ResearchSpace :: Browsing by Author "Roro, Kittessa T"

Browsing by Author "Roro, Kittessa T"

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Ammonia gas sensing characteristics of V2O5 nanostructures: A combined experimental and ab initio density functional theory approach
(Elsevier, 2020-04) Akande, Amos; Mosuang, T; Ouma, CNM; Benecha, EM; Tesfamicheal, T; Roro, Kittessa T; Machatine, AGJ; Mwakikunga, BW
A combined experimental and density functional theory study of NH(sub3) gas sensing and adsorption characteristics of a-V(sub2)O(sub5) synthesized from hydrated NH4VO3 in CVD at 400 °C (in N(sub2) for 12 and 24 h.) is presented. Highly crystalline orthorhombic a-V(sub2)O(sub5) nano-rods with dominant (001) and (110) planes/facets nano-rods were observed from XRD, SEM and TEM characterizations. Using VSM technique, para- to ferro- magnetic transition was observed in the a-V(sub2)O(sub5) nanoparticles synthesized at 24 h. Improved gas sensing was observed in case of paramagnetic a-V(sub2)O(sub5) nano-rods (nanoparticles synthesized at 12 h.) compared with the one synthesized at 24 h. Additionally, significant rise in gas sensing response was observed around the metal to insulator transition temperature. Calculation of adsorption of NH(sub3) molecule(s) on (001), (110), (200) and (400) facets showed that (001), (200) and (400) possessed more active sites than (110) surface. DFT calculations were also used to investigate NH3 adsorption on (110) surface of a-V(sub2)O(sub5) with the analysis showing exponential decrease in the electronic band gap of the material’s surface with the increasing numbers of NH(sub3) loadings.
Blue- and red-shifts of V2O5 phonons in NH3 environment by in situ Raman spectroscopy
(IOP Publishing, 2018-01) Akande, Amos A; Machatine, AGJ; Masina, Bathusile N; Chimowa, G; Matsoso, B; Roro, Kittessa T; Duvenhage, M-M; Swart, H; Bandyopadhyay, J; Ray, Suprakas S; Mwakikunga, Bonex W
A layer of ~30 nm V2O5/100 nm-SiO2 on Si was employed in the in situ Raman spectroscopy in the presence of NH3 effluent from a thermal decomposition of ammonium acetate salt with the salt heated at 100 °C. When the layer is placed at 25 °C, we observe a reversible red-shift of 194 cm−1 V2O5 phonon by 2 cm−1 upon NH3 gas injection to saturation, as well as a reversible blue-shift of the 996 cm−1 by 4 cm−1 upon NH3 injection. However when the sensing layer is placed at 100 °C, the 194 cm−1 remains un-shifted while the 996 cm−1 phonon is red-shifted. There is a decrease/increase in intensity of the 145 cm−1 phonon at 25 °C/100 °C when NH3 interacts with V2O5 surface. Using the traditional and quantitative gas sensor tester system, we find that the V2O5 sensor at 25 °C responds faster than at 100 °C up to 20 ppm of NH3 beyond which it responds faster at 100 °C than at 25 °C. Overall rankings of the NH3 gas sensing features between the two techniques showed that the in situ Raman spectroscopy is faster in response compared with the traditional chemi-resistive tester. Hooke's law, phonon confinement in ~51 nm globular particles with ~20 nm pore size and physisorption/chemisorption principles have been employed in the explanation of the data presented.
Carbon nanotube/nickel oxide nanocomposite thin films for selective solar absorber
(SAIP 2010, 2010-09) Roro, Kittessa T; Mwakikunga, Bonex W; Forbes, A; Tile, N; Yalisi, B
Multi-walled carbon nanotube (MWCNT)/Nickel oxide nanocomposites were prepared on aluminum substrates for selective solar absorber applications. MWCNTs are functionalized in order to disperse in water and ethanol. Results from the characterization of the composite samples using techniques such as transmission and scanning electron microscopies will be presented and discussed.
