Made available in DSpace on 2014-06-11T19:29:53Z (GMT). No. of bitstreams: 0 Previous issue date: 2008-07-07Bitstream added on 2014-06-13T18:47:28Z : No. of bitstreams: 1 santos_rs_me_guara.pdf: 3310410 bytes, checksum: dbdc747497f7d936a9a108f561ec367f (MD5) / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / No Brasil, devido ao forte crescimento da demanda de energia elétrica nas últimas décadas e com a necessidade de atender a esta procura, torna-se indispensável a inclusão no sistema elétrico brasileiro de programas de eficiência energética e a utilização de fontes alternativas de energia, pois com a construção de novas hidrelétricas e termoelétricas para atender o consumidor, ocorre inundações de grandes áreas e diminuição da emissão de gases para a atmosfera, o que causa fortes impactos ambientais. O uso de fontes alternativas também diminui a dependência do Brasil de combustíveis fósseis, pois futuramente ocorrerá o esgotamento de reservas naturais no país, tendo a necessidade de importar energia, o que elevará os preços das tarifas de energia elétrica. Para que haja a conservação de energia proveniente de fontes tradicionais e incentivar a implantação de energia solar no sistema elétrico brasileiro, desenvolveu-se um programa que analisa a viabilidade técnico-econômica de inclusão do sistema fotovoltaico e fototérmico ao sistema tradicional de distribuição de energia nas diversas regiões do Brasil em residências, pois é o setor que possui um maior potencial de economia de energia. O programa é uma ferramenta de fácil manuseio, de acesso rápido que permitem tomadas de decisões rápidas e seguras quanto à implantação de energia solar por parte do consumidor, além de fornecer em qual região é mais viável essa implantação. / In Brazil, due to a solid increasing in electrical energy demand at the last decades and due to the need to comply with this demand, became essential the inclusion of energetic efficiency programs and use of alternative energy resources in the Brazilian electrical supply system. However, the new hydro and thermo electrical power plants construction to attend the consumer cause strong environmental impacts, since they need big flooded lakes and eliminate theirs pollutants in the atmosphere, respectively. Moreover, the use of alternative energy resources decreases Brazil dependency of fossil fuels, besides it prevents the breakdown of the natural reserves and the need of energy importation, what would increase the charge for the electrical energy. To the energy conservation provided by traditional resources and to encourage the implementation of a solar energy based system in Brazil, it was developed a software that analyzes technical and economic feasibility of the photovoltaic and photothermical systems introduction in the traditional supply system on the several regions of Brazil to residential use, because this is a sector that has an elevated potential of energy saving. The software has a friendly interface, with quick access that allows the user to take quick and safe decisions, regarding to the solar energy implementation by consumers, and evaluate which area of the country is the most interesting to the implementation.

