First Generation TurbinesFirst Generation tidal stream turbines operate on a principle similar to that of wind turbines. The power available is proportional to the density of the water, times the swept area of the rotor, times the cube of the water velocity. While typical sea current velocities are lower than typical wind velocities the power density will be much greater for a marine turbine because the density of water is roughly a thousand times greater than that of air. Tidal stream turbines can therefore be much smaller than wind turbines and still generate equivalent amounts of power at lower rotational speeds.
First generation devices are either pile-mounted or employ gravity bases to fix them to the seafloor. Pile installation, particularly in deeper waters becomes logistically challenging and expensive. Gravity bases use the massive weight of the entire assembly to create negative buoyancy. Typically they will require further fixing to the seabed to ensure they remain in place.
A summary of three tidal devices currently deployed at EMEC is shown in the table below.
Counter-Rotating Marine Turbine (CoRMaT)According to Clarke et al, 2008 CoRMaT is a second-generation tidal stream device. It has been designed from first principles using a novel contra-rotating rotor concept with an onboard permanent magnet direct drive generator to produce electrical power. The device is neutrally buoyant allowing it to " fly " in the tidal stream with a single-point compliant mooring providing a tether to the seabed. The system can operate in any part of the water column in both deep and shallow waters.
The contra-rotation of the turbine eliminates any reactive torque thus providing greater stability. Drag forces imposed on the turbine, in the absence of torque, allows the device to align to the tidal flow.
Such an arrangement imparts a number of distinct advantages:
CoRMaT was originally developed in the Energy System Research Unit at the University of Strathclyde, and has resulted in the spin-out company Nautricity, who are working on the further development and commercialization of the turbine. Having been extensively tested to prove the concept, CoRMaT is now ready for full-scale deployment. A device has been deployed at the Thames tidal array in London as part of a scoping project.