Studying the influence of Angola’s tropical climatic conditions on the operational efficiency of Silicon photovoltaic solar cells and finding technological solutions to enhance their performance

24 (iii) The equivalent stress is found maximum at vertical bar of support of solar panel. The maximum value is about 7.46 × 104 Pa. This value is lower than the limit stress of aluminum alloy (7.1 × 109 Pa). 3. Fe doped- TiO2 thin film deposited by USPD: (i) Fe-doped TiO2 thin film formed anatase phase and tetragonal structure. Crystal size is about 15 ÷ 25 nm; Eg = 3.32 ÷ 3.43 eV. (ii) With 1.5% Fe-doped TiO2 sample have the best activity Photocatalytic in UV with value kapp = 0.016 min-1. 4. W-doped TiO2 thin film deposited by sol-gel mix hydrothermal method: (i) W-doped TiO2 thin film formed anatase phase and tetragonal structure. Wolfram formed with structure WO3. (ii) W-doped TiO2 thin films have thickness about 1.30 ÷ 1.50 µm. Crystal size is about 15 ÷ 25 nm. High transmittance and optical band gap are about Eg = 3.6 ÷ 3.5 eV. (iii) With 2% W-doped TiO2 sample have the best activity Photocatalytic in Vis with value kapp = 0.14 min-1. 5. The Al2O3 ultra-thin films have been grown by ALD technique Trimethyl Aluminum (TMA), and water have been used as the metal and oxygen precursors: (i) The estimated deposition growth rate was 1.0 Å/cycle at deposition temperature of about 200oC. The Al2O3ultra-thin films have refractive index n = 1.6 ÷ 1.75 in visible light. (ii) The best valuable is about eff ~ 700 µs with thickness is about 11nm. If used post-deposition annealing (PDA), the carrier lifetime is eff ~ 770 µs after PDA with thickness is about 11 nm. the efficiency is the best about 20.55% when the Al2O3 film thickness is about 11 nm. 1 INTRODUCTION Alternative energy sources have been promoted due to the diminishing resources, rising costs and sustainability concerns faced by carbon-based fossil fuels. Wind and solar energy are the most promising renewable energy sources that receive significant amount of research and development. These energy sources are safe and much less harmful to the environment. Solar energy is beginning to grow and is currently on the rise, even though the price is still much more expensive than traditional energy sources [1]. The photovoltaics (PV) are currently poised to be used to harness solar energy. Photovoltaics, commonly known as solar cells, convert sunlight directly to electricity via p-n junction [1, 2]. Angola is in the Western region of southern Africa, occupying an area of approximately 1.246.700 km2 area, that makes Angola the sixth largest country of Africa. Luanda is the largest city in Angola and is also its capital. The population of Luanda is about 2.8 million people. Luanda is also urbanizing at approximately 4% annually. In short, one of the issues that attracted tremendous attention in the world and especially in Angola, is to study the applicability of solar cells to one of the most environmentally friendly renewable energy sources in remote areas and to reduce the cost of solar cells. This is a real and urgent issue to solve the problem of energy security. This is also the basis for us to select the contents of this thesis. Thesis title “Studying the influence of Angola’s tropical climatic conditions on the operational efficiency of Silicon photovoltaic solar cells and finding technological solutions to enhance their performance”. Object and scope of research of the thesis: The aim of this Thesis is to review photovoltaic module technologies for increased performance in tropical climate. This research seeks to review the cell, technologies utilised in PV module manufacture. This section reviews the major environmental factors that affect the performance of solar PV system. Ambient conditions such as the intensity of solar radiation, temperature, wind speed, humidity and dust significantly influence the performance of PV panels. 2 The impact of solar systems on rural livelihoods and experiment influence of tropical climate in the operation of solar cells in Angola. The simulation of the interaction between wind and the solar panels in Angola by computational fluid dynamics. Study and investigate the effect of technological parameters on TiO2 thin film deposition on the anti-reflective glass layer for cleaning in PV system. Study and investigate the influence of atom layer deposited (ALD) technology parameters on the formation of Al2O3 thin films applied as passive layer of c-Si solar cell. Research Methods: In this study, we used the experimental method in combination with theoretical guess and simulation method as computational fluid dynamics. All samples in the thesis are samples that we built ourselves on the systems we built and developed (except the ALD). The deposition methods include the Ultrasonic spray pyrolysis deposited (USPD) method, hydrothermal hydrolysis method and ALD method. Sample quality was investigated by X-ray diffraction (XRD), Raman scattering spectroscopy, scanning electron microscope (SEM), atomic force microscope (AFM), The scientific and practical significance of the thesis Scientific significance: Survey and study the effect of real environmental conditions on the characteristics of c-Si solar cells in Angola. Simulate the effect of wind on c-Si solar cells in real conditions in Angola. Study and investigate the effect of technological parameters on TiO2 thin film deposition on the anti-reflective glass layer for cleaning in PV system. Study and investigate the effect of ALD technology parameters on the formation of Al2O3 thin films applied as passive layer of c-Si solar cell: Study the conditions necessary for low temperature depossition of Al2O3 thin films by the ALD technique and examines properties of the resulting films. The practical meaning of the thesis: The first time investigated the effect of real conditions on the activity of c-Si solar cells in Angola. The first-time study examines the effect of wind on off-grid solar 23 Figure 4. 