Understanding Wind Power Technology
Theory, Deployment and Optimisation

Wind energy technology has progressed enormously over the last decade. In coming years it will continue to develop in terms of power ratings, performance and installed capacity of large wind turbines worldwide, with exciting developments in offshore installations.

Designed to meet the training needs of wind engineers, this introductory text puts wind energy in context, from the natural resource to the assessment of cost effectiveness and bridges the gap between theory and practice. The thorough coverage spans the scientific basics, practical implementations and the modern state of technology used in  onshore and offshore wind farms for electricity generation.

Key features: 

• provides in-depth treatment of all systems associated with wind energy, including the aerodynamic and structural aspects of blade design, the flow of energy and loads through the wind turbine, the electrical components and power electronics including control systems 
• explains the importance of wind resource assessment techniques, site evaluation and ecology with a focus of project planning and operation 
• describes the integration of wind farms into the electric grid and includes a whole chapter dedicated to offshore wind farms 
• includes questions in each chapter for readers to test their knowledge

Written by experts with deep experience in research, teaching and industry, this text conveys the importance of wind energy in the international energy-policy debate, and offers clear insight into the subject for postgraduates and final year undergraduate students studying all aspects of wind engineering. Understanding Wind Power Systems is also an authoritative resource for engineers designing and developing wind energy systems, energy policy makers, environmentalists, and economists in the renewable energy sector.

