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.
The UK has by far the best tidal current resource in Europe, estimated at between 18 and 60 TWh (5GW according to the Carbon Trust to 16 GW according to Edinburgh University; or 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.
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. In the temperate waters of the Gulf Stream, marine fouling will form on the blades, diminishing power output, and will require at least yearly jet-cleaning on the surface. Access for this operation is simple with the TidalStream system, but it is not realistic to expect that such regular machine access could take place in shipping lanes.
The figure above represents a cross section through a tidal channel. The water velocity is generally at its maximum at the surface but at the sea bed 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. It shows how the seabed mounted single rotor unit loses out in terms of energy capture in comparison with the TidalStream multi-rotor system which captures more of the high energy flow associated with the near-surface layers.
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 could not be expected to survive. The Triton platforms 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 (see above), 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 TidalStream take their energy from all layers, thus ensuring good mixing of the flow.
The Pentland Firth helps fills and drain the North Sea twice a day - water flows of up to 3 million tons a second.
One of the best sites anywhere in the world for tidal power generation from tidal turbines is the Pentland Firth, between the north of Scotland and Orkney. This is a deep (60-90m) channel that carries water from the Atlantic Ocean to help fill and drain the North Sea and the Baltic twice a day, with current flows at up to 16 knots.
The attachment of secure mounting systems for tidal devices is limited by the operating environment found in the Firth. The seabed is generally considered to be a scoured rock layer that would require either a gravity base or drilled anchor system. However, the overall depth and high tidal flows preclude the use of divers and jack-up drilling. Gravity base designs are well understood and have been in use for many years in the marine sector. These are the most likely and lowest risk means of securing turbines in this environment. An alternative is to use dynamically-positioned drillships operating in neap tides at near to their limit of station-keeping ability.
The long-term effect on the environment from the operation of underwater turbines is not well understood though recent studies suggest that the rotating blades move too slowly to be a significant danger to marine animals.
One representation of this energy resource is shown in the hotspot chart alongside.
It can be seen that the best resource lies at the east end of the Firth, where the channel is narrowest, and especially between the islands of Stroma and Swona, the two islands straddling the main flow.
Stream depth effects
Resource
The Pentland Firth looking west
TidalStream TRITON
The enabling technology for tidal turbine deployment
Up to 10MW from a single installation
Float-out installation and recovery
Safe and efficient on-board maintenance
Developing concepts for harvesting tidal energy
