INNOVATION IN WINDTURBINE DESIGN

INNOVATION IN WINDTURBINE DESIGN

Peter JamiesonGarrad Hassan, UK

A John Wiley & Sons, Ltd., Publication

This edition first published 2011 2011, John Wiley & Sons, Ltd

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Library of Congress Cataloguing-in-Publication DataJamieson, Peter, 1946-

Innovation in wind turbine design / by Peter Jamieson. -- 1st ed.p. cm.

Includes bibliographical references and index.ISBN 978-0-470-69981-2 (hardback)

1. Wind turbines. I. Title.TJ828.J36 2011621.4’5 – dc23

2011013534

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To Adele and Rose

Contents

Acknowledgements xiii

Foreword xv

Preface xvii

Introduction 10.1 Why Innovation? 10.2 The Challenge of Wind 20.3 The Specification of a Modern Wind Turbine 20.4 The Variability of the Wind 40.5 Commercial Wind Technology 40.6 Basis of Wind Technology Evaluation 5

0.6.1 Standard Design as Baseline 50.6.2 Basis of Technological Advantage 60.6.3 Security of Claimed Power Performance 60.6.4 Impact of Proposed Innovation 6References 7

Part I DESIGN BACKGROUND

1 Rotor Aerodynamic Theory 111.1 Introduction 111.2 Aerodynamic Lift 121.3 The Actuator Disc 141.4 Open Flow Actuator Disc 15

1.4.1 Axial Induction 151.4.2 Momentum 16

1.5 Generalised Actuator Disc Theory 171.6 The Force on a Diffuser 231.7 Generalised Actuator Disc Theory and Realistic Diffuser Design 241.8 Why a Rotor? 241.9 Basic Operation of a Rotor 25

viii Contents

1.10 Blade Element Momentum Theory 271.10.1 Momentum Equations 271.10.2 Blade Element Equations 28

1.11 Optimum Rotor Theory 301.11.1 The Power Coefficient, Cp 331.11.2 Thrust Coefficient 361.11.3 Out-of-Plane Bending Moment Coefficient 36

1.12 Generalised BEM 381.13 Limitations of Actuator Disc and BEM Theory 41

1.13.1 Actuator Disc Limitations 411.13.2 Wake Rotation and Tip Effect 411.13.3 Optimum Rotor Theory 421.13.4 Skewed Flow 421.13.5 Summary 42References 43

2 Rotor Aerodynamic Design 452.1 Optimum Rotors and Solidity 452.2 Rotor Solidity and Ideal Variable Speed Operation 462.3 Solidity and Loads 482.4 Aerofoil Design Development 482.5 Sensitivity of Aerodynamic Performance to Planform Shape 522.6 Aerofoil Design Specification 54

References 55

3 Rotor Structural Interactions 573.1 Blade Design in General 573.2 Basics of Blade Structure 583.3 Simplified Cap Spar Analyses 60

3.3.1 Design for Minimum Mass with Prescribed Deflection 613.3.2 Design for Fatigue Strength: No Deflection Limits 61

3.4 The Effective t/c Ratio of Aerofoil Sections 623.5 Blade Design Studies: Example of a Parametric Analysis 643.6 Industrial Blade Technology 69

3.6.1 Design 693.6.2 Manufacturing 693.6.3 Design Development 70References 73

4 Upscaling of Wind Turbine Systems 754.1 Introduction: Size and Size Limits 754.2 The ‘Square-Cube’ Law 784.3 Scaling Fundamentals 784.4 Similarity Rules for Wind Turbine Systems 80

4.4.1 Tip Speed 804.4.2 Aerodynamic Moment Scaling 814.4.3 Bending Section Modulus Scaling 814.4.4 Tension Section Scaling 81

Contents ix

4.4.5 Aeroelastic Stability 814.4.6 Self Weight Loads Scaling 814.4.7 Blade (Tip) Deflection Scaling 824.4.8 More Subtle Scaling Effects and Implications 824.4.9 Gearbox Scaling 834.4.10 Support Structure Scaling 834.4.11 Power/Energy Scaling 834.4.12 Electrical Systems Scaling 844.4.13 Control Systems Scaling 844.4.14 Scaling Summary 84

