Ocean Current Energy

Ocean waters are constantly on the move. Ocean currents flow in complex patterns affected by the wind, water salinity and temperature, topography of the ocean floor, and the earth's rotation. The ocean currents are driven by wind and solar heating of the waters near the equator, though some ocean currents result from density and salinity variations of water. These currents are relatively constant and flow in one direction only, in contrast to the tidal currents closer to shore. Some examples of ocean currents are the Gulf Stream, Florida Straits Current, and California Current.

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 about the same amount of force as a constant 110 mph wind. Ocean currents thus 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.

Ocean Current Energy Technologies

The United States and other countries are pursuing ocean current energy, including Japan, China, and some European Union countries; however, marine current energy is at an early stage of development. There are no commercial grid-connected turbines currently operating; to date, only a small number of prototypes and demonstration units have been tested. Some of these technologies have been developed for use with tidal currents in near-shore environments; these near-shore tidal current energy technologies are not analyzed in the OCS Alternative Energy Programmatic EIS.

	Figure 1: Current Turbines Visualization

Under the most likely commercial development scenario, energy would be extracted from ocean currents by using submerged water turbines similar to wind turbines (See Figure 1). These turbines would have rotor blades, a generator for converting the rotational energy into electricity, and a means of transporting the electrical current to shore for incorporation into the electrical grid. Also, there would need to be a way to keep the turbines stationary, such as posts or cables anchored to the sea floor. Additional components might include concentrators (or shrouds) around the blades to increase the flow and power output from the turbine.

	Ocean Current Project Source: Minerals Management Service

In large open areas with fast currents, it would be possible to install water turbines in groups or clusters to make up a marine current "farm", with a predicted density of up to 37 turbines per square kilometer. Space would be needed between the water turbines to eliminate wake-interaction effects and to allow access by maintenance vessels.

Alternative Designs

Various alternatives to water turbine designs have been proposed, including having a barge moored in the current stream with a large cable loop to which "parachutes" are fastened. The parachutes would be pushed by the current, then closed on their way back, forming a loop similar to a large horizontal water wheel.

Technical Challenges

For ocean current energy to be utilized successfully at a commercial scale, a number of potential 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.

Environmental Considerations

Ocean current energy technology project planning will need to consider species protection (including fish and marine mammals) from injury from turning turbine blades. Consideration of shipping routes and present recreational uses, such as fishing and diving, will be required when considering siting of the turbines. Other considerations include risks from slowing the current flow by extracting energy. Local effects, such as changes of estuary mixing resulting in temperature and salinity modifications, might affect species in estuaries.