The goldenmean or golden middle way is the
desirable middle between two extremes, one of excess and the other of
deficiency. It appeared in Greek at least as early as the Delphic maxim "nothing in excess", which was discussed in Plato's Philebus. Aristotle analyzed the golden mean in the Nicomachean Ethics Book II: That virtues of character can be described as means. It was subsequently emphasized in Aristotelianvirtue ethics. For example, in the Aristotelian view, courage is a virtue, but if taken to excess would manifest as recklessness, and, in deficiency, cowardice. The middle way form of government for Aristotle was a blend between monarchy, democracy and aristocracy.
History
Western philosophy
Crete
The earliest representation of this idea in culture is probably in the mythological Cretan tale of Daedalus and Icarus.
Daedalus, a famous artist of his time, built feathered wings for
himself and his son so that they might escape the clutches of King Minos. Daedalus warns his beloved son whom he loved so much to "fly the middle course",
between the sea spray and the sun's heat. Icarus did not heed his
father; he flew up and up until the sun melted the wax off his wings.
For not heeding the middle course, he fell into the sea and drowned.
Delphi
Another early elaboration is the Doric saying carved on the front of the temple at Delphi: "Nothing in excess" ("Μηδὲν ἄγαν").
Cleobulus
To Cleobulus is attributed the maxim: Μέτρον ἄριστον ("Moderation is best").
Socrates
Socrates teaches that a man must know "how to choose the mean and avoid the extremes on either side, as far as possible."
In education, Socrates asks us to consider the effect of either
an exclusive devotion to gymnastics or an exclusive devotion to music.
It either "produced a temper of hardness and ferocity, (or) the other of
softness and effeminacy." Having both qualities, he believed, produces harmony; i.e., beauty and goodness.
Plato
Proportion's relation to beauty and goodness is stressed throughout Plato's dialogues, particularly in the Republic and Philebus. He writes (Phlb. 64d–65a):
Socrates: That any kind of mixture that does not in some
way or other possess measure of the nature of proportion will
necessarily corrupt its ingredients and most of all itself. For there
would be no blending in such a case at all but really an unconnected
medley, the ruin of whatever happens to be contained in it. Protarchus: Very true. Socrates: But
now we notice that the force of the good has taken up refuge in an
alliance with the nature of the beautiful. For measure and proportion
manifest themselves in all areas of beauty and virtue. Protarchus: Undeniably. Socrates: But we said that truth is also inclined along with them in our mixture? Protarchus: Indeed. Socrates: Well, then, if we cannot capture the good in one
form, we will have to take hold of it in a conjunction of three:
beauty, proportion and truth. Let us affirm that these should by right
be treated as a unity and be held responsible for what is in the
mixture, for goodness is what makes the mixture good in itself.
In the Laws,
Plato applies this principle to electing a government in the ideal
state: "Conducted in this way, the election will strike a mean between
monarchy and democracy …"
Aristotle
In the Eudemian Ethics, Aristotle
writes on the virtues. Aristotle’s theory on virtue ethics is one that
does not see a person’s actions as a reflection of their ethics but
rather looks into the character of a person as the reason behind their
ethics. His constant phrase is, "… is the Middle state between …". His
psychology of the soul and its virtues is based on the golden mean
between the extremes. In the Politics,
Aristotle criticizes the Spartan Polity by critiquing the
disproportionate elements of the constitution; e.g., they trained the
men and not the women, and they trained for war but not peace. This
disharmony produced difficulties which he elaborates on in his work. See
also the discussion in the Nicomachean Ethics of the golden mean, and Aristotelian ethics in general.
Each intellectual virtue is a mental skill or habit by which the
mind arrives at truth, affirming what is or denying what is not. In the Nicomachean Ethics he discusses 11 moral virtues:
Jacques Maritain, throughout his Introduction to Philosophy (1930), uses the idea of the golden mean to place Aristotelian-Thomist
philosophy between the deficiencies and extremes of other philosophers
and systems.
Eastern philosophy
Gautama Buddha (fl. 6th century BC) taught of the Middle Way, a path between the extremes of religious asceticism and worldly self-indulgence.
Zhuangzi was the Tao's most famous commentator (369–286 BC).
Tiruvalluvar (2nd century BC and the 8th century AD; date disputed) in his Tirukkural of the Sangam period of Tamilakam writes of the middle state
which is to preserve equity. He emphasises this principle and suggests
that the two ways of preserving equity is to be impartial and avoid
excess. Parimelalagar was the historical commentator of the Tirukkural.
One such instance is Ecclesiastes7:15-16, where the preacher admonishes his audience to "be not righteous over much" and to "be not over much wicked." Adam Clarke takes the phrase "righteous over much" to mean indulging in too much "austerity and hard study," and concludes that “there is no need of all this watching, fasting,
praying, self-denial, etc., you carry things to extremes. Why should you
wish to be reputed singular and precise?” Thus, the ideal of the golden mean may have existed as long as six
hundred years before Aristotle. However, some scholars, such as Albert
Barnes, hold a slightly different interpretation of Ecclesiastes
7:16-17.
Ahead of the times Rambam, 1138-1204 AD (probably due to Plato's and Aristotle’s engagement with Ethics),
determined that a person has to take care of his soul as well as his
body, and just as a person who is sick in his body turns to the doctor, a
person who has mental illness needs to go to the doctor of the soul,
which is, according to him, the philosopher or the sage. Rambam opposed the deterministic approach, arguing that a person has free will and the ability to change its properties.
The golden mean is also a core principle in Musar literature in which practitioners are encouraged to bring every character trait (middah; plural middot)
into a balanced place between extremes. For example, it is not good to
have too much patience, but it is not good to live without any patience
at all. Musar can be said to involve being mindful enough to bring one's
character traits, thoughts and desires into a balanced state in real
time; living one's life in accord with the golden mean.
Christianity
Thomas Aquinas, the medieval Catholicphilosopher and theologian, in his Summa Theologiae, Prima Secundæ Partis, Question 64,
argued that Christian morality is consistent with the mean: "evil
consists in discordance from their rule or measure. Now this may happen
either by their exceeding the measure or by their falling short of it[.]