4.5 Analysis of Commercial Data 854.5.1 Blade Mass Scaling 864.5.2 Shaft Mass Scaling 904.5.3 Scaling of Nacelle Mass and Tower Top Mass 904.5.4 Tower Top Mass 914.5.5 Tower Scaling 924.5.6 Gearbox Scaling 96

4.6 Upscaling of VAWTs 974.7 Rated Tip Speed 974.8 Upscaling of Loads 994.9 Violating Similarity 1014.10 Cost Models 1014.11 Scaling Conclusions 103

References 103

5 Wind Energy Conversion Concepts 105References 107

6 Drive Train Design 1096.1 Introduction 1096.2 Definitions 1096.3 Objectives of Drive Train Innovation 1106.4 Drive Train Technology Maps 1106.5 Direct Drive 1146.6 Hybrid Systems 1176.7 Hydraulic Transmission 1186.8 Efficiency of Drive Train Components 120

6.8.1 Introduction 1206.8.2 Efficiency Over the Operational Range 1216.8.3 Gearbox Efficiency 1226.8.4 Generator Efficiency 1226.8.5 Converter Efficiency 1236.8.6 Transformer Efficiency 1246.8.7 Fluid Coupling Efficiency 124

6.9 The Optimum Drive Train 1256.10 Innovative Concepts for Power Take-Off 126

References 129

x Contents

7 Offshore Wind Turbines 1317.1 Design for Offshore 1317.2 High Speed Rotor 132

7.2.1 Design Logic 1327.2.2 Speed Limit 1327.2.3 Rotor Configurations 1337.2.4 Design Comparisons 134

7.3 ‘Simpler’ Offshore Turbines 1387.4 Offshore Floating Turbine Systems 139

References 141

8 Technology Trends Summary 1438.1 Evolution 1438.2 Consensus in Blade Number and Operational Concept 1458.3 Divergence in Drive Train Concepts 1458.4 Future Wind Technology 146

8.4.1 Introduction 1468.4.2 Airborne Systems 1468.4.3 New System Concepts 147References 149

Part II TECHNOLOGY EVALUATION

9 Cost of Energy 1539.1 The Approach to Cost of Energy 1539.2 Energy: The Power Curve 1569.3 Energy: Efficiency, Reliability, Availability 161

9.3.1 Efficiency 1619.3.2 Reliability 1619.3.3 Availability 162

9.4 Capital Costs 1639.5 Operation and Maintenance 1649.6 Overall Cost Split 1649.7 Scaling Impact on Cost 1669.8 Impact of Loads (Site Class) 167

References 170

10 Evaluation Methodology 17310.1 Key Evaluation Issues 17310.2 Fatal Flaw Analysis 17410.3 Power Performance 174

10.3.1 The Betz Limit 17510.3.2 The Pressure Difference across a Wind Turbine 17610.3.3 Total Energy in the Flow 177

10.4 Drive Train Torque 17810.5 Representative Baseline 17810.6 Design Loads Comparison 179

Contents xi

10.7 Evaluation Example: Optimum Rated Power of a Wind Turbine 18110.8 Evaluation Example: The Carter Wind Turbine and Structural Flexibility 18310.9 Evaluation Example: Concept Design Optimisation Study 186

References 187

Part III DESIGN THEMES

11 Optimum Blade Number 19111.1 Energy Capture Comparisons 19111.2 Blade Design Issues 19211.3 Operational and System Design Issues 19411.4 Multi Bladed Rotors 199

References 199

12 Pitch versus Stall 20112.1 Stall Regulation 20112.2 Pitch Regulation 20312.3 Fatigue Loading Issues 20412.4 Power Quality and Network Demands 206

