38m wt blade design

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Post on 07-Apr-2018




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  • 8/3/2019 38m WT Blade Design


    3388 MMeetteerr WWiinndd TTuurrbbiinnee BBllaaddee DDeessiiggnn

    IInntteerrnnsshhiipp RReeppoorrtt

    MMiicckkaall EEddoonn

    June 2007

    Supervisor, French teacher: Florence TametMain supervisor: Jane KruseProject supervisor: Tupac Daz LopezProject co-worker: Joan Pros GarciaProject co-worker: Tong Wu

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  • 8/3/2019 38m WT Blade Design



    Firstly, I would like to thank the Nordic Folkecenter for Renewable Energy for theirhospitality and for having welcomed me in Ydby (North Denmark). I am especially gratefulto all the working staff that helped in my project. Without their help, it would not have been


    I am deeply indebted to my supervisors, Jane Kruse and Tupac Daz Lopez for theirfriendship and help throughout the project. Your organization, technical advice, and access tooutside resources have been a tremendous gift and I for one wont soon forget.

    I would also like to extend a special thank you to Joan Pros Garcia and Tong Wu, myco-workers on this project for their motivation and taste for hard work. I am particularlythankful to the Ingenirhjskolen i rhus and its teachers for having welcomed and trainedme in rhus (West Denmark).

    I am truly grateful to the Region Rhne-Alpes for according me the grant explora sup2006-2007

    Last but not least, I would like to thank the IUT of Annecy-le-Vieux and theMesures Physiques department teachers for the broad teaching I was given and for theenjoyment and interest they encouraged.

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    TTaabbllee ooffccoonntteennttss

    General Introduction...... 4

    1. Nordisk Folkecenter for Renewable Energy description...... 4

    2. Internship Description... 43. Project Description.... 5

    II Blade characterization.... 6

    III Introduction to wind turbine and wind classes.. 7

    1. Vertical axis turbines.... 72. Horizontal axis turbines.... 73. Wind speed classes... 8

    IV Dimensions..... 10

    1. Profile type........ 102. Chord lengths........ 113. Transition parts..... 134. Angle of attack...... 135. Twisting angles..... 146. Tip speed ratio ......... 16

    V Pitch control and power curves..... 17

    1. Why pitching?....... 172. Pitch control results....... 19

    VI Blade materials...... 22

    1. Materials .. 222. Inner support and shell thickness . 23

    VII Loads calculations........ 24

    VIII Wind Rotor Design system 2.0...... 26

    1. Programme outline.... 262. Instructions.... 28

    IX Future development.......


    X Conclusion.... 33

    XI Glossary...... 35

    XII References..... 37

    XIII Appendix..... 38

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    1. Nordisk Folkecenter for Renewable Energy description

    The Nordic Folkecenter for Renewable Energy (http://www.folkecenter.net/gb/) is anon-profit, independent, organization that provides research, development of technology,training and information for the manufacture, industrial innovation and implementation ofrenewable energy technologies and energy savings in Denmark and throughout the world.Folkecenter intends to achieve measurable increases in the utilization of renewable energytechnologies and thereby significant reductions in environmental pollution associated withenergy use in Denmark and elsewhere. It obtains support from local authorities, national and

    international agencies, and the industry. The ultimate long term purpose is a completereplacement of fossil fuels and atomic power with renewable energies.

    Folkecenter works on four major fronts: The research/development and the implementation of renewable energy systems such

    as small scale wind turbine power innovation and design, advanced generatorconstruction, farm biogas design and demonstration, CO2 neutral transportation withhydrogen and plant oil, solar architecture and integration of solar cells in buildings,wave energy testing.

    Consultation to manufacturers, local groups, and relevant initiators within renewable

    energy. Disseminating information on renewable energy in Denmark and elsewhere in the

    world, to trainees, concerned citizen groups, and political decision makers focusing ondecentralized solutions.

    The last front is the Village for Green Research, where Folkecenter is situated todemonstrate practical examples of integration of several energy solutions; solarhousing, water recycling systems etc. as experimental and functional examples of afuture ecological society.

    2. Internship Description

    My internship wasnt only about wind turbine blade designing. Because theFolkecenter has resources involving many different kinds of energy I learned a bit ofeverything and had to get to know the centre and all the different building in the park as well asthe different test sites. Of course, this was really interesting to me and I had the opportunity tolearn a lot, especially about solar cells, off-grid houses (autonomous houses), heating system,biogas, plant oil, and so on. But I devoted most of my time to wind energy and the project thatwas defined for me at the beginning of April.

    I had multiple opportunities to visit the large near shore wind turbine that FC ownswhich has an annual production of over 1.4 million KWh. After climbing to the top of the 40meter tower I viewed the nacelle and observed its contents and the inter-working of thegenerator, gearbox, shaft, yawing system and their associated systems. In addition to the largewind turbine systems had some practical work in the Folkecenter small to medium wind

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    turbine test site. The Folkecenter possesses several different small to medium wind turbines attheir test site and it was possible for me to make different experiments (power calculation,generator replacement work) on these turbines.

    The programme I used for designing the blade was made at the Folkecenter circa 1992.This programme is written in turbo Pascal and is working only with the first version of DOS soit is only working on only one of the computers at the centre. This programme is called WindRotor Design System (WRD) and a good part of the report is based on its output. I alsodesigned an other blade (29.140 m) on this programme, as training for the 38 meter blade anddid the power calculations for this blade. Because this report is limited in term of page numberit will only be about the 38 meter wind rotor blade design project. I presented the WRDprogramme to new trainees several times during my stay at the Folkecenter.

    3. Project Description

    I was given the project of designing a 38 meter blade for a 1.5 MW wind turbine at thevery beginning of my internship. I received documentation on aerodynamics, wind turbines,wind turbine design, and the computer where Wind Rotor Design system is installed. I was

    given the manual for the programme and I started studying about aerodynamics in general, inEnglish. Then my supervisor, a wind expert from Cuba, put me in touch with two studentsfrom the Ingenirhjskolen i rhus, an engineering school in the city of rhus. These twoErasmus students started the project at the same time as me and we then started to worktogether. They came to Ydby and I went to rhus for some weeks so we could realise and endthis project together. We did most of the research in rhus, and the blade design on WindRotor Design System was done at Folkecenter.

    The first two weeks I familiarized myself on the WRD programme, while my co-workers Joan and Tong worked on the profile research. What we call profile is the shape of thesection, all along the blade. They studied different profiles and chose the best one, the FX66-S196-V1. They did four different modifications on this profile to study their influences.

    Those modifications were shape modification: more or less thick on different point of theairfoil. Then, they built those airfoils in wood and, thanks to a wind tunnel, could appreciatethe result on the lift and drag coefficient. Because they didnt get good results their conclusionwas that the original airfoil is still the best, they could not improve it. So we decided to use theAirfoil FX66-S196-V1 unmodified for our blade. More explanations about this airfoil will begiven further in the report. Because I did not take part of those experimentations the results ofthe tests are not in this report. The characteristics of the profile we used are given, as they canbe found on the Ris National laboratory website. This report is personal but nevertheless, apresentation of our work. It is built in the order we did the blade design and investigations.The different chapters are linked together and for a quicker and better understanding this reportincludes a glossary at the end, page 35 and 36. The next chapter will present the methods steps

    we followed to design our blade.

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    IIII BBllaaddee cchhaarraacctteerriizzaattiioonn

    The purpose of this project is to design a blade for a big wind turbine, taking intoconsideration the aerodynamic and loads calculations, brake system, and dimension details.The blade description is what we want. The Method steps, described bellow shows in what

    order we designed our blade.Blade description:

    - 38 meters long.- Wind Turbine Class III.- Pitch controlled system- 1800kW of nominal power (mechanical power).

    Method steps:

    1. Profile research: search information about blade profiles, do wind tunnel experimentsand choose the best. This report only includes the presentation of the profile we used.

    2. Calculate the blade dimensions and angles: calculate the chord lengths, twistingangles.

    3. Power calculations: using a pitch controlled system, make the power calculations,keeping a nominal power of 1800kW.

    4. Choose the blade material: research about blade materials, find the properties, makea comparison and choose the best blade material. calculate the material layer thickness

    5. Loads calculations: calculate the forces and moments with high wind speeds to makesure that the wind turbine will work and survive.

