The Agulhas Current - a potentially viable alternative energy source?

By Laura Braby, SAEON Egagasini Node Tweet

Laura Braby recently submitted her PhD in Physical Oceanography at the University of Cape Town and hopes to pursue a postdoctoral fellowship at SAEON investigating the variability of the Agulhas Current.

The Agulhas Current flows along the east coast of Southern Africa and is one of the strongest ocean currents in the Southern Hemisphere.

On average, the Agulhas Current transports 77 million cubic metres of water southwestwards every second.

This powerful current is being investigated as a potentially viable alternative energy source for South Africa.

Globally, resource characterisation studies have been done in other major ocean currents including the Gulf Stream off the coast of the United States of America and the Kuroshio Current, which flows offshore of Japan and Taiwan. Whilst these currents share many similarities with the Agulhas Current, each current also has its own unique characteristics.

Energy extraction in the Agulhas Current

Possible sites for energy extraction in the Agulhas Current have been investigated using several different types of data to allow for the best possible understanding of the dynamics of South Africa’s narrow and fast current. These data sets include Acoustic Doppler Current Profilers (ADCPs) that were deployed in the ocean, satellite remote-sensing data and ocean models.

Strength and direction of flow are important factors to consider, with the northern sector of the Agulhas Current considered to be the most stable. Large eddies, 300–400 km in diameter, form in the Mozambique Channel and move towards the start of the Agulhas Current (shown in Figure 1).

The interaction of these eddies with the Agulhas Current can cause the current to meander and move offshore irregularly, every 50 to 150 days. These meanders, known as Natal Pulses, present a unique challenge for energy generation and improved data sets are critical to correctly predicting the energy potential of the Agulhas Current and informing the relevant stakeholders.

Figure 1: A schematic showing the main circulation features of the greater Agulhas Current system. Clockwise and anti-clockwise rotating eddies in the source region of the Agulhas Current are indicated (Source: Lutjeharms and Ansorge, 2001).

One area of fast (an average speed of 1.59m.s^-1) and relatively stable flow in the Agulhas Current has been identified, approximately 100 km from the coast of East London.

Using the satellite data it is possible to predict, with fairly good accuracy, the occurrence of a Natal Pulse approximately two weeks before it arrives at the proposed site. This would enable grid planners to prepare for periods of no power generation with sufficient warning and to plan for the utilisation of other energy sources.

This location for energy extraction is situated well with regards to avoiding global shipping routes, marine protected areas and important fishing regions whilst still maintaining a close proximity to the coastal substations (Figure 2).

Contribution to the electricity grid

One major challenge of harnessing the power of the Agulhas Current would be mooring the turbines in water depths in excess of 250 m. It has however been found that a turbine array in the Agulhas Current compares well with other alternative energy sources and could provide South Africa with significant contribution to the electricity grid.

Figure 2: The locations of Eskom substations relative to ADCP sites of interest (Source: Meyer et al., 2013).

Whilst there has been a huge amount of development in the renewable and alternative energy sector over the past three decades, offshore ocean technology remains the least developed. Due to the vastness of the resource, there are many aspects of the ocean still to be investigated and understood.

Further improved regional ocean models and data assimilation efforts are critical to reducing the barrier of entry of this resource into the offshore renewable energy market in South Africa.

Acknowledgements

This article is based on the book chapter “Meyer, I., L. Braby, M. Krug and B. Backeberg (2017), Mapping the ocean current strength and persistence in the Agulhas to inform marine energy development, In Marine Renewable Energy: Resource Characterization and Physical Effects, 179-215. Springer, Cham.”