Innovative Wind Turbine: An Illustrated Guidebook 9780367819316, 0367819317, 9781000765854, 1000765857, 9781000765977, 1000765970, 9781000766097, 1000766098, 9781003010883

Table of contents :
Content: Introduction. Horizontal Axis Wind Turbines. Ducted Wind Turbines. Vertical Axis Wind Turbines. Airborne Wind Energy. More Wind Turbines.

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Innovative Wind Turbines

Innovative Wind Turbines An Illustrated Guidebook

Vaughn Nelson

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2020 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed on acid-free paper International Standard Book Number-13: 978-0-367-81931-6 (Hardback) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged, please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Names: Nelson, Vaughn, author. Title: Innovative wind turbines : an illustrated guidebook / by Vaughn Nelson. Description: First edition. | Boca Raton, FL : CRC press/Taylor & Francis Group, [2020] Identifiers: LCCN 2019042277 (print) | LCCN 2019042278 (ebook) | ISBN 9780367819316 (hardback ; acid-free paper) | ISBN 9781003010883 (ebook ; acid-free paper) Subjects: LCSH: Wind turbines--Handbooks, manuals, etc. Classification: LCC TJ828 .N45 2020 (print) | LCC TJ828 (ebook) | DDC 621.4/8--dc23 LC record available at https://lccn.loc.gov/2019042277 LC ebook record available at https://lccn.loc.gov/2019042278 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

Contents Preface......................................................................................................................vii Acknowledgments......................................................................................................ix Author........................................................................................................................xi Chapter 1 Introduction...........................................................................................1 1.1 Drag Device................................................................................5 1.2 Lift Device..................................................................................7 1.3 Innovative Systems..................................................................... 9 1.4 Historical Innovative Systems.................................................. 10 1.5 United States Innovative Program............................................ 13 1.6 Background............................................................................... 17 References........................................................................................... 18 History Sources................................................................................... 19 Chapter 2 Horizontal Axis Wind Turbines.......................................................... 21 2.1 One Blade................................................................................. 22 2.2 Multiple Blades.........................................................................26 2.3 Sailwings.................................................................................. 29 2.4 Multiple Rotors......................................................................... 33 2.5 Multiple Turbines...................................................................... 36 References........................................................................................... 41 Chapter 3 Ducted Wind Turbines........................................................................ 43 3.1 Venturi Wind Turbines............................................................. 52 3.2 Acceleration.............................................................................. 55 References........................................................................................... 61 Chapter 4 Vertical Axis Wind Turbines.............................................................. 63 4.1 Darrieus.................................................................................... 63 4.2 H-Rotor, Giromill..................................................................... 69 4.3 Savonius.................................................................................... 75 4.4 Multiple Blades, Multiple Rotors............................................. 76 4.5 Squirrel Cage............................................................................ 82 4.6 Other VAWTs........................................................................... 85 References...........................................................................................90

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Chapter 5 Airborne Wind Energy........................................................................ 91 5.1 Generator Aloft.........................................................................92 5.2 Generator on Ground................................................................ 98 5.3 Comments............................................................................... 110 References......................................................................................... 112 Chapter 6 More Wind Turbines......................................................................... 115 6.1 Flexbeam................................................................................ 115 6.2 Blade, Spar Angle................................................................... 117 6.3 Other Rotors .......................................................................... 118 6.4 Lift Translators....................................................................... 124 6.5 No Rotor................................................................................. 124 6.6 Blades..................................................................................... 128 6.7 Magnus Effect........................................................................ 134 6.8 Other....................................................................................... 135 References......................................................................................... 139 Index....................................................................................................................... 141

Preface Innovative Wind Turbines is a tribute to the inventors, entrepreneurs, researchers, and companies who have envisioned, designed, and constructed models and prototypes through their efforts. There are numerous concepts and ideas on ways to convert wind energy into usable energy. I will look at innovative, novel, or unusual concepts with lots of photos and a few historical examples (from before the modern wind industry, which began after the first oil crisis in 1973). I will cover prototypes that have been constructed and a few design concepts. Almost all have not reached the commercial stage, although airborne wind energy systems have the potential to become commercial. Essentially, performance (power curves and energy production), problems, and failures will not be mentioned. Problems and failures are common with prototypes and first production units. The Alternative Energy Institute, West Texas A&M University (WTAMU) and the Agricultural Research Service (ARS), U.S. Department of Agriculture, Bushland, Texas had a cooperative agreement on rural applications for wind energy. From 1976 to 2015, we field-tested more than 80 wind turbines (most of them prototypes or first production units) from 50 W to 500 kW. All had failures and problems from within days to six months of operation. Much of the information gleaned from this project is available on the Internet; however, many of the businesses and web sites involved are now defunct, and it is difficult to get information on those units and permission to use the photos. A couple of entities refused to send photos with the primary reason that units were still in development. That is somewhat strange, as web sites and photos are available on the Internet. I apologize in advance if the source for a photo is not properly recognized. Many of the sites provide videos, and I have included a URL for those I found interesting.

CONTACT I am sure there are innovative or novel prototypes not mentioned, so if you know of any that have been constructed, please contact me or Ken Starcher. We plan on creating a web site; however, we have no fixed date on implementation. Vaughn Nelson; [email protected] Kenneth Starcher; [email protected]

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Acknowledgments I am deeply indebted to the many colleagues at the Alternative Energy Institute (AEI was terminated in 2015) of West Texas A&M University and at the Wind Energy Group (program canceled in 2012) at the Agricultural Research Service, U.S. Department of Agriculture, Bushland, Texas. The students in my classes and the students who worked at AEI provided many insights and valuable feedback. Others include the numerous international researchers and interns who worked with us on projects at AEI and USDA. Thanks also to Brandon Stienle, Creative Manager, Information Technology at WTAMU, for preparing the computer drawings. I appreciate all the entities and people that I was able to contact that graciously provided photos. Only two declined to provide photos, and there were a few where contact points were not functional or where I failed to obtain a reply. Special thanks to the reviewers: Kenneth Starcher, Paul Gipe, and David Carr. Ken (Instructor, Engineering Technology) has been at WTAMU since he was a freshman in 1976 and has worked in renewable energy, primarily wind energy, over that entire span. Paul is a long-time renewable energy advocate, founder and information guru of Wind-Works, and author of eight books on wind energy. David is the founder and chief officer of RenewTest; previously, he was Dean of Technology at Frank Phillips College, and before that he was at the Alternative Energy Institute, WTAMU, for 12 years serving in various roles. I want to express gratitude to my wife, Beth, who has put up with me all these years. As always, she is very supportive, especially in visiting all those wind farms to obtain information and take photos and in accompanying me on many trips to different parts of the world.

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Author Dr. Vaughn Nelson has been involved with renewable energy, primarily wind energy, since the early 1970s. He is the author of four books and has published more than 50 articles and reports. He also served as the principal investigator on numerous grants and conducted more than 60 workshops and seminars from local to international levels. Dr. Nelson’s primary work focused on wind resource assessment, education and training, applied research and development, and rural applications of wind energy. Presently, he is a professor emeritus of Physics at West Texas A&M University (WTAMU). He retired as the Dean of the Graduate School, Research, and Information Technology, in 2001. He was the founder of the Alternative Energy Institute and director from its inception in 1977 through 2003. He returned as director for another year in July 2009. Later, the Alternative Energy Institute was terminated in 2015. Dr. Nelson served on several State of Texas committees, most notably the Texas Energy Coordination Council for 11 years. He received three awards from the American Wind Energy Association, one of which was the Lifetime Achievement Award in 2003, was named a Texas Wind Legend by the Texas Renewable Industries Association in 2010, and received an award for Outstanding Wind Leadership in Education from Wind Powering America in 2013. He also served on the boards of directors for state and national renewable energy organizations. I developed and taught courses in wind energy and solar energy, first on campus and then online starting in 1998. From the material developed for those courses, I wrote two introductory textbooks, Wind Energy and Wind Turbines, and Solar Energy, and was the principal author on Wind Water Pumping (also available in Spanish). Those books (also on CD) plus Wind Characteristics: An Analysis for the Generation Wind Power by Janardan Rohatgi and Vaughn Nelson were available from AEI. As Dean of the Graduate School, Research, and Information Technology, I was instrumental in starting the online program for WTAMU with the first courses in 1997. When I returned as Director of AEI from 2009 to 2010, I developed the curriculum for a renewable energy emphasis in Engineering Technology. Ken Starcher developed and taught Bioenergy, and we then wrote an introductory textbook. In the series Energy and the Environment, Dr. Nelson is the author of Wind Energy (2009, 2nd Ed. 2013, and 3rd Ed. 2019 with Kenneth Starcher), Introduction to Renewable Energy (2011, 2nd Ed. 2015 with Kenneth Starcher), and, also with Kenneth Starcher, Introduction to Bioenergy (2016). Dr. Nelson earned a PhD in Physics from the University of Kansas, an EdM from Harvard University, and a BSE from Kansas State Teachers College in Emporia. He was a member of the Departamento de Física, Universidad de Oriente, Cumana, Venezuela for two years and then was at WTAMU from 1969 until his retirement.

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Introduction

This book is an illustration of innovative wind turbines with general information and lots of photos. However, it is impossible to be all inclusive, so I recommend searching the Internet for more images, especially for the few turbines I was unable to get permission for photos. Two of my requests were declined, and there were others where contact information was not available due to defunct businesses and web sites. I will not be reporting problems, failures, power curves, energy production, and scientific and technical information. Almost all the images and information will be about prototypes, although I will include a few design concepts. In some cases, I will include information for videos. Energy in the wind can be transformed to other forms of energy, and today the predominate application is for generating electricity. Other transformations are for sailing and pumping water. Historical uses were for powering ships, grinding grain, pumping water, and even sawing wood. Today, the primary application is for the production of electricity with megawatt wind turbines installed worldwide in wind farms—on land and offshore. It is important to know the difference between energy (ability to do work) and power. Power = energy/time (P = E/t) and thus if you know power and time you can calculate energy from power × time (E = P × t). For example, the energy in gasoline makes the car move, however, cars have different size motors, rated size, or power (horsepower in English units). For rating (power) of wind turbines I will use kilowatts (kW, 1,000 W), megawatts (MW, 1,000,000 W), and for installed capacity for countries and states, gigawatts (GW, 1,000,000,000 W). For example, the world installed wind capacity was approximately 600 GW at the end of 2018. One kW = 1.3 horsepower. For energy we will primarily use electric energy, kilowatt hour (kWh), which is what is purchased from the electric utility company. For more specific information on wind energy and wind turbines, see Vaughn Nelson and Kenneth Starcher, Wind Energy, 3rd Ed, CRC Press, 2019. The energy in the wind is proportional to the cube of the wind speed. If the wind speed is doubled, the energy available is eight times larger. At low wind speeds there is not much energy and at high wind speeds there is too much energy, so wind turbines have to be controlled not to use all of that energy. Too many web sites for innovative wind turbines tout their operation in low winds—a dubious claim since there is not much energy at those wind speeds. For a wind turbine, the two most important factors contributing to the amount of energy generated are a decent to excellent wind regime and the cross sectional area (area intercepted by the rotor). Of course, economic considerations—cost of materials, ability to manufacturer and assemble—and value of energy produced are the main factors for commercial wind turbines. However, as wind turbine prices have declined, turbines are now being sold in areas of medium winds.

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Innovative Wind Turbines

FIGURE 1.1  Dutch windmills, UNESCO World Heritage Site, Kinderdijk, The Netherlands. Rotor size approximately 25 m diameter.

Wind turbines are classified according to the interaction of the blades with the wind (aerodynamics, drag and lift), orientation of the rotor axis to the ground, and innovative or unusual types of machines. Of course, early wind turbines were innovative and then the designs evolved. Examples are the Dutch windmill and the American farm windmill, about which there is substantial information available from societies and museums, such as the Kinderdijk (www.kinderdijk.com) in the Netherlands (Figure 1.1), a UNESCO World Heritage Site since 1997. The Dutch used gangs of windmills to drain the polders of the Netherlands, and the 19 Dutch windmills at Kinderdijk used in tandem to drain a polder south of Rotterdam could be considered the world’s first wind farm. The windmills operated until 1950 and 15 were still inhabited in 2019. There are numerous images of the windmills on Wikimedia Commons under search category Kinderdijk windmills. The American farm windmill for pumping water was instrumental in the settlement of the Great Plains, and there are quite a few locations where these early historical windmills can be seen. The largest collection is at the American Windmill Museum (https:// windmill.com) in Lubbock, Texas (Figure 1.2). The aerodynamic interaction of the blades with the resultant wind is resolved into drag and lift forces (Figure 1.3). The orientation of the rotor axis designates a horizontal axis wind turbine (HAWT) or a vertical axis wind turbine (VAWT). The rotor on a HAWT is oriented (yaw) perpendicular to the wind by means of a motor on large units, by coning on some downwind units, or by a tail on small units. Some of the historical windmills and early wind turbines had a tail fan for orientation. An advantage of the VAWT is that it accepts wind from any direction without the need for yawing the wind turbine into the wind, and the gearbox and generator could be at ground level. The force of the wind (lift and drag) on a blade is applied at some radius from an axis, thus there is a torque on the blade causing it to rotate. The rotational speed is in revolutions per minute (rpm) or angular velocity (ω, radians/second). The power is equal to the torque times rotational angular velocity, P = Tω. Some terms and definitions follow: Tip Speed Ratio (TSR): Speed of the tip of the blade divided by the wind speed. Chord: Length of the airfoil from the nose to the trailing edge.

Introduction

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(a)

(b)

FIGURE 1.2  (a) A few of the many water pumping windmills outside at the American Windmill Museum. Vestas V47 in the background. (b) Some of the historical windmills inside the pavilion at the American Windmill Museum.

Pitch: Angle of the chord to the plane of rotation. For a blade, pitch is noted for airfoil at the end of the blade. Blades can be fixed pitched or variable. If variable, feather position is when the blade is parallel to the wind and stall position is perpendicular to the wind. Planform: Blade shape; can have variable or fixed chord (Figure 1.4). Solidity: Ratio of blade planform area to swept area of rotor. Twist: Different pitch along the blade. Blade may also have different airfoil shape along the blade. Yaw: Orientation (angle) about tower axis, usually referenced to ground wind.

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FIGURE 1.3  Forces on airfoil, lift and drag, due to airflow of relative wind.

FIGURE 1.4  Twist and planform for Carter 25 wind turbine blade. Blade was divided into 10 sections for analysis.

rpm: Revolutions per minute. Teeter: Rotor shaft connection to hub is flexible, like a teeter totter. Overspeed control and shutdown: All wind turbines must have a reliable way not to use all power at high wind speeds and a way to shut down the turbine if there is a loss of load from the utility grid. This book uses SI units, to convert to English units; 1 meter (m) = 3.28 feet; 1 m/s = 2.2 miles/hour, 1 kilogram (kg) = 2.2 pounds (lbs).

Introduction

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1.1  DRAG DEVICE In a drag device, the wind pushes against the blades or sails (Figure 1.5). Drag devices are inherently limited in efficiency since the speed of the device or blades is less than the wind speed. The wind pushes on the blades of a drag turbine, forcing the rotor to turn on its axis. Maximum theoretical efficiency is 16% and the maximum speed of the drag device or drag rotor is one-third the wind speed. Note that drag devices have large torque and small rpm. Too many inventors think the high torque means high power—a common misconception. High solidity and low rpm are major disadvantages for drag wind turbines. The earliest known windmills are the Persian windmills, and the ones at Nashtifan, Iran (Figure 1.6) are still operating, grinding grain, although future use is uncertain [1,2]. The windmills are made of wood, clay, and straw. The long axis of the building is erected perpendicular to the predominate winds and curved walls

FIGURE 1.5  Drag device. An example is a sailboat moving downwind.

FIGURE 1.6  Windmills for grinding grain. Height with mill room is 15–20 m height. In general, eight rotors, six blades per rotor, and half of rotor is shielded on upwind side. (https:// commons.wikimedia.org/wiki/Category:Nashtifan Wikimedia Commons).

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shield half of the rotor. For video see https://news.nationalgeographic.com/2017/01/ nashtifan-iran-windmills/. Examples of drag devices are cup anemometers; vanes and paddles that are shielded from the wind or change parallel to the wind on half the rotor cycle (Figure 1.7). No drag wind turbines for producing electricity have been produced commercially, however drag devices (Figure 1.8) are popular with inventors and homebuilders

FIGURE 1.7  Diagrams of some drag wind turbines.

FIGURE 1.8  Drag devices. Top left clockwise: (1) Around 10 m diameter, with flywheel and tanks (could be filled with water) which was supposed to store rotational energy and reduce variation in power. (2) Cups, 1.2 m diameter. Inventor predicted power output as 4 kW. (3) Shielded plywood sheets, 1.2 by 2.5 m. Notice the large wheel for speed increase to the generator. Inventor predicted output as 4 kW. (4) Panemone device, blades move parallel to wind when moving upwind. All the turbines were in locations near Amarillo, Texas.

Introduction

7

because they are easy to construct. Invariably, inventors become irate when they are told that the inefficient aerodynamics and amount of material required for blades limit commercialization of drag devices. Note the top left device in Figure 1.8 rotated at 2 rpm was essentially a kinetic sculpture. For a photo of a clam shell drag device go to http://savonius-balaton.hupont.hu/151/p2satop. The clam shells are open on the downwind side and closed on the upwind side of rotation. I could not find any information on power and dimensions.

1.2  LIFT DEVICE Using lift, the blades can move faster than the wind and are more efficient in terms of aerodynamics (maximum theoretical efficiency is 59%) and amount of material needed. The common operational tip speed ratio (TSR) is around 7–8 for a lift device and 0.3 for a drag device. The ratio of amount of power per planform area for a lift device is around 75 compared to a drag device, which is 0.5 or less. Again, this emphasizes why commercial wind turbines for producing electricity use lift. Most lift devices use blades similar to propellers or airplane wings, but other concepts have also been used (Figure 1.9). In 1927, Georges Jean Marie Darrieus invented a wind turbine whose blades were shaped like a jumping rope (Figure 1.10), however, the Darrieus wind turbine is not self-starting. He also patented straight blade vertical axis wind turbines, namely the H-rotor or giromill. The giromill (Figure 1.11)

FIGURE 1.9  Diagrams of different rotors.

