WIND TURBINE:
History
The world's first megawatt wind turbine on Grandpa's Knob, Castleton,
VermontWind machines were used for grinding grain in Persia as early
as 200 B.C. This type of machine was introduced into the Roman Empire
by 250 A.D. By the 14th century Dutch windmills were in use to drain
areas of the Rhine River delta. In Denmark by 1900 there were about
2500 windmills for mechanical loads such as pumps and mills, producing
an estimated combined peak power of about 30 MW. The first windmill for
electricity production was built in Cleveland, Ohio by Charles F Brush
in 1888, and in 1908 there were 72 wind-driven electric generators from
5 kW to 25 kW. The largest machines were on 24 m (79 ft) towers with
four-bladed 23 m (75 ft) diameter rotors.
By the 1930s windmills were mainly used to generate electricity on farms,
mostly in the United States where distribution systems had not yet been
installed. In this period, high-tensile steel was cheap, and windmills were
placed atop prefabricated open steel lattice towers. A forerunner of modern
horizontal-axis wind generators was in service at Yalta, USSR in 1931.
This was a 100 kW generator on a 30 m (100 ft) tower, connected to the local
6.3 kV distribution system. It was reported to have an annual load factor of
32 per cent, not much different from current wind machines.
Records
The world's largest turbines are manufactured by the Northern German companies
Enercon and REpower. The Enercon E112 delivers up to 6 MW , has an overall height
of 186 m (610 ft) and a diameter of 114 m (374 ft). The REpower 5M delivers up to
5-MW , has an overall height of 183 m (600 ft) and a diameter of 126 m (413 ft).
The turbine closest to the North Pole is a Nordex N-80 in Havoygalven near
Hammerfest,Norway. The ones closest to the South Pole are two Enercon E-30 in
Antarctica, used to power the Australian Research Division's Mawson Station
A wind turbine is a machine that converts the kinetic energy in wind into mechanical
energy. If the mechanical energy is used directly by machinery, such as a pump or
grinding stones, the machine is usually called a windmill. If the mechanical energy
is converted to electricity, the machine is called a wind generator, or more commonly
a wind turbine (wind energy converter WEC).
A wind turbine is old technology applied to meet new challenges. We need to adapt and
use every means at our disposal to combat global warming and carbon dioxide build up,
yet still provide energy for our modern (lavish) lifestyles. Whatever your views as
to the use of alternative energy, as an engineering student you will probably want to
know how these beautiful machines work. I know I have always been fascinated by them
This aerial view of a wind power plant shows how a group of wind turbines can make
electricity for the utility grid. The electricity is sent through transmission and
distribution lines to homes, businesses, schools, and so on.
These three-bladed wind turbines are operated "upwind," with the blades facing into
the wind. The other common wind turbine type is the two-bladed, downwind turbine.
So how do wind turbines make electricity? Simply stated, a wind turbine works the
opposite of a fan. Instead of using electricity to make wind, like a fan, wind
turbines use wind to make electricity. The wind turns the blades, which spin a shaft,
which connects to a generator and makes electricity. Utility-scale turbines range in
size from 50 to 750 kilowatts. Single small turbines, below 50 kilowatts, are used for
homes, telecommunications dishes, or water pumping.
TYPES OF WIND TURBINES:
Wind turbines can be separated into two types based on the axis about which the turbine
rotates. Turbines that rotate around a horizontal axis are more common. Vertical-axis
turbines are less frequently used.
Wind turbines can also be classified by the location in which they are to be used. Onshore,
offshore, or even aerial wind turbines have unique design characteristics.
Wind turbines may also be used in conjunction with a solar collector to extract the energy
due to air heated by the Sun and rising through a large vertical solar updraft tower.
Horizontal axis:
Horizontal-axis wind turbines (HAWT) have the main rotor shaft and electrical generator
at the top of a tower, and must be pointed into the wind. Small turbines are pointed by
a simple wind vane, while large turbines generally use a wind sensor coupled with a
servo motor. Most have a gearbox, which turns the slow rotation of the blades into a
quicker rotation that is more suitable for generating electricity.
Since a tower produces turbulence behind it, the turbine is usually pointed upwind of
the tower. Turbine blades are made stiff to prevent the blades from being pushed into
the tower by high winds. Additionally, the blades are placed a considerable distance
in front of the tower and are sometimes tilted up a small amount.
