Designing Wind Turbines: Engineering and Manufacturing Process in the Industrial Context 3031085485, 9783031085482

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Synthesis Lectures on Renewable Energy Technologies

Uwe Ritschel Michael Beyer

Designing Wind Turbines Engineering and Manufacturing Process in the Industrial Context

Synthesis Lectures on Renewable Energy Technologies Series Editor RICHARD DUNLAP, HALIFAX, NS, Canada

This series publishes short books analyzing and reviewing the past and present energy use of society and its future needs. A breakdown of current energy sources shows that approximately 80% of the world’s primary energy comes from fossil fuels. The book provides an assessment of the needs to change the way in which energy is produced and utilized.

Uwe Ritschel · Michael Beyer

Designing Wind Turbines Engineering and Manufacturing Process in the Industrial Context

Uwe Ritschel Wittenbeck, Germany

Michael Beyer Rostock, Germany

ISSN 2690-5000 ISSN 2690-5019 (electronic) Synthesis Lectures on Renewable Energy Technologies ISBN 978-3-031-08548-2 ISBN 978-3-031-08549-9 (eBook) https://doi.org/10.1007/978-3-031-08549-9 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

Wind has been tamed for a long time in human history for transportation and as source of mechanical power for grinding grain and pumping water. Since about three decades electricity from wind plays an important role in the transformation process towards a climate-friendly, CO2 -free energy supply. In addition, the energy payback time1 of wind turbines is short (less than 1 year) such that in many scenarios for the future energy supply wind contributes a significant share. The book is based on practical experience derived from development projects for complete designs of new wind turbines and parts of it. Two of those designs are used for illustration in this book. Focus of this book is on mechanical design of the drive train of a wind turbine and how a wind turbine is developed within the engineering process of a wind turbine manufacturer. Initially, we provide a concise account on basic concepts to generate electricity from a wind rotor. We are not aiming to provide a complete overview on wind energy and wind turbines. For this we refer to a number of excellent accounts on wind energy for which updated editions appear on a regular basis, like books of Hau [Hau 2014] and Manwell et al. [Manwell et al. 2009]. The wind turbine manufacturer is often called Original Equipment Manufacturer (OEM). Detailed descriptions and explanations are given here for those components of the wind turbine that are normally developed by the OEM of a particular type. The OEM needs to have full knowledge of the complete system that consists of all parts being rotor blades, nacelle, drive train, tower, and foundation including the dynamic properties and the response to controller action. This knowledge is called system competence. The drive train is the most important system. In wind turbines with gearbox, it consists of many components like shafts, bearings, gearbox, and generator. In systems without a gearbox, a large extraordinary generator has to be integrated into the drive train. Some components are normally designed by the OEM itself. Examples are hub and machine frame. These components will be treated in detail here. For other components preliminary sizing is presented, since they are merely specified by the OEM. The design is done by specialized industry companies. Examples are bearings and gearboxes. A detailed 1 The energy payback time indicates the operational time of a wind turbine needed to generate the

same amount of energy that was used for its manufacturing and installation.

v

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Preface

design is then provided by suppliers. The working principles of these components are explained. The main contents of typical specifications for those components are described. Other components like blades and towers are only treated briefly. The content of this book is largely covered by a lecture held at the University of Rostock and guest lectures at other places. During the last 20 years the Rostock area has become one of the centers of wind industry activities in Germany. About 10.000 jobs have been created both for blue- and white-collar personnel. Hence, there has been high demand for graduates from universities with knowledge in wind energy technology for many years. As a consequence, an institute for wind energy technology has been founded at the university as part of the mechanical engineering faculty. A curriculum was developed to provide an introduction to the field and further insights in more specialized topics. One of these more specialized topics is the one-semester course entitled “Designing wind turbines in the industrial context”, which has been taught every year since 2015. Wittenbeck, Germany Rostock, Germany February 2022

Uwe Ritschel Michael Beyer

Acknowledgments

We thank friends and colleagues from the wind industry for their support. Numerous pictures and illustrations were kindly made available to us for this book. Images come from the companies Deutsche Groβwälzlager GmbH, Eleon AS, Wind Projekt GmbH, and Windrad Engineering GmbH. The wind turbines used for illustration are the Windrad 2 MW and the Eleon 3.4 MW turbines, one is a typical geared machine and the other one an innovative gearless design. We thank both companies for providing material for educational purposes. Both designs have been used for many years also for educating students at the University of Rostock. We are particularly grateful to Carsten Hennhöfer, Mohsen Moomkesh, and Dhairya Solanki for helping us with the CAD models used for illustration. Carsten Hennhöfer is Chief Design Engineer of the Windrad 2 MW turbine. February 2022

Uwe Ritschel Michael Beyer

vii

Contents

1

Wind Energy Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Why Wind Turbines? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Concepts to Utilize Wind Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Drag-Type Wind Turbines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Lift-Type Wind Turbines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3 Power Performance of Different Concepts . . . . . . . . . . . . . . . . . 1.3 The c P (λ) Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Electrical Energy from Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 The Power Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 2 3 4 6 8 9 9

2

Modern Wind Turbines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Geometry with Upwind Rotor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Components and Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Rotor Blades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Towers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Power Generation System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Essential Control Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.1 Variable Speed Operation by Torque Control . . . . . . . . . . . . . . . 2.6.2 Power Regulation by Blade Pitch . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 Alternative Power Regulation Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 Operational States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8.1 Safety System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9 Why All Utility-Scale Wind Turbines Look Similar . . . . . . . . . . . . . . . . 2.10 Some Recent Wind-Industry Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.1 Trend to Lower Specific Power . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.2 Cost of Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.3 Trend to High Onshore Towers . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.4 Other Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.5 Onshore and Offshore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.11 Wind Turbines Used for Illustration in This Book . . . . . . . . . . . . . . . . . .

13 13 15 16 20 24 27 27 29 31 31 33 33 35 35 35 36 36 37 37 ix

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3

Development Process and Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Modern Development Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Engineering Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Standards, Guidelines, Certification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Load Cases, Ultimate and Fatigue Loads . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Vertical Integration of Manufacturing in Wind Industry . . . . . . . . . . . . . 3.6 Transport and Logistics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Requirements for Offshore Wind Energy . . . . . . . . . . . . . . . . . . . . . . . . . .

43 43 45 46 49 52 54 56

4

Drivetrain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Gearbox or No Gearbox? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Hub Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Rotor Bearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Separated or Integrated Drivetrain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Wind Turbines with Gearbox and High-Speed Generator . . . . . . . . . . . . 4.5.1 Low-Speed Side—Machine Frame and Rotor Bearing Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2 Gearbox and Its Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.3 High-Speed Shaft and Connection of Gearbox and Generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.4 High Speed Generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.5 Drivetrain Dynamics for Wind Turbine with Gearbox . . . . . . . 4.6 Concepts with Medium Speed Generator . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7 Direct Drive Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.1 Type of Generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.2 The Air Gap Challenge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.3 Position of the Generator and Integration Concept . . . . . . . . . . 4.7.4 Inner and Outer Rotor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.5 Eleon as an Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

57 57 59 60 61 63

Structural Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Materials and Manufacturing Process for Main Components . . . . . . . . . 5.1.1 Cast Iron and Casting Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2 Construction Steel and Welding . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.3 Forged Alloy Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.4 Glas Fiber Reinforced Plastic . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Hub . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Hub Design Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 2 MW Hub Design and Structural Analysis . . . . . . . . . . . . . . . . 5.2.3 Hub Stability Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Machine Frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Sizing and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

81 81 82 83 84 84 85 86 88 90 90 91

5

63 69 70 70 71 73 74 75 76 76 77 78

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5.3.2 Optimizing the Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Generator Frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Main Shaft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.1 Sizing and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . High Speed Shaft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rotor Lock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nacelle Cover and Spinner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.1 Nacelle Cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.2 Spinner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

97 99 101 101 105 106 108 109 111

6

Bearings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Main Bearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1 Bearing Housing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Pitch Bearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Yaw Bearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

113 113 117 119 121

7

Gearbox . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Gearbox Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.1 Alternative Gearbox Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Shrink Disc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Torque Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Slip Ring and Rotary Union . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

123 123 127 127 129 133

8

Bolt Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Basics Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Main Shaft to Hub . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Main Bearing Housing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Tower Top . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

135 135 140 142 144

9

Yaw and Pitch System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Yaw System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1.1 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1.2 Sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Pitch System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.1 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.2 Sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

147 147 147 148 152 152 153

10 Auxiliary Systems and Secondary Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1 Mechanical Brake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1.1 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1.2 Sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Meteorological System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3 Cooling and Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.1 Basics Heat Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

157 157 158 158 159 161 161

5.4 5.5 5.6 5.7 5.8

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10.3.2 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.3 HVAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Hydraulics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.5 Tower Internals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

162 163 165 167

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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About the Authors

Uwe Ritschel has studied physics and received his Ph.D. in 1989. After several years in fundamental research in 2000 he joined the engineering department of Nordex, a wind turbine manufacturer. In 2002, he established the Windrad Engineering GmbH and worked as Managing Director. Main business is the development of new wind turbines for international customers from wind industry. Since 2014 he holds the Chair of Wind Energy Technology at the University of Rostock. In 2019, he co-founded the independent research institute IWEN working more generally on renewable energies and energy transition. Michael Beyer received his Ph.D. in physics in 1985 at the University of Mainz. He held several research and teaching positions in different countries including Germany, U.S.A., Switzerland and several visiting and research grants on basic research. He entered wind business in 2008 and from 2014 until 2021 he was Managing Director of Windrad Engineering GmbH, an independent engineering office and design house for wind turbines. He shares his experience through consulting and is teaching mechanical aspects of wind energy at the University of Kassel.

xiii

1

Wind Energy Basics

During recent history of mankind primary energy was mainly based on fossil sources like coal, oil and natural gas. There is a broad consensus that a transition from fossil to renewable energies has to take place during the next three decades. Main reason for this transition is not the scarcity of fossil resources but the impending climate change caused by CO2 emission when the fossil fuels are burned. The two renewable technologies that that can be quickly scaled up to the generation capacity needed are photovoltaics and wind power.1 As argued for example recently by Schellnhuber et al. (2016) the upscaling of new renewables is the only viable way to keep the global warming below 2 ◦ C and, thus, prevent irreversible damage to our climate.

1.1

Why Wind Turbines?

Wind power, one can say without exaggeration, is so far the most successful of the renewable energies. By end of 2020 the wind power capacity installed worldwide was 743 GW, and about 75 GW new installation was forecasted for 2021 (GWEC 2019). Wind turbines (WT) have gone through an amazing development during the last three decades or so. Most obviously they have enormously grown in size. The first industrial wind turbines during the 1980s had a rotor diameter of 15 m. The latest offshore wind turbines’ diameter measures more than 200 m. In the course of this development a number of virtues of the technology have been achieved:

1 Of course hydro power and the use of biomass have long been used and are also renewable. The may

be called the traditional renewables. However, they cannot be scaled up to the necessary generation capacity. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 U. Ritschel and M. Beyer, Designing Wind Turbines, Synthesis Lectures on Renewable Energy Technologies, https://doi.org/10.1007/978-3-031-08549-9_1

1

2

1 Wind Energy Basics

• Electrical energy production by one wind turbine reaches several millions of kWh and thus can, on average, supply the electricity for thousands of households. The energy production of a large wind farm can be compared to a conventional coal power plant. • Cost of wind energy decreased since the 1980s to values that are similar or even below the figures of conventional power stations. The size of the individual plant is essential here. • Not unrelated to the cost of energy is the energy payback time, i.e., the energy needed to be produced during operation of the wind turbine to compensate the energy needed to manufacture and install it. For wind turbines it is a few months only, much less than the payback period of other technologies. As a consequence wind energy always plays a important role in the scenarios for the energy transition. In a cooperative study of several German research institutes wind contributes about one fifth of the global primary energy corresponding (under certain assumptions explained in the study) to 28.000 TWh/a2 (Schmid et al. 2011). This demand can the mapped to the needed wind capacity, which is a measure for the power (kW) installed. If we would assume that an average of 45% of the installed capacity is utilized,3 it would correspond to a total installation of about 7,000 GW compared to about 740 GW that have been installed during the past 30 years. This is a large number in view of the ambitious time scale needed to decelerate global warming. Due to the intermittent nature of wind a comparison between electricity from wind and, e.g., coal is misleading. Having a large amount of coal stored nearby the coal-fired power plant this can produce electricity whenever there is demand. In principle it can produce almost always rated power, i.e., the power for which its steam turbine and generator have been designed for. Wind power and photo voltaic on the other hand have to be backed up by some storage or by some other kinds of energy source that is available when wind and/or sun are not available. This is a special topic by itself not addressed in this book.

1.2

Concepts to Utilize Wind Energy

There is a long history of using wind energy for various purposes and with various concepts. A good overview is provided in Manwell et al. (2009) and Hau (2014). In this section we describe some basics of wind energy to then quickly arrive at three bladed turbines that we see now in many countries of the world. 2 Somewhat lower figures for wind power are provided for instance by the Global Wind Energy

Council (Sawyer 2016). 3 An average capacity factor of 0.45 has been assumed. This is too high for present day turbines

but taking into account the present trends it might be realistic. The capacity factor is explained in Sect. 2.10.

1.2

Concepts to Utilize Wind Energy

3

Fig. 1.1 Lift and drag on a body with arbitrary (here elliptical) shape

Force Lift

Wind speed v Drag

A moving fluid (air) will generate a force on a body (blade). If both air and blade move, the relative speed is relevant. In general the force will not be aligned with the direction of the relative speed. It can be decomposed into a component parallel to the relative speed, called drag, and a component perpendicular to the relative speed, called lift. The value of lift and drag and their ratio depends on the relative speed, the geometry of the body and the angle between speed v and the (main axis of the) body, the angle of attack. Only if we have symmetry with respect to reflection across the direction of v, lift is zero. A sphere would be such an object that produces no lift since it is symmetric with respect to any direction. In Fig. 1.1 the situation is depicted for two dimensions. The basic concept can be easily extended to three dimensions. Wind turbines can utilize drag or lift for operation, i.e., make the rotor turn dominantly by drag or lift force. They have a vertical or a horizontal axis of rotation. The operating principles of these turbines will be treated in the following sections. Almost all utility-scale wind turbines on the other hand have three blades, are of lift-type and have a (nearly) horizontal axis. What is the reason why all other concepts have not been successful when it comes to large wind turbines? In the next two section we will answer this question.

1.2.1

Drag-Type Wind Turbines

The archetype of a wind turbine operating on drag is the cup rotor. This is probably the most intuitive way to use wind to generate torque and rotation. It is based on our experience that wind generates a force on ourselves. Due to the shape of the cup the drag is direction dependent. This can be used to operate a rotor. Naturally the axis will be chosen vertical since then the rotor works for any wind direction as wind is approximately horizontal. A pictorial demonstration is given in Fig. 1.2, which provides a top view. The cup rotor generates a torque M = (D1 − D2 ) r as long as D1 > D2 (where D1 and D2 refers to drag forces and r to the radius, i.e., the distance from the rotational axis to the center of the cup). The open cup (D1 ) propels the rotor while the other cup (D2 ) hinders the movement. There will be a starting torque at speed zero. But obviously the cups cannot move faster than v, because then the drag on the propelling cup becomes zero. The power generated by the rotor is given by P = M , where  is the angular velocity of the rotor.

4

1 Wind Energy Basics

Fig. 1.2 Schematic sketch of cup rotor with two arms. Axis of rotation perpendicular to drawing plane

Drag 1

Wind speed v

Axis +

Drag 2

This concept is simple and robust and, in fact, widely used for anemometers. However, calculations (that we do not present here) show that the extracted power in relation to the wind power hitting the rotor is small. Further on, the surface from which power is extracted—it can be called aerodynamically active area—needs to be covered by some material which for larger rotors leads to weight and structural problems. In conclusion, this would be too costly for large utility-scale wind turbines. There are many variants of the drag concept, like the Savonius rotor. Wind turbines based on these concepts are sometimes used for powering lighting at remote places or similar. The power of these devices is typically some Kilowatts. Large wind turbines based to the drag concept are not built.

1.2.2

Lift-Type Wind Turbines

Utilizing the lift force is less intuitive but more efficient. The reason is that the ratio between lift and drag can reach high values well above 100 for certain types of airfoils. The high lift-to-drag ratio is the basis of airborne flight as well of the lift-type wind turbines. Figure 1.3 shows essentials on airfoils. The chord is a straight line that connects leading and trailing edge. The incident flow makes an angle with respect to the direction of chord. This is the angle of attack called α. The aerodynamic force that is caused by the flow can be decomposed as shown already in Fig. 1.1 into lift and drag. For airfoils the lift-to-drag ratio becomes high for certain angles of attack. For airfoils used in wind turbines the angle of attack with highest lift-to-drag ratio is small, at about 5◦ or so (depending on the detailed shape of the airfoil). Lift-type wind turbines can work with horizontal or vertical axes. The ones with vertical axis are also called Darrieus wind turbines. Darrieus rotors have not made it to series production of utility-scale wind turbines. So they will not be treated in detail in this book. A reference on this type of wind turbines is, e.g., Paraschivoiu (2002). To understand the aerodynamic concept of the horizontal axis wind turbines consider a rotor with one blade pointing vertically upwards as shown in the model of Fig. 1.3.

1.2

Concepts to Utilize Wind Energy

Fig. 1.3 Force on airfoil at model of a wind turbine with blade cut at a certain distance from the axis of rotation pointing upwards

5 Force on airfoil Lift Direction of flow

Airfoil Drag

Direction of chord Trailing edge

Leading edge

Angle of attack α Speed of flow

Blades are cut at a certain radius to show the orientation of the airfoil with respect to the rotor plane. The rotor plane is an imaginary plane perpendicular to the axis of rotation. We assume that the rotor has a certain angular velocity  and the rotor plane is oriented perpendicular to the wind direction, i.e., the rotor is facing the wind. The axis of rotation is parallel to the wind direction. In nowadays wind turbines this is roughly but not strictly true. The axis of rotation is pointing slightly upwards. This so-called tilt angle is normally around 5◦ . The reason why it is not simply zero will be explained in Sect. 2.1. Figure 1.4 shows the situation of the blade at this section in the two-dimensional projection. Wind speed v is perpendicular to the rotor plane. There is a second component that the moving blade experiences as a result of the rotation speed at the radius r . This speed vt =  r is parallel to the rotor plane and in three-dimensions tangential to the circle with radius r with the center at the axis of rotation. In a self-consistent (and more correct) treatment of the flow near a rotor blade, one has to introduce induction factors. For the sake of simplicity this is omitted here. In Fig. 1.4 the variable w is the relative flow speed, the vector sum of wind speed v and tangential speed vt . In contrast to Fig. 1.3 there is an additional angle between the chord line and the direction defined by the rotor plane. This angle is denoted by β in Fig. 1.4 and called local pitch (or twist) angle of the airfoil at radius r under consideration.

6

1 Wind Energy Basics

Fig. 1.4 Incident flow at airfoil and resulting forces. Decomposition in thrust and propulsion with respect to rotor plane

The aerodynamic force can be projected onto directions parallel and perpendicular to the rotor plane. Essential for making the rotor turn is the component parallel to the rotor plane, called propulsion in in Fig. 1.4. In order to generate a torque to maintain the rotation, the propulsion must point to the right in Fig. 1.4. In order to get a good performance, it should be as large as possible. The perpendicular component, the thrust, is not useful but can not be avoided. The thrust on the rotor blades is the crucial force sizing the blade root and eventually the lower part of the tower. As can be concluded from the previous remarks and Fig. 1.4 a high lift-to-drag ratio is important for large propulsion. If drag exceeds a certain limit at a given lift value, the propulsion vanishes or becomes negative such that the concept of the lift-type wind turbines fails. By turning (pitching) the complete blade around its longitudinal axis this can be used to regulate torque and power to a desired value, e.g., at high wind speeds or to stop the rotor. The relative wind speed w and its direction with respect to the rotor plane depends on and increases linearly with radius r . For a good performance the local pitch angle β has to be chosen in the design of the blade such that up to a certain wind speed the angle of attack at each radius operates at the maximum lift-to-drag ratio. Consequently, rotor blades have a twisted shape. This is further addressed in Sect. 2.3.

1.2.3

Power Performance of Different Concepts

To address power performance, let us consider an imaginary surface with area A that is oriented perpendicular to the wind direction, e.g., a circle. Air flowing through the circle has kinetic energy. The kinetic energy per unit of time, i.e., the wind power P, streaming through the circle is given by  P = A v3, (1.1) 2

1.2

Concepts to Utilize Wind Energy

7

where  is the air density. The value for the air density at sea level and 15 ◦ C temperature is  = 1.225 kg/m3 . For a wind speed of 10 m/s at which modern wind turbines start to generate rated power, the power per unit area is about 600 W/m2 . This means, if we want to design a wind turbine with access to power in the range of MWs, we need to use a rotor area of several thousand m2 . The important question is, how much of this power can be extracted from the wind and converted into electrical energy. The relevant quantity is the power coefficient c P defined as the ratio of the power supplied by the wind turbine and the wind power due to its kinetic energy. There is a relatively simple and famous way to determine an upper limit of c P , which is the actuator disk theory sometimes also called Betz theory.4 The result is that under certain assumptions the upper limit of c P is 16/27 or roughly 0.59. So for the example given above we would obtain a maximum power of roughly 360 W/m2 . In the previous sections we have discussed drag- and lift-type wind turbines. Drag-type wind turbines have a relatively low c P , much lower than the limit. Together with the fact that the surface where the wind power is extracted needs to be covered by material which would lead to extremely heavy and expensive structures for large rotors this concept can not be used for utility-scale wind turbines. Going beyond the simple actuator disc model, wind turbines are realized with vertical or horizontal rotor axes. In general the lift-type concepts (both horizontal and vertical axis) have better power performance than the ones operating on drag (vertical axis). Lift-type vertical axis wind turbines can reach values for c P of about 0.3, while horizontal axis wind turbines reach much higher values. In this case maximum c P values of slightly above 0.5 are possible, which are not so far from the Betz limit. As mentioned in Sect. 1.2.2 airfoils at each position of the blade (or radius measured from the axis of rotation) has to operate at high lift-to-drag ratio or, in other words, the angle of attack must have a certain value. For typical airfoils this value is roughly at 5◦ . One consequence of this is that the chord direction changes along the blade such that the blade has a twisted shape. However, also the chord length of the blade is changing, i.e., decreasing with increasing radius. This, however, requires more theoretical insight as it is an extension of Betz’ approach taking into account effects of rotation and radial dependence. Drag and lift-type concepts described here have one feature in common. A rotating device (simply called rotor) extracts power from wind. In principle, also a linear motion could be used. In fact, linear motion is used in novel ideas for wind power plants with kites, see Weber (2021). We do not treat this topic further in this book.

4 Many text books contain good accounts of the actuator disk theory, e.g., Hau (2014).

8

1.3

1 Wind Energy Basics

The c P (λ) Curve

So far we have only mentioned a c P value, which in fact is the maximum value of a more general c P (λ) curve classifying the rotor. An important quantity in the context of the power performance of a wind rotor is the tip-spead ratio. This quantity is often denoted by λ and defined as vtip · R λ·v λ= = →= (1.2) v v R wherein  denotes the angular velocity of the rotor, R rotor radius, and v the ambient wind speed (before kinetic energy is extracted by the rotor). The quantity  usually expressed in units of 1/s is related to the rotor rotational speed N by  = N π/30 often expressed in units of rounds per minute (rpm). For the common three-bladed wind turbines it is obvious what tip speed means. For a drag type wind turbine like sketched in Fig. 1.2 one can take the speed at the outer end of the cup. The rotor can actually operate at any λ which constitutes the c P (λ) curve. An example for such a curve for the rotor blades of one of the turbines we use in this book is shown in Fig. 1.5. Qualitatively the c P curves for other rotors are similar. They start from zero (no rotation no power) have a maximum that depends on the concept and then decrease again. For λ too large there must be a point where the rotor cannot generate power anymore and for even larger λ the power would be negative. Using this λ one can directly calculate the speed a rotor would reach in case it can turn freely. Somewhere in between is the maximum c Pmax . The respective tip speed ratio for maximum value is denoted as design tip-speed ratio λ D for the rotor, i.e., c Pmax = c P (λ D ). The maximum at λ D  9.5 in the Fig. 1.5 is quite common for modern wind turbines. In many textbooks the maximum for three-bladed rotors is still at lower λ values which is actually outdated. The design tip-speed ratio is related to the rotor solidity, defined as

Fig. 1.5 Power coefficient c P as a function of tip speed ratio λ at pitch angle θ = 0◦

1.5 The Power Curve

9

the projected area of all blades in relation to the total rotor area. As rotor blades are much slimmer now than some decades earlier (see also Sect. 2.10) the design tip-speed ratio has increased up to values of about 10. For more details we refer to Hau (2014).

1.4

Electrical Energy from Wind

The rotor can be connected to a rotating electrical machine in order to generate electric power. Electric machines in principle can work as a motor generating torque or as a generator absorbing torque or, in other words, generating a negative torque (brake torque). If for instance at a given wind speed the rotor operates at a certain c P it generates the torque M = P/ , where  is the angular velocity. If the electrical machine, now working as a generator, is able to generate the torque with the same magnitude, the system will operate at constant speed. Seen from the mechanical point, the generator works as a brake to keep the speed of rotation at a certain value. Without the generator torque the rotor would speed up further to a speed that is in accordance with c P (λ) curve explained in Sect. 2.6.1. For production of electricity an aerodynamic rotor connected to a generator is sufficient. Such a machine is called direct drive or sometimes gearless wind turbine. About a quarter of all wind turbines worlwide is of this type. Alternatively, many models use a gearbox situated between rotor and generator in the main drive train. The reason for this is that the rotor speed is slow, e.g., 15 rpm. With such a slow speed one needs a special generator design with higher number of so-called pole pairs. Hence, those direct-drive generators are large and heavy. In order to use a standard high-speed machine, designed, e.g., for 1500 rpm, one needs a suitable gearbox, in this particular example with a gear ratio of 100. In order to achieve this ratio one needs several gear stages. One can conclude that in wind turbines either one has to live with a complex and expensive generator (gearless) or with a complex and expensive gearbox (geared system).

1.5

The Power Curve

The power curve is the electrical power generated by the wind turbine as a function of wind speed. For all wind turbines that generate electrical energy there is a generic form of this curve. Key quantity is the power rating (or rated power) of the system PR . The generator will have a certain rated power mentioned on its type plate. If the sizing of other components is done according to modern engineering standards, effectively also many other component will be sized for the same power level. Depending on the size and the performance of the rotor at a certain wind speed the rated power will be reached. This is called rated wind (speed) or v R . The increase of power to that point will roughly be ∝ v 3 . For higher wind speeds the power has to be limited to PR . There are different concepts to limit or, as it is also called, regulate the power. In all modern utility-scale wind turbines this is done by pitching the blades and, thus, tuning the rotor to a lower c P level.

10

1 Wind Energy Basics

As an example we want to show the power curve of a 2 MW wind turbine, used in this book as an example. With its rotor we can reach c P = 0.5. Some of this mechanical power is lost on the way to the electrical grid. Mechanical and electrical losses amount to about 10% of the power extracted. Hence, for modern wind turbines we can assume c P,e = 0.45 for the ratio of electrical power supplied to the grid to wind power flowing through the rotor, where the suffix e stands for electrical. As a result a simple form of the power curve is given by   0.45 A v 3 v < v R 2 P(v) = (1.3) PR v ≥ vR For wind speeds v < 3 m/s the wind turbines do not feed power into grid. The power of the wind is quite low for these wind speeds and losses in the system are of the same magnitude. The speed when power production starts is called cut-in wind speed. At the other end, for high wind speeds, one stops the turbine when the wind speed exceeds 25 m/s. This is called the cut-out speed. In terms of power rating (in W), air density (in kg/m3 ) and rotor area (in m2 ) the rated wind speed (in m/s) is (approximately) given by  vR =

PR 0.225 A 

1/3 .

(1.4)

where we have used the value 0.45 for c P,e again. A power curve obtained with the above equation for the wind turbine with 93 m rotor and 2 MW rated power is shown in Fig. 1.6.

Fig. 1.6 Power curve of wind turbine with 2 MW rated power

1.5 The Power Curve

11

The power curve of other turbines, even for drag-type concepts, would look similar, the quantitative form depending on PR , c P , rotor area A and air density ρ. Most of these quantities we have in our hand. Only with the air density it is different. For instance at an altitude of 1000 m the air density is about 10% lower than at sea level with a significant impact on the power curve. In reality the situation is more complex. One reason is wind turbulence. Due to this below v R the power curve is typically above the simple form because positive gusts increase power more than negative gusts reduce it. On the hand near v R the power curve is below the simple form because positive gusts are regulated to rated power.

2

Modern Wind Turbines

Compared to early times of utilizing wind energy, modern wind turbines have experienced quite some changes. Basic elements are still present: Some device for capturing the wind (blades), a building (tower) and a foundation. But driven by structural integrity and costs modern wind turbines nowadays look different. Also the purpose has changed in time. Previously utilized mostly mechanical for pumping water, grinding grain, driving machines via belts, etc., modern wind turbines have been producing electrical energy since the middle of the last century.

