Environmental Engineering Reference
In-Depth Information
Tide height and current velocity - South Lundy Island
3.0
1.2
2.5
1.0
2.0
0.8
1.5
0.6
1.0
0.4
0.5
0.2
0.0
0.0
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23
-0.5
-1.0
Tide height
-1.5
Current velocity
-2.0
Time (24 hours)
Figure 2.19 Tide height and current velocity. (Reproduced from Sinden, G.E., 2007, DPhil Thesis
with permission of Environmental Change Institute, Oxford University Centre for the Environment)
The technology to extract energy from the tidal current is conceptually simple: a turbine
is placed in a suitable tidal fl ow, which turns the generator through a gearbox as in Figure
2.20. It is similar to a submerged wind turbine, except that the greater specifi c gravity of
seawater results in much higher energy densities in tidal streams than is found in winds of
the same velocity. However, the water velocities available in tidal streams (typically rated
velocities of 2-3 m/s on good sites) are much lower than the air velocities used by wind tur-
bines. Although power output is proportional to the cube of the velocity, tidal stream rotors
generally produce signifi cantly greater output than wind turbines of the same size because of
the massively increased water density. Compared to wind, tidal fl ow velocities are expected
to have little turbulence and thus vary in a smooth manner, reducing fatigue loads on the
rotor and generating electricity with little short term variation.
At different sites, the peaks and troughs occur at different times. Power being proportional
to the cube of current velocity, this implies that maximum and minimum electricity genera-
tion is possible at different times. By combining generation from sites that are out of phase
it is theoretically possible to smooth out the diurnal variability. The tidal stream resource is
highly site - specifi c, and there is a limited number of tidal stream sites around the world worth
exploring. The accessible tidal stream resource for the most suitable sites in the UK (includ-
ing the Channel Islands) is estimated to be approximately 36 TW h/year. The power variability
from such schemes will be similar to that from barrages.
A second type of variability also affects tidal velocities. This is the spring-neap cycle,
which occurs over 14 days and changes tide ranges (the difference in height between high
and low tides) from a minimum to a maximum, then back to a minimum. Unlike diurnal
variations, the spring-neap cycle affects all sites in the same way at the same time, so there
is no scope for smoothing by combining generation from many sites.
It is concluded that patterns of tidal stream power output differ signifi cantly from
electricity demand cycles; maximum potential output sometimes coincides with peak
demand but at other times minimum output does. However at the likely levels of penetration
 
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