About the Authors xiv

1 The History of Wind Energy 1
Jos Beurskens

1.1 Introduction 1

1.2 The First Windmills: 600–1890 2

1.2.1 Technical Development of the First Horizontal Windmills 5

1.3 Generation of Electricity using Wind Farms: Wind Turbines 1890–1930 10

1.4 The First Phase of Innovation: 1930–1960 16

1.5 The Second Phase of Innovation and Mass Production: 1960 to Today 25

1.5.1 The State-Supported Development of Large Wind Turbines 28

1.5.2 The Development of Smaller Wind Turbines 36

1.5.3 Wind Farms, Offshore and Grid Connection 38

1.5.4 International Grids 41

1.5.5 To Summarise 43

References 43

2 The International Development of Wind Energy 45
Klaus Rave

2.1 The Modern Energy Debate 45

2.2 The Reinvention of the Energy Market 48

2.3 The Importance of the Power Grid 50

2.4 The New Value-added Chain 53

2.5 International Perspectives 55

2.6 Expansion into Selected Countries 58

2.7 The Role of the EU 59

2.8 International Institutions and Organisations 61

2.8.1 Scenarios 64

2.9 Global Wind Energy Outlook 2012 – The Global View into the Future 65

2.9.1 Development of the Market in Selected Countries 65

2.10 Conclusion 71

References 71

3 Wind Resources, Site Assessment and Ecology 73
Hermann van Radecke

3.1 Introduction 73

3.2 Wind Resources 73

3.2.1 Global Wind Systems and Ground Roughness 73

3.2.2 Topography and Roughness Length 75

3.2.3 Roughness Classes 76

3.2.4 Contour Lines and Obstacles 79

3.2.5 Wind Resources with WAsP, WindPRO, Windfarmer 81

3.2.6 Correlating Wind Potential with Mesoscale Models and Reanalysis Data 84

3.2.7 Wind in the Wind Farm 90

3.2.8 Wind Frequency Distribution 95

3.2.9 Site Classification and Annual Energy Production 96

3.2.10 Reference Yield and Duration of Increased Subsidy 99

3.3 Acoustics 101

3.3.1 The dB(A) Unit 101

3.3.2 Sources of Noise 103

3.3.3 Propagation through the Air 105

3.3.4 Imission Site and Benchmarks 105

3.3.5 Frequency Analysis, Tone Adjustment and Impulse Adjustment 106

3.3.6 Methods of Noise Reduction 106

3.3.7 Regulations for Minimum Distances 107

3.4 Shadow 107

3.5 Turbulence 109

3.5.1 Turbulence from Surrounding Environment 110

3.5.2 Turbulence Attributed to Turbines 111

3.6 Two Comprehensive Software Tools for Planning Wind Farms 111

3.7 Technical Guidelines, Fgw Guidelines and IEC Standards 112

3.8 Environmental Influences Bundes-Immissionsschutzgesetz (Federal Imission Control Act) and Approval Process 113

3.8.1 German Imission Protection Law (BImSchG) 114

3.8.2 Approval Process 115

3.8.3 Environmental Impact Assessment (Eia) 115

3.8.4 Specific Aspects of the Process 118

3.8.5 Acceptance 121

3.8.6 Monitoring and Clarifying Plant-Specific Data 121

3.9 Example Problems 121

3.10 Solutions to the Problems 123

References 124

4 Aerodynamics and Blade Design 126
Alois Schaffarczyk

4.1 Summary 126

4.2 Horizontal Plants 126

4.2.1 General 126

4.2.2 Basic Aerodynamic Terminology 127

4.3 Integral Momentum Theory 130

4.3.1 Momentum Theory of Wind Turbines: the Betz Limiting Value 130

4.3.2 Changes in Air Density with Temperature and Altitude 132

4.3.3 Influence of the Finite Blade Number 133

4.3.4 Swirl Losses and Local Optimisation of the Blades According to Glauert 134

4.3.5 Losses Due to Profile Drag 136

4.4 Momentum Theory of the Blade Elements 137

4.4.1 The Formulation 137

4.4.2 Example of an Implementation: WT-Perf 139

4.4.3 Optimisation and Design Rules for Blades 139

4.4.4 Extension of the Blade Element Method: The Differential Formulation 140

4.4.5 Three-Dimensional Computational Fluid Dynamics (Cfd) 141

4.4.6 Summary: Horizontal Plants 142

4.5 Vertical Plants 142

4.5.1 General 142

4.5.2 Aerodynamics of H Rotors 144

4.5.3 Aeroelastics of Vertical Axis Rotors 149

4.5.4 A 50 kW Rotor as an Example 150

4.5.5 Design Rules for Small Wind Turbines According to H-Darrieus Type A 150

4.5.6 Summary: Vertical Rotors 151

4.6 Wind-Driven Vehicles with a Rotor 151

4.6.1 Introduction 151

4.6.2 On the Theory of Wind-Driven Vehicles 152

4.6.3 Numerical Example 153

4.6.4 The Kiel Design Method 153

4.6.5 Evaluation 154

4.6.6 Completed Vehicles 155

