Water power which is available continuously, or nearly so, is called primary or firm power. The amount of primary power which a given water power plant can produce is measured by the minimum flow of the stream, either natural or as regulated by storage. Power which can be supplied only pan of the year, when the flow of the stream is sufficient to produce it. is called secondary or surplus power. Primary power I rings the higher price of the two in the market per hone-power year. often about double that of surplus power. The amount of power available at a given kvation t rpm a natural variable stream ma% be increased by the use of pondage. or storage, or both.
use of pondage. Storage, on the other hand, is a term usually applied where water which is not otherwise usable,— that is, waters mainly derived from floods or stream flows in excess of the power plant capacities, are stored and held in reserve by natural lakes or artificial reservoirs. Such stored waters are used to make good part of the deficiency in supply of a natural stream during the low water periods. Storage thus increases the average available output of the plant, regardless of the mode of operation, whereas pondage only increases the available output when the demand for power varies from hour to hour during the day.
In the early days of water power develop ment, water power plant capacities were usually made a little greater than the dry weather ca pacities of the streams supplying them. For example. a common practice was to make a plant capacity equal to the flow of the stream during the third driest month, in other words. the stream would supply the full capacity of the water power plant during little months of the year, on as average When the use of auxiliary steam power became more general, and faauring demands increased, the plant capaci Des of newer power developments were often wade equal to the sixth or seventh month's flow ail order of dryness, so that the stream would supply the full capacities of the plants during Eve or six months of the year only, the defi csency during the remaining six or seven months being usually made up by the use of auxiliary steam where a constant power output was required.
Modern large capacity turbines, hydro-elm we developments, inter-connected transmission hues, and large demands for power at times for peak loads or break-down service, have brought about an increase in the economic ca pacity of hydro-electric power developments, until such plants are sometimes installed hav ing capacities equal to the flow of the stream the ninth driest month, or even greater, especially where there is extensive pondage or storage in connection with the plant, so that power in large quantities can be developed for short time use from impounded water quite in dependent of the volume of the natural flow at the time. It is seldom commercially feasible,
even on the steadiest natural stream, to install a water-power plant having a capacity adequate to utilize the full flow of the stream at all times. As a rule, a large portion of the natural flow during floods goes to waste; furthermore, few hydroelectric plants can utilize all the water which is actually available, but are re stricted in their output at times by low demand for power or by varying loads. The ratio of the average to the maximum or peak load out put of an hydro-electric power plant is called the load factor. If power is used mainly dur ing certain hours of the day only, as for elec tric lighting purposes, the load factor is usually relatively low. If power can be used 24 hours per day, the load factor is increased. The re duction of the operating hours from 10 to eight hours in a manufacturing plant, for t x ample, operated by water power without pond age, may reduce the load factor by about 20 per cent, and bring about a corresponding economic loss of water power.
Some of the advantages of water as com pared with steam power have been described. lu chief advantage is the saving in coal. This as so significant that in France and Switzerland water power is designated as 'white coal' Some of the disadvantages of water power are. (1) A larger investment is generally re quired per horse power of plant capacity than for steam power; (2) The power must be de veloped near the place where it naturally occurs, and if desired for distant use beyond a range of A few hundred feet, it must be transmitted by electricity, whereas steam power can he devel oped at the place where it is to be used; (3) Ice northern climates the use of water power in winter is sometimes seriously hampered• by the occurrence of ice, especially those varieties known as •needle.• or 'anchor ice; (4) Low head water-power developments are often hampered by variations in head, especially as a result of backwater during floods; (c) Water power can only be generated from the water as it comes to the power plant, and since most streams are %ariable in the amount of their flow, the power output also is variable, unless the stream is regulated by natural or artificial storage. Steam power can be gen erated at any rate required, up to the limit of the plant capacity.