Tides are created by the gravitational attraction of the moon
and the sun acting on the oceans of the rotating earth, the relative
motions causing the surface of the oceans to be raised and
lowered periodically [39]. Tidal power can be extracted by
impounding a tide with a barrage to recover potential energy,
or alternatively by extracting kinetic energy directly from
the tidal stream. A vertical-axis turbine (Darrieus turbine)
has a potential advantage over a horizontal turbine in that it is
able to extract power form any direction without adjustment.