Page 153 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

Other Forms of Ocean Energy Chapter | 6 143

Because it passes relatively close to the state of Florida, much research has
been invested in understanding and quantifying the Florida Current—the ocean
current that flows from the Straits of Florida, around the Florida Peninsula,
and along the southeastern coast of the United States before it merges with the
Gulf Stream. The volume of water transported in the Florida Current is around
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3
30 Sv (1 Sv = 10 m /s), in contrast to around 150 Sv in the Gulf Stream
itself [3]. By way of comparison, the mighty Amazon, the largest river in the
world, has an annual mean flow of 0.2 Sv. The mean energy flux in the Florida
Current has been estimated from numerical simulations as around 22 GW, with
a standard deviation of around 6 GW [3,4], and over half of the total kinetic
energy flux is concentrated in the upper 200 m of the water column. Therefore,
from both economic and technical perspectives, any technology that is to exploit
this resource would realistically need to be located in the upper part of the water
column, such as an array of floating turbines, and in relatively deep water (see
Section 6.2.2).
Observations have demonstrated that the northward extension of the Florida
Current—the Gulf Stream off the coast of North Carolina—has a fairly per-
sistent current speed of just under 1 m/s at a depth of 75 m, associated with a
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2
mean (and maximum) power density of 800 W/m (4500 W/m ) [5]. At such
water depths, subsurface turbines could become feasible as turbine and mooring
technologies develop.
The Agulhas Current flows southward along the east coast of South Africa.
As with other western boundary currents, it is relatively fast and narrow, with a
mean flow of around 70 Sv [6]. As with the Florida Current, its close proximity
to the coast and relative stability make it an attractive proposition for electricity
generation. However, the trajectory of the Agulhas Current is intermittently
interrupted by perturbations known as ‘Natal Pulses’—large solitary meanders
that form at the Natal Bight between 29 and 30 degrees S [7]. There are, on
average, 1.6 Natal Pulses per year, each lasting around 15–20 days. In the region
of the Agulhas Current that looks most promising for electricity generation
(around 28.8 degrees E and 32.5 degrees S), due to favourable velocity and
bathymetry characteristics, the current core is in close proximity to the coast.
At this location, any turbine deployed in the Agulhas Current would need to
operate in current speeds of 0.6–2 m/s (Fig. 6.2).

6.2.1 Variability

It is often thought that “unlike the wind, [ocean currents] are always flowing”
[8]. However, Bane et al. [8] propose an alternative statement: “the [ocean
currents are] always flowing, but not always in the same place.” Although
ocean currents are relatively persistent, they suffer from variability at a range
of temporal scales. For example, as mentioned earlier, the Agulhas Current is
characterized by ‘Natal Pulses’ lasting 15–20 days, with 1 or 2 events occurring
per year. Most variability in the Florida Current is found at ‘high’ frequencies,