CENTRE FOR RENEWABLE AND SUSTAINABLE ENERGY STUDIES

OCEAN

MARINE ENERGY

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: temperature- and 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.

ORIGIN 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. The amount of energy transferred, and the size of the resulting waves, depends on the wind speed, the length of time for which the wind blows, and the distance over which it blows, (the “fetch”). In oceanic areas, wind energy is transferred to wave energy and concentrated at each stage in the conversion process, so that original uniformly distributed solar radiation power levels of typically ~ 100 W/m2 of earth surface can be converted to waves with locally concentrated power levels in the order of 10 to 50 kW per meter of wave crest length, (the standard form of measurement) in ocean zones where relative high wave energy occurs.

The global distribution of wave power map and its close up of South Africa as presented in Figure 1 and Figure 2 show that South Africa has a significant offshore wave power resource compared to the rest of the world. Through analysis of wave conditions recorded and modelled along the South African coast the spatial distribution of wave power in the nearshore area is also known and presented in Figure 3and Figure 4 below.

Worls Waves Data
Figure 1: Global distribution of average annual wave power (World Waves data/OCEANOR/ECMWF)

Wave Power Map
Figure 2: Wave power map: Zoom on South Africa (World Waves data/OCEANOR/ECMWF)

Inshore Wave Power
Figure 3: Inshore wave power distribution derived from measured wave data (Retief et al)

Average Wave Power
Figure 4: Spatial distribution of mean annual average wave power of the South African south west coast derived from 10 years of hindcast wave data (Joubert, 2008)


WAVE ENERGY CONVERSION TECHNOLOGY

Mankind has been trying to harness the energy contained in ocean waves for centuries. 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. Due to the diversity of the designs it is very difficult to classify all these devices, but some examples of classification categories include: deployment location, size and orientation or power takeoff (PTO) and conversion principle.

SOME EXAMPLES OF EXISTING WEC TECHNOLOGY ARE PRESENTED BELOW:

OSCILLATING WATER COLUMN (OWC)

An Oscillating Water Column (OWC) WEC type essentially comprises of 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.

The first commercial scale, grid connected wave energy plant, called LIMPET, was commissioned in November 2000 off the Scottish Isle of Islay and is still operational today. The developers of LIMPET have recently incorporated its technology into a breakwater at Mutriku in Spain. Other examples of OWC devices include the Australian Oceanlinx and the Irish floating OE buoy.

1898 patent for a WEC
Figure 5: 1898 patent for a WEC

Wavegen's Mutriku breakwater
Figure 6: Wavegen’s Mutriku breakwater OWC WEC (www.wavegen.co.uk)

Oceanlinx in Port Kembla
Figure 7: Oceanlinx in Port Kembla (www.oceanlinx.com)

OE buoy at sea
Figure 8: OE buoy at sea (www.oceanenergy.ie)

RELATIVE MOTION

BUOY TYPE WECS

Buoy type WEC’s generally consist out 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. Examples of buoy type WEC’s include: PowerBuoy (Ocean Power Technologies) and CETO (Carnegie Wave Energy Ltd).

SURGE DEVICES

Surge type WEC devices are designed to convert the kinetic energy contained in the oscillatory motion of water particles in surge waves. Examples of surge type WEC devices include: Waveroller and Oyster (refer to Figure 10 below).

CETO buoy device
Figure 9: CETO buoy device (www.cweireland.ie)

Aquamarine's nearshore, surge device called Oyster
Figure 10: Aquamarine's nearshore, surge device called Oyster (www.aquamarinepower.com)

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 floats connected by a spine and deck structure on which three wind turbines are mounted.

offshore attenuator device
Figure 11: Pelamis: offshore attenuator device (www.pelamiswave.com)

offshore attenuator device
Figure 12: Pelamis: offshore attenuator device (www.pelamiswave.com)

SOUTH AFRICA'S WEC

South Africa has its very 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. In essence the SWEC’s wave energy conversion principle is a combination of that 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.

Full scale SWEC device
Figure 13: Full scale SWEC device and its operational principle

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 salinity- and 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 out 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.

Marine Current Turbines
Figure 14: Seagen (Marine Current Turbines, www.marineturbines.com)

FUTURE OF THE MARINE ENERGY INDUSTY

Ocean energy has the potential to make a significant contribution to satisfying the ever increasing global demand for energy. At present ocean energy technology is still expensive and relatively unproven, but global driving forces such population growth, need for energy security, technological advances and limited supply- 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.