Yaw system
Encyclopedia
The yaw system of wind turbines is the component responsible for the orientation of the wind turbine rotor
towards the wind
.
appeared in the mid-18th century. Their rotatable nacelle
s were mounted on the main structure of the windmill
using primitive wooden gliding bearings
lubricated with animal fat
. The necessary yawing torque was created by means of animal power
, human power
or even wind power
(implementation of an auxiliary rotor known as fantail).
Vertical axis wind turbines (VAWT) do not need a yaw system since their vertical rotors can face the wind from any direction and only their self rotation gives the blades a clear direction of the air flow. Horizontal axis wind turbines
however need to orient their rotors into and out of the wind
and they achieve that by means of passive or active yaw
systems.
Horizontal axis wind turbines
employ some sort of yaw system which can be passive or active. Both passive and active systems have advantages and disadvantages and various design solutions (both active and passive) are being tried in order to find the optimal design for each wind turbine depending on its size, cost and purpose of operation.
). The various components of the modern active yaw systems vary depending on the design characteristics but all the active yaw systems include a means of rotatable connection between nacelle and tower (yaw bearing
), a means of active variation of the rotor orientation (i.e. yaw drive
), a means of restricting the rotation of the nacelle (yaw brake) and a control system which processes the signals from wind direction sensors (e.g. wind vanes
) and gives the proper
commands to the actuating mechanisms.
The most common types of active yaw systems are:
rotor into the wind. In their simplest form these system comprise a simple roller bearing connection between the tower and the nacelle and a tail fin mounted on the nacelle and designed in such a way that it turns the wind turbine
rotor into the wind by exerting a "corrective" torque to the nacelle. Therefore the power of the wind is responsible for the rotor rotation and the nacelle orientation. Alternatively in case of downwind
turbines the tail fin is not necessary since the rotor itself is able to yaw
the nacelle into the wind. In the event of skew winds the "wind pressure" on the swept area causes a yawing moment
around the tower axis (z-axis) which orients the rotor.
The tail fin (or wind vane) is commonly used for small wind turbines since it offers a low cost and reliable solution. It is however unable to cope with the high moments required to yaw the nacelle of a large wind turbine. The self-orientation of the downwind
turbine rotors however is a concept able to function even for larger wind turbines. The French wind turbine manufacturer Vergnet
has several medium and large self-orientating downwind
wind turbines in production.
Passive yaw systems have to be designed in a way that the nacelle does not follow the sudden changes in wind direction
with too fast a yaw movement, in order to avoid high gyroscopic
loads. Additionally the passive yaw systems with low yaw-friction
are subjected to strong dynamic loads due to the periodic low amplitude
yawing caused by the variation of the inertia moment
during the rotor rotation. This effect becomes more severe with the reduction of the number of blades.
The most common passive yaw systems are:
. It can be of the roller or gliding type and it serves as a rotatable connection between the tower and the nacelle
of the wind turbine
. The yaw bearing
should be able to handle very high loads, which apart from the weight of the nacelle
and rotor (the weight of which is in the range of several tenths of ton
s) include also the bending moment
s caused by the rotor during the extraction of the kinetic energy
of the wind.
s exist only on the active yaw systems and are the means of active rotation of the wind turbine
nacelle
. Each yaw drive
consists of powerful electric motor
(usually AC
) with its electric drive and a large gearbox, which increases the torque
. The maximum static torque
of the biggest yaw drive
s is in the range of 200.000Nm with gearbox reduction ratios
in the range of 2000:1. Consequently the yawing of the large modern turbines
is relatively slow with a 360° turn lasting several minutes.
against rotation a means of braking is necessary. One of the simplest ways to realize that task is to apply a constant small counter-torque at the yaw drive
s in order to eliminate the backlash
between gear-rim
and yaw drive
pinions and to prevent the nacelle
from oscillating due to the rotor rotation. This operation however greatly reduces the reliability
of the electric yaw drive
s, therefore the most common solution is the implementation of a hydraulically actuated disk brake.
