India, South America, and other countries interested in the employment of sun power for mechanical purposes, have watched with great attention the result of recent experiments in France, conducted by M. Tellier, whose plan of actuating motive engines by the direct application of solar heat has been supposed to be more advantageous than the plan adopted by the writer of increasing the intensity of the solar rays by a series of reflecting mirrors. The published statements that “the heat-absorbing surface” of the French apparatus presents an area of:315 square feet to the action of the sun's rays, and that “the work done has been only 43,360 foot pounds per hour,” furnish data proving that Tellier's invention possesses no practical value. '['he results of protracted experiments with my sun motors, provided with reflecting mirrors as stated, have established the fact that a surface of 100 square feet presented at right angles to the sun, at noon, in the latitude of New York, during summer, develops a mechanical energy reaching 1,850,000 foot pounds per hour. The advocates of the French system of dispensing with the “cumbrous mirrors” will do well to compare the said amount with the insignificaut mechanical energy represented by 43,360 foot pounds per hour,de- veloped by 215 square feet of surface exposed to the sun by Tellier, during his experiments in Paris referred to. The following brief description will give a clear idea of the nature and arrangement of the reflecting mirrors adopted by the writer for increasing the intensity of the solar heat, which imparts expansive force to the medium propelling the working piston of the motive engine. Fig. 1 represents a perspective view of a cylindrical beater, and a frame supporting a series of reflecting mirrors composed of narrow strips of window glass coated with silver on the under side. The frame consists of a light structure of wrought iron or steel, provided with transverse ribs, as shown by the illustration, each rib being accurately bent to a parabolic curvature whose focus coincides with the axis of the cvlindrical heater. It need hardly be stated that the mirrors supported by the said transverse ribs continue from side to side of the frame, which accordingly resembles a para- bolic trough whose bottom is composed of mirrors. It will be readily understood that this trough with its bent ribs and flat minors forms a perfect parabolic reflector, to which a cylindrical heater, as stated, may be attached for generating steam or expanding the gases intended to actuate the piston of the motive engine. Regarding the mechanism for turning the reflector toward the sun, engineers are aware that various combinations based on the principle of the “universal joint” may be employed. Concerning previous attempts made in France to utilize solar energy for mechanical purposes, it is well known that practical engineers, having critically examined Mouchot's solar engine, which M. Tellier proposes to supersede, find that it is incapable of developing sufficient power for any domestic purpose. Again, the investigations carried out by order of the French government to ascertain the merits of Mouchot's invention show that, irrespective of the great expense of silver- lined curved metallic reflectors for increasing the insufficient energy of direct solar radiation, these reflectors cannot be made on a sufficient scale for motors having adequate power to meet the demands of commerce; nor is it possible to overcome the difficulty of rapid wear of the delicate silver lining of the metallic reflect ors consequent on atmospheric influence, which after a few hours of exposure renders their surface tarnished and ineffective unless continually polished. A glance at the accompanying illustration (Fig. 1) shows that the reflector constructed for my sun motor differs altogether from thatoriginated by Mouchot, which Tellier's apparatus, tested at Paris, was intended to displace. DESCRIPTION OF THE ILLUSTRATED REFLECTOR. (1) The mirrors which reflect the solar rays are devoid of curvature, being flat narrow strips of ordinary window glass, cut to uniform width and length, perfectly straight. (2) The under sides of said strips are coated with silver by a process which prevents the action of the sun's rays from destroying the silver coating as in ordinary looking glasses. (3) The mirrors supported by the bent metallic ribs, extending from side to sid6 of the parabolic trough, are held down by the heads of small screws tapped into the ribs. Thin slats of wool may be introduced between the mirrors and the ribs—an expedient of some importance in localities where the reflector is exposed to high winds. (4) It needs no explanation that the reflecting surface of the mirrors cannot become tarnished by atmospheric influence, since the bright side of the silver coating is permanently protected by the glass; hence it will be only necessary to remove dust from the mirrors—an operation readily performed by feather brushes secured to light handles of suitable length. (5) The frame of the reflector, being composed. of