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 Modeling of advanced heat pump cycles and aerodynamic design of a small-scale centrifugal compressor for electric vehicles(2023) mei, zhenyuan; Radermacher, Reinhard; Hwang, Yunho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Unlike conventional vehicles powered by internal combustion engines, electric vehicles do not have enough waste heat to provide sufficient heating to the cabin. Thus, an additional heating system, such as a heat pump, is needed. However, its performance decreases significantly when the ambient temperature is low. The new kangaroo heat pump cycle (KC) is proposed to increase the heating capacity in low-temperature climates. It is an enhanced flash tank-based vapor injection heat pump cycle (FT-VIC). A sub-cycle is added to the system to increase the refrigerant inlet quality entering the flash tank, which leads to a higher refrigerant mass flow rate and heating capacity. Because KC has a higher heating capacity, the heating needed from the electric heater can be reduced, thus reducing energy consumption and increasing the driving distance. In this study, thermodynamic models were developed for the basic heat pump cycle (BC), FT-VIC, and KC. And a new method evaluating the life cycle climate performance (LCCP) of electric vehicle heat pumps based on the SAE J2766 standard was proposed. Results show that KC is effective at low ambient temperatures. At -15°C, KC can save 13.8% energy compared to BC, and save 2.7% energy compared to FT-VIC. However, due to the additional weight, KC has a higher LCCP than other cycles. If the pressure ratio limit is removed and the compressor efficiencies are constant, KC can have a lower LCCP than other cycles in cold climates. Transient models were also developed to assess their performance in urban driving conditions. Results show that at the end of the simulation, the cabin room temperature of KC is 3.6°C and 7.0°C higher than that of FT-VIC and BC, respectively. However, due to the high pressure ratio and refrigerant mass flow rate, the accumulated power consumption of KC is 32.4% higher than FT-VIC and 64.4% higher than BC. Despite its high energy consumption, it is more efficient than adding heat from an electric heater. In addition, a small-scale centrifugal compressor was designed for an electric vehicle to reduce the compressor’s size and weight. Results show that its COP is 6.6% higher than that of the scroll compressor at the design point. However, its efficiency at the off-design point quickly drops. Future studies are needed to improve its off-design point performance.Item Harmonization of Life Cycle Climate Performance and Its Improvements for Heat Pump Applications(2016) Troch, Sarah Virginia; Hwang, Yunho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Life Cycle Climate Performance (LCCP) is an evaluation method by which heating, ventilation, air conditioning and refrigeration systems can be evaluated for their global warming impact over the course of their complete life cycle. LCCP is more inclusive than previous metrics such as Total Equivalent Warming Impact. It is calculated as the sum of direct and indirect emissions generated over the lifetime of the system “from cradle to grave”. Direct emissions include all effects from the release of refrigerants into the atmosphere during the lifetime of the system. This includes annual leakage and losses during the disposal of the unit. The indirect emissions include emissions from the energy consumption during manufacturing process, lifetime operation, and disposal of the system. This thesis proposes a standardized approach to the use of LCCP and traceable data sources for all aspects of the calculation. An equation is proposed that unifies the efforts of previous researchers. Data sources are recommended for average values for all LCCP inputs. A residential heat pump sample problem is presented illustrating the methodology. The heat pump is evaluated at five U.S. locations in different climate zones. An excel tool was developed for residential heat pumps using the proposed method. The primary factor in the LCCP calculation is the energy consumption of the system. The effects of advanced vapor compression cycles are then investigated for heat pump applications. Advanced cycle options attempt to reduce the energy consumption in various ways. There are three categories of advanced cycle options: subcooling cycles, expansion loss recovery cycles and multi-stage cycles. The cycles selected for research are the suction line heat exchanger cycle, the expander cycle, the ejector cycle, and the vapor injection cycle. The cycles are modeled using Engineering Equation Solver and the results are applied to the LCCP methodology. The expander cycle, ejector cycle and vapor injection cycle are effective in reducing LCCP of a residential heat pump by 5.6%, 8.2% and 10.5%, respectively in Phoenix, AZ. The advanced cycles are evaluated with the use of low GWP refrigerants and are capable of reducing the LCCP of a residential heat by 13.7%, 16.3% and 18.6% using a refrigerant with a GWP of 10. To meet the U.S. Department of Energy’s goal of reducing residential energy use by 40% by 2025 with a proportional reduction in all other categories of residential energy consumption, a reduction in the energy consumption of a residential heat pump of 34.8% with a refrigerant GWP of 10 for Phoenix, AZ is necessary. A combination of advanced cycle, control options and low GWP refrigerants are necessary to meet this goal.