Development of Cooling Systems With Active Elastocaloric Regenerators
by David Catalini
The vapor compression cycle (VCC) has been developed and optimized over a century to provide cooling in residential and commercial buildings, and transport systems. However, its usage has resulted in unpredicted environmental damage such as depleting the ozone layer and promoting global warming when its refrigerant leaks into the atmosphere. Because of this, it is important to develop a superior technological alternative without the environmental costs. One way to tackle this problem is to develop heat pumping cycles using solid-state refrigerant since a solid is incapable of leaking into the atmosphere. However, a solid-refrigerant cannot flow to deliver cooling the same way a fluid-refrigerant does. This requires a system conceptual redesign, which started with near-room temperature cooling with magnetocaloric materials in 1976 and elastocaloric materials in 2012. In this work, four different system configurations were studied with the following objectives: 1) maximizing the system’s temperature lift and 2) measuring the cooling capacity as a function of the useful temperature lift of the system when operating as a water chiller. The first configuration was based on the thermowave heat recovery strategy, while the other three were based on a single stage, two-stage and reciprocating variants of the active regeneration cycle. From the studied configurations the thermowave-based cycle achieved a system’s temperature lift of 8 K, at large average strain of 4.5%. It produced a maximum useful temperature lift of 5 K and a maximum cooling capacity of 125 W. All active regeneration-based cycles achieved similar final results while the best results was a system’s temperature lift of 21.3 K at a low average strain of 3.5% and a maximum useful temperature lift of 6.5 K and a maximum cooling capacity between 16 W and 25 W. The advantage of the reciprocating system integration is that it can achieve these results at lower strain than the one- stage and two-stage configurations. This dissertation identified a fundamental limitation of the active regeneration cycles using single composition elastocaloric materials. It is due to the fact that the local strain is larger than the average strain where the temperature is lower, which limits the maximum applicable average strain to prevent premature failure. This directly affects both the temperature lift and cooling capacity of the system. Different alternatives to address this issue, as well as how to improve the overall thermal and structural performance of the system within the constraints of the materials commercially available are suggested.
Doctoral Dissertation
http://hdl.handle.net/1903/27238
Development of a General-Purpose Steady-State Simulation Framework for Vapor Compression Systems
by Ransisi Huang
The vapor compression system is the dominating technology in heat pumping, air conditioning and refrigeration. Vapor compression is associated with significant energy consumption and high global warming potential. Steady-state simulation of vapor compression system is a crucial numerical technology that helps to assess and mitigate the energy and environmental impact of these systems. This dissertation aims to advance the steady-state modeling and simulation technologies for vapor compression systems toward higher level of flexibility, computational efficiency, and robustness, improving designs and reducing time to market.First, the dissertation proposes a generalized solution methodology for the steady-state analysis of arbitrary systems. A tripartite-graph based tearing algorithm is proposed to generically formulate the residual equations. The methodology was extensively validated by five test systems with capacities from 10 to 100 kW. The maximum simulation energy imbalance was 0.91%, and the maximum system performance deviation was 8.94%. The methodology was also applied to analyze two advanced vapor compression systems, presenting strong capability to contribute to the acceleration of their R&D stage. Second, the dissertation develops an approximation-assisted modeling methodology to speed up the steady-state system simulation. Three approximation-assisted heat exchanger models were compared in terms of accuracy and computational efficiency. Kriging metamodel presented the highest accuracy among the three. For heat exchanger performance approximation, its overall ∆P and ∆h mean absolute error (MAE) were 4.46% and 0.9%, respectively. For system simulations, the maximum COP and capacity errors with Kriging metamodel were 2.54% and 1.45%, respectively. System simulation was sped up by 10X - 600X, depending on the test conditions. Third, the dissertation proposes two convergence improvement approaches on the basis of nonlinear equation fundamentals, and assessed them on a standard vapor compression system as a first step, allowing for later application to more complex cycles. The assessment results show that a large initial Jacobian condition number presents low convergence probability at the current initial guess point. The results also indicate a correlation between component nonlinearity and simulation convergence. It was found that by changing the characterization methods in the heat exchanger models, 47 out of 51 originally non-converged cases were able to reach convergence.
Doctoral Dissertation
http://hdl.handle.net/1903/26721
Personal Cooling System with Phase Change Material
by Yiyuan Qiao
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.
Doctoral Dissertation
http://hdl.handle.net/1903/26588
Design and Experimental Characterization of Metal Additive Manufactured Heat Exchangers for Aerospace Application
by Fabio Battaglia
High temperature heat exchangers are key to the success of emerging high-temperature, high-efficiency solutions in energy conversion, power generation and waste heat recovery applications. When applied to the aerospace applications, the main objective is to develop heat exchangers that can realize significant performance improvement in terms of gravimetric heat exchange density (kW/kg). In the present study, two air-to-air crossflow heat exchangers were designed, built and tested to determine their potential for high performance, pre-cooling heat exchanger for aircraft applications. A novel design based on manifold-microchannel technology was chosen as it provided localized and optimum distribution of the flow among the heat transfer surface micro channels, offering superior heat transfer performance and low pressure drops, when compared to conventional, state of the art heat exchangers for the chosen application. However, fabrication of the manifold microchannel design for high temperature with super alloys as the heat exchanger material presents serious manufacturing challenges fabrication techniques. To overcome this limit, direct metal laser sintering (DMLS) additive manufacturing technique was selected for the fabrication of the Ni-based superalloy manifold-microchannel heat exchangers in the present study. Extensive work was performed to characterize the printing capability of different metal 3D-printers in terms of printing orientation, printing accuracy and structure density. Based on the knowledge acquired, two units were printed, with overall size of 4”x4”x4” and 4.5”x4”x3.5” and fin thickness of 0.220 mm and 0.170 mm, respectively. The printed units were the largest additively printed, superalloy-based manifold-microchannel heat exchangers found in the literature. The experimental characterization was carried at high temperature (600°C) and the model prediction of the performance was updated to characterize the behavior of the heat exchangers in this operational conditions. Based on the experimental results, a gravimetric heat duty of 9.4 kW/kg for an effectiveness (ε) of 78% was achieved, which corresponds to an improvement of more than 50% compared to the conventional designs. The characterization of the performance at high temperature was then completed by analyzing the thermo-mechanical stress generated by the simultaneous presence of temperature gradient and pressures. The current study is the first to characterize the behavior of manifold-microchannel heat exchanger under high temperature in terms of performance prediction and thermo-mechanical analysis.
Doctoral Dissertation
http://hdl.handle.net/1903/26619
Energy audit and modeling of two multi-purpose buildings on the College Park Campus of The University of Maryland
by Devashis Man Shrestha
Gossett Football House and Biology-Psychology Building are two high-energy consuming, multi-purpose buildings in the University of Maryland, College Park, Campus. This thesis details the energy analysis and energy model development for these buildings to identify energy savings opportunities. The research was conducted in three phases per building: (I)-A comprehensive summary of relevant building information collected from the energy audit walkthroughs and study of building records, followed by building utility analysis and benchmarking. (II)- Energy model development to simulate building energy consumption which was calibrated according to ASHRAE-14 guidelines. (III)- Analysis and simulation of savings from energy conservation measures to increase energy efficiency and reduce the carbon footprint of the respective buildings. Combined savings of 3,876 MMBtu, 787,290-gal water, and $100,800 per annum along with 515 MT CO2eq emission reductions per annum were projected for the two buildings. These savings directly contribute to the campus sustainability goals and increase the energy efficiency of the campus buildings.
Master's Thesis
http://hdl.handle.net/1903/26535
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