AUXILIARY STATIONS 695 



most efficient unit at practically the same cost as one of poorer 

 efficiency. A less boiler capacity is, of course, needed with a 

 higher turbine efficiency and consequently a plant of high effi- 

 ciency can, as a rule, be built at practically the same cost as one 

 of lower efficiency. 



Low-water Stations. The function of the auxiliary plant, 

 when used as supplemental capacity during low-water periods is 

 similar to that of the storage reservoir. It converts at least a 

 part of the secondary power, which would be available only part 

 of the year, into primary power available at all times, thus increas- 

 ing its sale value. It is also of value in making up shortage of 

 water power from loss of head during high back-water caused by 

 floods. Enough pondage can usually be provided to insure that 

 daily fluctuations can be taken care of, even though the peak load 

 is somewhat in excess of the power corresponding to the minimum 

 stream-flow. This, of course, necessitates that the average or 

 integrated load over the twenty-four-hour period must be within 

 the energy available from the minimum stream-flow. 



The problem, therefore, really resolves itself into two ques- 

 tions : First, in the case of a plant already in operation, to what an 

 extent shall an auxiliary supply be provided to convert the variable 

 power supply into a continuous supply? Second, in case of a 

 new development, for what capacity shall it be built? 



Both cases involve a study of the stream-flow and the load 

 conditions, the first cost and annual operating charges for the 

 hydro-electric plant of different capacities as well as the cor- 

 responding charges for auxiliary plants of the required capacities. 

 In the first case the cost of the auxiliary supply for various degrees 

 of insurance is determined and compared with the increased earn- 

 ings obtained by converting the secondary power into primary. 

 In the second case the problem may be considered from several 

 different points of view. So, for example, one may start out with 

 the assumption that the total cost per kilowatt and year shall be a 

 minimum, or, if all the power produced can be sold in the market 

 at a certain price, it should be investigated at what plant capacity 

 the profit becomes a maximum. In the case of a new develop- 

 ment, the cost per kilowatt decreases as the capacity increases, 

 and an increase in the annual cost per kilowatt of the auxiliary 

 plant is accompanied by a decrease in the annual cost of the 

 hydraulic plant, A point may, therefore, be reached at which 



