Studying the influence of angolas tropical climatic conditions on the operational efficiency of silicon photovoltaic solar cells and finding technological solutions to enhance their perfomance

This following analysis the effects of solar radiation, ambient temperature, the surface temperature of the PV panels, meteorological data, and relative humidity on the performance of a

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

MATEUS MANUEL NETO

DISSERTATION 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

Major: Engineering Physics

1 ASSOC PROF PhD NGUYEN NGOC TRUNG

2 PROF PhD VO THACH SON

HA NOI - 2018

i

CONTENTS

CONTENTS i

LIST OF SYMBOLS iv

LIST OF ABBREVIATIONS vi

LIST OF FIGURE viii

LIST OF TABLES xii

INTRODUCTION 1

CHAPTER 1 LITERATURE REVIEW 4

1.1 Overview of renewable energy use in the World - 4

1.2 Overview of solar cell use in the Angola - 6

1.3 The photovoltaic effect - 9

1.4 Physics of Solar Cells -11

1.5 Overview of Silicon Solar Cell Technologies -17

How does PERC technology improve performance? -20

1.6 Influence of tropical climate in the performance of PV panels -21

1.6.1 Spectrum -22

1.6.2 Irradiance -22

1.6.3 Module temperature -23

1.6.4 Wind speed -23

1.6.5 Incident angle -23

1.6.6 Effect of humidity on the solar panels -24

1.6.7 Effect of dust on the solar panels -25

CHAPTER 2 THE INFLUENCE OF ANGOLA TROPICAL CLIMATIC CONDITION ON THE PV SYSTEM PERFORMANCE 27

2.1 Experimental introduction -27

2.2 Influence of Solar radiation -29

2.3 Influence of Temperature -32

2.4 Influence of humidity -37

2.5 Effect of radiation on PV characteristics -40

2.6 Effect of an inverter -42

2.7 Effect of wind actions on PV panels -42

2.7.1 General -42

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2.7.2 Computational Fluid Dynamics (CFD) Procedure -47

2.7.3 Effect of wind actions on solar panels -51

Chapter summary -67

CHAPTER 3 STUDY ON TITANIUM DIOXIDE APPLICATION AS SOLAR CELL SELF-CLEANING LAYER 68

3.1 Settlement overview of dust on a solar panels glass cover -68

3.2 Thin film TiO2 using as self cleaning material of solar panels -69

3.2.1 The self-cleaning properties -69

3.2.2 Theoretical basis of self-cleaning -69

3.2.2 1.Young’s equation -70

3.2.2.2 Cassie model -70

3.2.2.3 Wenzel model -71

3.2.2.4 Cassie and Baxter’s Equation -71

3.2.3 Thin film TiO2 using as self-cleaning material -72

3.3 Nanocrystalline TiO2 thin film deposited by spray pyrolysis technique and sol gel –hydrothermal method -73

3.3.1 Experiment details -74

3.3.2 Characterizing of TiO2pure and doped iron or tungsten thin film 77

3.3.2.2 Characterizing of Fe-dopedTiO2 thin film -81

3.3.2.3 Characterizing of W-dopedTiO2 thin films -85

3.3.3 Self-cleaning properties of TiO2pure and doped iron or tungsten thin film -88

Chapter conclusion -93

CHAPTER 4 GROWTH AND CHARACTERIZATION OF Al2O3 ULTRA-THIN FILM AS A PASSIVATION LAYER FOR SILICON SOLAR CELLS 95

4.1 The need for silicon solar cells passivation layer Al2O3 -95

4.2 Carrier Recombination in Crystalline Silicon -96

4.3 Surface passivation -98

4.4 Surface passivation materials - 100

4.4.1 Silicon dioxide SiO2 - 101

4.4.2 Hydrogenated amorphous silicon nitride a-SiNx:H - 101

4.4.3 Hydrogenated amorphous silicon a-Si:H - 101

4.4.4 Aluminum oxide Al2O3 - 102

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4.5 Growth Al2O3 ultra-thin film by Atomic Layer Deposition - 102

4.5.1 Introduction ALD technique - 102

4.5.2 Growth process of thin film aluminum oxide by ALD - 104

4.5.3 Process for p-type Si solar cells(PERC) fabricated - 105

4.5.4 The investigated method - 107

CONCLUSION - 117

The scientific and practical significance of the thesis - 118

Recommendations for Future Studies - 119 REFERENCES Error! Bookmark not defined

iv

LIST OF SYMBOLS

v

47 Auger Carrier lifetime due to band to band i.e Auger recombination

vi

LIST OF ABBREVIATIONS

vii

viii

LIST OF FIGURE

Figure 1.1 Price comparison of energy sources [5] 4

Figure 1 2 PV module price over time [14] 5

Figure 1.3 Graph illustration absorption of a photon in a semiconductor with an Eg band gap [16] 10 Figure 1.4 Component parts of a typical PV cell 11

Figure 1.5 Graph illustration structure of a 1D p-n homogeneous solar cell 12

Figure 1.6 Ideal short-circuit current density of p-n junction solar cell as a function of Eg [22] 14

Figure 1.7 J- V curves of a solar cell in the dark and under illuminated condition [17] 15

Figure 1.8 Graph illustration equivalent circuit of a real solar cell [17] 15

Figure 1.9.Graph of p–n junction solar cell factor as a function of band gap 16

Figure 1.10 Market share of PV cells (%) [26] 17

Figure 1.11 Scheme of a modern crystalline silicon cell [29] 18

Figure 1.12 Imaging of Poly‐crystalline Si cell (a), and Mono-crystalline Si cell (b) [19] 20

Figure 1.13 The structure of a conventional cell (a) and the structure of a cell with PERC technology (b) [17] 20

Figure 1.14 A cell with PERC technology will generate more current due to the reflection of light at the backside of the cell 21

Figure 2 1 Average monthly insolation levels in Luanda 27

Figure 2 2 Diagram of collecting and monitoring data of operating solar panels 28

Figure 2 3 Kipp & Zonen’s Pyranometer Model CMP6, ISSO 9060 / WMO First Class Standard: (a) Solarimeters measurement system; (b) The anemometer 28

Figure 2 4 The software for collect data 29

Figure 2 5 The solar irradiance intensity through the experimental day in day 29

Figure 2 6 Maximum output power versus solar radiation 30

Figure 2 7 The PCE versus solar radiation 30

Figure 2 8 The open circuit voltage Voc(a) and the short circuit current Isc (b) with varying solar radiation 31

Figure 2.9 The ambient temperature and modules temperature on time in day 33

Figure 2.10 Average panel temperature versus ambient temperature 34

Figure 2.11 Open circuit voltage (a) and Short circuit current (b) versus ambient temperatures 35

Figure 2.12 The conversion efficiency versus ambient temperature 37

Figure 2.13 Maximum power output versus wind speed 37

Figure 2.14 The relationship between open circuit voltage and relative humidity 38

Figure 2 15 The relationship between short circuit current and relative humidity 38

Figure 2.16 The relationship between relative humidity and ambient temperature 39

Figure 2 17 The relationship between relative humidity and PV efficiency 39

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Figure 2 18 Equivalent circuit for amorphous solar cell The current sink Irec stands for

recombination losses in i-layer 40

Figure 2.19 Current-Voltage characteristics of PV cells before and after projecting by neutrons with different illumination dose :1-=0; 2- =6*108; 3-=1.2*109; 4-=1.8*109; and 5-=2.4*109 41

Figure 2.20 Comparison of power converted by inverter (Pac) and power produced by PV system (Pmax) 42

Figure 2 21 Location of the application point of the global wind force acting on monopitch canopies (SR EN 1991-1-4/2006) 44

Figure 2 22 Scheme of a free standing panel in the air flow (a, c [81]); (b) the resulting movement due to flow separation [82] 44

Figure 2 23 Illustration of lift and draft forces 45

Figure 2.24 Wind speed (km/h) in Luanda shows days per month 46

Figure 2 25 Solar panels placed at ground level 47

Figure 2.26 Generated model of solar panels(a) Support and (b) Support with solar panels 48 Figure 2.27 Computational domain 49

