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6 WIND POWER PLANTS
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6 WIND POWER PLANTS
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6.1 The wind
– as energy source
The basic principle of wind
- the wind is caused by the difference in pressure from one place to another.
- when warm air rises, it lowers the air pressure underneath it.
- up high, it cools off and sinks, raising the air pressure underneath it.
- in between the low pressure area and the high pressure area, air rushes from high to low,
causing wind.
Mastný, P. a kol.: Obnovitelné zdroje elektrické energie, ČVUT Praha
http://windeis.anl.gov/guide/basics/index.cfm
About 1-2 % of the solar energy is converted to kinetic energy of air and can be used in
wind turbines.
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Land and sea breezes
-land and sea breezes are basically caused by differential heating of the land and sea during the day and
night, creating differences in local air pressure, thus inducing winds to blow in different directions
During the day
when the sun heats up the earth's surface, the land heats up much faster than the sea.
Warm air above the land expands and rises. This creates a region of local low pressure. A sea breeze on
the other hand remains comparatively cool and is a region of high pressure. A sea breeze thus blows in
from the sea to the land. This wind we feel like cooler breeze that blows from the sea, ocean or other
larger water surface.
At night
the reverse takes place as the land cools down much faster than the sea the cooler and denser
air on the land creates a region of local high pressure. The sea on the other hand, conserves its heat
and is relatively warmer than the land. The air over the sea expands and rises creating a region of local
low pressure. A land breeze thus blows out from the land to the sea.
http://sk.wikipedia.org/wiki/Morsk%C3%BD_v%C3%A1nok
http://www.youtube.com/watch?v=ZQV72Yzmjyc
http://weather.blurtit.com/q186584.html
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6.2 Power and energy of wind
Energy of moving air masses can be expressed by the equation:
m
- mass of air
A
– area which the volume of air flowing (area wind passing through perpendicular to the wind)
s
– distance which surpasses moving air
V
– volume of air
Power of the wind blowing through unit area:
Power of the wind blowing through unit area
is directly-proportional to air density, and the
cube of the wind speed.
A 20% increase in the wind velocity -
increases the power generated with 73%
v
- air velocity
ρ
- density of air
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Wind speed and wind power are time variables.
Wind energy flowing per unit area for a given period is given by:
ρ
- density of air [kg/m
3
]
http://www.tzb-info.cz/tabulky-a-vypocty/70-hustota-sucheho-vzduchu
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6.3 Wind power plants
- systems, in which the kinetic energy of wind is converted into mechanical energy
(turbine), that is subsequently transformed into electrical energy (generator).
http://www.mechanicalengineeringblog.com/tag/introduction-to-wind-turbine/
http://turbinegenerator.org/wind/how-wind-turbine-works
Power of wind turbine
- kinetic energy of the wind after passing through the propeller wind turbine decreases,
because its part is changed to the mechanical energy
- Albert Betz in 1920 proved that the ideal wind power plant can convert to mechanical
energy up to 59,3% of the kinetic energy of wind, by slowing the speed to one-third
c
p
-
power factor - indicates how much of the kinetic energy of wind is converted into
mechanical energy
v
2
/v
1
-
the ratio of the wind speed behind the turbine to wind speeds before turbine
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provided that the swept area of blades in m
2
is:
http://www.mechanicalengineeringblog.com/tag/introduction-to-wind-turbine/
http://turbinegenerator.org/wind/how-wind-turbine-works
maximal power of wind power plant (WPP) is:
[W; kg.m
-3
, m, m.s
-1
]
D
WPP of company ENERCON –
influence of the size on performance
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6.4 Dividing of wind turbines
According to the performance
a) small
(about 20 kW for houses, farms, lighting, ...)
http://news.bbc.co.uk/2/hi/uk/6969865.stm
http://www.energor.com/Wind%20Power%20System.html
Off-grid wind turbine system
On-grid wind turbine system
b) medium
(20 - 50 kW)
c) large
(above 50 kW).
- above 20 kW are almost exclusively used for the power supply to the electrical network.
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According to rotor location
a) wind turbines with horizontal axis
- are currently the most widely used
- maximum use of performance can be achieved by 2 and 3 blades,
- in order to best using the wind energy, wind power head (called nacelle) is mounted
on a hub with possibility of yawing (using electric motor)
1. Wind causes blades to rotate
2. Shaft turns generator to produce electrical energy
3. A transformer turns this into high-voltage electricity
4. Electricity is transmitted via the power grid
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http://www.mechanicalengineeringblog.com/tag/introduction-to-wind-turbine/
Janíček, F.: Obnoviteľné zdroje energie 1. Bratislava, 2007
Composition of wind turbine
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http://www.ivt.ntnu.no/offshore2/?page_id=287
http://www.edinformatics.com/math_science/alternative_energy/wind/how_wind_turbines_work.htm
Nacelle: The nacelle is placed on the top of the tower and contains of the gear box, low- and high-speed shafts and the
generator.
