Ocean energy technologies are at the early stages of development compared with other, more well-established renewable and conventional generation technologies. The oceans contain a huge amount of energy that can theoretically be exploited for generating useful energy. Ocean energy technology could contribute to meeting cost-effective, sustainable and secure energy demands in the medium to long term. 

The different types of ocean technologies are the following:

• Ocean wave energy is the energy occurring from movements of water near the surface of the sea in an oscillatory or circular process that can be converted into electricity. Waves are a function of the energy transfer effected by the passage of wind over the surface of the sea. The distance over which this process occurs is called the ‘fetch’. Longer fetches produce larger, more powerful waves, as do stronger winds and extended periods of wind.

• Tidal current energy is energy contained in naturally occurring tidal currents which can be directly extracted and converted into electricity. Strong tidal currents are most frequently found near headlands and islands. These retard the progress of the tidal bulge as it moves around the earth, leading to head differences that can only be equalised by a flow of water around and between the land features. It is this flow that constitutes the tidal current. Energy can be extracted using devices that move in response to the forces the current exerts, and use this movement to drive an electrical generator. (Tidal current is also referred to as tidal stream.)

• Ocean thermal energy conversion (OTEC) is based on drawing energy from the thermal gradient between surface water temperature and cold deep-water temperature, by use of a power-producing thermodynamic cycle. A temperature difference of 20^o C (from surface to approximately 1 km depth) is commonly found in ocean areas within 20^o  of the Equator. These conditions exist in tropical areas, roughly between the Tropic of Capricorn and the Tropic of Cancer.

• Salinity gradient energy can take two forms. The first, commonly known as the solar pond approach, involves the application of salinity gradients in a body of water for the purpose of collecting and storing solar energy. Large quantities of salt are dissolved in the hot bottom layer of the body of water, making it too dense to rise to the surface and cool, causing a distinct thermal stratification of water that could be employed by a cyclic thermodynamic process similar to OTEC. The second application of salinity gradients (and the one most commonly referred to when describing electricity generation from salinity gradients) takes advantage of the osmotic pressure differences between salt and fresh water. The exploitation of the entropy of mixing freshwater with saltwater is often facilitated by use of a semi-permeable membrane, resulting in the production of a direct electrical current.

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