UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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 given thesis/dissertation in DRUM.
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Item Personal cooling system with phase change material(2020) Qiao, Yiyuan; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Personal cooling systems (PCS) are attracting more attention recently since they can set back building thermostat setpoints to achieve energy savings and provide high-level human comfort by focusing on micro-environment conditions around occupants rather than the entire building space. Thus, a vapor compression cycle (VCC)-based PCS with a condenser integrated with the phase change material (PCM) is proposed. The PCM heat exchanger (PCMHX) works as a condenser to store waste heat from the refrigerant in the cooling cycle, in which the PCM melting process can affect the system performance significantly. Different from most previous study, various refrigerant heat transfer characteristics along the condenser flow path can result in the uneven PCM melting, leading to the degradation of the system performance. Therefore, enhancing heat transfer in the PCM, investigating the proposed PCS performance, improving PCMHX latent heat utilization in terms of the distribution of PCM melting, and developing a general-purpose PCM model are the objectives of this dissertation. Five PCMHX designs with different heat transfer enhancements including increasing heat-transfer area, embedding conductive structures, and using uniform refrigerant distribution among condenser branches are introduced first. Compared with non-enhanced PCM, the graphite-matrix-enhanced PCMHX performs the best with 5.5 times higher heat transfer coefficient and 49% increased coefficient of performance (COP). To investigate the proposed system performance, a system-level experimental parametric study regarding the thermostat setting, PCM recharge rates, and cooling time was conducted. Results show that the PCS can work properly with a stable cooling capacity of 160 W for 4.5 hours. A transient PCM-coupled system model was also developed for detailed system performance, PCM melting process and heat transfer analysis. From both experiment and simulation work, the uneven PCM melting was presented, which could result in an increase of condenser temperature and a degradation of system COP with time. Results show that one significant reason for the uneven PCM melting is the variation of the refrigerant temperature and heat transfer coefficient. Therefore, through experimental analysis, several solutions were proposed to minimize the negative effect of the uneven PCM melting. In addition, to extend the PCMHX application, a multi-tube PCMHX model was developed for general-purpose design. A new multi-tube heat transfer algorithm was proposed, and variable tube shape, connection, and topology for tubes and PCM blocks were considered. The comparison with other PCMHX models in the literature shows that the proposed model exhibits much higher flexibility and feasibility for comprehensive multi-tube configurations. The PCS coupled with PCMHX could achieve energy savings for a range of 8-36% depending on the climate and building types in the U.S.Item Dynamics of Vapor Compression Cycle with Thermal Inertia(2018) Dhumane, Rohit; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The use of heating, ventilation, air conditioning, and refrigeration (HVACR) systems is always increasing. Reducing energy consumption has become necessary in modern times for environmental, economic and legislative reasons. Thus, there is ongoing research to improve the performance and reduce the negative environmental impact of these systems. HVACR systems are normally sized for peak load conditions. As a result, these systems operate under off-peak conditions most of the time by on-off cycling. The average efficiency of the system during cycling is lower due to transient losses caused by refrigerant migration and redistribution. This motivates a detailed understanding of the dynamics of vapor compression systems (VCS) for their improved design and performance. The dissertation contributes towards reducing energy consumption from HVACR by exploring both sides: improving the performance of current systems and developing highly energy efficient personal conditioning systems (PCS) which reduce the load of HVACR systems altogether. PCS reduce the energy consumption of building HVACR by up to 30%. Multi-physics modeling for thermo-fluid, electricity and mechanical domains is conducted to compare performance of four PCS employing different thermal storage options. The dissertation then focuses on vapor compression system based version of PCS called Roving Comforter, operating cyclically between its cooling and recharge mode. Exhaustive study of design space including optimization of thermal storage, operation with a natural refrigerant and alternate recharge modes is conducted to improve its overall coefficient of performance. The dissertation then presents comprehensive dynamic validated modeling of air-conditioning systems operating in cyclic operations to characterize cyclic losses. Parametric study with different operating conditions is carried out to provide guidelines for reduction of these cyclic losses. Secondly, a physically based model of the test setup for quantifying the cyclic losses of air-conditioning systems is developed and used to understand its influence on the cyclic losses. A new term called “Thermal Inertia Factor” is defined to enable more uniform rating of equipment from various test centers and help selection of actual energy efficient air-conditioners.Item MODELING AND OPTIMIZATION OF MICROGRID ENERGY SYSTEM FOR SHIP APPLICATIONS(2016) Cao, Tao; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Microgrid energy systems are widely used in remote communities and off-grid sites, where primary energy supplies are dominated by fuels. Limited attentions have been paid to ship applications, which require thorough and in-depth research to address their unique challenges and increasing pressure on reducing fuel consumptions. This dissertation presents comprehensive microgrid system studies for ship applications in four aspects: component modeling and study, dynamic system modeling on novel designs, novel optimization based system design framework development and investigations on two enhancement options: integrating with separate sensible and latent cooling systems, maximizing heat recovery through pinch analysis. Comprehensive component studies consist of new component models addressing unique features of ship applications. Desiccant wheels with new materials were investigated experimentally, especially under high humidity conditions for ship applications. Dynamic system modeling was conducted on several novel solar energy and waste heat powered systems, with a focus on their capabilities to reduce fuel consumptions and CO2 emissions. Results were validated against experimental data. Payload and economic studies were conducted to evaluate feasibilities of applying the designs to ship applications. A novel optimization based design framework was then developed. The framework is capable of conducting both system configuration and control strategy optimization under transient weather and load profiles, differentiating itself with current control strategy focused energy system optimization studies (Jradi and Riffat, 2014). It also extends Buoro et al. (2012)’s study on system configuration optimization to complete design from scratch with comprehensive equipment selections and integrating options. The design framework was demonstrated through a case study on container ships. Optimized systems and control strategies were found from three different scenarios: single-objective optimization, bi-objective optimization and optimization under uncertainty. Finally, two previously listed options were investigated to enhance microgrid system performance regarding thermal comfort and fuel savings. This research fills current research gaps on microgrid energy system for ship applications. It also serves as the basis for advanced microgrid system analysis framework for any applications. The dynamic system modeling platform, optimization based design framework and enhancement methods can help engineers develop and evaluate ultra-high efficiency designs, aiming to reduce energy consumptions and CO2 emissions.