Ocean Current Energy
|Sea Surface Temperatures show the Gulf Stream Current. This current carries warm water, shown in red, from the sunny tropics to higher latitudes of the British Isles.
The relatively constant flow of ocean currents carries large amounts of water across the earth’s oceans. Technologies are being developed so that energy that can be extracted from ocean currents and converted to usable power.
Ocean waters are constantly on the move. Ocean currents flow in complex patterns affected by wind, water salinity, temperature, topography of the ocean floor, and the earth's rotation. Most ocean currents are driven by wind and solar heating of surface waters near the equator, while some currents result from density and salinity variations of the water column. Ocean currents are relatively constant and flow in one direction, in contrast to tidal currents along the shore.
While ocean currents move slowly relative to typical wind speeds, they carry a great deal of energy because of the density of water. Water is more than 800 times denser than air. So for the same surface area, water moving 12 miles per hour exerts the same amount of force as a constant 110 mph wind. Because of this physical property, ocean currents contain an enormous amount of energy that can be captured and converted to a usable form. It has been estimated that taking just 1/1000th the available energy from the Gulf Stream would supply Florida with 35% of its electrical needs.
|Major Ocean Surface Currents (Source: NOAA)
Ocean Current Energy Technologies
|Artist rendering of ocean current turbines.
The United States and other countries are pursuing ocean current energy; however, marine current energy is at an early stage of development. Relative to wind, wave, and tidal resources, the energy resource potential for ocean current power is the least understood, and its technology is the least mature. There are no commercial grid-connected turbines currently operating, and only a small number of prototypes and demonstration units have been tested. More advanced technologies have been developed for use with tidal currents in near-shore environments.
There are a number of different current technology concepts under development. Prototype horizontal axis turbines, similar to wind turbines, have been built and tested, and over the next 5 to 7 years would be the most likely commercial development scenario.
Although ocean current technology is still in its early stages of development, several tidal and in-stream current turbine applications are near commercialization. These devices take advantage of the daily tidal cycles in near-shore ocean environments, or steady water flow from freshwater rivers.
Open Hydro system used for harnessing tidal energy.
|Seagen,the world's first commercial tidal energy turbine.
For ocean current energy to be utilized successfully at a commercial scale, a number of engineering and technical challenges need to be addressed, including:
- avoidance of cavitations (bubble formation);
- prevention of marine growth buildup;
- reliability (since maintenance costs are potentially high); and
- corrosion resistance.
Because the logistics of maintenance are likely to be complex and the costs potentially high, system reliability is of particular importance. At present no open-ocean current turbines are deployed in U.S. waters—this technology is truly in its infancy. However, there is interest in testing prototype turbines in U.S. waters in the near future, particularly off the coast of Florida.
2007, the Bureau published the Final Programmatic Environmental Impact
Statement for Alternative Energy Development and Production and Alternate Use
of Facilities on the Outer Continental Shelf. This document examines the potential environmental
impacts related to renewable energy development on the OCS for each phase of
development (technology testing, site characterization, construction,
operation, and decommissioning). Actual proposals will be evaluated in
project-specific analyses under the National Environmental Policy Act. Additional information regarding potential
environmental impacts from current energy development can be found in the U.S.
Department of Energy Report “ Report to Congress on the Potential
Environmental Effects of Marine and Hydrokinetic Energy Technologies” and at the Ocean Energy Systems Annex IV webpage.
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