This study deals with the thermodynamic analysis of a molten carbonate fuel cell (MCFC) hybrid system to determine its efficiencies, irreversibilities and performance.The analysis includes a performance investigation of a typical molten carbonate fuel cell stack, an industrial MCFC hybrid system, and an MCFC hybrid system deployed by Enbridge. A parametric study is performed to examine the effects of varying operating conditions on the performance of the system. Furthermore, thermodynamic irreversibilities in each component are determined and an optimization of the fuel cell is conducted. Finally, a simplified and novel method is used for the cost analysis of the Enbridge MCFC hybrid system.An exergy analysis of the hybrid MCFC systems demonstrates that overall efficiencies of up to 60 % are achievable. The maximum exergy destruction was found in components in which chemical reactions occur. In addition, the turboexpander is one of the major contributors to the overall exergy destruction of the system. The cost analysis of the Enbridge system illustrates that by merging the importance of “green” energy and rising costs of carbon offsets, this new technology could be a promising solution and substitute for future energy supply. / UOIT

In this thesis biomimetic design is applied to the redesign of a PEM fuel cell flow field. A number of designs inspired by biological phenomena were developed to address the problem of attaining a uniform current density distribution across a PEM fuel cell. These designs are evaluated using a numerical model. One design, inspired by Murray’s law of branching in plants and animals, is further evaluated using and a physical model and comparing it to a commercial triple serpentine flow field. Improvements in pressure drop were seen for the Murray’s law inspired flow field, however, it was found to be prone to flooding. If this flow field design were to be applied to high temperature membrane materials, materials that can operate above 100 °C where water is always in the vapor state, the mass transfer and reduced pressure drop advantages of the Murray flow field could be fully achieved.

In this thesis biomimetic design is applied to the redesign of a PEM fuel cell flow field. A number of designs inspired by biological phenomena were developed to address the problem of attaining a uniform current density distribution across a PEM fuel cell. These designs are evaluated using a numerical model. One design, inspired by Murray’s law of branching in plants and animals, is further evaluated using and a physical model and comparing it to a commercial triple serpentine flow field. Improvements in pressure drop were seen for the Murray’s law inspired flow field, however, it was found to be prone to flooding. If this flow field design were to be applied to high temperature membrane materials, materials that can operate above 100 °C where water is always in the vapor state, the mass transfer and reduced pressure drop advantages of the Murray flow field could be fully achieved.

The anisotropic mass transport issues inside a fuel cell membrane have been studied in this thesis using computer modelling. The polymer electrolyte membrane (PEM) conductivity of a PEM fuel cell (PEMFC) depends on the hydration state of the hydrophilic charged sites distributed in the pores of the membrane. Water humidification of these charged sites is crucial for sustaining the membrane conductivity and reducing concerning voltage losses of the cell. During the operation of a PEMFC, the transport of humidified inlet gases (fuel/oxidant) is influenced by external design factors such as flow field plate geometry of the gas circulating channels. As a result, there arises a distribution in the mass transport of water inside the membrane electrode assembly. A two-dimensional, cross-the-channel, fuel cell membrane layer mass transport model, developed in this work, helps the study of the impact of factors causing the distribution in the membrane ionic conductivity on ohmic losses.<p>The governing equations of the membrane mathematical model stem from the multicomponent framework of concentrated solution theory. All mass transport driving forces within the vapour and/or liquid equilibrated phases have been accounted in this research. A computational model, based on the finite control volume method, has been implemented using a line-by-line approach for solving the dependent variables of the mass transport equations in the two-dimensional membrane domain. The required boundary conditions for performing the anisotropic mass transport analysis have been obtained from a detailed agglomerate model of the cathode catalyst layer available in the literature.<p>The results obtained using boundary conditions with various flow field plate channel-land configurations revealed that the anisotropic water transport in the cathode half-cell severely affects the ohmic losses within the membrane. A partially humidified vapour equilibrated membrane simulation results show that a smaller channel-land ratio (1:1) sustains a better membrane performance compared to that with a larger one (2:1 or 4:1). Resistance calculations using the computer model revealed that ohmic losses across the membrane also depend on its physical parameters such as thickness. It was observed that the resistance offered by a thinner membrane towards vapour phase mass transport is comparatively lower than that offered by a thicker membrane. A further analysis accounting the practical aspects such as membrane swelling constraints, imposed by design limitations of a fuel cell, revealed that the membrane water content and ionic conductivity are altered with an increase in the compression constraint effects acting upon a free swelling membrane.

