Source: Contributions to the Centennial Exhibition, p 558
Title: Sun Power - The Solar Engine.
Author: John Ericsson
Ericsson purpose is general. He starts his presentation with his Solar Steam Engine, and only at the end of the text, present his solar Hot Air Engine
The illustration on Plate 66 derives its chief interest from the fact that it represents the first motor actuated by the direct agency of the sun's radiant heat.
It was constructed at New York, 1870, and intended as a present to the French Academy of Sciences. Apart from being a motor, this engine was designed to operate as a meter for registering the volume of steam generated by the concentrated heat of a pencil of solar rays of a given section.
Regarded as a steam-meter, it proved important, as it verified the results of privious experiments and previous calculations, based on the number of thermal units developed by the evaporation of water in a given time. Engineers will not fail to notice the unusual proportions of the working parts, nor will they fail to appreciate the object in view, that of reducing the friction to a minimum an indispensable condition in a meter.
The entire mechanism being shown with perfect distinctness in the perspective view of the engine on the plate referred to, it is only necessary to mention that the square pedestal which supports the steam cylinder (4.5 in. in diameter), the beam-centre, and the crank-shaft, conceals a surface-condenser.
Under a clear sun the engine performed its functions with perfect uniformity, at a velocity of 240 revolutions per minute. It consumed, at the stated rate, only part of the steam furnished by a solar steam-generator, temporarily employed, belonging to an engine of greater dimensions then in the course of construction.
With reference to ascertaining correctly the amount of mechanical power developed by the concentrated radiant heat applied to this engine, experts need scarcely be reminded that, by dispensing with a vacuum, the atmospheric resistance and back pressure exerted against the piston furnish elements for measuring, with critical nicety, the dynamic force transmitted by pencils of solar rays of definite sections.
Drawings and descriptions of the mechanism by which the sun's radiant heat has been concentrated in my experimental engines will not be presented in this work, nor will the form of the steam-generator which receives the concentrated heat be delineated or described. Experienced professional men will appreciate the motive viz., that of preventing enterprising persons from procuring patents for modifications.
With reference to the course thus adopted, it will be proper to mention that I have in several instances, notably in the case of the screw-propeller and the caloric engine, been prevented from perfecting my invention in consequence of conflicting privileges having in the meantime been granted to others.
Regarding the solar engine, it may be well to state that I shall not apply for any patent rights, excepting for the purpose of protecting the community, and that it is my intention to devote sufficient time and means to ensure its completion. Hence my anxiety to guard against legal obstructions being interposed before perfection of detail shall have been measurably attained.
In the meantime, let us hope that no exclusive privilege will be granted tending to throw obstacles in the way of an unrestricted manufacture and introduction of the solar engine in countries where a continuously clear sky warrants its adoption, especially in Upper Egypt and on the coast of Peru.
The experiments instituted show that the mechanism which I have adopted for concentrating the sun's radiant heat abstracts, on an average, during nine hours a day, for all latitudes between the equator and 45 deg., fully 3.5 units of heat per minute for each square foot of area presented perpendicularly to the sun's rays.
A unit of heat being equivalent to 772 foot-pounds, it will be perceived that, theoretically, a dynamic energy of 2,702 foot-pounds is transmitted by the radiant heat, per minute, for each square foot; hence, 270,200 foot-pounds for an area of 10 feet square. If we divide this sum by the adopted standard, 33,000, we ascertain that 100 square feet of surface exposed to the solar rays develop continuously 8.2 horse power during nine hours a day, within the limits of latitude before mentioned.
But engineers are well aware that the whole dynamic energy of heat cannot be utilized in practice by any engine or mechanical combination whatever, nor at all approached ; hence I have assumed, in order not to overrate the capability of the new system, that a solar engine of one horse-power demands the concentration of solar heat from an area of 10 feet square.
