The development, design, and modeling of a rapid continuous processing scheme is developed to economically manufacture conductive polymer composite bipolar plates for fuel cells. Bipolar plates are required to possess several important properties for fuel cell operation, with the most sought after being electrical conductivity and mechanical strength. The polymer composite material generated at Virginia Tech is based on material generated by a wet-lay process and uses polyethylene terepthalate (PET) or polyphenylene sulfide (PPS) as the binder, although PPS is mainly used. In order to reach sufficient conductivity for use in generating bipolar plates, the polymer is doped with high levels of conductive graphite particles in the range of 70-80 wt%. The polymer system is reinforced with 6-9 wt% glass or carbon fibers. When compression molded into a solid, flat preform, the wet-lay material exhibits excellent bulk (in-plane) conductivity (> 250 S/cm). The material also exhibits tensile and flexural strengths of 57.5 and 95.8 MPa, respectively, higher than other polymer composite material being considered for bipolar plate production. However, formability and through-plane conductivity needs improvement. The laminate bipolar plates developed at Virginia Tech are made using wet-lay material in the core and a thermoplastic/graphite mixture on the surfaces. The wet-lay material provides mechanical integrity, while a powder form of PVDF or PPS and graphite mixture added to the surfaces to improve through-plane conductivity and formability. The manufacturing scheme for the production of laminate bipolar plates is based on the pre-consolidation of the wet-lay material, which establishes a solid, flat surface for the continuous addition of laminate powder. Because the laminate powder only requires heating, radiation heating is used in the process design to pre-heat the preform prior to compression molding. The heated preform passes underneath a press, where forming of channels takes place along with cooling of the bipolar plate. It is estimated that the entire process can take one minute to produce a bipolar plate. The cost of manufacturing a bipolar plate is estimated to be $8/kW, below the goal of $10/kW. The annual production is determined to be 250,000, with over 500,000 possible depending on certain design factors. / Ph. D.

Proton exchange membrane fuel cells (PEMFCs) are one of the leading candidates in alternative energy conversion devices for transportation, stationary, and portable power generation applications. PEMFC systems with their own fuel conversion unit typically consist of several subsystems: a fuel processing subsystem, a fuel cell stack subsystem, a work recovery-air supply subsystem, and a power electronics subsystem. Since these subsystems have different physical characteristics, their integration into a single system/subsystem level unit make the problems of dynamic system synthesis/design and operation/control highly complex. Typically, the synthesis/design optimization of energy systems is based on a single full load condition at steady state. However, a more comprehensive synthesis/design and operation/control optimization requires taking into account part as well as full load conditions for satisfying an optimal efficiency/cost/environmental effect objective. Optimal couple of these various aspects of system development requires dynamic system/subsystem/component modeling and a multi-disciplinary approach which results in an integrated set of diverse types of models and highly effective optimization strategies such as decomposition techniques (e.g., Dynamic Iterative Local-Global Optimization: DILGO). In energy system synthesis/design and operation/control problems, system/ component models are typically treated deterministically, even though input values, which include the specific load profile for which the system or subsystem is developed, can have significant uncertainties that inevitably propagate through the system to the outputs. This deficiency can be overcome by treating the inputs and outputs probabilistically. In this work, various uncertainty analysis methodologies are applied; and among these traditional probabilistic approaches (e.g., Monte Carlo simulation) and the response sensitivity analysis (RSA) method are examined to determine their applicability to energy system development. In particular, these methods are used for the probabilistic (non-deterministic) modeling, analysis, and optimization of a residential 5 kWe PEMFC system, and uncertainty effects on the energy system synthesis/design and operation/control optimization have been assessed by taking the uncertainties into account in the objectives and constraints. Optimization results show that there is little effect on the objective (the operating cost and capital cost), while the constraints (e.g., on the CO concentration) can be significantly affected during the synthesis/design and operation/control optimization. / Ph. D.

