Popular Science Monthly/Volume 19/August 1881/The Electric Storage of Energy
|"THE ELECTRIC STORAGE OF ENERGY."|
SOME few weeks ago a letter appeared in the "Times," signed "F.R.S.," describing a "box of lightning" which the writer had brought over from Paris for the purpose of submitting it to Sir William Thomson. Since then a long discussion has taken place on the subject of the invention and its usefulness. To begin with, we fully share the regret of Professor Tyndall, who has written a letter on the matter, that so much loose nomenclature has been introduced into the subject. The term "electric storage of energy" appears to us to be singularly unhappy. What is known as a condenser, or a Leyden-jar, is truly an instrument for the electric storage of energy, because, when charged, its parts are in a condition of molecular strain, which is recognized as an electrical phenomenon; and the release of this state of strain invariably produces at first some of the phenomena of electricity in motion. But in the case of M. Faure's secondary battery, which is the invention under discussion, although it is charged by a current of electricity and gives out a current of electricity, the form of the store of energy which it contains is not that of electrical stress or strain, but that of chemical separation—a form of potential energy which can be caused, under certain circumstances, to become kinetic energy in the form of heat. However, the term has now become established, and, being convenient, will probably survive. But it is to be hoped that the real state of things will be thoroughly and publicly explained by our leaders of science, so that the use of this form of words may not cause a confusion in scientific ideas.
From the ease with which secondary batteries can be constructed of very low resistance, so that they will give for a short time what practical electricians call a quantity current, they have been for some time in use for certain special purposes, principally for heating the wire of the galvanic écraseur in surgical practice. By a secondary battery is meant a galvanic battery which, as at first put together, has no tendency to give a current at all; but, if a current of electricity be passed through it of sufficient tension to decompose the fluids which it contains, will give a current in the opposite direction, due to the recombination of the separated parts of the decomposed fluid. The older forms consisted of two plates of platinum, preferably coated with spongy platinum immersed in a weak mixture of sulphuric acid and water, the action in this case being that the charging current decomposes the water (either directly or as the result of a chemical action set up by decomposing the acid first) into oxygen and hydrogen, which gases are absorbed by the platinum plates, the oxygen by one and the hydrogen by the other. When the charging battery is removed, the secondary battery will give a powerful current until all the oxygen and hydrogen absorbed by the plates are recombined in the form of water. It was afterward found that satisfactory results could be got from plates of lead treated in the same way. Their employment, of course, reduced the first cost of the apparatus. M. Planté then produced his secondary battery, in which he obtained great surface, and consequently low internal resistance, and large current, by rolling into a spiral form two lead plates separated by pieces of insulating material placed between them at intervals. He further succeeded in greatly increasing the time for which the battery would yield a given current, or its capacity, by adopting an elaborate process for the "formation" of the plates, which consisted in charging the battery and discharging it, varying the direction of the exciting current, and leaving the battery undisturbed between the charging and discharging for gradually increasing intervals of time. This process added enormously to the expense of the apparatus, which was also too bulky and heavy.
M. Faure, however, has succeeded in increasing the capacity of the battery, and getting rid of the long and delicate process of formation. His battery, like M. Planté's, consists of two plates of lead rolled together into a spiral, but he coats each plate with a thin layer of red lead (one of the oxides of that metal), kept in its place by a piece of absorbent felt, which also keeps the two plates from touching. This felt is saturated with the weak acid. The effect of the exciting current in this case is to deposit spongy lead on one plate and to convert the red oxide on the other into puce-colored oxide which contains more oxygen than the red form; no doubt, also, the spongy lead at a late period of the charging becomes saturated with hydrogen. When the battery is now set in action, the spongy lead becomes reoxidized to red lead and the puce-colored oxide reduced to the same salt.
Sir William Thomson early in this month wrote to the "Times," pointing out the great advance which this invention had made in the practical and economical storage of energy. His letter was answered by Professor Osborne Reynolds, who, with the intention of preventing the public from being astonished at the storing of so much energy as one million foot-pounds in apparatus occupying a cubic foot of space and weighing about seventy-two pounds, proceeded—somewhat irrelevantly as we think—to discuss the energy contained in a pound of coal, and also to complicate the now inevitable controversy by referring to a totally different problem, the transmission of energy by electrical means. The controversy thus started has gone on. Sir William Thomson, Professor Osborne Reynolds, Professor Ayrton, and Professor Tyndall, taking part in it.