12.4.1 Grid Code Requirements and Implicationsfor Wind Turbine Design 206

References 208

13 HAWT or VAWT? 21113.1 Introduction 21113.2 VAWT Aerodynamics 21113.3 Power Performance and Energy Capture 21713.4 Drive Train Torque 21813.5 Niche Applications for VAWTs 22013.6 Status of VAWT Design 220

13.6.1 Problems 22013.6.2 Solutions? 221References 222

14 Free Yaw 22314.1 Yaw System COE Value 22314.2 Yaw Dynamics 22314.3 Yaw Damping 22514.4 Main Power Transmission 22514.5 Operational Experience of Free Yaw Wind Turbines 22614.6 Summary View 227

References 227

15 Multi Rotor Systems 22915.1 Introduction 22915.2 Standardisation Benefit and Concept Developments 22915.3 Operational Systems 230

xii Contents

15.4 Scaling Economics 23015.5 History Overview 23215.6 Aerodynamic Performance of Multi Rotor Arrays 23215.7 Recent Multi Rotor Concepts 23215.8 Multi Rotor Conclusions 237

References 238

16 Design Themes Summary 239

Part IV INNOVATIVE TECHNOLOGY EXAMPLES

17 Adaptable Rotor Concepts 24317.1 Rotor Operational Demands 24317.2 Control of Wind Turbines 24517.3 Adaptable Rotors 24617.4 The Coning Rotor 248

17.4.1 Concept 24817.4.2 Coning Rotor: Outline Evaluation – Energy Capture 25017.4.3 Coning Rotor: Outline Evaluation – Loads 25017.4.4 Concept Overview 251

17.5 Variable Diameter Rotor 252References 253

18 A Shrouded Rotor 255References 258

19 The Gamesa G10X Drive Train 259

20 Gyroscopic Torque Transmission 263References 268

21 The Norsetek Rotor Design 269References 271

22 Siemens Blade Technology 273

23 Stall Induced Vibrations 277References 280

24 Magnetic Gearing and Pseudo-Direct Drive 28324.1 Magnetic Gearing Technology 28324.2 Pseudo-Direct Drive Technology 286

References 288

25 Summary and Concluding Comments 289

Index 291

Acknowledgements

My professional life in wind technology began in 1980 in the employment of James Howdenand Company of Glasgow and I very much appreciate many colleagues who shared theseearly days of discovery. Howden regrettably withdrew from turbine manufacture in 1988but by then my addiction to wind was beyond remedy.

In those days I much admired a growing wind energy consultancy, now GL GarradHassan, and was delighted to join them in 1991 and, as it happened, founded their Scottishoffice. I felt that it would be great to have a working environment among such talentedpeople and that I would have a continuing challenge to be worthy of them. Nothing haschanged in that respect.

In particular I would very much like to thank Andrew Garrad and Dave Quarton, forencouragement, practical support and great tolerance over the past few years in the prepa-ration of this book.

Very recently, since 2009, I have additionally been employed in the Doctoral TrainingCentre for Wind Energy in the University of Strathclyde and I am indebted to Bill Leit-head also for supporting the book and for many valuable brainstorming sessions on windtechnology over the years.

I have a special thanks to the late Woody Stoddard who was an inspiring friend andenormously supportive, especially considering the few times we met.

Considering the very many times I have imposed on his good nature, I have equallyto thank Mike Graham for his freely given help in so many projects and as an excellent,unofficial aerodynamics tutor. Much thanks also to Henrik Stiesdal, who, as an extremelybusy man at the technical helm of a large wind turbine manufacturing company, found timeto contribute a chapter to this book.

