Tidal Power

(d) Two tidal basins of approximately equal areas are used. Turbines are installed in the walls dividing the sea from each basin. Fig. 4 shows the cycle of operations. From A to B the upper basin discharges through its turbines into the sea. From B to E the sea enters the lower basin through its turbines. The upper basin is filled from the sea through its sluice gates between C and D, and the lower basin is emptied through its sluice gates from F to G. The head varies from 0.25 H to 0.62 H, and the output is some 25% greater than in system (c) but the number of turbines required is much greater.

It is possible to arrange in each of these systems that the head shall be maintained constant during any one working period, but since this means that the working head is then limited to the mini mum obtaining during that period, a loss of energy is involved, with a great additional cost of construction and complication in manipulation and with little compensating advantage. For any scheme of development involving the use of a tidal estuary of such types as are found in the Severn or Dee, the cost of any of the multiple basin systems would appear to put them definitely out of court. A scheme involving operation only on a falling tide has the disadvantage that the output is only about 6o% of the output theoretically possible with double-way operation. On the other hand the output per unit of turbine capacity is sensibly the same, while it enables a much more efficient type of turbine set ting to be used, and halves the number of sluice gates.

The Severn Scheme.

Up to 1928 no tidal scheme of any magnitude had been constructed. The possibilities of such an installation on the estuary of the Severn are, however, now under consideration by a committee appointed by the British Govern ment. This would consist of a single tidal basin of about 25sq.m.

in area formed by a barrage probably in the neighbourhood of Sudbury, along with a storage reservoir some roof t. above sea level and about 5m. away above the Wye valley. It is estimated that the scheme would be capable of developing some soo,000h.p. over a Io-hour working day throughout the year.

Fundy Bay Scheme.

Two schemes, each of the two-basin type, have been suggested for utilizing the tides in the Bay of Fundy (q.v.). From one of these, at Passamaquoddy Bay, where the average spring tidal range is 23.2 ft., it is estimated that from 500,00o to 700,000 h.p. can be generated. The other, which is a smaller but relatively less costly scheme, is at Hopewell at the head of Shepody Bay, and utilizes the estuaries of the Petitcodiac and Memramcook rivers as the two tidal basins. Here the spring tidal range is 45.5 ft. and the available power about 200,000 h.p. Both schemes have been before the New Brunswick Government.

Aber-Vrach Scheme.

A combined tidal and river-power scheme to which the French Government is giving financial assis tance is projected at Aber-Vrach on the coast of Brittany. For the tidal station a barrage 49oft. long is to be constructed in the estu ary. Four turbines connected to synchronous generators are to be installed, each capable of developing r,2ooh.p., under the maxi mum head available at spring tides. The turbines are to operate both on the ebb and flow tides. This station will work in conjunc tion with an auxiliary water-power station some 4m. away on the river Diouris. The river is to be dammed at this point, forming a fresh water reservoir from which turbines having a maximum capacity of 2,7ooh.p. are to be supplied. Electrically driven cen trifugal pumps of a capacity of 3,2ooh.p. will also be installed.