Downwind machines have been built, despite the problem of turbulence, because they
don't need an additional mechanism for keeping them in line with the wind, and because
in high winds, the blades can be allowed to bend which reduces their swept area and
thus their wind resistance. Because turbulence leads to fatigue failures and reliability
is so important, most HAWTs are upwind machines.
There are several types of HAWT:
These four- (or more) bladed squat structures, usually with wooden shutters or fabric
sails, were developed in Europe. These windmills were pointed into the wind manually or
via a tail-fan and were typically used to grind grain. In the Netherlands they were also
used to pump water from low-lying land, and were instrumental in keeping its polders dry.
Windmills were also located throughout the USA, especially in the Northeastern region.
Modern Rural Windmills
These windmills, invented in 1876 by Griffiths Bros and Co (Australia), were used by
Australian and later American farmers to pump water and to generate electricity.
They typically had many blades, operated at tip speed ratios (defined below) not better
than one, and had good starting torque. Some had small direct-current generators used to
charge storage batteries, to provide a few lights, or to operate a radio receiver.
The American rural electrification connected many farms to centrally-generated power and
replaced individual windmills as a primary source of farm power in the 1950's. Such
devices are still used in locations where it is too costly to bring in commercial power.
Common modern wind turbines
Usually three-bladed, sometimes two-bladed or even one-bladed (and counterbalanced), and
pointed into the wind by computer-controlled motors. The rugged three-bladed turbine type
has been championed by Danish turbine manufacturers. These have high tip speeds of up to
6x wind speed, high efficiency, and low torque ripple which contributes to good reliability.
This is the type of turbine that is used commercially to produce electricity. The blades
are usually colored light gray to blend in with the clouds and range in length from 20 to
40 metres (60 to 120 feet) or more.
Cyclic stresses and vibration
Cyclic stresses fatigue the blade, axle and bearing material, and were a major cause of
turbine failure for many years. Because wind velocity often increases at higher altitudes,
the backward force and torque on a horizontal-axis wind turbine (HAWT) blade peaks as it
turns through the highest point in its circle. The tower hinders the airflow at the lowest
point in the circle, which produces a local dip in force and torque. These effects produce
a cyclic twist on the main bearings of a HAWT. The combined twist is worst in machines with
an even number of blades, where one is straight up when another is straight down. To improve
reliability, teetering hubs have been used which allow the main shaft to rock through a few
degrees, so that the main bearings do not have to resist the torque peaks.
When the turbine turns to face the wind, the rotating blades act like a gyroscope. As it
pivots, gyroscopic precession tries to twist the turbine into a forward or backward
somersault. For each blade on a wind generator's turbine, precessive force is at a
minimum when the blade is horizontal and at a maximum when the blade is vertical. This
cyclic twisting can quickly fatigue and crack the blade roots, hub and axle of the
turbine.
Vertical-axis
12 m Windmill with rotational sails in the Osijek CroatiaVertical-axis wind turbines
(or VAWTs) have the main rotor shaft running vertically. Key advantages of this arrangement
are that the generator and/or gearbox can be placed at the bottom, near the ground, so the
tower doesn't need to support it, and that the turbine doesn't need to be pointed into the
wind. Drawbacks are usually pulsating torque that can be produced during each revolution
and drag created when the blade rotates into the wind. It is also difficult to mount
vertical-axis turbines on towers, meaning they must operate in the often slower,
more turbulent air flow near the ground, resulting in lower energy extraction efficiency.
Windmill with rotational sails
This is a new invention. This windmillstarts making electricity above a windspeed of 2m/s.
Its sails contract and expand as the wind speed changes. This windmill has three sails of
variable surface area. The speed is controlled through a magnetic rev counter that expands
or contracts the sails according to windspeed. A (microprocessor type) control unit controls
the sails either manually or automatically. In case of a control unit failure, strong winds
would tear the sails, but the frame would remain intact.
Neo-AeroDynamic
This has an airfoil base designed to harness the kinetic energy of the fluid flow via an
artificial current around its center. It is differentiated from others by its capability to
unitize most of the air mass passing through redirecting it to flow over the upper chamber
of the airfoils, and causing a lift force all around. It is applicable not only to wind,
but also to a variety of hydroelectric applications, including free-flow (rivers, creeks),
tidal, oceanic currents and wave motion, via ocean wave surface currents. Views of Hydro model,
:Portable aero model
Darrieus wind turbine
"Eggbeater" turbines. They have good efficiency, but produce large torque ripple and cyclic
stress on the tower, which contributes to poor reliability. Also, they generally require some
external power source, or an additional Savonius rotor, to start turning, because the starting
torque is very low. The torque ripple is reduced by using 3 or more blades which results in a
higher solidity for the rotor. Solidity is measured by blade area over the rotor area. Newer
Darrieus type turbines are not held up by guy wires but have an external superstructure
connected to the top bearing.
