Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Solar energy is by far the greatest energy resource available to generate power. One of the difficulties of using solar energy is that it is not available 24 hours per day - some form of storage is required if electricity generation at night or during cloudy periods is necessary. If a combined cycle power plant is used to obtain higher efficiencies, and reduce the cost of electricity, storage will allow the secondary cycle to operate independently of the primary cycle. This study focuses on the use of packed beds of rock or slag, with air as a heat transfer medium, to store thermal energy in a solar thermal power plant at temperatures sufficiently high for a Rankine steam cycle. Experimental tests were done in a packed bed test section to determine the validity of existing equations and models for predicting the pressure drop and fluid temperatures during charging and discharging. Three different sets of rocks were tested, and the average size, specific heat capacity and density of each set were measured. Rock and slag samples were also thermally cycled between average temperatures of 30 ºC and 510 ºC in an oven. The classical pressure drop equation significantly under-predicts the pressure drop at particle Reynolds numbers lower than 3500. It appears that the pressure drop through a packed bed is proportional to the 1.8th power of the air flow speed at particle Reynolds numbers above about 500. The Effectiveness-NTU model combined with a variety of heat transfer correlations is able to predict the air temperature trend over the bed within 15 % of the measured temperature drop over the packed bed. Dolerite and granite rocks were also thermally cycled 125 times in an oven without breaking apart, and may be suitable for use as thermal storage media at temperatures of approximately 500 ºC. The required volume of a packed bed of 0.1 m particles to store the thermal energy from the exhaust of a 100 MWe gas turbine operating for 8 hours is predicted to be 24 × 103 m3, which should be sufficient to run a 25-30 MWe steam cycle for over 10 hours. This storage volume is of a similar magnitude to existing molten salt thermal storage. / AFRIKAANSE OPSOMMING: Sonenergie is die grootste energiebron wat gebruik kan word vir krag opwekking. ‘n Probleem met die gebruik van sonenergie is dat die son nie 24 uur per dag skyn nie. Dit is dus nodig om die energie te stoor indien dit nodig sal wees om elektrisiteit te genereer wanneer die son nie skyn nie. ‘n Gekombineerde kringloop kan gebruik word om ‘n hoër benuttingsgraad te bereik en elektrisiteit goedkoper te maak. Dit sal dan moontlik wees om die termiese energie uit die primêre kringloop te stoor, wat die sekondêre kringloop onafhanklik van die primêre kringloop sal maak. Dié gevalle studie ondersoek die gebruik van ‘n slakof- klipbed met lug as hitteoordragmedium, om te bepaal of dit moontlik is om hitte te stoor teen ‘n temperatuur wat hoog genoeg is om ‘n Rankine stoom kringloop te bedryf. Eksperimentele toetse is in ‘n toets-bed gedoen en die drukverandering oor die bed en die lug temperatuur is gemeet en vergelyk met voorspelde waardes van vergelykings en modelle in die literatuur. Drie soorte klippe was getoets. Die gemiddelde grootte, spesifieke hitte-kapasiteit en digtheid van elke soort klip is gemeet. Klip en slak monsters is ook siklies tussen temperature van 30 ºC en 510 ºC verkoel en verhit. Die klassieke drukverlies vergelyking gee laer waardes as wat gemeet is vir Reynolds nommers minder as 3500. Dit blyk dat die drukverlies deur ‘n klipbed afhanklik is van die lug vloeispoed tot die mag 1.8 as die Reynolds nommer groter as omtrent 500 is. Die ‘Effectiveness-NTU’ model gekombineerd met ‘n verskeidenheid van hitteoordragskoeffisiënte voorspel temperature binne 15 % van die gemete temperatuur verskil oor die bed. Doloriet en graniet klippe het 125 sikliese toetse ondergaan sonder om te breek, en is miskien gepas vir gebruik in ‘n klipbed by temperature van sowat 500 ºC Die voorspelde volume van ‘n klipbed wat uit 0.1 m klippe bestaan wat die termiese energie vir 8 ure uit die uitlaat van ‘n 100 MWe gasturbiene kan stoor, is 24 × 103 m3. Dit behoort genoeg te wees om ‘n 25 – 30 MWe stoom kringloop vir ten minste 10 ure te bedryf. Die volume is min of meer gelyk aan dié van gesmelte sout store wat alreeds gebou is.

