Tese de doutoramento. Engenharia Química e Biológica. Universidade do Porto. Faculdade de Engenharia. 2011

The driving force behind research in alternative clean and renewable energy has been the desire to reduce emissions and dependence on fossil fuels. In the United States, ground vehicles account for 30% of total carbon emission, and significantly contribute to other harmful emissions. This issue causes environmental concerns and threat to human health. On the other hand, the demand on fossil fuel grows with the increasing energy consumption worldwide. Particularly in the United States of America, transportation absorbs 75% of this energy source. There is an urgent need to reduce the transportation dependence on fossil fuel for the purpose of national security. Polymer electrolyte membrane (PEM) fuel cells are strong potential candidates to replace the traditional combustion engines. Even though research effort has transferred the fuel cell technology into real‒world vehicle applications, there are still several challenges hindering the fuel cell technology commercialization, such as hydrogen supply infrastructure, cost of the fuel cell vehicles, on‒board hydrogen storage, public acceptance, and more importantly the performance, durability, and reliability of the PEM fuel cell vehicles themselves. One of the key factors that affect the fuel cell performance and life is the run‒time thermal and water management. The temperature directly affects the humidification of the fuel cell stack and plays a critical role in avoiding liquid water flooding as well as membrane dehydration which affect the performance and long term reliability. There are many models exists in the literature. However, there are still lacks of control‒oriented modeling techniques that describe the coupled heat and mass transfer dynamics, and experimental validation is rarely performed for these models. In order to establish an in‒depth understanding and enable control design to achieve optimal performance in real‒time, this research has explored modeling techniques to describe the coupled heat and mass transfer dynamics inside a PEM fuel cell. This dissertation is to report our findings on modeling the temperature dynamics of the gas and liquid flow in the porous media for the purpose of control development. The developed thermal model captures the temperature dynamics without using much computation power commonly found in CFD models. The model results agree very well with the experimental validation of a 1.5 kW fuel cell stack after calibrations. Relative gain array (RGA) was performed to investigate the coupling between inputs and outputs and to explore the possibility of using a single‒input single‒output (SISO) control scheme for this multi‒input multi‒output (MIMO) system. The RGA analyses showed that SISO control design would be effective for controlling the fuel cell stack alone. Adding auxiliary components to the fuel cell stack, such as compressor to supply the pressurized air, requires a MIMO control framework. The developed model of describing water transport in porous media improves the modeling accuracy by adding catalyst layers and utilizing an empirically derived capillary pressure model. Comparing with other control‒oriented models in the literature, the developed model improves accuracy and provides more insights of the liquid water transport during transient response. / text

Tese de doutoramento. Engenharia Química e Biológica. Faculdade de Engenharia. Universidade do Porto. 2009

The development of improved water management strategies for the polymer electrolyte membrane fuel cell (PEMFC) could stand to benefit from an improved understanding of the surface and internal structure of the gas diffusion layer (GDL). The GDL is a fibrous porous material enabling mass transport between the PEMFC catalyst layer and flow fields. Fluorescence-based visualizations of liquid water droplet evaporation on GDL surfaces were performed to investigate water droplet pinning behaviours. The heterogeneous in-plane and through-plane porosity distributions of untreated GDLs were studied using computed tomography visualizations. The through-plane porosity distributions were utilized to calculate heterogeneous local tortuosity, relative diffusivity, and permeability distributions. Finally, the heterogeneous through-plane porosity distributions of GDLs treated for increased hydrophobicity were investigated. This work provides new insight into GDL material properties to better inform future PEMFC models.

The development of improved water management strategies for the polymer electrolyte membrane fuel cell (PEMFC) could stand to benefit from an improved understanding of the surface and internal structure of the gas diffusion layer (GDL). The GDL is a fibrous porous material enabling mass transport between the PEMFC catalyst layer and flow fields. Fluorescence-based visualizations of liquid water droplet evaporation on GDL surfaces were performed to investigate water droplet pinning behaviours. The heterogeneous in-plane and through-plane porosity distributions of untreated GDLs were studied using computed tomography visualizations. The through-plane porosity distributions were utilized to calculate heterogeneous local tortuosity, relative diffusivity, and permeability distributions. Finally, the heterogeneous through-plane porosity distributions of GDLs treated for increased hydrophobicity were investigated. This work provides new insight into GDL material properties to better inform future PEMFC models.

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.

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.

No description available.

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

This thesis presents a compendium of work related to performance analyses of a proton electrolyte membrane (PEM) fuel cell with two novel design configurations. The finite element based numerical analysis has been carried out to solve the numerical transport models involved in a PEM fuel cell coupled with the flow in a porous medium, charge balance, electrochemical kinetics and membrane water content. The scope of this research work focuses on improving the performance of the PEM fuel cell by optimizing the thermo-fluid properties of the reactant species instead of analysing the complex electro-chemical interactions. Two new design configurations have been numerically analyzed; in the first design approach, a perforated-type gas flow distributor is used instead of a conventional gas flow distributor such as a serpentine, straight or spiral shape; the second design approach examines the effect of reactant flow pulsation on the PEM fuel cell performance. Results obtained from the numerical analyses were also compared with the experimental data and a good agreement was found. Performance of the PEM fuel cell with a perforated-type gas distributor was analyzed at different operating and geometric conditions to explore the merits of this new design configuration. Two-dimensional numerical analyses were carried out to analyze the effect of varying the different operating parameters; threedimensional numerical analyses were carried out to study the variation of different geometric parameters on overall performance of the new design configuration of the PEM fuel cell. The effects of the reactant flow pulsation on the performance of PEM fuel cell were analyzed using a two-dimensional numerical approach where both active and passive design configurations were numerically simulated to generate the pulsations in the reactant flow. The results showed a considerable increase in overall performance of the PEM fuel cell by introducing pulsations in the flow.