The unit when completed will incorporate two buoyant 500kW generators joined together by a cross beam giving a total power capacity of 1MW (at 2.3m/s flow rates with a 0.7m/s cut in). The fixed pitch turbine blades will be 15 metres in diameter – three 8m blades on a 2.5m hub.
The design of TidEl and its anchoring mechanism leave it free to align itself downstream of the tidal flow as the tide changes direction with no need for re-orientation electronic controls. TidEl can be installed in deep waters (below the light depth) and so barnacle and other sealife damage can be avoided reducing maintenance intervals and downtime.
The closer a tidal turbine is the surface, the more the difference in water flow rates between the top and bottom of the turbine become problematic. By floating the TidEl 30 metres or more below the surface, this problem is greatly reduced.
Each TidEl can be remotely instructed to float to the surface by extending its mooring chains for maintenance or replacement. TidEl’s can be towed rather than carried on the service vessel and so there is no need for a lift vessel to be used reducing maintenance costs. The servicing schedule is predicted to be once every two years, with maintenance taking place at dockside rather than at sea. While the tide is turning (and no power is being generated) the TidEl to be serviced can be swapped out for a refurbished unit.
A one-tenth sized prototype system including generator, mooring, and grid integration controls recently underwent seven weeks of testing at NaREC (New and Renewable Energy Centre) in Blyth, Cumbria. The 10% scale version (partly funded by a DTI grant) succesfully coped with the combination of turbulence, waves, and tides. The next step is 12 months of testing of a full-sized version 1MW in Orkney part funded again by the DTI with a grant of £2.7 million (out of a project cost of £4.5 million). Details of this DTI Grant (PDF) can be viewed here.
Using prices from current suppliers it has been calculated that a 100MW tidal turbine farm of 100 TidEl’s would generated electricity at a cost of just 5.75p / kWh including the costs of constructing and maintaining 20 miles of 132kV grid connection to connect the units and to transmit generated electricity to the National Grid on the coast.
25 local employees would be required to run the tidal farm and a total of seven spare units would be required for swapping in and out during maintenance and for contingencies. The total capital cost of the 100MW farm would be around £140 million, with £5 million required per year for management and maintenance, and a further £14 million at most required for eventual de-commissioning in 20+ years time.
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