Figure 2.28 Mesh of computational domain 49

Figure 2.29 Mesh of computational domain 50

Figure 2 30 Distribution of pressure and streamline of fluid flow around solar panels at centered XY plan – Wind velocity 3m/s & Attack angle 0o 51

Figure 2 31 Distribution of pressure and streamline of fluid flow around solar panels at centered YZ plan – Wind velocity 3m/s & Attack angle 0o 52

Figure 2.32 Distribution of pressure and streamline of fluid flow around solar panels at centered XZ plan – Wind velocity 3m/s & Attack angle 0o 53

Figure 2.33 Distribution of pressure on solar panels – Wind velocity 3m/s & Attack angle 0o 54

Figure 2.34 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 55

Figure 2 35 Distribution of pressure and streamline of fluid flow around solar panels at centered XY plan – Inclined angle 30o& Attack angle 0o 56

Figure 2 36 Distribution of pressure and streamline of fluid flow around solar panels at centered YZ plan – Inclined angle 30o& Attack angle 0o 57

Figure 2 37 Distribution of pressure and streamline of fluid flow around solar panels at centered XZ plan – Inclined angle 30o& Attack angle 0o 58

Figure 2 38 Distribution of pressure on solar panels – Inclined angle 30o& Attack angle 0o 59 Figure 2 39 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 60

Figure 2 40 Distribution of pressure and streamline of fluid flow around solar panels at centered XY plan – Wind velocity 9m/s & Inclined angle 30o 61

Figure 2 41 Distribution of pressure and streamline of fluid flow around solar panels at centered YZ plan – Wind velocity 9m/s & Inclined angle 30o 62

Figure 2.42 Distribution of pressure and streamline of fluid flow around solar panels at centered XZ plan – Wind velocity 9m/s & Inclined angle 30o 63

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Figure 2.43 Distribution of pressure on solar panels – Wind velocity 9m/s & Inclined angle 30o 64 Figure 2.44 Effect of inclined angle of PV to aerodynamic characteristics - Wind velocity

9m/inclined angle 30o: a) Coefficient of lift and drag force and b) Aerodynamic quality 65

Figure 2.45 Mesh of strength analysis problem: a) Without solar panels and b) With solar panels 66 Figure 2.46 Total deformation - Wind velocity 9m/s; Attack angle 0o& Inclined angle 30: 66

Figure 2.47 Equivalent Stress - Wind velocity 9m/s; Attack angle 0o& Inclined angle 30o: 66 Figure 3 1.Contact angle platform 70

Figure 3.2 Wetting on flat and rough surfaces: (a) flat, (b) rough, Wenzel case; (c) Cassie and Baxter case 71

Figure 3.3 Type two self-cleaning mechanism exists of TiO2 thin films 72

Figure 3.4.The pyrolysis system at SEP, HUST 75

Figure 3.5 Raman spectra of the TiO2 films deposited at substrate temperatures 78

Figure 3 6.FESEM images of TiO2 films deposited at: a) 4000C; b) 4500C 78

Figure 3.7 AFM image of TiO2 films deposited at substrate temperatures: a) 400; b) 450 and c)5000C 79

Figure 3 8.The optical transmission spectra of TiO2 films deposited at substrate temperatures 80

Figure 3.9 The plots of (αh)2 versus h for optical band gap the calculation of TiO2 films deposited at substrate temperatures 81

Figure 3.10.XRD patterns of the F0, F15, F35, F55 and F75 samples 82

Figure 3.11 FESEM images of the Fe doped TiO2 thin films: (a) F15; (b) F35; (c) F55 (d) F75 83

Figure 3.12 Transmittance spectra of Fe doped TiO2 thin films: (a) F0; (b) F15; (c) F35; (d) F55 (e) F75 83

Figure 3.13 Plot of the band-gap and Fe3+ concentration of the doped samples 84

Figure 3 14.XRD patterns of the W doped TiO2 thin films: (a) W0; (b) W1; (b) W2 and(d) W5 85

Figure 3.15 Raman spectra of W doped TiO2 thin films: (a) W0; (b) W1; (b) W2 and(d) W5 86

Figure 3.16 SEM image of a) TiO2 thin film and b) its cross sectional 86

Figure 3.17 SEM images of W doped TiO2 thin films (a) W0; (b) W1; (b) W2 and(d) W5 87

Figure 3.18 Transmittance spectra of W doped TiO2 thin films: (a) W0; (b) W450; (c) W1; (d) W2 (e) W5 87

Figure 3.19 The plot of (h)2 vs hfor the W doped TiO2 thin films: (a) W0; (b) W450; (c) W1; (d) W2 (e) W5 88

Figure 3.20 Photocatalytic degradation rate of MB in the presence of samples 89

Figure 3.21 Linear transform ln(C0/C) = f(t) of the kinetic curves of MB degradation of TiO2 90

Figure 3.22 Photocatalytic degradation rate of MB in the presence of TiO2 91

Figure 3.23 Linear transform ln(C0/C) = f(t) of the kinetic curves of MB degradation of W doped TiO2 thin films: (a) W0; (b) W1; (b) W2and (d) W5 under visible light irradiation 92

Figure 3 24 Images observed superhydrophilic ability of samples 93

Figure 3.25 Images observed superhydrophilic ability of samples under visible light irradiation 93

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Figure 4 1 Schematic of recombination mechanism 97 Figure 4 2 Schematic drawing for ALD process in one cycle 104 Figure 4 3 The flow diagram for p-type Si solar cells(PERC) fabricated 106 Figure 4 4 (a) Schematic of the inductively coupled photoconductance apparatus used for measuring the effective lifetime (b) The Xenon photo flash lamp WCT-120 here used for QSSPC, fabricated by Sinton Consulting 109 Figure 4 5 Spectroscopic ellipsometry measurements for the sample deposited at 200 cycles: (left) amplitude ratio Psi (in percentage) with WVASE32 parameter fitting, (right) phase shift delta (in degree) with WVASE32 parameter fitting The measurements were carried out with different incident angles 110 Figure 4 6 Al2O3 film thickness as a function of the number of ALD cycles (Tdep ≈ 2000C) 111 Figure 4 7 Refractive index (n) and extinction coefficient (k) as functions of wavelength of the 200-cycle Al2O3 sample 111 Figure 4 8 SEM image of 30nm ALD Al2O3 on Si 111 Figure 4 9 XPS survey spectrum of Al2O3 film deposited at 200 cycles 112 Figure 4 10 High resolution XPS spectra of Al2p and O1s for Al2O3 film deposited at 200 cycles 113 Figure 4 11 Affection of substrate temperature on Al2O3 film growth by ALD technique 113 Figure 4 12 The plots effective carrier lifetime as a function of Al2O3 film thickness 114 Figure 4 13 The plots effective Voc as a function of Al2O3 film thickness 114 Figure 4 14 The plots effective efficiency as a function of Al2O3 film thickness 115 Figure 4 15 The plots effective Effective minority lifetime as a function of annealing temperature 115 Figure 4 16 Morphology of Al2O3 thin films before PDA and after PDA 116

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LIST OF TABLES

Table 1 1 Solar Village Program 8

Table 2 1 The influence of irradiation on Isc and Voc of experimental PV samples 41

Table 2 2 Terrain roughness, power-law exponent and boundary layer thickness values corresponding to different exposure categories [80] 43

Table 2 3 Characteristics of solar panels 48

Table 2.4 Boundary conditions 50

Table 3.1 The table of chemicals 74

Table 3.2 The factor investigation and samples 76

Table 3.3 The table of samples 76

Table 3 4 Band gap and roughness of samples at substrate temperatures 81

Table 3 5 Crystallite sizes, band-gap energies Eg and absorption wavelengths  of the undoped and Fe3+doped samples 84

Table 4 1 List of materials grown by ALD [139]–[143] 103

Table 4 2 Available reactant groups for specific elements [140]–[144] 103

Table 4 3 XPS main-peak parameters, atomic concentration, and O/Al atomic ratio of the 200-cycle sample 112

