Boosting Electrical Generation of a Photovoltaic Array by Thermal Harvest from p-Si Cells: An Experimental and Theoretical Study
by Joshua Kelley
Solar power generation deployment is increasing globally with photovoltaic modules. Most energy available to conventional PV is absorbed as heat or passes through. Performance of Photovoltaic Thermal (PVT) collectors which mimic currently available, polycrystalline, commercial PV modules was measured in the mid-Atlantic US. A linear model is developed for their performance which uses values available in Typical Meteorological Year files and shows daily accuracies to within 11%. Pressure losses for the collectors were measured and an empirical model established. Electrical generation is modeled by PVT in conjunction with an Organic Rankine Cycle. 20% - 45% boosts to electricity production in the Southwest are projected. 5%-15% boosts are projected in the mid-Atlantic.
Master's Thesis
http://hdl.handle.net/1903/17044
Process Intensification by Utilizing Multistage Manifold Microchannel Heat and Mass Exchangers
by Ratnesh Tiwari
Much of research and development work has been dedicated to implement the heat and mass transfer using microchannel technology; however, it is not yet cost effective and is limited to higher end applications such as electronics cooling and selected applications in automotive and aircraft heat exchangers. The work on mass transfer application of micro channels also has been very limited, despite the very high potential contribution of micro channels for mass transfer enhancement. Scaling up of microchannel equipment presents several manufacturing and process organization challenges such as flow distribution inside microchannels, cost, fouling, high pressure drops etc.
This thesis presents the development of a cost effective and compact tubular manifold microchannel heat and mass exchanger (MMHX) for industrial applications. A novel design for the flow distribution manifolds has been proposed. The proposed manifold helps in the enhancement of heat and mass transfer by creating better flow mixing. The MMHX is designed in such a way that the manifold causes the flow to break into multiple passes of very short flow lengths in the microchannels. These flow lengths are short enough such that the flow in the channels is always into entry zone (developing laminar zone) both hydrodynamically as well as thermally, resulting in higher heat transfer than that in the fully developed laminar flow in conventional microchannel heat exchangers. The pressure drop in the device is low as the fluid flow length into the microchannels is very short. While the manifold design helps in flow distribution, very short flow length inside the microchannels mitigates the problems of flow instability of two-phase heat transfer applications such as that in evaporators and condensers.
The mass transfer in gas liquid reaction applications is enhanced due to the multiple passes where continuous breaking of the gas liquid interface as well as mixing of the bulk liquid occurs. A multi-pass microchannel heat and mass exchanger prototype was designed, fabricated and was experimentally tested for the performance as liquid-liquid heat exchanger, evaporator, condenser and gas- liquid absorber. Experiments were carried out by changing the liquid and gas flow rates, geometry of the microchannels and the size of the manifold. Flow visualization studies were also performed to study two phase flow distribution and flow pattern in the manifold. Experimental results have shown that the mass transfer coefficient (using CO2 and DEA-water solution) for the microchannel absorber is 1 to 2 orders of magnitude higher than the conventional absorber. This increase in mass transfer is mainly attributed to high interfacial area to volume ratio of microchannels and good mixing in the manifold. Similarly, heat transfer coefficient for the single phase heat transfer as well as for two phase heat transfer (evaporator and condenser) is about 3 to 8 times higher than the conventional heat exchangers such as shell and tube or plate type heat exchanger. High transfer rates enable us to design compact heat and mass transfer devices for the industrial applications. Industrial processes, such as carbon capture, which are not economically viable due to their high cost, can be feasible with the development of these next generation heat and mass transfer equipment. Due to the simplicity of the component design and the assembly, cost of the industrial scale equipment can be substantially lower as compared other compact heat exchangers.
Current work is the continuation of heat and mass transfer work being carried out at the S2TS lab in University of Maryland. While Jha V.(2012) studied the first version of single pass manifold microchannel heat exchanger, Ganapathy H. (2014) studied the absorption of CO2 in DEA solution in single microchannel as well as in parallel microchannels. MMHX studied in this study builds on the previous work by introducing the multipass concept and utilizing commercially available fin tubes as microchannel surfaces.
Doctoral Dissertation
http://hdl.handle.net/1903/16504
Development of Thermoelastic Cooling Systems
by Suxin Qian
Thermoelastic cooling, or elastocaloric cooling, is a cutting-edge solid-state based alternative cooling technology to the state-of-the-art vapor compression cooling systems that dominate the world today. Being environment friendly without any global warming potential, these thermoelastic cooling systems could reduce energy expenses and carbon emission since they are potentially more efficient than the vapor compression cycle (VCC). Nevertheless, as a result of its immature nature, its realistic application potential requires comprehensive research in material fundamentals, cycle design, system simulation, the proof-of-concept prototype development and testing. Therefore, understanding the performance potential and limitations of this emerging new cooling technology, building the theoretical framework and guiding future research are the objectives of this dissertation.