2.1

Geometry with Upwind Rotor

In a horizontal axis wind turbine (HAWT) the rotor can be upwind or downwind of the tower, in other words, with respect to the wind direction in front of or behind the tower. The downwind position has a number of advantages, but nevertheless all present utility-scale wind turbines have upwind rotor. The main disadvantage of the downwind position is the strong tower shadow. In the following we focus on the upwind position. When the turbine is operating there is a thrust force on the blades bending them towards the tower. In order to avoid that the tip of a blade hits the tower, the distance of the rotor plane and the tips needs to be sufficiently large. How can this distance be obtained? One way is to increase the horizontal distance between tower and rotor which is called the rotor overhang. If the overhang is large enough, one can leave the rotor vertical. This method was used in older wind turbines but it is clearly expensive as it includes more structural material to hold the rotor in the overhang distance. A smarter way to achieve a sufficiently large tip to tower distance (TTD) is to change slightly the overall geometry. The result is sketched in Fig. 2.1. Firstly, the rotor axis is tilted by a small amount. Tilt angles of present-day wind turbines are about 5◦ or somewhat above. In addition to the tilt angle larger turbines have a rotor cone angle, the blade is pointing © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 U. Ritschel and M. Beyer, Designing Wind Turbines, Synthesis Lectures on Renewable Energy Technologies, https://doi.org/10.1007/978-3-031-08549-9_2

13

14

2 Modern Wind Turbines

slightly into forward direction. The cone angle is about 3◦ relative to the rotor plane. The rotor plane itself is defined perpendicular to the rotor axis. Additional a pre-bending of the blade can be utilized to increase the distance, meaning that the blades are bent relative to the blade axis away from the tower, see Fig. 2.1.

Fig. 2.1 Illustration of geometrical changes to increase tip to tower distance

2.2

Components and Systems

2.2

15

Components and Systems

A wind turbine consists of numerous components and systems. Some are essential, some are more auxiliary items. The key components of the wind turbine are blades, components of the drive train like gearbox and generator, as well as tower and foundation. The outer components are depicted in Fig. 2.2. Some of the components are manufactured by the OEM and some are obtained from suppliers depending on the degree of vertical integration in the company. It is useful to distinguish between components and systems. Systems are assemblies of components with a particular purpose where some party needs to have the competence. Systems are always more than the sum of their components. Of course, the wind turbine is a system. Other important systems are • • • • •

Drive train—consisting of rotor, shaft, gearbox, etc. Pitch system—consisting of pitch bearing, drive, energy storage, etc. Yaw system—consisting of yaw bearing, drives and brakes Control system—consisting of sensors and controller hardware and software Safety system—stand-alone system to stop the turbine in case of danger

Fig. 2.2 Components of a wind turbine

16

2 Modern Wind Turbines

Generator Gearbox Main shaft Main bearing Hub

Spinner Electrical Cabinets Generator frame Machine frame Yaw system

Fig. 2.3 Components and systems inside the nacelle

Other systems are • Cooling systems for gearbox and generator—consisting of pumps, fans, valves, etc. The key components of the wind turbine are hub, machine frame, bearings, shaft, etc. They are illustrated in Fig. 2.3.

2.3

Rotor Blades

Rotor blades are one of the key components of the wind turbine. They should have good aerodynamic performance, i.e., yield a high value of the power coefficient. Other aspects of the aerodynamics are noise and performance in power regulation. Further, the mechanical properties of the blades are important. One objective is to make light rotor blades. Since the rotor needs to be held in some distance in front of the tower-top, a light rotor reduces loads on all subsequent structural parts including the foundation. Not only the static loads due to the weight of the components are important, but also the dynamic loads occurring due to structural vibrations are reduced by low mass and inertial moment of the rotor. In modern wind turbines of earlier days there were attempts using blades made from steel and aluminum. Nowadays blades are made from composite materials where fibers like glass and carbon fibers are enclosed by a suitable resin, the so-called fiber reinforced plastics (FRP). FRP are light and strong enough to design slim blades that withstand loads from wind, gravity, and vibrations. In recent years the design has become slimmer. The chord-

2.3

Rotor Blades

17

Edgewise loads Flapwise loads carrying carrying laminate laminate: spare cap

Shear web Adhesive joint

Adhesive joint

Flapwise loads carrying Sandwich laminate: spar cap

Edgewise loads carrying laminate

Fig. 2.4 Basic structural design of rotor blades

length relative to the blade length has become smaller. The power performance of the rotor does not suffer from this change. Only the rotor has to run faster with slimmer blades. Slim blades are lighter, but of course the structural stability needs to be ensured by a suitable structural design. The standard internal structure of a rotor blade is shown in Fig. 2.4 (see also Sing et al. 2013). The shell is manufactured in two molds. It consists of an upper and a lower part where the division is approximately the chord line of the airfoils. Two shear webs that go through most of the blade keep the upper and lower part in their distance when the blade is bent due to wind loads. Spar caps are located at and in between the shear webs. They are made of stronger material with fibers mainly in the direction of the blade axis such that the stress is in direction of the fibers. The spar caps in some blades are made from carbon fibers which have higher strength than glass fibers. The shear webs together with the spar caps build a spar that is responsible for the stability of the blade. In some rotor blades—depending on the preference of the blade manufacturer— the spar is made first and then as a module attached to the shells. Alternatively the webs are attached to the shell that contains the spar caps already in the laminate. The parts of the blade shell in front and behind the spar caps is made from sandwich material which is light and stiff such that the airfoil retains its original shape. Shells and webs are connected to each other by glue. The matrix material are resins like polyester or epoxy. In summary: • Rotor blades are mainly fabricated from glass fiber reinforced plastic (FRP) • Some larger blades contain also carbon fibers in highly loaded parts (box girder, spar cap) • Carbon fiber is much more expensive than glass fiber • Shell of the blade partly consists of sandwich material (FRP, Balsa wood/foam) • Matrix material is polyester or epoxy resin Concerning the manufacturing procedure two molds are needed for the upper and lower side of the blade. The challenge is now to obtain a material where fibers are distributed

18

2 Modern Wind Turbines

evenly in the resin for instance 60% fibers immersed in 40% resin. One way of fabrication is hand laminate, where the fibers are laid into the mold and resin is distributed by hand with rollers. However, the quality as judged for instance by the uniformity of the material is typically not satisfactory with this method. A solution would be to add more material to compensate the shortcomings of the production process. But this would lead to higher mass which one does not want. For these reasons, hand laminate is no longer used for large rotor blades. The approved method for the mass production of rotor blades is vacuum infusion. In this process, fiber material is also placed into a mold. The form lined with fiber mats is then covered by an airtight foil. The mold is designed in such a way that resin can also be fed to one side at certain distances through pipes or hoses. On the opposite side there are lines to which vacuum pumps are connected, which suck the air out of the space between the foil and the mold. At the same time, the resin is then sucked into the space and gradually fills the space between fibers with resin. Either polyester or epoxy is used as the resin. Resins are liquid at low temperatures and then harden when the mold is heated. A disadvantage of the blades produced in this way is that they are difficult to recycle. Resin or the matrix and fibers made from it are difficult to separate from one another. How should one deal with many thousands of rotor blades that can no longer be used after about 20 or 30 years? One possibility, if not very nice, is to burn the material that has been cut into smaller pieces. Another possibility is shredding everything and placing them in concrete as a filler, for example in road construction. However, real opportunities for recycling are being worked on and there are promising approaches for a sustainable solution. An important part of the blade is the blade connection, i.e., the connection between the blade and the hub or, more precisely, between the blade and the blade bearing—usually called the pitch bearing—where the pitch bearing is then attached to the hub. High loads have to be transmitted over the blade connection on a relatively small circle of bolts. As seen in Fig. 2.3, the blades are cylindrical on the hub side. This part is called the blade root. For example, a 60 m long blade has a diameter in the connection area or around 2 m. There are around 70–100 threaded rods on the circle, which must be firmly anchored in the laminate. Two solutions have been established for the connection between the threaded rods and the blade root. One works with laminated bushings that are provided with internal threads. On the outside, the bushings have a wavy shape, so that a solid connection with the laminate is created. The alternative to this is the connection with the help of T-bolts, as is also used in a similar way in furniture and is therefore also called IKEA connection. In the process, holes are drilled in the axial direction for the threaded rods in the blade root. At the end of each axial bore, additional holes are drilled in the transverse direction for the T-bolts into which the threaded rods are screwed. The blade root is then connected to the bearing ring with nuts on the rods. By pretensioning the rods, the blade is firmly connected to the bearing. Since the composite material and structures made from it is regarded as less reliable than for instance steel structures, testing plays an important role in rotor blades. From each new blade one specimen has to be put on a test rig in order to prove the stability under extreme

2.3

Rotor Blades

19

loading. Blade test facilities need a large hall and a solid concrete wall or block where the blade can be mounted in horizontal position. Now the blade is pulled vigorously at various load application points, so that is corresponds to the extreme loading that may occur at the wind turbine. According to some standards, a test for fatigue loads must also be carried out. There are no comparable requirements for components made of steel or cast iron. A cross section of a typical airfoil was provided in Fig. 1.3. The outer shape of the blade follows an aerodynamic airfoil similar to those used for air crafts, however, adjusted to the necessities of wind turbine requirements. Since the air flow is not uniquely distributed across the blade (due to different relative wind velocity as a function of radius) the angle of attack varies along the rotor blade. Therefore the rotor blade has an intrinsic twist, which is different from wings of airplanes. Wind flow is going from leading to trailing edge passing along the suction side (upper surface) and the pressure side (lower surface) in Fig. 1.3. Chord line and max thickness (usually given in per cent of cord length) define the relevant areas that lead to actual forces (lift and drag) acting onto the blade structure and make the blade rotate. Airfoils used for blades are usually asymmetric. A measure of asymmetry is the camber line which is the middle line between upper and lower surface. Due to the different relative wind speeds across the radius the chord length changes as well. For an optimal performance it is larger at root and smaller at tip. At root, usually for technical reasons and reasons of rather low relative wind speed at blade root and hub, most of the blades have a cylindrical blade root. This is seen in Fig. 2.5 (right). A different concept to treat the blade root following more the theoretical concept is realized by Enercon in the same Fig. 2.5 (left). Design and structural analysis of blades is a chapter by its own and will not be tackled further in this book. From wind turbine design point of view the blade can be considered as a component that can be purchased from a supplier. Although very relevant for the design

Fig. 2.5 Examples of cord length distribution: Wide cord length at root by Enercon (left), normal cord length distribution of a blade by Nordex (right)

20

2 Modern Wind Turbines

of the turbine it is yet not a turbine specific design element as is the hub or the main frame. Hence for further reading see, e.g., Hau (2014).

2.4

Towers

Three different tower concepts are used for wind turbines, each of which comes with a number of advantages and disadvantages, the tubular steel tower, the lattice tower, and the concrete tower, see Fig. 2.6. The majority of towers for wind turbines are tubular steel towers. The tubular steel tower basically consists of a steel tube with a certain wall thickness. The tube can be conical or partly cylindrical and partly conical. The base diameter normally is larger than the diameter at the tower top such that some part of tower needs to be conical. The tube is made from steel plates that are bent and attached to each other by horizontal and vertical welding seems. The tower is highly loaded—statically and dynamically—such that quality requirements on welding seems are high. The necessary quality is achieved by automatic or semi automatic procedure and close examination of the seems.

Fig. 2.6 Tower concepts for wind turbines, left to right: Tubular steel tower, lattice tower, and hybrid tower of reinforced concrete and steel

2.4 Towers

21

Just due to the large amount of material, the tower is one of the most expensive components of a wind turbine. The tubular steel tower for a multi-MW wind turbine can weigh several hundred tons. For example the tower of the 2 MW turbine for hub height of 80 m has a mass of about 170 t. As a consequence, mass optimization is a important part of the engineering process. It turns out that the wall thickness of the tower needed for stability becomes lower when the diameter is larger. This is true up to a diameter after that the wall would start to buckle. As a results of this, the towers of recently installed wind turbines are cylindrical up to about 2/3 or more of their total height and then end in a conical part. Since a tubular steel tower is too long and too heavy for transport on roads, for onshore wind turbines they have to be divided into sections. The typical tubular steel tower is divided into three or more sections each of which can be moved from a manufacturing plant to the site with special transport vehicles. The size of the sections (diameter, length, and mass) is determined by transport restrictions. The tower sections are connected by flanges with bolts. Flanges are thin steel rings that have to be machined with high precision and handled with great care to avoid plastic deformation. In the tower manufacturing plant they are welded to the tubular sections. Especially the diameter is important for the stability of the tower. At the bottom the bending moments are highest. A larger diameter would be desirable. This is where a drawback of the tubular steel towers becomes relevant. It is the transport restriction concerning the diameter. In general, during the transport bridges have to be passed which are usually not more than 4.5 m high. Since tower sections so far cannot the fabricated at the wind-farm site the bridges are so to say a hard limit for the tower base diameter. Alternatively to increase the tower-base stability one could use thicker steel plates for the wall. But also sheet thickness is limited to some centimeters since the plates have to be bent. Effectively there is a limit for the size and hub height of wind turbines on tubular steel towers—about 140 m rotor diameter and not more than 140 m hub height—above which other tower concepts are needed. Tubular steel towers for offshore wind turbines where the transport restrictions do not exist have a larger diameter and are made in one piece. A tower concept for which comparable limits do not exist is the lattice tower. It consists of main beams—in most cases four main beams—connected by a lattice of thinner steel beams that prevent buckling of the main beams, very similar to masts of electrical transmission lines. The beams are attached to each other by bolts. The lattice tower can be assembled from the individual beams or pre-assembled sections that are easy to transport on roads. Consequently, no limits for hub height and rotor size exist for the lattice towers. On the other hand assembly time is much longer than for tubular towers, the tip to tower distance is demanding, torsional stiffness has to be carefully observed as a low torsional stiffness might lead to operational problems. Further the bolts—many more than in the tubular steel tower—have to be maintained and last but not least many people do not accept lattice tower for aesthetic reasons. Under the bottom line, although the lattice tower has its advantages, only very few wind turbines are installed on it.

22

2 Modern Wind Turbines

While the two tower concepts mentioned use steel as structural material the third concept is based on concrete. Concrete has the special feature that it can tolerate large pressure but little or no tensile stress. Like in bridges or tall buildings one can pre stress the concrete such that the concrete remains under pressure even when for instance a large bending moment at the tower bottom of the wind turbine would cause tensile stress at the wall. Further, concrete has good vibrations damping properties, i.e., higher structural damping than steel. For smaller wind turbines spun-concrete masts were quite popular for a while. More recently concrete towers became popular to overcome the limitations of the tubular steel tower but still provide the outside appearance of the latter. One can use in-situ concrete. But for many towers prefabricated elements are used which have a size suitable for transportation. The towers can be completely made from concrete. But very popular recently have been the so-called hybrid towers that have a concrete lower part up to a third or half of the total height and a tubular steel part on top. Again a restriction for the base diameter made from in-situ concrete or pre-fabricated parts does not exist such that the concept is suitable for large rotors and hub heights. New wind turbines with hybrid towers look more or less like tubular steel towers from outside where the concrete part is painted with the same color as the steel part. Only those who know the diameter limitation of steel tubes recognize the larger width of the tower base. Concerning stability the properties of concrete are different from steel. Pure concrete without steel reinforcement tolerates pressure but must not be loaded by tension stress. Especially the loading and sizing of the lower part of the tower is determined by the thrust on the rotor. In the tower wall a bending moment grows from top to bottom which leads to pressure at the downwind and to tension at the upwind side. For large heavily loaded structures it is not sufficient to include steel reinforcement into the wall. The tower needs to be pre-stressed in the vertical direction such that even when large bending moments occur the concrete always remains under pressure. This is achieved by steel strands that go from the bottom of the tower to the top of the concrete part. In some towers the steel strands are guided in channels inside the wall, in others the strands lie at the wall inside the tower. An important part in this concept is the component at the top end of the concrete part where the steel strands are connected and an adapter for the steel flange is provided where a steel tube for the uppermost part can be fixed. Although the hybrid tower is more expensive than a tubular steel tower, for large wind turbines with hub heights of 150 or more meters, it is regarded as the best choice by many project developers. As any other building wind turbines need a foundation. Unless in many smaller buildings the foundation is not sized by the weight of the structure on top but again by the bending moment. The so-called slab foundations are large and heavy plates of reinforced concrete that are sized in such a way that even the extreme thrust values cannot overturn the turbine. If the soil is stable enough the large slab is sufficient. If the soil is too soft one uses vertical piles beneath the slab that go down to more stable layers below the surface.

2.4 Towers

23

Fig. 2.7 Different type of foundation anchors for tower, insert or base can (left) anchor bolts (right)

Fig. 2.8 Construction of foundation with anchor bolts (top left to bottom right)

Between the tower and the concrete slab a reliable connection is needed. Here we discuss the connection to the tubular steel tower. There are two different types of connection used in wind industry. A sketch of both concepts in provided in Fig. 2.7. One is the connection with long anchor bolts shown in Fig. 2.7 (right side). The anchor bolts are extending from the bottom flange of the tower to a ring beneath the foundation. The bolts are not immersed in the concrete but separated from it by plastic sheaths. By

24

2 Modern Wind Turbines

stressing the bolts the foundation is clamped between tower base and the ring at the bottom. A realization is shown in Fig. 2.8. Another type of connection is the steel insert (or base can) shown in Fig. 2.7 (left side). It is a short steel tube that is set into concrete. It has a flange on top to which the base part of the tower can be attached.

2.5

Power Generation System

The generator converts mechanical to electrical power. The essential process for this conversion takes place in the air gap between generator rotor and stator. The relevant physical quantity is the torque in the air gap, where the mechanical and the electrical pictures match. On one hand, torque can be expressed in terms of mechanical quantities coming form the aerodynamic rotor of the wind turbine and, on the other hand, it can be expressed in terms of electrical quantities such as currents and voltage acting in the cabling and power electronics. Since we are focusing mostly on mechanical aspects of wind energy in this book, we refer to details of the electrical machines to Heier (2014). Various concepts for electrical machines that can be used in wind turbines are available. Widely used are synchronous and induction machines generating 3-phase alternating current (AC). Induction machines are also called asynchronous ones. They basically differ by realization of circuitry in the rotor, whereas the circuitry of the stator is alike. Synchronous and asynchronous refer to the rotational speed of the generator rotor in relation to the rotation of the magnetic field from AC in the stator of the generator. The rotor is synchronous, if it is rotating with the same speed and asynchronous, if its speed is (slightly) different. If the stator is directly coupled to an electrical grid, the rotational speed is determined by the grid frequency. In modern wind turbines, however, the rotation is freed from this limitation by using converters in one or another way. To run the wind turbine with adjustable or, as it is called often, variable speed is very important as we show in Sect. 2.6.1. To demonstrate, on a basic level, how variable speed works, we utilize the concept of a permanently excited synchronous generator. Magnets are attached to the generator rotor in a small distance (air gap) from stator coils. While turning the rotor voltage is induced in the stator coils with a frequency proportional to the rotational speed. Connecting this machine to the grid and feeding in electricity with the proper frequency (50 Hz or 60 Hz depending on the country) becomes possible with a converter. The converter consists of a rectifier or generator-side converter producing direct current (DC) from AC. Then another converter— normally called inverter–converts DC to AC with grid frequency. The connection between the converters is called the DC link. Thus, effectively we have an AC to AC converter that converts electrical current from any frequency to grid frequency. One can also state it in a slightly different way: The stator is still connected to the grid but the AC frequency is changed on its way to a value that is good for operating the aerodynamic rotor.

2.5

Power Generation System

25

The converter is not only good for changing frequencies. Even more important is that the torque can be controlled. With torque control it is possible to run the turbine at a speed with maximum power coefficient. In Fig. 2.9 the two concepts used for wind turbines are shown. The two upper panels correspond to a complete decoupling of grid frequency as described above. This is used for mechanical drive trains with gearbox (upper panel) and for direct drives (middle panel). The generator is connected to the so-called fully-fed converter. The power flow is simple. It goes from the generator through the converter to grid. The converter has to be sized for rated power. The lower panel is referred to as doubly feed induction generator (DFIG) concept. In this case the frequency of the rotor current is adjusted (i.e, detached from grid frequency) by a converter and the stator is still connected to grid. The rotor needs to be connected in this case by slip rings to the outside. The advantage of this concept is that the converter needs not to be sized for the full power rating. In existing machines the rating is about 30% of the system rating. Cost and power rating are roughly proportional. The speed range of the doubly-fed system is limited. It is related to the share of power that runs through the converter. A vital part of the power generation system is the transformer. Wind turbine generators often work on a voltage level of about 700 V. Some large machines also work at somewhat higher voltages. Also the converter is designed for the same level. But that voltage is not suitable for transferring the electrical energy over large distances. The reason is that the necessary cable cross sections would be too high. Thus, each wind turbine has a transformer to step up to a level of 30 kV for instance. As indicated in Fig. 2.9, the transformer is between converter and grid. From the design point of view the location and integration of all these components is relevant. The generator is the interface between mechanical and electrical engineering. The mechanical aspects like integration of the generator in the drive train design are described in Sect. 4. The task is quite different for geared and gearless systems. How about converter and transformer? In many wind turbines the converter is at the bottom of the tower. Accordingly, also the transformer is at the tower bottom or even in a separate housing outside the tower. The advantage of this concept is, that the access to converter and transformer for service and repair is easy (in case of land based wind turbines). The mass of these components does not contribute to the nacelle mass. On the other hand, the low voltage level extends from nacelle to tower bottom. For high towers and large power ratings this implies a large amount of copper in the cables that connect downwards. As a consequence, there is a trend to integrate the complete power generation system with transformer into the nacelle. To provide the complete picture we mention that wind turbines with generator directly connected to the grid have been built also. Until about 20 years ago these machines were quite successful due to their simplicity and lower cost. Many of those wind turbines have a concept with two rated speeds (one low, one high), as if there were two generators in the wind turbine. The relevance of this will be explained in the next Sect. 2.6. In fact, this concept is realized by switching the number of active pole pairs that are contained in one housing.

26

2 Modern Wind Turbines

Rotor

Converter

Smoothing inductor

Grid

Wind Gear box

3-phase line Generator DC link

Filters

Transformer

Rotor

Converter

Smoothing inductor

Grid

Wind Generator

3-phase line DC link

Filters

Transformer

Rotor Converter

DC link

Smoothing inductor Grid

Wind 3-phase line Gear box

Transformer

Generator

Fig. 2.9 Layout of full converter with gearbox (upper) and w/o (middle) as well as double feed induction generator (lower)

2.6

Essential Control Concepts

27

Another solution, is “stretching” the limits for slippage from few percent to few 10% by what is called “dynamic slip control” for induction machines, see, e.g., Heier (2014).

2.6

Essential Control Concepts

Modern wind turbines are operating automatically, i.e., align with the wind direction, run at the right rotor speed, pitch the blades to the proper angle. They are controlled by microprocessors like many other machines. In the following sections a brief account of operation and control concepts is given. For more details we refer to Burton et al. (2021), Gasch et al. (2012). We distinguish control strategies below rated power (equivalent to below rated wind speed) and at rated power (equivalent to above rated wind speed). Below rated we aim at extracting as much power as possible. Above rated wind speed there is enough power in the wind and hence the task is to keep the mechanical power extracted by the wind turbine constant. This is called power regulation. In the following section we shall focus on variable speed control and power regulation by pitch. This concept is used by all utility-scale wind turbines fabricated during the last two decades. The control scheme to realize such a concept is sketched in Fig. 2.10. In this figure we identify two control loops, one for torque demand, i.e., torque control, and one for pitch angle demand, i.e., pitch control. In former wind turbines a concept with constant speed and power regulation by stall, i.e., onset of turbulence at rotor blades at certain wind speed, was used frequently. This will be briefly explicated at the end of this subsection.

2.6.1

Variable Speed Operation by Torque Control

The basis of variable speed control is the c P (λ) curve as explained in Sect. 1.3. In today’s turbines blades can be turned around their axis. This is called pitching. In Fig. 1.5 the curve is shown for a pitch angle θ = 0 only, which (normally) refers to the respective maximum c P value of the rotor. If the pitch angle θ of the rotor blades is changed we get a set of c P (θ, λ)

Fig. 2.10 Control scheme pitch and torque control, see text

Wind

Rotor Pitch angle

Torque Power

Generator Converter Torque demand

Pitch actuator

Pitch demand

Speed transducer Controller

Speed

Power

28

2 Modern Wind Turbines

curves with lower maxima as θ deviates from zero. The variation on the pitch angle will be used below in the context of power regulation. In this paragraph we focus on the case θ = 0 only, or in other words optimal power performance. To appreciate the variable speed concept we calculate the mechanical power Pmech for θ = 0. With rotor area A and wind speed v the mechanical power that can be extracted from the wind is  (2.1) Pmech (λ) = c P (λ) Pwind = c P (λ) A v 3 . 2 It depends explicitly on the tip speed ratio λ. The result of this equation is a set of power curves for different wind speeds. These curves are shown in Fig. 2.11 as blue lines. Each blue line represents a certain wind speed v indicated in the figure. Since λ = vtip /v and vtip =  R, with the rotor radius R, for each wind speed v and rotor speed a λ can be determined to be used in Eq. 2.1 that lead to the blue set of curves. To get the respective power maximum for a given wind speed, the c P value has to be maximal. This is the case for the design tip speed ratio λ D which is constant for all wind speeds. In its turn, the rotor needs to be operated at a speed proportional to the wind speed, i.e., variable speed. In our particular example we find λ D = 9.5 and c P,max = 0.5 from Fig. 1.5. The curve connecting the respective power maxima is the green line in Fig. 2.11. The black vertical line refers to the rated rotor speed and its height (black dot) to the power set point, which is a bit higher than rated power (delivered to grid) to compensate for mechanical and electrical losses in the system.

3000 11m/s

Mechanical power in kW

2500 10m/s

2000

9m/s

1500

8m/s

1000

7m/s 500

0

6m/s 5m/s 4m/s 0

5

10 15 20 Rotor speed in rpm

25

30

Fig. 2.11 Mechanical power extracted from the wind as a function of generator speed for different wind speeds (blue), optimal power (green), inverter curve (red), rated speed (vertical black line), see text

2.6

Essential Control Concepts

29

The aim is to operate the wind turbine on this green line, as long as wind speed is low. How can we make the rotor to operate at λ D ? The method used in all modern wind turbines is torque control. The basic idea is as follows: We need to measure the rotational speed N of the rotor and read from Fig. 2.11 or, better, a table that contains this information, the respective maximum power that can be generated at this N . Then the power generation system is arranged such that is provides a generator torque T corresponding to T = P/ , where  = N π/30. In fact, the aerodynamic rotor may be at any N , not corresponding to λ D . In such a situation the aerodynamic torque is different from the counteracting generator torque. If N is too small (λ < λ D ) the aerodynamic torque is larger than the generator torque corresponding to this N and the rotor will speed up. Conversely, if N is too high, the aerodynamic torque is smaller and the rotor will slow down. With this concept we always end up at the point were aerodynamic and generator torque are the same and at the maximum c P if we have used the proper c P (λ) curve. This procedure is also called maximum power point tracking. Going back to Fig. 2.11, the variable speed concept allows the operational state to follow the green line, in this particular case, up to a wind speed of roughly 7.5 m/s, which is where rated rotor speed is reached. Since maxima of the blue set of curves that show the mechanical power are rather flat, the operational characteristic curve (power vs. speed or torque vs. speed) for the generator, sometimes called inverter curve allows some freedom to satisfy technical limitations such as technical inertness or physical limitations such as fluctuations due to turbulence. Hence the practical inverter curve (red dashed line in Fig. 2.11) can slightly deviate from exact optimal curve (green). To summarize, control philosophy below rated is as follows: Power transfer from wind to rotor depends on rotational speed. The generator is used as an electromagnetic brake to control rotational speed such that the power harvest is maximum. This way electrical power is produced. At a given time t the state of the turbine is defined by the rotational speed. Using measured rotational speed the brake torque of the generator is chosen such that the power taken form the wind is maximum. This is realized by so called “inverter curve” based on the conclusions from Fig. 2.11. The inverter curve reflects the properties of the aerodynamic rotor. Task of the electrical layout is that the inverter curve can be realized by the generator/inverter system.

