Tidal barrage
Around the same time, the 240 MW la Rance Tidal Power Station was built in Brittany, France, opened in November 1966.
Turbines installed in the barrage wall generate power as water flows in and out of the estuary basin, bay, or river.
Smaller plants include the Annapolis Royal Generating Station on the Bay of Fundy, and another across a tiny inlet in Kislaya Guba, Russia.
A number of proposals have been considered for a barrage across the River Severn, from Brean Down in England to Lavernock Point near Cardiff in Wales.
As people have become more aware of environmental issues, they have opposed barrages because of the adverse effects associated with changing a large ecosystem that is habitat for many varieties of species.
The turbine gates are kept closed until the sea level falls, in order to create sufficient head across the barrage.
Turbines are able to be powered in reverse by excess energy in the grid to increase the water level in the basin at high tide (for ebb generation).
In normal estuarine situations, however, two-basin schemes are very expensive to construct due to the cost of the extra length of barrage.
It is also the only site where a full-scale evaluation of the ecological impact of a tidal power system, operating for 20 years, has been made.
[8] French researchers found that the isolation of the estuary during the construction phases of the tidal barrage was detrimental to flora and fauna, however; after ten years, there has been a "variable degree of biological adjustment to the new environmental conditions.
Also as a result of the construction, sandbanks disappeared, the beach of St. Servan was badly damaged and high-speed currents have developed near sluices, which are water channels controlled by gates.
Unlike barrages, tidal fences do not interrupt fish migration or alter hydrology, thus these options offer energy generating capacity without dire environmental impacts.
If the turbines are moving slowly enough, such as low velocities of 25–50 rpm, fish kill is minimalized and silt and other nutrients are able to flow through the structures.
For example, a 20 kW tidal turbine prototype built in the St. Lawrence Seaway in 1983 reported no fish kills.
Even with the most fish-friendly turbine design, fish mortality per pass is approximately 15%[citation needed] (from pressure drop, contact with blades, cavitation, etc.).
have so far failed to solve this problem for tidal barrages, either offering extremely expensive solutions, or ones which are used by a small fraction of fish only.
The potential energy contained in a volume of water is:[11] where: The factor half is due to the fact, that as the basin flows empty through the turbines, the hydraulic head over the dam reduces.
As a result, a tidal power scheme may not produce returns for many years, and investors may be reluctant to participate in such projects.
[3] Governments may be able to finance tidal barrage power, but many are unwilling to do so also due to the lag time before investment return and the high irreversible commitment.
The UK government itself appreciates the technical viability and siting options available, but has failed to provide meaningful incentives to move these goals forward.
