|Fig. 1: Major Ocean Surface Currents (Source: Wikimedia Commons, courtesy of NOAA)|
|Fig. 2: Global Thermohaline Circulation.  (Source: Wikimedia Commons, courtesy of NASA)|
The relatively constant flow of ocean currents carries large amounts of water and energy across the earth's oceans. Although there has been no commercial development in the United States, technologies are being developed so that ocean currents may provide a source of renewable, clean energy that can be extracted from ocean currents and converted to usable power. 
Ocean currents flow in complex patterns and pathways and are affected by several elements such as wind, temperature, topography of the ocean floor, the earth's rotation and water salinity. Most ocean currents are driven by wind and solar heating of surface waters, while some currents result from density and salinity variations of the water column. The major surface currents are represented in Fig. 1.
Currents are also caused by Thermohaline circulation as shown in Fig. 2. Thermohaline circulation is part of the global ocean circulation that is driven by density gradients created by surface heat and freshwater fluxes. The term thermohaline is derived from thermo- having to do with temperature and -haline referring to salt content, factors that collectively define the density of ocean water.  Wind-driven surface currents such as the Gulf Stream travel polewards from the equatorial Atlantic Ocean, cooling along the way, and eventually sinking at high latitudes forming North Atlantic Deep Water. This dense water then flows into the ocean basins. While most of it upwells in the Southern Ocean, the world's oldest waters upwell in the North Pacific. Diffusion and mixing of water therefore takes place between the ocean basins, reducing differences between them and making the Earth's oceans a global system. On their journey, the water masses transport energy in the form of heat. 
Ocean currents have a relatively constant and directional flow, in contrast to tidal currents along the shore. While ocean currents may move slowly relative to wind speeds, due to the density of water, they carry a great deal of energy. 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. Due to this physical property, ocean currents contain an enormous amount of energy that can be captured and converted to a usable form. 
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. 
|Fig. 3: Horizontal Axis Turbines.  (Courtesy of the U.S. Department of Energy)|
The best sites for the harvesting of ocean current energy are between islands, around heavily indented coasts where there are strong tidal currents, or where the temperature difference between the warm surface water and the cold deep water is about 20°C or more.  Marine turbines are the most popular piece of technology being proposed. The blades of these turbines need to be about 20 meters, only one third the size of a wind generator to produce three times as much power. Each turbine will be mounted on a tower, which will connect to the grid by underwater just as with offshore wind towers. The towers will breach the surface of the water and will be illuminated to warn ships. There are also designs so these towers can be lifted out of the water to clean seaweed or other growths from the blades and for other maintenance. 
Current energy technologies, also called tidal or hydrokinetic technologies, convert the kinetic energy of moving water into electricity. Current energy technologies take advantage of the horizontal flow ocean currents to power a generator that converts mechanical power into electrical power. Current energy devices are often rotating machines similar to wind turbines with a rotor that spins in response to the speed of water currents with the rotational speed proportional to the velocity of the water. The rotor may have an open design comparable to a wind turbine or may be enclosed in a duct that channels the current and water flow. Current energy converters can be into four main types: 
Horizontal axis turbines typically look similar to wind turbines as shown in Fig. 3. They harvest kinetic energy from the moving water in the same way that wind turbines extract energy from the moving air.
The horizontal rotor is enclosed inside a duct. These ducts are often funnel-shaped and work by concentrating the current flow through the turbine. This technology may allow for function over a wider range of current velocities, thus producing more electricity per unit of rotor area.
In vertical axis turbines, the axis of the rotor is situated perpendicular to the current flow. These turbines may also be enclosed within a duct.
Oscillating hydrofoils pivot in response to tidal currents flowing over a wing or flap-like structure. The special situating achieve with this design drive fluid in a hydraulic system to generate electricity.
|Fig. 4: World Primary Energy Consumption.  (Converted at 4.2 × 1010 joules per TOE)|
For ocean current energy to be used 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
High maintenance costs
Generation of electromagnetic fields (EMF)
Toxicity of paints, lubricants, and antifouling coatings
Interference with animal movements and migrations, including entanglement and strike by rotor blades or other moving parts
|Ω||=||0.7 × 4π R2 d|
|=||0.7 × 4 π × (6.37 × 106 m )2 × 4.27 × 103 m|
|=||1.52 × 1018 m3|
Therefore, the total amount of energy available for extraction in the world's ocean is 2000 joules/m3 x 1.52 × 1018 m3 = 3.04 × 1021 joules. When you compare this total to the annual energy demand per year for the entire world, as shown in Fig. 4, you see that it's only 6 years worth.
© Bobby Zarubin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 "Report to Congress on the Potential Environmental Effects of Marine and Hydrokinetic Energy Technologies," U. S. Office or Energy Efficiency and Renewable Energy, December 2009.
 S. Rahmstorf, "Ocean Circulation and Climate During the Past 120,000 Years," Nature 419, 207 (2002).
 "BP Statistical Review of World Energy 2014," British Petroleum, June 2014.