2.6.2

Power Regulation by Blade Pitch

Above rated wind speed the rotor can generate more than rated power. One has to limit or regulate the power to rated. One possibility to do so is to pitch the blade and hence change the angle of attack and therefore local lift and drag coefficients of the blade airfoils and eventually the effective c P value of the rotor. This leads to limitation of mechanical power and can be used to keep the power at rated. This is reflected in Fig. 2.12 sometimes called

30

2 Modern Wind Turbines

mussel diagram that shows the power contour as a function of pitch angle and wind speed. The diagram is calculated from the c P (θ, λ) curves at rated rotor speed for the 2 MW wind turbine of this book. To keep the mechanical power at 2 MW we find that at about v = 11 m/s the pitch angle has to increase, when the wind speed increases. Modern large wind turbines use blade pitch to limit the mechanical power. The control philosophy above rated is as follows: Since power transfer from wind to rotor depends on blade pitch angle at rated power, we use rotational speed of the rotor to control the pitch angle. This means that the rotor is kept at an (almost) fixed speed, the rated speed. Hence at a given time t the state of the turbine above rated is defined by rotational speed and blade pitch angle. Using measured rotational speed a subsequent pitch angle will be predicted (set point) that is capable to keep the rotational speed at rated rotor speed, compare Fig. 2.10. This procedure is realized by a “control equation” that relates the actual set point of pitch angle θsp (t) to the previous set point at earlier time t − t, where t is called sample time

Power contour in kW 00

10

60

00

4000

24

50

00

40

0

500

00

22

00

20

0

0 00

3

00

00

10

3000

16

−2

00

−1 4000

0

0

18

2000

Wind speed in m/s

20

0

0

300

200

14 0

12 2000

0

100

−1

00

0

10

00

1000

−20

8 6 4 −5

0

0

0

5

10

15

20

25

30

Pitch angle in deg

Fig. 2.12 Power contour for fixed rotor speed as a function of wind speed and pitch angle, so-called mussel diagram. Red dashed line indicates operational path of the wind turbine

2.8

Operational States

31

of the controller. A simple example of a control equation for a so called PI control, using proportional and integral terms, reads as follows, θsp (t) = θsp (t − t) + [K P ((t) − (t − t)) + K I (t)t] f (θ),

(2.2)

where parameters K P and K I are freely chosen to govern the proportional and integral gain. The speed error at time t is (t) = (t) − ref . The measured speed (t) is taken, e.g., at low speed shaft, and normally filtered in subsequent processing, see also Fig. 2.10. The reference speed ref can be rated speed or close to it. The factor f (θ) refers to non-linearity. Reason for this is that the power contour is steeper at high wind speed and high pitch angle compared to small pitch angle and rated wind speed. This can be seen from inspection of Fig. 2.12. The contour lines are getting closer for θ going from zero to larger values. In the simplest case the factor f (θ) can be chosen as a gain divisor f ∝ θ−1 beyond a reasonable minimum value for θ0  2◦ · · · 4◦ . In order to prevent drift and other effect of the control signals, real controllers are more complex, however, these essential control loops are the relevant features of any controller.

2.7

Alternative Power Regulation Concepts

Before the variable speed concept has been thoroughly established, also other methods for power regulation have been utilized. In contrast to variable speed concept, there is also a constant speed concept. Although the constant speed concept is rather outdated, still some wind turbines run with this concept. Using an induction generator and constant speed the mechanical power extracted from the wind is not optimal. This concept requires the generator to be connected to the grid directly. Since the grid has a fixed frequency so the generator has to have a nearly constant speed. At and above rated, another possibility to limit the power is by using the stall effect. This requires a particular design of the blades. The physics effect behind is as follows: If a certain wind speed is reached the (otherwise avoided) stall effect sets in and torque remains constant or nearly constant while thrust further increases. However, since from a certain size of the turbine the loads on the blades and the thrust on the turbine through stall effect is getting rather large, so that the structure becomes expensive, stall regulation is limited to wind turbines with maximum rated power of 1 · · · 1.5 MW.

2.8

Operational States

To operate a wind turbine automatically, safety for human, machine and environment requires installation of two independent systems: (1) Safety system, (2) Operating system. There is a hierarchy required for these systems, namely that operations of the safety system are superior to the operating system at all times.

32

2 Modern Wind Turbines

Operation of a wind turbine typically distinguishes between about seven major operational states. These are collected in Table 2.1. Idling is a state in case wind too high, v ≥ vout , or too low, v ≤ vin . Production is referred to as normal operation of the wind turbine between wind speeds of vin < v < vout . Stop is a normal stop and could come in several variants in terms of stop speed by controller or manual action. An Emergency stop is usually triggered by the safety chain, automatically or by pushing the emergency stop button. Start is a transient state running up the rotor and connecting the generator to the grid. Waiting for wind is a stationary state when wind speed is too low for start and too high for stop. Service is stationary state initiated manually to maintain the turbine. The task of the controller is to automatically switch between all those states, if not prohibited by the hierarchy requirements mentioned above. The latter is the case, if the turbine was stopped by the safety system. In this case the wind turbine has to be reset manually. To this end authorized service personnel has to visit the wind turbine site, check the reason for trip of safety chain, and reset the system manually. The purpose of control is engineering integrity of the wind turbine and all of its subsystems by control and protection mechanisms. In order to operate the wind turbine automatically quite some operational data are processed by the control unit. The control functions may govern or otherwise limit functions or parameters. To this end sensors are employed. Operational data are rotational speed, electrical power, wind speed, generator temperature monitoring, condition of the braking equipment, wind direction including alignment to the wind direction, grid voltage, grid frequency, faults in machinery components as well as faults in electrical components such as short circuit and overload to name a few.

Table 2.1 Major operational states of a wind turbine State

System

Idling

Brakes off, Generator off, Blades feathering, i.e., Blade angle: about 90◦

Production

Brakes off, Generator and blade angle: Controlled by Main Controller

Stop

Braking by blade pitch and generator

Emergency stop

Braking by blade pitch, generator off

Start

Brakes off, Generator off, Blade angle: On ramp to zero

Waiting for wind

Brakes off, Generator off, Blade angle: 45◦ · · · 70◦

Service

Rotor locked, brake on, blade service position

2.9 Why All Utility-Scale Wind Turbines Look Similar

2.8.1

33

Safety System

Basis of the safety system is a safety concept. The safety concept is a part of the system concept intended to ensure that in the event of a malfunction the wind turbine remains in a safe condition. If any malfunction occurs it is the task of the safety system to ensure that the installation behaves in accordance with the safety concept. Examples for safety concepts are: Single failure concept or redundancy concept. The safety system is the hardware and the logics realized in a particular turbine to follow the safety concept. It is intended to keep the installation in a safe condition and is logically super ordinate to the control system. The safety system has to be activated after safetyrelevant limiting values have been exceeded and/or if the control system is incapable of keeping the installation within the normal operating limits. Further on the safety system should not be overwritten at any time, neither manually nor automatically. The safety system has to be protected against unauthorized access. The safety system must be fail safe. Components have be doubled or otherwise constructed in a fashion that allows functioning under all circumstances. And last but not least, the safety system should superseed the controller. Safety relevant limiting values are related to operation parameters that are monitored during operation, i.e., rotational speed, power, short circuit in the electrical power system, mechanical shock, cable twisting, malfunctioning of control system, emergency push button released. In practice there are more relevant processes related to the safety of a wind turbine system. Those processes that are accompanying the development are: Health, Safety and Environment (HSE) process initiated in order to omit risks for people safety, environment, work conditions, etc. A Design Failure Mode and Effect Analysis (D-FMEA) is performed to omit possible failure risks related to design and choice of components and systems and last but not the least modern wind turbine design is governed by a Quality Management System (QMS) to ensure realization of all the concepts related to quality of the processes in the most general fashion, e.g., according to ISO 9001:2015.

2.9

Why All Utility-Scale Wind Turbines Look Similar

Most of the wind turbines installed nowadays are of lift-type and have nearly horizontal axis and three blades. Is this just lack of innovation in wind turbine manufacturers or is there a technical reason for this? In fact there are good reasons why almost all wind turbines use this configuration. Firstly, horizontal axis wind turbine (HAWT) with aerodynamically well designed rotor blades have a high power efficiency c P . The value c P max = 0.5 reached by this type can not be reached with other concepts so easily.

34

2 Modern Wind Turbines

Secondly, the rotor has a relatively small amount of aerodynamically inactive material. Only a small hub is needed to keep the blades in position. Only a small area near the center which is about 1% of the rotor disk is not active. This also means that with little amount of structural material (the hub) really long aerodynamically active elements (the blades) can be kept in their position. Thirdly, the high c P can be achieved with relatively little amount of material (blades) extracting wind power from a large area (rotor area). The area covered by the blades divided by the rotor area is called the solidity. Lower solidity is one of the main trends in blade design during the last 10 years or so. By using carbon fiber in highly loaded parts of the blades and/or optimizing structural concepts, the blades are not only much longer but also more slender than 10–20 years ago. The solidity of the rotors is decreasing. In this way specific costs of the rotor per area have been reduced. Solidity is the essential parameter for cost reduction. The question which number of blades is suitable, three, two, or even just one, is an old one. From the mechanical point of view there is a big difference between two and three blades. Technically speaking, the three-bladed rotor (also rotors with more blades) has the same inertia with respect to any axis perpendicular to the axis of rotation. This is not the case for the two-bladed rotor. Therefore three blades are dynamically more benign than two. This is most probably the main reason, why almost all wind turbines have three blades. Another reason for three instead of two blades is the tip speed. Using two blades with the same dimensions as for three baldes the solidity of the rotor is reduced and the tip speed has to go up. This causes more noise and poses an environmental challenge for wind turbines at least on land. In spite of this, there are the famous early two-bladed experimental wind turbines1 and also new proposals for wind turbines with two blades. In conclusion, the present day wind turbines are the result of a development over many decades. Three-bladed wind turbines with slim blades were favored by some pioneers as early as in the 40s of the last century (Hau 2014). So it is not straightforward to present a better concept. An alternative that also comes with high c P and low solidity is the lift-type VAWT. There are new attempts to push the vertical-axis type for offshore application. The most important advantage of the VAWT is that they operate on any wind direction while the HAWT the rotor axis has to be aligned to the wind direction. On the other hand, the structural parts needed to form the rotor with an area comparable to a HAWT become large and heavy. Structural dynamics will be an issue and it might be challenging to create an economic design for a large VAWT to compete with the present-day HAWT. A potential alternative that may come up in the future is kites. Stated in a simplified fashion the kite is a single blade held in aerodynamically position by cables. The power generation system is located on the ground. Compared to nowadays wind turbines the kites do not need a tower and foundation, and the nacelle must not be lifted on top of the tower. This means a lot of savings and potentially lower cost of energy. The drawback of the kite is the missing stability that is provided by the tower. The kite must not get off control, hit the 1 See Hau (2014) for more details.

2.10

Some Recent Wind-Industry Trends

35

ground and get damaged. A long-term (many years) automatic operation for a kite power plant has not yet been demonstrated.

2.10

Some Recent Wind-Industry Trends

During the past 20 years or so wind industry developed from small businesses to mature medium size companies. For example the number one OEM in the western world, Vestas from Denmark, has revenues of 17 Bill. USD and 30.000 employees in 2020. Since 2000 the revenues have grown by a factor of 17. In Denmark and Germany wind industry has become a major factor on the job market having created many thousands of well paid jobs. If it comes to technical trends, quite obviously, wind turbines have grown in size. In 2000 the typical wind turbine had a power rating of 2 MW and a rotor diameter of 80 m. Recent new types for installation on land have a power rating of 5 MW and a rotor diameter of 150 m. The growth in size comes along with a reduction of cost of energy which is the driving force behind this development. For the wind turbines of the 2020s there is a clear distinction between land-based machines and offshore models. New offshore models have a rotor diameter of 200 m and more and power ratings around 10 MW. These turbines are developed only for offshore wind farms while the the prototypes used for certification and tests is usually installed at a site on land where the logistic issues with large and heavy components can be solved.

2.10.1 Trend to Lower Specific Power A trend that is also very essential is the development of the specific power during these years. The specific power is the power rating of the wind turbine divided by the rotor area. If we calculate this quantity for the two land based models mentioned in Sect. 2.11 we obtain 400 W/m2 for the small one and 280 W/m2 for the large one. A lower specific power means a lower rated wind speed, in other words at a site with a certain mean wind speed the time span while rated power is generated is larger for lower specific power. A quantity that is important for any power plant is the capacity factor simply defined as the average power produced during the year divided by rated power. At a given site lower specific power leads to higher capacity factor which, in other words, means that the power plant is utilized to a higher degree. Sometimes it is stated that wind turbines are more efficient now than 20 or 30 years ago. But in the sense that thermal losses are lower or aerodynamic efficiency is higher this is not correct. It is simply that the capacity is higher and the electrical energy produced per MW installed is larger.

2.10.2 Cost of Energy What does this mean for the cost of energy? If a larger rotor is used the blades are more expensive, the investment becomes larger, and the cost of energy might be the same or

36

2 Modern Wind Turbines

even higher than before. This is not the case since the trend towards larger rotors and lower specific power has been accompanied by the development of slimmer blades. The cost of blades is largely determined by the amount of material that is used for the shell and the internal structure. Besides the length the other main dimension is the chord length which varies along the blade. An important result of the aerodynamic optimization is that one can reach a high power coefficient for different chord length functions. For smaller chord length the rotor speed or, in other words, the tip-speed ratio has to be higher at a certain wind speed. A smaller chord length with same relative thickness means that the blade will be lighter than the one with larger chord length. Of course, this cannot be continued beyond a certain limit which is set by the stability of the blade and its bending when subject to extreme loading. During the years the blade structural design has been optimized and better materials and manufacturing procedures are used such that blades are slimmer now. Longer and slimmer blades that lead to lower specific power used lead to higher capacity factor and lower cost of energy.

2.10.3 Trend to High Onshore Towers One important challenge for a wind turbine concerns the hub height. The hub height is mainly determined by the tower height. The mean wind speed increases with increasing height above ground. The vertical wind gradient depends on the roughness of the surface. For a smooth surface, e.g., above water the wind speed approaches its surface-independent value (the so-called geostrophic wind) more quickly than for rough surfaces, e.g., on land with trees or other objects that are obstacles for the air flow near the surface. As a result, the hub-height for offshore turbines relative to the rotor diameter is lower than for onshore installations. The lower speed near the surface also means higher turbulence which is also detrimental for wind turbines. Onshore attractive hub heights are equal or more than the rotor diameter which means 150 m and more for the wind turbine of the early 2020s. As mentioned in Sect. 2.4 for the popular tubular steel tower there is a limit due to logistic reasons. Sufficiently high towers can be built in form of lattice of concrete-steel hybrid towers. As lattice towers are considered unsightly by many people there has been a trend to high hybrid towers during recent years.

2.10.4 Other Trends There are a number of other important trends to be mentioned. Control of wind turbines has been improved considerably in recent years. Whereas 20 years ago transient operational states like starts and stops where running rough with wind turbine towers vibrating, these procedures are very smooth nowadays. Further improvements can be achieved by more sophisticated control procedures like individual blade pitch and using remote sensing methods with LiDAR for short-term prediction of the wind speed. All this leads to reduced

2.11 Wind Turbines Used for Illustration in This Book

37

vibrations and to lower loading of the structure. Since fatigue loading is often sizing the components of wind turbines, the improved controllers lead to lighter components and lower costs. Advanced digitization leads to more information about the machines and can be used for remote control. Supervisory control and data acquisition (SCADA) systems are implemented to organize the data for high level supervision. Another trend in wind industry is improved service for wind turbines. OEM are offering customers long-term warranty and service agreements. This mitigates the risk for the investors and increases the value created for the OEM.

2.10.5 Onshore and Offshore So far we have not much explicated on differences between wind turbines sites on land and those in the sea. To date all offshore wind turbines that are fabricated on an industrial scale have three blades and also otherwise use the same concepts as their counterparts on land. There is only a growing difference concerning the size of machines. The present day onshore turbines have a power rating of 5 MW while the offshore turbines have 10 MW. Further, while on land gearboxes in the main drive train with high-speed generators are the dominating technology, the trend for offshore the wind turbines is a gear-less drive train. For more details we refer to Ritschel (2020).

2.11

Wind Turbines Used for Illustration in This Book

The 2 MW wind turbine used for illustration in this book is a geared machine using the 3point concept for the rotor bearing. The turbine was developed in the years 2009–2010 and built under a license agreement by the Chinese company Geoho. The authors participated in the development project and have permission to use design documents. Further the authors have permission on a set of photographs illustrating the wind turbine components and the assembly process. Figure 2.13 shows a wind farm in China, wherein a total of almost 500 wind turbines of this type and variants thereof have been installed. The nacelle of this type of wind turbine is completely assembled before installation takes place in a wind farm. To this end, all the components and systems have to be delivered to the assembly hall of the OEM. A few components at different assembly stages are shown in Fig. 2.14.

38

2 Modern Wind Turbines

Fig. 2.13 Wind farm with 2 MW wind turbines of this book located in Inner Mongolia (upper), Overview of wind turbine as used for building permission processes (lower)

2.11 Wind Turbines Used for Illustration in This Book

39

Fig. 2.14 Components of wind turbine in assembly hall. Bearing housings and main shafts, gear boxes, main frame with drive train, hubs, details of main shaft flange, and pitch bearings (left to right, top to bottom)

40

2 Modern Wind Turbines

Fig. 2.15 3.4M116 direct drive wind turbine of the Estonian company Eleon a/s

The Eleon 3.4M116 is a direct drive turbine developed from 2010 to 2012. The prototype was installed in 2013. Some details of this turbine are used for illustration in this book. Figure 2.15 shows the prototype located on Saaremaa island of Estonia. Figure 2.16 provides some insight into the drive-train concept.

2.11 Wind Turbines Used for Illustration in This Book

Fig. 2.16 Direct drive nacelle of Eleon 3.4M116 wind turbine

41

3

Development Process and Requirements

In the early days wind turbines were developed by small teams in small companies. Often startups pursued new concepts, developed a prototype and tried to market their product. Nowadays there are still new ideas and startups in the wind industry but the mainstream is different. The industry is dominated by relatively large companies with thousands of employees and several hundred engineers. The product development process (PDP) in this case needs to be well structured. Many different departments of a company have to contribute and cooperate in an efficient manner. In a highly competitive environment the speed of product evolution has been important in recent years. Wind turbines are still growing in size. Digitalization is of growing importance. Consequently, the speed with which new types of wind turbines are brought to the market is pivotal to keep up with the competition. The basic ideas of the PDP as briefly described here originated from other industries like automotive and has been adapted to wind. Further in this section, we are looking at the engineering process as such, certification as an important additional task in the PDP and other requirements for the development. The topics discussed in this chapter could fill alone a book. The details are important in the industrial environment. However, in the present text we want to focus on the wind turbine technology as such, the design of the components and the strength analyses that are normally done by the OEMs. So the topics in this chapter are reported in rather concise form.

3.1

Modern Development Process

In an industrial company many different aspects have to be taken into account when a new product is developed. A number of different units like sales, manufacturing, service, engineering, procurement, finance/controlling and others have to contribute and cooperate. Systematic management and documentation of the process plays a key role. In large com© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 U. Ritschel and M. Beyer, Designing Wind Turbines, Synthesis Lectures on Renewable Energy Technologies, https://doi.org/10.1007/978-3-031-08549-9_3

43

44

3 Development Process and Requirements Planning Gate Reviews

Detailing

Concept 1

2

Production ramp-up

Prototype 3

4

Time Sales Procurement Engineering Production

Fig. 3.1 Matrix of product development process with horizontal lines for sub-processes and phases/gates. Symbols are explained in text

panies this is done in the framework of a structured product development process which consists of a time line with the tasks of different units. The time line is grouped in phases like planning phase, concept phase, design phase, prototype phase, etc. So step by step the future product becomes more detailed. Each of the partial processes has its time schedule and milestones. At some point in the process also external parties have to be involved like for example component suppliers and a certification body. Certification is an important part of the development process and will be discussed separately in Sect. 3.3. The duration of the process is up to five years depending on the novelty of the design in the context of the firm. Sometimes only a variant of a already existing turbine, sometimes a radically new design is developed. An important task for the project management is that activities are synchronized periodically in the course of the process. This can be done for instance in form of gate reviews after each phase. A scheme illustration the product development process is depicted in Fig. 3.1. In Fig. 3.1 the milestones are symbolized by the black triangles (contents is not further specified). The gate reviews after each phase are symbolized by the diamonds. During the final phases the prototype of the new turbine type is built and during the industrialization phase requirements for a series production are established. An important result of the engineering process is the bill of material which contains all components of the product from large main components to small bolts and nuts. Important in the project plan is the critical path. It is the sequence of tasks built upon each other that determine the overall duration of the projects. For wind turbines the critical path is often determined by main components and their delivery times. Since the main components often integrate many functions, it takes a great deal of time and work to obtain the final design. At the same time these components have a long order time. Such components are called long-lead items. For instance lead times for components made from cast iron (see Sect. 5.1.1) may be one year, rotor blades if not already existing may even take longer.

3.2

Engineering Process

45

Important documents resulting from the PDP are specifications. On the one side there are requirement specifications which contain the features of a system or a component. The requirement specification contains the loading of the system or component, the external conditions under which it has to function, and also for example standards according to which it must be developed. But it may also include requirements from other processes. In brief, the requirement specification says “what is needed.” Taking into account the requirements, on the other side the feature specification contains a description of the component or system and explains “how it is done”. The requirement specifications are mainly resulting from the engineering process. Addressees can be colleagues in the same company working on the same project or external suppliers.

3.2

Engineering Process

In the PDP process the result of the planning phase is a description of the key data of the wind turbine, rotor diameter, power rating and some basic concepts (see, e.g., Sect. 4). Early in the concept phase or even in the planning phase one starts with a first CAD design and load simulations. In the course of the engineering project then an iterative procedure follows. With the loads stability analyses for components are carried out. Further in the project input from the other processes has to be included. Based on this design changes have to be made which leads to a updated simulation model. Depending on the starting point, the degree of novelty of the systems, a certain number of iteration steps have to be made. The final result of this procedure is the design of the prototype, consisting of drawings of all parts, a bill of material, specifications and documents for certification (see Sect. 3.3). In fact, the typecertification procedure can start well before the final results are obtained. It is reasonable to certify the loads before the final stability analysis is done. Designing the wind turbine is part of the engineering process. Important tools here are: • A software tool for carrying out simulations. Modern simulation codes allow to model the complete wind turbine from rotor blades to foundation and the elasticity of the supporting soil. External forces by wind and (offshore) waves as well as structural vibrations are taken into account. The model can be used on the one hand to study and optimize the control procedures of the turbine in various operational situations. On the other hand the model allows to calculate sectional loads in blades, drive train, tower and the foundation. The simulated loads are the basis for the design. It is the only quantitative information that is available at this point. A simulation tool for this purpose is, e.g., Flex5 originally developed by Stig Øye at the DTU. • A tool for 3D computer-aided design. With modern design tools one can systematically develop the components in 3-dimensional fashion and assemble the system. The design can be made more and more detailed, starting with a rough geometry, adding step by step

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3 Development Process and Requirements

Fig. 3.2 Processes involved in virtual prototyping, see text

smaller details like bolt holes. From the design model drawings and also visualizations of the turbine or parts of it can be made. A tool for this purpose is, e.g., the commercial software Solidworks. • Stability analysis of components is another essential task in the engineering process. In general the stability has to be proven with respect to ultimate (also called extreme) loads and fatigue loads. For the stability analysis we need the geometry of the component, the material parameters like yield strength and the loading obtained by simulation. Nowadays main components are analyzed with finite-element methods. A software used for this purpose is, e.g., ANSYS which is widely used in wind industry. As shown in Fig. 3.2 the three basic tasks in the engineering process are simulation, design and analysis.

3.3

Standards, Guidelines, Certification

The engineering process takes place within the framework of standards and guidelines. Roughly speaking the standards contain rules that have to be obeyed but leave some freedom to create new technology. The guidelines partly fill the gaps that are left open by the standards. The idea behind the procedure is that the wind turbine developed should be safe in various respects. The turbine as such should not fail when used in the planned way and it should be possible to operate it without major repairs for 20 or 25 years up to wear parts that have to be replaced. Another aspect is personal safety. When people are servicing a wind turbine they should work in a safe environment. Most of the time wind turbines are operating without

3.3

Standards, Guidelines, Certification

47

people in the machine or nearby. But of course there is an overlap between personal safety and technical safety. The wind energy standards have a long history and have evolved in form of many revisions over the years. In the past national standards played an important role. An example is the Danish Standard DS472 that had to be followed for many wind turbine projects in the 80s and 90s of the last century when Denmark was the forerunner in wind energy utilization. During the past 20 years the international wind turbine standard IEC 61400 became more and more important. For many parts of the world and locations onshore and offshore wind turbines have to be developed according to the IEC standard in order to get a type certification and to become bankable. The IEC 61400 has recently been integrated into a new set of standard that goes under the name IEC RE for renewable energies. Important for the designer of a wind turbine are • • • •

IECRE OD-501 Edition 2.0 for type and component certification, IEC 61400-1 for land based wind turbines, IEC 61400-2 for small wind turbines, and IEC 61400-3 for offshore wind turbines.

The standards 61400-1 to 3 formulate design requirements in a relatively broad sense. For instance also wind conditions that one has to assume and load situations in which the wind turbine has to be studied are described. So-called wind turbine classes are defined on the basis of a certain range of wind speeds and turbulence intensities for which the turbine is designed. Further requirements concern the design of all parts of the wind turbine and the safety system. We are not going into the details here. The standard on design requirements is accompanied by many other more specific standards that are treating particular technologies. The initial document in the list above, superior to the others, concerns the certification process as such. The engineering team that designs the wind turbine is mainly involved in the type certification, i.e. a certain design (type) is checked by a second party, the certification body. The certification body needs an accreditation for this work. The type certification consists of several different actions. First of all the design is checked as long as the wind turbine only exists on the paper (or on the computer) in form of drawings, specifications, and reports on operation, control and safety, loads and stability analyses. After the design has been approved or during the assessment when one has sufficient confidence in ones own work, the prototype can be fabricated. Testing the prototype, evaluating the manufacturing process and the quality management are further steps of the type certification. As sketch of a possible flow chart, when development and certification go hand-in-hand is shown in Fig. 3.3. Very important are data from the prototype. It is the first time the developer has the real machine and not its virtual counterpart. In the measurement many quantities are recorded for some time, normally for one year or so. Recorded quantities are power, rotor speed, blade pitch angle, yaw angle, and others. Wind speed and direction are not recorded at the

WT concept and operation&control procedures

Examination of operation concept and loads

Synchro Load simulation

Examination of components

plan Component design & analyses

Prototype measurements

Examination of production process and quality management

Certification

3 Development Process and Requirements

Development

48

Type approval

Series production

Fig. 3.3 Synchronization between development process and certification process and sub processes

wind turbine but at a nearby met mast. Each wind turbine has devices to measure wind speed and direction on the roof of the nacelle but due to the influence of the rotor the data are too much distorted. With the data from the prototype and mast one can check the performance of the wind turbine. Very important are the measured power curve and the functionality of the controller. Also loads are measured. This is done indirectly with strain gauges. Strain gauges that are attached to the surface of structural parts like blades, shafts, tower and others allow to determine bending moments which, as stated in Chap. 2, are often important for the sizing of the components. In this way the measurement closes the cycle. Simulated data which up to this point are often called load assumptions are validated by the measurements. Project certification is sometimes done in larger wind farm projects. Almost all offshore wind farms go through a project certification. The purpose of this action is that a second party checks all assumptions and derived data that result from the project development. An example is the wind turbine as such. It has a certain type certification, loads have been calculated for specific site conditions. So one task for the project certification is to compare the simulated loads as basis for the design with the real loads that are expected for the given project. For offshore wind turbines it is even more difficult since waves also have an impact on the loading of the system. So to carry the load comparison is a part of the certification process. An important role is also played by guidelines. Guidelines are issued, e.g., by certification bodies or other associations. A guideline that has been used a lot in wind industry is the one of the former German Lloyd, often called the GL guideline (DNV GL 2010). Since many years the German Lloyd is accredited for the certification of wind turbines among other activities. In 2013 it merged with the Scandinavian certification body Det Norske Veritas.

3.4

Load Cases, Ultimate and Fatigue Loads

49

The GL guideline is more comprehensive than the IEC standard, something like a hand book for the development of wind turbines. In a wind turbine development project the guideline is often used in combination with the standard. In recent years personal safety become a more and more important topic. There is a certain overlap of issues between technical safety (which is required by IEC 61400) and personal safety. But there are also many other aspects of personal safety that are not touched upon by the IEC standard. There was a discussion over several years whether wind turbines are machines comparable to other industry products like vacuum cleaners or dish washers. Finally for the European Union it was decided that the wind turbine is a functional machine for which a declaration of conformity to the machine directive 2006/42/EC in combination with EN 50308 has to be stated. As a result the manufacturer can use the CE mark (according to Regulation (EC) No 765/2008). With the CE mark, the manufacturer confirms the conformity of the machine with regard to the applicable European, i.e., EU directives and the implementation of all conformity assessment procedures prescribed for the machine. The general requirements with a focus on the protection of employees are aimed at avoiding risks to health and safety. Among other things, the standard regulates door openings, access routes, floors, platforms and railings, ladders with fall protection, passage and work areas, attachment points and handles, lighting, protection of moving parts, protection against electrical hazards, as well as noise levels and thermal insulation. It also includes statements on climbing equipment, moving parts of protective and blocking devices, emergency stops, power shutdowns, fire protection and warning signs. It is true that EN 50308 is not a harmonized standard within the meaning of the Machinery Directive, i.e., there is no presumption of conformity. However, it claims to take in, if possible, the ways to access the wind turbine, the tower, the nacelle, and the hub should be easy to reach and working there should be comfortable as much as possible.

