Ocean current energy

The use of ocean currents as an energy source carries great potential, but development has proceeded slowly because the cost is not competitive with that of other energy sources.

Background

Just as winds flow through the Earth’s atmosphere, currents flow throughout the world’s oceans. These currents are a potential power source as great as wind, although winds harnessed for power have greater speed than the currents. The energy available in a fluid flow varies both with velocity (by the square) and with density:

Kinetic Energy = (Density) × (Velocity)²

Because water is nearly eight hundred times denser than air (1,000 and 1.27 kilograms per cubic meter, respectively), a current of 1.6 kilometers per hour has as much energy as a wind of 45 kilometers per hour, which is considered an excellent average speed for wind energy. Furthermore, currents are more dependable than winds and flow in a constant direction.

Ocean Temperature and Salinity

Ocean currents are caused by differences in temperature and salinity. For example, as water near the poles is cooled, its increases, and much of this cooler water sinks toward the ocean floor. From there it flows toward the equator, displacing warmer water as it goes. Meanwhile, water near the equator is warmed, becoming less dense. It tends to flow along the surface toward the higher latitudes to replace the sinking denser water.

The Gulf Stream is such a current. It starts from an area of warm water in the equatorial Atlantic and in the Gulf of Mexico. This warm water flows generally northward parallel to the coast of North America and bends gradually to the right due to the rotation of the Earth. This tendency to curve (right in the Northern Hemisphere, left in the Southern Hemisphere) is called the Coriolis effect, and it bends the flow northeast as the West Wind Drift, bringing warm, moist air to Western Europe. It continues south as the Canaries Current (carrying cooler water) past western North Africa. Finally, the bending turns back west toward North America as the North Equatorial Current.

Similar circular patterns, or gyres, occur in all the world’s oceans, with many locations having great potential for electrical power generation. For instance, the Gulf Stream has more energy than all the world’s rivers combined. The area off Florida might yield 10,000 megawatts (10 billion watts) without observable change in the heat flow to Europe.

Salinity differences also cause major flows. The most easily tapped salinity currents are those between a sea with high evaporation and the open ocean. High-salinity water flows along the bottom from the Mediterranean Sea, for instance, while less saline Atlantic water flows in to replace it. (German submarines used these currents during World War II for drifting silently past the major British base at Gibraltar.) Two lesser potential sources of current power are tidal currents and the currents at the mouths of rivers.

Methods for Harnessing Ocean Currents

Electrical power generation from currents requires three things: mooring power stations to the ocean floor, generating power, and transmitting power to customers on shore.

Mooring and transmitting power are related economic constraints on ocean current power. Although an underwater cable from a mid-Atlantic power station could technically supply power, deeper mooring lines and longer cables eventually cost more than the power delivered. Thus, ocean current stations, if built, will tend to be near shore on the continental shelf and slope before investors attempt to moor a plant to the depths of the ocean floor.

Using currents in deeper and more distant waters will require some means of energy storage. This issue has been considered in design studies for ocean thermal energy conversion (OTEC) power stations, which would harness the temperature difference between warm tropical waters and the colder deep waters. Electricity could be used for some energy-intensive process (such as refining aluminum) or for electrolyzing hydrogen from water. Hydrogen could be used to synthesize chemical products, such as ammonia or methanol. Once the potential of current power is proven, investors may consider the second set of risks inherent in such mid-ocean ventures.

Among various proposals, two methods have been studied in detail: turbines and sets of parachutes on cables. Turbines were first proposed by William Mouton, who was part of a study team led by Peter Lissaman of Aerovironment, Inc. Their design is called Coriolis. In the study design, one 83-megawatt Coriolis station has two huge counter-rotating fan blades (so it does not pull to one side), roughly 100 meters in diameter. The blades move slowly enough for fish to swim through them.

With blades so large, neither rigid blades nor the central hub could be made strong enough without being too heavy and expensive. However, a catenary (free-hanging, like the cables of the Golden Gate Bridge), flexible blade can be held in the proper shape by the current while the generators are in a rim around the blades. The rim also acts as a funnel to increase current speed past the blades and as an air for raising the station when necessary.

Another concept is parachutes on cables, which was proposed by Gary Steelman. His water low-velocity energy converter (WLVEC) design is an endless loop cable between two pulleys, much like a ski-lift cable. Parachutes along the cable are opened by the current when going downstream and closed when coming back upstream. The WLVEC is cheaper than Coriolis, but there is a question of how well any fabric could withstand sustained underwater use.

Ocean currents are sufficiently powerful and predictable to supply electricity effectively. However, costs of competing fossil fuels must rise significantly before investors will overcome their timidity about constructing offshore power plants. However, test projects in the United States, China, Japan, and the European Union, in particular Britain, Ireland, and Portugal, continue with high expectations.

Bibliography

Charlier, Roger Henri, and John R. Justus. “Ocean Current Energy Conversion.” In Ocean Energies: Environmental, Economic, and Technological Aspects of Alternative Power Sources. New York: Elsevier, 1993.

Congressional Research Service. Energy from the Ocean. Honolulu: University Press of the Pacific, 2002.

Goldin, Augusta. Oceans of Energy: Reservoir of Power for the Future. New York: Harcourt Brace Jovanovich, 1980.

Lissaman, P. B. S. “The Coriolis Program.” Oceanus 22, no. 4 (Winter, 1979/1980): 23.

‗‗‗‗‗‗‗. “Tapping the Oceans’ Vast Energy with Undersea Turbines.” Popular Science 221, no. 3 (September 1980).

Noyes, Robert, ed. Offshore and Underground Power Plants. Park Ridge, N.J.: Noyes Data, 1977.

"Ocean Energy." International Renewable Energy Agency, 11 July 2024, www.irena.org/Energy-Transition/Technology/Ocean-energy. Accessed 27 Dec. 2024.