Made available in DSpace on 2014-06-11T19:30:09Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-02-09Bitstream added on 2014-06-13T20:39:45Z : No. of bitstreams: 1 mattos_sr_me_guara.pdf: 1275419 bytes, checksum: 6a6b8796ddc4c875ee2155aa72412e97 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Este aquecedor solar foi construído em uma estrutura metálica, que tem um revestimento de polietileno reflexivo parabólico cilíndrico, onde no foco da parábola passa uma serpentina com sete tubos em cobre de 9,5 mm e o projeto foi desenvolvido para obter temperaturas mais elevadas do que os aquecedores planos. O objetivo desta dissertação foi atingir temperaturas mais altas na fase líquida. As temperaturas alcançadas foram obtidas utilizando uma densidade de potência solar que variou entre (1.000±10) W/m2, nas vazões de: 66, 54, 42, 30, 27, 24, 21, 18, 15, 12 e 9 litros por hora. A máxima temperatura atingida utilizando-se a vazão de nove litros por hora foi de 71°C, permitindo uma diferença de temperatura, em relação a temperatura ambiente de 43°C. A máxima vazão utilizada no aquecedor solar parabólico cilíndrico foi de 66 litros por hora e obteve-se uma temperatura de 38°C, sendo a diferença em relação à temperatura ambiente de 6°C. Assim com o desenvolvimento do aquecedor solar parabólico cilíndrico obtiveram-se temperaturas maiores em uma área de captação menor, do que em aquecedor convencionais planos e com rendimentos superiores a 70% em sua maior faixa de operação / This solar heater is constructed of a metallic structure, which has a polyethylene coating reflective parabolic cylindrical, where the focus of the parable is a serpentine seven copper pipes of 9.5 mm and the project was designed to obtain higher temperatures than the heaters plans and the objective of this thesis was to reach higher temperatures in the liquid phase. The temperatures reached were obtained using a solar power density ranged from (1000±10) W/m2, the flow rates: 66, 54, 42, 30, 27, 24, 21, 18, 15, 12 and 9 liters per hour. The maximum temperature reached by using the flow of nine liters per hour was 71°C, allowing a temperature difference in relation to ambient temperature of 43°C. The maximum flow rate used in the cylindrical parabolic solar heater was 66 liters per hour and obtained a temperature of 38°C, and the difference in the ambient temperature of 6°C. So with the development of a cylindrical parabolic solar heater temperatures were obtained in a larger area smaller than in conventional heating plans and with performance up to 70% on the higher range of operation

M.Eng. / The need and problem was originated from the trends of the earths dwindling energy resource. As time progresses humans are becoming more aware of need to use so called “alternative energy sources” to alleviate the main energy converters i.e. power stations. The student was tasked with investigating the thermal performance of a solar powered refrigeration cycle (prototype) that could: produce enough refrigeration effect that it replaces the standard home vapour compression unit, used for cooling or freezing of foods, heats up a geyser sufficiently to have hot water for a common house hold, has excess energy to heat or cool liquid or air based environments and has the potential to lower the electrical bill of a house. The introductory step was to obtain the thermo fluid properties of aqua ammonia solutions. A setback came about when determining the aqua ammonia properties. There were too many conflicting properties being yielded by six different authors. In an attempt to gain confidence in only one author a comparison table was prepared. The table compared the six authors to each other. By looking at all the values compared it brought great clarity to the problem. When continuing the research into the fundamental law approach of solving for the cycle new findings were made. Initially very little comprehensive studies were done which explained in fundamental laws to solve for the absorption cycle. After extensive reviewing of a detailed study on how to solve for absorption refrigeration cycles, then it was able to begin improving on the thermo – fluid design of the cycle. As cycle and component design began to progress the train of thought began to steer in a direction. Each component needed to be detail designed. The advantage of having each component specifically catered for in the cycle design was that it would increase the cycle efficiency. In this way it would ensure that during the concept generation phase the functioning of each component was clear, thereby enabling a clear understanding of how components would compliment each other in a cycle. A mode of solving for the cycle was to endeavour that all parameters could be calculated unambiguously, with the aid of computerisation. Testing was carried out on a real life commercial thermal siphoning machine in order to realistically understand how absorption refrigeration works and gain experience. At the end of the study the most important result is that the dissertation research shows strong evidence that it will be possible to create a device which can fulfil the four tasks listed above. Another result is a program which is a refined cycle design of the pump absorption type refrigeration. The program solves for points along the cycle. Lastly it was found that even though EES was the simpler program to use for aqua ammonia solution properties it was the only program which catered for sub cooling and super heating.

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Water electrolysis conjugated with renewable energy sources potentially realizes a sustainable society. Although the current electrolyzers operate at extreme pH to maximize the electrolysis efficiency, near-neutral pH conditions may optimize the overall system operation when conjugated with renewable energy sources. In this context, a study on the electrolysis in the mild conditions is essential. The dissertation investigates the water electrolysis in various conditions, with a particular focus placed on milder conditions, to rationalize and improve its performance. Microkinetic analysis was performed for the cathodic half-reaction in conjugation with mass transport evaluation using various electrode materials. The analysis revealed a significant universal influence of electrolyte properties on the reaction performances at near-neutral pH. Investigation of the associated electrolyte properties (ion size, viscosity and activity/fugacity) rationally optimized the reaction conditions. Together with the separately performed studies on the anodic half-reaction and system configurations, the finding was successfully transferred to electrocatalytic and solar-driven water splitting systems. The presented herein is a fundamental yet crucial aspect of water electrolysis, which can advance the water electrolysis for the future.

