--- title: "Tidal versus wind power plant" subject: "Power Plant Design" chapter: 1 key: "semester_8/power_plant_design:tidal-vs-wind" source: https://www.bctengineer.com/q/semester_8/power_plant_design/tidal-vs-wind site: BCT Engineering --- # Tidal versus wind power plant Power Plant Design · Chapter 1: Energy Sources and Electric Power Generation - Asked in: 2070 Magh, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/tidal-vs-wind - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-1 ## Question Compare the operation characteristics of the tidal power and wind power plant regarding daily curve, stability, grid connection and operating range. ## Short answer Tidal plants provide predictable, periodic generation with synchronous machines giving good stability and few large HV connections, while wind plants have random output, low inertia from converters, many dispersed units needing grid codes. Tidal operates above ~1–2 m head (20–30% CF), wind between 3–4 and 25 m/s (25–40% CF). ## Answer Both tidal and wind plants convert the kinetic/potential energy of a natural moving fluid into electricity, but tidal power follows the **predictable lunar cycle** while wind power follows **random weather**. This makes their operating characteristics very different. #### Daily generation curve - **Tidal:** Output comes in blocks linked to the tide. A barrage with two tides a day (period about 12 h 25 min) gives 2 to 4 generating periods a day, with zero output near slack water while the head builds up. The pattern shifts by about 50 minutes each day and varies over the spring–neap cycle (about 14.8 days), but it can be predicted years ahead. - **Wind:** Output depends on wind speed, $P = \frac{1}{2}\rho A C_p V^3$, so it changes from minute to minute. The daily curve is irregular; at many sites wind is stronger in the afternoon or at night, but this cannot be scheduled. ```text P Tidal (barrage) P Wind | __ __ __ | /\ _/\ /\_ | | | | | | | | / \/ \__/ \ |_| |____| |____| |__ |__/ \_ +-------------------> t +-------------------> t 0 12h 24h 0 12h 24h ``` #### Stability - **Tidal:** Uses large, slow bulb or Kaplan turbines with synchronous generators. These have rotating inertia, governors and excitation control, so they support transient and voltage stability. The output change is gradual and known in advance. - **Wind:** Modern turbines are variable-speed (DFIG or full converter). The power electronics decouple the rotor inertia from the grid, so the plant gives little natural inertia. Sudden gusts or calm periods cause power swings and frequency/voltage fluctuations. Special controls (synthetic inertia, fault ride-through) are needed. #### Grid connection - **Tidal:** Few, large plants (hundreds of MW) at coastal estuaries; connected to HV transmission through a step-up substation, like a hydro plant. Because output is predictable, dispatch planning is easy. - **Wind:** Many small units (2–15 MW each) collected through a medium-voltage collector network and a pooling substation. Needs grid codes for reactive power, LVRT, harmonics and forecasting; high penetration needs storage or reserve. #### Operating range - **Tidal:** Generates only when the head across the barrage exceeds a minimum (about 1–2 m); output rises with head. Capacity factor is about 20–30%. - **Wind:** Generates between cut-in speed (about 3–4 m/s) and cut-out speed (about 25 m/s), with rated output from about 12–15 m/s. Capacity factor is about 25–40% (higher offshore). #### Summary | Aspect | Tidal plant | Wind plant | |---|---|---| | Daily curve | Periodic, predictable | Random, weather dependent | | Forecasting | Exact, years ahead | Short-term only | | Inertia / stability | Good (synchronous machines) | Low (converter based) | | Grid connection | Large HV plant at coast | Many units, MV collector + HV | | Operating range | Head > ~1–2 m | 3–4 m/s to ~25 m/s | | Capacity factor | 20–30% | 25–40% | | Reserve need | Low, schedulable | High, needs backup/storage | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How do the daily output curves of tidal and wind plants differ? A. Wind is periodic and predictable; tidal is random B. Tidal is periodic and predictable (blocks tied to tides); wind is irregular and weather dependent C. Both are constant D. Both follow the sun Answer: B. Tidal follows the lunar cycle and can be predicted years ahead; wind follows random weather. (Key point: Daily curve) 2. Why does a tidal plant support stability better than a wind plant? A. It uses power electronics to add inertia B. It has many small units C. It has no governor D. It uses synchronous generators with inertia, governors and excitation control Answer: D. Tidal turbines drive synchronous machines giving inertia and control, while converter-based wind turbines decouple inertia. (Key point: Stability) 3. What does wind power depend on according to P = 0.5 rho A Cp V^3? A. The cube of wind speed, so it changes quickly with speed B. Linearly on wind speed C. Square of wind speed D. Only on blade length Answer: A. Power varies with V^3, so small speed changes cause large power swings. (Key point: Wind power equation) 4. What operating limits apply? A. Tidal needs head above about 1-2 m; wind between cut-in 3-4 m/s and cut-out 25 m/s B. Tidal works at any head C. Wind works from 0 m/s D. Tidal cut-out is 25 m/s Answer: A. A barrage generates only when head exceeds a minimum, and wind turbines work between cut-in and cut-out speeds. 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