Department of Physics
Permanent URI for this community
Browse
Browsing Department of Physics by Author "Maluta, N. E."
Now showing 1 - 11 of 11
Results Per Page
Sort Options
Item Open Access Comparison and evaluation of empirical and machine learning models in estimating global solar radiation in Limpopo province(2023-10-05) Murida, Thalukanyo Witney; Mulaudzi, T. S.; Maluta, N. E.; Mphephu, N.This study investigated the performance of machine learning techniques as compared to the empirical models to forecast the global solar radiation in Limpopo regions. The machine learning techniques used in this study are Support Vector Machines, Random Forest, and Artificial Neural Network, and the empirical models used are the Clemence and Hargreaves- Samani models. To assess the efficiences of the machine learning models against the empirical models, the researchers calculated and compared the models performance evaluation using statistical equations such as Coefficient of determination, Mean Square Error, Mean Absolute Error, and Root Mean Square Error. Calibaration was done to improve performance of the empirical models. The present study found that machine learning techniques perform better than the empirical models when estimating the global solar radiation in the selected Limpopo regions.Item Open Access Computational study of low index surface of an anatase TiO2 doped with ruthenium (Ru) and strontium (sr) for application in Dye sensitized solar cells(2019-05-18) Nemudzivhadi, Hulisani; Maluta, N. E.; Maphanga, R. R.Titanium dioxide (TiO2) is considered to be an ideal semiconductor for photocatalysis because of its high stability, low cost and safety towards both humans and the environment. Doping TiO2 with different elements has attracted much attention as the most important way of enhancing the visible light absorption, in order to improve the efficiency of the dye sensitized solar cells (DSSCs). In this study, first principle density functional theory was used to investigate electronic and optical properties of bulk anatase TiO2, undoped, and ruthenium (Ru) and strontium (Sr) doped anatase TiO2 (1 0 0) surface. Two different doping approaches i.e., substitutional and adsorption mechanisms were considered in this study. The results showed that absorption band edges of Ru and Sr-doped anatase TiO2 (1 0 0) surface shift to the long wavelength region compared to the bulk anatase TiO2 and undoped anatase TiO2 (1 0 0) surface. Also, the results revealed that the band gap values and the carrier mobility in the valence band, conduction band and impurity energy levels have a synergetic influence on the visible-light absorption and photocatalytic activity of the doped anatase TiO2 (1 0 0) surface. Furthermore, according to the calculated results, we propose the optical transition mechanisms of Ru and Sr-doped anatase TiO2 (1 0 0) surface. Thus, we conclude that the visible light response of TiO2 can be modulated by doping with both Ru and Sr. However, Sr-doped system shows higher photocatalytic activity than the Ru-doped system. The study has successfully probed the interesting optical response mechanism of TiO2 (1 0 0) surface.Item Open Access Density functional theory study of adsorption of cronconate dyes on TiO2 Anatase (010) and (100) surfaces(2019-05-18) Ranwaha, Tshifhiwa Steven; Maluta, N. E.; Maphanga, R. R.Currently the dye sensitized solar cells have attracted more attention due to their low cost, transparency and flexibility. These types of solar cells use the dye molecule adsorbed on TiO2 semiconductor in Nano architecture with the role of absorbing photons, in recent research attempts are being made to shifts the absorption spectral of TiO2 to visible and near infrared–region of solar spectrum to achieve maximum photo absorption which yields to an increase in the efficiency of the dye sensitized solar cells. In the current study, density functional theory (DFT) was used to model two croconate dyes (CR1 and CR2), one with an electron donating methyl group (CR1) and the other with an electron –withdrawing caboxyl group (CR2). The geometric, electronic and optical properties of these dyes were compared. The adsorption behaviour of the two dyes on (010 and 100) anatase TiO2 surfaces were investigated in this study by employing first principle calculation based on DFT using a plane-wave pseudo potential method. The generalized gradient approximation (GGA) was used in the scheme of Perdew-Burke Ernzerhof to describe the exchange -correlation function as implemented in the CASTEP package in Material Studio of BIOVIA. The adsorption results shows a spontaneous electron injection followed by efficient regeneration of the oxidized dye molecules by the electrolyte and strong binding ability of CR2 to the TiO2 surface, but also shows a comparable binding strength of CR1. The results of this study will help in the design of high