Table 4 4 XPS binding energy (eV) and atomic ratio of Al-2p and O-1s core level for Al2O3 113

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 received 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 photovoltaic (PV) are currently poised to be used to harness solar energy Photovoltaic, commonly known as solar cells, convert sunlight directly to electricity via p-n junction [2]

Angola is in the Western region of southern Africa, occupying an area of approximately 1.246.700 km2, which makes Angola be the sixth largest country of Africa The extent of Angolan coastline is of more than 1.600 kilometers, bordering the Atlantic Ocean Angola has land borders

to the North with the Republic of Congo and the Democratic Republic of the Congo, to the East with the Democratic Republic of Congo and Republic of Zambia, and to the South with the Republic of Namibia, with an extension of more than 1.400 km [3]

The current population of Angola in 2018 is estimated at 29.25 million, up from 2014's estimated 25.6 million As one of the least densely populated countries in the world, Angola has a density of 14.8 people per square kilometer Luanda is capital in Angola The population of Luanda is about 6.5 million people Luanda is also urbanizing at approximately 4% annually Although Angola is a member of the Power Pool of South Africa (Southern African Power Pool, SAPP) and the Power Pool of Central Africa (Energy Pool of Central Africa-ECCP), the country has no current links with its neighbors, with exception of several isolated lines that supply several towns on the south with Namibia Today, more than 13 million Angolans, or about 26 percent of the population, have no access to electricity, according to the International Energy Agency (IEA) Rural areas, especially those in the rural south of the country, have no access to a grid

Angola is a Southern African nation whose varied terrain encompasses tropical Atlantic beaches, a labyrinthine system of rivers and Sub-Saharan desert that extends across the border into Namibia This fact benefits the existence of insulin throughout almost every year The global solar radiation is in the annual horizontal plane, between 1.370 and 2.100 kWh/m2 and year Due

to the exceptionally sunny climate, solar energy holds huge potential for Angola, especially in rural areas The Rural Electrification Program issued mid 2012 aims to benefit 8 million people in 2016 Solar generation systems play a large part in the second and third round of this program, which focuses on building mini systems and distribution of electricity This goal

is part of the Angeles 2025 goal to increase capacity by four times and to achieve

an electrification rate of 50-60% The World Energy Outlook 2011 of the IEA states that 13.7 million Angolans (26.2%) have no structural access to electricity, especially in the rural south

of Angola Because of the high irradiation levels (6.3 kWh/m2day in Angola), Solar PV has huge potential for providing a solution to Angola’s energy difficulties

The Ministry of Energy and Water (MINEA) is now in a phase of concluding the National Strategy for Renewable Energies Solar PV will be essential in short, medium and long term MINEA announced the installation of 142 solar PV systems worth of 534.6

MW from 70 solar PV villages (grid connected) Furthermore, a capacity building program conducted by Econ Pöyry emphasizes a large potential for solar PV in the south of the country, where hydropower resources are more limited The study includes a 3 MW solar plant in Tombua, with potentially further solar plants being installed in Namibia and Benguela However, this is just the beginning The Angolan government has stated that it still needs a lot of knowledge and experience to successfully implement more renewable energy installations

Due to the Angola’s climate and location, solar energy seems to be the primary choice for investigation This following analysis the effects of solar radiation, ambient temperature, the surface temperature of the PV panels, meteorological data, and relative humidity on the performance of a c-Si solar cells, grid-tied system 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 The study will focus on the technology available to harness it Details

on the PV technology and setup used in this research are discussed in the following sections

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 study an influence of Angola’s tropical climatic conditions on operating efficiency of Silicon photovoltaic and technological solutions to enhance their performance

This research seeks to review:

- The major Angola’s tropical environmental factors such as the intensity of solar radiation, temperature, and wind speed, humidity and dust that affect the performance

- Ultra-thin film Aluminum oxide deposited by atom layer deposition (ALD) application as passivation layer in thin film solar cell

3

Research Methods:

We have experimented on the effects of climate conditions in the Luanda, Angola on PV modules The weather parameters we studied as the intensity of sunlight, ambient temperature, humidity and wind speed in outdoor conditions

The flow of wind over the model of a photovoltaic structure exposed to wind flowing over

an open terrain has been investigated using ANSYS Fluent software

The deposition methods include the USPD method, sol gel mix hydrothermal method and ALD method

Sample quality was investigated by X-ray diffraction, Raman scattering spectroscopy, scanning electron microscope, atomic force microscope, UV-VIS spectra, variable-angle spectroscopic ellipsometry and X-ray photoemission spectroscopy

The scientific and practical significance of the thesis

We are analyzing the influence of Angolan tropical climate on performance of based photovoltaic system located in Luanda, Angola By analyzing the effects of different variables that affected the performance of silicon PV system located in an African tropical environment, it was observed that solar irradiance had the greatest impact on performance This implies that the unexpected factor such as dust accumulation will probably have a significant impact on the performance of the system by reducing the amount of sunlight that the PV panels are exposed to

silicon-The calculated results using ANSYS Fluent software show that, inclination of panels has the largest effect on the wind loads acting on solar panels Small changes in panel inclination are observed to result in significant changes in wind loads

We have deposited transparent TiO2 thin films using TiAcAc precursor by employing a simple and inexpensive spray pyrolysis and sol-gel-hydrothermal techniques Fe, W-doped TiO2 films are essential for application as a self-cleaning surface in solar panels

Study and investigate the effect of ALD technology parameters on the formation of Al2O3 ultra-thin films applied as passive layer of c-Si solar cell The Al2O3 ultra-thin films can be used as a passivation layer to improve performance of silicon based solar cells

The structure of the thesis

In addition to the "Introduction", "Conclusion", "List of symbols and abbreviations", "List

of tables", "List of images and graphs", and " References ", the thesis is presented in four chapters as follows:

Chapter 1: Literature review

Chapter 2: The influence of tropical climatic condition on the PV system performance in Angola Chapter 3: Experimental study on titanium dioxide application as solar cell self-cleaning layer Chapter 4: Growth and characterization of aluminum oxide ultra-thin film as a passivation

layer for silicon solar cells

4

CHAPTER 1 LITERATURE REVIEW

1.1 Overview of renewable energy use in the World

There are many options for reducing greenhouse gas emissions from energy systems while still meeting the global energy needs One of the options could be renewable energy, which is huge potential to mitigate climate change, can also provide people with utilities when using them Renewable energy can, if properly implemented, contribute to socio-economic development, access to energy as a safe source of energy, and reduction of negative impacts

on the environment and health

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 Additional policies are needed to attract the necessary increase in technology and infrastructure investment

Among the important renewable energy sources (hydropower, biomass, wind and solar), photovoltaic (PV) is the fastest growing sector with an average annual growth rate of about 60

% in the year 2012 [4]

Figure1.1 presents a plot of cost and price comparison of different energy sources in USD ($) It can be seen in the graph that the cost of 1kwh energy from PV is highest

Figure 1.1 Price comparison of energy sources [5]

The performance of a PV system can be characterized by its power output (PO), power conversion efficiency (PCE) and reliability [6] The PO measures the capacity of the module and the amount of electricity (in watts) it can generate On the other hand, the PCE quantifies the percentage of power generated by the module in comparison with the total solar energy available to the system Generally, solar module reliability measures the probability that it will perform the intended function over a specified interval understated conditions 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

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

5

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 determine the choice of energy for the individual, company, community or nation

