Tidal stream power
Electricity generated by underwater turbines placed in fast tidal currents, analogous to wind turbines but driven by water about 800 times denser than air.
Tidal stream devices extract kinetic energy from currents in straits, sounds and headlands without impounding water. Because seawater density is about 1,025 kg/m³, power density is very high:
At 2.5 m/s, a typical peak spring current, the flow carries about 8 kW per m², compared with roughly 0.5 kW/m² for wind at 12 m/s. Commercially interesting sites have peak spring currents above about 2–2.5 m/s and water depths of 25–50 m.
Device types include seabed-mounted horizontal-axis turbines (Meygen, Scotland: 6 MW, the largest array), floating platforms carrying turbines below the hull (Orbital O2, 2 MW), and vertical-axis or ducted designs. Rotors are typically 15–26 m in diameter and turn slowly, with tip-speed ratios limited by cavitation. The Betz limit applies, though channel blockage can raise the effective maximum.
- Capacity factor: roughly 30–45%, highly predictable from astronomical tides
- Global practical resource: estimated at 100–150 TWh/yr from well-studied sites; Pentland Firth, Raz Blanchard, Bay of Fundy and Indonesian straits are prime areas
- Cost: pre-commercial LCOE often reported around EUR 200–300/MWh (Ocean Energy Europe, JRC), with UK CfD awards in 2022–2024 at roughly GBP 180–200/MWh in 2012 prices
Challenges are biofouling, corrosion, very high turbulence loads and expensive marine operations in short slack-water windows.
Related terms
All terms →- Tidal range power
Electricity generated from the height difference between high and low tide, by impounding water behind a barrage or lagoon and releasing it through low-head turbines.
- Wave energy
Energy captured from the motion of ocean surface waves using devices such as point absorbers, oscillating water columns and attenuators; still at pre-commercial stage.
- Betz limit
The theoretical maximum fraction of kinetic energy an open rotor can extract from the wind: 16/27, or about 59.3%.
- Swept area
The area of the circle traced by a turbine's rotating blades; wind turbine power output is directly proportional to it, and it grows with the square of rotor diameter.
- Tip-speed ratio
The ratio of the linear speed of a turbine blade tip to the undisturbed wind speed; three-bladed utility turbines are designed for values of about 7 to 10.
- Contract for Difference (CfD)
A support scheme or contract in which a generator receives the difference between a fixed strike price and a market reference price, paying back when prices exceed it.