Photovoltaic Systems Engineering

Third Edition

Roger д. Messenger • Jerry ventre

( röC) CRC Press \ V J Taylor Si Francis Group

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

Contents

Preface xvii Acknowledgments xxi About the Authors xxiii

Chapter 1 Background 1

1.1 Introduction 1 1.2 Population and Energy Demand 2 1.3 Energy Units 2 1.4 Current World Energy Use Patterns 3 1.5 Exponential Growth 7

1.5.1 Introduction 7 1.5.2 Compound Interest 7 1.5.3 Doubling Time 8 1.5.4 Accumulation 9 1.5.5 Resource Lifetime in an Exponential Environment 10

1.6 Hubbert's Gaussian Model 11 1.7 Net Energy, Btu Economics, and the Test for Sustainability 13 1.8 Direct Conversion of Sunlight to Electricity with Photovoltaics 14 References 18 Suggested Reading 19

Chapter 2 The Sun 21

2.1 Introduction 21 2.2 The Solar Spectrum 21 2.3 The Effect of Atmosphere on Sunlight 23 2.4 Sunlight Specifics 25

2.4.1 Introduction 25 2.4.2 Definitions 26 2.4.3 The Orbit and Rotation of the Earth 27 2.4.4 Tracking the Sun 29 2.4.5 Measuring Sunlight 32

2.4.5.1 Precision Measurements 32 2.4.5.2 Less Precise Measurements 35

2.5 Capturing Sunlight 35 2.5.1 Maximizing Irradiation on the Collector 35 2.5.2 Shading 38

2.5.2.1 Field Measurement of Shading Objects 38 2.5.2.2 Computational Methods of Determining Shading 39

v

vi CONTENTS

2.5.3 Special Orientation Considerations 40 2.5.3.1 Horizontal Mounting 41 2.5.3.2 Non-South-Facing Mounting 41

References 46 Suggested Reading 46

Chapter 3 Introduction to PV Systems 47

3.1 Introduction 47 3.2 The PV Cell 49 3.3 The PV Module 52 3.4 The PV Array 55 3.5 Energy Storage 55

3.5.1 Introduction 55 3.5.2 The Lead-Acid Storage Battery 56

3.5.2.1. Chemistry of the Lead-Acid Cell 56 3.5.2.2 Properties of the Lead-Acid Storage Battery 57

3.5.3 The Nickel Cadmium Storage Battery 62 3.5.3.1 Chemistry of the Nickel Cadmium Storage Battery 62 3.5.3.2 Properties of the Nickel Cadmium System 63

3.5.4 Other Battery Systems 64 3.5.5 Hydrogen Storage 65 3.5.6 The Fuel Cell 66 3.5.7 Other Storage Options 67

3.6 PV System Loads 67 3.7 PV System Availability 69 3.8 Associated System Electronic Components 72

3.8.1 Introduction 72 3.8.2 Charge Controllers 72

3.8.2.1 Charging Considerations 73 3.8.2.2 Discharging Considerations 76

3.8.3 Maximum Power Point Trackers and Linear Current Boosters 76 3.8.4 Inverters 80

3.8.4.1 Square Wave Inverters 81 3.8.4.2 Modified Sine Wave Inverters 82 3.8.4.3 Pulse-Width-Modulated Inverters 85 3.8.4.4 Other Desirable Inverter Features 88

3.9 Generators 89 3.9.1 Introduction 89 3.9.2 Types and Sizes of Generators 90 3.9.3 Generator Operating Characteristics 91

3.9.3.1 Rotation Speed 91 3.9.3.2 Efficiency 91 3.9.3.3 Fuel Types 92

CONTENTS vii

3.9.3.4 Altitude Effects 93 3.9.3.5 Waveform Harmonie Content 93 3.9.3.6 Frequency Stability 93 3.9.3.7 Amplitude Stability 94 3.9.3.8 Noise Level 94 3.9.3.9 Type of Starting 94 3.9.3.10 Overload Characteristics 94 3.9.3.11 Power Factor Considerations 94

3.9.4 Generator Maintenance 95 3.9.5 Generator Selection 95

3.10 Balance of System (BOS) Components 95 3.10.1 Switches, Circuit Breakers, Fuses, and Receptacles 96 3.10.2 Ground Fault, Surge, and Lightning Protection 97 3.10.3 Inverter Bypass Switches and Source Circuit Combiner Boxes 98 3.10.4 Grounding Devices 98

