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Measurement of direct, diffuse, and total solar radiation with silicon photovoltaic cells
Measurement of Direct, Diffuse, and Total Solar Radiation with Silicon Photovohaic Cells Kudret Sel~uk Instructor in Mechanical Engineering, Middle East Technical University, Ankara, Turkey
John I. Yellott Director, Yellott Solar Energy Laboratory, Phoenix, Arizona, U. S. A.
The paper presents the results of tests carried o u t d u r i n g t h e s p r i n g o f 1962 a t t h e Y e l l o t t S o l a r Energy Laboratory to determine the suitability of commercially available silicon photovoltaic c e l l s for u s e i n s o l a r r a d i o m e t e r s . S i n c e t h e s h o r t c i r c u i t c u r r e n t o f t h e s i l i c o n cell h a s b e e n s h o w n to exhibit linear variation with varying intensity of incident radiation, this quantity was measured for six cells, s e l e c t e d a t r a n d o m , u n d e r w i d e l y varying temperatures, under both tungsten i l l u m i n a t i o n a n d s o l a r r a d i a t i o n . Over t h e r a n g e o f t e m p e r a t u r e s l i k e l y t o be e n c o u n t e r e d in s o l a r r a d i o m e t r y (40 t o 160 d e g r e e s F ) , t h e s h o r t c i r c u i t c u r r e n t varied f r o m 10 t o 15 p e r c e n t , under tungsten illumination. Under natural s u n l i g h t , t h e m a x i m u m v a r i a t i o n w a s 6.3 p e r c e n t . By measuring the millivolt drop across a small resistance, instead of determining the shortcircuit current directly with a milliammeter, t e m p e r a t u r e c o m p e n s a t i o n is s h o w n t o be a d e q u a t e l y a c c o m p l i s h e d by t h e u s e o f a t h e r m i s t o r and a Manganin wire shunting resistance, which a r e m a i n t a i n e d a t t h e t e m p e r a t u r e o f t h e cell.
T h e effect u p o n t h e cell c o e f f i c i e n t o f s o l a r altitude, in terms of deviation from the cosine r e s p o n s e a n d air m a s s effect, w a s t h e n i n v e s t i g a t e d , u s i n g a s i l i c o n cell r a d i o m e t e r w h i c h c o u l d serve b o t h as a p y r h e l i o m e t e r a n d as a p y r a n o m e t e r . It w a s f o u n d t h a t , for s o l a r a l t i t u d e s a b o v e 60 d e g r e e s , n o c o r r e c t i o n for c o s i n e d e v i a t i o n or air m a s s effect is n e c e s s a r y . F o r l o w e r s o l a r a l t i t u d e s , t h e s e f a c t o r s m u s t be t a k e n i n t o c o n s i d e r a t i o n by d e t e r m i n i n g t h e c o e f f i c i e n t at varying solar altitudes, through calibration against a standard thermopile-type instrument. A day-long comparison of the output of a s i l i c o n cell p y r a n o m e t e r w i t h t h e o u t p u t o f a n Eppley 180-degree horizontal pyrheliometer s h o w e d a n a g r e e m e n t in t o t a l r a d i a t i o n w i t h i n 1 p e r c e n t . It is c o n c l u d e d t h a t , w i t h p r o p e r c a l i b r a t i o n a n d a d e q u a t e a t t e n t i o n t o t h e effects c a u s e d by s o l a r a l t i t u d e , a t e m p e r a t u r e - c o m p e n s a t e d s i l i c o n - c e l l p y r h e l i o m e t e r c a n be u s e d with confidence to measure direct, total, and diffuse solar radiation with an accuracy of at l e a s t -4-3 p e r c e n t .
E A S U R E M E N T S of the total solar radiation reaching the earth's surface, and its two components, the direct b e a m from the sun and the diffuse radiation from the sky, are essential in most fields of solar energy research. Instantaneous values as well as day-long totals are needed to evaluate the performance of existing solar devices, and to estimate the probable effectiveness of future installations. I n other fields, such as meteorology and agriculture, where the energy of the sun is utilized b y natural forces, m e a s u r e m e n t
of the intensity of solar radiation is also vitally important. Thermopile-type pyrheliometers and pyranometers* have been in use for m a n y years for the measurement of solar radiation, and their characteristics 1, 2, 3 are too well known to require comment. However, because of the intricate and careful workmanship required to produce these instruments and the facilities and skill required in their calibration, their cost is necessarily high. M a n y laboratories have need of a versatile radiometer t h a t can be produced at low cost and used with confidence to measure solar radiation to an accuracy of =t=3 percent.
