STORAGE OF POWER.
The three principal forms of stored power which are now in sight above
the horizon of the industrial outlook are the electric storage
battery, compressed air, and calcium-carbide. The first of these has
come largely into use owing to the demand for a regulated and stored
supply of electricity available for lighting purposes. Indeed the
storage battery has practically rendered safe the wide introduction of
electric lighting, because a number of cells, when once charged, are
always available as a reserve in case of any failure in the power or
in the generators at any central station; and also because, by means
of the storage cells or "accumulators," the amount of available
electrical energy can be subdivided into different and subordinate
circuits, thus obviating the necessity for the employment of currents
of very high voltage and eluding the only imperfectly-solved problem
of dividing a current traversing a wire as conveniently as lighting
gas is divided by taking small pipes off from the gas mains.
Compressed air for the storage of power has hitherto been best
appreciated in mining operations, one of the main reasons for this
being that the liberated air itself--apart from the power which it
conveyed and stored--has been so great a boon to the miner working in
ill-ventilated stopes and drives. The cooling effects of the
expansion, after close compression, are also very grateful to men
labouring hard at very great depths, where the heat from the country
rock would become, in the absence of such artificial refrigeration,
almost overpowering. For underground railway traffic exactly the same
recommendations have, at one period during the fourth quarter of the
nineteenth century, given an adventitious stimulus to the use of
compressed air.
Yet it is now undoubted that, even in deep mining, the engineer's best
policy is to adopt different methods for the conveyance and storage of
power on the one hand, and for the ventilation of the workings on the
other. Few temptations are more illusory in the course of industrial
progress than those presented by that class of inventions which aim at
"killing two birds with one stone". If one object be successfully
accomplished it almost invariably happens that the other is
indifferently carried out; but the most frequent result is that both
of them suffer in the attempt to adapt machinery to irreconcilable
purposes.
The electric rock-drill is now winning its way into the mines which
are ventilated with comparative ease as well as into those which are
more difficult to supply with air. It is plain, therefore, that on its
merits as a conveyer and storer of power the electric current is
preferable to compressed air. The heat that is generated and then
dissipated in the compression of any gas for such a purpose represents
a very serious loss of power; and it is altogether an insufficient
excuse to point to the compensation of coolness being secured from the
expansion. Fans driven by electric motors already offer a better
solution of the ventilation difficulty, and the advantages on this
side are certain to increase rather than to diminish during the next
few years.
The electric rock-drill, which can already hold its own with that
driven by compressed air, is therefore bound to gain ground in the
future. This is a type and indication of what will happen all along
the industrial line, the electric current taking the place of the
majority of other means adopted for the transmission of power. Even in
workshops--where it is important to have a wide distribution of power
and each man must be able to turn on a supply of it to his bench at
any moment--shafting is being displaced by electric cables for the
conveyance of power to numerous small motors.
The loss of power in this system has already been reduced to less than
that which occurs with shafting, unless under the most favourable
circumstances; and in places where the works are necessarily
distributed over a considerable area the advantage is so pronounced
that hardly any factories of that kind will be erected ten years hence
without resort being had to electricity, and small motors as the means
of distributing the requisite supplies of power to the spots where
they are needed. It was a significant fact that at the Paris
Exposition of 1900 the electric system of distribution was adopted.
In regard to compressed air, however, it seems practically certain
that, notwithstanding its inferiority to electric storage of power, it
is applicable to so many kinds of small and cheap installations that,
on the whole, its area of usefulness, instead of being restricted,
will be largely increased in the near future. There will be an advance
all along the line; and although electric storage will far outstrip
compressed air for the purposes of the large manufacturer, the air
reservoir will prove highly useful in isolated situations, and
particularly for agricultural work.
For example, as an adjunct to the ordinary rural windmill for pumping
water, it will prove much more handy and effective than the system at
present in vogue of keeping large tanks on hand for the purpose of
ensuring a supply of water during periods of calm weather. Regarding a
tank of water elevated above the ground and filled from a well as
representing so much stored energy, and also comparing this with an
equal bulk of air compressed to about 300 pounds pressure to the
square inch, it would be easy to show that--unless the water has been
pumped from a very deep well--the power which its elevation indicates
must be only a small fraction of that enclosed in the air reservoir.
It will be one great point in favour of compressed air, as a form of
stored energy for the special purpose of pumping, that by making a
continuous small flow of air take the place of the water at the lowest
level in the upward pipe, it is possible to cause it to do the pumping
without the intervention of any motor.