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FIGURE 1.10  Horizontal axis wind turbine (HAWT) and vertical axis wind turbine (VAWT). Left: Carter, fixed pitch, constant rpm operation, one stop full blade pitch change to stall for overspeed control, 25 kW, diameter = 10 m, tower = 17 m. Right: Darrieus, 100 kW, constant rpm operation, mechanical brake, rotor diameter = 17 m, rotor height = 25 m, tower = 3 m. ARS, USDA Wind Test Facility, Bushland, Texas. Wind research program terminated in 2012.

FIGURE 1.11  Giromill, McDonnel Douglass, 40 kW, rotor is 18 m diameter by 12.8 m height, generator at ground level. Prototype unit at Rocky Flats, now the National Wind Technology Center, National Renewable Energy Laboratory, United States.

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FIGURE 1.12  Savonius wind turbine, 5 kW, each rotor was 3 m height by 1.75 m diameter, prototype at Kansas State University. (Photo courtesy of Gary Johnson.)

has straight blades and can be self-starting as blades can be articulated, or the blades can be articulated during each revolution to increase efficiency. In Finland, Sigurd Johannes Savonius built S-shaped rotors that resembled two halves of a cylinder separated by a distance smaller than the diameter (Figure 1.12). A rotating cylinder in an airstream will feel a force called the Magnus effect (Figure 1.13). For HAWTs the rotor can be upwind or downwind from the tower (Figures 1.14 and 1.15). The configuration (Figure 1.16) of most megawatt wind turbines is three blades, full span pitch control, rigid hub, yaw motor for orientation, and rotor upwind on a tubular tower (Figure 1.17). Sizes range from 1 to 8 MW, with diameters from 60 m to over 150 m on towers from 80 m to over 100 m. Megawatt wind turbines with direct drive (no gear box) and permanent magnet generators are available. One blade and two blade wind turbines may have teetering hubs or a flexbeam.

1.3  INNOVATIVE SYSTEMS Innovative or unusual wind systems must be evaluated in the same way as other wind turbines. The important parameters are system performance, structural requirements, characteristics of materials, system feasibility, and ease of construction. Innovative ideas include tornado types; tethered units to reach the high winds of the jet stream; tall towers that use rising air; tall towers for humid air; torsion flutter, electrofluid, diffuser augmented, Magnus effect, and other systems. Some of the concepts from 1972 to 1985 have been reported in Popular Science [3–8]. Most innovative concepts

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FIGURE 1.13  Magnus effect wind turbine, 100 kW, at Southern California Edison test site.

FIGURE 1.14  Diagram for downwind and upwind rotor. Note coning on a downwind rotor means it does not need a motor for orientation, passive yaw control. Upwind unit with tail for yaw control. Large units generally have a yaw motor.

remain at the experimental or feasibility stage and not all are recent inventions. For example, sail wings, wings on railroad cars, and the Magnus effect (the Madaras concept of rotating cylinders on railroad cars) have been around for a long time.

1.4  HISTORICAL INNOVATIVE SYSTEMS Of course, the early prototypes of today’s modern wind turbine were innovative. During the 1950s, Ulrich Hütter of Germany designed and tested wind turbines that remained the most technologically advanced for the next two decades (Figure 1.18). A design by Edouard Andreau, a French engineer, was built by Enfield and erected at

Introduction

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FIGURE 1.15  Left: Downwind, Enertech, 5 kW, 6.5 m diameter, fixed pitch, constant rpm operation. Note tip flap brakes for overspeed control. Right: Upwind, Hummingbird, 5 kW, 6 m diameter, fixed pitch, variable rpm. Units at Alternative Energy Institute Wind Test Center, West Texas A&M University, Canyon, Texas. AEI terminated in 2015.

FIGURE 1.16  Major components of large wind turbine.

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FIGURE 1.17  Large wind turbine, Vestas V90 near Gruver, Texas. Rated power 2 MW, diameter = 90 m, tower = 80 m. Blades are in the feathered position in this photo. Note the person at base of tower and minivan.

FIGURE 1.18  Hütter wind turbines: left 100 kW, right 10 kW. (Photo courtesy of NASA-Lewis.)

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FIGURE 1.19  Diagram of Enfield-Andreau wind turbine, power = 100 kW at 13 m/s, diameter = 24 m, tower = 30 m.

St. Albans in 1952. The unit was different in that the blades were hollow. When they rotated, the air flowed through an air turbine connected to an alternator at ground level and the air then exited from the tips of the blades (Figure 1.19). The unit was moved to Grand Vent, Algeria, for further testing in 1957. Frictional losses were too large for it to be successful. Denmark had the only successful program that began in 1947 with a series of investigations on the feasibility of using wind power, and continued until 1968 [9, pp.  229–240]. The results were encouraging and culminated in the Gedser wind turbine (Figure 1.20). The unit was erected in 1957, and from 1958 through 1967 it produced 2,242 MWh. It was shut down in 1967 when maintenance costs became too high. The rotor was upwind of the tower, and the blades were fixed pitch with tip brakes for overspeed control. The wind turbine had an asynchronous generator that provided stall control, and it also had an electromechanical yaw mechanism. Denmark and the United States furnished funds to place the Gedser wind turbine in operation for a short period in 1977 and 1978 for research that involved tests for aerodynamic performance and structural load limits.

1.5  UNITED STATES INNOVATIVE PROGRAM The Solar Energy Research Institute (SERI), later renamed the National Renewable Energy Laboratory (NREL), was the lead agency in innovative concepts (Table 1.1), and reports on projects funded by SERI are available in conference proceedings [10–12]. The U.S. Department of Energy (DOE) provided funding for this program

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FIGURE 1.20  Gedser 200-kW wind turbine, diameter = 24 m, tower (prestressed concrete) = 26 m. (Photo courtesy of Danish Wind Industry Assn.)

TABLE 1.1 Solar Energy Research Institute, Innovative Wind Program Project Innovative wind turbines (VAWT) Tornado type wind energy system Diffuser augmented wind turbine Wind/electric power charged aerosol Electrofluid dynamic wind generator Energy from humid air Madras rotor power plant, phase I Vortex augmenters Yawing wind turbine, blade cyclic pitch Oscillating vane Dynamic inducer

Contract West Virginia University Grumman Aerospace Grumman Aerospace Marks Polarized University of Dayton South Dakota School, M&T University of Dayton Polytechnic Institute, New York Washington University, St. Louis United Technologies Aerovironment

from 1978 to 1983. The idea of a confined vortex (tornado) was invented by T.J. Yen (Figure 1.21) of Grumman Aerospace. The DOE funded theoretical and model studies of the concept. Another concept was using unconfined vortices produced along the edges of a delta wing and placing two rotors at those locations, and the DOE funded these model studies as well.

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Introduction

TOP WIND STREAM

CLOSED VALVES

ADJUSTABLE VERTICAL VANES

WIND

INTERACTING REGION

BLADES

TURBINE EXHAUST

H

CABLES ANCHORED TO GRID STATIONARY STRUCTURE

BEARING

FLYWHEELS

TO GENERATOR

FIGURE 1.21  Vortex generator.

There have been several proposals for solar updraft wind turbines and generation of electricity from low temperature solar heat. The idea of a solar chimney power plant was proposed more than 100 years ago [13] by Isidoro Cabanyes, a colonel in the Spanish army. In this system, the sun heats the ground and air beneath a large transparent canopy and the warm air rises in the tower in the middle of the collector to drive a generator in the tower or around the tower base. The main advantage of solar updraft over photovaltaic panels is the ability to produce power after sundown. That’s because the energy that is absorbed by the land when the sun is shining keeps the air in the collector warm enough at night to keep the turbines spinning. Several prototype models of a solar updraft wind turbine have been built. A tower producing 75 kW was constructed in Spain in the 1980s, and there is a 200 kW experimental unit in China, however, no full-scale units are

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FIGURE 1.22  Left: Diagram of updraft tower with greenhouse at bottom. Right: Unit in Spain, tower diameter = 10 m, height = 200 m. (https://commons.wikimedia.org/wiki/ File:SolarChimneyManzanares_view_from_8km_south_direction.JPG).

operational as of 2019. There are proposals for larger systems consisting of a 732 m tower producing 200 MW, to a 1,000 m inflatable tower producing 170 MW with a collector 7 km in diameter [14]. The West German government funded the construction of the 200 m tall tower in Manzanares, Ciudad Real, Spain in 1982 [15]. A 240 m diameter greenhouse at the bottom provided the hot air to drive the air turbine rated at 75 kW located inside the tower (Figure 1.22). The collection diameter was 244 m, with an area of 46 hectares, and one section was used as an actual greenhouse. The unit operated for  approximately eight years. The tower in Jinshawan, Inner Mongolia, China started producing power in 2010. However, the tower was too short at 50 m height, and the many of the glass panels in the metal frames cracked and shattered in the heat. The Schachle wind turbine was somewhat unusual in two respects: tower and power transmission. The tower was a rigid triangular truss on a rotatable base, with a height of 30 m and a triangular base of 23 m on a side. The variable speed rotor powered a hydraulic drive and that hydraulic power was then transferred to a generator on the base. Schachle tested his first unit at Moses Lake, Washington. He then sold his design to Bendix and the Bendix-Schachle unit (Figure 1.23) was tested (commissioned in December 1980) at the Southern California Edison test site in San Gorgonio Pass near Palm Springs, California. The losses in the hydraulic drive were high and the unit reached a power output of only 1.1 MW rather than the design 3 MW, rated at 18 m/s. The tower could only rotate 330°, so the control system had to avoid rotating it through those stops. Jamieson’s comprehensive technical book covers the fundamentals of design, explains the reasons behind design choices, and describes the methodology for evaluating innovative systems and components [16]. The second edition has an expanded section on airborne wind energy systems.

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FIGURE 1.23  Bendix-Schachle wind turbine, upwind, D = 50 m, at Southern California Edison test site, San Gorgonia, California (1083). Blades (laminated wood and fiberglass) in feather position in photo.

There are many images of wind turbines on the Internet, and two sites of interest are: 1. Colorful catalog of vertical and horizontal wind turbines from around the world. Editorial: Peter Borsanyi VAWT-Techniker; SavoniusBalaton, Hungary 2008–2019. http://savonius-balaton.hupont.hu/50/ katalog-20082018 2. Colin Houghton. 2018. Wind turbines designs—the world’s most amazing windmills designs, My Wind Power Systems. https://www. mywindpowersystem.com/2018/04/03/amazing-wind-turbines-designs/

1.6 BACKGROUND I began my career at West Texas State University (WTSU), now West Texas A&M University (WTAMU) in 1969 teaching physics. In the early 1970s, Dr. Earl Gilmore, who was at Amarillo College, suggested we start doing research on wind energy, so we began analyzing wind data from the National Weather Service stations for Texas and surrounding states. We received our first grant at WTSU for wind resources assessment from the State in 1974. Then we were joined by Dr. Robert Barieau, and we also started field testing small wind turbines. In 1976, we signed a cooperative agreement for rural applications of wind energy with the Agricultural Research Services, U.S. Department of Agriculture, Bushland, Texas. The wind group at the

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USDA was directed by Dr. R. Nolan Clark. In 1976, Kenneth Starcher, a freshman at West Texas State University, started working with our research group. Ken’s entire career has been at WTAMU working in renewable energy, primarily wind energy. In 1977 we received line item funding at WTAMU from the State for wind energy and we formed the Alternative Energy Institute (AEI). Then we established the AEI Wind Test Center, adjacent to the WTAMU campus, because it was too expensive and time consuming to perform field testing of wind turbines at remote locations. At the same time, field testing of wind turbines was performed at the USDA Wind Lab. We have had numerous visiting researchers and interns at AEI and the USDA lab. In 2010, AEI was selected by the Wind Technology Center of the National Renewable Energy Laboratory as one of four regional wind test centers. Between AEI and the USDA lab, we field tested more than 80 wind turbines from 50 W to 500 kW. Most of the turbines were in the range of 5–100 kW. We did not test any airborne wind energy systems. Almost all the units were prototypes or first production units, and almost every unit had problems or failures within days to six months. We have seen our share of major failures featuring every component of a wind energy conversion system. That is the major reason for not reporting problems and failures of innovative wind turbines in this book, as failures and problems are common and expected with any prototype. The wind program at the USDA was terminated in 2012 and the Alternative Energy Institute was terminated in 2015.

REFERENCES

1. Tehran Times. http://www.tehrantimes.com/news/408532/Iranians-pioneers-of-windpower-harnessing 2. Historical Iranian Sites and People; Nashtifan Windmills. http://historicaliran.blogspot. com/2012/03/nashtifan-windmills.html 3. S. Kidd and D. Garr. 1972. Electric power from windmills? Popular Science, November, 70. 4. E. F. Lindsey. 1974. Windpower. Popular Science, July, 54. 5. V. Chase. 1978. 13 wind machines. Popular Science, September, 70. 6. J. Schefter. 1983. 5 wild windmills. Popular Science, June, 76. 7. B. Juchau. 1983. 650-foot power tower. Popular Science, July, 68. 8. J. Schefter. 1983. Barrel-bladed windmill—Power from the Magnus effect. Popular Science, August, 70. 9. United Nations. 1994. Proceedings of the United Nations Conference on New Sources of Energy: Wind Power, Vol. 7. 10. SERI (Solar Energy Research Institute.) 1980. In Proceedings of Second Wind Energy Innovative Systems Conference, Vols. I and II, pp. 635–638 and 938–1051. 11. SERI (Solar Energy Research Institute.) 1981. In Proceedings of Fifth Biennial Wind Energy Conference and Workshop, Vol. I, p. 415. 12. American Solar Energy Society. 1983. In Proceedings of Sixth Biennial Wind Energy Conference and Workshop. 13. Solar updraft tower. https://en.wikipedia.org/wiki/Solar_updraft_tower 14. T. Grose. 2014. Solar chimneys can convert hot air to energy, but is funding a mirage? National Geographic, April. https://news.nationalgeographic.com/news/ energy/2014/04/140416-solar-updraft-towers-convert-hot-air-to-energy/ 15. B. Juchau. 1983. A 650-foot power tower. Popular Science, July, 68. 16. P. Jamieson. 2018. Innovation in Wind Turbine Design, 2nd ed. Wiley.

Introduction

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HISTORY SOURCES R. W. Righter. 1996. Wind Energy in America: A History. University of Oklahoma Press, Norman, OK. D. G. Sheppard. 1994. Historical development of the windmill. In D. A. Spera. ed., Wind Turbine Technology, Fundamental Concepts of Wind Turbine Engineering. ASME Press: New York. D. J. Vargo. 1975. Wind energy developments in the 20th Century. NASA Technical Reports Series. https://www.sti.nasa.gov Danish Wind Energy Association. History of wind energy. www.windpower.org P. Gipe. 2016. Wind Energy for the Rest of Us. See Chs 3, 4. www.wind-works.org D. Dodge. Illustrated history of wind power development. https://www.telosnet.com/wind/

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Horizontal Axis Wind Turbines

Horizontal axis wind turbines (HAWTs) are the main type of wind turbine for wind farms, with around 600 GW installed by the end of 2018 [1,2]. Offshore accounted for around 24 GW, primarily installed in Europe, with China now at 4.6 GW. There were 51.3 GW installed in 2018, with 4.5 GW of that installed offshore—a market share of eight percent. The major markets are China, the United States, and Europe, however, wind farms are being installed in many countries across the globe. The Global Wind Energy Council expects new installations onshore and offshore of 55 GW per year from 2019 through 2023. The predominate model is three blade, upwind, full span pitch control, and a tubular tower ranging in size from 1 to 8 MW. The larger size is primarily for the offshore market. There have also been repowering projects, where smaller wind turbines have been replaced with fewer larger models. The new wind turbines are at higher hub heights, which also increases energy production. At the end of 2018 the number of utility scale wind turbines was estimated at 337,000, with an average size of 1.8 MW. I will use the example of Vestas, who is the world’s top producer of wind turbines with an installed capacity of 102,594 MW from 67,943 turbines, for how the size of wind turbines has increased over time. Vestas introduced the 2 MW platform in 2000, and there are now more than 22,000 turbines installed. Vestas introduced the 4 MW platform in 2010, and more than 7,000 turbines are now installed. As of 2019, the Haliade-X offshore wind turbine (https://www. ge.com/renewableenergy/wind-energy/offshore-wind/haliade-x-offshore-turbine) is the largest commercial wind turbine with a rated power of 12 MW, rotor diameter of 220 m, tower height of 150 m. A very rough estimate for small wind (less than 100 kW) for the world is 1.5 GW of installed capacity from 1.4 million wind turbines, with horizontal axis wind turbines dominating the market. The major market and manufacturing are in China, followed by the United States and the United Kingdom. In China and in developing countries, most of the wind turbines are stand-alone systems that generate 50–300 W. In the United States and Europe much of the small wind capacity is connected to the grid. A single blade rotating very fast can essentially extract as much power from the wind as many blades rotating slower. A wind turbine with one blade will save on material but a counterweight is needed for dynamic balance. Most modern wind turbines have two or three blades, however, wind turbines have been built with one to a number of blades. Rotors with larger diameters will rotate slower, and rotors with more solidity will rotate slower for the same swept area. Since the tip speed ratio is 7–8, the following wind turbines with diameters of 5, 10, 50, and 100 m would have rotor rpm around 250, 120, 25, and 10 respectively. Note that variable rpm units operate at higher efficiency than constant rpm units, and the rpm will increase as wind speed increases. 21

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2.1  ONE BLADE The Messerschmitt-Bölkow-Blohm (MBB) Monopteros and FLAIR designs were single-bladed wind turbines built in Germany. The Monopteros had full-span pitch control and the rotor was upwind. MBB developed turbines of 15–30 kW with rotor diameters from 12.5 to 17 meters. MBB also built the Monopteros 50 series rated at 475 kW (Figure 2.1) to 1 MW with rotor diameters from 47 to 56 meters. At a European wind conference, we toured the Monopteros and rode the 2-man elevator to the top. The teeter angle changed on ever rotation as the blade was angled away from the tower on the bottom of the cycle, so the top swayed by around 0.5 m on every rotation. Professor Franz Wortman and his students at the University of Stuttgart designed a one-bladed FLAIR or Flexible Autonomous 1-Bladed Rotor. In the mid1980s, a FLAIR prototype, 8 m in diameter, was tested at the university’s test site near Schnittlingen in Southern Germany. Riva Calzoni built the following single-bladed wind turbines: M5, M7, M30, and M33. Riva Calzoni built 25 of the M5 and another 25 of the M7 models by 1992, and then they abandoned the small turbine market. The 5 kW turbine (Figure 2.2) was tested by the Alternative Energy Institute (AEI), WTAMU. With the very low solidity, the angular speed could reach 350 rpm. For control in high wind speeds or loss of load, the blades moved to the stall position due to two small counterweights on a counterweight shaft (notice in figure) that were out of the plane of rotation. A 20 kW unit (Figure 2.3) was tested by the Atlantic Wind Test Site (now Wind Energy Institute of Canada), Prince Edward Island. Riva Calzoni built and installed more

FIGURE 2.1  Monopteros wind turbine, 475 kW, variable pitch blade, upwind, near Hamburg, Germany.

