Taken from Ocean Energy Systems website: (http://www.ocean-energy-systems.org/ocean_energy/)
The energy in the ocean waves is a form of concentrated solar energy that is transferred through complex wind-wave interactions. The effects of earth’s temperature variation due to solar heating, combined with a multitude of atmospheric phenomena, generate wind currents in global scale. Ocean wave generation, propagation and direction are directly related to these wind currents. On the other hand, ocean tides are cyclic variations in seawater elevation and flow velocity as a direct result of the earth’s motion with respect to the moon and the sun and the interaction of their gravitational forces. A number of phenomena relating to earth rotational tilt, rate of spinning, and interaction among gravitational and rotational forces cause the tide conditions to vary significantly over time. Tide conditions are more apparent in coastal areas where constrained channels augment the water flow and increase the energy density.
Ocean energy resources are contained in: (a) ocean waves & swells, (b) tidal range (rise & fall), (c) tidal currents, (d) ocean current, (e) ocean thermal energy and (f) salinity gradients
Tidal & Currents
Potential energy associated with tides can be harnessed by building barrage or other forms of construction across an estuary, while kinetic energy associated with tidal (marine) currents can be harnessed using modular systems.
Kinetic and potential energy associated with ocean waves can be harnessed using modular technologies.
Thermal energy due to the temperature gradient between the sea surface and deepwater can be harnessed using different Ocean Thermal Energy Conversion (OTEC) processes.
At the mouth of rivers where fresh water mixes with salt water, energy associated with the salinity gradient can be harnessed using pressure-retarded reverse osmosis process and associated conversion technologies.
Other renewable ocean resource concepts, such as hydrothermal vents, along with hybridization of the aforementioned schemes, are also being pursued. With the advent of various novel concepts and reported success of several deployments, the ocean renewable energy sector, especially the field of tidal current and wave energy conversion technology have gained significant attention throughout the world. Many technologies are also being explored for energy uses other than electricity generation, such as, producing drinking water through desalination, supplying compressed air for aquaculture, and hydrogen production by electrolysis.