... Therefore it is evident that moral virtue observes the mean."
Islam
Islam promotes the golden mean in many instances. The Quran
states an example in finance, in that a person should not spend all he
makes as not to be caught needing, and not to be stingy as to not live a
comfortable life. Muhammad
also had a saying "خير الأمور أوسطها" meaning the best choice is the
middle ground/golden mean one. In Quran (Chapter 'The Cow', verse number
143) it is said that, "We have made you a balanced, moderate nation".
Quran quotes the example of two groups of people, calling one of
them extremely greedy (Chasing the wealth of the world) in Chapter 'The
Cow' verse 96 and to the others as inventors of monasticism
(over-zealousness in religion) in Chapter Al-Hadeed verse number 27.
Islam counsels its followers to abstain from both these paths of
extremities and adopt moderation in chasing the world and practicing
religion alike.
Not the least the Quran emphasises that the Muslim community
(Umma) is a ’middle nation’ / a 'just community' / an Umma justly
balanced / a moderate nation / a midmost nation (ummatan wasaTan) in
verse 2-143: a middle between extremism and sloppiness.
Quotations
"In many things the middle have the best / Be mine a middle station." — Phocylides
"When Coleridge
tried to define beauty, he returned always to one deep thought; beauty,
he said, is unity in variety! Science is nothing else than the search
to discover unity in the wild variety of nature,—or, more exactly, in
the variety of our experience. Poetry, painting, the arts are the same
search, in Coleridge’s phrase, for unity in variety." — Jacob Bronowski
"…but for harmony beautiful to contemplate, science would not be worth following." — Henri Poincaré.
"If a man finds that his nature tends or is disposed to one of these
extremes..., he should turn back and improve, so as to walk in the way
of good people, which is the right way. The right way is the mean in
each group of dispositions common to humanity; namely, that disposition
which is equally distant from the two extremes in its class, not being
nearer to the one than to the other." — Maimonides
"What is wanted is a balance between extravagance and miserliness
through moderation, with the goal of distance between both extremes." — al-Ghazali
Wind turbine design is the process of defining the form and configuration of a wind turbine to extract energy from the wind. An installation consists of the systems needed to capture the wind's energy, point the turbine into the wind, convert mechanical rotation into electrical power, and other systems to start, stop, and control the turbine.
In 1919, German 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 wind's kinetic energy to be captured. This Betz' law limit can be approached by modern turbine designs which reach 70 to 80% of this theoretical limit.
In addition to the blades, design of a complete wind power system
must also address the hub, controls, generator, supporting structure
and foundation. Turbines must also be integrated into power grids.
Blade shape and dimension are determined by the aerodynamic
performance required to efficiently extract energy, and by the strength
required to resist forces on the blade.
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 that away
from the turbine. The way that energy is extracted from the air also
causes air to be deflected by the turbine. Wind turbine aerodynamics at
the rotor surface exhibit phenomena that are rarely seen in other
aerodynamic fields.
Power control
Rotation speed must be controlled for efficient power generation and
to keep the turbine components within speed and torque limits. The
centrifugal force on the blades increases as the square of the rotation
speed, which makes this structure sensitive to overspeed. Because power
increases as the cube of the wind speed, turbines must survive much
higher wind loads (such as gusts of wind) than those loads from which
they generate power.
A wind turbine must produce power over a range of wind speeds.
The cut-in speed is around 3–4 m/s for most turbines, and cut-out at
25 m/s. If the rated wind speed is exceeded the power has to be limited.
A control system involves three basic elements: sensors to
measure process variables, actuators to manipulate energy capture and
component loading, and control algorithms that apply information
gathered by the sensors to coordinate the actuators.
Any wind blowing above the survival speed damages the turbine.
The survival speed of commercial wind turbines ranges from 40 m/s
(144 km/h, 89 MPH) to 72 m/s (259 km/h, 161 MPH), typically around
60 m/s (216 km/h, 134 MPH). Some turbines can survive 80 metres per
second (290 km/h; 180 mph).
Stall
A stall on an airfoil occurs when air passes over it in such a way that the generation of lift rapidly decreases. Usually this is due to a high angle of attack
(AOA), but can also result from dynamic effects. The blades of a fixed
pitch turbine can be designed to stall in high wind speeds, slowing
rotation. This is a simple fail-safe
mechanism to help prevent damage. However, other than systems with
dynamically controlled pitch, it cannot produce a constant power output
over a large range of wind speeds, which makes it less suitable for
large scale, power grid applications.
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, the degree of blade pitch tended to increase noise
levels.
Vortex generators
may be used to control blade lift characteristics. 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.
Furling
Furling
works by decreasing the angle of attack, which reduces drag and blade
cross-section. One major problem is getting the blades to stall or furl quickly enough in a wind gust. A fully furled turbine blade, when stopped, faces the edge of the blade into the wind.
Loads can be reduced by making a structural system softer or more flexible. This can be accomplished with downwind rotors or with curved blades
that twist naturally to reduce angle of attack at higher wind speeds.
These systems are nonlinear and couple the structure to the flow field -
requiring design tools to evolve to model these nonlinearities.
Standard turbines all furl 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
blade. They have a battery-reserve in case of grid failure. Small wind turbines (under 50 kW) with variable-pitching
generally use systems operated by centrifugal force, either by
flyweights or geometric design, and avoid electric or hydraulic
controls.
Fundamental gaps exist in pitch control, limiting the reduction of energy costs, according to a report funded 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 on commercial turbines.
Significant load mitigation has been demonstrated in simulations for
blades, tower, and drive train. However, further research is needed to
increase energy capture and mitigate fatigue loads.
A control technique applied to the pitch angle is done by
comparing the power output with the power value at the rated engine
speed (power reference, Ps reference). Pitch control is done with PI
controller. In order to adjust pitch rapidly enough, the actuator uses
the time constant Tservo, an integrator and limiters. The pitch angle
remains from 0° to 30° with a change rate of 10°/second.
