Environmental Engineering Reference
In-Depth Information
privatized electricity supply industry. Furthermore, environmental considerations may present
a barrier to large scale developments.
2.6.4 Operational Strategies
Ebb generation is the simplest mode of operation for a tidal barrage scheme. The operating
cycle consists of four steps:
Sluicing on the fl ood tide, to fi ll the basin.
Holding the impounded water until the receding tide creates a suitable head.
Releasing the water from the basin to the sea via turbines, on the ebb tide, until the tide
turns and rises to reduce the head to the minimum operating point.
Holding until the tide rises suffi ciently to repeat the fi rst step.
Ebb generation with fl ood pumping is a modifi cation of this mode which allows increased
energy output. By using the turbines in reverse as pumps, the basin level and hence the gen-
erating head can be raised. The energy required for pumping must be imported but since the
pumping is carried out against a small head at high tide and the same water is released later
through the turbine at a greater head, this can produce a net energy gain with some limited
ability to re-time output. The energy gain through pumping could be small but useful and
typically in the range 3-13%.
Flood generation is the reverse of ebb generation and is rarely suggested alone, possibly
because it offers little storage opportunity as the basin is often being fi lled by a river, which
reduces the total energy capacity.
Two - way generation (ebb and fl ood) is possible with reversible turbines and is used at La
Rance (together with fl ood pumping). The additional energy recovered may not justify the
extra cost and complexity of the turbines.
Most schemes propose ebb-only generation. The full resource could only be captured if
the barrage of the scheme has a very large two-way generating fl ow capacity in order to
achieve rapid fi lling and emptying of the basin. In practice, it is neither economic nor is there
enough physical space to install enough turbines to come anywhere near full resource capture.
Typically only about one-third of the gross power can be harnessed, so after turbine and
generator losses the mean electrical output will at best be about a quarter of the gross
power.
The other main reason for preferring ebb generation is the nature of the amenity that results.
In such a scheme, the water level in the basin never falls below the mean sea level, so the
basin is available for recreational activity much like an inland reservoir. On the other hand,
it is bad news for the bird population for whom exposed mudfl ats at low tide provide a major
food resource. High initial capital cost and polarized opinions on the resulting environmental
changes are the main reasons why tidal schemes seldom get beyond the feasibility study stage.
Tidal energy barrages are expected to have very long lifetimes. Their design life could be
about 120 years, but with normal maintenance and replacement of turbine generators at 40
year intervals, their lifetime could effectively be unlimited.
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