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    IIIIII IInnttrroodduuccttiioonn ttoo wwiinndd ttuurrbbiinnee aanndd wwiinndd ccllaasssseess

    A wind turbine is a generic term for machines with rotating blades that convert thekinetic energy of wind into useful power. The basic idea has been around for a long time butmodern wind turbines are a far cry from the original designs. Towards the end of the 20th

    century, wind turbine designs followed three basics philosophies on supporting loads:- Withstanding loads.- Shedding or avoiding loads.- Managing loads mechanically, electrically, or both.

    Modern turbines evolved from the early designs and are typically classified as two orthree blade rotors. Most of the turbines used today have three blades. The rotational speed isalso a very important design factor. Turbines operating at a constant rotor speed have beenfomenting up to now, but turbines with variable rotational speed are becoming increasinglymore common with the desire to optimize the energy captured, to lower stress, and to obtain

    better power quality. There are many different wind turbine classes, but two stand out as thebest known: the vertical axis turbine, and the horizontal axis turbine.

    1. Vertical axis turbines

    Vertical axis turbines with C shaped blades wereturbines commonly used in the past century. They work likewater wheels which allow the water to arrive tangentially tothe wheel at a right angle to the rotational axis. These kindof wind turbines are designed to act towards the air.Commonly vertical axis turbines are mounted on the ground.

    This also makes them easily accessible and no yawmechanism is needed. However, the efficiency is muchlower, it needs total dismantling just to repair the mainbearing and the rotor is placed very close to the ground wherethere is less wind power.

    2. Horizontal axis turbines

    Horizontal axis wind turbines are the most commontype of wind turbine in use today. There are a lot of different

    classes with different numbers of blades, directions to thewind, and brake systems among other things.

    The advantage of a one or two blades rotor is thepossible savings in production costs and weight. However, theuse of fewer blades requires a higher rotational speed or alarger chord length to yield the same energy output. A threeblade design also decreases fluctuating loads from inertiavariation.

    Figure 1: Vertical axis wind turbine

    Figure 2: Horizontal axis wind turbines

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    3. Wind speed classes

    One of the most important considerations in wind turbine design is the environmentwhere it will be installed. Wind turbines can work in almost all the places, but the designdimensions shall be different depending on the design place. To classify those places, theIEC standards (International Electrotechnical Commission) provide four groups. Theseclassifications are dependent on wind velocity and the intensity of the turbulence.

    Wind turbine

    class I II III S

    Vref (m/s) 50 42,5 37,5 -A Iref 0,16 -B Iref 0,14 -C Iref 0,12 -

    Table1: wind turbine classes

    Vref is the reference wind speed average over 10 min.A designates the category for higher turbulence characteristics.

    B designates the category for medium turbulence characteristics.C designates the category for lower turbulence characteristics.Iref is the expected value of the turbulence intensity at 15 m/s.

    For the fourth wind turbine class, the class S, the manufacturer shall describe themodels used and provide values of design parameters in the design documentation (twoexamples of use are off-shore wind turbine and places with hurricanes).

    In accordance with the IEC standards, the average wind speed shall be chosen as


    The probability distribution of the wind speed over a time of 10 minutes is estimatedusing a Rayleigh distribution given by this expression:

    2)2/(1)( avehub VVhubR eVP


    The class we use for our 38 meter blade is class III. The wind turbine will be locatedin a place that doesnt have very strong winds. The wind speed average will be 7.5 m.s-1. Wewill have a cut-in between 2 and 3 m.s-1 and a Cut-out between 20 and 21 m.s-1. For a class Iwe have a cut-in between 3 and 4 m.s-1 and a Cut-out between 25 and 26 m.s-1

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    Figure 3 Figure 4

    Figure 3: probability of win speeds in class I (blue line) and class III (red line), Mathcad calculationsFigure 4: Energy output depending on wind speeds for class I (blue line) and class III (red line), Mathcad calculations

    On Figure 3 we can see that the maximum wind speed probability for a class III isbetween 3 and 7 m.s-1. This probability is equal to 0.114. A class I has this probability equalto 0.086 for wind speeds between 5 and 10 m.s-1. This means that a wind turbine working ina class III place is producing power, for a wind speed between 3 and 7 m.s-1, 6446 hours peryear. For a class I and between 5 and 10 m.s-1 the result is 5394 hours. We define thenominal wind speed as the one from which we get the most of its energy. It equals to 10 m.s-1for a class III and 13 m.s-1 for a class I. We can notice that the curve is sharper for class IIIthan for class I and this is this is the design condition for our blade. It will be designed for asmaller range of wind speed and we will be consequently more efficient on that range.

    The figure 4 shows us how the power is distributed, per wind speed. We can see that,for the nominal wind speed of the class I and class II we get more energy from the wind for aclass I than for a class III. The explanation is that the energy we can extract from the winddepends on the cubic of its speed.

    Taking into account that the wind speed is lower in a class III, we need a bigger bladethan class I or class II to achieve the same power. In fact, the figure 4 is helping us a lot fordesigning our blade. We want that our blade reaches its nominal power at the nominal windspeed which is 10 m.s-1. This is how my supervisor decided to make a 38 meter blade. Wewant to have 1800 kW of power, with an efficiency of 0.5 (this value is from average). The

    power formula is the following one:P=0.5*1.24*0.5*(*D2/4)*Vnom3

    1.24 is the air density, in kg.m-3 and D is the rotor diameter (38*2 + 3 meters of hub).Vnom is the nominal wind speed.

    The loads and momentums may be the same for a class I, II or III. The class III windturbines will start to produce power at lower wind speeds than classes I or II .Even if theprobability of getting wind speeds between 3 and 7 m.s-1 is larger than other speeds we dontuse a class III wind turbine because it is an advantage. We use the class III wind turbine

    because its a need. It can be used more widely but it has the main disadvantage of being lesscost effective that the class I ones.

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    IIVV DDiimmeennssiioonnss

    1. Profile type

    Two very important elements of a successful wind turbine are the blade and the power

    control system. In designing the blade, the most essential thing is to choose a good profile.A blade profile is typically similar to the wing of an aircraft. In fact, we used to

    choose a classical aircraft wing profile as a cross section in the past. We used them becausethere were not any other profiles available and the shape has a big similarity. In more recenttimes and thanks to new researches, engineers have begun to design profiles specific to windturbines.

    photo 1: blade example (2MW offshore wind turbine, in Copenhagen)

    It is important to say here that a wind turbine blade use lift at all moment. The liftmakes the rotor turn, even for low wind speed. If the wind is pushing the blade it means theblade turns because of drag. We can compare it to a sail boat. If the wind is pushing the boatfrom the rear, the boat will move thanks to the drag. If the boat moves perpendicularly to thewind it will use lift and will be ten times faster than the wind speed. That is why a blade isdesigned in order to use only the lift as a turning force.

    When we select a profile we must check several important criteria: it should have a

    high coefficient of lift while maintaining a low coefficient of drag. Consequently the CL/CDcoefficient should have a high value. Before designing the blade, a number of compromisesincluding good lift and stall characteristics are also taken into consideration. The blades mustbe easy to produce and resistant to the weather. The profile must be still efficient for theblade stability according to dust and dirt accretion

    Based on the above criteria, we picked the FX66-S196-V1 design in the book windturbine airfoil catalogue, from Ris National Laboratory (appendix page 39).

    Figure 5: FX66-S196-V1, WRD design

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    This profile was tested into a wind tunnel with a wooden model. The results were putinto an Excel programme and the different coefficients were checked (appendix page 42).This programme was made by Professor Sren, from the Ingenirhjskolen i rhus. Thosetests produced satisfactory results and we decided to go with the FX66-S196-V1 design.

    The FX66 shape is neither too thick nor too thin at the beginning or end of the blade.From the book wind turbine airfoil catalogue, it is also said that the FX66 is one of the bestairfoil profiles available.











    0 10 20 30 40 50 60 70 80 90

    Alfa []













    0 10 20 30 40






    Graph 1: CL and CD vs. angle of attack Graph 2: CL/CD vs. angle of attack

    CL can start from almost 0.6 and reach the peak at 1.413 when the alpha is 9 degree.The ratio value of CL/CD is also very high, up to 49 when alpha equals to 6.

