4. Electronic Theses and Dissertations (ETDs) - Faculties submissions
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Item Assessment of DC-DC Converter Selection Metrics(University of the Witwatersrand, Johannesburg, 2024) Letsoalo, Future Malekutu; Hofsajer, IvanThe exponential growth of Internet of Things (IoT) devices, powered by diverse energy sources, poses significant challenges in power electronics. Despite advances in DC-DC converter topologies, a gap remains in the literature regarding standardized performance metrics for selecting suitable converters, making the selection process complex. This study critically assesses metrics from seminal works of the 1960s to contemporary state-of-the-art, proposing a systematic approach to converter assessment. Two major categories of metrics are identified: averaging metrics and waveform-preserving metrics. Averaging metrics, grounded in Wolaver's foundational work, are effective for high-level comparisons among many converter options, establishing a performance baseline. The study introduces an average modeling tool to reveal core converter characteristics for objective comparison. Waveform-preserving metrics, on the other hand, provide detailed performance insights and are suitable for a narrower set of converter options. The study further categorizes these metrics to assess converter switches and reactive components. A new RMS metric is proposed, refining the existing processed power metric for better accuracy. By integrating both averaging and waveform-preserving metrics at relevant design stages, this study offers a systematic framework for converter assessment. This approach bridges the gap between high-level comparison and detailed performance evaluation, facilitating informed decision-making in converter selection.Item Analysing Test-Driven Development Adherence in Open-Source Projects Using Test-to-Code Traceability Links(University of the Witwatersrand, Johannesburg, 2024) Kirui, Gerald Kipruto; Levitt, StephenSeveral studies have been conducted to determine the impact of Test-Driven Develop- ment (TDD) on software quality. Many studies utilise test-to-code traceability tools and strategies to detect TDD adherence as part of their methodology. However, most test-to-code traceability tools rely on filename-based matching algorithms, which suffer from low recall; therefore, most TDD detection methods are not accurate. This study aims to assess the effectiveness of a statement-based matching algorithm over a filename-based one and whether it can be used to detect the TDD adherence of a software project. The filename-based and statement-based matching algorithms are implemented in Python. To evaluate these algorithms, 500 tests from sixteen Java projects (encompassing frameworks, libraries, and tools for data processing, testing, and web services) are used. These projects range in size from 5,115 lines of code (LoC) to 378,167 LoC. This evaluation helps in understanding the performance of the algorithms through their weighted F1-scores. A mathematical function is created from first principles to correlate method coverage and time deltas with the test-with-development (TWD) adherence of a project. Thereafter, 100 Java projects are used to demonstrate the utility of this function. The results show that the statement-based matching algorithm, with a weighted F1-score of 0.771 and a 95% confidence interval of [0.741, 0.786], is more accurate than the filename-based matching algorithm, with a weighted F1-score of 0.218 and a 95% confidence interval of [0.193, 0.246]. Additionally, the results show that the relative TWD adherence of eleven projects is found to be highly correlated with TDD scores from surveys (rs = 0.723 and p-value = 0.012), and the method coverage of eighteen projects is found to be highly correlated with code coverage obtained from Codecov (rs = 0.744 and p-value = 0.0003). The literature review reveals that no studies have explored the correlation between TDD adherence and the development step size in software projects. An investigation is conducted using a statement-based test-to-code traceability tool and relative TWD adherence values to determine if TDD projects have smaller development steps compared to non-TDD projects. The study finds no significant correlation between median production method churn and relative TWD adherence of projects in all but one out of eight cases.Item A Study of the Effect of Temperature on Cavity Partial Discharges in Polyethylene (PE) Insulation(University of the Witwatersrand, Johannesburg, 2024) Khangale, Mulovhela Kennedy; Nyamupangedengu, CuthbertSynthetic Polymers such as polyethylene are prevalent for high-voltage insulation applications as they offer remarkable insulating and dielectric properties. Notwithstanding precautionary measures made during manufacturing