Examine individual changes

'[[File:Windpark Berching01 verkleinert.jpg|thumb|An example of a [[wind turbine]], this 3 bladed turbine is the classic design of modern wind turbines]] [[File:Wind turbine int.svg|thumb| Wind turbine components : 1-[[Wind turbine design#Foundations|Foundation]], 2-[[Wind turbine design#Connection to the electric grid|Connection to the electric grid]], 3-[[Wind turbine design#Tower|Tower]], 4-Access ladder, 5-[[Wind turbine design#Yawing|Wind orientation control (Yaw control)]], 6-[[Nacelle (wind turbine)|Nacelle]], 7-[[Wind turbine design#Generator|Generator]], 8-[[Anemometer]], 9-[[Wind turbine design#Electrical braking|Electric]] or [[Wind turbine design#Mechanical braking|Mechanical]] Brake, 10-[[Gearbox]], 11-[[Wind turbine design#Blades|Rotor blade]], 12-[[Wind turbine design#Pitch control|Blade pitch control]], 13-[[Wind turbine design#The hub|Rotor hub.]]]] '''Wind turbine design''' is the process of defining the form and specifications of a [[wind turbine]] to extract energy from the [[wind]].<ref name="BERR"> {{cite web | publisher =UK Department for Business, Enterprise & Regulatory Reform | title =Efficiency and performance | url =http://www.berr.gov.uk/files/file17821.pdf | accessdate = 2007-12-29 }} </ref> A wind turbine installation consists of the necessary systems needed to capture the wind's energy, point the turbine into the wind, convert [[mechanical energy|mechanical rotation]] into [[electrical power]], and other systems to start, stop, and control the turbine. This article covers the design of [[horizontal axis wind turbine]]s (HAWT) since the majority of commercial turbines use this design. In 1919 the physicist [[Albert Betz]] showed that for a hypothetical ideal wind-energy extraction machine, 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. This [[Betz' law]] limit can be approached by modern turbine designs which may reach 70 to 80% of this theoretical limit. In addition to aerodynamic design of the blades, design of a complete wind power system must also address design of the hub, controls, generator, supporting structure and foundation. Further design questions arise when integrating wind turbines into electrical power grids. == Aerodynamics == {{main article|Wind turbine aerodynamics}} The shape and dimensions of the blades of the wind turbine are determined by the aerodynamic performance required to efficiently extract energy from the wind, and by the strength required to resist the forces on the blade. [[File:Wind rotor profile.jpg|thumb|Wind rotor profile]] The aerodynamics of a horizontal-axis wind turbine are not straightforward. The air flow at the blades is not the same as the airflow far away from the turbine. The very nature of the way in which energy is extracted from the air also causes air to be deflected by the turbine. In addition the aerodynamics of a wind turbine at the rotor surface exhibit phenomena that are rarely seen in other aerodynamic fields. In 1919 the physicist [[Albert Betz]] showed that for a hypothetical ideal wind-energy extraction machine, 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. This [[Betz' law]] limit can be approached by modern turbine designs which may reach 70 to 80% of this theoretical limit. == Power control == The speed at which a wind turbine rotates must be controlled for efficient power generation and to keep the turbine components within designed speed and torque limits. The centrifugal force on the spinning blades increases as the square of the rotation speed, which makes this structure sensitive to overspeed. Because the power of the wind increases as the cube of the wind speed, turbines have to be built to survive much higher wind loads (such as gusts of wind) than those from which they can practically generate power. Wind turbines have ways of reducing torque in high winds. A wind turbine is designed to produce power over a range of wind speeds. All wind turbines are designed for a maximum wind speed, called the survival speed, above which they will be damaged. The survival speed of commercial wind turbines is in the range of 40&nbsp;m/s (144&nbsp;km/h, 89 MPH) to 72&nbsp;m/s (259&nbsp;km/h, 161 MPH). The most common survival speed is 60&nbsp;m/s (216&nbsp;km/h, 134 MPH). If the rated wind speed is exceeded the power has to be limited. There are various ways to achieve this. A control system involves three basic elements: sensors to measure process variables, actuators to manipulate energy capture and component loading, and control algorithms to coordinate the actuators based on information gathered by the sensors.<ref name="univ2011">{{cite web |author1=Alan T. Zehnder |author2=Zellman Warhaft |lastauthoramp=yes |title=University Collaboration on Wind Energy |date= 27 July 2011 |url= http://www.sustainablefuture.cornell.edu/attachments/2011-UnivWindCollaboration.pdf |publisher= Cornell University [[Atkinson Center for a Sustainable Future]] |accessdate= 22 August 2011}}</ref> === Stall === Stalling works by increasing the angle at which the relative wind strikes the blades ([[angle of attack]]), and it reduces the induced drag ([[Aerodynamic drag|drag]] associated with [[Lift (force)|lift]]). Stalling is simple because it can be made to happen passively (it increases automatically when the winds speed up), but it increases the cross-section of the blade face-on to the wind, and thus the ordinary drag. A fully stalled turbine blade, when stopped, has the flat side of the blade facing directly into the wind. A fixed-speed HAWT (Horizontal Axis Wind Turbine) inherently increases its angle of attack at higher wind speed as the blades speed 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 blade sets, 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 placed on the airfoil to enhance the lift if they are placed on the lower (flatter) surface or limit the maximum lift if placed on the upper (higher camber) surface.<ref name=Sandia>{{cite web | last1 = Johnson | first1 = Scott J. | last2=van Dam |first2=C.P. |last3=Berg |first3=Dale E. | authorlink = | title = Active Load Control Techniques for Wind Turbines | work = | publisher = Sandia National Laboratory | year = 2008 | url = http://www.sandia.gov/wind/other/084809.pdf | doi = | accessdate =13 September 2009}}</ref> [[Reefing|Furling]] works by decreasing the angle of attack, which reduces the induced drag from the lift of the rotor, as well as the cross-section. One major problem in designing wind turbines is getting the blades to stall or [[Furl (sailing)|furl]] quickly enough should a gust of wind cause sudden acceleration. A fully 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.<ref name="univ2011"/> This could be accomplished with downwind rotors or with curved blades that twist naturally to reduce angle of attack at higher wind speeds. These systems will be nonlinear and will couple the structure to the flow field - thus, design tools must evolve to model these nonlinearities. Standard modern turbines all furl the blades in high winds. Since furling requires acting against the torque on the blade, it requires some form of pitch angle control, which is achieved with a [[slewing 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 that if hydraulic power fails, the blades automatically furl. Other turbines use an electric servomotor for every rotor blade. They have a small battery-reserve in case of an electric-grid breakdown. Small wind turbines (under 50&nbsp;kW) with variable-[[Pitching moment|pitching]] generally use systems operated by centrifugal force, either by flyweights or geometric design, and employ no electric or hydraulic controls. Fundamental gaps exist in pitch control, limiting the reduction of energy costs, according to a report from a coalition of researchers from universities, industry, and government, supported by the [[Atkinson Center for a Sustainable Future]]. Load reduction is currently focused on full-span blade pitch control, since individual pitch motors are the actuators currently available on commercial turbines. Significant load mitigation has been demonstrated in simulations for blades, tower, and drive train. However, there is still research needed, the methods for realization of full-span blade pitch control need to be developed in order to increase energy capture and mitigate fatigue loads. A control technique applied to the pitch angle is done by comparing the current active power of the engine with the value of active power at the rated engine speed (active power reference, Ps reference). Control of the pitch angle in this case is done with a PI controller controls. However, in order to have a realistic response to the control system of the pitch angle, the actuator uses the time constant Tservo, an integrator and limiters so as the pitch angle to be from 0° to 30° with a rate of change (± 10° per sec). [[File:Pitch Controller.jpg|thumb|Pitch Controller]] From the figure at the right, the reference pitch angle is compared with the actual pitch angle b and then the error is corrected by the actuator. The reference pitch angle, which comes from the PI controller, goes through a limiter. Restrictions on limits are very important to maintain the pitch angle in real term. Limiting the rate of change is very important especially during faults in the network. The importance is due to the fact that the controller decides how quickly it can reduce the aerodynamic energy to avoid acceleration during errors. <ref name="univ2011"/> == 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 