theory of wind turbine design

7/21/2019 Theory of Wind Turbine Design

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(01 ) An example of awind turbine,this 3 bladed turbine is the classic design of modern wind turbines

(02) Wind turbine components : 1-Foundation,2-Connection to the electric grid,3-Tower,4-Access

ladder, 5-Wind orientation control (Yaw control),6-Nacelle,7-Generator,8-Anemometer,9-Electricor

MechanicalBrake, 10-Gearbox,11-Rotor blade,12-Blade pitch control,13-Rotor hub.

Design specification

Thedesign specificationfor a wind-turbine will contain a power curve and guaranteedavailability.With the data from thewind resource assessmentit is possible to calculatecommercial viability.[1]The typicaloperating temperaturerange is 20 to 40C (4 to 104F).In areas with extreme climate (likeInner MongoliaorRajasthan)specific cold and hot weatherversions are required.

Wind turbines can be designed and validated according toIEC 61400standards.[2]

Low temperature

Utility-scale wind turbine generators have minimum temperature operating limits which apply inareas that experience temperatures below20 C. Wind turbines must be protected from iceaccumulation. It can makeanemometerreadings inaccurate and which can cause high structureloads and damage. Some turbine manufacturers offer low-temperature packages at a few percentextra cost, which include internal heaters, different lubricants, and different alloys for structuralelements. If the low-temperature interval is combined with a low-wind condition, the windturbine will require an external supply of power, equivalent to a few percent of its rated power,for internal heating. For example, theSt. Leon, Manitobaproject has a total rating of 99 MW andis estimated to need up to 3 MW (around 3% of capacity) of station service power a few days ayear for temperatures down to30 C. This factor affects the economics of wind turbine

operation in cold climates.

Aerodynamics

Main article:Wind turbine aerodynamics
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Wind rotor profile

The aerodynamics of a horizontal-axis wind turbine are not straightforward. The air flow at theblades is not the same as the airflow far away from the turbine. The very nature of the way inwhich energy is extracted from the air also causes air to be deflected by the turbine. In additionthe aerodynamics of a wind turbine at the rotor surface exhibit phenomena that are rarely seen inother aerodynamic fields.

In 1919 the physicistAlbert Betzshowed that for a hypothetical ideal wind-energy extractionmachine, the fundamental laws of conservation of mass and energy allowed no more than 16/27(59.3%) of the kinetic energy of the wind to be captured. ThisBetz' lawlimit can be approachedby modern turbine designs which may reach 70 to 80% of this theoretical limit.

Power control

A wind turbine is designed to produce a maximum of power at wide spectrum of wind speeds.All wind turbines are designed for a maximum wind speed, called the survival speed, abovewhich they do not survive. The survival speed of commercial wind turbines is in the range of40 m/s (144 km/h, 89 MPH) to 72 m/s (259 km/h, 161 MPH). The most common survival speed

is 60 m/s (216 km/h, 134 MPH). The wind turbines have three modes of operation:

Below rated wind speed operation Around rated wind speed operation (usually atnameplate capacity) Above rated wind speed operation

If the rated wind speed is exceeded the power has to be limited. There are various ways toachieve this.

A control system involves three basic elements: sensors to measure process variables, actuatorsto manipulate energy capture and component loading, and control algorithms to coordinate the

actuators based on information gathered by the sensors.[3]

Stall

Stalling works by increasing the angle at which the relative wind strikes the blades (angle ofattack), and it reduces the induced drag (dragassociated withlift). Stalling is simple because itcan be made to happen passively (it increases automatically when the winds speed up), but it
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increases the cross-section of the blade face-on to the wind, and thus the ordinary drag. A fullystalled turbine blade, when stopped, has the flat side of the blade facing directly into the wind.

A fixed-speed HAWT inherently increases its angle of attack at higher wind speed as the bladesspeed up. A natural strategy, then, is to allow the blade to stall when the wind speed increases.

This technique was successfully used on many early HAWTs. However, on some of these bladesets, it was observed that the degree of blade pitch tended to increase audible noise levels.

Vortex generators may be used to control the lift characteristics of the blade. The VGs are placedon the airfoil to enhance the lift if they are placed on the lower (flatter) surface or limit themaximum lift if placed on the upper (higher camber) surface.[4]

Pitch control

Furlingworks by decreasing the angle of attack, which reduces the induced drag from the lift ofthe rotor, as well as the cross-section. One major problem in designing wind turbines is getting

the blades to stall orfurlquickly enough should a gust of wind cause sudden acceleration. Afully furled turbine blade, when stopped, has the edge of the blade facing into the wind.