efficient dye for DSSCs.Item Open Access Density functional theory study of copper zinc tin (Cu2ZnSnS4) with Calcium and Barium(2020-08-24) Mlotshwa, Thokozane Mxolisi; Maluta, N. E.; Maphanga, R. R.; Kirui, J. KThe sun is the most important source of renewable energy today. Producing energy from sunlight using cheap, abundant and non-toxic materials is considered a major challenge in the field of solar-electrical energy conversion. Fossil fuel combustion, depletion of non-renewable sources, global warming and environmental degradation are some of the push factors towards clean, non-toxic and environmentally friendly methods of producing electrical energy. To harvest solar energy, a thin film solar cell composed of the Cu2ZnSnS4 (CZTS) semiconductor is a candidate, which can harvest useful amounts of energy. Some of its advantages are the optical direct band gap and high absorption coefficients. In this study, CZTS is investigated as a material for solar cells using first principle method. Thus, structural, electronic and optical properties of pure CZTS and doped CZTS (112) surface were investigated using the density functional theory as implemented in the Cambridge Serial Total Energy Package code. Alkali earth metals, Calcium (Ca) and Barium (Ba) were adsorbed on the CZTS (112) surface using the adsorption locator module. The results suggest that doping with barium rather than calcium could improve the photocatalytic activity on the CZTS based solar cells. Doping using different elements yielded improved optical and electronic properties of the CZTS based solar cells.Item Open Access Determination of global solar radiation using temperature-based model for different climate conditions for Limpopo Province of South Africa(2022-07-15) Mathebe, Sampie Mphagala; Maluta, N. E.; Mulaudzi, T. S.The research mainly focused on the determination of global solar radiation using temperature-based model by Hargreaves and Samani for the Northern regions of Limpopo Province of South Africa. The daily maximum and minimum temperature data measured at the following six (6) stations were used: Ammondale, Mutale, Nwanedi, Roedtan, Sekgosese and Xikundu for the period 2008 – 2010. The values of empirical coefficient Kr for the Inland stations of South Africa were computed and used as an input to the model. The observed and calculated global solar radiation data were compared on the basis of the statistical error tests that is mean bias error (MBE), the mean percentage error (MPE) and the root mean square error (RMSE). Based on the statistical results the model was found suitable to estimate monthly average daily global solar radiation for the regions listed above and elsewhere with similar climatic conditions and areas where the radiation data is missing or unavailable. Hence, the study will also help to advance the state of knowledge of global solar radiation to the point where it has applications in the estimation of monthly average daily global solar radiation across.Item Open Access Estimation of Global Solar Radiation from SAURAN stations using air temperature-based models Hargreaves and Samani and Clemence models(2020) Shabangu, Charlotte Beauty; Maluta, N. E.; Mulaudzi, T. S.Knowledge of the amount of solar radiation available in a location is important for solar energy systems, architectural designs, agronomy, and installation of pyranometers. Some developing countries do not have good quality meteorological stations that can directly measure global solar radiation. Thus, several empirical methods were developed to estimate global solar radiation. This study uses two temperature-based models which are Hargreaves - Samani and Clemence models. Four selected stations from the Southern African Universities Radiometric Network (SAURAN) for this study are University of KwaZulu–Natal, Howard college (KZH), University of Stellenbosch (SUN), Nelson Mandela University (NMU) and University of Venda (UNV). A three-year (2014-2016) temperature data for each station were sourced from SAURAN. The performance of the two models was validated using statistical analysis that is, Mean Percentage Error (MPE), Mean Bias Error (MBE), Root Mean Square (RMSE), Coefficient of Determination (R2) and t-statistical value (t). Both models obtained acceptable values of MBE, MPE, RMSE, R2 and t in KZH, NMU and UNV stations. Both models achieved the best values of MBE from 2014 to 2016, ranging from -0.0099 to 0.0147 in KZH station, followed by NMU with MBE values ranging from - 0.0293 to -0.0014, -0.0104 to 0.0330 for SUN station, 0.0241 to 0.0245 for UNV station. The models achieved MPE values between ± 10 % in all the stations. The R2 values for both models are close to 1, while the t-statistic values of one, which is less than critical value, was achieved by the models from all selected stations. This suggests that both models have got capacity to estimate global solar