Tidal Power: Turning a Predictable Tide into Electricity

Two different machines, one resource

Tidal energy is harvested in two mechanically unrelated ways. A tidal stream turbine looks and behaves like an underwater wind turbine, spun by the horizontal flow of water as the tide floods and ebbs through a strait or headland. A tidal barrage instead dams an estuary and generates from the vertical head difference between the impounded basin and the open sea as the tide falls or rises. Both are entirely predictable decades in advance, unlike wind or solar, because they are driven by celestial mechanics rather than weather.

Stream turbines: kinetic power and the Betz limit

The kinetic power available in a stream of cross-sectional area A moving at speed v is the same cubic law that governs wind:

P_available = 0.5 . rho . A . v^3

rho (seawater) ≈ 1025 kg/m^3  (about 800x the density of air)

Because that seawater is roughly 800 times denser than air, a tidal turbine extracts far more power than a wind turbine of the same swept area at the same speed — a modest 2-3 m/s tidal current can rival a much faster wind. No turbine can capture all of that kinetic power, though: it has to leave the water moving fast enough downstream to get out of its own way. The theoretical ceiling, the Betz limit, caps the fraction any open-flow turbine can extract at Cp = 16/27 ≈ 0.593; real turbines, once you include blade losses, generator efficiency and blockage from the seabed and surface, typically deliver an overall coefficient of about 0.35-0.45.

Because power scales with v³, output is extremely sensitive to the state of the tide: a turbine rated at peak spring-tide flow may produce only an eighth of its rated power at half that speed, and stream turbines therefore generate in four pulses a day, twice on the flood and twice on the ebb, dropping to near zero at slack water.

Barrages: potential energy from the tidal range

A barrage instead stores water at high tide behind a dam with sluice gates and turbines, then releases it through the turbines as the sea level falls, extracting potential rather than kinetic energy:

P = rho . g . Q . H

Q = volumetric flow rate through the turbines (m^3/s)
H = instantaneous head difference between basin and sea (m)

Total energy per tidal cycle is roughly proportional to the basin area times the square of the tidal range, so barrage sites are chosen for the largest possible range — the Bay of Fundy (up to about 16 m) and the Severn Estuary (up to about 14 m) are the two most-studied examples in the world, though only a handful of barrages (La Rance in France, Sihwa Lake in South Korea) have actually been built, because damming a whole estuary carries a heavy environmental and construction cost that stream turbines avoid.

The 12.4-hour clock and the spring-neap cycle

Most coastlines are dominated by the M2 lunar semi-diurnal tide, with a period of about 12 hours 25 minutes — two highs and two lows a day, arriving roughly 50 minutes later each day as the Moon's position relative to the observer slowly precesses. Layered on top is the roughly 14.77-day spring-neap cycle: when the Sun and Moon align at new and full moon their tidal forces add, giving the largest range (spring tides); at the Moon's quarters the two forces partly cancel, giving the smallest range (neap tides). Because power scales with the cube (stream) or square (barrage) of the tidal amplitude, a site's spring-tide output can be several times its neap-tide output on the very same turbine.

Frequently asked questions

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