and installation processes, insulation systems are always susceptible to defects for various reasons, which constitute a significant source of Partial Discharge (PD) activity. It is a precursor to insulation degradation leading to premature failure of high-voltage equipment. PD activity is complex due to its non-stationary behaviour and multi- variance dependence. Studies in partial discharge mechanisms have received significant attention over the years to improve phenomena understanding and, in some cases, to allow conclusions to be drawn on the parameters affecting PD mechanisms. These studies have shown that different mechanisms and parameters influence partial discharge activity. In this study , experimental and analytical modelling techniques are used to explore the behaviour of partial discharge mechanisms at varying temperatures. Experimental PD measurements were carried out in accordance with the IEC 60270 standard. A test voltage of 11 kV ac was used. The test temperatures studied were 15C, 40C, 50C, 60C, 70C, 80C and 90C. Test specimens with a cavity diameter of 2.5 mm were assembled using three 1.5 mm thick polyethylene sheets sandwiched between two flat brass electrodes. Partial discharge parameters such as the charge magnitude, inception voltage and PD phase resolved pattern (PDPRP) were measured and analysed at varying temperatures. For analytical modelling, the streamer-like discharge concept is adopted to model PDIV while the apparent charge magnitude is modelled based on the induced charge concept introduced by Pedersen in the 1980s. The curve fitting approach was adopted to replicate and explain the measured experimental data. Results showed that Partial Discharge Inception Voltage (PDIV) increased linearly with temperature for the entire test temperature range. PD charge magnitude initially decreased with temperature from 15°C to 60°C and then increased from 60°C to 90°C. The evolution of PD phase resolved pattern (PDPRP) with temperature was characterised by a turtle-like pattern at ambient temperature, which transitioned into a rabbit ear PDPRP as the temperature increased to 90°C. The findings are interpreted using the mean free path effect on ionisation probability as well as the residual charge dynamics in the cavity as a function of temperature. The overall conclusion is that in polyethylene, cavity discharge characteristics respond to temperature changes. The variations in PD characteristics iv are monotonous for PDIV and non-monotonous for apparent charge magnitude as well as PDPRP. The implications of the findings are that in PD diagnosis,temperature of the equipment under test must be taken into account in interpretation of PD measurements results.Item Characterisation of Standard Telecommunication Fibre Cables for Cost-Effective Free Space Optical Communication(University of the Witwatersrand, Johannesburg, 2024) Iga, Fortune Kayala; Cox, Mitchell A.In an era marked by an escalating demand for high-speed internet connectivity, optical communication plays a crucial role in meeting these needs. Free Space Optical (FSO) communication, which involves the wireless transmission of optical signals through the atmosphere, holds promise for extending existing fibre optic networks and connecting individuals beyond current coverage areas. Despite the potential, commercial FSO systems remain prohibitively expensive. A cost-effective FSO system can be achieved by utilising small form-factor pluggable (SFP) transceiver modules. These budget-friendly devices offer powerful transmit lasers and highly sensitive receiving photodiodes. To utilise these devices, optical signals are collimated out of a transmitting fibre into the atmosphere and coupled back into a receiving fibre. However, further investigation is needed to determine the optimal fibre cables for transmitting and receiving optical signals across the atmosphere to maximise received optical power and achieve efficient FSO communication. This study aims to characterise the light coupling performance of standard telecommunication fibre cables, with a focus on the optical power transmitted from and received by the fibre cable under atmospheric conditions. The methodology employed for characterising the power transmitted by the fibre cables involves mea- suring the optical power in the fundamental Gaussian mode. This mode optimises transmission through the atmosphere by minimising beam divergence. Subse- quently, light coupling from free-space is characterised by measuring the optical power coupled into the different fibre cables under non-ideal conditions, including misalignment and atmospheric turbulence. The findings of this research show notable correlations between the physical attributes of the fibre cables, namely refractive index profile, core size and numerical aperture, and their transmission