The disk brake requires a flat circular brake disk
and plurality of brake calipers with hydraulic piston
s and brake pads
http://svendborg-brakes.dk/uk/products/brakes/active_brakes.html. The hydraulic yaw brakes are able to fix the nacelle
in position thus relieving the yaw drive
s from that task. The cost however of the yaw brake in combination with the requirement of a hydraulic installation (pump
, valves, piston
s) and its installation in the vicinity of brake pads sensitive to lubricant
contamination
is often an issue.
A compromise that offers several advantages is the use of electric yaw brakes. These replace the hydraulic mechanism of the conventional brakes and with electro-mechanically actuated brake calipers. The use of electric yaw brakes eliminates the complexity of the hydraulic leakages and the subsequent problems that these cause to the yaw brake operation.
Several wind turbine design and manufacturing companies experiment with alternative yaw breaking methods in order to eliminate the drawbacks of the existing systems and to reduce the cost of the system. One of these alternatives involves the use of air pressure in order to achieve the necessary yaw braking moment. In this case, some of the gliding surface (usually the axial, due to higher available surface) is utilized in order to accommodate the yaw brake pads and the pneumatic brake mechanism. The pneumatic actuator can be a conventional pneumatic cylinder or even a flexible air chamber which inflates when supplied with pressurized air. Such a device is able to exert very high braking forces due to the high active surface. This is achieved with a simple industrial air pressure compression system (6-10 bar) which is a reliable and low cost solution. Furthermore in the even of leakage, the environmental impact is practically zero compared to hydraulic oil leakages. Finally brake actuators can be produced with very low cost from lightweight plastic materials thus significantly reducing the overall cost of the system.
by means of a long beam
. The combination of the large surface
of the fin and the increased length
of the beam create a considerable torque
which is able to rotate the nacelle
despite the stabilizing gyroscopic effects of the rotor. The required surface
however for a tail fin to be able to yaw
a large wind turbine
is enormous thus rendering the use of such a devise un-economical.
Rotor
Rotor may refer to:*A rotating part of a mechanical device, for example in an electric motor, generator, alternator or pump.In engineering:...
towards the wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
.
History
The task of orientating the rotor into the wind was a complicated issue already for historical windmills. The first windmills able to rotate in order to "face" the windWind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
appeared in the mid-18th century. Their rotatable nacelle
Nacelle
The nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
s were mounted on the main structure of the windmill
Windmill
A windmill is a machine which converts the energy of wind into rotational energy by means of vanes called sails or blades. Originally windmills were developed for milling grain for food production. In the course of history the windmill was adapted to many other industrial uses. An important...
using primitive wooden gliding bearings
Yaw bearing
The yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the...
lubricated with animal fat
Animal fat
Animal fats are rendered tissue fats that can be obtained from a variety of animals.- Pet nutrition :In pet nutrition, the source of animal fat concerns food manufacturers. AAFCO states that animal fat is "obtained from the tissues of mammals and/or poultry in the commercial processes of rendering...
. The necessary yawing torque was created by means of animal power
Working animal
A working animal is an animal, usually domesticated, that is kept by humans and trained to perform tasks. They may be close members of the family, such as guide or service dogs, or they may be animals trained strictly to perform a job, such as logging elephants. They may also be used for milk, a...
, human power
Human power
Human power is work or energy that is produced from the human body. It can also refer to the power of a human. Power comes primarily from muscles, but body heat is also used to do work like warming shelters, food, or other humans....
or even wind power
Wind power
Wind power is the conversion of wind energy into a useful form of energy, such as using wind turbines to make electricity, windmills for mechanical power, windpumps for water pumping or drainage, or sails to propel ships....
(implementation of an auxiliary rotor known as fantail).
Vertical axis wind turbines (VAWT) do not need a yaw system since their vertical rotors can face the wind from any direction and only their self rotation gives the blades a clear direction of the air flow. Horizontal axis wind turbines
Wind turbine
A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
however need to orient their rotors into and out of the wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
and they achieve that by means of passive or active yaw
systems.
Horizontal axis wind turbines
Wind turbine
A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
employ some sort of yaw system which can be passive or active. Both passive and active systems have advantages and disadvantages and various design solutions (both active and passive) are being tried in order to find the optimal design for each wind turbine depending on its size, cost and purpose of operation.