Pitch Controller
As in the figure at the right, the reference pitch angle is compared
with the actual pitch angle b and then the difference is corrected by
the actuator. The reference pitch angle, which comes from the PI
controller, goes through a limiter. Restrictions are important to
maintain the pitch angle in real terms. Limiting the change rate is
especially important during network faults. The importance is due to the
fact that the controller decides how quickly it can reduce the
aerodynamic energy to avoid acceleration during errors.
Other controls
Generator torque
Modern
large wind turbines operate at variable speeds. When wind speed falls
below the turbine's rated speed, generator torque is used to control the
rotor speed 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 between 6 and 7). This
means that rotor speed increases proportional to wind speed. 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. 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 adjusted accordingly.
One technique to control a permanent magnetsynchronous motor is field-oriented control.
Field-oriented control is a closed loop strategy composed of two
current controllers (an inner loop and cascading outer loop) necessary
for controlling the torque, and one speed controller.
Constant torque angle control
In this control strategy the d axis current is kept at zero, while the vector current aligns with the q axis in order to maintain the torque angle at 90o.
This is a common control strategy because only the Iqs current must be
controlled. The torque equation of the generator is a linear equation
dependent only on the Iqs current.
So, the electromagnetic torque for Ids = 0 (we can achieve that with the d-axis controller) is now:
Machine Side Controller Design
Thus, the complete system of the machine side converter and the
cascaded PI controller loops is given by the figure. The control inputs
are the duty rations mds and mqs, of the
PWM-regulated converter. It displays the control scheme for the wind
turbine in the machine side and simultaneously how the Ids to zero (the torque equation is linear).
Yawing
Large turbines are typically actively controlled to face the wind direction measured by a wind vane situated on the back of the nacelle. By minimizing the yaw
angle (the misalignment between wind and turbine pointing direction),
power output is maximized and non-symmetrical loads minimized. However,
since wind direction varies, the turbine does not strictly follow the
wind and experiences a small yaw angle on average. The power output
losses can be approximated to fall with (cos(yaw angle))3.
Particularly at low-to-medium wind speeds, yawing can significantly
reduce output, with wind common variations reaching 30°. At high wind
speeds, wind direction is less variable.
Electrical braking
2kW Dynamic braking resistor for small wind turbine.
Braking a small turbine can be done by dumping energy from the generator into a resistor bank, converting kinetic energy
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 slows the blades, which increases the stalling
effect and reducing efficiency. 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 disc brake
stops rotation in emergency situations such as extreme gust events. The
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 because mechanical brakes can ignite a fire
inside the nacelle if used at full speed. Turbine load increases if the
brake is applied at rated RPM.
Turbine size
Figure 1. Flow diagram for wind turbine plant
Turbines come in size classes. The smallest, with power less than
10 kW are used in homes, farms and remote applications whereas
intermediate wind turbines (10-250 kW ) are useful for village power, hybrid systems and distributed power. The world's largest wind turbine as of 2021 was Vestas'
V236-15.0 MW turbine. The new design's blades offer the largest swept
area in the world with three 115.5 metres (379 ft) blades giving a rotor
diameter of 236 metres (774 ft). Ming Yang in China have announced a larger 16 MW design.
A person standing beside 15 m long blades.
For a given wind speed, turbine mass is approximately proportional to
the cube of its blade-length. Wind power intercepted is proportional to
the square of blade-length. The maximum blade-length of a turbine is limited by strength, stiffness, and transport considerations.
Labor and maintenance costs increase slower than turbine size, so
to minimize costs, wind farm turbines are basically limited by the
strength of materials, and siting requirements.
Low temperature
Utility-scale
wind turbine generators have minimum temperature operating limits that
apply in areas with temperatures below −20 °C (−4 °F). Turbines must be
protected from ice accumulation that 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 extra cost, which include internal heaters,
different lubricants, and different alloys for structural elements. If
low-temperatures are combined with a low-wind condition, the turbine
requires an external supply of power, equivalent to a few percent of its
rated output, for internal heating. For example, the St. Leon Wind Farm in Manitoba,
Canada, has a total rating of 99 MW and is estimated to need up to 3 MW
(around 3% of capacity) of station service power a few days a year for
temperatures down to −30 °C (−22 °F).
The nacelle houses 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 RPM for a one-megawatt turbine) into the
1,800 (750-3600) RPM that the generator needs to generate electricity. Gearboxes are one of the more expensive components for installing and
maintaining wind turbines. Analysts from GlobalData estimate that the
gearbox market grew from $3.2bn in 2006 to $6.9bn in 2011. The market
leader for Gearbox production was Winergy in 2011. The use of magnetic gearboxes has been explored as a way of reducing maintenance costs.
For large horizontal-axis wind turbines (HAWT), the generator is mounted in a nacelle at the top of a tower, behind the rotor hub. Older wind turbines generate electricity through asynchronous machines
directly connected to the grid. The gearbox reduces generator cost and
weight. Commercial generators have a rotor carrying a winding so that a
rotating magnetic field is produced inside a set of windings called the stator. While the rotating winding consumes a fraction of a percent of the generator output, adjustment of the field current allows good control over the output voltage.
The rotor's varying output frequency and voltage can be matched
to the fixed values of the grid using multiple technologies such as doubly fed induction generators or full-effect converters, which converts the variable frequency current to DC and then back to AC using inverters.
Although such alternatives require costly equipment and cost power, the
turbine can capture a significantly larger fraction of the wind energy.
Most are low voltage 660 Volt, but some offshore turbines (several MW) are 3.3 kV medium voltage.
In some cases, especially when offshore, a large collector
transformer converts the wind farm's medium-voltage AC grid to DC and
transmits the energy through a power cable to an onshore HVDC converter station.
