Onshore and Offshore Wind Energy E-Book

Vasilis M. Fthenakis

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 634

Veröffentlichungsjahr: 2024

Beschreibung

Highly accessible and authoritative account of how wind energy is safely harnessed to address the ever-pressing climate and energy challenges

Onshore and Offshore Wind Energy provides an in-depth treatment of wind energy’s scientific background, current technology, and international status, with an emphasis on large turbines and wind farms, both onshore and offshore. In the newly revised second edition, highly qualified authors include technological advances in the field including offshore wind turbine structures, foundation design, installation, grid integration, and reliability, offering guidance on operation and maintenance.

The text is supported by copious illustrations and around 50 inspiring full-color photographs from around the world. To further aid in reader comprehension and information retention, questions with answers and problems are included in each chapter. An accompanying website includes figures, tables, and solutions of the problems.

The book is an essential primer for new entrants to the wind industry and to students on undergraduate and graduate courses on renewable energy. It also offers a unique treatise of the sustainability of emerging transformative technologies, which makes it useful to both system analysts and energy policy strategists.

In Onshore and Offshore Wind Energy, readers will find information on:

• Basics on wind energy capture and conversion by wind turbines
• Technology evolution and deployment experiences in the EU, China, Taiwan, and US wind farms, plus common access issues
• Production and installation techniques
• Operation, maintenance and risk mitigation
• Grid integration, synergies with other renewable energies, and green hydrogen production
• Life cycle sustainability, recycling, and the role of wind energy in addressing climate and energy challenges

Onshore and Offshore Wind Energy is aimed at a wide readership including professionals, policy makers, and employees in the energy sector in need of a basic appreciation of the underlying principles of wind energy, along with second and third year undergraduate and postgraduate students.

Leseprobe

Table of Contents

Cover

Table of Contents

Title Page

Copyright

About the Authors

Foreword

Preface

Acknowledgment to the Second Edition

Acknowledgment to the First Edition

About the Companion Website

1 Introduction

1.1 Wind energy and Planet Earth

1.2 Winds of the world

1.3 From windmills to wind turbines

1.4 Power, energy, and performance

1.5 Coming up-to-date

Appendix 1.A: Energy units and conversions

CO

2

Emissions per fuel type

CO

2

emissions in transportation

Self-assessment questions

Problems

Answers to questions

References

2 Capturing the wind

2.1 Wind speed and power

2.2 Wild wind

2.3 Predicting turbine output

Self-assessment questions

Problems

Answers to questions

References

3 Wind turbines

3.1 Turbine types and sizes

3.2 Aerodynamics

3.3 Mechanics

3.4 Electrics

3.5 Turbine control

3.6 Onshore wind farms

Self-assessment questions

Problems

Answers to questions

References

4 Fundamentals of offshore system

4.1 Introduction

4.2 Offshore wind power fundamentals

4.3 Tackling intermittency of wind power

4.4 Offshore systems and choice of foundations

4.5 Offshore wind farm case studies

4.6 Bringing the power ashore

Appendix 4.A: Specifications of turbines

Appendix 4.B: Details of offshore wind farms on different types of foundations

Self-assessment questions

Problems

Answers to questions

References

5 Offshore wind farm engineering

5.1 Conceptual design of wind arms

5.2 Cases studies

5.3 Detailed site investigation

5.4 Offshore construction

5.5 Engineering models of wind turbine systems

5.6 Loads on a wind turbine structure

5.7 Frequencies of loads acting on wind turbine structure

5.8 Design requirements

5.9 Summary of offshore wind turbine engineering

Appendix 5.A: Modes of vibrations

Appendix 5.B: Foundation stiffness

Appendix 5.C: Construction of wind and wave spectrum

5.10 Kaimal spectrum

5.11 Construction of wave spectrum

Self-assessment questions

Problems

Answers to questions

References

6 Operations and maintenance (O&M)

6.1 Introduction

6.2 Maintenance practices and tools

6.3 Wind turbine O&M data

6.4 Statistical tools of reliability analysis

6.5 Workforce education and training

6.6 Summary

Self-assessment questions

Problems

Answers to questions

Appendix 6.A: Hurricanes, typhoons, and cyclones

Appendix 6.B: Price of OWF: Normalized cost: LCOE breakdown for European wind farms:

References

7 Grid integration

7.1 Setting the scene—national grids

7.2 Electricity markets and types of power generators

7.3 Connecting to the grid

7.4 Electrical quality

7.5 Large-scale wind power

7.6 Intermittency and variability

7.7 Grid-friendly wind power plants

7.8 Capacity credit and backup generation

7.9 The variability challenge and solutions

7.10 Are 100 renewable energy grids feasible?

7.11 Grid reliability

7.12 The grid of the future

Self-assessment questions

Problems

Answers to questions

References

8 Wind energy growth and sustainability (cost, resources, environment)

8.1 Affordability

8.2 Resource availability

8.3 Environmental and social aspects

8.4 Land use

8.5 Water use

8.6 Life-cycle analysis

8.7 Landscape and amenity

8.8 Birds and bats

8.9 Farming

8.10 Seabirds, fish, and marine conservation

Self-assessment questions

Problems

Answers to questions

References

Notes

Index

End User License Agreement

List of Tables

Chapter 1

Table 1.1 Performance data for six wind farms.

Table 1.2 The “top ten” countries for cumulative installed capacity at the s...

Chapter 4

Table 4.A.1 Characteristics of wind turbines.

Table 4.B.1 UK wind farms.

Table 4.B.2 The diameter of monopiles for various UK projects.

Table 4.B.3 Details of offshore wind farm supported on GBS either operating ...

Table 4.B.4 Table of some key floating wind turbine projects (operating on p...

Chapter 5

Table 5.1 Wind farm specifications (tentative) in the East Coast of USA.

Table 5.2 Examples of wind farm systems (operating or planned).

Table 5.3 Typical turbine specifications.

Table 5.4 Load combination.

Table 5.5 ULS and SLS criteria.

Table 5.B.1 Formulae for stiffness for slender piles applicable to Jacket on...

Table 5.B.2 Stiffness of rigid piles based on Shadlou and Bhattacharya

22

.

Table 5.B.3 Vertical stiffness for shallow skirted foundations exhibiting ri...

Table 5.B.4 Stiffness for shallow skirted foundations exhibiting rigid behav...

Table 5.6 Details of the jacket structure.

Table 5.7 Foundation details for the example problem.

