Wind turbine Types of WECs Nature of wind Power in wind

1. Basic
Principles of
Wind Energy
Conversion

2. 1. The Nature of Wind
The circulation of air in the atmosphere is caused by the non-uniform heating of the earth’s surface by the sun. The air
immediately above a warm area expands, it is forced upward by cool, denser air which flows in from surrounding areas
causing wind.
The nature of the terrain, the degree of cloud and the angle of the sun in the sky are all factors which influences this
process.
In general, during the day the air above the land mass tends to heat up more rapidly than the air over water. In coastal
regions this manifests itself in a strong onshore wind. At night the process is reversed because the air cools down more
rapidly over the land and the breeze therefore blows off shore.
Despite the wind’s intermittent nature, wind patterns at any particular site remain remarkably constant year by year.
Average wind speeds are greater in hilly and costal area than they are well inland. The winds also tend to blow more
consistently and with greater strength over the surface of the water where there is a less surface drag.

3. 2. The Power in Wind
Wind possesses energy by virtue of its motion. Any device capable of slowing down the
mass of moving air, like a sail or propeller, can extract part of the energy and convert is into
useful work.
There are three factors determine the output power generated from the wind mill, they are
(1) The wind speed
(2) The cross section of wind swept by rotor, and
(3) The overall conversion efficiency of rotor, transmission system and generator or pump.
No device, however well-designed, can extract all of the wind’s energy because the wind
would have to be brought to a halt and this would prevent the passage of more air through
the rotor. The most that is possible is for the rotor to decelerate to whole horizontal column
of intercepted air to about one-third of its free velocity.
A 100% efficient aerogenerator would therefore only be able to convert up to a maximum of
around 60% of the available energy in wind into mechanical energy.

4. Types of Wind Turbines
• Windmills” are used to grind grain into flour
• Many “Wind Turbine” names –
- wind-driven generator
- wind generator
- wind turbine
- wind-turbine generator (WTG)
- wind energy conversion system (WECS)”
• Wind turbines characterized by turbine blade’s axis of rotation
- Horizontal axis wind turbines (HAWT)
- Vertical axis wind turbines (VAWT)
• Groups of wind turbines are located in what is called either a “wind farm” or a “wind park”

5. Principal components of most wind energy conversion systems.
Typical WECS Components

6. Turbine Blade – an Air Foil
(a) Lift in wing (b) wind turbine blade forces
Increasing the angle of attack can cause a wing to stall

7. Vertical Axis
Wind Turbines
• Darrieus rotor - the only vertical axis machine
with any commercial success
• Wind flowing by the vertical blades (aerofoils)
generates “force” producing rotation
• No yaw (rotation about vertical axis) control
needed to keep them facing into the wind
• Heavy machinery in the nacelle is located on the
ground
• Blades are closer to ground where wind-speeds
are lower

8. Horizontal Axis Wind Turbines
• “Downwind” HAWT – a turbine with the
blades behind (downwind from) the tower
• No yaw control needed –naturally orients
in line with the wind
• Wind’s “shadow” behind the vertical axis
produces turbulence  vibration  mech
stress on the blade and supporting
structure

9. Horizontal Axis Wind Turbines
• “Upwind” HAWT – blades are in front of
(upwind of) the tower
• Most modern wind turbines are this
type
• Blades are “upwind” of the tower
• Require somewhat complex yaw control
to keep them facing into the wind
• Operate more smoothly and deliver
more power

10. Number of Rotating Blades
• Windmills have multiple blades
 need to provide high starting torque to overcome weight of the pumping rod
 must be able to operate at low windspeeds to provide nearly continuous
water pumping
 a larger area of the rotor faces the wind
• Turbines with many blades operate at much lower rotational speeds - as the speed
increases, the turbulence caused by one blade impacts the other blades
• Most modern wind turbines have two or three blades

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