Tidal Power

Talk by Angela Robotham
Wednesday 13th August 2008

Review of the Presentation given by Angela Robotham of Marine Current Turbines Ltd. on SeaGen Project

On the 13th August the Angela Robotham the Engineering Director of Marine Current Turbines Ltd (MCTL), gave us a presentation of the companies SeaGen system. A synopsis is given below.

MCTL is a small, flexible locally based company aimed at developing and marketing equipment capable of exploiting the clean renewable energy potential of high flow . rate tidal and ocean currents. MCTL is orientated to technology development and the exploitation of the IPR it generates from its design activities. As such it has no manufacturing facilities of its own but sources components, sub-assemblies and sub¬systems from well selected sub-contractors. It retains responsibility for Quality assurance of the design implementation. Its current project is SeaGen

Seagen SeaGen is a Commercial Demonstration facility designed to generate and feed electricity amounting to 3,800 MW/hours to the grid each year by means of two 600 kW generators, driven through custom designed epicyclic gear boxes by two variable pitch two blade marine turbines of diameter 16 meters. The SeaGen system is currently in the late stages of commissioning in Strangford Lough. It has the following design features:
  • Single vertical 3.4 meter diameter monopile fixed into the sea bed. The monopile carries a horizontal 29 meter crossbeam structure with a generator pod at each end. Each exchangeable generator pod contains a turbine, gearbox and generator.
  • In operation the cross beam and its generator pods and turbine blades are below water level. The horizontal crossbeam can be raised up the monopile by hydraulic rams to bring the generator pods and turbine rotors clear of the water for easy and safe maintenance.
  • Composite twin, variable pitch turbine blades. The variable pitch allows the tidal energy to be exploited to the maximum whilst maintaining a safe turbine tip speed. It also enables both ebb and flow of the tide to be exploited by reversing the blade pitch rather than having to have an additional yaw control system and a more complex mechanical design.
  • High Efficiency submergible marine gear box to match the turbine max safe speed of 14 rpm to the 1000 rpm generator speed requirement. The safe speed of the turbine rotor is determined by cavitation considerations at the rotor tips.
  • The monopile also houses the following sub-systems:
    • Hydraulic System for raising and lowering the crossbeam with its generator pods
    • Generator output conditioning to bring the 690 volt approx output to 11 kV for grid connection and stabilise frequency variations resulting from turbulent tidal flow conditions.
    • Turbine blade pitch control systems
A system like SeaGen requires a flow rate in excess of 2 meters/sec. There are a number of possible sites around UK with an overall power potential of 18.3 GW. Such sites include Anglesey, St David's, Isle Man, Bristol Channel. The Strangford Lough location was chosen as an ideal location for a commercial demonstrator due to the following factors
  • Sheltered Location allowing easy access for performance monitoring and fine tuning
  • The high environmental diversity of the site enables a broadly based Environmental Impact Study to be carried out to confirm that the expected low environmental impact has been achieved
  • Proximity of a grid connection. ( 300 m - although a below seabed bore hole was required to protect it from scouring in the high flow conditions)
  • High Energy location ( flow can be greater than 3.5 m/s at spring tide) From the questions arising the following points emerged:
  • The fixed aspect SeaGen system is not suitable for sites where the flow direction varies considerably with time. Its efficiency is not generally affected by variations of flow direction up to 15 degrees about the axis.
  • Whilst wind turbines often use three blade rotors, MCTL believe that the relatively low efficiency gain achieved by adding an additional blade to their system is not cost effective given the increased complexity of the blade control systems and the reduction in the reliability resulting.
  • The Bristol Channel provides only limited opportunities for use of a tidal system such as SeaGen. The Severn Estuary area is not suitable due to the lack of water depth - a barrage/lagoon system is more suited. Deep water between Foreland Point near Lynton and Nash Point on the Welsh coast would be suitable for tidal systems.
  • To enable commercial operation without subsidy it would be necessary to export electricity at less than 5 p/kW. This is a long term aim - at present a double subsidy is available for tidal generated power.
  • There are 6 credible competitors to MCTL. Their offerings are less developed at the current time.
Our thanks go to Angela Robotham for an extremely interesting and clearly presented introduction to this project. Further information is available from the web sites below:
http://www.marineturbines.com
http://www.seageneration.co.uk
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