My warm thanks also go to very many other work colleagues and associates who, know-ingly or otherwise, have made valuable contributions to this book. Among them are:

Albert Su, Alena Bach, Alexander Ovchinnikov, Andrew Latham, Anne Telfer, BenHendriks, Bob Thresher, Chai Toren, Charles Gamble, Chris Hornzee-Jones, Chris Kirby,Christine Sams, David Banks, David Milborrow, David Sharpe, Ed Spooner, Emil Moroz,Ervin Bossanyi, Fabio Spinato, Fatma Murray, Jan Rens, Geir Moe, Georg Bohmeke, Ger-ard van Bussel, Herman Snel, Irina Dyukova, Jega Jegatheeson, Jim Platts, John Armstrong,Leong Teh, Lois Connell, Magnus Kristbergsson, Marcia Heronemus, Mark Hancock, Mar-tin Hansen, Masaaki Shibata, Mauro Villanueva-Monzon, Mike Anderson, Mike Smith, NickJenkins, Nils Gislason, Patrick Rainey, Paul Gardner, Paul Gipe, Paul Newton, Paul Veers,

xiv Acknowledgements

Peter Musgrove, Rob Rawlinson-Smith, Roger Haines, Roland Stoer, Ross Walker, RossWilson, Ruud van Rooij, Sandy Butterfield, Stephen Salter, Steve Gilkes, Tim Camp, TakisChaviaropoulos, Trevor Nash, Unsal Hassan, Varan Sureshan, Vidar Holmoy, Win Rampen,Wouter Haans.

Peter Jamieson

Foreword

Those of us who have been active in the wind energy for the last few decades have beenlucky. We have been involved in an industry that is technically fascinating, commerciallyexciting and thoroughly worthwhile. We have seen turbines increase in diameter from 10 to120 m and in power from 10 to 10 000 kW – what a fantastic journey!

The size of the turbines is the most obvious characteristic because it can be so clearlyseen – wind turbines are now by far the biggest rotating machines in the world. Less visibleis the ingenuity of the designs. Looking back a couple of decades there were many ‘whacky’ideas that were seriously contemplated and even offered commercially and some of thosewhacky ideas have become conventional. Superficially the latest generation of turbines mayall look the same but underneath the nacelle and inside the blades there are many fascinatingdifferences. For a long time the mantra of the wind turbine industry has been ‘bigger andbigger’ but now it has moved to ‘better and better’ and this change marks a change in theareas of innovation.

Peter Jamieson is one of the clearest thinkers in the industry and I am delighted andhonoured to have worked with him for almost 20 years. He is a real blue sky thinkerunimpeded by convention and driven by a strong sense of rigour. Innovation in wind turbinedesign is what Peter has been doing for the last 30 years and it is about time he wrote abook about it. I fully supported Peter’s idea that he should put his professional thoughts onrecord and now he has done so.

Anyone interested in the technical aspects of both the past developments and the excitingfuture of wind turbines should read this book carefully and be inspired. This is no aridtechnical text or history – this is real intellectual capital and, of course, innovation.

Andrew Garrad

Preface

This book is about innovation in wind turbine design – more specifically about theevaluation of innovation – assessing whether a new concept or system will lead toimproved design enhancing performance or reducing cost.

In the course of a working life in wind energy that began in 1980, the author’s workhas increasingly been, at the request of commercial clients and sometimes public author-ities, to evaluate innovative systems providing reports which may or may not encouragefurther investment or development. In some cases, the clients are private inventors witha cherished idea. Other cases include small companies strategically developing innovativetechnologies, major industrial companies looking for an entry to the wind turbine market ormajor established wind turbine companies looking to their next generation technology.

There is substantial conservatism in the wind industry as in most others and largely forthe best possible reasons. Products need to be thoroughly proven and sound whereas changeis generally risky and expensive even when there is significant promise of future benefit.To some extent change has been enforced by the demands year by year for larger windturbines and components, a situation which only now may be beginning to settle. There issome convergence in the preferred mainstream design routes. However as new players andnew nations enter the wind business, there is also a proliferation of wind technology ideasand demand for new designs. The expansion of wind energy worldwide has such impetusthat this book could be filled with nothing but a catalogue of different innovative designsand components.

It may initially seem strange that as much of the book is devoted to technology backgroundas to discussion of specific innovative concepts. However innovation is not a matter ofgenerating whacky concepts as an entertainment for bored engineers. The core justificationfor innovation is that it improves technology, solving problems rather than creating them.To achieve that, it is crucial that the underlying requirements of the technology are wellunderstood and that innovation is directed in areas where it will produce most reward.Hence is the emphasis on general technology background. Within that background somelong established theory is revisited (actuator disc and blade element momentum) but withsome new equations developed.

