RUDOLPH J. MARCUS and HENRY C. WOHLERS Chemistry Department, Stanford Research Institute, Menlo Park, Calif.
A New Solar Furnace Design a n d Operation Ultraviolet sunlight is concentrated and used in this furnace. Photochemical yields were increased by a factor of 10 over more conventional furnaces
RECENT
studies have shown that cerium(1V) perchlorate is photochemically reduced in aqueous solution a t the focus of a rear-silvered solar furnace (9) according to the equation :
Ce+++'
+
'1'1 € 3 2 0
hW jqq;+
+ H+ +
Ce++++
'/4
0
2
(1)
Because this reaction occurs a t the focus of a solar furnace a n entirely new photochemical technique is available-that of using a solar furnace as a light concentrator (9,70). T h e solar furnace used for the earlier experiments was constructed with backsilvered mirrors. Above 3500 A., no important difference was found between the distribution of energy a t the sun's image in the solar furnace and that of unconcentrated sunlight a t the earth's surface (70), despite the fact that the light passed through 0.75 inch of soft glass. This type of water-white polished plate glass, even when only 0.25 inch thick, transmits only 75yo of the incident radiation a t 3500 A., and this value decreases rapidly to practically zero a t 3000 A. ( 7 7 ) . As the lower limit of the solar spectrum a t the earth's surface is about 3000 A,, the energy in the 3000- to 3500-A. range was wasted for photochemical reactions. Many photochemical reactions require light in the 3000- to 3500-A. range. Therefore, a 2-foot solar furnace was constructed as a light source for photochemical experiments, consisting of a front-surfaced concentrator on a telescope-type mounting. I t is used without a heliostat, and thus is a considerably simpler and more reliable instrument for photochemical purposes than the one previously described.
This third report on photochemical reactions describes a new, smaller solar furnace. Previous studies on the utilization of solar energy for chemical reaction appeared in I/EC in November 1959, p. 1335, and M a y 1960, p. 377.
The Solar Furnace T h e mirror used for these studies is a 2-foot paraboloid of Stellite metal (cobalt-chromium-tungsten alloy). This mirror is a t least 10 years old, but there is no visible evidence of oxidation of the metal surface ; however, the surface is slightly scratched. T h e reflectivity of this mirror is 60 to 7oyo in the 3000- to 8000-A. range; this experimentally measured value is in good agreement with data supplied by the Haynes Stellite Co., Kokomo, Ind. T h e focal length of the 24-inchdiameter paraboloid is 9.5 inches; the diameter of the sun's image at the focus of this paraboloid is d = 2ftan16' =f/107.3 = 0.088inch T h e calculated rim angle is 64.5'. T h e flux available at the focal spot was
calculated by means of the formula of De L a R u e ( 7 ) and Hiester (7) to be 300 cal./sq. cm.-second. Based upon a n image area of 0.039 sq. cm., 49 watts are available a t the focus of the furnace; this value may be compared with the 204 watts available a t the focus of the 5foot-diameter SRI furnace (9) and is larger than might be expected from the diameters of the two paraboloids. T h e additional energy available in the smaller paraboloid is due to the front surfacing and the absence of a heliostat. T h e first of these factors is accounted for in calculating the flux by a higher reflectivity, the latter by fewer reflections. T h e 2-foot paraboloid is located on the roof of a two-story laboratory building. T h e mirror support is welded 4-inch iron pipe, suppoyied on the bu.ilding wall by a n overhang. Four supports on the
The paraboloid mirror was used with a Springfield equatorial mount to track the sun. Details of special castings are shown in rear view 1. Locking nuts for coarse azimuth. 2. Boston miter gear G 4 6 2 4 . 3. W a l l bracket. 4. Leveling bolts. 5. Counter weight. 6. Boston worm gear LVHB. 7. Azimuth adjustment. 8. Fine 10. Paraboloid cover. 1 1. Pin-hole aiming dedeclination adjustment. 9. Declination scole. vice. 12. Friction-bearing surface. 13. Boston worm gear G 1 0 4 8 8 teeth-inch. 1 4 . Casting C. 15. 5/32-inch ball bearings for 3.75-inch diameter. 16. Latitude. 17. Polar axis stud. 18. Casting B. 19. Casting A. 2 0 . Support for mount. VOL. 52, NO. 10
OCTOBER 1960
825
JOINT FOR ADDITIONAL GAS SAMPLING BULB\
x
GAS SCMPL'IhG
TYGON TUGING
3ULB
Reaction cell and monitoring system for liquid-phase photochemical reactions in 2-foot solar furnace Reaction cell was a b o v e the mirror
mounted
F R O N T SURFACED PARFB~L~IC
opposite face were constructed of heavy material to provide a firm base for the mirror and its mounting. Mount. T h e parabolic mirror is pivoted on an equatorial mount positioned on the upright support post shown. I n an equatorial mount, the main axis of the mounting is adjusted to be parallel to the axis of the earth, and the paraboloid can be turned about this axis, called the polar axis, and about a perpendicular axis, called the declination axis. For each equatorial mount the angle between the polar axis and the northern horizon must equal the latitude at that location. Initially the mount is adjusted to true north, which is most conveniently done at solar noon, and locked in place. Thereafter the declination is adjusted daily. With these two adjustments, the mirror will follow the sun as the declination axis of the furnace is revolved around the polar axis of the mount. T h e chief disadvantage of this type of mount is that it is not capable of continued use much past the meridian. Reversal is accomplished by rotating the furnace 