In this thesis study, fabrication and doping of silicon thin films prepared by electron beam evaporation equipped with effusion cells for solar cell applications have been investigated. Thin film amorphous Si (a-Si) layers have been fabricated by the electron beam evaporator and simultaneously doped with boron (B) and phosphorous (P) using effusion cells. Samples were prepared on glass substrates for the future solar cell operations. Following the deposition of a-Si thin film, crystallization of the films has been carried out. Solid Phase Crystallization (SPC) and Metal Induced Crystallization (MIC) have been employed to obtain thin film crystalline Si. Crystallization was performed in a conventional tube furnaces and Rapid Thermal annealing systems (RTA) as a function of process parameters such as annealing temperature and duration. Produced films have been characterized using chemical and structural characterization techniques such as Raman Spectroscopy, X-Ray Diffractometer and Secondary Ion Mass Spectrometer (SIMS). The electrical properties of the films have been studied using Hall Effect and I-V measurements as a function of doping. We have demonstrated successful crystallization of a-Si by SPC at temperatures above 600 &deg / C. The crystallization occurred at lower temperatures in the case of MIC. For doping, P was evaporated from the effusion cell at a temperature between 600 &deg / C and 800 &deg / C. For B, the evaporation temperature was 1700 &deg / C and 1900 &deg / C. The thickness and the band gap of the Si films were determined by ellipsometry method and the results were compared for different evaporation temperatures. The effect of doping was monitored by the I-V and Hall Effect measurements. We have seen that the doping was accomplished in most of the cases. For the samples annealed at relatively high temperatures, the measured doping type was inconsistent with the expected results. This was attributed to the contamination from the glass substrate. To understand the origin of this contamination, we analyzed the chemical structure of the film and glass by X-ray Fluorescence (XRF) and seen that the glass is the main source of contamination. In order to prevent this contamination we have suggested covering the glass substrate with Si3N4 (Silicon Nitride) which act as a good diffusion barrier for impurities.