Chapter 6

Table 6.1 Components of a wind turbine.

Table 6.2 Locations, causes, and effects of the failure which are included i...

Table 6.3 Definition of Hurricane.

Table A2 Taxonomy of systems.

Chapter 8

Table 8.1 Rare earth elements (symbols, atomic numbers, names).

Table 8.2 External and internal costs.

List of Illustrations

Chapter 1

Figure 1.1 Bring in the new: a scene in Portugal

Figure 1.2 New horizons: a Danish offshore wind farm

Figure 1.3 The renaissance of wind energy

Figure 1.4 Atmospheric cells and latitudinal wind belts.

Figure 1.5 Wind patterns of the world.

Figure 1.6 A wind pattern over the UK and Ireland.

Figure 1.7 Making the most of prevailing winds: a wind farm off the shores o...

Figure 1.8 The Spanish windmills made famous in Cervantes’

Don Quixote

Figure 1.9 A Dutch windmill dating from 1757 and an English tower windmill o...

Figure 1.10 An eight-bladed English windmill and a multibladed American wind...

Figure 1.11 The Brush Wind Turbine of 1888, with associated equipment

Figure 1.12 The 200-kW Gedser wind turbine in an old photograph of 1957a...

Figure 1.13 Main components of a typical modern high-power wind turbine.

Figure 1.14 Sky-high: working on the nacelle of a large wind turbine

Figure 1.15 Delivering power, energy, and performance: a 5-MW offshore wind ...

Figure 1.16 Electricity consumption and generation.

Figure 1.17 Global cumulative installed capacity, 1995–2010.

Figure 1.18 Wind energy moves offshore

Figure 1.19 Offshore total installation as of 2023

Figure 1.20 A bright future for wind power

Chapter 2

Figure 2.1 Power in the wind.

Figure 2.2 Typical power curve for a 2-MW wind turbine.

Figure 2.3 (a) A distribution of hourly average wind speeds over a complete ...

Figure 2.4 Different surfaces, different wind shear: scenes from England and...

Figure 2.5 Variation of wind power with height, depending on surface roughne...

Figure 2.6 Wind speed variations throughout the year.

Figure 2.7 High uplands, wild winds

Figure 2.8 Turbulence and gusts.

Figure 2.9 Lord Rayleigh and Waloddi Weibull

Figure 2.10 Rayleigh distributions for four average wind speeds.

Figure 2.11 Weibull distributions for an average wind speed of 6 m/s.

Figure 2.12 (a) Rayleigh pdf for an average wind speed of 8 m/s and (b) the ...

Figure 2.13 Gaussian distributions for short-term variability of wind speed....

Figure 2.14 Carl Friedrich Gauss and Emil Gumbel

Figure 2.15 The probability of peak wind gusts, based on a Gumbel distributi...

Figure 2.16 Large-scale wind resources of Western Europe. The wind speeds in...

Figure 2.17 Wind resources in northern California.

Figure 2.18 Wind speeds and power densities at heights of 50 and 125 m.

Figure 2.19 Estimated average wind speeds at 125 m above mean sea level in t...

Figure 2.20 Predicting a site’s wind regime by the wind atlas method.

Figure 2.21 Predicting turbine output over the course of a year.

Figure 2.22 Checking the anemometer on a high nacelle

Chapter 3

Figure 3.1 Established favorite: the three-bladed HAWT rotor

Figure 3.2 Typical dimensions of large HAWTs.

Figure 3.3 Four types of vertical-axis wind turbines.

Figure 3.4 A rotor and its stream tube.

Figure 3.5 Vortex formation in a turbine wake.

Figure 3.6 Forces on a thin plate in a steady wind.

Figure 3.7 Maximizing lift, minimizing drag:

Greenbird

on its record run

Figure 3.8 Cambered wings with plenty of lift: the

Wright Flier 2

takes to t...

Figure 3.9 Forces acting on an aircraft wing in an horizontal airstream.

Figure 3.10 Wind speeds and forces on a HAWT airfoil.

Figure 3.11 Wind speeds and directions at three distances from the hub.

Figure 3.12 Progressively twisted from hub to tip: the blades of a high-perf...

Figure 3.13 A large modern HAWT: high torque at low speed

Figure 3.14 Power coefficient and tip speed ratio.

Figure 3.15 Tracking wind speed to obtain maximum turbine output.

Figure 3.16 Preparing for duty off the coast of Scotland: three blades for a...

Figure 3.17 HAWT blade tips

Figure 3.18 As the angle of attack increases, stall develops.

Figure 3.19 Lift and drag coefficients.

Figure 3.20 Effects of wind speed on angle of attack.

Figure 3.21 Up it goes: the nacelle of a 5-MW HAWT

Figure 3.22 Gear arrangements for (a) a simple spur gearbox and (b) a planet...

Figure 3.23 Towers old and new in Portugal and Canada

Figure 3.24 About to go aloft a French wind turbine

Figure 3.25 Mechanical strength and resilience: a large modern HAWT

Figure 3.26 Simple vibrating systems.

Figure 3.27 Vibrations in systems with mass, stiffness, and damping.

Figure 3.28 Vibration amplitude versus frequency in a second-order system.

Figure 3.29 Designed to withstand forces and vibrations: the tower and blade...

Figure 3.30 Pioneers of electrical power: Tesla and Edison

Figure 3.31 AC voltage, current, and power.

Figure 3.32 The j-notation for AC circuits.

Figure 3.33 Three-phase voltages.

Figure 3.34 Three-phase AC power.

Figure 3.35 A simple AC generator.

Figure 3.36 A three-phase AC generator.

Figure 3.37 Cyclists on a “very long bike.”

Figure 3.38 Interaction between stator and rotor fields (a) when generating ...

Figure 3.39 Main elements of a variable-speed wind turbine (W): GB, gearbox;...

Figure 3.40 The rotor of this gearless wind turbine is directly coupled to a...

Figure 3.41 Assembling stators for high-power gearless wind turbines

Figure 3.42 A squirrel-cage rotor.

Figure 3.43 Typical torque-speed characteristic of an induction machine.

Figure 3.44 Main elements of a “fixed speed” wind turbine: W, turbine rotor;...

Figure 3.45 Typical effects of increasing the rotor resistance of a wound-ro...

Figure 3.46 Main elements of a variable-speed turbine using a doubly-fed ind...

Figure 3.47 The flow of real power in a doubly-fed induction generator.