    Even if the FX66-S196-V1 profile meets all the criteria above, it does have one bigdisadvantage; it doesnt have a shape family. Usually, blade manufacturers use severaldifferent profiles throughout the blade. They tend to be bigger near the root, flatter near the

    tip, and derive from the same profile family. In this case the profile has a thickness of 19,6 %of the chord, all along the blade. Engineers always choose one or more good profile family toapply to the whole blade. It is better to do so because on a blade the speed increases with theradius and not all the profiles have the best efficiency at the same speed (or angle of attack).We want to use only one family so we will use the FX66-S196-V1 profile all along the blade.

    2. Chord lengths

    When the wind passes through the rotor plane and makes contact with the movingrotor, it provides the lift force on the blade. This force can be found using the equationsbelow:



    WcCFLL =

    The drag is force:2


    WcCF DD =


    LC is the lift coefficient

    DC is the drag coefficient is the density of the air

    c is the chord lengthFigure 6: lift & drag forces

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    W is the relative wind speed (Vrel)

    If the chord length increases, the lift force also increases. We are not interested in theincrease of the drag coefficient because its applying bending moment at the blade.

    As we know, the force on the blade is related to the wind speed and the swept area (ofthe rotor). The forces within a blade are increasing with the radius and the loads in the rootare higher than the loads on the tip. This is why the chord length must change from the rootto the tip.

    One of the best ways to design the dimensions for a blade is to use an element method.Depending on the radius we can design different chord lengths in different sections,specifically bigger at the root and smaller at the tip.

    We calculated all the chord lengths and twist angle for a given radius with the Excelprogramme. It calculated the chord lengths for 9 sections, from the very root to the very tip ofthe blade. Some of the elements had chord dimensions larger than 4 meters. That seemed too

    big for a 38 meter blade, so we applied a reduction of 20 % at all the chord lengths. ProfessorSren agreed that we make this correction because his programme is only giving theoreticaldata and, in practical, if the root is too wide it makes the blade a lot heavier but does notincrease the efficiency so much.

    The Excel programme only gave us an estimation of the twist angle and the chordlengths for 9 elements (9 sections). After the reduction the maximum chord equals 3.92meters, at a distance of 4.75 meters from the root. At the tip this distance is 0.85 (See allchord lengths in appendix, page 41). For a 38 meter blade, 9 elements are very few. WRD 2.0gives us the opportunity to add as many elements as we want. We input the chord lengths andtwist angle from the Excel file and then the WRD 2.0 interpolated the data to create 40


    Figure 7: twist angle and chord ratio all along the blade

    Figure 7 clearly shows the chord length all along the blade as well as the twist angle.The twist angle will be described further in this chapter. We can notice that the decreasing ofchord length is not linear with the increasing of the radius. We could compare byproportionality those data to the 29 meter blade I had previously designed from confidentialdata of a real blade. The chords for our blade looked good and it mainly reassured us aboutour reduction coefficient of 20%.

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    3. Transition parts

    The transition part on our blade is the section between the circle at the root and thefirst shape of our FX66-S196-V1 airfoil. The root of the blade cannot have the shape of anairfoil and must have, for practical and technical reasons the shape of a circle. There will betoo much stress on the root if the connection to the hub was an airfoil shape. The circle shapeis also more practical for the pitching system.

    We measured this transition region, on an existing 20 meters blade at Folkecenter. Itslength, from the base of the cylinder to the first profile section is about 3 meters. Thetransition part will not produce any power for the wind turbine, so we want it to be as short aspossible. We decided that 5m for this transition section would be reasonable. In the Excelprogramme, we calculated 4.75m, as the transition section length.

    4. Angle of attack

    The angle of attack, or angle between the chord line and the relative velocity, is calculated bythis expression:

    )( P += Where, is the flow angle is the twist of the blade

    P is the pitch angle

    The angle of attack for a wind turbine may be determined by the coefficient of lift anddrag. The design angle should be the angle where the lift is the biggest, and the drag is thesmallest.

    C_lift C_drag

    0 0.500 0.0094 0.942 0.010

    6 1.160 0.012

    8 1.374 0.013

    10 1.518 0.018

    12 1.370 0.040

    14 1.225 0.065

    16 1.220 0.084

    18 1.233 0.105

    Best angle of attack








    0 5 10 15 20





    Figure 8: Flow around section of a wind turbine blade

    Figure 9: CL/CD vs. angle of attack

    Table2: Lift and Drag coefficient, fonction of

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    Based on this data, we determined the best angle of attack for an FX66-S196-V1 is 9degrees.

    5. Twisting angles

    There are some important angles in blade design.

    1. The angle of attack is the angle between the profile's chord line and the direction of the

    airflow wind.2. The flow angle is the angle between the relative velocity and the rotor plane.

    3. The pitch angle p is the angle between the tip chord and the rotor plane.

    4. The twisting angle , which is the angle measured relative to the tip chord. We can

    calculate this value using the expression = + +p.

    A rotor blade will stop providing enough lift once the wind hits the blade at a steeperangle of attack. The rotor blades must therefore be twisted to achieve an optimal angle ofattack throughout the length of the blade.

    Assume that the tip pitch angle p is zero. will equal + . The angle between theprofiles chord line and the rotor plane should be the twisting angle .

    Considering W, the induced velocity constant we can easily see from Figure 8 that ifwe grow the rotational speed, r it will reduce the flow angle consequently the blade needsto be twisted to keep the same angle of attack . This angle is equal to 9 degrees, from theprevious chapter. The twisted angles are not linear all along the blade (twist angle versus theblade length on figure 7, page 12.). The WRD makes the twisting linear all along the bladebut we kept the data from the Excel programme.

    For our design, the blade starts its FX66-S196-V1 profile at around 4.75 meters.Although the twisting begins after the cylinder part, during the transition part, we calculatedthe twisting angle from 4.75 meter as 25.3. We then got the values for at least 9 parts withthe Excel programme (see all twisting angles in appendix, page 41). At the tip there is notwist. We have set the twisting centre at 50% on the chord. The twisting centre is closer to60 to 70 % on a real blade but we didnt find any data about it but it makes no big difference.We put these values into WRD, exploded them into 40 sections, and received the followingfigures:

    Idem as figure 8

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    Figure 10: rotor view 1, WRD print screen

    Figure 11: rotor view 2, WRD print screen

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    6. Tip speed ratio

    The tip speed ratio is the ratio of the speed of the blades tip, over the wind speed.After choosing a satisfying profile, we had to design the blade with this profile. We tookeverything including the tip speed ratio, transition part length, angle of attack, chord lengths,twisting angles, shell thickness, and material into account .

    For the tip speed ratio, we used the formula:








    We went through several industry wind turbine papers and using WRD tested eachusing a similar blade. We assumed that our blade would be operated at rotational speed of22.5 rpm with a nominal wind speed of 11m/s. (in the following pages, we will prove thatthose assumptions are accurate. See further details in Wind speed classes and Pitchcontrol parts). If we implement our assumption in this formula, R becomes 38 meters, and

    we can get the tip speed ratio

    X = 8

    This ratio should not exceed 10, if we take in account noise consideration.

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    VV PPiittcchh ccoonnttrrooll aanndd ppoowweerr ccuurrvveess

    1. Why pitching?

    The following Figure 12 perfectly explains the need to change the angle of attack ofthe whole blade when the speed of the wind changes.

    Figure 12: velocity triangles for a section in a blade with constant pitch and constantrotational speed for two different wind speeds

    We can note on those drawings that the angle of attack of our blade is not the same ifthe wind speed, V0 changes. Vrel is the relative velocity. Consequently we need to pitch ourblade to keep the desired angle of attack (9, from the previous chapter).

    Actually, the pitch of the blade is not changing with the speed of the wind, the pitch ischanging with the output power of the generator. In order to avoid damage from the strongwinds, every wind turbine has a power control system. As it controls the wind turbine outputit produces stable power after the nominal wind speed is reached. There are several differentways to control the power. Some techniques include pitch control, active stall control,passive stall control and yaw control. We will discuss these different methods in this section.Our blade only uses pitch control. The other methods for controlling the power are alsoquickly explained because they help to understand why we pitch the blade for controlling theoutput power.

    a. Pitch control:

    If a wind turbine is pitch controlled, the turbines electronic controller will check theoutput power several times per second. When the power becomes too high, the controller will

    send an order to pitch the blade slightly into the wind until the output power reduces to anacceptable value. As we just saw, this will reduce the angle of attack of our blade. Wheneverthe wind goes down again, the blade will turn back to its original position to producemaximum power. Because the blade is twisted we need to put a reference for the zero of thepitching system. A pitching angle of zero degrees corresponds to an angle of zero degrees atthe tip. Before reaching the nominal wind speed the blade is pitched in order to get themaximum power (or efficiency). Once the nominal wind speed has been reached the blade ispitched in order to keep the maximum power until the Cut-out.

