1- BRIEF HISTORY OF WIND POWER 1.1 WIND ENERGY Wind energy is a converted form of solar energy which is produced by the nuclear fusion of hydrogen (H) into helium (He) in its core. The H → He fusion process creates heat and electromagnetic radiation streams out from the sun into space in all directions. Though only a small portion of solar radiation is intercepted by the earth, it provides almost all of earth’s energy needs. Wind energy represents a mainstream energy source of new power generation and an important player in the world's energy market. As a leading energy technology, wind power’s technical maturity and speed of deployment is acknowledged, along with the fact that there is no practical upper limit to the percentage of wind that can be integrated into the electricity system. It has been estimated that the total solar power received by the earth is approximately 1.8 × 10 11 MW. Of this solar input, only 2% (i.e. 3.6 × 10 9 MW) is converted into wind 1
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1- BRIEF HISTORY OF WIND POWER
1.1 WIND ENERGY
Wind energy is a converted form of solar energy which is produced by the nuclear fusion
of hydrogen (H) into helium (He) in its core. The H → He fusion process creates heat and
electromagnetic radiation streams out from the sun into space in all directions. Though
only a small portion of solar radiation is intercepted by the earth, it provides almost all of
earth’s energy needs. Wind energy represents a mainstream energy source of new power
generation and an important player in the world's energy market. As a leading energy
technology, wind power’s technical maturity and speed of deployment is acknowledged,
along with the fact that there is no practical upper limit to the percentage of wind that can
be integrated into the electricity system.
It has been estimated that the total solar power received by the earth is approximately 1.8
× 10 11 MW. Of this solar input, only 2% (i.e. 3.6 × 10 9 MW) is converted into wind
energy and about 35% of wind energy is dissipated within 1000 m of the earth’s surface.
Therefore, the available wind power that can be converted into other forms of energy is
approximately 1.26 × 10 9 MW. Because this value represents 20 times the rate of
the present global energy consumption, wind energy in principle could meet entire energy
needs of the world.
Compared with traditional energy sources, wind energy has a number of benefits and
advantages. Unlike fossil fuels that emit harmful gases and nuclear power that generates
radioactive wastes, wind power is a clean and environmentally friendly energy source. As
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an inexhaustible and free energy source, it is available and plentiful in most regions of the
earth. In addition, more extensive use of wind power would help reduce the demands for
fossil fuels, which may run out sometime in this century, according to their present
consumptions. Furthermore, the cost per kWh of wind power is much lower than that of
solar power. Thus, as the most promising energy source, wind energy is believed to play
a critical role in global power supply in the 21st century.
1.2 WIND GENERATION
Wind results from the movement of air due to atmospheric pressure gradients. Wind
flows from regions of higher pressure to regions of lower pressure. The larger the
atmospheric pressure gradient, the higher the wind speed and thus, the greater the wind
power that can be captured from the wind by means of wind energy-converting
machinery. The generation and movement of wind are complicated due to a number of
factors. Among them, the most important factors are uneven solar heating, the Coriolis
Effect due to the earth’s self-rotation, and local geographical conditions.
1.2.1 UNEVEN SOLAR HEATING
Among all factors affecting the wind generation, the uneven solar radiation on the earth’s
surface is the most important and critical one. The unevenness of the solar radiation can
be attributed to four reasons. First, the earth is a sphere revolving around the sun in the
same plane as its equator. Because the surface of the earth is perpendicular to the path of
the sunrays at the equator but parallel to the sunrays at the poles, the equator receives the
greatest amount of energy per unit area, with energy dropping off toward the poles. Due
to the spatial uneven heating on the earth, it forms a temperature gradient from the
equator to the poles and a pressure gradient from the poles to the equator. Thus, hot air
with lower air density at the equator rises up to the high atmosphere and moves towards
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the poles and cold air with higher density flows from the poles towards the equator along
the earth’s surface. Without considering the earth’s self-rotation and the rotation-induced
Coriolis force, the air circulation at each hemisphere forms a single cell, defined as the
Meridional Circulation.
