Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item IMPROVING THE PROCESS OF SUPERCRITICAL CARBON-DIOXIDE-ASSISTED LIQUEFACTION OF BIOMASS FOR THE PRODUCTION OF BIOFUELS(2024) Murray, Cameron; Gupta, Ashwani; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The United States has been looking for alternative energy sources to combat energy dependence and carbon emissions. There exists a multitude of methods attempting to achieve this effort while making use of existing infrastructure. One such novel method is the supercritical carbon-dioxide-assisted liquefaction of biomass. This method seeks to exploit a large supply of biomass waste in the US through the use of carbon dioxide, a readily available and nontoxic gas. This paper investigated two potential improvements for liquid yields in the supercritical carbon-dioxide-assisted liquefaction system. Those improvements were the effects of heating on the solid and liquid yields and the efficacy of supercritical carbon dioxide extraction of liquid products. Three specific aspects of heating were investigated: resident time, heating rate, and total time. Resident times of 10 minutes 20 minutes and 60 minutes were tested. High heating rates were achieved via the use of induction heating. Heating rates of 6, 12, and 250 ℃/min were tested. The effects of total reaction time were also investigated; however, this was dependent on the heating rate and resident time, thus it could not be independently controlled. The investigation found that neither resident time nor total reaction time has a significant impact on the solid or liquid yields. The heating rate, on the other hand, showed a good correlation with a proposed relationship of L = 13.01 · H0.1687 and S = 58.57 · H-0.08348, where L is the liquid yield in wt%, S is the solids yield in wt%, and H is the heating rate in ℃/min. This investigation had a stated goal of achieving a liquid yield of over 30% in under 45 minutes while maintaining a solids yield of less than 50%. It achieved this goal with a particular test having a liquid yield of 32% and a solids yield of 40% in under 12 minutes. Supercritical carbon dioxide extraction was proven to be effective at recovering liquid yields. It was not as successful as acetone-aided extraction; however, it shows promise, especially given its potential for overall process integration in the future. sCO2 extraction was seen to be most effective when conducted in conjunction with sCO2 liquefaction.Item Data Requirements to Enable PHM for Liquid Hydrogen Storage Systems from a Risk Assessment Perspective(2021) Correa Jullian, Camila Asuncion; Groth, Katrina M; Reliability Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantitative Risk Assessment (QRA) aids the development of risk-informed safety codes and standards which are employed to reduce risk in a variety of complex technologies, such as hydrogen systems. Currently, the lack of reliability data limits the use of QRAs for fueling stations equipped with bulk liquid hydrogen storage systems. In turn, this hinders the ability to develop the necessary rigorous safety codes and standards to allow worldwide deployment of these stations. Prognostics and Health Management (PHM) and the analysis of condition-monitoring data emerge as an alternative to support risk assessment methods. Through the QRA-based analysis of a liquid hydrogen storage system, the core elements for the design of a data-driven PHM framework are addressed from a risk perspective. This work focuses on identifying the data collection requirements to strengthen current risk analyses and enable data-driven approaches to improve the safety and risk assessment of a liquid hydrogen fueling infrastructure.Item Dynamics in Metal Halide Perovskites for Optoelectronics(2020) Howard, John Michael; Leite, Marina S; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A diverse portfolio of renewable energy technologies is required to limit global warming to less than 2 ◦C. Of the possible emissions-free options, photovoltaic (PV) technologies can be most widely deployed, given the abundance of the solar resource compared. As with all power generation sources, PV adoption is predicated on the availability of technology solutions that are both inexpensive and highly efficient. One solar cell material, the metal halide perovskites (MHP), may provide the ideal combination, with > 25% efficiency devices within the first decade since their invention fabricated through simple spin coating. Despite the unprecedented rise in MHP performance, stability remains a critical challenge with the most stable devices at the 1-year benchmark compared to the >25-year lifetime of Si-based PV. Further progress concerning enduring power output will require a fundamental understanding of the impact of environmental stressors (light, temperature, bias, oxygen, and water) on the basic physical processes governing solar cell operation. Therefore, my dissertation elucidates the interplay between the ambient