(a) A windmill requires wind of speed more than 15 km/h to generate electricity from wind energy also large numbers of windmills are rewuired to get feasible output which covers a large area.
(b) Vary strong ocean waves are required in order to extract energy from waves.
(c) Very high tides are rrquired in order to extract from energy from tides. Also occurrence of tides depends on the relative positions of the Sun, moon and the Earth.