    A turbine with a pitch controller requires smart engineering to make sure that the rotorblades pitch the exact amount. This means that the controller should be sensitive to wind

    changes. On a pitch controlled wind turbine, in order to keep the rotor blades at the optimum

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    angle and producing output no more than rated power through all wind speeds, the computerwill generally pitch the blades a few degrees every time the wind changes. The disadvantageis that the time scales for the fluctuations are so small compared to the time it takes tophysically pitch the blades. That is why the pitch controlled system must be accompaniedwith a variable speed asynchronous generator. After reaching the nominal wind speed therotor is supposed to keep a constant rpm but it will in fact accelerate in the case of a gust. Thespeed variation can be up to 35%. The variable speed reduces the fatigue loads and improvesthe power quality. The system needs converters to keep the same output frequency.Consequently the extra cost of the control system and the necessary converters has to beconsiderated.

    b. Active stall control:

    There is another power control system called an active stall control. When the windturbine reaches its rated power it will control the turbine in a very different manner than thepitch control does. If the generator is about to be overloaded, the controller will pitch itsblades in the opposite direction than a pitch controlled machine would. It will increase theangle of attack of the rotor blades in order to make the blades go into a deeper stall, not usingthe excess energy in the wind.

    The advantage of active stall is that the output power of the wind turbine can becontrolled more accurately than a pitch control. It can also run in very high wind situations.

    The disadvantage is that a turbine using active stall control will waste more power andwill be subjected to larger loads.

    c. Passive stall control:

    Passive stall controlled wind turbines have a fixed angle on the rotor blades, a constantrotational speed and a specific profile. When the wind speed is too high, the power iscontrolled through the blades profile and aerodynamic design.

    One of the advantages of stall control is that it avoids moving parts in the rotor itself.

    The disadvantage is that passive stall control requires a very complex aerodynamicdesign and that the blades have to support high loads. There are additional design relatedchallenges in the structural dynamics of the whole wind turbine. For example it requiresengineers to choose an outstanding performing profile and pay close attention to the blade andwind turbine vibration. If the rotor needs to be stopped because the wind is too high, the

    blade has an aerodynamic brake called tip break. It is in fact a flap that will be deployed atthe tip (as seen on following Photo 2) when the centrifugal force is too important. The discbrake, mounted on the shaft is only used when the rotor is stopped, it cannot stop a rotor thatis over speeding.

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    Photo 2: deployed tip break, Nordic Folkecenterswind turbine, in Hanstholm.

    d. Yaw control:

    There is another possible control that can be added to a wind turbine. This system willyaw the rotor partly out of the wind so it will also decrease the output power and forces on thesystem. Yawing the rotor will reduce its swept area. Yaw control is especially suitable for

    small wind turbines (1 kW or less), as it subjects the rotor to cyclically varying stress whichmay ultimately damage the entire structure. This kind of control is called furling.

    2. Pitch control results

    Following Folkecenters requirements, our 38 meter blade will operate with pitchcontrol and the wind turbine will have a 1.5 MW output. In our design we have to take intoaccount an efficiency of 80%. It means that our blade must have a nominal output power of1800 kW, if we want the generator to have an output of 1500 kW. This efficiency coefficientcomes from the fact that the gearbox has an efficiency of 0.9 and the generator has anefficiency of 0.9 also. Consequently we will control the wind turbine to produce an outputpower of 1.8 MW. From the wind class III and the wind characteristics, we determined thecut-in wind speed to be 2 m.s-1, and a cut-out wind speed of 25m.s-1. In this range we cantake advantage of almost all the power we can harvest.

    As a first experience we wanted to see how our blade would behave if we try to get themaximum power from the cut-in to the cut-out wind speed. We also wanted to know thenominal wind speed of our blade. The nominal wind speed is the speed of the wind when thenominal power of the blade is reached. We expected to have a large output power. But wedont want the maximum power of the blade and this experience will demonstrate the need fora pitch control and a constant rpm. As explained in the chapter dealing with future

    development we didnt take into consideration the gusts so we are only speaking about pitchcontrol. We dont need to use variable speed because our wind is constant, for each windspeed.

    As a second experience we want to use our blade as a stall controlled one. We knowthat keeping the rpm constant will prevent the power from increasing too strongly, andprobably will reach steadily at wind speed around 25 m.s-1. As it is a stall controlled bladethere will be no pitch. In this case we only keep the rpm constant once we have reached thenominal wind speed. The rpm is held constant, not by use of the mechanical breaks but byusing the generator itself. The generator in this kind of wind turbine is asynchronous and therpm can be changed by changing the frequency. For example an asynchronous generator of 6

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    poles has a rpm of 1000 with 50 Hz and 1200 with 60 Hz output. The following figurepresents the two curves of our experiments.

    Figure 13: Power curves with passive stall control (in pink)and free system (in blue), from WRD data

    For this first experience WRD calculated a maximum power output of 1509 kW at 10m.s-1 and a rotational speed of 22.5 rpm. When the wind speed is increased to 11 m.s-1, themaximum power goes to 2009 kW with a rotational speed of 24.7 rpm. Since we chose aWind Class III turbine, the nominal wind speed should be 10 m.s-1. We calculated thenominal wind speed of our blade to be 10.75 m.s-1. At this speed the power output is 1861kW and the rotational speed reaches 22.5 rpm. When the wind speed is 20 m.s-1, the outputpower can reach up to 12,047 kW with the rotational speed as high as 45 rpm. This highrotational velocity would burn the wind turbine up.

    If we use our blade as a stall regulated one we can notice on the graph that the outputpower at wind speed of 20 m.s-1 will still reach more than 5500 kW. This power is still toohigh and this kind of control will not avoid the damage on the wind turbine, it will ruin it. Bytesting these two situations we were able to verify the important role the pitch control plays inpower control and blade protection.

    The next thing to calculate is the output power, using the pitch control system. Nowwe want to keep it equal to 1800 kW, after reaching 11m.s-1, the nominal wind speed. With

    each wind speed, we tested multiple pitch angles in the WRD programme. WRD would thendisplay the output powers for each pitch. This allowed us to determine the most efficientpitch angle at each wind speed. After running this test multiple times, we verified that thecontrol could maintain an output power of around 1800 kW successfully. At the highest windspeed tested, 25m.s-1, we set the pitch angle to 24.4 degrees. For further explanations aboutthe way we worked on the WRD programme, see the WRD chapter, page 26. The next figureshows us the output power curve, from 2 m.s-1 to 25 m.s-1.

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    Figure 14: Power curve with pitch control, from WRD data

    We can see that it is possible the keep the output power constant to the wanted valueafter the nominal wind speed is reached. We can see that the pitching control system allowsus to keep the curve flat so the wind turbine is constantly producing 1500 kW of power afterthe nominal power has been reached. We compared the data we got from the programme toexisting blades. We obtained some confidential data from LM Glasfiber (worlds leadingmanufacturer of wind turbine blades) and their results were similar to ours. We alsocompared our results with a 1.8 MW wind turbine. This wind turbine is from Vestas (Vestasis the world leading and Danish company for wind turbine). The wind turbine we used forcomparison is the V90-1.8 MW. We were given its booklet and could make comparison withthe electrical power curve, the power curves had the same shape. The electrical power curvesdiffer from one generator to another. For example, a generator of 2 MW could be mounted on

    this Vestas wind turbine but most of the time blades and generators go together, they shouldbe designed for the same wanted power. The data for figure 13 and 14 are given in appendix,page 44 and 45.

    Power Coefficient








    0 10 20 30

    Wind speed in m/s

    Cp Power Coefficient

    Figure 15: Power coefficient of our blade

    The power coefficient tells us how efficiently a turbine converts the energy in the windto electricity. The average efficiency of this coefficient is somewhat above 20%. It mayseem little but we have to keep in mind that our fuel (the wind) is free so there is not reallyany need to save it. The optimal turbine is therefore not necessarily the turbine with thehighest energy output per year. When we did the power calculations on WRD, the blade was

    a wooden blade but it does not really matter because in this part of the experiment we are onlyinterested in the aerodynamics point of view.

