Tidal Energy

Tidal energy can be divided into two distinct forms:

1. Tidal rise and fall

2. Tidal streams or currents

26.3.2.1 Tidal Rise and Fall

Tidal rise and fall energy is potential energy derived by height changes in sea level, caused by the gravitational attraction of the moon, the sun, and to a lesser extent other astronomical bodies, on oceanic water bodies. The effects of these tides are complex and most major oceans and seas have internal tidal systems, called am-phidromic systems (Fig. 26.2). Each major ocean has its own internal circulation system, called 'gyres', which rotate anticlockwise in the northern hemisphere and clockwise in the southern hemisphere.

There are a number of different tidal components, which operate at different phases (called Kelvin waves), the largest being the M2 component, due to the moon. Tidal range energy can be best harnessed nearshore, particularly in estuaries, where tidal rise and fall can be amplified as coastal waters shallow towards the coast.

26.3.2.2 Tidal Stream Energy

The movement of ocean water volumes, caused by the changing tides, creates tidal stream energy. Kinetic energy can be harnessed usually near shore or particularly where there are constrictions, such as straits, islands and passes.

Tidal stream energy results from local regular (diurnal at roughly 24 h periods and semi-diurnal at 12 h 25 min periods) flows caused by the tidal cycle. Spring tides occur when the gravitational attraction of the sun and moon act in the same

GOT4.7

NA5A/GSFC

GOT4.7

NA5A/GSFC

Tide Current Map

0 Id 20 JO 10 SO 60 TO SO 90 100 110 120 130 cm

Fig. 26.2 Global distribution of tidal amplitude, i.e., M2 tidal constituent. (Ray 2007)

0 Id 20 JO 10 SO 60 TO SO 90 100 110 120 130 cm

Fig. 26.2 Global distribution of tidal amplitude, i.e., M2 tidal constituent. (Ray 2007)

direction, whilst neap tides result when the attraction of the sun and moon operate in opposite directions. These tides are forecastable on an 18.5-year cycle. These tides cause kinetic movements, which can be accelerated near coasts, where there is constraining topography, such as straits between islands (Soerensen and Weinstein 2008).

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