In concentrated solar power (CSP) stations, large arrays of mirrors which are capable of changing its orientation are used to reflect the incident solar energy to a stationary receiver kept at a distance. Such mirrors are often called as heliostats. The receiver contains a heat absorbing medium like molten salt. By absorbing the thermal energy reflected from thousands of heliostats, the temperature would reach around 6000C and the heat can be used in thermal power plants to generate steam and thus run a turbine to produce electricity. One of the biggest advantages of CSP over conventional energy harvesting from Sun is that it can generate electricity during night for long hours of time from the thermal energy stored during daytime. This eliminates the usage of batteries or any other energy storing methods. The conversion efficiency is also high in CSP due to the high temperature achieved. With prior knowledge of the station coordinates, viz., the latitude and longitude, the day of the year and time, the direction or the path of sun can be fully determined. Typically, the sun's motion is tracked by the azimuth-elevation (Az-El) or the target-aligned configuration heliostats. In both these approaches, the mirror needs to be moved about two axes independently using two actuators in series with the mirror effectively mounted at a single point at the centre. This arrangement causes the mirror to deform in presence of gusty winds in a solar field which results in loss of pointing accuracy. Typically a beam error of less than 2-3 mrad is desirable in a large solar field and this value also includes other sources of loss of pointing accuracy like gravity and wind loading. In order to prevent this, a rigid support frame is required for each of the heliostats. In this work, two three degree-of-freedom parallel manipulators, viz., the 3-UPU wrist and 3-RPS, have been proposed to track the sun in central receiver systems. The main reasons for choosing a parallel manipulator as heliostat are its desirable characteristics like large load carrying capacity, high accuracy in positioning the mirror and easy to obtain the inverse kinematics and convenient for real time control. The proposed parallel manipulators support the load of the mirror, structure and wind loading at three points resulting in less deflection and thus a much larger mirror can be moved with the required tracking accuracy and without increasing the weight of the support structure. The algorithm for sun tracking is developed, extensive simulation study with respect to actuations required, variation of joint angles, spillage loss and leg intersection has been carried out. Using FEA, it is shown that for same sized mirror, wind loading of 22 m/s and maximum deflection requirement (2 mrad), the weight of the support structure is between 15% and 60% less with the parallel manipulators when compared to azimuth-elevation or the target-aligned configurations. A comprehensive study on stroke minimization of prismatic joints is carried out. It is found that a stroke of 700 mm is required for a 2 m x 2 m heliostat at Bangalore when the farthest heliostat is at a distance of 300 m from the tower. Although, there is an extra motor required to track the sun, the 3-RPS manipulator is better than the conventional methods if the mirror area per actuator criteria is taken into consideration. Prototypes of the Az-El and 3-RPS heliostats were made with a mirror size of 1 m x 1 m. A PID controller implemented using MATLAB-Simulink and a low cost, custom made motor driver circuit is used to control the motion of the 3-RPS heliostat. The algorithm developed is tested on the prototype by tracking a point marked on the wall of the lab space and is found to have a tracking error of only 7.1 mrad. Finally, the actual sun tracking is carried out on the roof of a building reflecting the sun-light to a wall situated 6.72 m above and a distance of 15.87 m from the heliostats. The images are captured at various instances of time from 11:30 a.m. to 3:30 p.m. on October 15th and November 10th, 2016, tracking errors are quantified and it is demonstrated that the proposed 3-RPS parallel manipulator can indeed work as a heliostat in concentrated solar power plants.