Second, the earth’s self-rotating axis has a tilt of about 23.5° with respect to its ecliptic
plane. It is the tilt of the earth’s axis during the revolution around the sun that results in
cyclic uneven heating, causing the yearly cycle of seasonal weather changes.
Third, the earth’s surface is covered with different types of materials such as vegetation,
rock, sand, water, ice/snow, etc. Each of these materials has different reflecting and
absorbing rates to solar radiation, leading to high temperature on some areas (e.g. deserts)
and low temperature on others (e.g. iced lakes), even at the same latitudes. The fourth
reason for uneven heating of solar radiation is due to the earth’s topographic surface.
There are a large number of mountains, valleys, hills, etc. on the earth, resulting in
different solar radiation on the sunny and shady sides
1.2.2 CORIOLIS FORCE
The earth’s self-rotation is another important factor to affect wind direction and speed.
The Coriolis force, which is generated from the earth's self-rotation, deflects the direction
of atmospheric movements. In the north atmosphere wind is deflected to the right and in
the south atmosphere to the left. The Coriolis force depends on the earth’s latitude; it is
zero at the equator and reaches maximum values at the poles. In addition, the amount of
deflection of wind also depends on the wind speed; slowly blowing wind is deflected
only a small amount, while stronger wind deflected more.
In large-scale atmospheric movements, the combination of the pressure gradient due to
the uneven solar radiation and the Coriolis force due to the earth’s self-rotation causes the
single Meridional Cell to break up into three convectional cells in each hemisphere: the 3
Hadley cell, the Ferrel cell, and the Polar cell. Each cell has its own characteristic
circulation pattern. In the Northern Hemisphere, the Hadley cell circulation lies between
the equator and north latitude 30°, dominating tropical and sub-tropical climates. The hot
air rises at the equator and flows toward the North Pole in the upper atmosphere. This
moving air is deflected by Coriolis force to create the northeast trade winds. At
approximately north latitude 30°, Coriolis force becomes so strong to balance the
pressure gradient force. As a result, the winds are defected to the west. The air
accumulated at the upper atmosphere forms the subtropical high-pressure belt and thus
sinks back to the earth’s surface, splitting into two components: one returns to the equator
to close the loop of the Hadley cell another moves along the earth’s surface toward North
Pole to form the Ferrel Cell circulation, which lies between north latitude 30° and 60°.
The air circulates toward the North Pole along the earth’s surface until it collides with the
cold air flowing from the North Pole at approximately north latitude 60°. Under the
influence of Coriolis force, the moving air in this zone is deflected to produce westerlies.
The Polar cell circulation lies between the North Pole and north latitude 60°. The cold air
sinks down at the North Pole and flows along the earth’s surface toward the equator. Near
north latitude 60°, the Coriolis Effect becomes significant to force the air flow to
southwest.
1.2.3 LOCAL GEOGRAPHY
The roughness on the earth’s surface is a result of both natural geography and manmade
structures. Frictional drag and obstructions near the earth’s surface generally retard with
wind speed and induce a phenomenon known as wind shear. The rate at which wind
speed increases with height varies on the basis of local conditions of the topography,
terrain, and climate, with the greatest rates of increases observed over the roughest
terrain. A reliable approximation is that wind speed increases about 10% with each
doubling of height. In addition, some special geographic structures can strongly enhance
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the wind intensity. For instance, wind that blows through mountain passes can form
mountain jets with high speeds.
1.3 HISTORY OF WIND ENERGY APPLICATIONS
The use of wind energy can be traced back thousands of years to many ancient
civilizations. The ancient human histories have revealed that wind energy was discovered
and used independently at several sites of the earth.