References 101 Suggested Reading 102

Chapter 4 Grid-Connected Utility-Interactive PV Systems 103

4.1 Introduction 103 4.2 Applicable Codes and Standards 104

4.2.1 The National Electrical Code 104 4.2.1.1 Introduction 104 4.2.1.2 Voltage Drop and Wire Sizing 106

4.2.2 IEEE Standard 1547-2003 109 4.2.2.1 Introduction 109 4.2.2.2 Specific Requirements 109 4.2.2.3 Comparison of PV Inverters to Mechanically

Rotating Generators 112 4.2.2.4 Islanding Analysis 112

4.2.3 Other Issues 115 4.2.3.1 Aesthetics 115 4.2.3.2 Electromagnetic Interference 115 4.2.3.3 Surge Protection 115 4.2.3.4 Structural Considerations 116

4.3 Design Considerations for Straight Grid-Connected PV Systems 117 4.3.1 Determining System Energy Output 117 4.3.2 Array Installation 119 4.3.3 Inverter Selection and Mounting 119 4.3.4 Other Installation Considerations 120

4.4 Design of a System Based on Desired Annual System Performance 121 4.4.1 Array Sizing 121 4.4.2 Inverter Selection 122

viii CONTENTS

4.4.3 Module Selection 123 4.4.4 Balance of System 125

4.4.4.1 Module Extension Wire, Rooftop Junction Box, and Module and Array Frame Grounding 126

4.4.4.2 Wire and Conduit from Rooftop Junction Box to Inverter 127

4.4.4.3 DC and AC Disconnects and Overcurrent Protection.... 130 4.4.4.4 Point of Utility Connection 130 4.4.4.5 Final System Electrical Schematic Diagram 131

4.5 Design of a System Based on Available Roof Space 132 4.5.1 Array Selection 132 4.5.2 Inverter Selection 136 4.5.3 Balance of System 136

4.5.3.1 Module Extension Wire, Rooftop Junction Box, and Module and Array Frame Grounding 136

4.5.3.2 Wire and Conduit from Rooftop Junction Box to Inverter 137

4.5.3.3 DC and AC Disconnects and Overcurrent Protection.... 137 4.5.3.4 Point of Utility Connection 138 4.5.3.5 Estimating System Annual Performance 138 4.5.3.6 Final System Electrical Schematic Diagram 138

4.5.4 Extension of Design to Lower Wind Speed Region 139 4.6 Design of a Microinverter-Based System 140

4.6.1 Introduction 140 4.6.2 System Design 140 4.6.3 Bells and Whistles (i.e., Monitoring Possibilities) 142

4.7 Design of a Nominal 21 kW System That Feeds a Three-Phase Distribution Panel 142 4.7.1 Introduction 142 4.7.2 Array Configuration 143 4.7.3 System DC Wiring 144 4.7.4 System AC Wiring 146

4.7.4.1 Wire and Overcurrent Protection Sizing 146 4.7.4.2 Voltage Drop Calculations 148 4.7.4.3 Connecting to a 277/480 V Y-Connected Source 149

4.8 Design of a Nominal 250 kW System 150 4.8.1 Introduction 150 4.8.2 Configuring the Array 150 4.8.3 Source Circuit Voltage Drop Calculations 151 4.8.4 DC Disconnects and Overcurrent Protection 155 4.8.5 AC Wire Sizing, Disconnects, and Overcurrent Protection 155

4.9 System Performance Monitoring 158 References 161

CONTENTS ix

Chapter 5 Mechanical Considerations 163

5.1 Introduction 163 5.2 Important Properties of Materials 163

5.2.1 Introduction 163 5.2.2 Mechanical Properties 164 5.2.3 Stress and Strain 166 5.2.4 Strength of Materials 169 5.2.5 Column Buckling 170 5.2.6 Thermal Expansion and Contraction 170 5.2.7 Chemical Corrosion and Ultraviolet Degradation 172 5.2.8 Properties of Steel 174 5.2.9 Properties of Aluminum 176

5.3 Establishing Mechanical System Requirements 177 5.3.1 Mechanical System Design Process 177 5.3.2 Functional Requirements 178 5.3.3 Operational Requirements 178 5.3.4 Constraints 179 5.3.5 Trade-Offs 180

5.4 Design and Installation Guidelines 180 5.4.1 Standards and Codes 180 5.4.2 Building Code Requirements 181

5.5 Forces Acting on PV Arrays 182 5.5.1 Structural Loading Considerations 182 5.5.2 Dead Loads 183 5.5.3 Live Loads 183 5.5.4 Wind Loads 183 5.5.5 Snow Loads 191 5.5.6 Other Loads 192