Innovative ideas by definition break the mould. They often require new analytical toolsor new developments of existing ones and, in general, fresh thinking. They do not lendthemselves to a systematised, routine approach in evaluation. Evaluating innovation is anactive process like design itself, always in evolution with no final methodology. On theother hand, there are basic principles and some degree of structure can be introduced to theevaluation process.

xviii Preface

In tackling these issues a gap was apparent – between broad concepts and detailed design.This is territory where brain storming and then parametric analyses are needed, when purejudgement is too limited but when heavy weight calculation is time consuming, expensiveand cannot be focussed with any certainty in the right direction. This why ‘detailed design’is not much addressed. It is the subject of another book. This one concerns building bridgesand developing tools to evaluate innovative concepts to the point where investment indetailed design can be justified.

Innovation in wind energy expresses the idealism of the designer to further a sustainabletechnology that is kind to the planet.

Introduction

0.1 Why Innovation?

Fuel crises, concerns about global environmental threats, the urgent needs for energy inexpanding new economies of the former third world have all contributed to an ever increas-ing growth of renewable energy technologies. Presently, wind energy is the most matureand cost effective of these.

While other more diverse applications are discussed, this book keeps a main focus onwind energy converters that produce electricity. This is primarily because the greater partof the author’s experience is with such systems. However, in a more objective defence ofthat stance, it may be observed that by far the largest impact of wind technology on theworld’s energy supply presently comes from systems generating into electrical networks.

Innovation is about new ideas and some quite unusual designs are evaluated in this book.Why give attention to such designs which may not be in the mainstream? Exploring alterna-tive concepts not only deepens understanding of why the mainstream options are preferredbut also suggests where they should be challenged by alternatives that have significantpromise. In any case ideas are grist to the mill of technological progress and those whichfail in one embodiment may well later be adapted and successfully reincarnated.

As is discussed shortly, the generation of power from the wind presents unique challenges.Unlike cars and houses, for example energy is a commodity which has utilitarian value only.No one prefers a particular petrol because is has a nicer colour. The wealthy may indulge ingold or gold-plated bathroom taps but no one can purchase gold-plated electricity. Energymust meet generally stringent specifications of quality in order to be useful (voltage andfrequency levels particularly in the case of electricity). Once it does, the main requirementis that it is dependably available and as cheap as possible.

The end purpose of innovation in wind turbine design is to improve the technology.Usually this means reducing the cost of energy and this is the general basis of evaluatinginnovation in this book. However even this simply stated goal is not always the finalcriterion. In some instances, for example the objective is to maximise energy return from anavailable area of land. Sometimes capital cost has a predominant influence. The bottom lineis that any technology must be tailored accurately to an engineering design specificationthat may include environmental, market, cost and performance issues.

Innovation in Wind Turbine Design, First Edition. Peter Jamieson. 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd.

2 Innovation in Wind Turbine Design

The detailed design of a wind turbine system is not a minor or inexpensive task. By thetime an innovative design is subject of a detailed design study, although it may yet be someway from market, it has already received significant investment and has already passedpreliminary tests as to the potential worth of the new concepts.

Thus there is an intermediate stage between first exposure of a concept up to the stageof securing investment in a prototype when the concepts are examined and various levelsof design are undertaken. Usually a search for fatal flaws or obvious major shortcomings isthe first stage. The design may be feasible but have much more engineering content thanits competitors and it is therefore unlikely to be cost effective. More typically there is noclear initial basis for rejecting the new concepts and a second level of appraisal is required.A systematic method is needed to review qualitatively, and where possible quantitatively,how the design compares to existing technology and for what reason(s) it may have merit.At this stage detailed, expensive time consuming analyses are precluded, but there is a greatneed for parametric evaluations and simplified analyses that can shed light on the potentialof the new concepts.

