Photovoltaics

Professor Konrad Mertens, Department of Electrical Engineering and Computer Science, Münster University of Applied Sciences, Germany Konrad Mertens was appointed Professor at Muenster University of Applied Sciences in 2000. He covers the subjects of photovoltaics, sensor technology and optoelectronics. He is founder and head of the photovoltaic test lab and head of the lab of sensor technology and optoelectronics at his university. Previous to this, he worked in industry as project engineer and project manager in the fields of photovoltaivs and intelligent networks. Professor Mertens is author of the German text Photovoltaik - Grundlagen, Technologie und Praxis, and is court-appointed expert for photovoltaic plants.

Preface xi Abbreviations xiii 1 Introduction 1 1.1 Introduction 1 1.1.1 Why Photovoltaics? 1 1.1.2 Who Should Read this Book? 2 1.1.3 Structure of the Book 2 1.2 What is Energy? 3 1.2.1 Definition of Energy 3 1.2.2 Units of Energy 4 1.2.3 Primary, Secondary and End Energy 5 1.2.4 Energy Content of Various Substances 6 1.3 Problems with Today s Energy Supply 7 1.3.1 Growing Energy Requirements 7 1.3.2 Tightening of Resources 8 1.3.3 Climate Change 9 1.3.4 Hazards and Disposal 10 1.4 Renewable Energies 11 1.4.1 The Family of Renewable Energies 11 1.4.2 Advantages and Disadvantages of Renewable Energies 12 1.5 Photovoltaic The Most Important in Brief 12 1.5.1 What Does Photovoltaic Mean? 13 1.5.2 What are Solar Cells and Solar Modules? 13 1.5.3 How is a Typical Photovoltaic Plant Structured? 14 1.5.4 What Does a Photovoltaic Plant Bring? 14 1.6 History of Photovoltaics 15 1.6.1 How it all Began 15 1.6.2 The First Real Solar Cells 16 1.6.3 From Space to Earth 18 1.6.4 From Toy to Energy Source 18 2 Solar Radiation 21 2.1 Properties of Solar Radiation 21 2.1.1 Solar Constant 21 2.1.2 Spectrum of the Sun 22 2.1.3 Air Mass 23 2.2 Global Radiation 24 2.2.1 Origin of Global Radiation 24 2.2.2 Contributions of Diffuse and Direct Radiation 25 2.2.3 Global Radiation Maps 25 2.3 Calculation of the Position of the Sun 29 2.3.1 Declination of the Sun 29 2.3.2 Calculating the Path of the Sun 31 2.4 Radiation on Tilted Surfaces 33 2.4.1 Radiation Calculation with the Three-Component Model 33 2.4.2 Radiation Estimates with Diagrams and Tables 37 2.4.3 Yield Gain through Tracking 38 2.5 Radiation Availability and World Energy Consumption 40 2.5.1 The Solar Radiation Energy Cube 40 2.5.2 The Sahara Miracle 41 3 Fundamentals of Semiconductor Physics 43 3.1 Structure of Semiconductors 43 3.1.1 Bohr s Atomic Model 43 3.1.2 Periodic Table of the Elements 45 3.1.3 Structure of the Silicon Crystal 46 3.1.4 Compound Semiconductors 47 3.2 Band Model of the Semiconductor 47 3.2.1 Origin of Energy Bands 47 3.2.2 Differences in Isolators, Semiconductors and Conductors 48 3.2.3 Intrinsic Carrier Concentration 49 3.3 Charge Transport in Semiconductors 50 3.3.1 Field Currents 50 3.3.2 Diffusion Currents 52 3.4 Doping of Semiconductors 53 3.4.1 n-Doping 53 3.4.2 p-Doping 54 3.5 The p-n Junction 54 3.5.1 Principle