Figure 3.48 Sub- and super-synchronous operation of a DFIG.

Figure 3.49 These HAWTs are in the 2-MW class but have different generator c...

Figure 3.50 Typical power curve for a 2-MW turbine.

Figure 3.51 A simple feedback control loop.

Figure 3.52 Hierarchical control of a large HAWT.

Figure 3.53 Wind farm locations in north-western Germany

Figure 3.54 Plenty of scope for onshore wind: scenes from Canada

Figure 3.55 Heavy work in high places

Figure 3.56 An array of wind turbines on an extensive flat site.

Figure 3.57 A wind rose for Plymouth, England

Figure 3.58 Presenting a united front to the prevailing wind

Figure 3.59 A typical screen showing wind farm layout and operational status...

Chapter 4

Figure 4.1 Moving offshore (Figure 4.3 1st edition)

Figure 4.2 Moving offshore: new horizons for wind energy (Figure 4.1 1st edi...

Figure 4.3 Offshore wind farm locations in northwest Europe (Figure 4.2 1st ...

Figure 4.4 Cumulative offshore wind turbine installations per country.

Figure 4.5 Layout of an offshore wind farm.

Figure 4.6 Schematic of wind farm grid integration.

Figure 4.7 Aerial view of Dudgeon Wind Farm showing turbines and substation...

Figure 4.8 Offshore wind farm locations in the USA.

Figure 4.9 Offshore wind farm locations in China.

Figure 4.10 Global circulation of wind.

Figure 4.11 Variations of wind speed and power due to wind shear.

Figure 4.12 Catching the wind, resisting the waves

Figure 4.13 Wave patterns: (a) regular, (b) irregular, and (c) random.

Figure 4.14 Global mean wind speeds for 1979–2012.

Figure 4.15 Global offshore wind speeds. Open access: https://globalwindatla...

Figure 4.16 Power curves for turbines

Figure 4.17 Combining offshore wind with battery storage.

Figure 4.18 Use of offshore wind to produce hydrogen.

Figure 4.19 Turbine foundations: (a) monopile; (b) gravity; (c) tripod.

Figure 4.20 On the move: transition pieces with platforms

Figure 4.21 A jackup barge, its long legs extended down onto the sea floor, ...

Figure 4.22 A 6 MW turbine supported by a “jacket” off the coast of Scotland...

Figure 4.23 Floating turbines: (a) tension-legged platform and (b) spar buoy...

Figure 4.24 Types of systems (grounded and floating).

Figure 4.25 Typical bottom fixed and floating foundations. (1) Suction bucke...

Figure 4.26 Global trends in offshore wind farm systems.

Figure 4.27 Examples of bottom-fixed jacket on suction caisson foundation, A...

Figure 4.28 One of the 25 support vessels used during installation of

Horns

...

Figure 4.29 The

Poseidon

accommodation platform being installed next to the ...

Figure 4.30 Installation complete at

Horns Rev 2

Figure 4.31

London Array

: (a) location in the Thames Estuary and (b) turbine...

Figure 4.32 Onshore work in progress: groundworks for the Cleve Hill substat...

Figure 4.33 Offshore installation begins

Figure 4.34 Case study Hywind Wind Park (location & schematic).

Figure 4.35 Installation of the turbine: (a) The substructure being pulled t...

Figure 4.36 An offshore substation for a large wind farm

Chapter 5

Figure 5.1 Typical spacing of turbines

Figure 5.2 Water depth variation with distance from the shore

Figure 5.3 Dogger Bank Wind Farm.

Figure 5.4 Dogger Bank Wind Farm.

Figure 5.5 Location of four offshore wind farms including Westermost Rough....

Figure 5.6 Westermost Rough Wind Farm (Wiley)

Figure 5.7 Navigation channels around the Westermost Rough Offshore Wind Far...

Figure 5.8 Karehamn Wind Farm Array. It may be noted that this is a single c...

Figure 5.9 Layout of the West of Duddon Sands wind farm (Author’s figure).

Figure 5.10 US Atlantic OCS planning areas

Figure 5.11 Offshore Wind Farm development in East Coast of US

Figure 5.12 Wind speeds

Figure 5.13 Wind farm development in China (author figure). From Bhattachary...

Figure 5.14 Bathymetry (m) of the waters adjacent to China following Lian

et

...

Figure 5.15 Geology profile for 12 wind farms in Europe (Author’s own figure...

Figure 5.16 Development of the scour in the Robin Rigg Wind Farm between 200...

Figure 5.17 Scour issues in Robin Rigg Wind Farm (own figure).

Figure 5.18 Ground profile in Bohai Sea (Northen East China, see Figure 5.12...

Figure 5.19 Ground profile in offshore Fujian Sea or Taiwan Strait

Figure 5.20 Identified location on “green sand” or Glauconite in offshore lo...

Figure 5.21 Installation of GBS system

Figure 5.22 Installation of Seagreen Offshore Wind Farm (Scotland) Jacket sy...

Figure 5.23 Choosing foundation based on geology and water depth (Author’s o...

Figure 5.24 Different types of FOWTs and anchoring systems

Figure 5.25 Example of suction bucket jacket

Figure 5.26 Load transfer for (a) a monopile supported and (b) multiple supp...

Figure 5.27 Load transfer for floating offshore wind turbine system

Figure 5.28 Two types of floating systems (Catenary & Tension Leg Platform)....

Figure 5.29 TLP system

Figure 5.30 Sway-bending modes of vibration for offshore wind turbines suppo...

Figure 5.31 Rocking modes of vibration for offshore wind turbines supported ...

Figure 5.32 Mechanical model of a monopile system.

Figure 5.33 Mechanical model of a wind turbine structure

Figure 5.34 (a) Loads on offshore wind turbine foundations (b) Explanati...

Figure 5.35 Schematic diagram showing the loads on a wind turbine structure....

Figure 5.36 (a) Combination of wind and wave load. (b) Loading scenario for ...

Figure 5.37 Frequency range of the four main loads acting on the wind turbin...

Figure 5.38 Schematic diagram of the design choices.

Figure 5.39 Frequency range of the loads along with natural frequency of the...

Figure 5.40 Aspects governing the SLS requirements for monopile foundation...

Figure 5.A.1 Modes of vibration and the formulation can be obtained.

Figure 5.A.2 Modes of vibration for monopiles and jacket on piles.

Figure 5.A.3 Modes of vibration for a GBS structure.

