Waves
A lovely wave! We know from experience how powerful waves can be. Photo courtesy of Scott Shiffman
Waves are caused by strong winds blowing across the surface of the sea, thus wave power is an expression of wind energy which is itself an expression of solar energy, as the sun causes winds which drive the waves.
We can see how much power is contained in a wave when it crashes against the shore, but this power is actually dissipated already by the sloping land leading up to the beach.
Waves are affected by the depth of water beneath them, so waves that are out to sea are more powerful than those breaking on a beach.
How Does Wave Power Work?
We can extract energy from the motion of waves at the surface of the water. Surface ocean waves are both transverse and longitudinal in nature. This means the water moves both up and down, and forward and back.
The transverse element provides the vertical motion, and the longitudinal element provides the horizontal movement. In deep water, a particle on the surface of the water will trace out a circle, while a particle in shallow water will trace out an ellipse (because the shallow water reduces the transverse effect).
Here's an animation from Greenpeace, showing how this technology works:
We can use this motion to generate energy. Wave energy would be best applied far enough from land so as to get the most energy from the waves, but close enough to be practical. Essentially, a long "snake like" generator would "catch" the waves as they approach land.