and reception performances. The comprehensive characterisations of the standard fibre cables presented in this study provide insights into their suitability for distinct roles within a low-cost FSO system.Item Exploring the Performance of a PV-Supplied Cooking System with Hybrid Electric-Thermal Energy Storage(University of the Witwatersrand, Johannesburg, 2024) Chiloane, Learn LeagoThis research explores the extent to which the addition of a thermal storage tank can reduce the electrical cooking energy in solar Photo-Voltaic (PV) supplied cooking systems. The aim is to consider a smaller battery size to store the decreased electrical cooking energy and the rest of the solar energy is stored in a low-cost thermal storage tank. This is done to reduce the cost of the storage unit to address the challenges of clean-cooking access in rural African households without grid electricity. The energy performance of the system is evaluated considering a small-scale solar system that powers a storage water heater as the thermal storage tank and a slow cooker as the cooking appliance. A thermal-electrical analogy and experimental tests are employed to model and validate the effectiveness of the system under different cooking conditions. Rice is chosen as the cooking medium because it is the fastest growing staple in Africa, and its cooking process benefits from starting it with pre-heated water. It is shown that starting with pre-heated water accelerates the readiness of rice, and reduces the additional electrical energy to complete the cooking process which translates to a similar battery size reduction. The results from three cooking scenarios indicate that a hybrid cooking system incorporating a battery and thermal storage tank can reduce the battery capacity requirements by up to 24.3% for one meal, 14.7% for two meals, and 10.6% for three meals in comparison to a pure electrical cooking system with a battery only.Item Feasibility of region of interest selection preprocessing using a multi-photodiode fingerprint-based visible light positioning system(University of the Witwatersrand, Johannesburg, 2024) Achari, Dipika; Cheng, LingThis research presents a novel Multi-Photodiode Fingerprint-Based Visible Light Positioning (VLP) system aimed at improving the accuracy and reducing the computational expenses of indoor localization. The system leverages an advanced K-Nearest Neighbors (KNN) algorithm, enhanced by Signal Strength Clustering, alongside a region selection strategy based on frequency-modulated VLC encoded IDs. Through extensive simulations, the system demonstrated a notable reduction in Mean Absolute Error (MAE) to approximately 2.5 meters, with a Root Mean Square Error (RMSE) of around 3.0 meters. In addition, the system exhibited robustness across varying ambient light conditions and room sizes, maintaining an accuracy rate of 95%, even in challenging environments. Analysis revealed that error rates increased in larger rooms, with average errors ranging from 1.50 meters in smaller spaces to 3.51 meters in larger environments. This suggests that while the system is effective in smaller areas, its accuracy diminishes slightly as room size expands. However, integrating frequency domain analysis and region of interest (ROI) selection proved to be a practical approach, enhancing the overall performance of the VLP system by providing faster and more accurate indoor navigation. Future research includes exploring advanced modulation techniques integrating supplementary sensing technologies and fine-tuning the algorithm parameters to improve the system’s accuracy and reliability, especially in more complex or dynamic environments.Item Physical layer of Security with NOMA in mixed RF/FSO systems(University of the Witwatersrand, Johannesburg, 2024) Osman, Amani Alged Fetaha; Takawira, Fambirai; Moualeu, JulesThe ever-increasing demand for spectrum resources, driven by the surge in high data tra c poses a signi cant challenge to the e ciency of wireless communication systems. In response, non-orthogonal multiple access (NOMA) has been proposed as a pivotal technique designed to address the spectral challenges inherent to tra- ditional multiple access schemes. NOMA, through its ability to allow multiple users to share the same time-frequency resources non-orthogonally, represents a promising avenue for improving spectral e ciency (SE). To further improve connectivity and enhance e ciency, NOMA can seamlessly be integrated with multiple-input-multiple-output (MIMO) systems. MIMO, a cor- nerstone technology in modern wireless communication, utilizes multiple antennas at both the transmitter and receiver to exploit spatial diversity, thereby achieving substantial gains in terms of capacity and reliability. The integration of MIMO with NOMA not only addresses the escalating demand