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FIGURE 2.2  Riva Calzoni, MP5, 5 kW, 5 m diameter, at AEI Wind Test Center.

FIGURE 2.3  Riva Calzoni-MBB, M7, 20 kW, D = 7 m, at Atlantic Wind Test Site.

than 100 350 kW units in Italy (Figure 2.4), and then stopped building that model as well. Powerhouse Wind, New Zealand (http://www.powerhousewind.co.nz/index.html) has a one-bladed wind unit with two counterweights (Figure 2.5) for better balance and a teetering hub. The rotor is downwind and the operation is stall regulated. The teetering hub allows the blade’s angle to change in response to variations in wind speed, so as to harness more energy from wind gusts. The unit has a brake for regular

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FIGURE 2.4  Riva Calzoni wind turbines (350 kW, D = 33 m) in Campania, Southern Italy, late 1990s. (Photo: Paul Gipe. All rights reserved.)

FIGURE 2.5  Thinair turbine, 2 kW, D = 3.6 m, teeter hub, variable rpm operation 60–335 rpm. (Photo courtesy of Powerhouse Wind, New Zealand.)

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25

overspeed control. However, the teeter hub allows the rotor to swing into a horizontal position in extreme high winds. Powerhouse Wind’s original design team members Bill Currie, Wayne O’Hara, Richard Butler, and Peter Shaw all worked in a variety of creative engineering organizations. The ADES wind turbine was developed by the TEMPERO group, a technological Spanish engineering firm that aims to develop solutions that encourage the rational and efficient use of renewable energy. The ADES wind turbines are single-bladed downwind turbines with a variable pitch rotor and set or variable speeds, depending on the model. The ADES wind turbine has a unique design (Figure 2.6) that includes three passive mechanical systems to compensate for wind gusts: swiveling singlebladed rotor; pendulum power train; and self-steering nacelle. The effects of a wind gust are first absorbed by the swiveling rotor and then by the pendulum unit. The pendular power train compensates and accumulates power from wind speed variations, thus providing more even generator rotation and subsequently diminishing structural overload and power peaks caused by wind gusts. All components of the power train have internal components that rotate at different speeds and accumulate kinetic rotational energy that is much greater than that of the pendular unit itself. Available sizes are 60, 100, 200, and 335 kW with a design to 3 MW. ADES has a system for using the wind turbine to obtain water from the wind. Go to their web site to see video of the pendular operation (http://www.ades.tv/en/products/ pendular-wind-turbine/id/24).

FIGURE 2.6  ADES pendular wind turbine, downwind. Left: hub, drive train, and generator. Right: 200 kW, D = 36 m, hub height = 36 m. (Photos courtesy of ADES, Spain.)

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2.2  MULTIPLE BLADES Thomas O. Chalk invented a rotor with a large number of blades based on the design of a bicycle wheel (Figure 2.7). One of the reasons for the large number of blades is that theoretically the most aerodynamic efficiency rotor has an infinite number of infinitely thin blades. Chalk started with two hub plates and a holding fixture for the rim and then threaded thin, hollow airfoil blades on the spokes. When all the parts were in place, the hub plates were jacked apart and the spokes became taut. On one design the rim of the wheel was used for a belt drive for speed increase for the generator. He formed the American Wind Turbine Company, and appropriately the rotor blades were painted red, white, and blue (see the cover of the July 1974 issue of Popular Science [3]). Two units were tested by Dr. William Hughes at Oklahoma State University and another unit was tested at the Rocky Flats wind test center, now the NREL Wind Technology Center. WindTronics, a private technology company, built a similar unit with a stronger rim to address one of the flaws of the Chalk turbine. The tips of the blades were attached to a rim that had permanent magnets, and there were field poles in the outer non-moving rim (stator of the generator). This design reduced tip losses and the outer rim acted as a small shroud. The design evolved to more and differently shaped blades connected to the inside rim, and the unit eventually became the Honeywell WindTronics (Figure 2.8). Others have built wind turbines using bicycle wheels, and based on searches on the Internet, there are do-it-yourself turbines (one example can be found at http://nrgcycle.com). An interesting one used compact discs (CDs) attached to the spokes of a bicycle wheel. Dr. Vladimir Kliatzkin formulated the WinFlex design more than 40 years ago. The design is somewhat based on a bicycle wheel in that the outer part of the rotor is

FIGURE 2.7  Chalk wind turbine, 1.5 kW, 48 blades, blade length 1.5 m; D = 4.6 m, inside diameter = 1.5 m.

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FIGURE 2.8  Honeywell WindTronics, 1 kW, D = 2.2 m. Left: Early model, 10 blades. Right: 20 blades, 1 kW, D = 2.2 m on 10 m tower at American Windmill Museum, Lubbock, Texas.

like the inner tube of a bicycle tire. Kliatzkin’s son Eliezer , was cofounder and CEO of WinFlex from 2008–2016, and during that period they developed the WinFlex concept and built three prototypes: 10, 200, and 132 kW (Figure 2.9). The pressure in the tube (inflatable wheel) was maintained by a central inflationary system. The main idea was that their wind turbine would be cheaper to build because the flexible tube and blades made from composite fabric were much lighter in weight than conventional wind turbine rotors. In 2010, WinFlex received one of the prizes from the GE Ecoimagination Challenge, and in 2013, Winflex was awarded a BIRD (Binational Industrial R&D Foundation, Israel) energy grant for a joint development program with GE of the WinFlex turbine. A video of the installation and operation

FIGURE 2.9  WinFlex, inflatable tube rotor with sail blades, 132 kW at 10 m/s, D = 20 m, hub height = 24 m. (Photo courtesy of Elizier Kliatzkin.)

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of the 200 kW prototype is available here: https://finder.startupnationcentral.org/ company_page/winflex or on YouTube https://www.youtube.com/watch?time_ continue=44&v=W7XqvIsLjX4. What is interesting in the video is the result of the torque forces on the tube as the wind changes direction. You can see the deformation of the ring. In more technical terms, a change in angular momentum due to a change in wind direction results in forces perpendicular to the plane of rotation. One part of the video—caption Extreme Wind Survivability—shows the turbine folded in high winds (https://www.youtube.com/watch?v=ofv5PsAWygk). The rendered video shows the rotor folded in half about the vertical plane very high winds (https://www. youtube.com/watch?v=MfEsXrdUEGA). Horizontal axis wind turbines have been built with four to eleven blades. More blades increase the efficiency a little bit, however, in general the cost for extra blades exceeds the return from the increase in energy production. The Blue Max was a wind turbine similar to the Mehrkam wind turbine, and it had four extruded aluminum blades and a rotor with fairly small solidity. M.C. (Kip) Cheney was with United Technologies Research Center (UTRC) and worked on their passive pitch control design (see Section 6.1), and after UTRC retired from making wind turbines, Cheney formed Windtech. Several two-bladed wind turbines were installed in California using that technology. However, the last Windtech turbine had five pultruded fiberglass blades and a rotor with fairly small solidity (Figure 2.10). Terrence Mehrkam started designing and building prototype wind turbines in the early 1970s; first a two-bladed, 7.6 m diameter unit and then a 13.8 m diameter unit. In 1977, he developed a four-bladed, 12 kW unit and then a six-bladed, 40 kW unit for his rural Pennsylvania homestead. The blades were made from extruded aluminum, anodized or coated with polyurethane for weather protection. He sold a 225 kW unit, which was installed at the Allentown, Pennsylvania amusement park. Later he sold around 10, 40 kW turbines (Figure 2.11) for the California wind market. The turbines were six-bladed, fixed pitch, and downwind, however, the brake design

FIGURE 2.10  Windtech, five blades, 75 kW, D = 15.8 m. (Photo courtesy of Kip Cheney.)

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FIGURE 2.11  Mehrkam, six blades, 40 kW, D = 12 m, tower = 12 m, at Atlantic Wind Test Site, Prince Edward Island, Canada.

was not sufficient to stop the turbines in overspeed situations. Mehrkam was killed in 1981 at a wind farm in San Diego County while trying to stop a runaway rotor in high winds, a feat he had accomplished twice before. The California Department of Occupational Safety and Health concluded that Mehrkam climbed to the top of the tower without using any form of fall protection and either fell or was thrown off the tower. He violated two premier safety rules for working on wind turbines: (1) never climb a tower with the rotor in an overspeed condition (in fact, go far upwind and wait for the machine to collapse or for the winds to die down), and (2) never climb a wind turbine alone. Missouri Wind and Solar (https://mwands.com) offers wind turbines with five, seven, nine, and eleven blades (Figure 2.12) and power ratings from 0.5 to 2 kW, with 1.6 m diameter rotors. Note that for any wind turbine a larger number of blades generally means higher solidity, which results in lower rpm. However, performance (energy production) is generally not improved enough to cover the extra cost of more blades.

2.3 SAILWINGS Sailwings have a long history, particularly those on the Lasithi Plateau on the island of Crete, for pumping irrigation water (Figure 2.13) beginning in the twentieth century. There were more than 10,000 windmills installed on the plateau with four to eight blades, however, by 2020 most had been replaced by diesel and electrical

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FIGURE 2.12  Missouri Wind & Solar, Freedom II, 2 kW, D = 1.5 m, 11 blades. (Photo Courtesy of Missouri Wind and Solar.)

FIGURE 2.13  Water pumping windmills with sailwings on the Lasithi Plateau, Crete.

irrigation pumps. The larger windmills with stone towers had 10 blades, however some were fixed in orientation to the predominate wind direction. The original Cretan windmill had two problems: manual furling and loose sails that tended to flap in high winds. One of the blades had a whistle on it to notify the operator to change the sail area (furl) when the winds were too high. For more information search for Lasithi Plateau windmills or Crete windmills on the Internet, and for more images go to Wikimedia Commons at https://commons.wikimedia.org/wiki/ File:Agios_Georgios01.jpg. Others have tried the sailwing because of the lower cost, however, no units are in commercial production. A prototype for pumping irrigation water (Figure 2.14)

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FIGURE 2.14  (a) El Gavillero Wind Test Site, IIE. Left to right: Large sailwing for pumping large volumes of water, small 2.5 m diameter unit for pumping water, electric generation. (b) Large sailwing unit with tip brakes for overspeed control. The man on the tower is Marco Borja.

was tested at the El Gavilero Wind Test Site of the Instituto de Investigaciones Electricas, Mexico. The sailwing, with 11 m diameter on an 18 m tower, drove a 10 kW synchronous generator with permanent magnets, which was connected to the motor of a 6 kW irrigation pump. The unit had tip brakes for overspeed control, and the generator had a dynamic brake. Wecs-Tech built a number of sailwing units for wind farms in Texas and California. The wind farm near Dalhart, Texas originally had a few vertical axis wind turbines, however, they later changed to the horizontal axis sailwing (Figure 2.15). At that time, in the early 1980s, they could not connect to the utility grid, so they were going to make fertilizer. The downwind rotors had aluminum spars, Dacron cover for the sails, and a V-belt drive from the gearbox to the generator. At the Dalhart installation, high winds soon damaged most of the units, and no units operated very long. Wecs-Tech also built an 18 m diameter unit, rated at 150 kW. In the high altitude (1,160 m) of the Tehachapi Pass in California the sails were weakened by UV-radiation, and again no units operated very long. Note that the Dalhart wind farm is in the Southern High Plains and its elevation is a little bit higher, over 1,210 m. Professor Gordon Brittan, Montana State University, was instrumental in developing the Montana Wind Company’s sailwing turbine, the Windjammer. The Windjammer II was mounted on a circular track for orientation into the wind (downwind unit) and there were support cables from the central post to the rim (Figure 2.16). The sails were furled to control power in high winds and for shutdown. The rim of the rotor rested on two truck tires, which then drove generators located on the lower portion of the frame. The unit was rated at 100 kW at 11.4 m/s. The Windjammer III and IV models were erected in Altamont Pass, California. An infomercial video (2012) that shows the operation of Windjammer IV is on the Internet at https://www. youtube.com/watch?v=GiPEvBGhT_s. Windjammer V was an upwind unit without the support cables and central post.

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FIGURE 2.15  Wecs-techTech turbine at wind farm near Dalhart, Texas, 1978. Purported power was 70 kW. Later they added two booms with weights for balance to the front side of the turbine.

FIGURE 2.16  Windjammer II, location near Livingston, Montana. Carter 300 kW turbines are in the background.

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FIGURE 2.17  Sweeney prototype, D = 3 m. (Photo courtesy of T. Sweeney.)

A sailwing invented by Professor Thomas Sweeney, Princeton University, consisted of a sail stretched between a leading edge spar and a trailing cable, which made for an elegant design. A two-bladed test unit (Figure 2.17) was built at Princeton University [3], and later Grumman purchased the license and built a similar unit and then a three-bladed unit (Figure 2.18). However they were never placed into production. Grumman later received government funding for a conventional wind turbine with 25 kW, 7.6 m diameter (3 blades, variable pitch with tip brakes, downwind, passive yaw) called the Windstream. The second Windstream had a larger diameter (10 m) but less power (15 kW). The larger diameter with smaller rated power was to meet specifications of the request for proposal from the government. For several years, the Wind Technology Center, NREL, used the prototype Grumman turbine for visualization of aerodynamic flow through the rotor and to measure pressure changes in the airflow over the new airfoil developed specifically for wind turbines (thin airfoil, S809).

2.4  MULTIPLE ROTORS Wind turbines exhibiting both horizontal and vertical axes with multiple rotors on the same axis or on the same tower have also been built to date. For HAWTs with rotors on the same axis, the wind speed behind the first rotor is reduced by 2/3, so the amount of energy passing through a second rotor placed close to the first rotor is reduced by a significant factor. One solution is to place the rotors further apart or to

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Innovative Wind Turbines

FIGURE 2.18  Grumman, truck test of 5 kW unit, D = 7.6 m. (Photo courtesy of Grumman.)

align the rotor axis at an angle to the wind for inflow. In almost all cases it is cheaper to make the blades longer. Another solution is to place a smaller efficiency rotor on the upwind side with a length around 1/3 the length of the larger rotor on the downwind side. The large blade is less aerodynamically efficient at the root and inner part of the blade due to structural considerations. The NOAH wind turbine had two five-bladed rotors (Figure 2.19) placed close to one another. The wind rotors counter-rotated, with one connected to the stator and the other to the rotor of a generator so no gearbox was needed. The wind turbine had a unique overspeed control consisting of a counterweight that tilted the rotor assembly to the horizontal position and then had to be reset manually. An inventor in Florida built a smaller unit with the rotors further apart (Figure 2.20). Note the large number of blades at 12 per rotor. Kowintec, South Korea, built a prototype dual rotor wind turbine, a 30 kW proofof-concept turbine that has been in service since January 2001. Then UWinSys, Oregon, was founded in June 2001 to further develop the Kowintec turbine, and a 1 MW prototype (Figure 2.21) was installed in South Korea in August 2007. This was the largest dual rotor on the same axis, with the smaller rotor upwind. The smaller rotor was designed to capture the wind impacting the hub and root of the larger rotor, which had to be designed for strength rather than aerodynamic efficiency. The counterrotating rotors were connected to a dual right angle gearbox that drove a generator on the vertical axis. Some information on the 200 kW (front rotor D = 12 m, downwind rotor D = 24 m, tower = 35 m) and the 1 MW United Wind Systems is available

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35

FIGURE 2.19  NOAH wind turbine, 45 kW, D = 10 m, tower = 12 m. Notice cables connecting blades.

FIGURE 2.20  Inventor with prototype, 2 kW, counter-rotating rotors, no gearbox.

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FIGURE 2.21  UWinSys turbine (Kowintec), 1 MW, front rotor D = 25 m, downwind rotor D = 47 m, tower height = 45 m.

from Peter Borsanyi, Wind Turbine Catalog, 2008–2018, http://savonius-balaton. hupont.hu/111/united-wind-systems-oregonusa. Also, a high resolution photo of the 1 MW system is available at http://www.ibkjob.kr/ibk/jw/ComInfo?jw=read&corp_ sno=5058109183&flag=1. Airgensis (http://www.airgenesiswind.com) has designs for a 500 kW and a 11 MW dual rotor system with two rotors of equal diameter with the generators at ground level. Doug Selsam probably has built more wind turbines with multiple rotors on a coaxial shaft than any other inventor. The line of the rotors is kept at an angle to the wind to improve influx of the wind to the downwind rotors. He built units of two to seven rotors (two and three blades) with rated power from 2 to 4 kW (Figure 2.22). One unit contained thirteen two-bladed rotors, each with a diameter of 0.5 m, and rated at 400 W. For several rotors on a single shaft, the almost ultimate wind turbine is the Sky Serpent (Figure 2.23). See www.selsam.com for video of a unit in operation.

2.5  MULTIPLE TURBINES There have been many prototypes with multiple rotors on a tower. Lagerwey built units with two, four, and then six rotors (Figure 2.24) on one tower. Because the swept area increases as the square of the radius, multiple turbines on the same tower are generally not commercially viable as it is cheaper to increase the diameter. There is the problem of orientation of the rotors perpendicular to the wind. Do you rotate

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FIGURE 2.22  Selsam Supertwin; 2 kW, two rotors 3.7 m apart, D = 3 m. (Photo courtesy of Doug Selsam.)