the [[tip speed ratio]] is 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 the applied generator torque controls the rotor speed. If the generator torque is lower, the rotor accelerates, and if the generator torque is higher, the rotor slows down. Below rated wind speed, the generator torque control is active while the blade pitch is typically held at the constant angle that captures the most power, fairly flat to the wind. Above rated wind speed, the generator torque is typically held constant while the blade pitch is active. One technique to control a permanent magnet synchronous motor is [[Vector control (motor)|Field Oriented Control]]. Field Oriented Control is a closed loop strategy composed of two current controllers (an inner loop and outer loop 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 q axis in order to maintain the torque angle equal with 90^o. This is one of the most used control strategy because of the simplicity, by controlling only the Iqs current. So, now the electromagnetic torque equation of the permanent magnet synchronous generator is simply a linear equation depend on the Iqs current only. So, the electromagnetic torque for Ids = 0 (we can achieve that with the d-axis controller) is now: T_e= 3/2 p (λ_pm I_qs + (L_ds-L_qs) I_ds I_qs )= 3/2 p λ_pm I_qs [[File:Machine Side Controller.jpg|thumb|Machine Side Controller Design]] So, the complete system of the machine side converter and the cascaded PI controller loops is given by the figure in the right. In that we have the control inputs, which are the duty rations m_ds and m_qs, of the PWM-regulated converter. Also, we can see the control scheme for the wind turbine in the machine side and simultaneously how we keep the I_ds zero (the electromagnetic torque equation is linear). === Yawing === [[File:Yawanglecurve.jpg|thumb|Percent output vs. wind angle]] Modern large wind turbines are typically actively controlled to face the wind direction measured by a [[wind vane]] situated on the back of the [[Nacelle (wind turbine)|nacelle]]. By minimizing the yaw angle (the misalignment between wind and turbine pointing direction), the power output is maximized and non-symmetrical loads minimized. However, since the wind direction varies quickly the turbine will not strictly follow the direction and will have a small yaw angle on average. The power output losses can simply be approximated to fall with ([[Cosine|cos]](yaw angle))³. Particularly at low-to-medium wind speeds, yawing can make a significant reduction in turbine output, with wind direction variations of ±30° being quite common and long response times of the turbines to changes in wind direction. At high wind speeds, the wind direction is less variable. {{Clear}} === Electrical braking === [[File:Turbrake.jpg|thumb|upright|2kW Dynamic braking resistor for small wind turbine.]] Braking of a small wind turbine can be done by dumping energy from the generator into a [[resistor]] bank, converting the kinetic energy of the turbine rotation into heat. This method is useful if the kinetic load on the generator is suddenly reduced or is too small to keep the turbine speed within its allowed limit. Cyclically braking causes the blades to slow down, which increases the stalling effect, reducing the efficiency of the blades. This way, the turbine's rotation can be kept at a safe speed in faster winds while maintaining (nominal) power output. This method is usually not applied on large grid-connected wind turbines. === Mechanical braking === A mechanical [[drum brake]] or [[disk brake]] is used to stop turbine in emergency situation such as extreme gust events or over speed. This brake is a secondary means to hold the turbine at rest for maintenance, with a rotor lock system as primary means. Such brakes are usually applied only after blade furling and electromagnetic braking have reduced the turbine speed generally 1 or 2 rotor RPM, as the mechanical brakes can create a fire inside the nacelle if used to stop the turbine from full speed. The load on the turbine increases if the brake is applied at rated RPM. Mechanical brakes are driven by hydraulic systems and are connected to main control box. {{Clear}} == Turbine size == [[File:Flow diagram for wind turbine plant.jpg|thumb|Figure 1. Flow diagram for wind turbine plant]] There are different size classes of wind turbines. The smallest having power production less than 10&nbsp;kW are used in homes, farms and remote applications whereas intermediate wind turbines (10-250&nbsp;kW ) are useful for village power, [[hybrid systems]] and [[distributed power]]. The world's largest wind turbine, an 8-MW turbine located at the Burbo Bank Extension wind farm in [[Liverpool Bay]], [[United Kingdom|United Kingdom,]] was installed in 2016.<ref>{{Cite web|url=http://apps2.eere.energy.gov/wind/windexchange/filter_detail.asp?itemid=5827|title=WINDExchange: World's Largest Offshore Wind Turbine Installed in Liverpool Bay|website=apps2.eere.energy.gov|access-date=2017-02-27}}</ref> Utility-scale turbines (larger than one megawatt) are used in central station [[wind farm]]s, distributed power and community wind.<ref>{{Cite web|url=http://apps2.eere.energy.gov/wind/windexchange/utility-scale-wind.asp|title=WINDExchange: Utility-Scale Wind|website=apps2.eere.energy.gov|access-date=2017-02-27}}</ref> [[File:WindPropBlade.jpg|left|thumb|A person standing beside 15 m long blades.]] For a given survivable wind speed, the mass of a turbine is approximately proportional to the cube of its blade-length. Wind power intercepted by the turbine is proportional to the square of its blade-length.<ref>{{cite journal|last=Sagrillo|first=Mick|title=SMALL TURBINE COLUMN|journal=Windletter|year=2010|volume=29|issue=1|url=http://www.renewwisconsin.org/wind/Toolbox-Homeowners/Back%20to%20the%20basics%205-Collector%20Size.pdf|accessdate=19 December 2011}}</ref> The maximum blade-length of a turbine is limited by both the strength and stiffness of its material. Labor and maintenance costs increase only gradually with increasing turbine size, so to minimize costs, wind farm turbines are basically limited by the strength of materials, and siting requirements. Typical modern wind turbines have diameters of {{convert|40|to|90|m|ft}} and are rated between 500&nbsp;kW and 2 MW. As of 2014 the most powerful turbine, the [[Vestas V164|Vestas V-164]], is rated at 8 MW and has a rotor diameter of 164m.<ref>{{cite web|url=http://www.renewable-alternative.com/2014/02/vestas-worlds-biggest-wind-turbine.html/ |title=Vestas world's largest wind turbines |publisher=Renewableenergyfocus.com |date=2010-10-24 |accessdate=2013-11-06}}</ref> {{Clear}} == Nacelle == {{main article|Nacelle (wind turbine)}} The [[Nacelle (wind turbine)|nacelle]] is [[Enclosure (electrical)|housing]] the gearbox and generator connecting the tower and rotor. Sensors detect the wind speed and direction, and motors turn the nacelle into the wind to maximize output. === Gearbox === In conventional wind turbines, the blades spin a shaft that is connected through a gearbox to the generator. The gearbox converts the turning speed of the blades 15 to 20 rotations per minute for a large, one-megawatt turbine into the faster 1,800 revolutions per minute that the generator needs to generate electricity.<ref>{{Cite web|url=https://energy.gov/eere/wind/inside-wind-turbine-0|title=The Inside of a Wind Turbine|last=|first=|date=|website=U.S. Department of Energy|archive-url=|archive-date=|dead-url=|access-date=2017-02-27}}</ref> Analysts from GlobalData estimate that gearbox market grows from $3.2bn in 2006 to $6.9bn in 2011, and to $8.1bn by 2020. Market leaders were [[Winergy]] in 2011.<ref name=PT-GD>"[http://www.power-technology.com/features/feature-global-wind-energy-market-gears-growth/ The global wind energy market gears up for growth]" ''Power Technology'' / ''GlobalData'', 18 September 2013 . Accessed: 16 October 2013.</ref> The use of magnetic gearboxes has also been explored as a way of reducing wind turbine maintenance costs.<ref>{{cite web|url=http://machinedesign.com/motorsdrives/could-magnetic-gears-make-wind-turbines-say-goodbye-mechanical-gearboxes |title=Could Magnetic Gears Make Wind Turbines Say Goodbye to Mechanical Gearboxes? |publisher=machinedesign.com}}</ref> === Generator === [[File:Scout moor gearbox, rotor shaft and brake assembly.jpg|thumb|[[Gearbox]], rotor shaft and brake assembly]] For large, commercial size horizontal-axis wind turbines, the [[electrical generator]]<ref>{{cite journal | journal = International Journal of Dynamics and Control | title = A Review on the Development of the Wind Turbine Generators across the World | author = Navid Goudarzi | publisher = Springer | date = June 2013 | volume = 1 | issue = 2 | pages = 192–202 | url = http://link.springer.com/article/10.1007/s40435-013-0016-y | doi=10.1007/s40435-013-0016-y}}</ref> is mounted in a [[Nacelle (wind turbine)|nacelle]] at the top of a tower, behind the hub of the turbine rotor. Typically wind turbines generate electricity through [[induction motor|asynchronous machines]] that 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 network: typical rotation speeds for wind generators are 5–20 rpm while a directly connected machine will have an electrical speed between 750 and 3600 rpm. Therefore, a gearbox is inserted between the rotor hub and the generator. This also reduces the generator cost and weight. Commercial size generators have a rotor carrying a field winding so that a rotating [[magnetic field]] is produced