Solar collector absorber tubes play a critical role in converting incident solar heat flux into absorbed thermal energy and transferring it to a heat transfer fluid. In this study a single horizontally orientated absorber tube was investigated numerically in terms of the influence of different circumferential spans of symmetrical and asymmetrical heat flux distributions on buoyancy-driven secondary flow, internal heat transfer and friction factor characteristics. Three types of circumferential heat flux boundaries were considered, namely fully uniform, partial uniform and sinusoidal non-uniform heat flux distributions. Both gravitational symmetry and asymmetry for non-uniform heat flux distributions were investigated to cover symmetry angles in terms of the gravitational field (g) of 0° (symmetrical case), 20°, 30°, 40° and 60°. Different sized stainless steel absorber tubes having a length of 10 m, and inner diameters of 62.7 mm, 52.5 mm, 40.9 mm and 35.1 mm were considered. Three dimensional steady-state simulations were performed for water as working fluid, covering laminar flow Reynolds numbers ranging from 130 to 2200, as well as for turbulent flow Reynolds numbers ranging from 3030 to 202 600. Buoyancy effects, temperature dependent fluid thermal properties, tube-wall heat conduction and the external wall heat losses by convection and radiation were taken into consideration. Average internal heat transfer coefficients, local internal heat transfer coefficients, Richardson numbers and overall friction factors were obtained for different angular spans of incident heat flux, inlet fluid temperatures, heat flux intensities and outer wall thermal conditions Laminar flow results indicated that the angular span, angular position, and intensity of the applied external heat flux all have significant influences on the buoyancy induced mixed convection inside the tube. This resulted in significant variations in the internal heat transfer coefficients and the friction factor which are not well described by classical empirical correlations. Buoyancy induced secondary flow significantly enhanced the internal heat transfer coefficient and significantly increased the friction factor compared to forced convection cases. Higher heat transfer coefficients and friction factors were obtained for non-uniform heat flux distributions compared to uniform heat flux distributions and were found to be dependent on the angle span and position of the heat flux. Higher inlet temperatures resulted in increased Nusselt numbers and lower friction factors, while higher external heat loss resulted in lower Nusselts numbers and lower friction factors. An increase in the asymmetry of the heat flux distribution resulted in a reduction of the Nusselt number and friction factor. Even though turbulent flow cases with a Reynolds number range of approximately 3000 to 9000 were also influenced by buoyancy driven secondary flow, and followed the same parameter trends, it occurred to a lesser extent compared to the laminar flow cases. Turbulent flow cases with Reynolds numbers higher than 9100, exhibited little dependence on secondary flow effects and indicates the suitability of classical fully uniform heat flux heat transfer and friction factor correlations for highly turbulent flow irrespective of the distribution or intensity of the heat flux. / Thesis (PhD)--University of Pretoria, 2017. / Advanced Engineering Centre of Excellence at the University of Pretoria, NRF, TESP, NAC, and SOLAR Hub with the Stellenbosch University, EEDSM Hub and CSIR is highly acknowledged and duly appreciated. / Mechanical and Aeronautical Engineering / PhD / Unrestricted

