Wind Turbines: Fundamentals, Technologies, Application, Economics

886 Pages·2013·74.87 MB·English

by  Erich Hau

Wind Turbines
Erich Hau
Wind Turbines
Fundamentals, Technologies,
Application, Economics
Third, translated edition
With585 Figuresand 41 Tables
ABC
ErichHau
Dipl.–Ing.
Munich,Germany
Translation:
HorstvonRenouard
London,UK
ISBN978-3-642-27150-2 e-ISBN978-3-642-27151-9
DOI10.1007/978-3-642-27151-9
SpringerHeidelbergNewYorkDordrechtLondon
LibraryofCongressControlNumber:2011943801
(cid:2)c Springer-VerlagBerlinHeidelberg2000,2006,2013
Thisworkissubjecttocopyright. AllrightsarereservedbythePublisher, whetherthewholeorpartofthe
materialisconcerned,specificallytherightsoftranslation,reprinting,reuseofillustrations,recitation,broad-
casting,reproductiononmicrofilmsorinanyotherphysicalway,andtransmissionorinformationstorageand
retrieval,electronicadaptation,computersoftware,orbysimilarordissimilarmethodologynowknownorhere-
afterdeveloped.Exemptedfromthislegalreservationarebriefexcerptsinconnectionwithreviewsorscholarly
analysisormaterialsuppliedspecificallyforthepurposeofbeingenteredandexecutedonacomputersystem,
forexclusiveusebythepurchaserofthework.Duplicationofthispublicationorpartsthereofispermittedonly
undertheprovisionsoftheCopyrightLawofthePublisher’s location, initscurrentversion,andpermission
forusemustalwaysbeobtainedfromSpringer.PermissionsforusemaybeobtainedthroughRightsLinkatthe
CopyrightClearanceCenter.ViolationsareliabletoprosecutionundertherespectiveCopyrightLaw.
Theuseofgeneraldescriptivenames,registerednames,trademarks,servicemarks,etc.inthispublicationdoes
notimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfromtherelevantprotective
lawsandregulationsandthereforefreeforgeneraluse.
Whiletheadviceandinformationinthisbookarebelievedtobetrueandaccurateatthedateofpublication,
neithertheauthorsnortheeditorsnorthepublishercanacceptanylegalresponsibilityforanyerrorsoromissions
thatmaybemade.Thepublishermakesnowarranty,expressorimplied,withrespecttothematerialcontained
herein.
Printedonacid-freepaper
SpringerispartofSpringerScience+BusinessMedia(www.springer.com)
Foreword to the Third English Edition
This revised and greatly enlarged third edition is an attempt at keeping abreast of
developments in wind energy technology and especially of the meteoric expansion in
wind energy utilisation for generating power. Since the first edition appeared in 1988,
the book has kept pace with this development with five editions in German and now
three editions in the English language.
In 1988, the idea of utilizing the energy of the wind for generating electricity was
still the dream of s few enthusiasts and was not taken very seriously by the established
electricity industry. Today, generating power from wind energy has gained its rightful
place in the electricity supply spectrum. In almost all the countries, more and more wind
turbines are being used to supplement conventional power stations and their number is
increasing throughout the world. The rate of increase being counted in double figures.
In some countries, notably in Germany, the utilisation of wind energy has already
become an indispensable part of the effort of switching power generation increasingly to
renewable energies and bringing the age of atomic energy to an end.
However, it is not only ecological concerns – and in many cases also apprehensions
about the hazards of atomic energy – which provide the driving force for current
development and future hopes. By now, the profitability achieved by generating
electricity from wind energy is also playing an increasing role. In a fair comparison –
that is to say a comparison of the costs of generating electricity from power stations
newly to be built with the costs of generating electricity from wind energy, the latter are
found to be scarcely higher and, in the long term and under favourable conditions, even
prove to be the cheaper alternative. The large producers of electricity are increasingly
aware of this fact and, after initially hesitating for a long time, are now also investing in
wind energy. The visible result of this assessment is the construction of large offshore
wind farms.