inside a set of windings called the [[stator]]. While the rotating field winding consumes a fraction of a percent of the generator output, adjustment of the field current allows good control over the generator output voltage. Older style wind generators rotate at a constant speed, to match [[utility frequency|power line frequency]], which allowed the use of less costly induction generators{{Citation needed|date=March 2013}}. Newer wind turbines often turn at whatever speed generates electricity most efficiently. The varying output frequency and voltage can be matched to the fixed values of the grid using multiple technologies such as [[DFIG|doubly fed induction generators]] or full-effect converters where the variable frequency current produced is converted to DC and then back to AC. Although such alternatives require costly equipment and cause power loss, the turbine can capture a significantly larger fraction of the wind energy. In some cases, especially when turbines are sited offshore, the DC energy will be transmitted from the turbine to a central (onshore) [[Static inverter|inverter]] for connection to the grid. === Gearless wind turbine === {{anchor|Gearless wind turbine}} Gearless wind turbines (also called [[Direct drive mechanism|direct 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. [[Enercon]] and EWT (formerly known as Lagerwey) have produced gearless wind turbines with separately electrically excited generators for many years,<ref>{{cite web|author=Text und Photos: ENERCON Germany www.enercon.de |url=http://www.wwindea.org/technology/ch01/en/1_2_3_2.html |title=Anatomy of an Enercon direct drive wind turbine |publisher=Wwindea.org |accessdate=2013-11-06}}</ref> and [[Siemens]] produces a gearless "inverted generator" 3 MW model<ref name="trGearless">Fairly, Peter. [http://www.technologyreview.com/energy/25188/ Wind Turbines Shed Their Gears] ''[[Technology Review]]'', 27 April 2010. Retrieved: 22 September 2010.</ref><ref name="ingsie3">Wittrup, Sanne. [http://ing.dk/artikel/110879-gearloese-moeller-fra-siemens-bliver-solgt-for-foerste-gang First Siemens gearless] ''Ing.dk'', 11 August 2010. Retrieved: 15 September 2010.</ref> while developing a 6 MW model.<ref name="ingsie6">Wittrup, Sanne. [http://ing.dk/artikel/111960-siemens-udvikler-6-mw-gearloes-moelle 6MW Siemens gearless] ''Ing.dk'', 15 September 2010. Retrieved: 15 September 2010.</ref> To make up for a direct drive generator's slower spinning rate, the diameter of the generator's [[Rotor (electric)|rotor]] is increased so that it can contain more magnets to create the required frequency and power. Gearless wind turbines are often heavier than gear based wind turbines. A study by the [[European Union|EU]] called "Reliawind"<ref>[http://www.reliawind.eu reliawind.eu]</ref> based on the largest sample size of turbines has shown that the reliability of gearboxes is not the main problem in wind turbines. The reliability of direct drive turbines offshore is still not known, since the sample size is so small.<!-- 20% gearless market share sounds odd, must be of offshore only? <ref name=PT-GD/> --> Experts from [[Technical University of Denmark]] estimate that a geared generator with permanent magnets may use 25&nbsp;kg/MW of the [[rare earth element]] [[Neodymium]], while a gearless may use 250&nbsp;kg/MW.<ref name="ingPM">Wittrup, Sanne. [http://ing.dk/artikel/123609-permanente-magneter-volder-vestas-problemer-i-produktionen PMs cause production problems] [http://translate.google.dk/translate?sl=da&tl=en&js=n&prev=_t&hl=da&ie=UTF-8&layout=2&eotf=1&u=http%3A%2F%2Fing.dk%2Fartikel%2F123609-permanente-magneter-volder-vestas-problemer-i-produktionen English translation] ''Ing.dk'', 1 November 2011. Accessed: 1 November 2011.</ref> In December 2011, the [[United States Department of Energy|US Department of Energy]] published a report stating critical shortage of rare earth elements such as neodymium used in large quantities for permanent magnets in gearless wind turbines.<ref name=doeRem>[[Steven Chu|Chu, Steven]]. [http://energy.gov/sites/prod/files/DOE_CMS_2011.pdf Critical Materials Strategy] ''[[United States Department of Energy]]'', December 2011. Accessed: 23 December 2011.</ref> China produces more than 95%<!--p9--> of rare earth elements, while [[Hitachi]] holds more than 600 patents covering [[Neodymium magnet]]s.<!--p56--> Direct-drive turbines require 600&nbsp;kg of permanent magnet material per megawatt, which translates to several hundred kilograms of rare earth content per megawatt<!--p20-->, as neodymium content is estimated to be 31% of magnet weight<!--p89+155-->. Hybrid drivetrains (intermediate between direct drive and traditional geared) use significantly less rare earth materials<!--p20-->. While permanent magnet wind turbines only account for about 5% of the market outside of China, their market share inside of China is estimated at 25% or higher<!--p20-->. In 2011, demand for neodymium in wind turbines was estimated to be 1/5 of that in electric vehicles<!--p91-->.<ref name="doeRem"/><!--more EVs since then--> == Blades == === Blade design === [[File:Unlackierte Blattspitze.JPG|thumb|upright|Unpainted tip of a blade]] The ratio between the speed of the [[airfoil|blade]] tips and the speed of the wind is called [[tip speed ratio]]. High efficiency 3-blade-turbines have tip speed/wind speed ratios of 6 to 7. Modern wind turbines are designed to spin at varying speeds (a consequence of their generator design, see above). Use of [[aluminum]] and [[composite materials]] in their blades has contributed to low [[moment of inertia|rotational inertia]], which means that newer wind turbines can accelerate quickly if the winds pick up, keeping the tip speed ratio more nearly constant. Operating closer to their optimal tip speed ratio during energetic gusts of wind allows wind turbines to improve energy capture from sudden gusts that are typical in urban settings. And in contrast, older style wind turbines were designed with heavier steel blades, which have higher inertia, and rotated at speeds governed by the AC frequency of the power lines. The high inertia buffered the changes in rotation speed and thus made power output more stable. It is generally understood that noise increases with higher blade tip speeds. To increase tip speed without increasing noise would allow reduction the torque into the gearbox and generator and reduce overall structural loads, thereby reducing cost.<ref name="univ2011"/> The reduction of noise is linked to the detailed aerodynamics of the blades, especially factors that reduce abrupt stalling. The inability to predict stall restricts the development of aggressive aerodynamic concepts.<ref name="univ2011"/> Some blades (mostly on Enercon) have a [[winglet]] to increase performance and/or reduce noise.<ref>Hau, Erich. "Wind Turbines: Fundamentals, Technologies, Application, Economics" p142. Springer Science & Business Media, 26. feb. 2013. ISBN 3642271510</ref><ref>{{Cite news|url=http://www.windpowermonthly.com/article/1133706/enercons-direct-drive-evolution|title=Enercon's direct drive evolution|access-date=2017-02-27}}</ref> A blade can have a [[lift-to-drag ratio]] of 120,<ref name=cost3A>Jamieson, Peter. [https://books.google.dk/books?id=HyUIpGPO-k0C&printsec=frontcover&hl=da Innovation in Wind Turbine Design] sec11-1, ''John Wiley & Sons'', 5 July 2011. Accessed: 26 February 2012. ISBN 1-119-97545-X</ref> compared to 70 for a [[sailplane]] and 15 for an airliner.<ref name=kroo>Kroo, Ilan. [http://www.aeronautics.nasa.gov/pdf/23_kroo_green_aviation_summit.pdf NASA Green Aviation Summit] p9, ''[[NASA]]'', September 2010. Accessed: 26 February 2012.</ref> {{Clear}} === The hub === [[File:Connecting Hub to Turbine Tower No 11 - geograph.org.uk - 787410.jpg|thumb|A Wind turbine hub being installed]] In simple designs, the blades are directly bolted to the hub and are unable to pitch, which leads to aerodynamic stall above certain windspeeds. In other more sophisticated designs, they are bolted to the pitch mechanism, which adjusts their [[angle of attack]] according to the wind speed to control their rotational speed. The pitch mechanism is itself bolted to the hub. The hub is fixed to the rotor shaft which drives the generator directly or through a gearbox. === Blade count === {{No footnotes|section|date=August 2012}} [[File:Mod-5B Wind turbine.jpg|thumb|left|upright|The 98 meter diameter, two-bladed NASA/DOE [[Mod-5B]] wind turbine was the largest operating wind turbine in the world in the early 1990s]] [[File:Mod-0 Wind turbine.jpg|thumb|The NASA test of a one-bladed wind turbine rotor configuration at Plum Brook Station near Sandusky, Ohio]] The number of blades is selected for aerodynamic efficiency, component costs, and system reliability. Noise emissions are affected by the location of the blades upwind or downwind of the tower and the speed of the rotor. Given that the noise emissions from the blades' trailing edges and tips vary by the 5th power of blade speed, a small increase in tip speed can make a large difference. Wind turbines developed over the last 50 years have almost universally used either two or three blades. However, there are patents that present designs with additional blades, such as Chan Shin's Multi-unit rotor blade system integrated wind turbine.