On this basis, I will show presently that those regions of the earth which suffer from an excess of solar heat will ultimately derive benefits resulting from an unlimited command of motive power which will, to a great extent, compensate for disadvantages hitherto supposed not to be counterbalanced by any good.
But before estimating the magnitude of mechanical power which we may produce by availing ourselves of the fuel contained in that great storehouse from whence it may be obtained free of cost and transportation, let us consider the leading feature of the device resorted to, especially that by which I have succeeded in augmenting the comparatively low temperature developed by direct solar radiation sufficiently for the production of useful work.
The solar engine, when steam is employed as the medium for transmitting the radiant energy, is composed of three distinct parts the engine, the steam-generator, and the mechanism by means of which the inadequate energy of the sun's rays adverted to is increased to such a degree that the resulting temperature will exceed that corresponding with the steam-prressure necessary in an efficient engine.
The motor itself, when the acting medium under consideration is emploved, ressembles in all essential points a modern steam engine, utilizing to the fullest extent the mechanical energy of the steam admitted to the working-cylinder.
But when atmospheric air is employed as the medium for transmitting the solar energy to the motor, an entirely different combination of mechanism is called for, as will be seen hereafter.
Regarding the steam-generator, it will be superfluous to point out the advantages resulting from its not being exposed to the action of fire or soot ; hence that it can only suffer from the slow action of ordinary oxidation.
As the motor itself ressembles a steam-engine, we have of course merely to consider the nature of the mechanism by means of which the solar heat is concentrated and the temperature raised above that of the water in the steam-generator.
Regarding this mechanism - viz., the concentration apparatus - it has been asked, Is it costly? Is it heavy and bulky, so as to render transportation difficult? And, finally, the question has frequently been put, Is it liable to derangement and expensive to keep in order?
The cost is moderate. The weight is small; indeed, lightness is the most notable peculiarity of the concentration apparatus. As to bulk, it may be observed that this apparatus is composed of small parts readily put together.
With reference to durability, the fact need only be pointed out that certain metals, however thin, if keept dry, may be exposed to the sun's rays during an indefinite length of time without appreciable deterioration; hence, unlike the furnaces of steam-boilers, which soon become unserviceable, the concentration apparatus, as it consists of thin metallic plates, composed of durable materials, cannot be damaged by the mere action of the sun's rays.
Another question has been asked, Whether the solar engine will answer as well on a large as it does on a small scale?
The following reply disposes of this pregnant query: It is not necessary, nor intended, to enlarge considerably the size of the apparatus by means of which the comparatively feeble intensity of the sun's rays has been successfully concentrated, and the temperature sufficiently elevated to generate steam for actuating the solar engine.
The maximum size adopted has been adequate to utilize the radiant heat of a pencil of rays (sunbeam) of 35 square feet section. The employment of an increased number of such structures will, therefore, in most cases be resorted to when greater power is wanted, as we increase the number of hands when we desire to perform an additional amount of work.
The motor itself viz., - the steam-cylinder and the working parts - will obviously be proportioned, as at present, in accordance with the pressure of steam employed and the work to be done.
It should be clearly understood that I do not recommend the erection of solar engines in places where there is not steady sunshine, until proper means shall have been devised for storing up the radiant energy in such a manner that regular power may be obtained from irregular solar radiation.
Experienced engineers need not be told that formidable difficulties present themselves in storing up mechanical energy of any kind ; yet when coal can no longer be obtained, necessity, ingenuity, and increased experience will find means of overcoming obstacles which now appear insurmountable.
Before considering further the nature and capabilities of my solar engine, it will be proper to notice the result of the labors of Professor Mouchot of Tours, formerly of the Lycee of Alençon - France, who claims to have anticipated me in employing solar heat for the production of motive power.
Mouchot bases his claim on some experiments, made in 1866, intended to show that by the accumulation of heat which takes place when a blackened surface is surrounded by glass bells, steam may be generated for actuating machinery.