5.6 Array Mounting System Design 192 5.6.1 Introduction 192 5.6.2 Objectives in Designing the Array Mounting System 192

5.6.2.1 Minimizing Installation Costs 192 5.6.2.2 Building Integration Considerations 194 5.6.2.3 Costs and Durability of Array-Roof Configurations 195

5.6.3 Enhancing Array Performance 195 5.6.3.1 Irradiance-Enhancement 195 5.6.3.2 Shading 196 5.6.3.3 Array-Cooling 196 5.6.3.4 Protection from Vandalism 196

5.6.4 Roof-Mounted Arrays 196 5.6.4.1 Stand-Off Mounting 196 5.6.4.2 Rack Mounting 198

X CONTENTS

5.6.4.3 Integrated Mounting 198 5.6.4.4 Direct Mounting 199

5.6.5 Ground-Mounted Arrays 199 5.6.5.1 Rack Mounting 200 5.6.5.2 Pole Mounting 200 5.6.5.3 Tracking-Stand Mounting 200

5.6.6 Aesthetics 201 5.7 Computing Mechanical Loads and Stresses 202

5.7.1 Introduction 202 5.7.2 Withdrawal Loads 202 5.7.3 Tensile Stresses 203 5.7.4 Buckling 203

5.8 Stand-Off, Roof Mount Examples 205 5.8.1 Introduction to ASCE-7 Wind Load Analysis Tabular Method.... 205 5.8.2 Array Mount Design: High Wind Speed Case 207 5.8.3 Array Mount Design: Low Wind Speed Case 209 5.8.4 Exposure C, Exposure D, and Other Correction Factors 210

References 212 Suggested Reading 213

Chapter 6 Battery-Backup Grid-Connected PV Systems 215

6.1 Introduction 215 6.2 Battery-Backup Design Basics 216

6.2.1 Introduction 216 6.2.2 Load Determination 217 6.2.3 Inverter Sizing 217 6.2.4 Battery Sizing 217 6.2.5 Sizing the Array 219

6.3 A Single-Inverter 120 V Battery-Backup System Based on Standby Loads 220 6.3.1 Determination of Standby Loads 220 6.3.2 Inverter Selection 221 6.3.3 Battery Selection 221 6.3.4 Array Sizing 223 6.3.5 Charge Controller and Module Selection 224 6.3.6 BOS Selection and Completion of the Design 226

6.3.6.1 Array Mounting Equipment 226 6.3.6.2 Rooftop Junction Box 227 6.3.6.3 Source Circuit Combiner Box and Surge Arrester 227 6.3.6.4 Wire and Circuit Breaker Sizing: dc Side 227 6.3.6.5 Wire and Circuit Breaker Sizing: ac Side 229 6.3.6.6 Wiring of Standby Loads 230

CONTENTS xi

6.3.6.7 Equipment Grounding Conductor and Grounding Electrode Conductor Sizing 231

6.3.7 Programming the Inverter and Charge Controller 232 6.3.8 Fossil Fuel Generator Connection Options 234

6.4 A 120/240 V Battery-Backup System Based on Available Roof Space 235 6.4.1 Introduction 235 6.4.2 Module Selection and Source Circuit Design 235 6.4.3 Source Circuit Combiner Box and Charge Controller Selection... 236 6.4.4 Inverter Selection 238 6.4.5 Determination of Standby Loads and Battery Selection 238 6.4.6 BOS Selection and Completion of Design 239

6.4.6.1 Rooftop Junction Box 239 6.4.6.2 Source Circuit Combiner Box and Surge Arresters 240 6.4.6.3 Wire and Circuit Breaker Sizing: dc Side 241 6.4.6.4 Wire and Circuit Breaker Sizing: ac Side 242 6.4.6.5 Wiring of Standby Loads 242 6.4.6.6 Equipment Grounding Conductor and Grounding

Electrode Conductor Sizing 243 6.5 An 18 kW Battery-Backup System Using Inverters in Tandem 244

6.5.1 Introduction 244 6.5.2 Inverter and Charge Controller Selection 244

6.5.2.1 Inverter Selection 244 6.5.2.2 Charge Controllers 246

6.5.3 Module Selection and Array Layout 246 6.5.3.1 Module Selection 247 6.5.3.2 Array Layout 248 6.5.3.3 Array Performance 250

6.5.4 Battery and BOS Selection 251 6.5.5 Wire Sizing 251 6.5.6 Final Design 254

6.6 AC-Coupled Battery-Backup Systems 257 6.6.1 Introduction 257 6.6.2 A 120/240 V Battery-Backup Inverter with 240 V Straight

