Tides: How energy resides
Why dense seawater makes slow currents powerful
What this tab covers The physical form in which the energy is held or carried, where it comes from, the chain of transformations that put it there, and how concentrated it is.Tidal energy is the gravitational pull of the Moon and Sun on the oceans, held as the potential energy of water raised by the tide and the kinetic energy of the currents it drives. Its ultimate source is the rotation of the Earth and the orbit of the Moon, not the Sun's heat.
Form Gravitational potential energy of raised water (tidal range) and kinetic energy of moving water (tidal stream) Origin Gravitational and rotational (Earth-Moon-Sun system)How it got there
Rotation of the Earth and orbit of the Moon Gravitational pull of the Moon and Sun on the oceans Tidal bulges sweep round the planet twice a lunar day Water piles up in bays and estuaries (range) Water accelerates through straits and headlands (stream) Friction on shallow sea floors turns it to heatHow concentrated it is
| Measure | Value | Note |
|---|---|---|
| Tidal stream at 2.5 m/s (fast channel) | about 8 kW per m2 of flow cross-section4 | Calculated as 0.5 x 1,027 kg/m3 x (2.5 m/s) cubed; power rises with the cube of speed, and water is about 800 times denser than air |
| Tidal stream at 1.5 m/s (minimum useful speed) | about 1.7 kW per m2 of flow cross-section2 | Same formula; IRENA puts the minimum useful current at 1.5-2 m/s |
| Tidal range: La Rance barrage | about 2.8 W per m2 of basin3 | Calculated from IRENA's 540 GWh/yr over the 22 km2 basin; a reminder that barrages need large enclosed areas |
As the Earth turns beneath the Moon, the oceans are pulled into two bulges that sweep round the planet. In the open ocean the rise is modest, but in funnel-shaped bays and estuaries the water resonates and piles up, and in narrow straits it races through as a current. Two ways of capturing it follow: a barrage or lagoon traps water at high tide and releases it through turbines (tidal range), or turbines sit in the current like underwater wind turbines (tidal stream).
Because seawater is about 800 times denser than air, a current of 2.5 m/s carries roughly 8 kW through each square metre, several times the power of a strong wind. But per square metre of enclosed sea, a barrage yields only a few watts, so useful sites are rare and fixed by coastal geometry.
Sources
- Tidal Energy: Technology Brief IRENA, 2014
- La Rance Tidal Barrage (Annex IV site page) Tethys, Pacific Northwest National Laboratory / OES-Environmental, 2024
- Tidal stream generator Wikipedia, 2025