Oceans represent a huge, predictable resource for renewable energy. The main forms of ocean energy are waves, tides, marine currents, salinity gradient and temperature gradient. Wave and tidal energy are currently the most mature technologies.
Tidal current energy is created by local regular diurnal (24-hour) or semi-diurnal (12+ hour) flows of ocean water caused by the tidal cycle. Kinetic energy can be harnessed, usually nearshore and particularly where there are constrictions, such as straits, islands and passes.
Wave energy is created as kinetic energy from the wind is transmitted to the upper surface of the ocean. At present there are several different wave energy technology designs and some are at the cutting edge of engineering design.
Ocean thermal energy conversion (OTEC) uses the temperature difference between surface and deep water in a heat cycle to produce electricity. Although tropical areas are most favourable for the exploitation of this source of energy, the potential resources are enormous.
Osmotic power generation exploits the energy available from differences in the salt concentration in seawater and is especially suited to countries with abundant fresh water resources flowing into the sea. There are two practical methods for this - reversed electro-dialysis (RED) and pressure retarded osmosis (PRO).