A decomposition methodology based on the concept of "thermoeconomic isolation" applied to the synthesis/design and operational optimization of a stationary cogeneration proton exchange membrane fuel cell (PEMFC) based total energy system (TES) for residential/commercial applications is the focus of this work. A number of different configurations for the fuel cell based TES were considered. The most promising set based on an energy integration analysis of candidate configurations was developed and detailed thermodynamic, kinetic, geometric, and economic models at both design and off-design were formulated and implemented. A decomposition strategy called Iterative Local-Global Optimization (ILGO) developed by Muñoz and von Spakovsky was then applied to the synthesis/design and operational optimization of the fuel cell based TES. This decomposition strategy is the first to successfully closely approach the theoretical condition of "thermoeconomic isolation" when applied to highly complex, non-linear systems. This contrasts with past attempts to approach this condition, all of which were applied to very simple systems under very special and restricted conditions such as those requiring linearity in the models and strictly local decision variables. This is a major advance in decomposition and has now been successfully applied to a number of highly complex and dynamic transportation and stationary systems. This thesis work presents the detailed results from one such application. / Master of Science

Electrical shorting through the proton exchange membrane (PEM) is a form of early failure commonly found in PEM fuel cells. In order to improve the durability and thus the commercial potential for PEM fuel cells, this form of failure must be understood and mitigated. This research investigates whether complete penetration is the most likely cause of shorting and establishes general parameters (force, contact pressure, temperature, and time) that lead to shorting in a typical PEM material, Nafion® NRE211. Data was obtained from a novel indentation apparatus that was coupled with an electrical circuit to assess the force and depth of penetration at which shorting occurs in a PEM at temperatures ranging from 70ï °C to 100ï °C. The results show that shorting occurs when full penetration is reached, based on both displacement at shorting, and resistance of the electrical circuit at shorting. In addition, a finite element model was created in a commercial finite element tool (Abaqus) in an attempt to predict time to penetration under loads and geometric configurations typically found in PEM fuel cells. The finite element model was investigated for use with standard Abaqus material modules (e.g. two-layer viscoplastic and hyperelastic-viscoelastic) describing Nafion® behavior. The results suggest that the standard material models do not sufficiently describe Nafion® behavior in this particular application and suggest the need for alternative material models that capture both the viscous and plastic nature of Nafion®. / Master of Science

Fuel cell system models are key tools for automotive fuel cell system engineers to properly size components to meet design parameters without compromising efficiency by over-sizing parasitic components. A transient fuel cell system level model is being developed that includes a simplified transient thermal and parasitics model. Model validation is achieved using a small 1.2 kW fuel cell system, due to its availability. While this is a relatively small stack compared to a full size automotive stack, the power, general thermal behavior, and compressor parasitics portions of the model can be scaled to any number of cells with any size membrane area. With flexibility in membrane size and cell numbers, this model can be easily scaled to match full automotive stacks of any size. The electrical model employs a generalized polarization curve to approximate system performance and efficiency parameters needed for the other components of the model. General parameters of a stack's individual cells must be known to scale the stack model. These parameters are usually known by the time system level design begins. The thermal model relies on a lumped capacity approximation of an individual cell system with convective cooling. From the thermal parameters calculated by the model, a designer can effectively size thermal components to remove stack thermal loads. The transient thermal model was found to match experimental data well. The steady state and transient sections of the curve have good agreement during warm up and cool down cycles. In all, the model provides a useful tool for system level engineers in the early stages of stack system development. The flexibility of this model will be critical for providing engineers with the ability to look at possible solutions for their fuel cell power requirements. / Master of Science

Research and prototyping of a fuel cell stack system for micro aerial vehicles (MAVs) was conducted by Virginia Tech in collaboration with Luna Innovations, Inc, in an effort to replace the lithium battery technology currently powering these devices. Investigation of planar proton exchange membrane (PEM) and direct methanol (DM) fuel cells has shown that these sources of power are viable alternatives to batteries for electronics, computers, and automobiles. However, recent investigation about the use of microtubular fuel cells (MTFCs) suggests that, due to their geometry and active surface areas, they may be more effective as a power source where size is an issue. This research focuses on hydrogen MTFCs and how their size and construction within a stack affects the power output supplied to a MAV, a small unmanned aircraft used by the military for reconnaissance and other purposes. In order to conduct this research effectively, a prototype of a fuel cell stack was constructed given the best cell characteristics investigated, and the overall power generation system to be implemented within the MAV was modeled using a computer simulation program. The results from computer modeling indicate that the MTFC stack system and its balance of system components can eliminate the need for any batteries in the MAV while effectively supplying the power necessary for its operation. The results from the model indicate that a hydrogen storage tank, given that it uses sodium borohydride (NaBH4), can fit inside the fuselage volume of the baseline MAV considered. Results from the computer model also indicate that between 30 and 60 MTFCs are needed to power a MAV for a mission time of one hour to ninety minutes, depending on the operating conditions. In addition, the testing conducted on the MTFCs for the stack prototype has shown power densities of 1.0, an improvement of three orders of magnitude compared to the initial MTFCs fabricated for this project. Thanks to the results of MTFC testing paired with computer modeling and prototype fabrication, a MTFC stack system may be possible for implementation within an MAV in the foreseeable future. / Master of Science