Loads can be reduced by making a structural system softer or more flexible.[3]This could beaccomplished with downwind rotors or with curved blades that twist naturally to reduce angle ofattack at higher wind speeds. These systems will be nonlinear and will couple the structure to theflow field - thus, design tools must evolve to model these nonlinearities.

Standard modern turbines all furl the blades in high winds. Since furling requires acting againstthe torque on the blade, it requires some form of pitch angle control, which is achieved with aslewing drive.This drive precisely angles the blade while withstanding high torque loads. In

addition, many turbines use hydraulic systems. These systems are usually spring-loaded, so thatif hydraulic power fails, the blades automatically furl. Other turbines use an electric servomotorfor every rotor blade. They have a small battery-reserve in case of an electric-grid breakdown.Small wind turbines (under 50 kW) with variable-pitchinggenerally use systems operated bycentrifugal force, either by flyweights or geometric design, and employ no electric or hydrauliccontrols.

Fundamental gaps exist in pitch control, limiting the reduction of energy costs, according to areport from a coalition of researchers from universities, industry, and government, supported bytheAtkinson Center for a Sustainable Future.Load reduction is currently focused on full-spanblade pitch control, since individual pitch motors are the actuators currently available on

commercial turbines. Significant load mitigation has been demonstrated in simulations forblades, tower, and drive train. However, there is still research needed, the methods for realizationof full-span blade pitch control need to be developed in order to increase energy capture andmitigate fatigue loads.

A control technique applied to the pitch angle is done by comparing the current active power ofthe engine with the value of active power at the rated engine speed (active power reference, Psreference). Control of the pitch angle in this case is done with a PI controller controls. However,
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in order to have a realistic response to the control system of the pitch angle, the actuator uses thetime constant Tservo, an integrator and limiters so as the pitch angle to be from 0 to 30 with arate of change ( 10 per sec).

Pitch Controller

From the figure at the right, the reference pitch angle is compared with the actual pitch angle band then the error is corrected by the actuator. The reference pitch angle, which comes from thePI controller, goes through a limiter. Restrictions on limits are very important to maintain thepitch angle in real terms. Also, limiting the rate of change is very important especially duringfaults in the network. The importance is due to the fact that the controller decides how quickly itcan reduce the aerodynamic energy to avoid acceleration during errors.

[3]

Other controls

Generator torque

Modern large wind turbines are variable-speed machines. When the wind speed is below rated,generator torque is used to control the rotor speed in order to capture as much power as possible.The most power is captured when thetip speed ratiois held constant at its optimum value(typically 6 or 7). This means that as wind speed increases, rotor speed should increase

proportionally. The difference between the aerodynamic torque captured by the blades and theapplied generator torque controls the rotor speed. If the generator torque is lower, the rotoraccelerates, and if the generator torque is higher, the rotor slows down. In below rated windspeeds, the generator torque control is active while the blade pitch is typically held at theconstant angle that captures the most power, fairly flat to the wind. In above rated wind speeds,the generator torque is typically held constant while the blade pitch is active.

One example of a technique so as to control a permanent magnet synchronous motor is by usingtheField Oriented Control.Field Oriented Control is a close loop strategy and is composed bytwo current controllers (we have an inner loop and outer loop control design/cascade design)necessary for controlling the torque and one speed controller.

Constant torque angle control

In this control strategy the d axis current is kept zero, while the vector current is align with the qaxis in order to maintain the torque angle equal with 90o. This is one of the most used controlstrategy because of the simplicity, by controlling only the Iqs current. So, now theelectromagnetic torque equation of the permanent magnet synchronous generator is simply alinear equation depend on the Iqs current only.
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So, the electromagnetic torque for Ids = 0 (we can achieve that with the d-axis controller) is now:

Te= 3/2 p (pmIqs+ (Lds-Lqs) IdsIqs)= 3/2 p pmIqs

Machine Side Controller Design

So, the complete system of the machine side converter and the cascaded PI controller loops isgiven by the figure in the right. In that we have the control inputs, which are the duty rations mds

and mqs, of the PWM-regulated converter. Also, we can see the control scheme for the windturbine in the machine side and simultaneously how we keep the Idszero (the electromagnetictorque equation is linear).

