Environmental Engineering Reference
In-Depth Information
Water level in basin
5
-5
7
6
5
4
3
2
1
1
2
3
4
5
6
7
Time/days
Figure 2.18 Water level and electrical power output of the proposed Severn Barrage over a spring-
neap tide cycle. (Source: the Watt Committee on Energy now disbanded. Reproduced with permission
of Oxford University Press)
Power Variability
One of the main disadvantages of tidal schemes is the pulsed nature of their electrical output.
Figure 2.18 shows the expected electrical output from the 7 GW scheme proposed for the
Severn estuary assuming ebb generation only.
Power is generated for fi ve to six hours during spring tides and for three hours during neap
tides. This pattern may, but is unlikely to, match the pattern of demand in an integrated power
system. On the positive side the power availability from such a scheme is highly predictable.
In a large integrated system such as the UK's this predictability will ease the task of schedul-
ing alternative generation to take up the demand when the output from the scheme falls to
zero.
2.6.5 Tidal Current Schemes
The direct production of electricity from tidal streams is a relatively new concept. It relies
on a different approach to conventional tidal barrage schemes and is attractive in that it does
not require such massive infrastructural investment.
Tidal stream technology extracts energy directly from the currents that fl ow in certain
locations, driven by the rise and fall of tides in the vicinity. These currents usually have a
low velocity (1 m/s), though this can be enhanced by the local topography. In particular, the
velocity can be magnifi ed greatly in straits between islands or between islands and the main-
land. Tides can be predicted with very high accuracy; hence after measurements at a site, the
energy available for conversion can be forecast with confi dence.
Figure 2.19 shows a tidal cycle together with the associated stream current velocity. Tidal
height varies approximately sinusoidally and, as already mentioned, is more or less com-
pletely predictable. For any site the tidal current velocity has four peaks and four troughs per
day, as in the fi gure.
 
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