According to REN21 [7], depending on system size, local solar radiation conditions and other factors, solar cell prices range from  9-13 cents/kWh for large scale installations The price of solar cells is about  22-44 cents/kWh in European countries In the United States, the installation price on the roof is  20-37 cents/kWh In long-term calculations, the cost of solar power will fall below 10 cents/kWh IEA ETP (2012) [8], in their calculation, shows that PV cost in 2030 will be around 7-11 cents/kWh for large scale projects and 8-14 cents/kWh for installation of PV panels on the roof Martín-González in their study [9], [10], has announced that solar electricity costs will be 5 to 10 cents/kWh by 2030-2040, depending

on the region

In recent years, solar PV becomes the power supply in a wide variety of locations, this happened mainly due to the huge drop in the production costs, which greatly reduced the retail price of panels (from an average cost in 2006 of 4.5 $/Wp to less than 1 $/Wp in 2013) [2] Several factors have contributed to lowering the cost of solar energy such as the ability to deposit thin films over large areas, the ability to automate the technology, increase the photovoltaic efficiency, etc The trend of the achieved cost reduction of PV modules is depicted in Figure1.2 [11] To progress this trend, it is proposed that improvement of the performance and reliability of PV module in the World difference climatic zones would engineer its adoption and usage, especially in African country as Angola PV technology is poised to meet the energy needs of these regions owing to its decentralized, sustainable and renewable nature [2], [12], [13]

0 1 2 3 4 5 6

Figure 1 2 PV module price over time [11]

To achieve a full competitiveness of PV energy worldwide especially in those locations 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

6

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 Nowadays, photovoltaic 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

Our study, however, aims to review available PV cell technologies commercially available

as will be thoroughly discussed in this section

1.2 Overview of solar cell 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 cost

of PV modules in Angola are still too high for most rural farmers or potential solar home system (SHS) users

In June of 2003, the Angola PV LUATA project was started It is implemented by the Department of Renewable Energy (DRE), National Energy Directorate of the Ministry of Energy and Waters, Government of Republic of Angola It is supported financially by the British Petroleum Company (BP), as advisors to the DRE It is a pilot project with the aim to investigate whether the PV LUATA approach is appropriate and can be integrated in the rural electrification strategy Angola

The project has supported the formation and operation of three LUATA in the district of Piranhas Province of Bengo In all cases, the BP business is a subsidiary to an existing company with business activities in other fields: farm implements, waste management and a farmer’s cooperative

Specifications of the systems were based on experiences from other solar power projects, consideration of usefulness and affordability, and surveys of energy use and affordability in the Bengo Province The base system is composed of 28 modules with an output of 24V 200Wp Each grouped in 7 parallel of 4 modules connected in series to provide an up voltage 96V and intensity 112A, a total power 10.8 kWp; A charge controller battery of 112A 48 batteries of 2V, 900Ah wired in series to a voltage up to 96V An inverter is at 96Vc.c/220Vac and 12.000W ÷ 14.000W power

Angola’s Ministry of Energy and Water (MINEA) has announced a national strategy for renewable energy, with solar energy an essential component in the short, medium and long-term MINEA is targeting the installation of 142 solar photovoltaic systems providing 534.6 kilowatts, Solar Solutions West Africa says These installations will be distributed among medical centers, schools, administrative buildings and streetlights The long-term goal is to integrate renewable energy, including wind, biomass and solar, into the national grid

Angola is set to implement rural solar energy electrification by 2025 that will benefit several rural areas of the country According to the Angola’s minister of Energy and Water, João Baptista Borges, the solar energy deployment project will be keys and will add power supply in rural areas Solar energy projects will benefit many people, as well as schools and other institutions The minister said it will benefit the country’s current electrification process

7

since it was cheap, fast and effective “Angola is a tropical country where the sun appears during the year, so we should take more and better advantage of this natural resource”, said the minister Added João Baptista Borges

The Angolan government has established a strategy whose goal is that by 2025, at least 7.5% of the electricity generated in the country will be coming from new renewable energies, most prominent in Energy Photovoltaic that will represent about 800 MW With this goal, it is intended:

- Improve access to energy in rural areas (Solar Village)

- Develop the use of new network-linked renewable technologies: Creation of plant units

to produce Photovoltaic Energy Sources;

- Promote public and private investment, aiming at the promotion of new energy sources

In Angola, rural electrification is foreseen under three implementation models [15]:

- Rural electrification through grid extension

- Electrification through isolated systems

- Electrification through individual systems

Rural electrification through grid extension: The selected grid extension model contemplates already numerous distribution grids outside the large urban areas, essentially to electrify municipal townships Those grids reach 174 locations, which represent approximately 5% of the Angolan population Grid extension outside large urban areas will allow for the electrification of many municipal townships, in a total of approximately 1.7 million people All municipal townships will be connected to the main grid, including the border towns which will be serviced through Namibia The electrification model proposed is based mainly on the installation of 60kV substations, branching from existing or planned 220kV substations These substations are generally located in municipal townships which, in turn become points of departure to connect either other municipality townships or rural grids Furthermore, the connection to the main national grid reduces significantly the costs of powering these distribution networks, thus facilitating the involvement of the private sector Electrification through isolated systems: The electrification by means of isolated systems

is considered for 31 localities, which represents only 1% of the population These localities are preferentially supplied by competitive mini-hydro, and where these are not available, by diesel generators and PV systems The proximity of a municipal township to a competitive mini-hydro - for such level of consumption - reduces the gains which could be obtained from connecting it to the national grid and changes, in various situations, the economic rationale of interconnecting that location The hydropower atlas has made it possible to evaluate, on a preliminary basis, approximately 100 locations identified by the Ministry of Energy and Water The mini-hydro projects with lower generation costs have been resized and reappraised to the load of the close-by municipal townships and rural grids This appraisal has resulted in 7 mini-hydro selection which potential to supply 9 municipal townships through isolated systems Additionally, a medium sized hydro project has been identified in river Cuango, which has the potential to supply power to four municipalities with a population of more than 300,000 people while the connection between North and East systems is not built

8

About 21 municipal townships present high connection costs per energy unit Therefore, they should be electrified through isolated systems based on diesel with some solar power support which can reduce fuel costs

Finally, the concept of a 100% solar municipality - with photovoltaic technology and batteries will be tested in the locality where economic rationality is the highest, considering the high cost of transportation of diesel: The Municipality of Rivungo, in Cuando Cubango Should this solution be successful, it can be extended to other distant municipal townships that have good solar resources

Electrification through individual systems: This model comprises the creation of “solar villages” or small local networks in communal townships and settlements with more than 3,000 inhabitants “Solar villages” constitute an intermediate solution, which provides modern electricity services to communal infrastructures, such as schools, health units, administration and public lighting of main streets Small local networks can be developed by private initiative The National Strategy for New Renewable Energies foresees a target of 500 solar villages by

2025 Some remote villages could benefit from solar lanterns and improved cook-stoves Currently, Photovoltaic Energy still accounts for less than 1% of total national energy consumption A program for the implementation of photovoltaic systems for the electrification of rural areas (Solar Village Program) not covered by the transmission and distribution networks

is under way as show in table 1.1

Table 1 1 Solar Village Program

Angola, despite being in a tropical zone, has a climate that is not characterized for his region, due to the confluence of three factors:

(i) The Benguela Current, which is cold, lying along the south coast

(ii) The relief inside the country

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(iii) Influence of the Namib Desert, which is in the southeast

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, the area of the interior can be divided into three areas:

(i) North, with high rainfall and high temperatures

(ii) Central Plateau, with a dry season and average temperatures around 19° C

(iii) South with very sharp temperature variations due to the proximity of the Kalahari Desert and the influence of tropical air masses

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

The output from the technology creates new opportunities, but consequently also creates new daily routines and livelihoods Two-thirds of the client respondents stated that they had changed their daily routines because of the access to electric services through the PV Three changes were specifically mentioned, of which two are related to access to more and higher quality light sources; do domestic work at night and reading/studying at night

1.3 The photovoltaic effect Solar cells are semiconductor devices, which can convert Sun light into electricity directly The working principle of solar cells is based on the photovoltaic effect [16]–[19]

Photovoltaic effect was first reported in 1839 by French physicist Edmond Becquerel He conducted electrical experiments by placing two metal plates in a conductive liquid as electrodes, and when he accidentally exposed them to Sun exposure, he observed a small voltage appearing between the two electrodes

In 1955, Western Electric began to sell commercial licenses for silicon photovoltaic (PV) technologies