Hydraulic
Hydraulic
wind turbines perform the frequency and torque adjustments of gearboxes
via a pressurized hydraulic fluid. Typically, the action of the turbine
pressurizes the fluid with a hydraulic pump at the nacelle. Meanwhile,
components on the ground can transform this pressure into energy, and
recirculate the working fluid. Typically, the working fluid used in this
kind of hydrostatic transmission is oil, which serves as a lubricant,
reducing losses due to friction in the hydraulic units and allowing for a
broad range of operating temperatures. However, other concepts are
currently under study, which involve using water as the working fluid
because it is abundant and eco-friendly.
Hydraulic turbines provide benefits to both operation and capital
costs. They can use hydraulic units with variable displacement to have a
continuously variable transmission that adapts in real time. This
decouples generator speed to rotor speed, avoiding stalling and allowing
for operating the turbine at an optimum speed and torque. This built-in transmission is how these hydraulic systems avoid the
need for a conventional gearbox. Furthermore, hydraulic instead of
mechanical power conversion introduces a damping effect on rotation
fluctuations, reducing fatigue of the drivetrain and improving turbine
structural integrity. Additionally, using a pressurized fluid instead of
mechanical components allows for the electrical conversion to occur on
the ground instead of the nacelle: this reduces maintenance difficulty,
and reduces weight and center of gravity of the turbine. Studies estimate that these benefits may yield to a 3.9-18.9% reduction
in the levelized cost of power for offshore wind turbines.
Some years ago, Mitsubishi, through its branch Artemis, deployed
the Sea Angel, a unique hydraulic wind turbine at the utility scale. The
Digital Displacement technology underwent trials on the Sea Angel, a
wind turbine rated at 7 MW. This design is capable of adjusting the
displacement of the central unit in response to erratic wind velocities,
thereby maintaining the optimal efficiency of the system. Still, these systems are newer and in earlier stages of commercialization compared to conventional gearboxes.
Gearless
Gearless wind turbines (also called direct drive)
eliminate the gearbox. Instead, the rotor shaft is attached directly to
the generator, which spins at the same speed as the blades.
Advantages of permanent magnet direct drive generators
(PMDD) over geared generators include increased efficiency, reduced
noise, longer lifetime, high torque at low RPM, faster and precise
positioning, and drive stiffness. PMDD generators "eliminate the
gear-speed increaser, which is susceptible to significant accumulated
fatigue torque loading, related reliability issues, and maintenance
costs".
To make up for a direct-drive generator's slower rotation rate, the diameter of the generator's rotor
is increased so that it can contain more magnets to create the required
frequency and power. Gearless wind turbines are often heavier than
geared wind turbines. An EU study showed that gearbox reliability is not the main problem in wind turbines. The reliability of direct drive turbines offshore is still not known, given the small sample size.
In December 2011, the US Department of Energy announced a critical shortage of rare-earth elements such as neodymium. China produces more than 95% of rare-earth elements, while Hitachi holds more than 600 patents covering neodymium magnets.
Direct-drive turbines require 600 kg of permanent magnet material per
megawatt, which translates to several hundred kilograms of rare-earth
content per megawatt,
as neodymium content is estimated to be 31% of magnet weight. Hybrid
drivetrains (intermediate between direct drive and traditional geared)
use significantly less rare-earth materials. 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. In 2011, demand for neodymium in wind turbines was estimated to be 1/5 of that in electric vehicles.
Blades
Blade design
Unpainted tip of a blade
The ratio between the blade speed and the wind speed is called tip-speed ratio.
High efficiency 3-blade-turbines have tip speed/wind speed ratios of 6
to 7. Wind turbines spin at varying speeds (a consequence of their
generator design). Use of aluminum and composite materials has contributed to low 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.
Noise increases with tip speed. To increase tip speed without
increasing noise would reduce torque into the gearbox and generator,
reducing structural loads, thereby reducing cost. The noise reduction is linked to the detailed blade aerodynamics,
especially factors that reduce abrupt stalling. The inability to predict
stall restricts the use of aggressive aerodynamics. Some blades (mostly on Enercon) have a winglet to increase performance and reduce noise.
A blade can have a lift-to-drag ratio of 120, compared to 70 for a sailplane and 15 for an airliner. In order to optimize the lift-to-drag ratio of a blade, they are
typically designed with varying airfoil cross-sections along their
length, customized to the varying wind speeds and angles encountered
from root to tip.
An additional design improvement is the incorporation of vortex generators,
small fins mounted to the surface of the blade, the help to smooth the
airflow, preventing flow separation and reducing turbulence, both of
which contribute to reducing energy losses. All of these innovations have the end goal of increasing the efficiency of converting wind energy to electricity.
Applications of IMU in Wind Power Generation
Blade Dynamic Deformation and Load Monitoring
The role of the Inertial Measurement Unit (IMU) in wind power generation is to measure the three-axis acceleration and
angular velocity of wind turbine blades, hubs, and tower tops in
real-time. By using inertial navigation algorithms, it calculates the
motion states (position, velocity, and attitude) of these components.
IMU captures the global dynamic information of wind turbines and,
through data fusion with Kalman filters and GNSS data, reduces
cumulative errors. This enables high-precision estimation of blade
deflection and loads, providing critical support for monitoring the
operational loads of wind turbines.
IMUs measure angular velocity and acceleration, which, combined
with navigation algorithms, capture the flexural attitude and positional
changes of blades during operation in real time. Through the use of
Kalman filters (KF) to fuse data from multiple IMUs, and based on rigid
body geometric models and rotor angles, the position of each IMU is
determined. Precision is further enhanced by compensating for
differences between actual positions and the model.
GNSS Integration:
GNSS plays two key roles:
Time Synchronization: It provides a unified time reference for all IMUs, ensuring sensor data alignment.
Absolute Position Reference: By integrating IMU data, it
limits drift errors, ensuring convergence and accuracy in IMU navigation
solutions (position and attitude).
Structural Health Monitoring and Fault Prediction
IMU
can be combined with other sensors (e.g., vibration and stress sensors)
to improve fault detection sensitivity through multi-source data
fusion. For example, IMUs installed on the turbine main shaft can
extract tower acceleration signals through signal processing and use
azimuth information to identify specific faults. Multi-sensor fusion
technology can detect blade stress changes and crack risks, reducing
downtime losses.
