Mechanical Engineering
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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 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 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 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.Item UNDERSTANDING DIRECT BOROHYDRIDE - HYDROGEN PEROXIDE FUEL CELL PERFORMANCE(2013) Stroman, Richard O'Neil; Jackson, Gregory S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Direct borohydride fuel cells (DBFCs) generate electrical power by oxidizing aqueous BH4- at the anode and reducing an oxidizer, like aqueous H2O2 for an all-liquid fuel cell, at the cathode. Interest in DBFCs has grown due to high theoretical energy densities of the reactants, yet DBFC technology faces challenges such as side reactions and other processes that reduce cell efficiency and power generation. Relationships linking performance to cell design and operation will benefit from detailed and calibrated cell design models, and this study presents the development and calibration of a 2D, single-cell DBFC model that includes transport in reactant channels and complex charge transfer reactions at each electrode. Initial modeling was performed assuming ideal reactions without undesirable side reactions. Results were valuable for showing how design parameters impact ideal performance limits and DBFC cell voltage (efficiency). Model results showed that concentration boundary layers in the reactant flow channels limit power density and single-pass reactant utilization. Shallower channels and recirculation improve utilization, but at the expense of lower cell voltage and power per unit membrane area. Reactant coulombic efficiency grows with decreasing inlet reactant concentration, reactant flow rate and cell potential, as the relative reaction rates at each electrode shift to favor charge transfer reactions. To incorporate more realistic reaction mechanisms into the model, experiments in a single cell DBFC were performed to guide reaction mechanism selection by showing which processes were important to capture. Kinetic parameters for both electrochemical and critical heterogeneous reactions at each electrode were subsequently fitted to the measurements. Single-cell experiments showed that undesirable side reactions identified by gas production were reduced with lower reactant concentration and higher supporting electrolyte concentration and these results provided the basis for calibrating multi-step kinetic mechanism. Model results with the resulting calibrated mechanism showed that cell thermodynamic efficiency falls with cell voltage while coulombic utilization rises, yielding a maximum overall efficiency operating point. For this DBFC, maximum overall efficiency coincides with maximum power density, suggesting the existence of preferred operating point for a given geometry and operating conditions.Item A HYBRID AIR CONDITIONER DRIVEN BY A HYBRID SOLAR COLLECTOR(2012) Al-Alili, Ali; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The objective of this thesis is to search for an efficient way of utilizing solar energy in air conditioning applications. The current solar Air Conditioners (A/C)s suffer from low Coefficient of Performance (COP) and performance degradation in hot and humid climates. By investigating the possible ways of utilizing solar energy in air conditioning applications, the bottlenecks in these approaches were identified. That resulted in proposing a novel system whose subsystem synergy led to a COP higher than unity. The proposed system was found to maintain indoor comfort at a higher COP compared to the most common solar A/Cs, especially under very hot and humid climate conditions. The novelty of the proposed A/C is to use a concentrating photovoltaic/thermal collector, which outputs thermal and electrical energy simultaneously, to drive a hybrid A/C. The performance of the hybrid A/C, which consists of a desiccant wheel, an enthalpy wheel, and a vapor compression cycle (VCC), was investigated experimentally. This work also explored the use of a new type of desiccant material, which can be regenerated with a low temperature heat source. The experimental results showed that the hybrid A/C is more effective than the standalone VCC in maintaining the indoor conditions within the comfort zone. Using the experimental data, the COP of the hybrid A/C driven by a hybrid solar collector was found to be at least double that of the current solar A/Cs. The innovative integration of its subsystems allows each subsystem to do what it can do best. That leads to lower energy consumption which helps reduce the peak electrical loads on electric utilities and reduces the consumer operating cost since less energy is purchased during the on peak periods and less solar collector area is needed. In order for the proposed A/C to become a real alternative to conventional systems, its performance and total cost were optimized using the experimentally validated model. The results showed that for an electricity price of 0.12 $/kW-hr, the hybrid solar A/C's cumulative total cost will be less than that of a standard VCC after 17.5 years of operation.Item Numerical and Experimental Studies on Free Piston Stirling Engines(2012) Shrestha, Dibesh; Balachandran, Balakumar; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Free piston Stirling engine (FPSE) is a closed cycle engine that converts thermal energy into mechanical energy. The focus of this thesis is on understanding limit-cycle motions in FPSEs. First, making use of reduced-order models, parametric studies are carried out to understand what FPSE parameters affect the creation of oscillatory motions. It is shown that quasi-static variations of the stiffness and the damping terms on the power piston can lead to conditions for Hopf instabilities in the system. The effect of the inclusion of a nonlinear spring term to the system is also investigated through numerical studies. The nonlinear springs include hardening springs and magnetic springs. The results, which include first results for FPSEs with hardening springs, show that nonlinear springs can help these systems realize limit-cycle motions. Finally, preliminary experimental studies conducted to realize oscillatory motions in a FPSE with a magnetic spring are also reported.