Energy resources: tidal energy
Energy resources: tidal energy

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1 Energy resources: Tidal energy

1.1 Tidal energy

The rise and fall of ocean tides result from the combined gravitational pull on water by the Moon and, to a lesser extent, by the Sun, which exerts a force on water directed towards the two astronomical bodies. These gravitational effects combine with centrifugalo forces that result from the Earth and the Moon orbiting each other to make the details of tidal changes complex. The physics of this second contribution to tides is beyond the scope of this course.

Tides are not completely synchronized with the position of the Moon and Sun relative to the Earth because of viscous drag. Figure 1 shows how the gravitational force exerted by the Sun and Moon together affects tidal range, according to phases of the Moon. Tides are a great deal more complicated as regards their driving forces than the effects that we notice. However, the forces are enormous and serve to generate the energy bound up with the ebb and flow of the tides.

Figure 1
Figure When Earth, Moon and Sun are aligned, at full and new Moons, the tides have their highest range (spring tides). At half moons the tidal range is at its lowest (neap tides).

Globally, total tidal power has been estimated to be 2.7 TW. Far out at sea, the tidal range is not as great as that experienced in the UK, and is generally no more than 0.5 to 1.0 m. Where coastlines face the open ocean, there are two tidal cycles per day, separated by approximately 12 hours 25 minutes. Complex coastlines, such as those around the UK, and where narrow seaways constrict tidal flow, result in modifications to both the timing of tides and their range. These can result in bizarre modifications to the tidal cycle; for instance, the Solent has four noticeable tides each day. Similarly, coastal tide ranges vary markedly around the land, rising to more than 15 m in a very few places, such as the Bay of Fundy in Newfoundland. The higher the tidal range the greater the potential for electricity generation; tidal range is akin to the working head of hydropower schemes.

Figure 2 expresses available tidal power in terms of kW m−2 (i.e. the power per unit area of the sea surface) around the coast of the UK. The greatest potential coincides with narrow tidal straits, such as the Pentland Firth, constricted estuaries, such as that of the Severn, and areas where major tidal flows meet, as in the southern North Sea and the English Channel. The highest energies are associated with the many narrow channels separating the islands of the Inner and Outer Hebrides, which are notorious for their tidal rips and whirlpools. Because of its complex tidal flow at the margin of the North Atlantic, the UK coastline focuses about half the total tidal energy potential of Europe. This is estimated to be in the region of 53 TWh yr−1; potentially about 14% of total UK energy needs.

Figure 2
Figure 2 Potential tidal power per unit area around the UK coast.

Potential is one thing, but being able to harness it is quite another matter. The key factor, as well as tidal power per unit area, is the volume of tidal flow that can be constrained through turbines. Figure 2 shows one location that offers major possibilities, the Severn Estuary. In more detail, two other west-coast estuaries would also seem to be possible sites, the Solway and Morecambe Bay. The very high power potential along the intricate coastline of NW Scotland is associated with dangerous tidal currents, and presents construction problems that cannot be overcome by modern civil engineering practices.

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