Tides
The rise and fall of the sea caused by the pull of the Moon and Sun.
Natural sources · Earth, gravity and atoms · No. 16
- Natural sources
- Origin: Gravitational / orbital
- Renewable: Yes
Tides can be forecast years ahead, yet their peaks arrive about 50 minutes later each day.
Chart: Illustration: Tides
Where it comes from
Chart: Map of where it comes from
The pull of the Moon and Sun raises the seas and drives currents; open ocean tides rise well under a metre, but some bays amplify them past 14 m.
Three things to know
- Seawater is about 800 times denser than air, so even a modest tidal current carries a lot of power.
- Planned barrages in India, Korea, the Philippines and Russia once totalled about 115 GW, but few were built.
- Tidal energy comes ultimately from the Earth's rotation and the Moon's orbit, not from the Sun's heat.
The journey
- 1 Moon and SunGravity tugs on the oceans
- 2 TidesWater rises and currents race
- 3 Bays and straitsFunnel shapes amplify range and flow
- 4 Barrages and turbinesDams across estuaries or underwater rotors
- 5 ElectricityAbout 1.1 TWh a year today