The three types of device discussed above can all be exploited offshore provided they can be moored so that they remain stationary relative to the waves. However most offshore devices try to exploit the wave motion in different ways.
The hosepump, developed in Sweden, is based on an elastic tube that changes its internal volume as it is stretched. One end of the tube is sealed and attached to a float while the other end is open and, is connected to a moored plate close to the bottom of the sea. As waves pass the device, the float moves up and down, alternately stretching and relaxing the tube. This pumps water in and out of the lower end of the tube and the pressurised water is used in a hydraulic energy conversion system to generate electricity.
A wave power float pump developed in Denmark takes a slightly different approach. In this case a float at the surface is attached to a rod, which bears in turn on a shaft (like a crankshaft), attached to a piston-pump device. Movement of the float up and down causes the rod to rotate the (crank) shaft, turning the vertical motion into rotary motion from which electricity can be extracted, exactly as in a piston engine.
The Archimedes Wave Swing, developed in the Netherlands, adopts a similar principle but the movement up and down of a buoyant floater is converted into electricity by means of a linear generator.
2. Ducks, wave pumps and other water snakes
There are a number of wave energy devices designed in hinged sections which all float. As waves pass these devices, the different sections move relative to one another and this differential motion is used to derive hydraulic energy, which is then converted into electricity.
The first of these is called Salter's Duck after British designer Stephen Salter. The prototype appeared in the 1970s but the concept is still under development. The duck has a beak-shaped float, which is fixed by a hinge to a second anchored section. The beak moves in the waves relative to the anchored spine and this relative motion is used to extract energy.
The McCabe wave pump comprises three rectangular pontoons connected through hinges. The central pontoon has a damper plate attached to it, which slows its vertical motion relative to the two outer pontoons. This generates relative motion between the three sections, which again can be exploited to generate electricity.
The Pelamis looks like, and is named after a sea snake. It comprises a series of buoyant cylindrical sections joined end to end. The device is tethered by one end. As waves pass along it, the sections move relative to one another and hydraulic cylinders extract energy from this motion. A 750 kW prototype is under construction.
A US programme is developing a wave energy converter, which is based on a piezoelectric material in sheet form. The device, called the eel, can produce an electric current when bent by waves. It is still in a very early stage of development.
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