4.6.7 Summary: Wind Vehicles 156

4.7 Exercises 157

References 158

5 Rotor Blades 162
Lothar Dannenberg

5.1 Introduction 162

5.2 Loads on Rotor Blades 163

5.2.1 Types of Loads 163

5.2.2 Fundamentals of the Strength Calculations 165

5.2.3 Cross-Sectional Values of Rotor Blades 167

5.2.4 Stresses and Deformations 172

5.2.5 Section Forces in the Rotor Blade 176

5.2.6 Bending and Inclination 178

5.2.7 Results According to Beam Theory 179

5.3 Vibrations and Buckling 180

5.3.1 Vibrations 180

5.3.2 Buckling and Stability Calculations 183

5.4 Finite Element Calculations 184

5.4.1 Stress Calculations 184

5.4.2 Fem Buckling Calculations 185

5.4.3 FEM Vibration Calculations 186

5.5 Fibre-Reinforced Plastics 187

5.5.1 Introduction 187

5.5.2 Materials (Fibres, Resins, Additives, Sandwich Materials) 188

5.5.3 Laminates and Laminate Properties 192

5.6 Production of Rotor Blades 195

5.6.1 Structural Parts of the Rotor Blades 195

5.6.2 Composite Manufacturing Methods 198

5.6.3 Assembly of the Rotor Blade 199

References 200

6 The Drive Train 202
Sönke Siegfriedsen

6.1 Introduction 202

6.2 Blade Angle Adjustment Systems 203

6.3 Wind Direction Tracking 209

6.3.1 General 209

6.3.2 Description of the Function 209

6.3.3 Components 210

6.3.4 Variations in Wind Direction Tracking Arrangements 213

6.4 Drive Train Components 215

6.4.1 Rotor Locking and Rotor Rotating Arrangements 216

6.4.2 Rotor Shaft and Mountings 217

6.4.3 Gears 220

6.4.4 Brake and Coupling 223

6.4.5 Generator 225

6.5 Drive Train Concepts 227

6.5.1 Direct-Driven – Double Mounting 228

6.5.2 Direct-Driven – Torque Support 230

6.5.3 One–Two Step Geared Drives – Double Bearings 232

6.5.4 One–Two Step Geared Drives – Torque Support 234

6.5.5 Three–Four Step Geared Drives – Double Mountings 235

6.5.6 Three–Four Step Geared Drives – Three-Point Mountings 237

6.5.7 Three–Four Step Geared Drives – Torque Support 239

6.6 Damage and Causes of Damage 240

6.7 Design of Drive Train Components 241

6.7.1 LDD 244

6.7.2 RFC 244

6.8 Intellectual Property in the Wind Industry 246

6.8.1 Example Patents of Drive Trains 247

Further Reading 251

7 Tower and Foundation 253
Torsten Faber

7.1 Introduction 253

7.2 Guidelines and Standards 255

7.3 Tower Loading 255

7.3.1 Fatigue Loads 255

7.3.2 Extreme Loads 257

7.4 Verification of the Structure 258

7.4.1 Proof of Load Capacity 258

7.4.2 Proof of Fitness for Use 259

7.4.3 Proof of Foundation 259

7.4.4 Vibration Calculations (Eigen Frequencies) 260

7.5 Design Details 261

7.5.1 Door Openings in Steel Tube Towers 262

7.5.2 Ring Flange Connections 262

7.5.3 Welded Connections 262

7.6 Materials for Towers 263

7.6.1 Steel 263

7.6.2 Concrete 263

7.6.3 Timber 264

7.6.4 Glass Fibre-Reinforced Plastic 265

7.7 Model Types 265

7.7.1 Tubular Towers 265

7.7.2 Lattice Masts 266

7.7.3 Guyed Towers 266

7.8 Foundations for Onshore WTs 267

7.8.1 Force of Gravity 267

7.8.2 Piles 267

7.8.3 Cables 267

7.9 Exercises 268

7.10 Solutions 269

References 272

8 Power Electronics and Generator Systems for Wind Turbines 273
Friedrich W. Fuchs

8.1 Introduction 273

8.2 Single-Phase AC Voltage and Three-Phase AC Voltage Systems 275

8.3 Transformer 278

8.3.1 Principle and Calculations 278

8.3.2 Equivalent Circuit Diagram, Phasor Diagram 279

8.3.3 Simplified Equivalent Circuit Diagram 281

8.3.4 Three-Phase Transformers 282

8.4 Generators for Wind Turbines 283

8.4.1 Induction Machine with Short-Circuit Rotor 284

8.4.2 Induction Machine with Slip Ring Rotor 295

8.5 Synchronous Machines 303

8.5.1 General Function 303

8.5.2 Voltage Equations and Equivalent Circuit Diagram 304

8.5.3 Power and Torque 306

8.5.4 Models of Externally Excited Synchronous Machines 307

8.5.5 Permanently Excited Synchronous Machines 308

8.5.6 Variable Speed Operation of Synchronous Machines 309

8.6 Converter Systems for Wind Turbines 310

8.6.1 General Function 310

8.6.2 Frequency Converter in Two-Level Topology 311

8.6.3 Frequency Converter with Multi-Level Circuits 317

8.7 Control of Variable-Speed Converter-Generator Systems 318

8.7.1 Control of the Converter-Fed Induction Generator with Short-Circuit Rotor 319