This book is very much about these preliminary evaluation stages, how simple insightfulmethods can provide guidance at a point where the value of the innovation is too uncertainto justify immediate substantial investment or detailed design.

0.2 The Challenge of Wind

According to Murray [1], the earliest written reference to windmills of the fifth century BClists it, among other things, as something a devout Buddhist would have nothing to do with,albeit in the context of small air driven rotors for amusing children. The aerodynamic rotorconcept is therefore ancient.

To generate electricity (by no means the only use for a wind turbine but certainly a majorone under present consideration), requires the connection of such a rotor to an electricgenerator. Electric motor/generator technology began in Faraday’s discoveries in the mid-nineteenth century. About 60 years ago and preceding the modern wind industry, the averagehousehold in the US contained about 40 electric motors. The electric motor/generator istherefore not ancient but has been in mass production for a long time in recent history.What then is difficult about the marriage of rotor and generator into successful and economicpower generating systems? The challenge of modern wind technology lies in two areas, thespecification of an electricity generating wind turbine and the variability of the wind.

0.3 The Specification of a Modern Wind Turbine

Traditional ‘Dutch’ windmills (Figure 0.1) have proliferated to the extent of 100 000 overEurope in their heyday. Some have survived 400–600 years, the oldest still operating inthe UK being the post mill at Outwood, Surrey built in 1628. A short account of thehistory of early traditional wind technology in Eggleston and Stoddard [2] shows that theyexhibit considerable practical engineering skill and empirical aerodynamic knowledge intheir design and interesting innovations such as variable solidity blades (spilling the airthrough slats that can open or close) that have not surfaced in modern wind turbine design.However, these machines were always attended, were controlled manually for the most part,were integrated parts of the community, were designed for frequent replacement of certaincomponents, and efficiency was of little consequence.

Introduction 3

Figure 0.1 Jill Post Mill at Clayton Sussex. Reproduced by permission of Paul Barber

In contrast, to generate electricity cost effectively is the specification of a modern powergenerating wind turbine. To meet economic targets, it is unthinkable for the wind turbine tobe permanently attended, and unacceptable for it to be much maintained. Yet each unit is aself contained mini-power station, requiring to output electricity of standard frequency andvoltage into a grid system. Cost effectiveness is overriding but the efficiency of individualunits cannot be sacrificed lightly. As will be elaborated subsequently, energy is a prime valuewhilst the capital costs of any particular components are only some fraction of lifetime costsand have a lesser impact on cost of energy. Also the total land area requirements per unitoutput will increase as efficiency drops.

It should be clear that wind technology embraces what is loosely called ‘high tech’ and‘low tech’ engineering. The microprocessor plays a vital role in achieving self monitoringunmanned installations. There is in fact nothing particularly simple about any kind of systemfor generating quality electricity. Diesel generators are familiar but not simple, and have along history of development.

Thus it is by no means enough to build something ‘simple and rugged’ that will surviveany storm. Instead the wind turbine must be value engineered very carefully to generatecheap electricity with adequate reliability. This is the first reason why the technology ischallenging.

4 Innovation in Wind Turbine Design

0.4 The Variability of the Wind

The greatest gust on record was on 12 April 1934 at the peak of Mount Washington in theNorthern Appalachians [3]. ‘On record’ is a revealing phrase as anemometers have usuallyfailed in the most extreme conditions. At 103 m/s, a person exposing 0.5 m2 of frontal areawould have experienced a force equivalent to about 1/3 of a ton weight. In terms of annualmean wind speed, the windiest place in the world [3] is on the edge of Antarctica, on amountain margin of East Adelie land. At 18 m/s annual mean wind speed, the availablewind energy is about 200 times that of a typical European wind site. These are of courseextreme examples and there are no plans to erect wind turbines on either site.