3.4

Load Cases, Ultimate and Fatigue Loads

In Sect. 3.2 we have mentioned the simulation model. The model is good to study the system in operating situations and get realistic results for the loads. The question here is: What kind of situations should be studied? To answer this question we have to consult the IEC standard. It contains a list of so-called design load cases (DLC) for which the model has to be analyzed. The list is divided in eight different groups which are named: 1. 2. 3. 4. 5. 6.

Power production Power production with occurrence of failure Start up Normal shut down Emergency shut down Parked

50 Fig. 3.4 Design loads emerge from simulation with certain design load cases based on guidelines as minimum requirement

3 Development Process and Requirements GL2010 IEC 61400- 1,2, or 3 Simulation model

Load cases

Design loads

7. Parked with occurrence of failure 8. Transport, assembly, maintenance and repair. The procedure is illustrated in Fig. 3.4. A simulation model has to be combined with the proper loads cases to arrive at the design loads for a particular wind turbine. We are not going into further details on the simulation model to load cases of the guideline here. About 20 years ago a complete set of load cases consisted of about 100 situations. Nowadays the number can be in the order of 10 Thousands. The result of the simulation are time series of loads and other relevant parameters of the turbine. Duration of one time series is in many cases 10 min with a temporal resolution of some milliseconds. Loads cases with transient events are sometimes shorter or longer depending on the situation. When we talk about loads here in this Section we mean the mechanical sectional loads that are sizing the components. Full information on loading of the structure is provided by six components, i.e., three forces and three moments. When we consider, e.g., a shaft we have the axial force and two offaxial components, and, in addition, we have the torsional moment and two bending moments. When these sectional loads are known, an analysis of the shaft can be done. Time series from simulation are analyzed with respect to extreme (or ultimate) and fatigue loads. What are ultimate loads? During its service life the wind turbine has to withstand extreme situations with high wind speeds, strong gusts, and (offshore) high waves. Extreme loads could also occur in other situations like certain failures during operation. An example is a grid or load loss during operation which should not lead to any damage. The philosophy concerning these events is that extreme situations that might occur with a frequency of once per 50 years should be survived by the system. So one can regard the ultimate-load situations as rare or even singular events during the service life of the turbine. The results of the analysis are presented in form of a matrix. In its diagonal the matrix contains the ultimate loads. Tables of this type are provided in Tables 3.1, 5.2, 6.4, and 7.2. There are two figures in each field, one for the upper and one for the lower ultimate value. Loads are in general not symmetric with respect to changing “+ sign” to “- sign”. The horizontal lines in the matrix contain the values of other components at the same time when the ultimate value occurred, called simultaneous components. The figures extracted from simulation have to be multiplied by safety factors. The motivation for these factors is that firstly we are working with a model and the reality will be somewhat different, and secondly the results depend to a certain extent on statistics. The turbulent wind in combination with

3.4

Load Cases, Ultimate and Fatigue Loads

51

Table 3.1 Extreme loads at hub center R rotating coordinate system. Safety factors included as indicated in column SF, see text

structural dynamics of the system leads to a stochastic time series of loads. To cover both effects a safety factor of 1.35 is used, 35% higher ultimate loads that simulated. This is the factor for “normal” load cases that may occur under normal operating conditions. One has to apply a factor of 1.1 for “abnormal” load cases which have a lower probability to occur. Additional information in the matrix on the left side shows the coding for the load case were the ultimate value occurred and in the right column the wind speed in this moment of time. The result for such a matrix is provided in Table 3.1. It was obtained for the 2 MW wind turbine and provides loads for the center of the hub. It is an important point in the model of the turbine that essentially consists of an elastic beam that connects the tower axis and the the blade axes. Loads at this point are used to size main shaft, machine frame, main bearing and its housing as well as the gearbox. The second analysis that needs to be done is with respect to fatigue loads. The varying loads occurring every day, sometimes with higher, sometimes with lower range of change. In this way damage is accumulating over the years. It has been mentioned already in the summary that wind turbines see a large number of such load fluctuations compared with other machines. So analysis with respect to fatigue is essential. At this point we have to dig a little deeper into this matter. From the simulation model we obtain loads. What we have to analyze to assess fatigue is stress (S) in materials. For relevant materials an S(N ) curve is available. It describes how many load cycles (number N ) can be tolerated before a crack appears. A crack can proceed quickly to total failure of a component. So cracks should not occur in any structural component over the service life time.

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3 Development Process and Requirements

A method to proceed from time series of loads to stress fluctuations and fatigue is as follows. For the component under investigation an FE model is used. Reference load cases are calculated with a certain value of the six load components (e.g. unit loads) which leads to a stress tensor for this specific situation. Then at points for which fatigue is presumably high the actual stress tensor, in fact, the time series for it, is obtained by linear superposition of the reference cases. From the time series fatigue damage can be obtained with the critical plain method, see, e.g, Barkey and Lee (2012). Fatigue can be obtained for a single time series. What we want is fatigue for the service life of the turbines. This sounds tedious but in fact it is straightforward. According to wind frequency and other input one can extrapolate the damage from a set of time series to the complete service life. The key property of fatigue is linear accumulation. The damage related to certain operating situation can be multiplied by a factor to take into account the total amount of time the turbine operates in this situation and damage values of different situations can be simply added. The additional input needed is the wind speed distribution for one year.

3.5

Vertical Integration of Manufacturing in Wind Industry

Vertical integration means that the whole supply chain or parts of it is owned by the OEM. The wind turbine consists of a multitude of large and small components, so it is not realistic to operate a company that is 100% vertically integrated. But by manufacturing blades, the tower, the generator of a direct-drive concept, and some more items, the share of the total value is quite high already. There are pros and cons on a high degree of vertical integration. The wind turbine is a highly complex system. Key components and systems contribute to the dynamics of the whole system and the response to controller activities. So the in-depth knowledge and understanding of all relevant parts of the system is important. There are also economic reasons pro and contra vertical integration. A compact overview of respective advantages is given in the list below. Level of vertical integration High Dependence on suppliers outside is low Value created in company is high

Low Need for capital for investment in manufacturing facilities is low Higher flexibility concerning fluctuating demand on the market

Quality assurance is easier

As in many cases there is no simple answer to the question about vertical integration. As a consequence there are examples among wind turbine OEMs with very different strategies in that respect. There has been a clear trend over the last two decades or so to higher

3.5 Vertical Integration of Manufacturing in Wind Industry

53

vertical integration in wind industry. In the earlier days until about the year 2000 many OEM only assembled the nacelle in a manufacturing facility. Virtually all parts needed for nacelle and hub were purchased from suppliers. Also blades were bought from independent blade manufacturers. Even the controller hardware and software, so to say the brains of the turbine, were sourced from specialized suppliers. As the market volume was growing the OEM realized that strong dependence on external suppliers, especially when it comes to the key components, can be dangerous. In a situation with growing demand on the market, e.g., a blade supplier effectively determines how many pieces the OEM can sell. As a consequence, many OEM started their own blade development and manufacturing activities. Also the controller, at least the controller software, was declared as a key competence by many OEM. Other key components are discussed in the following list: • Hub and machine frame are either made from cast iron (see Sect. 5) or in case of the machine frame sometimes also a welded design is used. Most OEM purchase these items from a foundry or a metal processing company. Most of these suppliers are independent. On the other hand the German OEM Enercon, well-known for high vertical integration, operates its own foundry. • There are large bearings as main, yaw and pitch bearings in the system. These bearings are fabricated with high precision, materials need special treatment, such that the bearings are normally sourced from independent suppliers • The gearbox in the main drive train (not in direct-drive concepts) is a highly sophisticated component. There is a number of gearbox manufacturers for whom wind gearboxes are not the only but an important market. Concerning selection, treatment and machining of materials know-how and precision machinery is needed. The gearbox plays an important role for the drive-train dynamics in interplay with other components like rotor blades and the generator. Further the loading of the gearbox, especially the off-axial loads, need to be known and communicated to the gearbox manufacturer. So a detailed requirement specification is needed and a close cooperation between supplier and OEM has to be established. • The generator of a geared wind turbine is a standard industrial product that in principle as a very similar design is also used (often as a motor) for other purposes. This is different for direct-drive turbines. In this case the generator is a multi-pole machine exclusively designed for wind turbines. The worldwide most successful OEM with directdrive concepts (Enercon from Germany and Goldwind from China) fabricate the generator in-house. But there are also direct-drive designs offered by electrical machinery manufactures like ABB and Siemens. • Inverters in the power generation system are also key components of nowadays utilityscale wind turbines. Often they are provided by the suppliers of the generator. The interplay between generator and inverter is controlled by the inverter software which in most turbines is not to the full extent running on the main controller. So it is reasonable that the system, generator, inverter, and other components, are from the same source.

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3 Development Process and Requirements

• The tower has a big impact on the structural dynamics. Vibration frequencies of the tower have to be tuned to values that are not in conflict with multiples of the rotor frequency. As a consequence, the tower is often developed by OEMs while it is fabricated by a specialized steel processing company (in case of a tubular steel tower). • Rotor blades and the main controller have been discussed briefly already above in this Section. Depending to a large extent on the policy of an OEM and to some extent also on the concepts, the strategies concerning vertical integration are very different. One strategy followed by most big players is that, if a component is not made in-house, there should at least be two or better three suppliers. This obviously reduces risk of dependencies.

3.6

Transport and Logistics

Components of wind turbines are large (rotor blades) and heavy (nacelle, tower sections), see Fig. 3.5. Therefore transport restrictions and logistic issues in this context have to be discussed. If we are mainly focusing on the design of the nacelle we are talking about the width, height and weight of the nacelle. But we are also commenting on the tower and rotor blades. Not many components can be fabricated near wind farm sites. Hence transport, often on roads, sometimes by other means, from factory to site needs to be planned. For the transport special purpose vehicles are needed and permissions from the authorities have to be asked for. Since otherwise conflict with normal traffic would be a problem, the vehicles are only moved during the night, accompanied by additional safety vehicles. On roads the bottle necks are bridges. There are standards for vertical clearance of bridges in different countries. On freeways, which are used for long distance, the clearance is 4.9 m (or 16 ft) in the US and 4.5 m in Germany. This means that the height of a component should not be much more than 4 m. The width might be larger but then it is an obstacle for other traffic. Mass is also an issue. Roads and bridges are designed for a certain load capacity. Nacelle and tower are the heaviest parts. To reduce mass and also length of individual items the tower is divided in sections (see Sect. 2.4). Also the nacelle of large wind turbines is often modularized such that individual modules are not exceeding a certain mass. The mass limit often depends on individual bridges, so that the transport route has to be planned carefully. If not width or mass, it might also be just length that has a transport limit. This is the case for rotor blades which are relatively light, even in large turbines not more than 20 t, but long. Length is in particular an issue in curves. A method to get rid of problems with length is blade division. If the rotor blades were designed in two or more sections (like the tower), this problem would be solved. There are many ideas for divided blades, but it is more challenging to make divided blades compared with tower sections. Thus, so far most blades are fabricated and transported in one piece. But whereas one thought some ten years

3.6 Transport and Logistics

55

Fig. 3.5 Components of a wind turbine during transport, nacelle (upper), tower sections (middle) and rotor blades, transport during nights only (lower)

ago a blade length of 60 m for wind farms on land would be the maximum, we see now blades with 70 m and more. So the length limit has been pushed further by using improved transport vehicles and planning the routes very carefully. The discussion in this section holds mainly for wind turbines on land. For offshore machines there are different rules to be described in the next section.

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3.7

3 Development Process and Requirements

Requirements for Offshore Wind Energy

Offshore wind turbines are different in some respects from the land-based cousins. One is the size of the turbines. So far in the evolution of wind turbines larger size always meant lower cost of energy. Provided that the large components like tower, blades and nacelle come from a factory near a suitable harbor, size and mass limits that come from transport on roads are absent. This is the main reason for the size difference between land based and offshore wind turbines. There are logistic challenges as well. Cranes in the harbor need to be suitable, quays have to be sized for heavy loads, ships and installation vessels must be available at reasonable cost and need to have sufficient capacity. For some time ago high cost of installation vessels slowed down the realization of projects. In recent years the cost of energy has come down to a competitive level. A second aspect is corrosion protection that concerns all parts of the wind turbine (and its substructure). Salty water and higher humidity are affecting the outside steel structures. Better corrosion protection of all exterior parts needs to be better than on land. For the tower shell this implies that coating needs to be thicker. It is slightly different to the interior of the turbine, where some of the components emit waste heat from the surface. In a closed system the air would heat up with time. The standard solution for wind turbines is that air from outside is used to replace the inside air. Offshore it is not possible to use the salty outside air directly, e.g., by simply leaving a hole in the nacelle cover (passive concept) or using a fan (active). Before the air can be used to cool the system humidity and salt need to be removed. The clean air then generates some over-pressure inside the turbine such that there is a controlled inlet and at some point, e.g., the gap between nacelle cover and tower and between spinner and nacelle cover the air leaves the interior. This air exchange concept has nothing to do with the cooling system for gearbox and generator responsible for removing most of the heat from the components, see Sect. 10.3. Another difference between on- and offshore concerns service work on the wind turbine. On sea accessibility of the turbine is much more difficult than on land. Access from a boat is only possible when the waves are not too high. Depending on the location the time windows for boat access might be small since wind speeds are generally higher offshore and high waves are frequent. The trend here clearly is to carry people by helicopter and drop them on a hoisting platform. The transportation and hoisting by helicopter can be carried out at wind speeds more than 20 m/s. For an offshore site it is important that service is planned only and unplanned repair work remains rare if not absent. Thus, reliability of the wind turbine and all its systems has a higher status than on land. This may be achieved by including a higher degree of redundancy. For instance sensors needed for automatic operation can be redundant. Another strategy to avoid unplanned service is to install condition monitoring systems. While for onshore turbines condition monitoring is optional and not used in many turbines, for offshore sites it is mandatory.

4

Drivetrain

As mentioned Sect. 2.9 most wind turbines have three blades and a tower and therefore different types of different manufacturers look very much alike. Only from the shape of the nacelle in some cases we can suspect that internals are different. Inside the nacelle we have the drivetrain, introduced as a system in Sect. 2.2. This is where different manufacturers or developers leave their footprint. In this section we describe different ideas for the drivetrain. Different concepts for the rotor bearing (also called main bearing) are described and pro and cons for the particular solutions are mentioned. The individual components, connections between components, etc. will be described in the subsequent sections. An excellent account on drive trains is also given by Schaffarczyk (2014). Actually rotor blades are also part of the drivetrain. They are very important for instance for the drive train dynamics. As mentioned already we are not giving a detailed account on aerodynamics and blade design. We have given a short summary on rotor blades in Sect. 2.3, so the drivetrain in this section starts with the hub.

4.1

Gearbox or No Gearbox?

People often mention that all wind turbine look alike: A slim tower, three blades. It is not easy to distinguish different brands of wind turbines. At the first glance this is true. But looking more carefully we see a lot of differences. Not only sizes of rotors and towers are different. One feature that allows to distinguish wind turbines is the shape of the nacelle. Some models have a long box shape nacelle, others have a shorter nacelle with a large circular cross section. It is the drivetrain concept that to a certain extent determines the shape of the nacelle. To generate electricity or more precisely electrical power from wind we only need the aerodynamic rotor and the electrical generator. As described previously in a large HAWT the rotor speed is low, roughly in a range of 10–20 rpm. On the other side there are well proven © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 U. Ritschel and M. Beyer, Designing Wind Turbines, Synthesis Lectures on Renewable Energy Technologies, https://doi.org/10.1007/978-3-031-08549-9_4

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Gearbox

Low speed side

Generator

High speed side

Gearbox plus Medium speed generator

Gear Box

Low speed side

Generator

Gearbox plus High speed generator

Medium speed side

No gearbox Low speed generator

Generator

58

Low speed side

Fig. 4.1 Basic types of wind turbine drivetrains

industry motor/generator concepts1 that are designed for relatively high speed. If for instance a generator is equipped with two or three pole pairs for a 50 Hz grid it has a synchronous speed of 1500 or 1000 rpm respectively. We call these machines high-speed generators in the following. As we know already from Sect. 2.5 the generator speed is decoupled from the grid frequency by a converter. A machine designed for 1500 rpm will not perform well at a much lower speed. As a consequence, if one decides to use such a generator a gearbox is needed as schematically shown in Fig. 4.1 to operate the generator at or around the design range. The gear ratio needs to be approximately 100. It needs to convert from low speed to high speed. Most gearboxes for other application—in cars for example—are functioning into the other direction. As also shown in Fig. 4.1 we distinguish the low-speed side of the drive train, between rotor and gearbox, from the high-speed side from gearbox to generator. There will be more details about gearboxes in Chap. 7. If we do not want a gearbox the generator has to be designed for the slow rotational speed. In this case, roughly speaking, what was previously the “mechanical” gear ratio to increase speed is now the “electrical” gear ratio, by increasing the number of pole pairs instead. Typical direct drive generators have 50 · · · 100 pole pairs. The poles are arranged on a ring with large diameter (5–10 m), hence, these generators are also called ring generators. In an optimized design the diameter (perpendicular to the axis of rotation) is larger than the axial dimension. One of the challenges in designing direct-drive wind turbines in the arrangement of the generator relative to the rotor and the connection with the rotor, in other words, the integration of the generator. This will be elaborated in more detail in Sect. 4.7.3. Another idea is to choose a concept in between the high speed with gearbox and direct drive. Then instead of three gear stages one can use one or two stages. The required generator 1 Each electrical motor can also be operated as a generator to convert mechanical into electrical power.

4.2

Hub Concept

59

operates at medium speed. Sometimes these concepts are called hybrid, since elements of both, geared and direct-drive, machines are present. More on hybrid concepts is explained in Sect. 4.6. Having described different concepts now to set up the drive train of a wind turbine, one may ask: “What is the best solution?”. Aspects are performance, cost, and reliability. There is no simple answer to this question. Concepts have advantages and disadvantages. We mention some of them in the subsequent subsections. Wind industry had a lot of problems with gearboxes in the past. Especially when wind turbines were quickly growing in size and power rating from the late 1990s to the early 2000s manufacturers were desperately fighting against premature failures of gearboxes. For a company specializing on wind turbines with gearbox it is not an easy task to switch to direct drive. Therefore most established OEM worked on improving their design in cooperation with the gearbox manufacturer. Presently the majority of wind turbines is still with gearbox and high-speed generator. However, there is a trend towards direct drive and hybrid wind turbines. Only if we look at the new offshore wind turbines, direct drive and hybrid are already dominating. In the long term this will probably be the case also for wind turbines on land.

4.2

Hub Concept

The hub is the central part of the rotor that connects blades with the subsequent parts of the drive train. It is a special component in wind turbines and highly loaded. All aerodynamic loads on blades and in addition the weight of blades have to be supported by the hub. The main purpose of the rotor is to generate torque that can be converted to electricity. However (as explained in Sect. 1.2.2), blades experience also thrust (force perpendicular to the rotor plane) which can be larger then the propelling force and hence is dimensioning the hub. Details of the design process, materials etc. of the hub are discussed in Sect. 5.2. The hub is a component that is normally designed in-house by the manufacturer of the wind turbine. It is often also used as a housing for the pitch system. Interfaces of the hub to other parts of the drive train are blade flanges and the flange that connects, e.g., to a rotor shaft or directly to the rotor bearing. A picture of the structural part of the hub, i.e., the hub cast, is shown in Fig. 4.2 which also shows the mentioned flanges.

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Fig. 4.2 Picture of the 2 MW hub cast

4.3

Rotor Bearing

Independent of the fact whether there is a gearbox or not, we need a rotor bearing, often also called main bearing. The purpose of this bearing is to keep the rotor in its position and allow it to turn freely—there is always some friction, but it should be small—with respect to the rotor axis. A bearing has an inner and an outer ring. One can use models with roller elements between the rings, called roller bearings, or sliding bearings. Although sliding bearings are often used in large machinery2 they have never been used for the rotor of wind turbine. The reason is the loading situation. As will be explained in more detail later there are large bending moments (see Sect. 5.3.1) that have to be supported by the rotor bearing. Sliding bearings use a thin oil film between rings to avoid the direct contact between the solid surfaces. This concept is less tolerant with respect to off-axial loads (see Sect. 4.5). Bearing types used for wind turbines are spherical, cylindrical or tapered roller bearings. Some solutions frequently used in wind turbines are discussed below. In addition there are rotor bearing concepts using a shaft and concepts using an axle. In the former case the inner ring is rotating and the outer ring to the non-rotating structure, and in the latter case the outer ring is rotating and the inner ring is attached to an axle.

2 For example as the bearing for ship propeller shafts sliding bearings are used.

4.4

Separated or Integrated Drivetrain

61

In wind turbines with a gearbox only shaft-type rotor bearing concepts are used. The reason is that with an axle it is more difficult to guide the torque to gearbox and generator. On the other hand, axle-type bearings are used for several design-variants of direct-drive wind turbines for which the generator is directly attached to the rotor.

4.4

Separated or Integrated Drivetrain

In a classical wind turbine design with gearbox the drivetrain is separated. This means that different functionalities like rotor bearing, support of bearing, transfer of torque from rotor to gearbox, the gearbox itself and the generator are implemented in separate components. The separated drive train is shown in schematic form in Fig. 4.3. There are two bearings (2) that suspend the main shaft (1) and with it the rotor. The gearbox (3) is attached to the main shaft and connected to the generator (5) by the high-speed shaft (4). Details of this drivetrain concept and its variants will be shown later in this Sect. 4. The conclusion at this point is that the separate drivetrain will, in general, be longer since each component needs a certain space. The advantage is that the loading and sizing of the components in such a design is relatively straightforward. Each component gets its own specification and can be purchased from specialized suppliers. The wind turbine manufacturer than has to assemble all components to build the drivetrain. The opposite philosophy is integration of functionalities. Two bearings can be united in one housing. The bearing of the first stage of the gearbox can be the main bearing at the same time. Housings of gearbox and generator are firmly attached to each other such that no extra

Fig. 4.3 Schematic picture of wind turbine with gearbox and separated drivetrain with low-speed shaft (1), main bearing (2), gearbox (3), high-speed shaft (4), generator (5), machine frame (6), generator frame (7), torque support (8) and tower (9)

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Fig. 4.4 Schematic picture of a highly integrated design. Main bearing is inside gearbox. Number coding as in Fig. 4.3

high-speed shaft is needed. An example for an integrated drive train is shown schematically in Fig. 4.4. Herein the main shaft is part of the gearbox. It has also the role as the shaft of the first stage of the gearbox. So the housing of the gearbox and the bearing housings are integrated in one component. Further the generator is directly flanged to the gearbox. In this way the high-speed shaft and the generator frame are not needed. Obviously in the integrated design there are not only less components but it can also be much shorter in the axial direction. Consequently the whole nacelle will be more compact than in the separated design. This is quite attractive and promising with regard to potential cost saving. In fact, the attitude concerning integration is ambivalent in wind industry and some integrated wind turbine designs failed or vanished from the market. The reason is that with integration of different functions, like, for example, bearing and gearbox housing, also the loading situation becomes more complicated. With loads from the rotor directly acting upon the gearbox there might be deflections that, in turn, have impact on the contact between gear wheels. Hence integration is a good idea in principle but needs careful consideration of the loading situation. More detailed analyses of the multi-functional components are needed than in the case of separated design. If these analyses are not carried out or not carried out correctly then the OEM risks problems with the product. Therefore wind turbines with gearboxes nowadays show often a separated or a mildly integrated design. The 3-point

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concept explained in Sect. 4.5.1.2 is the most important example used by many OEM. In direct-drive wind turbine most recent designs have generator and rotor bearing integrated (see Sect. 4.7).

4.5

Wind Turbines with Gearbox and High-Speed Generator

As displayed in Fig. 4.5 about 70% of the wind turbines worldwide are of this type: Gearbox in combination with high-speed generator. In this section we describe the drivetrain concepts of such wind turbines. The drivetrain is a system that consists of several components. The components, or better some examples of components, will be described from Sect. 5 on. In the following we first focus on the main bearing, also called rotor bearing concept. In the mechanical design process the decision for a certain concept is an important step. It has impact on costs and the assembly process. Further in this section the “rear part” of the wind turbine will be described, i.e., the gearbox, the generator and the connection between the two.

4.5.1

Low-Speed Side—Machine Frame and Rotor Bearing Concepts

The first three bearing concepts described in the following can in principle be used for wind turbines with gearbox or with direct drive. The first two types, however, are mainly used for wind turbines with a gearbox. The concepts with axle—sometimes also called king pin—is only used in combination with direct-drive machine and will be described below. An account in the literature concerning the rotor bearing issue in wind turbines is given in Hart et al. (2020).

Fig. 4.5 Market share of wind turbine concepts with respect to gearbox concepts (GWEC 2019)

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4.5.1.1 Two-Bearing Concept This concept—often also called 2-point—has been used in many wind turbine and of course also in other contexts. In one realization it consists of two bearings and a main shaft supported by a suitable machine frame (MF). The machine frame is often also called main frame or bed plate. A concept design for a drive train with two bearings is shown Fig. 4.6. Each bearing has its own housing. The connection between machine frame and bearing housing is approximately at the level of the rotor axis. This way the connection itself—one of the highest loaded bolt connections in the wind turbine—is mainly loaded by a shear force and not by a bending moment. Consequently, the machine frame has the shape of a trough or tub. It has to reflect the tilt angle between rotor axis and the horizontal plane. In Fig. 4.6 the support structure consists of a front part for the bearings and the torque support for the gearbox (see also Sect. 4.5.2). Usually (not always) the generator is placed on a an extra component, the auxiliary frame (also called generator frame), attached to the machine frame. As seen in Fig. 4.6 the machine frame supports the bearings and the torque. Further in the lower part it provides flanges for installing yaw drives. Since it integrates many functions and complex loading it is often the component one has to devote a great deal of work in the engineering process. Concerning the manufacturing process the machine frame can be a cast or a welded design. Due to the complexity of the geometry of the machine frame, casting has advantages and most of the machine frames of wind turbine that are manufactured in larger numbers are castings. The classical concept with separate bearing housings uses spherical roller bearings (SRB) as rotor bearing. An SRB supports radial and axial forces with respect to the rotor axis. Since an SRB is used in the Windrad 2 MW design and also in many other wind turbine with gearbox it will be treated in somewhat more detail in Sect. 6.1 Another virtue of the SRB is that it is self-aligning such that some miss alignment of the bearing housings relative to each other can be tolerated. The two bearings can also be an SRB and a cylinder roller bearing. With cylinder rollers the configuration is much less tolerant. With this type of bearing the shaft should be aligned with high precision with the bearing and its housing. The solution

Fig. 4.6 Modern 2-bearing design. Main bearing is of spherical roller type

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65

then is a common housing for both bearings which reduces potential geometric deviations and constraint forces resulting from this otherwise. In principle also other roller bearings can be used. The requirement is only that at least one of the two bearings has to support the thrust force (in axial direction) and both have to support radial forces caused by the weight of the rotor and bending moments due to aerodynamic forces on the rotor. An example for a different realization of the 2-bearing concept are two tapered roller bearings at a distance that are prestressed against each other in a common housing. In the 2-bearing concept the weight of the gearbox is supported by the main shaft. Only the torque has to be supported directly by the machine frame. So the naïve picture is that with this concepts only torque is transferred from the rotor to the gearbox. The off-axial loads are absorbed by the two bearing and need not to be considered for the gearbox. Why is this picture naïve or even too naïve? The reason is that in the low-speed part of the drive train the bearing situation is statically indeterminate. There are three points (two bearings and the torque support) that are mounted on the machine frame and have to be aligned to the rotor axis. Howerver, two points already define the axis and any misalignment of one of the components with respect to the remaining one will lead to constraint forces. In addition deflections due to loading during operation can lead to constraint forces as well. If this is not under control and not taken into account in the design it may cause problems. The constraint forces and a method to avoid or at least mitigate them in the drive train will be discussed in more detail in Sect. 7.3. In conclusion there are advantages and disadvantages of the 2-bearing concept. Advantages are: • Long experience in wind industry and good reputation • Various standard bearings like spherical roller bearings can be used • With some care and additional measures only or mainly torque is transferred to the gearbox Disadvantages are: • There are many components like shaft, two bearings, bearing housing, that add to the costs of the product • The drivetrain is relatively long in axial direction • There are many components at or near the tower axis such that it may be difficult to create an access way from tower to nacelle that is consistent with HSE requirements (see Sect. 3.3) • The fact that drive train suspension is statically indeterminate entails a certain risk for this concept concerning constraint forces.