1.3.1 SAILING
As early as about 4000 B.C., the ancient Chinese were the first to attach sails to their
primitive rafts. From the oracle bone inscription, the ancient Chinese scripted on turtle
shells in Shang Dynasty (1600 B.C.–1046 B.C.), the ancient Chinese character sail - in
ancient Chinese) often appeared. In Han Dynasty (220 B.C.–200 A.D.), Chinese junks
were developed and used as ocean-going vessels. As recorded in a book wrote in the
third century there were multi-mast, multi-sail junks sailing in the South Sea, capable of
carrying 700 people with 260 tons of cargo. Two ancient Chinese junks are shown in
Figure. Fig (a) is a two-mast Chinese junk ship for shipping grain, quoted from the
famous encyclopedic science and technology book Exploitation of the works of nature.
Figure (b) illustrates a wheel boat in Song Dynasty (960–1279). It is mentioned in books
that this type of wheel boats was used during the war between Song and Jin Dynasty
(1115–1234). Approximately at 3400 BC, the ancient Egyptians launched their first
sailing vessels initially to sail on the Nile River, and later along the coasts of the. Around
1250 BC, Egyptians built fairly sophisticated ships to sail on the Red Sea. The wind-
powered ships had dominated water transport.5
Figure: Ancient Chinese junks (ships): (a) two-mast junk ship; (b) wheel boat
1.3.2 WIND IN METAL SMELTING PROCESSES
About 300 BC, ancient Sinhalese had taken advantage of the strong monsoon winds to
provide furnaces with sufficient air for raising the temperatures inside furnaces in excess
of 1100°C in iron smelting processes. This technique was capable of producing high-
carbon steel.
The double acting piston bellows was invented in China and was widely used in
metallurgy in the fourth century BC. It was the capacity of this type of bellows to deliver
continuous blasts of air into furnaces to raise high enough temperatures for smelting iron.
In such a way, ancient Chinese could once cast several tons of iron.
1.3.3 WINDMILLS
China has long history of using windmills. The unearthed mural paintings from the tombs
of the late Eastern Han Dynasty (25–220 AD) at Sandaohao, Liaoyang City, have shown
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the exquisite images of windmills, evidencing the use of windmills in China for at least
approximately 1800 years. The practical vertical axis windmills were built in Sistan
(eastern Persia) for grain grinding and water pumping, as recorded by a Persian
geographer in the ninth century. The horizontal axis windmills were invented in
northwestern Europe in 1180s. The earlier windmills typically featured four blades and
mounted on central posts – known as Post mill. Later, several types of windmills, e.g.
Smock mill, Dutch mill, and Fan mill, had been developed in the Netherlands and
Denmark, based on the improvements on Post mill. The horizontal axis windmills have
become dominant in Europe and North America for many centuries due to their higher
operation efficiency and technical advantages over vertical axis windmills.
1.3.4 WIND TURBINES
Unlike windmills which are used directly to do work such as water pumping or grain
grinding, wind turbines are used to convert wind energy to electricity. The first
automatically operated wind turbine in the world was designed and built by Charles
Brush in 1888. This wind turbine was equipped with 144 cedar blades having a rotating
diameter of 17 m. It generated a peak power of 12 kW to charge batteries that supply DC
current to lamps and electric motors. As a pioneering design for modern wind turbines,
the Gedser wind turbine was built in Denmark in the mid-1950s. Today, modern wind
turbines in wind farms have typically three blades, operating at relative high wind speeds
for the power output up to several megawatts.
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1.4 HISTORY OF WIND TURBINES
The first known use of wind power are placed, according to various
sources, in the area between today’s Iran and Afghanistan in the period from 7th to
10th century. These windmills were mainly used to pump water or to grind wheat. They
had vertical axis and used the drag component of wind power: this is one of the reason
for their low efficiency. Moreover, to work properly, the part rotating in opposite
direction compared to the wind had to be protected by a wall.
Figure: Persian Windmill
Obviously, devices of this type can be used only in places with a main wind direction,
because there is no way to follow the variations.
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The first windmills built in Europe and inspired by the Middle East ones had the same
problem, but they used a horizontal axis. So they substitute the drag with the lift force,
making their inventors also the unaware discoverer of aerodynamics.