In 1981, Paul Macready builds the first solar-powered aircraft-the Solar Challenger and flies it from France to England across the English Channel The aircraft had over 16,000 solar cells mounted on its wings, which produced 3,000 watts of power

Basically, the solar cell is made up of a very thin semiconductor layers, called a p-n junction Sun light can pass through a semiconductor p-layer of thickness  1÷5 μm, this layer

is used as an absorbing layer The generation of a potential is different at the p-n junction in response to electromagnetic radiation The photovoltaic effect is closely related to the photoelectric effect, where electrons are emitted from a material that has absorbed light with a frequency above a material-dependent threshold frequency In 1905, Albert Einstein understood that this effect can be explained by assuming that the light consists of well-defined energy quanta, called photons The energy of such a photon is given by h, where h is

10

Planck’s constant and is the frequency of the light For his explanation of the photoelectric effect Einstein received the Nobel Prize in Physics in 1921 [20] The photovoltaic effect can

be divided into three basic processes:

i) Generation of charge carriers due to the absorption of photons in the materials that form a junction

Absorption of a photon in a material means that its energy is used to excite an electron from an initial energy level Ei to a higher energy level Ef Photons can only be absorbed if electron energy levels Ei and Ef are present so that their difference equals to the photon energy, h = Ef - Ei The absorption of a photon in an ideal semiconductor is illustrated in figure 1.3 In an ideal semiconductor electron can populate energy levels below the so-called valence band edge, Ev, and above the so-called conduction band edge, Ec Between those two bands no allowed energy states exist, which could be populated by electrons Hence, this energy difference is called the band gap, Eg = Ec – Ev If a photon with energy smaller than Egreaches an ideal semiconductor, it will not be absorbed but will traverse the material without interaction

Figure 1.3 Graph illustration absorption of a photon in a semiconductor with an Eg band gap [16]

In a real semiconductor, the valence and conduction bands are not flat, but vary depending

on the so-called k-vector that describes the crystal momentum of the semiconductor If the maximum of the valence band and the minimum of the conduction band occur at the same k-vector, an electron can be excited from the valence to the conduction band without a change

in the crystal momentum Such a semiconductor is called a direct band gap material If the electron cannot be excited without changing the crystal momentum, a semiconductor is called

an indirect band gap material The absorption coefficient in an direct band gap material is much higher than in an indirect band gap material, thus the absorber can be much thinner [16] If an electron is excited from Ei to Ef, a void is created at Ei This void behaves like a particle with a positive elementary charge and is called a hole The absorption of a photon therefore leads to the creation of an electron-hole pair The radioactive energy of the photon

is converted to the chemical energy of the electron-hole pair The maximal conversion efficiency from irradiative energy to chemical energy is limited by thermodynamics This thermodynamic limit lies in between 67% for non-concentrated sunlight and 86% for fully concentrated sunlight [17]

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ii) Subsequent separation of the photo-generated charge carriers in the junction

The photovoltaic effect and solar cells are performing work in an external circuit When the solar cells are illuminated, photons with energy greater than the forbidden energy of the material are absorbed and generate electron- hole pairs Under the effect of the internal electric field of the p-n transition, electron-hole pairs are separated, accelerated to opposite electrodes, and generate a photovoltaic power [19], [21] The photovoltaic line generated in this case is direct current and can be used either by converting to alternating current or stored for future use Usually, the electron-hole pair will recombine, i.e the electron will fall back to the initial energy level Ei The energy will then be released either as photon (radioactive recombination) or transferred to other electrons or holes or lattice vibrations (nonradioactive recombination) If one wants to use the energy stored in the electron-hole, a solar cell must be designed such that the electrons and holes can reach to the corresponding electrode before they recombine, i.e the time it requires the charge carriers to reach the electrode must be shorter than their lifetime This requirement limits the thickness of the absorber

iii) Collection of the photo-generated charge carriers at the terminals of the junction Finally, the charge carriers are extracted from the solar cells with electrical contacts so that they can perform work in an external circuit The chemical energy of the electron-hole pairs is finally converted to electric energy

1.4 Physics of Solar Cells

It is possible to visualize the general picture of solar cells in figure 1.4 When PV module are exposed to light, several photo carriers are generated inside the photovoltaic cells; carrier separation within the device produces a photo voltage and charge motion produces a photocurrent, which runs in reverse through the diode junction Electrical power is harnessed

by combining photovoltaic devices with a suitably matched resistive load

Figure 1.4 Component parts of a typical PV cell

The performance of a PV module can be characterized by its power output (PO), power conversion efficiency (PCE) and reliability [21] The power output measures the capacity of the module and the amount of electricity (in watts) it can generate On the other hand, the power conversion efficiency quantifies the percentage of power generated by the module in

12

comparison with the total solar energy available to the module Thus, a module may generate more power than another module but possess a lower PCE Generally, module reliability measures the probability that it will perform the intended function over a specified interval under stated conditions

The performance of a solar cell is determined by how good a p-n junction can be produced, how effectively it collects the photocurrent generated, how much of the photon energy is preserved in the device, and how much power is lost due to defects in the material, intrinsic recombination mechanisms, and parasitic losses such as series resistances and shunts

To set up analytical expressions of solar cells current-voltage characteristics (j-v characteristic or current density versus voltage), we consider a one-dimensional p-n homogeneous juntion structure as shown in figure 1.5 below

Figure 1.5 Graph illustration structure of a 1D p-n homogeneous solar cell

According to references [22, ] when light enters from the emitter side, the number of electrons and holes are generated at a distance x with the generation rate of:

Where  is the absorption coefficient of the light, R the reflectivity of the light at the surface, F is the incident photon flux defined by the number of incident photons per unit area, unit time, and unit wavelength All these three variables depend on the wavelength Under steady state conditions, the continuity equation in the emitter layer is given by:

Where Dn is a diffusion coefficient of electrons, np is electron concentration in p-type semiconductors, npo is electron concentrations in the p-type semiconductor in thermodynamic equilibrium (cm-1) và n is the lifetime of electron (s)

We note that, the electrons in the region p near the charge space are accelerated by the electric field at the opposite side of the junction

So, it can be seen that:𝑛 = 𝑛 (1.3)

Considering the surface recombination velocity Sn of the excess electron atthe front surface [22]:

Technological boundaries

Space charge Region

w xp+xn

x Imbalanced

Region

Imbalanced Region

The short-circuit current (written as ISC) is the current through the solar cell when the voltage across the solar cell is zero (i.e., when the solar cell is short circuited) Usually, to facilitate the calculation short-circuit current density Jsc is used Thereby ignoring the recombination, the total short-circuits current density Jsc is calculated by integrating over the entire solar spectrum So that, we have:

Where min is the smallest occurring wavelength and max là is the largest occurring wavelength In this case, min is about  0.3 m for sunlight and max is the wavelength corresponding to the absorption edge of the semiconductor As we can see, photocurrent is proportional to the light intensity that emits to the semiconductor and strongly depends on the diffusion length and surface recombination velocity

In the ideal p-n junction, it is possible to calculate the upper limit of the short-circuit current density For simplicity, we assume that:

𝑑 = ∞, 𝑤 = 0, 𝑆 = 𝑆 = 0 𝑎𝑛𝑑 𝐿 = 𝐿 = 𝐿 (1.11)

If the diffusion length is long enough, that is αL >>1, the upper limit of 𝐽 + +𝐽 +

𝐽 is 𝑞𝐹(1 − 𝑅), Therefore, the upper limit of the short-circuit current density is given by:

𝐽 = 𝑞 ∫ (𝑞𝐹(1 − 𝑅))𝑑 (1.14)

0 10 20 30 40 50 60

Figure 1.6 Ideal short-circuit current density of p-n junction solar cell as a function of Eg [22]

Integrating from the lowest possible wavelength min to the wavelength corresponding to

the absorption edge max (mm) 1.2398/Eg (eV), the upper limit of the short-circuit current can

be calculated as a function of the band gap energy It is evident that the short-circuit current

density increases with the reducing band gap energy Eg The relationship between the short-circuit

current density and the band gap energy is shown in figure 1.6, assuming that R is zero