With the current interest in alternative methods of energy production and increased utilization of existing energy sources, microbial fuel cells have become an important field of research. Microbial fuel cells are devices which harvest electrons from microorganisms created by their enzymatic oxidation of complex carbon substrates or consumed by their reduction of chemical oxidants. Microbial fuel cells with photosynthetic biocathodes are of particular interest due to their ability to simultaneously produce electricity and hydrocarbons while reducing carbon dioxide. Most species of microorganisms including many bacteria and yeasts require exogenous electron transfer mediators in order to allow electron transfer with an electrode. While adding such chemicals is simple enough at a lab scale, problems arise with chemical costs and separation at a larger scale. The goal of this research was to develop electrodes composed of a robust material which will eliminate the need for added soluble electron mediators in a photosynthetic biocathode microbial fuel cell. Electrodes made from stainless steel 304L have been coated in a conductive polymer (polypyrrole) and an immobilized electron transfer mediator (methylene blue) and tested chemically for stability and in a microbial fuel cell environment for use in bioanodes and biocathodes. The use of these immobilized mediator in the photosynthetic biocathode increased the open circuit voltage of the cell from 0.17 V to 0.24 V and the short circuit current from 8 mA/m2 to 64 mA/m2 (normalized to the geometric surface area of the electrode) when compared to using the same mediator in solution. The opposite effect was seen when using the electrodes in a bioanode utilizing Saccharomyces cerevisiae. The open circuit voltage decreased from 0.37 V to 0.31 V and the short circuit current decreased from 94 mA/m2 to 24 mA/m2 when comparing the immobilized mediator to soluble mediators. The impact of the membrane and pH of the anode and cathode solutions were quantified and were found to have much less of an effect on the internal resistance than the microbial factors.

With the current interest in alternative methods of energy production and increased utilization of existing energy sources, microbial fuel cells have become an important field of research. Microbial fuel cells are devices which harvest electrons from microorganisms created by their enzymatic oxidation of complex carbon substrates or consumed by their reduction of chemical oxidants. Microbial fuel cells with photosynthetic biocathodes are of particular interest due to their ability to simultaneously produce electricity and hydrocarbons while reducing carbon dioxide. Most species of microorganisms including many bacteria and yeasts require exogenous electron transfer mediators in order to allow electron transfer with an electrode. While adding such chemicals is simple enough at a lab scale, problems arise with chemical costs and separation at a larger scale. The goal of this research was to develop electrodes composed of a robust material which will eliminate the need for added soluble electron mediators in a photosynthetic biocathode microbial fuel cell. Electrodes made from stainless steel 304L have been coated in a conductive polymer (polypyrrole) and an immobilized electron transfer mediator (methylene blue) and tested chemically for stability and in a microbial fuel cell environment for use in bioanodes and biocathodes. The use of these immobilized mediator in the photosynthetic biocathode increased the open circuit voltage of the cell from 0.17 V to 0.24 V and the short circuit current from 8 mA/m2 to 64 mA/m2 (normalized to the geometric surface area of the electrode) when compared to using the same mediator in solution. The opposite effect was seen when using the electrodes in a bioanode utilizing Saccharomyces cerevisiae. The open circuit voltage decreased from 0.37 V to 0.31 V and the short circuit current decreased from 94 mA/m2 to 24 mA/m2 when comparing the immobilized mediator to soluble mediators. The impact of the membrane and pH of the anode and cathode solutions were quantified and were found to have much less of an effect on the internal resistance than the microbial factors.