Thesis (MScEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Concentrating solar power technology is a modern power generation technology in which central receiver systems play a significant role. For this technology a field of heliostats is used to reflect solar irradiation to the receiver located on top of the tower. An extensive review has shown that contemporary receiver designs face geometric complexities, lack of thermal efficiency as well as issues with durability and cost. The purpose of this study is to develop a new receiver concept that can potentially reduce these issues. A parametric analysis was used to identify potential means of improvement based on an energy balance approach including sensitivities involved with convection and radiation heat transfer. Design criteria such as the use of headers to minimize pressure drop was also investigated. Based on these findings the hybrid pressurized air receiver was developed which is a combination of tubular and volumetric receiver technologies. The fundamental idea of the receiver was investigated by simulating the ray-tracing and coupled natural convection and radiation heat transfer. The ray-tracing results have shown that the use of quartz glass is a prospective solution to higher allowable flux densities, but with reflection losses in the order of 7 %. The coupled natural convection heat transfer simulation further revealed that the receiver concept effectively eliminates the escape of buoyant plumes and radiative heat losses are minimized. Empirical data was gathered from a medium flux concentrator and good agreement with the numerical results was obtained. The thesis therefore concludes that the research outcomes were met. Ongoing research aims to optimise the receiver concept for a 5MW pilot plant. / AFRIKAANSE OPSOMMING: Gekonsentreerde sonkrag tegnologie is ’n moderne kragopwekkingstegnologie waar sentrale ontvangersisteme ’n beduidende rol speel. Vir hierdie tegnologie word ’n veld heliostate gebruik om sonstraling na die ontvanger wat aan die bopunt van die toring geleë is te reflekteer. ’n Omvattende hersiening het daarop gewys dat kontemporêre ontwerpe van die ontvangers ’n aantal geometriese kompleksiteite, ’n tekort aan termiese doeltreffendheid sowel as probleme in terme van duursaamheid en koste in die gesig staar. Die doel van die studie is om ’n nuwe ontvangerskonsep te ontwikkel wat moontlik hierdie probleme kan verminder. ’n Parametriese analise is gebruik om potensiële maniere van verbetering aan te dui wat gebaseer is op ’n energiebalans benadering; insluitend sensitiwiteite betrokke by konvektiewe en stralingswarmteoordrag. Ontwerpkriteria soos die gebruik van spruitstukke om drukverliese te minimaliseer is ook ondersoek. Gebaseer op hierdie bevindinge is die hibriede saamgepersde-lug ontvanger ontwikkel. Laasgenoemde is ’n kombinasie van buis- en volumetriese ontvangertegnologie. Die fundamentele idee van die ontvanger is ondersoek deur straalberekening asook die gelyktydige natuurlike konveksie en stralingswarmteoordrag te simuleer. Die straalberekeningsresultate het getoon dat die gebruik van kwarts glas ’n moontlike oplossing is om hoër stralingsintensiteit te bereik, maar met refleksieverliese in die orde van 7 %. Die gelyktydige natuurlike konveksie en stralingswarmteoordrag simulasie het verder aan die lig gebring dat die ontvangerkonsep die ontsnapping

Thesis (MEng (Mechanical and Mechatronic Engineering))--Stellenbosch University, 2007. / ENGLISH: The design, construction and testing of a passive thermal system, a roof solar chimney, for a loft is considered. Unlike conventional solar chimneys the solar collector is constructed from corrugated iron roof sheets with the aim that it can be integrated into existing buildings at a lower cost or used in low cost housing developments. The main objective of the study was to determine the feasibility of such low-cost design to regulate thermal conditions in a loft, that is heating the loft during winter and enhancing natural ventilation during summer, by carrying out an experimental and analytical study. The results obtained from the experimental study showed that for winter the solar chimney, having a channel width, depth and length of 0.7 m, 0.1 m and 1.8 m respectively and with a peal solar radiation of 850 W/m², heated the room air 5°C higher than the ambient temperature during the hottest periods of the day, which is only marginally better than a loft with conventional roof insulation. At night, it was found that reverse airflow occurred through the chimney, cooling the loft down to ambient temperature, due to radiation heat loss from the roof collector to the night sky. For summer operation, the experimental data showed that the chimney was able to maintain the loft at ambient temperature and the analytical study found that the chimney was able to enhance natural ventilation effectively, reaching air exchange rate of 6.6 per hour for the 4.6 m³ volume space. It was also found that the chimney’s performance dropped rapidly and significantly during periods of low solar radiation and at night. A sensitivity analysis illustrated that for both summer and winter operation, the size, tilt angle and absorptivity of the roof collector greatly effected the efficiency and mass flow rates of the system, agreeing well with other literature. These results prove that this low cost solar chimney