The wind is caused by uneven heating of the Earth's surface creating pressure zones. Wind turbines convert the kinetic energy of wind into mechanical or electrical energy. Modern wind turbines have blades, a tower, a yaw mechanism, a generator, and sensors. Wind turbine efficiency is limited by the Betz limit of 59% of wind energy that can be captured. Turbines have cut-in, rated, and cut-out wind speeds that determine when they start and stop generating power for safety and efficiency.

1. WIND SYSTEM
The wind is a by-product of solar energy. Approximately 2% of the sun's energy
reaching the earth is converted into wind energy. The surface of the earth heats and cools
unevenly, creating atmospheric pressure zones that make air flow from high- to low-pressure
areas. The kinetic energy of the wind can be changed into other forms of energy, either
mechanical energy or electrical energy. Wind energy turns the rotor which turns a generator,
often through gearing which generates electricity. Some wind turbines have a narrow
"effective operating range".
Theoretically, about 1 to 2% of the sun‘s radiation turns into wind energy when it
arrives at the earth, which is about a hundred times of all the energy consumed on the
planet.
System Components
Tower
Wind turbine
Yaw mechanism
Mechanical gear
Electrical generator
Speed sensors and control
The modern system often has the following additional components:
Power electronics
Control electronics, usually incorporating a computer
Battery for improving the load availability in stand-alone mode

2. Tower
The construction can be tubular or lattice (or truss).
Both steel and concrete towers are available and are being used.
The tower height is typically 1 to 1.5 times the rotor diameter.
Towers must be at least 25 to 30 m high to avoid turbulence caused by trees and
buildings.
Figure: Representative size, height, and diameter of wind turbines
Spacing of the Towers
- The spacing depends on the terrain, the wind direction, the speed, and the turbine
size.
- The optimum spacing is found in rows 8 to 12-rotor diameters apart in the wind
direction, and 1.5 to 3-rotor diameters apart in the crosswind direction.
- A wind farm consisting of 20 towers rated at 500 kW each need 1 to 2 square
kilometers of land area.
Turbine Blades
Modern wind turbines have two or three blades.
The blades are made from composites, primarily fiberglass reinforced plastics
(GRP), and sometimes wood/epoxy laminates are used.
Extensive design effort is needed to avoid premature fatigue failure of the blade.
The mechanical stress in the blade under gusty wind is kept under the allowable
limit. This is achieved by controlling the rotor speed below the set limit (the stall
control).
This not only protects the blades, but also protects the electrical generator from
overloading and overheating.

3. Yaw Control
The yaw control continuously orients the rotor in the direction of the wind.
An active yaw drive contains one or more yaw motors, each of which drives a
pinion gear against a bull gear attached to the yaw bearing.
This mechanism is controlled by an automatic yaw control system with its wind
direction sensor usually mounted on the nacelle of the wind turbine.
However, rotating blades with large moments of inertia produce high gyroscopic torque
during yaw, often resulting in loud noise.
Speed control
no speed control
In this method, the turbine, the electrical generator, and the entire system are
designed to withstand the extreme speed under gusty wind.
yaw and tilt control
In this method the rotor axis is shifted out of the wind direction when the wind speed
exceeds the design limit.
pitch control
This changes the pitch of the blade with the changing wind speed to regulate the rotor
speed.
stall control
When the wind speed exceeds the safe limit on the system, the blades are shifted into
a position such that they stall. The turbine has to be restarted after the gust has gone.
Drive train
The drive train consists of the rotating parts of the wind turbine. These typically
include a low-speed shaft (on the rotor side), a gearbox, and a high-speed shaft (on the
generator side).
Other drive train components include the support bearings, one or more couplings, a
brake, and the rotating parts of the generator.
The purpose of the gearbox is to speed up the rate of rotation of the rotor from a low
value (tens of rpm) to a rate suitable for driving a standard generator (hundreds or
thousands of rpm).
Two types of gearboxes are used in wind turbines: parallel shaft and planetary.
For larger machines (over approximately 500 kW), planetary gearboxes become more
pronounced. Low-speed generators requiring no gearbox.
Generator
The conversion of the mechanical power of the wind turbine into the electrical power
can be accomplished by any one of the following types of the electrical machines.
1. Direct current (DC) machine.
2. Synchronous machine.
3. Induction machine.