Solid oxide fuel cells with graded electrodes consisting of multiple composite layers yield generally lower polarization resistances than single layer composite electrodes. Optimization of the performance of solid oxide fuel cells with graded electrode composition and/or microstructure requires an evaluation of both the three phase boundary length per unit volume and the effective diffusion coefficient in order to provide insight into how these properties vary over the design space. A numerical methodology for studying the three phase boundary length and effective diffusivity in composite electrode layers with controlled properties is developed. A three dimensional solid model of a sintered composite electrode is generated for which the mean particle diameter, composition, and total porosity may be specified as independent variables. The total three phase boundary length for the modeled electrode is calculated and tomographic methods are used to estimate the fraction of this length over which the electrochemical reactions can theoretically occur. Furthermore, the open porosity of the modeled electrode is identified and the effective diffusion coefficient is extracted from the solution of the concentration of the diffusing species within the open porosity. Selected example electrode models are used to illustrate the application of the methods developed, and the resulting connected three phase boundary length and diffusion coefficients are compared. A significant result is the need for thickness-specific effective diffusivity to be determined, rather than the general volume averaged property, for electrodes with porosity between the upper and lower percolation thresholds. As the demand for current increases, more of the connected three phase boundaries become active, and therefore a greater fraction of the electrode layer is utilized for a given geometry, resulting in a higher apparent effective diffusivity compared to the same electrode geometry operating at a lower current. The methods developed in this work may be used within a macroscopic electrode performance model to investigate optimal designs for solid oxide fuel cell electrodes with stepwise graded composition and/or microstructure.

en Español:Se propone un modelo de FC basado en ecuaciones electroquímicas para predicción del exceso de oxígeno y de la temperatura de la pila, permitiendo además una conexión circuital con la carga. Así mismo, se presenta una técnica de modelado basada en Fuzzy, orientada a la emulación, obteniendo gran precisión con carga computacional reducida. Usando este último modelo se diseña e implementa un emulador. Estos modelos y el sistema de emulación fueron validados usando un sistema experimental.Adicionalmente, diferentes topologías de sistemas de potencia basados en FC se proponen y analizan, obteniendo un criterio de selección dependiendo de la aplicación. Así mismo, se presentan criterios de control para una operación segura y eficiente del sistema. Finalmente, se proponen una metodología para la caracterización de los puntos óptimos de operación, y una estructura de control para operar en esas condiciones óptimas, siendo validados en un sistema experimental representativo del estado del arte. / in English:A new FC modeling approach based on electrochemical equations for thermal and oxygen excess ration prediction with a circuit-based load connection is introduced. A fuzzy-based modeling technique is also proposed for emulation purposes, it reproducing the fuel cell dynamics with a high accuracy and a short computational time. The implementation of a fuel cell emulation system, based on this model, is described and analyzed. The models and the emulation system are experimentally validated by using a benchmark fuel cell system.Different topologies for fuel cell-auxiliary storage devices interaction are also proposed and analyzed, thus giving an architecture selection criterion based on the load profile. Controllers, dynamic constrains and control objectives are designed for a safe and efficient fuel cell operation. Finally, a methodology for the identification of the fuel cell optimal operation conditions has been proposed, and a control strategy for operating in that optimal profile is introduced and validated.