Grid-Connected Inverter 258 6.6.3 A 120 V Battery-Backup Inverter with a 240 V Straight Grid-

Connected Inverter 260 6.6.4 A 120/208 V 3-Phase AC-Coupled System 261

6.7 Battery Connections 263 References 269

Chapter 7 Stand-Alone PV Systems 271

7.1 Introduction 271 7.2 The Simplest Configuration: Module and Fan 272

CONTENTS

A PV-Powered Water-Pumping System 274 7.3.1 Introduction 274 7.3.2 Selection of System Components 275 7.3.3 Design Approach for Simple Pumping System 277

7.3.3.1 Determination of Pump Operating Current and Voltage 277

7.3.3.2 Module Selection 278 A PV-Powered Parking Lot Lighting System 280 7.4.1 Determination of the Lighting Load 280 7.4.2 Parking Lot Lighting Design 282

7.4.2.1 Determination of Lamp Wattage and Daily Load Presented by the Fixture 283

7.4.2.2 Determination of Battery Storage Requirements 284 7.4.2.3 Determination of Array Size 284 7.4.2.4 Final System Schematic 285 7.4.2.5 Structural Comment 286

A Cathodic Protection System 287 7.5.1 Introduction 287 7.5.2 System Design 288 A Portable Highway Advisory Sign 291 7.6.1 Introduction 291 7.6.2 Determination of Available Average Power 292 A Critical-Need Refrigeration System 293 7.7.1 Introduction 293 7.7.2 Load Determination 294 7.7.3 Battery Sizing 294 7.7.4 Array Sizing 295

7.7.4.1 Fixed Array 295 7.7.4.2 Tracking Array Mount 297 7.7.4.3 Special Cases 297

7.7.5 BOS Component Selection 298 7.7.6 Overall System Design 299 A PV-Powered Mountain Cabin 300 7.8.1 Introduction 300 7.8.2 Load Determination 301 7.8.3 Battery Selection 303 7.8.4 Array Sizing and Tilt 304 7.8.5 Controller and Inverter Selection 308

7.8.5.1 Charge Controller 308 7.8.5.2 Inverter 308

7.8.6 BOS Component Selection 309 7.8.6.1 Wire, Circuit Breaker, and Switch Selection 309 7.8.6.2 Other Items 310

A Hybrid-Powered, Off-Grid Residence 311 7.9.1 Introduction 311

CONTENTS xiii

7.9.2 Summary of Loads 313 7.9.3 Battery Selection 314 7.9.4 Array Design 315 7.9.5 Generator Selection 318 7.9.6 Generator Operating Hours and Operating Cost 319 7.9.7 Charge Controller and Inverter Selection 320 7.9.8 Wire, Circuit Breaker, and Disconnect Selection 322 7.9.9 BOS Component Selection 324 7.9.10 Total System Design 324

7.10 Summary of Design Procedures 325 References 330 Suggested Reading 330

Chapter 8 Economic Considerations 331

8.1 Introduction 331 8.2 Life-Cycle Costing 332

8.2.1 The Time Value of Money 332 8.2.2 Present Worth Factors and Present Worth 333 8.2.3 Life-Cycle Cost 336 8.2.4 Annualized Life-Cycle Cost 340 8.2.5 Unit Electrical Cost 341

8.3 Borrowing Money 341 8.3.1 Introduction 341 8.3.2 Determination of Annual Payments on Borrowed Money 341 8.3.3 The Effect of Borrowing on Life-Cycle Cost 343

8.4 Payback Analysis 344 8.5 Externalities 347

8.5.1 Introduction 347 8.5.2 Subsidies 348 8.5.3 Externalities and Photovoltaics 348

References 350 Suggested Reading 350

Chapter 9 Externalities and Photovoltaics 351

9.1 Introduction 351 9.2 Externalities 352 9.3 Environmental Effects of Energy Sources 353

9.3.1 Introduction 353 9.3.2 Air Pollution 353

9.3.2.1 The Clean Air Act and the U.S. Environmental Protection Agency 353

9.3.2.2 Greenhouse Gases 356

xiv CONTENTS

9.3.3 Water and Soil Pollution 357 9.3.4 Infrastructure Degradation 358 9.3.5 Quantifying the Cost of Externalities 358

9.3.5.1 The Cost of C02 358 9.3.5.2 Sequestering C02 with Trees 359 9.3.5.3 The Clean Power Estimator 360 9.3.5.4 Attainment Levels as Commodities 361 9.3.5.5 Subsidies 361

