Environmental Engineering Reference
In-Depth Information
(47.7%) and France (42.1%) sharing the bulk of the resource and Eire (7.6%) accounting for
most of the rest.
It has been estimated that exploitation of all practicable estuaries in the UK could lead to
electricity generation of up to around 20% of demand in England and Wales.
2.6.2 Tidal Enhancement
Over much of the surface of the oceans, the
tidal range
(the vertical rise and fall) is rather
small, less than one metre, but in certain places, there is an enhancement of the range.
Enhancement may be due to the following.
Funnelling
The tide is gradually constrained from the sides and so increases in height - or the reverse,
later in the cycle.
Resonance
The estuary has a resonant period equivalent to the tidal period. The length and depth of the
estuary are very important for resonance.
Coriolis Effect
The spinning of the earth leads to the Coriolis effect mentioned earlier with respect to wind.
The tide is infl uenced by this and in some locations tends to increase in height at high tide
and be drawn away from the coast at low tide, with the net effect of enhancing the tidal
range.
These enhancements are highly predictable, allowing the output from a tidal scheme to be
determined many years ahead.
2.6.3 Tidal Barrages
Constructing a barrage across an estuary and allowing tidal waters alternately to fi ll the
estuary through sluice gates and then to empty it through turbines can generate energy. A
barrage constructed across an estuary is equipped with a series of gated sluices and a bank
of low head axial turbines. Where it is necessary to maintain navigation to the upper part of
the estuary, a ship-lock may be required.
Tidal barrages are a currently available technology, but very few exist worldwide. The best
known example is the 240 MW scheme at La Rance in France, and smaller installations have
been made in Nova Scotia, Russia and China. The UK has a number of attractive sites due
to its high tidal ranges, the largest potentials being on the Severn estuary (8600 MW capacity)
and the Mersey (700 MW capacity). However, the scale of these installations and the calcu-
lated long payback periods make the required investments unlikely in the context of the