During the following centuries many modifies were applied for the use in areas
where the wind direction varies a lot: the best examples are of course the Dutch
windmills, used to drain the water in the lands taken from the sea with the dams,
could be oriented in wind direction in order to increase the efficiency.
Figure: Dutch Windmill
The wind turbines used in the USA during the 19th century and until the
’30 of 20th century were mainly used for irrigation. They had a high number of steel-
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made blades and represented a huge economic potential because of their large quantity:
about 8 million were built all over the country.
Figure: American Multi Blade Windmill
The first attempt to generate electricity were made at the end of 19th century, and
they become more and more frequent in the first half of the following century.
Almost all those models had an horizontal axis, but in the same period (1931)
Georges Jean Marie Darrieus designed one of the most famous and common type of
VAWT, that still bears his name.
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Figure: Eole Darrius wind turbine
The recent development led to the realization of a great variety of types
and models, both with vertical and horizontal axis, with rated power from the few kW
of the beginning to the 6 MW and more for the latest constructions. In the electricity
generation market the HAWT type has currently a large predominance.
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1.5 TYPES OF WIND TURBINES
Wind turbines can be separated into two types based by the axis in which the
turbine rotates. Turbines that rotate around a horizontal axis are more common. A wind
turbine applicable for urban settings was also studied. All three types of wind turbines
have varying designs, and different advantages and disadvantages.
1.5.1 HORIZONTAL AXIS WIND TURBINE
Horizontal axis wind turbines, also shortened to HAWT are the most common type used.
A HAWT has a similar design to a windmill; it has blades that look like a propeller that
spin on the horizontal axis. All of the components (blades, shaft and generator) are on top
of a tall tower, and the blades face into the wind. The shaft is horizontal to the ground.
The wind hits the blades of the turbine that are connected to a shaft causing rotation. The
shaft has a gear on the end which turns a generator. The generator produces electricity
and sends the electricity into the power grid. The wind turbine also has some key
elements that add to efficiency. Inside the Nacelle (or head) are an anemometer, wind
vane, and controller that read the speed and direction of the wind. As the wind changes
direction, a motor (yaw motor) turns the nacelle so the blades are always facing the wind.
The power source also comes with a safety feature. In case of extreme winds the turbine
has a break that can slow the shaft speed. This is to inhibit any damage to the turbine in
extreme conditions. These are identified by the fact that the axis of rotation of the blades
are in a fixed horizontal position therefore the unit must be placed in the direction of the
wind, these are most popular in rural areas. 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. Since turbulence leads to
fatigue failures, and reliability is so important, most HAWTs are upwind machines.
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Currently Horizontal Axis Wind Turbines (HAWT or propellers) cover more than 90% of
wind turbine World Park. About 100 companies produce these machines.
HAWT ADVANTAGES:
The tall tower base 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%.
Variable blade pitch, which gives the turbine blades the optimum angle of
attack.
Blades are to the side of the turbines center of gravity, helping stability.
Tall tower allows placement on uneven land or in offshore locations can
be sited in forest above tree-line.
Most are self-starting.
Accept wind from any angle
HAWT DISADVANTAGES:
Massive tower construction is required to support the heavy blades,
gearbox, and generator.
Components of a horizontal axis wind turbine (gearbox, rotor shaft and
brake assembly) being lifted into position.
Their height makes them obtrusively visible across large areas, disrupting
the appearance of the landscape and sometimes creating local opposition.
HAWTs generally require a braking or yawing device in high winds to
stop the turbine from spinning and destroying or damaging itself.
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The tall towers and blades up to 90 meters long are difficult to transport.
Transportation can reach 20% of equipment costs.
Tall HAWTs are difficult to install, needing very tall and expensive cranes
and skilled operators.
Reflections from tall HAWTs may affect side lobes of radar installations
creating signal clutter, although filtering can suppress it.
1.5.2 VERTICAL AXIS WIND TURBINE
Vertical-axis wind turbines (or VAWTs) have the main rotor shaft arranged vertically.