FIGURE 2.23  Doug Selsam with his Sky Serpent, 26 rotors, 1.1 kW at 9 m/s, D = 1 m. (Photo courtesy of Doug Selsam.)

the tower or the individual turbine? The wind-lens was a shrouded wind turbine (see Chapter 3) developed at Kyushu University Research Institute. Three of the units (3 kW, rotor diameter = 2.8 m, diffuser diameter = 3.6 m) were mounted on a tower, resulting in the multi-lens turbine (Figure 2.25). Another idea is for the area to be covered by a hexagonal arrangement of wind turbines. Jetpro Technology, Taiwan, has shrouded wind turbines that can be cascaded into modules of 5, 9, and 14 turbines (Figure 2.26). Each turbine is framed within an extruded aluminum alloy hexagon. The base plate of an individual wind turbine was designed as a hinge to join to the supporting beam of the frame and included a slip ring for electric connection. Go to the Jetpro Technology web site and click on the gallery for more images of multiple turbine installations (http://www.jetprotech.

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FIGURE 2.24  Lagerwey wind units. Left: each turbine, 35 kW, D = 10 m, tower height = 25 m. Right: each turbine = 75 kW, D = 15 m, mast height = 56 m. (Photos courtesy of Lagerwey.)

FIGURE 2.25  Front view, RIAMWIND, 9 kW, width = 8.6 m, downwind shrouded, passive yaw control (oriented as one unit). Note the cross bracing between the units. (Courtesy of Kyushu University and Riamwind Corporation, www.riamwind.co.jp.)

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FIGURE 2.26  Units can be mounted in clusters. Each turbine mounted independently; 200 W, rotor diameter = 0.68 m.

com.tw/en/profile/index.htm). Jetpro also has shrouded turbines with rated power of 1, 5, and 50 kW. The 50 kW wind turbine is a 5-bladed turbine with rotor diameter of 8.0 m; the unit can also be cascaded. Vestas, which is the leading manufacturer of wind turbines in the world, built a prototype of 900 kW with four turbines (Figure 2.27). Individual nacelles are

FIGURE 2.27  Vestas, 900 kW, multi-rotor concept at Risø test site, Denmark. V29 is an upwind unit, full span pitch control. (Photo courtesy of Vestas.)

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refurbished V29–225 kW units (29 m diameter). The lower twin-rotor unit is at a height of 29 m and the second is at 59.5 m with the tip height at 75 m, which is tailored to the site-specific requirements of the Risø test site in Denmark. Clearance between the rotor and the tower is 1.5 m. The two nacelles at the same level are oriented perpendicular to the wind with a yaw motor. For more details, photos, and video go to https://www.vestas.com/en/about/discover_wind#!innovative. Graphically compare two and three wind turbines (each 25 kW, D = 10 m, clearance one meter) mounted on a single tower with the equivalent single rotor (Figure 2.28). You can see that the area for the single rotor is larger, so for units up to 10 MW the consensus is still that single rotors are better than multiple rotors on the same tower. Since weight becomes a problem with very large wind turbines, 20 MW up to 50 MW, there are design concepts for multiple wind turbines [4] on the same tower, primarily for the offshore market. The optimum blade has increased twist and chord length from the tip of the blade toward the root. This highly efficient blade has 22° twist at the root (Figure 1.4). Because of the constraints of materials, structural considerations, and economics, large blades have less twist and chord than optimum; approximately 25% of the blade length towards the root. One proposed solution [5,6] is two rotors: one small highly efficient rotor in front of the large rotor (Figure 2.29). Figure 2.20 shows this concept with the rotors on each end of the nacelle. A rough comparison of the swept area of the small rotor with the equivalent swept area for increased blade length for a 3 MW wind turbine shows that the new small rotor could be replaced by increased blade length around 2 m, because the swept area increases by the square of the radius. For more information go to the Internet and search multiple blade wind turbines, multiple rotor wind turbines, dual rotor wind turbines, or counter-rotating wind turbines. I am sure that there are other multiple rotor prototypes that I have missed. If you know of an interesting one, email me ([email protected]) or Kenneth Starcher ([email protected]).

FIGURE 2.28  Comparison of area for multiple rotors. Dashed line is for single rotor diameter, D = 14.1 m for two rotor system, D = 17.3 m for three rotor system. Small rotor D = 10 m.

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FIGURE 2.29  Dual rotors to increase energy production for large wind turbines.

REFERENCES 1. World Wind Energy Association. June 2019. Wind power capacity worldwide reaches 597 GW, 50,1 GW added in 2018. https://wwindea.org/blog/2019/02/25/ wind-power-capacity-worldwide-reaches-600-gw-539-gw-added-in-2018/ 2. GWEC: Global wind report 2018. April 2019. https://gwec.net/global-wind-report-2018/ 3. E. F. Lindsey. 1974. Windpower. Popular Science, July, 54. 4. T.E. Sweeney. March 1973. The Princeton Windmill Program. AMS Report No. 1093. http://www.dtic.mil/dtic/tr/fulltext/u2/a132117.pdf 5. A. Ozbay, W. Tian, H. Hu. An experimental investigation on the aeromechanics and near wake characteristics of dual-rotor wind turbines (drwts), in: AIAA Science and Technology Forum and Exposition (SciTech2014) 6. A. Rosenberg, S. Selvaraj, A. Sharma. 2014. A novel dual-rotor turbine for increased wind energy capture. Journal of Physics: Conference Series, 524.

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A duct, shroud, or diffuser increases the power/area for a rotor by accelerating the air passing through the rotor, thereby increasing the velocity, by accommodating the expanded wind stream behind the rotor (Figure 3.1), and it also reduces tip losses. The turbine is sometimes referred to as a diffuser augmented wind turbines (DAWT). Two aspects touted by DAWT manufacturers are less vibration and less noise. The maximum theoretical efficiency for a wind energy conversion device is the Betz limit, namely 59%. For a conventional wind turbine, the wind stream intercepted is the swept area of the rotor. Some inventors of ducted turbines claim their turbines exceed the Betz limit; however, they do not account for the full area of their turbines. The area that should be considered is larger than just the rotor swept area, and for ducted turbines the area should be the maximum size of the duct inlet, shroud, or diffuser. Therefore, no one has a prototype wind turbine where the calculated values of power produce an efficiency greater than 59%. Beware of promoters of wind turbines that claim efficiencies that exceed the Betz limit and also beware of those that claim how well their turbine performs in low winds. The problem there is that not much energy is produced in low winds, and in the final analysis it is the value of the amount of energy produced that is important. A toroidal wind turbine is another design option (Figure 3.2) that is similar to a ducted wind turbine. For an acceleration platform wind turbine, it is somewhat difficult to determine what should be the intercept area for the device. Should it be the total cross section of the platform or just a slice across the platform equal to the rotor diameter? A number of DAWTs have been designed and built, however, none have reached the stage of significant commercial production. The primary problem is the trade-off between increased production and the increased cost of material for the shroud, duct, or diffuser. Another problem is the orientation of the structure as the wind direction changes and what that does to the flow pattern through the duct. In general, wind speed increases with height, so ducted turbines need to be on towers or maybe on buildings. Ducted wind turbines are not a new idea as one was installed in San Gorgonia Pass, California, in the 1920s. Paul Gipe has a comprehensive section on Ducted or Augmented Turbines in Chapter 7 (“Novel Wind Turbines”) of his latest book [1]. Paul also has information and photos on six ducted wind turbines on his web site (www.wind-works.org/cms/index.php?id=637). One of the turbines is the Vortex 7, New Zealand. The unit was mounted on a circular track with a truss structure to support the shroud; the specifications were rotor diameter of 7.3 m, shroud diameter of 12 m, and height around 22 m, and power was overrated at 1 MW. In 2005, a Swiss Company named Enflo constructed a very small ducted turbine, the Windtec 0071. The turbine was rated at 0.5 kW with a shroud of 0.9 m diameter. The French firm Eléna Energie installed two Eléna 15 ducted turbines on top of the Maison de l’Air in Paris. The Eléna 30, which was overrated at 6.8 kW, had a 1.2 m diameter 43

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Innovative Wind Turbines 0

1 2 3

4

5

TURBINE Vo CENTERBODY

DIFFUSER

FIGURE 3.1  Diagram for ducted wind turbine. FRONT VIEW

Optional Shroud Shells

TOP VIEW Optional Shroud Shells

STREAMTUBE

V

FIGURE 3.2  Toroidal wind turbine.

rotor in a duct of 1.6 m diameter and 2 m in length. Catching the Wind turbines had a cone shaped duct with the large opening facing the wind and the rotor at the exit (https://www.designboom.com/technology/bird-friendly-compressed-air-windturbines-by-raymond-green/). The Marquiss Wind Power turbine (∼2008) was a little bit different in that the frame for the rotor was square. The unit was designed primarily for installation on the roofs of commercial buildings. The 3.ZERO of Venturicon Windturbines is a ducted wind turbine with a flange that was developed in collaboration with the Innerstaatliche Hochschule (University of Applied Sciences), Buchs, Canton of St. Gallen in Switzerland (https://www. venturicon.com/english-1/). Venturicon states their turbine is flow-optimized due to the flange, which they call Fowler technology. The rated power is 1.15 kW at 12 m/s and the diameter including spoiler is 1.9 m. An early diffuser wind turbine (Figure 3.3) was tested at the Beijing Badaling Wind Power Test Station, China. The test station, which was near the Badaling Gate of the Great Wall, ceased operation in 1996. I was fortunate to be able to attend a

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FIGURE 3.3  Diffuser wind turbine (1985) at Beijing Badaling Wind Power Test Station near the Great Wall, China.

wind conference in Beijing in 1985 and as part of the conference we toured the test station. The industrialization of China since then has been phenomenal. For instance, in 1985 there were less than 1,000 cars in Beijing and today there are major traffic problems and smog from more than 6 million cars. The Wind Lens (Figure 3.4) is a modification of the wind turbine technology developed by Professor Yuji Ohya at the wind energy engineering section at Kyushu

FIGURE 3.4  Wind Lens, 3 kW, rotor diameter = 2.8 m, diffuser diameter = 3.6 m. (Photo courtesy of Kyushu University and Riamwind Corporation, www.riamwind.co.jp.)

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University’s Research Institute for Applied Mechanics (RIAM, https://www.riam. kyushu-u.ac.jp/windeng/en_index.html) in Japan [2]. A brim on the shroud produces vortices that create a low-pressure region behind the turbine, which helps more wind to flow through the Wind Lens. Two mid-size (100 kW at 12 m/s) Wind Lens turbines were constructed at the Ito campus of Kyushu University (2011). The rotor diameter was 12.8 m, diffuser diameter was 15.4 m, and the whole structure was 34 m high. The technology was also tested on a hexagonal floating platform, 18 m in diameter, in Hakata Bay, Fukuoka Japan starting in 2012. The renewable energy farm experiment had two Wind Lens turbines (3 kW each) and photovoltaic (PV) panels (2 kW). Gerald Brock was the inventor and became the CEO of the Windtamer Corporation. The Windtamer was a ducted turbine with a shroud and a diffuser and had the appearance of a flower design (Figure 3.5). The Windtamer had a tube within a tube to enhance the flow of the wind. Their first residential unit was installed in 2009 in Perry, New York. Windtamer became Arista Power in May 2011 and then later SkyWolf Wind Turbine Corporation (http://www.skywolfwindturbines.com/). The SkyWolf wind turbine is similar to the Windtamer, however, the shroud is replaced by bi-facial photovoltaic panels for a solar-hybrid diffuser augmented wind turbine (DAWT). The shroud has eight PV panels and another eight panels can be attached to the tower (Figure 3.6). Predrag and Nenad Paunovic are the founders of Poduhvat, a Serbian company that is developing state of the art technology in renewable energy, transport, and

FIGURE 3.5  Windtamer at the Clarkson Wind Turbine Test Site in Potsdam, New York; 3.5 kW, tower = 12 m. Cameron Gibb and Stuart Wilson, visiting students from Australia working on turbine designs at Clarkson University. (Photo courtesy of Ken Visser.)

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FIGURE 3.6  SkyWolf, Solar-Hybrid Wind Turbine; wind = 3.5 kW, eight PV panels = 3.5 kW, rotor D = 2.4 m, tower height = 7.6 m. (Photo courtesy of SkyWolf Wind Turbine Corporation.)

buildings plus the integration of those technologies. Vetar is the wind turbine used for their wind technology projects (https://poduhvat.com/vetar.html). The Vetar wind turbine has a double shroud and dual counter-rotating rotors each connected to an independent permanent magnet generator (Figure 3.7). The counter-rotating rotors cancel each other’s rotational forces and there is a turbulence control ring behind the second rotor. Vetrosun combines a slanted platform with solar panels that deflect airflow toward the wind turbine, which should increase energy output by around 15%. The combined power for model 13i4.2 is 17.2 kW with 13 kW wind and 4.2 kW solar energy. Vetrolit combines 15 kW of wind, 12 kW of solar panels, and 1,000 kWh of storage. Vetar has a design concept for a living- or workspace of 236 m2 with Vertrosun on top of the structure. Vetrodom segments are made from composite concrete, and the segments are shaped and interlocked to provide a very strong construction unit. Ducted Turbines International (DTI) is a wind turbine company founded by Paul Pavone and Ken Visser, Associate Professor in the Department of Mechanical and Aeronautical Engineering at Clarkson University, Potsdam, New York. DTI started in an effort to commercialize technology developed by Visser at Clarkson University. Ducted turbines have traditionally placed the rotor at the throat of the duct, which is the location of highest velocity. Research teams at Ducted Turbines International (http://ductedturbinesinternational.com) discovered that if the rotor is moved farther into the duct (Figure 3.8), the power output increases dramatically as the flow field is

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FIGURE 3.7  Vetar, 15 kW, counter-rotating rotors, each 2 blades, rotor diameter = 2.5 m, outside diameter = 3.6 m, active yaw, overspeed control = yaw. Also have Vetar, 10 kW unit, rotor diameter = 2 m, outside diameter = 2.9 m. (Photo courtesy of Vetar.)

FIGURE 3.8  Ducted Turbines International, 3.5 kW, rotor D = 3 m, duct exit D = 3.7 m, tower height = 7 m. Turbine mounted on roof of Technology Advancement Center at Clarkson University. (Drone photo by Chris Lenny, Watertown Daily Times.)

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more uniform. DTI is currently focusing on developing commercial prototypes with 3.5 kW and duct exit diameters of 3.7 and 4.7 m. The FloDesign wind turbine was a concept based on the mixer-ejector technology (includes cowling) originally developed for jet engines (see video at https://www. youtube.com/watch?v=WB5CawKfE2M). One unit was installed on Deer Island, Massachusetts (see photo nine at http://www.wind-works.org/cms/index.php?id=541). The company later became Ogin and had a different mixer design. One unit was installed in Antelope Valley, California, and seven units (Figure 3.9) were installed in San Gorgonio Pass, California, in the Spring of 2015. They were removed in 2016 due to difficulties with the shroud. Then in 2017, Halo Energy (https://www. halo.energy) acquired the design, and now the unit has a dual chambered shroud with rated power = 6 kW, rotor diameter = 2.4 m, and shroud exit diameter = 3.7 m. Halo declined to provide a photo for this book as they were still fine tuning the final commercial product at that time (June 2019), however, you can see a video on their web site of the first commercial production unit from April 1, 2019. The Anakata is a ducted wind turbine with a cowl (Figure 3.10). The cowl is attached to the shroud with slots for inlet air. A smaller turbine (A007) had a somewhat similar design except the blades and rim were a solid piece that rotated (https://www.youtube.com/watch?v=spqKCgKQf2E). Today, Anakata primarily designs add on devices for wind turbines to increase energy capture and reduce noise, as well as vortex generators, winglets, lips, and serrations based on their experience in aerodynamic research and development in Formula One. Lips increase torque at

FIGURE 3.9  Ogin, San Gorgonia Pass, CA, nominal rating 100 kW, however, could reach peak power of 150 kW, rotor diameter = 12.5 m, shroud diameter ∼18 m, hub height = 46 m. (Photo courtesy of J. Smith.)

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FIGURE 3.10  Anakata, A018, 1.4 kW, rotor diameter = 1.8 m, cowl diameter = 2.4 m. (Photo courtesy of Ben Wood, Anakata Wind Power Resources.)

the blade root and serrations reduce sound emissions. West Wind Power (http://www. westwindpower.com) has a 3 m diameter windmill that can be coupled with solar panels. Its ducted turbine has slotted rings on the downwind side of the duct. They built a prototype twin turbine model where the diffuser did not have the slotted rings. Varan Sureshan is the founder of Katru Eco-Inventions and the inventor of IMPLUX (https://www.katru.com.au). IMPLUX has a seven-bladed rotor mounted on a vertical axis inside a shroud (Figure 3.11). The shroud captures wind from any direction and minimizes wind leakage via a gate within the shroud, and the shroud structure redirects the wind stream in the vertical direction upward to the rotor. In addition, the wind flowing over the shroud sucks more air through the rotor. IMPLUX should have even higher efficiencies when used for power generation on top of buildings as it captures and streamlines the upwardly skewed wind that is present on top of buildings. Dr. Leon Fan is the founder of Kenning Global Energy Technologies and the inventor of the Wind Funnel, which is drag device in a sheet metal box (Figure 3.12). At the air inlet, the vanes and curved baffles are designed to ensure that laminar flow acts directly on the front of the rotor blades and to shield half the rotor. Note that this unit has to be oriented into the wind. Units were erected in China (Figure 3.13) and Seymour, Texas (2010). The VQ WindJet is similar in design to the Wind Funnel with curved baffles (vanes) on the lower front inlet and then a rear baffle (vane) on top to direct air flow down onto the rotor (Figure 3.14). There are dual rotors on a horizontal axis, however, the unit is mounted on a vertical axis with a tail fin for orientation into the wind. For video of operation go to https://www.youtube.com/watch?v=WeltXli55fo. A unit was installed as part of A.L. Huber general contractor’s Renewable Energy Demonstration

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FIGURE 3.11  KATRU, I225, 2.3 kW, rotor diameter = 2.25 m, unit diameter (top/base) 4.0/1.9 m, unit height = 3.6 m, weight = 720 kg. (Photo courtesy of Katru Eco-Inventions.)

FIGURE 3.12  Kenning Global Energy Technology, Wind Funnel, rated power ∼35 kW, located in Panjin, China. Note that unit is on a rotatable base. (Photo courtesy of Charlie Dou.)