<ref>{{cite web|url=http://www.google.com/patents/US5876181 |title=Patent US5876181 - Multi-unit rotor blade system integrated wind turbine - Google Patents |publisher=Google.com |accessdate=2013-11-06}}</ref> Aerodynamic efficiency increases with number of blades but with diminishing return. Increasing the number of blades from one to two yields a six percent increase in aerodynamic efficiency, whereas increasing the blade count from two to three yields only an additional three percent in efficiency.<ref>Eric Hau (ed), ''Wind Turbines Fundamentals, Technologies, Applications, Economics 2nd Edition'' ,Springer 2006, ISBN 3-540-24240-6 page 121</ref> Further increasing the blade count yields minimal improvements in aerodynamic efficiency and sacrifices too much in blade stiffness as the blades become thinner.{{Citation needed|date=April 2013}} Theoretically, an infinite number of blades of zero width is the most efficient, operating at a high value of the tip speed ratio. But other considerations lead to a compromise of only a few blades.<ref>{{cite web|title=CAT windpower course Blade design notes|url=http://www.scoraigwind.com/wpNotes/bladeDesign.pdf|author=Hugh Piggott|year=1998}}. Course notes from [[Scoraig]] Wind Electric, used in courses at the [[Centre for Alternative Technology]].</ref> Component costs that are affected by blade count are primarily for materials and manufacturing of the turbine rotor and drive train. Generally, the lower the number of blades, the lower the material and manufacturing costs will be. In addition, the lower the number of blades, the higher the rotational speed can be. This is because blade stiffness requirements to avoid interference with 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 blades with higher rotational speeds reduce peak torques in the drive train, resulting in lower gearbox and generator costs. System reliability is affected by blade count primarily through the dynamic loading of the rotor into the drive train and tower systems. While aligning the wind turbine to changes in wind direction (yawing), each blade experiences a cyclic load at its root end depending on blade position. This is true of one, two, three blades or more. However, these cyclic loads when combined together at the drive train shaft are symmetrically balanced for three blades, yielding smoother operation during turbine yaw. Turbines with one or two blades can use a pivoting teetered hub to also nearly eliminate the cyclic loads into the drive shaft and system during yawing. A Chinese 3.6 MW two-blade is being tested in Denmark.<ref>{{cite web|last=Boel|first=Thomas|title=Two wings work|url=http://ing.dk/artikel/134314-testdata-afliver-fordom-moelle-med-to-vinger-fungerer-fint|publisher=[[Ingeniøren]]|accessdate=22 November 2012|date=22 November 2012}} [http://ing.dk/artikel/134390-se-detaljerne-paa-kinesisk-moelle-med-kun-to-vinger#0 Design]</ref> [[Mingyang Wind Power|Mingyang]] won a bid for 87 MW (29 * 3 MW) two-bladed offshore wind turbines near Zhuhai in 2013.<ref>"[http://www.wspa.com/story/23564370/my-secures-off-shore-tender-in-zhuhai-guangdong-province-china-with-3mw-scd-wind-turbine-generators-construction-to-begin-in-october-2013 MY Secures Off-Shore Tender in Zhuhai, Guangdong Province, China with 3MW SCD Wind Turbine Generators, Construction to Begin in October 2013]" ''WSPA'', 30 September 2013. Accessed: 22 November 2013.</ref><ref name=scd>"[http://www.mywind.com.cn/English/program/products.aspx?MenuID=05030301&ID=30 2.5/2.75/3.0MW Series Wind Turbine Generator]" ''Ming Yang''. Accessed: 22 November 2013.</ref><ref>"[http://www.4coffshore.com/windfarms/zhuhai-guishan-offshore-wind-farm-demonstration-project-china-cn86.html 4c Zhuhai]"</ref> 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 === [[File:Windturbine HamburgWasser Steinwerder 02.jpg|thumb|Several modern wind turbines use rotor blades with carbon-fibre girders to reduce weight.]] In general, ideal materials should meet the following criteria: * wide availability and easy processing to reduce cost and maintenance * low weight or density to reduce gravitational forces * high strength to withstand strong loading of wind and gravitational force of the blade itself * high fatigue resistance to withstand cyclic loading * high stiffness to ensure stability of the optimal shape and orientation of the blade and clearance with the tower * high fracture toughness * the ability to withstand environmental impacts such as lightning strikes, humidity, and temperature<ref name="Ma 2014">Ma, P., & Zhang, Y. ''Perspectives of carbon nanotubes/polymer nanocomposites for wind blade materials''. In: ''[[Renewable and Sustainable Energy Reviews]]'', 30, (2014), 651-660, {{DOI|10.1016/j.rser.2013.11.008}}.</ref> This narrows down the list of acceptable materials. Metals would be undesirable because of their vulnerability to fatigue. Ceramics have low fracture toughness, which could result in early blade failure. Traditional polymers are not stiff enough to be useful, and wood has problems with repeatability, especially considering the length of the blade. That leaves fiber-reinforced composites, which have high strength and stiffness and low density, as a very attractive class of materials for the design of wind turbines.<ref>http://www.uotechnology.edu.iq/dep-laserandoptoelec-eng/branch/lectures/solid%20state/chapter%201%20classification%20of%20materail.pdf</ref> Wood and canvas sails were used on early windmills due to their low price, availability, and ease of manufacture. Smaller blades can be made from light metals such as [[aluminium]]. These materials, however, require frequent maintenance. Wood and canvas construction limits the [[airfoil]] shape to a flat plate, which has a relatively high ratio of drag to force captured (low aerodynamic efficiency) compared to solid airfoils. Construction of solid airfoil designs requires inflexible materials such as metals or [[composite material|composites]]. Some blades also have incorporated lightning conductors. New wind turbine designs push power generation from the single [[megawatt]] range to upwards of 10 megawatts using larger and larger blades. A larger area effectively increases the tip-speed ratio of a turbine at a given wind speed, thus increasing its energy extraction.<ref>{{cite book |author=Zbigniew Lubosny |title=Wind Turbine Operation in Electric Power Systems: Advanced Modeling (Power Systems) |publisher=Springer |location=Berlin |year=2003 |pages= |isbn=3-540-40340-X |oclc= |doi= |accessdate=}}</ref> [[Computer-aided engineering]] software such as [[HyperSizer]] (originally developed for spacecraft design) can be used to improve blade design.<ref>{{cite web |title= Materials and design methods look for the 100-m blade |work= Windpower Engineering |date= 10 May 2011 |url= http://www.windpowerengineering.com/design/mechanical/materials-and-design-methods-look-for-the-100-m-blade/ |accessdate= 22 August 2011 }}</ref><ref>{{cite news |title= From Aircraft Wings to Wind Turbine Blades: NASA Software Comes Back to Earth with Green Energy Applications |author= Craig S. Collier |work= NASA Tech Briefs |date= 1 October 2010 |url= http://www.techbriefs.com/component/content/article/8602 |accessdate= 22 August 2011 }}</ref> As of 2015 the rotor diameters of onshore wind turbine blades are as large as 130 meters,<ref>[http://www.windpowermonthly.com/article/1333448/nordex-secures-first-n131-3000-finland Nordex secures first N131/3000 in Finland] In: [[Windpower Monthly]], Retrieved 22. February 2015.</ref> while the diameter of offshore turbines reach 170 meters.<ref>[http://www.erneuerbareenergien.de/weltgroesste-offshore-turbine-errichtet/150/469/74200/ ''Weltgrößte Offshore-Turbine errichtet'']. In: ''Erneuerbare Energien. Das Magazin'' Retrieved 22. February 2015.</ref> In 2001, an estimated 50 million kilograms of [[fibreglass]] laminate were used in wind turbine blades.<ref name=Griffin03>{{cite journal |doi=10.1115/1.1629750 |title=Alternative Composite Materials for Megawatt-Scale Wind Turbine Blades: Design Considerations and Recommended Testing |year=2003 |author=Griffin, Dayton A. |journal=Journal of Solar Energy Engineering |volume=125 |issue=4 |page=515 |last2=Ashwill |first2=Thomas D.}}</ref> 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.<ref>{{cite conference |first = T |last = Ashwill |author2=Laird D |date=January 2007 |title = Concepts to Facilitate Very Large Blades |conference = 45th AIAA Aerospace Sciences Meeting and Exhibit |url = http://www.sandia.gov/wind/asme/AIAA-2007-0817A.pdf |id = AIAA-2007-0817 }}</ref> Gravitational loads include axial and tensile/ compressive loads (top/bottom of rotation) as well as bending (lateral positions). The magnitude of these loads fluctuates cyclically and the edgewise moments (see below) are reversed every 180° of rotation. Typical rotor speeds and design life are ~10rpm and 20 years, respectively, with the number of lifetime revolutions on the order of 10^8. Considering wind, it is expected that turbine blades go through ~10^9 loading cycles. Wind is another source of rotor blade loading. Lift causes bending in the flapwise direction (out of rotor plane) while air flow around the blade cause edgewise bending (in the rotor plane). Flapwise bending involves tension on the pressure (upwind) side and compression on the suction (downwind) side. Edgewise bending involves tension on the leading edge and compression on the trailing edge. Wind loads are cyclical because of natural variability in wind speed and wind shear (higher speeds at top of rotation). Failure in ultimate loading of wind-turbine rotor blades exposed to wind and gravity loading is a failure mode that needs to be considered when the rotor blades are designed. The wind speed that causes bending of the rotor blades exhibits a natural variability, and so does the stress response in the rotor blades. Also, the resistance of the rotor blades, in terms of their tensile strengths, exhibits a natural variability.