Popular Science Monthly/Volume 9/September 1876/Industrial Applications of Solar Heat
THE history of burning-mirrors of brass is known. At Rome the sacred fire was lighted with apparatus of this kind, and Archimedes fired the ships which were blockading Syracuse by concentrating upon them the sun's rays by means of a large reflector. Buffon repeated successfully the experiments of Archimedes. With a mirror of very slight curvature, consisting of a number of pieces of looking-glass, he set fire, at some distance, to fir and beech planks, melted tin and silver, and brought iron to a red heat. Saussure later accumulated, by means of superimposed inclosures of glass, the sun's heat up to a temperature exceeding that of boiling water, and Sir John Herschel repeated these experiments at the Cape of Good Hope at various times between 1834 and 1838. At the same period the French physicist Pouillet was engaged at Paris in measuring the calorific intensity of solar radiation, arriving at the conclusion that the heat emitted from the sun and poured down upon the earth in one year would suffice to melt a sheet of ice thirty metres thick, and enveloping the entire globe.
About the year 1860, M. Mouchot, then Professor of Mathematics in the Lycée of Alençon, being stimulated by the researches of Pouillet as well as by those of Melloni, the ablest of Italian physicists, who has made experiments of incomparable precision upon the transmission of heat, boldly attacked the question of the utilization of the sun's heat. The mechanical equivalent of heat had at length been determined. Thanks to Melloni, we already knew the quantity of caloric which different bodies, as glass, when reduced to thin laminæ, suffer to pass through, as also the difference in the reflecting power of polished metallic surfaces according to the nature of the metals, employed. But to measure the amount of vis viva transmitted daily from the sun to the earth, and, more Utopian still, to concentrate, at little cost, the sun's rays, so as to realize all the effects of which they are capable, were objects the attainment of which was henceforth insured, though Buffon and Saussure had failed, owing to the insufficiency of the data at their command. The question is now merely a matter of calculation, an application of well-known physical laws.
In order to concentrate to any useful purpose the sun's rays, there was need of a receiver which should be of moderate size and reasonable cost. After sundry attempts, one of which was with an apparatus resembling that of Saussure, Mouchot contrived a vertical boiler of copper, blackened on the outside, covered with three concentric bell-glasses, and resting on some bad conductor of heat, as sand, brick, or wood. Soon he increased the power of his apparatus by the addition of a metallic reflector, which enabled him to dispense with two of the three bell-glasses. With this apparatus he considerably raised the temperature of the water in the boiler, reduced it to vapor, melted sulphur, the liquefaction temperature of which is 116° C., and after twenty minutes of insolation brought the empty boiler up to the temperature of 200° C.
With this reflector a few seconds suffice to set on fire a heap of shavings or a piece of board. In a glass vessel placed at the focus of the reflector and inclosed in another vessel of glass, one kilogramme of tin has been melted in two minutes; the same quantity of lead took five minutes, and of zinc, six. The fusion-point of these three metals is 235°, 335°, and 475° C. respectively. With spherical or parabolic mirrors, whose focus is a point, and not a line, as in the conical or cylindrical mirrors employed in the foregoing experiments, the concentration of solar heat would have been still stronger.
While engaged in these investigations, the ingenious experimenter brought out his Marmite Solaire, a cylindrical glass vessel, in which is placed another cylinder of copper or of wrought-iron blackened on the outside, and resting on the bottom of the glass receiver. The whole is covered with a glass lid. A cylindrical reflector of silver directs the sun's rays upon the apparatus. With this marmite it takes less than four hours to prepare an excellent pot-au-feu, consisting of one kilogramme of beef and a quantity of vegetables, the whole being perfectly cooked, and very palatable, owing to the fact of the heat being applied with great regularity.
