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Ocean and marine energy denote the diverse forms of renewable energy acquired from the ocean. Two kinds of ocean energy exist: mechanical and thermal. The movement of the earth and the moon's gravitational force leads to the mechanical forces experienced. The movements of the earth create wind in the oceans that ultimately produces waves and the gravitational pull of the moon causes the initiation of coastal tides and currents. Thermal energy is acquired from the sun, which increases the temperature of the ocean yet the depths are still of a lower temperature. Therefore, the temperature difference experienced allows for the energy to become trapped and converted to useful energy, typically, electrical energy. Five kinds of ocean energy transformation take place: wave energy, tidal energy, marine current energy, and ocean thermal energy conversion [33–37].

 Wave energy is produced by the motion of a device either floating on the surface of the ocean or fixed to the ocean floor and there are several methods for transforming wave energy to electric energy. Wave energy is recognized as the most commercially developed of the ocean energy technologies yet is still far from where it could be practically.

 Tidal energy, the tidal cycle takes place every 12 hours as a result of the gravitational force of the moon. The difference in water height from low and high tide is potential energy. Comparable to traditional hydropower produced from dams, tidal water can be trapped in a barrage across an estuary at periods of high tide and forced by a hydro‐turbine at periods of low tide.

 Current energy, marine current is ocean water moving in one direction. Kinetic energy of the marine current can be trapped with submerged turbines that are relatively comparable to wind turbines, where the marine current forces the rotor blades to move to produce electrical energy.

 Ocean thermal energy conversion (OTEC), utilizes ocean temperature variations from the surface to depths lower than 1000 m, to obtain energy. Research focuses on two types of OTEC technologies to obtain thermal energy and transform it to electrical energy: closed and open cycles.

 Salinity gradient power is the energy generated from the variation in salt concentration between two fluids, usually fresh and saltwater, e.g. when a river flows into the sea. Collision of fresh and saltwater delivers large amounts of energy, which this technology strives to capture.

Tidal power stations generate tens to hundreds of MW similar to hydropower stations, wave energy converters (WECs) from some kW to MW, salinity gradient power stations from some kW to MW, ocean thermal energy converters (OTECs) from kW to MW and ocean thermo‐electric generators (OTEGs) from some watts to kW.

Ocean energy is still in the process of development, and intensive research is required, progress and demonstration efforts needed for learning and cost reduction before it can contribute to the energy supply. Therefore, the ocean energy market is still in its infancy, and the sector must address many issues to confirm the reliability and affordability of its technologies. A number of barriers are present in ocean energy technologies, which include obtaining site permits, the environmental influence of technology implementation, and grid connectivity for transporting the energy generated.

In 2019, the global installed capacity of ocean energies was 536 MW, a rise of 32 MW relative to 2014. Ocean energies amount to 0.03% of the global renewable energies installed capacity. However, there is huge potential to increase the use of ocean energies. Theoretical ocean energy resources might be enough to meet present and future global electricity demand, with a range of 20 000–80 000 TWh/year, culminating up to 100–400% of the present global demand. Moreover, a global potential of 337 GW could be achieved by 2050, one third of this would be in Europe. For the 2020 milestone, approximations suggest an installed capacity ranging from some hundred MW to 2 GW. Figure 2.12 illustrates the global ocean energy forecast based on device technology and infrastructure available, [38, 39].