rolled bars of iron or steel, requires no finish, excepting the top of the transverse ribs, which must correspond accurately with a given parabolic curvature. It should be observed that the needed accuracy is readily attained by a cutting tool guided by a bar of proper form. (6) Regarding cost of construction, it will suffice to state that manufacturers of glass, both in the United States and Germany, supply the mirrors, cut to exact size and silvered, at a rate of 60 cents per square foot, the weight 106 pounds per 100 square feet. Consequently the cost of the reflector and heater for the sun motor will not much exceed that of a steam boiler and ap purtenances, including chimney. The cost of the engine apart from the reflector will not be greater than that of an ordinary steam engine. (7) With reference to durability, it will be evident that the light metallic frame with its mirrors, and a heater acted upon only by reflected solar heat, will last much longer than steam boilers subjected to the action of fire, soot, and corrosion. Let us now briefly consider the distinguishing feature of the sun motor—namely, the increase of the intensity of the sun's radiant energy by parallel ravs and flat reflecting surfaces permanently protected against atmospheric influence. It hai' been supposed that the lens and the curved reflecting surface, by converging the sun's rays, could alone increase the intensity of radiant heat. But Newton's demonstration, showing that the temperature produced by solar radiation is “as the density of the rays,” taught me to adopt in place of curved surfaces and converging rays, flat surfaces and parallel rays, as shown by Fig. 2, which represents a transverse section of part of the reflector. The direct vertical solar rays, it will be seen, act on the mirrors; while the reflected rays, divided into diagonal clusters of parallel rays, ad on the heater, the surface of which will thus be exposed to a dense mass of reflected rays, and consequently raised to a temperature exceeding 600° F. at noon during ordinary sunshine. The cost, durability, and mechanical energy of the sun motor being thus disposed of, it remains to be shown whether the developed energy is continuous, or whether the power of the engine changes with the increase and diminution of zenith distance and consequent variation of atmospheric absorption. Evidently an accurate knowledge of the diathermancy of the terrestrial atmosphere is indispensable to determine whether the variation of the radiant energy is so great that the development of constant power becomes impracticable. Of course, manufacture and commerce demand a motor developing/Mil power during a modern working day of eight hours. Observations relating to atmospheric diathermancy continued during a series of years enable me to assert that the augmentation of solar intensity during the middle of the day is so moderate that, by adopting the simple expedient of wasting a certain amount of the superabundant heat generated while the sun is near the meridian (as the steam engineer relieves the excess of pressure by opening the safety valve), a uniform working power will be developed during the stipulated eight hours. The opening of the safety valve, however, means waste of coal, raised from a great depth at great cost, and possibly transported a long distance, while the radiant heat was ted automatically by the sun motor is produced by fuel obtained from an inexhaustible storehouse, free of cost and transportation. It will be proper to mention that the successful trial of the sun motor described and illustrated in Nature, vol. xxxi., p. 217, * attracted the special attention of land owners on the Pacific coast then in search of power for actuating the machinery needed for irrigating their sunburnt lands. But the mechanical detail connected with the concentration at a single point of the power developed by a series of reflectors was not perfected at the time; nor was the investigation relating to atmospheric diathermancy sufficiently advanced to determine with precision the retardation of the radiant heat caused by increased zenith distance. Consequently no contracts for building sun motors could then be entered into, a circumstance which greatly discouraged the enterprising Californian agriculturists prepared to carry out forthwith an extensive system of irrigation. In the meantime a simple method of concentrating the power of many reflectors at a given point has been perfected, while the retardation of solar energy caused by increased zenith distance has been accurately determined, and found to be so inconsiderable that it does not interfere with the development of constant solar power during, the eight hours called for. The new motor being thus perfected, and first class manufacturing establishments ready to manufacture such machines, owners of the sunburnt lands on the Pacific coast may now with propriety reconsider their grand scheme of irrigation by means of sun power. John Ericsson.
This article was originally published with the title "The Sun Motor" in s , , 10592 (August 2013)