2. Morphology of Al2O3 thin films before PDA and after PDA 4.7. Chapter summary CONCLUSION 1. Experimented on the effects of climatic conditions in the Luanda, Angola on PV modules for a year was investigated. The weather parameters were studied as the intensity of sunlight, ambient temperature, humidity and wind speed in outdoor conditions. The investigation results show that: (i) When the ambient temperature rises by about 1o, the temperature of the c-Si solar cell system increases by nearly 10o. (ii) When the relative humidity of the environment increases from 60% to 85%, the efficiency of the PV decreases by 2.4%. 2. The simulated results shown that: (i) The inclined angle of solar panels β = 30o within velocity of wind 9m/s and horizontal wind direction (attack angle α equal zero degree) is the best choice of system. (ii) The lower left corner in the direction of the wind is the largest distortion of about 0.685 mm. 22 Al2O3 in the films. The Al2p binding energy of 74.1 ± 0.2 eV is within the range of values reported in references [186-188]. The major peak at the binding energy of 531.3 eV can be assigned to oxygen bonded to aluminum in Al2O3. Table 4. 1. XPS binding energy (eV) and atomic ratio of Al-2p and O-1s core level for Al2O3 Film Binding energy Al2p (eV) Binding energy O1s( eV) Atomic ratio O/Al Al2O3 74.1 531.4 1.79 The implied Voc increases dramatically from 652.6 mV to about 656.9 mV at 4nm of thickness. This result clearly showed the improved passivation performance of the ultrathin ALD-Al2O3 films. For 11nm of thickness, the maximum implied Voc, 659.1 mV, was obtained after PDA. Fig. 4.15 shows the effective minority carrier lifetime of p-type c- Si wafers passivated by 5, 10, 15 and 20 nm Al2O3. For original c-Si wafer, the eff was measured to be  6 ms. After depositing a 20 nm- thick Al2O3 layers, a higher eff of  900 ms is obtained. The results show that the effective minority carrier lifetime is improved when the Al2O3 thin films is deposited on c-Si surface. To study the full potential and the thermal stability of the surface passivation performances of Al2O3layers, the lifetime samples were exposed to PDA at temperature (Ta) ranging from 530 to 670 oC with PDA time of 5 min in nitrogen enviromental. The effective minority carrier lifetime increasing with increasing PDA of Al2O3 films and begin reduction at PDA about 600oC. The effective minority carrier lifetime is maximium value about 288.3µs at PDA about 600oC and 20nm of thickness Al2O3. The different thickness of Al2O3 thin films, effective minority carrier lifetime was effected. The blisters gas can be have before PDA,due to make reduction of effective minority carrier lifetime.Morphology of Al2O3 thin films before PDA and after PDA illutration in figure 4.16. 3 systems in Angola. The structure of the thesis In addition to the "Introduction", "Conclusion", "List of symbols and abbreviations", "List of tables", "List of images and drawings", and " references ", the thesis is presented in four chapters as follows: Chapter 1: Overview Chapter 2: Survey and study the effect of real environmental conditions on the characteristics of c-Si solar cells in Angola. Simulate the effect of wind on c-Si solar cells in real conditions in Angola. Chapter 3: Study and investigate the effect of technological parameters on TiO2 thin film deposition on the anti-reflective glass layer for cleaning in PV system. Chapter 4: Study and investigate the influence of ALD technology parameters on the formation of Al2O3 thin films applied as passive layer of c-Si solar cell. CHAPTER 1. LITTERATURE REVIEW 1.1. Overview of renewable energy use in the World In the most demanding conditions, increasing the share of renewable energy in the energy mix will require policies to stimulate changes in the energy system. The deployment of renewable energy technologies has increased rapidly in recent years. Cost has been identified as an important factor in the choice of energy sources especially in the developing countries like Vietnam or Angola. To increase the adoption of the PV module in the developing countries, the cost has to be as low as possible. We can identify that PV module’s cost plays an important role to determines the choice of energy for the individual, company, community or nation. To achieve a full competitiveness of PV energy worldwide (especially in those location where sun irradiation is lower) and to further reduce the price of electricity generated by PV modules, research is required to improve their conversion efficiency and reduce the material utilization, reducing in this way part of the production costs. In short, solar energy has become an increasingly important source of clean energy in the world. With significant advances in technology, solar cells can increase efficiency and promise to bring significant growth to this "green energy" industry in the future. 