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FIGURE 3.13  Diagram of airflow for Kenning Wind Funnel.

FIGURE 3.14  VQ WindJet, 5 kW, Length = 6.7 m, height = 4.5 m, rotor D = 3 m, swept area = 15 m2. Unit at A.L. Huber, Overland Park, Kansas. Unit installed in 2009 and removed in 2016. (Photo courtesy of A.L. Huber General Contractor.)

Initiative (REDI). REDI is a demonstration project established to provide educational opportunities in wind and solar energy to public and private companies or individuals who are interested in the future of energy. Components to the REDI project include: the VQ WindJet 5; SunPower photovoltaic panels; the Energy Center demonstration and monitoring room; a “green wall”; reclaimed wood siding; and other sustainable elements incorporated in the building and in the culture. A video of the installation at A.L. Huber is available at https://www.youtube.com/watch?v=AOHIyEQwINU.

3.1  VENTURI WIND TURBINES The Venturi effect is where the wind speed is increased due to a constriction of the wind stream. The Invelox (INcreased VELOcity) was invented by Daryoush

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Allaei, founder of the company SheerWind in 2010. Invelox is essentially constructed with two separated funnels that accept wind from any direction and direct the flow downward to a constricting tube. A number of prototypes were developed and constructed with rated power ranging from kilowatts (Figure 3.15) to megawatts. For calculating efficiency of the unit, the swept area is that of the inlet, which is the diameter of the top funnel times the distance between the two funnels. It is not the area calculated from the rotor diameter. For example, in the diagram (Figure 3.15) the swept area is 74 m2, not 2.5 m2 as calculated from the Venturi tube diameter. So the power of the unit could be 20–25 kW. The largest unit was rated at 2.2 MW (Figure 3.16) and was constructed by SheerWind China near Hangshui. Note from ID =12.2 m

6.1 m

Thickness = 1.3 cm

ID = 1.8 m 9.1 m

15.2 m

3 m

FIGURE 3.15  SheerWind wind turbine. Demo test with 0.6 kW wind turbine, D = 1.3 m. (https://commons.wikimedia.org/wiki/File:Sheerwind_generator.jpg) 28.7 m

45.7 m

Venturi diameter 3.0 m

10.7 m

57.3 m

FIGURE 3.16  Diagram of SheerWind Invelox wind turbine in China, 2.2 MW, 3 rotors.

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the diagram the swept area is around 410 m2 and a rough estimate for rated power at 12 m/s based on that area would be around 170 kW. I was unable to obtain images of the 2.2 MW unit in China from Allaei, Masoud Sedeki of Pacificwind, or from my contacts in China. Pacificwind, New Zealand is another defunct business, however, their web site was still available at the time of writing, and they have both photos and specifications of SheerWind projects across the world, including the large MW unit in China, http://pacificwind.co.nz. They also have a video showing the construction of the unit, https://www.youtube.com/watch?v=JpsnV-4vBpE. For another photo go to the following web site: https://twitter.com/hashtag/betterwaywind?src=hash. SheerWind used a quantity called speed ratio (wind speed in constricting tube over free wind speed) as indicator of performance, a term which is essentially meaningless for estimating power and energy production. The two units for the Michigan National Guard at Camp Grayling were rated at 100 kW each and the unit at Camp Fort Custer had a different design with two intake tubes (Figure 3.17). The Fort Custer unit was rated at 200 kW from three wind turbine generators in the Venturi tube. It appears that the intake tubes were fixed in orientation, but that would need to be verified. A unit with just a Venturi tube (Figure 3.18) was erected on Palmyra Atoll, an unorganized incorporated territory administered by the U.S. federal government, which is located 1,000 miles south of Hawaii. Cooper Island and ten other land parcels in the atoll are owned by The Nature Conservancy, which manages them as a nature reserve. SheerWind was chosen because of the company’s claim that it was bird safe. Fortunately the installation also included solar photovoltaic panels as the SheerWind unit produced little electricity. The unit is fixed in orientation toward the predominate wind direction. Another SheerWind unit with just a Venturi tube was installed in 2016 on the NedPower building in the Netherlands (Figure 3.19), however, this unit could be oriented into the wind. For a critique of SheerWind read Michael Barnard’s article of July 2014 [3] and/or Paul Gipe’s article of February 2017 [4].

FIGURE 3.17  SheerWind unit at Camp Custer, Michigan. Rated power ∼200 kW, 3 wind turbines; length was around 100 m, venturi D = 3 m, diffuser D = 11 m. Note PV array on the right. (Michigan National Guard photo by Sgt. 1st Class Helen Miller. Construction not complete in this photo.)

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FIGURE 3.18  SheerWind, Invelox near airfield, Palmyra Atoll. Length = 25 m. (Photo courtesy of Cindy Coker.)

FIGURE 3.19  Venturi wind turbine on building of NedPower, Netherlands, 220 kW, wind turbine on rotatable platform; rotor diameter = 2.25 m, intake D = 4.5 m, outflow D = 7 m, 21 m long. (Photo by Dronevanrron, courtesy of NedPower SWH.)

3.2 ACCELERATION Instead of using shrouds, other structures can be used to increase the wind speed (Figure 3.2). A problem might be yaw orientation of the structure or the rotors. Note that the IMPLUX (Figure 3.11) design is somewhat similar to a vortex tornado (Figure 1.21) in that the wind flow over the top creates a lower pressure region and should assist in the wind flow through the rotor.

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FIGURE 3.20  Wind amplification turbine system. Dimension of the wind system, height = 6.4 m, diameter = 6.7 m, inner spiral diameter = 4.6 m. Each wind turbine is 2 m diameter, rated power = 1.6 kW. Can change lighting. (Photo courtesy of Majid Rashidi.)

A wind amplification turbine system was designed and developed by Professor Majid Rashidi at Cleveland State University [5]. One prototype consisted of a cylinder with two wind turbines per side of the cylinder and was installed on top of the Plant Services Building in 2009. The cylinder was 7.6 m in diameter and the base was 9 m tall (https://www.csuohio.edu/sustainability/wind-amplification-turbine-system). Then professor Rashidi designed and developed a helical structure, which is similar to a toroidal acceleration platform. The structure (Figure 3.20) has four fixed wind turbines and the whole structure is rotated to keep the turbines perpendicular to the wind. An aluminum frame (1.4 metric tons) is covered with white plastic pieces to form a helix. One of these wind systems rises around 12 m above the upper concourse of Progressive Field, the Cleveland Indians’ ballpark. As an aside, the structure can be lighted at night from the inside with different colors of LED lights, so it has the appearance of a colored lamp shade. The Optiwind system consisted of a cylinder with ducted wind turbines on the outside of the cylinder, referred to as a compact wind acceleration turbine. After the truss tower was erected, the cylinder with wind turbines was assembled around the tower and then raised by winches (Figure 3.21). The cylinder rotated to keep wind turbines perpendicular to the wind and if they were stackable cylinders, they rotated in unison. The prototype at Klug Farm, Torrington, Connecticut, consisted of two turbines with a rating of 50 kW, and then another stackable unit was added for a total

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FIGURE 3.21  Construction phase of Optiwind at Klug Farm, Torrington, Connecticut, 50 kW, 2 turbines, tower height 60 m, rotor D = 6 m. (File photo - Hearst Connecticut Media.)

rated power of 100 kW. Images of the installation at the Klug Farm are available on the Optiwind Facebook page: https://m.facebook.com/Optiwind-275790861337/. YouTube video of the turbines at Klug Farm is available at https://www.youtube.com/ watch?v=w2K_rMM36_U. A number of designs and some installations have been developed by architects, inventors, and people selling wind systems to integrate wind turbines into building structures. They usually tout the increase of wind speed caused by the building because of blockage and height. However, in the real world, incorporating wind turbines into buildings is a difficult choice because of noise, vibration, and safety concerns. In some installations on buildings, the wind turbines must be mounted perpendicular to the predominant wind direction because the wind turbines are fixed in yaw. There is a web site for urban wind turbines and there are images of wind flow over buildings, example projects, and documents to download at http://www. urbanwind.net/wineur.html. Buildings block and accelerate wind flow, so there are a number of installations of turbines on parapets, facades, and as part of the original design of buildings. Fourteen wind turbines installed on a corner of the Energy Adventure Aquarium building (Figure 3.22) in California constitute a kinetic sculpture. A building in Chicago mounted eight units horizontally on top of a building (Figure 3.23) although other buildings mounted units vertically. The Greenway Self Park, Chicago, has 12 helix wind turbines in two parallel rows mounted in a chamfered corner (Figure 3.24). The chamfer increases the turbines’ wind exposure and it also enables them to visually anchor and define the corner. Each turbine rotates independently, and because the most efficient layout of parking spaces in a rectangular structure means no spaces in garage corners, the wind turbines did not cause any loss of usable space. A spectacular structure featuring integrated large wind turbines is the Bahrain World Trade Center (Figure 3.25). The two 240-m towers with sail silhouettes have

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FIGURE 3.22  Fourteen 1-kW wind turbines mounted on parapet of Energy Adventure Aquarium. Each turbine is mounted in a housing of approximately 1.2 by 1.2 m. (Photo courtesy of AeroVironment.)

FIGURE 3.23  Eight helical horizontal axis wind turbines, 1 kW each, on top of building. Each unit in a cage 1.5 by 3 m. For more information go to www.aerotecture.com. Installed in 2007. (Photos courtesy of Kurt Holtz, Lucid Dream Productions.)

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FIGURE 3.24  Greenway Self Park, Chicago. Twelve helix wind turbines; each turbine 4.5 kW, D = 1.2 m, H = 4.9 m. Installed in 2011. See https://www.archdaily.com/74468/ greenway-self-park-hok. (Photo, © John Picken, www.johnpicken.com.)

FIGURE 3.25  Bahrain World Trade Centre, wind turbines 225 kW each, diameter of 29 m, installed in 2008. (https://commons.wikimedia.org/wiki/File:Bahrain_Manama_World_Trade_Center.jpg)

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three cross bridges that carry wind turbines [6]. The aerodynamic design of the building funnels the prevailing onshore Persian Gulf breezes into the path of the wind turbines. The wind turbines were predicted to generate around 1,000–1,300 MWh per year—11%–15% of the electricity needed by the building. Strata Tower, London [7], a residential building, has three wind turbines, each 19 kW, five blades, diameter = 9 m (Figure 3.26). They were anticipated to produce a combined 50 MWh of electricity per year, around 8% of the building’s need for electricity. Strata SE1 also incorporates a number of sustainable innovations. One reason for the wind turbines was aesthetics; in the beginning Southwark Council granted planning permission for Strata Tower on the grounds that the wind turbines would “create a dramatic and highly recognisable building form that achieves one of the council’s plan objectives, which is to create landmark buildings as signifiers of the Elephant & Castle on the London skyline.” Besides generation of electricity, for some installations the wind turbines are a kinetic sculpture and a public attraction. In addition to the option of installing wind turbines on existing buildings there are a number of designs for combining wind

FIGURE 3.26  Strata Tower (148 m height, 43 story), London. Installed in 2011. Left: https:// commons.wikimedia.org/w/index.php?curid=39407450. Right: By Cmglee - Own work, CC BY-SA 3.0. (https://commons.wikimedia.org/w/index.php?curid=20745152); (https://commons. wikimedia.org/wiki/Category:Strata_(building)#/media/File:Cmglee_Strata_SE1_turbines.jpg)

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power with buildings. In many cases ducted wind turbines are selected because of less noise, ducts acting as a safety shield, and supposedly less vibration. For some of the problems of operation of a ducted wind turbine in the field, read Heiner Doerner’s article, “Concentrating Windsystems – Sense or Nonsense?” (www. heiner-doerner-windenergie.de/diffuser.html).

REFERENCES



1. P. Gipe. 2016. Wind Energy for the Rest of US, A Comprehensive Guide to Wind Power and How to Use it. Bakersfield, California: wind-works.org, 560 pages. ISBN: 978-09974518-1-8. $65. ISBN 978-0-9974518-0-1 (ebook). $30. 2. Y. Ohya, T. Karasudani, T. Hagai and K. Watanabe. 2017. Wind lens technology and its application to wind and water turbine and beyond. Renewable Energy Environmental Sustainability. 2, 2. https://www.rees-journal.org/articles/rees/full_html/2017/01/ rees160022-s/rees160022-s.html 3. M. Barnard. 2014. Sheerwind Invelox: All hype, no substance. Clean Technica. https:// cleantechnica.com/2014/07/08/invelox-ducted-turbine-latest-long-line-failures/ 4. P. Gipe. 2017. SheerWind-Invelox: Is the end nigh for another ducted turbine? http:// www.wind-works.org/cms/index.php?id=64&tx_ttnews%5Btt_news%5D=4577&cHa sh=282d6ad0aa89e6fb384b0a1aa6b6b8ea 5. Cleveland State University. Wind amplification turbine system. https://www.csuohio. edu/sustainability/wind-amplification-turbine-system 6. For design aspects, I. Milne, Bahrain world trade centre; harnessing wind energy a post–occupancy evaluation http://global.ctbuh.org/resources/presentations/bahrainworld-trade-center-harnessing-wind-energy-a-post-occupancy-evaluation.pdf 7. For design aspects, J Bennetsen, High-rise wind turbines. http://www.ansys.com/-/ media/Ansys/corporate/resourcelibrary/article/AA-V6-I1-High-Rise-Wind-Turbines.pdf

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Vertical axis wind turbines (VAWT) generally have two or three blades, however, several units with more blades, differently shaped blades, and different configurations have been constructed (Figure 1.9). Because wind speeds generally increase with height above the ground, structural and economic considerations preclude large VAWTs (>100 kW) at heights around 100 m, so they have trouble competing with today’s megawatt horizontal axis wind turbines (HAWT). This is only one of their limitations, but a significant one. It was Sandia Lab’s reason for proposing offshore VAWTS and the reason a few other companies are working on large VAWTs for the offshore market. There the rotors do not need to be at elevated heights, and generator and drive train at a lower height is an operation and maintenance (O&M) advantage. The advantages claimed by promoters of VAWTS are that they accept wind from any direction (no yaw orientation needed) and the generator can be at ground level. One advantage cited for large VAWTs is that they can be placed closer together than HAWTs in wind farms, however, field data are not available at this time since there are no wind farms with large VAWTs. A comprehensive review of vertical axis wind turbines of 100 kW or larger that have been installed is available via downloadable PDF [1]. Today, large VAWTS are limited to prototypes and small VAWTs are a niche market, and many are only prototypes. VAWTs are a popular design with inventors, and web sites appear frequently touting a new wind turbine and their advantages compared to HAWTs. Some mention how they can easily scale up to megawatt size to compete with HAWTs or just install a very large number of small wind turbines. The problem is that one 3 MW wind turbine would be equivalent to 1,000, 3 kW units and a 30 MW wind farm would need 10,000 units.

4.1 DARRIEUS In the 1920s, the French engineer Darrieus invented a wind turbine where the blades had a troposkein shape; a shape similar to a skipping rope (Figure 1.10). The advantage of that shape is that it only has tension along the blade; no shear forces. However, it was more difficult to have that shape for the entire rotor, so some Darrieus units had cross braces near the top and bottom of the torque tube or X-braces. Darrieus also patented a rotor with straight blades, namely the H-rotor or giromill, diamond shape, delta shape, and Y shape. In the 1960s, South, Rangi, and Templin [2] at the National Research Council of Canada (CNRC) reinvented the Darrieus wind turbine, and then DAF-Indal manufactured a range of prototypes with 6, 40, 230, and 500 kW. DAF-Indal installed the 230 kW prototype on Madeleine Island in the Gulf of Saint Lawrence in 1977. Even though Darrieus turbines are not self-starting, during 63

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swirling winds they may self-start. During routine maintenance on July 6, 1978, the unit was left unattended overnight with the brake off. The next morning the unit was running and the flap brakes at midpoint of the blade deployed due to centrifugal forces, however, as the rotor slowed down then the flap brakes returned to the run position. Then one became stuck in the run position and the rotor went into overspeed and collapsed. A lesson learned from this event and on overspeed on other units, including HAWTs, was that mechanical brakes need to be on the low speed shaft. DAF-Indal’s 500 kW prototypes were installed at the Atlantic Wind test site and the Southern California Edison test site. These turbines had two 260-kW induction generators coupled to a single-stage transmission. Project ÉOLE (Figure 4.1) was the largest Darrieus wind turbine built, with a rating of 3.8 MW. It was installed in Cap-Chat, Canada in 1987. The rotational speed and cut-out wind speed were limited so the unit was downrated to 2 MW. Between 1988 and 1993, the unit produced 12,800 MWh. It has not been operational since 1993 due to failure of the bottom bearing, and now it serves as a tourist attraction with a wind energy interpretation center.

FIGURE 4.1  ÉOLE, 4 MW; rotor D = 64 m, H = 94 m. Height to top = 109 m. Base structure height = 8.5 m. Direct drive ring generator at ground level, 12 m in diameter. Note how far out the anchors are for the guy wires. To the right, four of 76 Neg-Micon wind turbines, 750 kW, D = 55 m, tower = 55 m. (https://commons.wikimedia.org/wiki/File:Eoliennes_ Gaspesie.jpg). Bottom left: Size of base. Bottom right: Man on top of generator; note size of drive shaft, flexible coupling not yet installed. (Photos [1985] taken by author at construction phase, rotor not yet mounted on top of base.)

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FIGURE 4.2  Darrieus, 34-m test bed at USDA-ARS, rated at 500 kW, peak power 625 kW. Notice spiral staircase, which eliminates vortex shedding by the tower. Notice guy wires to top of torque tube. (Photo courtesy of USDA-ARS.)

