Energy can be extracted from the ocean in various forms: from swells (waves), from water movement caused by tides or ocean currents, from tidal rise and fall, and from temperature differences (ocean thermal energy conversion or OTEC).

Wave power

The shear force of wind on the ocean surface causes ocean swells, which can travel hundreds or even thousands of kilometres with very little energy loss. Waves are therefore effectively concentrated wind energy. The power in waves increases with the wave height squared and the time between successive wave crests.

Resource rich wave power locations

Wave power systems around the world have been designed to work both near the shore and offshore. The available power is reduced by bottom friction in shallow water so there is more power at offshore locations, but the cost of constructing offshore platforms and cabling to get the power to shore can offset the advantages. West coasts exposed to prevailing winds and ocean swells in mid latitudes, e.g. Scotland, Ireland, Portugal, western Canada, southwest Africa, Chile and southern Australia and New Zealand provide the best sites for wave power.

Technologies

There are a range of wave power systems under development or in use around the world. One of the best known is the oscillating water column (OWC), in which waves surge up inside a chamber which is open at the bottom, forcing air out through an opening near the top (like a blowhole) with a turbine in it. The turbine turns, driving a generator. As the wave recedes, air is sucked back in, again driving turbine and generator. The turbine is designed to turn in the same direction on both the surge and the ebb of waves. In Australia, Oceanlinx has developed and tested an innovative variable pitch air turbine and a reflecting wall to focus waves on an OWC at Point Kembla.

There are various floating or submerged devices which heave (move up and down) and/or pitch (rock backwards and forwards) as waves pass. There are also submerged devices which harness the changing pressure as waves crest and troughs pass over them. In most cases the motion drives a hydraulic pump or a linear electrical generator or other means to extract power. In Western Australia, Carnegie Corporation’s CETO is an example of a heaving and pitching device. Fully submerged devices are less likely to be damaged by storms but are less easily accessed for maintenance than shoreline or floating devices.

Overtopping devices such as the Norwegian shoreline Tapchan and the Danish/Irish floating Sea Dragon allow waves to surge up a sloping intake and fill a reservoir above sea level, then drain back via a turbine into the ocean.

Marine current energy

Ocean currents such as the Gulf Stream and the East Australian Current contain huge amounts of kinetic energy which could in principle be harnessed using turbines resembling underwater wind turbines. However these currents generally flow too slowly and are too variable to be viable sources of power at present.

Tidal Power

The gravitational pull of the moon and sun causes water to flow resulting in varying surface levels at most coastal locations, usually producing two high tides and two low tides per day. This tidal rise and fall can be many metres in some locations where water flows into and out of an estuary. By building a barrage across such an estuary, a difference in elevation between the open sea and the water inside the barrage can be created, which can be used to drive turbines. More recent tidal power developments use the kinetic energy of the moving water to drive turbines directly, in the same way as wind turbines use the kinetic energy of moving air. These turbines, known as ‘hydrokinetic turbines’, can also extract power from ocean currents and rivers and require no barrages or dams.

Technologies

Barrage schemes such as the Rance River estuary, the Severn Estuary, the Bay of Fundy in eastern Canada, and the Kimberley in Western Australia, which works on a difference in elevation

Hydrokinetic turbines work like underwater windmills and are turned directly by flowing water without the need for a barrage. These may be axial flow like a stubby version of a normal wind turbine, or cross flow like a Darrieus wind turbine.

Tidal current power technology operates on similar principles to wind power, so designers can learn from wind turbine experience

OTEC (Ocean Thermal Energy Conversion)

The temperature of deep ocean water (DOW) is about 4°C, while surface water in the tropics is warm at approximately 27°C. The warm surface water can be used to evaporate a volatile working fluid such as ammonia and drive a turbine similar to a steam turbine in a conventional power station, but at a much lower temperature and lower efficiency. The cold DOW is then used to condense the working fluid. If brought to the surface, this DOW can supply nutrients and support a marine food chain, thus enhancing fish stocks. Unlike wind, wave and tidal energy, OTEC offers continuous power, and as with ocean currents, offers a huge amount of energy. However it is very low grade energy and the cost of the technology to harness it is huge. Prototype OTEC plants have been built but to date there are no full sized installations.

Advantages and challenges

Wind, wave and tidal power all have the advantage of free, non-polluting energy with minimal environmental impact, but all suffer from fluctuating supply and expensive hardware to capture the energy. Wind power technology is more advanced and less expensive today, however just as has happened with wind, it is expected that wave and tidal power costs will reduce as the technology is refined.

Further reading

Boyle, G. (2004). Renewable energy: power for a sustainable future. Oxford University Press/The Open University, Milton Keynes, UK. Ocean power fact sheet: http://cleanenergycouncil.org.au/info/index.php