Se ha desarrollado y construido una infraestructura experimental orientada a la validación de modelos de intercambiadores compactos de aletas y tubos y sistemas de refrigeración con sobrealimentación de líquido. El objetivo ha sido la obtención de datos experimentales fiables, con condiciones geométricas y de contorno exactamente definidas, para poder compararlos inequívocamente con resultados de simulaciones numéricas. Se presentan los modelos matemáticos, objetivo de la validación, y una descripción detallada del circuito de aire, del refrigerante líquido, y del refrigerante de cambio de fase, que integran la infraestructura.Estos tres circuitos están encargados de asegurar condiciones estables y controladas para los prototipos ensayados y para el sistema de refrigeración con sobrealimentación de líquido, en un amplio rango de temperaturas, flujos másicos y potencias. El diseño permite el ensayo de prototipos de intercambiadores de calor con diferentes geometrías y dimensiones. Se presentan detalladamente los instrumentos de medida con sus precisiones, montaje, se describen también los componentes y los parámetros de la unidad de adquisición de datos.Especial atención se ha dedicado a la calibración de los instrumentos de medida como parte esencial del proceso de preparación de los ensayos. Se describe el proceso de estimación de las incertidumbres sistemáticas de los sensores calibrados. Se expone en detalle la formulación y la metodología adoptada para el análisis de incertidumbre de los resultados experimentales.El procesamiento y el análisis de los datos experimentales se ha realizado en forma automática con un código computacional especialmente desarrollado, encargado de calcular los resultados a partir de las variablas medidas, de llevar a cabo el análisis de incertidumbres detallado, y de comparar los resultados numéricos y experimentales.Se presentan resultados experimentales obtenidos con la infraestructura experimental desarrollada. Se presentan estudios detallados de intercambiadores de calor compactos en condiciones de enfriamiento de aire, utilizando refrigerante líquido y de cambio de fase. Se presentan también resultados del estudio experimental del sistema de refrigeración con sobrealimentación de líquido. Los resultados han sido comprobados y verificados a través de balances energéticos en todos los componentes, donde la misma magnitud física ha sido evaluada de mediciones independientes. Con el objetivo de permitir el uso mas general de los resultados experimentales se presentan también los datos crudos de las variables medidas durante los ensayos.Se ha propuesto una metodología de validación para el modelo de intercambiadores compactos, basada en comparaciones sistemáticas de resultados numéricos y experimentales. Estas comparaciones han sido analizadas en términos estadísticos con el objetivo de cuantificar las diferencias observadas y dar una evaluación global de las prestaciones del modelo numérico en las condiciones ensayadas. La metodología propuesta para la validación del modelo de intercambiadores compactos puede ser utilizada como base para metodologías de validación en general. / Experimental infrastructures intended for validation of compact heat exchanger models, and models of liquid overfeed refrigeration systems have been developed and constructed. The aim has been the obtaining of reliable experimental data from tests at exactly defined geometrical and boundary conditions, permitting the unequivocal comparisons with numerical simulation results. The mathematical models are presented and detailed description of the airhandling, the liquid refrigerant, and phase-changing refrigerant circuits integrating the experimental infrastructure is given.These three circuits are encharged to provide stable controlled conditions for the tested prototypes and the liquid overfeed system in the desired range of temperatures, fluid flows, and capacities. The design permits the accommodation of heat exchanger prototypes with different geometry and sizes.Detailed overview of the measuring instruments is presented, with their accuracies and mounting, and the components and parameters of the data acquisition system are described.Special attention has been paid to the calibration of the measuring instruments as an essential part of the test preparation. The process of estimation of the systematic uncertainties in the calibrated sensors measurements is described. The formulation and the methodology adopted for the uncertainty analysis of the experimental results is exposed in detail.The experimental data processing and analysis has been performed automatically with a specially developed program encharged with the calculation of the experimental results from the measured variables, the detailed uncertainty analysis, and the numerical to experimental results comparisons.Experimental results obtained with the developed infrastructure are presented. Detailed studies of compact heat exchangers under cooling conditions, using liquid and phase-changing refrigerants, are performed and presented. Results from the experimental studies of the liquid overfeed refrigeration system are also presented. The results have been checked and verified through energy balance checks for all the components where measurements of the same physical magnitude can be contrasted with independent measurements. In order to give more general use of the obtained experimental data, the raw measured variables during the tests are also presented.An experimental validation methodology for the compact heat exchanger model has been proposed, based on systematic comparisons between numerical and experimental results. The comparisons have been analysed in statistical terms in order to quantify the observed differences and to give global evaluation of the numerical model performance in the tested conditions. The methodology proposed for validation of the heat exchanger model can be used as a basis for validation methodology for numerical models in general.

It is well known that microprocessor performance can be improved by lowering the junction temperature. Two stage cascaded vapor compression refrigeration (VCR) is a mature, inexpensive, and reliable cooling technology that can offer chip temperatures down to ?? C. Recent studies have shown that for a power limited computer chip, there is a non-linear scaling effect that offers a 4.3X performance enhancement at ?? C. The heat transfer performance of a compact evaporator is often the bottleneck in sub-ambient heat removal. For this reason, the design of a deep sub-ambient compact evaporator is critical to the cooling system performance and has not been addressed in the literature. Four compact evaporator designs were investigated as feasible designs - a baseline case with no enhancement structures, micro channels, inline pin fin arrays, and alternating pin fin arrays. A parametric experimental investigation of four compact evaporator designs has been performed aiming at enhancing heat transfer. Each evaporator consists of oxygen free copper and has a footprint of 20 mm x 36 mm, with a total thickness of 3.1 mm. The micro channel evaporator contains 13 channels that are 400 um wide by 1.2 mm deep, and the pin fin evaporators contain approximately 80 pin fins that are 400 um wide by 1.2 mm tall with a pitch of 800 um. Two phase convective boiling of R508b refrigerant was investigated in each evaporator at flow rates of 50 - 70 g/min and saturation temperatures of ??to ??C. Pressure drop and local heat transfer measurements are reported and used to explain the performance of the various evaporator geometries. The results are compared to predictions from popular macro- and micro-channel heat transfer and pressure drop correlations. The challenges of implementing a two stage cascade VCR systems for microprocessor refrigeration are also discussed.