In this form of marmite, now superseded by a simple glass vessel fixed at the focus of a conical mirror of silver-plated brass, fruits, potatoes, all sorts of legumes, meats, and grains, are cooked by solar heat. So, too, an infusion of tea or coffee can be readily prepared, and for this purpose we may employ one of those bottles of colored glass in which Lyons beer is put up. To cook legumes or grains rapidly, a different course may be taken. A closed vessel containing water is set in the focus of the reflector, and, when the liquid begins to boil, the upper portion of the vessel is connected by a tube with the bottom of another containing the legumes or grains, which are quickly cooked by the steam.
To transform the marmite into an oven, a disk of wrought-iron is placed beneath the glass lid, and in less than three hours a kilogramme of bread is baked. The crust is hard and brown, and the pith light and well raised, as with bread baked in an ordinary oven.
The roasting of meat, not requiring the same amount of heat as does the vaporization of an equal weight of water, can be performed in the open air, by the action of the solar reflector alone, the piece of beef, veal, or mutton being fixed upon a spit. In less than an hour we have in this way a very fine roast. The use of butter must be avoided, lest the chemical rays, by transforming the butter into butyric acid, should spoil the flavor of the meat. By interposing a pane of green or red glass we can intercept the chemical rays which cause this fermentation, and then the result leaves nothing to be desired.
By substituting for the two lids of the solar marmite an alembic-head, the apparatus can be used for the purposes of distillation. To this end the alembic-head is connected, by an horizontal tube, with a worm, the latter descending in the form of a helix and dipping into a constant current of cold water, while the metallic vessel, containing two litres of wine, is inclosed in the glass cylinder and set in the focus of the reflector. The alcohol is collected after forty minutes of exposure to the sun. Inasmuch as the apparatus grows hot slowly and continuously, the alcohol is highly concentrated and possesses a very agreeable aroma.
In all the foregoing experiments, M. Mouchot at first used concave silver mirrors, cylindro-parabolic in form, i. e., cylindrical mirrors whose base-line is an open curve resembling a parabola. The reflecting power of cylindrical mirrors increases in proportion to their aperture, and hence the time required, for instance, to boil a litre of water is inversely as the aperture of the mirror, i. e., the greater the aperture the shorter the time. But later the inventor has employed only conical mirrors, and in these the insolation surface is quadrupled when the diameter of the mirror is doubled.
Mouchot's researches did not end here. He proposed further to obtain mechanical effects with solar heat, and in the beginning of August, 1866, he put in operation at Paris the first machine of this kind.
In the mean time Ericsson was studying these same problems, without knowing anything about Mouchot's experiments. Starting from the facts collected by Herschel and Pouillet, Ericsson, in the first place, estimated the action of the sun upon a surface of nine square metres to be sufficient to vaporize eight litres of water; consequently it would be equal to one horse-power. From these premises he deduces striking consequences, as, for instance, that the solar heat falling on the roofs of Philadelphia alone would suffice to drive 5,000 steam-engines of twenty horse-power each. Then, having demonstrated that upon one square mile, using only one-half of the surface and devoting the remainder to buildings, roads, etc., we can drive 64,800 steam-engines, each of a hundred horse-power, simply by the heat radiating from the sun, he adds these remarkable words: "Archimedes, having completed his calculation of the force of a lever, said that he could move the earth; I affirm that the concentration of the heat radiated by the sun would produce a force capable of stopping the earth in its course." Again: "In England they are beginning to calculate the time when the coal will give out, though coal-mines are, so to speak, of recent exploitation. A few thousands of years—drops in the ocean of time—will exhaust the coal-mines of Europe, unless, meanwhile, recourse is had to the aid of the sun. True, the sun's beams do not every day reach the surface of the earth; but, when the great magazine is opened which shall supply heat gratuitously without cost of transportation, the prudent engineer will know how to provide a reserve against cloudy days. At the same time we would observe that a large proportion of the earth's surface is illumined by an ever-radiant sun. The solar engine's sphere of activity is as great as its dynamic power is considerable." Mr. Ericsson, who, besides genius, possesses wealth and a long experience, will doubtless some day take up again his studies upon the mechanical application of solar heat. Meanwhile, we must state what has been done in this direction by a Frenchman.