Figure 5.B.1 Concentrated load at the free end.

Figure 5.B.2 Moment at free end.

Figure 5.C.1 Taylor’s frozen turbulence hypothesis: an eddy travels with the...

Figure 5.41 1P and 3P range of turbines.

Figure 5.42 Bathymetry depth for US waters.

Figure 5.43 Major steps in the installation of wind turbines.

Chapter 6

Figure 6.1 Average annual failure rate and downtime values of WT subsystems ...

Figure 6.2 Wind turbine failure rate and downtime results from various datab...

Figure 6.3 Availability of onshore wind turbines based on various databases...

Figure 6.4 Availability of offshore wind turbines based on various databases...

Figure 6.5 Critical subassemblies in terms of failure rate

Figure 6.6 Critical components in terms of downtime

Figure 6.7 Comparison of weighted failure rates between onshore and offshore...

Figure 6.8 Comparison of weighted stop rates between onshore and offshore da...

Figure 6.9 Stop time per event for onshore and offshore wind turbines

Figure 6.10 Onshore wind turbines average O&M costs, by commercial operation...

Figure 6.11 Onshore wind turbines. Median annual O&M costs by project age an...

Figure 6.12 Offshore helicopter service for accelerated crew transfer

Figure 6.13 Averaged failure rate and downtime per failure values for subsys...

Figure A1 North Atlantic hurricane track.

Figure A2 Locations of wind farms on the northeast of the USA.

Figure A3 (a) General understanding of hurricane tracks; (b) hurricane track...

Figure A4 Wind farm in Taiwan

Figure A5 Typhoon tracks

Figure A6 Seismicity in Taiwan

Figure A7 Schematic of a generic scour protection design.

Figure A8 Schematic of sample scour protection design for monopile foundatio...

Figure A9 No scour protection and scour development.

Figure A10 Sketch of major parameters influencing anchoring cable dimension....

Figure A11 Schematic of cable structure.

Figure A12 Sketch of cable connection.

Chapter 7

Figure 7.1 (a) The US electric grid: 3 major interconnects, 8 NERC subregion...

Figure 7.2 The large power plants that feed the UK’s national grid.

Figure 7.3 Example of a load duration curve.

Figure 7.4 Power generators for load balancing and regulation.

Figure 7.5 (a) A typical grid network and (b) a wind turbine connection.

Figure 7.6 (a) A sinusoidal voltage waveform and its approximation by (b) a ...

Figure 7.7 A typical weekly record of electricity demand and wind generation...

Figure 7.8 Power profile of wind turbine providing ancillary services

Figure 7.9 Synergy of PV and wind in New York State.

Figure 7.10 Effect of grid flexibility on PV energy delivery.

Figure 7.11 100% solar in Australia.

Chapter 8

Figure 8.1 The three major pillars of wind large-growth sustainability

Figure 8.2 Typical cost breakdowns for onshore and offshore wind (assuming t...

Figure 8.3 Life cycle of an offshore wind farm in the early years of develop...

Figure 8.4 Turbine development during 2004–2022 and future projections.

Figure 8.5 Positive and negative cash flows.

Figure 8.6 Initial investment, annual costs, and levelized cost of electrici...

Figure 8.7 Material intensities of current and potential future wind energy ...

Figure 8.8 Wind turbine decommissioning: (a) offshore vessel and crane posit...

Figure 8.9 Recycling strengthens the three pillars of WT sustainability.

Figure 8.10 Environmentally friendly use of land by wind turbines

Figure 8.11 The life-cycle stages of wind turbines.

Figure 8.12 Beauty in the eye of the beholder

Figure 8.13 Figures in a windswept landscape

Figure 8.14 Distant horizons

Figure 8.15 Areas of high sensitivity for rare and endangered wild birds in ...

Figure 8.16 Undisturbed pastures

Figure 8.17 Offshore wind: high hopes for environmental benefits

Guide

Cover

Table of Contents

Title Page

Copyright

About the Authors

Foreword

Preface

Acknowledgment to the Second Edition

Acknowledgment to the First Edition

About the Companion Website

Begin Reading

Index

End User License Agreement

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Onshore and Offshore Wind Energy

Evolution, Grid Integration, and Impact

Second Edition

Vasilis Fthenakis

Columbia University and Brookhaven National Laboratory

NY, USA

Subhamoy Bhattacharya

University of Surrey

England, UK

Paul A. Lynn

Formerly Imperial College

London, UK

This second edition first published 2025© 2025 John Wiley & Sons Ltd.

Edition History: John Wiley & Sons Ltd. (1e, 2011)

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Library of Congress Cataloging-in-Publication Data:

Names: Fthenakis, Vasilis M., 1951- author. | Bhattacharya, Subhamoy,  author. | Lynn, Paul A., author.

Title: Onshore and offshore wind energy : evolution, grid integration, and  impact / Vasilis Fthenakis Columbia University NY, USA, Subhamoy  Bhattacharya, University of Surrey, England, UK, Paul A. Lynn,  Butcombe, North Somerset, UK.

Description: Second edition. | Hoboken, NJ, USA : Wiley, 2025. | Includes  index.

Identifiers: LCCN 2024025371 (print) | LCCN 2024025372 (ebook) | ISBN  9781119854470 (hardback) | ISBN 9781119854487 (adobe pdf) | ISBN  9781119854494 (epub)

Subjects: LCSH: Wind power. | Offshore electric power plants. | Wind energy  conversion systems.

Classification: LCC TJ820 .L95 2025 (print) | LCC TJ820 (ebook) | DDC  621.31/2136–dc23/eng/20240802

LC record available at https://lccn.loc.gov/2024025371

LC ebook record available at https://lccn.loc.gov/2024025372

Cover Design: WileyCover Images: © Sharomka/Shutterstock, © zhengzaishuru/Shutterstock

About the Authors

Vasilis Fthenakis

Vasilis Fthenakis obtained his Diploma in Chemistry from the University of Athens, Greece; his MS in Chemical Engineering from Columbia University; and his PhD in Fluid Dynamics and Atmospheric Science from New York University. He is the founder and director of the Center for Life Cycle Analysis (CLCA) and an adjunct Professor at Columbia University, New York, USA. He is also a distinguished scientist emeritus at Brookhaven National Laboratory (BNL), where he conducted energy-environmental research for 36 years. Dr. Fthenakis is the coauthor and editor of six books and about 400 scientific publications on topics at the interface of energy and the environment.