Carbon-Nickel oxide nanocomposites: Preparation and charecterisation
(2011-07) Tile, N; Roro, Kittessa T; Forbes, A
Nanocomposite materials have wide range of applications in solar energy conversion. In this work, C-NiO nanocomposite coatings are prepared using sol-gel synthesis and deposited on aluminium substrates using a spin coater. The coatings are prepared from alcoholic sols based on Ni-acetate using diethalonamine as a chelating agent and polyethylene glycol (PEG) as organic template. Sucrose is used as a carbon source. Sols with different PEG and sucrose concentrations are prepared and coated on aluminium substrates. The effect of heat temperature on the properties is also investigated. The optical and structural changes of the nanocomposite coatings are characterized by UV-Vis, FTIR, thermal emissometer, Raman, and SEM techniques. It has been shown that the solar absorption increases with increasing the heating temperature in the temperature range 300-550 °C due to the increase in carbon content in the composite material. Preliminary durability studies on these samples will also be presented.
Characterization of solar photovoltaic modules in outdoor and indoor conditions
(CIGRE Southern Africa National Committee, 2019-10) Mkasi, Hlaluku W; May, Siyasanga I; Basappa Ayanna, Manjunath; Pratt, Lawrence E; Roro, Kittessa T
This paper summarizes the results of solar photovoltaic (PV) module electrical characterization under outdoor and indoor conditions in the Republic of South Africa. Under outdoor conditions, the bi-facial PV modules on fixed racks and dual-axis trackers yielded more energy than mono-facial modules on the same mounting configuration due to the active backside. The indoor current-voltage (IV) measurements conducted on an A+A+A+ sun simulator matched within ± 3% of the manufacturer’s nameplate ratings on a sample of crystalline PV modules, with some exceptions. The measured electrical characteristics from the outdoor system was corrected to standard test conditions (STC) and compared with indoor measurements at STC. The corrected maximum power from the outdoor system measured roughly 5% below maximum power as measured on the indoor system.
Cost efficient procurement of solar photovoltaic plant for embedded generation
(2017-11) Chiloane, Lehlogonolo D; Roro, Kittessa T; Bischof-Niemz, Tobias S
South Africa has one of the highest carbon emissions per capita in the world, and this has raised the need to reduce reliance on coal generated power which is carbon intensive. The South African government has focussed on introducing renewable energy technologies for power generation through the Renewable Energy Independent Power Producers Procurement Programme (REIPPPP) which is a renewable energy competitive bidding mechanism, which commenced in 2011. The success of the REIPPP, has seen the average prices of utility scale solar PV dropping by 83 % from R3.65/kWh in 2011 to R0.62/kWh in 2016. South Africa’s electricity tarrifs have increased by more than 300 % from 2008 to 2016, and this has caused various energy consumers to implement energy efficiency measures and reduce their electricity costs where possible. It has also raised the need for various energy consumers to look at alternative energy supply options, one of them being solar PV for own electricity use.
Cr/alpha-Cr2O3 monodispersed spherical core-shell particles based solar absorbers
(UNISA, 2011-07) Khamlich, S; Nuru, ZY; Nemutudi, R; Nemraoui, O; Roro, Kittessa T; Mongwaketsi, N; McCrindle, R; Cingo, N; Maaza, M
Monodispersed spherical core-shell particles of Cr/alpha-Cr2O3 cermet ACG coatings investigated within this contribution could be successfully employed in thermal converters. Their selectivity depends on their chemical, physical and structural characteristics and related optical properties like reflectance, emittance, solar light absorption, or absorptance and transmittance. Parameters such as particles size, aging-time and substrate roughening will greatly influence their relevant optical properties as reported. The coated Cr/alpha-Cr2O3 spherical particles on rough copper substrates by a simple self-assembly-like method were characterized by scanning electron microscopy, energy dispersive spectrometry, Raman spectroscopy, and diffuse reflectance UV-VIS-NIR spectroscopy. The samples aged for more than 40h at 75 degrees C exhibit the targeted high absorbing optical characteristic “Black chrome” while those aged for smaller or equal to 40h show a significant high diffuse reflectance.
