Energy can be generated from tidal or marine currents driven by the tides or by oceanic circulations, using completely submerged turbines comprised of rotor blades and a generator. Water turbines work on the same principle as wind turbines, by using the kinetic energy of moving fluid and transferring it into useful rotational and electrical energy. A marine current turbine looks like a smaller version of a modern windmill, and the principle is exactly the same. But whereas a windmill draws energy from the movement of air, the marine turbine uses currents in the water. Marine turbines can be smaller and slower moving than wind turbines and deliver the same power, because water is denser than air. The power and flow of marine turbines devices can be increased by using concentrators or shrouds around the blades to concentrate flow towards the turbine.
Marine current energy is at an early stage of development, with only a small number of prototypes and demonstration units having been tested to date. A number of configurations have been tested on a small scale, that are essentially wind turbines adapted to the marine environment.
Three factors govern the energy capture by any water current kinetic energy converter such as the SeaGen system:
- The swept area of the rotor(s) which governs the cross-section of natural resource that can be accessed.
- The speed of the flow (kinetic energy is proportional to the velocity cubed so the amount of energy captured is very strongly influenced by current flow patterns and strengths.
- The overall efficiency of the system
A 2008 study by the Department of Energy identified all the turbine and marine current projects in the development stages. As of November 2008 the study estimated that a total of one device was at the concept design stage, thirteen devices were at the detailed design stage, eight devices were at the tank testing stage, eighteen devices were at the sea trials stage and thirteen devices were at the full-scale prototype stage. Since then more projects have entered the final stages of deployment.