The Sun Motor

Source: Scientific American
Title: The Sun Motor
Date: February 2, 1884
Author: John Ericsson

Another name which deserves to be placed high on the roll of the fostering friends of solar enginery is that of Captain John Ericsson, of New York City, the renowned inventor and builder of the Monitor. He was a builder of hot-air engines and other mechanical constructions. He made elaborate experiments ; invented an instrument for the measurement of the heat which comes from the sun ; also designed a boiler and engine for the operation of machinery by solar heat, a cut of which we reproduce from the Scientific American of May 5, 1877, by permission of the publishers.

He calculated that “the heat radiated by the sun during nine hours per day, for all the latitudes comprised between the equator and the forty-fifth parallel, corresponds per minute and per square foot of normal surface to 3.5 thermo units of 772 foot pounds. This would give a power of 270.000 foot pounds, or from 8 to 9 horse power on a surface of 100 square feet. The engine illustrated was built on the caloric system, and had run at 420 revolutions per minute, with the sun near the zenith and during fine weather." (POPE, Solar Heat, and its applications).

The annexed illustration represents a perspective view of a sun motor constructed by the writer and put in operation last summer. This mechanical device for utilizing the sun’s radiant heat is the result of experiments conducted during a series of twenty years; a succession of experimental machines of similar general design, but varying in detail, having been built during that period.

The leading feature of the sun motor is that of concentrating the radiant heat by means of a rectangular trough having a curved bottom lined on the inside with polished plates, so arranged that they reflect the sun’s rays toward a cylindrical heater placed longitudinally above the trough.

This heater, it is scarcely necessary to state, contains the acting medium, steam or air, employed to transfer the solar energy to the motor; the transfer being effected by means of cylinders provided' with pistons and valves resembling those of motive engines of the ordinary type. Practical engineers, as well as scientists, have demonstrated that solar energy cannot be rendered available for producing motive power, in consequence of the feebleness of solar radiation.

The great cost of large reflectors, and the difficulty of producing accurate curvature on a large scale, besides the great amount of labor called for in preventing the polished surface from becoming tarnished, are objections which have been supposed to render direct solar energy practically useless for producing mechanical power.

The device under consideration overcomes the stated objections by very simple means, as will be seen by the following description.
Ericsson’s solar caloric engine
The bottom of the rectangular trough consists of straight wooden staves, supported by iron ribs of parabolic curvature secured to the sides of the trough. On these staves the reflecting plates, consisting of flat window glass silvered on the under side, are fastened. It will be readily understood that the method thus adopted for concentrating the radiant heat does not call for a structure of great accuracy, provided the wooden staves are secured to the iron ribs in such a position that the silvered plates attached to the same reflect the solar rays toward the heater.

Fig. 2 represents a transverse section of the latter, part of the bottom of the trough, and sections of the reflecting plates, the direct and reflected solar rays being indicated by vertical and diagonal lines.

Referring to the illustration, it will be seen that the trough, 11 feet long and 16 feet broad, including a parallel opening in the bottom, 12 inches wide, is sustained by a light truss attached to each end, the heater being supported by vertical plates secured to the truss.

The heater is 6 1/4 inches in diameter, 11 feet long, exposing 130 x 9.8 = 1,274 superficial inches to the action of the reflected solar rays.

The reflecting plates, each 3 inches wide and 26 inches long, intercept a sunbeam of 130 x 180 = 23,400 square inch section.

The trough is supported by a central pivot, round which it revolves. The change of inclination is effected by means of a horizontal axle - concealed by the trough - the entire mass being so accurately balanced that a pull of 5 pounds applied at the extremity enables a person to change the inclination or cause the whole to revolve.
A single revolution of the motive engine develops more power than needed to turn the trough and regulate its inclination so as to face the sun during a day’s operation.

The motor shown by the illustration is a steam-engine, the working cylinder being 6 inches in diameter with 8 inches stroke. The piston-rod, passing through the bottom of the cylinder, operates a force-pump of 5 inches diameter.

