133,015 research outputs found
Fuel processor - PEM fuel cell systems for energy generation
No full textIn the last few years, increasing attention has been paid to PEM fuel cells, as promising device for decentralized energy production, both in stationary and automotive field, thanks to high compactness, low weight (high power-to-weight ratio), high modularity, good efficiency and fast start-up and response to load changes. The high efficiencies that can be obtained with a PEM fuel cell, however, require a high purity hydrogen feed at the anode. Hydrogen, though, is not a primary source, but it is substantially an energy carrier, that can be stored, transported and employed as gaseous fuel, however, it needs to be produced from other sources. The main hydrogen source is actually represented by hydrocarbons, through classical Steam Reforming or Partial Oxidation industrial scale processes. However, the limitation of hydrogen storage and transport due to its chemico-physical properties has pushed toward the concept of decentralized hydrogen production; in this way, the hydrogen source, such as methane, is distributed through pipelines to the small-scale plant, installed nearby the users, and the hydrogen produced in situ is fed directly to the energy production system, avoiding hydrogen storage and transportation. In this sense, research is oriented toward the optimization of the decentralized hydrogen production unit, generally named as fuel processor, for residential and automotive applications, for achieving fuel conversion into hydrogen with high efficiencies and high compactness.
Since the efficiency of the integrated fuel processor – fuel cell system strongly depends on system configuration and on the heat integration, a system analysis of the most promising configurations is performed, in order to identify the best solution for energy production in a PEM fuel cell system. Analysis of global system efficiency of fuel processor – PEM fuel cell systems is performed by means of the software AspenPlus®, with identification of best configuration and best operating conditions.
Moreover, since the application of fuel processor – PEM fuel cell system is foreseen for small and medium scale, an important characteristic that must me associated to the high efficiency is the compactness of the system. The PEM fuel cell, indeed, is generally characterized by high efficiency and compactness, therefore, in order to keep its standard, also the fuel processor coupled with it must be efficient and as compact as possible. In order to have an idea of the encumbrance of the reactors, a detailed mathematical model for fixed bed reactors was developed in this work, in order to size and compare conventional fixed bed reactor and membrane catalytic reactors. The software employed was Mathematica®
Estudo teórico e experimental de célula-combustível tipo pem e perspectivas de aplicação em sistemas de geração distribuída
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico. Programa de Pós-Graduação em Engenharia Mecânica.Células-combustível se apresentam como alternativas promissoras para acionamento primário em sistemas de geração ou cogeração de energia. Caracterizam-se pela conversão direta da energia química do combustível em energia elétrica, o que geralmente lhes confere eficiência superior aos acionadores primários convencionais utilizados para geração termoelétrica. Dentre os diversos tipos de células-combustível existentes, as Células-Combustível do tipo Membrana de Troca de Prótons, comumente chamadas de PEMFC ou simplesmente células PEM, estão em crescente estágio de desenvolvimento. Como seu eletrólito é um polímero sólido e sua temperatura de operação é em torno de 80ºC, são adequadas para aplicação automobilística e em sistemas portáteis de geração de energia elétrica (celulares, laptops, etc). Atualmente, devido a outras características favoráveis tais como, tamanho compacto, baixo peso, partida rápida, longa vida útil dos "stacks" (blocos) e capacidade de trabalhar em regime descontínuo e com altas densidades de corrente, sua aplicabilidade foi estendida para os sistemas de geração distribuída de energia elétrica. O presente trabalho tem como objetivo avaliar experimentalmente a influência dos parâmetros de operação sobre o desempenho de uma célula-combustível tipo PEM de 15 W de potência, de fabricação brasileira e em operação no LabCET. A partir dos resultados obtidos, ações foram implementadas de forma a melhorar o desempenho desta célula-combustível. Trabalho complementar foi também realizado para avaliar a aplicabilidade dessa tecnologia como sistema compacto de geração de energia elétrica. Nesta etapa, foi realizada uma análise termoeconômica preliminar de uma célula PEM de 30 kW, comparando os resultados com valores obtidos de duas bancadas experimentais existentes no LabCET, uma constituída de uma microturbina de 28 kW e outra constituída por um motogerador de 30 kW, ambos operando com gás natural veicular
Análise e desenvolvimento de modelo de transporte de massa visando a aplicação em células a combustível tipo PEM
