Tidal Power

TIDAL POWER. The idea of utilizing the rise and fall of the tides for power purposes has long attracted the attention of inventors, and many ingenious schemes have been suggested. So far, however, the only practicable method is based on the use of one or more tidal basins, separated from the sea by dams or bar rages, and of hydraulic turbines through which the water is passed on its way between basin and sea or between one basin and another.

Types of Schemes.—Briefly outlined the more promising of the schemes of tidal power development are as follows : (a) A single tidal basin is used, divided from the sea by a dam in which are placed the turbines. The basin is filled through sluices during the rising tide. At high tide these are closed. When the tide has fallen through about one-half its range, the turbine gates are opened and the turbines operate on a more or less con stant head until low tide or slightly after. If A be the surface area of the basin in square feet; if H ft. be the tidal range; if a constant working head of Ii ft. be adopted; and if the turbines operate until low tide; the volume of water used during the fall ing tide will be A (H---h) cu.ft., and the energy available in the water will be This expression is a maximum when h=114- 2, that is, when the working head is one-half the tidal range, and then equals 16 foot-pounds.

If

A is in square miles, and if the efficiency of the turbines is 75%, the output in h.-p.-hours per tide is given by The cycle of operations is shown in fig. I where AB denotes the working period and CD the period of replenishment of the basin. The sine curve represents the level of the sea outside the basin. Owing to the great variation in the rate of efflux of the water, in this constant-head method of operation, the power out put varies very largely during the falling tide. This variation may be reduced considerably by allowing the turbines to operate on a more or less constant fall of level in the basin as shown by the straight full line AB. By this method of operation the necessary turbine capacity for a given output may be greatly reduced. By

extending the working period beyond low tide, as indicated by the light line AE, a greater amount of energy may be developed per tide and the idle period is diminished, but at the expense of an appreciably greater variation in head. The most efficient combina tion of working period and of working heads can only be deter mined by detailed examination of the particular site, and with a knowledge of the exact form of the tidal curve.

(b) A single tidal basin is used, with the turbines operating on both rising and falling tides. The cycle of operations is indicated in fig. 2. The working period per complete tide now extends from A to B and from C to D. Slightly before low water, at B, the basin is emptied through sluice gates, and at D, a little before high water, the basin is filled through the sluice gates. With a working head equal to one-half the tidal range, the period of operation is approximately 6o% greater than in system (a) with operation down to low tide, and the work done is some 6o% greater.

(c) Two basins of approximately equal areas are used, with turbines in the dividing wall. Each basin communicates with the sea through suitable sluice gates. In one of these basins called the upper, the water level is never allowed to fall below one-third of the tidal range, while in the lower basin the level is not allowed to rise above one-third of the tidal range. The working head then varies from 0.53 H to o•8o H, with a mean of approximately o.66 H, and operation is continuous as indicated in fig. 3 which shows the cycle of operations. Between A and B, the upper basin is filled from the sea through appropriate sluice gates, and the lower basin discharges into the sea from C to D. For a given total basin area and a given tidal range the output is only about one-half that obtained in system (a) and one-third that obtained in system (b), so that except where the physical configuration of the site is particularly favourable the cost per h.p. is likely to prove very high.