180° around both the polar axis and the declination axis. T h e Springfield Mounting ( 8 ) consists of three major castings (see front and rear views). Casting A was designed to fit the support pipe; this casting, after the polar axis is aligned to true north, is locked into place with four set screws. T h e polar axis casting, B, is designed for the particular latitude where the mount is to be used; the polar angle at Menlo Park is 37' 27'. Casting B is made level with four leveling nuts. T h e solar furnace is attached to casting C, which moves on casting B with ball bearings. For operation of the furnace, the coarse azimuth locking nuts are loosened, and the paraboloid is brought perpendicular to the sun. T h e locking nuts are tightened, and the sun is tracked by maintaining the light spot from a pinhole aiming device on a fixed point on the paraboloid frame. Although the sun is tracked manually, it would be easy to drive the azimuth adjustment with a clock motor. Reaction Cell. T h e reaction cell was made of 76Yo silica glass, whose light transmission is about 70% in the 3000to 3500-il.wave length region in the geometry used here. Light rays entered
826
the vessel from the paraboloid below and came to focus inside rhe space enclosed by cooling coils-about 1.25 inches above the bottom of the flask. N o mechanical stirring was provided, as it \vas expected that sufficient mixing would be obtained in the l7/8-inch diameter reaction cell by convection from the sun's image focused at the bottom of the cell and from gas bubbles rising to the surface. The ancillary equipment consisted of argon gas flushing, sampling, and pressure-measuring devices. Experimental Oxygen Evolution. Over 20 runs were made on the cerium(1V)-cerium(II1) perchlorate system (Reaction 1 ) . .41though the results were comparable with those obtained when using the rearsilvered 5-foot furnace ( 9 ) ,the mass balance obtained with the 2-foot furnace was much better (see graph). The spread between volumes of oxygen calculated from pressure rise, titration of cerium( I V ) perchlorate, and mass spectrometer anal sis of the product gas was less than Based upon equal reactant concentrations and light intensities (as recorded by the pyrheliometer), oxygen evolution was approximately 10 times faster in the frontsurfaced solar furnace. This increase in rate was due to the additional light energy in the 3000- to 3500-A. region. In addition to the increased light energy available with the front-surfaced parab-
*22.
oloid, the smaller furnace was much easier to handle than the 5-foot furnace. Hand tracking of the sun was simpler than the electronically controlled heliostat. One man operated the 2-foot furnace, whereas two men were necessary for experiments with the larger furnace. Hydrogen Evolution, The photochemical production of oxygen is of little use for storage on earth of the sun's energy, without the concomitant formation of a storable reducing substance which can react with oxygen and thereby produce either heat or electricity directly from a fuel cell. It has been reported that hydrogen could be produced by the photc-oxidation ofcerium(II1) to cerium(1V) ions at 2537A. (3, 4, 6, 7 7 ) and by much longer exposures to sunlight ( 5 ) .
Ce-+++
+ OH- +
H P (2)
The combination of Reactions 1 and 2 would constitute a method for storing the sun's energy on the earth's sxface if Reaction 2 is suffkiently rapid. Experir e n t s were designed to produce hydrogen by this reaction at the focus of a frontsurfaced solar furnace. A number of runs were made with the following initial cerium(IV) to cerium(II1) ion concentration ratios: 0.154 to 0.075, 0.098 to 0.022, 0.045 to 0.10, 0 to 0.200, and 0 to 0.231. The Ferchloric acid concentrations varied between 2.17 and 1.5M except for one run with 6M perchloric acid. Mass spectrometer analysis of the product gases showed that oxygen was produced according to Reaction 1 but that any hydrogen produced by Reaction 2 would have been present at concentrations lower than 0.01 yo,which is the lower limit of the sensitivity of the SRI mass spectrometer. This concentration of hydrogen would correspond to a maximum possible production of 2.5 X 10-5 moles of hydrogen in a 2-hour exposure at the focus of a 2-foot diameter solar furnace. Acknowledgment
T h e 2-foot paraboloid was made available through the courtesy of S. W. Grinnell and C. Alvarez-Tostado of Stanford University. References (1) De La Rue, R. E., Stanford Research
~
KE
m
""
8~
Typical run with 0.1 54M cerium(lV) perchlorate shows mass balance measurements. Oxygen evolution rate was 10 times faster than with previous furnace
INDUSTRIAL AND ENGINEERING CHEMISTRY
X.
Pressure
0. Cerium(lV) ion concentration
0. Mass spectrometer measurements
Institute, Menlo Park, Calif., private communication (1959). (2) Gaertner, R. F., Kent, J. A . , IND.ENG. CHEY.50, 1223 (1958). (3) Heidt, L. J., Berestecki, J., J . A m . Chem. SOC.77. 2049 (1955). (4) Heidt, L. j., McMillan: A. F., Ibid., 76, 2135 (1954). (5) Heidt, L. J., McMillan, A. F., Scicncc 117, 75 (1953). (6) Heidt, L. J., Smith, M. E., J. Am. Chem. SOC. 7 0 , 2476 (1948). (7) Hiester, N. K., Tietz, T. E., Loh, E., Jet Propulsion 27, 507 (1957). (8) Ingalls, A. G., ed., "Amateur Telescope Making," pp. 20-34, Scientific ,4merican. Inc.. New York, 1955. (9) Marcus,' R. J., Wohlers, H. C., IND. ENG.CHEM.51, 1335 (1959). (10) Ibid., 52, 377 (1960). (11) Stewart, D. C., U . S. Atomic Energy Comm. AECD-2389 (1948). RECEIVED for review March 2, 1960 ACCEPTED July 19, 1960 Work supported by U. S. Air Force Research Center, contract AF 19(604)-3477.