M
* See Appendix I for recommended terminology. Manuscript received June 20, 1962. Vol. 6, No. ~,, 196'2
155
The availability at relatively low cost of the silicon photovoltaic cells, which were invented in 1954 by the Bell Laboratories team' of Chapin, Fuller, and Pearson, has aroused much interest in the possibility of using these devices as the sensitive elements of radiometers. In their earliest papers), 5 the inventors of the silicon photovoltaic cell showed that, at constant temperature, the short-circuit current of the cells had a linear response to solar radiation of varying intensity. Recently, thanks to the dramatic success of silicon cells in the space program, cells of good efficiency have become commercially available in large numbers. Their cost has been reduced ten-fold since the first cells were offered for sale. A preliminary investigation in the spring of 1961 at the Yellott Laboratory 1° gave such encouraging results that a detailed study was undertaken early in 1962. The purpose of the work was to determine the effectiveness of typical, "off-the-shelf" silicon cells in measuring direct, diffuse, and total solar radiation, under a wide range of solar altitudes, angles of incidence, cell temperatures, and cell orientations. Basic measurement of the incoming radiation was made with the well-known Eppley normal incidence and horizontal pyrheliometers. These were, in turn, compared with an Abbot silver-disc instrument at the Laboratory of Solar Energy of the University of Arizona, Tucson. The results of this investigation, which are given in detail below, show that, with proper precautions, the temperature-compensated silicon cell can indeed be used with confidence as the sensing element for both pyrheliometers and pyranometers. Since the measured quantity is the short-circuit current produced by the silicon photovoltaic cells, either directly as milliamperes, or as millivolts emf across a low resistance, the name Sol-A-Meter has been coined to denote the combination of a silicon cell and a temperature-compensated resistance that is in thermal but not electrical contact with the cell.
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FIG. 1 ( A ) - - V a r i a t i o n of short-circuit current with i n t e n s i t y of incident solar radiation, a d a p t e d from Pearson. FIG. l ( B ) - - T e m p e r a t u r e v a r i a t i o n of short-circuit c u r r e n t and open-circuit voltage for a typical silicon cell u n d e r c o n s t a n t t u n g s t e n illumination.
The linear variation of short-circuit current for an 11 percent efficiency silicon cell as the incident solar radiation is increased from zero to 10O0 watts per sq meter or 317 Btu per hr per sq ft is shown in Fig. I(A). Under constant illumination with tungsten light, there is a six-fold reduction in the open-circuit voltage from a typical solar cell as its temperature rises from - 2 0 to +180 degrees C, while its short-circuit current varies over a range of less than 15 percent, as shown in Fig. I(B). The relatively small variation of short-circuit current over the range of temperatures encountered in solar radiometry led to the concept of measuring the cell's output in terms of the voltage drop across a small resistance, which would have a negative temperatureresistance characteristic to offset the rising cell current 7"=mper=ture Controlled and lllumlMafeol
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Silicon P h o t o v o l t a i c Cell Characteristics The silicon cell has been the subject of many papers that have covered various aspects of its performance under a wide variety of operating conditions 6' 7, s Much of the later literature is devoted to the performance of these cells in space applications, where their unique ability to convert sunlight directly into electricity has made them extremely valuable as power sources for satellites. Their first cost still is far too high for terrestrial power applications, where economics must prevail, but their property of producing relatively high short-circuit currents which are directly proportional to the intensity of the radiation falling on them makes them exceptionally useful as the sensitive elements of solar radiometers. 156
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FIG. 2 - - D i a g r a m of a p p a r a t u s used to determine t e m p e r a t u r e v a r i a t i o n of short-circuit c u r r e n t of six silicon cells under b o t h t u n g s t e n and solar illumination.
Solar Ene~'gy
at elevated temperatures. A preliminary qualitative investigation 1° carried out with tungsten light during the spring of 1961, showed t h a t the t e m p e r a t u r e dependence of a silicon c e l l - - M a n g a n i n wire combination was small indeed. The present work began with a quantitative s t u d y of the temperature-current characteristic of a n u m b e r of silicon cells under both artificial (tungsten) illumination and under natural sunlight. The latter p a r t of the study was carried out after Dr. Bernd Ross of H o f f m a n n Electronics Corporation 15 had called the writers' attention to the substantial difference between the behavior of silicon cells under sunlight and under artificial illumination.
