How a tidal turbine works
A tidal turbine is a rotor on a shaft driving a generator, like a wind turbine but underwater. Because water is dense, the rotor is short and slow, and the hard part is the structure that holds it still in a reversing current.
A tidal stream turbine looks like a wind turbine that someone dropped in the sea, and for a first approximation that is what it is. The comparison holds right up until the numbers, and then it breaks in an interesting way.
Why the blades are so short
Seawater is about eight hundred times denser than air, and the energy in a moving fluid rises with its density and the cube of its speed. A current of a few knots therefore carries force that would take a gale to match.
A tidal rotor of ten to twenty metres can match a wind turbine several times its diameter, and it turns slowly, which matters both for fatigue and for the marine life question.
Holding it still in a moving ocean
The hard engineering is not the rotor, it is everything that keeps the rotor where you put it. Three approaches are in use.
| Foundation | How it works | Needs |
|---|---|---|
| Gravity base | Sits on the seabed under its own weight | A flat, firm bed |
| Piled | Driven into the seabed | Ground you can drive into |
| Floating | Moored platform, turbine slung below | Station keeping in a reversing flow |
Turning with the tide
The flood and the ebb run opposite ways, so the machine has to work in both. Some yaw the whole nacelle around; some pitch the blades to reverse their angle of attack, which avoids moving the heavy parts; some use symmetrical blades that do not care.
Where it stops being like wind
The real cost driver
A wind turbine can be climbed on a calm day. A tidal turbine must be recovered to the surface or dived on during the slack water between tides, a window that can be under an hour. Almost every design decision is bent around making that window long enough to be useful.
For where the energy comes from in the first place, see what tidal energy is. For the different technologies, see the three types.