The traveler who visits the library of Tours sees in the court-yard in front a strange-looking apparatus. Imagine an immense truncated cone, a mammoth lamp-shade, with its concavity directed skyward. This apparatus is of copper, coated on the inside with a very thin silver-leaf. On the small base of the truncated cone rests a copper cylinder, blackened on the outside, its vertical axis being identical with that of the cone. This cylinder, surrounded as it were by a great collar, terminates above in a hemispherical cap, so that it looks like an enormous thimble, and is covered with a bell-glass of the same shape.
This curious apparatus is nothing else but a solar receiver, or, in other words, a boiler, in which water is made to boil by the heat-rays of the sun. This steam-generator is designed to raise water to the boiling-point and beyond, by means of the solar rays, which are thrown upon the cylinder by the silvered inner surface of the conical reflector. The boiler receives water up to two-thirds of its capacity through a feed-pipe. A glass tube and a steam-gauge communicating with the inside of the generator, and attached to the outside of the reflector, indicate both the level of the water and the pressure of the steam. Finally, there is a safety-valve to let off the steam when the pressure is greater than is desired. Thus the engine offers all desirable safety, and may be provided with all the accessories of a steam-boiler.
The reflector, which is the main portion of the generator, has a diameter of 2.60 metres at its large, and one metre at its small base, and is eighty centimetres in height, giving four square metres of reflecting surface, or of insolation. The interior walls are lined with burnished silver, because that metal is the best reflector of the heat-rays: still brass with a light coating of silver would also serve the purpose. The inclination of the walls of the apparatus to its axis measures 45°. Even the ancients were aware that this is the best form for this kind of metallic mirrors with linear focus, inasmuch as the incident rays parallel to the axis are reflected perpendicularly to the same, and thus give a focus of maximum intensity.
The boiler is of copper, which of all the common metals is the best conductor of heat; it is blackened on the outside, because black possesses the property of absorbing all the heat-rays, just as white reflects them; and it is inclosed in a glass envelope, glass being the most diathermanous of all bodies—that is to say, the most permeable by the rays of luminous heat. Glass further possesses the property of resisting the exit of these same rays after they have been transformed into dark rays on the blackened surface of the boiler. None of these applications of physical laws present any novelty; people reduced them to practice instinctively, as it were, before men of science could assign the reasons. Here the arts of cookery and of gardening, and the processes for warming our rooms, did not wait for the experiments of the physicist. Saussure himself started from these data in his researches; but the inventor needed the discoveries of modern physics in order to give to these applications a rigorous formula.
The boiler proper of the Tours solar engine consists of two concentric bells of copper, the larger one, which alone is visible, having the same height as the mirror, i. e., eighty centimetres, and the smaller or inner one fifty centimetres; their respective diameters are twenty-eight and twenty-two centimetres. The thickness of the metal is only three millimetres. The feed-water lies between the two envelopes, forming an annular envelope three centimetres in thickness. Thus the volume of liquid is twenty litres, and the steam-chamber has a capacity of ten litres. The inner envelope is empty. Into it pass the steam-pipe and the feed-pipe of the boiler. To the steam-pipe are attached the gauge and the safety-valve. The bell-glass covering the boiler is eighty-five centimetres high, forty centimetres in diameter, and five millimetres in thickness. There is everywhere a space of five centimetres between its walls and those of the boiler, and this space is filled with a layer of very hot air.
The earth, owing to its diurnal and annual revolution, does not occupy the same position with regard to the sun at all hours of the day, or in all seasons of the year. This being the case, the generator is so contrived as to revolve 15°, or one twenty-fourth of its circumference, hourly around an axis parallel to the earth's axis, i. e., so as to follow the apparent diurnal motion of the sun, and to incline gradually on this axis in proportion to the solar declination. Hence the intensity of the utilized heat is always nearly the same, whatever the hour of the day or the season of the year, inasmuch as the apparatus is always so arranged as to reflect with the least possible loss all the rays emitted by the sun. This double motion of the generator is effected by a very simple contrivance.