One means of effecting this may be simply indicated. The air under
pressure is admitted from a very small air pipe and the bubbles, as
they rise, fill the hollow of an inverted iron cup rising and falling
on a bearing like a hinge. Above and beneath the chamber containing
this cup are valves opening upwards and similar to those of an
ordinary force or suction pump. The cup must be weighted with
adjustable weights so that it will not rise until quite full of air.
When that point is reached the stroke is completed, the air having
driven upwards a quantity of water of equal bulk with itself, and, as
the cup falls again by its own weight, the vacuum caused by the air
escaping upwards through the pipe is filled by an inrush of water
through the lower valve. The function of the upper valve, at that
time, is to keep the water in the pipe from falling when the pressure
on the column is removed. The expansive power of the air enables it to
do more lifting at the upper than at the lower level, so that a larger
diameter of pipe can be used at the former place.
Cheap motors working on the same principle--that is to say through the
upward escape of compressed air, gas or vapour filling a cup and
operating it by its buoyancy, or turning a wheel in a similar
manner--will doubtless be a feature in the machine work of the future;
and for motors of this description it is obvious that compressed air
will be very useful as the form of power-storage. Excepting under
very special conditions, steam is not available for such a purpose,
seeing that it condenses long before it has risen any material
distance in a column of cold water.
"The present accumulator," remarked Prof. Sylvanus P. Thompson in the
year 1881, referring to the Faure storage batteries then in use,
"probably bears as much resemblance to the future accumulator as a
glass bell-jar used in chemical experiments for holding gas does to
the gasometer of a city gasworks, or James Watt's first model
steam-engine does to the engines of an Atlantic steamer." When Faure,
having in 1880 improved upon the storage battery of Plante, sent his
four-cell battery from Paris to Glasgow, carrying in it stored
electrical energy, it was found to contain power equal to close upon a
million foot-pounds, which is about the work done by a horse-power
during the space of half an hour. This battery weighed very nearly 75
lb. It nevertheless represented an immense forward step in the problem
of compressing a given quantity of potential power into a small weight
of accumulator.
The progress made during less than twenty years to the end of the
century may be estimated from the conditions laid down by the
Automobile Club of Paris for the competitive test of accumulators
applicable to auto-car purposes in 1899. It was stipulated that five
cells, weighing in all 244 lb., should give out 120 ampere-hours of
electric intensity; and that at the conclusion of the test there
should remain a voltage of 1.7 volt per cell.
Very great improvements in the construction of electric accumulators
are to be looked for in the near future. Hitherto the average duration
of the life of a storage cell has not been more than about two years;
and where impurities have been present in the sulphuric acid, or in
the litharge or "minium" employed, the term of durability has been
still further shortened. It must be remembered that while the
principal chemical and electrical action in the cell is a circular
one,--that is to say, the plates and liquids get back to the original
condition from which they started when beginning work in a given
period,--there is also a progressive minor action depending upon the
impurities that may be present. Such a reagent, for instance, as
nitric acid has an extremely injurious effect upon the plates.
During the first decade after Plante and Faure had made their original
discoveries, the main drawback to the advancement of the electric
accumulator for the storage of power owed its existence to the lack of
precise knowledge, among those placed in charge of storage batteries,
as to the destructive effects of impurities in the cells. It is,
however, now the rule that all acids and all samples of water used for
the purpose must be carefully tested before adoption, and this
practice, in itself, has greatly prolonged the average life of the
accumulator cell.
The era of the large electric accumulator of the kind foreshadowed by
Prof. Sylvanus P. Thompson has not yet arrived, the simple reason
being that electric power storage--apart from the special purposes of
the subdivision and transmission for lighting--has not yet been tried
on a large scale. For the regulation and graduation of power it is
exceedingly handy to be able to "switch-on" a number of small
accumulator cells for any particular purpose; and, of course, the
degree of control held in the hands of the engineer must depend
largely on the smallness of each individual cell, and the number which
he has at command. This fact of itself tends to keep down the size of
the storage cell which is most popular.
But when power storage by means of the electric accumulator really
begins in earnest the cells will attain to what would at present be
regarded as mammoth proportions; and the special purpose aimed at in
each instance of power installation will be the securing of
continuity in the working of a machine depending upon some
intermittent natural force. Windmills are especially marked out as the
engines which will be used to put electrical energy into the
accumulators. From these latter again the power will be given out and
conveyed to a distance continuously.
