Tidal stream systems

Tidal stream systems utilise the kinetic energy from water currents to turn turbines, in the same way that wind mills use air currents. This method is becoming increasingly favoured because it is cheaper to impliment and maintain and has less impact on the environment than tidal barriages.
These "underwater windmills" resemble conventional modern windmills and essentially functions like them, with the obvious exceptions that they are entirely submerged underwater, and their blades are turned by the force of tidal currents rather than wind. Their blades are shorter and stronger than a conventional windmills to withstand extreme underwater tidal forces, and the direction of the blades adjust automatically depending on the prevailing current, to optimise power output.
The technology works best in areas where tidal ranges are greatest, such as the Norwegian coastline, and the coast of Britain, which has some of the strongest tidal zones in the world.
This is a dynamic area of research and the technology is improving all the time, but it is estimated that a full-scale "underwater windmills" will be able to produce 1 megawatt of electricity at an expected cost of 4-7 pence per kilowatt hour.
>Click here for a video showing these systems in action (scroll to the bottom of the page)