Thermodynamic fundamentals of elastocaloric materials are introduced first. Cycle designs and theoretical performance evaluation are presented together with a detailed physics-based dynamic model for a water-chiller application. The baseline system coefficient of performance (COP) is 1.7 under 10 K system temperature lift. To enhance the system performance, a novel thermo-wave heat recovery process is proposed based on the analogy from the highly efficient “counter-flow” heat exchanger. Both the theoretical limit of the “counter-flow” thermo-wave heat recovery and the practical limitations by experimentation have been investigated. The results indicated that 100% efficiency is possible in theory, 60% ~ 80% heat recovery efficiency can be achieved in practice. The world first of-its-kind proof-of-concept prototype was designed based on the dynamic model, fabricated and tested using the proposed heat recovery method. Maximum cooling capacity of 65 W and maximum water-water temperature lift of 4.2 K were measured separately from the prototype. Using the validated model, performance improvement potentials without manufacturing constraints in the prototype are investigated and discussed. The COP is 3.4 with the plastic insulation tube and tube-in-tube design, which can be further improved to 4.1 by optimizing the system operating parameters. A quantitative comparison is made for thermoelastic cooling and other not-in-kind cooling technologies in order to provide insights on its limitations, potential applications, and directions for future research. Though under current research status, the system efficiency is only 0.14 of Carnot efficiency, which is less than 0.21 for conventional VCC systems, the framework carried out in this dissertation shows a technically viable alternative cooling technology that may change the future of our lives.
Doctoral Dissertation
http://hdl.handle.net/1903/17329
Application of Ejector Cycle on Household Refrigerators
by Manjie Li
In this study, an enhanced single-evaporator refrigeration system with an ejector was studied, which can recover part of the expansion loss from the system. The focus of the study was applying the ejector cycle to the household refrigerator experimentally and numerically, which had a small refrigerant mass flow rate and a sub-zero evaporating temperature. The ejector designed from the previous work was fabricated and its test facility was constructed and instrumented. The test results were analyzed with the partial help of simulation software. The achieved maximum COP of ejector cycle was 2.4 at evaporating temperature of -10°C. Test results show that the pressure drop along the ejector suction loop needs to be carefully designed and poor mixing inside the ejector significantly impairs the ejector and overall system performance. More research work is needed to further prove the potential of ejector cycle in refrigerator system.
Master's Thesis
http://hdl.handle.net/1903/17129
Dynamic Modeling of Vapor Compression Systems for Residential Heat Pump Applications with Alternative Low-GWP Refrigerants
by Viren Bhanot
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.
Master's Thesis
http://hdl.handle.net/1903/17219
Velocity based defrost of evaporator coil of heat pumps
by Kamalakkannan Muthusubramanian
Much of research and development work has been dedicated to implement the heat and mass transfer using microchannel technology; however, it is not yet cost effective and is limited to higher end applications such as electronics cooling and selected applications in automotive and aircraft heat exchangers. The work on mass transfer application of micro channels also has been very limited, despite the very high potential contribution of micro channels for mass transfer enhancement. Scaling up of microchannel equipment presents several manufacturing and process organization challenges such as flow distribution inside microchannels, cost, fouling, high pressure drops etc.
This thesis presents the development of a cost effective and compact tubular manifold microchannel heat and mass exchanger (MMHX) for industrial applications. A novel design for the flow distribution manifolds has been proposed. The proposed manifold helps in the enhancement of heat and mass transfer by creating better flow mixing. The MMHX is designed in such a way that the manifold causes the flow to break into multiple passes of very short flow lengths in the microchannels. These flow lengths are short enough such that the flow in the channels is always into entry zone (developing laminar zone) both hydrodynamically as well as thermally, resulting in higher heat transfer than that in the fully developed laminar flow in conventional microchannel heat exchangers. The pressure drop in the device is low as the fluid flow length into the microchannels is very short.
While the manifold design helps in flow distribution, very short flow length inside the microchannels mitigates the problems of flow instability of two-phase heat transfer applications such as that in evaporators and condensers. The mass transfer in gas liquid reaction applications is enhanced due to the multiple passes where continuous breaking of the gas liquid interface as well as mixing of the bulk liquid occurs.
A multi-pass microchannel heat and mass exchanger prototype was designed, fabricated and was experimentally tested for the performance as liquid-liquid heat exchanger, evaporator, condenser and gas- liquid absorber. Experiments were carried out by changing the liquid and gas flow rates, geometry of the microchannels and the size of the manifold. Flow visualization studies were also performed to study two phase flow distribution and flow pattern in the manifold.