Defect-induced room temperature ferromagnetic properties of the Al-doped and undoped ZnO rod-like nanostructure
(Elsevier, 2017-07) Jule, L; Dejene, F; Ali, AG; Roro, Kittessa T; Mwakikunga, Bonex W
In this work, electron paramagnetic resonance (EPR) experiment on undoped ZnO and Al doped ZnO (AZO) nanoparticles prepared by facile sol–gel method were investigated. The effects of the Al concentration on room temperature ferromagnetic (FM) properties of the AZO (Zn1-xAlxO,0.1x0.30) are reported. EPR signal reveals the origin of FM is purely intrinsic and related with interaction of clusters carrying net magnetic moment coming from electron spin trapped in defect states. Increasing Al concentration results in reducing FM ordering which are likely due to reduction in amount of oxygen adsorption. Defect analysis based on PL, indicates zinc interstitials (Zni) and singly oxygen vacancies (Vo+) are responsible for mediating ferromagnetism in the undoped ZnO. The assertion was supported by Raman spectra and EPR analysis. Moreover, the present work suggests the potential applications of AZO in future spintronics and speculate the origin of ferromagnetism in AZO nanoparticles.
Determination of the laterally homogeneous barrier height of palladium Schottky barrier diodes on n-Ge (111)
(Elsevier, 2011-05) Chawanda, A; Roro, Kittessa T; Auret, FD; Mtangi, W; Nyamhere, C; Nel, J; Leach, L
The authors have studied the experimental linear relationship between barrier heights and ideality factors for palladium (Pd) on bulk-grown (1 1 1) Sb-doped n-type germanium (Ge) metal-semiconductor structures with a doping density of about 2.5×1015 cm-3. The Pd Schottky contacts were fabricated by vacuum resistive evaporation. The electrical analysis of the contacts was investigated by means of current–voltage (I–V) and capacitance–voltage (C–V) measurements at a temperature of 296 K. The effective barrier heights from I–V characteristics varied from 0.492 to 0.550 eV, the ideality factor n varied from 1.140 to 1.950, and from reverse bias capacitance–voltage (C-2–V) characteristics the barrier height varied from 0.427 to 0.509 eV. The lateral homogenous barrier height value of 0.558 eV for the contacts was obtained from the linear relationship between experimental barrier heights and ideality factors. Furthermore the experimental barrier height distribution obtained from I–V and (C-2-V) characteristics were fitted by Gaussian distribution function, and their mean values were found to be 0.529 and 0.463 eV, respectively.
Development of a smart monitoring and evaluation framework for hybrid renewable mini-grids
(2020-05) Ravanbach, B; Kühnel, M; Hanke, B; Von Maydell, K; Van Dyk, EE; Vumbugwa, M; Makaka, G; Lesala, ME; Shambira, N; Roro, Kittessa T
Hybrid renewable mini-grids have emerged as a viable solution for providing reliable, environmentally friendly electricity to remote communities. An affordable and grid-quality supply of energy can open new possibilities for socioeconomic progress. As part of a joint project between South Africa's Eastern Cape province and Germany's state of Lower Saxony a Photovoltaic (PV) hybrid mini-grid is developed in the municipality of Upper Blinkwater in Eastern Cape for a rural community of 70 households with 90% living off of social grants and no access to the main national grid. The aim of this work is to develop a smart Monitoring and Evaluation Framework (MEF) for hybrid renewable mini-grids by integrating cutting-edge technologies in a scalable platform of replicable solutions towards connecting the mini-grid with diverse stakeholders with enhanced observability of both generation and consumption profiles. The MEF provides the opportunity to streamline the flow of real-time energy data (generation, consumption, and storage) from the system to generate accurate and high-resolution data-driven load profiles for rural households. Simultaneously, the interrelation between energy access and social development will be studied and analyzed.
