Ocean energy generation potential in South Africa
http://www.urbanearth.co.za/articles/ocean-energy-generation-potential-south-africa
Submitted by: Jonathan Ramayia, Friday, May 18, 2012
World’s ocean currents (Source: Wikipedia)
Ocean energy generation has been spoken of for many decades, but unlike other renewable forms of energy generation like solar and wind, it’s never quite taken off. In spite of this, attempts to develop technologies that harness the immense power of the sea haven’t stopped. There are a number of ways which energy could be harnessed from the sea and intertidal area. This article overviews these as well as key projects around the world that have been proposed or are in operation. In addition to this, South Africa’s outlook for each of these ocean energy resources are highlighted.
Wave Energy
Energy can be obtained from the movement of the ocean by surface wave action where the kinetic and/or potential energy stored in waves are converted to mechanical and/or electrical energy. An essential technology to achieve this is a wave energy converter, an example of which is the Pelamis wave energy converter which flexes and bends as waves pass through them. The Pelamis unit is much like a snake with several connected ‘sections’ which move as waves pass through them. Hydraulic cylinders resist the flex and bend of the sections and then pump high pressure oil through hydraulic motors which drive electrical generators to produce electricity. A myriad of other technologies exist, including the ProteanTM; the Surge Drive; and PowerBuoy.
Pelamis prototype machine at EMEC, Scotland in 2004 [Source: Wikipedia]
Like wind farms, the idea of a ‘wave farm’ has also been proposed and developed with the first one being north of Porto, Portugal totalling 2.25MW installed capacity obtained from three Pelamis units. A second phase will see a further 21MW added with 25 more Pelamis units. The UK has also announced the development of a Wave Hub off the North Coast of Cornwall which will make use of a variety of wave converters and could potentially add 40MW onto the grid.
Currently, the United Kingdom is considered the world leader in piloting wave energy projects with several projects connecting to the grid and key responses from the government to exploit the island nation’s competitive advantage in the ocean. UK politicians have predicted that wave energy could ultimately provide them with 20% of their energy needs and create 10,000 jobs.
The South African potential for wave energy is high given the fact that South Africa has an extensive coastline of over 2,500 km and a sophisticated manufacturing economy. An indicator of wave power generation is kW per m which is basically a measure of the amount of electricity that can be generated for a ‘metre of wave’. Alternatively, this could be read as the energy potential of the wave. Research conducted by the Centre for Renewable and Sustainable Energy Studies (CRSES) at the University of Stellenbosch shows that the Western Cape has the highest kW/m along the South African coast ranging from 35 at the border with the Eastern Cape and 38 near Saldanha Bay to a peak of 40 kW/h off the coast of Cape Town. The KwaZulu-Natal coast near Durban has recorded an annual mean rating of 15 kW/h which is significantly less than the rest of the South African coast.
Current Energy (Marine and Tidal)
Another significant source of marine energy is current energy. In and along the ocean there are two main source of current energy, marine current and tidal current.
Tidal currents refer to the phenomenon where water from the sea flows into and out of an area of land depending on the time of day. As water moves in and out the movement generates a significant amount of energy. The Rance Tidal Power Station on the Rance River in Brittany, France was the first project in the world to harness the power of tidal currents in 1966. The power station’s installed capacity is 240MW. Tidal projects are usually situated at estuaries and river mouths so the scope for tidal energy generation in South Africa is limited due to environmental regulation and the value and sensitive nature of the estuarine environments along the coast.
A more viable option for South Africa is harnessing the energy from ocean currents. South Africa has two ocean currents that flow up and down the coast, namely the Agulhas (east coast) and Benguela Current (west coast). The Agulhas Current has been identified as one of the five major ocean currents in the world based on the power, speed and consistency at which they flow. Ocean currents flow continuously like large conveyer belts underneath the surface of the ocean over large regions of the ocean. An example of this is shown in the image below. Capturing the energy from ocean currents basically means capturing the kinetic energy from the flow of these currents which in the case of the Agulhas Current can flow as fast as 2.5m/s. Few applications have demonstrated that large amounts of energy can effectively be transmitted onto the grid but a recent project proposed by US-based company HAE off the coast of Durban, South Africa could be one of the first in the world to do so. In general, a number of units have been designed and tested to harness the kinetic energy of the sea ranging from shrouds to turbines.
Other sources for ocean energy
Apart from the above marine sources of energy two other notable sources are OTEC (ocean thermal energy conversion) and salinity gradients both of which have only been tested in laboratory conditions. Ocean thermal energy conversion uses temperature differences between cooler water in the depths and warmer water at the surface to run a heat engine that can produce electricity. Another form of harnessing energy from the ocean is through salinity gradients in the ocean (osmotic or salinity power). This occurs where energy is available through a difference in salt concentration of water e.g. seawater and river water where osmotic pressure is created and converted to electricity. Two methodologies used in this process are reverse electrodialysis (RED) and pressure-retarded osmosis (PRO).