Key advantages of this arrangement are that the turbine does not need to be pointed into
the wind to be effective. This is an advantage on sites where the wind direction is highly
variable. VAWTs can utilize winds from varying directions.
It is difficult to mount vertical-axis turbines on towers, meaning they are often installed
nearer to the base on which they rest, such as the ground or a building rooftop. This can
provide the advantage of easy accessibility to mechanical components. However, wind
speed is slower at a lower altitude, so less wind energy is available for a given size
turbine. Air flow near the ground and other objects can create turbulent flow, which can
introduce issues of vibration, including noise and bearing wear which may increase the
maintenance or shorten the service life. In designs that do not have helical rotors
significant torque variation will occur.
VAWT ADVANTAGES:
A VAWT can be located nearer the ground, making it easier to maintain
the moving parts.
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VAWTs have lower wind startup speeds than the typical the HAWTs.
VAWTs may be built at locations where taller structures are prohibited.
A massive tower structure is less frequently used, as they are more
frequently mounted with the lower bearing mounted near the ground,
making it easier to maintain the moving parts.
VAWTs situated close to the ground can take advantage of locations
where mesas, hilltops, ridgelines, and passes funnel the wind and increase
wind velocity.
VAWT DISADVANTAGES:
Most VAWTs have an average decreased efficiency from a common
HAWT, mainly because of the additional drag that they have as their
blades rotate into the wind. Versions that reduce drag produce more
energy, especially those that funnel wind into the collector area.
Having rotors located close to the grounds where wind speeds are lower
due and do not take advantage of higher wind speeds above.
While the parts are located on the ground, they are also located under the
weight of the structure above it, which can make changing out parts nearly
impossible without dismantling the structure if not designed properly.
Having rotors located close to the ground where wind speeds are lower
due to wind shear, they may not produce as much energy at a given site as
a HAWT with the same footprint or height.
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1.5.3 EARLY VAWT DESIGNS
VAWTs appear to have been developed long before their horizontal axis cousins. One of
the reasons for this is that the VAWT has a number of inherent advantages including the
fact that a drive shaft may be connected directly from the rotor to a mechanical load at
ground level, eliminating the need for a gearbox. The early pioneers involved in the
development of wind turbines many centuries ago applied VAWTs to the milling of
grain, an application where the vertical axis of the millstone could be easily connected to
the VAWT rotor. Quite a number of excellent review articles have been published in the
past detailing the historical development of wind turbines of all types Virtually all of
these reviews suggest that the very earliest wind turbines were indeed VAWTs and it is
thought that these were first used in Persia for milling grain more than 2000 years ago.
These early wind turbines were essentially drag devices with a rotor comprising a number
of bundles of reeds, or other simple blades, on a timber framework. The rotor was housed
within a walled enclosure that channeled the flow of wind preferentially to one side of the
rotor thereby generating the torque necessary to rotate the millstone. This type of device
was still in use during the latter half of the 20th century and an example located in the
border region of Afghanistan and Iran. The Persian and Sistan VAWTs had rigid vanes
to generate torque whereas other designs have used sails that can effectively pitch with
respect to their alignment on the rotor and thus can potentially increase efficiency. An
example of a Chinese VAWT of the type used for many years for pumping applications,
and which was described by King for pumping brine for salt production, is illustrated in
Figs below.
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Figure 1: An example of VAWTs in the Sistan Basin in the border region of Iran and
Afghanistan. Note in the right hand image how the upstream wall is used to expose only
one half of the rotor to the wind (photographs taken in 1971 near Herat, Afghanistan,
copyright: Alan Cookson).
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Figure 2: A Chinese VAWT used for pumping brine (photo taken in early 20th
Century) from King
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2- TYPES OF VERTICAL AXIS WIND TURBINES
A wide variety of VAWTs have been proposed over the past few decades and a number
of excellent bibliographies on VAWTs have been published that summarize research and
development of these devices. Some of the more important types of rotor design are
highlighted in the following sections.