cooling design was feasible to enhance natural ventilation mainly during hot summer conditions with high solar radiation. Compared to a loft with only conventional roof insulation, the chimney did not perform effectively during the winter to heat the loft up, meaning that winter operation for this specific design is not feasible. Possible improvements to the design include using construction materials with higher thermal capacities to retain heat energy and ensure continued operation during periods of low solar radiation, as well as using selective absorber coatings on the collector surface. It is recommended that further work on the project include the integration of these improvements into the present design and to use the findings obtained from the sensitivity analysis to improve system efficiencies. CFD analysis of the test-rig will be insightful as an additional means to validate and compare with the analytical and experimental data obtained in this report. With the continuation of these studies, this low-cost solar chimney design can be optimised, validated on a commercial scale and built into existing and new housing developments. Incorporating such a passive thermal device will aid homeowners in air regulation and thermal comfort of their living space as well as saving on energy requirements. / Sponsored by the Centre for Renewable and Sustainable Energy Studies, Stellenbosch University

Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Concentrating solar power plants are a promising means of generating electricity. However, they are dependent on the sun as a source of energy, and require thermal storage to supply power on demand. At present thermal storage – usually molten salt – although functional, is expensive, and a cheaper solution is desired. It is proposed that sensible heat storage in a packed bed of rock, with air as heat transfer medium, is suitable at temperatures of 500 – 600 °C. To determine if this concept is technically feasible and economically competitive with existing storage, rock properties, packed bed pressure drop and thermal characteristics must be understood. This work addresses these topics. No previously published data is available on thermal cycling resistance of South African rock, and there is limited data from other countries in the proposed temperature range for long-term thermal cycling, so samples were thermally cycled. There is rock which is suitable for thermal storage applications at temperatures of 500 – 600 °C. New maps of South Africa showing where potentially suitable rock is available were produced. Dolerite, found extensively in the Karoo, is particularly suitable. Friction factors were measured for beds of different particles to determine the importance of roughness, shape, and packing arrangement. Five sets of rock were also tested, giving a combined dataset broader than published in any previous study. Limitations of existing correlations are shown. The friction factor is highly dependent on particle shape and, in the case of asymmetric particles, packing method. The friction factor varied by up to 70 % for crushed rock depending on the direction in which it was poured into the test section, probably caused by the orientation of the asymmetric rock relative to the air flow direction. This has not been reported before for rock beds. New isothermal correlations using the volume equivalent particle diameter are given: they are within 15 % of the measurements. This work will allow a techno-economic evaluation of crushed rock beds using more accurate predictions of pumping power than could previously be made. Thermal tests below 80 °C show that bed heat transfer is insensitive to particle shape or type. A heat transfer correlation for air in terms of the volume equivalent diameter was formulated and combined with the E-NTU method. The predicted bed outlet temperatures are within 5 °C of the measurements for tests at 530 °C, showing that the influence of thermal conduction and radiation can be reasonably negligible for a single charge/discharge cycle at mass fluxes around 0.2 kg/m2s. A novel method for finding the optimum particle size and bed length is given: The Biot number is fixed, and the net income (income less bed cost) from a steam cycle supplied by heat from the bed is calculated. A simplified calculation using the method shows that the optimum particle size is approximately 20 mm for bed lengths of 6 – 7 m. Depending on the containment design and cost, the capital cost could be an order of magnitude lower than a nitrate salt system. / AFRIKAANSE OPSOMMING: Gekonsentreerde son-energie kragstasies is n belowende manier om elektrisiteit op te wek, maar hulle is afhanklik van die son as n bron van energie. Om drywing op aanvraag te voorsien moet hulle energie stoor. Tans is termiese stoor – gewoonlik gesmelte sout – hoewel funksioneel, duur, en n goedkoper oplossing word gesoek. Daar word voorgestel dat stoor van voelbare warmte-energie in n gepakte rotsbed met lug as warmteoordrag medium geskik is by temperature van 500 – 600 °C. Om te bepaal of dié konsep tegnies gangbaar en ekonomies mededingend met bestaande stoorstelsels is, moet rotseienskappe, gepakte bed drukval en hitteoordrag verstaan word. Hierdie werk spreek hierdie aspekte aan. Geen voorheen gepubliseerde data is beskikbaar oor die termiese siklus weerstand van Suid-Afrikaanse rots nie, en daar is beperkte