T3.1

• Windmills have been used as an energy source since antiquity by various civilizations such as the Chinese, Egyptians, Persians, and Babylonians
• European windmills were used in the 1700s and 1800s to grind grains and pump water
• Two basic windmill designs that developed by the 1700s were the post mill and the tower mill
• The post mill consisted of a large house and a tail pole to turn the structure into the wind
• The tower mill was more modern, with a rotor and tail mounted on a fixed tower
• Early windmill rotors were made of wood, reeds, and canvas, with sails up to 12m long and 3m wide providing 30 kW of power
• Wind power was used for electricity generation by 1930, with systems having two or three blades connected to a generator
• Factors accelerating wind power technology development include high-strength fiber composites, falling prices of power electronics, variable-speed generators, improved plant operation, economies of scale, and accumulated field experience
• Wind is created by the rotation of the earth, uneven heating of the atmosphere, and irregularities of the ground surface
• Wind turbines capture wind's kinetic energy and convert it into electrical power through rotating blades connected to a generator
• Wind turbines are classified into horizontal axis and vertical axis types, with horizontal axis being more common commercially
• Vertical axis wind turbines have advantages like omnidirectional wind acceptance but disadvantages such as poorer performance and reliability compared to horizontal axis turbines
• The Savonius turbine is S-shaped and useful for tasks like grinding grain and pumping water, but not efficient for large-scale electricity generation
• The Darrieus turbine has C-shaped rotor blades, operates on aerodynamic lift principles, and has a rotor efficiency of 40% to 45%
• The giromill turbine has vertical aerofoils attached to a central mast and works well in turbulent wind conditions
• Horizontal Axis Wind Turbines are the most common design, with the blade rotation axis parallel to the wind flow
• Some wind turbines operate in an upwind mode to avoid wind shade caused by the tower, improving power quality and reducing power spikes
• Up-Wind Turbines:

  • Improve the power quality of the generated voltage
  • Reduce spikes in power when blades move in front of the tower, especially in constant speed systems
  • Require a rigid hub, which must be away from the tower to prevent blades from hitting it
  • Dominant design for most wind turbines in the MW-range
• Down-Wind Turbines:

  • Wind passes the tower before striking the blades
  • Without a tail vane, the machine rotor naturally tracks the wind in a downwind mode
  • May be built without a yaw mechanism if the nacelle has a streamlined body to follow the wind
  • Rotor can be more flexible, allowing blades to bend at high speeds and take load off the tower
• Shrouded Wind Turbines:

  • Some turbines have an added structural design feature called an augmentor to increase wind passing through the blades
• Parts of a Wind Turbine:

  • Nacelle contains key components including gearbox and electrical generator
  • Gearbox increases rotational speed of the shaft for the generator
  • Generator converts mechanical energy of the rotating shaft to electrical energy
  • Tower carries the nacelle and rotor, advantageous to have a high tower for increased wind speeds
  • Rotor blades capture wind energy and transfer power to the rotor hub
• Yaw system:

  • Yaw drive used to keep rotor facing into the wind as wind direction changes for upwind turbines
  • Downwind turbines don't require a yaw drive, wind blows rotor downwind
  • Yaw motor powers the yaw drive
  • Almost all HAWT use forced yawing with electric motors and gearbox
• Airfoil Shape:

  • Wind turbine blades use airfoil shape to create lift and maximize efficiency
  • Lift force is perpendicular to the direction of motion, while drag force is parallel
  • Effect of angle of attack on airfoil lift is crucial
• Braking Mechanism:

  • Essential for turbines to stop automatically in case of malfunction
  • Two types of braking: aerodynamic braking system and mechanical braking system
• Control Mechanisms:

  • Optimize aerodynamic efficiency, keep generator within limits, enable maintenance, and reduce noise
• Stalling Principle:

  • Increased angle of attack results in decreasing lift-to-drag ratio
  • Different types of stalling control regulators: passive, active, and hybrid
• Pitch Control:

  • Decreasing angle of attack results in decreasing lift-to-drag ratio
  • Blades rotate out of the wind when wind speeds are too high
  • Advantages include good power control and no need for startup means
• Furling Principle:

  • Moving the axis out of the wind decreases angle of attack and cross-section
  • Requires active pitch control, with vertical or horizontal furling options
• Variations of Wind Speed:

  • Interannual, annual, diurnal, and short-term variations in wind speed
  • Interannual variations occur over timescales greater than 1 year and can affect long-term turbine production
  • Annual variations refer to significant variations in seasonal or monthly averaged wind speeds
  • Large diurnal variations occur due to differential heating of the earth's surface