Cooling of electronic equipments has become an important issue as the advances in technology enabled the fabrication of very small devices. The main challenge in cooling is the space limitation. The use of miniature refrigerators seems to be a solution alternative for the cooling problem. The objective of this study is to design and simulate a vapor compression refrigeration cycle for a micro-scale refrigerator. A MATLAB code is developed for the simulations. The four components of the refrigerator, namely, the condenser, evaporator, compressor and the capillary tube are designed separately. The cycle is successfully completed nearly at the same point where it begins. The cold space temperature, ambient air temperature, condensation and evaporation temperatures, and the evaporator heat load are the predetermined parameters. A fan is used to cool the condenser, and the compressor is selected as isentropic. R-134A is selected as the refrigerant and a simple interpolation code is developed to obtain the thermophysical properties of R-134A. The original design is carried out with an isentropic compressor. For the purpose of comparison, a cycle with a polytropic compressor is also considered. Similarly, two alternative designs for the evaporator are developed and simulated. A second law analysis is performed at the end of the study.

Die Erfindung der Kältemaschine macht es möglich, dass heute zu beliebiger Zeit Kälte in benötigter Menge und gewünschter Temperatur erzeugt werden kann. Aber für die Kälteerzeugung braucht man Antriebsenergie. Das Ziel dieser Doktorarbeit besteht darin, mit einer intelligenten Kombination von thermostatischem Expansionsventil (TEV) und innerem Wärmeübertrager (IWÜ) den Energieverbrauch von Kälteanlagen deutlich zu verringern, ohne dass dies zu einer Verschlechterung der Zuverlässigkeit oder wesentlichen Mehrkosten führt. Bis heute gelten die beiden bekannten Komponenten TEV und IWÜ als inkompatibel miteinander, weil die einfache Aneinanderreihung zu einem instabilen Regelverhalten führt. Nach detaillierter Analyse der Ursachen dieser Instabilität wird die Hypothese aufgestellt, dass durch eine absichtliche „Verschlechterung“ des IWÜ ein stabiler Arbeitsbereich gefunden werden kann. Bei dieser „Verschlechterung“ kann es sich z. B. um den Bypass eines Teils des Hochdruckstromes oder um eine Gleichstromanordnung der Ströme im Wärmeübertrager handeln. Diese Hypothese wurde primär durch Experimente, aber zusätzlich auch durch Simulationsrechnungen bestätigt. Dafür wurde ein Versuchsstand aufgebaut, mit welchem verschiedene Konfigurationen und Regelstrategien getestet wurden. Es wurde eine deutliche Verringerung des Energieverbrauchs gegenüber herkömmlichen Anordnungen erreicht.

Several steps are followed in order to evaluate the cycle as the title suggests. The diffusion absorption refrigerator (DAR) cycle performance is evaluated when using helium or hydrogen as auxiliary gas. A slight increase in COP is found when using helium, but it is not sufficient to justify the cost. A secondary simulation of an alternate dual-pressure cycle using a pump is done as feasibility comparison with the same parameters as the diffusion cycle. It was found that the second cycle is not acceptable due to high evaporator temperatures needed to ensure liquid enters the pump instead of partially evaporated solution. This would greatly increase the work input required for what essentially becomes a compressor. Optimisation of the DAR is evaluated by simulating the use of a rectification column and the effects of different design points on overall performance. Meteorological data for Potchefstroom, South Africa is used to perform a yearly analysis on the simulated cycle and to specify a suitable design point. The use of a radiative cooling system as heat sink for the system is then investigated and incorporated into the system model. Finally, the performance characteristics of the simulated DAR cycle are discussed, verified and compared with available data from similar research. It is shown that a 40% solution aqua-ammonia-hydrogen cycle driven by 526 kW of solar thermal energy at 130°C and a system pressure of 1.5 MPa can easily achieve a COP over 0.4 with an air-cooled absorber at 40°C and a water-cooled condenser at 35°C. A 231 kW refrigeration capacity at an average evaporator temperature of –20°C is achieved, satisfying the requirements for a domestic refrigeration system. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013.