Temperature Compensation Six T y p e 120-C standard silicon cells were purchased from the Semiconductor Division of Hoffman Electronics Corporation. These cells, which are 1 cm wide and 2 cm long, have a nominal active surface area of 1.8 sq cm and a rated efficiency of 6.0 percent. At 82.4 degrees F (28 degrees C), their short-circuit current is rated at 40 milliamperes (ma) for a tungsten lamp illumination of 100 milliwatts per sq cm. These cells were soldered to a copper disk t h a t served both as a t e m p e r a t u r e equalizer and as a comm o n negative terminal. The positive leads from the individual cells were formed b y copper wires soldered to the thin strips of solder on the upper surfaces of the cells, and, as Fig. 2 shows, these leads were connected to one set of terminals of a double-pole multi-point r o t a r y switch. The short-circuiting resistance, a 12.1 in. (30.7 cm) length of 30-gage copper wire, was immersed in an ice b a t h to eliminate t e m p e r a t u r e variations t h a t would otherwise m a s k the changes in shortcircuit current. The voltage drop across the resistance (0.118 ohm) was measured with a calibrated 0-10 m v recorder with available chart speeds of 2 in. per hour and 2 in. per minute (Varian T y p e G-10). Cell temperatures were measured b y 30-gage copperconstantan thermocouples t h a t were cemented to, but electrically insulated from, the upper surface of each cell. The thermocouples were shielded from the incident radiation b y small pieces of black tape, which also reduced the active areas of the cells in varying degrees. All of the thermocouples were connected through a common constantan lead to the cold junction, also maintained in the ice bath. The copper (positive) leads were connected through the second pole of the rotary switch to another 0-10 m y Varian potentiometer recorder. The copper disk with the attached silicon cells was placed in a double-walled aluminum vessel t h a t was set on an electric heater. A thin Pyrex plate (92 percent normal transmissivity) was used as a cover to minimize convection currents. During the first part of the ternVol. 6, No. 4, 1962
perature-compensation study, constant illumination was provided b y a 100-watt tungsten lamp, which received its energy through a constant-voltage transformer. During the second part of the program, the double-walled vessel and the electric heater were fastened to an altazimuth mounting so t h a t t h e y could follow the sun and tests were run between l1:00 a.m. and 1:00 p.m., when the direct solar b e a m was virtually constant in its intensity. T e m p e r a t u r e control was accomplished during both series of tests b y first filling the space between the two aluminum vessels with a mixture of ice, water, and salt. The copper disk was then taped to the b o t t o m of the smaller vessel, whereupon the temperatures of the disk and the attached silicon cells immediately fell to
TABLE 1--Variation of Short-Circuit Current of Un-
compensated Silicon Cells for Temperatures from 32 to 194° F, Expressed as Departure from Current at 77° F, for Tungsten Light and Solar Radiation Cell No.
Temp. OF
Illumination Source 2
32 50 68 77 86 104 122 140 158 176 194
Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight
4
-5.63 -2.62 --0.87 0
-6.00 -2.65 -3.28 -1.64 --1.4 -0.40 o
+1.74 +3.5 +5.65 + -+6.95 + -+8.26 + -+10.5
+11.0 +
--
-5.5 -2.68 -3.52 -1.24 --1.5 -0.62
3
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:3:65 1.63 5.05 2.24 7.3 2.65 9.15 3.05 0.9 3.65 1.5 3.45
-6.9 -2.61 -3.47 -1.4 -0.87 -0.4 0 0 +1.15 +o.4 +2.9 +0.8 +4.05 +1.6 +5.2 +2.01 +5.5 +2.21 +5.5 +2.41 +4.87 +2.01
a b o u t 27 degrees F. Cell temperatures and shortcircuit currents were then determined at frequent intervals as the ice melted under the combined influence of the incident illumination and the electric heater. The results of typical tests with b o t h tungsten lamp illumination and natural sunlight are given in Table 1 in terms of the departure of the short-circuit current at cell temperatures from 32 to 194 degrees F from the current observed at a cell temperature of 77 degrees F. (Cell No. 1 was damaged during the soldering process and gave inconsistent results; Cell No. 2 was altered after the tungsten lamp tests b y the addition of a compensating resistance and so its results are not reported for the sunlight conditions). The variation of output in millivolts for cells No. 2, 3, and 6 is given in 157
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FIG. 3--Variation of short-circuit current for typical silicon cells under constant tungsten illumination over the temperature range 40 to 195 degrees F.
FIG. 4--Percentage variation in short circuit current of typical silicon cell over the temperature range 40 to 195 degrees F for both tungsten illumination and solar radiation.
