An Overview of (Non-Barrage) Tidal Power Technologies
Horizontal axis turbines mounted on towers
These are close in concept to traditional windmills operating under the sea and have the most prototypes currently operating. A fullsize prototype, called SeaGen, was installed by Marine Current Turbines Ltd in Strangford Lough in Northern Ireland in April 2008. The blades can be pitched through 180° to allow operation on the ebb and flood. It is currently the only commercial scale device to have been installed anywhere in the world.
Horizontal axis turbines - semi-submersible installation
An interesting concept for mounting turbines in deep water where fixed towers would be expensive or difficult to construct. A tethered, buoyant structure with a largely submerged configuration avoids storm-force loads. This technology favours float-out installation without barges, cranes, jack-ups or divers This is the option favoured by TidalStream Ltd.
Open Centre Turbines
This type of horizontal turbine is closer in design to those used in hydro power applications. It is a rugged technology with a slow-moving rotor and lubricant-free construction and operation. The self-contained rotor has a solid state permanent magnet generator encapsulated within the outer rim, minimising maintenance requirements.It is the technology favoured by ARE.
Venturi / Shrouded Turbines
Bi-symmetrical horizontal axis turbine in a symmetrical venturi duct. The shroud increases teh water flow through the turbine, and as it is bi-directional now yawing is required for ebb and flow. This is the technology favoured by Lunar Energy, who have a contract for the construction of a 300GW scheme in Korea.
Vertical axis turbines
Vertical axis turbines are relatively rugged and eady to engineer. Their design makes them suitable for incorporating in existing structures such as bridge piers. An organisation trying to develop this type of turbine is Tidal Turbines Ltd
These don't use rotary devices at all but rather aerofoil or wing sections which are pushed sideways by the flow. They are considered particularly suitable for shallwo water installations because the swept area is a rectangle whose length is independent of water depth. Pulse Tidal are one comapany pursuing this concept.