Wind energy, factors and scope

Wind energy is the kinetic energy of moving air, caused by uneven heating of the earth's surface by the sun and by the earth's rotation. A wind turbine converts it into mechanical or electrical energy.

The power available in wind: P = ½ ρ A v³, where ρ = air density (about 1.2 kg/m³), A = swept area of rotor (π D²/4) and v = wind speed. The actual power extracted is limited by the Betz limit (maximum 59.3% of wind power), P = Cp · ½ ρ A v³.

Factors determining available wind energy

1. Wind speed: the biggest factor, as power ∝ v³; doubling speed gives 8 times the power.
2. Swept area / rotor diameter (power ∝ D²).
3. Air density: depends on altitude, temperature and pressure (lower at high altitude).
4. Height above ground: speed increases with height (wind shear); tower height matters.
5. Terrain and obstacles: hills, ridges and valleys; trees and buildings cause turbulence.
6. Wind duration and direction frequency (wind rose, Weibull distribution).
7. Climate and season (monsoon, pre-monsoon).
8. Efficiency of the machine (Cp, generator and gearbox).

Scope

• Wind is free, clean and renewable, and one of the fastest-growing energy sources in the world (onshore and offshore).
• Nepal: limited but real potential in the Kali Gandaki valley, Mustang, Siwalik ridges, and hills; useful for remote areas as small and hybrid systems. The estimated commercial potential is about 3,000 MW.
• Employment, reduction of imported fuels and CO₂.

Wind energy sources and generation

Sources: global winds (trade winds, westerlies), local winds (sea/land breeze, mountain/valley breeze in the Himalayas, monsoon winds). Generation:

 Wind -> Rotor blades -> Low-speed shaft -> Gearbox
      -> High-speed shaft -> Generator -> Transformer -> Grid

Controls: yaw drive to face the wind, pitch control for power regulation, brake for safety. Cut-in speed about 3–4 m/s, rated about 12–15 m/s, cut-out about 25 m/s.