4. 1. DC machine
- The rotor carries the permanent magnet poles and the stator carries the wound
armature which produces AC current.
- All machines are internally alternating current (AC) machines because of the
conductor rotation in the magnetic flux of alternate north and south polarity.
- The AC is then rectified using the solid state rectifiers. Such machines do not need the
commutator and the brushes. The brushless DC machine is expected to be limited to
ratings below one hundred kW.
2. synchronous machine
- The machine works at a constant speed related to the fixed frequency. Therefore, it is
not well suited for variable-speed operation in the wind plants.
- Moreover, it requires DC current to excite the rotor field, which needs sliding carbon
brushes on slip rings on the rotor shaft.
- This poses a limitation on its use. The need of the DC field current and the brushes can
be eliminated by using the reluctance rotor, where the synchronous operation is
achieved by the reluctance torque.
- The machine rating, however, is limited to tens of kW. The reluctance synchronous
generator is being investigated at present for small wind generators.
- When the synchronous machine used in the grid-connected system, it does not require
the reactive power from the grid. This results in a better quality of power at the grid
interface.
3. Induction machine
- The primary advantage of the induction machine is the rugged brushless construction
and no need for separate DC field power.
- The disadvantages of both the DC machine and the synchronous machine are
eliminated in the induction machine, resulting in low capital cost, low maintenance,
and better transient performance.
- For these reasons, the induction generator is extensively used in small and large wind
farms and small hydroelectric power plants.
- The machine is available in numerous power ratings up to several megawatts
capacity, and even larger.
- The induction machine needs AC excitation current. The machine is either self-excited
or externally excited.
- Since the excitation current is mainly reactive, a stand-alone system is self-excited by
shunt capacitors.
- The induction generator connected to the grid draws the excitation power from the
network.
- The synchronous generators connected to the network must be capable of supplying
this reactive power.
- For economy and reliability, many wind power systems use induction machines as the
electrical generator.

5. WORKING PRINCIPLE
Wind is the motion of the atmosphere, which is a fluid. As the wind approaches an
airfoil-shaped object, the velocity changes as the fluid passes the object, creating a pressure
gradient from one side of the object to the other. This pressure gradient creates a net force on
one side of the object, causing it to move in the fluid.
When wind hits the airfoil-shaped blade of a turbine, the lift force that is created
causes the blade to rotate about the main shaft. The main shaft is connected to an electric
generator. When the rotor spins due to forces from the wind, the generator creates electricity
that can be fed directly into the electric grid or into a system of batteries.
TYPES OF TURBINES
VAWT (Vertical Axis Wind Turbine)
 Drag is the main force
 Nacelle is placed at the bottom
 Yaw mechanism is not required
 Lower starting torque
 Difficulty in mounting the turbine
 Unwanted fluctuations in the power output
HAWT (Horizontal Axis Wind Turbine)
 Lift is the main force
 Much lower cyclic stresses
 95% of the existing turbines are HAWTs
 Nacelle is placed at the top of the tower
 Yaw mechanism is required
Offshore turbines
 More wind speeds
 Less noise pollution
 Less visual impact
 Difficult to install and maintain
 Energy losses due long distance transport

6. Mathematical Expression Governing Wind Power
The wind power is generated due to the movement of wind. The energy associated
with such movement is the kinetic energy and is given by the following expression:

7. OPERATING CHARACTERISTICS
All wind machines share certain operating characteristics, such as cut-in, rated and cut-out
wind speeds.
Cut-in Speed
Cut-in speed is the minimum wind speed at which the blades will turn and generate
usable power. This wind speed is typically between 10 and 16 kmph.
Cut-out Speed
At very high wind speeds, typically between 72 and 128 kmph, most wind turbines
cease power generation and shut down. The wind speed at which shut down occurs is
called the cut-out speed.
Having a cut-out speed is a safety feature which protects the wind turbine from
damage. Shut down may occur in one of several ways.
- An automatic brake is activated by a wind speed sensor.
- Some machines twist or "pitch" the blades to spill the wind.
- Still others use "spoilers," drag flaps mounted on the blades or the hub which
are automatically activated by high rotor rpm's, or mechanically activated by a
spring loaded device which turns the machine sideways to the wind stream.
Rated Speed
The rated speed is the minimum wind speed at which the wind turbine will generate
its designated rated power (i.e., a "10 kilowatt" wind turbine may not generate 10
kilowatts until wind speeds reach 40 kmph).
Rated speed for most machines is in the range of 40 to 55 kmph.
Betz Limit
It is the flow of air over the blades and through the rotor area that makes a wind
turbine function.
The theoretical maximum amount of energy in the wind that can be collected by a
wind turbine's rotor is approximately 59%. This value is known as the Betz limit.
If the blades were 100% efficient, a wind turbine would not work because the air,
having given up all its energy, would entirely stop.
Advantages
Wind energy doesn't pollute the air
Wind turbines don't produce atmospheric emissions.
The wind blows day and night, which allows windmills to produce electricity throughout
the day. (Faster during the day)
Energy output from a wind turbine will vary as the wind varies, although the most rapid
variations will to some extent be compensated for by the inertia of the wind turbine rotor.

8. Disadvantages
It is fluctuating in nature.
Wind energy needs storage device.
Wind Turbines generate sound (i.e., noise)
- Increasing tip speed  less sound
- The closest neighbor is usually 300 m  experiences almost no noise
However, birds are seldom bothered by wind turbines.
Prepared by
R. RamaRaj, KCET
J. S. Sakthi suriya raj, KCET
K. Selva Narayanan, KCET