# Marine energy

Marine energy, also known as ocean energy, ocean power, or marine and hydrokinetic energy, refers to energy harnessed from waves, tides, salinity gradients, and temperature differences in the ocean. The movement of water in the world's oceans stores vast amounts of kinetic energy, which can be converted into electricity to power homes, transportation, and industries.
Marine energy includes wave power, which is derived from surface waves, and tidal power, which is obtained from the kinetic energy of moving water. Offshore wind power, however, is not considered marine energy because it is generated from wind, even if the wind turbines are located over water.
The oceans have a tremendous amount of energy and are close to many if not most concentrated populations. Ocean energy has the potential of providing a substantial amount of new renewable energy around the world.

## Global potential

The global potential for marine energy is significant, with estimates suggesting that 20,000 to 80,000 terawatt-hours per year (TWh/y) of electricity could be generated from ocean temperature differences, salinity gradients, tides, currents, waves, and swells.
Global potential :
| Form | Annual generation |
| Tidal energy | >300 TWh |
| Marine current power | >800 TWh |
| Osmotic power Salinity gradient | 2,000 TWh |
| Ocean thermal energy Thermal gradient | 10,000 TWh |
| Wave energy | 8,000–80,000 TWh |
| Source: IEA-OES, Annual Report 2007 ||
Indonesia, as an archipelagic country that is three quarters ocean, has 49 GW recognized potential ocean energy and has 727 GW theoretical potential ocean energy.

## Forms of ocean energy

The oceans are a vast, largely untapped source of energy, including surface waves, fluid flow, salinity gradients, and thermal differences.
Marine and Hydrokinetic (MHK) or marine energy development in U.S. and international waters includes projects using the following devices:
* Wave power converters in open coastal areas with significant waves;
* Tidal turbines placed in coastal and estuarine areas;
* In-stream turbines in fast-moving rivers;
* Ocean current turbines in areas of strong marine currents;
* Ocean thermal energy converters in deep tropical waters.

### Marine current power

Strong ocean currents are driven by temperature, wind, salinity, bathymetry, and the rotation of the Earth. The Sun acts as the primary driving force, causing winds and temperature differences. Because there are only small fluctuations in current speed and stream location with no changes in direction, ocean currents may be suitable locations for deploying energy extraction devices such as turbines.
Ocean currents are instrumental in determining the climate in many regions around the world. While little is known about the effects of removing ocean current energy, the impacts of removing current energy on the farfield environment may be a significant environmental concern. The typical turbine issues with blade strike, entanglement of marine organisms, and acoustic effects still exists; however, these may be magnified due to the presence of more diverse populations of marine organisms using ocean currents for migration purposes. Locations can be further offshore and therefore require longer power cables that could affect the marine environment with electromagnetic output.

### Osmotic power

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. Another system is based on using freshwater upwelling through a turbine immersed in seawater, and one involving electrochemical reactions is also in development.
Significant research took place from 1975 to 1985 and gave various results regarding the economy of PRO and RED plants. Small-scale investigations into salinity power production take place in other countries like Japan, Israel, and the United States. In Europe the research is concentrated in Norway and the Netherlands, in both places small pilots are tested. Salinity gradient energy is the energy available from the difference in salt concentration between freshwater with saltwater. This energy source is not easy to understand, as it is not directly occurring in nature in the form of heat, waterfalls, wind, waves, or radiation.

### Ocean thermal energy

Water typically varies in temperature from the surface warmed by direct sunlight to greater depths where sunlight cannot penetrate. This differential is greatest in tropical waters, making this technology most applicable in water locations. A fluid is often vaporized to drive a turbine that may generate electricity or produce desalinized water. Systems may be either open-cycle, closed-cycle, or hybrid.

### Tidal power

The energy from moving masses of water – a popular form of hydroelectric power generation. Tidal power generation comprises three main forms, namely tidal stream power, tidal barrage power, and dynamic tidal power.

### Wave power

Solar energy from the Sun creates temperature differentials that result in wind. The interaction between wind and the surface of water creates waves, which are larger when there is a greater distance for them to build up. Wave energy potential is greatest between 30° and 60° latitude in both hemispheres on the west coast because of the global direction of wind. When evaluating wave energy as a technology type, it is important to distinguish between the four most common approaches: point absorber buoys, surface attenuators, oscillating water columns, and overtopping devices.
The wave energy sector is reaching a significant milestone in the development of the industry, with positive steps towards commercial viability being taken. The more advanced device developers are now progressing beyond single unit demonstration devices and are proceeding to array development and multi-megawatt projects. The backing of major utility companies is now manifesting itself through partnerships within the development process, unlocking further investment and, in some cases, international co-operation.
At a simplified level, wave energy technology can be located near-shore and offshore. Wave energy converters can also be designed for operation in specific water depth conditions: deep water, intermediate water or shallow water. The fundamental device design will be dependent on the location of the device and the intended resource characteristics.

## Environmental effects

Common environmental concerns associated with marine energy developments include:
* the risk of marine mammals and fish being struck by tidal turbine blades
* the effects of EMF and underwater noise emitted from operating marine energy devices
* the physical presence of marine energy projects and their potential to alter the behavior of marine mammals, fish, and seabirds with attraction or avoidance
* the potential effect on nearfield and farfield marine environment and processes such as sediment transport and water quality
The Tethys database provides access to scientific literature and general information on the potential environmental effects of marine energy.