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    VVII BBllaaddee mmaatteerriiaallss

    1. Materials

    The blade endures many loads during its life. A list of those loads is given in the next

    chapter. The two main loads are gravity and centrifugal forces. The rotating mass load is avery important factor to take into consideration when designing a blade. The inertia dependson the mass thus we want a light blade. Therefore the choice of material is very important.The blades must be lightweight but strong enough to resist the different loads. This sectionwill explain why we chose the carbon fibre.

    We have the choice between different material for our blade, fibre glass, FibreReinforced Polyester (FRP), epoxy and carbon fibre. A rotor blade can be made fromdifferent materials. The most commonly used materials are fibre glass and FRP (used tomanufacture both, internal and external parts of the blade). Each one of these materials hasdifferent characteristics, they have two different chemical compositions and mechanical

    properties. Carbon fibre is the strongest, and has the best fatigue strength (the fatigue strengthis the stress level that a material can endure for N cycles. For a wind turbine blade it is about108). The prices of these two kinds of fibres may vary depending on the manufacturer.Typically the carbon fibre is more expensive than the glass fibre.

    After much discussion and calculations, we realised that a fibre glass blade neededmore thickness to reach the fatigue limits of the carbon fibre ones. This meant that we needmore material to build the same blade; this would make the blade heavier.

    In the end we decided to design a carbon fibre reinforced epoxy blade with theseproperties:

    Material Carbon Fiber Fiber glass Steel

    Density (g/cm^3) 1,76 2 7,85Tensile modullus or

    Youngs modulus(Gpa)235 20 210

    Tensile strength (Mpa) 3920 ----- 1860Fatigue strength (Mpa) ----- 300 1200

    Table3: Blade materials comparison.

    Because we could not find the fatigue strength of the carbon fibre, we used the data forsteel. Professor Sren told us that the carbon fibre has a better fatigue strength than the steel

    and because we obtained very good results it was a good solution. The data of the carbonfibre are given in appendix, page 45.

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    2. Inner support and shell thickness

    To withstand the loads and stresses, the blade needs beams and an appropriate shellthickness. The stresses in a blade are bigger in the middle and lower in the tip and the root(see the loads calculation page 25). Consequently, the material in the middle part must bethicker than the other parts of the blade. We do not want to make the blade overly thick at theroot also in order to save the expensive carbon fibre. We can make the blade thinner at the tipbecause the bending moment is very low. The figure 16 shows the thickness dimensions andthe figure 17 shows the characteristics of a typical blade section.

    Figure 16: thickness, function of the radius Figure 17: Shell and support, WRD design

    The inner elements (the inside rectangle) were designed from examples taken from theinternet. We also did measurements on a 20 meter blade, in the blade exposition at theFolkecenter. We decided, after having tested different shapes that the one in Figure 17 is thebest. It made the blade the strongest. We used the same element all along the blade bykeeping the proportionality.

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    VVIIII LLooaaddss ccaallccuullaattiioonnss

    As part of the design process, a wind turbine must be analysed for the different loads itwill experience during its life. The first purpose is to be sure that the turbine will be able tosupport the loads with a sufficient safety margin.

    The external loads acting on a wind turbine are mainly wind forces. In most cases theloads on a wind turbine can be classified as follows:

    - Aerodynamic blade loads.- Gravity loads on the rotor blades.- Aerodynamic drag forces on nacelle and tower.- Gyroscopic loads caused by the yaw motion.- Centrifugal forces and Coriolis forces during the rotor rotation.

    In this part we are going to focus on the loads affecting the blades, especially

    aerodynamic and gravity loads.

    Figure 18: Projection of lift and drag to calculate normaland tangential load on blade section

    We can see on Figure 18 how the normal (Pn) and tangential (Pt) loads can be foundby projecting the lift and drag into a direction normal to and tangential to the rotor plane. Thenormal load is responsible for the thrust force and the integrated tangential loads give themechanical shaft torque.

    The gravity loads on the rotor blades cause bending moments in the blades in theedgewise direction.

    All the graphs and calculations in the Figure 19 show a blade sketch, thebending moments (Max_M) diagram and all the stresses along the blade.

    Wind loads distribution:The blade loads are related to the bending momentum and the area A where they are

    applied. The section of the blade closest to the tip experiences a higher wind speed. Its areaA is small but the bending momentum is minimum so the loads are little in this area. In the

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    middle section though, the increase in wind speed is greater than the decrease in area. Thisleads to higher stress in the middle section, making it the most likely to fail.

    Figure 19: loads graph, print screen of WRD

    WRD calculated the different momentums for us; these included the operating

    condition, the yawing condition and the case of the extreme condition. The maximumbending moment is 4145.242 kNm and its situated in the root of the blade. This value iscalculated with the Danish security coefficients. Further explanations are given in the chapterdealing with WRD, page 30 (more data are given in appendix, page 43).

    We can see that the maximum stress is in the middle of the blade and its value is 279Mpa. This value is a lot under the fatigue strength so we can be sure that our blade wontbreak for a period for 20 years.

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    VVIIIIII WWiinndd RRoottoorr DDeessiiggnn ssyysstteemm 22..00

    Figure 20: First screen of WRD

    Wind Rotor Design System (WRD) is a powerful European design tool. The firstversion was created in 1992 and the second version, which is the one we used, was introducedin 1995. This programme was created at the Nordic Folkecenter for renewable energy and isused to design wind rotor blades and do power and loads calculations. This programme canbe, and has been, compared to AeroDyn. AeroDyn is not a programme though; it is a libraryof subroutines that can be called from a structural-dynamics code such as FAST. WRD ismore precise than AeroDyn and is much more user friendly. The only weak point of WRDis that it doesnt take aeroelastics into consideration. Because of this, WRD should be usedalongside the Fast programme, a free software that also includes AeroDyn, to get moreaccurate results. It is possible with WRD to output results in a table or graph. It is also

    possible to have different views of the rotor and to upload the drawing in AutoCad. WRD canbe described as a virtual wind tunnel because it is very easy to change different parametersand see the affect. Parameters such as the tip angle (min and max), rpm (min and max), windspeed (min and max) can be set very easily in the programme. WRD was not designed forpitch controlled blades, but only for stall controlled blades. It can be used for pitch controlledblade though by going from curve to curve in the result graph. A notable characteristic of theWRD system is the convenience and reliable data treatment. The time spent on inputting datainto the system is reduced by using user-friendly interface and logical steps for rotor design.

    1. Programme outline:

    The hierarchical structure of this programme consists of the following units:

    1. Design2. Power calculation3. Load calculation4. Airfoil library

    a) The Design menu allows us to set up all the parameters of the rotor, the blade, and theblade section.

    This is the correct order for Blade Data Editing:

    - Input the rotor diameter or blade length.

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    - Input the hub diameter.- Modify the radius, chord, twist, twist centre, profile ordinates and C L/CD data at the bladeroot and tip.- Input the section data for multiple airfoils.- Change the sections number to the number of real airfoil sections- Modify the radius, chord, twist, twist centre and airfoil of the real airfoil sections and insertthe interpolated sections using the insert option.- Modify the radius, chord, twist and twist centre of interpolated sections.

    It is possible in this section to determine the size of the rotor, the number of blades, thesize of blades, and the construction of the blade section. It is also possible to have a 2D or 3Dview of the blade. This makes an excellent interface in designing the geometry of blade andeven its twist angle. There is a built-in mini CAD for designing the geometry of the blade,including the twist.

    The blade section can be one section with the option of holes and different materials.The blade section can also be a shelled (thin walled) section. In this case, the programmeautomatically calculates the coordinates of the sectional internal profile. The moments ofinertia, the mass centre, and the relative elastic centre and other properties of the blade arealso calculated automatically.

    b) In the power calculation unit, we can calculate the power efficiency with varying windspeeds and different rotor rotational velocities. It is important to mention here that the powermentioned is the mechanical power. In order to get the electrical power we divide themechanical power by a coefficient of 1.2. The blade we designed in this project is for a 1.5MW wind turbine so we should reach a mechanical power of 1.8 MW. With this calculatedresult, it is possible to see under which conditions the rotor performs well. We can thendesign the rotor according to the wind conditions (see wind speed classes, page 8). The

    results can be displayed on the screen as a graphic picture or a data table. We are also giventhe efficiency (CP). The output curves are RPM vs. Power and RPM vs. CP. These curves arecalculated for different wind speeds and we have the possibility to sweep different pitchangle. We used this part to determine the pitch angle that our blade should reach, for a givenwind speed. This way we can get the best efficiency from the blade. After having reachedthe wanted power, we need to keep the RPM constant and only make the pitch change. Wechose a maximum RPM of 22.5 by comparing data from existing LM blades.