Types
Active yaw systems
The active yaw systems are equipped with some sort of torque producing device able to rotate the nacelle of the wind turbine against the stationary tower based on automatic signals from wind direction sensors or manual actuation (control system override). The active yaw systems are considered to be the state of the art for all the modern medium and large sized wind turbines, with a few exceptions proving the rule (e.g. VergnetVergnet
Vergnet is a French wind turbine manufacturer headquartered in Orléans. The company also produces water pumps installed in Africa.Vergnet wind turbines are small to mid size designed for operation in tropical countries: Caribbean, Africa, parts of Asia and South America...
). The various components of the modern active yaw systems vary depending on the design characteristics but all the active yaw systems include a means of rotatable connection between nacelle and tower (yaw bearing
Yaw bearing
The yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the...
), a means of active variation of the rotor orientation (i.e. yaw drive
Yaw drive
The yaw drive is an important component of the horizontal axis wind turbines' yaw system. To ensure the wind turbine is producing the maximal amount of electric energy at all times, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. This only applies for...
), a means of restricting the rotation of the nacelle (yaw brake) and a control system which processes the signals from wind direction sensors (e.g. wind vanes
Weather vane
A weather vane is an instrument for showing the direction of the wind. They are typically used as an architectural ornament to the highest point of a building....
) and gives the proper
commands to the actuating mechanisms.
The most common types of active yaw systems are:
- Roller yaw bearingYaw bearingThe yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the...
- Electric yaw driveYaw driveThe yaw drive is an important component of the horizontal axis wind turbines' yaw system. To ensure the wind turbine is producing the maximal amount of electric energy at all times, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. This only applies for...
- Brake: The nacelle is mounted on a roller bearing and the azimuth rotation is achieved via a plurality of powerful electric drivesElectric motorAn electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
. A hydraulic or electric brakeBrakeA brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The rest of this article is dedicated to various types of vehicular brakes....
fixes the position of the nacelleNacelleThe nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
when the re-orientation is completed in order to avoid wearWearIn materials science, wear is erosion or sideways displacement of material from its "derivative" and original position on a solid surface performed by the action of another surface....
and high fatigue loadsFatigue (material)'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.Fatigue occurs...
on wind turbine components due to backlashBacklash (engineering)In mechanical engineering, backlash, sometimes called lash or play, is clearance between mating components, sometimes described as the amount of lost motion due to clearance or slackness when movement is reversed and contact is re-established...
. Systems of this kind are used by most of the wind turbine manufacturers and are considered to be reliable and effective but also quite bulky and expensive.
- Roller yaw bearingYaw bearingThe yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the...
- Hydraulic yaw driveYaw driveThe yaw drive is an important component of the horizontal axis wind turbines' yaw system. To ensure the wind turbine is producing the maximal amount of electric energy at all times, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. This only applies for...
: The nacelle is mounted on a roller bearing and the azimuth rotation is achieved via a plurality of powerful hydraulic motorHydraulic motorA hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into torque and angular displacement . The hydraulic motor is the rotary counterpart of the hydraulic cylinder....
s or ratchetingRatchet (device)A ratchet is a device that allows continuous linear or rotary motion in only one direction while preventing motion in the opposite direction. Because most socket wrenches today use ratcheting handles, the term "ratchet" alone is often used to refer to a ratcheting wrench, and the terms "ratchet"...
hydraulic cylinderHydraulic cylinderA Hydraulic cylinder is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in engineering vehicles.- Operation :...
s. The benefit of the yaw system with hydraulic drivesHydraulic motorA hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into torque and angular displacement . The hydraulic motor is the rotary counterpart of the hydraulic cylinder....
has to do with the inherent benefits of the hydraulic systems such as the high power-to-weight ratioPower-to-weight ratioPower-to-weight ratio is a calculation commonly applied to engines and mobile power sources to enable the comparison of one unit or design to another. Power-to-weight ratio is a measurement of actual performance of any engine or power sources...
and high reliabilityReliability engineeringReliability engineering is an engineering field, that deals with the study, evaluation, and life-cycle management of reliability: the ability of a system or component to perform its required functions under stated conditions for a specified period of time. It is often measured as a probability of...
