1. UNIT II WIND ENERGY COLLECTORS
Wind Turbines – Classification – Working principle – Horizontal axis and vertical axis machines – Application of wind turbines – Power coefficient – Betz coefficient by momentum theory.

2. Wind Energy and Wind Power
Wind is a moving air or flow of gases on a large scale
Winds are caused by the uneven heating of the atmosphere by the sun, the irregularities of the earth's surface, and rotation of the earth.
Wind flow patterns are modified by the earth's terrain, bodies of water, and vegetative cover.
This wind flow, or motion energy, when "harvested" by modern wind turbines, can be used to generate electricity.

3. WIND ENERGY AND POWER
The kinetic energy (KE) (Energy-motion)of an object (or collection of objects) with total mass M and velocity V is given by the expression:
KE = ½ * M * V^2
To find the kinetic energy of moving air molecules (wind), consider a large air parcel with the shape of a huge hockey puck: that is, it has the geometry of a collection of air molecules passing though the plane of a wind turbines blades (which sweep out a cross-sectional area A), with thickness (D) passing through the plane over a given time.

4. The volume (Vol) of this parcel is determined by the parcel's area multiplied by its thickness: Vol= A * D
Let ρ represent the density of the air in this parcel. Note that density is mass per volume and is expressed as: ρ = M / Vol and M = ρ * Vol
Now let's consider how the velocity (V) of our air parcel can be expressed. If a time T is required for this parcel (of thickness D) to move through the plane of the wind turbine blades, then the parcel's velocity can be expressed as V = D / T, and D = V * T.
KE = ½ *M * V^2 Substitute for M = ρ * Vol to obtain
KE = ½ * (ρ * Vol) * V^2 and Vol can be replaced by A * D to give
KE = ½ * (ρ * A * D) *V^2 and D can be replaced by V * T to give
KE = ½ * (ρ * A * V * T) * V^2 leaving with: KE = ½ *ρ*V^3*A*T

5. Now, power is just energy divided by time, so the power available from our air parcel can be expressed as:
Pwr = KE / T = (½ *ρ * V^3 * A * T) / T = ½ * ρ * V^3 * A
And if we divide Pwr by the cross-sectional area (A) of the parcel, then we are left with the expression: Pwr / A = ½ * ρ * V^3
Pwr / A only depends on (1) the density of the air and (2) the wind speed. In fact, there is no dependence on size, efficiency or other characteristics of wind turbines when determining Pwr / A.
The term for the quotient Pwr / A is called the "Wind Power Density" (WPD)

6. How Wind Power Is Generated
The terms "wind energy" or "wind power" describe the process by which the wind is used to generate mechanical power or electricity. wind turbines convert the kinetic energy in the wind into mechanical power. This mechanical power can be used for specific tasks (such as grinding grain or pumping water) or a generator can convert this mechanical power into electricity to power homes, businesses, schools, and the like

7. Wind Turbines
Wind turbines, like aircraft propeller blades, turn in the moving air and power an electric generator that supplies an electric current.
Simply stated, a wind turbine is the opposite of a fan. Instead of using electricity to make wind, like a fan, wind turbines use wind to make electricity.
The wind turns the blades, which spin a shaft, which connects to a generator and makes electricity.

8. Wind Turbine Types
Modern wind turbines fall into two basic groups; the horizontal-axis variety, like the traditional farm windmills used for pumping water, and the vertical-axis design, like the eggbeater-style Darrieus model, named after its French inventor. Most large modern wind turbines are horizontal-axis turbines.

9. Wind Turbine Subsystems
Foundation
Tower
Nacelle
Hub & Rotor
Gearbox
Generator
Electronics & Controls
Yaw
Pitch
Braking
Power Electronics
Cooling

12. Horizontal Axis Wind Turbines (HAWT)
Horizontal axis wind turbines, also shortened to HAWT, are the common style that most of us think of when we think of a wind turbine.
A HAWT has a similar design to a windmill, it has blades that look like a propeller that spin on the horizontal axis.
Horizontal axis wind turbines have the main rotor shaft and electrical generator at the top of a tower, and they must be pointed into the wind.
Small turbines are pointed by a simple wind vane placed square with the rotor (blades), while large turbines generally use a wind sensor coupled with a servo motor.
Most large wind turbines have a gearbox, which turns the slow rotation of the rotor into a faster rotation that is more suitable to drive an electrical generator.

13. Since a tower produces turbulence behind it, the turbine is usually pointed upwind of the tower. Wind turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds.
Additionally, the blades are placed a considerable distance in front of the tower and are sometimes tilted up a small amount.
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.

16. HAWT Advantages
Blades are to the side of the turbine’s center of gravity, helping stability.
Allowing the angle of attack to be remotely adjusted gives greater control, so the turbine collects the maximum amount of wind energy.
The ability to pitch the rotor blades in a storm so that damage is minimized.
Most of them are self-starting.
Can be cheaper because of higher production volume.

17. 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.
Downwind variants suffer from fatigue and structural failure caused by turbulence when a blade passes through the tower's wind shadow (for this reason, the majority of HAWTs use an upwind design, with the rotor facing the wind in front of the tower).
HAWTs require an additional yaw control mechanism to turn the blades toward the wind.
HAWTs generally require a braking device in high winds to stop the turbine from spinning and destroying or damaging itself.