Micro air vehicles (MAVs) are small remote controlled aircraft used by military personnel for reconnaissance and are currently powered by batteries. The MAVs rely on the battery for propulsion, navigation, and reconnaissance equipment. The thrust of this research is to develop a fuel cell system capable of higher power densities, higher power to weight ratios, and increased overall power output than the batteries in use today. To this end, a feasibility study is first conducted to determine if fuel cells could be used to replace batteries as the MAV power source and what fuel cell configurations would show the best performance. Hydrogen, methanol, and formic acid fuel cells are considered, using a conventional flat-plate design and a novel micro-tubular design. Several micro-tubular fuel cells (MTFCs) are tested to show that these cells are a possibility for power production in MAVs. Those tested are developed and improved in collaboration between Luna Innovations, Inc. and the Center for Energy Systems Research at Virginia Tech and then manufactured by Luna Innovations, Inc. Also, an isothermal, lumped-parameter (LP) model for MTFCs is developed to predict behavior. The use of this LP model aids in understanding the dominant losses of the cell and ways of improving cell performance. Results from the feasibility study indicate that by using methanol powered MTFCs a 50% increase in overall energy output is possible, while also decreasing the mass of the power production system. Through testing and an iterative design process, an increase of three orders of magnitude of the maximum power production of the MTFCs constructed by Luna Innovations, Inc., has been realized. Results of the LP MTFC model are compared with the experimental results from the MTFC testing and tubular cells from the literature. / Master of Science

Catalyst layer modeling can be a useful tool for fuel cell design. By comparing numerical results to experimental results, numerical models can provide a better understanding of the physical processes occurring within the fuel cell catalyst layer. This can lead to design optimization and cost reduction. The purpose of this research was to compare, for the first time, a direct numerical simulation (DNS) model for the cathode catalyst layer of a PEM fuel cell to a newly developed experimental technique that measures the ionic potential through the length of the catalyst layer. A new design for a microstructured electrode scaffold (MES) is proposed and implemented. It was found that there is a 25%-27% difference between the model and the experimental measurements. Case studies were also performed with the DNS to compare the effects of different operating conditions, specifically temperature and relative humidity, and different reconstructed microstructures. Suggested operating parameters are proposed for the best comparison between numerical and experimental results. Recommendations for microstructure reconstruction, MES construction and design, and potential measurement techniques are also given. / Master of Science