The enormous expansion of wind energy utilisation would not have been possible
without an improved technology of the wind turbines. Even though the basic concept of
the wind turbines used commercially has remained unchanged for several decades,
significant advances have been achieved in their detailed development and contribute
decisively to improved efficiency, increased reliability and more advantageous production
costs. Examples of this are the evolved aerodynamics of the rotors or the development of
gearless designs with electrical generators driven directly by the rotor, or control
technology with ever-increasing intelligence which, coupled with modern frequency
inverters, improves the quality of the electricity delivered and reduces the system loading.
The increasing size of the turbines also requires novel technical solutions, for example for
assembling turbines having a rotor diameter of more than 150 meters and towers of
corresponding height. The development of offshore technology has only just begun. In the
coming years, large offshore wind farms will be erected which will require new designs
VI FOREWORD TO THE THIRD ENGLISH EDITION
and considerable investments for the transmission and distribution of power on land to
consumers located far in the interior. The present third edition also attempts to analyse the
current state of the art in these areas and to clarify it by quoting examples.
These general remarks about the present state of wind energy utilisation apart, I will
refrain from commenting further on the self-evident importance of wind energy for the
global energy crisis and for the environmental crisis. However, I do feel that some
helpful hints on what this book is about will not be misplaced.
The title wind turbines refers to industrially developed and manufactured machines
for generating electrical power. Those looking for a do-it- yourself manual for putting
together a windwheel, as interesting a hobby as this may be, will, therefore, be
disappointed. Instead, this book is intended to give a general overview of modern wind
turbine technology and to provide a means of orientation in the associated technical and
economical fields and its contents and form of representation are structured with this
objective in mind.
I have tried to analyse and to describe the problems involved and their technical
solutions phenomenologically and to avoid mathematical equations as much as possible.
Equations have been included only where a presentation of fundamental principles was
deemed to serve a better understanding but for those interested, I have indicated relevant
literature at the end of each chapter, which would provide the first steps to the
mathematical treatment of the problems described. Those who are responsible for
decisions about investments or steer the technical and scientific work do not commonly
sit at the computer themselves. However, they must have a clear picture of the state of
the art, what technical options are available and what lessons are to be learnt.
A book of this scope which touches on a variety of special disciplines cannot be
written without assistance. I must, therefore, thank the many people who have helped
me along the way and without whose contributions the book could not have been
completed. Primarily, I am very much obliged to my friend and colleague of many
years, Gerald Huß, who supported me very much in the first phase of the work, as early
as 1980. For this third edition he recalculated and refreshed numerous diagrams and
supported me in the final editing of the entire book.
Finally, I wish to thank all those who have assisted me in my work on these recent
editions. In particular, I am indebted to Horst von Renouard in London who translated
the second and this third edition and who has also contributed to the clarity of the text
and its representation with numerous suggestions. As for all previous editions Tanja
Rüth again edited the diagrams for this present edition. Lea Baumstark prepared the
tables and, together with Roland Schmalzl, supported me in typing the additional texts.
Last, but not least, I owe thanks to my publisher Springer Verlag for publishing this
voluminous book in its third edition.