    The needed data for power calculation are as follows:- Name of rotor: Mickal-Joan-Tong 38 m blade project- Name of file with airfoil coordinates/A (multi)

    - Name of file with CL and CD data: N/A (multi)- Airfoil (shape) [single/multi type]: MULTI- Airfoil (CL/CD) [single/multi type]: MULTI- Blade length (mm): 3.8000000000E+04- Zero-line of the blade (% of chord): 0.0000000000E+00- Rotor diameter (mm): 7.8000000000E+04- Hub diameter (mm): 2.0000000000E+03- Number of blades: 3.0000000000E+00- Air density (kg/m**3): 1.2400000000E+00- Min. tip angle (deg): 0.0000000000E+00- Max. tip angle (deg): 2.4400000000E+01

    - Tip angle: Scale Division (deg): 5.0000000000E+00

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    - Min. wind speed (m/sec): 0.0000000000E+00- Max. wind speed (m/sec): 2.5000000000E+01- Wind speed: Scale Division (m/sec): 5.0000000000E+00- Min. rotational speed limit (rpm): 5.3000000000E+00- Max. rotational speed limit (rpm): 2.2500000000E+01- Rotational speed: Scale Division (rpm) 4.0000000000E+00

    c) The load calculation part executes the calculations of loads and stresses along the bladeand the rotor shaft. The stress calculations allow for massive and lightweight blades with theuse of different materials in the blade structure. These results are also shown as a graphicpicture or a data table. The load results are shown and explained in the chapter concerningload in this report.

    The extra data necessary for load calculation are as follows:- Hub height (mm): 8.0000000000E+04- Nominal electrical output (kW): 1.5000000000E+03- Electrical efficiency: 8.0000000000E-01- Angle of attack at max. lift (deg): 9.0000000000E+00- Max. lift coefficient (CL): 1.4899000000E+00- Max. drag coefficient (CD): 1.0000000000E+00- Dist. from R_Cent. to the bearing(mm) 0.0000000000E+00- Dist. from R_Cent. to the T_Cent.(mm) 0.0000000000E+00- Dist. from R_Cent. to the B_Cent.(mm) 0.0000000000E+00- Maximum yawing speed (rad/sec): 1.0000000000E+00- Nominal yawing speed (rad/sec): 1.7453000000E-02- Max. rotational speed ( % of nominal) 1.0000000000E+02- Rated wind speed (m/sec): 7.5000000000E+00- Mass of blade (kg) [calculated if 0 ] 0.0000000000E+00

    - Mass of rotor (kg) [calculated if 0 ] 0.0000000000E+00- Blade load factor, kbx: 1.0000000000E+00- Blade load factor, kby: 1.0000000000E+00- Rotor load factor, kx: 1.7000000000E+00- Rotor load factor, ky: 1.0000000000E+00- Rotor load factor, kz: 1.7000000000E+00

    d) There is a large airfoil library in this system. The library contains the data for the NACA63-1, 2, 3, 4 series airfoils which are widely used for modern wind rotors. It is also possibleto add customized airfoils if the lift/drag coefficient data and the airfoil coordinates areknown. We wrote new files for our FX66-S196-V1 airfoil and could easily insert them into

    the WRD programme. It is possible to have a view of the airfoil, see its CL/CD coefficients,and see function of the angle of attack. It is consequently very easy to detect any mistakes inthe data.

    2. Instructions:

    a) First, design of the blade.

    The first thing we needed to input in the programme was the airfoil coordinates. Ifdesired, its possible to write a file similar the ones used in WRD using Notepad. We took thedata FX66-S196-V1 - Re=1.5x106 (Measured at Stuttgart) from the Ris website (See

    appendix in page 38):

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    We then checked to make sure there were no mistakes in the airfoil by drawing it onthe built-in mini-cad.

    We also entered the data for the CL and CD coefficients. We knew those coefficientsfor angles between -2.8 degrees and 10.3 degrees. The angle scale may seem small, but WRDcan make predictions with angles up to 90 degrees.

    Once the airfoil coordinates and the associated CL and CD data are in the programme,we can start to design the blade. First, we need to set up the blade length. We alreadydetermined this length to be 38 meters. Next, we need to input the hub diameter (we havechosen 3 meters). After that, we need to change the section number to 10. We know the data(radius, chord and twist angle) for 9 sections between the root and the tip, using the excelprogramme. We need to add a tube before the airfoil. This tube will be the connection to thehub and we set its length to 1 meter. This tube used to be long on the old blades but it isbetter if the blade takes the shape of the airfoil at a smaller radius r, even if the moment is notvery important for such radius.

    Before the next step, we need to enter all the parameters for each section: radius, chordand twisting angle. After that, we need to make an interpolation line between the circle andthe airfoil. This is calculated automatically with WRD. Finally we need to tell the programmethat we want more sections to make the power calculations more precise. We chose to have40 sections all along the blade. This number is a compromise between good precision andcalculation time (which can be very important depending on the parameters). Now the bladeexists and we do not need to input more parameters because the power calculation is completefrom an aerodynamics point of view.

    Idem as figure 10 idem as figure 11

    b) Second, power calculation of the blade.

    Now we can start to calculate the mechanical power of the blade for each wind speed.We set up the Cut-in wind speed at 2 m/s and the Cut-out wind speed at 25 m/s. For thesecalculations, we can change the tip angle (which is the same as the pitch angle), the windspeed, and the rotational speed limit (rpm of the rotor). The objective in these calculations isto reach the nominal power as quick as possible by finding the best combination of rpm andtip angle. There are different combinations of rpm and tip angle for a given power, but weshould find the ones that make the pitch angle increase with the wind speed so the blade is notpitching all the time. We copied results for each wind speed and did the power graph onexcel:

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    Idem as figure 14

    c) Third, Load Calculation of the Blade.

    To make the load calculation, we now need to enter more data in WRD. First, weenter the hub height. We set ours to 80 meters by examining existing Vestas wind turbinedesigns. We then set up the nominal electrical output at 1500 kW and the electrical efficiencyat 0.8. We also need to enter the angle of attack at the maximum lift. This is equal to 9

    degrees and can be determined from the CL graph. Then we entered the maximum lift anddrag coefficient and the maximum yawing speed. The maximum yawing speed is equal to 1radian/sec (approximately 57,3 degrees/second). This value may look very big, but it is usedto account for bad gusts, earthquakes, or both. This value is used in the Danish safetystandard. We also enter the nominal yawing speed which is 1.75E-02 radian/sec (1degree/sec).

    We now need to define the blade material. We chose to use carbon fibre because ithas a great strength/weight ratio. We need to know the following data of the carbon fibre:elastic modulus, fatigue strength, partial coefficient for fatigue load (1.5) and partialcoefficient for extreme load (1.5).

    We included supports in our blades to increase their strength. We used someexamples found on the internet to design these supports along with taking measures of realblades in the blade exposition park at Folkecenter, as said page 23.

    Idem as figure 17

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    IIXX FFuuttuurree ddeevveellooppmmeenntt

    Many of our specifications derive from calculations and theories. We tried our best toproduce accurate estimations, but its difficult to produce a complicated design without atleast small errors. In our blade design, we tried to make our blades as close to industry

    designs as close as we could, but they will still has some errors from the manufacturingprocess. Those errors can be seen as uncertainty, but are more likely future development.

    1. Chord lengths.From the aerodynamic theory, the chord length c is calculated by the following formulas:



    xxxxC L =++= 432


    Here, c is the chord length, C is factor, x is the speed ratio, B is the number of blade, R isthe blade radius, CL is the maximum lift coefficient and X is the tip speed ratio.

