--- title: "Wind energy, factors and scope" subject: "Energy Environment and Society" chapter: 3 key: "semester_7/energy_environment_and_society:wind-energy-factors-scope" source: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/wind-energy-factors-scope site: BCT Engineering --- # Wind energy, factors and scope Energy Environment and Society · Chapter 3: Renewable Energy Sources - Asked in: 2072 Chaitra; year not given - Video: [How do wind turbines work? - Rebecca J. Barthelmie and Sara C. Pryor](https://www.youtube.com/watch?v=xy9nj94xvKA) — TED-Ed - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/wind-energy-factors-scope - Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-3 ## Question What do you understand by wind energy? Write down the factors that determine the available wind energy in any area. Also write down its scope. Describe wind energy sources and wind generation. ## Short answer Wind energy is kinetic energy of moving air converted by turbines; available power is ½ρAv³, so wind speed, swept area, air density, height, terrain, and wind duration determine it. Scope: free, clean, renewable; in Nepal useful for remote/hybrid systems, with about 3,000 MW estimated potential. ## Answer **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:** ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. If wind speed doubles, how does available wind power change? A. Doubles B. Becomes 4 times C. Becomes 8 times D. Becomes 16 times Answer: C. P = ½ ρ A v³, so power is proportional to v³ and doubling speed gives 2³ = 8 times. (Key point: Wind power equation) 2. What does the Betz limit state? A. A turbine can extract at most 59.3% of the wind power B. A turbine must extract 100% of wind power C. Air density must be 1.2 kg/m³ D. Rotor diameter must be below 100 m Answer: A. Cp cannot exceed 0.593, so P = Cp · ½ ρ A v³ with Cp ≤ 0.593. (Key point: Betz limit) 3. How does altitude affect available wind energy, other factors equal? A. Higher altitude gives lower air density and so less power B. Higher altitude raises density C. Altitude has no effect D. Higher altitude removes the v³ dependence Answer: A. Air density falls with altitude, so P = ½ ρ A v³ falls. (Key point: Factors affecting wind energy) 4. Which is the correct generation chain? A. Generator - gearbox - rotor - grid B. Rotor blades - low-speed shaft - gearbox - high-speed shaft - generator - transformer - grid C. Rotor - transformer - generator - gearbox D. Tower - generator - rotor - shaft Answer: B. The rotor drives a slow shaft, the gearbox speeds it up, the generator makes electricity and the transformer connects to the grid. 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