668 



NATURE 



[August 28, 1913 



the wave increases as the opening- narrows, and 

 the particles of water composing it acquire a hori- 

 zontal motion. In fact, the tidal wave, after 

 entering an estuary, may be considered to be a 

 stream, which, while the crest of the wave is 

 passing, becomes quiescent for a time, and then 

 flows in the opposite direction until the trough 

 of the wave, in its turn, causes another period of 

 quiescence. These periods of quiescence are 

 called the tidal intervals. 



Now it has been found that the potential energy 

 of a river may be converted to useful energy by 

 damming the stream at a convenient place to 

 obtain a working head for turbines ; the difference 

 in level of the stream above and below the dam 

 being but a few feet. The economy of this 

 method of generating electricity has been estab- 

 lished by experience, and it is clear that if the 

 utilisation of the tidal stream could be effected on 

 somewhat similar lines, similar results might 

 reasonably be expected. This in effect constitutes 

 the problem which has to be solved. 



The chief difficulty which has to be surmounted 

 in utilising the tidal stream for power purposes i- 

 the tidal interval, and this difficulty must be con- 

 sidered as a problem to be solved for every 

 location. In one case it might happen that it 

 would be found cheaper to form one tidal reser- 

 voir and another reservoir inshore above the level 

 of the highest tide. 



The inshore reservoir would be filled twice daily 

 by means of pumps actuated by the tidal reservoir, 

 to serve as the supply to an ordinary hydro-electric 

 installation which could be operated when required. 

 In another case it might be found that the diffi- 

 culty could be best dealt with by forming an auxili- 

 ary reservoir al a convenient spot higher up a neigh- 

 bouring river, thus providing a separate unit to 

 carry the load over the tidal intervals. In another 

 case, again, it is possible that the intermittent 

 operation of the turbines would not be found in- 

 convenient. But speaking generally, the difficulty 

 can be surmounted by the formation of two or 

 more reservoirs connected to the tide and to 

 a central turbine chamber by means of sluice 

 valves ; the feed to the turbines springing altern- 

 ately from the main tidal stream and from the 

 reservoirs in such a manner that a working head of 

 water might be continuously maintained. 



But it cannot be too strongly insisted upon that 

 the first requirement for a tidal installation is a 

 suitable site, the peculiarities of which will deter- 

 mine the character of the system adopted. For, 

 since the success of such an undertaking depends 

 on the cost per unit of power for structural work 

 and equipment, it is evident that advantage must 

 be taken of every favourable peculiarity, and that 

 the system adopted will be dependent on the site. 



In a case where the tidal interval is to be 

 bridged by means of three tidal reservoirs the 

 sequence of flow between the reservoirs and the 

 tidai way is somewhat difficult to follow, but may 

 readily be understood from the tabular descrip- 

 tion below. The three reservoirs are severally de- 

 noted by the letters "a," "b," and "c." 

 NO. 2287, VOL. 91] 



The tide rising from low water to one-third of 

 its range : — 



"a' : Turbines fed "b" Standing "c" Emptying into 



from reservoir empty the tidal way 



The tide rising from one-third of its range to 

 high water : — 



"a" Filling to high- "b" limply "c" Turbines fed 



water level from the tidal way 



into the reservui 



The tide falling from high water to one-third 

 of range : — 



"a" Standing full "b" Turbines fed "c" Filled up lo 

 from the tidal way tide level 



into the reservoir 



The tide falling from one-third of its range to 

 low water : — 



Emptying to 

 /-tide level 



p" Turbines fed 

 from reservoir 



It has been practically established that the basis 

 of calculation for the power of a tidal installation 

 should be one-third of the range of the minimum 

 tide; hence when high tide is reached, water flow- 

 ing from the tidal way may be passed through 

 the turbines into the empty reservoir "b," the 

 capacity of which must be such that the water 

 level inside will have risen to the height of one- 

 third of the range of the tide when the tide has 

 fallen that distance. Similarly at low tide, water 

 impounded in reservoir " a " to the level of the last 

 high tide may be discharged through the turbines 

 into the tidal way, the capacity of reservoir 

 " a " being such that the tide will have risen one- 

 third of its range before the water level inside has 

 fallen an equal distance. 



A third reservoir "c" must be provided to re- 

 ceive the water flowing through the turbines from 

 the tidal way, during the time that the tide is 

 rising from one-third of its range to high tide. 

 It must then be filled to high-tide level direct from 

 the tide, so that when the latter has fallen one- 

 third of its range, the contents mav be discharged 

 through the turbines into the tidal way. 



By an arrangement of valves and sluices all the 

 reservoirs can be controlled automatically to per- 

 form their several functions as and when required. 



The technical description of the several sluice 

 valves or gates, of the structural details of the 

 supply, or turbine chamber, and of the embank- 

 ment walls was given in detail in the author's 

 paper on tidal waters as a source of power, which 

 was read and discussed before the Society of 

 Engineers on May 5. It is, however, necessary 

 to state that the cost of these details has been 

 exhaustively considered, and it has been found 

 that though the expense entailed is very heavy, it 

 cannot be considered prohibitive unless the 

 capacity of the installation is less than it should 

 be to justify the outlay. For it will readily be 

 seen that the length of the embankment walls 

 will not increase directly as the area of the reser- 

 voirs, and hence that within defined limits, the 

 greater the capacity of the installation the smaller 

 will be its cost per unit of horse-power. 



