/ MEDIA STATEMENT / This content is not written by Creamer Media, but is a supplied media statement.
By ZLM Project Engineering Pty, Ltd
Ocean currents provide the largest potential for harnessing energy from the sea as one of the primary advantages of this technology is the energy density. While solar and wind systems are well-suited for remote off grid locations, ocean energy is ideal for large-scale developments in the multiple gigawatt range. Sea water is 832 times as dense as air, providing a 5-knot ocean current, as occurs off the coast of KwaZulu-Natal, with more kinetic energy than a 350 km/h wind.
A recent prefeasibility study commissioned by the eThekwini municipality into harnessing the Agulhas current for power generation found that it is the largest Western boundary ocean current in the southern hemisphere, transporting about 70 X 106 m3/sec of water at an average velocity of 1.2 m/sec with velocities of up to 1.8 m/sec. This also makes it one of the largest ocean currents on earth.
This volume of moving water gives an average energy potential of 50.4 GW of power that can be harnessed from the coast of KZN alone!
Not only is the Agulhas current extremely powerful, it is also very stable mainly due to the stabilizing effect of the steep continental slope. The continental shelf on the East Coast of Southern Africa is very narrow with a steep drop off. This edge occurs along the 50 m contour approximately 8 km off the coast north of Cape St Lucia and along the 100 m contour in the southern part of the province and down toward Port Alfred. Between Richards Bay and Durban, the shelf widens to about 45 km to form the “Natal Bight” after which it narrows again.
The stability of the current flowing down the coast of KwaZulu-Natal is affected by the “Natal Pulse” The Natal Pulse is the term used to refer to large solitary meanders that have been measured to move slowly (about 21 cm/sec) down the East Coast slowing down to about 5 cm/sec where the continental shelf broadens at Algoa Bay.
These meanders are said to originate in the Natal Bight and to be caused by cyclonic eddies that break off the inside edge of the main current which, in turn, push the current flow further offshore. This sporadically gives rise to a North flowing counter current at the shore whilst the eddy is passing down the coast.
The meanders occur intermittently and grow as they progress down the coast extending up to 170 km offshore at Algoa Bay. These meanders occur about 5 times per year, each meander lasting for about 4-5 months on its journey down the coast. This indicates that the current is stable for about 80% of the time in any one site. These pulses can be predicted at any point off the coast using SST measurements taken by satellite to track their movement down the coast.
Any site selection process for offshore generating stations off the coast of KwaZulu-Natal must take the following marine reserves into account.
a. The Maputaland and St Lucia Marine reserve
b. The Tugela reserve
c. The Trafalgar reserve
Unless legislation changes to allow modification of these reserve boundaries or the placement of generating stations within these reserves, they appear to be inviolate and any studies will have to take this into account.
A suitable placement for a generating station would be under the surface and fixed to the seabed, rather than floating on the surface. This is mainly due to the East coast of Southern Africa being home to some of the most dangerous waters on Earth. Any permanent structure that is floating on the surface in the Agulhas current will be subject to storms, high seas and rogue waves. These rogue waves are reported to be up to 20 m high with steep fronts and with deep troughs in front of them. They have caused severe damage to large ships such as the Wafra which had its bow section buckled by a wave off the South-Eastern coast in about 1965. Relatively recently the tug HAWK disappeared off this coast and is also suspected to have been hit by a rogue wave.
Ocean current generation is still in its infancy with no commercial plant yet installed although several types of plant have been proposed using axial turbines, crossflow turbines, both in various guises, as well as oscillating devices.
The closest comparison to ocean currents is the use of tidal current energy and these schemes, which are more developed than ocean currents schemes, can be used, probably with modifications, for ocean current generation. Tidal currents are similar to ocean currents in that they utilize the current flow but need to account for the fact that the flow reverses direction every six hours. This technology is also the closest to mainstream commercial operation with some pilot plants currently in operation. There are two main types of plant harnessing tidal currents: tidal barrages and open water systems. For KwaZulu-Natal the open water system is preferable as no suitable sites for the barrage type systems are available. While the technology used for open ocean tidal energy is adaptable it is generally deployed in much shallower water, up to about 40 m deep, so would have to be adapted for the deeper waters utilized in ocean current harvesting, at about 100 m to 200 m.
There are two basic designs of underwater generator and these, as in wind turbines, are horizontal and vertical flow turbines. Almost all the current designs are based on one of these two systems with detail differences and mounting or fixing methods differing. The two main fixing methods are by means of a tower fixed to the bed or the turbine tethered by means of cables fixed to the sea bed. Another method is to hang or fix the turbine under a floating platform but this brings weather conditions and freak waves into play which is a problem on the KwaZulu-Natal coast.
Tethered or kite like structures appear to be the most suitable for the KwaZulu-Natal coastline. These have been designed specifically for ocean currents and have the advantage that they can swing as the current changes direction. The design is semi-submersible and can thus be adjusted for various depths in order to optimise the placement of the turbines in the current.
Legislative requirements for an offshore system are the same as for any other independent power producer and have already been explored.
It can therefore be seen that offshore energy, although in its infancy, is certainly feasible with the additional advantage of being able to provide baseload generation for South Africa and deserves to be explored further.