2.1 DARRIEUS
2.1.1 HISTORICAL BACKGROUND
French aeronautical engineer Georges Jean Marie Darrieus patented in 1931 a “Turbine
having its shaft transverse to the flow of the current”, and his previous patent (1927)
covered practically any possible arrangement using vertical airfoils.
It’s one of the most common VAWT, and there was also an attempt to implement the
Darrieus wind turbine on a large scale effort in California by the FloWind
Corporation; however, the company went bankrupt in 1997. Actually this turbine
has been the starting point for further studies on VAWT, to improve efficiency.
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2.1.2 USE AND OPERATION
The fundamental step forward made by Darrieus was to provide a means of raising the
velocity of the VAWT blades significantly above the free stream wind velocity so that lift
forces could be used to significantly improve the coefficient of performance of VAWTs
over previous designs based primarily on drag. Darrieus also foresaw a number of
embodiments of his fundamental idea that would be trialed at large scale many decades
later. These included use of both curved-blade (Fig. a) and straight blade versions of his
rotor. He also proposed options for active control of the pitch of the blades relative to the
rotor as a whole, so as to optimize the angle of attack of the wind on each blade
throughout its travel around the rotor circumference (as shown in Fig. b ).
Figure: Images from the Darrieus VAWT patent: (a) curved-blade rotor
embodiment; (b) plan view of straight-blade rotor showing an optional active
blade pitching mechanism
The Darrieus turbine can take a number of forms but is most well known in the geometry
sometimes called the “egg-beater” shown in Fig. a , where the two or three blades are
curved so as to minimize the bending moments due to centrifugal forces acting on the
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blade. The shape of the curved blade is close to that taken by a skipping rope in the
absence of gravity and is known as the Troposkein (“spinning rope”).
One of the characteristics of the Darrieus family of turbines is that they have a limited
self-starting capacity because there is often insufficient torque to overcome friction at
start-up. This is largely because lift forces on the blades are small at low rotational speeds
and for two-bladed machines in particular the torque generated is virtually the same for
each of the stationary blades at start-up.
The swept area on a Darrieus turbine is A=23
. D 2, a narrow range of tip speed ratios
around 6 and power coefficient Cp just above 0.3.
Figure: Cp- λ diagram for different types of wind turbines
Each blade sees maximum lift (torque) only twice per revolution, making for a
huge torque (and power) sinusoidal output that is not present in HAWTs. And the
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long VAWT blades have many natural frequencies of vibration which must be avoided
during operation.
Figure: Forces that act on the turbines.
One problem with the design is that the angle of attack changes as the turbine spins, so
each blade generates its maximum torque at two points on its cycle (front and back of the
turbine). This leads to a sinusoidal power cycle that complicates design.
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Another problem arises because the majority of the mass of the rotating
mechanism is at the periphery rather than at the hub, as it is with a propeller. This leads
to very high centrifugal stresses on the mechanism, which must be stronger and heavier
than otherwise to withstand them. The most common shape is the one similar to an egg-
beater that can avoid in part this problem, having most of the rotating mass not far from
the axis. Usually it has 2 or 3 blades, but some studies during the ’80 demonstrate that the
2 bladed configurations have a higher efficiency.
Figure: Three bladed Derrius wind turbine
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2.1.3 EXAMPLES
The biggest example of this type of turbine was the EOLE, built in Quebec
Canada in 1986. Its height is about 100 m, the diameter is 60 m and the rated
power was about 4 MW, but due to mechanical problems and to ensure longevity the
output was reduced to 2.5 MW. It was shut down in 1993.
2.2 SAVONIUS
2.2.1 HISTORICAL BACKGROUND
Savonius wind turbines were invented by the Finnish engineer Sigurd J. Savonius
in 1922, but Johann Ernst Elias Bessler (born 1680) was the first to attempt to build a
horizontal windmill of the Savonius type in the town of Furstenburg in Germany in 1745.