Sir John Herschel, it is well known, elaborated the old idea of concentrating solar radiation, and conducted a series of experiments at Cape Town, in 1838, showing that not only was it possible to produce boiling heat by accumulating solar heat as described, but he succeeded in elevating the temperature sufficiently for roasting meat.
Some time previous to 1870, Mouchot made a small model engine, a mere toy, actuated by steam generated on the plan of accumulation by glass bells ; but finding the heat insufficient, he added a polished metallic reflector.
The increase of temperature resulting from this expedient rendered his steam-generator more effective, and it was found that under favorable circumstances sufficient steam could be produced to actuate his small model.
The Conseil Général of Indre-et-Loire having subsequently provided Professor Mouchot with necessary means, he put up a steam generator at Tours in 1872, which he deems a perfect machine, its action being based on the results of his previous experiments.
The accompanying diagram, Fig. 1, represents a vertical section of the said steam-generator, thus described by M. L. Simonin in Revue des Deux Mondes : "The traveller 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 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 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 deg. 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 enclosed 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 steamchamber 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 deg., 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 its 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 foregoing description of the Solar Steam-Generator of Mouchot is so lucid that it requires no explanation.
Mr. Simonin, however, erroneously supposes that the power developed by the apparatus is nearly the same at all hours of the day, the fact being that the energy developed by the concentrated solar heat varies with the depth of atmosphere penetrated by the rays.
The latter evidently depends on the sun's zenith distance; hence at Paris, where the maximum solar intensity during the summer solstice is 65°.0 Fah, at noon (see diagram on Plate 9), it scarcely reaches 52°.0 F. at five o'clock in the afternoon, owing to the increased zenith distance, and consequent increase of the depth of atmosphere to be penetrated by the sun's rays.
Obviously, the efficiency of the solar generator will be diminished in the same ratio as the stated intensities.
Mr. Simonin states that on some occasions, when the sun has been exceptionally clear, the solar generator at Tours has evaporated five litres of water per hour, which he assumes equivalent to half a horse-power. The reflector producing this result - a truncated cone - being 2.6 metres (8 feet 6 inches) in diameter, it will be found that in order to double the reflective area necessary to generate steam for an engine of one horse-power, a truncated cone of 3.6 metres (11 feet 9 inches) aperture will be required.
Practical engineers are aware that an inverted conical body whose base is nearly 12 feet in diameter, swinging round an inclined axle at least 60 deg. on each side of the vertical line, presents a structure so formidable, even if counterpoised, that it would not be prudent to increase its size.
Accordingly, one hundred of Mouchot's solar generators would be needed to furnish steam for an o engine of 100 horse a very moderate power, if employed for manufacturing or other industrial purposes.
Referring to the diagram, Fig. 2, representing a bird's-eye view of the aperture of the conical reflector at Tours in three different positions - viz., a in the morning, b at midday, and c during the afternoon - it will be seen that each instrument, owing to the necessary change of position, demands a front space of nearly twenty feet.
If placed side by side, the conical solar generators required for an engine of 100 horse-power would therefore occupy a front of 2,000 feet from east to west. If arranged in four lines, with sufficient space north and south to prevent interference, a distance of 500 feet by 200 feet would be required.
Now, let us consider that the scheme calls for 100 separate boilers, to be continually fed with water, the height of which can only be known by the indication of outside gauges, while the steam from the scattered boilers must be conveyed by a series of flexible tubes to the motive engine.
The hundred glass bells can, no doubt, be dusted and kept clean with moderate exertion ; but the hundred silver-plated reflectors, which Mr. Simonin says must be exposed to the vicissitudes of the atmosphere, cannot be kept bright without herculean labor, since silver tarnishes in a few hours.
In view of the foregoing statement, which embraces only the chief difficulties attending Mouchot's system, the most sanguine might well despair of rendering sun-power available for practical purposes.