. On the downside however the hydraulic systems are always troubled by leakages of hydraulic fluidHydraulic fluidHydraulic fluids, also called hydraulic liquids, are the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water...
and clogging of their high pressure hydraulic valvesValveA valve is a device that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category...
. The hydraulic yaw systems often (depending on the system design) also allow for the elimination of the yaw brake mechanism and their replacement with cut-off valvesGate valveThe gate valve, also known as a sluice valve, is a valve that opens by lifting a round or rectangular gate/wedge out of the path of the fluid. The distinct feature of a gate valve is the sealing surfaces between the gate and seats are planar, so gate valves are often used when a straight-line flow...
.
- Gliding yaw bearingYaw bearingThe yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the...
- Electric yaw driveYaw driveThe yaw drive is an important component of the horizontal axis wind turbines' yaw system. To ensure the wind turbine is producing the maximal amount of electric energy at all times, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. This only applies for...
: The nacelle is mounted on a friction based gliding bearingYaw bearingThe yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the...
and the azimuth rotation is achieved via a plurality of powerful electric drivesElectric motorAn electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
. The need for a yaw brake is eliminated and depending on the size of the yaw system (i.e. size of the wind turbineWind turbineA wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
) the gliding bearingYaw bearingThe yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the...
concept can lead to significant cost savings.
- Gliding yaw bearingYaw bearingThe yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the...
- Hydraulic yaw driveYaw driveThe yaw drive is an important component of the horizontal axis wind turbines' yaw system. To ensure the wind turbine is producing the maximal amount of electric energy at all times, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. This only applies for...
: The nacelle is mounted on a friction based gliding bearingYaw bearingThe yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the...
and the azimuth rotation is achieved via a plurality of powerful hydraulic motorHydraulic motorA hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into torque and angular displacement . The hydraulic motor is the rotary counterpart of the hydraulic cylinder....
s or ratchetingRatchet (device)A ratchet is a device that allows continuous linear or rotary motion in only one direction while preventing motion in the opposite direction. Because most socket wrenches today use ratcheting handles, the term "ratchet" alone is often used to refer to a ratcheting wrench, and the terms "ratchet"...
hydraulic cylinderHydraulic cylinderA Hydraulic cylinder is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in engineering vehicles.- Operation :...
s. This system combines the characteristics of the aforementioned gliding bearingYaw bearingThe yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the...
and hydraulic motorHydraulic motorA hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into torque and angular displacement . The hydraulic motor is the rotary counterpart of the hydraulic cylinder....
systems.
Passive yaw systems
The passive yaw systems utilize the wind force in order to adjust the orientation of the wind turbineWind turbine
A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
rotor into the wind. In their simplest form these system comprise a simple roller bearing connection between the tower and the nacelle and a tail fin mounted on the nacelle and designed in such a way that it turns the wind turbine
Wind turbine
A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
rotor into the wind by exerting a "corrective" torque to the nacelle. Therefore the power of the wind is responsible for the rotor rotation and the nacelle orientation. Alternatively in case of downwind
Windward and leeward
Windward is the direction upwind from the point of reference. Leeward is the direction downwind from the point of reference. The side of a ship that is towards the leeward is its lee side. If the vessel is heeling under the pressure of the wind, this will be the "lower side"...
turbines the tail fin is not necessary since the rotor itself is able to yaw
Yaw
The word yaw can refer to:* Yaw angle, one of the Tait-Bryan angles, describing the heading of a vehicle or machine, and some other related elements:**Yaw system, component responsible for the orientation of a wind turbine towards the wind....
the nacelle into the wind. In the event of skew winds the "wind pressure" on the swept area causes a yawing moment
Moment (physics)
In physics, the term moment can refer to many different concepts:*Moment of force is the tendency of a force to twist or rotate an object; see the article torque for details. This is an important, basic concept in engineering and physics. A moment is valued mathematically as the product of the...
around the tower axis (z-axis) which orients the rotor.