Yawing

Percent output vs. wind angle

Modern large wind turbines are typically actively controlled to face the wind direction measuredby awind vanesituated on the back of thenacelle.By minimizing the yaw angle (themisalignment between wind and turbine pointing direction), the power output is maximized andnon-symmetrical loads minimized. However, since the wind direction varies quickly the turbinewill not strictly follow the direction and will have a small yaw angle on average. The poweroutput losses can simply be approximated to fall with (cos(yaw angle))3. Particularly at low-to-medium wind speeds, yawing can make a significant reduction in turbine output, with winddirection variations of 30 being quite common and long response times of the turbines tochanges in wind direction. At high wind speeds, the wind direction is less variable.
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Electrical braking

2kW Dynamic braking resistor for small wind turbine.

Braking of a small wind turbine can also be done by dumping energy from the generator into aresistorbank, converting the kinetic energy of the turbine rotation into heat. This method isuseful if the kinetic load on the generator is suddenly reduced or is too small to keep the turbinespeed within its allowed limit.

Cyclically braking causes the blades to slow down, which increases the stalling effect, reducingthe efficiency of the blades. This way, the turbine's rotation can be kept at a safe speed in fasterwinds while maintaining (nominal) power output. This method is usually not applied on large

grid-connected wind turbines.

Mechanical braking

A mechanicaldrum brakeordisk brakeis used to stop turbine in emergency situation such asextreme gust events or over speed. This brake is also used to hold the turbine at rest formaintenance as a secondary mean, primarily mean being the rotor lock system. Such brakes areusually applied only after blade furling and electromagnetic braking have reduced the turbinespeed generally 1 or 2 rotor RPM, as the mechanical brakes can create a fire inside the nacelle ifused to stop the turbine from full speed. Also the load on turbine increases if brake is applied onrated RPM. These kind of mechanical brake are driven by hydraulic systems and connected to

main control box.

Turbine size
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Figure 1. Flow diagram for wind turbine plant

There are different size classes of wind turbines. The smallest having power production less than10 kW are used in homes, farms and remote applications whereas intermediate wind turbines(10-250 kW ) are useful for village power,hybrid systemsanddistributed power.The largestwind turbines (660 kW2+MW) are used in central stationwind farms,distributed power andcommunity wind.[5]

A person standing beside 15 m long blades.

For a given survivable wind speed, the mass of a turbine is approximately proportional to thecube of its blade-length. Wind power intercepted by the turbine is proportional to the square ofits blade-length.[6]The maximum blade-length of a turbine is limited by both the strength andstiffness of its material.

Labor and maintenance costs increase only gradually with increasing turbine size, so to minimizecosts, wind farm turbines are basically limited by the strength of materials, and sitingrequirements.

Typical modern wind turbines have diameters of 40 to 90 metres (130 to 300 ft) and are ratedbetween 500 kW and 2 MW. As of 2011 the most powerful turbineEnercon E-126is rated at 7.5MW.[7]

Generator
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Gearbox,rotor shaft and brake assembly

For large, commercial size horizontal-axis wind turbines, thegenerator[8]is mounted in anacelleat the top of a tower, behind the hub of the turbine rotor. Typically wind turbines generateelectricity throughasynchronous machinesthat are directly connected with the electricity grid.Usually the rotational speed of the wind turbine is slower than the equivalent rotation speed of

the electrical networktypical rotation speeds for wind generators are 520 rpm while a directlyconnected machine will have an electrical speed between 750-3600 rpm. Therefore, a gearbox isinserted between the rotor hub and the generator. This also reduces the generator cost andweight.

In conventional wind turbines, the blades spin a shaft that is connected through a gearbox to thegenerator. The gearbox converts the turning speed of the blades 15 to 20 rotations per minute fora large, one-megawatt turbine into the faster 1,800 rotations per minute that the generator needsto generate electricity.[9]

Analysts from GlobalData estimate that gearbox market grows from $3.2bn in 2006 to $6.9bn in2011, and to $8.1bn by 2020. Market leaders wereWinergyin 2011.[10]

Older style wind generators rotate at a constant speed, to matchpower line frequency,whichallowed the use of less costly induction generators[citation needed]. Newer wind turbines often turn atwhatever speed generates electricity most efficiently. The varying output frequency and voltagecan be matched to the fixed values of the grid using multiple technologies such asdoubly fedinduction generatorsor full-effect converters where the variable frequency current produced isconverted to DC and then back to AC. Although such alternatives require costly equipment andcause power loss, the turbine can capture a significantly larger fraction of the wind energy. Insome cases, especially when turbines are sited offshore, the DC energy will be transmitted fromthe turbine to a central (onshore)inverterfor connection to the grid.