Swept area for a Darrieus rotor is approximately 0.65 × rotor diameter × rotor height (0.65 D × H). Other similarly shaped turbine rotors may be more circular or more elliptical, resulting in a larger or smaller swept area. Sandia National Laboratories, United States, managed a VAWT research program from the early 1970s until it was discontinued in the 1990s. They developed software analysis programs and designs of Darrieus wind turbines, and prototypes of 60, 100 (Figure 1.10), and 500 kW (Figure 4.2) were constructed. The last two were tested at the Wind Test Center, ARS-USDA, Bushland, Texas. Today, Sandia National Laboratories’ wind program is primarily focused on rotor and blade development for HAWTs (https://energy.sandia.gov/energy/renewable-energy/wind-power/). The Darrieus type is the only large VAWT that reached commercial production, and two companies in the United States, FloWind and VAWTPower (Table 4.1), installed units in wind farms in California. FloWind (Figure 4.3) installed more than 500 turbines in the Altamont and Tehachapi passes (95 MW by end of 1985). The specific performance for 1989 of 250 turbines (FloWind-19) was 410 kWh/m 2, which was below the value for the best HAWTs, which produced more than 1,000 kWh/m 2 [3]. TABLE 4.1 FloWind and VAWTPower specifications Rated Wind Speed

FloWind 17 FloWind 19 VAWTPower

Capacity (kW)

Size, D–H (m)

m/s

mph

170 250 200

17–23 19–25 17–25

19.6 17 17

44 38 38

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FIGURE 4.3  FloWind wind turbines, 170 kW, D = 17 m, H = 23 m, in Altamont Pass, California.

During the years of operation, structural failures at the strut-blade joints meant turbines were decommissioned and by the end of 2004 most were scrapped. For more information visit www.wind-works.org, then navigate along the following menu path: WIND > SMALL WIND TURBINES > Vertical Axis Wind Turbines > FloWind. The high rated wind speed, especially for the FloWind 17, was due to early tax credits being based on Dollars ($) invested, not rated power or energy produced. However, promoters sold the turbines to unsophisticated investors on $/kW of rated power, therefore a high kW lowered the $/kW. The FloWind VAWT seemed like a better deal than more reasonably rated wind turbines. The promoters could always argue that VAWTs had a higher peak power than HAWTs, and again this was misleading. Fayette (HAWT) was another vastly overrated wind turbine unit that was installed in large numbers in the early wind farm market in California. The unit was 25 m in diameter with a 100 kW generator. Later the production tax credit was based on energy produced, which presented a significant improvement. Alcoa built some prototypes, of which the largest was a 500 kW unit with three blades. The unit at the SCE test site in California failed from brake burnout during high wind speed shutdown due to a problem with the software control program. At high rotor rpm, a blade came loose and cut the guy wire and the unit collapsed. Alcoa abandoned the development of Darrieus wind turbines and the program manager Paul Vosburgh set up VAWTPower, which installed 40 turbines in San Gorgonio pass, California. The company went out of business a short time later, and Vosburgh developed another company, VAWTPower Management, which built a 60 kW prototype that was cantilevered; it did not have guy wires at the top of the

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FIGURE 4.4  VAWTPower Management, 60 kW, at Clines Corner, New Mexico. Height = 25 m, base support = 9.5 m; rotor D = 15 m, H = 15 m, swept area = 142 m2.

tower (Figure 4.4). Now the unit is not operational, however, the truck stop at Clines Corner where the turbine stands does not want the unit removed as it serves as a tourist attraction. Adecon used an external lattice frame to replace the guy wires to the top of the unit (Figure 4.5), and this support could also raise the rotor higher above the ground. Notice that the torque tube has been replaced by a truss lattice. Canadian CWT power took over Adecon, and in the early 1990s they installed 10 150 kW units at Pincher Creek, Alberta, Canada. These units have four blades, which gives a more even power generation per cycle as two-bladed turbines go through a short negative power on every rotation. The trade-off is more even power generation per rotation for three and four blades versus the expense for more blades. A DAF-Indal 40 kW unit was tested in a wind assist system for irrigation at the USDA, for a mechanical connection to an electric motor, and later for a mechanical connection to a diesel engine [4]. The smaller units of DAF-Indal had blade flaps (air brakes) at the midpoint of each blade for overspeed control. Darrieus units are not self-starting, however, in unusual conditions such as swirling winds they can selfstart. On a small unit being tested by AEI-USDA, blade flaps saved the unit in high winds during a brake malfunction. One of the reasons that DAF-Indal and Alcoa became interested in the Darrieus turbines was the large market potential for extruded aluminum blades. For blades with large chords, separate dies were needed, and then the blades sections were bolted together. For example, for the 500 kW test bed (Figure 4.2) the chord for the

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FIGURE 4.5  Left: Adecon at Atlantic Wind Test Site, 125 kW, two blades, rotor D = 17 m, H = 43. Height to bottom of rotor = 26 m. Note man beneath the rotor. Right: CWT at Pincher Creek, four blades, D = 21.5 m, H = 30.5 m. Note that generator is close to the ground compared to unit on left. (Right Photo, Paul Gipe. All rights reserved.)

center section was 1.2 m long and it was made from three extrusions. The extruded straight blades were then bent for the correct curvature. FloWind had a test unit that had fiberglass reinforced plastic blades. A common solution to the problem of the Darrieus rotors not being self-starting was to include smaller Savonius rotors on the torque tube to provide startup (Figure 4.6). Hi-VAWT installed 432 turbines on a fish farm near Wanggong Fishing Port in Taiwan, probably the largest wind farm of small units in the world. Note that the total of 1.3 MW is the power of one large wind turbine.

FIGURE 4.6  Hi-VAWT, 3 kW, D = 4 m, H = 4.2 m, Darrieus and two stacked Savonius rotors, tower height = 6 m. Changhwa County, Taiwan. (Photo courtesy of Hi-VAWT.)

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4.2  H-ROTOR, GIROMILL The advantages of the H-rotor or giromill turbines (Figures 1.9, 1.11) over the Darrieus turbine are that they can be placed on a higher base tower, the rotor swept area is larger for a given diameter, and their straight blades are easier to manufacture [5]. The disadvantages are increased bending moments and the struts increase drag losses, so overall efficiency is a little less. The blades can be fixed pitch or they can be articulated for starting or changed on each rotation to improve efficiency. An early small turbine developed by Herman Drees, the Pinson Cycloturbine (Figure 4.7) had articulating blades, which changed pitch angle during every rotation (the mechanism was a major point for wear). Around 140 Pinson Cycloturbines were installed before the company went out of business. Cornell University tested a Pinson unit that drove a churn for heating water for a dairy. The churn is a good load match for the variable torque-rpm characteristics of the turbine. An extensive program began in the 1980s in the United Kingdom on fixed pitch, straight-blade vertical axis wind turbines. Peter Musgrove, professor at Reading University, developed the concept of a variable geometry rotor for controlling power by changing the swept area [1, section 3.1]. By increasing the chord of the blades, thereby creating a larger solidity, the units would start in low winds. Sir Robert McAlpine & Sons Ltd. and Northern Engineering Industries formed a subsidiary, VAWT Ltd., that developed prototypes. Their model numbers reflect the swept area

FIGURE 4.7  Pinson Cycloturbine, 4 kW, three blades, rotor D = 3.6 m, H = 2.4 m, tensegrity tower = 10 m. Note the wind vane on top which controls the blade pitch. (Photo, Paul Gipe. All rights reserved.)

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FIGURE 4.8  VAWT Ltd. H-450, 130 kW; two blades, rotor D = 25 m, H = 18 m, tower height  = 25 m. (Photo, Jos Beurskens. All rights reserved.)

of the rotor. The VAWT-450, a 130 kW unit (Figure 4.8) was installed on Carmarthen Bay in Wales in 1986. The VAWT-260 was a smaller unit developed for isolated grids and that 100 kW unit (D = 19.5 m, H = 13.3 m) was installed on the Isles of Scilly off the coast of Cornwall in 1987. The turbine had two generators, 30 and 75 kW. The smaller generator was used for start-up and in low winds, then both generators were used in strong winds. In 1988, the turbine was modified to a fixed geometry. The next larger unit (VAWT-850, 500 kW unit, D = 38 m, H = 22.5 m) had fixed blades and was also installed at Carmarthen Bay in 1990. The UK Department of Energy ended the program because they concluded that H-rotor turbines could not compete economically with the existing HAWTs in the market. Presently, VertAx (established 2007), UK, is designing a multi-megawatt H-rotor turbine for offshore. Götz Heidelberg at Heidelberg Motor GmbH developed a variable speed H-rotor with direct drive permanent magnet generators. A large 300 kW prototype was developed, where the entire tower rotated to drive a ring generator at ground level (Figure 4.9). The turbine was erected in 1991 in Kaiser-Wilhelm-Koog on the German North Sea coast. The turbine was dismantled in 1993 and moved to Münster, where it operated for a number of years. For a video of the operating turbine, go to https:// www.youtube.com/watch?time_continue=4&v=0KyXIPVsXaQ. In 1994, they installed five 300 kW turbines, now non-rotating tripod towers with the generator

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FIGURE 4.9  Heidelberg Motor GmbH. Top: Tower rotated with generator at bottom of tower, 300 kW, D = 32 m, H = 21 m, tower height = 50 m. Bottom: 300 kW, D = 34 m, H = 21 m, tower height = 50 m. Notice ring generator at top of tower. (Photos courtesy of Felix Heidelberg.)

at hub height in Kaiser-Wilhelm-Koog (Figure 4.9). In January 1995, there was a problem with a welding seam on the strut blade and one unit collapsed. The other four units were taken out of operation and then taken down in 1997. Heidelberg Motor GmbH installed some three-bladed, 20 kW units in extreme environments and the one at the German research facility in Antarctica operated for 15 years before the station was decommissioned in 2008. Uppsala University in Sweden developed prototype H-rotors in the early 2000s. Then Vertical Wind AB was formed and they installed a 200-kW prototype in 2010 near Falkenberg on the west coast. After Vertical WindAB discontinued development,

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the unit was sold to Uppsala University. The turbine was mounted on a wooden tower and guy wires were added. The guy wires altered the vibration frequencies of the rotor and tower, so the operational range was limited. The ANew Institute (https://www.anew-institute.com) in Poland, developed prototypes of 15 kW (three blades), 20 kW (four blades), and 200 kW (three blades). The blades were slightly V-shaped and attached with a single strut and the unit was mounted on a tripod tower with the generator near the top of the tower. A very interesting prototype was the 24 kW unit, which consisted of two levels, three-bladed rotors (Figure 4.10a). The ANew-MI rated power is 200 kW at 12 m/s, with three fiberglass blades, and was designed for low and medium wind speeds. See video the “VAWT Size Comparison,” which shows the prototypes operating at http://www.

FIGURE 4.10  ANew Institute. (a) Left to right, 15 kW, 20 kW, stacked 24 kW, and 200 kW. (b) 1.5 MW, rotor D = 52 m, H = 32.5 m, tower height = 50 m; generator at ground level. (Photos courtesy of ANew Institute.)

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anew-institute.com/ or https://www.youtube.com/watch?v=wVP4Gnj8uVg. In 2017, they constructed a 1.5 MW unit (ANew-B1) also with the single strut to the blades and now a steel lattice tower (Figure 4.10b) with the generator at ground level. The power was transferred to a permanent magnet generator on the ground. The ANew Institute is working on an offshore version as a stand-alone floating unit or units to be based on old floating rigs or platforms. Nénuphar-Wind was created in 2006 by Charles Smadja and Frédéric Silvert in Lille, France, where their first small-scale 35 kW prototype was built. Nénuphar’s objective was to build megawatt wind turbines for the offshore market. First, they built three onshore prototypes at a test site in Fos-sur-Mer, Aix-en-Provence. The VERTIWIND 1H (2014) model had the blades angled at 120° (Figure 4.11), however, the blades were costly to manufacture and induced high torsion loads in the struts. The 1HS (2015) was the same as the 1H except with straight blades [1, section 3.4] as their objective was the offshore market with a floating 2 MW wind turbine. The 1HB (2017) had variable pitch blades for control and to improve aerodynamic performance. The company went out of business in April 2018 after failing to find further investors. A V-shaped rotor changed to a more horizontal position (less swept area) for control in high winds and overspeed (Figure 4.12). The centrifugal force worked against a spring in the tower top. Note that the unit in the figure had a gin pole for installation and maintenance at ground level.

FIGURE 4.11  Nenuphar, VERTIWIND-1H, 600 kW, D = 50 m, H = 40 m, hub height = 27 m. (Photo courtesy of Denis Pitance.)

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FIGURE 4.12  V-shape rotor, 5 kW, D = 3.8 m, that can change swept area due to high winds. (Photo courtesy of Derek Taylor.)

There are several VAWTs with curved or helical blades (Figure 4.13). The Hercules wind turbine by ENESSERE has blades that are covered with wood, which is an attractive design feature (http://enessere.com/en/products/hercules-wind-turbine/). An oddity was the Broadstar Wind Systems AeroCam turbine, which was an H-rotor with 10 blades on a horizontal axis with a cam for changing the pitch angle

FIGURE 4.13  Left to right: Aquilo Power, 5 kW. Urban Green Technology, 1 kW, D = 0.9, H = 1.4 m. VWTPower, Qr6, 7.5 kW, D = 3.1 m, H = 5.5 m. (Photo courtesy of VWT Power Ltd.)

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on each cycle. With the horizontal axis the unit would need to be perpendicular to the predominate wind direction, as on buildings, or have a mechanism to orient the unit. An infomercial from Boadstar shows renderings of a wind turbine with dual horizontal rotors that was oriented about the vertical tower (http://www.youtube. com/watch?v=De_vZtw8UMQ). A small prototype (1 kW, eight blades, diameter ∼2.5 m, length ∼2 m) was tested at the AEI Wind Test Center in 2007. I was unable to obtain a photo of the larger AeroCan, but images can be found via an Internet search of Broadstar Wind Turbine. A video is available of three Broadstar units, the AeroCam-1, on the JC Penney distribution center building, Reno, Nevada (https://www.youtube.com/watch?v=vXfSK4goiO8). Each unit was rated at 11 kW, diameter = 3.6 m, blade length = 3.8 m. Information found on the Internet indicated that Broadstar had a 250 kW unit, but I did not find any information about an actual prototype.

4.3 SAVONIUS Savonius wind turbines (Figure 4.14, see Figures 1.9, 1.12) have some characteristics of drag devices such as high solidity, however, they can reach an efficiency of 30%. Configurations include helical blades, different offsets, and differently shaped blades (Figure 4.15). The helix shape was developed for use as a wind turbine by Risto Joutsiniemi in 1978. Using that design, Oy Windside Production, Finland (https:// www.windside.com), has been producing Windside turbines with helix shaped blades since 1982 for the stand-alone market. They offer units with swept areas from 0.15 m2 to 12 m2; their model numbers reflect the swept area. Their latest unit is the WS-30 (rated power ∼10 kW, diameter = 3 m, length = 10 m) for connection to the grid. Their gallery page shows units in urban, remote, and harsh environments and there are also art and eco-images as turbines can be ordered in two or several colors. O

D

r

d

S

FIGURE 4.14  Left: Diagram of Savonius rotors. Right: Savonius turbine for pumping water, Brazil, 1970s.

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FIGURE 4.15  Left: Windside WS-4B, two stacked units plus PV at Rouse Hill Shopping Centre, Sydney, Australia. Each unit, 1 kW, D = 1 m, H = 4 m (© Copyright Oy Windside Production Ltd). Right: One of two Helix Wind units on ship Hornblower Hybrid, 5 kW, D = 0.6 m, H = 3.4 m (Photo, Dale Frost for the Port of San Diego, Wikimedia Commons).

The Flower Turbine (http://flowerturbines.com) is available with 2 (Figure 4.16) or 3 blades, rated power of 0.1, 0.25, 2, 3, or 5 kW, depending on rotor size. EcoQuest’s WindTree had a shroud around two Savonius vertical rotors with stated power of 2–3 kW at a wind speed of 5 m/s. Note that at 0.30 efficiency, the swept area would need to be 90 m2, which is very much larger than their prototype size of 2 m by 2 m. I have a slide of the WindTree obtained in the 1980s, but as the copyright holder could not be ascertained, so it could not be included here.

4.4  MULTIPLE BLADES, MULTIPLE ROTORS There are VAWTs with low solidity to solidity greater than one and units with many blades per rotor (Figure 4.17). There are two different connections for stacked rotors, individual units that operate independently, or sets of rotors connected to a common drive shaft (Figure 4.18). Typically, the generator is at the bottom of the rotor attachment to the tower or at ground level. Wind Harvest International turbines had an external frame for support with stators to increase the swept area and multiple stacked rotors (modules or sets). The stators were aerodynamically shaped, and the modules or sets were connected to a common drive shaft. The first prototype (1976) had five stators and two modules (rotors), with rotor diameter = 4.9 m, two to four blades per module, blade length = 2.4 m. On

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FIGURE 4.16  Flower Turbines, two-bladed Tulip, 3 kW, D = 2.5 m, H = 4 m. (Photo courtesy of Daniel Farb, Flower Turbines.)

the Windstar 480 (1984, Figure 4.19) the stators were stationary and on the 1066s (1991) they could rotate with the wind. With the Windstar 530G (2002) the external frame was abandoned for a guyed tower because the extra swept area did not cover the added expense of the external frame support and stators. The latest model (2018) is a cantilevered, three-bladed H-rotor; 50–70 kW, D = 12 m, blade length = 14 m. The McCamley wind turbine (https://www.mccamleypower.com) is another VAWT with a stator on the outside to focus wind onto the rotor. Blades are selfstarting and self-feather in high winds, and the top and bottom rings of the stator are shaped so they may provide some increased flow. The NT01 Mk2 model was rated at 1.5 m/s in a 12 m/s wind. The reason I included this turbine is because the sleek design is suitable for urban locations, either on existing buildings or incorporated into the design of new buildings. Although their web site was still active as of July 2019, I was unable to obtain an image despite email and letter requests. Visit their web site for images of units in Bulgaria and England. A video of the unit in operation atop a tower block in Lyaskovets, Bulgaria, is available at https://www.youtube.com/watch?time_ continue=64&v=0M-w5y5l8yk. In 2012, a MT01 MK3 turbine was installed at the car park at Keele University, Staffordshire, UK. There is an interesting article on the development and design changes from inception in 1990 to the Mk3 in 2012 (https:// melindaskea.com/2012/10/03/flat-pack-turbine-targets-urban-landscape/).

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FIGURE 4.17  Left: Mariah Windspire, 1 kW, D = 0.6 m, H = 3 m, at American Windmill Museum, Lubbock, Texas. Right: Century Wind, five blade H-rotor, D = 5 m, H = 5.2 m. (Right photo taken at Renewable Energy Roundup & Green Living Fair, Fredericksburg, Texas, 2003.)