High ridges and eminences of all kinds will in the future be selected
as the sites of wind-power and accumulator plants. In the eighteenth
century, when the corn from the wheat-field required to be ground into
flour by the agency of wind-power, it was customary to build the mill
on the top of some high hill and to cart all the material laboriously
to the eminence. In the installations of the future the power will be
brought to the material rather than the material to the power. From
the ranges or mountain peaks, and also from smaller hills, will
radiate electrical power-nerves branching out into network on the
plains and supplying power for almost every purpose to which man
applies physical force or electro-chemical energy.
The gas-engine during the twentieth century will vigorously dispute
the field against electrical storage; and its success in the
struggle--so far as regards its own particular province--will be
enhanced owing to the fact that, in some respects, it will be able to
command the services of electricity as its handmaid. Gas-engines are
already very largely used as the actuators of electric lighting
machinery. But in the developments which are now foreshadowed by the
advent of acetylene gas the relation will be reversed. In other words,
the gas-engine will owe its supply of cheap fuel to the electric
current derived at small expense from natural sources of power.
Calcium carbide, by means of which acetylene gas is obtained as a
product from water, becomes in this view stored power. The
marvellously cheap "water-gas" which is made through a jet of steam
impinging upon incandescent carbons or upon other suitable glowing hot
materials will, no doubt, for a long time command the market after the
date at which coal-gas for the generation of power has been partially
superseded.
But it seems exceedingly probable that a compromise will ultimately be
effected between the methods adopted for making water-gas and calcium
carbide respectively, the electric current being employed to keep the
carbons incandescent. When power is to be sold in concrete form it
will be made up as calcium carbide, so that it can be conveyed to any
place where it is required without the assistance of either pipes or
wires. But when the laying of the latter is practicable--as it will be
in the majority of instances--the gas for an engine will be obtainable
without the need for forcing lime to combine with carbon as in calcium
carbide.
Petroleum oil is estimated to supply power at just one-third the price
of acetylene gas made with calcium carbide at a price of L20 per ton.
This calculation was drawn up before the occurrence of the material
rise in the price of "petrol" in the last year of the nineteenth
century; while, concurrently, the price of calcium carbide was
falling. A similar process will, on the average, be maintained
throughout each decade; and, as larger plants, with cheaper natural
sources of energy, are brought into requisition, the costs of power,
as obtained from oil and from acetylene gas, will more and more
closely approximate, until, in course of time, they will be about
equal; after which, no doubt, the relative positions will be reversed,
although not perhaps in the same ratio. Time is all on the side of the
agent which depends for its cheapness of production on the utilisation
of any natural source of power which is free of all cost save
interest, wear and tear, and supervision.
Even the steam-engine itself is not exempt from the operation of the
general law placing the growing advantage on the side of power that is
obtainable gratis. One cubic inch of water converted into steam and at
boiling point will raise a ton weight to the height of one foot; and
the quantity of coal of good quality needed for the transformation of
the water is very small. One pound of good coal will evaporate nine
pounds of water, equal to about 250 cubic inches, this doing 250
foot-tons of work. But Niagara performs the same amount of work at
infinitely less cost. However small any quantity may be, its ratio to
nothing is infinity.
It has been the custom during the nineteenth century to institute
comparisons between the marvellous economy of steam power and the
expensive wastefulness of human muscular effort. For instance, the
full day's work of an Eastern porter, specially trained to carry heavy
weights, will generally amount to the removal of a load of from three
to five hundred-weight for a distance of one mile; but such a labourer
in the course of a long day has only expended as much power as would
be stored up in about five ounces of coal.
Still the fact remains that one of the greatest problems of the future
is that which concerns the reduction in the cost of power. Hundreds
of millions of the human race pass lives of a kind of dull monotonous
toil which develops only the muscular, at the expense of the higher,
faculties of the body; they are almost entirely cut off from social
intercourse with their fellow-men, and they sink prematurely into
decrepitude simply by reason of the lack of a cheap and abundant
supply of mechanical power, ready at hand wherever it is wanted.
Scores of "enterprises of great pith and moment" in the industrial
advancement of the world have to be abandoned by reason of the same
lack. In mining, in agriculture, in transport and in manufacture the
thing that is needful to convert the "human machine" into a more or
less intelligent brainworker is cheaper power. All the technical
education in the world will not avail to raise the labourer in the
intellectual scale if his daily work be only such as a horse or an
engine might perform.
The transmission of power through the medium of the electric current
will naturally attain its first great development in the
neighbourhoods of large waterfalls such as Niagara. When the
manufacturers within a short radius of the source of power in each
case have begun to fully reap the benefit due to cheap power,
competition will assert itself in many different ways. The values of
real property will rise, and population will tend to become congested
within the localities' served.