The generator just described is the one which M. Mouchot was enabled three years and a half ago to set up at Tours, the Conseil Général of Indre-et-Loire having provided the funds. It has yielded curious results, some of which are worthy of being recorded here, though before long they will be surpassed, when some improvements have been made in the apparatus. On May 8, 1875, the weather being fine, twenty litres of water at 20° C. temperature was introduced into the boiler at 8.30 a. m., and took only forty minutes to produce steam with a pressure of two atmospheres; in other words, a temperature of 121° C. was obtained, which is 21° centigrade degrees above boiling-point. This steam then quickly acquired a pressure of five atmospheres. This was the safety limit of the strength of the apparatus: if the process had been carried any further the boiler would have exploded. Toward noon on the same day, with fifteen litres of water in the boiler, steam at 100° C., i. e., a pressure of one atmosphere, was raised in less than fifteen minutes to five atmospheres—a temperature of 153° C. Finally, on July 22d, about one hour after mid-day, the heat being exceptionally great, the apparatus reduced to vapor five litres of water per hour, which is equal to one hundred and forty litres of steam per minute, or half a horse-power.
A steam-engine consists of two principal parts, the boiler and the engine proper, or motor. We suppose that with the boiler employed at Tours we can use the common motors; this is one of the advantages possessed by the solar apparatus, viz., that it does not require a special form of motor. At first the inventor employed for his demonstrations a double-acting engine, without either condensation or detention of steam, the cylinder of which had a capacity of one-third of a litre. This engine performed eighty strokes per minute, with a steady pressure of one atmosphere; it continued to work even under a slightly-clouded sun. This was later superseded by a rotary engine, that is, an engine with revolving cylinder, which avoids all transmission of movement; but the system is faulty. Yet this engine worked very well, driving at high velocity a little pump for raising water; the pump, however, being of weak construction, became disabled. It is a pity that the inventor has never measured the real work performed by his engine, by means of a dynamometer.
The solar reflector, being first of all a furnace using fuel that costs nothing, is not only of use as a means of developing motive force, but can also be employed for a multitude of purposes—for instance, distilling water to make it fit for drinking, concentrating and crystallizing saline solutions, preparing alcohol, etc. Five litres of wine can be distilled in a quarter of an hour by passing the vapor from the apparatus into a still. The manufacture of alcohol from grain, sugarcane, or beet-root, would be equally easy. The steam generated by this apparatus can also be employed for cooking fodder for cattle. M. Mouchot has devised a form of small marmites, quite different from his large steam-generator. These can be used by hunters for preparing their meals, and explorers of great deserts will now have something besides camel or buffalo chips for cooking their victuals.
Many and varied are the uses of this curious invention. The aëronaut can with its aid propel his air-ship. Hot-air motors and ammonia engines will be benefited by the use of the solar receiver; but it is especially in tropical countries that it is destined to find immediate employment, in driving the various kinds of machinery used in sugar and cotton plantations, in distilling impure water to make it fit for drinking, in crystallizing saline and saccharine solutions, in pumping water of irrigation, in manufacturing ice by means of the Carré machine, etc. In those countries fuel is scarce, firewood is not abundant, and coal, which has to be imported from a distance, often from the mines of England, commands an exorbitant price. Already in southern countries sea-salt is obtained purely by the action of solar heat. In Chili and in Mauritius, salt-marshes are divided into compartments, with walls and roof of glass, in order to promote evaporation; so in the famous nitre-beds of Iquique, on the coast of Peru, the salt might be crystallized by solar heat alone.
The cost of a solar apparatus of half a horse-power, like that at Tours, does not exceed fifteen hundred francs, and, when the manufacture is carried on upon a large scale, will he much less. By substituting for the silver plate, which is the most costly portion of the reflector, brass with a thin coat of silver, which will serve the purpose equally well, a considerable reduction of cost is effected.
