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The actual cost in a given case will, of course, vary according to the size of the station and the local conditions, which latter include the costs of coal and water, the cost of skilled and rough labour, also transport and import charges.
The Table shows costs applicable to a large power distribution scheme on the Rand. Whatever adjustments might be necessary for any given scheme, as between work's costs, distributing costs, and capital charges, it is not considered probable that a public supply will be given throughout this district at lower charges than are here shown.
A public supply system must include capital charges on a higher scale than would be required in the case of a private plant, in order to recover in later years the losses incurred during the time taken in building up the load from small beginnings to the scale that is necessary in order to reach the low working costs per unit sold, corresponding to the low tariff, which must be low from the outset, in order to justify consumers in adopting the public supply in preference to their own plant.
It has also to be borne in mind that public electric power supply undertakings are never completed. They are bound to keep ahead of the demand, which results in new capital always being spent, and, for the time being, unproductive.
In estimating the cost of supplying electric energy to a group of mines, it must not be overlooked that the "diversity factor" in such cases is somewhere near unity, as the maximum loads on the several mines practically occur simultaneously. The supplier cannot therefore in such a case secure the advantage, previously referred to, of being able to meet the consumer's requirements, with a lower capital outlay per Kilowatt than the consumers would themselves incur in adopting their own independent power plant.
Where the load is practically constant-as, for example, a battery of stamp mills-there is no economical advantage in transmitting the power electrically. This would only be done when electric energy could be bought at a price which would show an advantage compared with the cost of applying the power direct from one large engine.
The following table (B) has been prepared in order to establish comparison between the assumed prices at which electrical energy may be purchased (Table A) and the cost of supplying power from the consumer's own plant, with or without electrical transformation.
The working costs are given for four sizes of engine-ranging from 125 Brake H.P. to 1000 B. H. P.-and for two load factors, namely 25 per cent. and 50 per cent.
The estimates are each based on the use of a single gas engine with gas producer plant, suitable for the use of bitumenous coal.
The cost of coal is assumed to be 12/6 per ton (200 lbs.) for the largest engine, rising to 14/- for the smallest engine, to allow for the smaller consumer buying less cheaply than the larger one.
The Public Supply Station is assumed to buy coal at 10/6 per ton.
Estimated cost of Power on the Rand, developed by Gas Engines and Producer Gas on the Consumer's premises.
I & III. With direct transmission.
II & IV. With electrical transmission.
Cost per B.H.P. hour at 25% Load Factor with direct transmission.
Costs per B.H.P. at 25% Load Factor with electrical transmission.
Cost per B.H.P. hour at 50% Load Factor with direct transmission.
Cost per B.H.P. at 50% load factor with local electrical transmission.
The conclusions to be drawn from comparing Tables A and B are the following :—
(a) That for fairly steady loads during ordinary factory hours (25 per cent. load factor), it is cheaper for the Power-User to employ his own engine applied direct --that is to say, without the intervention of electrical transmission-even below 125 H.P., which is estimated to cost 0.99d. per B.H.P. hour, and 1000 H. P. 0.65d. per B.H.P. hour against 1.06d. per B.H.P. hour purchased.
(b) That for ordinary factory hours, where the diversity factor would justify electrical transmission in any case, a private plant of 250 H.P. and upwards can compete successfully with the assumed purchased supply of energy.
(c) That for a 50 per cent. load factor a local 500 H.P. engine, applied to its load direct, would cost less than the assumed purchase of electric energy.
(d) That for a 50 per cent. load factor, when the diversity factor justifies electrical transmission in any case, a 1000 H.P. private plant (or larger) can successfully compete with the assumed purchased supply, after covering motor losses.
It will be noticed that the advantages of increased load factor are not so marked in the case of the private plant as with the public supply undertaking. This is due to the latter requiring in any case to employ staff for a 24 hours' service per day, whatever the load factor may be, whereas the private plant requires only one shift, so long as the running can be confined to ordinary factory hours.
The advantages of gas plant compared with steam plant, particularly for high load factors, are:
(1) The thermal efficiency is approximately twice as high as in a steam plant, consequently only about half the coal is consumed for a given duty.
(2) The incidental and stand-by losses of a gas producer plant are less than in a steam plant.
(3) The water consumption of a gas engine and a producer plant combined is far less than with steam plant.
(4) The repairs required by producers are less than the repairs required by boilers.
(5) The attendance required for a battery of producers is less than for a battery of boilers doing the same duty in H.P. hours.
The disadvantages of producer gas plant are:
(1) That-omitting types designed solely for the use of anthracite or coke the capital outlay is somewhat higher than for steam plant of similar capacity.
(2) When constructed for the use of bitumenous coal they occupy more space than steam plants.
(3) They are not so well understood by the average attendant as the steam engines and boilers.
There is little doubt that with the increasing demand for large gas engines and producers, their costs will in the near future be materially reduced, whilst the one other disadvantage of importance, namely, want of knowledge of gas plant on the part of attendants, will quickly pass away with their increasing use.
The losses through radiation and condensation of steam in long ranges of steam pipes is sometimes enormous. There are factories using a number of scattered steam engines consuming an average of 15 lbs. of coal per B.H.P. hour, whereas there are no such losses from gas mains.
Just as it took ten years to establish the Parsons steam turbine, so may several years elapse before the use of producer gas for the development of power becomes general, but the author is convinced that the vastly superior thermal efficiency of the gas engine compared with the steam engine will certainly secure for it the most prominent place in the second stage of the "Age of Mechanical Power "-the stage of the Internal Combustion Engine.