environment and MHP composition on both the optical and electrical behavior using in situ methods. The first part of my thesis elucidates the time-dependent optical and elec- tronic response of different MHP compositions using different in situ microscopy techniques. I capture the transient photovoltage of both Br- and I-containing per- ovskites for different photon energies using heterodyne Kelvin probe measurements. My measurements demonstrate that the voltage rise (light ON) is 104× faster than the subsequent decay (light OFF). Uniquely, the decay time for the residual voltage depends on the excitation wavelength, but only for the MAPbBr3 thin film. Next, I spatially and temporally resolve the relationship between radiative recombination and relative humidity (rH) for multi-cation films. The time-dependent photolumi- nescence (PL) indicates that the Cs-Br ratio impacts the magnitude of light emission hysteresis across an rH cycle. Further, I establish the existence of a repeatable and reversible ≈25× PL gain for multiple moisture cycles up to 70% rH. The second part of my thesis establishes the ability of machine learning (ML) models to predict the time-dependent behavior of perovskite material properties. I collect a comprehensive set of humidity-dependent PL data for both MAPbBr3 and MAPbI3 perovskites. I then use that data to train recurrent neural networks to forecast light emission based on only the recorded rH values. Using Echo State Networks, I achieve a normalized root-mean-squared error of <11% for both compositions for a 12+ h prediction win- dow. Further, I use a Long Short Term Memory network to predict the PL from a degrading sample, achieving <5% error. My in situ measurements and predictive ML models provide a powerful framework for identifying structure-property rela- tionships and can help accelerate the development of long-term stable perovskite materials.Item Sizing Tool for Quadrotor Biplane Tailsitter UAS(2017) Strom, Eric; Baeder, James; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Quadrotor-Biplane-Tailsitter (QBT) configuration is the basis for a mechanically simplistic rotorcraft capable of both long-range, high-speed cruise as well as hovering flight. This work presents the development and validation of a set of preliminary design tools built specifically for this aircraft to enable its further development, including: a QBT weight model, preliminary sizing framework, and vehicle analysis tools. The preliminary sizing tool presented here shows the advantage afforded by QBT designs in missions with aggressive cruise requirements, such as offshore wind turbine inspections, wherein transition from a quadcopter configuration to a QBT allows for a 5:1 trade of battery weight for wing weight. A 3D, unsteady panel method utilizing a nonlinear implementation of the Kutta-Joukowsky condition is also presented as a means of computing aerodynamic interference effects and, through the implementation of rotor, body, and wing geometry generators, is prepared for coupling with a comprehensive rotor analysis package.Item IGNITION QUALITY TESTER CHARACTERIZATION WITH PURE COMPONENT AND CONVENTIONAL NAVY FUELS(2016) Mendelson, Jacob Lee; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The U.S. Navy is attempting to reduce dependence on conventional diesel fuels as a part of the environmental initiative commonly referred to as “The Great Green Fleet”. The purpose of this research was to characterize the measurements of ignition delay gathered by the Advanced Engine Technology Ignition Quality Tester (IQT) with conventional Navy diesel fuels, pure component biodiesel fuels, primary cetane standards, and toluene-hexadecane blends. The use of computational analysis with pressure traces gathered from the IQT allowed for the comparison of IQT ignition delay results with various methods of calculating start of combustion for various fuels. Physical and chemical ignition delays of each fuel were also calculated using different separation techniques and the chemical ignition delay results were compared with prior academic literature and with chemical ignition delays calculated with Lawrence Livermore kinetic theory.Item EXPERIMENTAL DEMONSTRATION OF LIGHT TRAPPING AND INTERNAL LIGHT SCATTERING IN SOLAR CELLS(2016) Murray, Joseph; Munday, Jeremy N; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Renewable energy technologies have long-term economic and environmental advantages over fossil fuels, and solar power is the most abundant renewable resource, supplying 120 PW over earth’s surface. In recent years the cost of photovoltaic modules has reached grid parity in many areas of the world, including much of the USA. A combination of economic and environmental factors has encouraged the adoption of solar technology and led to an annual growth rate in photovoltaic capacity of 76% in the US between 2010 and 2014. Despite the enormous growth of the solar energy industry, commercial unit efficiencies are still far below