2.2.2 USE AND OPERATION
The Savonius is a drag-type VAWT, so it cannot rotate faster than the wind speed. This
means that the tip speed ratio is equal to 1 or smaller, making this turbine not
very suitable for electricity generation. Moreover, the efficiency is very low compared
to other types, so it can be employed for other uses, such as pumping water or grinding
grain. Much of the swept area of a Savonius rotor is near the ground, making the
overall energy extraction less effective due to lower wind speed at lower heights.
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Figure: Savonius Rotor
Its best qualities are the simplicity, the reliability and the very low noise production. It
can operate well also at low wind speed because the torque is very high especially in
these conditions. However the torque is not constant, so often some improvements like
helical shape are used.
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2.2.3 EXAMPLES
The Savonius can be used where reliability is more important than efficiency:
• Small application such as deep-water buoys
• Most of the anemometers are Savonius-type
• Used as advertising signs where the rotation helps to draw attention
2.3 GIROMILL
2.3.1 HISTORICAL BACKGROUND
The straight-bladed wind turbine, also named Giromill or H-rotor, is a type of
vertical axis wind turbine developed by Georges Darrieus in 1927.
This kind of VAWT has been studied by the Musgrove’s research team in the
United Kingdom during the ’80.
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In these turbines the “egg beater” blades of the common Darrieus are replaced with
straight vertical blade sections attached to the central tower with horizontal supports.
These turbines usually have 2 or 3 vertical airfoils. The Giromill blade design is much
simpler to build, but results in a more massive structure than the traditional
arrangement and requires stronger blades. In these turbines the generator is located at
the bottom of the tower and so it can be heavier and bigger than a common
generator of a HAWT and the tower can have a lighter structure. While it is cheaper and
easier to build than a standard Darrieus turbine, the Giromill is less efficient and requires
motors to start. However these turbines work well in turbulent wind conditions and
represent a good option in those area where a HAWT is unsuitable.
2.3.2 USE AND OPERATION
The operation way of a Giromill VAWT is not different from that of a common
Darrieus turbine. The wind hits the blades and its velocity is split in lift and drag
component. The resultant vector sum of these two components of the velocity makes the
turbine rotate.
The swept area of a Giromill wind turbine is given by the length of the blades
multiplied for the rotor diameter. The aerodynamics of the Giromill is like the one of the
common Darrieus turbine.
Kirke conducted an in-depth study of a number of three-bladed Giromill with
aerodynamic/mechanical activation of the blade pitch mechanism. These devices were
referred to as “self-acting variable pitch VAWTs” with straight blades. Each blade was
mounted at its mid-span on the end of the rotor radial arm and counterweighted so the
mass center coincided with the pivot axis, located forward of the aerodynamic center.
The pitch mechanism was activated by the moment of the aerodynamic force about a
pivot, opposed by centripetal force acting on a “stabilizer mass” attached to the radial
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arm, such that the aerodynamic force overcomes the stabilizer moment and permits
pitching before stall occurs.
Figure: Three blade variable pitch VAWT developed by Kirke clearly
showing the counterweights incorporated in the blade pitch mechanism
(photograph − copyright Brian Kirke).
2.3.3 EXAMPLES
The VAWT-850 was the biggest H-rotor in Europe when it was built in UK in
the 1989. It had a height of 45m and a rotor diameter of 38m. This turbine had a gearbox
and an induction generator inside the top of the tower. It was installed at the Carmarthen
test site during the 1990 and operated until the month of February of 1991, when one
of the blades broke, due to an error in the manufacture of the fiberglass blades.
In the 90’s the German company Heidelberg Motor GmbH developed and built
several 300 kW prototypes, with direct driven generators with large diameter. In
some turbines the generator was placed on the top of the tower while in others turbines it
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was located on the ground. In 2010 the VerticalWind AB, after a 12 kW prototype
developed in Uppsala, Sweden, has developed and built in Falkenberg the biggest
VAWT in Sweden: it’s a 3 blades Giromill with rated power of 200 kW, with a tower
built in with a wood composite material that make the turbine cheaper than other