Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Central receiver power plants generate renewable electricity by exploiting the energy provided by the sun. The conditions experienced in the Northern Cape region of South Africa provide the ideal conditions for the development of these plants. Without a storage medium these plants have capacity factors in the range of 25-30%. The inclusion of a thermal energy storage medium provides the ability to increase the capacity factors of these plants. Although storage increases the costs, it results in better utilisation of the power block and a decrease in the levelised electricity cost (LEC). Eskom intends building a 100MWe central receiver dry cooled power plant in the Upington region. This research identifies the appropriate storage medium and ideal storage capacity to achieve the lowest LEC. A literature survey was performed to identify the different methods of storage that are available. The different storage methods were evaluated and the best storage medium for a central receiver power plant based on the developments of the various storage technologies was identified. To determine the costs associated with a central receiver power plant, data published by NREL was used. Different plant parameters were required to evaluate the costs. A power plant model based on efficiencies and energy balances was created to determine the required plant parameters. It provided the ability to determine the effect of changing different plant parameters on the LEC and estimate the plant output. The power block parameters were initially varied to determine the most efficient power block configuration. Once the most efficient power block configuration was identified the solar field and storage parameters were varied to determine the plant configuration which resulted in the lowest LEC. The most efficient power block configuration of 0.4206 was found for a system comprising of six feedwater heaters with the feedwater temperature of 230°C, main steam pressure 140 bar and an exit steam generator salt temperature of 290°C. A solar multiple of 3.0 with 16 hours of storage resulted in a LEC of R1.41/kWh with no system constraints. A capacity factor constraint of 60% resulted in a solar multiple of 1.8 with 8 hours of storage and a LEC of R1.78/kWh. / AFRIKAANSE OPSOMMING: Sonkragaanlegte met sentrale ontvangers wek hernubare elektrisiteit op deur sonenergie te ontgin. Die klimaat in die Noord Kaap-streek van Suid-Afrika is ideaal vir die oprigting van hierdie aanlegte. Sonder ’n bergingsmedium is die kapasiteitsfaktore van sulke aanlegte ongeveer 25-30%. Met die insluiting van ’n bergingsmedium vir termiese energie kan die kapasiteitsfaktore egter verhoog word. Hoewel berging aanlegkoste verhoog, lei dit terselfdertyd tot beter aanwending van die kragblok en ’n afname in die konstante eenheidskoste van elektrisiteit (LEC). Eskom beplan om ’n droogverkoelde kragaanleg van 100 MW met ’n sentrale ontvanger in die Upington-streek op te rig. Hierdie navorsing was dus daarop toegespits om die mees geskikte bergingsmedium en ideale bergingskapasiteit te bepaal om die laagste moontlike LEC uit die aanleg te verkry. ’n Literatuurstudie is onderneem om die verskeie beskikbare bergingsmetodes te bestudeer. Die verskillende metodes is beoordeel, waarna die beste bergingsmedium vir ’n kragaanleg met ’n sentrale ontvanger op grond van die ontwikkelings in die verskillende bergingstegnologieë bepaal is. Om die koste van ’n kragaanleg met ’n sentrale ontvanger te bepaal, is gepubliseerde data van die Amerikaanse Nasionale Laboratorium vir Hernubare Energie (NREL) gebruik. Verskillende aanlegparameters was egter nodig om die koste te beoordeel. Dié parameters is gevolglik bepaal deur ’n kragaanlegmodel op grond van doeltreffendheidsfaktore en energiebalanse te skep. Sodoende kon vasgestel word watter uitwerking veranderinge in die verskillende parameters op die LEC sou hê, en kon die aanleguitset geraam word. Die kragblokparameters is aanvanklik afgewissel om die doeltreffendste kragbloksamestel te bepaal. Nadat dít bepaal is, is die sonenergieveld en bergingsparameters weer afgewissel om vas te stel watter aanlegsamestel die laagste LEC tot gevolg sou hê. Die beste termiese benuttingsgraad is behaal vir ʼn stoom siklus met ses water verhitters en ʼn water temperatuur van 230 °C by die ketel se inlaat, ʼn stoom druk van 140 bar, en sout uitlaat temperatuur van 290 °C. ʼn Vermenigvuldigingsfaktor van drie vir die heliostaat veld, en 16 uur termiese energie storing gee ʼn opwekkingskoste van R 1.41/kW/h indien daar geen beperkings op die grootte of koste van die stelsel geplaas word nie. Indien die kapitaal uitgawe ʼn perk van 60 % op die kapasitiet van die stelsel plaas, verander die optimale ontwerpspunt na ʼn vermenigvuldigingsfaktor van 1.8, en die termiese stoorkapasitiet verlaag na 8 uur. In hierdie geval is die opwekkingskoste R 1.78/kWh.