In 2008, he co-authored “Grand Plan for Solar Energy,” a leading study published in Scientific American in 11 languages, showing the technical, economic, and geographical feasibility of solar with other renewables to satisfy 69% of the electricity needs of the USA by 2050.

Since 2002, to safeguard the environmental friendliness of photovoltaics, he has been defining and promoting a proactive, long-term environmental strategy, including recycling of photovoltaics at the end of their useful lives. His work produced a patented technology for CdTe recycling and models for optimizing the collection of end-of-life photovoltaics. He started the International Energy Agency (IEA) PV-EH&S Task (Task 12) on LCA and Recycling and led it as the U.S. Operating Agent from 2007 to 2012.

He is a Fellow of the American Institute of Chemical Engineers (AIChE) “in recognition and appreciation of superior attainments, valuable contributions, and service to Chemical Engineering,” a Fellow of the Institute of Electrical and Electronic Engineers (IEEE) for “outstanding contributions to photovoltaics technology,” and recipient of a number of awards including the 2006 US DOE Certificate of Appreciation “for superior technical, management and communications skills exhibited in photovoltaic environmental research,” the 2018 IEEE William Cherry Award for “pioneering research at the interface of energy and the environment that catalyzed photovoltaic technology advancement and deployment world-wide,” and the 2022 Karl Boer Medal of Merit for “distinguished contributions to quest for sustainable energy.” Currently, he is leading nationally and internationally research on solar desalination, solar hydrogen, energy systems modeling, life-cycle analysis, and photovoltaic recycling.

Subhamoy Bhattacharya

Subhamoy Bhattacharya, a Chartered Engineer (CEng) and Fellow of ICE (Institution of Civil Engineers, London), holds the Chaired Professor in Geomechanics at the University of Surrey since 2012, where he leads the Geotechnical Research group and directs the SAGE (Surrey Advanced Geotechnical Engineering) laboratory. He is also the Co-founder and Chief Scientific Officer of Renew Risk™ (based at Lloyd’s of London), a university spin-off company specializing in risk analytics for renewable energy/energy transition, which build the world’s first commercially available catastrophe models for offshore wind farms.

He is an expert in geotechnical and earthquake engineering, where his main interests are offshore foundations and earthquake engineering. He commenced his undergraduate education at the Indian Institute of Technology (IIT-Kharagpur) in geology and geophysics, and after one year of completion switched to civil engineering degree program at Indian Institute of Engineering Science and Technology (IIEST, Shibpur) and graduated with a first-class degree in civil engineering. Following his undergraduate education, he worked at Consulting Engineering Services (Jacobs) designing buildings and bridges before moving to Cambridge for doctoral studies.

He earned his PhD from the University of Cambridge (as a Cambridge Nehru Commonwealth Scholar) in 2003, investigating the failure of pile-supported structures (bridges and buildings) during seismic liquefaction and proposing a new theory of pile failure during earthquakes. Following his PhD, he worked at Offshore Geotechnical Consultancy Fugro designing foundations for offshore oil and gas structures, including the anchors for FPSO’s (Floating Production Storage and Offloading). In 2005, he transited to academia, first at the University of Oxford, where he was a Departmental Lecturer in Engineering Science and JRF (Junior Research Fellow) at Somerville College, and then at the University of Bristol, where he was a Senior Lecturer in Soil Dynamics before taking up the Chair at Surrey in 2012. He regularly lectures in the UK and abroad and acts as a consultant in civil, earthquake, geotechnical, and offshore engineering, including legal and arbitration cases.

Paul A. Lynn

Paul A. Lynn obtained his BSc (Eng) and PhD degrees from Imperial College London, UK. After five years in the electrical/electronics industry, mainly as a radar engineer with the Marconi Company, he lectured at Imperial College and the University of Bristol, latterly as a Reader in Electronic Engineering with a special interest in digital signal processing. A strong and growing interest in renewable energy then led him to accept an invitation to become the founding managing editor of Wiley’s journal “Progress in Photovoltaics,” a role he held for 14 years. He also wrote a trilogy of Wiley books on solar, wind, and wave/tidal energy. In retirement, he designed and built a 22-ft solar-powered catamaran that carried him and his wife on the first-ever solar voyage along the entire River Thames from Gloucestershire to London, and in recent years he has continued as an author, with numerous books, papers, and articles.

Foreword

Wind energy is one of only two renewable energy sources that can potentially power the world for all purposes multiple times over. The other is solar energy. The growth of wind has been remarkable, as has its decline in cost. Transforming the world’s energy infrastructure to address global warming, air pollution, and energy security problems requires the continued growth of this clean, renewable energy resource. To that end, this book is an essential and outstanding tool for professionals, technicians, policymakers, students, and interested lay readers who are in the wind energy industry, work on wind-related topics, need to make decisions regarding wind energy, or want to learn about it. This book contains not only case studies but also appendices, questions and answers, and problems. An instructor’s manual with solutions to the problems is also available.

The book methodically marches through the important aspects of wind energy. First up, in Chapter 1, is a discussion of the difference between windmills and wind turbines and of wind turbine development and installation to date. Chapter 2 details the power in the wind, wind speed probability distributions, and wind resources. Chapter 3 covers everything you want to know about wind turbine design and efficiency. Chapter 4 focuses on offshore wind systems, including their foundations and transmission systems. Chapter 5 examines the engineering of an offshore wind farm, including the spacing needed between wind turbines. Chapter 6 moves on to discuss wind turbine operation and maintenance. Integrating wind into the grid and dealing with wind’s variability are the subjects of Chapter 7. Lastly, Chapter 8 is on the cost, material needs, and impacts on the environment and wildlife of wind turbines.

Through this book, readers will gain the essential knowledge needed to help accelerate the growth of wind energy but also to bust myths about it often amplified by its detractors. One such myth is that wind turbines cause substantial bird deaths. Another is that large-scale wind development will cause blackouts due to wind’s variability. A third is that wind farms take up more land than coal generators and mines.

Readers will also have access to important data and equations necessary to analyze the efficiency of wind turbines, to integrate wind farms into the electrical grid, and to ensure that wind contributes to a reliable, clean, renewable energy future. Those who finish this book will have gained a wealth of knowledge, arming them with the information needed to speak accurately and authoritatively about all aspects of wind energy. It is time to start your learning experience with this fantastic book. There is no time to waste!

