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THE STORAGE OF ENERGY ESSENTIAL TO
ECONOMY OF WORKING IN
Mr. President and Gentlemen of the National Electric Light Association: The Committee on Data, reporting at the Washington meeting, stated that 1.5 pounds of coal per indicated horse-power, with a combined engine and dynamo efficiency of eighty-one per cent, would produce 402.84-watt hours per pound of coal. In the tabulated statement of data collected by this same committee, the highest results of practice given were 208-watt hours per pound of coal. The next highest was 192, the lowest twenty-five, and the average 91.7-watt hours, or only 22.7 per cent of that which the best practice, under favorable conditions, should give.
By far the larger number of stations reporting are small ones, in which the highest economies would not be expected, and we find, as a matter-of-fact, that most of these report results below the average. Discarding all results based on a consumption of less than 10,000 pounds of coal, we have an average for the largest stations of 127.7-watt hours per pound of coal. This is equivalent to a consumption of about five pounds of coal per indicated horse-power. The best results given (208) are equivalent to about three pounds per indicated horse-power, or more than twice as much as the machinery involved requires for the
best results. Bad as this showing is, it seems
compare favorably with foreign practice, for Professor Unwin says: * "Probably up to the present (1893), the consumption has in no case been less than 3.3 pounds per indicated horse-power."
When we consider how enormously short the results of actual practice in lighting stations are of those which are obtainable under the most favorable conditions with the same machinery, it behooves us to inquire into the cause of this deficiency and to see how far we can remedy it.
Professor Kennedy has made some very careful tests to indicate the effect of variable load upon coal consumption. Dividing the day into three portions, he determined the fuel consumption, the feed water evaporated and the indicated and electric horse-power developed during each period in an English lighting station. It will be seen that during the periods of light load the fuel consumption per horse-power is very large.
Comparing the consumption of steam and coal during mean load with that during the hours of
*On the Development and Transmission of Power, page 28.
heavy load (6 P. M. to midnight), we find that the mean steam consumption per electric horse-power hour was twenty-four per cent greater and the mean coal consumption per electric horse-power hour was forty-six per cent greater the difference of twenty-two per cent being attributable to waste at boilers, due to irregular working. During the whole twenty-four hours the mean evaporation in pounds of water per pound of coal was only eighty-five per cent of the evaporation during the period of maximum load.
Captain Sankey has applied Mr. Willans' formula to find the steam consumption of one or more engines working against a variable load in an electric lighting station. He takes a normal midwinter load curve and examines how the necessary current could be supplied during the twenty-four hours (1) with an engine capable of exerting the maximum power required; (2) with smaller engines. The results. rearranged and a little modified, are given by Unwin in the following table. It is assumed for convenience that the maximum load is 500 electric horse-power and that the engines are non-condensing.
We thus see how closely these results agree with Professor Kennedy's figures, derived from direct tests.
which gave twenty-four per cent increased consumption at mean load over that at maximum load.
The efficiencies of engines and dynamos in lighting stations are maintained quite high, notwithstanding the variable output of the establishment, by throwing in and out units as the load varies, so that at most only one unit need be working at less than its best load, and the variation in efficiency of this one unit is chiefly due to the larger proportion of the whole that the friction losses bear at light load. Almost the whole loss of efficiency, due to variable load, is, therefore, attributable to the boiler.
There is still another class of losses which is not amenable to calculation, viz., those due to leaks from boiler fittings and steam pipes, with condensation losses in the steam pipes, and due to the wasteful use of steam by all present known means of boiler feed. As these losses continue practically constant throughout the twenty-four hours, they assume especial importance where the load factor is low.
To show more clearly at what relative cost we produce power in our lighting stations with their variable loads and under other commercial conditions where the load is fairly constant, I quote Professor Unwin :
I. Engines working on a very regular load, in conditions similar to those of an engine pumping to a reservoir. Here for one effective horse-power exerted during 7,884 hours annually, engines of 1.47 indicated horse-power must be provided. Such engines may be taken to use fourteen pounds of steam per indicated horse-power hour in the test trials, but in ordinary work seven and one-half per cent more must be allowed for leakage, working auxiliary engines and less careful attention; this makes the consumption fifteen
pounds per indicated horse-power, or 15 × 1,176 (· g15) · eighteen pounds per effective horse-power hour. At nine pounds of steam per pound of coal, allowing also five per cent for lighting and banking fires, the engine would use 2.1 pounds of coal per effective horse-power hour.
COST OF INSTALLATION PER EFFECTIVE HORSE-POWER.
Cost of engines or with reserve 1.47 indicated
horse-power = 1.47 X $44.5 ·
Cost of boilers 1.47 X $24
Cost of buildings 1.47 X $30.
ANNUAL COST OF WORKING PER EFFECTIVE HORSE-POWER.
Interest on $144.80 at 4 per cent...
Maintenance and depreciation-buildings at 2
Machinery at 72 per cent..
Total fixed annual cost.....
Coal, 2.1 pounds for 8,760 hours at $1.75 per ton
STEADY WORKING COST OF ONE ELECTRICAL HORSE-POWER.