No full textTese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Mecânica, Florianópolis, 2012O atual cenário mundial na área de energia demanda o desenvolvimento tecnológico de alternativas sustentáveis, e de menor impacto ambiental. O uso eficiente de fontes de energia renováveis para a produção de energia elétrica em sistemas descentralizados e isolados, bem como para o setor de mobilidade, destaca-se como um ingrediente capaz de mitigar a agressão ambiental dos sistemas de energia. A célula a combustível é um dispositivo eletroquímico que converte diretamente a energia interna de ligação química de combustíveis em energia elétrica e calor com alta eficiência global, ausência de ruído e emissões. O elevado custo de desenvolvimento destes sistemas sugere que estratégias que combinem medições e previsões teóricas apresentem a maior chance de atingir os desenvolvimentos necessários. O principal objetivo da presente tese é desenvolver uma teoria para o transporte de massa em uma célula a combustível tipo PEM a partir de uma análise fenomenológica com base nos fundamentos do transporte de massa multicomponente, multifásico em meios porosos. O modelo tem por objetivo prever o comportamento do transporte elétrico e de massa com uma formulação adequada. Para este fim, foram revisadas as escalas de comprimento característicos dos diferentes componentes e fenómenos dentro da célula a combustível visando determinar as relações entre os processos termodinâmicos, eléctricos e eletroquímicos em uma célula de combustível tipo PEM. Foi revisada a grande quantidade de informações sobre teoria, modelagem e simulação da célula a combustível tipo PEM, a fim de classificar os diferentes modelos, ressaltar sua aplicabilidade e definir as necessidades de melhoria. A curva de polarização de um sistema de célula de combustível foi medida com o objetivo de identificar os fenómenos que controlam o transporte e a fenomenologia química, avaliar a aplicabilidade dos modelos globais disponíveis e determinar a ordem de grandeza dos parâmetros característicos globais da operação da célula de combustível. Então, foram revisadas as teorias fundamentais de transporte de massa e carga em duas fases, em fluxo multicomponente em meios porosos, focando na base do continuo e da termodinâmica para o tratamento de Maxwell-Stefan do transporte de massa. Finalmente, foi proposto um modelo fenomenológico geral para transferência de massa e carga aplicável às células a combustível tipo PEM. O modelo foi comparado com outros modelos da literatura e alguns problemas mais simples fundamentais foram resolvidos.Abstract : The present world energy scenario requires the development of alternative and sustainable energy sources and conversion systems that also result in an overall smaller impact in the environment. The efficient use of renewable energy sources for the production of electrical power in decentralized and isolated systems, as well as for the mobility sector, stands out as a possible ingredient to mitigate the environmental aggression from energy systems. Fuel cells are electrochemical devices that convert internal energy of chemical bond in electricity and heat power in an efficient, noiseless and lower emissions form. The relative high cost of system development suggests that a combined measurement, theoretical and simulation effort is the way to achieve the required breakthroughs. The main objective of the present thesis is to develop a theory for mass transport in a PEM fuel cell from a phenomenological analysis based on the fundamentals of the multicomponent, multiphase mass transport in porous media. The model aims at predicting the electric and mass transport behaviors with a formulation suitable for solution with current computational resources. To this end, the characteristic length scales of the different components and phenomena within the fuel cell were revised aiming at determining the relations between thermodynamic, electric and electrochemical processes in a PEM fuel cell. The vast amount of information on PEM fuel cell theory, modeling and simulation was reviewed with a view to classify the different models, point out their applicability and define the needs for further improvements. The polarization curve for a fuel cell system was measured with the purpose of identifying the controlling transport and chemical phenomena, assess the applicability of the available lumped models and to determine the orders of magnitude of global parameters characteristic of the fuel cell operation. Then, the fundamental theories of mass and charge transport in two-phase, multicomponent flow in porous media were reviewed, focusing on the continuum and thermodynamic basis for the Maxwell-Stefan treatment of mass transport. Finally, a general phenomenological model for mass and charge transfer applicable to PEM fuel cells was proposed, compared to other models from the literature and a few simpler fundamental problems were solved
The State of the Art in Fuel Cell Condition Monitoring and Maintenance
No full textFuel cell vehicles are considered to be a viable solution to problems such as carbon emissions and fuel shortages for road transport. Proton Exchange Membrane (PEM) Fuel Cells are mainly used in this purpose because they can run at low temperatures and have a simple structure. Yet to make this technology commercially viable, there are still many hurdles to overcome. Apart from the high cost of fuel cell systems, high maintenance costs and short lifecycle are two main issues need to be addressed. The main purpose of this paper is to review the issues affecting the reliability and lifespan of fuel cells and present the state of the art in fuel cell condition monitoring and maintenance. The Structure of PEM fuel cell is introduced and examples of its application in a variety of applications are presented. The fault modes including membrane flooding/drying, fuel/gas starvation, physical defects of membrane, and catalyst poisoning are listed and assessed for their impact. Then the relationship between causes, faults, symptoms and long term implications of fault conditions are summarized. Finally the state of the art in PEM fuel cell condition monitoring and maintenance is reviewed and conclusions are drawn regarding suggested maintenance strategies and the optimal structure for an integrated, cost effective condition monitoring and maintenance management system
Degradations and Improvements in PEM Fuel Cell Materials: A Computational Study
No full textThe advantages of Proton Exchange Membrane (PEM) fuel cells include lower operating temperature than other fuel cells and size small enough to fit into a car. Improving the cost and durability of PEM fuel cell materials is a hot topic of research today.