Several steps are followed in order to evaluate the cycle as the title suggests. The diffusion absorption refrigerator (DAR) cycle performance is evaluated when using helium or hydrogen as auxiliary gas. A slight increase in COP is found when using helium, but it is not sufficient to justify the cost. A secondary simulation of an alternate dual-pressure cycle using a pump is done as feasibility comparison with the same parameters as the diffusion cycle. It was found that the second cycle is not acceptable due to high evaporator temperatures needed to ensure liquid enters the pump instead of partially evaporated solution. This would greatly increase the work input required for what essentially becomes a compressor. Optimisation of the DAR is evaluated by simulating the use of a rectification column and the effects of different design points on overall performance. Meteorological data for Potchefstroom, South Africa is used to perform a yearly analysis on the simulated cycle and to specify a suitable design point. The use of a radiative cooling system as heat sink for the system is then investigated and incorporated into the system model. Finally, the performance characteristics of the simulated DAR cycle are discussed, verified and compared with available data from similar research. It is shown that a 40% solution aqua-ammonia-hydrogen cycle driven by 526 kW of solar thermal energy at 130°C and a system pressure of 1.5 MPa can easily achieve a COP over 0.4 with an air-cooled absorber at 40°C and a water-cooled condenser at 35°C. A 231 kW refrigeration capacity at an average evaporator temperature of –20°C is achieved, satisfying the requirements for a domestic refrigeration system. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013.

A need was identified for a set of basic design tools for Multi-Effect Distillation (MED) plant evaporators. This led to an investigation into the different types of evaporators as well as further research on horizontal falling film evaporators as used in the MED process. It also included the theory on these types of evaporators. In order not to duplicate existing design tools, an investigation was also performed on some of the tools that are currently available. The first set of tools that were developed were tools, programmed in EES (Engineering Equation Solver), for the vacuum system and the evaporator. These programs can be used to simulate different parameters (like different mass flows and temperatures). That enables the correct selection of components for the vacuum system and can be used to address sizing issues around the evaporator. It can also be used to plan the layout of the plant. The second of the design tools was developed by designing and building a flow pattern test section. From the flow pattern test section a set of curves for the wetted length under different conditions was obtained which can be used in order to design the sieve tray. This set of curves was found to be accurate for municipal as well as seawater and can be used in the design of the sieve tray of the evaporator. Further development can be done by implementing the figures of the wetted length into a simulation package like, for example, Flownex (a system CFD (Computational Fluid Dynamics) code that enables users to perform detail design, analysis and optimization of a wide range of thermal-fluid systems). The background gained from the study done on the evaporator can also be implemented into such a package. This could solve the problem of different design packages by creating a single design package with all of the above mentioned options included. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2009.