4 Nowadays, photovoltaics research will continue intense interest in new materials, cell designs, and novel approaches to solar material and product development. The price of photovoltaic power will be competitive with traditional sources of electricity. 1.2. Overview of solar cells use in the Angola In recent years, the Angolan government has been doing several efforts to popularize solar photovoltaic (PV) technology to supply energy services to people without access to an electric grid connection. Although the price for solar PV panels has decreased over the years, the costs of PV modules in Angola are still too high for most rural farmers or potential solar home system (SHS) users. Consequently, the climate of Angola is characterized by two seasons: the rainy season, from October to April and dry, known for Cacimbo, from May to August, drier, as the name implies, and at lower temperatures. Moreover, while the coast has high rates of rainfall, which will decrease from north to south and 800 mm for 50 mm, this area has annual temperature above 23 ° C. A wide range of benefits can be obtained from PV technology and solar systems, apart from light, for instance, operation of radio and TV sets, communication equipment, water pumps, fans, etc. Almost 50% of the households stated that the children benefited most from the PV System. Half of the respondents stated that having light was the best thing with a PV, and almost as many (43%) mentioned new possibilities for entertainment. The possibility to read and study at night was the greatest benefit by about a third of the respondents. 1.3. The photovoltaic effect 1.4. Physics of Solar Cells 1.5. Overview of Solar Cell Technologies 1.6. Influence of tropical climate in the performance of PV panels This section reviews the major environmental factors that affect the performance of solar PV system. Ambient conditions such as the intensity of solar radiation, temperature, wind speed, humidity and dust significantly influence the performance of PV panels. This Thesis focuses on the tropical climate like in Vietnam and Angola. These zones are characterised by high-temperature and humidity, heavy cloud cover and high rates of precipitation. The ambient temperatures range from 15 to 45 °C and it can provide the PV modules temperature to rise to 80 °C or higher. 21 CHAPTER IV- GROWTH AND CHARACTERIZATION OF AL2O3 ULTRA-THIN FILM AS A PASSIVATION LAYER FOR SILICON SOLAR CELLS 4.1. The need for silicon solar cells passivation layer AL2O3 4.2. Carrier Recombination in Crystalline Silicon 4.3 Surface passivation 4.4. Surface passivation materials 4.5. Growth Al2O3 ultra-thin film by Atomic Layer Deposition 4.6. Results and discussion From fitting results, the thickness of the 200-cycle film was estimated of 19.95  0.01 nm. VASE measurements and fitting were also done for other samples deposited at 50, 100 and 150 cycles, resulting thicknesses of 4.94 ± 0.01, 10.03 ± 0.01, 15.16 ± 0.01, respectively. The recorded binding energy profiles are shown in figure 4.6. The survey spectrum shows the peaks corresponding to the binding energies of Al, O and C. The presence of the C1s peak is advantageous as contaminant carbon can compensate the surface charging effect. The peaks observed at binding energies of 74.1 ± 0.2 eV and 531.4 ± 0.2 eV can be attributed to Al2p and O1s, respectively. These binding energies are in good agreement with the binding energies of Al2O3 films reported in literature [183-185]. The atomic concentration is calculated by using the following equation (4.12) with Ai the peak area off a photoelectron peak and Si the relative sensitivity factor of the peak (SO=0.711, and SAl=0.234). It can be found that the Al2p peak could be fitted with only one peak, suggesting that aluminum may be present only in the form of Figure 4. 1. XPS survey spectrum of Al2O3 film deposited at 200 cycles. 20 particles (W0; W1; W2 and W5) exhibit. The value of kapp can be calculated by the slope of linear plot. We can obtain that the kapp value of 2 wt% WO3/TiO2 is 0.14 min-1, 1 wt% WO3/TiO2 and 5 wt% WO3/TiO2 is about 0.11min-1 being approximately four times for pure TiO2 (0.004 min-1), respectively. Therefore, it dares say that coupling with an effective metal oxide is a promising selection to significantly enhance the photocatalytic activity of TiO2 under visible light illumination. 3.3.2.3. Superhydrophilic properties Superhydrolysis properties of samples were observed under UV irradiation. Images observed superhydrophilic ability of samples: F15, F35, F55 and F75 shown in figure 3.23. As we can see, in part glass substrate had not coated TiO2, the blue ink droplet was still shrink and almost no spread. In part of coated doped Fe, the blue ink droplet was spreader and formed thin film. In all four cases Fe doping, the survey results are almost same. Combining with the above result on UV-Vis spectra, the better hydrophilicity for Fe doped TiO2 thin film can be attributed to the doped TiO2 thin film with a narrower band gap. Hence, the narrower the band gap of the film is, the more the film accepts photon energy, and the greater the film creates superhydrophilicity. Figure 3.34. Images observed superhydrophilic ability of samples 3.3.3. Chapter conclution F15 F35 F75 F55 5 The relative humidity is in the range of 45 ÷ 95% with wind speeds of 0.2 m/s and