FIGURE 4.18  Left: Eight stacked H-rotors, 80 kW. Right: Base structure, height = 5 m. Note shaft at top of base. (Photo courtesy of Charlie Dou.)

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FIGURE 4.19  Windstar 480-4, 25 kW, Modules 3, # stators 5, blades/module 4, blade length 3 m, D = 5 m, turbine height 14 m, bottom blades pitch for turbine braking mode. (Photo courtesy of Wind Harvest International.)

Wind Engineering built a stacked Darrieus turbine (Figure 4.20) that had a common drive shaft to a 15 kW generator for a newspaper in Clearwater, Florida. The top of the unit was 50 m high as it was on top of the building. Even though the rotors were offset in orientation, it was not self-starting. Hi-VAWT Technology Corporation, Taiwan, installed a 12 kW unit, which was composed of four stacked individual units (DS3000) in South Korea (Figure 4.20). At night the unit was lit with colored lights. The company also has hybrid systems with photovoltaic and hybrid streetlight systems. A different design had two vertical axis Savonius rotors in parallel with a deflector shield in front (Figure 4.21). The rotors had helical blades, and it is assumed that the swept area is the distance from edge to edge of the two rotors. A few prototypes had multiple blades at the same location (Figure 4.22). The Beijing Badaling Wind Power Test Station was northwest of the Great Wall, Badaling, China, however, it ceased operation around 1996. SunSurfs Solar web site (https://www. solarstore.co/SunSurfs-WT3-Vertical-Axis-Wind-Turbine-200000W-200KW-_p_70. html) shows units with three and four blades at the end of the struts, articulated so it was self-starting, adjusted pitch on each cycle for better performance, and used for control for high winds and overspeed (Figure 4.23). I received no response from SunSurfs Solar Store to email, letter, or phone inquiries to obtain an image of the WT3-200 kW, and I could not find any information on the manufacturer of the wind turbines.

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FIGURE 4.20  Left: Three stacked Darrieus wind turbines (Dynergy), Clearwater, Florida, total 15 kW, each rotor set was 5 m diameter by 5 m height. Coy Harris is on top of the tower. Now not operating. Right: Hi-VAWT, four stacked Darrieus individual turbines, total 12 kW, South Korea. (Photo courtesy of Hi-VAWT.)

FIGURE 4.21  Dual rotors that counter-rotate, 10 kW. Photo taken at Renewable Energy Roundup & Green Living Fair, Fredericksburg, Texas, 2003.

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FIGURE 4.22  Left: Darrieus, Vestas, designed and built by Leon Bjervig, Lem, Denmark (1978). Note tapered torque tube. (Photo, Paul Gipe. All rights reserved. Photo taken 1980.) Right: Dual blade H-Rotor, 1.5 kW, D = 4.8 m, H = 2.3 m at Beijing Badaling Wind Power Test Station. (Photo taken 1985.)

WIND

FIGURE 4.23  Diagram of blade pitch position versus wind flow for articulating giromill with multiple blades on the same strut.

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4.5  SQUIRREL CAGE A single squirrel cage is somewhat similar to a drag device, with a high solidity rotor design that catches more wind on one side than the other (Figure 4.24). Squirrel cage blowers are common for air movement in ducts for heating, ventilation, and air conditioning systems (HVAC). Most wind turbines with squirrel cage rotors are mounted on a vertical axis. One design is to have a squirrel cage stator on the outside to funnel wind to an inner squirrel cage rotor (Figure 4.25). Squirrel cage units can

FIGURE 4.24  Single squirrel cage rotor. ∼4 kW, rotor D = 6 m, blade length = 3 m, Hereford, Texas.

FIGURE 4.25  Diagrams of stator and rotor. Left: 2-D. Right: 3-D, blue stator, red rotor. (https://commons.wikimedia.org/wiki/File:BR_T01.jpg).

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FIGURE 4.26  Prosto Triad, 300 kW, Koden, Poland. Tower height ∼30 m. Per tower, nine stackable units, stator D = 3 m, H = 3 m; rotor D = 2.5 m. (Photo courtesy of Prosto Wind Power.)

be stacked as individual units or with a common shaft. Prosto Wind Power (https:// prostowind.com) has 10 kW units that are stackable (Figure 4.26) with connected shafts. Prosto installed its first turbine of 30 kW in Gdansk, Poland in 2014 and since then has installed over 2 MW of units in Poland, with additional projects in other countries. The first U.S. installation, 40 kW, was at a farm in Union Springs, New York in 2018. Their units are bright green with a bit of yellow for braces and the ladder. Sergey Bolotov designed and developed the wind turbine Windrotor-Bolotov (WB), previously named WRTB (https://www.windrotor-bolotov.com). The Windrotor is a unique design (Figure 4.27) that consists of outside stator fins to direct flow to two rotors that counter-rotate (3 kW, D = 2 m, H = 5 m) with the generator between the rotors. Up to five Windrotors can be stacked for a 15 kW system. The first turbine was manufactured and installed in 1997 in Kazakhstan and it was still in operation in 2019. Since 2007, more than 200 Windrotors that were manufactured in Russia and South Korea have been installed across the world. See a map of their locations on Windrotor-Bolotov’s web site. The blades and stators are fiberglass reinforced plastic rather than metal. The web site has an excellent selection of photos, with some turbines in remote areas. The Waters Turbine (https://cureforglobalcrises.weebly.com/waters-turbine.html) exhibits the opposite reaction, with the squirrel cage mounted on a horizontal axis facing the wind (Figure 4.28). The wind then causes the squirrel cage fan to rotate and to drive a generator. The design idea was that the flat plate forces the air to divert and accelerate before passing through the energy extraction blades at the perimeter. Additionally, a low pressure void is created behind the obstacle, creating vortices, so

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FIGURE 4.27  Bolotov Windrotor. Left: WRTB H4 3000, stacked two modules, total rated power = 6 kW, D = 2 m, H = 10.5 m, base platform = 4.5 m. Astana, Kazakhstan, installed 2010. Right: Hybrid WRTB S4 8000, total rated power = 8 kW (wind 6 kW, PV 2 kW), D = 2 m, H = 12.5 m, base platform = 6 m. Astana, Kazakhstan, installed 2012. (Photos courtesy of Bolotov Windrotor.)

FIGURE 4.28  Left: Diagram of wind flow for Waters wind turbine. Right: One design of the Waters wind turbine.

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the air should be accelerated even more. The problem is the passage of air through the perimeter of the turbine and the interaction with the air stream outside the turbine.

4.6  OTHER VAWTS Skydrill was a large vertical axis drag device (Figure 4.29) with two stacked panels offset by 180° on the same drive shaft. Aluminum sheets were attached to spars to obtain a curved paddle shape for the rotors. The gearbox and generator were in the housing below the bottom rotor. The prototype was installed at the AEI Wind Test center at WTAMU. During high winds with the turbine shut down, aluminum sheets tore off and some even landed in the yard of the President’s residence across the road from the test center. Another drag device (Figure 4.30) had plastic U-shaped slats with a cantilevered rotor. The shaft sheared at the bottom of the prototype’s rotor, so an external frame was added to support the top of the rotor. During operation on the upwind part of the cycle, the slats vibrated in the vertical direction due to vortex shedding.

FIGURE 4.29  Left: Installation of Skydrill at AEI Wind Test Center, WTAMU. Rated power = 50 kW, rotor D = 11 m, H = 22 m, tower height = 45 m (includes housing; gearbox and generator inside). Right: View of assembly of one panel of the rotor. Note man in the photo.

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FIGURE 4.30  CM Energy, 20 kW, D = 7.3 m, H = 7.6 m, base tower = 5.0 m. Blades had short angle section on end. Close up of blade shape.

Eugen Gubajudlin of Germany invented a drag turbine with two counter-rotating rotors with U-shaped blades (Figure 4.31) made of sheet metal that drove a ring generator between the two rotors (http://www.egwind.de). A video of construction and operation is available at https://www.youtube.com/watch?v=V-hFDnfgXjE.

FIGURE 4.31  EG Wind, Germany. 80 W, dual counter-rotating rotors. One rotor, D = 1.5 m, H = 0.3 m, three blades. Generator is below top flange between the two rotors. (Photo courtesy of Eugen Gubajudlin.)

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FIGURE 4.32  FST Disk Blade Windmill. Plastic vane on top regulates the pitch angle of the disks. (Photo courtesy of David W. Williams.)

A wind turbine with disk blades (Figure 4.32) was developed by David Williams, Wingham, New South Wales, Australia. The technology relied on a combination of three features: vertical axis rotation of the blades, horizontal axis rotation of the disks, and an offset disk angle of 45° relative to the radial support arm. All three features were synchronized so the blades were adjusted to the correct position in relation to the wind direction by a wind vane via a single set of bevel gears. Disks of rough cut, flat Styrofoam with 0.6 m diameter were attached to a radial support arm at an angle of 45°. The swept area was 1.3 m2. The rated power was estimated at 140 W at 12 m/s from experimental power, which was 40 W at 8 m/s. There were two weights for balance at 180° to the rod connecting the disks to the central gear. Williams is developing a three-bladed prototype. A video of the operation is available at https://www.youtube.com/watch?v=5cfUNjmgcDs. The design uses lift forces but also positive drag forces on part of the cycle. In the video, a diagram shows the relative blade angles during one cycle; an operation similar to a panemone drag device. Lift and drag forces for flat plates have been measured in wind tunnels for angles of attack from zero to 90°. In the early days of the wind industry, those values for high angles of attack were used for other airfoils as data were unavailable for angles larger than when stall occurs. Global Wind Energy Systems, Cheyenne, Wyoming, constructed a few systems with housings with deflector vanes to direct wind to a vertical rotor (eight blades). The base was covered with slanted panels to also help direct the wind into the rotor (Figure 4.33). The Alternative Energy Institute was terminated in 2015 and the data for Global Wind Energy Systems were discarded, so other information about the systems was lost.

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FIGURE 4.33  Global Wind Energy Systems. 250 kW each. D = 10.9 m, H = 23 m, eight solid blades, base height 3.6 m.

IceWind (http://icewind.is/en/) is appropriately named since the company, which started in 2012, is based in Reykjavik, Iceland. They started constructing prototypes in 2009, and the early IceWind prototypes were essentially drag devices with solidity higher than one, so rpm was low. The latest units (2019) are Darrieus designs (Figure 4.34) with Savonius rotors on the inside. The residential (RW) models are for residential applications (homes, cabins, and farms) both on and off the grid and the commercial (CW) models are for telecommunication towers. A V-rotor mimics the lower half of a Darrieus rotor and avoids the struts (and resulting drag forces) of the H-rotors. A design of a novel vertical axis (NOVA) wind

FIGURE 4.34  IceWind, 2019. RW100, Darrieus and Savonius blades, 100 W, H  = 1.3 m, D = 0.9 m. (Photo courtesy of IceWind.)

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FIGURE 4.35  Prototype V-rotor, 5 kW. (Photo courtesy of Theodore Bird.)

turbine for offshore consisted of several blades positioned along the span of the V [6], and a 5 kW prototype was constructed (Figure 4.35). Commercial offshore wind power is currently foundation-based, which limits installation in depths to 50 m. Floating wind power is primarily at the research and development stage and focused on horizontal axis wind turbines [7]. In 1917, the first floating wind farm was installed 24 km off the Aberdeenshire Coast, Scotland. The Hywind Scotland Pilot Park has five, 6 MW Hywind turbines, which are tethered to the sea floor by floating chains, with a weight of 1,323 tons. The total length of the turbine is 253 m, of which 78 m is submerged, and the water depth is 100 m. I strongly recommend watching the video of the Hywind project development and installation at https://www.youtube.com/watch?v=PUlfvXaISvc. The SeaTwirl (https://seatwirl.com) is a floating wind turbine (Figure 4.36) with a unique design in that the rotor, tower, and the submerged keel are one unit that rotates, while the housing and generator are stationary (see video “This is how SeaTwirl works” on YouTube). The S1, 30 kW, was installed off the coast of Lysekil, Sweden in 2015. The SeaTwirl company is developing a 1 MW unit, S2, to be released in 2020. The photos of different prototypes and concepts for VAWTS in this book are not inclusive, as there are many other images on the Internet (search for vertical axis wind turbines and click on homemade). One example is the Jellyfish, 400 W, later renamed the SmartBox, which consisted of a hemispherical cap with three straight blades attached to the bottom of the cap (https://earthtechling.com/2011/02/home-windpower-simple-as-plug-and-play/). It is easy to predict that there will be press releases for new designs and prototypes of VAWTs claiming that their turbine outperforms conventional wind turbines. As of 2019, there are no wind farms with large vertical axis wind turbines (individual units rated at ≥100 kW).

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FIGURE 4.36  SeaTwirl, S1, 30 kW, height above sea level 13 m, length below sea level 18 m, depth of water 35 m. (Photo courtesy of SeaTwirl.)

REFERENCES 1. E. Möllerström, P. Gipe, J. Beurskens and F. Ottermo. 2018. A historical review of installed vertical axis wind turbines rated 100 kW and above. PDF download available, https://www.sciencedirect.com/science/article/pii/S1364032118308153 2. R.J. Templin & R.S. Rangi. 1984. Vertical-axis wind turbine development in Canada. IEE Proceedings A (Physical Science, Measurement and Instrumentation, Management and Education—Reviews), 130, 555–561. 10.1049/ip-a-1:19830085. 3. V. Nelson and K. Starcher. 2019. Wind Energy, 3rd ed. CRC Press. 4. V. Nelson, R.N. Clark, and R. Foster. 2004. Wind Water Pumping. Alternative Energy Institute, West Texas A&M University. 5. T.J. Price. 2006. UK Large-scale wind power programme from 1970 to 1990: The Carmarthen Bay experiments and the Musgrove vertical-axis turbines. Wind Engineering, 30(3). https://core.ac.uk/download/pdf/388349.pdf 6. E. Shires. 2013. Design optimization of an offshore vertical axis wind turbine. Energy, 166, ICE, EN1. http://eprints.whiterose.ac.uk/79230/1/ener166-007.pdf 7. Floating foundations: A game changer for offshore wind power. IRENA. https://www. irena.org/-/media/Files/IRENA/Agency/Publication/2016/IRENA_Offshore_Wind_ Floating_Foundations_2016.pdf.

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Airborne Wind Energy

In general, wind speed increases with height, so there is the trade-off of increased energy production versus the cost for taller towers. There is another aspect in that the diurnal wind pattern of low winds at night changes at heights of 40–50 m, so there are also more consistent winds and much more energy. Some people refer to this a lowlevel jet. Therefore, in some locations such as the Great Plains of the United States you do not need altitudes of 10 kilometers to reach the high wind speeds of the jet stream. These low-level jets also occur in the tropics and over water. Note that today most large commercial wind turbines have towers from 60 to over 100 m, and in 2017 Max Bögll Wind installed 3.4 MW wind turbines on 178 m towers in Germany (https://electrek.co/2017/11/02/worlds-tallest-wind-turbine-builtin-germany/). One aspect that was different is that the bottom section of the tower was a storage tank that was part of a pumped hydro system. There are around fifty companies and research institutes [1–3] pursuing airborne wind energy systems (AWES). The main advantages are stronger and more consistent winds at higher altitudes, less structure aloft, and no tower. They have operating heights from 50 to 1,000 m. For kites, balloons, and drone-like systems, the wind turbines and generators can be aloft, and power is transmitted through the tether to the ground, or the generator can be on the ground. In general, the kite or wing flies crosswind in a loop or a figure-eight. Kites can be soft or hard, which is essentially a wing or a wing with a tail. Drone technology (three to four small propellers) for controlling flight and vertical takeoff and landing is combined with wing technology for some AWESs. Of course, the control systems need to be autonomous, which means the software must be robust. This is a development mainly since 2005, and many company web sites feature informative videos. For information purposes, URLs for several of the videos are included in the following sections. For models with the generator on the ground, the rising (reel-out) portion of the flight loop provides the power as there are no rotors. There can be small rotors for takeoff and landing for airframes even with the generator on the ground. Another idea is to have a rail track on the ground where the kite pulls a wagon (also referred as cart or bogie) with a generator connected to the wheels, namely a lift translator (see Chapter 6, section 4). Professor Wubbo Ockels, Delft University of Technology envisioned wings on a ladder like a giant clothesline (LadderMill), which was essentially a lift translator with the upper end held aloft by a sail or balloon (http://www.energykitesystems.net/LadderMill/index.html). Wings on one side are positioned for maximum lift while the winds on the downside are positioned so that they bear their own weight. Later the single kite project at Delft University of Technology was called the LadderMill project [4,5]. Note: A sailboat is a lift translator, which moves faster than the wind when it sails perpendicular to the wind and moves slower than the wind when it sails downwind (drag device). 91

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5.1  GENERATOR ALOFT JoeBen Bevirt started Joby Energy [6,7] in 2008 and by 2010 they had constructed a number of prototypes. After testing more than 20 designs, the company selected a 30 kW system (Mercury 5 had eight turbines) for evaluation and testing. The wind turbines (motor/generator) were for vertical takeoff and landing and then they generated power when the airframe reached operating altitude, where the unit flew in a circular (crosswind) path. A computer controlled the flight of the AWES by driving aerodynamic surfaces on the wings and by differentially controlling the speed of the rotors. The main concept was a modular system; a multiple wing supporting several wind turbines. The Mercury 9 prototype was a flying box with eight turbines [8]. The original goal was to fly the systems in the jet stream, however, the Federal Aviation Authority rejected the idea, as the airborne wind turbine at altitudes less than 610 m (2,000 ft) were an obstruction to the FAA, similar to radio and TV towers. Access a video of a 2 MW, dual wing unit with 12 turbines at https://phys.org/news/201006-airborne-turbines-power-high-video.html. Joby Energy merged with Makani and Bevirt and started another company in 2009, Joby Aircraft, to develop a vertical takeoff and landing electric airplane, namely an air taxi. Repeated attempts to contact Bevirt at Joby Aircraft failed, so I do not have any images. A slide presentation on the Internet has images of some of the early prototypes and includes the video of the 2 MW design (https://prezi.com/omawfcjkz-zl/joby-energy-the-wind-turbine-kite/). An article in Popular Mechanics [9] has quite a bit of information on the early days of Joby Energy and it also covers the early days of Makani Power. Makani, which is now part of Google X.Company (https://makanipower.com/ journey/), has a tethered wing that moves in a loop. Makani started with a soft kite with a rated power of 2 kW in 2008. Because they wanted more efficiency and control, they developed rigid kites with a wing that could carry onboard rotors. The first wing (2010) had a wingspan of 5.5 m with two rotors (motor-generator) and rated power of 10 kW, and then Wing 7 (2013) had a wingspan of 8 m with four rotors and rated power of 30 kW (Figure 5.1). The prototype, M600 (2016), has eight rotors, wingspan of 26 m, and rated power of 600 kW. The kite takes off and lands vertically (Figure 5.2) and climbs to a height of 300 m at which point it begins a loop path

FIGURE 5.1  Makani Wing 7, 30 kW, wingspan 8 m, four turbines, rotor diameter ∼0.75 m. (Photo courtesy of Makani.)