By Mark Z. JacobsonProf. of Civil and Environmental Engineering, Stanford University

Preface

The wind energy industry, offshore as well as onshore, is growing at a remarkable pace. To many of us, it symbolizes a desire to harness one of nature’s most widespread sources of renewable energy, exploiting a “fuel” that is eternal and carbon-free. This book presents a concise account of large turbines and utility-scale wind energy aimed at a wide readership including professionals, policymakers, and employees in the energy sector needing an appreciation of the basic principles underlying wind energy or a quick update. Its style and level will also appeal to undergraduate and postgraduate students, as well as the large and growing number of thoughtful people who are interested in onshore and offshore wind farms and the contribution they are making to electricity generation in the 21st century. The first edition of this book, which was published in 2012, was one of the first books to emphasize today’s exciting developments in offshore wind, and it was designed as an appetizer rather than a formal textbook, with copious color photographs to illustrate the industry’s progress as it moves, apparently inexorably, toward 1000 GW of global installed capacity.

The current second edition describes fundamental technical aspects of offshore wind with new chapters on “Fundamentals of Offshore Systems” and on “Offshore Wind Engineering”. In addition, a new chapter on wind turbine “Operation and Maintenance” and greatly expanded and updated chapters on “Grid Integration” and “Wind Energy Growth and Sustainability” holistically cover the techno-economic and environmental-social aspects of wind energy for the high growth envisioned in climate change mitigation scenarios.

The effective harnessing of wind power involves many aspects of engineering science, from rotor aerodynamics to electrical generators, control systems, foundation structures, resilience, reliability, and grid networks. We have tried to introduce this huge field in a way that explains the essential theoretical background and indicates the main engineering challenges. The overall tone is deliberately accessible rather than overly technical, but several key topics, especially those pertaining to the emerging offshore wind turbine technologies, are covered in sufficient technical detail for students of wind energy.

We hope the new edition will serve as an essential primary resource for entrants to the wind energy industry needing an up-to-date appreciation of the subject. It also offers a unique treatise on the sustainability of emerging transformative technologies, making it valuable to system analysts and energy policy strategists. Last but not least, we have included end-of-chapter questions and problems to support instructors and the ever-increasing number of college and university students taking courses in renewable energy technologies.

Vasilis FthenakisColumbia University, New York, NY, USASubhamoy BhattacharyaUniversity of Surrey, SurreyEngland, UKPaul A. LynnImperial College (Retired), London, UKJanuary 2024

Acknowledgment to the Second Edition

Following Paul Lynn’s lead in including color photographs in the first edition, illustrating wind turbine beauty and extraordinary promise, we have added plenty more in this new edition.

We also added a lot of material on off-shore wind and problems and solutions in each chapter of this new edition.

Many of the materials presented in this book are based on the research work of our past and current students, which includes undergraduate, master’s, and PhD students. A special mention goes to Dr. Samet Ozturk, Zuoran Zhang, Ulvi Rahmanli, Sayan Bhattacharyya, Dr. Aleem Mohammad, and Dr. Georgios Nikitas.

This second edition is dedicated to our wives, Christina and Paromita, for their love and encouragement. Onshore and Offshore Wind Energy defines a pragmatic renewable energy solution to the risks that climate change presents to our children and the generations that follow, and we thank our children, Antonia, Menelaos, and Ishan, for being our constant source of motivation for a book that is more about their generation than it is about ours.

This is a new area, and the technology development is very fast. Offshore turbines may not only be sited in deeper waters and further offshore but also in seismic and typhoon zones. Much of the information presented is expected to be outdated in the next few years, and the book could need a new edition. There can also be errors and omissions in the book, and we would like to know them. Please email us at [email protected] and [email protected]; your comments will be duly acknowledged in the next edition.

It has been a pleasure for the two of us and Paul Lynn to work together on this new edition, a transatlantic link appreciated by the three of us.

Vasilis FthenakisSubhamoy Bhattacharya

Acknowledgment to the First Edition

I am grateful to a number of wind energy companies and organizations for permission to use their excellent color photographs. In alphabetical order, including the names of individuals who have offered generous help and advice, they are:

Canadian Wind Energy Association, Ottawa, Canada (Lejla Latifovic)

DONG Energy A/S, 2820 Gentofte, Denmark (Kathrine Westermann)

Ecotricity Ltd., Stroud, England (Mike Cheshire)

ENERCON GmbH, 26581 Aurich, Germany (Anne-Kathrin Gilberg)

French Wind Energy Association, 75008 Paris, France (Benoit Seveno)

London Array Ltd., London SW1H 0RG, England (Joanne Haddon)

REpower Systems AG, 22297 Hamburg, Germany (Caroline Zimmermann)

Vestas Wind Systems A/S, 8940 Randers SV, Denmark (Michael Holm)

The publishers acknowledge the use of the above photographs, which are reproduced with the permission of the copyright holders.

The book also includes 80 color illustrations by David Thompson, who has interpreted my sometimes rough and ready sketches with great skill. Dave worked closely with me on my previous book, Electricity from Sunlight, and it has been a pleasure to repeat the collaboration.

The author of a short but wide-ranging book on wind energy inevitably draws on many sources for information and inspiration. In my case, various books, articles, and websites have helped clarify the subject’s scientific basis, technological development, and current worldwide status, and I have tried to cite them adequately in the chapter reference lists. A special mention should be made of two books that have proved invaluable for clear explanations of difficult concepts that I have attempted to summarize:

Wind Energy Explained: Theory, Design and Application by J.F. Manwell, J.G. McGowan and A.L. Rogers (Wiley 2009)

Wind Power in Power Systems, edited by T. Ackermann (Wiley 2005)

I freely acknowledge the debt I owe the authors and recommend the books to anyone wishing to take their understanding of wind energy to a higher level.

Paul A. Lynn

About the Companion Website

This book is accompanied by a companion website.

www.wiley.com/go/fthenakis/windenergy2e

This website includes solution manual.

1Introduction

1.1 Wind energy and Planet Earth

Half a century ago, it would have taken a brave person to predict today’s extraordinary renaissance of machines powered by the wind. Traditional windmills for milling grain and pumping water had been largely consigned to technological history, overtaken by electric motors fed from centralized power plants burning fossil fuels. But by a curious twist of history, large numbers of wind turbines, installed both onshore and offshore, are today injecting energy into electricity grids for the benefit of us all and helping usher in a new age of renewable energy.

