Tidal Stream Turbines

By far 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 key to success is how to support 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 simplest of all configurations is a rotor on a pole fixed to the seabed:

Later, this design acquired top-sides...

Just as on a wind turbine, the single rotor turns upwind of a tube tower cantilevered from the base. The stall-regulated rotor is designed for a tip speed of 10 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.

In this design, the sleeve carrying the rotor provides 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.

The next image shows our 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. This idea was awarded a UK patent in 2003. The design is known as the SST - the Semi-Submersible Turbine.

Cable tethers are another option for supporting a buoyant submersible turbine that have been evaluated - however these give the generating system too much freedom and the result can be instability during operation. River tests on tidal models and experimental operation of large downwind free-yaw free-nod wind turbines such as the WEG MS4 indicate that the swing-arm provides an appropriate restriction of freedom so that the turbine runs stably and follows the flow direction accurately.

One drawback with the single rotor is that its torque has to be resisted by the righting moment of the buoy - not impossible, but adding complication. Also the rotor has to run in the wake of the main structural components. A better idea is contra-rotating twin rotors:

The rotors now 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. (This image is of an earlier design stage).

Today, this 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. The seabed anchorage is now shown with a gravity base, and the swinging arm ball-joint is attached to the base by a three-axis swivel assembly. The swinging arm is hinged at its upper end to the main spar buoy so that it can be stowed easily for installation / removal. During operation it is held in place by a cranked strut. The installation and maintenance systems are described on other pages - see menu at the left.

In today's design of Pentland Firth turbine, the 60m deep water flow is covered by four 20m rotors rather than a pair, in order to keep blade loads within practical limits. The whole turbine power output is 4MW. Once rolled over into its maintenance position, the main swing-arm can be stowed for float-out removal / installation.

A patent was awarded for the operating principle of the SST in 2003, and further applications are in train.

The tidal turbine is shown for comparison against an offshore wind turbine of the same power rating. The wind turbine is shown in its more common 25m water depth for offshore applications, the SST in 60m of water. The wind turbine with a 100m diameter rotor captures the same energy at a 10 m/s hub height wind speed. The gravity base needed for the wind turbine to withstand overturning moments is 25% larger than the tidal turbine base, which has to cope with higher direct loads but no overturning moment. The steelwork requirements of each turbine is roughly equivalent.