In 2021, Chinese researchers proposed an innovative multi-IMU
data fusion algorithm for wind turbine blade dynamic deformation
sensing. This algorithm uses a relative motion sensing fusion method
that employs an improved Kalman filter and a feedback-based distributed
structure to achieve multi-node data fusion.
High-Precision and Low-Precision IMU Collaboration:
High-precision IMUs (main nodes) are placed at the blade root
base, serving as a global reference point to provide information on the
overall torsional attitude and positional changes of the blade.
Low-precision IMUs (sub-nodes) are distributed at different positions along the blade, sensing local dynamic deformations.
Data from the high-precision IMU is filtered and fused to correct
the measurement errors of low-precision IMUs, significantly improving
the system's overall measurement accuracy and fault tolerance. Each
sub-node independently processes local data, and redundant information
is integrated through a global fusion layer to enhance fault tolerance.
Even if a single IMU fails, the system can maintain high accuracy.
Application in Blade Dynamic Testing
During wind turbine blade dynamic testing, blades undergo
continuous motion under external forces. By combining global reference
data from high-precision IMUs with local measurements from low-precision
IMUs, multi-node data fusion is achieved through a federated Kalman
filter. This enables precise perception of the blade's flexural attitude
and position in three-dimensional space.
Simulation results show that the fusion algorithm effectively
reduces the measurement errors of low-precision IMUs, significantly
decreasing the relative position and attitude errors of local blade
nodes while maintaining the accuracy of high-precision IMU nodes.
Particularly for complex motions at the blade's middle and tip, the
fusion algorithm demonstrates strong robustness and accuracy.
Hub design
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 more sophisticated designs, they are bolted to the pitch bearing, which adjusts their angle of attack with the help of a pitch system according to the wind speed. Pitch control is performed by hydraulic or electric systems (battery or ultracapacitor). The pitch bearing is bolted to the hub. The hub is fixed to the rotor
shaft, which drives the generator directly or through a gearbox.
Blade count
The 98 meter diameter, two-bladed NASA/DOE Mod-5B wind turbine was the largest operating wind turbine in the world in the early 1990sThe 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
rotor speed. 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 dramatically increases noise.
Wind turbines almost universally use either two or three blades.
However, patents present designs with additional blades, such as Chan
Shin's multi-unit rotor blade system. Aerodynamic efficiency increases with number of blades but with
diminishing return. Increasing from one to two yields a six percent
increase, while going from two to three yields an additional three
percent. Further increasing the blade count yields minimal improvements and
sacrifices too much in blade stiffness as the blades become thinner.
Theoretically, an infinite number of blades of zero width is the
most efficient, operating at a high value of the tip speed ratio, but
this is not practical.
Component costs 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. In addition, fewer blades allow higher rotational
speed. Blade stiffness requirements to avoid tower interference limit
blade thickness, but only when the blades are upwind of the tower;
deflection in a downwind machine increases tower clearance. Fewer blades
with higher rotational speeds reduce peak torque 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. However, these cyclic loads when combined at the drive train
shaft are symmetrically balanced for three blades, yielding smoother
operation during yaw. One or two blade turbines can use a pivoting
teetered hub to nearly eliminate the cyclic loads into the drive shaft
and system during yawing. In 2012, a Chinese 3.6 MW two-blade turbine
was tested in Denmark.
Blade size
Increasing blade length pushed power generation from the single megawatt
range to upwards of 10 megawatts. A larger area effectively increases
tip-speed ratio at a given wind speed, thus increasing its energy
extraction. Software such as HyperSizer (originally developed for spacecraft design) can be used to improve blade design.
As of 2015 the rotor diameters of onshore wind turbine blades reached 130 meters, while the diameter of offshore turbines reached 170 meters. In 2001, an estimated 50 million kilograms of fiberglass laminate were used in wind turbine blades.
Blade weight
An
important goal is to control blade weight. Since blade mass scales as
the cube of the turbine radius, gravity loading constrains systems with
larger blades. 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 ~10 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 flatwise direction (out of rotor plane) while airflow
around the blade cause edgewise bending (in the rotor plane). Flaps
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. Given the increasing size of production wind turbines, blade failures
are increasingly relevant when assessing public safety risks from wind
turbines. The most common failure is the loss of a blade or part
thereof. This has to be considered in the design.
In light of these failure modes and increasingly larger blade
systems, researchers seek cost-effective materials with higher
strength-to-mass ratios.
Blade materials
Several modern wind turbines use rotor blades with carbon-fiber girders to reduce weight.
In general, 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 wind and gravitational loading
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
History
Wood
and canvas sails were used on early windmills due to their low price,
availability, and ease of manufacture. 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 composites.
Advances in turbine blade materials mirrored the progression of
materials science as a broader subject. The first large turbine blades
were predominantly made from metals like steel and aluminum due to their
availability and robustness. However, their heavy weight and low flexibility restricted turbine size
and decreased efficiency, requiring more energy to maintain blade
rotation.
The wind energy sector eventually moved onto lighter materials,
namely fiberglass, a marked improvement over the excessive weight of
metals. However, fiberglass possessed its own set of disadvantages,
notably durability and sustainability issues. They were susceptible to environmental damages including UV radiation
and moisture, leading to delamination and loss of structural integrity.
Additionally, fiberglass is difficult to recycle, making the end-of-life
impact of fiberglass blades quite high.
As a response to these challenges, the wind energy industry
looked to carbon fiber as a blade material, the specific stiffness and
durability of which are greater than both metal and carbon fiber. The
superior stiffness-to-weight ratio allows for the use of larger blades,
increasing efficiency (see size section). In recent research,
bio-based composites and nanostructure enhancements have been utilized
to further reduce weight and increase strength and stiffness.
Polymer
The majority of 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. Material indices based on maximizing power efficiency, high fracture
toughness, fatigue resistance, and thermal stability are highest for
glass and carbon fiber reinforced plastics (GFRPs and CFRPs).
