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.
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Modeling of HVAC Configurations for De-Carbonization in a Mid-Size Hospital(2022) Grant, Zachary; Hwang, Yunho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)As the threat of climate change becomes more imminent, there has been increasing emphasis on technologies that reduce carbon emissions in the HVAC sector. The clear path forward given existing technologies is electrification since electricity production has future potential to become cleaner. In terms of building type, high ventilation requirements and near continuous occupancy make healthcare facilities some of the highest energy users. HVAC equipment runs all day and night in these facilities with little change. Conventional HVAC equipment such as a boiler is proven to consume more energy than heat pump systems. More specifically, the Variable Refrigerant Flow (VRF) heat pump and the Ground Source Heat Pump (GSHP) are areas of ongoing research. This analysis included creating whole-building energy models using EnergyPlus and OpenStudio to compare the energy consumption for these heat pump configurations and some cheaper electrification alternatives. The results suggested that the GSHP system possessed the greatest potential for energy savings and thus decarbonization given its higher efficiency during times of extreme ambient temperatures compared to other options.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 VENTILATION IMPACT ON AIRBORNE TRANSMISSION OF RESPIRATORY ILLNESS IN STUDENT DORMITORIES(2018) Jenkins, Sara T; Srebric, Jelena; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work presents a study of the effect of ventilation rates on the bioaerosols that cause upper respiratory illness. A network of 147 sensors was placed in a pair of dormitories on a college campus to measure carbon dioxide concentrations over two semesters. The concentration results served as input into multi-zone ventilation models of the two buildings, which had different heating, ventilation, and air conditioning (HVAC) systems. The dormitory with a central mechanical ventilation system had, as expected, a higher turnover of fresh air compared to the other, which relied on exhaust fans and infiltration. This well-ventilated building also contained far fewer occupants with recorded upper respiratory illness incidence in comparison to the poorly ventilated building. The central ventilation system increased dorm room ventilation rates by 500%, while decreasing respiratory illness incidence by over 85%. Comparative studies have shown similar findings with increased ventilation reducing incidence of upper respiratory illness by an order of magnitude.Item PERFORMANCE AND APPLICATIONS OF RESIDENTIAL BUILDING ENERGY GREY-BOX MODELS(2013) Siemann, Michael James; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The electricity market is in need of a method to accurately predict how much peak load is removable by directly controlling residential thermostats. Utilities have been experimenting with residential demand response programs for the last decade, but inconsistent forecasting is preventing them from becoming a dependent electricity grid management tool. This dissertation documents the use of building energy models to forecast both general residential energy consumption and removable air conditioning loads. In the models, complex buildings are represented as simple grey-box systems where the sensible energy of the entire indoor environment is balanced with the flow of energy through the envelope. When internet-connected thermostat and local weather data are inputs, twelve coefficients representing building parameters are used to non-dimensionalize the heat transfer equations governing this system. The model's performance was tested using 559 thermostats from 83 zip codes nationwide during both heating and cooling seasons. For this set, the average RMS error between the modeled and measured indoor air temperature was 0.44°C and the average daily ON time prediction was 1.9% higher than the data. When combined with smart power meter data from 250 homes in Houston, TX in the summer of 2012 these models outperformed the best traditional methods by 3.4 and 28.2% predicting daily and hourly energy consumption with RMS errors of 86 and 163 MWh. The second model that was developed used only smart meter and local weather data to predict loads. It operated by correlating an effective heat transfer metric to past energy data, and even further improvement forecasting loads were observed. During a demand response trial with Earth Networks and CenterPoint Energy in the summer of 2012, 206 internet-connected thermostats were controlled to reduce peak loads by an average of 1.13 kW. The thermostat building energy models averaged forecasting the load in the 2 hours before, during, and after these demand response tests to within 5.9%. These building energy models were also applied to generate thermostat setpoint schedules that improved the energy efficiency of homes, disaggregate loads for home efficiency scorecards and remote energy audits, and as simulation tools to test schedule changes and hardware upgrades.Item HIGH-PERFORMANCE TUBULAR EVAPORATOR UTILIZING HIGH ASPECT RATIO MANIFOLD MICROCHANNELS(2012) Jha, Vibhash Chandra; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Heat recovery using absorption chillers has not been economical for small scale applications due to high capital requirements and heavy weight/volume as deterring factors for its expanded use in waste heat-to-cooling applications. Development of advanced, high performance heat and mass exchanger components can significantly improve the competitive edge of heat activated absorption cooling systems, particularly with respect to weight reduction and size/volume of these systems. The main contribution of this thesis is demonstration of a novel high performance micro-grooved evaporator, as well as a solution heat exchanger, for use in a small-scale ammonia-water absorption cooling system. A compact tubular evaporator was developed which uses an innovative manifold/fluid feed system combined with a micro-grooved evaporator to realize substantially higher (4 to 5 fold) overall heat transfer coefficient of the evaporator; while requiring much less refrigerant charge per ton of cooling, when compared to conventional state of the art systems. The experimentally measured heat transfer coefficients reported in this study are record high, while pressure drops for the given capacity are modest. Additional contributions of the study included a detailed numerical study of single- stage