Giromill
A type of Darrieus turbine, these lift-type devices have vertical blades. The cycloturbine
variety have variable pitch to reduce the torque pulsation and are self-starting [1].
The advantages of variable pitch are: high starting torque; a wide, relatively flat torque
curve; a lower blade speed ratio; a higher coefficient of performance; more efficient
operation in turbulent winds; and a lower blade speed ratio which lowers blade bending
stresses. Straight, V, or curved blades may be used.
Savonius wind turbine
These are drag-type devices with two- (or more) scoops that are used in anemometers, the
Flettner vents (commonly seen on bus and van roofs), and in some high-reliability
low-efficiency power turbines. They always self-starting if there are at least three scoops.
They sometimes have long helical scoops to give a smooth torque. The Banesh rotor and especially
the Rahai rotor improve efficiency with blades shaped to produce significant lift as well as drag.
Windstar turbines
These lift-type devices made by Wind Harvest have straight, extruded aluminum blades attached
at each end to a central rotating shaft and are operated as Linear Array Vortex Turbine Systems
(LAVTS). Vertical-axis rotors each with their own 50-75kW generator are placed in three to any
number of rotors in linear arrays with each rotor’s blades passing within two feet of its neighbor.
In this configuration, the center rotors gain an increase in output and efficiency
(reaching the high efficiencies of HAWTs). This increased efficiency is protected under patent
(number 6784566) as the "vortex effect". Each rotor unit has a dual braking system of pneumatic
disc brakes and blade pitch. The newest Windstar LAVTS stand 50 feet tall, have 1500 and 3000
square feet of swept area per rotor and are designed to be placed in the turbulent winds within
the understory of wind farms.
Offshore
Offshore wind turbines near CopenhagenOffshore wind development zones are generally considered to
be ten kilometers or more from land. Offshore wind turbines are less obtrusive than turbines on land,
as their apparent size and noise can be mitigated by distance. Because water has less surface
roughness than land (especially deeper water), the average wind speed is usually considerably higher
over open water. Capacity factors (utilisation rates) are considerably higher than for onshore and
near-shore locations which allows offshore turbines to use shorter towers, making them less visible.
In stormy areas with extended shallow continental shelves (such as Denmark), turbines are practical
to install — Denmark's wind generation provides about 25-30% of total electricity demand in the
country, with many offshore windfarms. Denmark plans to increase wind energy's contribution to as
much as half of its electrical supply.
Locations have begun to be developed in the North American Great Lakes - with one project by
Trillium Power approximately 20 km from shore and over 700 MW in size. Ontario, Canada is
aggressively pursuing wind power development and has many onshore wind farms and several proposed
near-shore locations but presently only one offshore development.
In most cases offshore environment is more expensive than onshore. Offshore towers are generally
taller than onshore towers once the submerged height is included, and offshore foundations are
more difficult to build and more expensive. Power transmission from offshore turbines is generally
through undersea cable, which is more expensive to install than cables on land, and may use high
voltage direct current operation if significant distance is to be covered — which then requires
yet more equipment. The offshore environment can also be corrosive and abrasive in salt water
locations but locations such as the Great Lakes are in fresh water and do not have many of the
issues found in the ocean or sea. Repairs and maintenance are usually much more difficult, and
generally more costly, than on onshore turbines. Offshore wind turbines are outfitted with extensive
corrosion protection measures like coatings and cathodic protection however some of these measures
may not be required in fresh water locations.
While there is a significant market for small land-based windmills, offshore wind turbines have
recently been and will probably continue to be the largest wind turbines in operation, because larger
turbines allow for the spread of the high fixed costs involved in offshore operation over a greater
quantity of generation, reducing the average cost. For similar reasons, offshore wind farms tend to
be quite large—often involving over 100 turbines—as opposed to onshore wind farms which can operate
competitively even with much smaller installations.
There are some conceptual designs that might make use of the unique offshore environment.