In turbine blades, matrices such as thermosets or thermoplastics are used; as of 2017, thermosets are more common. These allow for the fibers to be bound together and add toughness. Thermosets make up 80% of the market,
as they have lower viscosity, and also allow for low-temperature cure,
both features contributing to ease of processing during manufacture.
Thermoplastics offer recyclability that the thermosets do not, however
their processing temperature and viscosity are much higher, limiting the
product size and consistency, which are both important for large
blades. Fracture toughness is higher for thermoplastics, but the fatigue
behavior is worse.
Manufacturing blades in the 40 to 50-metre range involves proven
fiberglass composite fabrication techniques. Manufacturers such as Nordex SE and GE Wind use an infusion process. Other manufacturers vary this technique, some including carbon and wood with fiberglass in an epoxy
matrix. Other options include pre-impregnated ("prepreg") fiberglass
and vacuum-assisted resin transfer moulding. Each of these options uses a
glass-fiber reinforced polymer
composite constructed with differing complexity. Perhaps the largest
issue with open-mould, wet systems is the emissions associated with the volatile organic compounds
("VOCs") released. Preimpregnated materials and resin infusion
techniques contain all VOCs, however these contained processes have
their challenges, because the production of thick laminates necessary
for structural components becomes more difficult. In particular, the
preform resin permeability dictates the maximum laminate thickness;
also, bleeding is required to eliminate voids and ensure proper resin
distribution. One solution to resin distribution is to use partially impregnated
fiberglass. During evacuation, the dry fabric provides a path for
airflow and, once heat and pressure are applied, the resin may flow into
the dry region, resulting in an evenly impregnated laminate structure.
Epoxy
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 passed
60 metres, infusion techniques became more prevalent, because
traditional resin transfer moulding injection times are too long
compared to resin set-up time, limiting laminate thickness. Injection
forces resin through a thicker ply stack, thus depositing the resin in
the laminate structure before gelation occurs. Specialized epoxy resins
have been developed to customize lifetimes and viscosity.
Carbon fiber-reinforced load-bearing spars can reduce weight and
increase stiffness. Using carbon fibers in 60-metre turbine blades is
estimated to reduce total blade mass by 38% and decrease cost by 14%
compared to 100% fiberglass. Carbon fibers 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 benefit from the trend of decreasing carbon
fiber costs.
Although glass and carbon fibers have many optimal qualities,
their downsides include the fact that high filler fraction (10-70 wt%)
causes increased density as well as microscopic defects and voids that
can lead to premature failure.
Carbon nanotubes
Carbon nanotubes
(CNTs) can reinforce polymer-based nanocomposites. CNTs can be grown or
deposited on the fibers or added into polymer resins as a matrix for
FRP structures. Using nanoscale CNTs 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 low filler contents (typically < 5 wt%). They have
low density and improve the elastic modulus, strength, and fracture
toughness of the polymer matrix. The addition of CNTs to the matrix also
reduces the propagation of interlaminar cracks.
Research on a low-cost carbon fiber (LCCF) at Oak Ridge National Laboratory gained attention in 2020, because it can mitigate the structural damage from lightning strikes. On glass fiber wind turbines, lightning strike protection (LSP) is
usually added on top, but this is effectively deadweight in terms of
structural contribution. Using conductive carbon fiber can avoid adding
this extra weight.
Bio-composites
A significant concern in materials criteria for a turbine blade
is its manufacturing and end-of-life environmental impact, as well its
recyclability. While there are methods for manufacturing of fiberglass
and carbon fiber composites into turbine blades have a lower carbon
footprint than aluminum, for example, they still have a noticeable
impact (30–100 kg CO2 equivalent per kg). Additionally, fiberglass is incredibly difficult to recycle and carbon
fiber composites, while possible to recycle, require additional research
to yield fibers that are suitable for reusing as turbine materials (as
opposed to the fibers being so degraded that they are only suitable for downcycling). The development of bio-composite materials with sufficient mechanical properties aims to address these issues.
Bio-composite materials use natural fibers and fillers as
reinforcement instead of synthetic glass or carbon fibers. Approaches
vary from partial to complete replacement of synthetics, with varying
levels of success. Unfortunately, plant-based natural fibers, while having extremely low
environmental impact, possess issues in their structural properties.
Namely, they have high cellulosic content and large oxygen reaction
sites, both of which contribute to issues in mechanical and thermal
performance. As such, other natural fibers, such as non-moisture attractive basalt, have become the focus of bio-composite research.
Research
Some polymer composites feature self-healing properties. Since the blades of the turbine form cracks from fatigue due to
repetitive cyclic stresses, self-healing polymers are attractive for
this application, because they can improve reliability and buffer
various defects such as delamination. Embedding paraffin wax-coated copper wires in a fiber reinforced polymer creates a network of tubes. Using a catalyst, these tubes and dicyclopentadiene
(DCPD) then react to form a thermosetting polymer, which repairs the
cracks as they form in the material. As of 2019, this approach is not
yet commercial.
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 sand-laden wind, which
increases roughness and decreases aerodynamic performance. The particle
erosion resistance of fiber-reinforced polymers is poor when compared to
metallic materials and elastomers. Replacing glass fiber with CNF on
the composite surface greatly improves erosion resistance. CNFs provide
good electrical conductivity (important for lightning strikes), high
damping ratio, and good impact-friction resistance.
For wind turbines, especially those offshore, or in wet
environments, base surface erosion also occurs. For example, in cold
climates, ice can build up on the blades and increase roughness. At high
speeds, this same erosion impact can occur from rainwater. A useful
coating must have good adhesion, temperature tolerance, weather
tolerance (to resist erosion from salt, rain, sand, etc.), mechanical
strength, ultraviolet light
tolerance, and have anti-icing and flame retardant properties. Along
with this, the coating should be cheap and environmentally friendly.