The tail fin (or wind vane) is commonly used for small wind turbines since it offers a low cost and reliable solution. It is however unable to cope with the high moments required to yaw the nacelle of a large wind turbine. The self-orientation of the downwind
Windward and leeward
Windward is the direction upwind from the point of reference. Leeward is the direction downwind from the point of reference. The side of a ship that is towards the leeward is its lee side. If the vessel is heeling under the pressure of the wind, this will be the "lower side"...
turbine rotors however is a concept able to function even for larger wind turbines. The French wind turbine manufacturer Vergnet
Vergnet
Vergnet is a French wind turbine manufacturer headquartered in Orléans. The company also produces water pumps installed in Africa.Vergnet wind turbines are small to mid size designed for operation in tropical countries: Caribbean, Africa, parts of Asia and South America...
has several medium and large self-orientating downwind
Windward and leeward
Windward is the direction upwind from the point of reference. Leeward is the direction downwind from the point of reference. The side of a ship that is towards the leeward is its lee side. If the vessel is heeling under the pressure of the wind, this will be the "lower side"...
wind turbines in production.
Passive yaw systems have to be designed in a way that the nacelle does not follow the sudden changes in wind direction
Wind direction
Wind direction is reported by the direction from which it originates. For example, a northerly wind blows from the north to the south. Wind direction is usually reported in cardinal directions or in azimuth degrees...
with too fast a yaw movement, in order to avoid high gyroscopic
Gyroscope
A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...
loads. Additionally the passive yaw systems with low yaw-friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
are subjected to strong dynamic loads due to the periodic low amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
yawing caused by the variation of the inertia moment
Moment of inertia
In classical mechanics, moment of inertia, also called mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass, is a measure of an object's resistance to changes to its rotation. It is the inertia of a rotating body with respect to its rotation...
during the rotor rotation. This effect becomes more severe with the reduction of the number of blades.
The most common passive yaw systems are:
- Roller Bearing (free system): The nacelleNacelleThe nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
is mounted on a roller bearing and it is free to rotate towards any direction. The necessary momentMoment (physics)In physics, the term moment can refer to many different concepts:*Moment of force is the tendency of a force to twist or rotate an object; see the article torque for details. This is an important, basic concept in engineering and physics. A moment is valued mathematically as the product of the...
comes from a tail fin or the rotor (downwindWindward and leewardWindward is the direction upwind from the point of reference. Leeward is the direction downwind from the point of reference. The side of a ship that is towards the leeward is its lee side. If the vessel is heeling under the pressure of the wind, this will be the "lower side"...
wind turbines) - Roller Bearing - Brake (Semi-active system): The nacelleNacelleThe nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
is mounted on a roller bearing and it is free to rotate towards any direction, but when the necessary orientation is achieved an active yaw brake arrests the nacelleNacelleThe nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
. This prevents the uncontrolled vibrationVibrationVibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
and reduced gyroscopic and fatigue loadsFatigue (material)'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.Fatigue occurs...
. - Gliding Bearing/Brake (Passive system): The nacelleNacelleThe nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
is mounted on a gliding bearingYaw bearingThe yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the...
and it is free to rotate towards any direction. The inherent frictionFrictionFriction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
of the gliding bearingYaw bearingThe yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the...
achieves a quasi-active way of operation.
Components
Yaw bearing
One of the main components of the yaw system is the yaw bearingYaw bearing
The yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the...
. It can be of the roller or gliding type and it serves as a rotatable connection between the tower and the nacelle
Nacelle
The nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
of the wind turbine
Wind turbine
A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
. The yaw bearing
Yaw bearing
The yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the...
should be able to handle very high loads, which apart from the weight of the nacelle
Nacelle
The nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
and rotor (the weight of which is in the range of several tenths of ton
Ton
The ton is a unit of measure. It has a long history and has acquired a number of meanings and uses over the years. It is used principally as a unit of weight, and as a unit of volume. It can also be used as a measure of energy, for truck classification, or as a colloquial term.It is derived from...
s) include also the bending moment
Bending Moment
A bending moment exists in a structural element when a moment is applied to the element so that the element bends. Moments and torques are measured as a force multiplied by a distance so they have as unit newton-metres , or pound-foot or foot-pound...
s caused by the rotor during the extraction of the kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
of the wind.