of Method of Operation 55 3.5.2 Band Diagram of the p-n Junction 56 3.5.3 Behavior with Applied Voltage 58 3.5.4 Diode Characteristics 59 3.6 Interaction of Light and Semiconductors 60 3.6.1 Phenomenon of Light Absorption 60 3.6.2 Light Reflection on Surfaces 64 4 Structure and Method of Operation of Solar Cells 67 4.1 Consideration of the Photodiode 67 4.1.1 Structure and Characteristics 67 4.1.2 Equivalent Circuit 69 4.2 Method of Function of the Solar Cell 69 4.2.1 Principle of the Structure 69 4.2.2 Recombination and Diffusion Length 70 4.2.3 What Happens in the Individual Cell Regions? 71 4.2.4 Back-Surface Field 73 4.3 Photocurrent 73 4.3.1 Absorption Efficiency 74 4.3.2 Quantum Efficiency 75 4.3.3 Spectral Sensitivity 76 4.4 Characteristic Curve and Characteristic Dimensions 77 4.4.1 Short Circuit Current ISC 78 4.4.2 Open Circuit Voltage VOC 78 4.4.3 Maximum Power Point (MPP) 79 4.4.4 Fill Factor FF 79 4.4.5 Efficiency h 80 4.4.6 Temperature Dependency of Solar Cells 80 4.5 Electrical Description of Real Solar Cells 82 4.5.1 Simplified Model 82 4.5.2 Standard Model (Single-Diode Model) 83 4.5.3 Two-Diode Model 83 4.5.4 Determining the Parameters of the Equivalent Circuit 85 4.6 Considering Efficiency 87 4.6.1 Spectral Efficiency 87 4.6.2 Theoretical Efficiency 90 4.6.3 Losses in Real Solar Cells 92 4.7 High Efficiency Cells 95 4.7.1 Buried-Contact Cells 96 4.7.2 Point-Contact Cell 96 4.7.3 PERL Cell 97 5 Cell Technologies 99 5.1 Production of Crystalline Silicon Cells 99 5.1.1 From Sand to Silicon 99 5.1.2 From Silicon to Wafer 103 5.1.3 Production of Standard Solar Cells 104 5.1.4 Production of Solar Modules 106 5.2 Cells of Amorphous Silicon 108 5.2.1 Properties of Amorphous Silicon 108 5.2.2 Production Process 108 5.2.3 Structure of the pin Cell 109 5.2.4 Staebler Wronski Effect 110 5.2.5 Stacked Cells 112 5.2.6 Combined Cells of Micromorphous Material 113 5.2.7 Integrated Series Connection 114 5.3 Further Thin Film Cells 115 5.3.1 Cells of Cadmium-Telluride 115 5.3.2 CIS Cells 116 5.4 Hybrid Wafer Cells 118 5.4.1 Combination of c-Si and a-Si (HIT Cell) 118 5.4.2 Stacked Cells of III/V Semiconductors 119 5.5 Other Cell Concepts 120 5.6 Concentrator Systems 120 5.6.1 Principle of Radiation Bundling 120 5.6.2 What is the Advantage of Concentration? 120 5.6.3 Examples of Concentrator Systems 122 5.6.4 Advantages and Disadvantages of Concentrator Systems 123 5.7 Ecological Questions on Cell and Module Production 123 5.7.1 Environmental Effects of Production and Operation 123 5.7.2 Availability of Materials 124 5.7.3 Energy Amortization Time and Yield Factor 126 Summary 129 6 Solar Modules and Solar Generators 133 6.1 Properties of Solar Modules 133 6.1.1 Solar Cell Characteristic Curve in All Four Quadrants 133 6.1.2 Parallel Connection of Cells 134 6.1.3 Series Connection of Cells 135 6.1.4 Use of Bypass Diodes 136 6.1.5 Typical Characteristic Curves of Solar Modules 141 6.1.6 Special Case Thin Film Modules 143 6.1.7 Examples of Data Sheet Information 145 6.2 Connecting Solar Modules 145 6.2.1 Parallel Connection of Strings 145 6.2.2 What Happens in Case of Cabling Errors? 