higher. Consequently, PV modules operating in the tropical climatic conditions seem to be possessed higher failure rates than those in other climates. The next Sections discuss the major environmental factors that affect the performance of solar PV system. Chapter Sumary CHAPTER II - THE INFLUENCE OF TROPICAL CLIMATIC CONDITION ON THE SOLAR PV PERFORMANCE IN ANGOLA 2.1. Experimental introduction In this Chapter, we are analyzing the influence of Angolan tropical climate on performance of small scale, grid connected, silicon-based photovoltaic system located in Luanda, Angola from September 2011 to September 2012. The outputs of PV system under real working conditions are influenced by some environmental factors as solar radiation, ambient temperature, the surface temperature of the PV panels, meteorological data, and relative humidity. The importance of this study is in the analysis of a PV system in the first year of operation, to understand the initial performance and losses occurring in the beginning of the lifetime of the system, and to rank the factors that affect its performance. A PV system is in Ya Hoji Henda Central, Luanda, Angola, and the average daily irradiance in this region in Luanda is around 4.5 kW h/m2/day, and changes slightly from season to season. To test the performance of the system, the data recorded every second. The recordings of these data were collected several times a week at different time intervals throughout the day. 2.2. Influence of Solar radiation One of the main factors affecting the performance of PV systems is the amount of radiation to which cells are exposed. The irradiation solar on time in day in month in Angola are corresponding to the measurement. The amount of incoming solar irradiance is much higher in duration 11 a.m. until 4 p.m. which can be determined as the peak sun during the day. The irradiance intensity incident on a PV module in the field is not constant and may only reach 1100 W/m2 around solar noon. The maximum power output of the system with solar irradiance 6 was measurement. The data points fit a linear relationship seems to be the only solar irradiation variable which significantly determines the output of the system. It is shown that the output of the system is directly related to the amount of radiation reaching the PV panels. Thereby, it can be considered the site location has a big potential in installing the PV system. The highest amount of solar irradiance was found at 13.41 p.m (28.2.2012) with 1200 W/m2 where the minimum was 125.50 W/m2 at 9.49 a.m (04.1.2012). The low solar irradiance intensity might be due to a very short and a sudden blockage of the Sun disk with too heavy clouds. It can be noted from the results that irradiation solar is about 100W/m2 ÷1000 W/m2 in time in day. The irradiation solar are not more different between month in year. Addition, irradiation solar in horizontal and vertical is different. So, the irradiation solar is about 100W/m2 ÷1000 W/m2 respectively has been observed. 100 200 300 400 500 600 700 800 900 4 6 8 10 12 14 Pm ax (W ) Solar radiation (W/m2) 100 200 300 400 500 600 700 800 900 4 6 8 10 12 14 16 Ef fic ie nc y (% ) Solar irradiation(W/m2) Experiment fit y = 5.740+0.011x R2= 0.941 Figure 2.1, 2.7 Maximum output power and PCE versus solar radiation Figure displays the effect of solar irradiance on output power of PV panel at PV panel temperature about 45 °C. The highest output power of PV panel will be produced by a high solar irradiance. As illustrated in this figure, the most efficient power production by PV panel was 15.04 % when solar irradiance was 900 Wm-2. Unfortunately, the efficiency of PV panel was decreased when it was exposed to lower solar irradiance. The efficiency was found in the worst condition by 5.5 % when solar irradiance was 100 Wm-2. Ideally, the power output of a solar panel is proportional to the incident irradiance since the photo-current is proportional to the irradiance. However, the irradiance intensity will affect the conversion efficiency of a PV module due to the parasitic resistances 19 values up to 88 %. The optical band gaps of 3.6 to 3.51 eV were estimated for as-deposited and annealed films respectively. As it can be seen, in both cases, the gap energies obtained by this method are bigger than the value in stoichiomteric bulk TiO2 (3.20 eV for anatase phase and 3.11 eV for rutile) which is probably due to the polycrystalline nature and to the reduced grain size of the films. 3.3.2.2. Photocatalysis properties The photocatalytic activities of samples were determined by measuring the degradation of MB under visible light irradiation. The photocatalytic activities of pure TiO2 and Fe doped TiO2 as a function of reaction time. Among five tested, only the F15 (Fe doped 1.5%) showed the highest photocatalytic activity and as the concentration of Fe increased in TiO2, the photocatalytic efficiency was gradually decreased. Figure 3.21 shows the related kinetics data over the catalysts under visible light irradiation. The regression curve of natural logarithm of normalized MB concentration versus reaction time approximates linear, indicating that the kinetics of MB degradation over the photocatalysts follows first- order reaction kinetics: ln (C/C0) = kappt where C is the concentration of solute remaining in the solution at irradiation time of t and C0 is the initial concentration at t = 0. kapp denotes the degradation