Ocean energy technology

Ocean energy is the power derived from the movement of the water, either tides, currents or waves or the temperature of the water (thermal energy). 

One of the benefits of ocean energy over other renewable energy sources is it easier to forecast and has greater availability (tides are predictable over all time-frames, and waves have a forecast horizon up to three times more than wind).

Ocean energy technologies include:

Tidal: In recent years, new technologies have been developed to extract tidal energy, they consist of tidal turbines which can be located individually or in an open fence configuration. These technologies do not restrict the flow of water, and thus have a much reduced environmental impact.

Wave: There are three classes of wave energy converters, which can harness the power of the waves in different ways:

• Point absorbers have a float that is small compared to the swell wavelength. The float is free to follow the movement of the wave and accept wave energy from any direction. The float can be tethered so that it is submerged and moved by the pressure of the wave passing overhead, or it can float on the surface and track or ‘heave’ with the movement of the sea surface. 
• Linear attenuators also incorporate a float, or a number of floats that are shaped or distributed to be aligned in the direction of wave travel. Their overall length may be large compared to the swell wavelength. However, they are also wavelength dependent. Unlike a point absorber they need to be slack moored so that they can turn to maintain their principal axis normal to the oncoming waves. 
• Terminators are designed to collect energy from waves by directly facing them. They may include passive devices such as a tapered channel to focus energy from a wide section of wave front.

Thermal: Ocean thermal energy operates by using warm sea water to vaporise a working fluid so that the relatively high pressure vapour can turn a turbine. It is also possible to use warm sea water as the working fluid and this is known as an open cycle system. A fraction of the warm sea water is evaporated by passing it through jets into a chamber with a pressure lower than the saturation pressure for the sea water temperature.

Currents: Ocean water is in continuous motion and this motion can be captured much like wind currents are with wind turbines. Technologies that would be used to extract energy from ocean currents would be similar to those used for extracting tidal energy, i.e. turbine-type devices that would be moored to the sea floor and would be free to move with the direction of the current. Extraction of ocean current energy is at the very early stages of development, as the resources and devices require much greater characterisation.