Gearless wind turbine

Commercial size generators have a rotor carrying a field winding so that a rotatingmagneticfieldis produced inside a set of windings called thestator.While the rotating field windingconsumes a fraction of a percent of the generator output, adjustment of the field current allowsgood control over the generator output voltage.Enerconand EWT (Formerly known asLagerwey) have produced gearless wind turbines with separately electrically excited generators
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for many years,[11]andSiemensproduces a gearless "inverted generator" 3 MW model[12][13]while developing a 6 MW model.[14]

Gearless wind turbines (often also calleddirect drive)get rid of the gearbox completely. Instead,the rotor shaft is attached directly to the generator, which spins at the same speed as the blades.

In a turbine generator, magnets spin around a coil to produce current. The faster the magnetsspin, the more current is induced in the coil. To make up for a direct drive generator's slowerspinning rate, the diameter of the generator'srotoris increased hence containing more magnetswhich lets it create a lot of power when turning slowly. To reduce the generator weight someconstructors usepermanent magnets(PM) in the generators' rotor, while conventional turbinegenerators useelectromagnetsfed with electricity from the generator itself. To enhance theircompetitiveness, the design of smaller generators with improved torque is still an active researcharea.

Gearless wind turbines are often heavier than gear based wind turbines. A study by theEUcalled

"Reliawind"

[15]

based on the largest sample size of turbines, has shown that the reliability ofgearboxes is not the main problem in wind turbines. The reliability of direct drive turbinesoffshore is still not known, since the sample size is so small.

Experts fromTechnical University of Denmarkestimate that a geared generator with permanentmagnets may use 25 kg/MW of therare earth elementNeodymium,while a gearless may use250 kg/MW.[16]

In December 2011, theUS Department of Energypublished a report stating critical shortage ofrare earth elements such as neodymium used in large quantities for permanent magnets ingearless wind turbines. China produces more than 95% of rare earth elements, whileHitachi

holds more than 600 patents coveringNeodymium magnets.Direct-drive turbines require 600 kgof PM material per megawatt, which translates to several hundred kilograms of rare earth contentper megawatt, as neodymium content is estimated to be 31% of magnet weight. Hybriddrivetrains (intermediate between direct drive and traditional geared) use significantly less rareearth materials. While PM wind turbines only account for about 5% of the market outside ofChina, their market share inside of China is estimated at 25% or higher. Demand for neodymiumin wind turbines is estimated to be 1/5 of that in electric vehicles.[17]

Blades

Blade design
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Unpainted tip of a blade

The ratio between the speed of thebladetips and the speed of the wind is calledtip speed ratio.

High efficiency 3-blade-turbines have tip speed/wind speed ratios of 6 to 7. Modern windturbines are designed to spin at varying speeds (a consequence of their generator design, seeabove). Use ofaluminumandcomposite materialsin their blades has contributed to lowrotational inertia,which means that newer wind turbines can accelerate quickly if the winds pickup, keeping the tip speed ratio more nearly constant. Operating closer to their optimal tip speedratio during energetic gusts of wind allows wind turbines to improve energy capture from suddengusts that are typical in urban settings.

In contrast, older style wind turbines were designed with heavier steel blades, which have higherinertia, and rotated at speeds governed by the AC frequency of the power lines. The high inertiabuffered the changes in rotation speed and thus made power output more stable.

The speed and torque at which a wind turbine rotates must be controlled for several reasons:

To optimize the aerodynamic efficiency of the rotor in light winds. To keep the generator within its speed and torque limits. To keep the rotor and hub within their centrifugal force limits. The centrifugal force from

the spinning rotors increases as the square of the rotation speed, which makes thisstructure sensitive to overspeed.