The ideal solar cell is represented by the J-V characteristics of a semiconductor diode in

dark and illuminated conditions and is shown in figure 1.7 It is easy to see that in the ideal

case, the light J-V curve is shifted from the dark J-V curve follow axis the J with JL J

J-V characteristic is following to [17] :

While, q is the charge of electron, V is bias voltage, T is the absolute temperature, k is

Boltzmann constant, “A” is the quality factor of the diode, J0 is the saturation current density

Since qV >> AkT at room temperature, the expression (1.15) can be approximated as follows:

15

Figure 1.7 J- V curves of a solar cell in the dark and under illuminated condition [17]

However, real solar cells cannot be described by equation (1.16) The reason is that, we cannot ignore the resistor of buck semiconductor material, the metal and semiconductor contacts resistor and metal contacts resistor together The sum of these resistances is called the RS serial resistor In addition, special attention should be paid to the presence of Rsh short-circuit resistors The existence of Rsh is determined by the impurities in the absorption layer, the imperfections surface and leakage current in the surface of the solar cell Thus, the equivalent circuit of a real solar cell is defined as shown in figure 1.8 It is easy to see that for ideal solar cells, the value of RS 0 and the value Rsh

Figure 1.8 Graph illustration equivalent circuit of a real solar cell [17]

Due to the losses caused by the RS and Rsh resistors, the equation for J-V characteristic of the actual solar cell is corrected from equation (1.15) and has the following to [23]:

The open-circuit voltage, (written asVOC), is the maximum voltage available from a solar cell, and this occurs at zero current The open-circuit voltage corresponds to the amount of forward bias on the solar cell due to the bias of the solar cell junction with the light-generated current The open-circuit voltage of the real p-n junction solar cell is given by:

RS

where To is solar cell temperature, Ts is the temperature of the sun and  the solid angle which the solar cell receive the incident irradiation from the sun [23] The upper limit of the open-circuit voltage increases with the band gap energy as shown in figure 1.9a

Figure 1.9.Graph of p–n junction solar cell factor as a function of band gap

Ideal open-circuit voltage (a) and conversion efficiency (b) [22]

The fill factor, (known by its abbreviation "FF"), is a parameter which, in conjunction with

Voc and Isc, determines the maximum power from a solar cell The fill factor is defined as a function of the open-circuit voltage and is approximately expressed by:

out

P

FF J V P

P 

(1.22)

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The upper limit of the conversion efficiency is a strong function of the semiconductor band gap energy The optimum efficiency of 30% occurs when the band gap is between 1.4 eV and 1.6 eV, as shown in figure 1.9b at AM 1.5 and 1 sun The band gap energy between 1 eV and

2 eV is suitable for solar cell to achieve relatively high efficiency

1.5 Overview of Silicon Solar Cell Technologies

Solar cells have been rapidly improved in the following years in both directions: Efficiency, and the variety of materials used to make cells [9]

Different PV materials have different energy band gaps which characterize their absorption capacity Photons with energy equal to the band gap energy of the PV cell material are absorbed to create free electrons while photons with less energy than the band gap energy pass through the material On the other hand, photons that possess higher energy than the band gap energy release excess energy in the form of heat as they are absorbed [24], [25] Advanced solar photovoltaic technology has been challenged by improvement of the absorption capacity of the materials and thus the conversion efficiency of PV cells Therefore, scientists around the World focus on research and development of advanced materials which possess wide energy band gap to be used as PV cells Figure 1.10 presents the market share of the six common PV cell materials It can be seen in the figure that poly-crystalline silicon demonstrates having the highest share with value of 54%

Figure 1.10 Market share of PV cells (%) [26]

The first solar PV device was made from crystalline silicon (c-Si) by Chapin et al in 1954 and showed an energy conversion efficiency of 6% [27] The theoretical limit for a single junction solar cell made of c-Si material with a band gap of 1.12 eV was calculated by Shockley and Queasier in 1961 to be 33.7% [28] According to the authors shown in figure 1.11, mono-crystalline PV module has market share of 30% of all PV cells The cell is produced by silicon wafer from a single high purity cylindrical crystal ingot To optimize cell density, the wafers are cut into octagonal shape [25]

Figure 1.11 illustrates the design of a modern wafer-based silicon solar cell It consists of p-type mono-crystalline or poly-crystalline silicon wafers The top of the wafer then is highly doped so that it becomes n+-type Similarly, the bottom of the wafer is made p+-type The central p-type region is the absorber, the n+- and the p+ region form the membranes that are needed to separate the electrons from the holes They are produced from 100 to 200 µm thick wafers sliced from bulks of solar grade silicon [25] They are also called conventional or

Gallium Arsenide

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traditional or wafer-based solar cells The cells could be mono-crystalline (mono-Si) or polycrystalline (multi-Si) in nature depending on the mode of production As mentioned above, hence the maximum of the valence band and the minimum of the conduction band occur at the difference k-vector, the electron cannot be excited without changing the crystal momentum, so the c-Si is an indirect semiconductor, i.e a sufficient absorber thickness of at least several tens of micrometers is required to ensure the absorption of a sufficiently large fraction of the incident light However, for commercial purposes, the industry is very interested in making the silicon wafers thinner to reduce material consumption and therefore cost The current record single-junction mono-crystalline cell on laboratory scale has an efficiency of 25.0 % [29]

Figure 1.11 Scheme of a modern crystalline silicon cell [29]

Technological requirements for increasing the c-Si solar cell efficiency are mainly based

on an optimal usage of solar irradiation and on reducing electronic losses in solar cells Optimal usage of incoming solar irradiation is guaranteed by a double layer antireflection coating, a pyramid surface texture at the front of the solar cell, and a SiO2 reflector at the rear side 10 of the c-Si solar cell The lightly P-doped front and the undiffused rear surface are passivized by thermal SiO2 The bulk lifetime is maximized by the usage of relatively high resistivity floating zone p-type c-Si The metal contact fingers at the front side are optimized for an optimal electrical performance by a local heavy p-diffusion with a minimal surface coverage to minimize shading The electrical contact at the rear side of the solar cell is optimized by a local heavy B-diffusion

Today, many cost-effective methods to optimize the solar cell efficiency are employed in innovative solar cell designs Technological requirements to support the cost driven trends of c-Si PV modules as following to [30]:

a) Reduction of the solar cell thickness

- Production and handling of thinner c-Si wafers

- Improvement of light trapping properties

- Good surface passivation of front and rear side

- Avoidance of wafer bow

- Development of metal pastes for thin c-Si wafers

b) Alternative c-Si feedstock

- Production of high electronic quality c-Si

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- New feedstock technologies

- Alternative solar cell designs when using n-type c-Si

c) Higher conversion efficiencies

- Improvement of light trapping properties

- Reduction of losses in emitter region

- Improvement of c-Si bulk lifetime

- Improved surface passivation of front and rear side

- Alternative solar cell designs (e.g back contact or heterojunction concept)

- Reduction losses due to the front and rear side electrical contacts (e.g shading and electrical losses)

- Photon up- and down-conversion

All these technological requirements are improved c-Si solar cell efficiency and its effective optimization Reduction of the solar cell thickness will significantly increase the surface to volume ratio, and therefore electronic losses at the front and the rear surface of the c-Si solar cell will become more detrimental for its performance Consequently, surface passivation will reduce surface recombination adequately Light-trapping schemes apply for thinner c-Si solar cells to achieve a similar optical performance when compared to thicker c-

cost-Si solar cells The significant difference in the thermal expansion coefficient of Al and cost-Si results in an unacceptable metal-semiconductor contact The reduction of the c-Si solar cell thickness could lead to an advanced technology soon