Super hydrophobic surfaces (SHS) cause water droplets to bead, and roll off the blades. SHS prevents ice formation, up to -25 C, as it changes the ice formation process.; specifically, small ice islands form on SHS, as opposed to a large ice
front. Further, due to the lowered surface area from the hydrophobic
surface, aerodynamic forces on the blade allow these islands to glide
off the blade, maintaining proper aerodynamics. SHS can be combined with
heating elements to further prevent ice formation.
Lightning
Lightning damage over the course of a 25-year lifetime goes from surface level scorching and cracking of the laminate
material, to ruptures in the blade or full separation in the adhesives
that hold the blade together. It is most common to observe lightning strikes on the tips of the
blades, especially in rainy weather due to embedded copper wiring. The most common method countermeasure, especially in non-conducting
blade materials like GFRPs and CFRPs, is to add lightning "arresters",
which are metallic wires that ground the blade, skipping the blades and
gearbox entirely.
Blade repair
Wind
turbine blades typically require repair after 2–5 years. Notable causes
of blade damage comes from manufacturing defects, transportation,
assembly, installation, lightning strikes, environmental wear, thermal cycling, leading edge erosion, or fatigue. Due to composite blade material and function, repair techniques found in aerospace applications often apply or provide a basis for basic repairs.
Depending on the nature of the damage, the approach of blade
repairs can vary. Erosion repair and protection includes coatings,
tapes, or shields. Structural repairs require bonding or fastening new
material to the damaged area. Nonstructural matrix cracks and delaminations
require fills and seals or resin injections. If ignored, minor cracks
or delaminations can propagate and create structural damage.
Four zones have been identified with their respective repair needs:
Zone 1- the blade's leading edge. Requires erosion or crack repair.
Zone 2- close to the tip but behind the leading edge. Requires aeroelastic semi-structural repair.
Zone 3- Middle area behind the leading edge. Requires erosion repair.
Zone 4- Root and near root of the blade. Requires semi-structural or structural repairs
After the past few decades of rapid wind expansion across the globe,
wind turbines are aging. This aging brings operation and
maintenance(O&M) costs along with it, increasing as turbines
approach their end of life. If damages to blades are not caught in time,
power production and blade lifespan are decreased. Estimates project that 20-25% of the total levelized cost per kWh produced stems from blade O&M alone.
Blade recycling
The Global Wind Energy Council (GWEC) predicted that wind energy will supply 28.5% of global energy by 2030. This requires a newer and larger fleet of more efficient turbines and
the corresponding decommissioning of older ones. Based on a European Wind Energy Association study, in 2010 between 110 and 140 kilotonnes of composites were consumed to manufacture blades. The majority of the blade material ends up as waste and requires
recycling or downcycling. As of 2020, most end-of-use blades are stored
or sent to landfills rather than recycled. It is also important to note that recent studies predict that nearly
52,000 tons of turbine blades are to be decommissioned every year until
2030. Typically, glass-fiber-reinforced polymers (GFRPs) comprise around 70%
of the laminate material in the blade. GFRPs are not combustible and so
hinder the incineration of combustible materials. The following methods are the major EOL paths for turbine blades, with
methods varying depending on whether individual fibers are to be
recovered and the requisite temperature/catalysts.
Mechanical recycling: This method doesn't recover
individual fibers. Initial processes involve shredding, crushing, or
milling. The crushed pieces are then separated into fiber-rich and
resin-rich fractions. These fractions are ultimately incorporated into
new composites either as fillers or reinforcements.
Pyrolysis: Thermal decomposition of the composites recovers individual fibers. For pyrolysis,
the material is heated up to 500 °C in an environment without oxygen,
causing it to break down into lower-weight organic substances and
gaseous products. The glass fibers generally lose 50% of their strength
and can be downcycled for fiber reinforcement applications in paints or
concrete. This can recover up to approximately 19 MJ/kg at relatively high cost. It requires mechanical pre-processing, similar to that involved in purely mechanical recycling.
Solvolysis: This method involves the polymer matrix
undergoing chemical decomposition via solvents including but not limited
to acetone, nitric acid, ammonia, and alcohols. Advantages of solvolysis include a lower operation compared to
pyrolysis and its ability to yield fibers with favorable surface and
mechanical properties. Solvolysis has a significant number of
operational considerations, including solvent flow, solvent diffusion,
phase transitions, etc., that depend heavily on the polymer structure of
the blades, which are notably heterogeneous and contain relatively high
numbers of defects and voids. As such, current research focuses on computational modeling of solvolysis to allow for more complete and efficient recycling.
Direct structural recycling of composites: The general idea
is to reuse the composite as is, without altering its chemical
properties, which can be achieved especially for larger composite
material parts by partitioning them into pieces that can be used
directly in other applications.
Start-up company Global Fiberglass Solutions claimed in 2020 that it
had a method to process blades into pellets and fiber boards for use in
flooring and walls. The company started producing samples at a plant in
Sweetwater, Texas.
Tower
Height
Wind velocities increase at higher altitudes due to surface aerodynamic drag (by land or water surfaces) and air viscosity. 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 tower
diameter, increasing the amount of material by a factor of at least
four. As of July 2025, the world's tallest wind turbine, at 300 metres (980 ft) hub height with 3.8 MW capacity, was under construction in Germany.
During the night, 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 m/s) the stable atmosphere is disrupted because of
friction turbulence and the atmosphere turns 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). The 1/7 power law is a good approximation of the
wind profile. Indiana
was rated as having a wind capacity of 30,000 MW, but by raising the
expected turbine height from 50 m to 70 m raised the wind capacity to
40,000 MW, and could be double that at 100 m.
For HAWTs, tower heights approximately two to three times the
blade length balance material costs of the tower against better
utilisation of the more expensive active components.
Sections of a wind turbine tower, transported in a bulk carrier ship
Road restrictions make tower transport with a diameter of more than
4.3 m difficult. Swedish analyses showed that the bottom wing tip must
be at least 30 m above the tree tops. A 3 MW turbine may increase output from 5,000 MWh to 7,700 MWh per year by rising from 80 to 125 meters. 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 bolted 18 mm 'plank' shells at the wind turbine test center Høvsøre in Denmark was certified by Det Norske Veritas, with a Siemens nacelle. Shell elements can be shipped in standard 12 m shipping containers.