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2007. / The focus of this project was to investigate the working of a liquefied gas micro satellite thruster. An introduction is given in which the significance of the project in relation to the literature is stated. The objectives of the project are also stated. In the literature survey the historical development and design specifications of some relevant thruster systems is discussed. An experimental model was designed and built to test the working of a thruster system. Attention is also given to the measurement and calibration techniques used to obtain experimental data. A computer program was written to simulate the thruster system. The experimental set-up was designed so that an accumulator could be charged with liquid butane from a storage tank. The accumulator was charged with 13 ml of liquid butane, which was heated and then exhausted through a nozzle. Copper mesh was placed in the accumulator to improve the heat transfer to the butane vapour before it was exhausted through the nozzle. A cantilever beam was used to measure the thrust of the system. The system was tested under atmospheric conditions of 100 000 Pa as well as under vacuum conditions of 20 Pa. Two nozzles were also tested: nozzle-1 with a throat diameter of 1 mm and an exit diameter of 5 mm and nozzle-2 with a throat diameter of 1 mm and an exit diameter of 1.6 mm. A computer program was written to simulate the flow of the butane vapour through the nozzle, as well as the complex two-phase behaviour of the butane in the accumulator. Traditional gas dynamic theory was used to model the flow through the nozzle. The transient behaviour of the system was modelled to predict the rate of liquid to vapour mass transfer in the accumulator. Additionally, the computer program was developed to simulate the system with copper mesh placed in the accumulator. From the experimental results it was shown that the addition of copper mesh in the accumulator improved the total thrust achieved with a 13 ml charge of liquid butane by more than 50 %. Under atmospheric conditions shockwaves were present in both of the two nozzles tested. Nozzle-2 showed an increase of 91 % in the total thrust achieved over a 5 second burst compared to the total thrust achieved using nozzle-1. With no copper mesh in the accumulator and using nozzle-1 a peak thrust of 39 mN was achieved under atmospheric conditions while under vacuum conditions a peak thrust of 495 mN was achieved. This resulted in a total thrust of 0.365 Ns under atmospheric conditions and 4.88 Ns under vacuum conditions with a 13 ml charge of liquid butane. Using the total thrust achieved the specific impulse of the system was calculated as 5 seconds under atmospheric conditions and 67.5 seconds under vacuum conditions with no mesh in the accumulator and using nozzle-1. The theoretical model compared well with the experimental results except when nozzle-1 was modelled under atmospheric conditions. Under vacuum conditions the results obtained from the theoretical model compared well with the experimental results using both of the nozzles. In the modelling of the mesh in the accumulator an overall heat transfer factor was incorporated into the model to take into account the uncertainty of the heat transfer area as well as the overall heat transfer coefficient. The theoretical model and experimental test results are discussed and thereafter conclusions are also drawn. There are also recommendations made for future work that could be done in the further development of a liquefied gas micro satellite thruster system. It is recommended that a “resistojet” type thruster should be tested at the University of Stellenbosch and that further testing be done with mesh in the accumulator to find the optimum amount of mesh that should be placed in the accumulator.