9.3.6 Health and Safety as Externalities 362 9.4 Externalities Associated with PV Systems 363

9.4.1 Environmental Effects of PV System Production 363 9.4.2 Environmental Effects of PV System Deployment

and Operation 364 9.4.3 Environmental Effects of PV System Decommissioning 365

References 366

Chapter 10 The Physics of Photovoltaic Cells 369

10.1 Introduction 369 10.2 Optical Absorption 369

10.2.1 Introduction 369 10.2.2 Semiconductor Materials 370 10.2.3 Generation of EHP by Photon Absorption 371 10.2.4 Photoconductors 374

10.3 Extrinsic Semiconductors and the pn Junction 376 10.3.1 Extrinsic Semiconductors 376 10.3.2 The pn Junction 377

10.3.2.1 Drift and Diffusion 377 10.3.2.2 Junction Formation and Built-in Potential 379 10.3.2.3 The Illuminated pn Junction 382 10.3.2.4 The Externally Biased pn Junction 383

10.4 Maximizing PV Cell Performance 386 10.4.1 Introduction 386 10.4.2 Minimizing the Reverse Saturation Current 386 10.4.3 Optimizing Photocurrent 387

10.4.3.1 Minimizing Reflection of Incident Photons 387 10.4.3.2 Maximizing Minority Carrier Diffusion Lengths 388 10.4.3.3 Maximizing Junction Width 390 10.4.3.4 Minimizing Surface Recombination Velocity 393 10.4.3.5 A Final Expression for the Photocurrent 394

10.4.4 Minimizing Cell Resistance Losses 396 10.5 Exotic Junctions 398

10.5.1 Introduction 398 10.5.2 Graded Junctions 398

CONTENTS xv

10.5.3 Heterojunctions 399 10.5.4 Schottky Junctions 400 10.5.5 Multijunctions 402 10.5.6 Tunnel Junctions 403

References 405

Chapter 11 Present and Proposed PV Cells and Systems 407

11.1 Introduction 407 11.2 Silicon PV Cells 409

11.2.1 Production of Pure Silicon 409 11.2.2 Single-Crystal Silicon Cells 410

11.2.2.1 Fabrication of the Wafer 410 11.2.2.2 Fabrication of the Junction 411 11.2.2.3 Contacts 413 11.2.2.4 Antireflective Coating 416 11.2.2.5 Modules 416

11.2.3 A High-Efficiency Si Cell with All Contacts on the Back 417 11.2.4 Multicrystalline Silicon Cells 418 11.2.5 Other Thin Silicon Cells 419 11.2.6 Amorphous Silicon Cells 420

11.2.6.1 Fabrication 420 11.2.6.2 Cell Performance 422

11.3 Gallium Arsenide Cells 423 11.3.1 Introduction 423 11.3.2 Production of Pure Cell Components 423

11.3.2.1 Gallium 423 11.3.2.2 Arsenic 424 11.3.2.3 Germanium 424

11.3.3 Fabrication of the Gallium Arsenide Cell 425 11.3.4 Cell Performance 427

11.4 Copper Indium (Gallium) Diselenide Cells 428 11.4.1 Introduction 428 11.4.2 Production of Pure Cell Components 428

11.4.2.1 Copper 428 11.4.2.2 Indium 429 11.4.2.3 Selenium 430 11.4.2.4 Cadmium 431 11.4.2.5 Sulfur 431 11.4.2.6 Molybdenum 432

11.4.3 Fabrication of the CIS Cell 432 11.4.4 Cell Performance 433

11.4.4.1 Transient Effects 434

xvi CONTENTS

11.5 Cadmium Telluride Cells 435 11.5.1 Introduction 435 11.5.2 Production of Pure Tellurium 435 11.5.3 Production of the CdTe Cell 436 11.5.4 Cell Performance 437

11.6 Emerging Technologies 438 11.6.1 New Developments in Silicon Technology 438 11.6.2 CIS-Family-Based Absorbers 440 11.6.3 Other III-V and II-VI Emerging Technologies 441 11.6.4 Other Technologies 442

11.6.4.1 Thermophotovoltaic Cells 442 11.6.4.2 Intermediate Band Solar Cells 442 11.6.4.3 Supertandem Cells 442 11.6.4.4 Hot Carrier Cells 442 11.6.4.5 Optical Up- and Down-Conversion 443 11.6.4.6 Organic PV Cells 443

11.7 New Developments in System Design 443 11.7.1 Micro Grids 443 11.7.2 Smart Grids 444 11.7.3 Inverter Performance Enhancement 445

11.8 Summary 445 References 446

Appendix A Average Daily Irradiation for Selected Cities 449

Appendix В Design Review Checklist 461

Index 463