absorption cycle with multiple cycle design enhancements to identify the controlling system parameters. A single-phase numerical study for manifold microchannel design was carried out to understand the effect of important geometrical parameters in support of design and development of the evaporator. The tubular evaporator was successfully fabricated and tested to the system pressure of 500 psi on the refrigerant-side and was experimentally evaluated with several microgroove surface made of aluminum and nickel alloys, and also with different flow header enhancements using R134a/water pair. For the experiments conducted, the microchannel width was typically in the range of 30-100 µm with a maximum aspect ratio of 10. The refrigerant flow rate was varied within 5-30 g/s and water flow rate was varied within 150-600 ml/s obtaining wide range of cooling capacity between 1- 5 kW for 2-12 °C LMTDs. The overall heat transfer coefficients greater than 20,000 W/m2-K was obtained which is roughly 4-5 times higher than state of art for given application. A maximum pressure drop of 200 mbars on water-side and 100 mbars on the refrigerant-side was observed at maximum mass flow rates. An alternative method for the evaporator design was also explored in form of flat plate evaporators which can further provide improved overall heat transfer coefficients. Manifold microchannels were used on both sides of the plates, with the aim to achieve overall heat transfer coefficient greater than 50,000 W/m2-K. The new micro-grooved evaporator has the potential to introduce a game-changing evaporative surface, with precise flow delivery and high heat transfer coefficients, driven by a combination of thin film evaporation, as well as convective boiling on the heat transfer surface.Item Experimental Study of Hybrid Cooled Heat Exchanger(2011) Tsao, Han-Chuan; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A test system for a hybrid cooled heat exchanger was designed, and the test facility was constructed based on ASHRAE Standard 41.2-1987. A conventional air-cooled tube-fin heat exchanger was tested with and without application of wetting water. The baseline tests were conducted to investigate the heat exchanger performance improvement by applying evaporative cooling technology. The heat exchanger capacity and air side pressure drop were measured while varying operating conditions and heat exchanger inclination angles. The results show the heat exchanger capacity increased by 170% with application of the hybrid cooling technology, but the air side pressure drop increased by 130%. Additional research investigating air fan power was also conducted, which increased 120% from the dry condition to the hybrid cooled condition. In summary, the potential for improving the heat exchanger performance by applying hybrid cooling is shown in this research.Item MODELING OF A COMBINED HEAT AND POWER UNIT AND EVALUATION OF SYSTEM PERFORMANCE IN BUILDING APPLICATIONS(2010) Bush, John; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents a validated model of a 4 kilowatt combined heat and power (CHP) system derived from laboratory experiments. The model is tuned to match steady state experimental tests, and validated with transient experimental results. Further simulations are performed using a modeled thermal storage system, and implementing the CHP system into a building model to evaluate the feasibility of CHP in the mid-Atlantic region, as well as the Great Lakes region. The transient simulation outputs are within 4.8% of experimental results for identical load profiles for a simulated summer week, and within 2.2% for a spring or autumn week. When integrated with a building model, the results show 23.5% cost savings on energy in the mid-Atlantic region, and 29.7% savings in the Great Lakes region.Item THE DEVELOPMENT OF AN AIR-COOLED ABSORPTION CHILLER CONCEPT AND ITS INTEGRATION IN CHP SYSTEMS(2004) Liao, Xiaohong; Radermacher, Reinhard K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation focuses on the feasibility, crystallization issues, and the integration of LiBr-H2O air-cooled absorption chillers into Cooling, Heating and Power (CHP) systems. The concept of an air-cooled system is attractive because the cooling tower and the associated installation and maintenance issues can be avoided. However, crystallization of the LiBr-H2O solution then becomes the main issue in the operation of the unit, since the air-cooled absorber tends to operate hotter than the water-cooled absorber due to the relative heat transfer characteristics of the coolant leading to crystallization of the working fluid. Differently from the conventional approaches to air-cooled absorption chillers, novel temperature control strategies in conjunction with a specialized application is proposed. This prevents crystallization but presents unique system integration challenges and opportunities. A model to accurately reflect the thermodynamic characteristics of air-cooled absorption chillers and to facilitate control is developed as part of this research, and field experiments that simulate air-cooled conditions with a water-cooled absorption chiller, which was driven by the waste heat of a microturbine, were conducted to validate the feasibility of the air-cooled concept and the accuracy of computer model. While CHP provides a good opportunity for the application of air-cooled absorption chillers, system integration issues need to be investigated. The capital cost of CHP equipment and the load fluctuation of a commercial building restrict the advantage of designing a unit sized for peak load. Therefore, the conventional Heating Ventilation and Air Conditioning (HVAC) system is needed to pick up the residual loads. Thus, the result of an extensive system integration analysis is that CHP should be arranged in series with the HVAC system to ensure obtaining more operating hours at its full capacity, so that the cost savings achieved through the recovery of waste heat are fully realized to repay its higher initial capital cost. The primary energy savings are presented for all potential configurations. As a part of this research a fully integrated CHP system has been installed and instrumented at the Chesapeake Building. It is a commercial office building on the University of Maryland campus. The experimental setup, data processing, and experience gained are detailed here. Based on the computer simulation, extensive experiments, first hand installation, operation and maintenance experience, valuable guidelines on the integration of an air-cooled absorption chiller in CHP are developed. All the guidelines are also applicable to water-cooled absorption chillers.