To keep the rotor and tower within their strength limits. Because the power of the windincreases as the cube of the wind speed, turbines have to be built to survive much higherwind loads (such as gusts of wind) than those from which they can practically generate

power. Since the blades generate moretorsionaland vertical forces (putting far greaterstress on the tower and nacelle due to the tendency of the rotor toprecessandnutate)when they are producing torque, most wind turbines have ways of reducing torque inhigh winds.

To enable maintenance. Since it is dangerous to have people working on a wind turbinewhile it is active, it is sometimes necessary to bring a turbine to a full stop.

To reduce noise. As a rule of thumb, the noise from a wind turbine increases with thefifth power of the relative wind speed (as seen from the moving tip of the blades). In
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noise-sensitive environments, the tip speed can be limited to approximately 60 m/s(200 ft/s).

It is generally understood that noise increases with higher blade tip speeds. To increase tip speedwithout increasing noise would allow reduction the torque into the gearbox and generator and

reduce overall structural loads, thereby reducing cost.

[3]

The reduction of noise is linked to thedetailed aerodynamics of the blades, especially factors that reduce abrupt stalling. The inabilityto predict stall restricts the development of aggressive aerodynamic concepts. .[3]

A blade can have alift-to-drag ratioof 120,[18]compared to 70 for asailplaneand 15 for anairliner.[19]

The hub

A Wind turbine hub being installed

In simple designs, the blades are directly bolted to the hub and hence are stalled. In other moresophisticated designs, they are bolted to the pitch mechanism, which adjusts theirangle of attackaccording to the wind speed to control their rotational speed. The pitch mechanism is itselfbolted to the hub. The hub is fixed to the rotor shaft which drives the generator through agearbox. Direct drive wind turbines (also called gearless) are constructed without a gearbox.Instead, the rotor shaft is attached directly to the generator, which spins at the same speed as theblades.

Blade count

This section includes alist of references,related reading orexternal links,but the sources

of this section remain unclear because it lacksinline citations.Pleaseimprovethisarticle by introducing more precise citations. (August 2012)
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The 98 meter diameter, two-bladed NASA/DOEMod-5Bwind turbine was the largest operatingwind turbine in the world in the early 1990s

The NASA test of a one-bladed wind turbine rotor configuration at Plum Brook Station nearSandusky, Ohio

The determination of the number of blades involves design considerations of aerodynamicefficiency, component costs, and system reliability. Noise emissions are affected by the locationof the blades upwind or downwind of the tower and the speed of the rotor. Given that the noiseemissions from the blades' trailing edges and tips vary by the 5th power of blade speed, a smallincrease in tip speed can make a large difference.

Wind turbines developed over the last 50 years have almost universally used either two or threeblades. However, there are patents that present designs with additional blades, such as ChanShin's Multi-unit rotor blade system integrated wind turbine.[20]Aerodynamic efficiencyincreases with number of blades but with diminishing return. Increasing the number of bladesfrom one to two yields a six percent increase in aerodynamic efficiency, whereas increasing theblade count from two to three yields only an additional three percent in efficiency.[21]Further
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increasing the blade count yields minimal improvements in aerodynamic efficiency andsacrifices too much in blade stiffness as the blades become thinner.[citation needed]

Theoretically, an infinite number of blades of zero width is the most efficient, operating at a highvalue of the tip speed ratio. But other considerations lead to a compromise of only a few

blades.

[22]

Component costs that are affected by blade count are primarily for materials and manufacturingof the turbine rotor and drive train. Generally, the fewer the number of blades, the lower thematerial and manufacturing costs will be. In addition, the fewer the number of blades, the higherthe rotational speed can be. This is because blade stiffness requirements to avoid interferencewith the tower limit how thin the blades can be manufactured, but only for upwind machines;deflection of blades in a downwind machine results in increased tower clearance. Fewer bladeswith higher rotational speeds reduce peak torques in the drive train, resulting in lower gearboxand generator costs.

System reliability is affected by blade count primarily through the dynamic loading of the rotorinto the drive train and tower systems. While aligning the wind turbine to changes in winddirection (yawing), each blade experiences a cyclic load at its root end depending on bladeposition. This is true of one, two, three blades or more. However, these cyclic loads whencombined together at the drive train shaft are symmetrically balanced for three blades, yieldingsmoother operation during turbine yaw. Turbines with one or two blades can use a pivotingteetered hub to also nearly eliminate the cyclic loads into the drive shaft and system duringyawing. A Chinese 3.6 MW two-blade is being tested in Denmark.[23]Mingyangwon a bid for87 MW (29 * 3 MW) two-bladed offshore wind turbines near Zhuhai in 2013.[24][25][26]

Finally, aesthetics can be considered a factor in that some people find that the three-bladed rotor

is more pleasing to look at than a one- or two-bladed rotor.