The manufacturing procedure of mono-crystalline solar cell as see in figure1.12b is relatively complicated, and expensive, so its price relatively high in comparison with the other solar cell types Mono-crystalline cell possess a power conversion efficiency (PCE) up to 25% [29], and energy payback time (EPBT) of 4 years with a designed operational lifetime of 30 years The solar cell demonstrates best performance at standard test conditions (STC) Poly-crystalline (multi-Si) PV cell as see in figure1.12a dominates the PV cell market with a market share of 54% Figure 1.10 presents statistics which shows that the cell share is the highest This type of solar module achieved the highest market share because it experienced accelerated growth in efficiency and decrease in cell cost in recent time The wafers are used

in poly-crystalline PV technology have square shaped hence less silicon is wasted during manufacturing compared to the production of mono-crystalline cell [29] The manufacturing process is more cost effective and less sophisticated than that of monocrystalline PV cell Polycrystalline cells demonstrate best performance at STC and moderately elevated temperatures Poly-crystalline cell possess power conversion efficiency (PCE) up to 20% The most intriguing facet of such a technology is the significant decline in their efficiency during the first few hundred hours of illumination This is normally reduced to approximately 30% of its initial efficiency after 1000 h [31] which is mostly due to the Staebler-Wronski effect [32] For the increasing efficiency of silicon cell, more and more study on reduced surface recombination velocity A reduction in surface recombination is called surface passivation At present, only a fraction of industrial solar cells has effective passivation scheme simple mended, which explains significant part of the efficiency gap between industrial cells and

20

high-efficiency laboratory cells [33] One of the technologies is PERC technology (Passivized Emitter Rear Cell)

Figure 1.12 Imaging of Poly‐crystalline Si cell (a), and Mono-crystalline Si cell (b) [19]

What is PERC technology?

Based on a change in the design of the rear of the cell which improves the capture of light falling on its surface, REC has introduced PERC technology (also known as backside passivation) into its cell production process and been able to bring it to full production level

In a conventional solar cell, there is an aluminum metallization layer which makes contact across the full area of the back of the cell REC’s PERC technology first coats the backside of the cell with a special dielectric layer that has tiny holes made by a laser The aluminum metallization is then applied on top of the dielectric layer and contacts the silicon wafer only through the microscopic holes (as see in figure1.13)

Figure 1.13 The structure of a conventional cell (a) and the structure of a cell with PERC technology (b)

[17]

How does PERC technology improve performance?

PERC technology increases the overall panel performance by increasing a cell’s ability to capture light A regular solar cell consists of two layers of silicon with different electrical properties- known as the base and the emitter A strong electrical field is generated where the two layers meet, which pulls negatively charged particles (electrons) into the emitter when they reach this interface The electrons are generated by light entering the cell and releasing electrons from the silicon atoms Electrons travel freely through the cell and contribute to the electrical current only if they can reach the interface

Different wavelengths of light generate electrons at different levels of the cell structure, shorter wavelengths (blue light) will generate more electrons near the front of the cell, compared to longer wavelengths (red light) which will generate electrons at the back of the cell or even pass through the wafer without generating current

Emitter layer

Base layer (silicon wafer)

Aluminum metallization

Emitter layer Base layer (silicon wafer) Aluminum metallization

Dielectric PERC layer

Small metal contacts

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The introduction of PERC technology increases the cell efficiency through the dielectric layer that back reflects into the cell any light that has passed through to the rear without generating electrons Through this reflection, the photons are essentially given a second chance to generate current (as see in figure1.14)

Figure 1.14 A cell with PERC technology will generate more current due to the reflection of light at the

backside of the cell

The extra energy yield of cells with PERC technology is added to by the improved ability

to capture light at longer wavelengths, e.g., when the sun is at an angle (early mornings and evenings) or under cloudy conditions At such times a higher quantity of blue light (wavelengths between 450 to 495 nm) is absorbed by the atmosphere as it has a longer path to travel to the Earth’s surface than when the Sun is directly overhead Blue light is generally converted to energy near the top of the cell, whereas red light (wavelengths between 620 to

750 nm) penetrates further through the cell and is converted to energy near the bottom Red light is less easily absorbed by the Earth’s atmosphere and as a result, cells which capture more red lights are generally more powerful The ‘reflective’ properties of the PERC technology ensure increased absorption of red light, even in weak or diffuse light conditions, delivering better energy yields

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

high-80 °C or higher 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

From the references, a number of researches have focused on the performance of PV modules in tropical climatic conditions [34] In the study of Ike C.U [35], the effect of

Dielectric PERC layer

Small metal contacts

Ligh is absorbed by the

aluminum metallization

Reflected light will generate additional current

The next Sections discuss the major environmental factors that affect the performance of solar PV system

1.6.1 Spectrum

For most solar cell measurements, the spectrum is standardized to the AM1.5G spectrum (as defined and tabulated in IEC standard 60904-3 [39]) to allow comparison of photovoltaic devices from different manufacturers and measured with different solar simulators However, the outdoor spectrum is also a variable and location-dependent parameter

The Air Mass (AM) in the “AM1.5G” is the path length that light goes through the atmosphere normalized to the shortest possible path length when the sun is directly overhead AM1.5 was chosen because it represents the average air mass (AM) at solar noon for optimally tilted PV arrays at latitudes in the continental USA For a specific location, a higher air mass corresponds to a red-shifting of the solar spectrum and vice versa [40]-[42] Besides air mass, the spectral distribution is also influenced by other meteorological factors such as the relative humidity and the aerosol content of the air [43]

1.6.2 Irradiance

The influence of spectral irradiance distributions has been studied widely in mid-latitude regions e.g., the performances of a-Si and multi c-Si modules on the basis of two-year accumulated outdoor data in Japan [44] The results show that the efficiency difference of the a-Si module between summer and winter was about 15% The research showed that direct normal irradiance has a major role in affecting the performance of concentrated PV module Latitude controls the duration of daylight and the path oblique rays travelled from the sun High-latitude regions usually have lower levels of sunshine and low-latitude regions have higher levels [45]-[47]

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Studies from the same group calculated the average photon energy (APE) and compared the influence of spectrum and temperature on the performance of a-Si and multi c-Si [48] The results indicate that the output energy of a-Si modules depends more on the spectral distribution and is less sensitive to the module temperature than for multi c-Si modules A linear relation between the average photon energy and the energy yield of a-Si modules was found in Thailand, and the authors suggested that a-Si PV modules might be better suited for tropical climates considering the blue-rich spectrum [49]

1.6.3 Module temperature

The module temperature is one of the most important parameters (total solar irradiance, incidence angle of the incident solar radiation and air mass) affecting the power output of PV modules The efficiency of c-Si modules decreases with increasing temperature, while thin-film modules show a less predictable trend, with additional dependence on the operating histories [50] The effect of temperature originates from the semiconductor properties, whereby the band gap decreases with increasing temperature With decreasing band gap short-circuit current increases slightly, since lower-energy photons may excite an electron across the band gap However, with the decrease in band gap, the quasi-Fermi-level splitting also decreases, hence the VOC of the device decreases The temperature sensitivity of a solar cell depends on its open-circuit voltage (Voc) The decrease in voltage is inversely proportional to the increase in temperature, while the increase in current is only logarithmically proportional

to the increase in temperature [51] Thus, the VOC effect dominates, and the net effect is a reduction in PV efficiency Solar cells with a higher VOC are less affected by temperature As

an example, HIT modules show a lower temperature-dependence with a correspondingly higher open-circuit voltage [52]

1.6.4 Wind speed

Wind has a cooling effect and can help the ventilation of PV system [53], [54] Their study showed that wind speed can greatly affect the operating performance of a PV system, especially in windy locations However, Garcıa et al [55] claimed that the positive cooling effect of wind speed is relatively small comparing to the negative effect of temperature and solar insolation on PV efficiency

1.6.5 Incident angle

PV modules are rated under standard test conditions (STC) with normally incident light, while under outdoor conditions photons arrive on a PV module surface at various angles Irradiance coming at high angles of incidence can be reflected significantly from the module’s front surface and depends, to some extent, on the surface type and soiling [56], [57] On the other hand, for thin-film modules with very thin absorber layer, large incidence angles caused

by diffuse light can lead to longer optical path length in the solar cells and therefore better light absorption [58] The effect of the incident angle on the PV performance in Singapore was studied and a theoretical annual angular loss of 3.3% was calculated [59] Since in the measurement setups of this work a c-Si irradiance sensor is used to measure the in-plane irradiance, it is reasonable to assume that the irradiance sensor and the test modules experience similar angular loss Thus, angular loss is not considered further in this work

