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WAVE ENERGY: THE TECHNOLOGY

The ocean covers an estimated 70% of the earth's surface and provides a vast resource with virtually unlimited untapped energy potential (US Myriad, 2000). Several Ocean Energy Conversion Technologies (OECTs) are at various stages of development ranging from theoretical concepts to fully demonstrated commercial plant, i.e. La Rance. These OECTs comprise various technologies, ie tidal power, wave power, ocean currents, salinity gradients and desalination capability.

The ocean intercepts solar radiation passing through the atmosphere and although a large proportion is re-radiated, a significant portion is retained by seawater particularly in the tropical regions. This consequently heats the upper mixed layer to an average year round temperature of 280C. Ocean thermal conversion technologies (OTEC) indirectly convert this stored energy to electricity. OTEC power cycles function as continuous heat engines driven by the transfer of energy between a thermal source and sink (Takahashi et al., 1997). This sink accepting waste heat is provided by the reservoir of cold water descending from the colder polar regions, flowing along the sea floor toward the equator.

The rise and fall of tides is a predictable and renewable source of energy initially exploited on a very small scale for milling purposes dating back to the twelfth century (Davis, 1991). The best method of operating a tidal barrage is to trap incoming water at high water behind a barrage and release the water by way of horizontal axis turbines, from the basin to the sea during the second part of the ebb tide and first part of the next flood.

Ocean waves are one of the world's most abundant sources of renewable energy, essentially comprising a concentrated form of solar energy. Waves are generated on the surface of the oceans by wind systems, which results from the differential heating of the earth (Whittaker et al., 1997). Wave energy converters generate electricity by driving generators at high rotational speed and make use of air or hydraulic systems often coupled to gearboxes to achieve this (Duckers, 2000).

The two components of energy within waves are potential energy and kinetic energy. Potential energy refers to the form or elevation of the wave, while kinetic energy is associated with the velocity of the water particles within the wave. Ocean wave energy conversion technologies therefore makes use of the kinetic energy trapped within the ocean's waves to produce electricity (Bregman, 1996).

In this respect two broad categories of wave devices are available, ie shoreline devices and offshore devices:

  • Surge devices: they utilise the forward horizontal force of the waves ·

  • Heaving floats: makes use of the vertical motion of relatively small buoys

  • Heaving and pitching floats: absorb energy from heaving and pitching motions

  • Pitching devices: harness energy from the pitching movement of rotary pumps

  • Heave and surge devices: they make use of heave and surge to pump water

  • Oscillating water columns (OWC): involves the conversion of wave induced fluctuations to energy (SEA Technology, 1996).

Simple  Wave Characteristics

  1. Deep Water Waves

    Water waves can be considered to travel along the surface of the sea with an approximate sinusoidal profile. They can be characterised (Southgate, 1981) in terms of the distance between successive crests (the wavelength, l ) and the time between successive crests (the period, T). In deep water these parameters are related as follows:

     

                Eqn. 1

    where g is the acceleration due to gravity.

    The velocity of the waves, C, is given by the following relationship

                        Eqn. 2

    Hence, longer waves travel faster than shorter ones. This effect is seen in hurricane areas, where long waves generally travel faster than the storm generating them and so the arrival of the hurricane is often preceded by heavy surf on the beaches.

     

    The energy, E, and power, P, in such waves can also be described by use of these parameters and the wave height, H:

         Eqn. 3

     

         Eqn. 4

    where r is the density of sea water and E and P are expressed per unit crest length of the wave.

    Most of the energy within a wave is contained near the surface and falls off sharply with depth. Therefore, most wave energy devices are designed to float (or in the case of bottom standing devices to be in shallow water) and so pierce the water surface in order to maximise the energy available for capturing.

     

  2. Shallow Water Waves

As waves approach the shore (i.e. H < l /2) they are no longer considered to be deep water waves and, as such, they can be modified in various ways (McCormick, 1981; Southgate, 1981 and 1987):

  1. Shoaling. The height of a wave varies with the depth of water in which the wave is travelling. In very shallow water this can result in an increase in wave height or shoaling. This results in increased energy and power densities in shallower waters close to shore.

  2. Friction and Wave Breaking. The increase in wave height produced by shoaling can be offset by other mechanisms. As waves become steeper they can break thereby losing both height and energy in turbulent water motion. In shallower areas the water disturbance caused by surface wave motion can extend down to the seabed. In these cases friction between the water particles and the seabed can result in energy loss.

  3. Refraction. As the waves propagate into shallow waters near to the coast, the wave fronts are bent so that they become more parallel to the depth contours and shoreline. Clearly this change of direction is of great importance to those shallow water Wave Energy Devices whose capture efficiency is orientation dependent.

  4. Diffraction and Reflection. The phenomenon of the refraction of sea waves is similar to the optical refraction of light. Other effects analogous to optical behaviour occur such as diffraction (waves bending around and behind barriers) and reflection. All these types of behaviour are dependent on the detailed variation of seabed topography and can lead to the focusing of wave energy in concentrated regions called ‘hot spots’.