Nevertheless, it underlines that there is enormous variation in wind conditions. Thisapplies both on a worldwide basis but also in very local terms. In the rolling hills of theAltamont Pass area of California, where many wind farms were sited in the 1980s, thereare large differences in wind resource (100% say in energy terms) between locations nomore than a few hundred metres apart. Wind turbines are situated right at the bottom of theearth’s boundary layer. Their aerofoils generally travel much more slowly than aircraft orhelicopter rotors, and the effect of wind turbulence is much more consequential for design.The crux of this is that it is hard to refine a design for such potentially variable conditions,and yet uneconomic to design a wind turbine fit to survive anywhere. Standardisation ismuch desired to cheapen production, but is in conflict with best economics at specific localsites. Designs often need to have adaptive features to accommodate larger rotors, upratedgenerators or additional structural reinforcement as necessary.

Anemometry studies, both to determine suitable sites and for the micro-siting of machineswithin a chosen area, are not academic exercises. Because of the sensitivity of wind energyto wind speed and wind speed to short and longer term climatic patterns, the developer whois casual about wind resource estimates is playing a game of roulette on profit margins.Thus the variability of the wind is the second major reason why wind turbine design ischallenging.

0.5 Commercial Wind Technology

In the twentieth century, wind technology headed towards mainstream power generationbeyond the water pumping and milling applications that had been exploited for severalthousand years. The Gedser wind turbine is often credited as the seminal design of themodern wind industry. With assistance from Marshall Plan post war funding, a 200 kW,24 m diameter, three bladed wind turbine was installed during 1956–1957 on the island ofGedser in the south east of Denmark. This machine operated from 1958 to 1967 with about20% capacity factor.1

In the early 1960s Professor Ulrich Hutter developed high tip speed designs, which hada significant influence on wind turbine research in Germany and the US.

In the early 1980s, many issues of rotor blade technology were investigated. Steel rotorswere tried but rejected as too heavy, aluminium as too uncertain in the context of fatigueendurance. Wood was a logical natural material designed by evolution for high fatigue bend-ing strength to weight ratio. The problem of moisture stabilisation of wood was resolved in

1 Capacity factor is the ratio of energy output produced over a period to that which would be produced if thesystem operated always at its full nameplate rating over the same period. Providing the wind turbine is reliable,capacity factor in the context of wind energy is primarily a measure of how good the site wind conditions may be.

Introduction 5

the wood-epoxy system developed by Gougeon Brothers in the US. This system has sincebeen employed in a number of small and large wind turbines (e.g. the former NEG-MiconNM82). Wood epoxy blade technology was much further developed in the UK, latterly byTaywood Aerolaminates who were assimilated by NEG-Micon and then in turn by Vestas.The blade manufacturing industry was, however, dominated by fibreglass polyester con-struction which evolved from a boat building background, became thoroughly consolidatedin Denmark in the 1980s and has since evolved into more sophisticated glass compositetechnologies using higher quality fibres (sometimes with carbon reinforcement), and moreadvanced manufacturing methods such as resin infusion.

During the 1980s, some megawatt scale prototypes had appeared and this history is welldocumented by Spera [4] and Hau [5]. In general these wind turbines had short lives andin some cases fatal flaws in design or manufacture. Thus, although information was gainedof some research value, in many respects these designs mostly followed technology routesrather disconnected from the emerging commercial wind turbine market. In contrast to this,in Denmark during the 1970s and 1980s, a gradual development of wind technology hadoccurred. This was a result of public pressures to develop renewables and to avoid nuclearenergy combined with a lack of indigenous conventional energy sources. Wind turbinedesign development proceeded with incremental improvement of designs which were beingmaintained in commercial use and with gradual increase in scale into ratings of a few100 kw. And a much more successful technology resulted.

Just as the first generation commercial Danish designs were emerging in the early 1980s, acombination of state and federal, energy and investment tax credits had stimulated a rapidlyexpanding market for wind in California. Over the period 1980–1995 about 1700 MW ofwind capacity was installed, more than half after 1985 when the tax credits had reduced toabout 15%.

