696 ECONOMICAL ASPECTS 



the sum of the two is a minimum, and this would fix the most 

 economical capacity of the development and, hence, the point of 

 greatest profit for a given market price of energy. The entire 

 problem of determining the economical capacity of a combined 

 hydro-electric and steam-power plant is very complicated. An 

 excellent treatise on this subject, offering a new method of solution, 

 was presented by Dr. C. T. Hutchinson before the A.I.E.E., 

 February, 1914, and the reader is referred to the same for further 

 information. 1 



The size of the auxiliary station is determined by the differ- 

 ence between the demand curve and the stream-flow curve, 

 except where storage is available, in which the stream-flow as 

 affected by the same should be used. 



In order to obtain the best results, the method of operation 

 also deserves a careful consideration. In this connection R. C. 

 Muir in the General Electric Review for June, 1913, makes the fol- 

 lowing recommendations: " In order to get the best economy out 

 of the steam station it must operate at practically a constant load 

 corresponding to full load on one or more units. In order to get 

 the best economy out of the water-power station with the water 

 available during low-water periods, the highest water level attain- 

 able in other words the maximum head must be maintained at 

 all times. 



"It is impossible to conform to both of these requirements, 

 especially where the minimum stream-flow capacity and the 

 steam-station capacity combined are not sufficient to carry the 

 peak load. In this case the steam plant can be operated at prac- 

 tically a constant load, using the water power during the peaks 

 and storing water during the balance of the time. With high- 

 head plants the head gained by storage is not of importance; 

 so that the steam plant can be operated most economically on 

 constant load, allowing the water power to take the peaks. With 

 low-head plants having considerable storage capacity both plants 

 can be operated advantageously during the low- water period. 

 Here again the water power should carry the peaks, and the steam- 

 plant operated at constant load -over a sufficient part of the day 

 so that the water level will not be materially affected. This 

 method of operation will prove much more economical, both as 

 regards fuel used and labor required, than the method of carrying 

 1 See also an article by H. S. Putnam, A.I.E.E. June., 1917. 