rate constant which enable to determine the photocatalytic activity. The correlation coefficients R2 are 0.96, 0.97, 0.78, 0.69 and 0.95 for F0, F15, F35, F55 and F75, respectively, at testing that the straight lines fit experimental data well. The value of kapp can be calculated by the slope of linear plot. From Fig.3.20, we can obtain that the kapp value of 1.5 wt% Fe doped/TiO2 is 0.016 min-1 respectively. The photocatalytic activities of pure TiO2 và WO3/TiO2 nano 0 20 40 60 80 100 120 140 160 180 200 0.0 0.5 1.0 1.5 2.0 2.5 3.0 ln (C /C o) Irradiation time, min F0 F15 F35 F55 F75 Figure 3.2. Linear transform ln(C0/C) = f(t) of the kinetic curves of MB degradation of TiO2: F0;F15;F35;F55 and F75 18 with the correlation coefficient r2 = 0.97. The relative error on Eg found is only 3 % on the average. Thin film TiO2 synthersized by sol gel hydrothermal method The XRD patterns and Raman spectra observed band positions are in accordance with the previous reports on the anatase phase [147]. No peaks correspond to the rutile phase was observed. The deposited films were uniform and highly adherent. . Figure 3.1. SEM images of W doped TiO2 thin films (a) W0; (b) W1; (b) W2 and(d) W5. SEM images of the WO3/TiO2 samples as well as pure TiO2 are shown in Fig. 3.17. The surface of pure TiO2 sample is smooth with nanoparticles size (Fig. 3.17a). An addition of WO3 affects the morphology of the sample and their surface becomes laced with irregularities. Also, pores and voids appear on the surface and became more pronounced with higher WO3 loadings. This effect is probably caused by the fast hydrolysis of the tungsten salt leaving holes behind them that create micron-sized cavities and pores. The films are highly transparent in the visible range of the electromagnetic spectrum with an average transmittance reaching a ) b ) c ) d ) 7 and the diode quality of the solar cells [106]. The dependence of the open circuit voltage Voc with solar radiation for the PV system is shown in Figure 2.8. The dependence between short circuit current and illumination intensity is linear. It can be easily to say that open circuit voltage are increases dependent on the radiation level. 300 400 500 600 700 800 900 49.0 49.5 50.0 50.5 51.0 O pe n ci rc ui t v ol ta ge , ( V ) Solar radiation, (W/m2) 100 200 300 400 500 600 700 800 900 5 10 15 20 25 30 Sh or t c irc ui t c ur re nt , ( A ) Solar radiation, (W/m2) Figure 2.2, 2.9 The open circuit voltage Voc and short circuit current Isc with varying solar radiation. Figure 2.9 illustrates the dependence of short circuit current Isc on the light intensity. It has been found that short circuit current demonstrated a small rise with increasing light intensity. The values of Isc for light intensity about 650W/m2 was 0.26 A, respectively. The value was slightly raised to be 0.275 A for light intensity about. It can be noted from the results that light intensity level has a crucial impact on current parameters of solar module rather than the voltage parameters. Such as the solar irradiance intensity exposed to PV system increases, the open circuit voltage and the short circuit current increase. As a result, the performance of the PV system is also increased. 2.3. Influence of Temperature As observed, the ambient temperature was at low temperature in the early morning with 23.2 °C caused by the low solar irradiance intensity of 92.50 W/m2. The ambient temperature during the experimental day starts to be increase with the increasing solar irradiance. The maximum ambient temperature was found at 1015 W/m2 by 31.8 °C, while the average for a day was 26.4 °C. In real working operation, most of solar energy productions occurs when the PV panel temperature is mostly higher than ambient temperature. The PV panel temperature starts to increase when solar irradiance was increased as well as the ambient temperature. There are so much differed between these two temperatures during peaks sun duration. 8 The maximum PV panel temperature found at 1015 W/m2 by 50.5 °C while the minimum was 24.6 °C at the lowest solar irradiance. Besides, the average PV panel temperature throughout the day was determined by 38.4 °C. The increase of PV panel temperature was due to higher insolation heating, low wind speed with the consequent low heat transferred from the panel to the ambient. To further understand the behavior of PV systems in varying temperatures, a relationship between the ambient temperature and temperature of the solar cell is considered. Thus, PV cells/modules operating in the tropical condition tend to generate low magnitude of electric current which leads to low power output and low performance. It was found that the open circuit voltage in figure 2.12, and the short circuit current in figure 2.13 did not decrease when the temperature increased. It was observed that the open circuit voltage reduce rapidly as the ambient temperature increases up to 42oC. Figure 2.13 shows the short circuit current density for an ambient temperature range of 30-42◦C. For semiconductors, the bandgap decreases as the temperature increases the open circuit voltage and the short circuit current have maximuum value in the ambient temperature range from 30oC to 36oC. In our experimental study, open circuit voltage Voc, and the short circuit current Isc did not decrease when the temperature increased were due to a difference in irradiation levels. Whilst analysing