Operation

The turbines in operation float submerged in the middle of the tidal stream, free to swing and heave in the turbulent water flow. Loads are several times those on a wind turbine, and the sheer size of the turbine, combined with the 60m water depth, make any kind of fixed structure almost unthinkable. But provided a seabed anchorage can be made, and the problems of cable flexing and hook-up solved, then a semi-submersible concept such as this becomes entirely plausible.

Shown below is a view of a 7 x 4 array (ie 4 diameters - 160m across the stream and 7 downstream) of 4 MW SSTs in the Pentland Firth, ie 22 turbines, 88 MW per square kilometre. So tidal turbines of capacity equivalent to a 1200 MW nuclear power station would take a sea area of 14 square km. In contrast, 88 MW of wind farms would occupy roughly 4 times as much upland or sea area and generate two thirds of the energy - see Cost Effectiveness.

In operation, these turbines will be silent and unobtrusive compared to wind turbines. They will produce up to 50% more energy per installed kW and do it predictably. Because the rotors turn so slowly (10 m/s or so compared to 60 m/s for a wind turbine) they are likely to present no more danger to sea fauna than the keel of a yacht (and much less than the danger of propellor impact from ships). If anything, the bases will provide shelter for seabed creatures that would otherwise be swept away by the tidal flow (in fact marine growth on the turbines themselves is likely to be a major reason for maintenance work).

Shipping will be adversely affected, and sea traffic - apart from maintenance workboats - will have to be kept completely out of tidal turbine areas (tidefarms?). In the Pentland Firth shipping goes both along the channel and across it, and careful planning of exclusion zones will be necessary.

The turbines would, of course, swing round to follow the flood and ebb tides, see below. There are various possibilities for ensuring that this is done reversibly, to avoid excess cable wind-up.