Tidal versus wind power plant

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.

 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