radiation in all the selected areas of study. However, the higher values of MBE and RMSE also revealed high level of overestimation by the models in SUN station. Therefore, this study has found evidence that both Hargreaves - Samani & Clemence models can be best recommended for estimating global solar radiation in KZH, NMU and UNV stations and areas with similar climatic and meteorological conditions.Item Open Access Evaluation of the regression coefficients for South Africa from solar radiation data(2019-09-20) Mulaudzi, Tshimangadzo Sophie; Maluta, N. E.; Kirui, J. K.The knowledge of solar radiation in this dispensation is crucial. The lack of grid lines in the remote rural areas of South Africa necessitates the use of solar energy as an alternative energy resource. Solar radiation data is one of the primary factors considered for the installation of renewable energy devices and they are very useful for solar technology designers and engineers. In some developing countries, estimation of solar radiation becomes a challenge due to the lack of weather data. This scenario is also applicable to South Africa (SA) wherein there are limited weather stations and hence there is a dire need of estimating the global solar radiation data for all climatic regions. Using a five year global solar radiation (𝐻) and bright sunshine (𝑆) data from the Agricultural Research Council (ARC) and South African Weather Service (SAWS) in SA, linear Angstrom – Prescott solar empirical model was used to determine regression coefficients. MATLAB interface was used whereby the linear regression plots were drawn. Annual empirical coefficients of 22 stations were determined and later the provincial values. The range of the regression coefficients, a and b were 0.216 – 0.301 and 0.381 – 0.512 respectively. The 2006 estimated global solar radiation per station in a province calculated from the modified models were compared with the observed and statistically tested. The root mean square errors were less than 0.600 MJm−2day−1 while the correlation relation ranged from 0.782 – 0.986 MJm−2day−1. The results showed the regression coefficients performed well in terms of prediction accuracy.Item Open Access First-principles study of Hematite (α-Fe2O3) surface structures doped with Copper (Cu), Titanium (Ti), nickel (Ni) and manganese (Mn)(2023-05-19) Mabaso, Clarence Vusi; Maluta, N. E.; Maphanga, R. R.Hematite has attracted research interest for many years due to its application in water splitting. Despite its attractive characters such as a reasonable optical band gap, the semiconductor is still faced with great uncertainty for the accomplishment of hematite based photoelectrochemical cells for water splitting. Doping with transition metals has shown to be a practical solution to overcome some of the limitations faced with hematite by modifying the energy band to improve its photo-electrochemical (PEC) activity. This study explored two surface structures of pure and transition metals (Ti, Cu, Ni and Mn) doped- α-Fe2O3 oriented in the directions (001) and (101). Calculations via the first principle using the density functional theory (DFT) were adopted, the results show that the doping of transition metals in α-Fe2O3 has an effect in modifying both the valence and conduction band edges. Specifically, doping Ti introduces more electrons in the conduction band and fills the unoccupied 3d states, which could improve the rate of charge transportation and likely enhance the electrical conductivity of α-Fe2O3. Doping with Mn, Ni, and Cu has effectively improved the absorption coefficient for α-Fe2O3 (001) and (101) surfaces, in the visible light region. The overall analysis of the results shows an opportunity for a successful photo-electrochemical water splitting application.Item Embargo Investigation of covariability between energy fluxes and CO2 exchange over a semi-arid savanna (Kruger National Park) by Eddie Covariance Technique(2024-09-06) Takalani, Lufuno; Mulaudzi, T. S.; Maluta, N. E.; Mateyisi, M.; Thenga, H.South Africa faces climate change, natural disasters, and rising temperatures due to increased levels of carbon dioxide in the atmosphere, primarily caused by deforestation, burning fossil fuels, and releasing carbon dioxide into the atmosphere without additional carbon sinks. The gap in understanding lies in studying the connection between energy flows and Net Ecosystem Exchange (NEE) at the semi-arid savanna of Kruger National Park to gain a more detailed and accurate understanding of these processes, especially in semi-arid savannas that are susceptible to changes in environmental factors. By studying energy fluxes and NEE at Kruger National Park using the eddy covariance technique, the dissertation seeks to deepen our understanding of the mechanisms driving carbon exchange in semi-arid savannas and provide insights into the impact of environmental