June 3, 1920] 



NATURE 



429 



the cycle of operations. From A to B the uppei 

 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 062 H, and the output is some 

 25 per cent, greater than in system (d), but the 

 number of turbines required is much greater than 

 in (d). 



It is possible, at the expense of additional com- 

 plication, 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 must then be the minimum 

 olitaining during the period, a loss of energy is 

 involved, with a great additional cost of construc- 

 tion and complication in manipulation, and with 

 little compensating advantage. 



The great difficulty in developing a tidal scheme 

 as compared with an orthodox low head water- 

 power scheme arises from the relatively great 

 fluctuations in head. In any scheme in which the 

 working head is a definite fraction of the tidal 

 range, the working head at spring tides is much 



V 



greater than at neap tides. In the case of the 

 Severn, for example, the working head at springs 

 would be twice as great as at neaps, and the 

 energy output per tide would be four times as 

 great at springs as at neaps, while at St. Malo 

 the output would be 55 times as great at springs 

 as at neaps. 



Not only is the installation subject to this 

 cyclical fluctuation of head, but in any simple 

 scheme the turbines also cease to operate for a 

 more or less extended period on each tide ; and as 

 this idle period depends on the time of ebb or flood 

 tide it gradually works around the clock, and 

 will, at regular intervals, be included in the 

 normal industrial working day. It is true that 

 schemes of operation such as have been indicated 

 are feasible in which this idle period may be 

 eliminated and continuous operation ensured, but 

 only at a considerable reduction of output per 

 square mile of tidal basin area. Even in such 

 schemes, unless the working head is fixed with 

 reference to the tidal range at neap tides, the 

 variation of head between springs and neaps 

 causes the output to be very variable. 



In any installation, then, designed for an 

 ordinary industrial load, unless the output is cut 

 down to that obtainable under the minimum head 

 NO. 264.0. VOL. \0^ 



available at the worst period of a neap tide, in 

 which case only a very small fraction of the total 

 available energ7 is utiFised and the cost of the 

 necessary engineering works per horse-power will, 

 except in exceptionally favourable circum- 

 stances, be prohibitive, some form of storage 

 system forms an essential feature of the scheme. 



Various storage systems have been suggested. 

 Electrical accumulators must be ruled out, if only 

 on account of the cost, and the same applies to 

 all systems making use of compressed air. The 

 only feasible system appears to consist of a stor- 

 age reservoir above the level of the tidal basin. 

 Whenever the output of the primary turbines 

 exceeds the industrial demand, the excess energy 

 is utilised to pump water into the reservoir, and 

 when the demand exceeds the output from the 

 primary turbines it is supplied by a series of 

 generators driven by a battery of secondary tur- 

 bines operated by the water from the storage 

 reservoir. 



Evidently this method is available only when 

 the physical configuration of the district affords 

 a suitable reservoir site within a reasonable 

 distance of the tidal basin. Unfortunately also, 

 considerable losses are inevitable in the process, 

 and the energy available at the switchboard of this 

 secondary station is only about 50 per cent, of 

 the energy of the water utilised by the primary 

 turbines. Where two tidal schemes at some 

 distance apart differ sufficiently in phase, it is 

 possible, by working the two in conjunction, to 

 reduce or eliminate the idle period between tides, 

 and thus to reduce the necessary storage some- 

 what ; but this does not affect the necessity of 

 storage as between spring and neap tides. 1 



Since storage reduces the available output by 

 one-half, and at the same time complicates the 

 system, besides adding considerably to the first 

 cost and maintenance charges, the prospects of 

 tidal-pow^er schemes would be much more promis- 

 ing if the whole of the output could be utilised 

 as it is generated. By feeding into a distributing 

 main in conjunction with a large steam station 

 and /or inland water-power scheme, and delivering 

 to an industrial district capable of absorbing a 

 comparatively large night load, such a state of 

 affairs might be realised, at all events approxi- 

 mately. There is also the possibility that the 

 intermittent operation of certain electro-chemical 

 processes may be developed so as to enable any 

 surplus power to be absorbed as and when avail- 

 able, and, if so, power developed tidally will 

 probably prove cheaper in this country than that 

 developed from any other source. 



Owing to the relatively large variations in 

 working head in any simple scheme, and to the 

 small working heads, the design of hydraulic 

 turbines capable of giving constant speed with 

 reasonable efficiencies, and of moderately high 

 speeds of rotation, is a matter of considerable 

 difficulty. Modern developments, however, 

 promise much better results in both these 

 respects than would have appeared possible only 

 a few years ago, and turbines are in existence 