for connectivity but also unlocks the potential for massive connectivity and enhanced spectral e ciency in wireless networks. However, the distribution of radio resources within the MIMO- NOMA framework brings forth security challenges owing to the intrinsic charac- teristics of wireless transmissions. In this regards, this thesis tackles the security aspect of MIMO-NOMA systems from an information theoretic viewpoint. The information theoretic security or physical layer security (PLS) e ectively mitigates vulnerabilities stemming from eavesdropping and interception, thereby fortifying the overall security of wireless communication in the present case of MIMO-NOMA systems. Another challenge that requires some attention is ensuring reliability over long distances. Although the radio frequency (RF) spectrum band is recognized for its reliability in diverse weather conditions, its inherent nature renders it sus- ceptible to security concerns. To address the imperative for secure long-distance communications, it is essential to explore innovative solutions that harness the inherent reliability of the RF spectrum while concurrently mitigating its vulner- abilities through enhanced security measures. Integrating technologies such as free-space optical (FSO) communication, acknowledged for its secure transmission capabilities, along with the implementation of advanced security protocols stands as a promising solution to achieving reliability and security in long-distance com- munications. In this work, PLS of a MIMO-NOMA based system in a mixed RF-FSO com- munications is investigated. Additionally, this thesis delves into the injection of vi arti cial noise (AN) in an e ort to explore other features of the proposed sys- tem from a PLS perspective. The last part of the work investigates the secrecy performance of the proposed system in the presence of multiple colluding and non- colluding eavesdroppers. To this end, the statistical properties are examined by deriving the statistical characteristics of the underlying system model, which will subsequently be used to establish both the analytical and asymptotic frameworks for evaluating secrecy performance measures. Finally, Monte Carlo simulations are adopted to validate the accuracy of our proposed mathematical derivations.Item Prospects of gibbsite-rich laterite as a source of aluminosilicates in geopolymerisation(University of the Witwatersrand, Johannesburg, 2024) Gilayeneh, Victor Saye; Nwaubani, Sunday; Otieno, MikeLaterite, an iron-rich soil widely found in the tropical and subtropical regions of the world, has shown promise for the development of eco-friendly construction materials through geopolymerisation. However, this material varies greatly in composition based on location, prevailing climate conditions, and even in depth within a given lateritic profile. The top layer of most lateritic profiles is usually low in kaolinite but rich in aluminium or iron hydroxide minerals. Despite these variations, research on the use of laterite in geopolymerisation has predominantly focused on materials rich in kaolinite. Therefore, this study explores the potential of aluminium-rich laterite as a source of aluminosilicates in geopolymerisation. In this study, the reaction kinetics, setting times, flow behaviour, strength development, phase composition, and pore structure of geopolymer derived from aluminous laterite were examined. This study also considered both calcined and uncalcined laterite as well as the influence of calcium minerals, namely calcium carbonate (CaCO3) and Portland cement, which replaced 40% of the laterite. In addition, the influence of the laterite’s properties on the performance of the derived geopolymer was also examined. The flow behaviour of the paste was found to be influenced by the viscosity of the activating solution, while the setting times and heat of reaction varied according to the type of laterite and the presence of calcium carbonate or Portland cement, which reduced the setting times and accelerated the rate of heat liberation within the first hour of the isothermal calorimetry test. The geopolymer mix based on calcined laterite displayed the highest amount of heat liberated, while its uncalcined laterite counterpart showed the lowest. All mixes within the calcined laterite series exhibited higher compressive strength than those in the uncalcined series, but only the calcined laterite mixes containing calcium minerals achieved structural strength. The uncalcined laterite mixes experienced strength regressions, with samples of the uncalcined laterite mix containing calcium carbonate developing cracks and subsequently disintegrating. The phase assemblage, porosity and pore structure were also influenced by the type of laterite and the presence of calcium carbonate or Portland cement. However, the