S.E. Electrical Engineering UNIT IV Wind Energy System
Slide 2
Introduction Wind is an indirect form of solar energy since
wind is introduced chiefly by the uneven heating of the earths
crust by sun. Conversion of this wind energy into electrical energy
can reduce the power deficit.
Wind Energy It is estimated that India has potential about
20000 MW. There are various wind potential stations are identified
: Tamilnadu-39, Gujrat-36, AP-30, Maharashtra-27, Karnataka-26,
Kerala-16, Lakshadweep-8, Rajasthan-8, MP-7, Orisa-7, WB-2 and 1
each in Anadman and UP Out of 208 stations 7 stations have shown
wind power density more than 500W/sq.m.
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State wise potential in India, 2005
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Advantages and Disadvantages 1. Advantages :- It is available
free and is inexhaustible. It is clean and non polluting. Have low
maintenance cost. Has low cost of power generation of about Rs.
2.25/kWh
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2. Disadvantages Capital cost is high. Wind energy available in
fluctuating in nature. Large variation in wind during cyclones,
tornadoes may cause damage to installation. Design of system is
difficult due to large variation in wind speed. It causes sound
pollution in large unit.
Slide 10
Classification of wind Planetary winds :- are caused due to
greater solar heating of the earths surface near the equator as
compared to solar heating near to poles. Local winds :- are caused
due to differential heating of land and water in costal areas.
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Principle Of Wind Power Generation Total wind power is
proportional to the cube of incoming wind velocity, density of air
and the cross sectional area of wind stream. Total wind power
density- it is defined as the total wind power per unit area of
wind stream.
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Site selection Four types of site are suitable for Wind Energy
Conversion System (WECS) 1. Plane land sites 2. Hill tops sites 3.
Sea-shore sites 4. Off-shore shallow water sites
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Some important criteria for site selection Located where the
high avg. wind velocities available are in the range of 6 m/s to 30
m/s through out the year. The WECS must be located far away from
cities and forests. There should no tall structures in 3 km radius.
The wind farms are located in flat open areas, deserts, seas,
shores and off shores sites since wind velocities are high in these
location. Wind velocity of wind must measures at different heights,
as height increases wind velocity increases. This helps in
selection of appropriate height of wind tower.
Slide 14
Some important criteria for site selection Historical data of
wind mean speed must be collected for avg. velocities during the
year for proper selection of the site. Ground surface should have
high soil strength to reduce cost of foundation. Transportation
facilities should be constructed for site.
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Classification of Wind Mills :- Based on Orientation of the
Axis of Rotor :- 1. Horizontal axis 2. Vertical axis Based on Type
of Rotor :- Propeller type (Horizontal axis) Multiple Blade type
(Horizontal axis) Savonius type (Vertical axis) Darrieus type
(Vertical axis)
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Horizontal Axis Wind Mill :- Horizontal wind mills may be type
Propeller or multi- bladed. The orientation of the axis of rotors
are along the horizontal axis, so that it is parallel to the
direction of wind stream. Most commonly used wind mills are
propeller type. Single bladed, Two bladed and Three bladed. Two
bladed 2 MW to 3 MW capacity. Three bladed 3 MW to 15 MW capacity.
The rotational speed range 300-400 rpm. For multi blade type range
60-80 rpm. Material used for blades is glass fibre reinforced
plastic.
Slide 18
Horizontal Axis Wind Mill :- Multiblade rotor consist of number
of curved sheet metal blades with increasing chord length away from
the centre. No of blades used 12- 20, inner and outer end are
fixed. Diameter varies 2 m to 5 m.
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Vertical Axis Wind Mill :- Savonius rotor, a hollow elliptical
cylinder is sliced into two pieces and each of these halves fixed
to a vertical axis with a fixed gap. It forms S shape due to this
it is also called as S-rotor. Darrieus rotor consists of two or
three convex blades with aerofoil cross-section. Along length
blades are curved into shape called troposkein. The blades are
mounted symmetrically vertical.
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Vertical Axis Wind Mill :- Advantage of this type of mill is no
orientation of vanes of the mill is required according to direction
of wind. Can generate power with any direction. No requirement of
tall structure. Low cut in speed as compare to horizontal axis
mill. 8 kmph in vertical and 16 kmph in horizontal.
Horizontal Axis Wind Generator :- Used all over the world. The
main component are Usually have two or three blades. Diameter of
rotor 2 m-25 m. Mounted on tower top, designed to with stand with
wind load during abnormal condition. Electromagnetic breaks are
provided for automatic application of break if wind speed exceed
the designed speed. The hub, breaks, gear box generator with
electrical controls are housed in box called nacelle.
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Horizontal Axis Wind Generator :- A yaw control mechanism is
provided to adjust the nacelle around vertical axis. In small wind
turbines, a flap or vanes is provided on the nacelle for automatic
moving according to direction of wind Generator provided for
generation of power.
Vertical axis wind turbine generator It consist of hollow
vertical shaft mounted between the top and bottom bearings. The
tower height about 100 m. Two thin and curved symmetrical blades
are attached at the rotor shaft at the top and bottom. The turbine
shaft coupled to the generator having in between breaks, gear box
and electrical control.
