Scotland's Renewable Energy Blog

An Overview of  Tidal Power Technologies

‘Conventional’ wind-turbine-underwater’ concept

IN the rush to develop commercial tidal power the obvious way to go is to develop devices that are very similar to wind turbines but that operate underwater using the flow of water rather than the flow of air to turn the blades. This is the technology that will be used for Scotland’s first tidal farm in the Sound of Islay. While this is the most obvious design there are several competing technologies in development, some of which we take a brief look at below:

Horizontal axis turbines mounted on towers

This is a variation on the  traditional ‘undersea windmill’ concept, where the rotors are mounted on towers that project above the water. A full size 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. The arm the rotors are mounted on can be raised clear of the water for maintenance. It is currently the only commercial scale device to have been installed anywhere in the world. A 4-turbine 5MW installation is planned for Kyle Rhea off the Isle of Skye, with the intention of it becoming operational some time in 2012

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 would allow float-out installation without barges, cranes, jack-ups or divers. Again, there is a substantial above water structure, creating potential obstructions to  navigation. 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  Open Hydro , who are currently testing at EMEC in Orkney. 

Venturi / Shrouded Turbines

THis is a bi-symmetrical horizontal axis turbine in a symmetrical venturi duct. The shroud increases the water flow through the turbine, and as it is bi-directional now yawing is required for ebb and flow. This is the technology used by the Rotech tidal turbine, developed in Aberdeen in partnership with Lunar Energy.  This very promising looking project seems to be stalled at the moment, with no further news on the previously announced commercial deployment in Korea.

Vertical axis turbines

Vertical axis turbines are relatively rugged and easy to engineer. Their design makes them suitable for incorporating in existing structures such as bridge piers.  One organisation trying to develop this type of turbine is  Tidal Turbines Ltd . We are not aware of any vertical axis devices currently under test or planned for Scotland.

Oscillating devices

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 shallow water installations because the swept area is a rectangle whose length is independent of water depth. Pulse Tidal are one company pursuing this concept. Pulse have previously announced plans for a commercial deployment in Kyle Rhea, although this is another project with no recent news.