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Background

Tidal Stream Turbines

The simplest way to develop tidal stream energy is to borrow from  horizontal axis wind turbines, where the technology, components and  know-how have been developed over the last 30 years. A tidal stream  turbine is just like a wind turbine underwater, except that the density  of seawater is 800 times greater than air, and flow rates typically one  fifth. So a properly rated tidal turbine would have a rotor diameter about half that of a wind turbine of the same rated power. The blades, hub and transmission can all be borrowed from wind turbine components, properly sized and marinised for long-term underwater use.

The real problem is supporting the rotor-transmission so that it follows the water flow and can be installed and maintained easily and cheaply.  For deepwater sites, where two thirds of the resource lies, submerged floating designs are necessary. These avoid the storm vulnerability of surface floating devices, and the impracticality of seabed-mounting.

The section below will take you through our thinking process to the final design shown on the Home Page.

The starting point is as on a wind turbine, with the single rotor turning  upwind of a tube tower cantilevered from the base. The rotor is designed for a tip speed of 12 m/s or less to keep the tips clear of cavitation, and the blade design is suitably chunky because of the large loads (the blade root and hub proportions need to be equivalent to those of a wind turbine rotor of roughly twice the diameter). The blades would be  moulded in glass-carbon composite and the hub and gearbox suitably size (about four times the torque / power ratio of a comparable wind turbine  design) and suitably marinised.

The simplest of all configurations is  a rotor on a pole fixed to the seabed  In the design, right, the sleeve carrying the rotor provides both slewing so that the rotor can be yawed to follow the current, and  the means of removal / maintenance, though in practice this would be  extremely difficult. This design would not be suitable for deep water  because of the huge tube base loads and difficulty of access to the  seabed.

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The image, left, shows the first floating design. The swinging arm design with a  spar buoy support whose buoyancy can be controlled allows the turbine to roll over and swing up to the surface for maintenance. The middle  turbine is running - downstream of its anchorage - with the thrust on  the rotor keeping the rotor well down in the stream.

The left hand  turbine has stopped its rotor, removal of thrust then causing it to rise a little towards the surface. In the right hand turbine, the water has  been pumped out of the spar so that the turbine rises still further -  and as it does so, rolls on to its back presenting the rotor and  machinery pod above the surface of the sea for maintenance.

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The swinging-arm concept is preferable to designs that tether the floating  turbine with cables - because the latter give the turbine too much freedom, resulting in potential instability during operation. River and tow-tank tests show that the swing-arm provides an adequate restriction of freedom so that the turbine runs stably and follows the flow direction accurately.

This initial idea was awarded a UK patent in 2003. The design was then known as the TidalStream SST - the Semi-Submersible Turbine.

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One drawback with the single rotor is that its torque has to be resisted  by the righting moment of the buoy - not impossible, but undesirable.  Also the rotor must run in the wake of the main structural components, which would increase fatigue loading. A better solution is to adopt contra-rotating rotors, as shown on the right.

Now the rotors run in relatively clean water flow, contra-rotating so that their torques cancel out. The main spar is extended to provide a  'bottom-stop' to prevent blade strike on the seabed. (However, the  anchorage is still too small and the swing-arm ball-joint also rather  impractical! The man walking topsides is at serious risk!)

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The evolution, left, of the twin turbine design carries two 20m rotors is rated at 1 - 2 MW depending on current speed, and operates in 30 - 50m  water depths. Each rotor runs in clean water upstream of its support arm. As an alternative to a gravity base, the seabed anchorage is a  structural frame piled to the seabed, and the swinging arm ball-joint is attached to the base by a properly engineered three-axis swivel assembly. The swinging arm is hinged at its upper end to the main spar buoy so that it can be stowed for installation / removal. During operation it is held in place by a cranked strut.

Subsequent developments of Triton have all been based on a twin-hull catamaran concept, as illustrated on other pages.

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