Yaw drives
The yaw driveYaw drive
The yaw drive is an important component of the horizontal axis wind turbines' yaw system. To ensure the wind turbine is producing the maximal amount of electric energy at all times, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. This only applies for...
s exist only on the active yaw systems and are the means of active rotation of the wind turbine
Wind turbine
A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
nacelle
Nacelle
The nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
. Each yaw drive
Yaw drive
The yaw drive is an important component of the horizontal axis wind turbines' yaw system. To ensure the wind turbine is producing the maximal amount of electric energy at all times, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. This only applies for...
consists of powerful electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
(usually AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
) with its electric drive and a large gearbox, which increases the torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
. The maximum static torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
of the biggest yaw drive
Yaw drive
The yaw drive is an important component of the horizontal axis wind turbines' yaw system. To ensure the wind turbine is producing the maximal amount of electric energy at all times, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. This only applies for...
s is in the range of 200.000Nm with gearbox reduction ratios
Gear ratio
The gear ratio of a gear train is the ratio of the angular velocity of the input gear to the angular velocity of the output gear, also known as the speed ratio of the gear train. The gear ratio can be computed directly from the numbers of teeth of the various gears that engage to form the gear...
in the range of 2000:1. Consequently the yawing of the large modern turbines
Wind turbine
A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
is relatively slow with a 360° turn lasting several minutes.
Yaw brake
In order to stabilize the yaw bearingYaw bearing
The yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the...
against rotation a means of braking is necessary. One of the simplest ways to realize that task is to apply a constant small counter-torque at the yaw drive
Yaw drive
The yaw drive is an important component of the horizontal axis wind turbines' yaw system. To ensure the wind turbine is producing the maximal amount of electric energy at all times, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. This only applies for...
s in order to eliminate the backlash
Backlash (engineering)
In mechanical engineering, backlash, sometimes called lash or play, is clearance between mating components, sometimes described as the amount of lost motion due to clearance or slackness when movement is reversed and contact is re-established...
between gear-rim
Yaw drive
The yaw drive is an important component of the horizontal axis wind turbines' yaw system. To ensure the wind turbine is producing the maximal amount of electric energy at all times, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. This only applies for...
and yaw drive
Yaw drive
The yaw drive is an important component of the horizontal axis wind turbines' yaw system. To ensure the wind turbine is producing the maximal amount of electric energy at all times, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. This only applies for...
pinions and to prevent the nacelle
Nacelle
The nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
from oscillating due to the rotor rotation. This operation however greatly reduces the reliability
Reliability engineering
Reliability engineering is an engineering field, that deals with the study, evaluation, and life-cycle management of reliability: the ability of a system or component to perform its required functions under stated conditions for a specified period of time. It is often measured as a probability of...
of the electric yaw drive
Yaw drive
The yaw drive is an important component of the horizontal axis wind turbines' yaw system. To ensure the wind turbine is producing the maximal amount of electric energy at all times, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. This only applies for...
s, therefore the most common solution is the implementation of a hydraulically actuated disk brake.
The disk brake requires a flat circular brake disk
Disc brake
The disc brake or disk brake is a device for slowing or stopping the rotation of a wheel while it is in motion.A brake disc is usually made of cast iron, but may in some cases be made of composites such as reinforced carbon–carbon or ceramic matrix composites. This is connected to the wheel and/or...
and plurality of brake calipers with hydraulic piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
s and brake pads
Brake pads
Brake pads are a component of disk brakes used in automotive and other applications. Brake pads are steel backing plates with friction material bound to the surface that faces the disk brake rotor.- Function :...
http://svendborg-brakes.dk/uk/products/brakes/active_brakes.html. The hydraulic yaw brakes are able to fix the nacelle
Nacelle
The nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
in position thus relieving the yaw drive
Yaw drive
The yaw drive is an important component of the horizontal axis wind turbines' yaw system. To ensure the wind turbine is producing the maximal amount of electric energy at all times, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. This only applies for...
s from that task. The cost however of the yaw brake in combination with the requirement of a hydraulic installation (pump
Pump
A pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...