Experimental results have shown that the mass transfer coefficient (using CO2 and DEA-water solution) for the microchannel absorber is 1 to 2 orders of magnitude higher than the conventional absorber. This increase in mass transfer is mainly attributed to high interfacial area to volume ratio of microchannels and good mixing in the manifold. Similarly, heat transfer coefficient for the single phase heat transfer as well as for two phase heat transfer (evaporator and condenser) is about 3 to 8 times higher than the conventional heat exchangers such as shell and tube or plate type heat exchanger. High transfer rates enable us to design compact heat and mass transfer devices for the industrial applications. Industrial processes, such as carbon capture, which are not economically viable due to their high cost, can be feasible with the development of these next generation heat and mass transfer equipment. Due to the simplicity of the component design and the assembly, cost of the industrial scale equipment can be substantially lower as compared other compact heat exchangers.
Current work is the continuation of heat and mass transfer work being carried out at the S2TS lab in University of Maryland. While Jha V.(2012) studied the first version of single pass manifold microchannel heat exchanger, Ganapathy H. (2014) studied the absorption of CO2 in DEA solution in single microchannel as well as in parallel microchannels. MMHX studied in this study builds on the previous work by introducing the multipass concept and utilizing commercially available fin tubes as microchannel surfaces.
Master's Thesis
http://hdl.handle.net/1903/16963
Separation of Fine Liquid Droplets From High Speed Air Utilizing the Electrohydrodynamics Technique
by Ning Yang
With constant process intensification in recent years, the separation of fine micron and submicron size liquid droplets from gaseous flow mediums has become an important subject for the process and aerospace industries. While conventional technologies are not effective in this droplet size range, electrostatic separation demonstrated remarkable effectiveness and reliability while lowering maintenance and operation cost. However, it is commonly used for low droplet concentration in relatively low velocity gas flow. This current experimental study is focused on electrostatic separation of high concentration of fine electrically conductive droplets from high velocity gas flow. Different separators including wire-to-plate, wire-to-cylinder, single stage, and multi-stage separators were designed, built and tested at gas velocities up to 15 m/sec and droplet concentration up to 22,000 ppm. The results demonstrated that two-stage plate, as well as tubular separators provides maximum separation efficiency at minimum power consumption. However, the tubular separator is easier to package in the required space envelope and 1-inch diameter tubes are more efficient at high velocity and droplet concentrations.
Master's Thesis
http://hdl.handle.net/1903/16955
Direct Numerical Simulation of Incompressible Multiphase Flow With Phase Change
by Moon Soo Soo Lee
Phase change problems arise in many practical applications such as air-conditioning and refrigeration, thermal energy storage systems and thermal management of electronic devices. The physical phenomenon in such applications are complex and are often difficult to be studied in detail with the help of only experimental techniques. The efforts to improve computational techniques for analyzing two-phase flow problems with phase change are therefore gaining momentum.
The development of numerical methods for multiphase flow has been motivated generally by the need to account more accurately for (a) large topological changes such as phase breakup and merging, (b) sharp representation of the interface and its discontinuous properties and (c) accurate and mass conserving motion of the interface. In addition to these considerations, numerical simulation of multiphase flow with phase change introduces additional challenges related to discontinuities in the velocity and the temperature fields. Moreover, the velocity field is no longer divergence free. For phase change problems, the focus of developmental efforts has thus been on numerically attaining a proper conservation of energy across the interface in addition to the accurate treatment of fluxes of mass and momentum conservation as well as the associated interface advection.
Among the initial efforts related to the simulation of bubble growth in film boiling applications the work in \cite{Welch1995} was based on the interface tracking method using a moving unstructured mesh. That study considered moderate interfacial deformations. A similar problem was subsequently studied using moving, boundary fitted grids \cite{Son1997}, again for regimes of relatively small topological changes. A hybrid interface tracking method with a moving interface grid overlapping a static Eulerian grid was developed \cite{Juric1998} for the computation of a range of phase change problems including, three-dimensional film boiling \cite{esmaeeli2004computations}, multimode two-dimensional pool boiling \cite{Esmaeeli2004} and film boiling on horizontal cylinders \cite{Esmaeeli2004a}. The handling of interface merging and pinch off however remains a challenge with methods that explicitly track the interface. As large topological changes are crucial for phase change problems, attention has turned in recent years to front capturing methods utilizing implicit interfaces that are more effective in treating complex interface deformations.
