CORIOLIS FORCE OCEAN CURRENTS

The Gulf Stream and the Kuroshio are Coriolis Force Currents

                Although most of the efforts for producing power from renewable energy have been concentrated on capturing energy from the wind and sun, there is a much better source of renewable energy that is available to our southeastern states that can produce huge amounts of electricity:  the Gulf Stream.

                The Gulf Stream Turbines’ ability to produce a steady supply of cheap electricity depends on the characteristics of the current that drives it.  With the exception of the tides, which are produced by the gravitational pull of the moon and the sun, most of the ocean’s energy comes from the sun’s radiant energy.  Although some of that energy is converted by algae into carbohydrates, most of it is stored as heat and it is the warmed water that activates the global winds that produces the oceans' waves and currents.

 In both the Atlantic and Pacific oceans the two trade winds drive an immense body of water westwards over a width of some 50o of latitude, broken only by the narrow belt of the east-going Equatorial Counter-current which is found a few degrees north of the equator in both of these oceans.  A similar westward flow of water occurs in the South Indian Ocean, driven by the southeast trade wind.  Because of the Coriolis effect, caused by the Earth’s eastward rotation, these winds do not move the water directly toward the east or west, but nudge it off to the right in the Northern Hemisphere and to the left in the Southern Hemisphere; thereby producing giant eddies that are centered in latitudes of approximately 30o N. and S. that rotate clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. 

In addition to producing the Coriolis effect that produces the oceans’ gyres, another consequence of the Earth’s eastward rotation is that the center of each of those gyres is offset toward the western edge of the ocean basin that confines it.  Because the volume of water flowing toward the poles along the narrower western sides of the gyres is the same as that circulating back toward the Equator down the much broader eastern expanses, the constricted western currents are forced to flow much faster than their eastern counterparts.  This results in such powerful currents as the Gulf Stream in the western North Atlantic, and the Kuroshio in the North Pacific.

Ocean Passages of the World (published by the Hydrographic Department of the British Admiralty, 1950) lists 14 currents that exceed 3 knots (3.45 mph), a few of which are in the open ocean.  The Gulf Stream and the Kuroshio are the only two currents that the book lists that have velocities above 3 knots that flow throughout the year.  The book gives the strongest currents recorded for the Gulf Stream and the Kuroshio in nautical miles per day.  These speeds are equivalent to 156.5 statute miles per day (6.52 mph) for the Gulf Stream, and 133 statute miles per day (6.375 mph) for the Kuroshio.  Because these speeds were determined by how far the current carried a floating object in 24 hours, they do not reflect the maximum current speeds at specific times or places.  Although most ocean currents are generally too slow or are too far from shore to justify the costs of capturing their kinetic energy, this is not true of the Gulf Stream and the Kuroshio. 

The Gulf Stream

The Gulf Stream starts roughly where the Gulf of Mexico narrows to form a channel between Cuba and the Florida Keys.  From there the current flows northeast through the Straits of Florida between the mainland and the Bahamas, flowing at a substantial speed for some 400 miles.  The following satellite photo shows the Gulf Stream, with the red being the warmest water and blue being the coolest.  The three blue lines in the chart on the right show the location of the Gulf Stream’s central axis. The best sites for the Gulf Steam Turbines would be along this axis that is located between 15 and 191/2 miles off of Florida’s east coast, between North Palm Beach and Key Largo. As can be seen in the satellite photo, it is well north of this area where the current’s path starts to meander.

  

            Although the peak current velocity of the Gulf Stream may at times reach more than 8 miles per hour in its narrow axis off of Miami, the most likely velocities for those turbines placed along the current’s central axis would probably be between 5 and 6.5 mph. If asynchronous generators are used, as long as the current exceeds about 3.5 mph, constancy of speed can actually produce more power than faster currents with large variations of velocities if the rotors, gearbox, and generator are properly sized for the slower current.  This is because the turbine generators in the slower but steadier currents can have higher capacity factors.  The efficiency of the turbines and generators will depend on the distribution of the current’s velocities over time.   

The Kuroshio

 

Submersible turbines can also produce power from the Kuroshio, the Pacific Ocean’s equivalent to the Gulf Stream.  As previously stated, the Kuroshio’s maximum flow rate is only slightly slower than that of the Gulf Stream.  Although the Gulf Stream Turbine power plants can also generate low-cost electricity from the Kuroshio, they would not operate at quite the same high capacity factors there as would in the Gulf Stream’s central axis.  This is because the velocity of the Kuroshio fluctuates more due to both seasonal and tidal effects, flowing slower in the fall and with rising tides.  The current has two stable path patterns south of Honshu, the largest island of Japan.  It has a straight path that flows eastward, after passing the tip of the Kii Peninsula, and it has a large meandering path that flows around a large cold water mass that can form to the southwest of that peninsula.  Either pattern can persist for periods ranging from several months to several years.  Because of the Kuroshio’s changing paths and extreme water depths in the area, there is just one place where the turbines can consistently produce power.  That is just south of the Izu Peninsula and Sagami Bay, where both current paths pass over the Izu-Ogasawara Ridge, where the Pacific Plate subducts under the Philippine Sea Plate.  The yellow circle shows that location.   