, valves, piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
s) and its installation in the vicinity of brake pads sensitive to lubricant
Lubricant
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...
contamination
Contamination
Contamination is the presence of a minor and unwanted constituent in material, physical body, natural environment, at a workplace, etc.-Specifics:"Contamination" also has more specific meanings in science:...
is often an issue.
A compromise that offers several advantages is the use of electric yaw brakes. These replace the hydraulic mechanism of the conventional brakes and with electro-mechanically actuated brake calipers. The use of electric yaw brakes eliminates the complexity of the hydraulic leakages and the subsequent problems that these cause to the yaw brake operation.
Several wind turbine design and manufacturing companies experiment with alternative yaw breaking methods in order to eliminate the drawbacks of the existing systems and to reduce the cost of the system. One of these alternatives involves the use of air pressure in order to achieve the necessary yaw braking moment. In this case, some of the gliding surface (usually the axial, due to higher available surface) is utilized in order to accommodate the yaw brake pads and the pneumatic brake mechanism. The pneumatic actuator can be a conventional pneumatic cylinder or even a flexible air chamber which inflates when supplied with pressurized air. Such a device is able to exert very high braking forces due to the high active surface. This is achieved with a simple industrial air pressure compression system (6-10 bar) which is a reliable and low cost solution. Furthermore in the even of leakage, the environmental impact is practically zero compared to hydraulic oil leakages. Finally brake actuators can be produced with very low cost from lightweight plastic materials thus significantly reducing the overall cost of the system.
Yaw vane (passive systems)
The yaw vane (or tail fin) is a component of the yaw system used only on small wind turbines with passive yaw mechanisms. It is nothing more than a flat surface mounted on the nacelleNacelle
The nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
by means of a long beam
Beam (structure)
A beam is a horizontal structural element that is capable of withstanding load primarily by resisting bending. The bending force induced into the material of the beam as a result of the external loads, own weight, span and external reactions to these loads is called a bending moment.- Overview...
. The combination of the large surface
Surface
In mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R3 — for example, the surface of a ball...
of the fin and the increased length
Length
In geometric measurements, length most commonly refers to the longest dimension of an object.In certain contexts, the term "length" is reserved for a certain dimension of an object along which the length is measured. For example it is possible to cut a length of a wire which is shorter than wire...
of the beam create a considerable torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
which is able to rotate the nacelle
Nacelle
The nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
despite the stabilizing gyroscopic effects of the rotor. The required surface
Surface
In mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R3 — for example, the surface of a ball...
however for a tail fin to be able to yaw
Yaw
The word yaw can refer to:* Yaw angle, one of the Tait-Bryan angles, describing the heading of a vehicle or machine, and some other related elements:**Yaw system, component responsible for the orientation of a wind turbine towards the wind....
a large wind turbine
Wind turbine
A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
is enormous thus rendering the use of such a devise un-economical.
See also
- Wind turbineWind turbineA wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
- WindmillWindmillA windmill is a machine which converts the energy of wind into rotational energy by means of vanes called sails or blades. Originally windmills were developed for milling grain for food production. In the course of history the windmill was adapted to many other industrial uses. An important...
- Yaw bearingYaw bearingThe yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the...
- Yaw driveYaw driveThe yaw drive is an important component of the horizontal axis wind turbines' yaw system. To ensure the wind turbine is producing the maximal amount of electric energy at all times, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. This only applies for...
- Wind powerWind powerWind power is the conversion of wind energy into a useful form of energy, such as using wind turbines to make electricity, windmills for mechanical power, windpumps for water pumping or drainage, or sails to propel ships....
- Wind turbine designWind turbine designWind turbine design is the process of defining the form and specifications of a wind turbine to extract energy from the wind. A wind turbine installation consists of the necessary systems needed to capture the wind's energy, point the turbine into the wind, convert mechanical rotation into...
Further reading
- Wind Power Plants, R. Gasch and J. Twele, Solarpraxis, ISBN 3-934595-23-5
- Wind Energy Handbook, T. Burton [et al.], John Wiley & Sons, Ltd, ISBN 0-471-48997-2
- How to build a wind turbine - The axial flux windmill plans, H. Piggott Scoraigwind