Marine Energy
Author: Centre for Renewable and Sustainable Energy Studies - Stellenbosch University
( Article Type: Explanation )
The oceans of the world are a huge source of untapped energy that, if properly harnessed, could greatly contribute to the ever increasing global energy demand. The ocean contains energy in various forms a few of which include: temperatureand salinity gradients, tides, tidal streams, ocean currents and waves. Due to South Africa’s close proximity to the storm generation zones in the south Atlantic ocean and the Agulhas ocean current, the country as an abundant resource of wave- and ocean current energy.
Origins of wave power
Wave energy is an indirect result of solar radiation. Winds are generated by the differential heating of the earth’s atmosphere, and as they blow over large areas of ocean, part of the wind energy is converted to water waves.
Wave energy conversion technology
The first patent for a wave energy converter or WEC dates back to the start of the 19th century. Today there are hundreds if not thousands of different devices at various stages of development
Some examples of existing WEC technology are presented below:
Oscillating Water Column (OWC)
An Oscillating Water Column (OWC) WEC type comprises a partly submerged structure, open below the water surface, inside which air is trapped above the free water surface. Incident waves cause the water surface to oscillate, and the air can be channelled through a bi-directional turbine to drive an electric generator. Relative motion Buoy type WECsBuoy type WEC’s consist of a free floating buoy and a power take off system. Incident waves displace the floating buoy relative to the power take off system and the potential energy in the vertical displacement of the wave is converted to electricity.
Surge devices
Surge type WEC devices are designed to convert the kinetic energy contained in the oscillatory motion of water particles in surge waves. Pelamis The Pelamis WEC is a floating device consisting of four tubular sections connected at three hinges. These tubular sections move relative to each other as a wave propagates along it and power is generated through a digitally controlled hydraulic power conversion system.
Hybrid systems
Floating Power Plant is developing a hybrid device called the Poseidon which consists of a multiple fl oats connected by a spine and deck structure on which three wind turbines are mounted.
South Africa’s WEC
South Africa has its own WEC device specifically designed to operate optimally in the local wave conditions. The
device is called the Stellenbosch Wave Energy Converter or SWEC and consists of series of OWC chambers coupled in a V-formation. The SWEC is fully submerged and founded on the sea floor making it a robust structure with very few moving parts. The SWEC’s wave energy conversion principle is a combination of an OWC and an attenuator device such as the Pelamis. It forms a closed loop air pump system, providing smooth airflow that can be fed to a conventional air turbine. In order to demonstrate the technical feasibility of the SWEC in the ocean it is proposed to incorporate the SWEC into a coastal structure such as a breakwater. Such a configuration is currently being investigated at Stellenbosch University.
Origins of ocean current energy
Ocean currents are generated by forces (such as the rotation of the planet, the gravitational pull of the moon and salinityand temperature differences) acting on the water body of the ocean. The strength and direction of ocean currents are influenced by local conditions such as water depth, the coastline and other ocean currents. Devices designed to harness ocean current energy generally consist of axial rotors which drive a turbine through a gearbox similar to wind turbines. The energy potential in ocean currents is far greater than wind energy due to the greater density of seawater compared to air.
Future of the marine energy industry
Ocean energy has the potential to make a significant contribution to satisfying the global demand for energy. At present ocean energy technology is still expensive and relatively unproven, but globally, driving forces such population growth, need for energy security, technological advances and limited supply of and the negative impacts of fossil fuels will see this free, non-polluting energy source and its utilisation play a more prominent role in the world’s energy supply in the near- to long-term future.
