It's all there, just waiting to be tapped
Oceans contain 97% of the earth's water and cover 71% of its surface; they are sources of huge kinetic energy from waves and swells, currents and tides, and of thermal energy in the form of the heat they harness from the sun. The difference in salinity between seawater and fresh river water creates a chemical pressure potential (salinity gradient power) that can also be used to generate electricity.
All these approaches could, in theory, provide a sizeable share of the world's energy needs. However, currently they only make up a tiny percentage of the energy extracted from renewable sources. Of this, 90% comes from two tidal range barrages: one in France (240 MW), operational since 1967, the other in the Republic of Korea (254 MW), in operation since August 2011.
Marine energy is still in its infancy, with many technologies still at a research or testing stage aimed at finding the best possible systems for converting the various types of marine energy.
Getting in the motion
Marine kinetic energy is very powerful as the density of water is roughly 850 times that of air.
As it emanates from different sources that vary in their power and predictability, its conversion into electrical energy requires a wide range of devices so as to cover all these aspects.
Waves and swells are generated primarily by winds; they are intermittent and vary in intensity.
Tides are driven by the gravitational pull of the moon, while currents result from the effects of tides as well as from other factors such as the mixing of different water temperatures and degrees of salinity. Both tidal and current resources are more predictable and less intermittent than waves or swells.
The main criteria for selecting the optimum sites at which to tap into marine kinetic energy sources are tidal current velocity, wave formation and turbulence, water depth and bathymetry and access to grid connection.
Wave and tidal converters may be fixed to the seabed, tethered or floating.
On top of the crest
Tapping into wave energy is particularly challenging. Waves are driven primarily by winds that blow across oceans; they combine and continue to gain energy over long, open stretches of water. Some of the best locations for wave energy converters are the Atlantic coastline in Europe and the Pacific coast states in the US.
As wave energy resources are diverse in their nature, it is likely that a number of different devices, rather than a single concept, will be deployed to harness them. These currently include more than half a dozen designs, including:
- Attenuators – long floating structures deployed to catch the waves' motions so as to produce energy
- OWCs (oscillating water columns) – partially submerged structures that can be installed on- or offshore. In OWCs a piston moves up and down with the waves, compressing and decompressing air that is fed into bidirectional air turbines in order to produce electricity
- Overtopping devices that capture water as waves break into a raised storage reservoir. The water is then returned to the sea, passing through a conventional low-head turbine which generates power. An overtopping device may use ‘collectors’ to concentrate the waves’ energy
- OWSCs (oscillating wave surge converters) – general seabed mounted devices that extract energy from the surge motion in the waves
- Point absorbers – floating structures that absorb energy from all directions and convert the motion of the buoyant top relative to the base into electrical power
- Pressure differential devices that capture energy from pressure changes as the wave moves over them
Catching the tide
The technologies used in harnessing tidal and current sources are often similar. Tides and currents are quite predictable and flow in a predictable direction. Tidal and current converters are installed under the surface and include:
- Horizontal and vertical axis turbines – these work in a manner comparable to that of land-based and offshore wind turbines. They are placed in the water; currents or tidal streams cause the rotors to spin around their horizontal or vertical axes and generate power
- Venturi effect devices – systems that funnel the water through a duct, increasing its velocity and driving a turbine to produce electricity
- Tidal kites – these are tethered to the seabed and carry a turbine below a wing. They fly in the tidal stream, swooping in a figure-of-eight shape to increase the speed of the water flowing through the turbine
- Oscillating hydrofoils, which are attached to an oscillating arm and operate like an aircraft wing with the tidal current flowing either side of it creating lift. This physical motion feeds into a hydraulic power system to be converted into electricity
- Archimedes screws, which are corkscrew-shaped devices with a helical surface surrounding a central cylindrical shaft. They draw power from the tidal stream as the water moves up/through the spiral, turning the turbines
Rivers also offer significant potential; certain types of turbines deployed in the marine environment are being installed in rivers.
Hot and cold don't mix
OTEC (ocean thermal energy conversion) uses the temperature difference between cold deep waters and the warmer waters near the surface to run heat engines that produce electricity. OTEC works best when the temperature difference is around 20o C, as is typically found in tropical coastal areas. It can be attractive for small remote island communities who use it to replace diesel generators or to provide air-conditioning and desalinated water.
These applications and others such as aquaculture or the retrieval of cooler water and nutrients from deep waters for a variety of fish and shellfish species can boost the cost effectiveness of OTEC, according to an Indonesian study.
OTEC has a substantial potential; however, what is currently technically recoverable is much less significant. Japan, for instance, assesses OTEC potential in its territorial waters and exclusive economic zone (220 nautical miles or 370 km from its coast) at 904 232 MW, but feasible OTEC potential (i.e. recoverable in a zone 30 km off its coast) at just 5 952 MW. Indonesia estimates its OTEC potential to be 222 GW.
The major capital investment required for OTEC installations may slow down their deployment. Currently only a few sites are operational – most are experimental or pilot projects.
Marine energy conversion is still at an early stage of development and faces a number of challenges. International Standards will prove essential to the expansion of the industry.
IEC TC (Technical Committee) 114: Marine energy – Wave, tidal and other water current converters, prepares International Standards for all these converters. Its work programme includes assessment of various parameters such as resources, performance, measurement and testing.
The future of the marine energy sector does not depend on technological solutions alone, but also on environmental and economic concerns.
The environmental impact of marine energy converters, which may be deployed in sensitive marine environments, must be low. This is the objective of thorough risk assessments that cover various aspects such as the impact turbine blades may have on marine mammals and fish and the effects of the acoustic output of turbines or of changes in water flow and energy removal. The results of surveys so far are encouraging, showing that marine mammals will avoid large, slow moving turbines and that fish are largely unaffected.
However, more research is required and environmental concern may slow, or even prevent, the installation of marine energy converters in certain zones.
As costs for developing technologies are often a matter of concern and uncertainty, marine energy conversion, like other renewable energy sources, will certainly require financial support from governments and interested stakeholders, such as utilities. This support may take the form of direct investment, subsidies, cost-levelling mechanisms or guaranteed feed-in tariffs, as the cost of electricity produced by marine energy conversion initially will be higher than that produced by other means, including well-established renewables like solar and wind. As worries about large subsidies for renewable energies mount, funding may prove an issue in the future.
The overall return of marine energy conversion is likely to translate into large volumes of additional clean energy resources in coming decades. The IEA forecasts that "by 2050 ocean energy will have grown to 337 GW of installed wave and tidal energy capacity", from well under 1 GW today. This expansion will be made possible in no small part by the pioneering standardization work carried out by TC 114.