Rotor: Attached to the nacelle and includes:
Blades, generally made of glass-reinforced fiber. For larger blades, lighter and stronger carbon fibers are commonly
used.
Extenders, which attach the blades to the central hub
Pitch drive to control the angle of the blades
Hub: A simple mechanism that connects the motor with the blades using a gear to move the motor. The hub of the rotor is
attached to the low speed shaft of the wind turbine.
Low-speed shaft: The rotor turns the low-speed shaft, which is a long pole, at about 20 to 60 rotations per minute.
Gearbox serves to increase the relatively low rotor speed (20-30 rpm) to high nominal speed generator (for example, 1500
rpm). It contains a system for dynamic change of gear ratio, which allows to keep a constant rotor speed of the generator at
full operating speed wind turbine. This regulation allows to operate the plant at variable speed rotor, which are dependent on
the wind speed and allows to keep constant output speed to drive the generator.
In 1992 the company Enercon introduced technology without gearbox . By using multi-pole generator, it is not necessary to
convert speed of the main shaft into high-speed. Generated current with variable parameters is transformed and frequency
adapted (using power electronics) to network parameters to which the power plant is connected.
Coupling: Attaches the gearbox to the generator. Flexible couplings may be used to reduce oscillating loads that could
otherwise cause component damage.
High-speed shaft: Drives the generator.
Generator: The generator transforms the rotational energy into electrical power.
Transformer: The transformer converts the electrical power from low voltage to a higher voltage suitable for grid
connection.
Wind vane: An instrument that indicates wind direction using a vane that rotates around a vertical axis.
Anemometer: Instrument that measures wind speed using cups that rotate around a mobile shaft at varying speeds.
Mechanical brake: The mechanical brake is placed on the small fast shaft between the gearbox and the generator. It is
used as an emergency brake, in case the blade tip brake should fail.
Yaw rotor: The yaw motor turns the nacelle so that the rotor faces the wind.
The controller starts up the machine at wind speeds of about 4 m/s and shuts off the machine at about 25 m/s. Turbines do
not operate at wind speeds above about 25 m/s because they might be damaged by the high winds.
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- the minimal limit of wind speed for energy using of wind turbine is about 4 m.s
-1
,
- optimal wind speed is about 14 m.s
-1
.
- after exceeding this speed is the power provided by the wind limited and part of its energy is
unused
- maximal limit of wind speed is about 25 m.s
-1
.
- higher speeds are dangerous because they can cause damage to the wind power plants.
- therefore, wind power plants at such speeds stop and orient in a safe position.
The power diagram of wind turbine
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The impact of height of towers to power wind turbines
http://www.solarwindtek.com/site/windFAQs.shtml
1 foot = 0,3048 meters
1 miles per hour = 0,44704 meters / second
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Development of the performance of wind turbines
http://nextbigfuture.com/2012/06/800-
foot-tall-wind-turbines-are.html
Note: Currently, the world’s largest wind turbine is
manufactured by Enercon and produces 7,5 MW
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b) wind turbine with a vertical axis
- principle of operation of the wind facility is the same as for wind turbines with a horizontal
axis of rotation (that is, speed of rotation of the turbine into generator is transmitted
through the gearbox)
- machine box with generator is located on the land. It is considered as a major advantage in
compare with the wind power plants with a horizontal axis of rotation (mounting and statics
is easier).
- great advantage of these rotors is - they do not need to moved in the direction of the wind
- disadvantage: lower efficiency in compared with turbines with horizontal axis
- utilization for smaller performance
Rotors that are most commonly used in wind turbines with a vertical axis of rotation are:
- Savonius rotor
- Darrieus rotor
- H rotor
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Tip speed ratio: ratio of the speed of the windmill rotor tip,
at radius R when rotating at ω radians/second, to the speed
of the wind, V, and is numerically:
6.5 Wind farms
- whereas the wind energy has relatively low concentrations, wind turbines are
concentrated in the so-called wind farms in appropriate locations.
- individual wind plants (turbines) are localized so that, as each other do not reduce
performance
- the minimal spacing in a direction perpendicular to the wind has be min. 4xD, in direction
of wind min. 6xD.
Example of turbine
spacing in a wind farm
Connecting the wind farms to the grid
- installed capacity of wind farm is a decisive factor in
determining the voltage level of the network to which
the wind farm is connected.
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Methods of interconnection of individual wind turbines:
b) circle connection
a) radial connection
Circle connection is characterized by higher investment costs, but higher reliability and
lower energy losses in the internal network.
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Location of wind farms
a) on-shore
b) off-shore
Concrete foundation of wind trbine
F
our main types of wind turbine foundations —
monopile, jacket, tripile and gravity base
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6.6 Installed capacity of wind power plants
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