their theoretical limits. A push for thinner cells may reduce device cost and could potentially increase device performance. Fabricating thinner cells reduces bulk recombination, but at the cost of absorbing less light. This tradeoff generally benefits thinner devices due to reduced recombination. The effect continues up to a maximum efficiency where the benefit of reduced recombination is overwhelmed by the suppressed absorption. Light trapping allows the solar cell to circumvent this limitation and realize further performance gains (as well as continue cost reduction) from decreasing the device thickness. This thesis presents several advances in experimental characterization, theoretical modeling, and device applications for light trapping in thin-film solar cells. We begin by introducing light trapping strategies and discuss theoretical limits of light trapping in solar cells. This is followed by an overview of the equipment developed for light trapping characterization. Next we discuss our recent work measuring internal light scattering and a new model of scattering to predict the effects of dielectric nanoparticle back scatterers on thin-film device absorption. The new model is extended and generalized to arbitrary stacks of stratified media containing scattering structures. Finally, we investigate an application of these techniques using polymer dispersed liquid crystals to produce switchable solar windows. We show that these devices have the potential for self-powering.Item Boosting Electrical Generation of a Photovoltaic Array by Thermal Harvest from p-Si Cells: An Experimental and Theoretical Study(2015) Kelley, Joshua; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Solar power generation deployment is increasing globally with photovoltaic modules. Most energy available to conventional PV is absorbed as heat or passes through. Performance of Photovoltaic Thermal (PVT) collectors which mimic currently available, polycrystalline, commercial PV modules was measured in the mid-Atlantic US. A linear model is developed for their performance which uses values available in Typical Meteorological Year files and shows daily accuracies to within 11%. Pressure losses for the collectors were measured and an empirical model established. Electrical generation is modeled by PVT in conjunction with an Organic Rankine Cycle. 20% - 45% boosts to electricity production in the Southwest are projected. 5%-15% boosts are projected in the mid-Atlantic.Item INCREASING THE SUSTAINABILITY OF PSYCHROPHILIC SMALL-SCALE ANAEROBIC DIGESTERS(2015) Witarsa, Freddy; Lansing, Stephanie; Environmental Science and Technology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The research was aimed at increasing the energy production efficiency of small-scale anaerobic digesters in temperate climates while quantifying their environmental impacts. Biochemical methane potential tests were used to quantify methane (CH4) production from separated and unseparated manure during psychrophilic digestion, and compare CH4 production when pre-incubated alternative inocula (wetland sediment (WS), landfill leachate (LL), mesophilic digestate (MD)) were used. Methanogenic and Archaeal communities were analyzed using T-RFLP and qPCR. At 24 ºC, unseparated manure produced significantly higher (40%) quantity of CH4 than separated manure due to higher volatile solids (VS) content, but differences were insignificant at digestion times of ≤16 days. At lower digestion times, farmers could digest liquid, separated manure without sacrificing CH4 production, but at longer digestion times, the VS in unseparated manure has the time necessary for CH4 conversion. The alternative inocula studies showed that LL inoculum after incubation for 91 days at 25 ºC produced significantly higher quantity (≥20%) of CH4 than MD and WS during digestion at the same temperature, and was not significantly different in CH4 quantity than MD that was incubated and digested at 35 ºC (202 ± 4 L/kg VS). Methanosarcinaceae was dominant in the LL reactor, while the other reactors were abundant in Methanosaetaceae, indicating that inoculum rich in Methanosarcinaceae may be beneficial for starting digestion at lower mesophilic temperature ranges. Longer incubation time generally reduced the inoculum amount needed for batch digestion and prevention of volatile fatty acids accumulation. In batch systems with long digestion time (90 days), MD inoculum from well-established digesters, 35% inoculum to substrate ratio, and 35 ºC operation temperature are recommended for highest CH4 production per unit of digester volume. Additionally, life cycle assessments (LCA) were conducted to compare the sustainability of an unheated Chinese fixed-dome digester with a heated and insulated small-scale plug-flow digester in the US. The LCA showed that the US plug-flow digester was more sustainable than the Chinese fixed-dome system only in climate change category, but contributed negatively towards 17 impact categories. Digester