Effect of accelerated thermal ageing on the selective solar thermal harvesting properties of multiwall carbon nanotube/nickel oxide nanocomposite coatings
(Hindawi Publishing Corporation, 2012-05) Roro, Kittessa T; Mwakikunga, Bonex W; Tile, N; Yalisi, B; Forbes, A
Varying amounts of dispersed multiwalled carbon nanotubes in NiO have been used to develop composites that absorb the solar energy very well but lose very little through emission. Determination of absorptance, asol, and emissivity, ether, from such selective solar absorbers shows that the optimum efficiency of 71% can be attained when about 10 mg of MWCNTs are composited with NiO. One such absorber was subjected to thermal ageing tests. The performance criterion (PC) limit for passing the test when simulated for 25 years is (- a+0.25 e)=0.05. It was found that the typical absorber had a PC value of -0.01. This value is much better than the passing limit. Raman spectra of the typical absorber before and after the thermal ageing test showed a reduced intensity in the D and G bands of disordered and graphitic carbon, respectively but an enhancement of the NiO bands indicating loss of carbon atoms due to thermal ageing tests. Simple equations are derived determining the proportion of carbon atoms that are lost and the proportion of carbon atoms that remains in the absorber; both of these are in agreement with the original carbon composition before the thermal ageing test. It is reported that the typical absorber will retain 63% of the carbon after 25 years.
Effect of heating temperature on the optical properties of sol-gel synthesized C/NiO nanocomposite thin films
(2011-05) Roro, Kittessa T; Tile, N; Yalisi, B; Forbes, A
For efficient photo-thermal conversion of solar energy a spectrally selective solar absorber surfaces are widely used. An ideal selective surface should have a high absorptance (low reflectance) in the solar wavelength range (0.3 to 2.5 µm) and low emittance (high reflectance) in the infrared wave length range (2.5 to 20 µm) in order to minimize heat losses. One of the designs that can be used to achieve a combination of high solar absorptance with low emittance is a tandem absorber. This can be realized by using a coating which contains metal particles embedded in an oxide matrix on a highly reflecting metal substrate. Due to its high absorptance in the visible and its metal-like characteristics, carbon is an excellent choice of material for solar absorber application. However, there are very few reports on carbon containing metal oxide materials. Sol-gel technique has been widely used to prepare spectrally selective coatings on aluminium substrate for solar absorber application in low temperature photo-thermal conversion have shown the feasibility of the C/NiO composite coatings for a selective solar absorber application, nonetheless, a detailed systematic investigation on the effect of the sol-gel fabrication process parameters on the structural and optical properties were not reported. It is expected that the heating temperature has a crucial effect on the properties of the final composite coatings. It is therefore the purpose of this work to investigate the effect of heating temperature on the optical properties of the C/NiO composite coatings.
The effect of soiling on the PV performance ratio for different PV systems
(SASEC 2018, 2018-06) Mienie, S; Basappa Ayanna, Manjunath; Pratt, Lawrence E; Roro, Kittessa T; Diale, M
Soiling is an important and unpredictable factor that affects PV plant performance. Soiling is dependent on so many factors making it hard to quantify and accurately incorporate in PV prediction models and levelized cost of electricity (LCOE). In this experiment, a comparison was done to establish the effect of soiling for 2 different PV plants installed at the Council of Scientific and Industrial Research (CSIR) campus in Pretoria. The soiling rate for the performance ratio (PR) was 1.2 % per week on the 558 kWp DC single-axis tracker system and 0.4 % per week on the 202.3 kWp DC dual-axis plant over the period from June 2017 to August 2017. Soiling has a smaller effect on dual-axis systems as compared to single-axis systems due to the soiling accumulation mechanics and PV system elevation above ground level.
