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, 2012. / ENGLISH ABSTRACT: This dissertation is basically a summary, with some interpretation, of published research by the author. The scope is limited to the fields of turbo-machinery, computational fluid dynamics and solar chimney power plants. The main contribution in the field of turbo-machinery in general is in the development of a through-flow method that automatically satisfies mass conservation. Concerning fan design, the contributions are the realization of the importance of the exit kinetic energy in the determination of the efficiency of rotor-only axial flow fans, and the quantification of the effect of off-axis inflow into cooling system fans on their performance. In the field of centrifugal fans and compressors an original, unifying model for the prediction of slip factor was developed. To investigate accident scenarios in closed cycle gas turbine nuclear reactors, all possible operational modes of multi-stage axial compressor operation caused by flow and rotation direction were investigated experimentally and computationally. Spanning the fields of turbo-machinery and solar chimneys, the basic theory of solar chimney turbines was developed, showing that high turbine efficiency was possible. In the field of solar chimneys, an original thermodynamic approach was developed to predict the main relationships that govern solar chimney performance, and to solve the through-flow equations for non-ideal systems with losses. Equations for the accurate determination of all the thermodynamic variables in a solar chimney as dependent on chimney height, wall friction, additional losses, internal drag and area change were derived and solved. Coefficients of wall friction, bracing wheel loss and exit kinetic energy were determined experimentally, and empirical equations were developed to predict the loss coefficient of the collector to turbine transition section and and the turbine inlet flow angle. A simple power law approach allowed the calculation of the optimal turbine pressure drop in solar chimney power plants. A comparison of two sets of equations used to calculate the heat fluxes into, inside and leaving the solar collector, resulted in similar air temperature rises in the collector, and similar produced power. It turned out however that the optimal flow for minimal turbine pressure drop was dependent on the heat transfer models. Investigation of the performance of various solar chimney turbo-generator layouts using analytical models and optimisation techniques showed that the optimal number of turbines varies with plant size, but the individual turbine size, the number of blades and even the efficiency remains close to constant. It was found that the cost of a turbogenerator system, however, varies significantly with size. A joint paper with several German universities and institutions did a comparative cost analysis of solar chimney power plants / AFRIKAANSE OPSOMMING: Hierdie verhandeling is basies ’n opsomming, met interpretasie, van gepubliseerde navorsing deur die outeur. Die omvang is beperk tot die gebiede van turbomasjinerie, berekeningsvloeidinamika en sonskoorsteenkragstasies. Die hoof bydrae op die gebied van turbomasjinerie in die algemeen is in die ontwikkeling van ’n deurvloeimetode wat outomaties massabehoud bevredig. Wat waaierontwerp betref is die bydrae die besef van die belangrikheid van die uitlaat kinetiese energie in die bepaling van waaierbenuttingsgraad, en die kwantifisering van die effek van af-as invloei in verkoelingswaaiers op hulle gedrag. Op die gebied van sentrifugaalwaaiers en -kompressors is ’n oorspronklike, samevattende model vir die voorspelling van glipfaktor ontwikkel. Om ongeluk-scenario’s in geslote kringloop gasturbine kenreaktors te ondersoek is al die moontlike werksmodusse veroorsaak deur vloei en rotasie rigting van ’n multistadium aksiaalkompressor eksperimenteel en numeries ondersoek. As brug tussen turbomasjinerie en sonskoorstene is die basiese teorie van sonskoorsteenturbines ontwikkel met die aanduiding dat hoë turbine benuttingsgraad moontlik is. Op die gebied van sonskoorstene is ’n oorspronklike termodinamies benadering ontwikkel om die hoofverwantskappe te voorspel wat sonskoorsteen gedrag bepaal, en om die deurvloei vergelykings op te los vir nie-ideale stelsels met verliese. Vergelykings vir die akkurate bepaling van al die termodinamiese veranderlikes in ’n sonskoorsteen soos afhanklik van skoorsteenhoogte, wandwrywing, bykomstige verliese, interne sleur en oppervlakte verandering is afgelei en opgelos. Koëffisiënte vir wandwrywing, verstywingswiel-verlies en uitlaat kinetiese energie is eksperimenteel bepaal, en empiriese vergelykings is ontwikkel om die verlieskoëffisiënt van die kollektor-tot-skoorsteen oorgang en die turbine inlaatvloeihoek te bepaal. ’n Eenvoudige magswet benadering het dit mootlik gemaak om die optimum turbine-drukval in sonskoorsteen aanlegte te bepaal. ’n Verglyking van twee stelle vergelykings om warmtevloede in, binne en uit die sonkollektor te bereken het gelei na soortgelyke temperatuurstygings en gelewerde drywing. Die optimale vloei vir maksimum drywing was egter afhanklik van die warmteoordrag modelle. Ondersoek van die gedrag van verskeie turbo-generator uitlegte, deur gebruik van analitiese modelle en optimeringstegnieke het getoon dat die optimale aantal turbines wissel met aanleg grootte, maar die individuele turbine grootte, die aantal lemme en selfs die benuttingsgraad bly feitlik konstant. Daar is egter gevind dat die koste van ’n turbogenerator stelsel beduidend wissel met grootte. ’n Gesamentlike artikel met verskeie Duitse universiteite en instansies het ’n vergelykende koste analise van sonskoorstene gedoen.