The background to this development is, of course, the massive redirection of energy policy that most experts and politicians now agree is essential if Planet Earth is to survive the 21st century in reasonable shape. For the last few hundred years, humans have been using up fossil fuels that nature took around 400 million years to form and store underground. A huge effort is now underway to develop and install energy systems that make use of natural energy flows in the environment including wind and sunlight, with a major contribution from large wind turbines. This is not simply a matter of fuel reserves, for it is becoming clearer by the day that, even if those reserves were unlimited, we could not continue to burn them with impunity. Today’s scientific consensus assures us that the resulting carbon dioxide emissions would lead to a major environmental crisis. So the danger is now seen as a double-edged sword: on the one side, fossil fuel depletion; and on the other, the increasing inability of the natural world to absorb emissions caused by the burning of what fuel remains, leading to accelerated climate change.

Back in the 1970s, there was very little public discussion about energy sources, including electricity. In the industrialized world, we had become used to the idea that electricity is generated in large, centralized power plants, preferably out of sight as well as mind, and distributed to factories, offices, and homes by a grid network with far-reaching tentacles. Few people had any idea how the electricity they took for granted was produced, or that the burning of coal, oil, and gas was building up global environmental problems. Those who were aware tended to assume that the advent of nuclear power would prove a panacea; a few even claimed that nuclear electricity would be so cheap that it would not be worth metering! It was all very reassuring and convenient—but, as we now realize, dangerously complacent.

Yet, even in those years, a few brave voices suggested that all was not well. In his famous book Small is Beautiful,1 first published in 1973, E.F. Schumacher poured scorn on the idea that the problems of production in the industrialized world had been solved. Modern society, he claimed, does not experience itself as part of nature, but as an outside force seeking to dominate and conquer it. And it is the illusion of unlimited powers deriving from the undoubted successes of much of modern technology that is the root cause of our present difficulties. We are failing to distinguish between the capital and income components of the Earth’s resources. We use up capital, including oil and gas reserves, as if they were a steady and sustainable income. But they are once-and-only capital. It is like selling the family silver and going on a binge.

Schumacher’s message, once ignored or derided by the majority, is now seen as mainstream. For the good of Planet Earth and future generations, we have started to distinguish between capital and income and to invest heavily in renewable technologies—including wind energy—that produce electricity free of carbon emissions. The message was powerfully reinforced by former US Vice President Al Gore, whose inspirational video in 2006 An Inconvenient Truth2 has been watched by many millions of people around the world.

The fossil fuels laid down by solar energy over hundreds of millions of years must surely be regarded as capital, but the winds that blow over the world’s land surfaces and oceans day by day, year by year, and century by century are effectively free income to be used or ignored as we wish. Nothing is “wasted” or exhausted if we don’t use it because it is there anyway. The challenge for the future is to harness such renewable energy effectively, designing and creating efficient and hopefully inspiring machines to serve humankind without disabling the planet.

This is a good moment to consider the meaning of renewable energy a little more carefully. It implies energy that is sustainable in the sense of being available in the long term without significantly depleting the Earth’s capital resources or causing environmental damage that cannot readily be repaired by nature itself. In his excellent book A Solar Manifesto3, German politician Hermann Scheer considered Planet Earth in its totality as an energy conversion system. He noted how, in its early stages, human society was itself the most efficient energy converter, using food to produce muscle power and later enhancing this with simple mechanical tools. Subsequent stages—releasing relatively large amounts of energy by burning wood and focusing energy where it is needed by building sailing ships for transport and windmills to grind grain and pump water—were still essentially renewable activities in the above sense (Figure 1.1).

What really changed things was the 19th-century development of the steam engine for factory production and steam navigation. Here, almost at a stroke, the heat energy locked in coal was converted into powerful and highly concentrated motion. The industrial society was born. And ever since, we have continued burning coal, oil, and gas in ways which pay no attention to the natural rhythms of the Earth and its ability to absorb wastes and by-products, or to keep providing energy capital. Our approach has become the opposite of renewable, and it is high time to change priorities (Figure 1.2).

It would, however, be unfair to pretend that renewable energy is an easy answer. For a start, it is diffuse and intermittent. Often, it is unpredictable. And although the “fuel” is free and the waste products are minimal, up-front investment costs tend to be large. There are certainly major challenges to be faced and overcome as we move toward a new energy mix for the 21st century.

Figure 1.1 Bring in the new: a scene in Portugal

(Source: with permission of Repower).

Our story now moves on to modern wind energy, which is already one of the most mature of the renewable technologies, and still advancing rapidly. But before getting involved in the details, we should consider the gift of a global wind resource that is helping wean us away from our addiction to fossil fuels (Figure 1.3).

1.2 Winds of the world

The winds of the world are produced by the Sun’s uneven heating of the Earth’s atmosphere and may be thought of as a form of solar energy. Variations in atmospheric pressure caused by differential heating propel air from high-pressure to low-pressure regions, generating winds that are also greatly affected by the Earth’s rotation and surface geography.4 On a large scale, they may be broadly divided into latitudinal and longitudinal patterns.

The most consistent latitudinal wind patterns are found over the great oceans of the world, well away from large land masses and mountain ranges. For many centuries, the captains of sailing ships depended on reliable trade winds to speed them on their way, trying to avoid the horse latitudes at around 30° north and south and the equatorial doldrums that threatened to becalm them for days on end. It is hardly surprising that wind meteorology exercised some famous minds throughout the great age of sail. Edmond Halley (1656–1742), an English astronomer best known for computing the orbit of Halley’s comet, published his ideas on the formation of trade winds in 1686, following an astronomical expedition to the island of St. Helena in the South Atlantic. The atmospheric mechanism proposed by George Hadley (1685–1768), a lawyer who dabbled productively in meteorology, attempted to include the effects of the Earth’s rotation—a theory that was subsequently corrected and refined by American meteorologist William Ferrel (1817–1891).

Figure 1.2 New horizons: a Danish offshore wind farm

(Source: with permission of Orsted).

Figure 1.3 The renaissance of wind energy

(Source: with permission of VESTAS).

Figure 1.4 Atmospheric cells and latitudinal wind belts.