the effect of temperature on the PV system, it was observed that both the open circuit voltage and short circuit current slightly increases as the temperature increases up to 34oC, and then decreases as the temperature increases. The higher solar irradiance had a larger influence on the performance of the modules than an increase in the ambient temperature. The open circuit voltage and the short circuit current have maximuum value in the ambient temperature range from 30oC to 36oC. 17 different from that observed on F15. It reveals that the transmittance of TiO2 thin films has an abrupt decrease when wavelengths are below 355 nm. This indicates a shoulder at near 355 nm and a base which approaches near zero at about 300 nm. The transmittance quickly decreases when below 355 nm due to the absorption light caused by the excitation electrons from the valence band to the conduction band of TiO2. Moreover, the wavy nature of transmittance between 355 and 900 nm is due to the interference between the TiO2 thin films and substrate. Similar type of behaviour observed for other samples. From these spectra it seen that the average transmittance films increase with substrate temperatures. Increase in transmittance with substrate temperature to attributed to the thickness of the film, and nature of microstructure and surface morphology. The Eg values are about 3.32; 3.4eV and 3.43 eV with substrate temperatures at 400; 450 and 500oC. The obtained Eg value matches with spray pyrolysis TiO2 thin films reported by N. Mahdjoub et al [142]. Band gap and roughness of samples deposited at different substrate temperatures shown in the following (Table 3.5). Table 3. 4. Band gap and roughness of samples at substrate temperatures TT Substrate Temperature, (0C) Band gap, (eV) Roughness, (nm) 1 400 3.32 2.97 2 450 3.4 7.74 3 500 3.43 7.92 The calculated band-gap energies and the corresponding wavelengths are presented in table 3.5. The values indicate that the absorbance in the visible region of the doped samples increases with the concentration of the Fe3+ dopant which is consistent with the changes in the color of the samples from white to brownishbeige. It reveals that the band-gap energy decreases steeply at low Fe3+ concentrations. At high Fe3+ concentrations Eg continues decreasing but it changes much more slowly. In contrast to the results of earlier studies, an intensive study of all possible combinations of Eg and Fe3+ concentration (c) resulted with an exponential equation of the form [147]: Eg = 2.77e−0.16c 16 3.3.2.1. Characterzation material Thin film TiO2 synthersized by spray pyrolysis method Raman spectra and XRD patterns of the F0, F15, F35, F55 and F75 thin films belong to the anatase TiO2. In addition, no characteristic peaks of iron oxides phases appeared for all samples. The estimated crystalline size of the F0, F15, F35, F55 and F75 thin films was about 26 nm; 26 nm; 23,4; 21.1nm; and 18.9 nm. The error in this estimation was ± 0.3 nm. The addition of Fe3+ could occupy regular lattice site of TiO2 and distorted crystal structure, because the decrease in crystalline grain sizes of TiO2 (from 26 to 18.9 nm) when the Fe content increased (from 0 % to 7.5 % atm %) it can be caused by a number of defects in the anatase crystallites produced by the substitution of part of the Ti4+site by Fe3+ions. The results are consistent with the R. Meshach et al. [145]. Table 3. 3. Crystallite sizes, band-gap energies Eg and absorption wavelengths  of the undoped and Fe3+doped samples. Samples Particle zise D, nm Wave length , nm Band gap Eg, eV F0 26 360 3.4 F15 26 405 3.06 F35 23.4 415 3 F55 21.1 425 2.9 F75 18.9 440 2.8 3D and 2D AFM image of TiO2 films deposited under different substrate temperatures are presented. The TiO2 films composed of irregular grains with grain sizes of 30-60 nm. According to 3D AFM image, the average grain size increases with the increased substrate temperatures. In AFM analysis, rms is the most widely used value to characterize surface roughness [146]. The rms value of the TiO2 film is found to increase to the range of 3÷8 nm when the films deposited at substrate temperatures 400; 450 and 5000C. The increase in the roughness is due to the increase in the grain size. The samples were a uniform film with nanosize and non-cracks, consisting of nearly spherical nanoparticles of 10÷30 nm. The particle form and size of these samples were like those of the F15 sample. In the case of F35, F55 and F75, the morphology was 9 30 32 34 36 38 40 42 45.0 47.5 50.0 52.5 55.0 Ambient temperature, (oC) O pe n ci rc ui t v ol ta ge , ( V ) 24 26 28 30 32 34 36 38 40 42 44 15.0 17.5 20.0 22.5 25.0 27.5 30.0 Sh or t c irc ui t c ur re nt , ( A ) Ambient temperature, (oC) Figure 2.3, 2.4. Open circuit voltage and Short circuit current versus ambient temperature. 2.4. Influence of humidity High temperature-humidity study was performed on c-Si PV modules, and their performance degradations due to moisture inception are discussed in this section. When PV cells are exposed to humidity for long term there will be some degradation in performance. It has been observed that the high content of water vapour in the air causes encapsulant delamination. 55 60 65 70 75 80 85 90 46.0 46.5 47.0 47.5 48.0 48.5 49.0 Experiment Fit O pe n cir cu it vo lta ge , ( V) Relative humidity, (%) y = 50.27-0.04x R2 = 0.978 55 60 65 70 75 80 85 20 21 22 23 24 25 26 Experiment Fit Sh or t c irc ui t c ur en t, (A ) Relative humidity, (%) y = 29.76-0.082x R2 = 0.979 Figure 2.5, 2. 