The hidden power of the oceans

Potential contribution to the energy mix still unknown

There is a growing consensus as to the degree renewable energy sources will be able to contribute to the energy mix in the next 20 years. However there is one element that may be underestimated; the power of the oceans.

Approximately two-thirds of the earth is covered by oceans and seas. This large surface area captures a huge amount of solar heat naturally. This results in a thermal gradient between the top and the bottom of the oceans which can be harvested in a variety of different ways for the generation of electricity. Local variations in sunlight lead to the geographical temperature differences that power ocean currents and the displacement of air. Those can be harvested by marine current power stations and off-shore wind farms. The wind also creates waves which can be used to generate electricity. Finally, the gravity of the moon moves the sea causing tidal variances; movements that can be harvested by tidal power stations.

The combined potential of all those generation systems in the coming decades is still largely unknown.

Offshore wind power

Offshore wind farms are already big business around the world. If Europe is to maintain its present pattern of continuous growth in wind energy over the coming decades, it will have to rely mainly on off-shore wind farms. Many offshore wind farms are already operational and an even larger number of projects are under construction, in the design phase, or under consideration.

Tidal power

The commissioning of the first commercial tidal power plant, at La Rance in France, dates back to 1968. Although the technology is easy and proven, the hoped-for breakthrough did not occur. This was primarily due to its high capital cost and long pay-back period. But given today’s energy context, the long delayed breakthrough could very well happen quite soon. Large-scale projects are being considered (see table) in Argentina, Canada, India, Russia, the UK, and USA. Some novel technologies are being suggested in order to reduce the high capital costs. Those include compressed air to drive air turbines instead of conventional hydro turbines; using magneto-hydrodynamic generators for direct conversion of the energy of a tidal current into electricity; and replacing a conventional rigid dam with a flexible barrier to concentrate the tidal current utilized by the two previous possibilities.

Ocean wave power

Various technological concepts to harvest wave power exist. Some of them are being proved in operational prototypes; others are still in a research or development phase. This year, the first commercial wave power plant will go into operation in northern Portugal (see blog post). It makes use of the Pelamis power generating device built by Ocean Power Delivery in Scotland.

The technologies to harvest wave power can be grouped as follows:

• Tapered channel or reservoir systems. These are shoreline devices that drive the waves into an elevated reservoir. Water flow out of this reservoir is used to generate electricity, using a standard hydroelectric turbine.
• Oscillating column systems, using air pressure generated by the wave movement to drive turbines. This must be used on or near the shore.
• Fixed or semi-fixed offshore devices which make use of the pressure differential in the water column that occurs at some particular submerged point as the wave passes over that point. The pressure differential is used by a variety of means to cause a fluid to flow in a circuit, which is then used to drive a turbine and generator.
• Offshore devices which utilize their buoyancy to cause movement in a part of the device as it moves up and down in the wave. This movement can then be used directly in a linear generator, or indirectly through a hydraulic system and a turbine such as in the Pelamis device.
• Piezoelectric systems

Marine current power

Harvesting the energy of marine currents is still at an early stage of development. There are no commercial grid-connected turbines currently operating, and only a small number of prototypes and demonstration units have been tested. The most likely type to be commercially developed is submerged water turbines similar to wind turbines, though various alternative designs have been proposed as well.

Ocean Thermal Energy Conversion (OTEC)

OTEC makes use of the temperature difference between the warm surface water and the cold deep water to drive a heat engine. For the system to be efficient, a minimum temperature difference of 20°C is required, which restricts the technology to use in tropical oceans. OTEC is particularly promising as an alternative energy resource for tropical island communities that rely heavily on imported fuel. The Sea Solar Power Company has designed an OTEC system that is said to be economically efficient. A prototype of this system has been installed in Hawaii.

References

• The National Renewable Energy Centre (USA) on OTEC
• A market overview of ocean power systems on EcoBusinessLinks
• Article on the ocean energy permanent magnet linear generator buoy, in ScienceDirect
• The Offshore Alternative Energy Guide of the US Department of the Interior, Minerals Management Service (MMS)