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4.5.1.2 Partly Integrated Concept—3-Point Suspension On the first glance quite similar to the previous concept is the 3-point suspension, see Fig. 4.7. In this concept there is one main bearing, an SRB, and the second main bearing of the main shaft is absent. It is replaced by the bearing of the first stage of the gearbox. In this way the function of the shaft bearing and gearbox bearing (see also Sect. 7.1) are integrated and compared to the 2-point suspension one bearing and its housing are saved. The low-speed part of the drive train is supported at three points, the shaft bearing and the two arms of the gearbox, therefore the name 3-point suspension. The price for saving one bearing is that on one hand loads on the gearbox are quite different. On the other hand, constraint forces are absent. The loading situation does not depend on the deformation of the machine frame. But now the gearbox has to support offaxial loads from the rotor. The arms of the gearbox that in the 2-bearing concept only support the torque are now also supporting off-axial loads. Also, the main loads from the rotor that have to be guided to the tower are partially going through the gearbox. Consequently, the 3-point suspension was criticized in the context of premature gearbox failures. For some time it was unpopular in wind industry. Some manufacturers kept this concept and worked in cooperation with gearbox manufacturers on improving the gearbox. After a learning curve it is still used in many wind turbines nowadays. Advantages of the concept are: • • • •

Second shaft bearing not needed Standard spherical roller is used as main bearing Statically determinate concerning loading Long experience with the concept and learning curve

Disadvantages are: • Off-axial loads on the gearbox • Large axial dimension of drive train The Windrad 2 MW turbine used as an example uses a 3-point suspension, Fig. 4.8.

Fig. 4.7 Concept design of wind turbine with 3-point rotor bearing

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Fig. 4.8 Machine frame with 3 point drive train in assembly hall

4.5.1.3 Moment Bearing The moment bearing is quite different from the previously described 2- and 3-point bearing concepts. Generally we need at least two support points along the shaft. As mentioned in the context of the 2-point suspension we can have a low-speed shaft and two tapered roller bearings. They need to be prestressed against each other. In a moment bearing the two rows are moved close to each other while the diameter becomes larger. This way one can arrange the two rows of roller elements in one inner and one outer ring such that the two main bearings eventually become one component that support axial and radial forces and even the bending moments. This is why it is called moment bearing. A simple 3D model to illustrate the concept is shown in Fig. 4.9. If for instance the outer ring of the bearing is attached to a suitable machine frame, the inner ring is connected with the hub on the one side and with the gearbox on the other side. A sketch of such a configuration is depicted in Fig. 4.10. In the sketch a short shaft connects hub and gearbox. One might also think of a design where the inner ring of the bearing is directly attached, e.g., to a component of the first planetary stage (see Fig. 7.2). In such a design one can say that the role of the main shaft is played by the inner bearing ring. Hence with such a bearing one can save costs for other components (shaft, bearing housing). Furthermore the nacelle can be shorter with a moment bearing since the axial distance between hub and gearbox is smaller which has some impact on logistics. A disadvantage is that the moment bearing is more expensive than the bearings used for the other concepts.

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Fig. 4.9 Schematic 3D picture of a moment bearing

Fig. 4.10 Sketch of drivetrain with moment bearing

For wind turbines with gearbox the moment bearing has first been used by Vestas in a design with 90 m rotor and 3 MW power rating. However, this minimal design with the gearbox directly attached to the moment bearing was not used in subsequent designs. Other designers decided to attach a short main shaft and a low-speed shaft coupling in between bearing and gearbox (Rachholz et al. 2012). Using this type of bearing, the machine frame looks quite different than for the other concepts. It is sometimes called of cradle type. The bearing ring is embedded like in a

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cradle. The frame has to provide a material efficient connection between a flange at the bottom, where the yaw bearing is attached, and the flange where the moment bearing is attached. For geared wind turbines with high speed generator the moment bearing has not been used much. But for direct drive concepts it is used in a number of designs. Also the Eleon wind turbine used for illustration in this book has a moment bearing. Advantages are: • Short in axial direction • In principle no main shaft needed • All loads supported by one bearing Disadvantages are: • Higher costs of bearing • Connection to gearbox tricky • Limited experience.

4.5.2

Gearbox and Its Interfaces

The key component of wind turbine with high-speed generator is the gearbox. It is also the most expensive component of the drive train. The gearboxes with gear ratios of 100 or so have several stages. One stage has a gear ratio in the range of four to six. Gearboxes with three stages are very widespread. In Sect. 7 more details about gearboxes will be provided. Another important task in the design process is to connect the rotor to the gearbox. With the two-bearing concept a frequently used solution is that the gearbox has a hollow shaft that is slipped over the main shaft. Then the assembly is compressed with a shrink disk. The shrink disk is usually a quite massive component that can generate a high radial force. It will be described in more detail in Sect. 7.2. With the two-bearing concept the weight of the gearbox is carried by the main shaft and its bearings. Therefore there is no weight carrying connection between gearbox and machine frame. However, without any connection between gearbox and machine frame the concept would not work. Any gearbox needs a torque support, otherwise it would simply rotate with the speed of the main shaft. The reason is that the gearbox converts the torque from a high to a relatively low level inversely proportional to the gear ratio. This means for the concept with high-speed generator that almost all the torque needs to be guided from the gearbox to the machine frame. The common concept for the torque support with the two-bearing concept is that the gearbox housing has two arms in horizontal direction. The arms are prevented to move up or down by elastic bearings that are inside a frame attached to the machine frame.

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For the 3-point suspension the main shaft is also connected with a shrink disk. But of course in this case the flow of loads is different. Not only torque has to be supported but also the aerodynamic loads from the rotor and all weights of components, including the one of the gearbox, that are in the low-speed part of the drive train. For the 3-point concept the gearbox support often is also simply called torque support. The gearbox support prevents not only vertical movement but also displacement in sideways direction. The thrust (and therewith movement in axial direction) is supported by the main bearing. In the case of the moment bearing the situation is again different. A classical shaft is not needed. The rotating part of the first gear stage can be connected directly to the inner ring of the bearing. Other concepts use a shaft-like component and/or a cardanic connection to decouple the gearbox from the main bearing and the loads from the rotor.

4.5.3

High-Speed Shaft and Connection of Gearbox and Generator

The standard solution to connect gearbox and generator is via a high-speed shaft. This component is connected to the output shaft of the gearbox on one side and to the generator shaft on the other side. In the separated design this connection must not be stiff in order to avoid constraint forces. A frequently used design works with two lamellar couplings with a light shaft made of composite materials in between. This concept is tolerant with respect to misalignment. In the separated design the generator is installed on special bearings that provide a flexible connection between auxiliary frame and generator and also reduces structure born sound. There are also concepts with gearbox and high-speed generator where the generator is directly flanged to the gearbox. Then the weight of the generator is supported by the gearbox housing.

4.5.4

High Speed Generator

High speed generators are designed not only for wind turbines but also for other industrial applications. The design is rather straight forward and therefore high speed generators in combination with proper gear boxes are widely used. This makes them rather attractive from cost point of view. Integration into the wind turbine design needs careful consideration of two interfaces: (1) connection to high speed shaft, i.e., torque transfer, (2) mounting to auxiliary frame, which influence the total alignment. Both interfaces are described in next Sect. 5. It is advised to check total alignment and possible imbalance after assembly is completed. Unlike other industrial application utilized as ground based generators, design of generators for wind turbines require further attention. One is related to rather high acceleration in

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71

the nacelle and another one to the efficiency of the generator/converter system as a whole. Namely, it is not necessary to optimize the electrical performance at rated speed but rather below rated speed due to the particular power characteristics of wind turbines. Rated speed is defined through grid frequency which is usually 50 or 60 Hz, or in other units 3000 or 3600 rpm. High speed generators utilized in wind turbines usually have rated speed half or one third of that, depending on the number of pole pairs implemented within the electrical circuitry. The number of pole pairs is chosen with respect to the gear ratio to end up with an economical solution for the complete drive train.

4.5.5

Drivetrain Dynamics for Wind Turbine with Gearbox

One particular requirement for certification is the so called drive train analysis. This is important, because of all the rotating parts inside the nacelle, which is on top of an elastic structure, i.e., the tower. The basic idea is to identify possible resonant behavior in the structure through start-up, stop or operation of the wind turbine. Excitation of the drive train could be by rotor frequency or multiples of it, e.g., 1P, 3P, 6P for a three blade rotor, and by tooth engagements wherever those occur, in particular inside the gearbox. Drive train analyses addresses the response of the drive train as a whole and in particular within the rotational degree of freedom. This way possible torsional vibrations can be identified and if relevant, prevented at an early stage of development by design, by damping measures and/or change of design parameters to shift resonances into an uncritical region. Elements to be considered are: Blades, hub, main shaft, shrink disc, gearbox, brake disc, high speed coupling, and the generator rotor, Fig. 4.11. The whole system is embedded into the main frame including the torque supports, the generator frame with the generator housing/stator. Within the gearbox further rotating and supporting structures that participate

Fig. 4.11 Components considered for drive train analysis; blades are not shown

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in the dynamics are planetary stages consisting of ring gear, planet carrier, planet wheels, sun wheel on the entry side and the intermediate/high speed gears including gears and pinions at the rear side all embedded into the gear housing that is connected to the main frame via the torque support. All parts are coupled to each other like a chain, either stiff or elastically via force elements including stiffness and damping to build up the dynamic equation for the torsional mode (in the simple case of one dimension). This also includes tooth engagement, which requires notion of meshing stiffness. From a practical point of view the challenge is to draft all the relevant parameters for completion of the analysis. What is needed are the moments of inertia of the components, torsional stiffness of rotating parts, meshing stiffness as mentioned above. The result will be shown in a Campbell diagram which provides intersections between excitation frequency (mostly given in units of rpm) and system frequencies (normally given in units of Hz). An example of such a diagram being a part only of quite a few of those diagrams for a full analysis is shown in Fig. 4.12. Only the lower frequencies are presented here. The complete analysis set includes frequencies up to about 500 Hz or so. Excitation frequencies are lines through the origin with different slope depending on the excitation mechanism: The two black solid lines are related to the rotor speed (1P and 3P), whereas the two black dashed (1P, 2P of Planet II) and dashed dotted (1P, 2P of output shaft) lines refer to an excitation from tooth engagement. Details and nomenclature of a gear box are described in Sect. 7. The vertical black dashed dotted lines confine the operational region of the wind turbine, whereas the remaining vertical lines provide the normal shut down speed (orange) and the one triggered by the safety chain (red). The horizontal lines constitute the system frequencies that could possibly be excited. They emerge form a coupled chain model of the torsional degree of freedom of the complete drive train. The modes are simply numbered according to the eigenvalues as they appear and refer to 1st, 2nd, 3rd etc. In the legend the subindex refers to the dominating (isolated) system frequency, e.g. drive train means torsional mode, edge/flap refers to blade modes. Focus is on the intersection between the horizontal lines (system modes) and the sloping lines (excitation). If the wind turbine is operating close to any of those points there is excitation possible. If this excitation is relevant or not depends on the actual dynamics of the whole system and has to be investigated in a separate so called transient analysis. However, most of the intersecting points are harmless, since damping is strong or coupling between excitation and response is low. Nevertheless, if an intersection is identified that might lead to challenges for the design, mild design changes might help, e.g., change of parameters, meaning change of material and/or components used, change of sizes or masses. If this does not lead to satisfactory results, from a development point of view, it might still be time for a design change. Finally, if there is no design related solution possible additional elements like dampers might be useful. This would/could be implemented, e.g., in blades or the tower, depending on the actual position of the intersection, but only, if there is no other solution available. In other cases, there might also be a software solution possible, in order to make

4.6

Concepts with Medium Speed Generator

73

Fig. 4.12 Example of one of the Campbell diagrams necessary for a complete drive train analysis, see text

the wind turbine move “quickly” through the critical region to prevent this region being part of the regular operational region.

4.6

Concepts with Medium Speed Generator

As explained in Sect. 4.1, in a sense, a wind turbine with medium speed generator is positioned in between a classical gearbox and direct drive wind turbine. Like in case of direct

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drive, one has a generator with a relatively high number of pole pairs which axial dimension is shorter than the radial one. Also the gearbox with one or two planetary stages is shorter than the conventional three-stage gearbox. Therefor a natural design idea is to attach gearbox and generator directly to each other or even integrate them into a common housing. There have been a number of wind turbine designs to exploit advantages of the medium speed concept. A famous design with a long history in wind industry is the multibrid concept. The multibrid design consists of a so-called 1 1/2 stage planetary gearbox directly connected to the medium speed generator. In addition, the bearing of the planetary stage is integrated with the rotor bearing. So in one housing we have the rotor bearing, the gearbox and the generator (Siegfriedsen and Böhmeke 1998). The advantage of a highly integrated concept is the potential for cost reduction in the drive train. The number of components is reduced. On the other hand integration also increases the technological risk. Loads lead to deformations of components and some parts like the gear wheal pairing are sensitive with respect to deformation. As a consequence a careful deformation analysis of the design has to be made taking into account all potential loading situations. The Multibrid concept was used for the offshore wind turbine Areva 5M. About 200 of this wind turbine with different rotor diameters (115 and 135 m) were built and installed in a number of German offshore wind farms, i.e. Alpha Ventus, Borkum West II (now named Trianel), Global Tech I and Wikinger. After a turbulent business history the company Multibrid GmbH then Areva Wind GmbH and now Adven Offshore (a Siemens Gamesa subsidiary) decided to discontinue manufacturing wind turbines including the multibrid concept from 2017 on and is now mainly focusing on offshore service and maintenance. Nevertheless, medium speed concepts have nowadays a market share of about 4%, see Fig. 4.5, but are promising for the future. For instance, the latest Vestas offshore wind turbines use a medium speed generator, a 2-point suspension, and gearbox and generator are attached to each other. Between main shaft and gearbox a flexible coupling is used.

4.7

Direct Drive Concepts

The idea to design a wind turbine without any gearbox is long standing and many attempts to realize such a concept have been made (Klinger 2012). Presently about 20% of wind turbines worldwide are direct drive, Fig. 4.5. They are also called gearless. There are different types of generators and many ways to arrange the generator relative to the rotor. In addition many different ways of integration exist. As a consequence, there is a large number of different direct-drive design concepts. Some have been successful in the market, some not. Some are used for the latest offshore wind turbine and some have vanished from the market. In the following an overview on basic design concepts is given and some examples provided. A sketch of a particular configuration is shown in Fig. 4.13.

4.7

Direct Drive Concepts

75

Fig. 4.13 Sketch of direct drive concept with axle and moment bearing

4.7.1

Type of Generator

On direct-drive wind turbines synchronous generators are used, both with electrical and permanent excitation. In permanently excited machines the rotor is equipped with permanent magnets and the stator carries coils made from copper. In electrically excited machines copper is on both sides. Which concept is used depends on costs, mass and to a lesser degree size where these aspects are often related to each other. The main objectives of the generator design are to keep mass and costs low and the size of the generator such that it can be transported on roads. For large wind turbines the generator may reach a size where this is not feasible. The solution to this issue then is to divide the generator in smaller segments that could be moved to and assembled at a suitable location. For offshore wind turbines this is a plant near a harbor. Also machines with super conducting coils have been suggested for wind turbines (Abrahamsen 2012). In such machines higher flux densities can be achieved and, as a consequence, weight and size reduction is possible. Although promising, this technology is not yet in a mature industrial state. For electrical machines also the geometry of the air gap is important. It determines the direction of the magnetic flux (field lines) relative to the axis of rotation. Electrical machines can have radial, axial and transverse flux. While generators with all flux directions have been

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suggested for wind turbines, the existing models have all radial flux. Also for machines with medium and high speed radial flux is used.

4.7.2

The Air Gap Challenge

As mentioned already in Sect. 4.1 the direct drive generator has a large diameter. For rotating electrical machines it is necessary that coils or magnets are moving (rotor) with respect to other coils (stator) to induce a voltage and generate electric current and power. Stator and rotor are separated by the air gap. In order to exploit the electrically active material with high efficiency the air gap of the generator must be small. In state-of-the-art designs the air gap is about 5 mm while, as mentioned in Sect. 4.1, the diameter of the generator, more specifically the ring diameter, is 5–10 m. Since torque (and hence power) increases quadratic with radius and only linear with length of the generator in most cases diameter is larger than the length of the direct drive generator. As a consequence design of the generator is highly demanding with respect to size accuracy and quality of its components. More importantly if the generator is connected in some way to the aerodynamic rotor it will also experience loads and, in turn, deformations. It is of utmost importance to have possible deformations under control in the design process such that the air gap never vanishes completely or, better, a safety distance remains in all situations. If the rotor would touch the stator during operation it would cause damage. The air gap failure is one of the worst-case scenarios for direct-drive wind turbines. In the design process of a wind turbine with gearless concept one needs to take into account the air gap challenge from the very beginning. One of the objectives is to reduce loading of the generator structure that can have impact on the air gap. In the ideal case only torque would be transferred from the aerodynamic rotor to the generator. In reality this is not possible; but there are strategies to reduce deformation. An obvious (but also expensive) solution is to include an elastic coupling between hub and generator (Baseer et al. 2020). In any case, a deformation analysis needs to be done. For this purpose finite-element tools are used (e.g., ANSYS), where the relevant structural properties of the generator can be modeled, wind loads and electromagnetic air gap forces are applied, and realistic deflections can be obtained. In this analyses one assumes also additional safety factors for instance for the loads such that eventually, if everything has been done correctly, one can be confident that no air gap problems will occur.

4.7.3

Position of the Generator and Integration Concept

A generator has to be positioned relative to rotor and tower and connected with the hub. There are three positions that have all been used in different wind turbine designs. The generator can be “in front of” the tower (i.e., on the same side as the rotor), above the tower, or behind

4.7

Direct Drive Concepts

77

the tower. One aspect of the position is the location of the center of mass of components above the upper tower flange. In general, the center of mass is not at or near the tower axis. If the generator is behind the tower one expects that the arrangement of nacelle and rotor can be balanced to some extent. However, mass or weight balance is only one aspect of the loading of the components at the tower top, the flanges, bolt connection, and the yaw bearing. Loads relevant for sizing at the tower top are dominated by aerodynamic loads from the rotor. Gravitational loads, of course, must be taken into account, but are not decisive. Since the location of the center of mass is at our disposal, we might place the generator also at the same side as the rotor. The center of mass for this design will be in front of the tower. The resulting loads, such as a bending moment at the tower top, can be carried by the appropriate design of yaw bearing, the bolt connections and the uppermost part of the tower. All direct-drive designs that are successful on the market have generator in front of the tower. An example will be given in Sect. 4.7.5 for a new innovative gearless design. The main reason that the position in front of the tower is more successful is its level of integration. Obviously, the rotor bearing can also be used as the generator bearing. In this case the hub is directly connected to the generator rotor and no additional shaft or coupling is needed. Another possible integration is that the housing of the generator plays the role of the machine frame. If the generator is attached at one side to a suitable structure above the tower and on the other side the rotor is attached to the generator shaft, e.g., then not only shaft and its bearing has double functionality but also the housing of the generator. Integration saves components and costs. On the other hand, the air gap analysis is more demanding than in a design where the generator has its own bearing and is simply a separate component. Nevertheless to use one main bearing for aerodynamic and generator rotor is mainstream for gearless wind turbines. Similar to a separated design of a geared wind turbine is positioning the generator behind the tower. In this case the generator would have its own bearing and a main shaft is needed to connect rotor and generator. The obvious advantage of this design concept is that the generator can be exchanged without removing nacelle and rotor from the tower. This concept was considered as promising for direct-drive wind turbine some 20 years ago but was never successful on the market. In our sketch Fig. 4.13 the generator is in front of the tower. A single moment bearing is used. There is a short axle attached to the machine frame such that in principle the generator can be transported as a complete unit. Such a unit could be also put on a test bench.

4.7.4

Inner and Outer Rotor

Another choice that needs to be defined during design process is whether the generator rotor is outside or inside the stator. In high-speed machines the rotor is always inside the stator, closer to the axis of rotation than the stator. This arrangement is called inner or internal rotor.

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But in general, depending on the geometry of the generator, it could also be the other way around; the rotor could be outside the stator. This case is called outer or external rotor. Both, rotor and stator, are arranged on a ring with a radius of several meters and need to be connected to structural components that are closer to the axis of rotation (like bearing or shaft for example). This connecting structure is called the shield. For large ring generator the shields can be one-sided. This allows one to arrange the rotor either inside or outside the stator. In the sketch Fig. 4.13 the generator has an inner rotor.

4.7.5

Eleon as an Example

Concerning the integration concept the Eleon wind turbine introduced in the Sect. 2.11 also uses a single moment bearing. The basic concept is depicted in Fig. 4.14. The bearing is the blue object (no roller elements shown). The special feature of this concept is that the stator housing is separated from the machine frame. Both stator an rotor are attached to the outer and inner ring of the moment bearing. The deflection from off-axial loads that effects the generator now is reduced to the deflection of the bearing. Since we need a rotary bearing this cannot be avoided. In that respect the Eleon design concept is unique. There is another special feature at the rear end of the generator. It is an auxiliary bearing that additionally stabilizes the air gap. As a consequence, the structural design is sized only by the air gap forces which is much different from other existing direct drive concepts.

Fig. 4.14 Sketch of Eleon concept

4.7

Direct Drive Concepts

79

Fig. 4.15 Eleon generator during installation process

One important detail remains to be discussed. With the purpose to support all loads by the moment bearing—except torque of course—the whole configuration is statically indeterminate through the second bearing. It turns out by careful analyses and fabrication of the components that the constraint forces remain small enough to pose no risk to the bearings. The generator of the 3.4 MW turbine before and during installation is shown in Fig. 4.15.

5

Structural Components

In this section we focus on main structural components for nacelle and drive train. These are components like hub, machine frame, main shaft that are individually designed for a particular type of wind turbine. Since these are pivotal components, they need to be designed by the party that has the overall system competence. To a certain extent this is also true for the nacelle housing and the spinner or hub cover. Therefore housings are also included in this Section. Some of the components include several functions. For example the machine frame provides support for rotor bearings, gearbox and yaw drives and has to fulfill further requirements explained in Sect. 5.3. Therefore development needs quite some effort and is timeconsuming. Further on, the machine frame is a so-called long-lead item. This means that it has a long delivery time, i.e., this type of components determines the schedule from kick-off event to prototype installation in a new wind turbine development project. Other key structural components are rotor blades and towers. They are mentioned briefly in Sects. 2.3 and 2.4. They are not treated in anymore detail here.

5.1

Materials and Manufacturing Process for Main Components

There are only few types of materials that are used for structural components. For instance the DNV GL guideline (DNV GL 2010) provides information about these materials. Of particular importance are • • • •

Cast iron for hub, machine frame and other parts Construction steel for machine frame and other supporting structures Forged alloy steel for the main shaft, if the wind turbine needs one Fiber reinforced plastic as the materials for nacelle cover and spinner.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 U. Ritschel and M. Beyer, Designing Wind Turbines, Synthesis Lectures on Renewable Energy Technologies, https://doi.org/10.1007/978-3-031-08549-9_5

81

82

5 Structural Components

In the following some properties of these materials are discussed. The materials for structural parts—except the FRP that is also mentioned here—are iron based. The amount of material needed and the pressure on general cost reduction in wind industry does not allow materials like aluminum or titan alloys for structural parts, although they have some favorable properties.

5.1.1

Cast Iron and Casting Process

Spheroidal or ductile cast iron is the most important structural material of the drive train. It mainly consists of iron and contains a 3.0–3.7% fraction of carbon besides some other minor admixtures. As shown in Fig. 5.1 the material contains small nodules that consist of graphite. This has an impact on the mechanical properties. While other cast iron materials are brittle, spheroidal cast iron is ductile and chosen for structural parts subject to volatile loads. The casting process is most suitable for components like the hub with a relatively complex geometry. It is possible to create a geometry that is optimized for given design loads. Limitations to the geometry are defined by the casting process itself. Designers need to have good knowledge about the casting process and they should communicate with the foundry to optimize the design for the manufacturing process. One example of a rule that should be followed in the design process is to avoid material accumulations, in other words, avoid that some parts of the structure are thicker than others. In the fabrication process accumulations would cool down more slowly than other parts of the structure which would lead to stress and potential damage. Another rule is caused by the fact that the material can tolerate more compressive than tensile stress. Consequently one should design the geometry such that for loads applied to the component the structure should lead to compressive stress, if possible.

Fig. 5.1 Cross section through spheroidal cast iron

5.1

Materials and Manufacturing Process for Main Components

83

A typical material accumulation results in case several reinforcing ribs meet at one point. To improve the design one could introduce a ring rib where the reinforcing ribs end. Or one can move the ribs in such a way that they do not meet at one point. The large structural components of wind turbine are produced in the sand-casting process. After the design is completed and a 3D-model is available (see Sect. 3), the first step in this process is to produce the pattern. The pattern is a replica of the component made from wood or foam. The pattern is used to form the mold from a mixture of sand and glue that consist of lower and upper parts (called drag and cope) and the core inside a flask. Further the core is needed that fills the space inside the mold corresponding to the hollow parts of the component. More details on the casting process can be found, e.g., in Campbell (2015). The pattern and suitable flasks can be normally reused. The mold and the core are destroyed in the process. Also after cleaning the sand can be reused. Patterns are expensive such that casting is not cost efficient for one or a small number of items. Since most wind turbines are developed for mass production casting has become the standard manufacturing process for structural components like hub and machine frame. An important issue in the manufacturing process is the quality of resulting items. In the casting process one has to make sure that the material has uniform properties and no cavities or other defects result in the process. Quality depends on know-how of the foundry in casting large items. A standard is also to carry out measurements with ultrasonic or x-ray devices to assure quality.

5.1.2

Construction Steel and Welding

The metallic material that is most widely used worldwide is so-called construction steel. Its chemical composition is slightly different from that of casting material. It contains 1.6% manganese and no or very little carbon. It is available as half-finished material in shape of plates of different thickness. Construction steel is ductile like spheroidal cast iron and suitable for welding. Further on, plates can be bent up to a certain curvature using the property that steel can be plastically deformed to a certain extent without being destroyed. However, the need to start a design from plates or bent plates implies a strong limitation to the geometry of the component to be manufactured. It is, e.g., cumbersome to produce a component like the hub that approximates a sphere. For manufacturing a component made of construction steel also starts from a computer aided 3D design. It needs to take into account properties of the plates like size, available thickness and restrictions due to the fact that the geometry has to be obtained from flat or rounded sections. Further also requirements due to the welding process need to be taken into account. Compared to components for the casting process this limits the geometry. Plates need to be cut in parts that are attached to each other by welding to build the component.

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5 Structural Components

The quality of welded components to a large extent depends on the quality of the welding seams. They need special care and control after completion. E.g., it needs to be checked whether the complete welding seam is filled with material. A side remark concerning the price of the raw or half-finished material may be in place. World-market prices for iron-based materials like steel plates have varied greatly over time depending on the economic situation. During the last 20 years price for steel plates made in the USA varied from USD 500 to USD 2000 in February 2022 per metric ton.1 The price of the finished component is determined by half-finished material (plates) and manufacturing cost. In large and heavy components material costs are very important or even dominating.

5.1.3

Forged Alloy Steel

Another manufacturing process that is used for components of wind turbine is forging. For example main shafts used for 2- and 3-point suspensions in wind turbines with gearbox are often made from forged alloy steel. Alternatively, main shafts can also be made by casting. Here we utilize the shaft of the Windrad 2 MW wind turbine as an example for a forged main component. The material in this case is alloy steel, containing iron and small admixtures of chromium, molybdenum, and nickel in the order of 1%. The material is delivered to the forgery in the form of a slug. Then it is heated and hammered to obtain the trumped-like shape of the main shaft. After forging the item is slowly cooled down and later machined. A demanding part of the machining process is preparing the bore through the main shaft. In comparison, with a cast main shaft the bore could be implemented in the casting process. The forged alloy steel has a high strength, significantly higher than that of cast iron or construction steel. This is an important advantage for the main shaft. The diameter of the main shaft influences the bearings. The bore of the main bearings is sized by the diameter of the main shaft. So it is an advantage to have a small main shaft diameter.

5.1.4

Glas Fiber Reinforced Plastic

The fiber reinforced plastic is mentioned here as the only non-metallic material, since most wind turbine nacelle covers and spinners are made of this material. They are not structural components in the sense that the main loads are going through but are important in the engineering process. Often they are designed by engineers of the wind turbine manufacturer. The material used here is a cheaper version of the composite material used in blades. The half finished material in this case are randomly ordered fiber mats. In the design process nacelle cover and spinner are divided into suitable sections. For each section a mold is prepared. The items are then produced by hand lamination where the mats are put into the molds and resin is distributed manually with rollers. 1 Price from http://steelbenchmarker.com/history.pdf.

5.2

Hub

85

Table 5.1 Material properties of some essential materials used in wind turbine design, cast for EN GJS 400-18-LT and Alloy for 34CrNiMo6 Parameter

Unit

Cast

Alloy

Steel

GFRP

Modulus of Elasticity, E

N/mm2

165,000. . . . . . 185,000

210,000

210,000

8,500

Poisson number, ν

–

0.3

0.3

0.32

Tensile strength, Rm Tensile yield strength, R p=0.2

N/mm2

400

740

510

100

N/mm2

250†

540

355

100

Density, 

kg/mm3

7.85 · 10−6

7.85 · 10−6

1.57 · 10−6

† Value for R p=0.1

Main loads for nacelle cover and spinner are direct wind loads. The main parameter for the strength of the component at a certain location is given by the plate thickness. Also sandwich material with glass fiber at upper and lower side and foam in between is used. Some essential properties of materials for design of wind turbines are collected in Table 5.1. Note that parameters depend on material thickness as well, which is not shown here to keep the table concise. Exact values are provided in the relevant standards and guidelines.