As the insolation surface, and consequently the power of the apparatus, is quadrupled when the diameter of the mirror is doubled, it will be easy to construct large generators without adding very much to the cost or complicating the mechanism. The one thing to be avoided in this case will be too great intensity of heat. It cannot be objected that the conical reflector takes up too much room, for a common steam-engine occupies considerable space likewise with its long boilers and its high chimney; as for the motor, properly so called, and the contrivances for transmitting the power, they are the same in both cases.
The strongest winds, at least in our latitudes, have no action on the reflection of the solar heat, or upon the mirror itself, which is not shaken by them. This is an important point, for this is an apparatus which must always be exposed in the open air. In regions where the wind-storms are more severe than they are here, the reflector might be staid and strengthened with iron ribs, so as to resist the most violent cyclones. It has been demonstrated that the bell-glass, even when highly heated by the direct radiation from the boiler, is in no danger of breaking, even when a cold rain falls upon it, and that it is even proof against hailstones; and now that a process has been. invented for tempering glass and making it almost unbreakable, we can without difficulty obtain bell-glasses strong enough for any emergency.
Experience will hereafter lead to many improvements now unthought of; but even as it stands to-day the solar engine at Tours is ready to pass from the speculations of theory to the application of practice. It is neither over-costly, nor difficult to set up, nor so complicated as to require great skill in managing it; and, from whatever point of view we regard it, it meets and overcomes all objections. We may say that it lends itself to every industrial use in which solar heat can be employed, especially in tropical countries where the absence of all kinds of fuel for industrial uses is severely felt. In the not distant future, in other countries, too, there will exist no other fuel than the sun, no other engines than those driven by solar heat. By that time no doubt the means of storing up this heat will have been discovered, for in our latitudes we shall have to make provision against cloudy days and seasons of rain, which unfortunately constitute the major part of the year.
It may appear to be a pleasant paradox to say that future generations, after the coal-mines have been exhausted, will have recourse to the sun for the heat and energy needed in manufacture and in domestic economy. Still, nothing could be plainer than this. In our day, when it is probable that force, motion, gravity, heat, light, electricity, magnetism, are simply modifications of one and the same agent, and the effect of the vibrations of that impalpable and invisible fluid known as ether, the assertion that the sun is the only fuel, the only force, must not call forth anywhere the smile of incredulity. All fuels, all forces, are to be regarded as only parts of the sun's heat. What is coal? Fossil carbon. And was not this carbon fixed in plants by the sun's heat, of which it is the equivalent? Under the action of solar radiations the carbonic acid in the atmosphere is decomposed on contact with plants; the carbon is fixed in the plant, and the oxygen goes back into the air to serve for the respiration of animals. Hence, no sun, no vegetation; no vegetation, no carbon; no carbon, no coal. Coal, in burning, gives up the solar heat which was stored up in it, and therefore it was that, on seeing a locomotive engine move, Stephenson said: "It is not the coal that drives this engine, it is the sun's heat stored up in the coal thousands of ages ago; locomotives are but the horses of the sun." We might make a like comparison with respect to wine and the alcohol it contains; and the Bordelais use no mere figure of speech when they speak of their admirable Sauterne wine as being "bottled sunshine."
When water rises in the shape of vapor, what is it that causes it to ascend? The heat of the sun. If it comes down as rain, forming torrents and brooks which feed our mill-races and drive our mills, what is it that turns the wheel? The sun, for it was the sun that in the first place raised the water. When the wind blows upon the sails of a windmill, or on the sails of a ship, what is it that drives the mill or propels the ship? The sun, for wind is simply an atmospheric current produced by the heating of a stratum of air which, being dilated by the sun, tends to an equilibrium with strata of the same density, and hence rises, while a volume of cooler air takes its place. And what are the tides, the propulsive power of which there is some thought of utilizing, whether directly by means of water-wheels, or indirectly by compressing air and so producing a constant supply of force? They are a portion of the heat of the sun, for the seas are formed by the coming together of all those torrents and rivers which descend into their common reservoir, the ocean. Then, too, the tides are the result of the combined attraction of sun and moon upon the earth. Thus we find that the sun is always and everywhere active.
