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RUDOLPH J. MARCUS and HENRY C. WOHLERS Chemistry Department, Stanford Research Institute, Menlo Park, Calif.
Photochemistry in the Solar Furnace Parallel Light and Spectral Distribution Last November the intricacies of the solar furnace were described to I/EC’s readers. Here is a further discussion on the subject showing just how the furnace could be used to promote specific chemical reactions
IN
USING THE solar furnace as a light source for photochemical reactions, diverging rays of light from its focal spot can cause considerable difficulty. When solutions are illuminated ( 6 ) by placing reaction vessels so that the focal spot falls within the solution, complete light absorption can be obtained if the vessel is big enough and if the extinction coefficient of the absorbing substance is high enough. With a diverging light source this is difficult, but if the light is parallel, it is fairly easy. Furthermore, if the light is parallel, complete absorption can be ensured by increasing the length of the sample cell. Thus, parallel light from the solar furnace may be important in utilizing solar energy (2). Diverging light which has been concentrated by a paraboloid can be made parallel by placing a second paraboloid of the same optical characteristics on the other side of the common focus of the two mirrors. T h e concentrator of the solar furnace in these laboratories ( 6 ) is a parabolic mirror having a diameter of 5 feet and a focal length of 26 inches, so that its f number is 0.4. I t is backsilvered, so that we are limited to wave lengths above 3500 A. T h e secondary paraboloid is a front-aluminized mirror having a diameter of 1 foot and a focal length of 4.9 inches, so that its f number is the same as that of the concentrator (Figure 1). T h e intensity of the parallel light beam was determined with the apparatus shown in Figure 2, and Figure 3 shows the results a t 3 different slit widths. These curves show roughly the same maxima and minima as those shown by Shaw (8) a t the earth’s surface. T h e maximum energy occurred a t about 6000 A. Other maxima were noted at 10,000 16,000, and 21,000 A. T h e water absorption bands were noted at wave lengths of 8000, 11,000, and 19,000 A. Above 3500 A. there is no significant difference in the distribution of energy a t the sun’s image in the solar furnace from
been determined a t various wave lengths ( 7 ) . To obtain true energies a t a particular wave length, the energies shown in Figure 3 were divided by the theoretical half-intensity band width a t the corresponding wave length. T h e result of this calculation for the 2-mm. slit-width data is shown in Figure 4. This curve shows the true energy relationship much better than do the raw data of Figure 3. In particular, the predominance of visible light in sunshine is more readily apparent. Thus we find that 5070 of the
that received directly a t the earth’s surface. This appears to be true despite the fact that the light undergoes 5 reflections and passes through 3 / 4 inch of soft glass. T h e cut-off a t 3500 A . was to be expected because of the back-silvered optics (7). Although each point on the curves of Figure 3 shows an amount of energy a t a nominal wave length, this amount is really spread out over a wave-length band. The width of this wave-length band changes with wave length, and has
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Figure 1. Orthogonal arrangement o f parabolic mirrors. The reflected light i s not exactly parallel; the focal image, 8 inches in diameter, i s 1 1 feet from the secondary paraboloid
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Intensity of the parellel light beam was measured, using this method VOL. 52, NO. 5
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Figure 3. Spectral distribution of energy in the solar furnace shows minima and maxima roughly the same as those shown a t the earth’s surface ( 8 )
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Figure 4. Normalized thermopile output. This curve shows the true energy relationship much better than raw data o f Figure 3
Thermopile output vs wave length
radiation on the solar furnace has wave lengths of 3500 to 7000 =I=500 A. This point is explored more quantitatively in Figure 5, which shows the cumulative percentage of energy in the furnace as a function of wave length. T h e thermopile can be used directly, without the intervening spectrometer, to determine the total energy available in the furnace. The 8-inch-diameter spot was scanned with the thermopile, which gave readings of 8.4, 7.8, 7.7, and 5.3 mv. a t distances of l/Z, 1 1 / 2 , 2 I / Z i and 3 l / 2 inches from the center of the circle. These readings were integrated and multiplied by the calibration factor of the thermopile. This calculation showed that 51 watts of energy were available in the 8-inch circle of parallel light-Le., a t a distance of 11 feet from the focus of the furnace. This result can also give us a measurement of the energy available at the focal spot itself by comparing the areas subtended by the t\vo paraboloids as shown in Figure 1. This comparison shows that the secondary paraboloid reflects one fourth of the light a t the common focus into the 8-inch circle. Direct multiplication of the above experimental results shows that 204 watts of energy are available a t the focus of the furnace. This figure is to be compared lvith a value of 214 watts calculated by De La Rue (3) from formulas developed by himself and Hiester ( 4 ) . These figures take into account losses due to geometrical imperfection, reflectivity imperfections, and absorption by the atmosphere. These energies agree with those obtained by Trombe for his 2.5-kw.furnace (9). This investigation has shown that parallel light can be obtained from the focus of the solar furnace. Total amount of energy available in the furnace (204 watts) and in parallel light from the
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Figure 5. Fraciional solar energy. The curve represents percentages a t the earth’s surface (5,8 ) and the points r e p r e s e n t percentages for the solar furnace
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furnace (51 watts) has been determined, together with wave-length distribution of this light. Thus, a t least rough quantum yields can bc determined when total light absorption occurs in the wave-length range under consideration. Shape of the solar spectrum does not appear to change in the solar furnace except for the short wave-length cut-off. TWOcomparisons with other experimental data were made. The total amount of energy found to be available in the furnace (204 watts) is comparabIe to engincering calculations for this quantity (214 watts). Comparison of the amount of energy in the 3500 to 4000 A. range present in the furnace (3.1 watts) with the 2.6 watts consumed in the photoreduction of ceric ion (6) yields a quantum efficiencv of 0.84 for that reaction.
Acknowledgment W e lvish to thank Arthur G. Brown and Robert E. De La Rue for valuable help and suggestions.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Literdure Cited (1) Beckman Instruments, Inc., South Pasadena, Calif.. Bull. 91-G, “Operating Instructions for Beckman Quartz Spectrophotometer,” November 1950. (2) Daniels, F., Duffie, J. -1. (eds.), “Solar Energy Research,” pp. 175-224, Univ. of Wisconsin Press, Madison, Wis., 1955. (3) De La Rue, R. E., Stanford Research Institute, private communication. (4) Hiester, N. K., Tietz, T. E., Loh, E., Jet Probulsion. 507-13., 546 (Mav 1957). (5) LeviAe, S.: Halter, H.; Mannis, ’F., J . Solar Energy 2, Iio. 2, 11-21 (1958). ( 6 ) Marcus, R. J., Wohlers, H. C., IKD. ENG.CHEM.51, 1335 (1959). (7) Morikofer, W., Trans. Conf. on Use of Solar Energy, Tucson, Ark.: vol. 1, pp. 63-9, Oct. 31-Nov. 1, 1955. (8) Shaw, J. H., O h 0 J . Sci. 53, 258-71 \
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(9) Thuman, W. C., Stanford Research
Institute, private communication. RECEIVED for review September 3, 1959 ACCEPTEDFebruary 29, 1960 Work supported by the U. S. Air Force Cambridge Research Center undvr Contract AF 19(604)-3477.
