Harnessing the ocean current

RECENTLY the media covered the story of a small fleet of Turkish electricitygenerating ships that could be the short-term solution to our power outages in this country.

The ships are floating power stations running on ordinary heavy fuel oil such as is used by merchant ships the world over. This is used to power the generators as well as the ship and works out cheaper than having a power station that burns diesel to run its generators – what Eskom has been relying on to supplement its coal-fired power stations.

The Turkish company Karadeniz has secured a contract to provide two of these ships to Ghana where they will plug into the country’s constrained electricity grids.

To show that it is not just another crazy idea, Karadeniz has already established itself in Lebanon, where it supplies 25% of the Mediterranean country’s electricity needs, as well as in Iraq where it provides 16% of the power required to keep Iraq’s electrical requirements up and running.

The beauty of such an arrangement is that at the end of a contract, if not wanted any further, the power station ship can sail away.

According to the reports the Turkish company Karpowership, which is a division of Karadeniz, has made approaches to Eskom, which is studying a proposal of having four of these ships, each capable of generating 500MW of electrical energy.

As a short-term opportunity the ability to anchor ships of this nature off the South African coast and “plug in” to the nation’s electricity grid seems an appealing one, especially for areas a great distance from the nearest power station.

While these ships normally burn heavy fuel oil, they can apparently be converted to burn natural gas instead, making for a much cleaner environment.

Now comes news that a wellknown Dutch shipbuilder, Damen Shipbuilding, which, incidentally, owns a shipyard in Cape Town, is involved with developing a power generating system of barges or platforms that use the ocean current to create electricity, as opposed to burning oil or gas.

In this sense the ships are similar to land-based windmills, except they are anchored at sea and their blades, or turbines, are underwater instead of protruding more than a hundred metres into the sky.

The use of harnessing the ocean currents has long been an exciting challenge but one that has not been actively pursued with any real determination. Off the South African coast is one of the world’s most powerful and regular ocean currents, moving water down the African coast with enormous energy, yet we have studiously ignored it except perhaps to study and utilise its ability to influence ship movements.

Many ships will sail further out to sea in order to take advantage of the strong Agulhas current when sailing in a south-westerly direction, while ships heading northeastwards will hug the coast to take advantage of a counter-current flowing north and also to avoid having to sail into the Agulhas current going the other way.

Other than that, the current is mainly ignored, and the tremendous energy it offers goes unused.

The new Dutch platforms are not floating power stations as are the Turkish vessels described above. They are essentially floating barges at sea, holding tidal turbines slung underneath, which are anchored to the ocean bed. The flow of water current causes the turbines to rotate, which in turn, drives the generator, generating electricity which is then supplied to the electricity grid on the mainland via a power cable. The platform is anchored just like a boat.

A potential problem will be the sheer strength of the ocean current and the ability to hold platforms in position while resisting the force of the current. The Dutch concept is, initially at first, aimed at harnessing the movement of tidal flow closer to the coast.

The company developing this idea with Damen, BlueTEC will be targeting remote islands around the world where they can be deployed in large farms to provide the islands with electrical power.

They will make use of tidal energy (currents) to rotate the turbine propellers below the sea surface.

In the first such use of these new craft, a platform is being positioned offshore of the island of Texel in the North Sea and will be connected to the Dutch electricity grid. Additional platforms will be added as the project is tried and tested.

The units are small, able to be loaded and shipped anywhere in the world in ordinary containers. Damen has used three standard container-sized modules to construct the Texel platform, which provides for efficiency in transportation and assembly.

BlueTEC says the Texel platform is just the first step and is looking at platforms that can generate between 2MW and 3MW.

This would put them on a rating similar to land-based wind farms, but unlike the wind farms the seagoing turbines will always have a current to utilise for generating energy.

They can be placed in a tidal farm configuration that can generate several hundred megawatts.

BlueTEC clams its tidal flow platform is cheaper than the cost of diesel-powered electricity generation.

This is just a first step in this direction, and of far greater importance, it marks the first step in using an energy source available to almost all ocean-facing countries, and is certainly applicable to South Africa, for harnessing a naturedriven reliable source of energy, with little or no harmful effect on the surrounding environment.

For the technically minded, the Texel platform consists of two standard 12m containers measuring a total of 24m x 2.4m wide.

The turbine’s rotating underwater blades can vary between four and 10 metres depending on the type of turbines used. They turn slowly and the effect on fish and animal life is considered negligible. Being a low 2m above the water they do not obstruct the view of an open ocean, although this has the potential of becoming a navigational hazard. They compare with landbased wind farms where the wind turbines are up to 150m high with multiple turbines.

A 200kW undersea turbine will supply electricity for 200 normal households making use of a predictable reliable source.