The Professor of the Lycée of Alencon, in claiming to have anticipated me, has done so ignorant of the fact that sun-power has been the study of my whole professional life - a life the early part of which was chiefly devoted to the production of a cheaper motive power than steam.
The industrious scientist, if he had been correctly informed on the subject, would no doubt have perceived the advantages resulting from such antecedents, with reference to a successful practical solution of the problem of utilizing solar heat.
On grounds already fully explained, minute plans of my new system of rendering sun-power available for mechanical purposes will not be presented in this work.
The occasion, however, demands that I should present an outline of the concentration apparatus before referred to.
It consists of a series of polished parabolic troughs, in combination with a system of metallic tubes charged with water under pressure, exposed to the influence of converging solar rays, the augmented molecular action produced by the concentration being transferred to a central receiver, from which the accumulated energy is communicated to a single motor.
Thus the mechanical power developed by concentrated solar heat is imparted to the solar steam-engine without the intervention of a multitude of boilers, glass bells, gauges, feeders, etc.
Moreover, the concentration apparatus, unlike the instrument of Mouchot, requires no parallactic motion, nor does its management call for any knowledge of the sun's declination from day to day. Its position is regulated by simply turning a handle, until a certain index coincides with a certain bright line produced by the reflection of the sun's rays.
Plate 67 represents a perspective view of a solar engine, in which the concentrated energy of the sun's rays is communicated to the motor by means of heated atmospheric air, instead of being communicated by water heated under pressure and expanded into steam.
A glance at the illustration shows that the upper end of the working-cylinder is heated by the sun's rays reflected by a curved mirror.
It will be seen by careful examination that the solar rays converge at a point beyond the axis of the reflector; hence that the form of the latter is not parabolic, but composed of an irregular curve. The object is that of spreading the converging rays over a greater length of the cylinder than possible with the divergence which would result from employing a reflector of true parabolic curvature.
It will be perceived on inspection that the upper end of the cylinder will be subjected to a concentration of heat many times greater than the concentration at the lower end.
Referring to the accompanying diagram representing a vertical section of the machine, it will be seen that the working-cylinder, open at the lower end, contains two pistons, a working piston a and an exchange-piston b.
The working-piston is connected with the crank-shaft d by the beam c and the connecting-rod g. The exchange-piston b is connected with the crank-shaft by the bell-crank f f and connecting-rod h. An annular space is formed round the exchange-piston, admitting of a free passage of the air from end to end of the cylinder during the motion of this piston.
It will be readily understood that during the downward motion of the exchange-piston the cold air from the lower end of the cylinder will be transferred to the upper end, heated by the concentrated solar rays ; hence internal pressure will be produced tending to force the working-piston down.
By a careful examination of the combination of the several working parts, it will be easily comprehended how the working piston is actuated by the confined air, heated and cooled alternately by the peculiar motion of the exchange-piston.
It will be evident that the large surface presented by the outside of the exchange-piston, and inside of the cylinder, will cause a rapid change of temperature of the air while circulating from end to end of the latter.
The upper end of the cylinder being heated by the concentrated solar rays, the cold air from the lower end will, during its transfer to the upper end caused by the downward motion of the exchange-piston, become heated and expanded; while during the upward motion of the said piston the air, in being transferred to the lower end of the cylinder, becomes cooled and contracted.
It will be found on due consideration that the exchange-piston thus performs the office of a regenerator. The engine, therefore, is capable of operating for a considerable time by exposing the upper end of the cylinder to the reflected solar heat during a few minutes at starting.
By continuous exposure to the concentrated solar rays, the engine performs fully 400 turns per minute. It should be observed that concentrated solar radiation supplies heat with such extraordinary rapidity that the apparently insufficient amount of heating surface presented by the cylinder has proved adequate, notwithstanding the great speed of the engine.
