Perhaps the most widely tested of shore and near-shore devices is the oscillating water column. If a tube, sealed at one end, is placed so that its open end is just beneath the surface of the sea, as waves pass the tube, the level of water inside the tube will rise and fall, alternately compressing and expanding the air column within the tube.
If, instead of a seal, the upper end of this tube is open and houses a device that acts like a wind turbine, then the moving water will cause the air to move in the tube and this will make the turbine rotor turn backwards and forwards. This air movement forms the basis for an oscillating water column wave energy converter.
Oscillating water columns can either be shore or bottom mounted. They normally comprise some form of concrete structure, which is designed to create an enclosure containing air, which is open to the sea at the bottom. A special type of turbine called a wells turbine is frequently mounted at the top. This can derive continuous power from movement of air both up and down without the need for a complex arrangement of valves.
Oscillating water columns have been tested in many parts of the world including Europe and Japan. The wells turbine, developed in the 1970s, has most usually been employed in these prototypes but newer bidirectional turbines with greater efficiency are under development. The economics of oscillating water column converters and of other shoreline devices can be improved if they are built into breakwaters.
Another approach to wave energy conversion uses and amplifies the height of a wave in order to create a head of water which can be used to drive a conventional low-head hydro turbine. Devices like this usually employ a tapered channel with its mouth open to the sea. The side walls of the channel rise above the normal sea level and beyond them is a reservoir.
Waves travelling towards the coast are focussed into the channel. As these waves flow along the channel they become more and more restricted by the taper and this forces the height of the wave to increase, until water starts to fall over the upper edges of the channel walls. This water is captured, creating a reservoir of water, which is above the sea level. This water can then be run back into the sea through a hydro turbine, generating electricity.
A system of this type, called tapchan, was built on the Norwegian coast in the late 1980s. The technology is relatively simple but construction costs can be high. Deployment is restricted by the need for a relatively low tidal range. Otherwise the operation of the converter is compromised.
The energy contained in moving waves is sufficient to cause a pendulum or flap to move backwards and forwards, and this too has been used as the basis for a shore-based wave energy convertor. The best known converter of this type is a Japanese device called the pendulor which comprises a box, open to the sea on one side, but with the open side closed using a flap hinged horizontally from the top. When waves strike the flap they cause it to oscillate to and fro like a pendulum and this motion can be converted into electricity using hydraulic9 rams. Small devices of this type have been built and tested and a plant of around 200 kW has been designed for a site in Sri Lanka.
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