Fuel cells (FC) are electrochemical devices that directly convert the chemical energy of a fuel into electricity. Power systems based on proton exchange membrane fuel cell (PEMFC) technology have been the object of increasing attention in recent years as they appear very promising in both stationary and mobile applications due to their high efficiency, low operating temperature allowing fast startup, high power density, solid electrolyte, long cell and stack life, low corrosion, excellent dynamic response with respect to the other FCs, and nonpolluting emissions to the environment if the hydrogen is obtained from renewable sources. The output-voltage characteristic in a PEMFC is limited by the mechanical devices which are used for regulating the air flow in its cathode, the hydrogen flow in its anode, its inner temperature, and the humidity of the air supplied to it. Usually, the FC time constants are dominated by the fuel delivery system, in particular by the slow dynamics of the compressor responsible for supplying the oxygen. As a consequence, a fast load transient demand could cause a high voltage drop in a short time known as oxygen starvation phenomenon that is harmful for the FC. Thus, FCs are considered as a slow dynamic response equipment with respect to the load transient requirements. Therefore, batteries, ultracapacitors or other auxiliary power sources are needed to support the operation of the FC in order to ensure a fast response to any load power transient. The resulting systems, known as FC hybrid systems, can limit the slope of the current or the power generated by the FC with the use of current-controlled dc-dc converters. In this way, the reactant gas starvation phenomena can be avoided and the system can operate with higher efficiency. The purpose of this thesis is the design of a DC-DC converter suitable to interconnect all the different elements in a PEMFC-hybrid 48-V DC bus. Since the converter could be placed between elements with very different voltage levels, a buck-boost structure has been selected. Especially to fulfill the low ripple requirements of the PEMFCs, but also those of the auxiliary storage elements and loads, our structure has inductors in series at both its input and its output. Magnetically coupling these inductors and adding a damping network to its intermediate capacitor we have designed an easily controllable converter with second-order-buck-like dominant dynamics. This new proposed topology has high efficiency and wide bandwidth acting either as a voltage or as a current regulator. The magnetic coupling allows to control with similar performances the input or the output inductor currents. This characteristic is very useful because the designed current-controlled converter is able to withstand shortcircuits at its output and, when connected to the FC, it facilitates to regulate the current extracted from the FC to avoid the oxygen starvation phenomenon. Testing in a safe way the converter connected to the FC required to build an FC simulator that was subsequently improved by developing an emulator that offered real-time processing and oxygen-starvation indication. To study the developed converters and emulators with different brands of PEMFCs it was necessary to reactivate long-time inactive Palcan FCs. Since the results provided by the manual reactivation procedure were unsatisfactory, an automatic reactivation system has been developed as a complementary study of the thesis. / En esta tesis se avanzo en el diseño de un bus DC de 48 V que utiliza como elemento principal de generación de energía eléctrica una pila de combustible. Debido a que la dinámica de las pilas de combustible están limitadas por sus elementos mecánicos auxiliares de control una variación rápida de una carga conectada a ella puede ocasionar daños. Es por esto que es necesario utilizar elementos almacenadores de energía que puedan suministrar estas rápidas variaciones de carga y convertidores para que gestionen de una forma controlada la potencia del bus DC. Durante la realización de pruebas de los convertidores es de gran importancia utilizar emuladores o simuladores de pilas de combustibles, esto nos permite de una forma económica y segura realizar pruebas criticas antes de conectar los convertidores a la pila. Adicionalmente una nueva topologia de convertidor fue presentada y ésta gestionará la potencia en el bus

In its present form, polymer electrolyte membrane fuel cell (PEMFC) technology requires some method of humidification to ensure that high performance and long life of the fuel cell membrane is maintained. External humidification utilizing ‘gas to gas’ membrane based planar humidifiers is one method of humidifying fuel cell reactant gases. This type of humidification offers the benefit of recycling heat and moisture from the fuel cell exhaust, and returning it to the reactants entering the fuel cell. In designing a planar membrane based fuel cell humidifier the two important areas to be considered are: - humidifier channel and plate design; and - humidifier membrane selection. In this work a humidifier design procedure was developed based on prototype humidifier testing. This design procedure involves selection of design parameters based on a dimensionless parameter which describes the ratio of gas residence time, and water diffusion time from the membrane surface. Humidifiers of different flow channel geometries were created with a rapid prototyping technique. These humidifier units were tested at different operating conditions in an attempt to validate the design equations involving a design parameter which is the ratio between the residence times of gas in the humidifier over the diffusion time of water from the surface of the membrane into the channel. This parameter offers a good starting point for humidifier design, the target value of this parameter was found to be between 2.0 and 4.0, with a desired value of 3.0. A fuel cell stack humidifier design procedure and suggestions are presented based this parameter. The design also considers designing a humidifier on limited volume constraints in which the humidifier would have to fit into the fuel cell system. A membrane selection procedure was developed based on design criteria requirements developed during this work for the fuel cell humidifier. This criterion includes high water permeation, low air permeation, good mechanical strength, robust handling, and long lifetime under various operating conditions. . Specific values for membrane selection included a water flux of greater than 14 kg m-2 h-1 in a water permeation test, less than 3 cm3 min-1 cm-2 kPa-1 air permeation when the membrane was dry, and a lifetime of at least 1500 hours of operation without performance degradation. Sixty membranes from various sources were screened for candidacy for use in the humidifier application. Membranes which passed the initial screenings were tested for durability at high and moderate temperature conditions. These membranes were operated until failure, at which time analysis was completed to determine the failure modes of the membrane. Mitigation strategies were proposed when applicable. Recommendations were made for membrane materials for the proposed operating requirements. Suggested membranes materials included those based on UHMWPE and inorganic additives, as well as homogenous membranes based on Nylon 6,6, PEEK, and PFSA.