The focus of investigations by most countries which have looked into the potential exploitation of tidal energy has been on using barrages to create artificial impoundments that can be used to control the natural tidal flow. Barrage developers in the UK and elsewhere have concluded that building a permeable barrage across an estuary minimises the cost of civil structures for the quantity of energy that can be realistically extracted.Construction of barrages across estuaries with high tidal ranges would be challenging but technically feasible. In shallow water, armored embankment would be used, but in deeper water this method would be impractical and too expensive because of the quantity of material required. Complete closure of estuaries would be achieved by emplacing a series of prefabricated sections, or caissons, made from concrete or steel which could be floated and then sunk into position.
The technique has been used in the Netherlands to close the Schelde Estuary. A large steel caisson was also used in the construction of the Vadalia power station on a tributary of the Mississippi.
Tidal barrages are comprised of sluice gates and turbine generators. Large-scale structures like the Severn Barrages would also include blank caissons and ship-locks. During the ebb tide, water is allowed to flow through the sluices and the turbine draft tubes to ensure the maximum possible passage of water into the impounded basin. At or close to high water, the sluice gates are closed.
At this stage of the cycle, the turbines can be used in reverse as pumps to increase the amount of water within the basin. Although there is an obvious energy demand, the amount of water transferred can provide an additional increase in energy output of up to 10% compared with a cycle where no pumping is used. The actual increase in energy output from pumping depends on the estuary and the tidal conditions.
Retention of water allows a head of water (i.e. a difference in vertical height between water levels) to be created as the flood tide progresses seaward of the barrage. Once a sufficient head has been created, water is allowed to flow back through the turbines to generate electricity.
In this respect, a tidal energy barrage is no different from a low-head hydroelectric dam. The large volumes of water and the variation in head require the use of double regulation, or Kaplan turbines. These turbines have guide vanes and blades that can be moved by hydraulic motors. This allows turbine operation, and therefore energy conversion efficiency, to be optimised through each generation cycle as the reservoir head drops.