Munich, October 2012 Erich Hau
Contents
1 Windmills and Windwheels .................................................................................... 1
1.1 The Origins of Windmills .................................................................................. 1
1.2 European Windmills .......................................................................................... 4
1.3 Economic Importance of Historical Windmills ............................................... 11
1.4 Scientific and Technical Development of Windmills ...................................... 13
1.5 The American Wind Turbine ........................................................................... 17
References ............................................................................................................... 21

2 Electrical Power from the Wind - The First Attempts ....................................... 23
2.1 Poul La Cour - A Pioneer in Denmark ....................................................... ..... 23
2.2 Large Wind Power Plants - Ambitious Projects in Germany ..................... ..... 28
2.3 1250 kW from the Wind - The First Large Wind Turbine in the US ......... ..... 33
2.4 Wind Turbines in the Fifties - Before the "Energy Crisis" ........................ ..... 36
2.5 After the Energy Crisis - A New Start toward Modern Wind Power ......... ..... 43
2.6 The Large Experimental Turbines of the Eighties ..................................... ..... 46
2.7 First Successes with the Small Wind Turbines in Denmark ...................... ..... 57
2.8 The Wind Farms in the United States ........................................................ ..... 58
References ......................................................................................................... ..... 63

3 Basic Concepts of Wind Energy Converters ...................................................... 65
3.1 Rotors with a Vertical Axis of Rotation........................................................... 66
3.2 Horizontal Axis Rotors .................................................................................... 69
3.3 Wind Energy Concentrators ............................................................................. 72
3.4 Terms and Expressions .................................................................................... 77
References ............................................................................................................... 78

4 Physical Principles of Wind Energy Conversion................................................. 79
4.1 Betz’s Elementary Momentum Theory ............................................................ 79
4.2 Wind Energy Converters Using Aerodynamic Drag or Lift ............................ 84
References ............................................................................................................... 87

5 Rotor Aerodynamics ............................................................................................. 89
5.1 Mathematical Models and Calculations ........................................................... 90
5.1.1 Blade Element Theory .......................................................................... 91
5.1.2 Vortex Model of Rotor Flow ................................................................ 96
VII I CONTENTS
5.1.3 Numeric Flow Simulation ..................................................................... 97
5.1.4 The Rotor Wake .................................................................................... 99
5.2 Rotor Power Characteristics .......................................................................... 102
5.2.1 Power and Torque Characteristics ...................................................... 102
5.2.2 Power Characteristics of Various Rotor Configurations ..................... 104
5.3 Aerodynamic Power Control ......................................................................... 106
5.3.1 Power Control by Rotor Blade Pitching .............................................. 107
5.3.2 Passive Stall Control with Fixed Blade Pitch...................................... 111
5.3.3 Active Stall Control ............................................................................ 115
5.3.4 Transient Aerodynamic Effects and Boundary-Layer Control ........... 117
5.3.5 Turning the Rotor Out of the Wind ..................................................... 119
5.4 The Aerodynamic Airfoil ............................................................................... 120
5.4.1 Characteristic Properties ..................................................................... 120
5.4.2 Airfoil Geometry and Classification ................................................... 123
5.4.3 Laminar Airfoils ................................................................................. 128
5.4.4 Influence on the Power Coefficient of the Rotor ................................ 132
5.5 Rotor-Design Features and Power Characteristics ......................................... 134
5.5.1 Number of Rotor Blades ..................................................................... 135
5.5.2 Optimum Shape of the Rotor Blades .................................................. 136
5.5.3 Rotor Blade Twist ............................................................................... 143
5.5.4 Blade Thickness .................................................................................. 145
5.5.5 Design Tip-Speed Ratio of the Rotor .................................................. 146
5.6 Existing Rotor Blade Designs ........................................................................ 148
5.7 Yaw Control of the Rotor .............................................................................. 152
5.8 Aerodynamics of Vertical-Axis Rotors .......................................................... 156
5.9 Experimental Rotor Aerodynamics ................................................................ 160
5.9.1 Measurements on Models in the Wind Tunnel ................................... 160
5.9.2 Measurements on Site ......................................................................... 163
References ............................................................................................................. 164

6 Loads and Structural Stresses ........................................................................... 167
6.1 Loads on the Wind Turbine ........................................................................... 168
6.2 Coordinate Systems and Terminology ........................................................... 170
6.3 Sources of Loading ........................................................................................ 171
6.3.1 Gravity and Inertial Loads .................................................................. 172
6.3.2 Uniform and Steady-State Air Flow ................................................... 173
6.3.3 Vertical Wind Shear and Cross Winds................................................ 177
6.3.4 Cross Wind on the Rotor ..................................................................... 178
6.3.5 Tower Interference .............................................................................. 179
6.3.6 Wind Turbulence and Gusts ................................................................ 183
6.4 Design Load Assumptions ............................................................................. 187
6.4.1 International and National Design Standards ...................................... 188
6.4.2 IEC Classes of Wind Turbines and German Wind Zones ................... 191
6.4.3 Normal Wind Conditions .................................................................... 193
CO NTENTS IX
6.4.4 Extreme Wind Conditions ................................................................... 194
6.4.5 Other Climatic and Environmental lnfluences .................................... 195
6.4.6 Other External Conditions .................................................................. 197
6.4.7 Safety Factors ..................................................................................... 198
6.5 Operational Status and Load Cases ................................................................ 199
6.5.1 Normal Operation ............................................................................... 199
6.5.2 Technical Faults .................................................................................. 201
6.6 Structural Stresses in the Wind Turbine ........................................................ 203
6.6.1 Kind of Stressing ................................................................................ 203
6.6.2 Load Spectra ....................................................................................... 205
6.7 Mathematical Models of Structural Dynamics .............................................. 208
6.7.1 Functional and Structural Modelling of the Wind Turbine ................. 208
6.7.2 Representation of the Wind Turbulence ............................................. 209
6.7.3 Analytical Approaches and Numerical Computer Codes.................... 213
6.8 Conceptual Design Features and Structural Stresses ..................................... 215
6.8.1 Number of Rotor Blades ..................................................................... 215
6.8.2 Rotor Hub Hinges in Two-Blade Rotors ............................................. 217
6.8.3 Stiffness of the Rotor Blades .............................................................. 220
6.8.4 Power Control System ........................................................................ 221
6.8.5 Rotor Speed Flexibility and Variable-Speed Operation ...................... 223
6.9 Measuring the Structural Stresses .................................................................. 226
6.9.1 Rotor Blade Testing ............................................................................ 227
6.9.2 Data Acquisition Systems and Field Measurements ........................... 228
References ............................................................................................................. 230