Effect of using global horizontal or plane of array irradiance for monitoring sun tracking solar photovoltaic plants performance
(Southern African Solar Energy Conference (SASEC), 2018-06) Basappa Ayanna, Manjunath; Pratt, Lawrence E; Mienie, S; Roro, Kittessa T
The performance ratio (PR) is a metric commonly used for assessing the performance of a photovoltaic (PV) plants worldwide. The standard PR calculation method (PRPoA) used in solar PV industry as per the International Electro-technical Commission (IEC) standard uses the solar irradiance in the Plane of Array (PoA) as reference. The CSIR PR calculation method (PRGHI) used Global Horizontal Irradiance (GHI) as a reference for the first phase of PV plants realized as part of the Energy Autonomous Campus program. The PRGHI holds the EPC contractor liable for any underperformance due to poor layout or tracker operations. The PRGHI is variable in the short term and highly variable across the seasons compared with the PRPoA. The temperature corrected PRGHI and PRPoA are calculated to minimize any biases due to temperature which may arise from different weather conditions. The PRPoA is less variable in the short term and across the seasons, so any loss or gain in performance is more easily identified for both the single axis and dual axis tracker systems. The seasonal effect on PRGHI is larger for the dual axis tracker system compared to single axis tracker PV system. The increasing PRGHI between summer and winter solstice for the dual axis tracker system is due to a decrease in the reference irradiance caused by the higher angle of incidence of the sun on the GHI reference sensor and is not due to an increase in PV electrical output. The predicted and actual PRGHI and PRPoA are compared for the single and dual axis tracker systems for a period of one year without temperature correction. The absolute delta between the predicted and actual PR for the single axis tracker was -10% for PRGHI and -4.5% for PRPoA. The absolute delta for the dual axis tracker was -4.5% for PRGHI and +0.4% for PRPoA. The large absolute PRGHI delta between the actual and predicted PRs for the single axis tracker is investigated and this paper focuses only on the tracker performance. The actual tracker tilt angle performed optimally during the summer solstice period but suboptimally during the early morning and late afternoon between the summer and winter equinoxes. Future work will characterize the effect of the sub-optimal tracker performance in terms of PoA irradiance and energy production.
Electroluminescence imaging of photovoltaic modules
(2019-10) Malefafana, MT; Pratt, Lawrence E; Basappa Ayanna, Manjunath; Roro, Kittessa T
Electroluminescence (EL) imaging is a non-destructive characterization technique used widely across the solar PV industry. Similar to an x-ray image, the EL image allows the analyst to peer inside the PV module and see features that are otherwise impossible to view with the naked eye. Those features include grain boundaries, interconnects, defects in the silicon, cracked cells and inactive cell areas. The integrity of the solar cells is critical because the cells are the generators that convert solar radiation into direct current electrical energy. Therefore, EL is used at a number of points in the supply chain to minimize the negative impact from cracked cells and inactive areas. EL images are collected at the factory during manufacturing processes, quality assurance batch testing, routine operations and maintenance as well as during failure analysis of fielded modules in real world conditions. In this paper, the basic techniques used to capture EL images at the CSIR are presented. Some of the typical features that can be seen on multi-crystalline silicon and mono-crystalline silicon photovoltaic (PV) modules, as well as bi-facial modules are presented. Two case studies have been analyzed as part of the failure analysis. One shows the impact that a handling mishap can have on the integrity of the solar cells. The second case study shows the use of EL, infrared thermography and the electrical characterization to investigate a module with burned back sheet.
Fabrication of MWCNT/NiO nanocomposite thin films for optically selective solar absorbers
(2011-07) Roro, Kittessa T
This presentation explores solar thermal energy for harnessing solar energy for thermal energy (heat). Solar thermal collectors for water heating use a spectrally selective surface that absorb sunlight and convert it to heat. The presentation also gives an overview of the sol-gel synthesis used in this study.
The field performance of a 558 kWp ground mounted single-axis PV system in Pretoria, South Africa
(2017-12) Roro, Kittessa T; Basappa Ayanna, Manjunath; Pratt, Lawrence E; Lekoloane, Gaoshitwe A; Klein, Peter; Koopman, Stephen J
According to a recent Green Tech Media (GTM) publication [Moskowitz, 2017], 23% of all ground-mounted photovoltaic (PV) systems are installed on trackers. Trackers are especially beneficial for commercial and utility scale applications, particularly in sunniest parts of the world where the direct normal irradiance (DNI) component of sunlight is relatively high. However, very few studies have been reported on the field performance of single axis tracking systems installed and operated in the Southern Africa region. In this study, therefore, the real-world performance of a 558 kWp ground mounted single-axis PV system at the Council for Scientific and Industrial Research (CSIR) Pretoria campus over a period of 21 months was analyzed. The plant performance was evaluated in terms of the standard performance ratio based on the irradiance in the plane of array and in terms of the performance ratio based on the Global Horizontal Irradiance. It is anticipated that the result of the study will provide useful inputs in assisting system designers, researchers and operators of solar PV plants in South Africa in understanding the optimal operation of solar tracking systems.