The question, as far as the public are concerned, is a purely commercial one. As yet, of course, the data of the cost of the battery and its durability are not yet ascertained; but, in any future discussion on the subject, the question of convenience, as well as that of absolute expense, wall have to be taken into consideration. At present we know that, at some expense, probably not too great, we can utilize a source of energy of feeble power for many purposes by allowing it to act for a long time, collecting its energy, and using it quickly, and that the loss in the process wall be but small; and that, further, if it be desired to use the electric light temporarily, it can be produced conveniently, if not economically, by the use of M. Faure's invention. Sir William Thomson in his first letter points out many practical uses for the new invention; we may supplement them by pointing out how the new secondary battery may be applied conveniently for many purposes. Three ordinary Daniell's cells will charge an element of the new battery easily, so that, if there be plenty of time for preparation, we can, by the aid of Faure's batteries, use this cleanly apparatus, which gives off no noxious fumes and needs but little attention, for all the purposes for which, up to the present time, we were obliged to employ the costly and troublesome Grove's or Bunsen's batteries, which contain violent caustic poisons, and give off irritating and unwholesome fumes.
The whole discussion about the mechanical value of coal seems to us mistaken; neither Sir William Thomson nor any other physicist proposes to use the new battery universally, and, at present, our cheapest way of charging it is by the use of a dynamo-electric machine, driven by a steam-or gas-engine—i. e., by making use of the mechanical power of coal and the oxygen of the air; setting aside, of course, the exceptional cases where water-power is to be obtained. Sir William Thomson himself gave, we think, the coup de grâce to any attempt at comparing the relative values of transmitting electric currents through conductors from the source of energy to a distant station where energy is wanted, and conveying energy by exciting Faure's batteries at the one place and conveying them to another, when he wrote in one of his letters of "Professor Reynolds's disappointment with M. Faure's practical realization of electric storage, because it does not provide a method of porterage superior to conduction through a wire." This is "like being disappointed with an invention of improvements in water-cans and water-reservoirs because the best that can be done in the way of movable water-cans and fixed water-reservoirs will never let the water-carrier supersede water-pipes wherever water pipes can be laid." If we may venture to extend the great electrician's metaphor, it is like finding fault with the Great Eastern Railway Company's service of sea-water brought to London in cans, on the ground that it is just possible to obtain sea-water by a large main laid down to the coast, and that such a scheme is now under consideration. Another valuable property of the new battery is pointed out by Sir William Thomson. If it were to be used either at a fixed station to work an electric railway, such as the firm of Siemens have already brought into practical use, or to be carried on an ordinary carriage to drive it, the energy developed by the vehicle in running down-hill would be stored up ready to be used for its propulsion when it again reached a level or an ascending incline.
In the course of the correspondence Professor Ayrton has again mentioned the experiments which he and Professor Perry are carrying out with the view of using coal or coal-gas instead of zinc in a primary battery. Should he succeed in doing so, we should obtain a source of energy about ten times cheaper in working than the best-known steam engine, and M. Faure's invention may very likely be the means of making it a commercial success; for, should Messrs. Ayrton and Perry, or any other physicist, succeed in making a coal or coal-gas battery giving a good proportion of the theoretical energy of the coal or gas, should it have a high internal resistance, it would be difficult to use it in practice; but, by the aid of Faure's batteries, in cases where work was only wanted to be done for a few hours a day, as in the case of electric lighting, the comparatively feeble current of the primary battery might be collected and stored for fifteen or sixteen hours, and then allowed to run out again in the eight or nine hours for which the source of energy is practically wanted.
The subject of this new secondary battery is one of great scientific importance. As the writer of a leader in the "Times" points out, it is by no means unlikely that a similar piece of apparatus may be made of some metal, and its appropriate salt, which shall be cheaper and lighter than one of M. Faure's form of similar powers; at all events, the invention and its results are pretty sure to turn the attention of inventors and investigators toward batteries both secondary and primary—a branch of inquiry which has for so many years been quite thrown aside in favor of endeavors to improve the dynamo-machine. Now, a primary battery is theoretically the most economical artificial source of energy, and it is only the comparatively high cost of the fuel generally used in these—zin—which prevents them from being practically useful. A galvanic battery gives out very nearly the whole energy due to the chemical combinations which take place in it; so that it is hardly too much to say that, were a battery to be employed to drive an electro-motor, under suitable conditions, we could obtain at least sixty per cent, of the chemical energy, while the best-known steam-engine will only give about ten per cent, of the chemical energy of the coal and air consumed in its furnace. There is thus a large margin for the first cost of the substance to be consumed in the battery.—Saturday Review.