5.2

Hub

The hub of a wind turbine is a special component that we do not have in a similar form in other machinery. The main properties of the hub are • Provide stable support for a certain number of rotor blades. Blades are directly connected to the hub or, as in all wind turbine of the last 20 years, a bearing that is then attached to the hub. Blades have a certain weight and experience aerodynamic forces that are dimensioning for the hub. • The hub needs to be connected to a shaft or in another way connected to the subsequent parts of the drive train such that the torque can be transferred to the generator. • The hub needs to be accessible up to a certain extent. The historic Dutch wind mills had four blades. With this rotor concept it was possible to use two wooden beams that cross each other in the center. This way one avoids to design a connection between blade root and hub near the rotor axis where high loads are expected. In two-bladed designs it is also possible to use a bar that crosses the rotor axis. For the

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tree-bladed rotor such solutions are not possible. All hubs need to have three flanges with 120◦ distance.

5.2.1

Hub Design Concept

Two concepts for designing the hub are described here. Firstly, we consider the tubular hub. As shown in Fig. 5.3 the hub consists of three tubes that cross each other in the rotor axis in a distance of 120◦ . There will be a fourth tube along the rotor axis. Such a hub can be a welded design with prefabricated tubes. At the ends of the tubes will be flanges to attach the blades and the shaft. Also the rotor cone can be realized in such a design by pointing the blade tubes slightly in forward direction. Wall thickness of the turbines has to be such that they are providing stable support for the blades. Quite different from the tubular design is the spherical hub design. We start with a sphere (1). As further shown in Fig. 5.2 the next steps are to cut off caps from the sphere, one for the connection to the main shaft (2) and three for connection to the blades (3), (4). Again the blade axis are positioned at an angle of 120◦ . Then one adds a narrow flange to the cuts (5), (6). By defining the wall thickness the object will become hollow and resembles a spherical shell with openings (7). Finally, rounding are added since any sharp edges could lead to sources of cracks (8).

Fig. 5.2 Basic design steps for spherical hub concept

5.2

Hub

87

Material thickness can be made variable, e.g., by placing an inner sphere and by slightly shifting the centers of the outer and inner sphere relative to each other. Also more holes may be necessary to enter the hub for servicing the pitch system depending on the concept. For instance the hub of the Eleon 3.4 MW machine does not have further openings. The hub can be accessed easily from the nacelle through the generator and the main bearing. But most wind turbines have these access openings. In some more detail we discuss the hub of the Windrad 2 MW turbine in the next Sect. 5.2.2. Fig. 5.3 Tubular hub design

1

2

3

4

5

6

7

8

88

5.2.2

5 Structural Components

2 MW Hub Design and Structural Analysis

The hub of the Windrad 2 MW turbine is a modern design based on the spherical hub approach. Clearly the closed sphere would be the most stable geometry. Cutting of parts for blade connection and main shaft connection the geometric stability is reduced. To increase the stability one can partly fill the opening with a reinforcement structure. But this structure must not be too large since access to the blade root should be possible. Furthermore openings are needed to remove the sand kernel in the casting process. In Fig. 5.4 two 3D views of the 2 MW hub are displayed. The figure (left side) shows the front view. The large opening is the flange for connecting the pitch bearing and through this bearing the blade. Also seen are the blind holes for the bolts that connect the pitch bearing to the hub. The reinforcement structure shown in the hub in Fig. 5.4 is a compromise between the different requirements mentioned in the previous paragraph. The other opening at the front side is the one to enter the hub for service. It needs to have sufficient size to allow service personal to enter the hub. The rear view on the right hand side shows the bolt hole circle for attaching the main shaft. The drawings, see Fig. 5.5 with measures are the reference for the manufacturing of the component. The object can be reconstructed from the drawings. The drawings are derived from the 3D model and equipped with the measures. For the casting process 3D models are also used directly. The drawings also contain information about details of the manufacturing process, like surface treatment and required roughness, rounding of edges, etc.

Fig. 5.4 2 MW hub design. Left side shows the front view with service opening, blade flange with blind holes for bolts that connect the hub with the pitch bearing. Right side shows rear view with main-shaft flange. For more details see text

5.2

Hub

Fig. 5.5 Overview drawing for the hub

89

90

5 Structural Components

Fig. 5.6 Result of FE extreme load analysis

5.2.3

Hub Stability Analysis

The stability analysis of the hub is carried out with FE methods (see Sect. 3.2). The hub is a component for which a great deal of analysis is done for structural optimization before the final proof of stability is carried out and documented for the certification process. The loads dimensioning the hub originate from the blades. On the rear end the hub is attached to the main shaft. In an approximate approach this joint can be treated as a stiff wall. In reality it is not absolutely stiff but flexible due to the elasticity of main shaft, machine frame, etc. Taking into account this realistic treatment of stiffness may reduce the calculated stresses in the hub, on the expense of more computation time of course. Concerning the load transfer to the component to be analyzed, sectional loads can be uniformly distributed across the blade flanges. Near the flanges this may, however, not lead to a realistic stress distribution. The reason is the bolt connection. Bolt connection cause a characteristic stress pattern in the clamped parts near the bolts. For a more precise analyses also the bolts with pre-stress have to be modeled. This makes models very cumbersome. The same holds for the bolt connection between hub and main shaft. As bolt connections extend to the vicinity of the hub (pitch bearings, blade roots, main shaft) in this approach parts of the vicinity of the hub have to be modeled as well. The result of the simplified approach for the van Mises stress and one of the extreme load cases is shown in Fig. 5.6.

5.3

Machine Frame

The machine frame is the most challenging component in the design process. It fulfills the following requirements:

5.3

Machine Frame

91

Fig. 5.7 Machine frame of the Windrad 2 MW wind turbine

• Provide together with main bearing and main shaft a firm connection between the upper end of the tower and the rotor. • Provide support for the elements of the drive train. For wind turbine with gearbox these are the bearing(s) and the gearbox or only the torque of the gearbox. For direct-drive wind turbine this is the stator part of the generator. • Provide flanges and surfaces where drives and brakes of the yaw system are attached to. • Since the machine frame is located above the tower the design also needs to take account of access ways for service personal that connect tower and nacelle. • It needs to integrate the rotor lock that allows to fix the rotor in particular positions. • Also points where the nacelle cover can be attached to are needed. It is made either from cast iron or steel plates as welded design. The frame of the 2 MW wind turbine of this book is shown in Fig. 5.7. The machine frame waiting in the assembly hall for further processing is shown in Fig. 4.8. A drawing of the 2 MW machine frame is provided in Fig. 5.8.

5.3.1

Sizing and Analysis

For sizing and structural proof loads have to be introduced onto the machine frame. For the three point bearing the points are the flanges of the main bearing housing and the torque arm supports on both sides of the gearbox. For the 2 MW sample wind turbine the load

92

Fig. 5.8 Drawing of the machine frame (Main sheet)

5 Structural Components

5.3

Machine Frame

93

Fig. 5.9 FE Model with load introduction areas indicated as red surfaces

introduction points are indicated by red areas in Fig. 5.9 which is a screen shot of the FE model geometry. For completeness also the loads introduced by the yaw drive and other attachments into the main frame have to be calculated. However, this might be separated from the main loads. In order to quantify the loads and to get an impression of the size and dimension Fig. 5.10 provides a schematics with distances in mm between key points of the hub nacelle structure. Those key points are denoted as follows: R for hub center (rotating with the hub), N for center of main bearing (fixed to the nacelle), K for intersection between tower axis and rotor axis (moving with nacelle), K 2 center of yaw bearing (fixed to tower top). The hub center could also be an origin of non-rotating coordinate system, i.e., not fixed to the hub, but to the nacelle. The coordinate origin is denoted as R N and shown in Fig. 5.11. For the 2 MW wind turbine of this book the extreme dynamical loads with respect to this coordinate system are given in Table 5.2. For each component the maximum loads are given in the matrix diagonal element. Lines show the isochronous load of the respective other components. Altogether each line constitutes a load case. DLC stands for Design Load Case and the acronyms in that row refer to particular loads cases mentioned in the guidelines for the two different rotor diameters (93 and 87 m) used in the layout of this turbine. The last two columns refer to the pitch angle and the wind speed at hub height, when the extreme event occurs.

94

5 Structural Components

Fig. 5.10 Scheme of hub nacelle assembly with key points and dimensions for a 2 MW wind turbine

Fig. 5.11 Nonrotating RN (left) and rotating R (right) coordinate system at hub center fixed to the nacelle, z axis points along rotor axis, x axis downwards (left) or along one blade (right) and y axis chosen to form a right handed system Fig. 5.12 Sketch of three point bearing with bearing L1 and L2 at positions N and N3 respectively

5.3

Machine Frame

95

In order to determine loads at the points of load introduction of the bearings of Fig. 5.9 from the loads given at the hub center the later ones have to be rearranged to determine the reaction forces at the bearings. For the 3-point suspension we have to take into account force  bending moment M,  and torque. To simplify this a little we leave away the torque for the F, time being and come back to it in Sect. 7.3. The situation is sketched in Fig. 5.12. Force F  are given in term of components by and bending moment M F = Fz eˆz + Fx eˆx

(5.1)

 = M eˆ y M

(5.2)

where we have chosen coordinate directions for convenience. In order to determine the reaction forces F1 and F2 at bearings L 1 and L 2 , respectively, we note that L 1 is a spherical roller bearing (fixed bearing) and L 2 which is inside the gearbox a floating bearing (eˆz axis is unbound). Hence reaction force are F1 = F1z eˆz + F1x eˆx

(5.3)

F2 = F2x eˆx

(5.4)

Balance of forces lead to equations for the components Fz + F1z = 0

(5.5)

Fx + F1x + F2x = 0

(5.6)

In order to balance moments we introduce a arbitrary reference point z 0 along the eˆz axis. The balance of moments than reads M + Fx (z 1 − z 0 ) + F1x (z 2 − z 0 ) + F2x (z 3 − z 0 ) = 0

(5.7)

Table 5.2 Extreme loads at hub center RN. Safety factors included as indicated in column SF, see text

96

5 Structural Components

where z 1 , z 2 , and z 3 denote z coordinates at R, N (= L1), N 3(= L2) respectively. For definiteness we choose the arbitrary reference point at the coordinate system N (main bearing L1), i.e., z 2 − z 0 = 0 and find (5.8) M − Fx a + F2x b = 0 where we have introduced a = z 2 − z 1 and b = z 3 − z 2 . Utilizing also Eq. 5.6 we can convert to 1 (M − Fx a) − Fx b 1 = (Fx a − M) b

F1x = F2x

(5.9) (5.10)

and from Eq. 5.5 we get F1z = −Fz that completely determines the reaction forces at bearing L1 and L2. In a simplified example we use values for load components in the vicinity of those given in Table 5.2. Ultimate gravitational force (FXRN) is chosen Fx = 700 kN. Thrust (FZRN) is chosen Fz = 650 kN. Bending moment (MYRN) is chosen M y = 6000 kN. The values for a and b can be read off Fig. 5.10. We choose a = 2.0 m and b = 2.3 m to make it simple. With these values components of the reaction forces are F1z = −650 kN 1 F2x = (700 · 2.0 − 6000) kN = −2000 kN 2.3   1 (6000 − 700 · 2.0) − 700 kN = 1300 kN F1x = 2.3

(5.11) (5.12) (5.13)

Summarizing our sizing calculation we conclude • • • •

From aerodynamic moments the machine frame receives sizable stress Those moments size the machine frame Reaction forces are much larger than gravitational forces Distance b is an important quantity as Fx ∝ 1/b

In both cases, with 3-point and 2-point suspension, the bending moment is supported by a pair of radial forces. A large distance between the bearings, the parameter b in equation above, reduces the radial forces. This situation is shown on the left side of Fig. 5.13. What happens if we use a moment bearing? In this bearing there are two rows of rollers very close to each other. So the parameter b is small. On the other hand, the individual rows do not support only a radial force but also forces in other directions. If the diameter of the bearing is large enough the moment can be supported by a pair of horizontal forces with similar magnitude as in the previous case. This is depicted on the right side of the Fig. 5.13. This leads to the ring-type geometry of the momentum bearing which makes it suitable for a direct drive wind turbine that have generators with a large diameter.

5.3

Machine Frame

97

Fig. 5.13 Support of M with two spherical roller bearings (left) and moment bearing (right)

5.3.2

Optimizing the Structure

Due to the complex load introduction and the design restrictions finding the optimal design is not straight forward, in particular for the main frame. It needs insight and experience to optimize a design with respect to minimize mass of the structure and costs. Typical restrictions are related to geometry, stress, damage, deformation, frequencies, serviceability, and maybe more features. To start with the available space has to be filled with material in a smart way to fulfill the restrictions and match the objective. Can this be automatized? The answer is yes, but partially only, because the methods known are not yet fully developed. If the structure can be optimized by considering, e.g., local stress then a method sometimes called “pixel method” can be introduced. However, damage related to fatigue is not a local property but needs analysis of the structure as a whole. Nevertheless, the pixel method is helpful to get some insight into the stress distribution. The idea is to take out material, if the local stress is below a certain limit. If fully automatized an algorithm depicted in Fig. 5.14 (left side) would lead to an optimized structure if the loop in the lower part of the diagram converges

Fig. 5.14 Simplified diagram of optimization algorithm without (left) and with human interface (right)

98

5 Structural Components

Fig. 5.15 Computer aided optimization process, iterations from top left to bottom right, see text

5.4

Generator Frame

99

and provided the above mentioned conditions (restrictions) are implemented correctly. As already mentioned, no conclusive algorithm has been developed to date, hence the designer has to intervene at certain stages of optimization in the loop. This is symbolized in Fig. 5.14 (right side) by mentioning the “human interface”. An example is provided in Fig. 5.15. From top left (definition of geometry) to bottom right (final result) iterations according to the algorithm mentioned Fig. 5.14 (right side) are shown. The top left pair shows a reasonable starting point for a direct drive machine frame. The green model provides the grid for FE analysis with a red area showing the load introduction at the main bearing and the colored model to its left provides the analysis result. Color coding for FE analysis is as follows: Blue means little stress, then green to yellow to orange to red symbolizes gradually higher stress values. The second pair on the left shows the (grayish) CAD model to the right and the respective (colored) FE analysis result to the left. This is from a different perspective as the first pair, but the same volume. The next pair shows the first iteration in the CAD model leaving out the parts that have small stress values and so forth until the the final result is seen in the bottom right pair of models. Additional openings seen in the final CAD model are due to necessity of a rotor lock and service access.

5.4

Generator Frame

The generator frame sometimes noted as auxiliary frame is mounted behind the main frame and carries the generator and other utilities, e.g., switch boards or control cabinets as well as power electronic equipment. Other than the main frame the generator frame has no structural function with respect to the wind loads from the rotor etc. Hence requirements to structure are less ambitious. Material choice could be steel, type S355, and due to simple geometry the auxiliary frame is mostly realized as a welded structure. Besides structural stability to carry the loads placed onto the generator frame, from a structural point of view, the most important item is the connection between generator frame and main frame that is usually realized by a bolted joint. An example of a generator frame is shown in Fig. 5.16 whereas the realization in the assembly hall shown in Fig. 5.17. The generator frame is already fixed to the main frame. The masses placed on the generator frame are, e.g., for the 2 MW wind turbine of this book: Generator 7,400 kg, Cooler 400 kg, Controller cabinet 540 kg, Nacelle housing 3,000 kg. In order to get the loads those masses are subject to accelerations (forwardbackward and side-side acceleration) experienced by the wind turbine nacelle; up-down movement are confined due to mounting to the tower from below and can be neglected. Including safety factors, ultimate accelerations found for the 2 MW wind turbine are about 3.5 m/s2 for side-side and about 4.7 m/s2 forward-backward. In addition also possible short circuit in the electrical equipment has to be considered which could lead to a torque at the high speed shaft of about 50 kNm (including safety factors).

100

5 Structural Components

Fig. 5.16 Generator frame seen from different angles

Fig. 5.17 Generator frame in assembly hall

Structural proofs will utilize FE techniques for generator frame as for the other structural parts as well. For the bolt FE analyses are used to check sliding and gaping of the connection between generator frame and main frame. Also here high quality bolts are needed with a bolt grade 10.9.

5.5

Main Shaft

101

Fig. 5.18 Main shaft in assembly hall

5.5

Main Shaft

The purpose of the main shaft is to fix the rotating hub to the fixed main frame via the main bearing. To this end, the main shaft of a wind turbine can have many different shapes. In some designs it is just the inner ring of a bearing that is connected to the hub. In this section we focus on a less compact realization of the main shaft suitable for the 2 MW wind turbine used in this book and shown in assembly hall in Fig. 5.18. A 3D model is shown in Fig. 5.19.

5.5.1

Sizing and Analysis

To determine loads we utilize again Fig. 5.12 for illustration and the results for F1 and F2 provided in Eqs. 5.9 and 5.10. What is needed are the sectional loads along the rotor axis z, i.e., Fx (z) and M y (z). From the results previously obtained for the machine frame, we obtain

102

5 Structural Components

Fig. 5.19 Model of a main shaft

 Fx (z) =  Mx (z) =

z≤a Fx 1 (M − F a) z >a x b

(5.14)

z≤a M − Fx z M − Fx a + b1 (Fx a − M)(z − a) z > a

(5.15)

In Fig. 5.20 the sectional loads are shown with Fx neglected as the bending moment dominates the main shaft design. The linear decrease of the moment motivates the geometry of the shaft. As we will see below, at the rear end of the shaft it is the torque that sizes the component. A side remark concerning alternative concepts may be appropriate at this point. For two-bladed wind turbines teeter hinges between rotor and main shaft have been used. One can imagine that for a large wind turbine the teeter hinge is a demanding concept. But the effect would be that the bending moment that is sizing the machine frame, main shaft and other components in the conventional design is largely decoupled from the structure. Blades are still bent of course, but the resulting moment is not transferred to the shaft. This would lead to a lighter main shaft. Coming back to our concept, to determine the rough size of the main shaft at the main bearing we first consider the bending moment which we assume to be about M = 6000 kNm

Fig. 5.20 Sketch of loads (forces and moments) as a function of z, distance along the main shaft

5.5

Main Shaft

103

for the 2 MW wind turbine. With a yield strength for material 34CrNiMo6 of R p0.2 = 540 MPa the limiting yield is σmax = 491 MPa. Employing the section modulus of a circular disc—neglecting the bore for simplicity −, W = π D 3 /32 we equate M/W to compare with the yield strength kN 32 6000 kNm = 491 · 103 2 → 3 π D m 32 6 3 3 D = m π 491 D ≈ 0.5 m

M/W =

(5.16) (5.17) (5.18)

Concerning the loading by torque we have to analyze the shear stress, i.e., τ = Mtorsion /W P , wherein W P = 2W = π D 3 /16 denotes the respective section modulus. Assuming Mtorsion = 2200 kNm we find τ=

N 2200 · 103 Nm = 90 D 3 π/16 mm2

(5.19)

for D = 0.5 m. This value needs to be compared to allowed τmax . Using σmax = 491 N/mm2 the maximum shear stress is τmax ≈ 0.65 σmax = 320 N/mm2 (Wittel et al. 2021). In its turn the minimum diameter can be calculated using Eq. 5.19.  Dmin =

16 · 2200 · 103 π · 320 · 106

1/3 m = 0.33 m

(5.20)

The geometry of the main shaft is much simpler than that of the main frame, however, loads are still rather demanding. Hence material choice is usually high quality steel such as 34CrNiMo6 and the main shaft is forged to withstand the loads. For the 2 MW wind turbine of this book an overview drawing is shown in Fig. 5.21 which also includes a cross section (Sectional view A-A). The main shaft is hollow to allow for supply lines between hub and nacelle. Also from structural point of view the inner part is not important because it is close to the neutral axis. Results of an FE analysis for ultimate loads is shown in Fig. 5.22. Result of an FE analysis for fatigue damage is shown in Fig. 5.23. Fatigue damage depends on surface roughness Rz (sometimes Ra ).2 Surface roughness is a source of micro cracks that leads to larger damage the larger the surface roughness is. Damage values shown in Fig. 5.23 are for Rz = 100 µm. In order to improve resilience against damage, alternatively to increasing the size or improve material quality, the surface can be smoothed at rather reasonable expenses. The surface area in Fig. 5.23 that is grayed has been treated to have a surface roughness of Rz = 32 µm. The respective FE analysis for fatigue damage is shown in insert of Fig. 5.23. 2 There is a subtle difference between R and R , which is not elaborated here. Both definitions are a z used in industry, hence care has to be taken, if one wants to compare results.

104

Fig. 5.21 Overview drawing of the main shaft

5 Structural Components

5.6

High Speed Shaft

105

Fig. 5.22 Van Mises stress for load case with maximum bending moment at flange

Fig.5.23 Result of fatigue analysis for surface roughness Rz = 100 µm. Area with surface roughness Rz = 35 µm are shown in grey with respective fatigue analysis result shown in inset

5.6

High Speed Shaft

The purpose of the high speed shaft is to transfer torque from gearbox to generator with an alignment tolerance of about 1◦ and a deflection tolerance of a few millimeter. Such a tolerance is necessary, since the frames used to attach main shaft, gearbox and generator are flexible structures and may lead to misalignment of the mechanical drive train and additional reaction forces. Due to gear ratio rated torque for a 2 MW wind turbine is about 14 kNm, extreme torque about 32 kNm. In order to protect the gearbox against possible short circuits

106

5 Structural Components

Fig. 5.24 High speed shaft mounted between generator (left) and gearbox (left) with brake disc on right side

in the generator (leading to instant high brake torque) high speed shafts usually have a torque limiter, in this case a reasonable value would be about 50 kNm. A high speed shaft employed in the 2 MW wind turbine is shown in Fig. 5.24. Material of the shaft besides steel is also glass fibre plastics. In order to allow slight misalignment the bearings on both sides possess a gimbal expansion joint.

5.7

Rotor Lock

The rotor lock is employed during service work to fix the rotor at a certain position so that hub and nacelle are form-closed and not moving against each other. In this case, the rotor has to be moved into the proper position for locking. This can be achieved manually or automatically. The rotor lock itself can be actuated hydraulically, electrically, or manually. The rotor lock has to withstand loads occurring during maintenance (maintenance design load cases). Since the loads are high for the 2 MW wind turbine of this book two rotor locks have been installed, see Fig. 5.25. Extreme loads only are considered. Fatigue life time is not relevant, since the rotor lock is only used during maintenance. The extreme torque including a safety factor of 1.5 is 1307 kNm. This should be compared to the extreme value that occurs during operation of the wind turbine of more than 2000 kNm and a rated torque of about 1300 kNm. Since there are two rotor locks installed a torque of 653 kNm has been introduced to the hub flange of Fig. 5.26 that provides a simplified FE model of the situation with only one rotor lock. Due to symmetry the result is also valid for the second rotor lock. Most important is that no shearing-off happens, however, a slight plastic deformation is

5.7

Rotor Lock

107

Fig. 5.25 Part of machine frame with main shaft showing hub flange and rotor lock bushing (2 of 3 visible) Fig. 5.26 FE model of shaft and main frame with rotor lock

allowed. A drawing of the pinion (sectional view) to be engaged into the rotor lock bushing is shown in Fig. 5.27, the realization of the rotor lock in Fig. 5.28. In this case the actuation is manually. High quality material of the bushing is 34CrNiMo6 with a tensile yield strength of R p0.2 = 700 MPa (same as main shaft) and for the pin 42CrMo4 with a tensile yield strength of R p0.2 = 650 MPa.

108

5 Structural Components

Fig. 5.27 Sectional view of rotor lock pinion with hand rail for manual activation

Fig. 5.28 Manual rotorlock pinion implemented into the machine frame

5.8

Nacelle Cover and Spinner

Nacelle and hub are covered by a housing. The purpose is to protect components and systems inside the nacelle and hub against environmental influence like water, dirt and dust. In addition the housing also serves as lightning protection and protects people during maintenance work. The housing for nacelle is usual called nacelle cover and for the hub spinner. Hubs of some wind turbines go without a spinner. In this case the hub cast is already closed against the mentioned influences. The housing has to fulfill quite a few requirements which are more or less all related to the loads that could occur during the life time of the wind turbine. Those loads are due to wind, snow and ice as well as other live loads. Examples are loads related to safety equipment (such as arrestors) or loads from a crane, if integrated into the nacelle housing.

5.8

Nacelle Cover and Spinner

5.8.1

109

Nacelle Cover

Most of the wind turbines use glass fiber reinforced plastic for nacelle covers, however, in some cases also alternatives are used. Aluminum has been utilized, e.g., by Enercon. Also, the housing could be integrated into the support structure. This was mentioned already for the hub but is also possible for the nacelle and in this case steel is used as a material choice. The laminate lay-out of a housing build of glass fiber reinforced plastic is provided in Table 5.3. In Table 5.4 a comparison of material properties between steel and typical glass fiber reinforced plastic (GFRP) used for housing is provided. An example of a nacelle cover is shown in Fig. 5.29. It consists of three main shells. A top cover encoded by brownish color and a bottom cover that is parted in the middle and color coded as greenish and blueish in the Figure. This way it is easy to attach the cover after assembly of the machine is completed. The yellowish pads seen at the cover are reinforcements to fix the shells to the support structure which shown in Fig. 5.30. A drawing of the housing is provided in Fig. 5.31. In order to estimate the loads from wind we utilize a reference wind speed of VRef = 50 m/s (i.e. wind class 1 according to IEC classification). The maximum wind speed (gust) is vmax = 1.4 VRef . The air pressure p due to this wind speed applied to the nacelle is p = cD

1.225 2 N kN  2 vmax = 1.0 70 2 ≈ 3 2 2 2 m m

(5.21)

Table 5.3 Layout of laminate for nacelle cover Laminate structure

Number of lay-ups

Gel coat (unsaturated polyester) in acc. to DIN 16946

0.5 · · · 1.0 mm

Reinforcing layer of glass fiber, cms 300 g/mm2 (mat)

1 layer

Reinforcing layer of glass fiber, cms 450 g/mm2 (mat)

11 layers

Top coat (unsaturated polyester) in acc. to DIN 16946

0.5 mm

Minimum sum of wall thickness

10 mm

Table 5.4 Material properties of steel and glass reinforced plastic (GRFP) Parameter

Symbol

Unit

Young modulus

E

N/mm2

210000

8500

Poisson number

μ

–

0.3

0.32

Tensile strength

Rm

N/mm2

510

100

Tensile yield strength

R p0.2

N/mm2

355

100



kg/mm3

7.85 · 10−6

1.75 · 10−6

Density

Steel

GFRP

110

5 Structural Components

Fig. 5.29 Nacelle cover for a 2 MW wind turbine consisting of three shells Fig. 5.30 Support structure for a nacelle cover

The nacelle size is approximately 4 m times 10 m which leads to a perpendicular force of F⊥ = 120 kN. This is quite large and requires a robust nacelle design. Due to the different loading mentioned above, structural analyzes are rather complex. They have to include stress analysis, buckling due to all load cases for housings, i.e., GFPR cover for nacelle and spinner. Also the support structure itself and all joints between housings, support structure and machine frame have to be considered. Typical load cases that need to be considered are, e.g., wind loads from all sides for upwind (pressure), downwind (suction) and lateral sides, live loads, snow and ice loads, dead loads of nacelle frame and crane, the

5.8

Nacelle Cover and Spinner

111

Fig. 5.31 Overview drawing of a nacelle with cover, side view (left), front view (right)

maximum crane capacity, gravity, to name a few. Not all loads act simultaneously, hence one has to proof load combinations and in addition each particular design load comes with a safety factor that vary from 1.5 up to 1.5 depending on the assumed uncertainty and the appearance probability.

5.8.2

Spinner

Most of the modern hubs have a spinner to protect the interior of the hub. Nevertheless there are also without spinner. This has the advantage of saving one additional component, i.e., the spinner, however, on he other hand maintenance work could more challenging, because service personnel has to get outside. A picture of hub including the spinner is shown in Fig. 5.32 (left). The hub made of cast is blue and the spinner being glass fiber reinforced plastic is white. In a maintenance situation the spinner is large enough to carry service personnel. Hence to maintain the hub it is not necessary to leave the inner area, which makes service more comfortable. An FE model for the spinner is shown in Fig. 5.32 (right). Besides wind loads also live load of service personnel standing inside has to be considered for structural analysis. This is indicated by the red arrow seen in Fig. 5.32 (right).

112

5 Structural Components

Fig. 5.32 Hub and spinner placed in the assembly hall (left) FE model of spinner for hub, red arrow indicates life load on plate (right), see text

6

Bearings

A wind turbine includes a number of large bearings. Typically those bearings are not available on shelf, but must be fabricated according to the specification provided by the design house or the manufacturer. Bearings to be considered are blade bearings connecting the blade to the hub, main bearing that carries the rotor, and yaw bearing that enables alignment of nacelle to the wind. In order to size bearings ultimate and fatigue loads are relevant.

