Energy

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.

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 twice-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.

Energy Capture

The illustration 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 and the energy content of a stream varies with the cube of the flow velocity. The figure 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.

The figure below shows how seabed-mounted turbines will inevitably miss a substantial part of the available energy flow in the tidal stream, and would require a significantly larger area of seabed to product the same total energy output.