heating and heating infrastructure were the main contributors towards the negative impacts observed in the US plug-flow digester.Item THE ECONOMICS OF WIND POWER AND WHISKY(2015) Page, Ian; Williams, Roberton; Agricultural and Resource Economics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)For many goods, economists tend to ignore the time between the onset of production and the final sale of the good. In some instances, economists do model production with intertemporal considerations such as the extraction of groundwater, population dynamics in fisheries, and manufacturing with costs characterized by learning-by-doing; but even in these cases output at any point in time tends to be unconstrained except when production is limited according to resource availability. In the following three essays, I examine the implications for agent and market behavior when producers cannot perfectly adjust output over time. In the first two essays, I focus on the relationship between electricity markets and both conventional and renewable power producers. Specifically, in the first essay, I quantify the effect a large level of installed wind power capacity (an intermittent renewable energy) has on power market conditions. Because wind power has virtually no marginal costs for generation, and its output cannot be perfectly controlled, a high penetration level of wind power could potentially lower average prices while also impacting price volatility. In the second essay, I construct a computational model of a conventional power producer that cannot perfectly adjust its output over time and faces prices that change according to a stochastic process. Then, I measure the impact price volatility has on producers in two ways. First, I analyze changes in their optimal generation strategies in light of price volatility, and then I simulate and track changes in output, profit, and emissions over time. My third essay pertains to production of whisky. While there are other examples of vintage goods that require a significant amount of maturation, the existing literature typically assumes that there is a unique optimal maturation age for a producer's inventory. However, many Scottish distilleries produce a line of whiskies that vary primarily according to age. I demonstrate that it is possible for a profit-maximizing distillery to mature multiple ages of whisky without market power, but a further exploration of distilleries' product lines indicates the market is far from perfectly competitive.Item Modeling of Falling-Particle Solar Receivers for Hydrogen Production and Thermochemical Energy Storage(2014) Oles, Andrew; Jackson, Gregory S.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)One of the most important components in a solar-thermal power plant is the central receiver where concentrated solar energy is absorbed in a medium for storage and eventual use in power generation or fuel production. Current state-of-the-art receivers are not appropriate for future power-plant designs due to limited operating temperatures. The solid-particle receiver (SPR) has been proposed as an alternative architecture that can achieve very high temperatures (above 1500 °C) with high efficiency, while avoiding many of the thermal stress issues that plague alternative architectures. The SPR works by having a flow of solid particles free-fall through a cavity receiver while directly illuminated to absorb the solar energy. Because of the high operating temperatures that can be achieved, along with the ability to continuously flow a stream of solid reactant, the SPR has the potential for use as a reactor for either chemical storage of solar energy or fuel production as part of a solar water-splitting cycle. While the operation of the SPR is relatively simple, analysis is complicated by the many physical phenomena in the receiver, including radiation-dominated heat transfer, couple gas-particle flow, and inter-phase species transport via reaction. This work aims to demonstrate a set modeling tools for characterizing the operation of a solid particle receiver, as well as an analysis of the key operating parameters. A inert receiver model is developed using a semi-empirical gas-phase model and the surface-to-surface radiation model modified to account for interaction with the particle curtain. A detailed thermo-kinetic model is developed for undoped-ceria, a popular material for research into solar fuel production. The inert-receiver model is extended to integrate this kinetic model, and further used to evaluate the potential of perovskite materials to enhance the storage capability of the receiver. A modified undoped ceria model is derived and implemented via custom user functions in the context of a computational fluid dynamics simulation of the receiver using the discrete-ordinates method for radiation transfer. These modeling efforts provide a basis for in-depth analysis of the key operating parameters that influence the performance of the solid-particle receiver.