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 DEVELOPMENT OF MULTI-STAGE ELASTOCALORIC COOLING DEVICES(2022) Emaikwu, Nehemiah; Radermacher, Reinhard; Takeuchi, Ichiro; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Elastocaloric solid-state refrigerants have lower environmental impact compared to conventional vapor compression refrigerants, but they require significant advancements to gain widespread implementation. Two barriers that prevent adoption are low temperature lift and poor fatigue life. This dissertation addresses those challenges through a single, scalable architecture with the objectives of 1) designing high-performing elastocaloric devices, and 2) maximizing temperature lift. The developed prototype consists of twenty-three 17 mm long, thermally insulated Ni-Ti tubes in a staggered pattern that exchange heat with the surrounding fluid medium through their external surface areas. They are contained inside a 3D-printed plastic that provides alignment and restricts heat transfer to other components. A top loader and fixed bottom plate transfer compressive loads to the tubes, and a 3D-printed housing encapsulates all components. Single, two, and three-stage configurations were experimentally investigated. A sensitivity analysis was conducted on the single-stage device and identified fluid-solid ratio, loading/unloading time, and strain as three parameters that could increase temperature span by over 1.5 K each. The combination of these findings resulted in a maximum steady-state temperature span of 16.6 K (9.7 K in heating and 6.8 K in cooling) at 4% strain and under zero load conditions. The temperature lift was increased in the two and three-stage configurations which achieved 20.2 K and 23.2 K, respectively, under similar operating conditions. Validated 1D numerical models developed for this work confirm that the multi-staging approach positively impacts thermal response, though with decaying significance as the number of banks increases. By further optimizing the operation condition which minimized the water volume in the fluid loop, the three-stage device was ultimately able to develop the largest lift of 27.4 K. The tubes used in the single and two-stage tests also withstood over 30,000 cycles without failure, showing promising fatigue life behavior and emphasizing the viability of this alternative cooling technology.Item NEXT GENERATION HEAT PUMP SYSTEM EVALUATION METHODOLOGIES(2021) Wan, Hanlong; Radermacher, Reinhard K.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Energy consumption of heat pump (HP) systems plays a significant role in the global residential building energy sector. The conventional HP system evaluation method focused on the energy efficiency during a given time scale (e.g., hourly, seasonally, or annually). Nevertheless, these evaluation methods or test metrics are unable to fully reflect the thermodynamic characteristics of the system (e.g., the start-up process). In addition, previous researchers typically conducted HP field tests no longer than one year period. Only limited studies conducted the system performance tests over multiple years. Furthermore, the climate is changing faster than previously predicted beyond the irreversible and catastrophic tipping point. HP systems are the main contributor to global warming due to the increased demands but also can be a part of the solution by replacing fossil fuel burning heating systems. A holistic evaluation of the HP system’s global warming impact during its life cycle needs to account for the direct greenhouse gas (GHG) emissions from the refrigerant leakage, indirect GHG emissions from the power consumption and embodied equipment emissions. This dissertation leverages machine learning, deep learning, data digging, and Life Cycle Climate Performance (LCCP) approaches to develop next generation HP system evaluation methodologies with three thrusts: 1) field test data analysis, 2) data-driven modeling, and 3) enhanced life cycle climate performance (En-LCCP) analysis. This study made following observations: First, time-average performance metrics can save time in extensive data calculation, while quasi-steady-state performance metrics can elucidate some details of the studied system. Second, deep-learning-based algorithms have higher accuracy than conventional modeling approaches and can be used to analyze the system's dynamic performance. However, the complicated structure of the networks, numerous parameters needing optimization, and longer training time are the main challenges for these methods. Third, this dissertation improved current environmental impact evaluation method considering ambient conditions variation, local grid source structure, and next-generation low-GWP refrigerants, which led the LCCP results closer to reality and provided alternative methods for determining LCCP input parameters with limited-data cases. Future work could be studying the uncertainty within the deep learning networks and finding a general process for modeling settings. People may also develop a multi-objective optimization model for HP system design while considering both the LCCP and cost.Item Dynamic Modeling of Vapor Compression Systems for Residential Heat Pump Applications with Alternative Low-GWP Refrigerants(2015) Bhanot, Viren; Hwang, Yunho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)With the increased focus on reducing greenhouse gas emissions, low-GWP refrigerants, R32 and D2Y60, have been proposed as drop-in replacements for R410A in residential heat pumps. This thesis presents the development of a modeling framework in Simulink® for the dynamic simulations of such residential heat pumps. The framework is component-based, allowing arbitrary cycle configurations, and includes most of the relevant components. Finite-volume method has been applied to the heat exchanger. Compression and expansion processes are treated as quasi-steady state. The framework has been used to study the performance of the system using the baseline refrigerant and charge-optimized alternatives at ASHRAE test conditions, and the results have been compared against experimental data. Steady-state COP values fall within ±8% of experimental data. For the cyclic tests, the pressure and temperature behaviors compare well and accumulated capacity and power consumption errors are found to be within ±9%. Relative differences between the refrigerants are consistent between simulations and measurements. The framework shows potential for being used to simulate the operation of residential heat pumps under dynamic conditions.Item Velocity based defrost of evaporator coil of heat pumps(2015) Muthusubramanian, Kamalakkannan; Ohadi, Michael M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Minimization of frost formation on the outdoor coils of residential heat pumps and subsequent defrost cycles to remove the frost in an energy efficient manner remains an active area of research and development in the HVAC industry. Inverting the cycle to reject heat from the outdoor coils is the most common method to run defrost cycles of the residential heat pumps. However, these defrost cycles are energy intensive. This research proposes a novel method that can substantially reduce the energy consumed in such defrost cycles. The method involves controlled use of reverse air flow on the outdoor coil surface during the defrost cycle, resulting in fewer defrost cycles needed for a given duration of heat pump operation. The method further provides better cleaning of the coil surface by improving the draining of the melted frost. It also allows part of the frost to be removed without the need to melt it. In this research, the proposed methods are evaluated experimentally by simulating the frost formation, and defrost cycle in a controlled environment. Defrost cycles were run with two of the most popular control strategies that are in use, time-based defrost and control-based defrost. Experimental results demonstrate that with the use of the proposed method, energy savings of 56% and 31% are possible for the above- and below-freezing environments respectively, as compared to a baseline that represents the ASHRAE recommended operating conditions.