Marine energy (also known as "ocean energy") refers to the renewable energy that can be generated from the world's oceans, seas and rivers. The marine energy resources are contained in waves, tides, the difference in salinity gradients, and the heat stored in surface waters. This energy can be converted into reliable, sustainable and cost-competitive electricity that can be used to power homes, transport, and industries.

A variety of technologies that can harvest the vast amount of energy stored in oceans and seas have been deployed or are in development. These are grouped into the four resource types: wave energy converters (WECs), tidal range and tidal stream devices, ocean thermal energy converters, and salinity gradient technologies. Read more about these technologies below.

Ocean Thermal Converters

Ocean Thermal Energy Conversion (OTEC) technologies exploit the temperature difference between cold ocean water at a depth of 800 – 1000 metres (about 5 ⁰C) and warm surface water (about 25 ⁰C) to produce electricity. The warm surface water of 25 ⁰C is used to evaporise a working fluid with a low boiling point. The resulting vapour pressure drives a turbine-generator which produces electricity. After generation of energy in the turbine-generator, the working fluid is cooled using the cold ocean water of 5 ⁰C and the energy generation circle can start again. OTEC technologies are therefore generating energy in a closed loop, 24 hours a day, all year round.

OTEC technology requires a temperature difference of 20 ⁰C to achieve significant energy yields. This means that OTEC technologies can only be efficiently deployed in equatorial and tropical seas and oceans.

Theoretical global potential OTEC

Wave energy converters

Wave energy converters (WECs) capture the kinetic energy of ocean waves to generate electricity. The amount of energy that can be extracted from waves is dependent on the height of the waves. WEC devices can be located flexibly - on the shoreline, near-shore, and offshore – to harness the available energy most efficiently. WECs are intended to be modular and deployed in arrays to obtain a significant combined power output. Due to the diverse nature of waves in different regions in the world, it's likely that several different technologies are needed to exploit the potential in all oceanic regions.

Theoretical global potential wave energy

Salinity gradient technologies

Salinity gradient – also referred to as "blue energy" – technologies generate power from the chemical pressure difference between two bodies of water. Energy is generated based on the difference in ionic concentration between fresh water and salt water. Two main technology types, Reverse Electro Dialysis (RED) and Pressure-Retarded Osmosis (PRO), can be deployed in deltas or fjords to generate a steady flow of electricity. These technologies make use of a semi-permeable membrane, which generates an osmotic potential that can be used to generate electrical energy. Salinity gradient technology can either be implemented as a standalone power plant, or as a hybrid energy generation process focusing on energy recovery, for example from a desalination or water treatment plant.

Theoretical global potential salinity gradient energy

Tidal energy converters

Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of Earth. The tides also create ocean currents that can reach high flow speeds in certain areas around the world. Both forms of energy can be used to generate renewable electricity.

Tidal range technologies generate energy from the vertical head difference between the high tide and the succeeding low tide. The tidal range is the vertical difference between the high tide and the succeeding low tide. Tidal range technology uses the same principles as conventional hydropower and requires a natural or a man-made structure (e.g. a dam or barrier) to impound a large body of water. The difference between the tide height inside and outside the impounded area causes water to be discharged from one side to the other. This water is forced through hydro turbines inside the structure to generate energy.The difference between high and low tide, as well as the size of the area of enclosed water, influence the power output of tidal range technologies. They can be deployed in locations where large water masses flow into compounded areas, such as bays or estuaries.

Tidal stream technologies convert the kinetic energy in tidal currents into electrical energy. The energy output of these tidal energy converters (TECs) is determined by the speed of the currents. The devices can either be implemented in existing civil structures such as storm surge barriers, fixed directly to the seabed, or can be buoyant and tethered to the seabed. TECs are generally modular and intended for array deployment to obtain a significant combined energy output.

Theoretical global potential tidal energy