Fig. 3. The behavior of all cells follows the predicted pattern of a gradual rise in short-circuit current as the temperature increases up to about 170 degrees F, which is well above the range encountered in solar radiometry. At higher temperatures, the short-circuit current begins to diminish again. Each cell has its own particular temperature-current characteristic, however, with Cell No. 5 showing the widest variation (15.95 percent), and No. 3 the smallest variation (10.2 percent) as the temperature rose from 32 to 158 degrees F, under tungsten illumination. Under sunlight, No. 5 showed a total rise of 5.90 percent, while the short-circuit current of No. 3 rose by 4.67 percent. A comparison of the performance of Cell No. 6 under tungsten and solar illumination is presented in Fig. 4. It is evident that the temperature
coefficient is considerably higher for tungsten light than for sunlight. This is apparently caused by the increase in red response of the cell at elevated temperatures. ''~ For many practical applications in the measurement of solar radiation, acceptable accuracy can be obtained by using Manganin wire as the short-circuiting or metering resistance, since this alloy has virtually zero temperature-resistance coefficient in the range from 70 to 120 degrees F. For more accurate work, and partieularly in bright sunshine where high cell temperatures are likely to be encountered, and generally in the measurement of tungsten illumination, more effective temperature compensation is required. This can be accomplished by using a suitable low-resistance thermistor in parallel with a shunting resistance of Manganin wire. The combination can be caused to have virtually any desired negative temperatureresistance characteristic by proper selection of the thermistor. Cell No. 2 was selected for compensation since its current-temperature characteristic was typical of the other cells under consideration. Over the temperature range from 32 to 158 degrees F, its short-circuit current rose by 13.9 percent, under constant tungsten illumination. A Fenwal bead thermistor (type KD05L1, 5 ohms resistance at 77 degrees F) was chosen since its resistance-temperature characteristic, shown in Fig. 5, when connected in parallel with a 0.25-ohm Manganin wire, gave the desired reduction in total metering resistance. A test of the compensation circuit was made by cementing a thermistor and a 2 in. length of 30-gage Manganin wire to the copper disk immediately adjacent to Cell No. 2. Once again, the annular volume of the double-walled aluminum vessel was filled with crushed ice, salt, and water, and the output of the compensated cell was measured at constant ilhunination (tungsten) as its temperature rose from 36 to 195 degrees F. As Fig. 6 shows, the compensation was excellent over the
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Fro. 5--Resistance-temperature characteristics of 5-ohm (at 77 degrees F) thermistor alone and with 0.25-ohm Manganin shunt. 158
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FIG. 6--Temperature variation of short-circuit current from Cell No. 2 when compensated by shunted thermistor (tungsten illumination).
range from 40 to 140 degrees F; above 150 degrees F the metering resistance falls too rapidly, and the compensation is no longer satisfactory. Fortunately, silicon cells in bright sunlight in the earth's atmosphere usually remain below 140 degrees F. Hence adequate compensation can be provided by the combination of a low-resistance thermistor and a Manganin shunting resistance. The selection of the thermistor and the shunting resistance can be accomplished with the aid of Fig. 7 when the temperature-current characteristics of a particular cell are known, and the output in millivolts is also determined. The ratio of the short-circuit currents at the lower and higher temperature limits is found, and its reciprocal is evidently the required resistance ratio. The magnitude of the Manganin shunting resistance is determined by dividing the desired output in millivolts by the short-circuit current in milliamperes. The thermistor that gives the desired resistance ratio can then be selected from Fig. 7. As an example, the successful compensation of Cell No. 2 for tungsten light, as shown in Fig. 6, was accomplished with a 5-ohm (at 77 degrees F) thermistor and a shunting Manganin resistance of 0.25 ohms. This was too drastic a compensation for use with sunlight, for which the short-circuit current ratio was 0.96, and in this case a thermistor with a resistance at 77 degrees F of 15 ohms should be used with a Manganin shunting resistance of 0.2 ohms.
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FIG. 7--Variation of combined resistance ratio with resistances of Manganin shunt and thermistor.