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2007. / Air-cooled steam condensers (ACSC’s) are increasingly employed to reject heat in modern power plants. Unfortunately these cooling systems become less effective under windy conditions and when ambient temperatures are high. A better understanding of the fundamental airflow patterns about and through such air-cooled condensers is essential if their performance is to be improved under these conditions. For known flow patterns, improved fan designs are possible and flow distortions can be reduced by means of extended surfaces or skirts, windwalls and screens. Spray cooling of the inlet air or the addition of an evaporative cooling system can also be considered for improving performance under extreme conditions. The present numerical study models the air flow field about and through an air-cooled steam condenser under windy conditions. The performance of the fans is modeled with the aid of a novel numerical approach known as the “actuator disc model”. Distorted airflow patterns that significantly reduce fan performance in certain areas and recirculatory flows that entrain hot plume air are found to be the reasons for poor ACSC performance. It is found that the reduction in fan performance is the main reason for the poor ACSC performance while recirculation of hot plume air only reduces performance by a small amount. Significant improvements in ACSC performance are possible under these conditions if a cost effective skirt is added to the periphery of the ACSC while the installation of a screen under the ACSC has very little effect.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2006. / Successful housing and breeding of exotic animals or plants often requires an environment that is quite different to the ambient conditions present. The current study approached the problem of sub-optimal water temperatures experienced by Central African Bream (Tilapia) housed within a South African greenhouse during winter months. A theoretical and experimental study of fundamental heat and mass transfer processes relevant to an aquacultural greenhouse was conducted. Experimental results were generally in agreement with those of previous researchers; while evaporation tests were found to concur particularly well with an analytical equation developed. The experimental results were used to develop a simple glass greenhouse model to evaluate the expected thermal behaviour during the coldest time of the year. Manipulation of the model revealed that water has the ability to absorb large quantities of solar radiation and regulate temperature fluctuations within such a system, and that the appropriate use of thermal insulation during both the night and day can maintain acceptable water temperatures for extended periods of time. With the conclusions drawn from the experimentation and modelling done, an optimised conceptual greenhouse design was presented, along with associated guidelines and principles for attaining the required water temperatures, and consequently providing the exotic fish specie with a healthy environment.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2005. / Parabolic trough solar collectors (PTSCs) constitute a proven source of thermal energy for industrial process heat and power generation, although their implementation has been strongly influenced by economics. In recent years, environmental concerns and other geopolitical factors have focused attention on renewable energy resources, improving the prospects for PTSC deployment. Further work is needed to improve system efficiencies and active areas of research include development of advanced heat collecting elements and working fluids, optimisation of collector structures, thermal storage and direct steam generation (DSG). A parabolic trough collector, similar in size to smaller-scale commercial modules, has been developed locally for use in an ongoing PTSC research programme. The aim of this study was to test and fully characterise the performance of the collector. Specialised logging software was developed to record test data and monitor PTSC performance in real-time. Two heat collecting elements were tested with the collector, one unshielded and the other with an evacuated glass cover. Testing was carried out according to the ASHRAE 93-1986 (RA 91) standard, yielding results for the thermal efficiency, collector acceptance angle, incidence angle modifier and collector time constant. Peak thermal efficiency was 55.2 % with the unshielded receiver and 53.8 % with the glass-shielded unit. The evacuated glass shield offered superior performance overall, reducing the receiver heat loss coefficient by 50.2 % at maximum test temperature. The collector time constant was less than 30 s for both receivers, indicating low thermal inertia. Thermal loss tests were conducted and performance of the trough’s tracking system was evaluated. The measured acceptance angles of 0.43° (unshielded) and 0.52° (shielded) both exceeded the tracking accuracy of the PTSC, ensuring that the collector operated within 2 % of its optimal efficiency at all times. Additionally, experimental results were compared with a finite-volume thermal model, which showed potential for predicting trough performance under forced convection conditions.

