ABSTRACT
The level of water in the large oceans of the Earth rises and falls according to predictable patterns. The main periods of these tides are diurnal at about 24 h and semidiurnal at about 12 h 25min. The change in height between successive high and low tides is the range, R. This varies between about 0.5m in general and about 10m at particular sites near continental land masses. The movement of the water produces tidal currents, which may reach speeds of ∼5ms−1 in coastal and inter-island channels. The seawater can be trapped at high tide in an estuarine basin of area
A behind a dam or barrier to produce tidal range power. If the water of density runs out through turbines at low tide, it is shown in Section 13.5 that the average power produced is
P¯ = AR2g/2 For example, if A = 10km2, R = 4m, = 12h 25 min, then P = 17MW. Obviously sites of large range give the greatest potential for tidal power, but other vital factors are the need for the power, and the costs and secondary benefits of the construction. The civil engineering costs charged to a tidal range power scheme could be reduced if other benefits are included. Examples are the construction of roads on dams, flood control, irrigation improvement, pumped water catchments for energy storage and navigation or shipping benefits. Thus the development of tidal power is very site-specific. The power of tidal currentsmay be harnessed in a manner similar to wind
power; this is also called ‘tidal stream power’. It is shown in Section 13.4 that the average power per unit area q in a current of maximum speed u0 is q ∼ 01u30. For u0 = 3ms−1, q ∼ 14kWm−2. In practice, tidal current is likely to be attractive for power generation only where it is enhanced in speed by water movement in straights between islands and mainland, or between relatively large islands. Therefore the opportunities for viable commercial sites are unusual. Where it is possible however, much of the
discussion concerning the use of the power is in common with tidal range power. We may also note that the flow power in a river has similar characteristics, but without the temporal variation. Harnessing river-stream power in the same manner as tidal stream power is certainly possible, but seldom considered. The harnessing of tidal range power (henceforward called ‘tidal power’ as
distinct from ‘tidal current/stream power’) has been used for small mechanical power devices, e.g. in medieval England and in China. The best-known large-scale electricity generating system is the 240MWe ‘La Rance’ system at an estuary into the Gulf of St Malo in Brittany, France, which has operated reliably since 1967, thereby proving the technical feasibility of this technology at large scale. However, economic and environmental constraints have meant that very few similar systems have been constructed since; see Section 13.6. Other sites with large tidal range, such as the Severn estuary in England and the Bay of Fundy on the eastern boundary between Canada and the United States, have been the subject of numerous feasibility studies over the past hundred years. The range, flow and periodic behaviour of tides at most coastal regions
are well documented and analysed because of the demands of navigation and oceanography. The behaviour may be predicted accurately, within an uncertainty of less than ±4%, and so tidal power presents a very reliable and assured form of renewable power. The major drawbacks are:
1 The mismatch of the principal lunar driven periods of 12 h 25min and 24 h 50min with the human (solar) period of 24 h, so that optimum tidal power generation is not in phase with demand.
