FRANCE’S RANCE SCHEME TIDES’ EBB AND FLOW BEHIND 240 MEGAWATT POWER STATION

Press, Volume CIII, Issue 30541, 9 September 1964, Page 16

FRANCE’S RANCE SCHEME TIDES’ EBB AND FLOW BEHIND 240 MEGAWATT POWER STATION

IBy

C. L. BOLTZ,

, Scientific Editor of the "Financial Times'] (Reprinted by arrangement)

A unique power-station is now more than half-way to completion in France in the lower part of the river Rance in Brittany, a mile or so from Dinard and St. Malo. Its uniqueness is in the fact that it will generate electricity from the energetic ebb and flow of ocean tides, and it will do so in commercial quantities contributing a worth-while proportion of France s electrical power. Already, it is true, a small tidal power station is in operation at St. Malo, acting as the experimental prototype to give data 1 or the Rance scheme. But its yield of nine megawatts is negligible.

The Rance scheme is of a different order altogether. It will generate 240 megawatts and estimates now made give a figure of 544 million kilo-watt-hours, or units, every year for the energy to be supplied. [This is rather more than a quarter of Benmore’s output.] The cost is given as 42,000 million old francs, equivalent to about £3l million. Thus the cost per kilowatt installed is roughly £l3O, higher than for the present generation of commercial nuclear power installations in Britain but less than for the earlier ones. The cost per unit sent out has not been revealed, even if calculated, but this could be considerably smaller than the high capital cost suggests because the running costs must be extremely low, the energy being supplied by nature free of charge. Store Of Energy It comes from the tides. They represent, owing to the resonant flicks, so to speak, given twice a day by the gravitational attraction of sun and moon, a store of energy of astronomical size for which nobody has to pay. As far as the channel is concerned it has been calculated that 28 million H.P. are dissipated round the French coast. This is equivalent to 21,000 megawatts, roughly equal to the installed capacity for the whole of France. It is a tiny fraction of this that E.D.F. hopes to tap at Rance, The site for such a tidal power station has been carefully chosen. Though it is possible to give simplified accounts of tidal motion all over the World, tidal phenomena are much affected by local conditions and there are anomalies, such as the maintenance of high tide at Le Havre for three hours. The tidal range between depth at low water and at high also varies rhythmically between minimum range (neap tides) and maximum (spring tides) ■ over a period of about a fortnight. And in various parts of the world there are very big ranges. In the Bay of ‘Fundy the range is 50 feet, the biggest in the world, due to resonance between the entrapped water and the ocean tides. There is also an enormous range of 45 feet in the estuary of the Severn, which separates England from Wales. Fundy Scheme An enormous Fundy scheme was abandoned some years ago largely because of the economics of transmission of the power over long distances

to where it could be used. A Severn scheme has also been abandoned. Only France is carrying a scheme to completion. The tidal range in the bay of Mont-Saint-Michel is about 41 feet and that at the mouth of the River Rance about 37. feet. After imaginative schemes for the bay, engineers have decided on a cautious first scheme on the Rance, where there seems to be less uncertainty about the result. The basic method of transforming the tidal ebb and flow into electricity is to make it turn a water wheel and so drive a turboalternator. This is simple in concept, but complex to put into operation in an economically satisfactory way. Methods must be found of getting the maximum use of tidal energy on both ebb and flow. In addition, time and tide, as everyone knows, wait for no man: so the output of electricity from the Rance station will, come at different times of the day because a tide is about 40 minutes later each day. Barrage Across River The resulting system is to have a barrage of three parts (excluding a lock to permit navigation) right across the river. One part contains the power station of 24 submarine “bulb sets.” The second part is a fixed dike to a small island. The third is a movable dam with roller gates. Each bulb set has an- outside water wheel or giant propeller 174 feet in diameter rotating at 94 revolutions per minute. The generating plant is inside watertight buildings

above. Each propeller can have its four blades feathered to allow the free flow of water when required. When the tide flows it turns the propellers and electricity is generated. Just before high tide when the speed of flow is slowing, the sets are disconnected and the movable gates are opened to allow free flow into the basin created by the barrage. At the last moment the gates are closed and the sets are switched on and used as pumps, getting electricity from the national grid. Thus an overfill of the basin is achieved, giving an artificially-increased head of water when the ebb starts. Over-emptying is also done towards the end of the ebb. Electricity Next Year The fixed dike is finished. The section with the roller gates has been commissioned since March, 1963. The main dike containing gates and power generating sets was closed off on July 20. 1963. The upstream part of the river is thus a lake with a permanent depth of 28 feet for navigation. Work on the power station, in a dry area cut off by temporary coffer dams, began this year. The total operation, an engineering project of great size, is expected to take six years from the beginning in January, 1961, and the first electricity should flow into the grid late next year. There is no doubt that electricalsupply authorities all over the world will be watching keenly. Who knows but that Britain’s own Central Electricity Generating Board may give a glance at the Rance?