    Based on these formulas, we can get chord length 4.902m at the root and 1.064m at the tip. Inthe real commercial industry though, a chord length near 5 meters at the root is too large.Although it would have the highest efficiency, we needed to scale it down by applying anarbitrary coefficient of 0.8. A possible improvement here would be to do more precisecalculations for the chord lengths. It could be by using different mathematical formula forexample.

    2. Profile

    As said before for many modern and commercial blades, more than one profile is usedall along the blade. All the NACA profiles, for example, have their families. It means thatengineers can use several profiles based on one type all along the blade they are designing.

    When we look at the CL coefficient (page 11) we can see that the curve is too steeparound the maximum lift. We know that the pitching system is accurate but the wind is notand because of that it creates vibrations on the tip. We need to use a profile with a CLcoefficient flatter in this area, even we are sacrificing power.

    Figure 21: example with different profiles, from root to tip

    It would also be an idea to make tests without the tube at the root. This is done todaywith companies like Enercon who design blades which have no tube at the root but startdirectly with a very large profile. In this case the hub is completely different. Other research

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    is currently being undertaken to put not a tube but a cone at the root. It could be stronger andmore stable.

    3. Shell thickness.

    For the blade to perform well, it should have good aerodynamics features and it shouldhave a good compromise between weight and wall thickness. If we want the blade to be lightwe should make the shell as thin as possible. Considering the loads on the blade we shouldalso make sure that the blade will not collapse in extreme conditions.

    In the loads distribution section of this paper, we reported that the most fail pronepoint is not at the tip, but in the middle. When the wind speed becomes higher the stressincreases in this section. In order to keep the blade safe, we have to increase the thicknessinside. This increases the blades strength but also increases the weight and consequently thedistance to the centre of mass. Increasing this distance makes the centre of gravity moving tothe tip. Some researches may be done in this direction.

    4. Turbulence

    In our study we only made the first approach with laminar flow. We did not includeturbulence into considerations in our design but at some point though, the wind turbine designshould contain them. This will lead to more efficient, longer lasting wind turbines. Takingthis into consideration will make us take into account the variable speed of the generator. Thewind turbine will still be pitch controlled so the rotor will have a constant rpm after thenominal wind speed is reached. In fact this rpm will be constant within 35%. That is why thevariable speed generator is needed, in the case of turbulence

    5. Vortex generators

    We have noticed that the blade manufactures are putting some fins on a certain lengthof the blade. Those ones create a thin layer of turbulence and curiously it makes the bladestall at a steeper angle. This technology has been used on aircraft for a long time. With moretime we could have included this in our researches.

    Photo3: Vortex generators, Nordic Folkecenters wind turbine.

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    XX CCoonncclluussiioonn

    In a word, our blade finally comes out as a success, with following features:

    The blade is designed to run under wind class III, whose reference wind speed is 37.5



    , the average wind speed is 7.5m.s


    . The nominal wind speed for class III is 10 m.s


    (theoretical value). The blade length is 38 meters, the hub is 3 meters wide. It makes a 78meters diameter rotor, as seen on the following picture:

    Figure 22: rotor view 3, WRD print screen

    Selected blade profile: FX66-S196-V1

    Best angle of attack for the profile: 9 degrees

    Designed rotor rotation: clockwise

    Profiles chord length at root: 3.92 meters

    Profiles chord length at tip: 0.85 meters

    Biggest twisting angle: 24.4 degrees at 3.92 meters from the root

    Twisting angle at the tip: none

    Blades material: carbon fibre reinforced with epoxy

    Density: 1,76 g/cm3

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    Tensile modulus (or youngs modulus): 235 GPa

    Ultimate strength: 3920 MPa

    Shell thickness at the root: 40 mm

    Shell thickness in the middle: 78 mm

    Shell thickness at the tip: 12 mm

    Cut-in wind speed: 2m.s-1

    Cut-out wind speed: 25 m.s-1

    Practical nominal speed, for pitch control: 10.75 m.s-1

    Rated rotational speed: 22.5 rpm

    Output mechanical power: 1860 kW

    Output electrical power: 1500 kW

    Gearbox+generator efficiency: 80%

    First pitching: 11m.s-1

    First pitch angle: 2 degrees

    Maximum pitch angle: 24.5 degrees, for the cut-out wind speed.

    Maximum stress : 279 Mpa (in the middle of the blade), value under the limits.

    This internship was a great experience for me, for many different reasons. Firstly Ihad the opportunity to work on a completely new subject. Working on wind turbines andstudying aerodynamics was a real pleasure. I enjoyed working using English for nearly threemonths. I did have very enriching exchanges with people coming from many places aroundthe world. I could discover a new country where there are wind turbines everywhere. I was

    extremely happy visiting in person the large wind turbine at the Folkecenter many times andvisiting off-shore wind farm and large wind turbine plants. I had the chance to work in twodifferent places.

    In the Folkecenter I had the opportunity to learn a lot about renewable energy. Inaddition to learning a lot about wind power I learnt about wave energy, heating systems, solarcells, passive houses, water recycling systems and so on. I did enjoy being taught by expertsin this area.

    In rhus, a big student city, I could work in different conditions. I had the realchance work in the Ingenirhjskolen i rhus, with two Erasmus students and I used theschools equipment and was taught by their professors. This internship has beenunforgettable for me.

    This report is available on the Folkecenters website for downloading.

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    XXII GGlloossssaarryy

    Airfoil. (profil daile) The cross section profile of the leeward side of a wind generator blade.Designed to give low drag and good lift. Also found on an airplane wing.

    Angle of Attack. (angle dattaque) The angle of relative air flow to the blade chord.Blade. (pale)The part of a wind generator rotor that extracts the wind energy.

    Chord. (corde) The distance between the trailing edge and the leading edge of a profile.

    Cut-In. (vitesse minimale pour produire une puissance) The wind speed when the windturbine starts operating.

    Drag. (trane)In a wind generator, the force exerted by an object because of the moving airresults it to move in the opposite direction. Also refers to a type of wind generator or

    anemometer design that uses cups instead of a blades with airfoils.

    Efficiency. (rendement)The ratio of energy output to energy input in a device.

    Epoxy. A 2-part adhesive system consisting of resin and hardener. It does not start to hardenuntil the elements are mixed together.

    Fatigue. (force de fatigue)Stress that causes material failure from repeated, cyclic vibrationor stress.

    Furling. (carter le rotor de la direction du vent pour diminuer sa surface balaye, voir page

    19)The act of a wind generator Yawing out of the wind either horizontally or vertically toprotect itself from high wind speeds.

    Gearbox. (multiplicateur de vitesse) This device is making the connection between the lowspeed of the rotor and the high speed of the generator.

    Generator. (gnrateur) A device that produces electricity (DC or AC) from a rotating shaft.

    Horizontal Axis Wind Turbine (HAWT). (olienne horizontale) Also called axial flowwind turbine. The shaft has the same direction as the wind come.

    Hub. (moyeu) The center of a wind generator rotor, which holds the blades in place andattachs to the shaft.

    Leading Edge. (partie frontale du profil) The edge of a blade that faces toward the directionof rotation.

    Lift. (portance) The force exerted by moving air on asymmetrically-shaped wind generatorblades at right angles to the direction of relative movement. Ideally, wind generator bladesshould produce high Lift and low Drag.

    Load. (charge) A force or moment applied on the blade.

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    Losses. (pertes) Power that is harvested by a wind generator but is not transferred to a usableform. Losses can be from friction, electrical resistance, or other causes.

    Moment. (moment) Action of a force attempting to produce motion around an axis.

    Pitch. (angle adjustable de la pale)See Setting Angle.

    Rated Power Output. (puissance de sortie nominale) Maximum power (electrical ormechanical) to be obtained from the wind turbine.

    Root. (racine)The area of a blade nearest to the hub. Generally the thickest and widest part ofthe blade.

    Setting Angle. (angle adjustable de la pale)The angle between the blade Chord and the planeof the blade's rotation. Also called Pitch or blade angle. A blade carved with a Twist has adifferent setting angle at the Tip than at the Root.

    Shaft. (axe liant le moyeu la bote de vitesse)The rotating part in the centre of a rotor of awind generator or motor that transfers power.

    Start-Up. (vitesse minimale de rotation du rotor) The windspeed at which a wind turbinerotor starts to rotate. It does not necessarily produce any power until it reaches cut-in speed.

    Thrust. (puissance) In a wind generator, wind forces pushing back against the rotor. Windgenerator bearings must be designed to handle thrust or else they will fail.