147 6.2.3 Losses Due to Mismatching 148 6.2.4 Smart Installation in Case of Shading 148 6.3 Direct Current Components 150 6.3.1 Principle Plant Build-Up 150 6.3.2 Direct Current Cabling 151 6.4 Types of Plants 153 6.4.1 Open Air Plants 153 6.4.2 Flat Roof Plants 155 6.4.3 Pitched Roof Systems 157 6.4.4 Facade Systems 159 7 Photovoltaic System Technology 161 7.1 Solar Generator and Load 161 7.1.1 Resistive Load 161 7.1.2 DC/DC Converter 162 7.1.3 MPP-Tracker 167 7.2 Grid-Connected Systems 168 7.2.1 Feed-In Variations 169 7.2.2 Installation Concepts 169 7.2.3 Structure of Inverters 171 7.2.4 Efficiency of Inverters 177 7.2.5 Dimensioning of Inverters 181 7.2.6 Measures for Increasing Self-Consumption 184 7.2.7 Requirements of Grid Operators 186 7.2.8 Safety Aspects 188 7.3 Stand-Alone Systems 189 7.3.1 Principle of the Structure 189 7.3.2 Batteries 190 7.3.3 Charge Controllers 194 7.3.4 Examples of Stand-Alone Systems 197 7.3.5 Dimensioning Stand-Alone Plants 199 8 Photovoltaic Metrology 205 8.1 Measurement of Solar Radiation 205 8.1.1 Global Radiation Sensors 205 8.1.2 Measuring Direct and Diffuse Radiation 207 8.2 Measuring the Power of Solar Modules 208 8.2.1 Buildup of a Solar Module Power Test Rig 209 8.2.2 Quality Classification of Module Flashers 210 8.2.3 Determination of the Module Parameters 211 8.3 Peak Power Measurement at Site 212 8.3.1 Principle of Peak Power Measurement 212 8.3.2 Possibilities and Limits of the Measurement Principle 213 8.4 Thermographic Measuring Technology 214 8.4.1 Principle of Infrared Temperature Measurement 214 8.4.2 Bright Thermography of Solar Modules 215 8.4.3 Dark Thermography 217 8.5 Electroluminescence Measuring Technology 218 8.5.1 Principle of Measurement 218 8.5.2 Examples of Photos 219 9 Design and Operation of Grid-Connected Plants 223 9.1 Planning and Dimensioning 223 9.1.1 Selection of Site 223 9.1.2 Shading 224 9.1.3 Plant Dimensioning and Simulation Programs 228 9.2 Economics of Photovoltaic Plants 230 9.2.1 The Renewable Energy Law 230 9.2.2 Return Calculation 231 9.3 Surveillance, Monitoring and Visualization 235 9.3.1 Methods of Plant Surveillance 235 9.3.2 Monitoring PV Plants 235 9.3.3 Visualization 238 9.4 Operating Results of Actual Installations 239 9.4.1 Pitched Roof Installation from 1996 239 9.4.2 Pitched Roof Installation from 2002 240 9.4.3 Flat Roof from 2008 241 10 Outlook 243 10.1 Potential of Photovoltaics 243 10.1.1 Theoretical Potential 243 10.1.2 Technically Useful Radiation Energy 243 10.1.3 Technical Electrical Energy Generation Potential 245 10.1.4 Photovoltaics versus Biomass 246 10.2 Efficient Promotion Instruments 247 10.3 Price Development 248 10.4 Thoughts on Future Energy Supply 249 10.4.1 Current Development in Renewable Energies 249 10.4.2 Consideration of Future Scenarios 249 10.4.3 Options for Storing Electrical Energy 251 10.4.4 Requirements of the Grids 254 10.5 Conclusion 255 11 Exercises 257 Appendix A 267 Appendix B 269 References 271 Index 277