6.1

Main Bearing

Main bearings are all rolling bearings, e.g., spherical roller bearings. Friction bearings (as, e.g., used in propulsion unit of ships) are not utilized in wind turbines. The purpose of the main bearing is to support the rotor and enable rotation. The 3D model of shaft and bearing housing carrying the main bearing is shown in Fig. 6.1. It carries aerodynamic loads and weight of the rotor. Rotational speed of modern, large wind turbines is rather low, i.e., 10 · · · 20 rpm. The bearing is chosen to fit the rotor shaft. In other words the diameter of the rotor shaft, which is designed to carry the rotor during life time under all circumstances, dimensions the main bearing. Lubrication is mostly by grease, rarely by oil and in modern wind turbines fully automated through a lubrication unit. Bearing used for 3-point suspension are spherical roller bearings (SRB). Many wind turbines use SRB. A bearing consists of two rings—outer and inner ring—and roller elements in between. The roller elements can have a point contact with the raceway (like balls) or a line contact (like a cylinder). Spherical roller bearings used as main bearings should be self aligning. To this end the raceway is double pitched and the rolling elements are slightly barrel-shaped. Hence the raceway of the outer ring is concave, the inner ring has two raceways each shaped to match the barrel-shaped rollers. In addition, the bore hole could be cylindrical or tapered depending on the design of the local shape of rotor shaft. An example of a SRB arrangement is shown © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 U. Ritschel and M. Beyer, Designing Wind Turbines, Synthesis Lectures on Renewable Energy Technologies, https://doi.org/10.1007/978-3-031-08549-9_6

113

114

6 Bearings

Fig. 6.1 Main bearing at low speed shaft including main bearing housing

in Fig. 6.2. Some companies that deliver such bearings also provide catalogs to determine the preliminary dimension. These are helpful prior to contracting a particular company. Nevertheless, final sizing of the bearing will be done by the supplier based on the complete loads to be provided by the designer. When identifying relevant loads we distinguish between equivalent dynamic loads, equivalent static loads and maximum radial loads. Input are radial Fr and axial Fa loads, for static loads with maximum values denoted by F0r and F0a . The general structure of the formulas used is Fig. 6.2 CAD model of a spherical roller bearing

6.1

Main Bearing

115

P = X Fr + Y Fa

(6.1)

P0 = X 0 F0r + Y0 F0a

(6.2)

wherein P(0) denotes the equivalent dynamic (static) load and X /Y(0) the radial/axial factors of the bearing given in the catalogs. However, also a minimum radial load is relevant to keep the rolling elements from sliding. The minimum radial load scales with the basic dynamic load rating C, which is also indicated in the bearing catalogs. By use of the equivalent static load and the static safety factor s0 the static load rating C0 is defined by C0 = s0 P0

(6.3)

Guiding value for normal operation of a (spherical) roller bearing for structural safety is s0 = 3.5. Finally, we consider durability equations. According to ISO 281 the nominative lifetime in hours L 10h is (C/P) p 60n = 106 (C/P) p

L 10h = 106 L 10

(6.4) (6.5)

wherein the exponent p = 3 for point contact and p = 10/3 for line contact and n is the rated speed in rpm, hence 60 n denotes the number of rotations in one hour. The second equation then provides the number of turns to failure. The equation provides the lifetime with failure probability of 10%. To give an example we need to determine loads and the geometry of the shaft. The geometry of the shaft is also determined by the loads, however, choice of material can change the size. Since the shaft is highly loaded and the bearing costs are strongly increasing with size, some optimal value has to be found. For the 2 MW sample wind turbine used in this book, the size of the shaft requires a bore diameter of the bearing of 670 mm. The static loads at the position of the bearing are F0r = 2000 kN, F0a = 600 kN for radial and axial loads respectively. With this information we utilize the bearing catalog and find a preliminary bearing, e.g. by FAG with the designation “240/670-E1A-MB1” (FAG Schaeffler Catalog 2022). The catalog provides all needed data for preliminary calculation. i.e. Y0 = 2.34 for static axial factor (the radial axial factor is always chosen X 0 = 1.0), The basic load ratings C = 10.5 · 106 N for dynamical loading and C0 = 21.1 · 106 N for static case. From Eq. 6.2, the equivalent static load P0 is then P0 = 1.0 · 2000 kN + 2.34 · 600 kN = 3400 kN Using a guiding value of s0 = 3.5 for normal rotating roller bearings and Eq. 6.3 we get for the actual basic load rating C0 = 3.5 · 3400 kN = 11.9 · 106 N < 21.1 · 106 N

116

6 Bearings

which is smaller than the basic load rating and hence form static point of view the bearing can be used. To evaluate the dynamic load we utilize Eq. 6.1, which refers to the number or rotations that 90% of the bearing survive. The ratio of radial to axial forces is Fr /Fa = 600/2600 = 0.3 This value is larger than the design value e = 0.28 from the catalog table, which implies factors of X = 0.67 and Y = 3.56, i.e., use of Eq. 6.1 leads to P = 0.67 · 2000 kN + 3.56 · 600 kN = 3476 kN Using this value along with the basic dynamical load rating C in Eq. 6.5 we find for the allowed number of rotations L 10 =

10.5 · 106 · 106 = 39.8 · 106 3.476 · 106

This has to be contrasted with the number of rotations during the 20 years lifetime of the wind turbine. With rated speed of 15 rpm the maximum number of turns adds up to N = 15 · 60 · 24 · 365 · 20 = 158 · 106 , which appears roughly 4 times larger than the allowed value. Note that so far we have assumed the ultimate value for P only. Instead of using one value for 20 years only, we use a spectrum Fri for each time interval qi = t/20a. The idea behind is that the load is not always maximum but varies from zero to maximum values. The resulting histogram for the 2 MW wind turbine is a load duration distribution given in Fig. 6.3. The equivalent effective load Peff is given by  Peff =



3/10 qi ·

10/3 Fri

(6.6)

i

Fig. 6.3 Load duration distribution of radial dynamical loads as result of load simulation in linear (left) and logarithmic (right) scale

6.1

Main Bearing

117

Table 6.1 Lifetime and total revolutions for main bearing of 2 MW wind turbine depending on assumptions on load rating and probability of operation experience Probability

90%

95%

99%

Load rating in kN C

9520

Rotations L x%

2.45 · 109

1.52 · 109

5.14 · 108

Lifetime in years

261.2

162.0

54.9

Load rating in kN C

8000

Rotations L x%

1.37 · 109

8.50 · 109

2.88 · 108

Lifetime in years

146.3

90.7

30.7

In this example Peff = 916 kN. This value is smaller, since during 20 years the wind turbine load is not always a constant maximum value, but varies due to wind conditions. To find those quantitative values (of the histogram), we need to make a dynamical load simulation. Depending on the basic dynamical load rating C we find the allowed number of rotations and the respective life time. They are shown in Table 6.1. Although preliminary sizing of bearings helps to speed up development process, final proof has to be performed by the supplier. To this end FE models are utilized modeling the bearing including details of the roller bodies and in cases also the surrounding structure as the vicinity is not rigid, but reacts elastically.

6.1.1

Bearing Housing

The main bearing is hold by a bearing housing that is fixed to the main frame. The main bearing housing attached to the main frame is shown in Fig. 5.9. A drawing of the main bearing housing for the 2 MW wind turbine of this book is provided in Fig. 6.4. For sizing, loads are introduced into the main bearing housing and analyzed by FE methods. A result for extreme value analysis is provided in Fig. 6.5. The machine frame is cast from EN-GJS-350-18U-LT material. With a material safety factor of γ M = 1.1 this leads to an elastic strength of σ R = R p0.2N /γ M = 200 MPa/1.1 = 182 MPa. As usual, for ductile materials, the von Mises yield criterion can be used. The maximum stress appearing in the analysis is 289 MPa which is above the yield value of 182 MPa. In this particular case the finding is not critical, since all stresses above 182 MPa occur only in very small areas with notches on single nodes. Therefore, these stresses will relieve due to plastification of the material and do not influence the durability of the structure.

118

Fig. 6.4 Drawing of main bearing housing

6 Bearings

6.2

Pitch Bearing

119

Fig. 6.5 Extreme value analysis (van Mises stress) of main bearing housing

6.2

Pitch Bearing

The blade or pitch bearing is the component between blade and hub which allows that the blade can be turned around the blade axis. In early wind turbines the connection between blade and hub was a fixed connection, however, modern wind turbines usually have a bearing for pitching the blade, e.g., if the power available in the wind becomes larger than the rated power of the wind turbine. In case of electrical pitch drive one of the bearing rings is geared, which is not necessary for a hydraulic pitch drive. The bearing rings are fixed via bolt connections to blade and hub respectively. The blade bearing utilizes single-row or double-row ball-bearing slewing ring. A sample is illustrated in Fig. 6.6. Material for both bearing rings (with or without gear) of the 2 MW wind turbine of this book is 42CrMo4 V. Further parameters are listed in Table 6.2. Sizing of the blade bearing relies on loads at the blade root. The relevant extreme loads for the 2 MW wind turbine are given in Table 6.3.

120

6 Bearings

Fig. 6.6 Illustration of one-row (left) and two-row (right) ball-bearing slewing ring. Example from Deutsche Großwälzlager GmbH, Rostock Table 6.2 Main parameters of the blade bearing for the 2 MW wind turbine Bearing

Value

Bolt circle

Outer

Inner

Outer diameter D A

2404 mm

Connection to

Hub

Blade

Inner diameter D I

1896 mm

Hole circle diameter

2324 mm

2110 mm

Height

H

199 mm

Number of bolts

79

60

Mass

m

1904 kg

Bolt size

M 36

M 36

6.3 Yaw Bearing

121

Table 6.3 Extreme loads at blade flange. Values are given with respect to the coordinate system B which is the blade coordinate system fixed at hub. Safety factors included as shown. Rotor diameter and blade number where extreme values occur are given in parenthesis at the respective design load case (DLC) identifier

6.3

Yaw Bearing

The yaw bearing is the component between nacelle and tower which allows that the nacelle can be turned around the tower axis. The yaw bearing is a slow moving bearing, i.e., for large modern wind turbines rotational speed is about 0.5◦ /s. The purpose of yawing is that the rotor can be aligned to the wind direction. Therefore the nacelle might turn several times around the tower axis. In order to prevent damage from the electrical cabling system the nacelle will stop after 2 · · · 3 turns, the wind turbine rotor will stop and the nacelle will turn back into the original position while the power production is interrupted. To drive the system one of the rings is geared. Besides single-row or double-row ball-bearing slewing ring along with a braking system also friction bearings are utilized. Material for both bearing rings (with or without gear) of the 2 MW wind turbine of this book is 42CrMo4 V. Further parameters are listed in Table 6.4. Sizing of the yaw bearing relies on loads at the tower top flange (yaw bearing K2). The relevant extreme loads for the 2 MW wind turbine are given in Table 6.5.

122

6 Bearings

Table 6.4 Main dimensions and material data of the yaw bearing for the 2 MW wind turbine Bearing

Value

Bolt circle

Outer

Inner

Outer diameter D A Inner diameter D I

3096 mm

Connection to

Hub

Blade

2617 mm

Hole circle diameter

2908 mm

2698 mm

Height

H

159 mm

Number of bolts

64

64

Mass

m

1894 kg

Bolt size

M 36

M 36

Table 6.5 Extreme loads at tower top flange. Values are given with respect to the coordinate system K2 which is the tower coordinate at the top flange. Safety factors included as shown. Rotor diameter where extreme values occur are given in parenthesis at the respective design load case (DLC) identifier

7

Gearbox

Implementation of a gearbox into a wind turbine opens the possibility to utilize high speed generators. High speed generators have the advantage that they are lightweight and can be produced in a standard fashion. They are inexpensive to be fabricated, can be unitized and easily exchanged in case of malfunction. The special challenges occurring in wind turbines through high loading and high number of load cycles are then bound to the gearbox and its integration into the rotor drive train and load flow. There have been many gearbox failures in the early history of utilization in wind turbines. However, by now, most of the challenges are understood and solved. Nevertheless, in order to build a proper gearbox for any of the concepts mentioned in this section, knowledge and experience within a manufacturer is a mandatory prerequisite for a successful implementation into the whole structure. The load situation for the gearbox is complex and need careful analyses in each case.

7.1

Gearbox Concepts

The purpose of a gearbox is to increase rotational speed, typically from about 10 . . . 20 rpm which is the typical speed range of modern aerodynamic rotors to about 1000 . . . 2000 rpm of the generator. In turn, the torque acting is reduced linearly with respect to the gear ratio involved. We distinguish two basic elements that are used to alter the gear ratio. One is by spur gear the other by planetary gear. For wind turbine gear boxes these elements are combined to optimize performance. A spur stage inside a wind turbine gearbox consists, e.g., of two gear wheels, i.e., a drive gear with large diameter and many teeth and a driven gear with smaller diameter and less teeth. A sketch is shown in Fig. 7.1. Sometime double helical gears are used, since they produce less noise.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 U. Ritschel and M. Beyer, Designing Wind Turbines, Synthesis Lectures on Renewable Energy Technologies, https://doi.org/10.1007/978-3-031-08549-9_7

123

124

7 Gearbox

Fig. 7.1 Sketch of spur gear (left) and planetary gear (right)

A planetary gear is more complex. A simple sketch is shown in Fig. 7.1 (right). It consists of a drive wheel (green), so-called planetary carrier which carries several intermediate pinions (blue), so-called planet pinions or simply planets. The blue wheels are kept in concurrence by a fixing outer ring gear (dark grey) and drive the central pinion (yellow) which is the so-called sun-wheel. Alternatively, one could keep the green element fixed to have the planet wheels concur and use the dark grey ring gear as a drive gear. Also for planetary gears helical gearing is utilized to reduce noise. Planetary stages realize a gear ratio of about 1:6. Based on the power of the wind turbine a modern gearbox for rated power above about 2.5 MW mostly consists of two planetary stages and one spur stage. This concept is a result of cost and efficiency studies.Total gear ratio is about 1:70 up to 1:110. Gear boxes for smaller wind turbine predominantly consist of one planetary stage and two spur stages. A schematic cross section of a gearbox with two planetary stages and one spur stage, is shown in Fig. 7.2. The first planet carrier (green part) is connected to the main shaft via a shrink disc. Also indicated are torque arms. The planet pinions are shown in blue and the ring gear (dark grey) is connected to the gearbox housing. The sun pinion of the first planetary stage (yellow) constitutes the planet carrier of the second stage. The planet pinions of the second stage are shown in green. The sun pinion of the second stage is the constitutes the high speed gear driving the high speed shaft. This is connected to the generator with a (preferably) flexible coupling. Depending on the exact internal design high speed shaft and low speed shaft may have different rotational orientation. This has to be checked before adding the generator to complete the drive train. An example of such a gearbox for a 2 MW wind turbine is shown in Figs. 7.3 and 7.4. This gearbox is for a 2 MW wind turbine and based on the three point bearing concept. It utilized two planetary stages and one helical stage. With a mild redesign the helical stage can be slightly adjusted to change the final gear ratio. So the same concept can be utilized for slightly different rotor speeds. For the 2 MW wind turbine a top view of the outline of the gearbox with the relevant components mentioned in this section is displayed in Fig. 7.5. Total length given in the

7.1

Gearbox Concepts

125

Fig. 7.2 Schematics of gearbox with two planetary stages and one spur stage

Fig. 7.3 Gearbox used in the 2 MW GeoHo wind turbine as seen from low speed side

drawing is about 3.5 m. From left to right we recognize the shrink disc to take the main shaft of the rotor, the torque arms to be connected to the torque support via two pins. The main body of the gearbox carries the different gear stages (not shown). To the right the cooling system is shown and at the far right end the slip ring mounted to the low speed shaft and on top the parking brake and the high speed shaft.

126

Fig. 7.4 Gearbox used in the 2 MW GeoHo wind turbine as seen from high speed side

Fig. 7.5 Outline of gearbox with low speed side (left) and high speed side (right)

7 Gearbox

7.2

Shrink Disc

7.1.1

127

Alternative Gearbox Concepts

Beside those mainstream gear boxes used many times in standard wind turbines there are more concepts that meet challenges and also have some advantages. Nevertheless, production has not yet reached industrial scales as is the case for the concepts mentioned before. The fusion drive by Moventas utilizes two planetary stages to arrive at a medium gear ratio. The respective generator built by TheSwitch is arranged around the second planetary stage which then builds a very compact unit. The companies offers two versions one for 3 · · · 4.1 MW and one for 6 · · · 7.7 MW rated power output, depending on the actual rotor size and hence rotational speed of the aerodynamic rotor. The diameter of the arrangement is 2.3 and 2.8 m, which is rather small indeed, compared to direct drives. Another innovative concept is provided by Voith, i.e. the Voith WinDrive. It utilizes a gear box with two planetary stages and a planetary-type stage between gearbox and generator wherein the ring gear is used to adjust the gear ratio to eventually keep the generator rotor at constant speed. This is realized by a hydraulic torque converter that is driven by the same sun wheel and drives the ring wheel. As a consequence the generator can be kept at constant speed, even for variable speed of the aerodynamic rotor. A further gearbox concept named AeroGears™ was introduced by Renk for the Areva offshore wind turbine. The basic concept was developed for a 1.5 MW machine. Integration of this concept into the 5 MW Areva off shore wind turbine drive train is referred to as the multibrid concept, which is highly integrated, uses a single stage planetary gearbox, a medium speed generator and a single main bearing instead of a main shaft. This way tower top mass is reduced. At times when is was build is was the lightest nacelle of 5 MW wind turbine.

7.2

Shrink Disc

The purpose of shrink discs is to connect the low speed or main shaft to the gearbox entry unit. The coupling has to be capable to withstand high torque. Method of choice in such case is a frictional joint. Form-fit joints, e.g., by bolts or tongue-and-groove joints are not used for such high torque connections. The rear end of the main shaft is positioned into a hollow entry shaft of the gearbox. The hollow shaft of the gearbox is braced by a ring that can be tensed up and this way the hollow shaft is pressed against the main shaft end. The ring is called shrink disc. A shrink disc for this purpose is shown in Fig. 7.6. The mode of action is demonstrated in Fig. 7.7. The red and blue rings are the main parts of the shrink disc. They are moved against each other by tightening the bolts. Due to inclination between the red and blue rings the diameter is diminished and as a consequence the hollow shaft is compressed. This is indicated by the arrows pointing to the green area. Due to friction between the surfaces of main shaft (not shown in the figure) and hollow shaft (green) of gearbox the joint is fixed.

128

7 Gearbox

Fig. 7.6 Shrink disk pre-assembled to the hollow shaft of the gearbox ready to connect the main shaft of the gearbox

Fig. 7.7 Mode of action of a shrink disk, arrows indicate forces

The shrink disc is sized by the maximum torque. An impression on dimensions can be given through a preliminary sizing. To this end manufacturers provide torque tables along with reasonable diameters and powers that could be hold by a shrink disc. E.g. rough values for the shrink disc of the 2 MW sample wind turbine used in this book are: Diameter of hollow shaft d = 700 mm, diameter of the main shaft that fits into the hollow shaft dw = 570 mm, total diameter of the shrink disc D = 1900 mm, bolt circle diameter A = 800 mm, total length H = 430 mm, and a total possible length contraction of shrink disc e = 50 mm, to

7.3 Torque Support

129

name a few parameters. As a consequence the torque that can be hold by the shrink disc is about 6800 kNm. Final sizing will be provided by the manufacturer based on a detailed load simulation of the design house or/and OEM of the wind turbine.

7.3

Torque Support

The purpose of torque support is to keep the gearbox from rotation due to the torque imposed to its entry shaft. Depending on the bearing concept, i.e., two bearing or three point support, the torque support has additional tasks rather than just torque support. In the simplest case, when the wind turbine rotor is carried by two bearings the gearbox is fastened to the main shaft via previously mentioned shrink disk and the torque support just needs to cope for the reactive forces induced to the gearbox by the torque. In case of three point concept the torque support also carries off axial loading, which makes sizing more demanding. The torque support is part of the main frame as shown in Fig. 7.8. The gearbox carries pins that fit into the torque support, see, e.g., Figs. 7.3 and 7.4. Pins and support are both made of steel and an intermediate elastic material is used. This could be elastomers or a sandwich of several steel shells and elastomers. The right choice is not so easy and requires some experience, in order not to generate unnecessary resonances and/or constraint forces, depending on the drive train concept. A detailed view of the bushing is provided in Fig. 7.9. Process of structural sizing of torque support uses finite element method and includes the vicinity of the support as seen in Fig. 7.10. To do so typical material properties for a 2 MW wind turbine are provided in Table 7.1. Loads used refer to a 2 MW wind turbine with three point support. The residual forces are provided in Table 7.2. Vertical residual forces (x

Fig. 7.8 Torque support for gearbox

130

7 Gearbox

Fig. 7.9 Detail of bushing for the torque support with shells of steel and elastomers

Fig. 7.10 Finite element model for torque support with part of machine frame

Table 7.1 Material properties for sizing of torque support Poisson ratio

ρ (kg/mm3 )

Minimum Rm (MPa) yield strength (MPa)

169000

0.275

7.20E-06

200

320

Ø36

210000

0.3

7.85E-06

–

–

67 < t ≤ 200

169000

0.275

7.20E-06

220

370

Element

Material

Thickness E (N/mm2 )

Machine frame

EN-GJS- 60 < t ≤ 350-22-LT 200

Bolts

Steel

Torque supports

EN-GJS400-18ULT

7.3 Torque Support

131

Table 7.2 Residual forces parallel to rotor axis (x) and in vertical direction (y)

Fig. 7.11 Maximum equivalent stress (van Mises) for FXNR component on left and right side of torque support

direction) are generated by torque, horizontal perpendicular to the rotor axis (y direction) are generated by the bending moment. The example is for two rotor diameter 87 and 93 m. Results of the structural analysis (in this case van Mises stress) is provided in Fig. 7.11. Bolts are pre-tensioned and the load case for maximum vertical component on the right side (FXNR) is applied. Equivalent (van Mises) stress is exceeding the allowed yield, hence in this case equivalent plastic strain has to be analyzed. This is shown in Fig. 7.12. The small plastic deformation found are allowed in this case. In case of rotor support with two bearings, loads are induced by torque only, since off axial loads from the rotor are taken care of by the two bearing support. Hence layout of torque support is less demanding. However, since the complete layout is statically indeterminate, additional loads are induced by reactive forces coming from elastic deformation of the system. This is depicted in Fig. 7.13. Due to deformation of the mainframe the rotor axis pitches at the location of the torque support by amount , which amounts to a few millimeters. Modulus of resilience of typical elastomer bushings is about 100 kN/mm. Hence

132

Fig. 7.12 Equivalent plastic strain of torque support for the same load case Fig. 7.13 Demonstration of reactive forced due to deformation of the drive train

Fig. 7.14 Hydraulic torque support with blue and red lines symbolizing two independent hydraulic liquid circuits arranged to counterbalance the gearbox torque

7 Gearbox

7.4

Slip Ring and Rotary Union

133

the reactive forces on the gearbox add up to a few 100 kN. Those reactive forces can be reduced by employing hydraulic torque supports. The mode of action, e.g., as shown by ESM Mitch GmbH, of a hydraulic torque support is demonstrated in Fig. 7.14. Constraint forces are annihilated due to use of hydraulic reservoir connected cross wise. Of course elastomer elements for damping are still needed. However, the residual constraint forces are much smaller.

7.4

Slip Ring and Rotary Union

“Slip ring” or “rotary union” is situated at the rear end of the low speed shaft of the gearbox, see Fig. 7.15. The purpose of the slip ring is to supply all components situated in the hub and the blade with power and controller signals. The shaft is hollow to incorporate the cables and/or pipes from slip ring to hub. Components in the rotor are electrical pitch drives that need several kW of power, or in case that pitch is actuated via hydraulics, the hydraulic medium is feed through the rotary union and pipes. Further on heating of blade needs several 10 kW power that will also be provided from the nacelle to the hub. Information signals need a different cabling with rather low power level and could also be optical and contact free. Sizing of slip ring and number of ports have to be determined by the designer of the wind turbine. Figure 7.15 (left side) shows the final position of the slip ring at the rear end of the gearbox. The slip ring is a highly charged component as it is almost permanently running and hence needs careful maintenance. On the other hand it is a relatively inexpensive component that should be exchanged in case of malfunction, in order to prevent any more severe damage.

Fig. 7.15 Rear end of gearbox. Right side: rear end of (hollow) low speed shaft (left pinion), high speed shaft (right pinion). Left side: Slip ring mounted to low speed duct, high speed with brake disk

8

Bolt Connections

Bolt connections are substantial for wind turbines. In this chapter we focus on bolt connections between main structural components. These are joints between blade and hub, between hub and rotor bearing, rotor bearing housing and main frame as well as main frame and tower. Up to the bearing housing the mentioned connection are adjacent to rotary bearings and also involve connections to the respective slewing rings. In addition there are bolts connecting the tower sections.

8.1

Basics Concepts

Since the wind turbine is complex and hence addressed by many disciplines, as a consequence, we are facing two ways of treating bolts inside a wind turbine structure. Bolts treated according to mechanical engineering are all bolts inside the nacelle and the connection to the tower top flange. They are subject to small tolerances and their structural proof is according to (VDI 2230) (or similar) addressing systematic calculation of high duty bolted joints. Bolts treated according to civil engineering are those of the tower (except the top flange). They allow for larger tolerances and proofs are done according to Eurocode 3 (2005) addressing the design of steel structures for buildings and other civil engineering works using the limit state design philosophy. Ultimate proof of bolts refer to compression strength and tensile strength. Bolts are not allowed to exceed respective nominal stress values. High loading pressure of all participating elements such as bolts, washers, nuts, flanges should stay below the limiting values. Sliding of bolted connections is not allowed. In order to achieve proof against sliding surface treatment is allowed, if necessary. Friction coefficients can be enhanced by application of protective coating, e.g., utilizing Interzinc™. Proof against shearing-off has to be provided as well.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 U. Ritschel and M. Beyer, Designing Wind Turbines, Synthesis Lectures on Renewable Energy Technologies, https://doi.org/10.1007/978-3-031-08549-9_8

135

136

8 Bolt Connections

Fatigue proof has to be done for the total life time of the structure, e.g., 20 or 25 years. Damage should be 1 and are given in Table 8.2. For fatigue load analysis we utilize load spectra for the single bolt connection. In this case only axial loads need to be considered. The load spectrum, i.e., histogram of load cycles binned according to their occurrence is provided in Fig. 8.8 (left). The proof utilizes a reference stress curve according to (VDI 2230), which is a certain Wöhler line, dark black line, in Fig. 8.8 (right).

142

8 Bolt Connections

Table 8.2 Results of joint analyses No

Slipping

Overload

Pressure

Shearing

1

1.57

1.15

1.25

24.91

2

1.11

1.18

1.29

8.19

3

1.01

1.15

1.25

16.01

Fig. 8.8 Left: Load cycle frequency for axial force, Right: Number of load cycles as a function of stress cycles (S/N curve or Wöhler curve), actual stress thin line, reference bold line

8.3

Main Bearing Housing

The main bearing housing holds the main bearing in the defined position. It is connected to the main frame by bolts. The geometrical situation is shown in Fig. 8.9, which displays the geometry for use in a finite element model. For proof all materials should be defined. In the sample case these are given in Table 8.3. The relevant parameters for proof of this joint are shown in Table 8.4. For this joint we only show the result of proof against slipping. The pictorial result in shown in Fig. 8.10. From inspection of the color coding, slipping is proven to be smaller than 0.1 mm, which is sufficient. Slipping is different at different places of the connecting surfaces, since material is not stiff, but responses elastically to stress and connecting surfaces can elastically and locally be deformed.

8.3

Main Bearing Housing

143

Fig. 8.9 Finite element model of machine frame with bearing housing and bolts Table 8.3 Material of components involved in the joint between bearing housing and machine frame Part

Material

Thickness/ Diameter (mm)

Module of Elasticity (N/mm2 )

Poisson Ratio (–)

Density (kg/mm3 )

Minimum Yield Strength (N/mm2 )

All casted parts

EN-GJS350-22-LT

60 < t ≤ 200

1.69E+05

0.275

7.10E-06

n/a

Bolts

10.9

n/a

2.1E+05

0.3

7.85E-06

940

Outer ring of the bearing

Steel

n/a

2.1E+05

0.3

7.85E-06

n/a

Table 8.4 Material of components involved in the joint Bolt

STD - WE - M36 x 333 - 10.9 - 00

Washer

EN 14399-6-36

Coefficient of friction at the interface

0.5 (using Interzinc 698)

Tightening factor

1.2 (hydraulic tightening)

Minimum pretension

580 kN

Maximum pretension

700 kN (υ ∗ As ∗ R p0.2min = 0.91∗ 817 mm2 940 N/mm2 )

144

8 Bolt Connections

Fig. 8.10 Slipping of surfaces connected by bolts against each other

8.4

Tower Top

Connection between tower and nacelle is a slewing ring since the nacelle and hence the rotor has to be aligned to the wind direction. A schematic drawing of the 2 MW sample turbine is shown in Fig. 8.11. The tower top flange (yellow) is connected to the yaw bearing (red) and the yaw bearing to the nacelle (purple). In addition, there might be a break disc (green) clamped in either of the bolted connections, which is normally the one adjacent to the tower flange (yellow). Because of this complex mechanical functionality the tower top joint to the nacelle is considered a topic of mechanical engineering rather than civil engineering which is the case for the rest of the tower. In fact, only one joint is symbolized by a bolt, i.e., the connection of the top flange to the outer ring of the yaw bearing. A bolt of the connection between inner ring of yaw bearing and machine frame (main frame) is not displayed. The classification of bolt in this joint is given in Table 8.5 for bolts on both, inner and outer rings.

