Oceans cover more than 70% of Earth's surface; they are sources of huge kinetic energy from waves, currents and tides, and of thermal energy in the form of heat they harness from the sun. They could, in theory, cover a sizeable share of the world's energy needs. However, technical and other issues greatly limit the amount of energy that can actually be recovered from the various marine resources.
Technologies are being developed to find the best possible systems to convert the various types of marine energy into electrical energy.
Tapping marine kinetic resources
Marine kinetic energy is very strong as the density of water is roughly 850 times that of air.
It comes from different sources – waves, tides or current – some more powerful and predictable than others. Therefore, its conversion into electrical energy requires a wide range of technologies to cover all its aspects.
Waves 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.
The main criteria for selecting sites at which to tap into marine energy sources are tidal current velocity, wave formation and turbulence, water depth and bathymetry, and access to grid connection.
Wave and tidal converters can be fixed to the seabed, tethered or floating and rely on different technologies, some of which are still at the research or early development stages.
Riding on 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.
Extraction of wave energy at useful scales is proving challenging owing to the nature and intensity of waves which vary according to the distance from shore and depth of water. A number of devices are being considered to convert wave energy. They include, among others, oscillating devices, floating absorbers that take in wave energy from all directions, pressure differential devices that capture energy from pressure change as the wave moves over them, and overtopping devices that have a wall over which waves break into a storage reservoir where the resulting higher level of water will drive a turbine and generate power when released back into the sea.
Going with the tide
Tides and currents are quite foreseeable and flow in a predictable direction. Many of these devices, as well as wave converters, are being tested at EMEC (the European Marine Energy Centre). Established in 2003 it is the only centre of its kind in the world to provide developers of wave and tidal energy converters with purpose-built open-sea testing facilities spread across the Orkney Islands (UK).
Tidal converters are installed under the surface and include:
- Horizontal and vertical axis turbines that work in a manner comparable to that of land-based and offshore wind turbines. They are placed in the water and currents or tidal streams cause the rotors to spin around their horizontal or vertical axes and generate power
- Venturi effect devices, which are systems that funnel the water through a duct, increasing its velocity and driving a turbine to produce electricity
- Tidal kites that 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
- 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.
The heat is on!
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, typically found in tropical coastal areas.
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 5 952 MW.
OTEC also requires major capital investments and there are few, mainly experimental or pilot projects, in operation. OTEC also has other benefits: it can be used to produce desalinated water, to cool nearby buildings or for aquaculture providing cooler water and nutrients from deep waters for a variety of fish and shellfish species.
TC 114 has set up a Project Team to look at guidelines for design assessment of OTEC systems.
Marine energy conversion is still at an early stage of development and faces a number of challenges. Its future does not depend on technical questions alone, which are the primary focus of IEC TC 114 work, but also on a number of environmental, political and economic issues.
The environmental impact of marine energy converters, which may be deployed in sensitive marine environments, must be low. It is the object of thorough risk assessments that cover various aspects such as the effects of turbine blades on marine mammals and fish, the effects of acoustic output of turbines or changes in water flow and energy removal. Results of some of these surveys so far are encouraging, but more research is required and environmental concern may slow down, or even prevent the installation of marine energy converters in certain zones.
Political obstacles may include the location of installations, particularly when these are in overlapping or disputed zones, and issues related to funding. As costs for developing technologies are often a matter of concern and uncertainty, marine energy conversion will certainly require, like other renewable energy sources, financial support from governments and interested stakeholders, such as utilities. This support may take the form of direct investments, subsidies, cost-levelling mechanisms or guaranteed feed-in tariffs as the cost of electricity produced by marine energy conversion will be initially higher than that produced with other means, including well-established renewables like solar and wind.
The overall return of marine energy conversion in terms of large volumes of additional clean energy resources suggests that a number of governments and other stakeholders are likely to provide substantial support to the sector, as can already be observed in many countries and regions, and as was the case for solar and wind energy projects before.