--- title: "Wind energy conversion options and characteristics" subject: "Power Plant Design" chapter: 1 key: "semester_8/power_plant_design:wind-conversion-options" source: https://www.bctengineer.com/q/semester_8/power_plant_design/wind-conversion-options site: BCT Engineering --- # Wind energy conversion options and characteristics Power Plant Design · Chapter 1: Energy Sources and Electric Power Generation - Asked in: 2070 Bhadra, 3+5 marks - 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_8/power_plant_design/wind-conversion-options - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-1 ## Question What are the two basic options for wind energy to electric energy conversion? Discuss the characteristics of wind power on the basis of turbine efficiency, energy cost, capacity and annual energy output. ## Short answer The two options are constant-speed (squirrel-cage induction generator, simple, lower efficiency) and variable-speed (DFIG or full converter, captures 5-15% more energy, higher cost). Wind turbine efficiency is limited by Betz limit (59.3%), practical 35-45%; energy cost is capital-intensive; capacity from small to multi-MW; annual output E = P_rated × 8760 × CF, CF 20-40%. ## Answer Wind energy is converted to electricity by a wind turbine driving a generator. The two basic options are classified by how the turbine speed relates to the grid frequency. #### Two basic options **1. Constant (fixed) speed system** - Turbine runs at nearly constant speed set by the grid frequency. - Uses a squirrel-cage **induction generator** connected directly to the grid through a gearbox. - Simple, robust, cheap. - Cannot follow the optimum tip-speed ratio, so efficiency is lower at varying wind speeds; draws reactive power (capacitor banks needed); mechanical stress from gusts. **2. Variable speed system** - Rotor speed changes with wind speed to keep the optimum tip-speed ratio and maximum $C_p$. - Uses a **doubly fed induction generator (DFIG)** with a partial converter, or a **synchronous / permanent magnet generator** with a full power converter (AC–DC–AC). - Captures about 5–15% more energy, lower mechanical stress, controllable reactive power. - Higher cost and complexity due to power electronics. ```text Fixed speed: Rotor -> Gearbox -> SCIG ------------------> Grid | Capacitors Variable speed: Rotor -> (Gearbox) -> Generator -> AC/DC/AC -> Grid ``` (Some textbooks also state the two options as **grid-connected** versus **stand-alone (with battery)** systems.) #### Characteristics of wind power | Basis | Characteristic | |---|---| | Turbine efficiency | Maximum theoretical $C_p$ = 16/27 = 0.593 (Betz limit). Practical: 0.35–0.45 for large HAWT; lower for small and vertical-axis machines. Gearbox and generator losses add a further 5–10% | | Energy cost | No fuel cost; levelised cost depends on capital cost, wind speed and capacity factor. Large wind farms on good sites are competitive with conventional plants; small units are costly | | Capacity | Unit sizes from below 1 kW to over 10 MW (offshore). Rated output is reached at rated speed; output is zero below cut-in (about 3–4 m/s) and above cut-out (about 25 m/s) | | Annual energy output | $E = P_{rated} \times 8760 \times CF$. Capacity factor is usually 20–40%. Since $P \propto v^3$, mean wind speed strongly decides yearly energy | #### Power curve ```text P | ___________ | / | | / | | _/ | |_____/ |____ +-----+-----+---------+----> v cut-in rated cut-out ``` Wind power is clean and has low running cost, but its variability needs backup or storage. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A fixed speed wind system uses A. A squirrel-cage induction generator connected directly to grid through a gearbox B. A full AC-DC-AC converter C. A battery only D. A DC generator Answer: A. Simple and cheap but cannot track optimum tip-speed ratio and draws reactive power. (Key point: Fixed speed system) 2. Variable speed operation A. Keeps rotor speed constant B. Keeps optimum tip-speed ratio and max Cp, capturing about 5-15% more energy C. Needs no power electronics D. Is cheaper Answer: B. Cost is higher due to converters, but energy capture and stress are better. (Key point: Variable speed advantage) 3. Annual energy output of a wind plant is A. P_rated x CF B. P_rated x 8760 x 0.593 C. P_rated x 8760 x CF D. P_rated x 365 Answer: C. CF is usually 20-40%. (Key point: Energy output) 4. Wind turbine output is zero A. Between rated and cut-out B. Below cut-in (about 3-4 m/s) and above cut-out (about 25 m/s) C. At rated speed D. Only at night Answer: B. Below cut-in there is too little power; above cut-out the turbine shuts down for protection. 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