The Geography Department
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The Geography Department |
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8-5 6 Renewable energy
Geog123 book 2 pages 80-81 5.7 Green electricity pages 82-83 5.8 Is solar power the answer?
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case study: the Rance tidal power station
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A barrage has been constructed across the River Rance in Brittany to use the power of the tides to generate electricity. The tidal range at the mouth of the Rance is normally 11.4 m., but can rise to 13.50 metres at Spring Tide. However, the finest site for this type of station is in Britain in the Bristol Channel, with a tidal range of 13.8 m., which is over 40'.
The high tides in Brittany are caused by water being funnelled by the Contentin peninsula to the east. The result is that the tide rushes very quickly up the River Rance, between the towns of St. Malo and Dinard at a speed of 20 km/hr.
The barrage has three parts; the dam itself in which are 24 turbines, the rolling barrage which allows excess water to escape from the river and the lock gates on the left bank for navigation.
The barrage is designed to harness the tidal energy by allowing turbines to generate electricity as the tide rushes up the river and by generating again as it rushes out again at low tide. Although at 40', the fall of water is quite small, there is a considerable volume to use, about three times the flow of the River Rhone in flood at Avignon. The 24 sets of turbines are also reversible to produce current in both directions of flow and can act as pumps when required. Each set produces 10 megawatts each, a total of 240 MW.
Sometimes the tide is not high enough for the turbines to be used as the rising waters enter the basin; they are simply allowed to pass via the sluices near the right bank and power generated as they escape low tide. At such times the turbines can be used to pump in extra water from the sea to increase the head of water passing through the turbines. This is usually done on off-peak hours with power from electricity stations that have spare current. The water is then allowed to escape through the turbines when demand is highest. Luckily the tides can be predicted accurately and with the use of a computer the barrage produces power when needed and stores water when power consumption in the area is low.
Opposite is a plan of the Rance Barrage; Translation of the key labels is:
ecluse - locks
mer - sea
bassin - basin
puits d’acces du material - entrance shaft
salle de commande - control room
digue morte - coffer dam
barrage mobile - rolling sluices
The maximum power produced to date by the installation is 544 gigawatt hours, that is 1 million kilowatt hours of electricity (1 million electric fires on for an hour). The pumps have consumed 64.5 gigawatt hours in the last year, but remember that normal power stations cannot shut down completely at night and have a base load that must produce to keep going …it is really free.
The harnessing of tidal power is a field in which the French have a lead over the rest of the world. However, there are few other sites for this kind of station in France, even if there are possibilities in Britain. A hydro-electric dam on the River Rhone might well produce as much electricity more cheaply and European natural gas is now a competitor in new generating stations.
The experiment will have succeeded if, or when, the next tidal station is built
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