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    Tidal Energy 

    The use of devices which harness tidal energy to make electricity is becoming increasingly popular in the UK. The RYA explains how these devices work and how they can affect recreational boating.

    Marine renewable energy covers a range of technologies including wave, tidal and ocean thermal energy conversion. Unlike the three-bladed turbines used to capture wind energy, there is a large range of technologies used to harness tidal energy. The process by which the most popular technologies work can be seen below, but in practice, they could look very different both in size and shape.

    How do tidal energy devices work?

    The Government is legally committed to meeting 15% of the UK's energy demand from renewable sources by 2020. As a result, exploiting renewable energy resources by harnessing tidal energy is becoming increasingly important in the UK. Tidal energy may have a crucial role to play in the UK energy mix, with many of these innovative technologies already being trialled in UK waters.

         
         
    Tidal barrages involve building a dam across (1) or near (2) an estuary with a high tidal range. This generates energy by allowing water to flow in and/or out of the estuary or bunded tidal barrage (2)through low head hydro turbines. Single (3) and multiple (4)basin offshore tidal lagoons are built on a tidal flat in areas with high tidal ranges.   Tidal impoundments such as the four methods described opposite generate energy by allowing water into the storage reservoir when the tide is rising which creates a head of water. The water is held within the reservoir until the tide has fallen on the outside of the enclosure and then the water is released back to the sea through a turbine(s) to generate power.
         
     
    Horizontal axis turbines work in a similar manner to wind turbines.  The turbine is placed in the water and the tidal stream causes the rotors to rotate around the horizontal axis and generate power.   Vertical axis turbines work in a similar manner to horizontal axis turbines but the tidal stream causes the rotors to rotate around the vertical axis and generate power.
         
     
    Reciprocating Hydrofoils have a hydrofoil attached to an oscillating arm.  The lift caused by the tidal stream causes the arm to oscillate and generate power.   Venturi Effect Devices are devices which funnel the water through a duct, increasing the water velocity. The resultant flow can drive a turbine directly or the induced pressure differential in the system can drive an air turbine.
         
     
    A tidal kite is tethered to the sea bed and carries a turbine below the wing. The kite ‘flies’ in the tidal stream, swooping in a figure-of-eight shape to increase the speed of the water flowing through the turbine.   The Archimedes Screw is a helical corkscrew-shaped device (a helical surface surrounding a central cylindrical shaft). The device draws power from the tidal stream as the water moves up/through the spiral turning the turbines.

     

    Images and descriptions courtesy of Aquatic Renewable Energy Technologies (Aqua-RET) www.aquaret.com 

    Tidal Energy Sites in the UK

    There are a number of tidal energy sites already leased within UK waters. The Crown Estate website lists information about each project, including test sites, and where each site is in the planning process. You can also click on the image below for a larger view of The Crown Estate map.

     Wave and Tidal Energy Map

    This map is courtesy of The Crown Estate.

    The RYA Position

    Whilst the RYA acknowledges the Government's desire to promote renewable energy, it is keen to ensure that the navigational safety of recreational craft is safeguarded around the coast.

    The RYA has developed a detailed position statement on offshore renewable tidal energy developments, based on recreational craft data, which it provides to every developer during the consultation process. The RYA believes that the impact that tidal energy installations have on recreational boating can be minimised provided developers fully consider the following key points:

    • Collision risk. The RYA believes that the collision risk to recreational craft can be minimised by specifying a minimum safe water clearance over submerged structures, moving components and associated infrastructure determined in accordance with the MCA Under Keel Clearance policy paper.
    • Charting, marking and lighting. The RYA supports the guidance provided by the MCA, UKHO and GLAs and works with them to identify site specific issues that may occur.
    • Navigational and communication equipment. Any proposed development should account for any effect on small craft navigation and communication equipment in detail.
    • Location. Recreational routes, general sailing areas, racing areas and access to boating facilities and anchorages must be considered when examining the impacts of tidal energy devices and their associated infrastructure. Poorly sited tidal energy devices and arrays and those that are built within the 12nm limit may increase the risk to navigational safety and discourage visiting boaters to the area.  This would have an adverse effect not only on visitors but also on the local economy.
    • Sailing and racing areas. Any interference or turbulence or adverse impact on tidal streams created by tidal energy devices and arrays in a sailing or racing area would create a significant negative impact on the site and diminish its value for recreation.
    • Cumulative and in-combination effects. The RYA expects development site plans to include all adjacent developments that may have cumulative and in-combination effects on shipping and navigation. 

    The RYA has also carried out an extensive mapping project to identify the main cruising routes, sailing and racing areas to better inform the management process. Initially this culminated in a detailed description of cruising routes, racing and sailing areas for the three strategic wind farm development areas in 2004 and is reported in the document 'Sharing the Wind'.

    This project then evolved to cover the whole of the UK following considerable demand from the offshore renewable energy industry. In 2005, with funding from Trinity House, the RYA produced the first UK Coastal Atlas of Recreational Boating. This dataset has since been updated and is now regularly incorporated into developers’ consultations and navigational risk assessments.

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