<ref>Ronold, K. O., & Larsen, G. C. (2000). Reliability-based design of wind-turbine rotor blades against failure in ultimate loading. Engineering Structures, 22(6), 565-574.</ref> In light of these failure modes and increasingly larger blade systems, there has been continuous effort toward developing cost-effective materials with higher strength-to-mass ratios. In order to extend the current 20 year lifetime of blades and enable larger area blades to be cost-effective, the design and materials need to be optimized for stiffness, strength, and fatigue resistance.<ref name="Ma 2014" /> The majority of current commercialized wind turbine blades are made from fiber-reinforced polymers (FRPs), which are composites consisting of a polymer matrix and fibers. The long fibers provide longitudinal stiffness and strength, and the matrix provides fracture toughness, delamination strength, out-of-plane strength, and stiffness.<ref name="Ma 2014" /> Material indices based on maximizing power efficiency, and having high fracture toughness, fatigue resistance, and thermal stability, have been shown to be highest for glass and carbon fiber reinforced plastics (GFRPs and CFRPs).<ref>Bassyouni, M., & Gutub, S. A. (2013). Materials selection strategy and surface treatment of polymer composites for wind turbine blades fabrication. Polymers & Polymer Composites, 21, 463-471.</ref> {{wide image|Fiberglass-reinforced epoxy blades of Siemens SWT-2.3-101 wind turbines.jpg|1100px|align-cap=center|[[Fiberglass]]-reinforced [[epoxy]] blades of Siemens SWT-2.3-101 wind turbines. The blade size of 49 meters<ref>{{cite web|title=Aerodynamic and Performance Measurements on a SWT-2.3- 101 Wind Turbine|url=http://www.nrel.gov/docs/fy12osti/51649.pdf|work=WINDPOWER 2011|publisher=National Renewable Energy Laboratory|accessdate=14 October 2013|page=1|date=22–25 May 2011}}</ref> is in comparison to a [[Electrical substation|substation]] behind them at [[Wolfe Island Wind Farm]].|alt=Fiberglass-reinforced epoxy blades of Siemens SWT-2.3-101 wind turbines.}} Manufacturing blades in the 40 to 50 metre range involves proven fibreglass composite fabrication techniques. Manufactures such as [[Nordex SE]] and [[GE Wind]] use an infusion process. Other manufacturers use variations on this technique, some including [[carbon]] and [[wood]] with fibreglass in an [[epoxy]] matrix. Other options include preimpregnated ("prepreg") fibreglass and vacuum-assisted resin transfer molding. Each of these options use a glass-fibre reinforced [[polymer]] composite constructed with differing complexity. Perhaps the largest issue with more simplistic, open-mould, wet systems are the emissions associated with the volatile organics released. Preimpregnated materials and resin infusion techniques avoid the release of volatiles by containing all VOCs. However, these contained processes have their own challenges, namely the production of thick laminates necessary for structural components becomes more difficult. As the preform resin permeability dictates the maximum laminate thickness, bleeding is required to eliminate voids and ensure proper resin distribution.<ref name=Griffin03/> One solution to resin distribution a partially preimpregnated fibreglass. During evacuation, the dry fabric provides a path for airflow and, once heat and pressure are applied, resin may flow into the dry region resulting in a thoroughly impregnated laminate structure.<ref name=Griffin03/> Epoxy-based composites have environmental, production, and cost advantages over other resin systems. Epoxies also allow shorter cure cycles, increased durability, and improved surface finish. Prepreg operations further reduce processing time over wet lay-up systems. As turbine blades pass 60 metres, infusion techniques become more prevalent; the traditional resin transfer moulding injection time is too long as compared to the resin set-up time, limiting laminate thickness. Injection forces resin through a thicker ply stack, thus depositing the resin where in the laminate structure before gelation occurs. Specialized epoxy resins have been developed to customize lifetimes and viscosity.<ref>{{cite journal |doi=10.1016/S0034-3617(07)70148-0 |title=Advanced materials for turbine blade manufacture |year=2007 |author=Christou, P |journal=Reinforced Plastics |volume=51 |issue=4 |page=22}}</ref> Carbon fibre-reinforced load-bearing spars can reduce weight and increase stiffness. Using carbon fibres in 60 metre turbine blades is estimated to reduce total blade mass by 38% and decrease cost by 14% compared to 100% fibreglass. Carbon fibres have the added benefit of reducing the thickness of fiberglass laminate sections, further addressing the problems associated with resin wetting of thick lay-up sections. Wind turbines may also benefit from the general trend of increasing use and decreasing cost of carbon fibre materials.<ref name=Griffin03/> Although glass and carbon fibers have many optimal qualities for turbine blade performance, there are several downsides to these current fillers, including the fact that high filler fraction (10-70 wt%) causes increased density as well as microscopic defects and voids that often lead to premature failure.<ref name="Ma 2014" /> Recent developments include interest in using carbon nanotubes (CNT’s) to reinforce polymer-based nanocomposites. CNT’s can be grown or deposited on the fibers, or added into polymer resins as a matrix for FRP structures. Using nanoscale CNT’s as filler instead of traditional microscale filler (such as glass or carbon fibers) results in CNT/polymer nanocomposites, for which the properties can be changed significantly at very low filler contents (typically < 5 wt%). They have very low density, and improve the elastic modulus, strength, and fracture toughness of the polymer matrix. The addition of CNT’s to the matrix also reduces the propagation of interlaminar cracks which can be a problem in traditional FRP’s.<ref name="Ma 2014" /> Further improvement is possible through the use of carbon nanofibers (CNFs) in the blade coatings. A major problem in desert environments is erosion of the leading edges of blades by wind carrying sand, which increases roughness and decreases aerodynamic performance. The particle erosion resistance of fiber-reinforced polymers is poor when compared to metallic materials and elastomers, and needs to be improved. It has been shown that the replacement of glass fiber with CNF on the composite surface greatly improves erosion resistance. CNF’s have also been shown to provide good electrical conductivity (important for lightning strikes), high damping ratio, and good impact-friction resistance. These properties make CNF-based nanopaper a prospective coating for wind turbine blades.<ref>Zhang, N., Yang, F., Guerra, D., Shen, C., Castro, J., & Lee, J. L. (2013). Enhancing particle erosion resistance of glass-reinforced polymeric composites using carbon nanofiber-based nanopaper coatings. Journal of Applied Polymer Science, 129(4), 1875-1881.</ref><ref>Liang, F., Tang, Y., Gou, J., & Kapat, J. (2011). Development of multifunctional nanocomposite coatings for wind turbine blades. Ceramic Transactions, 224, 325-336.</ref> === Blade recycling === The Global Wind Energy Council (GWEC) predicts that wind energy will supply 15.7% of the world’s total energy needs by the year 2020, and 28.5% by the year 2030.<ref>{{Cite web|url=http://www.gwec.net/publications/global-wind-energy-outlook/gweo-2008/|title=GLOBAL WIND ENERGY OUTLOOK 2008 {{!}} GWEC|website=www.gwec.net|access-date=2016-11-07}}</ref> This dramatic increase in global wind energy generation will require installation of a newer and larger fleet of more efficient wind turbines and the consequent decommissioning of aging ones. Based on a study carried out by the European Wind Energy Association, in the year 2010 alone, between 110 and 140 kilotons of composites were consumed by the wind turbine industry for manufacturing blades.<ref>{{Cite web|url=http://www.ewea.org/fileadmin/files/our-activities/policy-issues/environment/research_note_recycling_WT_blades.pdf|title=Research note outline on recycling wind turbines blades|last=The European Wind Energy Association|first=|date=|website=}}</ref> The majority of the blade material will eventually end up as waste, and in order to accommodate this level of composite waste, the only option is recycling. Typically, glass-fibre-reinforced-polymers (GFRPs) compose of around 70% of the laminate material in the blade. GFRPs hinder incineration and are not combustible.<ref>{{Cite journal|last=Duflou|first=Joost R.|last2=Deng|first2=Yelin|last3=Acker|first3=Karel Van|last4=Dewulf|first4=Wim|date=2012-04-01|title=Do fiber-reinforced polymer composites provide environmentally benign alternatives? A life-cycle-assessment-based study|url=https://www.cambridge.org/core/journals/mrs-bulletin/article/do-fiber-reinforced-polymer-composites-provide-environmentally-benign-alternatives-a-life-cycle-assessment-based-study/6B4BA944EF6BB811E727BC38BF5CFABA|journal=MRS Bulletin|volume=37|issue=4|pages=374–382|doi=10.1557/mrs.2012.33|issn=1938-1425}}</ref> Therefore, conventional recycling methods need to be modified. Currently, depending on whether individual fibres can be recovered, there exists a few general methods for recycling GFRPs in wind turbine blades: * Mechanical Recycling: This method doesn't recover individual fibres. Initial processes involve shredding, crushing, and/or milling. The crushed pieces are then separated into fibre-rich and resin-rich fractions. These fractions are ultimately incorporated into new composites either as fillers or reinforcements.