A need was identified for a set of basic design tools for Multi-Effect Distillation (MED) plant evaporators. This led to an investigation into the different types of evaporators as well as further research on horizontal falling film evaporators as used in the MED process. It also included the theory on these types of evaporators. In order not to duplicate existing design tools, an investigation was also performed on some of the tools that are currently available. The first set of tools that were developed were tools, programmed in EES (Engineering Equation Solver), for the vacuum system and the evaporator. These programs can be used to simulate different parameters (like different mass flows and temperatures). That enables the correct selection of components for the vacuum system and can be used to address sizing issues around the evaporator. It can also be used to plan the layout of the plant. The second of the design tools was developed by designing and building a flow pattern test section. From the flow pattern test section a set of curves for the wetted length under different conditions was obtained which can be used in order to design the sieve tray. This set of curves was found to be accurate for municipal as well as seawater and can be used in the design of the sieve tray of the evaporator. Further development can be done by implementing the figures of the wetted length into a simulation package like, for example, Flownex (a system CFD (Computational Fluid Dynamics) code that enables users to perform detail design, analysis and optimization of a wide range of thermal-fluid systems). The background gained from the study done on the evaporator can also be implemented into such a package. This could solve the problem of different design packages by creating a single design package with all of the above mentioned options included. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2009.

[pt] O presente trabalho diz respeito ao desenvolvimento de um modelo de simulação para evaporadores do tipo casco e serpentina, com aplicações para resfriadores de água. O modelo considera o trocador de calor como divido em duas zonas: uma de evaporação e outra de superaquecimento. Diferentes mecanismos de transferência de calor, existentes em cada zona, foram, portanto, levados em consideração. Definidas as condições de entrada de ambos os fluidos e a geometria do evaporador,um programa escrito em FORTRAN foi desenvolvido para calcular o desempenho térmico do evaporador, incluindo a capacidade térmica, condições de saída do refrigerante e água e distribuição de área de troca pelas duas zonas. Propriedades do refrigerantes foram calculadas com subrotinas do pacote REFPROP, desenvolvido no NIST, EUA, permitindo a modelagem de equipamentos operando com uma vasta gama de refrigerantes, incluindo hidrocarbonetos e misturas não azeotrópicas. Em virtude da ausência de informação na literatura, um aparato experimental foi construído para a determinação do coeficiente de transferência de calor no lado da carcaça, por onde escoa a água. / [en] The present work is concerned with the development of a simulation model for shell and coil heat exchangers with first application in water-chillers. The model considers the heat exchanger as divided into two zones: two-phase region (boiling) and superheating region. Different refrigerant properties and heat transfer mechanisms are thus taken into account for each zone. For prescribed heat exchanger's geometry and water and efrigerant inlet conditions, a program in FORTRAN calculates the evaporator's performance which includes: refrigerant and water outlet conditions, evaporator's thermal capacity, and tube distribution for each zone. The REFPROP code programmed in FORTRAN was used for getting the local refrigerant properties, and the new kattan et al. s model for refrigerants boiling, including the new flow pattern map, was used for predicting the internal convective coefficient of heat transfer at the boiling region. A simple correlation was obtained for predicting the water convective coefficient depending on Reynolds number for the specific geometry at tested ranges of temperature. An evaporator was manufactured and an accurate test rig that included two controlled temperature baths and a data acquisition system was used for obtaining used data. The model is able to deal with a number of pure refrigerants and refrigerant blends. / [es] El presente trabajo discute el desarrollo de un modelo de simulación para evaporadores de tipo casco y serpentina, con aplicaciones para refrigeradores de agua. El modelo considera que el agente de intercambio de calor está divido en dos zonas: una de evaporación y otra de supercalentamiento. Por esto, se consideraron los diferentes mecanismos de transferencia de calor existentes en cada zona. Definidas las condiciones de entrada de ambos fluidos y la geometría del evaporador, se desarrolló un programa en FORTRAN para calcular el desempeño térmico del evaporador, incluyendo la capacidad térmica, condiciones de salida del refrigerante y agua y distribución de área de intercambio térmico por las dos zonas. Las popriedades del refrigerante fueron calculadas con subrutinas del paquete REFPROP, desarrollado en el NIST, EUA, lo que permitió modelar los equipos operando con una vasta gama de refrigerantes, incluyendo hidrocarbonetos y mezclas no azeotrópicas. En virtud de la ausencia de información en la literatura, se construyó un aparato experimental para la determinación del coeficiente de transferencia de calor en el lado del casco, por donde sale el agua.