The Nafion membrane and cathode catalysts are two areas where PEM fuel cells have issues of cost, durability, and efficiency. In order to improve these materials, researchers need a better understanding of the detailed mechanisms for basic operation and degradation. Computational quantum mechanics has improved in recent years to the point where it can provide accurate potential energy maps of reactions that are difficult to determine by laboratory experiments alone. With the basic understanding of mechanisms, experimentalists can make educated predictions of ways to improve fuel cell materials.
Experimental studies suggest that Nafion degradation is caused by generation of trace radical species (such as OH●, H●) when in the presence of H2, O2, and Pt. We use density functional theory (DFT) to construct the potential energy surfaces for various plausible reactions involving intermediates that might be formed when Nafion is exposed to H2 (or H+) and O2 in the presence of the Pt catalyst. We find that OH● can be generated in trace amounts on the Pt surface from HOOH and OOHad. Next, we look at various ways in which the OH● can attack the Nafion sidechains or endgroups on the backbone.
Researchers are looking for ways to replace the Pt cathode catalyst, due to the preciousness of Pt and the low efficiency of the oxygen reduction reaction (ORR) on Pt, among other things. Alloying Pt with non-precious Co greatly increases the ORR efficiency. However, Pt3Co was reported to not withstand long-cycle testing due to the migration of Co metals onto the catalyst surface and leaching of Co into the electrolyte. To overcome these challenges, we first study Pt3Co to find out what makes these alloys so special in improving fuel cell efficiency, as well as what causes degradation to occur. Then, we apply the principles we learned in proposing improved fuel cell alloy catalysts.</p
Experimental validation of equilibria in fuel cells with dead-ended anodes
No full textThis paper investigates the nitrogen blanketing front during the dead-ended anode (DEA) operation of a PEM fuel cell. Surprisingly the dynamic evolution of nitrogen and water accumulation in the dead-ended anode (DEA) of a PEM fuel cell arrives to a steady-state suggesting the existence of equilibrium behavior. We use a multi-component model of the two-phase one-dimensional (along-the-channel) system behavior to analyze and exploit this phenomenon. Specifically, the model is first verified with experimental observations, and then utilized for showing the evolution towards equilibrium. The full order model is reduced to a second-order ordinary differential equation (ODE) with one state, which can be used to predict and amalyse the surprising but experimentally observed steady state DEA behavior
Fundamental Study of nanostructured electro-catalysts with reduced noble metal content for PEM based water electrolysis
No full textIdentification and development of non-noble metal based electro-catalysts or electro-catalysts with significant reduction of expensive noble metal contents (E.g. IrO2, Pt) with comparable electrochemical performance as the standard noble metal/metal oxide for proton exchange membrane (PEM) based water electrolysis would constitute a major breakthrough in the generation of hydrogen by water electrolysis. Accomplishing such a system would not only result reduction of the overall capital costs of PEM based water electrolyzers, but also help attain the targeted hydrogen production cost [< $ 3.0 / gallon gasoline equivalent (gge)] comparable to conventional liquid fuels.
In line with these goals, it was demonstrated that fluorine doped IrO2 thin films and nanostructured high surface area powders display remarkably higher electrochemical activity, and comparable durability as pure IrO2 electro-catalyst for the oxygen evolution reaction (OER) in PEM based water electrolysis. Furthermore, corrosion resistant SnO2 and NbO2 support has been doped with F and coupled with IrO2 or RuO2 for use as an OER electro-catalyst. A solid solution of SnO2:F or NbO2:F with only 20 - 30 mol.% IrO2 or RuO2 yielding a rutile structure in the form of thin films and bulk nanoparticles displays similar electrochemical activity and stability as pure IrO2/RuO2. This would lead to more than 70 mol.% reduction in the noble metal oxide content. Novel nanostructured ternary (Ir,Sn,Nb)O2 thin films of different compositions have also been studied. It has been shown that (Ir0.40Sn0.30Nb0.30)O2 shows similar electrochemical activity and enhanced chemical robustness as compared to pure IrO2. F doping of the ternary (Ir,Sn,Nb)O2 catalyst helps in further decreasing the noble metal oxide content of the catalyst.
