Currents can be powerful enough to tip canoes, damage docks and even topple bridges. The force of all that moving water can also provide a clean, affordable and unobtrusive source of renewable energy

Both river and ocean currents can be converted into electricity.

Ocean currents are driven by wind and solar heating of the waters near the equator,
although some ocean currents result instead from variations in water density and salinity. These
currents are relatively constant and flow in one direction only, in contrast to the tidal currents
closer to shore where the varying gravitational pulls of the sun and moon result in diurnal high
tides. Some examples of ocean currents are the Gulf Stream, Florida Straits Current, and
California Current (Figure 1). The Florida Straits Current starts only 8 km offshore in the
southern part of Florida, close to Miami, and sustains relatively large speeds over significant
distances in relatively unchanging patterns. In contrast, the California Current has relatively slow
speeds and shifts periodically. Ocean currents tend to be concentrated at the surface, although
significant current continues at depths below ships’ drafts. The Aleutian passages have also been identified as an area for potential development of ocean current energy extraction.

Ocean current speeds are generally lower than wind speeds. This is important because the
kinetic energy contained in flowing bodies is proportional to the cube of their velocity. However,
another more important factor in the power available for extraction from a flowing body is the
density of the material. Water is about 835 times denser than wind, so for the same area of flow
being intercepted, the energy contained in a 12-mph water flow is equivalent to that contained in
an air mass moving at about 110 mph. Thus, ocean currents represent a potentially significant,
currently untapped, reservoir of energy.

The total worldwide power in ocean currents has been estimated to be about 5,000 GW,
with power densities of up to 15 kW/m2. The relatively constant extractable energy density near
the surface of the Florida Straits Current is about 1 kW/m2 of flow area. It has been estimated
that capturing just 1/1,000th of the available energy from the Gulf Stream, which has
21,000 times more energy than Niagara Falls in a flow of water that is 50 times the total flow of
all the world’s freshwater rivers, would supply Florida with 35% of its electrical needs.
Countries that are interested in and pursuing the application of ocean current energy technologies
include the European Union, Japan, and China.

Ocean current energy is at an early stage of development, with only a small number of
prototypes and demonstration units having been tested to date. One such technology involves
submerged turbines. Energy can be extracted from the ocean currents using submerged turbines
that are similar in function to wind turbines, capturing energy through the processes of
hydrodynamic, rather than aerodynamic, lift or drag. These turbines would have rotor blades, a
generator for converting the rotational energy into electricity, and a means for transporting the
electrical current to shore for incorporation into the electrical grid.
Turbines can have either horizontal or vertical axes of rotation (Figure 2). Mechanisms
such as posts, cables, or anchors are required to keep the turbines stationary relative to the
currents with which they interact. Prototype horizontal axis turbines, similar to wind turbines,
have been built and tested. Vertical axis turbines are either drag or lift designs. The lift devices
seem to offer more potential (e.g., the Darrieus-design turbine design with three or four thin
blades of aerofoil cross-section has been tested in the Kurushima Straits off Japan) (WEC 2001).
Turbines may be anchored to the ocean floor in a variety of ways. They may be tethered
with cables, with the relatively constant current interacting with the turbine used to maintain
location and stability. Such a configuration would be analogous to underwater kite-flying where
the kite would be a turbine designed to keep upright and the kite flyer would be the anchor.
Additional components may include concentrators (or shrouds) around the blades to increase the
flow and power output from the turbine. Various alternative designs have been proposed,
including the use of a barge moored in the current stream with a large cable loop to which waterfilled parachutes are fastened. The parachutes would be pushed by the current, and then closed on their way back, forming a loop similar to a large horizontal waterwheel.
In large areas with powerful currents, it would be possible to install water turbines in
groups or clusters to create a “marine current facility,” similar in design approach to wind turbine
facilities. Turbine spacing would be determined based on wake interactions and maintenance needs. A 30-MW demonstration array of vertical turbines in a tidal fence is being investigated in
the Philippines (WEC 2001).
For marine current energy to be utilized, a number of potential problems would need to
be addressed, including avoidance of drag from cavitations (air bubble formation that creates
turbulence and substantially decreases the efficiency of current-energy harvest), prevention of
marine growth buildup, corrosion control, and overall system reliability. Because the logistics of
maintenance are likely to be complex and the costs potentially high, system reliability is of
particular importance.

ENVIRONMENTAL CONSIDERATIONS

Potential environmental impacts that would need to be considered with the development
and utilization of ocean current energy on the OCS include impacts on marine ecology and
conflicts with other potential uses of the same area of the ocean. Resource requirements
associated with the construction and operation of these technologies would also need to be
addressed. Regardless of the size and nature of the anticipated environmental impacts, project
planning would need to consider the protection of species, particularly fish and marine
mammals. The slow blade velocities should allow water and fish to flow freely and safely
through the structure. Protective fences and sonar-activated brakes could prevent larger marine
mammals from harm. In the siting of the turbines, consideration of impacts on shipping routes,
and present as well as anticipated uses such as commercial and recreational fishing and
recreational diving, would be required. Additional considerations include the need to introduce
possible mitigating factors, such as the establishment of fishery exclusion zones.
Concerns have been raised about risks from slowing the current flow by extracting
energy. Local effects, such as temperature and salinity changes in estuaries caused by changes in the mixing of salt and fresh waters, would need to be considered for their potential impact on
estuary ecosystems (Charlier and Justus 1993).

ECONOMIC CONSIDERATIONS

Because no commercial turbines are currently in operation, it is difficult to assess the
costs of current-generated energy and its competitiveness with other energy sources. Initial
studies suggest that for economic exploitation, velocities of at least 2 m/s (4 knots) would be
required (a 5-knot current has the kinetic energy equivalent of wind at more than 100 mph),
although it is possible to generate energy from velocities as low as 1 m/s. Major costs of these
systems would be the cables to transport the electricity to the onshore grid.
Although tidal-current energy extraction is not within the scope of this paper, there are
many similarities and common problems. Tidal-energy extraction is being explored in many
areas of the world. In 2003, the world’s first commercial grid-connected tidal-current plant
opened in Hammerfest, Norway, as a 300-kW plant generating 700 MW hours of power
annually. Other locations pursuing tidal-generated power include San Francisco; Devon,
England; and British Columbia.

SUMMARY

Ocean-currentв?’generated energy technologies have many favorable characteristics,
including the following:
• Water currents have a relatively high energy density.
• Some ocean currents are relatively constant in location and velocity, leading
to a large capacity factor (fraction of time actively generating energy) for the
turbines.
• Because they are installed beneath the water’s surface, water turbines have
minimal visual impact.
• The largest current energy resource on the U.S. OCS, the Florida Straits
Current, is relatively close to heavily populated areas of Miami that have high
power demands.

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