7 Vibration Characteristics ................................................................................... 233
7.1 Exciting Forces and Vibrational Degrees of Freedom ................................... 234
7.2 Aeroelastic Stability of Rotor Blades............................................................. 236
7.2.1 Static Divergence ................................................................................ 236
7.2.2 Natural Frequencies and Vibration Modes ......................................... 237
7.2.3 Typical Rotor Blade Vibrations .......................................................... 239
7.3 Torsional Vibration of the Drive Train .......................................................... 242
7.3.1 Mathematical Model ........................................................................... 242
7.3.2 Equivalent Mechanical Models for the Electrical Grid Coupling ....... 246
7.3.3 Natural Frequencies and Vibration Modes ......................................... 247
7.3.4 Excitations and Resonances ................................................................ 249
7.4 Dynamics of the Yaw System ........................................................................ 252
7.4.1 Modelling and Moments Around the Yaw Axis ................................. 252
7.4.2 Excitation and Resonances.................................................................. 254
7.5 Vibration of the Whole Wind Turbine ........................................................... 256
7.5.1 Tower Stiffness ................................................................................... 256
7.5.2 Vibrational Characteristics of Existing Wind Turbines ...................... 258
7.6 Mathematical Simulation ............................................................................... 264
References ............................................................................................................. 267
X CONTENTS
8 Rotor Blades........................................................................................................ 269
8.1 Materials ........................................................................................................ 270
8.2 Aircraft Wings as Model ................................................................................ 272
8.3 Experimental Designs of Rotor Blades in the Past ........................................ 274
8.3.1 Revited Aluminium Construction ....................................................... 275
8.3.2 Steel Designs ....................................................................................... 277
8.3.3 Traditional Wood Construction .......................................................... 280
8.3.4 Previous Designs with Fibre-Reinforced Composites ........................ 281
8.3.5 Wood/Epoxy Composites ................................................................... 286
8.4 Modern Fibre-Reinforced Composite Blades ................................................ 287
8.4.1 Fibre Composites Technology ............................................................ 287
8.4.2 Structural Design of the Rotor Blades ................................................ 289
8.4.3 Manufacturing Methods ...................................................................... 290
8.5 Blade Connection to the Rotor Hub ............................................................... 293
8.6 Comparison of Rotor Blade Designs ............................................................. 296
8.7 Aerodynamic Brakes on Stall-Controlled Rotors .......................................... 300
8.8 Lightning Protection ...................................................................................... 302
8.9 Ice Warning and De-icing .............................................................................. 303
References ............................................................................................................. 304