8.4 Tower Top

145

Fig. 8.11 Cross section of tower top joint to nacelle of 2 MW turbine

Table 8.5 Characteristics of bolt for top flange joint Bolts outer ring

Bolts inner ring

Connection to

Tower

Nacelle

Hole circle diameter (mm)

2908

2698

Number of bolts

64

64

Bolt size

M 36

M 36

Borehole distribution

Equal

Equal

Clamping length (mm)

180

196

Bolt class

10.9

10.9

Bolt yield strength Rp0.2 min. (N/mm)

940

940

Nominal preload (%)

70

70

Tightening procedure

Torque

Torque

Tightening factor α A

1.6

1.6

9

Yaw and Pitch System

Yaw and pitch system are essential for the operation of the wind turbine. The pitch system is also pivotal for the safe operation. Since they actively move other things, the yaw system the nacelle and the pitch system the blades, they are also called actuators. Design of actuators require knowledge in mechanical and electrical layout. In addition actuators are (mostly) driven by closed loop control that are supervised by the wind turbine controller. Therefore actuator systems are sometimes referred to as mechatronic systems. In the following two sections we discuss different concepts for these systems and the route to the right sizing.

9.1

Yaw System

The yaw system is designed to keep the rotor, i.e., the nacelle of the wind turbine, aligned to the wind direction. From control point of view only slow control is necessary for the actuator, unlike, e.g. for the pitch actuator (see next Sect. 9.2). In fact, two operational states are distinguished: yawing and rest. During a lifetime one can assume that only 10% of the time the wind turbine is yawing whereas for most of the time the nacelle is at rest and the task of the yaw system is to keep the nacelle fixed in the required position.

9.1.1

Layout

From mechanical point of view the yaw system includes a bearing consisting of a slewing ring with one or two rows of roller elements. Yawing is performed by 2 · · · 8 drives. The yaw motion during yawing is damped by brakes or by controlled interlocking some of the drives. During rest the yaw system is kept in position by brakes. Brakes are either calipers acting on a disc attached adjacent to the bearing or by brakes mounted inside the drives, or,

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 U. Ritschel and M. Beyer, Designing Wind Turbines, Synthesis Lectures on Renewable Energy Technologies, https://doi.org/10.1007/978-3-031-08549-9_9

147

148

9 Yaw and Pitch System

Fig. 9.1 Sectional drawing of yaw system carrier of the 2 MW GeoHo turbine

due to the high brake torque demand, by both. Also, if the number of drives is large enough fixing is also possible by complete interlocking the drives. A modern yaw system is shown in Fig. 4.8 for a 2 MW wind turbine. A respective sectional drawing is shown in Fig. 9.1. In this particular case the yaw drives are (almost) equally distributed. Drives are mounted to the main frame and will turn the nacelle with respect to the large gear ring seen in the lower part of Fig. 4.8. The gear ring will be fixed to the tower top flange. Figure 9.2 displays the same yaw system from below. In this case the calipers of the yaw brake system and part of the brake disk can be recognized. Alternatively to the above described yaw system, a friction bearing utilizing sliding (brake) pads can be used. The sliding pads are pre-stressed and the drives need to overcome the initial brake torque.

9.1.2

Sizing

The high and unsteady loads in different directions (bending and torsional w.r.t. the tower axis) lead to high demands for the design of the yaw system. Utilizing a moderate damping

9.1 Yaw System

149

Fig. 9.2 Yaw brakes of the 2 MW GeoHo turbine in assembly hall

by brake, when the nacelle is yawing, reduces small load amplitudes (all the ones that are smaller than the brake torque) that would otherwise add to fatigue damage of the gear rim. For sizing the yaw system the relevant load utilized is the torsion moment here denoted by MXK2. Normally MXK2 is rather large during normal operation for wind speeds above rated wind speed. In particular this quantity is used to size drives, brakes, gears, mounting of drives, and bolting of bearings. The extreme (or ultimate) torsional load for the 2 MW wind turbine is about 5800 kNm coming from normal operation at high wind speed. Fatigue loads can be extracted from extrapolating design load cases to the design life time of the wind turbine. To this end loads are binned into intervals and the respective exposure time added. The result is known as load duration distribution shown in Fig. 9.3. Note that the distribution is given in a logarithmic scale. In practice two operational variants have been utilized for yaw systems for the two operational states. Variant 1 consists of drives (D), motor brakes (B1) and mechanical brakes (B2). During rest brakes B2 keep the nacelle in position up to a certain MXK2. If this value is exceeded brakes B1 will add additional brake torque. During operation B1 will be released first, then B2 will be released up to a certain residual torque. The residual torque is useful in order to suppress small movements of the nacelle due to back lash. The drives D will move the nacelle against this residual torque. Drives D are sized such that they are able to move the nacelle at all times up to few rare load peaks.

150

9 Yaw and Pitch System

Fig. 9.3 Load duration distribution for torsional loads MXK2 around the tower axis of a 2 MW wind turbine

Variant 2 consists of drives (D) with controllable torque and motor brakes B1. During rest motor brakes B1 are engaged as well as drives are interlocked by using the brake torque of the motors to keep the nacelle in position. During operation first the motor brakes are released and some part of the drives move the nacelle into the desired position whereas other parts of the drives act against the movement to keep a small residual brake torque to damp the small movements due to back lash. Besides structural layout to secure mechanical integrity of the yaw system, also sizing of the drives and brakes is necessary. This will be addressed in the following. In fact, the operational strategy influences the sizing of those parts of the yaw system. We explain the decisions to be made for the 2 MW wind turbine used in this book. From inspection of Fig. 9.3 we find that MXK2 above, e.g., ±2500 kNm appear only few hours during the design life time of the wind turbine. More precisely, by summing up all the time bins above a limit of |MXK2| > 2,500 kNm we find that the total time, where the wind turbine experiences loads above this limit is roughly 11 h in 20 years. Looking more closely into the times series, we additionally find that the events with loads higher than the limit last few seconds only. This is generally the case and due to turbulence of wind. During this time above the given limit the wind turbine can slip a few degrees which is acceptable. Hence, we use the mentioned limit as a ultimate aerodynamic load assumption for sizing of brakes and drives. In addition to the aerodynamic load assumption mentioned before, the bearing also build up a brake torque. This slewing torque can be calculated using an empiric equation usually provided by the bearing manufacturer. As an example we show a formula originally provided

9.1 Yaw System

151

for the 2 MW turbine. MXK2R =

μ (4.37 MBK2 + FXK2 D + 2.185 FSK2 D) 2

(9.1)

wherein D is the raceway diameter provided in Fig. 9.1, μ = 0.006 the friction coefficient, FXK2 the axial force, FSK2 the radial force, MBK2 the residual bending moment and MXK2R the resulting friction torque. The mean friction torque can be calculated by using the mean values for the quantities on the right hand side. As an example we give results for two different versions of the 2 MW turbine with 87 and 93 m rotor diameter in the Table 9.1 Finally, for sizing the drives we need to include the residual brake torque engaged during operation for reason mentioned above. A reasonable value for this wind turbine is 500 kNm, however, the value might be slightly adjusted during initial operation of the wind turbine, in case the brakes develop undesired sound during operation. The result for the 2 MW wind turbine versions is shown in Table 9.2. From a practical and commercial point of view one would demand the same total drive torque for both variants which would be the maximum of the design values found for the two variants. In order to demonstrate how the previously mentioned slippage can be estimated prior to operation, we use a simplified approach. In reality many factors influence slippage such as inertia, deformation in the system, initial break loose torque, etc. To get an idea of the order of magnitude, as a role of thumb, in a simplified approach, Newton’s law for rotating systems can be used, i.e., ¨ M(t) = I  (9.2)

Table 9.1 Mean loads to determine friction loads Sensor

87 m

93 m

Unit

FXK2

1235.6

1259.4

kN

FSK2

143.5

139.0

MBK2

1346.6

1460.3

kNm

30.6

32.1

kNm

MXK2R

kN

Table 9.2 Total design loads for sizing of drives in kNm for two different rotor diameters, 87 and 93 m Description

87 m

93 m

Aerodynamic load

2500

2500

31

32

Bearing slewing mean torque Operational residual brake torque Total design torque

500

500

3031

2882

152

9 Yaw and Pitch System

where  is the yaw angle, I the moment of inertia of the rotor nacelle assembly around the tower axis and M(t) the excess of the torsional torque MXK2 as provide by the design load calculation. Since M(t) is given at discrete values for certain discrete time, the equation can be integrated numerically and as a result we find for the 2 MW wind turbine in question a few degrees of possible slippage which is acceptable. As soon as the external torque M(t) would be below the limit of |MXK2| = 2500 kNm the “uncontrolled” movement will stop. Finally, the gear ratio has to be determined. One gear ratio stems from the gear rim of the bearing w.r.t. the drive pinion and in addition a mechanical gearbox is needed, since commercial drives usually operate at about 1500 rpm. The gear rim contains 211 teeth that are driven by a pinion with 13 teeth. If we request a angular velocity of 0.5 ◦ /s (or 1/12 rpm), which is typical, an additional gearbox is needed with a gear ratio of about 1100.

9.2

Pitch System

The pitch system is located in the hub adjacent to the blade root. For safety reasons, modern wind turbines have independent pitch systems for each blade, i.e., each blade has its own drive, energy storage, and control. The pitch system allows to turn the blade around the blade axis (pitch axis) and as such has several purposes • Adjusting the global pitch angle • Controlled start-up of the system with the help of pitching the blade from larger to smaller angles • Adjustment of pitch angle with a pitch speed of up to about 5 ◦ /s during operation at wind speeds v > v R to keep the rotor speed at rated speed • In some cases adjustment of pitch angle below rated wind speed to optimize performance or reduce loads (so-called fine pitching) • Perform normal and controlled (aerodynamic) stop with about 5 ◦ /s • Perform emergency stop with about 7 ◦ /s as part of the safety system with the help of energy storage, i.e., pressure storage for hydraulic pitch and batteries or capacitor storage (so-called ultracaps) for electric drives.

9.2.1

Layout

A design example for electrical pitch is given in Fig. 9.4. Position of electric drives inside hub represents the most common arrangement. In few cases pitch drives are positioned outside the hub, e.g., at blade root and hence the gear ring is not inside but outside of the bearing. If the hub is large enough, maintenance appears more comfortable for the arrangement inside, as shown in Fig. 9.4.

9.2

Pitch System

153

Fig. 9.4 Exploded view of electrical pitch arrangement of 2 MW turbine

A 3D drawing of the 2 MW wind turbine is shown in Fig. 9.6. Numbers refer to Pitch box (1) containing the pitch controller, mounting for pitch drive including gearbox (2), and pitch motor (3). Letters are referring to the rest of the drawing collection, not shown here. The hub related to this drawing and ready for further processing during manufacturing of wind turbine in the assembly hall is shown in Fig. 9.6. Since the blade pitch angle moves only within a quarter circle use of a pinion driven by hydraulics is also possible. A design example for hydraulic pitch is shown in Fig. 9.7. An actuator of hydraulic pitch is shown in Fig. 9.8. For maintenance the pitch system should be equipped with a brake or lock to fix the blade angle at a certain position during maintenance (90◦ or 0◦ depending on maintenance concept).

9.2.2

Sizing

For sizing the pitch system the relevant load utilized is the torsion w.r.t. to the blade axis denoted by MXB. This quantity is used to size drives, gears and mounting of drives. The extreme (or ultimate) torsional load for the 2 MW wind turbine is about 130 kNm coming from normal operation at high wind speed with occurrence of gusts. This is about the load expected once in 50 years, which is the event horizon for a wind turbine with a design life time of 20 years. For safe operation at all times the pitch drives have be designed such that this ultimate torque MXB can be handled at all times. Since the pitch system is decisive for the performance of the wind turbine, modern simulation codes also include the pitch dynamically as, e.g., required by GL2010 guideline. This means that in a simulation the blade turns to the new pitch position under influence

154

9 Yaw and Pitch System

Fig. 9.5 Hub and spinner of 2 MW wind turbine in assembly hall

Fig. 9.6 Drawing of pitch arrangement for the 2 MW wind turbine, see text

9.2

Pitch System

155

Fig. 9.7 Design sample of hydraulic pitch arrangement

Fig. 9.8 Pinion for hydraulic pitch as shown during Husum Wind Fair

of external forces (in particular torque around the blade axis) acting on the blade and the friction of the blade bearing (similar to the treatment for yaw bearing) as well as the inertia of the whole system. In this case sizing of the drives can be done directly. In other words a particular pitch system arrangement can be validated for the particular wind turbine in question by simulation. In less sophisticated approaches that are still compatible to the IEC 61400 guideline the response of the pitch actuator to the pitch demand of wind turbine controller is modeled by a PT1 element (first order response). This means that the new pitch position is reached in the simulation irrespective of control details and external forces with a certain delay. From an academic point of view this is less desirable. From a practical point

156

9 Yaw and Pitch System

of view this requires some room for safety margins left to the experience of designers or suppliers. In practice only few types of electrical drives have been realized. Those are DC motors, asynchronous motors, and servo motors. Since the pitch system is an important component of variable-speed wind turbines, pitch speed and pitch angle must be precisely adjustable. This qualifies the drives to be used. In addition safety function, i.e., features of emergency drive, must be reliable. Due to the complexity of the pitch system and its importance it makes sense to engage experienced partners for design of pitch system for a wind turbine. Partners that could also serve as system providers and have the necessary equipment, e.g., test facilities, to qualify the pitch system selected. Since the pitch systems are independent of each other due to safety reasons, one could as well provide individual pitch demands for each of the blades instead of one global pitch demand provided by the main controller. Whereas the latter solution is well established and widely used the individual pitch control (often abbreviated as IPC) is relatively new. Individual pitch control utilizes measurements of the actual blade bending moments at or close to the blade root, e.g., by fiber optic sensing or strain gauges. From the stress data an algorithm determines an individual correction to the global blade pitch demand and eventually correct the (global) pitch demand accordingly. This could be done independently of the main controller.

Auxiliary Systems and Secondary Steel

10

Auxiliary systems are necessary to help the wind turbine being operated and maintained. Example for auxiliary systems are mechanical brake and various cooling systems. Some components such as fans are also consumers as they need power for operation. Auxiliary systems may run permanently or occasionally. From an electrical point of view, in order to dimension the transformer for the auxiliary systems this is recognized by partial average load ranging from few percent (e.g., roughly 10% for the yaw system) up to 100% during life time and on average. This information is needed to layout the auxiliary transformer used to provide power to the consumers. In this section we also include some remarks on secondary steel. Examples for secondary steel are ladders and platforms inside the tower. The layout of secondary steel is also part of the engineering process.

10.1

Mechanical Brake

Since the main brake system of modern wind turbines is based on aerodynamic brake by pitching the blades, the mechanical brake system plays a supplementary role and is hence sometimes just called parking brake. It is used during maintenance to hold the rotor in a certain position or to align the rotor to the rotor lock device, since the hub can only be entered, if the rotor lock is engaged. The mechanical brake is not sufficient to secure the hub. The second task is related to the safety chain or emergency stop of the rotor. At the end of emergency stop case the rotor has to be brought to a complete standstill. The brake is then activated, if the rotor speed is, e.g., below 0.5 rpm. Only in design of some anterior wind turbines a mechanical brake was used as a main brake system. Those brakes have been part of the safety system by “fail safe” layout. This means that the brake are passively closed and opened actively. Hence in case any failure occurs in the system leads to a sudden mechanical closing of the brake. Notably the structural © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 U. Ritschel and M. Beyer, Designing Wind Turbines, Synthesis Lectures on Renewable Energy Technologies, https://doi.org/10.1007/978-3-031-08549-9_10

157

158

10 Auxiliary Systems and Secondary Steel

stress imposed to the drive train by this procedure is very high and therefore alternatives have been developed.

10.1.1 Layout The mechanical brake is normally positioned at the high speed shaft, after the gearbox. Behind the gearbox the torque is reduced proportional to the gear ratio and hence the mechanical brake can have a relatively small size. The mounting place for the calipers is shown in Fig. 10.1 and the complete mechanical brake integrated into the high speed shaft is shown in Fig. 10.2. For direct drives, i.e., wind turbines without a gearbox, the mechanical brake can be integrated at different places, e.g., adjacent to the main bearing or within the generator. The operational concept is the same as before. Mechanical brakes are usually operated by hydraulic pressure, however, also electrical solution are utilized.

10.1.2 Sizing Since the mechanical brake is not engaged during operation, but just used to stop the rotor during idling or smaller rotor speed, the thermal and mechanical stress is rather low. Sizing is according to design load cases occurring during maintenance. For a wind turbine with

Fig. 10.1 Position of mechanical brake at the rear of gearbox

10.2

Meteorological System

159

Fig. 10.2 Mechanical brake mounted at high speed shaft and gearbox

gearbox the holding torque is about 10 kNm at the high speed shaft. The brake is closed actively.

10.2

Meteorological System

Each wind turbine has a meteorological station to measure wind speed, wind direction and ambient temperature. The meteorological station is normally placed on the roof top of the nacelle, fairly behind the rotor. Examples are shown in Fig. 10.3. What is actually needed is the wind speed and the direction far in front of the rotor. However, since obviously the rotor is the far most structure into upwind direction there is literally no place for any component or system in front of the rotor. To compensate the disturbance and the deceleration of the wind due to the rotating(!) rotor, signals from the sensors behind the rotor need to be corrected. Compensation is an art by itself based on parametric formulas where the parameters are mostly adjusted from experience and separate flow investigations. This will not be further addressed in this book. Since the wind measurements (speed and direction) are not part of the (fast) control algorithm, but merely needed for slower control (yawing) and/or operational alerts (cut-in, cut-out wind speeds) placement of sensor behind the rotor is acceptable. Nevertheless, correction of signals is important, in particular for aligning the rotor to the wind direction, and not to reduce efficiency of energy yield of the wind turbine due to misalignment.

160

10 Auxiliary Systems and Secondary Steel

Fig. 10.3 Examples of different realization of meteorological station on nacelle roof

In order to improve on that situation, what would be needed is knowledge of wind speed and direction at some distance in front of the rotor. This is subject of ongoing research and development and mostly based on utilizing Lidars.1 Such a Lidar might be placed on the nacelle with beam directed into upwind direction. Since the blades are relatively small free area between the blades appear to be enough space to detect the wind speed some 20 · · · 200 m in front of the rotor. There are quite a few challenges in utilizing such a Lidar

1 Light detection and ranging is using Laser emission to determine velocity of aerosols and this way the speed of wind.

10.3

Cooling and Heating

161

system in replacement of the standard MET system and therefore as of today, such a system is not used in series production of wind turbines. Temperature is also part of slow control and measured to check operation against allowed upper and lower limits. In some cases temperature is also an ingredient to determine air density to switch between different operational modes, if implemented and needed. Finally, the roof top is also a suitable place for aviation lights as can be identified in Fig. 10.3 in the lower right picture.

10.3

Cooling and Heating

Cooling systems are required to cope with heat related to losses in the system. Losses occur in mechanical and electrical components. They are unavoidable and the related energy is eventually converted to heat. Total loss starting from aerodynamic power captured by the rotor to electrical power delivered to the grid is about 10%. Wind turbines with a gearbox have a little more losses and direct drive wind turbines a little less. In Table 10.1 major losses of components and systems are collected.

10.3.1 Basics Heat Exchange Irrespective of details in layout, a heat exchanger brings two media close together, so they can thermodynamically react to reduce their temperature difference and, due to heat transfer, exchange heat. A rough sketch is provide in Fig. 10.4. Heat flow of a warm medium (w) can be described by the following formula Q˙ = −ρw V˙w c p (w) (T1 − T2 )

(10.1)

whereas the heat flow to the cold medium (c) is Q˙ = −ρc V˙c c p (c) (T1 − T2 )

(10.2)

Table 10.1 Major losses in a wind turbine of 3 MW rated power Component

Loss in %

Main bearing

T2 > T1 .

10.3.2 Layout Layout of cooling systems are usually done by specialized companies according to the requirements that have to be stated in the technical specification.

10.3.2.1 Cooling of Gearbox Primary cooling medium inside the gearbox is usually oil that is also used for lubrication. This oil is cooled preferably through oil/air or alternatively by oil/liquid plus liquid/air heat exchanger. The oil/air heat exchanger is mostly situated above the gear box with pipes rigidly coupled to the gearbox housing. The alternative cooling concept is used, if the outer cooler is at a distance to the gearbox, e.g., roof top. However, it is obvious that though utilization of two heat cycles this system is less effective. Simplified schematic diagrams are shown in Figs. 10.5 and 10.6.

10.3.2.2 Cooling of Generator For cooling of generator we distinguish high speed generator (utilized with a gearbox) and direct drive low speed generators. The electrical structure of a high speed generator is preferably cooled by an air/liquid plus liquid/ambient air heat exchanger or alternatively by a liquid/ambient air exchanger. Heat transfer through the surface of the generator is rather small and simply taken care of by the nacelle ventilation. Smaller wind energy converter up to about 500 kW rated power

10.3

Cooling and Heating

163

Fig. 10.5 Schematic diagram oil/air cooling of gear box

Fig. 10.6 Schematic diagram of oil/liquid plus liquid/air cooling of gearbox

Fig. 10.7 Schematic diagram of air/liquid plus liquid/air cooling of generator

are just equipped with cooling fins and heat transfer is also into the nacelle. A simplified schematic diagram is shown in Fig. 10.7. Due to the size of direct drive generators that are slowly rotating units cooling can be realized by direct air cooling with purified ambient air. Such a solution needs filters to take out dust and humidity. Such filters are widely used for gas turbines, even in an offshore environment. No such filters are necessary for an air/ambient air heat exchange system as the air inside the generator is not exchanged. Alternatively the generator is also cooled by water and heat exchange is then via water/ambient air cooling system.

10.3.3 HVAC HVAC stands for Heating, Ventilation and Air Conditioning. Heating may be relevant for cold climate conditions, ventilation and air conditioning is utilized inside the nacelle or other

164

10 Auxiliary Systems and Secondary Steel

closed rooms, such as tower basement, to provide a well defined operational environment, e.g., for the electrical and electronic components. Nacelle ventilation is necessary to cope with the surface heat emerging from the various components and systems in the nacelle, such as gearbox, generator, electrical cabinets, etc. Ventilation is mostly passive through openings. In difficult environmental conditions such as dusty areas active ventilation is used to produce a slight overpressure in the nacelle by using filtered air intake. This way uncontrolled air intake via rotary junctions or leakage is suppressed. Similar for offshore ventilation is active to keep salty air outside the nacelle as part of corrosion protection measures. Passive heat exchangers, e.g., for water/ambient air, are placed outside the nacelle on the roof top. No vent is used, hence there is no additional energy consumption for this concept. However, the system is subject to environmental effects. In particular, cooling capacity increases with increasing wind speed, which is advantages. However, special care is needed to secure corrosion protection. An example for passive cooling system is shown in Fig. 10.8, which shows the recent Nordex wind turbine N131. Active cooling systems are more independent of the outer conditions. An vent is activated, if necessary, which is also an additional consumer and potentially reduced the efficiency of the wind turbine. Usually the cooling system is arranged in the rear part of the nacelle rather than on the roof top. An older example of an active cooling system on the roof top was used by Nordex up to about 2010 and shown in Fig. 10.9 whereas a more recent solutions of the same company with the cooling cabinet integrated in the rear of the nacelle is shown in Fig. 10.10.

Fig. 10.8 N149/5.7 MW with passive cooling system on roof top during installation in Santow/Grevesmühlen (Germany) by Wind-Projekt GmbH

10.4

Hydraulics

165

Fig. 10.9 Active cooling solution of Nordex wind turbines on roof top prior to 2010

Fig. 10.10 Integrated active cooling system in the Nordex N117 wind turbine

Finally, there exist also solution with a primary passive cooling system supported by a booster (active cooler) in case the passive cooling were not sufficient.

10.4

Hydraulics

Hydraulics is an effective and inexpensive method to actuate pitch, brakes in drive train and yaw system, and rotor lock. Hydraulics in wind turbines are useful, in case a linear movement can be used for actuation. In this case large forces can be applied.

166

10 Auxiliary Systems and Secondary Steel

Fig. 10.11 Schematic diagram of hydraulic system for pinion actuator

Fig. 10.12 Hydraulic brake used in a GE 1.5 MW wind turbine (left) visible brake disc (right)

A hydraulic systems includes pumps, storage, valves, control, pinions, and cylinders. A schematic diagram is provided in Fig. 10.11. Some wind turbines have a centralized hydraulic system that provides pressure to all the hydraulic actuators, but also stand alone hydraulic units for each task are utilized. Hydraulic systems have quite a few advantages compared to alternative solutions, e.g., electrical brakes. Main advantage is the usually compact design compared to the large forces transferred within limited space. The technology itself is very mature and total cost

10.5 Tower Internals

167

level rather moderate. The obvious disadvantages is related to possible leakage and related environmental damage. An example of a hydraulic brake in a GE 1.5 MW wind turbine is shown in Fig. 10.12 (left). Figure 10.12 (right) shows a hydraulic brake including a view onto the adjacent brake disc.

10.5

Tower Internals

The space inside the tower, close to the tower bottom, is widely used to place important components such as converter cabinets, switch boards, control cabinets, transformers, etc. In some wind turbines the mentioned components are located in part or all inside the nacelle. A decision, where to place those components is usually due to superior business or technical reasons.

Fig. 10.13 Sketch of tower section with internals distributed on one level (left) and two levels (right)

168

10 Auxiliary Systems and Secondary Steel

Figure 10.13 shows arrangements of the lowest section of the 2 MW wind turbine. On the left side the cabinets are arranged on one platform at the tower bottom at the same level as the door opening. Control cabinets are placed in the tower bottom to locally control the wind turbine from the ground floor and to ease the interface to the outside world. Alternatively cabinets can also be distributed across two levels. This is sketched in Fig. 10.13 on the right side. Besides the cabinets in Fig. 10.13 (left) also a ladder is shown opposite to a service lift. Both equipment can be used to enter the nacelle. Power bars (alternative to power cables) are also seen leading from the nacelle all the way down to the grid-side converter cabinet. Due to the large weight of the components, the bottom platform is based on the concrete foundation of the tower rather than connected to the tower shells. A second platform is about 1.5 m below the upper flange of the section. This one is attached to the tower shell and used for regularly maintaining the bolt connections and hence named service platform. Generally tower shells are highly loaded structures and hence any attachment reduces the load rating. Attachments have to be considered very carefully. There exist several possibilities to connect ladders, platforms etc. to the tower shell mentioned in the following list.

Fig. 10.14 Part of service ladder with fixations and center rail as part of fall arresting device

10.5 Tower Internals

169

• Since shell material is steel magnets can be used • Brackets of platforms can be bolted to the flanges • Support of platforms in the tower bottom area can be done directly to the foundation, see Fig. 10.13 (right) for support of a second floor • If brackets are welded to the tower wall, round bars or threaded rods are better than flat bars, in terms of resilience against fatigue loading Note that as for the rest of the turbine, but also in particular for design and sizing tower internals, HSE regulations must be taken into account, i.e., access widths, safety measures, warning notices, etc. This holds more so for service ladder and service lift. Service ladder should be equipped with a fall arresting device. This can be seen as a center rail in Figs. 10.14 and 10.15. Service personnel has to carry personal safety equipment (PSE) with proper adapter to the local system. Figure 10.15 also shows the service platform from below with bars connecting the platform to the tower shell. A service lift is shown in Fig. 10.16. The platform opening has to be secured by bars (left side). A typical lift by a supplier is shown on the right side of Fig. 10.16. There are climbing alternatives to use of lifts and ladders which actively assist climbing the ladder and are available, e.g., under the brand name “Highstep” or similar.

Fig. 10.15 View into inside of tower with service ladder and service platform

170

10 Auxiliary Systems and Secondary Steel

Fig. 10.16 Service lift, 3D model (left side) and possible realization in the wind turbine (right side)

Fig. 10.17 Cable loop in Enercon tower

10.5 Tower Internals

171

At the tower top the challenge is to lead the power cable from the nacelle to the tower although the nacelle can be rotated around the tower axis. This is solved by using a cable loop. This way the nacelle can be turned about 2 · · · 3 times before it has to be brought back to original position by yawing the nacelle. A typical realization of the cable loop is shown in Fig. 10.17, in this case by Enercon.

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