Marine Current Turbines - http://www.marineturbines.com

The leader in terms of getting a large machine running in realistic conditions at a viable site (off Lynmouth in the Bristol Channel).

The 300kW 'Seaflow' prototype shown has a single rotor (for simplicity), variable pitch blades and fixed yaw - the blades reverse pitch when the tide changes direction. The tower is grouted into a socket drilled into the seabed.

MCT have a DTI grant for construction of a 1MW prototype 'Seagen' - with two contra-rotating rotors each 16m in diameter - again on a fixed monopole tower. Consents have been requested for a site at Strangford Lough in Northern Ireland.

MCT have not announced any plans for floating designs.

The Engineering Business - http://www.engb.com

The 150kW 'Stingray' demonstrator shown here ran in 2003 for 3 months in Yell Sound, Shetland, with grant support from the DTI. It operates like a whale's tail, with the yellow part in the picture flexing up and down in the prevailing current.

The Engineering Business say they have no plans to build larger machines at present.

SMD Hydrovision

http://www.smdhydrovision.com

This floating concept for two contra-rotating rotors has been tested at the National Renewable Energy Centre in Blyth, and has won a number of awards for innovation as well as financial support from the DTI.

Lunar Energy

http://www.lunarenergy.co.uk

This ducted fan approach works on the basis that the duct both accelerates the water flow rate into the turbine and smooths out small directional changes in flow that would otherwise disturb the rotor.

University of Swansea

www.swanturbines.co.uk

This concept appears to be similar to MCT's, with turbines rigidly mounted on gravity bases. The tower is evidently telescoped upwards to raise the turbine machinery out of the water for maintenance.

Blue Energy

www:bluenergy.com

Horizontal axis turbines are not the only way to capture flow energy, although they have dominated wind turbine technology. Energy can also be captured with vertical axis turbines, which have the advantage of not needing to yaw around as the flow direction changes.