armónico. Asimismo, un buen acuerdo es obtenido para la región bajo el canal cuando las correlaciones locales son escaladas a datos globales de pilas en funcionamiento reportados en la literatura. Sin embargo, los resultados indican que la obstrucción local del transporte difusivo bajo la costilla es mayor, probablemente debido a la condensación de agua y efectos de interferencia causados por las paredes de la costilla en el transporte de agua. Ambos fenómenos no fueron considerados en las imágenes XCT presentes, ya que los experimentos corresponden a procesos puramente de invasión capilar, y deberán estudiarse en el futuro.Programa Oficial de Doctorado en Ingeniería MatemáticaPresidente: Pedro L. García Ybarra.- Secretario: Alfredo Iranzo Paricio.- Vocal: Jens Elle