    Tip. (extrmit de la pale)The end of a wind generator blade farthest from the hub.

    Tip Speed Ration. (ratio de la vitesse de lextrmite de la pale/vitesse du vent) The ratio ofhow much faster than the windspeed that the blade tips are moving. Abbreviation TSR.

    Torque. (couple) See moment.

    Trailing edge. (partie arrire du profil)The edge of a blade that faces away from the directionof rotation.

    Twist. (vrille) In a wind generator blade, the difference in Pitch between the blade root andthe blade tip. Generally, the twist allows more Pitch at the blade root for easier Startup, andless Pitch at the tip for better high-speed performance.

    Wind Turbine. (olienne) A machine that extracts the force of the wind. Called a WindGenerator when used to produce electricity. Called a Windmill when used to crush grain orpump water.

    Windward. (dans la direction du vent) Toward the direction from which the wind blows

    Yaw. (rotation de la nacelle, la tour tant laxe) Rotation parallel to the ground. A windgenerator yaws to face winds coming from different directions.

    Yaw axis. (plus ou moins laxe de la tour) Vertical axis through the center of gravity.

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    XXIIII RReeffeerreenncceess

    Books and documents:

    1. Wind turbine aerodynamics : A state-of-the-art from Karman Institute if Fluid

    Dynamics. Lecture Series (March 19-23, 2007).2. Guidelines for design of wind turbines from Ris Laboratory.3. Wind turbine types Author: Martin O.L.Hansen from DTU.4. Aerodynamics of wind turbines Author Sren Gudntoft.5. IEC 61400-1 Danish Standarts.6. Preliminary Structural Design of Composite Blades for two and three blade rotors.

    Authors G. Bir and P. Migliore from NREL7. Wind turbine airfoil catalogue from Ris Laboratory.

    Web sites:

    1. http://www.grc.nasa.gov/WWW/K-12/airplane/short.html 2. http://www.windpower.org/en/core.htm 3. http://www.risoe.dk/vea/profcat/WWW/HTML/a_index.htm 4. http://www.mh-aerotools.de/airfoils/javafoil.htm 5. http://www.yccarbon.com6. www.wikipedia.com

    Drawings, figures and tables:

    Figure 1:http://www.dpa.unina.it/adag/eng/images/renewable_energy/002_turbine_vertical_schematic_a.png

    Figure 2:http://ec.europa.eu/research/energy/nn/nn_rt/nn_rt_wind/images/wind_en_1370.gif

    Table 1: Danish Standards IEC 61400-1 page 8.

    Figure6: Wind turbine types Author: Martin O.L.Hansen from DTU page 11

    Figure 8: Wind turbine types Author: Martin O.L.Hansen from DTU page 13.

    Figure 12: Wind turbine types Author: Martin O.L.Hansen from DTU page 17.

    Figure 21 :http://www.lr.tudelft.nl/live/binaries/7860c285-a659-498d-97fa-729fdefc229d/img/blade48_small.gif

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    XXIIIIII AAppppeennddiixx

    A. The FX66-S196-V1 airfoil. 39

    B. Excel Design programme.. 40

    a) Info... 40b) Design...... 40c) Power... 40d) Blade profile.... 42e) Example... 43

    C. Rotor data, from WRD. 43

    D. Power curves Excel table... 44

    a) Without control 44b) Stall control.. 44c) Pitch control. 45

    E. Carbon fibre properties 45

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    A. The FX66-S196-V1 airfoil (References form Ris laboratory)

    Reynolds number = 1.5 x 10^6Measured in the Laminar Wind Tunnel at the Institut for Aerodynamics and Gasdynamics ofStuttgart

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    B. Design Excel programme

    a) Info

    b) Design

    c) Power

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    d) Blade profil

    The data in this board go up to alpha=90

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    e) Example

    C. Rotor data, from WRD

    1 . Name of rotor................ ........loadcalc2 . Name of file with airfoil coordinatesN/A (multi)3 . Name of file with CL and CD data.....N/A (multi)4 . Airfoil (shape) [single/multi type]: MULTI5 . Airfoil (CL/CD) [single/multi type]: MULTI6 . Blade length (mm)................. ... 3.8000000000E+04

    7 . BLADE GEOMETRY//Number of sections = 4.0000000000E+018 . Zero-line of the blade (% of chord).. 0.0000000000E+009 . Rotor diameter (mm).................. 7.8000000000E+04

    10 . Hub diameter (mm).................... 2.0000000000E+0311 . Number of blades .................. .. 3.0000000000E+0012 . Air density (kg/m**3)................ 1.2400000000E+0013 . Min. tip angle (deg)............... .. 0.0000000000E+0014 . Max. tip angle (deg)................. 2.4400000000E+0115 . Tip angle: Scale Division (deg)...... 5.0000000000E+0016 . Min. wind speed (m/sec).............. 0.0000000000E+0017 . Max. wind speed (m/sec).............. 0.0000000000E+0018 . Wind speed: Scale Division (m/sec)... 0.0000000000E+0019 . Min. rotational speed limit (rpm).... 5.3000000000E+0020 . Max. rotational speed limit (rpm).... 2.2500000000E+0121 . Rotational speed: Scale Division(rpm) 4.0000000000E+0022 . Hub height (mm)...................... 8.0000000000E+0423 . Nominal electrical output (kW)....... 1.5000000000E+0324 . Electrical efficiency....... ......... 8.0000000000E-0125 . Angle of attack at max. lift (deg)... 9.0000000000E+00

    26 . Max. lift coefficient (CL)........... 1.4899000000E+0027 . Max. drag coefficient (CD)........... 1.0000000000E+0028 . Dist. from R_Cent. to the bearing(mm) 0.0000000000E+0029 . Dist. from R_Cent. to the T_Cent.(mm) 0.0000000000E+0030 . Dist. from R_Cent. to the B_Cent.(mm) 0.0000000000E+0031 . Maximum yawing speed (rad/sec)....... 1.0000000000E+0032 . Nominal yawing speed (rad/sec)....... 1.7453000000E-0233 . Max. rotational speed ( % of nominal) 1.0000000000E+0234 . Rated wind speed (m/sec)............. 7.5000000000E+0035 . Mass of blade (kg) [calculated if 0 ] 0.0000000000E+0036 . Mass of rotor (kg) [calculated if 0 ] 0.0000000000E+0037 . Blade load factor, kbx............. .. 1.0000000000E+0038 . Blade load factor, kby .............. 1.0000000000E+0039 . Rotor load factor, kx................ 1.7000000000E+0040 . Rotor load factor, ky................ 1.0000000000E+0041 . Rotor load factor, kz................ 1.7000000000E+00

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    D. Power curves Excel table

    a) Without control

    b) Stall control

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    c) Pitch control

    E. Carbon fibre properties

    We were given the following data by You Chang Carbon co., Ltdhttp://www.yccarbon.com/This material has been tested in accordance with the Pyrofil test methods and confirms to therequirements of specification reference.

    Fiber type: 3KNumber of Filament: 3000Yield tex: 200Tensile strength (MPa): 3920Tensile strength (kgf/m m2): 400

    Tensile modulus (GPa): 235Tensile modulus (tonf/mm2): 24.0Elongation (%): 1.7Filament dia. (micrometer): 7.0Density (g/cm3): 1.76Electrical resistivity (ohm.cm) : 1.5 x 10-3

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    After weeks of research I decided to locate my internship in the North of Denmark, ina None Governmental Organization called Nordic Folkecenter for Renewable Energy. Their

    focus is on renewable energies both technical and social aspects. I was given the project ofdesigning a 38 meters wind turbine blade with two other Erasmus student from theIngenirhjskolen i rhus. We did researches together in order to know more about blades,and how to design ours, which parameters to set up, materials to use. We designed our bladefrom two different programmes. The main one we used is called Wind Rotor Design System.This programme was created at Folkecenter and because it is quite an old one its working ononly one computer. We could get access on the second programme, in rhus. Thisprogramme mainly helped us to get the dimension of the blade (radius, chord, twist angle).We used then WRD to make power calculations and load calculations.

    We had opportunities to visit real wind turbines so it helped us a lot to understand

    whats happening on the blade. This internship was incredibly enriching for me, I coulddiscover a new country, where there are wind turbines everywhere. I could also work inEnglish, for the very first time.