6 The relationship between open circuit voltage, short circuit current and relative humidity The open circuit voltage of PV module reduced with increasing relative humidity, as figure 2.16 represents. The relative humidity has an adverse impact on solar radiation so that the resultant negative influence reflects on the PV cell output voltage. The open circuit voltage was reduced by 2.1% with relative humidity increasing 60% to 85% respectively. Relationship of open circuit voltage with relative humidity trend of linear with R2 = 0.978. This trend of linear relationship may be influenced by the ambient temperature and windy velocity which had a strong inverse relationship with humidity 10 in this data set. Figure 2.17 illustrates the effect of relative humidity on the short circuit current produced. Increasing relative humidity reduced current highly. Increasing relative humidity from 60 to 85% reduced the short circuit current by 8.2%. The obtained results illustrated that at low relative humidity conditions the open circuit voltage, and short circuit current increase. This results egree with Kazem [109]. Addition, the extent of this linear relationship with R2 about 0.979. The Luanda characterized by its medium temperatures that it ranges from 26 to 40°C. Figure 2.18 shows the relation between relative humidity and air temperature for the studied period. Relative humidity increase causes a relative reduction in air temperature. The recorded air temperatures in the Luanda summertime were suitable for the PV arrays operation. However, the relative humidity seems as the dominated affecting factor in this city that reduces the PV performance. The relative humidity causes a reduction in the solar intensity that reduced the resulted efficiency of a PV panel, as figure 2.19 reveals. The reductions in the tested PV panel efficiency were about 18 % respectively. When moisture entered the solar cell, it provides an additional shunt path for the output current. This is equivalent to the reduction of the Rp which a significant reduction can be. When the output is shorted circuited to measure the Isc, this shunt resistance will shunt away some of the photon current Iph generated from the photon absorption, resulting in a reduction in the Figure 2.7. The relationship between relative humidity and ambient temperature 24 26 28 30 32 34 36 38 40 42 44 20 30 40 50 60 70 80 90 R el at iv e hu m id ity , ( % ) Ambient temperature, (oC) Figure 2.8. The relationship between relative humidity and PV efficiency 60 65 70 75 80 85 11.2 12.0 12.8 13.6 14.4 15.2 Experiment Fit Ef fic ie nc y, (% ) Ralative humidity, (%) y = 21.1 - 0.1x R2 = 0.984 15 contact angle is formed (θ≤5°), as the water tends to spread completely across the surface rather than forming droplets. Nanostructures of these surfaces could enhance contact angle over 150°. So the raindrops, immediately after reaching the surface, would roll on it and wash away dust particles. [130]. Titanium dioxide nanofilms has super-hydrophobic properties for self cleaning surfaces due to its photocatalytic activity and photo-induced super- hydrophilicity. [131]. 3.3. Characterization of nanocrystalline titania thin film deposited by spray pyrolysis technique 3.3.1. Experiment 3.3.1.1. Prepare 3.3.1.2. Synthesized thin film TiO2 by spray pyrolysis method Table 3. 1. The factor investigation and sample TT Sample Factor 1 T-40 [Fe] = 0 [Ti]/[AC] = 1 : 2; V = 20 ml v = 1.0 ml/min; L = 30 cm Calcinal: TC = Ts oC, t = 20 mins; Environmental: air 2 T-45 [Fe] = 0 3 T-50 [Fe] = 0 4 F-15 [Fe] = 1.5 [Ti]/[AC] = 1 : 2 ; T = 450 oC V = 20 ml ; v = 1,0 ml/min L = 30 cm Calcinal: TC = 450 oC, t = 20 min; Environmental: air 5 F-35 [Fe] = 3.5 6 F-55 [Fe] = 5.5 7 F-75 [Fe] = 7.5 3.3.1.3 Synthesized thin film TiO2 by sol gel –hydrothermal method Table 3. 2. The table of samples Number Sample Factor 1 W-0 [W] = 0 Sol gel, Hydrothermal T = 140oC t = 24h Dry : T = 90oC ; t = 24h Calcined : T = 550oC ; t = 2h (for particles) 2 W-1 [W] = 1 3 W-3 [W] = 3 4 W-5 [W] = 5 3.3.1.4. Investigate characteration of materials 3.3.1.5. Photocatalysis 3.3.1.6. Superhydrophilic 3.3.2. Results and discussion 14 velocity of wind increases from 3 to 6 m/s, the lift force increases but the drag forces decreases. For wind velocity from 6 to 15m/s, lift and drag forces vary with a very small difference as see in figure 2.39a. The aerodynamic quality of solar panels slight with increasing of wind velocity from 3 to 15m/s. The variable of aerodynamic characteristics of solar panel is quite small. So, the 9m/s of wind velocity is choosing to estimate the simulation about direction of wind. c. Effect of wind direction At 45o direction of wind, the lift force is negative but aerodynamic quality of solar panels is smallest. At 135o direction of wind, both lift force and drag force is negative. It seems that the solar panels could not keep its fixed position. d. Strength analysis of solar panels The solar panels are deformed at four corners. In it, the lower left corner in the direction of the wind is the largest distortion of about 0.685mm. Equivalent stress is found maximum at vertical bar of support of solar panel. The maximum value is about 7.46x104Pa. This value is lower than the limit stress of aluminum alloy (7.1x109Pa). Thus, we could conclude that solar panels are durable with wind velocity 9 m/s, attack angle 0o and 30o inclined angle of solar panels. CHAPTER III- EXPERIMENTAL STUDY ON PHOTOCATALYTIC PROPERTIES OF TITANIUM DIOXIDE APPLICATION AS SOLAR CELL SELF CLEANING LAYER 3.1. Settlement overview of dust on a solar panels glass cover Perhaps the most optimal method to remove dirt from surfaces is to modify the surface to obtain self-cleaning properties. 