MScEng / This project entails the design and development of a new 2-DOF articulated dump truck (ADT) suspension seat. A study of the ADT vibration environment was conducted using data measured with accelerometers inside the cabin. With the system’s required operational capabilities determined, the concept design phase resulted in a feasible concept. The first prototype was manufactured based on the initial set of specifications. A variety of numerical modelling techniques were used to analyse and evaluate the seat’s dynamic response. Vertical and lateral laboratory tests of the suspension seat with human occupants were completed. The vertical lumped parameter model of the suspension seat with a human occupant gave good correlation with the laboratory measured frequency response. A broad band input signal, and not the ISO 7096 (2000) EM1 signal, was used to obtain the frequency response used to verify the lumped parameter model. The SEAT values for the ISO 7096 (2000) EM1 signal and various ADT road conditions were calculated using the lumped parameter models for a small, medium and large subject, the same three subjects used in the laboratory tests. SEAT values using the ISO 7096 (2000) EM1 signal of 0.94, 0.93 and 0.88 were obtained for the small, medium and large subjects. The lowest SEAT values obtained using the road data were 0.63, 0.56 and 0.48 for the small, medium and large subjects. The transmissibility curves determined from the lateral laboratory tests were used to calculate the SEAT values for the lateral ADT cabin vibrations. The lowest SEAT values obtained were 0.83, 0.83 and 0.82 obtained for the small, medium and large subjects. After all the results from the testing and modelling were evaluated the design was assessed. All the data and information collected was used as input for the design of a second prototype, which was not manufactured. Not all the set specifications were achieved for the first prototype, but the new suspension seat gave comparable vertical vibration isolation performance to that of expensive commercially available ADT suspension seats. The lateral suspension demonstrated good lateral vibration isolation and is a feature not currently available in current ADT suspension seats.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2005. / The objective of the thesis is to improve the performance prediction of axial compressors, using a streamline throughflow method (STFM) code by modelling the hub and casing wall boundary layers, and additional flow mechanisms that occur within a blade row passage. Blade row total pressure loss and deviation correlations are reviewed. The effect of Mach number and the blade tip clearance gap are also reviewed as additional loss sources. An entrainment integral method is introduced to model the hub and casing wall boundary layers. Various 1-dimensional test cases are performed before implementing the integral boundary layer method into the STFM. The boundary layers represent an area blockage throughout the compressor, similar to a displacement thickness, but affects two velocity components. This effectively reduces the compressor flow area by altering the hub and casing radial positions at all stations. The results from the final STFM code with the integral boundary layer model, Mach number model and tip clearance model is compared against high pressure ratio compressor test cases. The blockage results, individual blade row and overall performance results are compared with published data. The deviation angle curve fits developed by Roos and Aungier are compared. There is good agreement for all parameters, except for the slope of deviation angle with incidence angle for low solidity. For the three compressors modelled, there is good agreement between the blockage prediction obtained and the blockage prediction of Aungier. The NACA 5-stage transonic compressor overall performance shows good agreement at all speeds, except for 90% of design speed. The NACA 10-stage subsonic compressor shows good agreement for low and medium speeds, but needs improvement at 90% and 100% of design speeds. The NACA 8-stage transonic compressor results compared well only at low speeds.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2006. / The main objective of this thesis was to design a hydrofoil system without a trim and ride height control system and investigate the change in resistance of a representative hull across a typical speed range as a result of the addition of the hydrofoil system, while retaining adequate stability. The secondary objectives were as follows: Find a representative hull of sailing catamarans produced in South Africa, and to establish an appropriate speed range for that hull across which it is to be tested. Test and explain the drag characteristics of this hull. Find a suitable configuration of lifting foils for this hull that would not require any form of trim or ride height control to maintain stability throughout the speed range. Test and compare the resistance characteristics with and without the assistance of lifting foils. Test and explain the effects of leeway and heel on the total hydrodynamic resistance both with and without lifting foils. A representative hull (RH1), based on a statistical analysis of sailing catamarans produced in South Africa and an existing hull design of suitable size, was designed. A speed