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Interest on $182.38 at 4 per cent...

Maintenance and depreciation-buildings at 2

per cent....

$7.30

.93

Machinery at 71⁄2 per cent

Total fixed annual cost.....

Cost of coal, 2.21 pounds, 8,760 hours, at $1.75

per ton....

Cost of petty stores

Cost of driving, stoking and cleaning.

Total annual cost per electric horse-power,

I0.20

$18.43

$16.94

1.64

11.67

30.25

$48.68

II. Engines working with variable load under conditions similar to those of an electric lighting station. Here, for one effective horse-power supplied on the average throughout the year, engines of 5.87 indicated horse-power have to be provided. On account of the inefficiency and waste, due to variation of the load, it is best to estimate the steam and coal from experience in similar cases. Probably no electric lighting works with quite so so low a pounds of coal per hour per A consumption of nine pounds. is probably much more common in the best managed stations. Six pounds of coal per electrical unit corresponds to 3.8 pounds per effective horse-power hour.

station at present consumption as six electric unit supplied.

COST OF INSTALLATION.

Cost of engines for one average effective horse

power with reserve, 5.87 indicated horse

power 5.87 X $44.5..

Cost of boilers = 5.87 X $24.

Cost of buildings = 5.87 X $30

Total....

$261.22

140.88

176.10

$578.20

ANNUAL COST OF WORKING PER EFFECTIVE HORSE-POWER.

Interest on $578.20 at 4 per cent......
Maintenance and depreciation-machinery at

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Cost of coal, 3.8 pounds for 8,760 hours, at $1.75

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$23.13

30.25
3.50

$56.88

$26.00

1.64

11.67

39.31

$96.19

In other words, our power costs us 2,138 times as much in an electric light station as it should. If we figure out the relative costs per electrical horse-power, the disparity will be still greater.

Using these same data, but carrying the results out to electrical horse-power, assuming a dynamo efficiency of ninety-five per cent, we have four pounds of coal per electrical horse-power.

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$73-51

Cost of coal, 4 pounds for 8,760 hours, at $1.75

per ton...

Cost of petty stores...

Cost of driving, stoking and cleaning..

$30.63
1.64

$542.64, at 71⁄2 per cent.....

Maintenance and depreciation

Buildings, $185.10, at 2 per cent..

Total fixed annual cost....

Total annual cost electric horse-power,

12.00

44.27

$117.78

When we compare this with the cost resulting from steady working, viz., $48.68, we see what an effective method of storage means in our central stations. If our storage cost us nothing, it would result in a saving for each electrical horse-power generated per year of $117.78 — $48.68 $69.10.

But any system of storage which we may employ will involve expenditures, both in the way of losses in storage, which will have to be provided against by an increased plant to provide for the same ultimate output, and in the way of interest on the additional investment required by the storage plant. As a matter of fact, these additional costs must in all cases be less than the saving otherwise effected, else we are paying more for our economy than it amounts to. In the present case, our storage must cost us less than $69.10.

If we could take the variable load off between the boiler and engine, viz., devise some means by which the same amount of energy could be supplied by the continuous operation of a boiler plant working at its most economical rate, we should at once do away with one of the most serious losses due to irregular working. Let us see how much the saving would be if the irregular working of the boilers could be obviated without cost.

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Interest on $551.69 at 4 per cent...

Maintenance and depreciation machinery,

$431.69, at 71⁄2 per cent..

Buildings, $120, at 2 per cent.

Total fixed annual cost..

$274.56

37.13

I 20.00

120.00

$551.69

22.07

32.38

2.40

$56.85

Total fixed annual cost brought forward,

Cost of coal, 2.21 pounds for 8,760 hours, at

$1.75 per ton..

Cost of petty stores.

Cost of driving, stoking and cleaning.

Total annual cost per electrical horse-power,

$56.85

$16.93

1.64

11.67

30.24

$87.09

If the storage cost us nothing, and were applied between the boiler and the engine, it would affect a saving of $117.78 $87.09 $30.69. The limiting expense, therefore, to which we can go for storage at this point is $30.69 per annual electrical horse-power.

If we move our storage forward one step and introduced it between our engine and dynamo, so that everything previous to the dynamo would work continuously at its maximum economy, the account would stand thus:

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per ton....

Cost of petty stores...

$226.97, at 71⁄2 per cent....

Buildings, $120, at 2 per cent..

Total fixed annual cost.

Cost of coal, 2.21 pounds, 8,760 hours, at $1.75

Cost of driving, stoking and cleaning..

Total annual cost per electrical horse-power,

17.02

2.40

$33.28

$16.94

1.64

11.67

30.25

$63.53

=

The saving by introducing storage at this stage would be $117.78 - $63.53 $54.25, or a gain by moving the storage forward one step of $54.25 $30.89 $23.56. If we move it forward still another step, and bring it between the dynamo and distribution, we have an additional possible gain of $69.10$54.25 $14.85.

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Recapitulating, we have possible gains by introducing the storage by successive steps forward.

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Supposing, in the case assumed, we endeavor to equalize the load by means of a storage battery. This will allow the boilers, engines and dynamos to work continuously at their most economical rate, and would seem to provide ideal conditions of working. We

not, however, storing the product we have to sell, as is the gas manufacturer-we are not storing electrical energy, as some suppose, but chemical energy, which must be transformed again into electrical energy before we can distribute it. These two transformations, of course, involve a loss of energy which must be provided for in boilers, engines and dynamos and also in coal. To be entirely fair with the storage battery, let us assume that its efficiency under all conditions is seventy-five per cent. Then, in order that we may have at average working one electrical horsepower, we must provide:

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