data van ander lande in die voorgestelde temperatuurbereik, dus is monsters onderwerp aan termiese siklusse. Daar bestaan rots wat geskik is vir termiese stoor toepassings by temperature van 500 – 600 °C. Nuwe kaarte van Suid-Afrika is opgestel om te wys waar potensieel geskikte rots beskikbaar is. Doleriet, wat wyd in die Karoo voor kom, blyk om veral geskik te wees. Wrywingsfaktore is gemeet vir beddens van verskillende partikels om die belangrikheid van grofheid, vorm en pak-rangskikking te bepaal. Vyf rotsstelle is ook getoets, wat n saamgestelde datastel gee wyer as in enige gepubliseerde studie. Beperkings van bestaande korrelasies word aangetoon. Die wrywingsfaktor is hoogs sensitief vir partikelvorm en, in die geval van asimmetriese partikels, pakkings metode. Die wrywingsfaktor het met tot 70 % gevarieer vir gebreekte rots, afhanklik van die rigting waarin dit in die toetsseksie neergelê is. Dit is waarskynlik veroorsaak deur die oriëntasie van die asimmetriese rots relatief tot die lugvloei rigting, en is nie voorheen vir rotsbeddens gerapporteer nie. Nuwe isotermiese korrelasies wat gebruik maak van die volume-ekwivalente partikel deursnee word gegee: hulle voorspel binne 15 % van die gemete waardes. Hierdie werk sal n tegno-ekonomiese studie van rotsbeddens toelaat wat meer akkurate voorspellings van pompdrywing gebruik as voorheen moontlik was. Termiese toetse onder 80 °C wys dat die warmteoordrag nie baie sensitief is vir partikelvorm en -tipe nie. n Warmte-oordragskorrelasie vir lug in terme van die volume-ekwivalente deursnee is ontwikkel en met die E-NTU-metode gekombineer. Die voorspelde lug uitlaat temperatuur is binne 5 °C van die meting vir toetse by 530 °C. Dit wys dat termiese geleiding en straling redelikerwys buite rekening gelaat kan word vir n enkele laai/ontlaai siklus by massa vloeitempos van omtrent 0.2 kg/m2s. n Oorspronklike metode vir die bepaling van die optimum partikelgrootte en bedlengte word gegee: Die Biot-getal is vas, en die netto inkomste (die inkomste minus die bed omkoste) van n stoomsiklus voorsien met warmte van die bed word bereken. n Vereenvoudigde berekening wat die metode gebruik wys dat die optimum grootte en lengte ongeveer 20 mm en 6-7 m is. Afhangende van die behoueringsontwerp en koste, kan die kapitale koste n orde kleiner wees as dié van n gesmelte nitraatsout stelsel

The percentage of renewable energy within the global electric power generation portfolio is expected to increase rapidly over the next few decades due to increasing concerns about climate change, fossil fuel costs, and energy security. Solar thermal energy, also known as concentrating solar power (CSP), is emerging as an important solution to new demands for clean, renewable electricity generation. Dish-Stirling (DS) technology, a form of CSP, is a relatively new player in the renewable energy market, although research in the technology has been ongoing now for nearly thirty years. The first large plant utilizing DS technology, rated at 1.5 MW, came online in January 2010 in Peoria, AZ, and plants rated for several hundred MW are in the planning stages. Increasing capacity of this technology within the utility grid requires extensive dynamic simulation studies to ensure that the power system maintains its safety and reliability in spite of the technological challenges that DS technology presents, particularly related to the intermittency of the energy source and its use of a non-conventional asynchronous generator. The research presented in this thesis attempts to fill in the gaps between the well established research on Stirling engines in the world of thermodynamics and the use of DS systems in electric power system applications, a topic which has received scant attention in publications since the emergence of this technology. DS technology uses a paraboloidal shaped dish of mirrors to concentrate sunlight to a single point. The high temperatures achieved at the focal point of the mirrors is used as a heat source for the Stirling engine, which is a closed-cycle, external heat engine. Invented by the Scottish clergyman Robert Stirling in 1816, the Stirling engine is capable of high efficiency and releases no emissions, making it highly compatible with concentrated solar energy. The Stirling engine turns a squirrel-cage induction generator, where electricity is delivered through underground cables from thousands of independent, autonomous 10-25 kW rated DS units in a large solar farm. A dynamic model of the DS system is presented in this thesis, including models of the Stirling engine working gas and mechanical dynamics. Custom FORTRAN code is written to model the Stirling engine dynamics within PSCAD/EMTDC. The Stirling engine and various other components of the DS system are incorporated into an electrical network, including first a single-machine, infinite bus network, and then a larger 12-bus network including conventional generators, loads, and transmission lines. An analysis of the DS control systems is presented, and simulation results are provided to demonstrate the system's steady state and dynamic behavior within these electric power networks. Potential grid interconnection requirements are discussed, including issues with power factor correction and low voltage ride-through, and simulation results are provided to illustrate the dish-Stirling system's capability for meeting such requirements.