Blade materials

Wood and canvas sails were used on early windmills due to their low price, availability, and easeof manufacture. Smaller blades can be made from light metals such asaluminium.Thesematerials, however, require frequent maintenance. Wood and canvas construction limits theairfoilshape to a flat plate, which has a relatively high ratio of drag to force captured (lowaerodynamic efficiency) compared to solid airfoils. Construction of solid airfoil designs requiresinflexible materials such as metals orcomposites.Some blades also have incorporated lightningconductors.

New wind turbine designs push power generation from the singlemegawattrange to upwards of10 megawatts using larger and larger blades. A larger area effectively increases the tip-speedratio of a turbine at a given wind speed, thus increasing its energy extraction.[27]Computer-aidedengineeringsoftware such asHyperSizer(originally developed for spacecraft design) can beused to improve blade design.[28][29]
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As of 2013, production wind turbine blades are as large as 120 meters in diameter[30][31][32][33]with prototypes reaching 160 meters.[34][35][36][37]In 2001, an estimated 50 million kilograms offibreglasslaminate were used in wind turbine blades.[38]

An important goal of larger blade systems is to control blade weight. Since blade mass scales as

the cube of the turbine radius, loading due to gravity constrains systems with larger blades.

[39]

Gravitational loads include axial and tensile/ compressive loads (top/bottom of rotation) as wellas bending (lateral positions). The magnitude of these loads fluctuates cyclically and theedgewise moments (see below) are reversed every 180 of rotation. Typical rotor speeds anddesign life are ~10rpm and 20 years, respectively, with the number of lifetime revolutions on theorder of 10^8. Considering wind, it is expected that turbine blades go through ~10^9 loadingcycles. Wind is another source of rotor blade loading. Lift causes bending in the flapwisedirection (out of rotor plane) while air flow around the blade cause edgewise bending (in therotor plane). Flapwise bending involves tension on the pressure (upwind) side and compressionon the suction (downwind) side. Edgewise bending involves tension on the leading edge andcompression on the trailing edge.

Wind loads are cyclical because of natural variability in wind speed and wind shear (higherspeeds at top of rotation).

Failure in ultimate loading of wind-turbine rotor blades exposed to wind and gravity loading is afailure mode that needs to be considered when the rotor blades are designed. The wind speed thatcauses bending of the rotor blades exhibits a natural variability, and so does the stress responsein the rotor blades. Also, the resistance of the rotor blades, in terms of their tensile strengths,exhibits a natural variability.[40]

Fiberglass-reinforcedepoxyblades of Siemens SWT-2.3-101 wind turbines. The blade size of 49meters[41]is in comparison to asubstationbehind them atWolfe Island Wind Farm.

Manufacturing blades in the 40 to 50 metre range involves proven fibreglass compositefabrication techniques. Manufactures such asNordexandGE Winduse an infusion process.Other manufacturers use variations on this technique, some includingcarbonandwoodwithfibreglass in anepoxymatrix. Options also include prepreg fibreglass and vacuum-assisted resintransfer molding. Each of these options use a glass-fibre reinforcedpolymercompositeconstructed with differing complexity. Perhaps the largest issue with more simplistic, open-mould, wet systems are the emissions associated with the volatile organics released.
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Preimpregnated materials and resin infusion techniques avoid the release of volatiles bycontaining all reaction gases. However, these contained processes have their own challenges,namely the production of thick laminates necessary for structural components becomes moredifficult. As the preform resin permeability dictates the maximum laminate thickness, bleeding isrequired to eliminate voids and insure proper resin distribution.[38]One solution to resin

distribution a partially preimpregnated fibreglass. During evacuation, the dry fabric provides apath for airflow and, once heat and pressure are applied, resin may flow into the dry regionresulting in a thoroughly impregnated laminate structure.[38]

Epoxy-based composites have environmental, production, and cost advantages over other resinsystems. Epoxies also allow shorter cure cycles, increased durability, and improved surfacefinish. Prepreg operations further reduce processing time over wet lay-up systems. As turbineblades pass 60 metres, infusion techniques become more prevalent; the traditional resin transfermoulding injection time is too long as compared to the resin set-up time, limiting laminatethickness. Injection forces resin through a thicker ply stack, thus depositing the resin where inthe laminate structure before gelatin occurs. Specialized epoxy resins have been developed to

customize lifetimes and viscosity.