3.2. Thin film TiO2 using as self-cleaning material of solar panels Super-hydrophobic surfaces have the same properties as lotus leaves. They show extremely low wetting property and greatly high hydrophobicity. On the super-hydrophilic surface, a very small water 11 measured Isc as observed experimentally. The reduction will be even more significant if Rs increases due to the corrosion of the contact pads under the exposure of moisture. As the degradation of Voc is small as observed experi mentally, the reduction in Isc will result in a reduction in the Pmax. Hence, the degradation trend of Pmax is like that of Isc. 2.5. Effect of radiation on PV characteristics The effect of neutron radiation has been investigated in solar cells. C-Si n+-p solar PV samples have equal surface area about 4 cm2. I-V characteristics are measured in the dark and illuminated before and after projecting by neutrons. All samples were irradiated at room temperature with different time and radiation fluencies. Samples irradiated by intensity of 103 particles per second at the dose  (6*108; 1.2*109; 1.8*109; and 2.4*109 particles). Table 2.1. The influence of irradiation on Isc and Voc of experimental PV samples. Before projecting by neutrons After projecting by neutrons with different illumination dose  (particles) 6*108 1.2*109 1.8*109 2.4*109 Isc (mA) 13.45 9.93 8.69 7.79 7.45 Voc (V) 0.475 0.455 0.44 0.43 0.41 2.6. Effect of an inverter Figure 2.22 displays the power produced and converted with a calculated average of 50% conversion efficiency. It was too low to what was rated by the manufacturer. Thus, when the efficiency of the system was considered with the inverter, it dropped by almost 12%, from 4.9% to 4.2%. This was twice the amount of reduction in efficiency caused by a combination of factors that include, wind speed, increasing ambient temperature, humidity, real power conversion 21-Aug--27-Aug-- 7-Sep --11-Sep --18-Sep -- 0 2 4 6 8 10 12 14 Po w er (W ) Date Pmax Pac Figure 2. 9. Comparison of power converted by inverter (Pac) and power produced by PV system (Pmax). 12 efficiency of the inverter, and a general degradation of the system. 2.7. Effect of wind actions on PV panels 2.7.1. General The two most common methods of solar panel deployment are on the ground or on the roof of building. These systems are sensitive to wind loading but design standards and codes of practice offer little assistance to the designers regarding provisions for wind-induced loading. The effect of wind on the solar panels located in the space is as follows: i) Wind speed; ii) Wind direction; iii) Height of the building and the presence of the parapet; iv) Angle of inclination of the panels. 2.7.2. Computational Fluid Dynamics (CFD) Procedure Aiming to determine the simulations of the interaction between the wind and solar panels in this study, we use the computer program ANSYS 18.2 fluent, which is used essentially, to calculate the possible flows of winds and the complex pressures that act on its surface. The proceeds to simulate the fluids flow problem by using CFD tool in ANSYS software include five basic steps below: - Identify computational domain; - Mesh computational domain; - Set up numerical conditions; - Solve occurring problems; - Analyze the results. 2.7.3. Effect of wind actions on solar panels a. Effect of inclined angles According to the calculated results as shown in figure 2.34, the wind affects a negative lift to solar panels. It means that the solar panels adhere with ground. When the solar panels are inclined with increased angle, the lift and drag forces vary with a little difference but aerodynamic quality (CL/CD) increases in the absolutely value. The wind acts on the solar panel with a minimum force, and solar panels have less damage by wind. 13 The variable of aerodynamic characteristics of solar panel is quite small. So, we choose the solar panels installed with inclined angle of 30o. This is also accordant with sunlight conditions in Angola. 20 25 30 35 40 -0.3 -0.2 -0.1 0.0 0.1 0.2 a) CD CL Co ef fic ie nt o f l ift a nd d ra g fo rc e Inclined angle (degree) 20 25 30 35 40 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 b) Inclined angle (degree) C L/ C D Figure 2.10. Effect of inclined angle of PV to aerodynamic characteristics - Wind velocity 3m/s & Attack angle 0o: a) Coefficient of lift and drag force and b) Aerodynamic quality b. Effect of wind velocity According to the calculated results as shown in figure 2.39, the wind affects a negative lift to solar panels. 0 3 6 9 12 15 18 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 a) CD CL Wind velocity (m/s) C oe ffi ci en t o f l ift a nd d ra g fo rc e 0 3 6 9 12 15 18 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Wind velocity (m/s) CL /C D b) Figure 2. 11. Effect of inclined angle of PV to aerodynamic characteristics - Inclined angle 30o, Attack angle 0o: a) Coefficient of lift and drag force and b) Aerodynamic quality It means that the solar panels adhere with ground. When the