Master of Agribusiness / Department of Agricultural Economics / Jeffery R. Williams / The objective of this thesis is to conduct a net present value analysis of installing a solar power generation system on company owned turkey grow out ranches. This research project provides information regarding the systems power production capacity, investment cost, maintenance requirements, amount of energy saved, useful life of the equipment, marginal state and federal tax brackets for the company. The investment cost of the system includes the price of the equipment and installation service. Many of the system costs may be offset by rebates, tax credits and grants from various government agencies. These must also be included in the financial analysis as they can greatly affect the financial viability of the project. The system is projected to have a useful life of 30 years with an inverter replacement planned for year 15. Four scenarios were evaluated using two levels of rebates and two electrical rate inflation levels. The evaluations conducted showed positive after tax NPV evaluations on three of four scenarios reviewed with the most financially attractive options available when the rebates, tax credits and grants were maximized. This was the case at both electrical rate inflation scenarios. These same scenarios produced favorable results when looking at reduction of live production ranch costs. The system effectively locked in electrical rates below current rates for the 30 year life of the system. This reduced ranch live production cost by as much as 11.73 percent. It also gives the company an advantage over the competition when used as a marketing tool due to the use of green technology in company production practices.

Poster exhibited at GPSC Student Showcase, February 24th, 2016, University of Arizona. / To make photovoltaics (PV) and concentrated solar thermal power (CSP) more practical forms of alternative energy, creative innovations to current solar energy methods must be employed. The PVMirror – a new technology – is our solution to that problem. The PVMirror combines PV and CSP technologies by splitting the spectrum of sunlight using a dichroic mirror film. Light that is not transmitted to the solar cell is to instead be reflected to a focus, by way of a curved mirror. This hybrid of technologies is competitive, as it is both efficient and affordable compared to many other innovations in renewable energy. The PVMirror is designed to be an easy and cost-effective replacement to the reflectors used in existing CSP plants. Ultimately, we hope to demonstrate this PVMirror technology on a large scale and market it. As validated from interviewing with industry professionals, this technology has the potential to impact the world of solar energy because many industrial companies and utility-scale CSP project developers are interested in pairing CSP and PV to increase efficiency. Currently, using an outdoor sun tracker, we are testing an 18 inch by 18 inch prototype with four solar cells.