Tax credits fuelled the indiscriminate overpopulation of various areas of California(San Gorgonio, Tehachapi and Altamont Pass) with wind turbines many of which wereill-designed and functioned poorly. It was the birthplace and graveyard of much more orless casual innovation. This created a poor image for the wind industry which took time toremedy. However, the tax credits created a major export market for European, especiallyDanish, wind turbine manufacturers who had relatively cost effective, tried and testedhardware available. The technically successful operation of the later, better designed windturbines in California did much to establish the foundation on which the modern windindustry has been built.2

0.6 Basis of Wind Technology Evaluation

A summary of some of the key issues in the evaluation of new wind technology is presentedhere. These topics are addressed in more detail in Chapters 9 and 10.

0.6.1 Standard Design as Baseline

The most straightforward way to evaluate new technology is to set it alongside existing stateof the art technology and conduct a side by side comparison. This is particularly effective

2 A historical review of wind technology (also written by the author) similar to the text up to this point may befound in the EWEA publication, Wind Energy the Facts .

6 Innovation in Wind Turbine Design

in the case of isolated components which are innovative and different in themselves fromthe standard solution but have little direct impact on the rest of the system. It is thenreasonable to assume that all other components in the system have the same costs as in thestandard design and conduct a cost of energy analysis in which only the new component isdifferentiated. Other innovations may be much more challenging. For example a new rotorconcept can have wide ranging implications for system loads. In that case one approach isto tailor certain key loads to be within the same level as the standard design and thereforeto have no impact on the components designed by those loads. Another more challengingroute is to develop analyses where the impact of loads on component cost is considered.The development of a baseline standard, state of the art design will be seen as a key elementin most of the evaluations of innovative technology.

0.6.2 Basis of Technological Advantage

If an innovative system is feasible in principle, the next obvious question is why is it betterthan anything that precedes it? Does it offer performance gains or cost reduction, does itenhance reliability? In the first instance it is not a matter of assessing the level of merit orthe realism of the claim so much as confirming that there is a core reason being offeredwhy the system may have merit.

0.6.3 Security of Claimed Power Performance

Especially with radically new system designs there may be a question mark over the likelylevel of performance. The evaluation of this is clearly critical for the system economics anda number of evaluations go no further than consideration of whether the proposed system hasa sufficiently good power performance coefficient. This is particularly the case in systemsthat sacrifice performance for simplicity. Sometimes the illusion of a very simple and cheapsystem will persuade an uncritical inventor that an idea is very promising when, in fact,the system in its essential concepts sacrifices so much energy that the considerable capitalcost savings that it may achieve are not enough to justify the concept and the cost of energyis high.

0.6.4 Impact of Proposed Innovation

Where can successful innovation make the greatest impact?This will be addressed by looking at the relative costs of components in a wind turbine

system and any impacts they have on system productivity through efficiency or reliability.Innovation is disruptive and needs to offer sufficient benefit to be worth the trouble. Thecapital cost of a yaw system of a large horizontal axis wind turbine is typically around 3 or4% of total wind turbine capital cost. About 1/2 to 2/3 of this cost is in the yaw bearing.This major component is generally not dispensable and so it is clear that no yaw systemsolution, however innovative, can make large capital cost savings in relation to wind turbinecapital cost. On the other hand if a new yaw system has improved reliability, its total valuein terms of impact in cost of energy is enhanced.

Introduction 7

References[1] Murray, H.J.R.A. A History of Chess , Oxford University Press, London, (1913); Benjamin Press, Northampton,

MA (1985). ISBN: 0-936317-01-9.[2] Eggleston, D.M. and Stoddard, F.S. (1987) Wind Turbine Engineering Design, Kluwer Academic Publishers.

ISBN: 13; 9780442221959.[3] Watson, L. (1984) Heavens Breath: A Natural History of the Wind , Hodder General Publishing Division.

ISBN: 0340430982 (0-340-43098-2).[4] Spera, D.A. (ed.) (2009) Wind Turbine Technology: Fundamental Concepts in Wind Turbine Engineering ,

2nd edn, ASME Press.[5] Hau, E. (2006) Wind Turbines: Fundamentals, Technologies, Application, Economics , Springer-Verlag.

ISBN: 13 978-3-540-24240-6.

Part IDesign Background