The figure on the left represents a cross section through a tidal channel. The water velocity is generally at its maximum at the surface but at the seabed itself it will be near to zero. The generally accepted velocity shear relationship with depth follows a one-seventh power rule. As with wind the energy content of a stream varies with the cube of the flow velocity.
The figure on the left indicates that a seabed-mounted single rotor unit may only capture 25% of the available energy, missing out on the more energetic flows near the surface. The Triton multi-turbine system captures energy from all flow layers including those near the surface.
Some turbine systems rely on floating surface support structures, and would appear attractive in that they gather only the most energetic surface layers. However, such systems are vulnerable to storms and the resulting severe sea states, and would not be suitable to the most energetic and exposed sites.
Stream depth effects
The Triton platform system is designed to strike a balance between being far enough above the seabed to capture the higher energy streams, and being sufficiently well submerged to minimise surface effects
There is actually the prospect that in a tidefarm of many fully submerged, seabed-mounted turbines, the velocity shear would be increased with further energy loss, as most of the water flow prefers to pass over the turbines rather than pass through the ‘rough’ bottom layers. In contrast, full-depth turbine systems such as Triton take their energy from all layers, thus ensuring good mixing of the flow.
The UK has by far the best tidal current resource in Europe, estimated at between 18 and 60 TWh (from 5GW according to the Carbon Trust to 16 GW according to Edinburgh University; from 5 to 16% of UK electricity demand).
Over 90% of the extractable energy contained in the top 10 UK mainland sites lies in waters 40m or more deep (The Carbon Trust). Over 50% of this deep-water resource lies in the 60m deep Pentland Firth, between the Scottish mainland and the Orkney isles. This is the channel that helps fill and drain the northern part of the North Sea and Baltic twice a day from the Atlantic Ocean. Our estimate is that 3 million tons of water a second flows through this channel at peak times.
The European and global resources are put by the Carbon Trust to be twice and 7-10 times respectively that of the UK.
The DTI in 2002 put the global resource at 3% of the 3000 GW of tidal stream energy ‘available’. ie at 90 GW half as much again as the UK’s total power capacity of 60GW
However, the main argument in favour of fully submerged turbines — namely that shipping can safely pass overhead — is negated as soon as the maintenance implications are considered. With mature technology offshore wind turbines still requiring three scheduled and three non-scheduled visits per turbine per year, it is unlikely that tidal turbines will be significantly less demanding. In the temperate waters of the Gulf Stream, marine fouling will form on the blades, diminishing power output, and will require regular cleaning. On a close array of seabed-mounted turbines, the almost constant access for such cleaning and other maintenance will be incompatible with location in a shipping lane.
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