Thesis (MScIng)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: This study investigated the flow field above a proposed solar chimney power plant, without a cross wind, using a commercial numerical solver in the form of CFX 4-4 by AEA Technology plc. The governing equations solved are for an incompressible steady state solution. Variation in density due to buoyant effects is modelled with the Boussinesq approximation, and turbulence is approximated by the k-ε model with modifications due to buoyancy. The effect of different turbulence conditions at atmospheric inflows is also investigated. Modifications to the k-ε turbulence model in the context of atmospheric turbulence are investigated and applied to the standard k-ε model. These modifications include the addition of source terms in the turbulence kinetic energy and the turbulence dissipation rate equations to allow for the production/destruction of turbulence due to buoyancy. Other modifications include an additional source term in the turbulence dissipation rate equation accounting for atmospheric stability and the specification of model constants relevant to atmospheric flows. Initial results for the flow field using the Boussinesq approximation show reasonable correlation between the current study and the study by Thiart (2002) whereby the flow field exhibits characteristics of the axially symmetric turbulent jet. One of the primary and most noticeable differences between the current study and that of Thiart (2002) is the difference in height at which air is drawn into the collector. In order to account for variation in density with height, a modification on the Boussinesq approximation, the Deep Boussinesq model, Montavon (1998), is applied to a simplified 100 m by 100 m rectangular geometry. The results obtained are compared to a similar model using the Boussinesq approximation and it is seen that the average velocities reached in the deep Boussinesq model are significantly larger than those obtained in the Boussinesq approximation. / AFRIKAANSE OPSOMMING: Hierdie studie ondersoek die vloeiveld bo 'n aangewese son-skoorsteen kragstasie, deur die gebruik van 'n kommersiële numeriese oplosser in die vorm van CFX 4-4; van AEA Technology plc. Die heersende vergelykings wat opgelos moet word, is vir 'n niesaampersbare, tyd-onafhanklike oplossing. Die wisseling in digtheid as gevolg van saamdrukbaarheids effekte, word gemodelleer met die Boussinesq benadering en turbulensie-skatting deur die k-ε model met aanpassings as gevolg van dryfkrag. Die effek van die verskillende turbulensie-toestande teen atmosferiese invloeing word ook ondersoek. Aanpassings op die k-ε turbulensie-model in die konteks van atmosferies turbulensie word ondersoek en toegepas op die standaard k-ε model. Hierdie veranderinge sluit die byvoeging van bron terme in die turbulente kinetiese energie en die turbulensieverspillings vergelykings om ruimte te laat vir die produksie/afbreking van turbulensie as gevolg van dryfkrag in. Ander aanpassings sluit in 'n bykomende bron term in die turbulensie-verspillings vergelyking wat rekenskap gee vir atmosferiese stabiliteit en die spesifikasie van model onveranderlikes met betrekking tot atmosferiese vloei. Aanvanklike resultate vir die vloeiveld met die gebruik van die Boussinesq benadering dui op geredelike ooreenstemming tussen de huidige studie en die studie van Thiart (2002), waar die vloeiveld eienskappe toon van 'n simmetriese turbulente spilpunt. Een van die mees primêre en duidelikste verskille tussen die huidige studie en diè van Thiart (2002) is die verskil in die hoogte waarteen die lug in die samesteller ingesuig word. Om rekenskep te kan gee vir die verskil in digtheid met betrekking tot hoogte is 'n aanpassing van die Boussinesq benadering, die Diep Bussinesq model, Montavon (1998), aangebring. Hierdie aanpassing word toegepas op ‘n vereenvoudigde 100 m by 100 m reghoek. Die resultate wat verkry word, word vergelyk met 'n soortgelyke model waar die algemene snelheid wat met die diep Boussinesq model bereik word merkbaar groter is as die in die Boussinesq benadering.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2000. / A solar chimney power plant consists of a central chimney that is surrounded by a transparent canopy located a few meters above ground level. The ground beneath this canopy or collector as it is known is heated by the solar radiation that is effectively trapped by the collector. This in turn heats the air in the collector, which flows radially inwards towards the chimney. This movement is driven by the difference between the hydrostatic pressure of the air inside- and outside the solar chimney system. The energy is extracted from the air by a turbine driven generator situated at the base of the chimney. The performance of such a solar chimney power plant is evaluated in this study making use of a detailed mathematical model. In this model the relevant discretised energy and draught equations are deduced and solved to determine the performance of a specific plant referred to as the "reference plant". This plant is to be located at a site near Sishen in the Northern Cape in South Africa where meteorological data is available. The performance characteristics of this plant are presented using values from the 21 st of December as an example. These characteristics include the instantaneous and integrated power output, as well as the absorption of the solar radiation of each of the parts of the collector. The air temperatures throughout the plant and the convective heat transfer coefficients in the collector in the region of developing and fully developed flow are presented. The pressure of the air throughout the system is presented as well as the pressure drop over the turbine. Temperature distributions in the ground below the collector are also presented and discussed.