8.7.2 Control of the Doubly-Fed Induction Machine 325

8.7.3 Control of the Synchronous Machine 326

8.7.4 Control of the Grid-Side Converter 326

8.7.5 Design of the Controls 329

8.8 Compliance with the Grid Connection Requirements 329

8.9 Further Electronic Components 331

8.10 Features of the Power Electronics Generator System in Overview 332

8.11 Exercises 333

References 338

9 Control of Wind Energy Systems 340
Reiner Johannes Schütt

9.1 Fundamental Relationships 341

9.1.1 Allocation of the WTS Automation 341

9.1.2 System Properties of Energy Conversion in WTs 344

9.1.3 Energy Transformation at the Rotor 344

9.1.4 Energy Transformation at the Drive Train 347

9.1.5 Energy Conversion at the Generator-Converter System 348

9.1.6 Idealised Operating Characteristic Curves of WTs 351

9.2 WT Control Systems 352

9.2.1 Yaw Angle Control 352

9.2.2 Blade Angle Control 353

9.2.3 Active Power Control 354

9.2.4 Reactive Power Control 357

9.2.5 Summary of the Control Behaviour and Extended Operating Ranges of the WT 358

9.3 Operating Management Systems for WTs 358

9.3.1 Control of the Operating Sequence of WTs 359

9.3.2 Safety Systems 362

9.4 Wind Farm Control and Automation Systems 363

9.5 Remote Control and Monitoring 365

9.6 Communication Systems for WTS 366

References 368

10 Grid Integration 369
Sven Wanser and Frank Ehlers

10.1 Energy Supply Grids in Overview 369

10.1.1 General 369

10.1.2 Voltage Level of Electrical Supply Grids 370

10.1.3 Grid Structures 370

10.2 Grid Control 372

10.2.1 Controlling the Power Range 373

10.2.2 Compensating Power and Balancing Grids 373

10.2.3 Base Load, Medium Load and Peak Load 374

10.2.4 Frequency Stability 375

10.2.5 Primary Control, Secondary Control and Tertiary Control 376

10.2.6 Voltage Stability 378

10.2.7 System Services by means of Wind Turbines 378

10.3 Basic Terminology of Grid Integration of Wind Turbines 380

10.3.1 Basic Electrical Terminology 380

10.3.2 Grid Quality 384

10.4 Grid Connections for WTs 387

10.4.1 Rating the Grid Operating Media 388

10.4.2 Checking the Voltage Changes/Voltage Band 390

10.4.3 Checking the Grid Reaction ‘Fast Voltage Change’ 395

10.4.4 Checking the Short-Circuit Strength 396

10.5 Grid Connection of WTs 397

10.5.1 Switchgear 398

10.5.2 Protective Equipment 399

10.5.3 Integration into the Grid System 401

10.6 Further Developments in Grid Integration and Outlook 401

10.6.1 Grid Expansion 402

10.6.2 Load Displacement 404

10.6.3 Energy Storage 404

References 405

11 Offshore Wind Energy 406
Lothar Dannenberg

11.1 Offshore Wind Turbines 406

11.1.1 Introduction 406

11.1.2 Differences between Offshore and Onshore WTs 407

11.1.3 Environmental Conditions and Nature Protection 409

11.2 Currents and Loads 409

11.2.1 Currents 409

11.2.2 Current Loads 410

11.2.3 Vortex Shedding of Bodies Subject to Flows 412

11.3 Waves, Wave Loads 413

11.3.1 Wave Theories 413

11.3.2 Superposition of Waves and Currents 423

11.3.3 Loads Due to Waves (Morison Method) 425

11.4 Swell 430

11.4.1 Regular Swell 430

11.4.2 Irregular or Natural Swells 430

11.4.3 Statistics 431

11.4.4 Swell Spectra 432

11.4.5 Influence of Currents 436

11.4.6 Long-Term Statistics of the Swell 436

11.4.7 Extreme Waves 436

11.5 Scouring Formation, Growth, Corrosion and Ice 437

11.5.1 Scouring 437

11.5.2 Marine Growth 438

11.5.3 Ice Loads 439

11.5.4 Corrosion 439

11.6 Foundations for OWTs 441

11.6.1 Introduction 441

11.6.2 Fixed Foundations 442

11.6.3 Floating Foundations 447

11.6.4 Operating Strength 448

11.7 Soil Mechanics 450

11.7.1 Introduction 450

11.7.2 Soil Properties 450

11.7.3 Calculation of Load-Bearing Behaviour of the Sea Bed 451

References 454

Index 455

Professor Alois Schaffarczyk, Kiel University of Applied Sciences, Kiel, Germany

Professor Schaffarczyk is a founding member and previous manager of CEwind eG, the consortium for wind energy research between Schleswig-Holstein’s Universities in Germany. He has worked in the field of wind turbine aerodynamics since 1997 and currently teaches courses in the CEwind MSc. Wind Engineering program.