By means of an ordinary cross-head se cured to the piston-rod below the steam cylinder, and by ordinary connecting rods, motion is imparted to a crank shaft and fly wheel, applied at the top of the engine frame; the object of this arrangement being that of showing the capability of the engine to work either pumps or mills.
It should be noticed that the flexible steam-pipe employed to convey the steam to the engine, as well as the steam-chamber attached to the upper end of the heater, have been excluded in the illustration.

The average speed of the engine during the trials last summer was 120 turns per minute, the absolute pressure on the working piston being 35 pounds per square inch. The steam was worked expansively in the ratio of 1 to 3, with a nearly perfect vacuum kept up in the condenser enclosed in the pedestal which supports the engine frame.

In view of the foregoing, experts need not be told that the sun motor can be carried out on a sufficient scale to benefit very materially the sunburnt regions of our planet.

With reference to solar temperature, the power developed by the sun motor establishes relations between diffusion and energy of solar radiation, which show that Newton’s estimate of solar temperature must be accepted.

The following demonstration, based on the foregoing particulars, will be readily comprehended. The area of a sphere whose radius is equal to the earth’s mean distance from the sun being to the area of the latter as 214.5 x 214.5, while the reflector of the solar motor intercepts a sunbeam of 23,400 square inches section, it follows that the reflector will receive the heat developed by 0.508 square inch of the solar surface.

Hence, as the heater of the motor contains 1,274 square inches, we establish the fact that the reflected solar rays, acting on the same, are diffused in the ratio of 1,274 / 0.508 = 2,507. Practice has now shown that, notwithstanding this extreme diffusion, the radiant energy transmitted to the reflector, by the sun, is capable of imparting a temperature to the heater of 520° F. above that of the atmosphere.

The practical demonstration thus furnished by the sun motor enables us to determine with sufficient exactness the minimum temperature of the solar surface.

It also enables us to prove that the calculations made by certain French scientists, indicating that solar temperature does not exceed the temperatures produced in the laboratory, are wholly erroneous. Had Pouillet known that solar radiation, after suffering a two thousand live hundred and sevenfold diffusion, retains a radiant energy of 520° F., he would not have asserted that the temperature of the solar surface is 1,760° C.

Accepting Newton’s law that ‘the temperature is as the density of the rays,’ the temperature imparted to the heater of the sun motor proves that the temperature of the solar surface cannot be less than 520° x 2,507 = 1,303,640° F.

Let us bear in mind that, while attempts have been made to establish a much lower temperature than Newton’s estimate, no demonstration whatever has yet been produced tending to prove that the said law is unsound.

On the contrary, the most careful investigations show that the temperature produced by radiant heat emanating from incandescent spherical bodies diminishes inversely as the diffusion of the heat rays.

Again, the writer has proved by his vacuum actinometer, enclosed in a vessel maintained at a constant temperature during the observations, that for equal zenith distance the intensity of solar radiation at midsummer is 5.48° F. less than during the winter solstice.

This diminution of the sun’s radiant heat in aphelion, it will be found, corresponds within 0.40° of the temperature which Newton’s law demands.

It is proposed to discuss this branch of the subject more fully on a future occasion.

The operation of the sun motor, it will be well to add, furnishes another proof in support of Newton’s assumption that the energy increases

as the density of the rays. The foregoing explanation concerning the reflection of the rays — see Fig. 2 — shows that no augmentation of temperature takes place during their transmission from the reflector to the heater. Yet we find that an increase of the number of reflecting plates increases proportionably the power of the motor.

Considering that the parallelism of the rays absolutely prevents augmentation of temperature during the transmission, it will be asked : What causes the observed increase of mechanical power? Obviously, the energy produced by the increased density of the rays acting on the heater.
The truth of the Newtonian doctrine, that the energy increases as the density of the rays, has thus been verified by a practical test which cannot be questioned.

It is scarcely necessary to observe that our computation of temperature - 1,303.640° F.- does not show maximum solar intensity, the following points, besides atmospheric absorption, not having been considered :

  1. The diminution of energy attending the passage of the heat rays through the substance of the reflecting plates;
  2. The diminution consequent on the great amount of heat radiated by the blackened surface of the heater
  3. The diminution of temperature in the heater caused by convection.

J. Ericsson