42i 



NATURE 



[June 3, 1920 



Many schemes of tidal-power development have 

 been suggested from time to time. Briefly out- 

 lined, the more promising of these are as 

 follows : — 



(a) A single tidal basin is used, divided from 

 the sea by a dam or barrage, in which are placed 

 the turbines. The basin is filled through sluices 

 during the rising tide. At high tide the sluices 

 are closed. When the tide has fallen through a 

 height the magnitude of which depends on the 

 working head to be adopted, the turbine-gates 

 are opened, and the turbines operate on a more 

 or less constant head until low tide. The maxi- 

 mum output from a given area of basin is 



obtained when the working head is approximately 

 one-half the tidal range, and the cycle of 

 operations under these conditions, and with a 

 constant rate of fall in the tidal basin, is shown 

 in Fig. I. Here the dotted sine curve represents 

 the level of the sea on a time base. The working 

 period extends from A to B. 



(b) A single tidal basin is used, with the tur- 

 bines operating on both rising and falling tides. 

 The cycle of operations is now indicated in Fig. 2. 

 The working period per complete tide extends 

 from A to B and from C to D. Slightly before 

 low water, at B, the basin is emptied through 



Fig. 2. 



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 

 50 per cent, greater than in system (a), and the 

 work done per complete tide is approximately 

 50 per cent, greater. 



(c) A single tidal basin is used with the turbines 

 operating on both rising and falling tides. Instead 

 of filling and emptying the tidal basin through 

 sluice-gates at high and low water,' ^nd working 

 under an approximately constant head, the water 

 is allowed to flow through the turbines and to 

 NO. 2640, VOL. 105] 



adjust its own level. Under these conditions the 

 rise and fall inside the basin are cyclical, with the 

 same period as the tide, but with a smaller rise 

 and fall and with a certain time-lag. The range 

 in the basin and the time-lag depend on the ratio 

 of the surface area of the basin and of the effective 

 discharge area of the turbines. The working 

 head during each tide varies from zero to a maxi- 

 mum. The cycle of operations is shown in Fig. 3. 

 The working period is from A to B and from 

 C to D, where the head at the points A, B, C, 

 and D is the minimum under which the turbines 

 will operate. The total working period per tide 

 is greater than with either of the preceding 



Fiq.5. 



systems, and the possible output is somewhat 

 greater. On the other hand, the variation of head 

 during any one tide is very large. 



(d) Two tidal 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 080 H, and 

 operation is continuous, as indicated in Fig. 4, 

 which shows the cycle of operations. The upper 



basin is filled from the sea through the appro- 

 priate sluice-gates from A to B, 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 systems 

 (b) and (c), so that, except where the physical 

 configuration of the site is particularly favour- 

 able, the cost per horse-power is likely to prove 

 very high. 



(e) Two tidal basins of approximately equal 

 size are used. Turbines are installed in the walls 

 dividing the sea from each basin. Fig. 5 shows 