<ref>{{Cite journal|last=Pickering|first=S. J.|date=2006-08-01|title=Recycling technologies for thermoset composite materials—current status|url=http://www.sciencedirect.com/science/article/pii/S1359835X05002101|journal=Composites Part A: Applied Science and Manufacturing|series=The 2nd International Conference: Advanced Polymer Composites for Structural Applications in Construction|volume=37|issue=8|pages=1206–1215|doi=10.1016/j.compositesa.2005.05.030}}</ref> * Chemical Processing/[[Pyrolysis]]: Thermal decomposition of the composites is used to recover the individual fibres. For [[pyrolysis]], the material is heated up to 500&nbsp;°C in an environment without oxygen, thus causing it to break down into lower weight organic substances and/or gaseous products. The glass fibres will generally loose 50% of their initial strength and can now be downcycled for fibre reinforcement applications in paints or concrete.<ref>{{Cite web|url=http://www.appropedia.org/Recycling_of_wind_turbine_blades#cite_note-.5B4.5D-4|title=Recycling of wind turbine blades - Appropedia: The sustainability wiki|website=www.appropedia.org|access-date=2016-11-08}}</ref> Research has shown that this end of life option is able to recover up to approximately 19 MJ/kg.<ref>{{Cite journal|last=Duflou|first=Joost R.|last2=Deng|first2=Yelin|last3=Acker|first3=Karel Van|last4=Dewulf|first4=Wim|date=2012-04-01|title=Do fiber-reinforced polymer composites provide environmentally benign alternatives? A life-cycle-assessment-based study|url=https://www.cambridge.org/core/journals/mrs-bulletin/article/do-fiber-reinforced-polymer-composites-provide-environmentally-benign-alternatives-a-life-cycle-assessment-based-study/6B4BA944EF6BB811E727BC38BF5CFABA/core-reader|journal=MRS Bulletin|volume=37|issue=4|pages=374–382|doi=10.1557/mrs.2012.33|issn=1938-1425}}</ref> However, this method has a relatively high cost and requires similar mechanical pre-processing. In addition, it has not yet been modified to satisfy the future need of large scale wind turbine blade recycling.<ref>{{Cite web|url=http://www.refiber.com/technology.html|title=ReFiber ApS Wind Turbine Blade Recycling Technology}}</ref> == Tower == Two main types of towers exist: [[Floating wind turbine|floating towers]] and land-based towers, which are usually more common. === Tower height === Wind velocities increase at higher altitudes due to [[Planetary boundary layer#Cause of surface wind gradient|surface aerodynamic drag]] (by land or water surfaces) and the viscosity of the air. The variation in velocity with altitude, called [[wind shear]], is most dramatic near the surface. Typically, the variation follows the [[wind profile power law]], which predicts that wind speed 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 avoid [[buckling]], 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. At night time, or when the atmosphere becomes '''stable,''' wind speed close to the ground usually subsides whereas at turbine hub altitude it does not decrease that much or may even increase. As a result, the wind speed is higher and a turbine will produce more power than expected from the 1/7 power law: doubling the altitude may increase wind speed by 20% to 60%. A stable atmosphere 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&nbsp;m/s) the stable atmosphere is disrupted because of friction turbulence and the atmosphere will turn '''neutral'''. A daytime atmosphere is either neutral (no net radiation; usually with strong winds and heavy clouding) or '''unstable''' (rising air because of ground heating—by the sun). Here again the 1/7 power law applies or is at least a good approximation of the wind profile. [[Indiana]] had been rated as having a wind capacity of 30,000 MW, but by raising the expected turbine height from 50 m to 70 m, the wind capacity estimate was raised to 40,000 MW, and could be double that at 100 m.<ref>{{cite web|url=http://www.indianacleanpower.org/renewableresources.html |title=Indiana's Renewable Energy Resources |publisher=Indianacleanpower.org |date=2013-08-07 |accessdate=2013-11-06}}</ref> For [[Horizontal axis wind turbine|HAWT]]s, tower heights approximately two to three times the blade length have been found to balance material costs of the tower against better utilisation of the more expensive active components. [[Image:Windkraftwerk in Schiff.jpg|thumb|Sections of a wind turbine tower, transported in a [[bulk carrier]] ship]] Road size restrictions makes transportation of towers with a diameter of more than 4.3 m difficult. Swedish analyses show that it is important to have the bottom wing tip at least 30 m above the tree tops, but a taller tower requires a larger tower diameter.<ref name=iba/> A 3 MW turbine may increase output from 5,000 MWh to 7,700 MWh per year by going from 80 to 125 meter tower height.<ref name=twrshell1>Wittrup, Sanne. [http://ing.dk/artikel/ny-type-vindmolletarn-samles-af-lameller-123516 Ny type vindmølletårn samles af lameller], [[Ingeniøren]], 29. October 2011. Accessed: 12 May 2013.</ref> A tower profile made of connected shells rather than cylinders can have a larger diameter and still be transportable. A 100 m prototype tower with [[TC bolt]]ed 18&nbsp;mm 'plank' shells has been erected at the wind turbine test center Høvsøre in Denmark and certified by [[Det Norske Veritas]], with a [[Siemens Wind Power|Siemens]] nacelle. Shell elements can be shipped in standard 12 m [[shipping container]]s,<ref name=iba>Emme, Svend. [http://www.jernindustri.dk/artikel/VisArtikel.aspx?SiteID=JM&Lopenr=108300013&newsletterRefID=6173 New type of wind turbine tower] ''Metal Industry'', 8 August 2011. Accessed: 10 December 2011.</ref><ref>"[http://andresen-towers.com/concept The shell tower in brief]". ''Andresen Towers''. Retrieved: 13 November 2012.</ref> and 2½ towers per week are produced this way.<ref name=twrshell2>Lund, Morten. [http://ing.dk/artikel/robotter-bag-dansk-succes-med-vindmoelletaarne-158563 Robotter bag dansk succes med vindmølletårne], [[Ingeniøren]], 12 May 2013. Accessed: 12 May 2013.</ref> As of 2003, typical modern wind turbine installations use towers about 210&nbsp;ft (65 m) high. Height is typically limited by the availability of [[crane (machine)|cranes]]. This has led to a variety of proposals for "partially self-erecting wind turbines" that, for a given available crane, allow taller towers that put a turbine in stronger and steadier winds, and "self-erecting wind turbines" that can be installed without cranes.<ref> [http://www.nrel.gov/docs/fy01osti/29493.pdf "WindPACT Turbine Design: Scaling Studies Technical Area 3 -- Self-Erecting Tower and Nacelle Feasibility"]. 2001. </ref><ref> R. D. Fredrickson. [https://www.xcelenergy.com/staticfiles/xe/Corporate/Renewable%20Energy%20Grants/BlattnerSelfErectingWindTurbine2005Report.pdf "A self-erecting method for wind turbines."]. 2003. </ref><ref> Nic Sharpley. [http://www.windpowerengineering.com/featured/business-news-projects/whats-holding-up-tower-technology/ "What’s holding up tower technology?"]. 2013. </ref><ref> [http://www.renewableenergyworld.com/articles/2002/01/self-erecting-wind-turbine-designed-for-remote-sites-5785.html "Self-Erecting Wind Turbine Designed for Remote Sites"]. 2002. </ref> === Tower materials === Currently, the majority of wind turbines are supported by conical tubular steel towers. These towers represent 30% – 65% of the turbine weight and therefore account for a large percentage of the turbine transportation costs. The use of lighter materials in the tower could greatly reduce the overall transport and construction cost of wind turbines, however the stability must be maintained.<ref>Ancona, Dan, and Jim McVeigh. (2011): Wind Turbine - Materials and Manufacturing Fact Sheet. Princeton Energy Resources International, LLC, 19 Aug. 2001. Web. 21 Oct. 2015. <http://www.perihq.com/documents/WindTurbine-MaterialsandManufacturing_FactSheet.pdf>.</ref> Higher grade S500 steel costs 20%-25% more than S335 steel (standard [[structural steel]]), but it requires 30% less material because of its improved strength. Therefore, replacing wind turbine towers with S500 steel would result in a net savings in both weight and cost.<ref>""Steel Solutions in the Green Economy." (2015): Wind Turbines. World Steel Association, 2012. Web. 21 Oct. 2015. <https://www.worldsteel.org/dms/internetDocumentList/bookshop/worldsteel-wind-turbines-web/document/Steel%20solutions%20in%20the%20green%20economy:%20Wind%20turbines.pdf>.</ref> Another disadvantage of conical steel towers is that constructing towers that meet the requirements of wind turbines taller than 90 meters proves challenging. High performance concrete shows potential to increase tower height and increase the lifetime of the towers. A hybrid of [[prestressed concrete]] and steel has shown improved performance over standard tubular steel at tower heights of 120 meters.<ref>Quilligan, Aidan, A. O’Connor, and V. Pakrashi. "Fragility analysis of steel and concrete wind turbine towers." Engineering Structures 36 (2012): 270-282.</ref> Concrete also gives the benefit of allowing for small precast sections to be assembled on site, avoiding the challenges steel faces during transportation.