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FIGURE 5.2  Makani M600 on stand. 600 kW, wingspan 26 m, eight wind turbines, rotor diameter = 2.3 m. (Photo courtesy of Makani.)

“crosswind flight” to generate energy (Figure 5.3). The average size of the loop is 250 m diameter, and the relative wind at the wing can be 8–10 times the actual wind speed. Onboard computers guide the autonomous kite’s flight path. A video (https:// makanipower.com/journey/) shows the crosswind flight of the M600 in December 2016. In 2019 they were flight testing the system offshore in deep water. The unit would be connected to a tubular steel spar buoy moored with a single line and a gravity anchor. They also run thousands of scenarios in flight simulation software to improve the algorithms for the kite performance. Altaeros Energies (www.altaeros.com) was spun out of MIT in 2010 to develop the world’s first autonomous platform for the telecom and renewable energy sectors. Altaeros Energies initially focused on developing the buoyant airborne turbine (BAT), which was a helium filled inflatable shell with a wind turbine (Figure 5.4). The prototype had a 3 kW Skystream wind turbine inside the shell. The shell is essentially a shroud (see Chapter 3) so there should also be some increase of wind speed. Altaeros Energies discontinued testing and production of the BAT in 2014. Now, Altaeros is focused on the Super Tower, a buoyant tethered aerostat at 800 m height that is autonomous, so there is no need for a ground crew. Watch the video of the BAT in operation (2014) as they were testing a unit in Alaska, https://www. youtube.com/watch?v=kldA4nWANA8. Fred Ferguson received a patent in the 1980s for a Magnus airship, which was a large, round helium filled sphere that rotated backwards as the airship flew forward, producing lift due to the Magnus effect. Ferguson realized that the airship concept

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FIGURE 5.3  Makani M600 aloft. (Photo courtesy of Makani.)

FIGURE 5.4  Altaeros BAT, 10.5 m diameter, 3 kW Skystream wind turbine inside the shell. (Photo courtesy of Altaeros.)

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FIGURE 5.5  Magenn Air Rotor System, early prototype. Balloon was 4 by 12 m, rated power = 4 kW. (Photos courtesy of Magenn Power Inc.)

was a potential wind turbine, so the Magenn Air Rotor System (MARS) was borne. MARS is a tethered system where a helium balloon on a horizontal axis rotates to drive generators on the ends of the axis. Magenn Power built and tested small MARS prototypes from 2004–2011 (Figure 5.5). The rudder in the center provided orientation perpendicular to the wind. The Magnus effect also provided additional lift, kept the device stabilized, positioned MARS within a controlled and restricted location, and assisted the rotating balloon to be more overhead at higher altitude. The balloon had an inner laminated coating of Mylar to reduce porosity and an exterior coating of Tedlar to reduce damage from ultra-violet radiation. Operational height was 60 m with optional length to 140 m. Note that a low cut-in wind speed of 3 m/s does not mean much as there is very little energy at those low values. The rotational speed of the turbine was 30–60 rpm, so a gear box was needed to increase the rotational speed to the generator. The vanes or sails for catching the wind essentially made this a drag device, and therefore the prototype only generated 2 kW of power. A gallery of photos is available at the New Atlas web site (https://newatlas.com/ magenn-mars-floating-wind-generator/11109/#gallery). A video of the test flight of the Alpha prototype (balloon only, 9 m by 17 m) is available at https://www.youtube. com/watch?v=I6ZFcKnP2AM. For the MARS 100 kW (Figure 5.6) the sails were changed to clam-shape and large circular stabilizers were added to the ends of the balloon for orientation. Now the operational height is 180–230 m (optional to 460 m) to capture the energy of the lowlevel jet at night. The cut-in wind speed is still 3 m/s, and again, it is not very important in overall energy production. The unit was designed to operate in winds up to 30 m/s. Helium is very difficult to contain and leaks at a rate of 0.5% per month, so the balloon had to be refilled with helium every six months. The company envisioned units of 1 MW and 5 MW from a balloon of 60 m in length and longer, however, the company has ceased operation. There are articles about MARS available on the Internet [10–12]. Kite-X (http://kitex.tech) is using drone technology for control of the kites and has tested a small system (Figure 5.7) of two wings (four rotors on each) connected to a common tether. The rotors are for take-off and landing and then they produce power during crosswind flight. In a single wind kite, the whole cable dissipates power through aerodynamic drag, and the optimal flight altitude for single kites is lower. In a dual wind

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FIGURE 5.6  Magenn Air Rotor System, 100 kW prototype. Balloon was 14 by 30 m, however height of sails for rotation and end circular stabilizers make overall dimension 27 by 30 m. Note two men on right of rotor on the ground. (Photos courtesy of Magenn Power Inc.)

FIGURE 5.7  Two Kite-X prototypes, wingspan = 1.4 m, chord 0.14 m, tether length = 80 m. (Photo courtesy of Andreas Okholm, KiteX.)

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FIGURE 5.8  Kiwee One, kite plus wind turbine, 100 W. Note tail on wind turbine for orientation. (Photo courtesy of Kitewinder.)

kite arrangement, only the top moving cables dissipate power. The company was founded in 2016, and by 2018 they had tested the dual kites, although the kites needed to fly in opposite phases in order to achieve better efficiency. The Kite-X web site has a page on prototype developments and there are videos (also on YouTube) associated with the tests. Olivier Normand and Dominique Rochier founded Kitewinder (https:// kitewinder.fr) in 2016, although Normand started with the idea in 2003 and in 2013 decided to pursue the design. Kitewinder’s Kiwee One uses a kite to raise a small wind turbine, 100 W, into the air (Figure 5.8). The operation is simple; it is launched by positioning the sail face to the wind and Kiwee One does the rest. Push the start button and Kiwee One unwinds the loop and starts the propeller to begin production. If the wind weakens, Kiwee One detects it thanks to a sensor placed in the handle and rewinds the loop. Thus, the propeller lands within a 5 m radius around its starting point. In addition, the sail and the propeller float so the Kiwee One also works on a boat. The weight of the Kiwee One is 5 kg, the kite area is 4 m2, and the nominal operational altitude is 50 m. They have a small video showing assembly, and with practice assembly should take around five minutes. Sky WindPower (http://skywindpower.com) developed a tethered system with four rotors (Figure 5.9), which they refer to as a flying electric generator or rotorcraft.

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FIGURE 5.9  Sky WindPower prototype tethered system, four rotors. Second prototype had a tail on the system. (Photo courtesy of Sky Wind Power.)

They also refer to it as a Wind Airborne Tethered Turbine System (WATTS), a great acronym. Instead of cyclic pitch as on helicopters, the rotors use collective pitch, where the blade pitch remains constant until pairs of rotors change pitch when a direction change is desired. The rotors lift the vehicle to altitude and then the rotors both maintain position and generate electricity. The unit can carry a payload, such as surveillance, communication, or monitoring equipment, and still send electrical energy to the ground. Units are being designed to operate at altitudes up to 5 km. The company’s web site hast two videos of the Jabiru II prototype using a tow truck to provide wind speed, and there is a rendering of a proposed flying electric generator rated at 240 kW with rotor diameters of 10 m.

5.2  GENERATOR ON GROUND In the generator-on-ground operation, the winch is attached to a motor/generator, so there is no need for a rotor on the kite or airframe. This also makes the airborne part simpler and less weight, however the winch power is cyclic, providing generation on reel-out and having to motor on reel-in (also referred to as pumping or yo-yo). On reel-out the flight path may be circular or figure-eight. However small rotors may be used for take-off and landing, and for some systems the propellers are folded back after the kite is aloft. Kitepower (https://kitepower.nl) was founded by Johannes Peschel and Roland Schmehl in January 2016 as a result of the work done by TU Delft’s pioneering kite power research group of the former astronaut Wubbo Ockels. By 2007, the first 20 kW Kitepower system at TU Delft demonstrated the proof of concept. The unit is a softwing kite, leading edge inflatable, with its bridle system controlled by an airborne kite control unit (KCU). The KCU has high precision motors, which pull the bridles, and it incorporates a small wind turbine to power its on-board electronics. The reel-out flight path is a figure-eight helix. In 2018, they were testing a 100 kW unit (Figure 5.10) with operational altitude of 70–450 m. The dual generators are for redundancy, and one motor is used to hold the kite while switching from slow to fast reeling mode, which requires zero torque. The control software includes two alternating autopilots, one for the figure-eight maneuvers during tether reel-out and one for the reel-in phase. The 100 kW system that Kitepower is currently developing will be one of the first airborne wind energy systems available on the market. By 2019, they had developed kites of 20, 40, and 60 m2 (Figure 5.11) with the 60 m2 for a 100 kW system. Kitepower is a growing team of TU Delft researchers and strong industry

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FIGURE 5.10  Kitepower. Top: TU Delft Leading Edge Inflatable V2 kite, 40 m2; Bottom: ground station, 100 kW net power. (Photos © Kitepower.)

partners with a collective vision to reinvent wind energy. On the Tech section of their web site is a banner video of the operation, and in the Press section there are 46 photos and 5 video files, both downloadable. KPS (http://www.kps.energy) uses twin kites that share a common foundation and generator such that as one kite is ascending the other is retracting for more constant power generation (Figure 5.12). A tele-operated airborne unit controls the flight path of the kite (up to a height of 450 m) and it achieves flight speeds of up to 40 m/s in a 9 m/s wind. KPS is developing a 500 kW system. A video on their web site shows the operation of the twin kites (https://youtu.be/yiqW_YlX9uA).

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FIGURE 5.11  Kitepower kites as of 2019; 25, 40, and 60 m2. The large kite is for the 100 kW unit. The man in the photo is Bert Buchholz, Kitepower Engineer. (Photo © Kitepower.)

FIGURE 5.12  KPS kite with controller, 40 kW, wingspan = 8 m, height to 450 m. (Photo courtesy of Kite Power Systems Ltd.)

Windswept and Interesting (https://windswept-and-interesting.co.uk) has tested a system of kites on a ring; a rotor kite, also called a Daisy kite (Figure 5.13). Roderick Reed refers to this as a kite tethering network. There is one single skin, a lifting kite, three toy kites per ring, and three stacked rings to make the turbine set. The toy kites (HQ symphony beach 3) are two line parafoil kites, 90 cm by 60 cm. The

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FIGURE 5.13  Windswept and Interesting Daisy kite, 2017. Unit produced around 800 W in a 10 m/s wind. (Photo courtesy of Rod Read, Windswept & Interesting.)

leading lower edge was stiffened with 4 mm carbon rod. Each blade kite is attached to a Dacron cuff, which compresses the ring and is made of six carbon tubes at 7 mm outer diameter. The overall force vector of the blade kite lifts, expands, and rotates the stack, which allows the torque to be transmitted down shared lines to a generator on the ground. Their small-scale prototypes are open source hardware and they are now testing rigid blade sets. The site has more than 100 video clips in their Designs and Test Results sections. Kitemill (http://www.kitemill.com) was established in 2008 in Voss, Norway, and by 2015 they had a demonstration of a fully autonomous production. The kite, an airframe, flies in an ascending helical path with the winch reeled out at 1/3 of the wind speed. The kite is reeled in several times faster than the reel-out rate. Four propellers are used for vertical takeoff and landing. Kitemill tested a small prototype (Figure 5.14) and the next prototype is 30 kW (Figure 5.15). The tether length was 750 m and the operating altitude was around 400 m.

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FIGURE 5.14  Kitemill, 5 kW, span = 3.8 m. Note 4 propellers for vertical takeoff and landing. Kite on landing pad and preparing for test flight. Chris Forette and Jo Grini. (Photos courtesy of Kitemill.)

A kite can be a lift translator so generation is less cyclic, however, on part of the track, the kite is downwind (drag device), and then on the upwind part it would need to use energy (Figure 5.16). Uwe Ahrens is the founder of X-Wind Power Plants. The company’s web site (www.x-wind.de/en/) has more details on the concept of single or multiple kites and bogies with generators. A side-rail track would be needed for O&M on individual kite bogies for a large energy system with multiple carts. In 2012, X-Wind conducted test flights on a straight track (Figure 5.17), and in 2019 they were constructing a demonstration project. A video of the 2012 test is available at https://www.youtube.com/watch?annotation_id=annotation_3883640407&featur e=iv&src_vid=pRRFaf2GiuU&v=swr-Nq7S3KU

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FIGURE 5.15  Kitemill, 30 kW, wingspan = 7.8 m. Top: Preparing for test flight: Left to right: Thomas Hårklau, Lode Carnel (behind drum), Christer Svenkerud, Sture Smidt, and Sindre Devik. Bottom: On landing pad. Note four propellers for vertical takeoff and landing. (Photos courtesy of Kitemill.)

Ampyx Power (https://www.ampyxpower.com), under founder and CEO Richard Ruiterkamp, developed prototype aircraft (AP0, AP1, and AP2). AP2 had a wingspan of 5.5 m (Figure 5.18) and flight tests began in 2012. The prototype was lifted to height by propellers, which then folded back to reduce drag (see video of the flight test on the company’s web site). A third generation prototype, AP3, 250 kW, dual fuselage and horizontal tail, with a wingspan of 12 m (Figure 5.19) is currently being built with its first test flight planned at the end of 2019. This prototype is designed to demonstrate the safety and autonomous operation of the system. Operational wind speed is 6 m/s with

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Energy consumption Kite pulled by bogie

WIND

FIGURE 5.16  Diagram of kite pulling cart or wagon on racetrack.

FIGURE 5.17  X-Wind test flight, Hermine 2, 10 kW, kite area = 20 m2. (Photo courtesy of X-Wind.)

a maximum of 18 m/s with flight control of ailerons, rudder on vertical tail and moveable horizontal tail, and pitch control (nose up-down). The design for AP3 is to move in a regular crosswind pattern at an altitude from 100 to 465 m. Once the tether is reeled out to a predefined length of about 750 m, the aircraft automatically returns towards a lower altitude and the tether is reeled in. Then it ascends and repeats the process. The design is for the aircraft to take off (using a conventional glider winch launch or catapult) and land into the wind from a platform (diameter of 20 m, height of 4.5 m) by utilizing an array of sensors, which means the platform rotates for the needed wind direction. Two propellers are used for the tethered climb, however, the aircraft can return to base using only one propeller. Ampyx Power is designing a commercial prototype, AP-4A with a wingspan of 35 m and a power rating of 2 MW. Note the mass of the AP-4A is estimated to be 3,500 kg. The estimated masses of the ground station, tether, and landing platform are unknown, but it will be much less

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FIGURE 5.18  Ampyx Power, prototype tethered airframe, 10 kW, wingspan = 5.5 m. Note: prototype has propellers for takeoff and landing. (Photo courtesy of Ampyx Power.)

FIGURE 5.19  Ampyx Power, AP3, 250 kW, wingspan = 12 m. (Photo courtesy of Ampyx Power.)

than for a conventional 2 MW wind turbine. For comparison note the large masses for a conventional wind turbine: “A Vestas, 90-m diameter, 3 MW wind turbine was installed near Gruver, Texas (Figure 6.23). For installation, an 800-metric ton crane was needed. Twenty trucks were needed to haul the crane to the site and another ten were required to transport the turbine and tower. The nacelle weighed 70 metric tons; the rotor, 41 metric tons; and the tower, 160 metric tons. The 80-m tall tower consisted of four sections on a foundation that required 460 m3 of concrete including a small pad for the transformer.” Vaughn Nelson and Kenneth Starcher, Wind Energy, 3rd Ed, 2019, CRC Press, p. 141.

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FIGURE 5.20  Twingtec prototype, 2.5 kW cycle power, wingspan = 3 m, kite area = 1 m2, operation altitude = 150 m, tether length = 200 m. (Photo courtesy of Twingtec.)

Twingtec (http://twingtec.ch/product/) was founded by a group of engineers and scientists from leading research and academic institutes in Switzerland who developed an autonomous energy drone; a Twing, which is the abbreviated version of tethered wing. The Twing takes off and lands vertically using three propellers. The 2.5 kW prototype (Figure 5.20) has a wingspan of 3 m, and the TT100 (100 kW, wingspan = 15 m) with an operational altitude of 300 m is under development (Figure 5.21). EnerKite (https://www.enerkite.de/en/) flies crosswind in a figure-eight helical path and then returns to the starting point. Its kite control is different as there are three tethers and the ground station steers the wing using differential drum drives. The EK30 is a mobile research and development platform (Figure 5.22). The EK200 was designed for on-site power or within isolated grids, with rated power of 100 kW, wing area of 30 m2, operational altitude to 300 m, and tether length of 600 m. There are design specifications for EK1M with rated power of 500 kW, wing area of 125 m2, operational altitude of 300 m, and tether length of 800 m. Skypull (https://www.skypull.technology), another Swiss company, started in 2013. Their unit is a box-wing drone, dual-winged with multi-element airfoils (Figure 5.23) and control ailerons on the two connecting sections. The double wing was chosen to maximize life with reduced wingspan and the multi-element airfoil maximizes lift and minimizes drag, similar to the wing of a commercial airplane during take-off and landing. The unit takes off and lands vertically and the propellers fold after reaching operating altitude. The SP0, 1.25 m (Figure 5.24), had its first flight in 2016; the demonstration unit SP2, 5.9 m, is scheduled for 2020, and there are design specifications for SP1700, 17 m wingspan, 1 MW. A video of the unit in flight is available at https://www.youtube.com/watch?time_continue=97&v=6s-Izqb_GVs.

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FIGURE 5.21  Rendering of TT100, power 100 kW cycle power, wingspan = 15 m, operation altitude