As of 2003, typical modern wind turbine installations used 65
metres (213 ft) towers. Height is typically limited by the availability
of cranes.
This led to proposals for "partially self-erecting wind turbines" that,
for a given available crane, allow taller towers that locate a turbine
in stronger and steadier winds, and "self-erecting wind turbines" that
could be installed without cranes.
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 transport costs. The use of
lighter tower materials could reduce the overall transport and
construction cost, as long as stability is maintained. Higher grade S500 steel costs 20%-25% more than S355 steel (standard structural steel),
but it requires 30% less material because of its improved strength.
Therefore, replacing wind turbine towers with S500 steel offer savings
in weight and cost.
Another disadvantage of conical steel towers is meeting the
requirements of wind turbines taller than 90 meters. High performance
concrete may increase tower height and increase lifetime. A hybrid of prestressed concrete and steel improves performance over standard tubular steel at tower heights of 120 meters. Concrete also allows small precast sections to be assembled on site. One downside of concrete towers is the higher CO 2
emissions during concrete production. However, the overall
environmental impact should be positive if concrete towers can double
the wind turbine lifetime.
Wood
is another alternative: a 100-metre tower supporting a 1.5 MW turbine
operates in Germany. The wood tower shares the same transportation
benefits of the segmented steel shell tower, but without the steel. A 2 MW turbine on a wooden tower started operating in Sweden in 2023.
Another approach is to form the tower on site via spiral welding
rolled sheet steel. Towers of any height and diameter can be formed this
way, eliminating restrictions driven by transport requirements. A
factory can be built in one month. The developer claims 80% labor
savings over conventional approaches.
Grid connection
Grid-connected
wind turbines, until the 1970s, were fixed-speed. As recently as 2003,
nearly all grid-connected wind turbines operated at constant speed
(synchronous generators) or within a few percent of constant speed
(induction generators). As of 2011, many turbines used fixed-speed induction generators (FSIG). By then, most newly connected turbines were variable speed.
Early control systems were designed for peak power extraction, also called maximum power point tracking—they attempted to pull the maximum power from a given wind turbine under the current wind conditions. More recent systems deliberately pull less than maximum power in most
circumstances, in order to provide other benefits, which include:
Spinning reserves to produce more power when needed—such as when some other generator drops from the grid
Variable-speed turbines can transiently produce slightly more power
than wind conditions support, by storing some 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, or to compensate for variable windspeeds.
A useful technique to connect a PMSG (Permanent Magnet
Synchronous Generator) to the grid is via a back-to-back converter.
Control schemes can achieve unity power factor
in the connection to the grid. In that way the wind turbine does not
consume reactive power, which is the most common problem with turbines
that use induction machines. This leads to a more stable power system.
Moreover, with different control schemes a PMSG turbine can provide or
consume reactive power. So, it can work as a dynamic capacitor/inductor bank to help with grid stability.
Grid Side Controller Design
The diagram shows the control scheme for a unity power factor :
Reactive power regulation consists of one PI controller in order to achieve operation with unity power factor (i.e. Qgrid = 0 ). IdN has to be regulated to reach zero at steady-state (IdNref = 0).
The complete system of the grid side converter and the cascaded PI controller loops is displayed in the figure.
Construction
As
wind turbine usage has increased, so have companies that assist in the
planning and construction of wind turbines. Most often, turbine parts
are shipped via sea or rail, and then via truck to the installation
site. Due to the massive size of the components involved, companies
usually need to obtain transportation permits and ensure that the chosen
trucking route is free of potential obstacles such as overpasses,
bridges, and narrow roads. Groups known as "reconnaissance teams" will
scout the way up to a year in advance as they identify problematic
roads, cut down trees, and relocate utility poles. Turbine blades
continue to increase in size, sometimes necessitating brand new
logistical plans, as previously used routes may not allow a larger
blade. Specialized vehicles known as Schnabel trailers are
custom-designed to load and transport turbine sections: tower sections
can be loaded without a crane and the rear end of the trailer is
steerable, allowing for easier maneuvering. Drivers must be specially
trained.
Foundations
Wind turbine foundations
Wind turbines, by their nature, are very tall, slender structures, and this can cause a number of issues when the structural design of the foundations are considered. The foundations for a conventional engineering structure are designed mainly to transfer the vertical load
(dead weight) to the ground, generally allowing comparatively
unsophisticated arrangement to be used. However, in the case of wind
turbines, the force of the wind's interaction with the rotor at the top
of the tower creates a strong tendency to tip the wind turbine over.
This loading regime causes large 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 foundation will resist this tipping tendency.
One of the most common foundations for offshore wind turbines is the monopile,
a single large-diameter (4 to 6 metres) tubular steel pile driven to a
depth of 5-6 times the diameter of the pile into the seabed. The
cohesion of the soil, and friction between the pile and the soil provide
the necessary structural support for the wind turbine.
In onshore turbines the most common type of foundation is a
gravity foundation, where a large mass of concrete spread out over a
large area is used to resist the turbine loads. Wind turbine size &
type, wind conditions and soil conditions at the site are all
determining factors in the design of the foundation. Prestressed piles or rock anchors are alternative foundation designs that use much less concrete and steel.
Costs
LiftraBlade Dragon installing a single blade on wind turbine hub.
A 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,
surviving variable loading and environmental conditions. Turbine
systems with fewer failures require less maintenance, are lighter and last longer, reducing costs.
The major parts of a turbine divide as: tower 22%, blades 18%, gearbox 14%, generator 8%.
Wind turbines can be designed and validated according to IEC 61400 standards.
RDS-PP (Reference Designation System for Power Plants) is a
standardized system used worldwide to create structured hierarchy of
wind turbine components. This facilitates turbine maintenance and
operation cost, and is used during all stages of a turbine creation.