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1.6.6 Effect of humidity on the solar panels

The moisture content of the atmosphere is commonly expressed by relative humidity, defined as the ratio of actual water vapor pressure contained in the air to the saturated water vapor pressure at the same temperature Air with constant water vapor content will lead to a decrease in relative humidity with a rise in temperature Water vapor in the atmosphere is invisible to human eye but is seen by solar cells The primary effect of humidity on terrestrial solar cells is corrosion, especially in the simultaneous presence of high temperature The most corrosive phenomenon on a solar cell array is the potential deterioration of the titanium-silver contact on silicon solar cells in humid environments Conditions of high temperature (>40 oC) and humidity (>60 %) can cause long-term deterioration of these contacts Typically, higher temperatures and higher humidity levels accelerate the corrosion process, as may be the presence of minute quantities of ionization contaminate (such as salts) Other effects include a growth of fungi, the growth rates are the highest at relative humidity levels between 75 % and

95 % and at temperatures between 20 oC and 40 oC and the formation of a sticky surface film

of moisture that catches dust and dirt particles [60]

Makhila [36] measured the impact of humidity on the degradation in solar cell efficiency The study observed that increasing wind velocity caused a reduction in the atmospheric air relative humidity that resulted in a better efficiency Gwandu [61] indicated that the humidity influences the sunlight irradiance level resulted in reducing the radiations subjected to the PV panel due to refraction, reflection or diffraction caused by water vapor particles The reduction in the received solar radiation level causes non-linearly irradiance alterations A little non-linear variation in open circuit voltage accompanied with significant linear variations in short circuit current Kazem [62] investigated the impact of relative humidity on the performance of the three types PV at Oman’s climate The obtained results illustrated that

at low relative humidity conditions the output current, voltage, and power increase Panjwani [63] studied the effect of humidity ranges between (40 to 78%) The study results indicated that there is an estimated loss of about 15-30% of the PV power Humidity brought down the utilized solar energy about 55-60% from just 70% of utilized energy The reason for this reduction resulted from the basal layer of water vapor lied at the front of the solar cell directly facing the sun Where the solar irradiance strikes the solar cell, face suffers from a loss in absorption/reflection energy occurs Omubo [64] investigated the relative humidity effects on the efficient electricity conversion from solar energy The results clarified a direct proportionality between solar flux, output current and efficiency of the solar cell The solar flux affected by relative humidity and both have a negative impact on the PV cell output voltage The maximum achieved PV power was at an operating temperature of 43ºC and relative humidity of about 77%

M.T.Cher et al [65] in their study, investigated influence of high temperature-humidity on

PV cells, and their performance degradations due to moisture inception They found that the degradation is directly related to the passivation integrity, and the inception of moisture causes a significant degradation in the short circuit current and maximum power output The effect of moisture on the Voc is less severe, and in some cases, enhancement of Voc can be observed because moisture acts as light trapping layer

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1.6.7 Effect of dust on the solar panels

Dust accumulation, which is always depicted by soiling ratio, is a vital factor influencing the performance of PV system as it blocks the insolation transmitted through the PV module and thus the solar energy converted by the modules It has been proven that the conversion efficiency of PV modules drops progressively as dust is accumulated on its surface In his study, Jiang et al showed that dust accumulation has a negative linear relationship with generation efficiency of solar panels [66] Paudyal et al [67] have conducted outdoor experiments and found that the high dust concentration at bottom could result in hot spot and ultimately permanent module damage Adinoyi et al in his research works [68], concluded that the decrease in electricity generation of solar PV modules is the result of dust frequency and intensity They testified that precipitation improves the generation of solar PV modules in dusty areas but cannot be relied upon for cleaning Trackers and more frequent cleaning schedules can help to improve generation efficiency in dusty conditions Cleaning is recommended immediately after dust period, with running water and use of a sponge [69] Researches have also investigated how different physical properties of soiling affect the performance of PV modules Micheli et al in their studies [70], observed that precipitation raise the soiling ratio after a long dry season or dusty period Soiling ratio is more correlated with frequency of rain than the amount of precipitation

The accumulation of dust on the solar panels will lead to reductions in the panels’ transmittance For long-term operation of PV arrays, it will be necessary to develop techniques to remove the deposited dust on the solar panel surface There was a wide range of studies carried out on the impact of dust worldwide although with different setting, environment and time frame In a pioneer work on the impact of dust on solar PV [71], degradation in performance of up to 4.7% was recorded with an average loss in incident solar radiation of less than 1% Wakim [72] in Kuwait City recorded a reduction in PV power by 17% due to sand accumulation after six days He also indicated that the influence of dust on

PV performance would be higher in spring and summer as compared to that in autumn and winter In a research work of Sayigh et al [73] on the effect of dust accumulation on tilted glass plates, they showed a reduction in plate- transmittance ranging from 64% to 17%, for tilt angles ranging from 0o to 60o, respectively, after 38 days of exposure It was revealed that dust particles would result in significant drop in the PV short-circuit voltage Interestingly, it was found that the smaller the particle size of a fixed deposition density, the greater would be the reduction in solar intensity received by the solar PV panels This was probably due to the greater ability of finer particles to minimize inter-particle gaps and thus obscuring the light path more than that for larger particles, which was verified in another work of similar nature

An experiment to investigate the effect of wind velocity and airborne dust concentration on the drop of PV cell performance by using Aeolian dust deposition on photovoltaic solar cells

by Goossens et al [74] showed that significant drop on the output power of photovoltaic due

to accumulation of dust

From the references, the following removal methods are studied and proposed for cleaning

of solar panels:

1) The natural method (wind lift, wind induced vibration);

2) The electromechanical method (shaking by sound our mechanical actuators);

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3) The mechanical method (cleaning tools, cleaning robot systems);

4) The electrical method (electrostatic and electrodynamics); and

5) The physical-chemical method (self-cleaning materials)

Thus, the objective of this work was to assess synthesized and the influence of cleaning materials on the solar cell

self-Chapter Summary

In this chapter we have explored:

Understand the principle of solar cell activity as well as the influence of technological on the performance of solar cells

PV cells are exposed to extreme climatic conditions and climatic fluctuation during operation and their reliability is primarily affected by the environmental factors However, the reliability of PV cells is rarely reported despite its importance It is important to conclude that the power delivered by the PV systems at a certain instant is still very much a function of environmental factors The efficiency of the module has dependency on the environmental parameters Meteorological data such as solar radiation, ambient temperature, relative humidity, dust and wind speed is accepted as dependable and widely variable renewable energy sources

In developing country as Angola, most of using PV cells is c-Si type, they have low maintenance and operating costs, so the lifetime of a PV cell plays a decisive role in determining the price per kWh of electricity, and thus the reliability of PV cells is important

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CHAPTER 2 THE INFLUENCE OF ANGOLA TROPICAL CLIMATIC CONDITION ON THE PV SYSTEM PERFORMANCE

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 about 4.5 kWh/m2/day, and changes slightly from season to season Average monthly insolation levels in Luanda show in figure 2.1

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 2.0

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

Figure 2 1 Average monthly insolation levels in Luanda

Solar radiation is measured in kWh/m2/day onto a solar panel set at an 81o angle (For best year-round performance) The level of solar radiation can be a problem during the rainy season, when the sky can be overcast for several days in a row In the rainy season, battery may not be fully charged in one day

The observation point is about Latitude: 8º 51' 25'' S and Longitude: 13º 17' 13'' E (S is south, and E is east), which almost corresponds to that of the main land of Angola Therefore,

it is expected that standard data of our country is collected In this PV system, c-Si PV modules are facing due north installed with angle about 30o

Figure 2.2 shows the system diagram for collecting and monitoring data of operating solar panels This system consists of various peripherals connected to a computer through an interface Communication among computer and peripherals is facilitated by converted