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How does tidal power work?

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Quick answer

Tidal power turns the movement of the tides into electricity. A barrage across an estuary traps seawater at high tide and releases it through turbines as the sea falls, or underwater turbines sit in fast tidal currents and spin like windmills. Because tides follow the Moon, output is predictable far into the future.

Tidal power generates electricity from the regular rise and fall of the sea, which is caused by the Moon's and Sun's gravity. Turbines capture energy either from water held behind a barrage and released through it, or from fast tidal currents flowing past underwater rotors.

Step by step

  1. The Moon's and Sun's gravity pull ocean water into bulges, creating rising and falling tides along coasts.
  2. At a barrage, sluice gates let the rising tide fill the basin behind the dam, then close.
  3. As the sea outside drops, a height difference builds between the basin and the open water.
  4. Water rushes back through turbines in the barrage, spinning them and driving generators that feed the grid.
  5. Tidal stream devices skip the dam and simply turn in currents that reverse direction with each tide.

Key things to understand

  1. The Moon's gravity, helped by the Sun's, raises bulges of ocean water that give most coasts two high tides a day.
  2. A tidal barrage is a dam across an estuary that traps water at high tide and lets it out through turbines.
  3. The height difference between trapped water and the falling sea drives water through low-head turbines that spin generators.
  4. Tidal stream turbines work like underwater wind turbines, turning in fast currents that flow through narrow channels.
  5. Because tides follow astronomical cycles, output can be predicted years ahead, though it rises and falls through each day.

Labelled diagram

  1. Rising tide

    The sea rises outside the barrage as the Moon's gravity raises water levels.

  2. Sluice gates

    Open to fill the basin, then close to trap water at high tide.

  3. Head difference

    The sea falls outside, leaving the basin several metres higher than the ocean.

  4. Bulb turbines

    Water flows back through turbines in the barrage, spinning the generators.

How a tidal barrage makes electricity

Real-world examples

  • The La Rance barrage in Brittany, France, has generated tidal electricity since 1966.
  • South Korea's Sihwa Lake station uses a seawall built for land reclamation to house its tidal turbines.
  • The MeyGen project in Scotland's Pentland Firth places turbines on the seabed in strong tidal currents.
  • Gujarat's Gulf of Khambhat, with tidal ranges of several metres, has been surveyed as a possible Indian tidal site.

Common mistake

✗ Myth: Tidal power is unreliable because the sea is always changing.

✓ Fact: Tides are among the most predictable natural events, set by the Moon and Sun. Output varies within each day, but the timing and size of that variation can be forecast years in advance.

People also ask

How does a tidal barrage generate electricity?
A barrage is a dam built across a tidal estuary. Gates let the incoming tide fill the basin, then close to hold the water as the sea outside falls. Once a large enough height difference forms, water is released through turbines in the dam, spinning generators. Some barrages also generate on the incoming tide.
What are the disadvantages of tidal energy?
Suitable sites with large tidal ranges or fast currents are scarce, and barrages are costly to build. They can alter sediment flow, mudflat habitats and fish movement in estuaries. Output also peaks and drops with the tidal cycle, so it does not always line up with the hours of highest electricity demand.
How many times a day can tidal power generate?
Most coasts have two high and two low tides roughly every 24 hours and 50 minutes. A barrage that generates only on the ebb produces power for several hours twice a day, while two-way barrages and tidal stream turbines can generate on both incoming and outgoing flows, giving four generating periods.
Is tidal power the same as wave power?
No. Tidal power uses the slow, predictable rise and fall of the sea caused by gravity, while wave power uses surface waves raised by wind. Tides run on a fixed astronomical schedule; waves depend on weather.
Why are there so few tidal power plants?
Good sites need an unusually large tidal range or very fast currents, and these are rare. Barrages are also expensive to build and can change mudflats, fish migration and sediment in the estuary, so many proposals have stalled.
How predictable is tidal energy?
Very predictable. Tide times and heights depend on the positions of the Moon and Sun, so they can be calculated decades in advance. Output still varies within each day and between spring and neap tides.
Does India have tidal power potential?
Yes. The Gulf of Khambhat and Gulf of Kutch in Gujarat have some of the largest tidal ranges in India and have been studied for tidal schemes, though cost and environmental concerns have kept large projects from going ahead.

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