[42]

Carbon fibre-reinforced load-bearing spars can reduce weight and increase stiffness. Usingcarbon fibres in 60 metre turbine blades is estimated to reduce total blade mass by 38% anddecrease cost by 14% compared to 100% fibreglass. Carbon fibres have the added benefit ofreducing the thickness of fiberglass laminate sections, further addressing the problems associatedwith resin wetting of thick lay-up sections. Wind turbines may also benefit from the generaltrend of increasing use and decreasing cost of carbon fibre materials.[38]

Tower

External images

Tower production

Typically, 2 types of towers exist:floating towersand land-based towers.

Tower height

Wind velocities increase at higher altitudes due tosurface aerodynamic drag(by land or water

surfaces) and the viscosity of the air. The variation in velocity with altitude, calledwind shear,ismost dramatic near the surface.

Typically, in daytime the variation follows thewind profile power law,which predicts that windspeed rises proportionally to the seventh root of altitude. Doubling the altitude of a turbine, then,increases the expected wind speeds by 10% and the expected power by 34%. To avoidbuckling,doubling the tower height generally requires doubling the diameter of the tower as well,increasing the amount of material by a factor of at least four.
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At night time, or when the atmosphere becomes stable,wind speed close to the ground usuallysubsides whereas at turbine hub altitude it does not decrease that much or may even increase. Asa result the wind speed is higher and a turbine will produce more power than expected from the1/7 power law: doubling the altitude may increase wind speed by 20% to 60%. A stableatmosphere is caused by radiative cooling of the surface and is common in a temperate climate: it

usually occurs when there is a (partly) clear sky at night. When the (high altitude) wind is strong(a 10-meter wind speed higher than approximately 6 to 7 m/s the stable atmosphere is disruptedbecause of friction turbulence and the atmosphere will turn neutral. A daytime atmosphere iseither neutral (no net radiation; usually with strong winds and heavy clouding) or unstable(rising air because of ground heatingby the sun). Here again the 1/7 power law applies or is atleast a good approximation of the wind profile.Indianahad been rated as having a wind capacityof 30,000 MW, but by raising the expected turbine height from 50 m to 70 m, the wind capacityestimate was raised to 40,000 MW, and could be double that at 100 m.[43]

ForHAWTs,tower heights approximately two to three times the blade length have been found tobalance material costs of the tower against better utilisation of the more expensive active

components.

Sections of a wind turbine tower, transported in abulk carriership

Road size restrictions makes transportation of towers with a diameter of more than 4.3 mdifficult. Swedish analyses show that it is important to have the bottom wing tip at least 30 mabove the tree tops, but a taller tower requires a larger tower diameter.[44]A 3 MW turbine mayincrease output from 5,000 MWh to 7,700 MWh per year by going from 80 to 125 meter towerheight.[45]A tower profile made of connected shells rather than cylinders can have a largerdiameter and still be transportable. A 100 m prototype tower withTC bolted18mm 'plank' shellshas been erected at the wind turbine test center Hvsre in Denmark and certified byDet NorskeVeritas,with aSiemensnacelle. Shell elements can be shipped in standard 12 mshippingcontainers,[44][46]and 2 towers per week are produced this way.[47]

Woodis being investigated as a material for wind turbine towers, and a 100 metre tall towersupporting a 1.5 MW turbine has been erected in Germany. The wood tower shares the sametransportation benefits of the segmented steel shell tower, but without the steel resourceconsumption.[48][49]
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Connection to the electric grid

All grid-connected wind turbines, from the first one in 1939 until the development of variable-speed grid-connected wind turbines in the 1970s, were fixed-speed wind turbines. As of 2003,nearly all grid-connected wind turbines operate at exactly constant speed (synchronous

generators) or within a few percent of constant speed (induction generators).[50][51]As of 2011,many operational wind turbines used fixed speed induction generators (FSIG).[52]As of 2011,most new grid-connected wind turbines arevariable speed wind turbinesthey are in somevariable speed configuration.[52]

