Energy - ocean

Ocean wave

Marine renewable energy

Marine renewable energy covers a range of technologies including, wave, tidal and ocean thermal energy conversion.

Wave energy

The prime energy source of wave energy is the sun. The sun heats the Earth's surface, the resulting temperature differences create winds and the wind blowing over ocean surfaces creates waves. A number of devices are being developed to convert wave energy into electrical energy. These wave power devices, depending on type, are located off shore, near shore or shore-based. Types include the oscillating water column, hinged contour device, buoyant moored device and overtopping device.

• oscillating water column – this has a chamber with one end open to the sea the other end vented to an air turbine. A rising wave acts like a piston compressing air in the chamber and forcing it out through the turbine. When the wave recedes, air is sucked back through the turbine and into the chamber. The turbine – a Wells turbine – is able to drive an alternator - generating electricity, on both cycles.

• hinged contour device - these devices comprise a series of articulated floating sections moored to the sea bed. Following the contours of the waves, the movement of the hinges - linked to hydraulic rams, drive hydraulic motors which in turn drive alternators to produce electricity.

• buoyant moored device - with these devices, the main structure is anchored to the sea bed whilst a moving part rises and falls with wave motion. Various mechanisms are employed to generate electricity via turbines. These include hydraulic pumps and pumps supplying pressurised seawater to drive the turbines.

• overtopping device - this is a floating device which channels waves up and over a ramp (overtop) into a reservoir area higher than the surrounding sea level. The resulting head of water is then used to drive low head turbines.

Tidal energy

The gravitational pull of the moon, and to a lesser extent the sun, on the Earth gives rise to a bulge in the oceans on the side facing the moon. On the opposite side of the Earth a second bulge is created by the centrifugal force due to the rotation of the Earth. The two bulges are the high tides and the areas between the bulges, the low tides. Tidal energy harnesses the kinetic energy of the ebb and flow of the tides.

• tidal flow turbines - These devices are ideally located where tidal flows or currents are strongest - normally where the current sweeps around headlands or where concentrated by estuaries or islands, etc. Most of these devices resemble wind turbines mounted on the sea bed.

• tidal barrages - exploit the difference in height between high and low tides on river estuaries which experience a sufficiently high tidal range. A dam is constructed across the mouth of the estuary; on the incoming tide, water flows through the dam via sluice gates. As the high tide peaks, the gates are closed holding a head of water (potential energy) behind the barrage. When the tide recedes, the trapped water is allowed to pass back through the dam, the kinetic energy of the moving water turning turbines and generating electricity.

Ocean thermal energy

Covering around 71% of the planet's surface, the oceans are the Earth's largest solar collectors storing energy in the form of heat. The sun's energy, absorbed by the ocean, creates temperature differences between layers of water. Thermal gradients between large masses of water, can be harnessed by ocean thermal energy conversion (OTEC) techniques to produce electricity.

FURTHER INFORMATION

Wave & tidal energy information

  • Danish wave energy server
    Information on wave theory, wave energy articles, wave energy devices, wave energy companies, links to related sites. www.waveenergy.dk/
  • RenewableUK - wave, tidal and wind energy
    (Formerly British Wind Energy Association) Useful resources on UK wind and tidal resources and technologies. www.renewableuk.com/