Developing the next generation of hydrogen generation and conversion technology
This thesis is divided into 3 chapters. The first two are related to novel techniques in hydrogen generation with solar input, while the last is focused on hydrogen conversion to electricity for use in vehicle applications.
Chapter 1: Design and optimization of a high-temperature concentrated solar collector for dry methane reforming.
This study details the modeling of a solar thermal collector designed to maximize absorption of solar heat. The heat is used to drive a dry methane reforming reaction to produce H2 and CO from CH4 and CO2. The finite element method based numerical model takes into account all modes of heat transfer, fluid flow, and chemical reaction. The goal of this study was to improve a previously designed collector to optimize absorber temperature. Various operational parameters were varied to obtain the final simulated results of optimized collector design. Different tube materials were considered to improve insulation given the high temperatures of the collector. Catalyst characteristics were tuned to accurately match experimental data. Absorption coating optical properties were optimized to maximize temperature. The results show a vacuum-insulated, high-temperature solar thermal collector for dry methane reforming can be manufactured, and is capable of achieving high temperatures even with low concentration ratios.
Chapter 2: Developing a high-temperature multilayer solar selective absorption coating for flat plate solar thermal collectors.
Absorption coatings are essential for use in solar thermal collectors to ensure heat provided by sunlight is adequately absorbed. Selective absorption coatings are unique in absorbing wavelengths in the visible spectrum while not emitting in the infrared spectrum, which is ideal for solar applications. Moderate-temperature selective absorption coatings have been extensively researched and many are available commercially, but their material integrity and performance characteristics degrade under high temperatures. Therefore, this study aims to fabricate a selective coating that remains functional and efficient at high temperatures. The design of this high-temperature coating, proposed and modeled by the National Renewable Energy Laboratory (NREL), uses multiple layers of TiSi, TiO2, and SiO2 to achieve optimal performance. A model was first created in MATLAB to verify the results published by NREL and to determine the effects of minor thickness changes (~2 nm) in each layer on the overall optical performance of the coating. The model results show a simplified design with more room for error does not significantly impact optical performance. The multilayer coating was then fabricated using physical and chemical vapor deposition with intermediate annealing steps. Finally, the coating characterized using the UV-Vis-NIR spectrophotometer to determine its optical performance properties, namely absorptivity and reflectivity at various wavelengths. The UV-Vis results for the best sample showed an absorption of 79.9% in the UV/Vis range with an emissivity of 4.95% in the IR, for an overall selectivity of 16. Future work is necessary in characterization of this coating in a thermal system. The ultimate goal is to use this high-temperature solar absorption coating in a flat plate concentrated solar thermal collector used for steam reforming of methane derived from biogas to generate hydrogen renewably.
Chapter 3: Optimizing the operational efficiency of a PEM fuel cell for use in high-efficiency vehicles.
Proton exchange membrane (PEM) hydrogen fuel cells are growing in popularity as an energy conversion technology, generating electricity from chemical energy. One barrier to large-scale deployment is efficiency, as most PEM fuel cells only achieve ~40% at nominal operating conditions. This study aims to increase this efficiency by considering several operating variables (cathode and anode side gas pressures, gas and membrane temperature/humidity, load point, fan speed, short circuiting, anode purging) and determining their effects on maximizing experimental efficiency of a commercially available PEM fuel cell. While previous studies focus on each specific variable described above, this work is novel in considering the effects of all variables. A robust gradient descent optimization is conducted to solve the multidimensional problem of maximizing efficiency. The overall system efficiency is considered paramount, so all power input is considered (i.e. fan power) and no external heating is used. As a primarily experimental study, we hope to spur further data acquisition in fuel cell characterization as a way to validate modeled systems. The study was successful in increasing the nominal 40% datasheet efficiency to 61.1% in bench testing.