Field performance of bifacal modules in South Africa
(SASEC, 2019-11) Basappa Ayanna, Manjunath; Mkasi, Hlaluku W; Pratt, Lawrence E; Roro, Kittessa T
This study presents the initial results of field performance analysis conducted on bi-facial modules installed on a fixed tilt rack and dual axis tracker system in Pretoria, South Africa. Bifacial modules produce better energy yield compared to monofacial modules depending on the geographical location, albedo of the back surface, height of a module from the surface and tilt angle. Currently multiple standards are under development to predict the bifacial gains and the field performance data is required to validate the models. This paper presents the bifacial gains over the monofacials without a high albedo back surface in a fixed tilt system, intercomparison of the grass cover, unpainted and white painted pavement and overall bifacial gain over the monofacials in dual axis tracker system. The bifaciality factor and efficiency curve of two bifacial types measured on indoor sun simulator is presented.
Global irradiance on photovoltaic array
(2018-06) Mkasi, Hlaluku W; Basappa Ayanna, Manjunath; Pratt, Lawrence E; Roro, Kittessa T
The amount of available solar irradiance is the most important parameter to determine solar energy generation for any given location. Satellite-based irradiance data is often used as a basis for predicting the energy generation for any location, as ground-based irradiance data is only available for specific sites. This paper presents a comparison of ground-based measured irradiance and satellite-based predicted irradiance and the corresponding modelled alternating current (AC) energy generation from a photovoltaic power plant. The 558 kW DC single-axis tracked photovoltaic (PV) plant located at the Council for Scientific and Industrial Research (CSIR) Pretoria campus is used for the case study. This paper briefly describes the techniques used to determine the Plane of Array (POA) irradiance from the Global Horizontal Irradiance (GHI) as well as the theory of operation for ground-based GHI sensors. The annual ground-based measured GHI at two locations with the CSIR campus were 3.1% and 4.9% lower compared to PVGIS Typical Meteorological Year (TMY) (1998-2011) and 1.9% and 3.6% lower compared with Meteonorm TMY (1960-2000) data, respectively. System Advisor Model (SAM) was used to model the AC energy output from the PV system. A strong correlation between the POA irradiance and alternating current (AC) power generation was calculated (R-squared = 0.987). The annual AC energy generation based on ground-based GHI measurements predicted 3.6% and 5.9% lower compared to PVGIS and 2.4% and 4.7% lower compared to Meteonorm TMY data, respectively.
A guideline for public entities on cost-efficient procurement of PV assets
(2015-09) Bischof-Niemz, T; Roro, Kittessa T
Traditionally, when public entities buy an asset under an Engineering, Procurement and Construction (EPC) contract, the screening of the responses to the request for proposals is done in two stages: In a filter stage, all proposals that are technically not acceptable because they do not fulfil certain firm criteria with regard to scope, quality or performance are excluded. In the second stage, proposals are then evaluated according to the financial offer (in most cases this will be the EPC lump-sum price in million Euros). In the case of buying a PV asset, this approach can be problematic, because evaluating proposals based on EPC price will inevitably lead to the tenderers offering only the minimum required installed capacity (in kWp) at the minimum quality, because any “over-delivery” in terms of scope or quality will lead to an increase in EPC price and thus to a lower chance of winning the tender. The CSIR Energy Centre therefore developed a methodology that can be applied by public entities in South Africa and elsewhere in the world to allow the procurement of PV assets at the lowest possible lifetime cost, measured in Levelised Cost of Electricity (LCOE). The methodology was successfully implemented in the procurement of the CSIR’s first 560 kWp, ground-mounted, single-axis tracker PV system on the main campus of the CSIR in Pretoria, South Africa. A very competitive LCOE and a very high quality PV system were achieved.

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