Along with many other solar energy conversion processes, research on photocatalytic water splitting to generate hydrogen and oxygen has experienced rapid major development over the past years. Developing an efficient visible-light-responsive photocatalyst has been one of the targets of such research efforts. In this regard, nitride materials, particularly Ta3N5, have been the subject of investigation due to their promising properties. This dissertation focuses on the fundamental parameters involved in the photocatalytic processes targeting overall water splitting using Ta3N5 as a model photocatalyst. The discussion primarily focuses on relevant parameters that are involved in photon absorption, exciton separation, carrier diffusion, carrier transport, and catalytic efficiency. A collection of theoretical and experimental studies of properties associated with Ta3N5 have been utilized to obtain a comprehensive understanding of this material. The fundamental structural and optoelectronic properties of Ta3N5 have been addressed. From the electronic properties, the dielectric constant and effective masses have been calculated. Because of its high dielectric constant and relatively low effective masses, Ta3N5 is promising for photocatalytic reaction applications. Studies of lattice dynamics, optical properties, and band positions have been able to clearly show that the synthesized Ta3N5 is essentially non-stoichiometric and that a truly pure phase of Ta3N5 has never been achieved, even though XRD has shown a pure phase sample. The photophysical properties of Ta3N5, such as the absorption coefficient, carrier mobility, and carrier lifetime, have been experimentally measured by synthesizing Ta3N5 thin films. Very low kinetic properties with very low transport properties and fast carrier recombination explained why overall water splitting has never been achieved with Ta3N5 as a photocatalyst to date. The extent to which the surface states of Ta3N5 photocatalysts affect photocatalytic performance has been investigated. The surface topmost layer is demonstrated to play a critical role in the photocatalytic activity of Ta3N5; further research on the surface properties of Ta3N5 should be conducted to understand and improve charge separation and the resulting photocatalytic activity. Finally, a remarkable improvement in the photocatalytic OER has been achieved with the addition of cobalt as a cocatalyst. There is a trade-off between the optimum contact of hole transfer from bulk Ta3N5 to the surface of the cobalt cocatalyst and providing active sites for the electrochemical reaction. Knowing the characteristics of cobalt on the Ta3N5 surface, further improvement was attempted by adding a noble metal to the CoOx/Ta3N5 photocatalyst system, where a synergetic effect of CoOx and noble metals was observed.

The recurring theme of this dissertation is the correlation between FeS2 surface chemistry and key electrical and electronic properties of FeS2. Efforts have been made to identify and characterize the FeS2 surface, investigate the photoelectrochemistry of FeS2 photoanodes under anhydrous and anoxic conditions, and investigate the influence of deliberate surface chemistry on FeS2 photoelectrochemistry. Infrared reflection-absorption spectroscopy (IRRAS) was used to investigate a thin adsorbate layer on pyrite. The results showed that the combination of angle-dependent studies and computational efforts are a powerful tool for characterizing the pyrite surface. The photoelectrochemistry of FeS2 photoanodes was investigated in an I¯/I3¯ acetonitrile electrolyte acetonitrile electrolyte. The results revealed that the non-aqueous system was suitable for strictly anhydrous and anoxic photoelectrochemical studies. A model was proposed to explain the observed influence of concentration of dissolved I2 on the photovoltage. A central component of the proposed model was that shunting was assumed to take place at physically distinct regions of the electrode and that mass-transport to and from these regions could be treated separately from mass-transport to the regions responsible for the rectifying behavior of the FeS2/liquid junction. The implication of the agreement between experimental and calculated J-E curves is that macroscopic photoelectrochemical investigations may underestimate the quality of FeS2 photoanodes due to the presence of defects. The influence of surface treatments on FeS2 photoelectrochemistry was further studied using non-coordinating redox species. A statistically significant increase of photovoltage was observed after treating FeS2 surfaces with KCN. X-ray photoelectron spectroscopy was used to study chemical bond formation between the electron donating ligands and iron(II) centers on the pyrite surface. The results were discussed in terms of charge recombination models and surface coordination chemistry. Unfinished work is also presented. Cathodic polarization in acidic media is a prerequisite for any detectable photoresponse. The exact function of the electrochemical activations was further investigated by electropolishing pyrite electrode under different experimental conditions including etchant identity and applied bias. The results suggested that the electrochemical treatment removes the damaged surface layer caused by mechanical polishing, and might also stabilize the surface states. Further experiments can be focus on anhydrous etching of pyrite photoanode. The research presented in this dissertation guides future studies of thin film FeS2 photovoltaics.

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