Unit 5 Wind Energy

Wind Energy

• Wind is air in motion, and its energy is proportional to the cube of its speed.
• Wind energy is an indirect form of solar energy, generated by the differential heating of the earth's surface.

Origin of Winds

• Global winds are caused by the differential heating of the earth's surface at the equator and polar regions.
• The rotation of the earth creates the Coriolis force, which makes global winds move towards the westerly direction.
• Local winds are generated by uneven heating of land and water surfaces.

Nature of Wind

• Wind varies from place to place, depending on the climate, geography, and terrain.
• Knowledge of wind behavior and structure is essential for installing wind turbines.

Wind Speed Variations

• Wind speed increases with height, but falls to zero at the earth's surface.
• The rate of change of wind speed with height is called wind shear.
• The gradient height is the height at which wind speed no longer increases with height.

Wind Turbine Siting

• Key considerations for selecting a wind turbine site include:
  • High annual mean wind speed
  • No obstruction within a certain distance
  • Open plain or coastal area
  • Height to take advantage of increased wind speed
  • Proximity to load centers and transportation links
  • Availability of wind data

Rotor Design

• A rotor extracts energy from the wind by transforming kinetic energy into rotational motion.
• Rotor design parameters include:
  • Solidity: the ratio of projected blade area to swept area
  • Chord: the width of the blade
  • Angle of incidence: the angle between the blade and the wind direction
  • Pitch angle: the angle between the blade and the direction of motion
  • Different velocities: wind velocity, incident wind velocity, blade linear velocity, and relative velocity

Wind Energy Conversion

• The principle of wind energy conversion is to extract energy from the wind by partially decelerating and expanding the airstream.
• The rotor collects wind from the whole area swept by the rotor, and the energy is extracted through the deceleration of the airstream.

Aerodynamic Considerations

• Lift and drag forces are created when wind moves over an aerofoil.
• The shape of the aerofoil determines the relative magnitude of lift and drag forces.
• Lift devices are more efficient due to their lower drag forces.

Types of Windmills

• Horizontal Axis Wind Turbine (HAWT): axis of rotation is parallel to the airstream.
• Vertical Axis Wind Turbine (VAWT): axis of rotation is perpendicular to the airstream.
• HAWTs are commonly used for electric power generation.
• VAWTs are suitable for applications where low speeds are required, such as piston pumps.

Rotors of HAWT and VAWT

• HAWT rotors can be single-bladed, two-bladed, three-bladed, or multi-bladed.
• VAWT rotors can be cup-type, Savonious, Darrieus, Musgrove, or Evans-type.
• Darrieus rotors are commonly used for large power generation due to their good power coefficient.### Advantages of Darrieus Wind Turbine
• Simple in construction
• Low cost of construction and installation
• Higher power coefficient and tip speed ratio compared to S-rotor

Disadvantages of Darrieus Wind Turbine

• Not self-starting machine
• Works on drag force, leading to low efficiency in converting wind energy
• Limited height, unable to utilize high wind speeds available at higher levels
• Unable to yaw out of the wind, requires special high torque braking system during high wind speeds

Starting Arrangements for Darrieus Rotor

• Attaching S-rotors at the top and bottom of the shaft to help start up
• Designing the generator to run as a motor to start up the wind turbine initially
• Partly shielding the rotor from the wind stream to behave as an unsymmetrical rotor and develop starting torque

Wind Energy Storage

• Chemical energy storage through batteries
• Thermal energy storage through heating water
• Compressed air storage
• Electrolysis of water to produce hydrogen and oxygen gases
• Pumping water to a high tank for storage
• Integration to electric grid

Environmental Impacts of Wind Turbine

• Zero emission during operation
• Threat to bird's life due to rotating rotors
• Noise disturbance due to aerodynamic noise
• Interference to transmission of TV and communication signals
• Visual intrusion due to high towers
• Safety concerns due to rotating blades
• Impact on ecosystem due to large-scale interception and use of wind energy

Recent Developments in Wind Turbines

• Yaw and tilt control for safety and speed control
• Pitch control to regulate rotor blades for maximum output
• Stall control to shift blades to a safe position during high winds
• Fixed blades with constant pitch for constant turbine speed
• Eddy current braking system to control speeds
• Variable speed drive system to capture maximum power

Factors Accelerating Wind Power Development

• Improved materials for large-sized rotor blades
• Power electronics for regulation and control
• Variable speed drive system for maximum energy capture
• Larger wind turbines for economical power output
• Short energy payback period
• Improved plant operations for high power supply
• Increased expertise in development and operation
• Renewable resource competing with conventional power sources