Other Currents

Other possible sites include the East Australian Coast current, which flows at a top rate of 110.47 statute miles per day (4.6 mph), and the Agulhas current off the tip of South Africa, which flows at a top rate of 139.2 statute miles per day (5.8 mph).  Still another possibility is the current that flows through the Strait of Messina, that narrow constriction that separates Sicily from Italy’s “toe.”  Interestingly, this current flows counter-clockwise, the reverse of what one might expect in the Northern Hemisphere.  Instead of being wind-driven, this current is produced by the evaporation of the salt water in the Mediterranean and the resulting changes in the water densities – combined with the distortions of the current’s path that are produced by the Coriolis effect and the shape of the land.  Another place where currents flow in one direction is through the Strait of Gibraltar, where there is a constant inflow in the upper layer and outflow in the lower layer.  These two opposing currents are the result of the same evaporation in the Mediterranean that produces the current that flows through the Strait of Messina. 

Comparing the Coriolis Currents to the Tidal Currents

            The reason that we are interested in the tidal current turbines is that they also involve the generation of electricity underwater and the transmission of that power to shore.  Another reason for our interest is that serious money is being invested in their development – even though their capacity factors are awful.

            The tides are the periodic motion of the water caused by the differences in the gravitational attractive forces of the moon and the sun acting on all the different parts of the rotating Earth.  As the tides rise and fall, they are accompanied by periodic horizontal movements of water, the tidal currents.  The tide is the vertical rise and fall of the water; the current is the horizontal flow.  The tide rises and falls, the tidal current floods and ebbs.  The variation in speed of the tidal currents from place to place is not consistent with the range of the tides and, depending on the shape of the coastline; it can even be the reverse.  Although there is presently no interest in producing more power from a tide’s changing water level, there is a growing interest in the tidal currents.

The first tidal current plant began commercial operation in Norway on November 13, 2003.  Located near Kvalsund, this plant is supplying power to a grid at Hammerfest.  Although this first turbine is only a 300-kW plant that will generate only 700 megawatt-hours per year, the operators are planning to install a much larger second-generation plant within 2 years.  The Kvalsund tidal currents flow at about 2.5 meters per second (5.59 mph) at their peak that comes midway between the high and low tides.  Like all semidiurnal tides, the tides rise and fall every 12 hours and 25 minutes.  The Kvalsund turbines have single rotors that have three variable-pitch blades that can be rotated 180 degrees to produce power from the tidal current’s ebb and flow.  This first machine weighs 200 metric tons (220.5 U.S. tons).  It is projected that the Kvalsund system will eventually have 20 megawatts of capacity.

            Marine Current Turbines Ltd., a British company, is building a tidal current system off Lynmouth, Devon, in southern England.  Similar to submerged wind turbines, the first experimental monopile-mounted machine has a single two-bladed variable-pitch rotor having a diameter of 11 meters (36.1 feet) that is mounted on a thick post set in the ocean floor.  The posts extend above the surface to allow the turbines to be raised from the water.  Its generating capacity is 300 kW and it generates power only when the current is flowing in one direction.  The cost of this first phase is £3.3 million (about $6.12 million).  An initial grant of 1 million Euro (about $1.3 million) was received from the European Commission towards the R & D costs and this has been followed by a grant towards the cost of the first Phase work from the UK government worth £960,000 (about $1.788 million). The German partners also received a grant worth approximately 150,000 Euro (about $196,027) from the German government.  The British company is now building the first “full size” twin rotor system, to be rated at 750 to 1200 kW, with each rotor being slightly larger than the rotor on the machine just described.  Unlike that experimental machine, this unit is to be connected to the grid and will produce power from both the tide’s ebb and flow.  This machine is expected to cost about £4.5 million (about $5.9 million), including the grid connection.  These turbines will utilize tidal currents, which move at velocities of between 2 and 3 meters per second (4.6 to 6.9 mph), to generate between 4 and 13 kW per square meter of sweep area.  Currents moving faster than 3 meters per second (6.9 mph) can cause undue stress on the blades in the same way that gale-force-winds can damage traditional wind turbine generators. 

As with all turbines, the ability to produce electricity is dependent on the speed and steadiness of the fluid driving it.  Although the usable current velocities that drive the tidal turbines are about the same as those that will drive turbines placed in the Gulf Stream, the tidal currents oscillate and can produce at their rated power only between the high and low tides.  The following graphs show the potential generating potential for those tidal turbines that are being planned for Dover, England.  The graph on the right shows the tidal turbines operating at their rated capacities for only a very small percentage of the time.  According to the graph on the left, the peak power would be about 3000 MW during the spring tides, with the average being about 1600 MW.  The peak power during neap tides would average about 750 MW, with the average being about 200 MW.  That means that the average of the high and low averages would be about 450 MW. Consequently, the system’s capacity factors would be incredibly poor.  If all the generators had total rated capacities of 3000 MW, the capacity factors would only be 15 percent.  If we were to superimpose the power of the Gulf Stream Turbine onto the graph on the left, its power would be shown as a horizontal line near the top of the graph.

      

If a group of Gulf Stream Turbines were to have a total rated generating capacity of 3000 MW and if they were to operate at capacity factors of 90 percent, they would produce 6 times as much the power per year and that power would be steady.  Despite the tidal turbine’s inability to generate steady power and the fact that other sources of electricity will be needed to carry the loads between the tidal turbines’ generating cycles, serious money is being invested in the development of these machines.

 

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