As a result, these reduced noble metal oxide catalyst systems would potentially be preferred as OER electro-catalysts for PEM electrolysis. The excellent performance of the catalysts coupled with its robustness would make them great candidates for contributing to significant reduction in the overall capital costs of PEM based water electrolyzers. This thesis provides a detailed fundamental study of the synthesis, materials, characterization, theoretical studies and detailed electrochemical response and potential mechanisms of these novel electro-catalysts for OER processes
2-D + 1-D PEM fuel cell model for the integration in fuel cell system simulations
No full textZiel dieser Arbeit ist es, eine simulationsbasierte Untersuchung des Betriebs eines PEM-Brennstoffzellensystems mit besonderem Fokus auf den Wasserhaushalt zu ermöglichen. In PEM-Brennstoffzellen sind die Wechselwirkungen in Bezug auf den Feuchtehaushalt z.B. von Transportmechanismen, Degradation und Leistung abhängig. Im PEM-Brennstoffzellensystem werden die Wechselwirkungen der Feuchte durch eine mögliche Rezirkulation und/oder eine passive Befeuchtung der Kathode erweitert,
Um diese Abhängigkeiten korrekt darzustellen, wurde ein PEM-Brennstoffzellen Stack-Modell entwickelt, das einerseits einen hohen Grad an Detailtreue und andererseits hohe Anforderungen an die Laufzeit erfüllt, um akzeptable Simulationszeiten für Brennstoffzellensystemsimulationen zu ermöglichen. Der Kern des Modells ist eine neuartige 2-D + 1-D Struktur, die flächenspezifische Bedingungen, wie z.B. trockene Kathodeneinlass- und feuchte Kathodenauslassbedingungen in Abhängigkeit von Gleich- oder Gegenstromstrukturen, löst. Um diesen Anforderungen an die Laufzeit gerecht zu werden, wurde ein numerischer Löser entwickelt, der speziell an die Struktur und den Inhalt angepasst ist.
Das Brennstoffzellenmodell und dessen Möglichkeiten zur Integration in eine Systemsimulation werden in dieser Arbeit vorgestellt.The aim of this work is to enable simulation based investigation of the operation of a PEM fuel cell system, with the special focus on its water management. For PEM fuel cells their multi-level interaction regarding humidity is characteristic depending e.g. on transport mechanisms, degradation and performance. In the PEM fuel cell system, the interaction of the humidity is further enhanced by a possible recirculation and passive humidification systems of the cathode.
In order to display these dependencies correctly, a PEM fuel cell stack model has been developed, which on the one hand meets high degree of resulting details and on the other hand meets high requirements concerning its runtime, to enable acceptable simulation times for fuel cell system simulations. The core of the model is a novel 2-D + 1-D structure that resolves area specific conditions, such as dry cathode inlet and wet cathode outlet conditions in dependence of co- or counter flow fields. To meet those requirements regarding its runtime a numerical solver has been developed that is specially adapted to the structure and content.
The fuel cell model as well as its possibilities with respect to the integration in system simulation is presented in this work
Reduced-dimensional models for straight-channel proton exchange membrane fuel cells
No full textA comprehensive description of proton exchange membrane fuel cell (PEMFC) performance includes the transport phenomena, phase change and electrochemical reaction inside the several components, which possess disparate characteristics and together form a complex three-dimensional geometry. Much of the modelling work in this area has, therefore, relied on the techniques of computational fluid dynamics (CFD). The comprehensive three-dimensional (3D) approach can, however, be prohibitively time consuming. Consequently, it is not the ideal basis for a rapid screening tool that operates under a wide range of design options and operating conditions. Mathematical models and solution procedures using simplified models with reduced dimensions have been proposed to address this issue. Such approaches are computationally efficient, but no systematic study has been conducted to qualitatively or quantitatively assess the impact of the neglected dimensionality on the accuracy of the resulting model. In this paper, we compare results from a hierarchy of reduced-dimensional models to the results from a comprehensive 3D CFD model for a single, straight-channel unit cell. The quality of the simulation results from reduced-dimensional models, including the cell voltage and the distributions of current density and relative humidity, are assessed. We demonstrate that the 2 + 1D approach, which includes mass transport in the 2D cross-section of the channel and membrane electrode assembly and integrates along the flow channel, is optimal in terms of both efficiency and accuracy. It provides a sound basis for a simulation tool that can be used in the early stages of a unit-cell design cycl
Electrochemical H/D isotope effects in PEM fuel cell
No full textAn electrochemical H/D separation system consisting of electrolyzer and PEM fuel cell has been proposed. Isotope separation could be important as a part of the energy saving process in an energy-hydrogen-energy cycle. Any transfer of energy into hydrogen or vice versa induces change of the H/D isotope ratio, which can be considered, as a method to produce heavy water as by-product. In this way, the separation efficiency can contribute to the overall efficiency of the cycle. (c) 2008 Elsevier B.V. All rights reserved
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