The VOF (Volume of Fluid) method was adopted in \cite{Welch2000} to simulate the one-dimensional Stefan problem and the two-dimensional film boiling problem. The approach employed a specific model for mass transfer across the interface involving a mass source term within cells containing the interface. This VOF based approach was further coupled with the level set method in \cite{Son1998}, employing a smeared-out Heaviside function to avoid the numerical instability related to the source term. The coupled level set, volume of fluid method and the diffused interface approach was used for film boiling with water and R134a at the near critical pressure condition \cite{Tomar2005}. The effect of superheat and saturation pressure on the frequency of bubble formation were analyzed with this approach. The work in \cite{Gibou2007} used the ghost fluid and the level set methods for phase change simulations. A similar approach was adopted in \cite{Son2008} to study various boiling problems including three-dimensional film boiling on a horizontal cylinder, nucleate boiling in microcavity \cite{lee2010numerical} and flow boiling in a finned microchannel \cite{lee2012direct}. The work in \cite{tanguy2007level} also used the ghost fluid method and proposed an improved algorithm based on enforcing continuity and divergence-free condition for the extended velocity field. The work in \cite{sato2013sharp} employed a multiphase model based on volume fraction with interface sharpening scheme and derived a phase change model based on local interface area and mass flux.
Among the front capturing methods, sharp interface methods have been found to be particularly effective both for implementing sharp jumps and for resolving the interfacial velocity field. However, sharp velocity jumps render the solution susceptible to erroneous oscillations in pressure and also lead to spurious interface velocities. To implement phase change, the work in \cite{Hardt2008} employed point mass source terms derived from a physical basis for the evaporating mass flux. To avoid numerical instability, the authors smeared the mass source by solving a pseudo time-step diffusion equation. This measure however led to mass conservation issues due to non-symmetric integration over the distributed mass source region. The problem of spurious pressure oscillations related to point mass sources was also investigated by \cite{Schlottke2008}. Although their method is based on the VOF, the large pressure peaks associated with sharp mass source was observed to be similar to that for the interface tracking method. Such spurious fluctuation in pressure are essentially undesirable because the effect is globally transmitted in incompressible flow. Hence, the pressure field formation due to phase change need to be implemented with greater accuracy than is reported in current literature.
The accuracy of interface advection in the presence of interfacial mass flux (mass flux conservation) has been discussed in \cite{tanguy2007level,tanguy2014benchmarks}. The authors found that the method of extending one phase velocity to entire domain suggested by Nguyen et al. in \cite{nguyen2001boundary} suffers from a lack of mass flux conservation when the density difference is high. To improve the solution, the authors impose a divergence-free condition for the extended velocity field by solving a constant coefficient Poisson equation. The approach has shown good results with enclosed bubble or droplet but is not general for more complex flow and requires additional solution of the linear system of equations.
In current thesis, an improved approach that addresses both the numerical oscillation of pressure and the spurious interface velocity field is presented by featuring (i) continuous velocity and density fields within a thin interfacial region and (ii) temporal velocity correction steps to avoid unphysical pressure source term. Also I propose a general (iii) mass flux projection correction for improved mass flux conservation. The pressure and the temperature gradient jump condition are treated sharply. A series of one-dimensional and two-dimensional problems are solved to verify the performance of the new algorithm. Two-dimensional and cylindrical film boiling problems are also demonstrated and show good qualitative agreement with the experimental observations and heat transfer correlations. Finally, a study on Taylor bubble flow with heat transfer and phase change in a small vertical tube in axisymmetric coordinates is carried out using the new multiphase, phase change method.
Doctoral Dissertation
http://hdl.handle.net/1903/18132
Analysis of Heat Pump Clothes Dryer
by Zhilu Zhang
Clothes dryers (CD) offer a rapid means to dry laundry in households, but consume a large portion (4%) of residential electricity. Heat pump clothes dryers (HPCD) can be much more energy-efficient than conventional electric CDs, but have not emerged in the U.S. market yet. In this study, experiments were conducted for a state-of-the-art commercial hybrid HPCD from the European market with two different operational modes followed by Department of Energy's test procedure. The HPCD's system performances were analyzed through measurements on humidity ratio (HR), temperature and power consumption for both Eco and Speed Modes. About 70% energy consumption reduction potential was observed as compared with a typical electric CD in the United States. The heating and cooling capacities during the Eco Mode were 1.48 kW and 1.18 kW, respectively, and the dehumidification rate was 0.372 g/s. The heat exchangers were modeled with CoilDesigner and their performances were simulated. The UA of the evaporator was mainly affected by the air flow rate (AFR), inlet air HR and refrigerant MFR while that of the condenser was mainly affected by the condensing temperature, AFR, and refrigerant MFR. The air leakage was estimated to be 24% to 45% in which the water vapor leakage was 26% and the energy loss was 5%. The mass transfer process through the drum was discussed and the mass transfer coefficient k between the cloth surface and air was calculated to be 0.237 g/m²·s. This study provides the performances of HPCDs and their design analysis, which can be used for developing improved HPCDs.
Master's Thesis
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