9 Mechanical Drive Train and Nacelle ............................................................... 305
9.1 Fundamental Considerations of Power Transmission .................................... 306
9.2 Previous Experimental Designs ..................................................................... 309
9.2.1 Generator in the Tower Base .............................................................. 310
9.2.2 Vertically Positioned Generator in the Tower Head ........................... 310
9.3 Current Standard Designs .............................................................................. 312
9.3.1 Gearbox between Rotor and Generator ............................................... 312
9.3.2 Direct Rotor-Driven Generator ........................................................... 314
9.4 Rotor Hub ...................................................................................................... 318
9.4.1 Cast Steel Rotor Hubs for Three-Blade Rotors ................................... 320
9.4.2 Rotor Hub Concepts for Two-Bladed Rotors ...................................... 321
9.5 Blade Pitch Mechanism ................................................................................. 325
9.5.1 Rotor Blade Bearings .......................................................................... 327
9.5.2 Blade Pitch Systems with Hydraulic Drive ......................................... 329
9.5.3 Electrically Driven Blade Pitch Systems ............................................ 333
9.5.4 Passive Blade Pitching ........................................................................ 335
9.5.5 Redundancy and Safety Issues ............................................................ 336
9.6 Rotor Shaft and Bearing Assembly ................................................................ 338
9.6.1 Bearing Technology ............................................................................ 338
9.6.2 Rotor Shaft with Separate Bearings .................................................... 341
9.6.3 Three-Point Suspension of Rotor Shaft and Gearbox ......................... 343
9.6·4 Rotor Shaft Integrated into the Gearbox ............................................. 344
9.6.5 The “Single-Bearing” Concept ........................................................... 345
9.6.6 Rotor Support on a Fixed Axle ........................................................... 346
CO NTENTS XI
9.7 Rotor Brake .................................................................................................... 347
9.8 Gearbox.......................................................................................................... 350
9.8.1 Gearbox Configurations ...................................................................... 350
9.8.2 External Load Specifications for the Gearbox .................................... 354
9.8.3 Internal Gearbox Dimensioning and Design ....................................... 357
9.8.4 Efficiency and Noise Emission ........................................................... 358
9.9 Variable-Speed Power-Splitting Gearboxes .................................................. 361
9.10 Torsional Flexibility in the Mechanical Drive Train ................................... 363
9.11 Installation of the Electric Generator ........................................................... 366
9.12 Nacelle ......................................................................................................... 368
9.12.1 Auxiliary Systems ........................................................................... 368
9.12.2 Load Carrying Concept ................................................................... 371
9.12.3 External Shape - Aesthetic Aspects ................................................ 374
9.13 Yaw System ................................................................................................. 377
9.14 Assembly and Functional Testing ................................................................ 380
References ............................................................................................................. 382

10 Electrical System .............................................................................................. 385
10.1 Synchronous and Asynchronous Generator ............................................... 386
10.1.1 Synchronous Generator ................................................................. 386
10.1.2 Induction Generator ...................................................................... 390
10.1.3 Generator with Permanent Excitation ........................................... 394
10.2 Assessment Criteria for Using Generators in Wind Turbines .................... 398
10.3 Fixed-Speed Generator Systems ................................................................ 401
10.3.1 Synchronous Generator Directly Coupled to the Grid .................. 401
10.3.2 Induction Generator Directly Coupled to the Grid ........................ 403
10.3.3 Variable-Slip Induction Generator ................................................ 405
10.3.4 Multi-Speed Generator Systems ................................................... 406
10.4 Variable Speed Generator Systems with Inverter ...................................... 408
10.4.1 Frequency Inverters ...................................................................... 409
10.4.2 Synchronous Generator with Inverter ........................................... 411
10.4.3 Induction Generator with Oversynchronous Cascade ................... 413
10.4.4 Double-Fed Induction Generator .................................................. 414
10.5 Directly Rotor-Driven Variable-Speed Generators .................................... 417
10.5.1 Synchronous Generator with Electric Excitation .......................... 417
10.5.2 Direct-Drive Generators with Permanent-Magnet Excitation ....... 419
10.6 Complete Electrical System of a Wind Turbine ........................................ 421
10.6.1 Large Turbines .............................................................................. 421
10.6.2 Small Wind Turbines .................................................................... 424
10.7 Comparison of Electrical Concepts ............................................................ 426
References ........................................................................................................... 428

11 Control Systems and Operational Sequence ................................................. 429
11.1 Input Data Measurement ............................................................................ 430
11.1.1 Wind Measuring System ............................................................... 431
11.1.2 Measuring Electric Power ............................................................. 434
11.1.3 Other Input Data ........................................................................... 435