HOW DOES IT WORK?
A barrage, which is in fact a huge dam, is built across a river estuary or bay. This barrage has gates in it which allow the water to flow into the barrage with the incoming tide. These gates are then closed when the tide begins to go back out. This water which is now trapped inside the barrage is now called a ‘hydrostatic head’. The greater the head the more power can be generated from the outflowing water. There are other gates within the barrage which are now opened; these gates contain hydro-electric generators, very similar to the ones used in Hydropower. These generators are now turned by the outflowing water and power is generated.

The tidal range has to be sufficient in order for this to be a practical means to generate power. This range should be in excess of 5 metres otherwise the power generated is not sufficient.

The main downfall of tidal power generation is the capital needed at the beginning of the project to construct the barrages and the effect on the environment by the change in the water levels. But once the barrages have been built there is a very low maintenance cost. The generators only need changing once every 30 or so years and there is very little work needed to be done.

IS IT RENEWABLE?
Yes, the tides will continue to ebb and flow, thus there will always be power being generated. The water is not used up, it stays in the water cycle, and can therefore be used over and over again without the need for replenishment.

APPLICATIONS OF TIDAL POWER
Small-scale tidal mills were used in the Middle Ages for grinding corn. The barrages which are built can be used as a means to cross the estuary with much greater ease. The main application of tidal power is as an additional means of generating renewable, sustainable energy which does not affect the environment in a negative way.

ADVANTAGES
· It is very cheap to maintain
· There is no waste or pollution
· Very reliable
· We can predict when tides will be in or out
· The barrage can help to reduce the damage of very high tidal surges or storms on   the land

DISADVANTAGES
· It changes the coastline completely and the estuaries are flooded so any mud flats   or habitats that birds or animals live on are destroyed
· Initial building cost is very expensive
· Water is not replenished, it cannot flow away so any dirt or pollution lingers around   the coast much longer
· Silt builds up behind the barrage
· Disrupts creatures’ migration in the oceans
· Needs a very big piece of sea to be cost effective
· Not many sites suitable for this kind of power generation; building the barrage
· Only produces power for about 10 hours of the day

TIDAL POWER IN SOUTH AFRICA
South Africa is a country with a huge coastline; there are many bays and lagoons which can be used to generate this kind of electrical power. We have many big cities that are located on the coast and would benefit greatly from this kind of power generation.

South Africa has only one nuclear power station, and therefore we rely a lot on fossil fuels for our electrical power. Thus we would benefit significantly from a renewable energy source such as tidal power.

INTERESTING INFORMATION
The capital necessary to build the barrage is very high and is its main downfall. But once it is built it is very cheap to maintain. The turbines only need to be changed every 30 years.

Ships cannot get into the bay unless provision is made for them in the form of a gate.
The barrage can be used as a road to cross the bay easily.
There will be a calm lot of water behind the wall; this can be used for recreational purposes such as yachting or swimming.