<ref>http://www.ecocem.ie/downloads/Concrete_Windmills.pdf</ref> One downside of concrete towers is the higher CO2 emissions during concrete production as compared to steel. However, the overall environmental benefit should be higher if concrete towers can double the wind turbine lifetime.<ref>Levitan, Dave. "Too Tall for Steel: Engineers Look to Concrete to Take Wind Turbine Design to New Heights." IEEE Spectrum, 16 May 2013. Web. 21 Oct. 2015. <http://spectrum.ieee.org/energywise/green-tech/wind/too-tall-for-steel-engineers-look-to-concrete-to-take-wind-turbine-design-to-new-heights>.</ref> [[Wood]] is being investigated as a material for wind turbine towers, and a 100 metre tall tower supporting a 1.5 MW turbine has been erected in Germany. The wood tower shares the same transportation benefits of the segmented steel shell tower, but without the steel resource consumption.<ref>McGar, Justin. "[http://designbuildsource.com.au/wind-power-revolution-worlds-timber-turbine Wind Power Revolution: The World’s First Timber Turbine]" ''Design Build Source'', 13 November 2012. Retrieved: 13 November 2012.</ref><ref>RICHARDSON, JAKE. "[http://cleantechnica.com/2012/10/18/99-natural-timber-tower-provides-wind-power/ 99% Natural Timber Tower for Wind Turbines]" ''Clean Technica'', 18 October 2012. Retrieved: 13 November 2012.</ref> ==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 recently as 2003, nearly all grid-connected wind turbines operated at exactly constant speed (synchronous generators) or within a few percent of constant speed (induction generators).<ref> P. W. Carlin, A. S. Laxson, and E. B. Muljadi. [http://geosci.uchicago.edu/~moyer/GEOS24705/Readings/Carlin_VariableSpeed.pdf "The History and State of the Art of Variable-Speed wind Turbine Technology"]. 2003. p. 130-131. </ref><ref> Murthy, S.S.; Singh, B.; Goel, P.K.; Tiwari, S.K. [http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=4487785&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D4487785 "A Comparative Study of Fixed Speed and Variable Speed Wind Energy Conversion Systems Feeding the Grid"]. 2007. doi: 10.1109/PEDS.2007.4487785 </ref> As of 2011, many operational wind turbines used fixed speed induction generators (FSIG).<ref name="caliao" > Nolan D. Caliao. [http://www.sciencedirect.com/science/article/pii/S0960148111000048 "Dynamic modelling and control of fully rated converter wind turbines"]. "Renewable Energy" 2011. doi: 10.1016/j.renene.2010.12.025 </ref> As of 2011, most new grid-connected wind turbines are [[variable speed wind turbine]]s—they are in some variable speed configuration.<ref name="caliao" /> Early wind turbine control systems were designed for peak power extraction, also called [[maximum power point tracking]]—they attempt to pull the maximum possible electrical power from a given wind turbine under the current wind conditions.<ref> Ali M. Eltamaly, A. I. Alolah, and Hassan M. Farh. [http://www.intechopen.com/books/new-developments-in-renewable-energy/maximum-power-extraction-from-utility-interfaced-wind-turbines "Maximum Power Extraction from Utility-Interfaced Wind Turbines"]. 2013. DOI: 10.5772/54675 </ref> More recent wind turbine control systems deliberately pull less electrical power than they possibly could in most circumstances, in order to provide other benefits, which include: * [[spinning reserve]]s to quickly produce more power when needed—such as when some other generator suddenly drops from the grid—up to the max power supported by the current wind conditions.<ref> E. Muljadi, M. Singh, and V. Gevorgian. [http://www.nrel.gov/docs/fy13osti/56817.pdf "Fixed-Speed and Variable-Slip Wind Turbines Providing Spinning Reserves to the Grid"]. In [http://www.intechopen.com/books/new-developments-in-renewable-energy "New Developments in Renewable Energy"]. 2013. </ref> * Variable-speed wind turbines can (very briefly) produce more power than the current wind conditions can support, by storing some wind energy as kinetic energy (accelerating during brief gusts of faster wind) and later converting that kinetic energy to electric energy (decelerating, either when more power is needed elsewhere, or during short lulls in the wind, or both).<ref> E. Muljadi and C.P. Butterfield. [http://www.nrel.gov/docs/fy00osti/27143.pdf "Pitch-Controlled Variable-Speed Wind Turbine Generation"]. 1999. </ref><ref> E. Muljadi, K. Pierce, and P. Migliore. [https://calpoly-wind-turbine.googlecode.com/hg/Research/A%20Conservative%20Control%20Strategy%20for%20Var%20Speed%20Stall%20Reg%20WT.pdf "A Conservative Control Strategy for Variable-Speed Stall-Regulated Wind Turbines"]. 2000. </ref> * damping (electrical) subsynchronous resonances in the grid<ref> Ewais, A.M.; Liang, J.; Ekanayake, J.B.; Jenkins, N. [http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=6303160&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D6303160 "Influence of Fully Rated Converter-based wind turbines on SSR"]. 2012. doi: 10.1109/ISGT-Asia.2012.6303160 </ref> * damping (mechanical) resonances in the tower<ref> Mate Jelavić, Nedjeljko Perić, Ivan Petrović. [http://act.rasip.fer.hr/materijali/11/EVER07-paper-34.pdf "Damping of Wind Turbine Tower Oscillations through Rotor Speed Control"]. 2007. </ref><ref> A. Rodríguez T., C. E. Carcangiu, I. Pineda, T. Fischer, B. Kuhnle, M. Scheu, M. Martin. [http://link.springer.com/chapter/10.1007/978-1-4419-9316-8_12 "Wind Turbine Structural Damping Control for Tower Load Reduction"]. 2011. doi: 10.1007/978-1-4419-9316-8_12 </ref> The generator in a wind turbine produces [[alternating current]] (AC) electricity. Some turbines drive an [[AC/AC converter]]—which converts the AC to [[direct current]] (DC) with a [[rectifier]] and then back to AC with an [[Inverter (electrical)|inverter]]—in order to match the frequency and phase of the grid. However, the most common method in large modern turbines is to instead use a [[DFIG|doubly fed induction generator]] directly connected to the [[Electrical grid|electricity grid]]. A useful technique to connect a permanent magnet synchronous generator to the grid is by using a back-to-back converter. Also, we can have control schemes so as to achieve unity [[power factor]] in the connection to the grid. In that way the wind turbine will not consume reactive power, which is the most common problem with wind turbines that use induction 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 consume reactive power. So, it can work as a [[Variable capacitor|dynamic capacitor]]/[[Electrical reactance#Inductive reactance|inductor]] bank so as to help with the [[Utility frequency#Stability|power systems' stability]]. [[File:Grid Side Controller.jpg|thumb|Grid Side Controller Design]] Below we show the control scheme so as to achieve unity power factor : [[Volt-ampere reactive|Reactive power]] regulation consists of one [[PID controller#PI controller|PI controller]] in order to achieve operation with unity power factor (i.e. Q_grid = 0 ). It is obvious that I_dN has to be regulated to reach zero at steady-state (I_dNref = 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 == [[File:Concrete base for turbine 23 - geograph.org.uk - 517353.jpg|thumb|Wind turbine foundations]] Wind turbines, by their nature, are very tall slender structures,<ref>Lombardi, D. (2010). Long Term Performance of Mono-pile Supported Offshore Wind Turbines. Bristol: University of Bristol.</ref> this can cause a number of issues when the structural design of the [[Foundation (engineering)|foundations]] are considered. The foundations for a conventional [[structural engineering|engineering structure]] are designed mainly to transfer the vertical [[Structural load|load]] (dead weight) to the ground, this generally allows for a comparatively unsophisticated arrangement to be used. However, in the case of wind turbines, due to the high wind and environmental loads experienced there is a significant horizontal dynamic load that needs to be appropriately restrained. This loading regime causes large [[torque|moment loads]] to be applied to the foundations of a wind turbine. As a result, considerable attention needs to be given when designing the footings to ensure that the turbines are sufficiently restrained to operate efficiently.<ref>Cox, J. A., & Jones, C. (2010). Long-Term Performance of Suction Caisson Supported Offshore Wind Turbines. Bristol: University of Bristol.</ref> In the current [[Det Norske Veritas]] (DNV) guidelines for the design of wind turbines the angular deflection of the foundations are limited to 0.5°.<ref>{{cite book |author=Det Norske Veritas|title=Guidelines for Design of Wind Turbines |publisher=Det Norske Veritas |location=Copenhagen |year=2001 |pages= |isbn= |oclc= |doi= |accessdate=}}</ref> DNV guidelines regarding [[earthquakes]] suggest that horizontal loads are larger than vertical loads for offshore wind turbines, while guidelines for [[tsunami]]s only suggest designing for maximum sea waves.<ref name=dnvQuake>[http://exchange.dnv.com:6389/dynaweb/offshore/os-j101/@Generic__BookTextView/11341;hf=0;cs=default;ts=default DNV-OS-J101 Design of Offshore Wind Turbine Structures] ''[[Det Norske Veritas]]''. Accessed: 12 March 2011.</ref> In contrast, IEC suggests considering tsunami loads.<ref name=iec61400/><!--7.3.6 Other loads, page 34--> [[Scale model]] tests using a 50-[[g-force|g]] [[centrifuge]] are being performed at the [[Technical University of Denmark]] to test [[monopile foundation]]s for offshore wind turbines at 30 to 50-m water depth.