It only remains to be stated that the body m m represents a radiator carrying off the heat which is not taken up by the circulating air during the motion of the exchange-piston. Of course, the amount of heat carried off by the radiator furnishes a nearly correct measure of the solar energy not converted into mechanical work.
Engineers need not be reminded that the form of the solar engine thus described is applicable only for purposes requiring moderate power. In the largest class of solar engines actuated by atmospheric air, in which the radiator is incapable of abstracting the superfluous heat, I employ valves, and take in fresh air at each stroke of the machine, precisely as in the caloric engine delineated on Plate 46.
Having thus cursorily examined the construction of the solar engine actuated by the intervention of atmospheric air, and briefly adverted to the steam solar engine and the mode adopted in concentrating the molecular motion imparted by solar radiation, and also pointed out the nature of the expedient resorted to in transferring the said concentrated molecular motion to mechanical motors, let us now consider the stupendous amount of the energy at our command.
It has already been stated that the result of repeated experiments with the concentration apparatus shows that it abstracts on an average, during nine hours a day, for all latitudes between the equator and 45 deg., fully 3.5 units of heat per minute for each square foot of area presented perpendicularly to the sun's rays.
Theoretically, this indicates the development of an energy equal to 8.2 horsepower for an area of 100 square feet.
On grounds before explained, our calculations of the capabilities of sun power to actuate machinery will, however, be based on one horse power developed for 100 square feet exposed to solar radiation.
The isolated districts of the earth's surface suffering from an excess of solar heat being very numerous, our space only admits of a glance at the sunburnt continents.
There is a rainless region extending from the northwest coast of Africa to Mongolia, 9,000 miles in length and nearly 1,000 miles wide. Besides the North African deserts, this region includes the southern coast of the Mediterranean east of the Gulf of Cabes, Upper Egypt, the eastern and part of the western coast of the Red Sea, part of Syria, the eastern part of the countries watered by the Euphrates and Tigris, Eastern Arabia, the greater part of Persia, the extreme western part of China, Tibet, and, lastly, Mongolia.
In the western hemisphere, Lower California, the table-land of Mexico and Guatemala, and the west coast of South America, for a distance of more than 2,000 miles, suffer from continuous intense radiant heat.
Computations of the solar energy wasted on the vast areas thus specified would present an inconceivably great amount of dynamic force.
Let us, therefore, merely estimate the mechanical power that would result from utilizing the solar heat on a strip of land a single mile in width, along the rainless western coast of America; the southern coast of the Mediterranean before alluded to ; both sides of the alluvial plain of the Nile in Upper Egypt; both sides of the Euphrates and Tigris for a distance of 400 miles above the Persian Gulf; and, finally, a strip one mile wide along the rainless portions of the shores of the Red Sea, before pointed out.
The aggregate length of these strips of land, selected on account of being accessible by water communication, far exceeds 8,000 miles. Adopting the stated length and a width of one mile as a basis for computation, it will be seen that this very narrow belt covers 223,000 millions of square feet.
Dividing the latter amount by the area of 100 square feet necessary to produce one horse-power, we learn that 22,300,000 solar engines, each of 100 horse-power, could be kept in constant operation, nine hours a day, by utilizing only that heat which is now wasted on the assumed small fraction of land extending along some of the water-fronts of the sunburnt regions of the earth.
Due consideration cannot fail to convince us that the rapid exhaustion of the European coal-fields will soon cause great changes with reference to international relations, in favor of those countries which are in possession of continuous sun-power.
Upper Egypt, for instance, will, in the course of a few centuries, derive signal advantage and obtain a high political position on account of her perpetual sunshine and the consequent command of unlimited motive force. The time will come when Europe must stop her mills for want of coal.
Upper Egypt, then, with her never-ceasing sun-power, will invite the European manufacturer to remove his machinery and erect his mills on the firm ground along the sides of the alluvial plain of the Nile, where an amount of motive power may be obtained many times greater than that now employed by all the manufactories of Europe.