For example,
a floating turbine might orient itself downwind of its anchor, and thus avoid the need for
a yawing mechanism. One concept for offshore turbines has them generate rain, instead of electricity.
The turbines would create a fine aerosol, which is envisioned to increase evaporation and induce
rainfall, hopefully on land.
Near-shore
Near-shore turbines are generally considered to be within a zone that is on land three kilometers of
a shoreline and on water within ten kilometers of land. Wind speeds in these zones share wind speed
characteristics of both onshore wind and offshore wind. Issues that are shared within near-shore wind
development zones are ornithological (including bird migration and nesting), aquatic habitat,
transportation (including shipping and boating) and visual aesthetics.
Sea shores also tend to be windy areas and good sites for turbine installation, because a primary
source of wind is convection from the differential heating and cooling of land and sea over the course
of day and night. Winds at sea level carry somewhat more energy than winds of the same speed in
mountainous areas because the air at sea level is denser.
Near-shore wind farm siting can sometimes be highly controversial as coastal sites are often
picturesque and environmentally sensitive (for instance, having substantial bird life).
Onshore
Wind turbines near Walla Walla in WashingtonOnshore turbine installations in hilly or mountainous
regions tend to be on ridgelines generally three kilometers or more inland from the nearest shoreline.
This is done to exploit the topographic acceleration where the hill or ridge causes the wind to
accelerate as it is forced over it. The additional wind speeds gained in this way make large differences
to the amount of energy that is produced. Great attention must be paid to the exact positions of the
turbines (a process known as micro-siting) because a difference of 30 m can sometimes mean a doubling
in output. Local winds are often monitored for a year or more with anemometers and detailed wind maps
constructed before wind generators are installed.
For smaller installations where such data collection is too expensive or time consuming, the normal way
of prospecting for wind-power sites is to directly look for trees or vegetation that are permanently
"cast" or deformed by the prevailing winds. Another way is to use a wind-speed survey map, or historical
data from a nearby meteorological station, although these methods are less reliable.
Wind farm siting can sometimes be controversial, particularly as the hilltop, often coastal sites
preferred are often picturesque and environmentally sensitive (for instance, having substantial bird life).
Local residents in a number of potential sites have strongly opposed the installation of wind farms,
and political support has resulted in the blocking of construction of some installations.
Turbine-design-and-construction
Advantages of vertical wind turbines
Easier to maintain because most of their moving parts are located near the ground. This is due to the
vertical wind turbine’s shape. The airfoils or rotor blades are connected by arms to a shaft that sits on
a bearing and drives a generator below, usually by first connecting to a gearbox. As the rotor blades are
vertical, a yaw device is not needed, reducing the need for this bearing and its cost.
Vertical wind turbines have a higher airfoil pitch angle, giving improved aerodynamics while decreasing
drag at low and high pressures. Mesas, hilltops, ridgelines and passes can have higher and more powerful
winds near the ground than up high because of the speed up effect of winds moving up a slope or funneling
into a pass combining with the winds moving directly into the site. In these places, VAWTs placed close
to the ground can produce more power than HAWTs placed higher up.
Low height useful where laws do not permit structures to be placed high. Smaller VAWTs can be much easier
to transport and install.
Does not need a free standing tower so is much less expensive and stronger in high winds that are close to
the ground. Usually have a lower Tip-Speed ratio so less likely to break in high winds.
Disadvantages of vertical wind turbines
Most VAWTs produce energy at only 50% of the efficiency of HAWTs in large part because of the additional
drag that they have as their blades rotate into the wind. This can be overcome by using structures to
funnel more and align the wind into the rotor (e.g. "stators" on early Windstar turbines) or the "vortex"
effect of placing straight bladed VAWTs closely together (e.g. Patent # 6784566).
There may be a height limitation to how tall a vertical wind turbine can be built and how much sweep
area it can have.
Most VAWTS need to be installed on a relatively flat piece of land and some sites could be too steep
for them but are still usable by HAWTs.
Most VAWTs have low starting torque.
A VAWT that uses guyed wires to hold it in place puts stress on the bottom bearing as all the weight of
the rotor is on the bearing. Guyed wires attached to the top bearing increase downward thrust in wind gusts.
Solving this problem requires a superstructure to hold a top bearing in place to eliminate the downward
thrusts of gust events in guyed wired models.
Advantages of horizontal wind turbines
Blades are to the side of the turbine's center of gravity, helping stability.