range was then established (0 – 25 knots) based on the statistics of the original (existing) design. A scaled model (of RH1) of practical and suitable dimensions was designed and manufactured, and its characteristics determined through towing tank testing. A hydrofoil system was then designed and during testing, was adjusted until a stable configuration was found. This resulted in a canard type configuration, with the front foil at the bow and the main foil between the daggerboards. Although a stable configuration was achieved, it was noted that any significant perturbation in the trim of the boat would result in instability and some form of trim control is recommended. The main objective was achieved. The experimental results concluded that a canard configuration was found to be stable for the RH1 (foil positioning already mentioned) and the addition of the hydrofoils provided a significant improvement only above a displacement Froude number of 2, which for our full scale prototype, is equivalent to approximately 14.2 knots. This is in agreement with the results of several other research projects that investigated hydrofoil supported catamarans with semi‐displacement type demi‐hulls. Below displacement Froude number of 2, a significant increase in total hydrodynamic resistance was observed. Since the speed of sailing craft is dependent on wind speed, there will often be conditions of relatively low boat speed (below displacement Froude number of 2). So it was recommended that a prototype design would have a retractable hydrofoil system which could be engaged in suitable conditions (sufficient boat speed). The effects of leeway and heel on the total hydrodynamic resistance were determined experimentally, but it was found that these trends were affected by the resulting changes in wave interference resistance. Since wave interference depended strongly on the hull shape, it was therefore concluded that no universal trends can be determined regarding the effects of heel and leeway on the total hydrodynamic resistance. These effects were determined for RH1 and it was shown that these effects are drastically altered by the addition of the lifting foils.

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.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2005. / This work is concerned with the design and hydrodynamic aspects of a hydrofoil-assisted trimaran. A design and configuration of a trimaran is evaluated and the performance of a hydrofoil-assisted trimaran is effectively compared to the performance of a hydrofoil-assisted catamaran with similar overall displacement and same speed. The performance of the trimaran with different outrigger clearances are also evaluated and compared. The hydrodynamic aspects focuses mainly on the performance and to a lesser extend on the sea-keeping and stability of a hydrofoil-assisted trimaran. The results were determined by means of experimental testing, theoretical analysis and numerical analysis. The project was initiated as a result of the success of the hydrofoil-assisted catamarans and due to the fact that there does not exist a hydrofoil-assisted trimaran (to the author’s knowledge) where the main focus of the foils is to significantly reduce the resistance. A brief history, recent developments and associated advantages regarding trimarans are discussed. A complete theoretical model is presented to evaluate the lift and drag of the hydrofoils, as well as, the resistance of the trimaran. The data so obtained is then used to compare the reliability and feasibility of the numerical and experimental predicted values. The design of the trimaran and hydrofoil system is explained, together with the problems associated with the final design of the trimaran. The design of a trimaran is much more complicated than a catamaran due to more design variables being associated with trimarans. The selection of the trimaran configuration is done in a logical manner considering stability and hydrodynamics. However, the hydrofoil-assisted trimaran is closely adapted to the main dimensions of the comparable hydrofoil-assisted catamaran. An in-depth discussion of the testing technique used and the problems that are associated with towing tank testing will facilitate similar tests in the future. The scaling method of Froude was modified to account for the different sized hulls. The numerical methods are explained, with emphasis on accuracy, limitations, feasibility and the time required to complete a calculation. The results are presented in an order suggested by the experimental and numerical work carried out. The resistance, trim and rise/sinkage results are presented with speed for both the trimaran and catamaran with and without the addition of foils. The addition of the foils supplies results based on the amount of lift the foils carry and therefore can easily clarify the significant resistance advantage the foils offer the trimaran and the catamaran evaluated in this project. The final design and results of the evaluated trimaran are discussed. It is concluded that the catamaran with similar displacement and speed is still superior to the trimaran, with and without foils in both cases. The addition of foils to the trimaran does decrease the resistance significantly. The conclusions regarding these results are presented, together with recommendations for future work.