Early wind turbine control systems were designed for peak power extraction, also calledmaximum power point trackingthey attempt to pull the maximum possible electrical powerfrom a given wind turbine under the current wind conditions.[53]More recent wind turbinecontrol systems deliberately pull less electrical power than they possibly could in mostcircumstances, in order to provide other benefits, which include:

spinning reservesto quickly produce more power when neededsuch as when someother generator suddenly drops from the gridup to the max power supported by thecurrent wind conditions.[54]

Variable-speed wind turbines can (very briefly) produce more power than the currentwind conditions can support, by storing some wind energy as kinetic energy (acceleratingduring brief gusts of faster wind) and later converting that kinetic energy to electricenergy (decelerating, either when more power is needed elsewhere, or during short lullsin the wind, or both).[55][56]

damping (electrical) subsynchronous resonances in the grid[57]

damping (mechanical) resonances in the tower[58][59]

The generator in a wind turbine producesalternating current(AC) electricity. Some turbinesdrive anAC/AC converterwhich converts the AC todirect current(DC) with arectifierandthen back to AC with aninverterin order to match the frequency and phase of the grid.However, the most common method in large modern turbines is to instead use adoubly fedinduction generatordirectly connected to theelectricity grid.

A useful technique so as to connect a wind turbine with a permanent magnet synchronousgenerator to the grid is by using a back-to-back converter. Also, we can have control schemes soas to achieve unitypower factorin the connection to the grid. In that way the wind turbine willnot consume reactive power, which is the most common problem with wind turbines that useinduction machines. This leads to a more stable power system. Moreover, with different control

schemes a wind turbine with a permanent magnet synchronous generator can provide or consumereactive power. So, it can work as adynamic capacitor/inductorbank so as to help with thepower systems' stability.
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Grid Side Controller Design

Below we show the control scheme so as to achieve unity power factor :

Reactive powerregulation consists of onePI controllerin order to achieve operation with unitypower factor (i.e. Qgrid= 0 ). It is obvious that IdNhas to be regulated to reach zero at steady-state(IdNref= 0).

We can see the complete system of the grid side converter and the cascaded PI controller loops

in the figure in the right.

Foundations

Wind turbine foundations

Wind turbines, by their nature, are very tall slender structures,[60]this can cause a number ofissues when the structural design of thefoundationsare considered.

The foundations for a conventionalengineering structureare designed mainly to transfer theverticalload(dead weight) to the ground, this generally allows for a comparativelyunsophisticated arrangement to be used. However in the case of wind turbines, due to the highwind and environmental loads experienced there is a significant horizontal dynamic load thatneeds to be appropriately restrained.

This loading regime causes largemoment loadsto be applied to the foundations of a windturbine. As a result, considerable attention needs to be given when designing the footings toensure that the turbines are sufficiently restrained to operate efficiently.[61]In the currentDetNorske Veritas(DNV) guidelines for the design of wind turbines the angular deflection of the
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foundations are limited to 0.5.[62]DNV guidelines regardingearthquakessuggest that horizontalloads are larger than vertical loads for offshore wind turbines, while guidelines fortsunamisonlysuggest designing for maximum sea waves.[63]In contrast, IEC suggests considering tsunamiloads.[2]

Scale modeltests using a 50gcentrifugeare being performed at theTechnical University ofDenmarkto testmonopile foundationsfor offshore wind turbines at 3050 m water depth.[64]

Costs

LiftraBlade Dragoninstalling a single blade on wind turbine hub.[65][66]

The modern wind turbine is a complex and integrated system. Structural elements comprise themajority of the weight and cost. All parts of the structure must be inexpensive, lightweight,durable, and manufacturable, under variable loading and environmental conditions. Turbinesystems that have fewer failures, require less maintenance, are lighter and last longer will lead toreducing the cost of wind energy.

One way to achieve this is to implement well-documented, validated analysis codes, according toa 2011 report from a coalition of researchers from universities, industry, and government,supported by theAtkinson Center for a Sustainable Future.[3]

The major parts of a modern turbine may cost (percentage of total) : tower 22%, blades 18%,gearbox 14%, generator 8%.[67][68]

Efficiency of wind turbines

A wind turbine is an exclusive form ofturbo machinewhich often operates at relatively lowerspeeds. Efficiency of wind power pl

theory of wind turbine design

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