Wind sources, turbines, parks, power control

Availability of wind energy sources

Wind arises from uneven heating of the earth by the sun. Good wind sources are:

• Coastal areas and offshore (sea breezes), open plains, and hill tops/ridges.
• Mountain passes and valleys where wind speeds up (Kali Gandaki valley in Mustang, Nepal).
• Average speed of at least 5–6 m/s at hub height is generally needed for economic power generation.
• Availability is mapped using meteorological data, anemometer masts, and remote sensing. The world has a vast technical potential (many times current demand); Nepal's estimated commercial potential is about 3,000 MW (based on earlier studies).

Wind turbines

A wind turbine converts the kinetic energy of wind to electricity: wind turns the rotor blades, which drive a shaft, gearbox and generator located in the nacelle on a tower. Types are horizontal axis (common; two or three blades) and vertical axis (Savonius, Darrieus). Large turbines are rated 2–15 MW. Power extracted = Cp · ½ ρ A v³ (Cp ≤ 0.593, Betz limit).

Wind parks (wind farms)

A group of many wind turbines installed at one site, connected to the grid via a common substation. Turbines are spaced 5–10 rotor diameters apart to avoid wake losses. They can be onshore or offshore. Benefits: economies of scale in transmission, installation and maintenance. Requirements: good wind, enough land or sea area, and a grid connection.

 T1  T2  T3  T4
  \  |   |  /
   [Collector cables]
         |
   [Substation] --> grid

Power control systems

Wind power must be limited at high speeds to protect the machine and to keep the output constant:

1. Pitch control: blades rotate about their axis to change the angle of attack; used in large modern turbines. Pitching out of the wind reduces power.
2. Stall control (passive): blade shape causes stall in high winds, so power is limited automatically; simple, but less accurate.
3. Active stall control: combination of both.
4. Yaw control: turns the rotor away from the wind in storms.
5. Variable-speed operation with power electronics (converter) to maximise Cp at low speeds.
6. Brakes (mechanical and aerodynamic) for emergency stop. Cut-in at about 3–4 m/s, rated at 12–15 m/s, cut-out at about 25 m/s.