Turbines are then placed at these sluices to capture the energy as the water flows in and out. An artistic impression difference between dam and barrage pdf a tidal barrage, including embankments, a ship lock and caissons housing a sluice and two turbines.
Turbines installed in the barrage wall generate power as water flows in and out of the estuary basin, bay, or river. When the water level outside of the basin or lagoon changes relative to the water level inside, the turbines are able to produce power. Embankments seal a basin where it is not sealed by caissons. The sluice gates applicable to tidal power are the flap gate, vertical rising gate, radial gate, and rising sector. Only a few such plants exist. France, which has been operating since 1966, and generates 240MW.
Barrage systems are affected by problems of high civil infrastructure costs associated with what is in effect a dam being placed across estuarine systems, and the environmental problems associated with changing a large ecosystem. The basin is filled through the sluices until high tide. Then the sluice gates are closed. The turbine gates are kept closed until the sea level falls to create sufficient head across the barrage, and then are opened so that the turbines generate until the head is again low.
Then the sluices are opened, turbines disconnected and the basin is filled again. The basin is filled through the turbines, which generate at tide flood. Rivers flowing into the basin may further reduce the energy potential, instead of enhancing it as in ebb generation. Of course this is not a problem with the “lagoon” model, without river inflow. Much of this energy is returned during generation, because power output is strongly related to the head. Another form of energy barrage configuration is that of the dual basin type.
With two basins, one is filled at high tide and the other is emptied at low tide. Turbines are placed between the basins. Two-basin schemes offer advantages over normal schemes in that generation time can be adjusted with high flexibility and it is also possible to generate almost continuously. In normal estuarine situations, however, two-basin schemes are very expensive to construct due to the cost of the extra length of barrage.
There are some favourable geographies, however, which are well suited to this type of scheme. Two lagoons operating at different time intervals can guarantee continuous power output, around 4. These independent barrages do not block the flow of the river. The placement of a barrage into an estuary has a considerable effect on the water inside the basin and on the ecosystem.
Many governments have been reluctant in recent times to grant approval for tidal barrages. Through research conducted on tidal plants, it has been found that tidal barrages constructed at the mouths of estuaries pose similar environmental threats as large dams. The La Rance plant, off the Brittany coast of northern France, was the first and largest tidal barrage plant in the world. Some species lost their habitat due to La Rance’s construction, but other species colonized the abandoned space, which caused a shift in diversity. 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.