presence of calcium carbonate also led to severe efflorescence and subflorescence, which negatively impacted the porosity and structural integrity. Also, the dissolution of gibbsite initiated the development of unstable phases in the uncalcined laterite mixes within the geopolymer and hybrid categories. Conversely, in the absence of activators, especially sodium hydroxide, as demonstrated in the binary mix containing uncalcined laterite, the dissolution of gibbsite is inhibited, resulting in the formation of stable phases.Item Improving bulk water pump station policies and operations in conditions of uncertain and changing demand(University of the Witwatersrand, Johannesburg, 2024) Mudau, Thendo DavidSouth Africa is facing growing and complex changes in water demand; furthermore, acute water scarcity challenges due to drought and degradation of surface water resources threaten its ability to manage the demands. Industrial and power generation sectors in some regions are plagued with water shortages, requiring inter-basin transfer schemes for bulk water transfer to these regions. The complexity in the water demand is further exacerbated when the demand that bulk water transfer systems are designed to manage varies from the demand projections. This has resulted in inefficient pumping operations and unnecessary costs, which are causing great concern as a result of the current water crisis and the increasing cost of electricity in South Africa. This was an investigation to find out how the increasing energy costs and inefficiencies due to uncertainties in demand could be mitigated in bulk water transfer schemes in South Africa. This was done through determination of the impact demand changes and uncertainties have on bulk water transfer pump stations’ adherence to the operational policy, performance, and costs. Approaches to reduce costs associated with the changes in the demand and improve the operational performance of the pump station were proposed. The Jericho pump station in the Mpumalanga province, South Africa, was used as a case study to assess the impact demand changes have on the pump station’s operation and control, as well as the operational energy costs. The results of the assessment showed that all these three aspects of the pump station had been negatively affected by the variability in the demand, particularly the energy costs with operational costs due to pumping operations during peak periods contributing to 33% of the estimated energy costs during the study period. Optimal pump scheduling and a capital investment in the form of a booster pump station to assist the Jericho pumping system and changes to the pump station control policy are proposed, and it is determined that they would ensure a 17.02% saving in annual energy costs. Furthermore, it is shown that operational costs due to pumping operations during peak Time- of-Use (TOU) periods are more than three times the standard and off-peak tariffs, and as such pumping operations during peak periods should be minimized. This research report showed the significance of optimal pump scheduling and how operational policies of bulk water transfer systems should continuously be reviewed and improved if required, especially with the ever-changing demands.Item Using a hybrid adsorption-membrane filtration system to produce biologically stable drinking water(University of the Witwatersrand, Johannesburg, 2024) Omalanga, Landry Sendango; Biyela , Precious; Simate, Geoffrey S.The purpose of water treatment is to produce clean and safe drinking water, for consumers. Water quality, both during treatment and distribution, is greatly affected by the presence of natural organic matter (NOM). The presence of NOM affects the effectiveness of water treatment processes and sometimes increases the cost of water treatment and leads to operational problems. Furthermore, the presence of biodegradable organic matter (BOM), which is a fraction of NOM, can degrade water quality during distribution resulting in the loss of biological stability. The excessive presence of BOM can be addressed using advanced water treatment processes or by relying on systems which combine multiple water treatment processes to increase treatment efficiency. The main aim of this study was to evaluate the effectiveness of a hybrid adsorption- membrane filtration system in lowering the bacterial regrowth potential in water. Ready-made multi-walled carbon nanotubes (MWCNTs) were used as adsorbents in this study. MWCNTs were chosen because they exhibit high adsorption properties mainly because of their fibrous shape and external surface accessibility. MWCNTs have hydrophobic characteristics and a propensity to aggregate due to the presence of electrostatic interactions among them, therefore, functionalization of MWCNTs was required to improve their dispersion