<ref name="ing50g">Rasmussen, Daniel. [http://ing.dk/artikel/113038-centrifuge-paa-dtu-tester-moellefundamenter-ved-50-g Wind turbine foundations at 50g] (in Danish) ''Ing.dk'', 26 October 2010. [http://ing.dk/artikel/113041-se-dtus-centrifuge-skabe-50-g 6minute Video] Retrieved: 25 November 2010.</ref> == Costs == [[File:Blade Dragon, Installing a single blade, Liftra.jpg|thumb|[[Liftra]] ''Blade Dragon'' installing a single blade on wind turbine hub.<ref>{{cite web|title=Blade Dragon|url=http://www.stateofgreen.com/en/Profiles/Liftra/Products/Blade-Dragon|publisher=State of Green|accessdate=13 December 2012}}</ref><ref>{{cite web|last=R. Simonsen|first=Torben|title=Liftra indstiller Blade Dragon|url=http://ing.dk/artikel/131279|accessdate=13 December 2012}}</ref>]] The modern wind turbine is a complex and integrated system. Structural elements comprise the majority of the weight and cost. All parts of the structure must be inexpensive, lightweight, durable, and manufacturable, under variable loading and environmental conditions. Turbine systems that have fewer failures,<ref name=cost1>Budny, Rob. [http://machinedesign.com/mechanical-drives/bearing-failures-cause-serious-problems-wind-turbines-there-are-solutions Bearing Failures Cause Serious Problems for Wind Turbines, but There Are Solutions] | Machine Design Magazine, 26 June 2014.</ref> require less maintenance, are lighter and last longer will lead to reducing the cost of wind energy. One way to achieve this is to implement well-documented, validated analysis codes, according to a 2011 report from a coalition of researchers from universities, industry, and government, supported by the [[Atkinson Center for a Sustainable Future]].<ref name="univ2011"/> The major parts of a modern turbine may cost (percentage of total): tower 22%, blades 18%, gearbox 14%, generator 8%.<ref name=cost3B>Jamieson, Peter. [https://books.google.dk/books?id=qCAwt6Tgga4C&printsec=frontcover&hl=da Innovation in Wind Turbine Design] p155, ''John Wiley & Sons'', 7 July 2011. Accessed: 26 February 2012. ISBN 0-470-69981-7</ref><ref name=cost2>Jamieson, Peter. [https://books.google.dk/books?id=rf9C33rGR1wC&printsec=frontcover&hl=da Innovation in Wind Turbine Design] sec9-1, ''John Wiley & Sons'', 7 July 2011. Accessed: 26 February 2012. ISBN 1-119-97612-X</ref><!--two editions of the same book, shown for accessibility--> == Efficiency and wind speed == The efficiency of a wind turbine is maximum at its design wind velocity, and efficiency decreases with the fluctuations in wind. The lowest velocity at which the turbine develops its full power is known as rated wind velocity. Below some minimum wind velocity, no useful power output can be produced from wind turbine. There are limits on both the minimum (2–5&nbsp;m/s) and maximum (25–30&nbsp;m/s) wind velocity for the efficient operation of wind turbines.<ref>{{cite book|title=Large wind turbines|author= Hau, E.--(Erich),Snel, Herman|edition=|isbn=0471494569|year=2000|publisher=Wiley, Chichester, New York}}</ref><ref name=Enercon/> [[Conservation of mass]] requires that the amount of air entering and exiting a turbine must be equal. Accordingly, [[Betz's law]] gives the maximal achievable extraction of wind power by a wind turbine as 16/27 (59.3%) of the total kinetic energy of the air flowing through the turbine.<ref>{{cite web |url=http://apps.carleton.edu/campus/library/digitalcommons/assets/pacp_7.pdf |title=The Physics of Wind Turbines Kira Grogg Carleton College, 2005, p.8 |format=PDF |accessdate=2013-11-06}}</ref> The maximum theoretical power output of a wind machine is thus 0.59 times the kinetic energy of the air passing through the effective disk area of the machine. If the effective area of the disk is A, and the wind velocity v, the maximum theoretical power output P is: :<math> P=0.59\frac{1}{2}\rho v^3 A </math> where ''ρ'' is [[air density]] As wind is [[Gratis|free]] (no fuel cost), wind-to-rotor efficiency (including rotor blade [[friction]] and [[drag (physics)|drag]]) is one of many aspects impacting the final [[price]] of wind power.<ref>{{cite web |url=http://windeis.anl.gov/guide/basics/ |title=Wind Energy Basics |publisher=[[Bureau of Land Management]] |accessdate=23 April 2016}}</ref> Further inefficiencies, such as gearbox losses, generator and converter losses, reduce the power delivered by a wind turbine. To protect components from undue wear, extracted power is held constant above the rated operating speed as theoretical power [[Cube (algebra)|increases at the cube]] of wind speed, further reducing theoretical efficiency. In 2001, commercial utility-connected turbines deliver 75% to 80% of the Betz limit of power extractable from the wind, at rated operating speed.<ref name=Enercon>[http://www.enercon.de/p/downloads/EN_Productoverview_0710.pdf "Enercon E-family, 330 Kw to 7.5 Mw, Wind Turbine Specification"] {{webarchive |url=https://web.archive.org/web/20110516022444/http://www.enercon.de/p/downloads/EN_Productoverview_0710.pdf |date=May 16, 2011 }}</ref><ref>Tony Burton et al., (ed), ''Wind Energy Handbook'', John Wiley and Sons 2001 ISBN 0471489972 page 65</ref>{{Update inline|date=April 2016}} All power plants have some consumption when they produce power, and some [[Idle (engine)|standby consumption]] when they are turned on without producing power. For a modern 3 MW wind turbine, the consumption may be 6-58&nbsp;kW depending on circumstances.<ref>[http://www.tu.no/artikler/her-far-vindmollene-penger-for-a-skru-seg-av/358610 Her får vindmøllene penger for å skru seg av] ''[[Teknisk Ukeblad]]'', September 2016.</ref> == Design specification == The [[design specification]] for a wind-turbine will contain a power curve and guaranteed [[availability]]. With the data from the [[wind resource assessment]] it is possible to calculate commercial viability.<ref name="BERR"/> The typical [[operating temperature]] range is {{convert|-20|to|40|C|F}}. In areas with extreme climate (like [[Inner Mongolia]] or [[Rajasthan]]) specific cold and hot weather versions are required. Wind turbines can be designed and validated according to [[IEC 61400]] standards.<ref name=iec61400>[http://webstore.iec.ch/preview/info_iec61400-1%7Bed3.0%7Den.pdf International Standard IEC 61400-1, Third Edition] ''[[International Electrotechnical Commission]]'', August 2005. Accessed: 12 March 2011.</ref> == Low temperature == Utility-scale wind turbine generators have minimum temperature operating limits which apply in areas that experience temperatures below –20&nbsp;°C. Wind turbines must be protected from ice accumulation. It can make [[anemometer]] readings inaccurate and which, in certain turbine control designs, can cause high structure loads and damage. Some turbine manufacturers offer low-temperature packages at a few percent extra cost, which include internal heaters, different lubricants, and different alloys for structural elements. If the low-temperature interval is combined with a low-wind condition, the wind turbine will require an external supply of power, equivalent to a few percent of its rated power, for internal heating. For example, the [[St. Leon, Manitoba]] project has a total rating of 99&nbsp;MW and is estimated to need up to 3&nbsp;MW (around 3% of capacity) of station service power a few days a year for temperatures down to –30&nbsp;°C. This factor affects the economics of wind turbine operation in cold climates. == See also == * [[Brushless wound-rotor doubly fed electric machine]] * [[Floating wind turbine]] * [[Vertical-axis wind turbine]] * [[Wind-turbine aerodynamics]] * [[Copper in renewable energy#Wind|Copper in renewable energy, section Wind]] == References == {{reflist|colwidth=30em}} == Further reading == * Robert Gasch, Jochen Twele (ed.), ''Wind power plants. Fundamentals, design, construction and operation'', Springer 2012 ISBN 978-3-642-22937-4. *{{cite book|title=Wind Power: Renewable Energy for Home, Farm, and Business|editor=Paul Gipe| edition= second|ISBN=978-1-931498-14-2|year=2004|publisher=Chelsea Green Publishing Company}} * Erich Hau, ''Wind turbines: fundamentals, technologies, application, economics '' Springer, 2013 ISBN 978-3-642-27150-2 (preview on Google Books) * Siegfried Heier, ''Grid integration of wind energy conversion systems'' Wiley 2006, ISBN 978-0-470-86899-7. * Peter Jamieson, ''Innovation in Wind Turbine Design''. Wiley & Sons 2011, ISBN 978-0-470-69981-2 * David Spera (ed,) ''Wind Turbine Technology: Fundamental Concepts in Wind Turbine Engineering'', Second Edition (2009), ASME Press, ISBN 9780791802601 * Alois Schaffarczyk (ed.), ''Understanding wind power technology'', Wiley & Sons 2014, ISBN 978-1-118-64751-6. *{{cite book|title=Wind Power Generation and Wind Turbine Design|editor=Wei Tong|ISBN=978-1-84564-205-1|year= 2010|publisher= WIT Press}} * Hermann-Josef Wagner, Jyotirmay Mathur, ''Introduction to wind energy systems. Basics, technology and operation''. Springer 2013, ISBN 978-3-642-32975-3. == External links == {{commons category|Wind turbines}} * [http://www.brighthub.com/environment/renewable-energy/articles/63997.aspx Offshore Wind Turbines - Installation and Operation of Turbines] * [http://www.eere.energy.gov/ Department of Energy- Energy Efficiency and Renewable Energy] * [http://www.bwea.com/ref/faq.html#efficient/ RenewableUK - Wind Energy Reference and FAQs] * [http://www.madehow.com/Volume-1/Wind-Turbine.html How is Wind turbine made] {{wind power}} {{DEFAULTSORT:Wind Turbine Design}} [[Category:Wind turbines]]'