Floating tethered turbines
Advantages
*TheDisadvantages
*Floating devices are subject to ocean wave action which can induce motions that will impact on the performance of the turbines they support; semi-submerged devices such as Evopod are designed to be a stable platform in waves so that they can operate for longer and extract more energy from the wave particle velocity. Waves large enough to have an adverse effect on Evopod would affect turbines in all parts of the water column. * Ocean waves create an orbital movement of the water particles which will add or subtract from the steady ocean current or tidal stream velocity as the wave passes the turbine. Without proper blade pitch or power take-off control systems this could lead to the blade stalling and loss of power output. With proper control systems it is possible to extract this kinetic energy from the waves, much as a wind turbine does in response to wind gusts. The wave particle velocity for short wave reduces with water depth and is therefore less of an issue for deeply submerged turbines. As the wavelength gets longer than it becomes a shallow water wave where there is little change in velocity in the water column. *The vertical component of the mooring load induced by the drag of the turbine can pull a floating platform under the water unless it is adequately compensated for by change in buoyancy forces, e.g. the submergence of the struts on Evopod, or hydrodynamic lift forces (lifting foils). Tests have shown residual buoyancy in Evopods is sufficient to withstand these forces with the extra bonus of improve system stability. *Floating devices have to be robust enough to withstand impact from flotsam and in Northern latitudes may need to be designed to cope with ice floes. This is however true for all turbines as flotsam may be fully submerged and therefore impact any seabed turbine.Design features
Hull design and rotating midwater buoy
The device differentiates itself from other tidal turbines in that the turbine is mounted on a floating, semi-submerged body that is tethered to the seabed. The power generation equipment is similar to that of a wind turbine and is housed in the cylindrical shaped watertight lower hull, which is deeply submerged below the water line and supported by small waterplane area surface piercing struts. One variant of this patented hull concept has three vertical struts that pierce the water surface, much like a multi-hull SWATH design. The two transversely separated aft struts provide the stability that is needed to resist the torque reacted by the single turbine/generator unit. The configuration of the struts also ensure that the device weathervanes about its midwater mooring buoy such that it always points into the direction of the current. The device is moored by a mid-water buoy, which is fixed to the seabed by four spread mooring lines which are anchored to the sea-bed by pile or gravity anchors. The buoy design is also unique in that it encompasses a geo-fixed part that is anchored to the seabed and a rotating part that is linked to Evopod by a rigid yoke. The turbine drag forces are therefore transmitted through a bearing system linking the fixed and rotating parts of the buoy. A slip ring power export swivel is located in the buoy so that twist is not imparted into the umbilical cable that takes the power from the midwater buoy to the seabed. A subsea power export cable links the umbilical’s seabed connection point to the shore. With the weather-vaning hull design and rotational midwater buoy, Evopod generates electricity with both the ebb and flood tides by always pointing into the tide’s direction of flow. This gives it a generating time of roughly 20 hours per/lunar day (approx 24hrs 50 minutes). In comparison to other marine bodies that float on the surface of the ocean, Evopod’s semi-submerged hull form is hardly affected by the passing waves. It is also designed to be readily detachable from the mid-water buoy for recovery operations. Developing safe installation, maintenance and recovery operations in the hazardous environment of fast flowing currents is one of the biggest challenges facing tidal energy device developers. The device is designed for deep water sites, such as the Pentland Firth (Up to 60meters water depth, flow speed 6 m/s). Deep water sites in UK waters have the fastest flow speeds and have the greatest potential for electricity generation.Testing and collaboration
Evopod 1/40th scale tank testing, Newcastle University, England
A 1/40th scale model of Evopod was initially tested in the test tank of1kW Evopod tidal test facility demonstration, Tees Barrage, England
The 1/10 scale device was initially used to demonstrate the tidal test facilities at the Tees Barrage in Thornaby-on-Tees near Middlesbrough, UK by Narec (National Renewable Energy Centre).1kW Evopod sea testing, Portaferry, Northern Ireland
In 2008 a 1/10 scale Evopod device was installed and tested in the tidal flow through35kW Evopod sea testing, Sanda Sound, Scotland
In 2010 Oceanflow Energy were awarded a Scottish WATERS grant to "Build and deploy the ‘Evopod’, a 35 kilowatt floating grid connected tidal energy turbine at Sanda Sound in South Kintyre".Awards
Oceanflow Energy and Evopod have won several awards, the most recent being the Shell Springboard Regional award in February 2009. It has also won awards for “innovation of the year” and “green business of the year” in the North East of England.See also
* Tidal Power * Tees Barrage * Strangford Lough * Electricity Generation * NaRECReferences
External links
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