Marine energy: diverse technologies that harness the power of the sea
Marine energies harness the power of ocean waves, tides, currents and thermal and saline gradients to produce electricity or heat. They are renewable sources that have great potential to be used in combination with wind and solar energy, contributing to the stability of the electricity system and the development of port and coastal economies.
In broad terms, marine energies can be classed as follows:
Wave energy converts wave motion into electricity. The principle is the same in all technologies: the displacement of the waves is transformed into mechanical and then electrical energy. Wave energy devices include oscillating water columns or OWCs (which consists of a chamber where the force of the waves compresses and decompresses air to move a turbine), point absorber buoys and surface attenuators (floating devices that convert the movement of the water particles into rotary or flexing motions) and overtopping systems that lift the water into a reservoir from which it drains out through turbines. Prototypes and grid-connected pilot plants are already in operation, although the technology has not yet reached full commercial maturity. Full roll-out will require robust and reliable designs that can withstand extreme weather conditions, as well as optimised maintenance strategies. Possible applications include hybridisation with offshore wind and the powering of specifically coastal processes, such as desalination.
Tidal energy harnesses the energy from the large-scale movement of the water mass caused by tides. Two systems are being developed: the use of barrages to harness the head (the difference in height between high and low tide), and more importantly, the use of tidal stream generators, either floating or fixed to the sea bed. These are installed in channels or estuaries with sufficiently swift currents. The first commercial multi-megawatt stream-based projects are now being developed, offering stable and predictable year-round generation.
Most potential thermal or saline gradient in the world is concentrated in the tropics and these technologies are still at a preliminary research phase.
Wave and tidal power constitute an abundant and (in the case of tides) a highly predictable resource. However, they are still at pre-commercial stage. The main challenges are to cut investment and operating costs and to improve reliability and durability of the components in the demanding ocean environment. With RDI programmes, sea trials and collaboration with the ship-building/port value chain, these technologies can potentially make an important contribution to a more diversified and resilient energy mix.
