TIDAL ENERGY

Tides are caused by the gravitational attraction of the moon and the sun acting upon the oceans of the rotating earth. The relative motions of these bodies cause the surface of the oceans to be raised and lowered periodically, according to a number of interacting cycles. These include:

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a half day cycle, due to the rotation of the earth within the gravitational field of the moon
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a 14 day cycle, resulting from the gravitational field of the moon combining with that of the sun to give alternating spring (maximum) and neap (minimum) tides
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a half year cycle, due to the inclination of the moon's orbit to that of the earth, giving rise to maxima in the spring tides in March and September
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other cycles, such as those over 19 years and 1 600 years, arising from further complex gravitational interactions.

The range of a spring tide is commonly about twice that of a neap tide, whereas the longer period cycles impose smaller perturbations. In the open ocean, the maximum amplitude of the tides is about one meter. Tidal amplitudes are increased substantially towards the coast, particularly in estuaries. This is mainly caused by shelving of the sea bed and funneling of the water by estuaries. In some cases the tidal range can be further amplified by reflection of the tidal wave by the coastline or resonance. This is a special effect that occurs in long, trumpet-shaped estuaries, when the length of the estuary is close to one quarter of the tidal wave length. These effects combine to give a mean spring tidal range of over 11 m in the Severn Estuary (UK). As a result of these various factors, the tidal range can vary substantially between different points on a coastline. The amount of energy obtainable from a tidal energy scheme therefore varies with location and time. Output changes as the tide ebbs and floods each day; it can also vary by a factor of about four over a spring-neap cycle. Tidal energy is, however, highly predictable in both amount and timing.

The available energy is approximately proportional to the square of the tidal range. Extraction of energy from the tides is considered to be practical only at those sites where the energy is concentrated in the form of large tides and the geography provides suitable sites for tidal plant construction. Such sites are not commonplace but a considerable number have been identified in the UK, France, eastern Canada, the Pacific coast of Russia, Korea, China, Mexico and Chile. Other sites have been identified along the Patagonian coast of Argentina, Western Australia and western India.

Tidal energy can also be exploited directly from marine currents induced by the combined lunar and solar gravitational forces responsible for tides. These forces cause semi-diurnal movement in water in shallow seas, particularly where coastal morphology creates natural constrictions, for example around headlands or between islands. This phenomenon produces strong currents, or tidal streams, which are prevalent around the British Isles and many other parts of the world where there are similar conditions. These currents are particularly prevalent where there is a time difference in tidal cycles between two sections of coastal sea. The flow is cyclical, increasing in velocity and then decreasing before switching to the opposite direction. The kinetic energy within these currents could be converted to electricity, by placing free standing turbo-generating equipment in offshore areas.
Different technical concepts for exploiting tidal energy

Most countries which have investigated the potential exploitation of tidal energy have concentrated on the use of barrages to create artificial impoundments that can be used to control the natural tidal flow. Barrage developers in the UK and elsewhere concluded that building a permeable barrage across an estuary minimises the cost of civil structures for the quantity of energy that can be realistically extracted. Construction of barrages across estuaries with high tidal ranges would be challenging but technically feasible. In shallow water armoured embankment would be used, but in deeper water this method would be impractical and too expensive because of the quantity of material required. Complete closure of estuaries would be achieved by emplacing a series of prefabricated sections, or caissons, made from concrete or steel which could be floated and then sunk into position. The technique has been used in the Netherlands to close the Schelde Estuary. A large steel caisson was used in the construction of the Vadalia power station on a tributary of the Mississippi.

Tidal barrages would comprise sluice gates and turbine generators. Large scale structures like the Severn Barrages would also include blank caissons and ship-locks. During the ebb tide water is allowed to flow through the sluices and the turbine draft tubes to ensure the maximum possible passage of water into the impounded basin. At or close to high water the sluice gates are closed. At this stage of the cycle the turbines can be used in reverse as pumps to increase the amount of water within the basin. Although there is an obvious energy demand, the amount of water transferred can provide an additional increase in energy output of up to 10% compared with a cycle where no pumping is used. The actual increase in energy output from pumping depends on the estuary and the tidal conditions.