It will be found, however, that facilities for shipment will to a
large extent perpetuate the advantage at present held by manufactories
situated on ports and harbours; and this, of course, will apply with
peculiar force to the cases of articles of considerable bulk. Where a
very great deal of power is needed for the making of an article or
material of comparatively small weight and bulk proportioned to its
value--such for instance as calcium carbide or aluminium--the
immediate vicinity of the source of natural power will offer
superlative inducements. But an immense number of things lie between
the domains of these two classes, and for the economical manufacture
of these it is imperative that both cheap power and low wharfage rates
should be obtainable.
An increasingly intense demand must thus spring up for systems of long
distance transmission, and very high voltage will be adopted as the
means of diminishing the loss of power due to leakage from the cables.
Similarly the "polyphase" system--which is eminently adapted to
installations of the nature indicated--must demand increasing
attention.
Taking a concrete example, mention may be made of the effects to be
expected from the proposed scheme for diverting some of the headwaters
of the Tay and its lakes from the eastern to the western shores of
Scotland and establishing at Loch Leven--the western inlet, not the
inland lake of that name--a seaport town devoted to manufacturing
purposes requiring very cheap supplies of power. It is obvious that
the owners of mills in and around Glasgow, and only forty or fifty
miles distant, will make the most strenuous exertions to enable them
to secure a similar advantage.
It is already claimed that with the use of currents of high voltage
for carrying the power, and "step-down transformers" converting these
into a suitable medium for the driving of machinery, a fairly
economical transmission can be ensured along a distance of 100 miles.
It therefore seems plain that the natural forces derived from such
sources as waterfalls can safely be reckoned upon as friends rather
than as foes of the vested interests of all the great cities of the
United Kingdom.
The possibilities of long distance transmission are greatly enhanced
by the very recent discovery that a cable carrying a current of high
voltage can be most effectually insulated by encasing it in the midst
of a tube filled with wet sawdust and kept at a low temperature,
preferably at the freezing point of water.
Wireless transmission of a small amount of power has been proved to be
experimentally possible. In the rarefied atmosphere at a height of
five or ten miles from the earth's surface, electric discharges of
very high voltage are conveyed without any other conducting medium
than that of the air. By sending up balloons, carrying suspended
wires, the positions of despatch and of receipt can be so elevated
that the resistance of the atmosphere can be almost indefinitely
diminished. In this way small motors have been worked by discharges
generated at considerable distances, and absolutely without the
existence of any connection by metallic conductors. Possibilities of
the exportation of power from suitable stations--such as the
neighbourhoods of waterfalls--and its transmission for distances of
hundreds or even thousands of miles have been spoken of in relation to
the industrial prospects of the twentieth century.
Comparing any such hypothetical system with that of sending power
along good metallic conductors, there is at once apparent a very
serious objection in the needless dispersion of energy throughout
space in every direction. If a power generator by wireless
transmission, without any metallic connection, can work one motor at
a distance of, say, 1,000 miles, then it can also operate millions of
similar possible motors situated at the same distance; and by far the
greater part of its electro-motive force must be wasted in upward
dispersion.
The analogy of the wireless transmitter of intelligence may be
misleading if applied to the question of power. The practicability of
wireless telegraphy depends upon the marvellous susceptibility of the
"coherer," which enables it to respond to an impulse almost
infinitesimally small, certainly very much smaller than that
despatched by the generator from the receiving station. From this it
follows, as already stated, that the analogy of apparatus designed
merely for the despatch of intelligence by signalling cannot safely be
applied to the case of the transmission of energy.
Making all due allowances for the prospects of advance in minimising
the resistance of the atmosphere, it must nevertheless be remembered
that any wireless system will be called upon to compete with improved
means of conveying the electric current along metallic circuits.
Electrical science, moreover, is only at the commencement of its work
in economising the cost of power-cables.
The invention by which one wire can be used to convey the return
current of two cables very much larger in sectional area is only one
instance in point. The two major cables carry currents running in
opposite directions, and as these currents are both caused to return
along the third and smaller wire their electro-motive forces balance
one another, with the result that the return wire needs only to carry
a small difference-current. The return wire, in fact, is analogous to
the Banking Clearing House, which deals with balances only, and which
therefore can sometimes adjust business to the value of many millions
with payments of only a few thousands. Later on it may fairly be
expected that duplicate and quadruplicate telegraphy will find its
counterpart in systems by which different series of electrical
impulses of high voltage will run along a wire, the one alternating
with the other and each series filling up the gaps left between the
others.