factors on ecosystem processes. The eddy covariance technique is a powerful tool that directly measures energy and carbon dioxide exchange between the land surface and the atmosphere. The study shows that the correlation between NEE and latent heat flux (LE) and net radiation (Rn) is generally the strongest, while ground heat flux (G) and sensible heat flux (H) have little impact on NEE. The dataset provides insight into the biometeorological and flow dynamics of the Skukuza ecosystem and how it responds to climate change. The study emphasizes the importance of considering seasonality, climatic variability, and precipitation when studying the surface energy balance and its components. The findings have implications for understanding the complex interactions between ecosystem processes and environmental factors.Item Embargo Studies on structural, electronic and optical properties of SnO2 doped with nitrogen, chloeine, antimony and indium(2024-09-06) Nekhwevha, Nditsheni; Maluta, N. E.; Maphanga, R. R.SnO2 has recently attracted a great deal of interest due to its many technological applications, including in solar cells as it possesses advantageous optical and electrical characteristics, outstanding chemical stability, and thermal stability. However, the photocatalytic activity and charge carrier mobility are constrained by the large band gap. A cost-effective and efficient method for reducing the SnO2 band gap and increasing the potential for photocatalytic applications is doping with different elements. Examining how mono-doping and co-doping impact the electronic, structural, electrical, and optical characteristics of the SnO2 supercell structure, the current theoretical study used Density Functional Theory (DFT) calculations of different metal and nonmetals (N, Cl, In, and Sb) and (N-Cl and In-Sb) as dopants and co-dopants, respectively. The results show that due to the band gap narrowing and the existence of impurity levels in the band gap, all mono-doped and co-doped SnO2 exhibit some small redshift. The results of the trials and the calculated optical characteristics, such as the dielectric function, reflectivity, absorption coefficient, and energy-loss spectrum, are in good agreement. According to the predicted absorption coefficient, doped SnO2 has a noticeable band of absorption. Doped SnO2 exhibits superior absorption in the visible area of the electromagnetic spectrum than undoped, In-doped, Sb-doped, and In-Sb co-doped SnO2.Item Embargo The adsorption of bidens pilosa dye molecules onto TiO2 nanoparicle surfaces for optimization of light harvesting efficiency in dye sensitized solor cell: an experimental and theory study(2024-09-06) Randela, Ronel Ronella; Maluta, N. E.; Mathomu, L. M.; Maphanga, R. R.The availability and high demand for electrical energy is a key global concern, as a result, Dye sensitized solar cells (DSSCs) have attracted a lot of attention in recent years due to their ease of preparation, low toxicity, and environmental friendliness. The current study describes the green synthesis of TiO2 nanoparticles as well as their characterization using ultraviolet-visible, Fourier transformed infrared spectroscopy and X-ray diffraction. Furthermore, the study used Density Functional Theory to describe the optical characteristics of produced nanoparticles. The UV-Vis results showed that the dye extracted using solvents such as water, methanol, and ethanol had a common absorbance at 665 nm among the solvents used ethanol had the highest absorption. The molecules responsible for a broader range of absorbance are known to be pheophytin and porphyrin, which are found in chlorophyll extracted from the B. pilosa plant. FTIR analysis of the prepared TiO2 revealed the absorbed functional groups of the synthesized B. pilosa extracts and confirmed the formation of TiO2 NPs with a vibrational band at 497 cm 1. The TiO2 NPs were heterogeneous in shape under TEM and SEM but spherical under SEM, indicating the formation of paste during agglomeration. XRD analysis confirmed that the polymorph formed is anatase with the highest peak of (101) surface, which was used to computationally adsorb the dye molecule. Pheophytin and porphyrin characteristics were optimized using DFT. For both experimentally and computationally, the UV-vis absorbance was found to be between 420 nm and 665 nm with a higher light harvesting efficiency. pheophytin and porphyrin exhibited energy gaps of 2.1 eV and 2.8 eV respectively. This study demonstrates that the dye molecule synthesized from B. pilosa is an efficient sensitizer for DSSCs. The adsorption results substantiate the spontaneous electron injection and subsequent efficient regeneration of oxidized dye molecules and the strong binding ability of porphyrin dye molecules to the TiO2 surface. The results of this study will be useful for the development of highly efficient organic dyes for DSSCs.