in the organic and inorganic solvents. A non-covalent functionalization process was employed using cetyltrimethylammonium bromide (CTAB) as a cationic surfactant to ameliorate the stability and dispersibility of MWCNTs in aqueous solution. The non-covalent functionalization was preferred to sustain the functionalities needed for BOM capture enhancement and environmental safety. Polysulfone (PSF) membranes were produced by phase inversion method using N, N- dimethylformamide as solvent for the removal of BOM from water. The phase inversion method was chosen in this study due to its simple processing, flexible production scales, and low cost. The MWCNTs and PSF membranes were characterized using microscopy techniques such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), X ray diffraction (XRD), Raman spectroscopy, tensile strength test, and the hydrophilicity (contact angle) test. These techniques were selected because they enable the evaluation of the morphology, composition, physical characteristics, and dynamic behavior of nanostructured materials. iv Batch adsorption experiments were employed to investigate the adsorption properties of functionalized MWCNTs for BOM removal. Four different concentrations of functionalized MWCNTs were tested to determine the ideal conditions for the adsorption of two forms of BOM; assimilable organic carbon (AOC) and biodegradable dissolved organic carbon (BDOC), from water. The concentrations of functionalized MWCNTs used were 4, 8, 12, and 16 mg in 100 mL of BOM solution. Furthermore, the cross-flow filtration mode, also known as tangential flow filtration, was used to separate the remaining BOM in water by passing water along the surface of the NF membrane using pressure difference. Cross-flow filtration was chosen because it removes the buildup from the surface of the membrane and provides the benefit of an improved membrane lifespan by helping to prevent irreversible fouling. A mathematical model of membrane filtration process in continuous system was also developed to better understand the correlations between the different variables of the membrane filtration process such as the inlet (feed) concentration (Cin) and flow rate (Qin), and the outlet (retentate) concentration (Cout) and flow rate (Qout), and the permeate concentration Cp. Results obtained after the functionalization process of MWCNTs showed an improvement in their stability and dispersibility in aqueous solution. The characterization of both MWCNTs and PSF membranes showed some interesting features. For example, morphological and structural studies show that MWCNTs possess fibrous shapes with a high aspect ratio, and a hollow structure with an inner diameter. The finger-like structures found on the surfaces of PSF membranes play a crucial role in their adsorption capabilities. These structures, which vary in pore size, contribute to the overall capacity of the membranes to absorb BOM from water. During adsorption experiments, it was observed that the removal of BOM from water increased with an increase in the adsorbent (functionalized MWCNTs) concentration. This is likely due to high concentration gradient which acts as a driving force to overcome resistances to mass transfer of dye ions between the aqueous phase and the solid phase. However, the maximum removal of both AOC and BDOC was recorded at a concentration of functionalized MWCNTs of 12 mg, at a contact time of 4 hours and at an agitation speed of 180 rpm. The PSF membrane produced by phase inversion method demonstrated the highest flux of 0.0091 ml/cm2.min at room temperature (25°C) and after a filtration time of 90 minutes. The selectivity and permeate flux were increased with forward flushing and backwashing processes of the PSF membranes because it flushes out accumulated debris and particles on the surface and inside the pores of the membranes. After using the hybrid adsorption-membrane v filtration system, BDOC concentrations dropped to an average of 65% of the initial raw water BDOC and the AOC concentrations dropped to approximately 80% of the initial raw water AOC. Outputs from the mathematical model demonstrated that the change in initial conditions (Cin and Qin) is responsible for the transient response (changes from one steady state to another) in these membranes. The adsorption and membrane nanofiltration hybrid system adopted in this study, effectively removed both AOC and BDOC from water, and can therefore be used to produce biologically stable drinking water. The outcome of this study could be the application of the combination of BOM targeting strategies and residual disinfection to better control bacterial regrowth in drinking water distribution systems (DWDSs). This in turn could help water utilities with meeting distribution systems, water quality guidelines, and protect public health