How does a tidal current turn predictable motion into electricity?

A tidal turbine extracts energy from water moving through a channel, rather than from a large visible wave. Gravitational interactions involving Earth, the Moon and the Sun produce tides; coastlines and the seafloor shape the currents at each site. Pressure and shear forces on an underwater rotor's shaped blades create a turning moment. A shaft, sometimes connected through a gearbox, drives a generator, and power electronics and cables make its electrical output usable.

Water transfers some mechanical energy to the rotor, slows and forms a turbulent wake downstream. The machine cannot remove all the passing water's kinetic energy while preserving the flow that supplies it. Blade performance, generator efficiency and operating limits restrict the useful output. Water continues through the channel; it is not consumed as fuel.

For a simple undisturbed flow, the incoming kinetic-energy flux through area A is ½ρAv³, where ρ is water density and v is speed. Faster water brings more mass each second, and each kilogram also has more kinetic energy. Doubling speed with other conditions fixed therefore multiplies this ideal incoming flux by eight. Actual output cannot keep rising that way indefinitely: equipment may limit generation or stop under unsuitable conditions.

Tidal currents commonly reverse and become weak near slack water. A turbine may turn to face the current or operate in both directions. The fastest current need not occur at local high tide, because water levels and channel flow respond through the basin's geometry and inertia. Predictable timing helps planning, but does not mean constant power; other resources and demand still need to be coordinated with the changing generation.

Saltwater corrosion, organisms growing on surfaces, debris and underwater maintenance make this a demanding setting. Designers also investigate shipping, sediments, sound and animal movements. Multiple turbines change currents and wakes differently from an isolated machine. The conversion chain joins astronomical timing, local geography, fluid forces and electrical machinery; each part affects how much usable energy a real site can supply.