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Vertical axis wind turbine generator It does not need any yaw
control mechanism, mill can accept wind from any direction. It does
not need to support the nacelle on the top of tower, since gear
box, breaks, generator on ground. Overall cost is less. Maintenance
cost is low. Design is easier.
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Types of Wind Turbine Generator Technologies Presently four
major types of WTG Technologies used: 1. Squirrel Cage Induction
Generators driven by fixed-speed, stall-regulated wind turbines 2.
Induction Generators with variable external rotor resistance driven
by a variable-speed, pitch regulated wind turbines 3. Doubly-Fed
Induction Generators driven by variable-speed, pitch regulated wind
turbines 4. Synchronous or Induction Generators with full converter
interface (back-to-back frequency converter), driven by
variable-speed, pitch regulated wind turbines
Slide 28
Doubly Fed Induction Generator (DFIG) Wound rotor induction
generator with slip rings Rotor is fed from a three-phase variable
frequency source, thus allowing variable speed operation reduction
of mechanical stress; higher overall efficiency, reduced acoustical
noise The variable frequency supply to rotor is attained through
the use of two voltage-source converters linked via a capacitor
Note: A more appropriate designation for this type of generator is:
Doubly Fed Asynchronous Generator
Slide 29
Performance of wind mills Power coefficient C p The coefficient
of performance, C p is defined as ratio of power, P delivered by
the rotor to the maximum power, P max i.e coefficient of
performance of wind turbine.
Slide 30
Tip-Speed Ratio Tip-speed ratio is the ratio of the speed of
the rotating blade tip to the speed of the free stream wind. There
is an optimum angle of attack which creates the highest lift to
drag ratio. Because angle of attack is dependant on wind speed,
there is an optimum tip-speed ratio R V TSR = Where, = rotational
speed in radians /sec R = Rotor Radius V = Wind Free Stream
Velocity R R
Slide 31
Solidity, Ratio of the blade area to the swept frontal area of
wind turbine. For vertical axis
Slide 32
Dependence of power coefficient, C p on Tip Speed ratio, Each
type of wind mill has some optimum tip speed to wind speed ratio,
at which it gives maximum C p. C p is lowest for Savonius and Dutch
of blades. C p is maximum for propeller type of blades. Ideal value
of C p is about 0.59.
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Dependence of Solidity , on Tip Speed ratio Rotor with high tip
speed ratio have low value of solidity. Rotors turn at high speed.
Rotor with tip low speed ratio have low value of solidity. Rotors
turn at low speed. The torque generated by Propeller and Darrieus
type of rotor is low, therefore they are suitable for electrical
power generation. The torque generated by multi bladed and Savonius
type of rotor is high, therefore they are suitable for application
like pumping of water.
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Methods of overcoming Fluctuation of power Pitch control.
Passive stall control. Active stall control. Yaw control.
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Pitch control :- The power output continuously checked by
electronic measuring unit. When power output becomes high, it
actuates the blade pitch mechanism which turns the blades out of
wind. This reduces the power output. When wind velocity reduces and
power falls, the blade is turned back to the original position and
power increases.
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Passive stall control In this control the blade is fixed at a
fix angle. When increase it creates the turbulence on the side
opposite to that facing the wind reduces the angle of attack. This
will reduces the lift force developed and hence reduces the power.
Simpler than pitch control.
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Active stall control There are two blades with changeable pitch
angles. When the wind speed reduces, it pitches the blades, so that
wind power output increases. When power reaches the rated power,
the blade is pitched in opposite direction to that of pitch
control.
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Yaw control Turbine rotates about vertical axis facing or away
from the wind Used only in small turbine
Slide 39
Grid connected wind energy conversion system
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Constant Speed drive DC generator :- A permanent magnet type dc
generator is used for small and medium power generation upto 100kW.
Synchronous generators :- are used for medium and large power
generation having constant speed with 1 % speed fluctuation.
Induction generators :- are used for variable shaft speed upto 10%.
These are preferred because of low cost.
Slide 41
Variable Speed Drive Scheme Variable voltage and Variable
frequency output of synchronous or induction generator converted
into DC by converter and then into fixed voltage and fixed
frequency AC by using inverter.
Slide 42
Wind Energy conversion system for battery charging Wind energy
converted into DC by DC generator and Charges the batteries. Charge
controller is used to control the charging and discharging. Can be
used as standalone system.
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Stand alone system with AC DC load Can be supply power AC as
well as DC. Needs inverter to converter DC to AC.
Slide 44
Wind Energy direct and indirect effect Indirectly the CO2 and
other emissions are released during the course of their
construction. But very negligible amount as compared to fossil fuel
power plant. It causes noise pollution. For this wind mills located
at least 2 to 3 km away from city. It poses threat to bird life due
to collision of birds with blades or wind tower. Mechanical failure
of wind mill may cause its parts to fly and harm the people working
around them.
Slide 45
Wind Energy direct and indirect effect Electromagnetic waves of
TV signals are obstructed by wind turbines. It may causes the poor
quality of radio and TV. Ecosystem is affected by the use of large
scale wind generators since, the nearby lakes and rivers will
becomes warmer caused by reduced evaporation from their
surface.