The contributions of Hadley and Ferrel to our understanding of latitudinal wind patterns are acknowledged in the names given to atmospheric “cells” shown in Figure 1.4, which illustrates major wind belts encircling the planet. Essentially, these are generated by the steady reduction in solar radiation from the equator to the poles. The associated winds, rather than flowing northward or southward as we might expect, deflect to the east or west in line with the Coriolis effect, named after French engineer Gaspard Coriolis (1792–1843), who showed that a mass (in this case, of air) moving in a rotating system (the Earth) experiences a force acting perpendicular to both the direction of motion and the axis of rotation.4

The Hadley cells—closed loops of air circulation—begin near the equator as warm air is lifted and carried toward the poles. At around 30° latitude, north and south, they descend as cool air and return to complete the loop, producing the northeast and southeast trade winds that have had such a major historical impact on ships sailing between Europe and the Americas. A similar mechanism produces polar cells in the Arctic and Antarctic regions, giving rise to polar easterlies. If you live in northwest Europe, you will know all about freezing winter winds from Siberia!

The Ferrel cells of the mid-latitudes, sandwiched between the Hadley and polar cells, are less well-defined and far less stable. Meandering high-level jet streams tend to form at their boundaries with the Hadley cells, generating localized passing weather systems. This makes the coastal wind patterns of countries such as Denmark, Germany, and Britain famously variable. So although the prevailing winds are westerlies, they are often displaced by flows from other points of the compass, especially during the winter months.

Figure 1.5 Wind patterns of the world.

Temperature and pressure gradients caused by the Sun also drive longitudinal “cells” that produce their own wind patterns. For example, there is a vast loop of winds over the Pacific Ocean known as the Walker Circulation, named after Sir Gilbert Walker, who, in the early 20th century, tried to predict Indian monsoon winds. The loop is caused by differences in surface temperature between the eastern and western Pacific and normally produces easterly trade winds, exerting major influences on the climates of southeast Asia and the western coasts of the Americas.

In general, we see that the complexity of the world’s major wind patterns, illustrated in Figure 1.5, is the result of air rising over warmer areas of oceans and continents and subsiding over cooler ones. The effects occur at all scales over the Earth’s surface, from the vastnesses of the Atlantic, Pacific, and Southern Oceans and the Sahara Desert down to mid-scale phenomena that generate famous winds such as the Mistral in France, the Chinook in North America, and the Harmattan in West Africa.

When we come to consider the wind regimes of smaller regions or countries, the patterns of Figure 1.5 are increasingly modified by local geography. Although it certainly helps to keep the big picture in mind, conditions are often affected by hills and mountains, valleys, forests, and variations in terrain—as well as by the time of day and season of the year.[4, 5] Coastal areas often experience “sea breezes” that carry air ashore during the day, followed by opposite “land breezes” at night, powered by the fluctuating temperatures caused by the different thermal capacities of the land and the sea.

Some of the above points are illustrated in Figure 1.6, which shows a typical wind distribution during the summer months over the UK and Ireland, two of the windiest nations in Europe. The main flow is from the Atlantic, corresponding to the prevailing westerlies, but a complex low-pressure feature (a depression) over Scotland, driven by a high-altitude Jetstream, produces localized winds that tend to circle anticlockwise. On a day such as this, not all UK and Irish wind turbines face the same way! In winter, the main flow quite often swings easterly and comes from the Arctic via continental Europe. On top of these broad seasonal effects, winds in a particular location can be greatly affected by local geography.

Figure 1.6 A wind pattern over the UK and Ireland.

What does this highly complex story tell us about generating large amounts of electricity from the wind? There are several points that bear on turbine design and installation. Highly variable wind patterns such as those of northwest Europe demand turbines that can align themselves easily with the flow. If turbines are placed in clusters, the shadowing effect of each on its neighbors may be quite serious when the wind blows from certain directions—a point needing careful consideration during the planning phase. Generally speaking, offshore wind farms are less problematic than onshore locations, for if installed well out to sea they are more likely to find themselves in the consistent company of prevailing winds, undisturbed by land features (Figure 1.7).

The unpredictability of wind means that commercial developers of large turbines go to considerable lengths to assess a site’s actual potential,5 monitoring and recording variations in speed and direction over a year or more before proceeding with installation. The winds of the world remain wild and free—and it will always be a challenge to harness their power as efficiently and economically as possible.

1.3 From windmills to wind turbines

Windmills have a long and venerable history. Some of the earliest practical machines, developed in Persia around the 10th century, were based on rectangular sails rotating about vertical axis. Windmill technology subsequently spread through the Middle East into southern Europe, and by the late 12th century, windmills were being built in England, Holland, and Germany, where horizontal axis machines were always preferred.4

Figure 1.7 Making the most of prevailing winds: a wind farm off the shores of Denmark

(Source: with permission of Orsted).

Although “windmill” implies a machine devoted to the milling or grinding of wheat and other grains, another application proved extremely valuable over the centuries—pumping water to drain low-lying land. The Dutch became world-renowned for using windmills to help reclaim large areas of land from the North Sea, and English farmers drained land in East Anglia. Many other nations contributed to developing highly effective windmills for grinding grain, pumping water, and other mechanical tasks. Around a quarter of a million windmills were installed over the centuries in Western Europe, and although many experts put their heyday in the years 1750–1850, there were still tens of thousands in operation at the start of the 20th century.

We see that windmills were a major source of mechanical energy in Europe before the Industrial Revolution gathered pace. Replacing or supplementing the muscle power of humans and animals, they were an important part of the economic, social, and cultural landscape. The flour miller was a key member of many communities; stories of millers and their families abounded in local folklore; and windmills entered the canon of European literature—never more famously than in Cervantes’ Don Quixote (1605/15), whose romantic but delusional hero attacked a set of Spanish windmills believing them to be ferocious giants. Don Quixote’s imaginary enemies survive to this day in the central Spanish region of La Mancha, carefully restored and eagerly sought by tourists (see Figure 1.8).

The highly variable winds of northwest Europe, already mentioned in the previous section, demanded windmills that could easily be turned to face the wind. In early post mills, the complete timber structure turned on a vertical post; in later tower mills, also known as smock mills, most of the building remained stationary, and only the top section, or cap, rotated. Tower mills could be built higher and heavier than post mills, allowing them to support larger sails. As the years went by, increasingly sophisticated features were incorporated: secondary rotors, known as fantails, to turn the cap into the wind automatically; sails (or blades) with a degree of twist to increase efficiency; and in some advanced designs, speed governors (Figure 1.9).

Figure 1.8 The Spanish windmills made famous in Cervantes’ Don Quixote

(Source: Lourdes Cardenal/Wikimedia Commons/CC BY SA 3.0).