Les piles à combustible PEM sont des systèmes complexes faisant intervenir de nombreux auxiliaires. Ces auxiliaires ont un impact sur le rendement global du système. Une des possibilités pour améliorer le rendement global du système est d'améliorer le rendement de ces auxiliaires et notamment celui du système de recirculation d'hydrogène.Le système de recirculation d'hydrogène permet à la fois l'apport de l'hydrogène à l'anode mais il permet également de gérer l'eau présente. Deux solutions sont proposées pour modifier le système de recirculation.La première solution utilise la consommation d'hydrogène à l'anode comme une pompe à vide permettant alors grâce à l'ouverture et à la fermeture cyclique d'une électrovanne, la mise en mouvement du gaz dans la boucle de recirculation. La deuxième solution consiste à utiliser un stack de pile à combustible comme une pompe à hydrogène qui s'intègre alors dans une boucle de recirculation classique.Ces deux solutions ont été étudiées analytiquement, numériquement (MATLAB©, SIMULINK©) et expérimentalement.La première solution a montré sa capacité à se substituer à un système de recirculation à pompe classique en ayant des performances de recirculation et une compacité équivalente mais surtout en améliorant le rendement global du système pile de 2.5% à sa puissance nominale.La seconde solution a montré une bonne capacité à gérer la recirculation d'hydrogène et sa spécificité de pompage de l'hydrogène seul ouvre la possibilité à de nombreux autres systèmes. / PEM fuel cells are complex systems involving many auxiliaries. These auxiliaries have an impact on the overall system performance. One possibility to improve the overall performance of the system is to improve the performance of these auxiliaries and in particular the system of hydrogen recirculation.The hydrogen recirculation system allows both the supply of hydrogen to the anode but also the management of the water present within the system. Two solutions are proposed to modify the recirculation system.The first solution uses the hydrogen consumption at the anode as a vacuum pumping mechanism used as the motive power of the recirculation loop. The second solution uses a fuel cell stack as a hydrogen pump which is then integrated into a conventional recirculation loop.Both solutions have been studied analytically, numerically (MATLAB ©, SIMULINK ) and experimentally.The first solution has demonstrated its ability to replace a conventional recirculation pump having equivalent recirculation performances and an equivalent compactness. It permits to improve the overall performance of the fuel cell by 2.5% at nominal power.The second solution has shown a good ability to manage the hydrogen recirculation. Its specificity to solely pump hydrogen opens up the possibility of many other systems. Anodic gases purification appears to be highly promising.

La pile à combustible (PàC) est un dispositif qui transforme l'énergie chimique en énergie électrique. Ce dispositif nécessite un certain nombre d’auxiliaires pour son fonctionnement. Afin d'assurer des performances en termes de sécurité, de fiabilité et de durée de vie de la PàC, des systèmes de diagnostic et de commande adéquats sont indispensables. Ainsi, cette thèse est une contribution au problème du diagnostic de défauts et à la commande de la PàC de type PEM. Le premier volet de ce travail est consacré au développement de méthodes de diagnostic appliquées à la PàC. Pour ce faire, deux approches ont été proposées. La première concerne la synthèse d’un filtre H-/H∞ permettant la détection de défauts capteurs et actionneurs tout en assurant un niveau de robustesse vis-à-vis d'éventuelles perturbations. En tenant compte des caractéristiques du système de la PàC, les conditions d'existence et de stabilité du filtre sont données sous la forme de LMI. La seconde approche traite le problème de la détection des défauts paramétriques. Pour cet objectif, un observateur adaptatif a été proposé, ce dernier permet d'estimer la valeur du paramètre susceptible de présenter une défaillance. Cette méthode donne la possibilité d'estimer simultanément les défauts paramétriques et les états non mesurés du système. Le deuxième volet de la thèse est dédié à la commande du système PàC. Le but est de synthétiser une loi de commande par rétroaction qui permet de répondre à la sollicitation de la charge tout en respectant une contrainte de fonctionnement nominal de la PàC. La synthèse de cette loi de commande a été réalisée en prenant compte tout le caractère non linéaire du système / The fuel cell is a device that transforms the chemical energy in electricity. This device requires some auxiliaries for its operation. In order to ensure the performances in terms of security, reliability and life cycle of the fuel cell, adequate diagnostic and control systems are indispensables. This thesis is a contribution to the problem of faults diagnosis and control of PEM Fuel Cell. The first part of this work is dedicated to the development of diagnostic methods applied to the fuel cell. To do this, two approaches are proposed. The first one concerns the synthesis of a H-/H∞ filter allowing the sensors and actuators faults detection while ensuring a level of robustness towards disturbances. Taking into account the fuel cell system characteristics, the conditions for existence and stability of the filter are given in the form of Linear Matrix Inequalities (LMI). The second approach deals with the problem of parametric faults detection. For this purpose, an adaptive observer has been proposed, which makes it possible to estimate the value of the parameter likely to exhibit a failure. This method gives the possibility of simultaneously estimating the parametric faults and the unmeasured states of the system. The second part of the thesis is dedicated to the control of the fuel cell system. The purpose is to synthesize a feedback control law that allows to answer the load request while respecting a nominal operating constraint of the fuel cell. Such a functioning allows to have a better efficiency while preserving the state of health of the fuel cell. The control law is obtained by using the original nonlinear model and without any kind of linearization