Near-shore devices

Near-shore devices are situated in shallow waters (typically 10 to 25 m water depth). Again the OWC is the main type of device, with several designs - such as the OSPREY (Ocean Swell Powered Renewable EnergY) - having been deployed worldwide. It is a steel OWC designed for bottom mounting in 14-m water depth at Dounreay in the North of Scotland and expected to generate 2 MW of electric power from four Wells turbines, while it could act as a support structure for a 1.5 MW wind turbine.

Another 60 MW floating OWC, the Mighty Whale, is to be deployed in the seas to the east of Japan in the near future (if the device achieves its predicted performance and reliability, it is intended that this will be the start of a large, national deployment programme). A floating OWC known as the Backward Bent Duct Buoy was deployed in Japan in 1988. This is similar to a conventional OWC but the opening faces towards the shoreline.

Offshore devices

Offshore devices exploit the more powerful wave regimes available in deep water (typically more than 40 m water depth). In order to extract energy from the waves, the device needs to be at or near the water surface and requires flexible moorings and electrical transmission cables. Several methods have been proposed to convert the oscillating motion of the body into useful mechanical energy and prototypes of some of these concepts have been deployed, such as:

ˇ         The Swedish Hose-pump consists of a specially reinforced elastomeric hose (whose internal volume decreases as it stretches), connected to a float, which rides the waves. The rise and fall of the float stretches and relaxes the hose thereby pressurising seawater, which is fed (along with the output from other Hose-pumps) through a non-return valve to a central turbine and generator unit.

   

 

ˇ         The McCabe Wave Pump consists of three rectangular steel pontoons, which move relative to each other in the waves. The key aspect of the scheme is the damper plate attached to the central pontoon, which ensures that it stays still as the fore and aft pontoons move relatively to the central pontoon by pitching about the hinges. Energy is extracted from the rotation about the hinge points by linear hydraulic pumps mounted between the central and two outer pontoons near the hinges. The device was developed to supply potable water (by reverse osmosis) but can also be used to generate electricity (via a hydraulic motor and generator).

   

 

ˇ         The Floating Wave Power Vessel is a steel platform containing a sloping ramp, which gathers incoming waves into a raised internal basin. The water flows from this basin back into the sea through low-head turbines. In these respects it is similar to an offshore Tapchan but the device is not sensitive to tidal range.

ˇ         The Danish Wave Power float-pump device uses a float, which is attached to a seabed mounted piston pump. The rise and fall motion of the float causes the pump to operate driving a turbine and generator mounted on the pump. The flow of water through the turbine is maintained as unidirectional through the incorporation of a non-return valve.