Ocean currents: How energy resides
Why only a few fast currents are worth tapping
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.Ocean currents hold kinetic energy of moving seawater, driven mainly by wind stress on the sea surface and shaped by the Earth's rotation, with differences in temperature and salinity adding a slower deep circulation. Most currents are slow, so usable power is confined to a few fast boundary currents such as the Gulf Stream.
Form Kinetic energy of moving seawater Origin Indirect solarHow it got there
Fusion in the Sun Sunlight Uneven heating of the Earth Winds Wind stress drags the sea surface (plus heating, cooling and evaporation changing water density) Large-scale ocean gyres and circulation Fast, narrow currents along western ocean boundaries (e.g. the Gulf Stream)How concentrated it is
| Measure | Value | Note |
|---|---|---|
| Power in a typical wind-driven current (about 0.2 m/s) | about 4 W per m2 of cross-section4 | Calculated as 1/2 x seawater density (1,025 kg/m3) x speed cubed; most US non-tidal currents flow at about 0.2 m/s or less. |
| Power in a current of 1 m/s | about 510 W per m2 of cross-section5 | Same calculation. Seawater is about 840 times denser than air, so 1 m/s of water carries as much power as about 9.4 m/s of wind. |
| Power in a current of 2 m/s | about 4,100 W per m2 of cross-section5 | Same calculation; doubling speed gives eight times the power. |
| Gulf Stream average speed | 6.4 km/h (about 1.8 m/s); up to about 9 km/h (2.5 m/s) near the surface speed7 | At 1.8 m/s the flow carries about 2,900 W per m2 of cross-section (calculated with the same formula). |
Winds dragging on the ocean surface set the upper ocean moving. The Earth's rotation turns these flows into vast basin-wide gyres, and the gyres are squeezed into narrow, fast jets along the western edges of ocean basins: the Gulf Stream off Florida and the Carolinas is the best known. Below the surface, water that is cooled and made saltier at high latitudes sinks and spreads slowly through the deep ocean.
Because water is about 840 times denser than air, even a modest current carries a lot of power per square metre of cross-section. But most of the ocean moves at walking pace or slower, where the power is only a few watts per square metre. Only the fast, steady cores of a few boundary currents are worth harnessing, and they flow far offshore over deep water, which makes anchoring turbines difficult and expensive.
Sources
- Marine Energy in the United States: An Overview of Opportunities (Kilcher, Fogarty and Lawson, NREL/TP-5700-78773) National Renewable Energy Laboratory / US DOE, 2021
- Power in the wind (power = 1/2 x density x swept area x speed cubed; the same kinetic-flux law applies to flowing water) Electrical and Mechanical Services Department, Hong Kong SAR Government (HK RE Net), 2026
- How fast is the Gulf Stream? NOAA National Ocean Service, 2026