It is, therefore, no paradox to regard the sun as the one source of fuel in the future, and as the reservoir of force to which generations to come will at no distant day have recourse. Hence it is that savants and great engineers, as Euclid, Archimedes, Hero, Salomon de Cans, Buffon, Saussure, Bélidor, Evans, Herschel, Pouillet, Ericsson, have in every age put to themselves the question how it might be possible to take from the sun a part of its heat for the benefit of this poor globe.
The world will not perish for want of coal, yet the coal-supply will fail, and that much sooner than Ericsson estimated, for the production doubles every ten or fifteen years. It will not take thousands of years to exhaust the European coal-mines, but only hundreds, and not very many hundreds either. In England, as appears from recent calculations, the supply will have been consumed in two or three centuries at the farthest. Belgium, Germany, France, and the other countries of Europe, are no better off. The United States of America and the north of China have coal enough to last for one thousand years, and that is all. We must then have recourse to the sun.
It will perhaps be said, "There is electricity." Electricity, as a mechanical agent, is too costly; to produce electricity we have to consume copper, zinc, and acids. Now, one kilogramme of copper, zinc, or acid represents several kilogrammes of coal expended in procuring it. In reducing copper-ore according to the Welch method, sixteen kilogrammes of coal is consumed for each kilogramme of copper obtained. Hence it were reasoning in a vicious circle to suppose that electrical or electro-magnetic machines can usefully or economically take the place of steam-engines. There is only one case in which this conclusion would be weakened; namely, if with a thermo-electric pile we should succeed in decomposing water into its elements, oxygen and hydrogen, at little or no expense. The problem would then be solved, for this would place in the hands of all the two greatest sources of light, heat, and force—oxygen and hydrogen. But, even then, to what should we owe this unexpected solution? To the sun, for it is only by the aid of a thermo-electric pile (wherein we suppose electricity to be produced by solar heat) that we could economically decompose oxygen and hydrogen; else it would require at least as much heat to dissociate them as they would yield on recombining—a petitio principii overlooked by those simple inventors who persist in attempting, by means of ordinary electric piles, to solve the great problem of economical motors and the fuel of the future.
As for directly storing up solar heat in good conductors or absorbents of heat which are then to be insulated—for instance, receiving the heat in porous black stones which are first exposed to the sun and afterward thrown into a great reservoir, just as snow is piled up in the ice-house—it involves no impossibility. These stones could be thrown into water, if needs were, and in this way we might easily attain or surpass the temperature of boiling water.
Straw, sawdust, wool, feathers, confined air, are insulating substances which retain heat. We might surround with a double envelope of this kind the reservoir holding the sun-heated stones, and in this way we might have our store of solar heat, as now we have our store of ice. It is one problem whether we have to retain cold or to retain heat. Now, ice keeps very well even when stowed in the hold of a vessel; a little sawdust and careful stowage do the whole work. The same means will serve in storing solar heat, and, if need he, shipping it to a distance. We have barely outlined the idea, but certain we are that at the proper time the scientific man will appear who shall discover a practical method of doing this.
The sun, as it would appear, will be the fuel of the future, and one might say that this was foreseen by the great encyclopædic scholar of the middle ages, Dante, when in his incomparable poem he said, "Guarda il calor del sol che si fa vino"—"Look at the sun's heat which changes into wine"—as though he meant to say, into all that is force, all that is life, all that is light.—Revue des Deux Mondes.
- Translated from the French, by J. Fitzgerald, A.M.
- A maker of instruments of precision, J. Salleron, who constructed the solar apparatus which was presented to the Institut last year, lately wrote to me as follows: "I have driven a small model steam-engine, with the steam generated in the boiler of this new generator, and M. Noel, Professor of Physics in the Vendôme Lycée, put the same engine in operation on January 5th last. The water began to boil after twenty-eight minutes, the hour being noon, and the temperature of the surrounding air near 0° C."