Ability to wing warp, which gives the turbine blades the best angle of attack. Allowing the angle of attack
to be remotely adjusted gives greater control, so the turbine collects the maximum amount of wind energy for
the time of day and season.
Ability to pitch the rotor blades in a storm, to minimize damage.
Tall tower allows access to stronger wind in sites with wind shear. In some wind shear sites, every ten
meters up, the wind speed can increase by 20% and the power output by 34%.
Tall tower allows placement on uneven land or in offshore locations.
Can be sited in forests above the treeline.
Most are self-starting.
Can be cheaper because of higher production volume, larger sizes and, in general higher capacity factors
and efficiencies.
Disadvantages of horizontal wind turbines
HAWTs have difficulty operating in near ground, turbulent winds because their yaw and blade bearing need
smoother, more laminar wind flows.
The tall towers and long blades (up to 180 feet long) are difficult to transport on the sea and on land.
Transportation can now cost 20% of equipment costs. Tall HAWTs are difficult to install, needing very tall
and expensive cranes and skilled operators.
Supply of HAWTs is less than demand and between 2004 and 2006, turbine prices increased up to 60%. At the
end of 2006, all major manufacturers were booked up with orders through 2008. The FAA has raised concerns
about tall HAWTs effects on radar in proximity to air force bases. Their height can create local opposition
based on impacts to viewsheds.
Offshore towers can be a navigation problem and must be installed in shallow seas. HAWTs can't be floated
on barges.
Downwind variants suffer from fatigue and structural failure caused by turbulence.
Horizontal-axis wind turbine aerodynamics
The aerodynamics of a horizontal-axis wind turbine are complex. The air flow at the blades is not the same
as the airflow far away from the turbine. The very nature of the way in which energy is extracted from the
air also causes air to be deflected by the turbine. In addition, the aerodynamics of a wind turbine at the
rotor surface include effects that are rarely seen in other aerodynamic fields.
Special wind turbines
One E-66 wind turbine at Windpark Holtriem, Germany carries an observation deck, open for visitors to see.
Another turbine of the same type, with an observation deck, can be located in Swaffham, England.
A series of floating wind turbines utilizing the Magnus Effect are in development in Canada by Magenn Power.
They deliver power to the ground by a tether system.
Wind Turbine Glossary
Anemometer: Measures the wind speed and transmits wind speed data to the controller.
Blades: Most turbines have either two or three blades. Wind blowing over the blades causes the blades to
"lift" and rotate.
Brake: A disc brake which can be applied mechanically, electrically, or hydraulically to stop the rotor
in emergencies.
Controller: The controller starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and
shuts off the machine at about 65 mph. Turbines cannot operate at wind speeds above about 65 mph because
their generators could overheat.
Gear box: Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speeds
from about 30 to 60 rotations per minute (rpm) to about 1200 to 1500 rpm, the rotational speed required
by most generators to produce electricity. The gear box is a costly (and heavy) part of the wind turbine
and engineers are exploring "direct-drive" generators that operate at lower rotational speeds and don't
need gear boxes.
Generator: Usually an off-the-shelf induction generator that produces 60-cycle AC electricity.
High-speed shaft: Drives the generator.
Low-speed shaft: The rotor turns the low-speed shaft at about 30 to 60 rotations per minute.
Nacelle: The rotor attaches to the nacelle, which sits atop the tower and includes the gear box, low- and
high-speed shafts, generator, controller, and brake. A cover protects the components inside the nacelle.
Some nacelles are large enough for a technician to stand inside while working.
Pitch: Blades are turned, or pitched, out of the wind to keep the rotor from turning in winds that are too
high or too low to produce electricity.
Rotor: The blades and the hub together are called the rotor.
Tower: Towers are made from tubular steel (shown here) or steel lattice. Because wind speed increases with
height, taller towers enable turbines to capture more energy and generate more electricity.
Wind direction: This is an "upwind" turbine, so-called because it operates facing into the wind. Other
turbines are designed to run "downwind", facing away from the wind.
Wind vane: Measures wind direction and communicates with the yaw drive to orient the turbine properly with
respect to the wind.
Yaw drive: Upwind turbines face into the wind; the yaw drive is used to keep the rotor facing into the wind
as the wind direction changes. Downwind turbines don't require a yaw drive, the wind blows the rotor downwind.
Yaw motor: Powers the yaw drive
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