The cell was soldered to a small brass disk which was mounted within but insulated from an aluminum cap. A thermistor (Fenwal type KD05L1) was inserted into a small hole drilled in the lower portion of the disk,
Th eemisfor_~ ~ , / 3 i l / c o n
Cell
Cosine Response and Air Mass Effect For use with sunshine in determining the cosine response and the dependence of cell output upon air mass, a new radiometer was constructed as shown in Fig. 8. A type l l 0 C Hoffman silicon cell (l cm square, 0.9 sq cm active area) was used as the sensitive element, since the instrument was designed to serve both as a pyranometer for measuring the total radiation from sun and sky, and as a normal incidence pyrheliometer. V o l . 6, N o . 3, 1962
7-/T e r m o c o c , p / e s Fie-. 8--Cross-seetion of temperature-compensated silicon cell pyrheliometer. ]59
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blackened in order to keep it as closely as possible at the temperature of the silicon cell. The tube of the radiometer shown in Fig. 8 had a length of 5.5 in., while the diameter of the window at the upper end was 0.55 in., giving the ten-to-one ratio of length to diameter recommended for pyrheliometers. 1 The instrument was designed so that the tube was removable; thus, the instrument could also be used as a pyranometer in the next series of tests to determine the cosine response of typical silicon cells. Determination of Cosine R e s p o n s e - - T h e intensity of solar radiation reaching a surface on the earth is the sum of the direct beam component, [D~ cosine 6, and the diffuse radiation from the sky, Id. Expressed as an equation, this is:
FIG. 9 - - V a r i a t i o n of cosine response for silicon cell at, incident angles from 0 to 90 degrees.
with the upper lead of the thermistor also soldered to the disk. The lower lead was insulated and led out through a Lucite spacer ring. The Manganin shunting resistor was wound around the outer diameter of the brass disk, with one end soldered to the disk and the other end soldered to the positive lead from the upper surface of the silicon cell. For test purposes, one copper-constantan thermocouple was cemented to the thermistor, while a second thermocouple was cemented to the lower surface of the silicon cell. When the instrument was exposed to bright sunlight, the two temperatures were virtually identical. The exposed portion of the brass disk was
Ito = Ia + IoN COS
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where 0 is the angle of incidence between the solar beam and the cell surface. When a pyranometer is used to measure total horizontal insolation, the angle of incidence between the sun's rays and the cell surface is the complement of the solar altitude, ~. In order to be able to cover the entire range of incident angles from 0 to 90 degrees, the lower portion of the radiometer shown in Fig. 8 was mounted FIG. 10--Spectral d i s t r i b u t i o n of solar r a d i a t i o n i n t e n s i t y in near space and on e a r t h for v a r y i n g air mass; spectral response of typical silicon cell.
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on an Astro Compass, or star finder, with the cell surface normal to the line of sight of the instrument. B y following the sun's motion in azimuth, and setting the altitude of the cell b y means of the vernier adjustment on the star finder, the angle of incidence between the solar beam and the cell could be varied accurately from 0 to 90 degrees by increments as small as 1 degree. If the exposed surface of the cell could absorb all of the solar radiation falling upon it, the ratio that constitutes the first part of Eq. (2) would always be exactly equal to the cosine of the incident angle. Since the reflectivity of the cell surface varies with the incident angle, a series of tests was run at mid-day when the solar altitude was approximately 68 degrees. The direct normal insolation, I . ~ , was measured with an Eppley normal incidence pyrheliometer on a suntracking mount, while the output of the silicon-cell pyranometer was read as the incident angle was varied in 10 degree increments from 0 to 90 degrees; in each setting, the output was measured with the cell both shaded and unshaded, to give both the diffuse or sky radiation, Id, and the total insolation at the prevailing incident angle, Lo. For each value of the incident angle, the agreement with the cosine law was determined by rearranging Eq. (2) to give: (Ito -- Id)
Percent true response - IDN × COS0 X 100
(3)
The results of this test are shown in Fig. 9, which gives both the cosine response and the Sol-A-Meter constant for incident angles from 0 to 83 degrees. The cell evidently follows the cosine law almost exactly for incident angles from 0 to 20 degrees; at higher angles the reflectivity of the cell's surface increases rapidly and the cell's response deviates increasingly from the true value. The performance of the Sol-A-Meter can be expressed in terms of an instrument constant or coefficient, giving the incident insolation in Btu per hr per sq ft per millivolt of output. For the instrument of Fig. 8, the coefficient at 0 = 0 degree was 82; as the incident angle increased, the coefficient also increased to a value of 90 at 0 -- 62 degrees and 100 at 0 = 74 degrees. The departure of the Sol-A-Meter response from the cosine law is considerably greater than that experienced with thermopile-type pyranometers, which generally adhere quite closely to the predicted performance until the incident angle exceeds 60 degrees. TM Effect of A i r M a s s V a r i a t i o n - - T h e spectral response of a typical silicon photovoltaic cell shown by the dashed line in Fig. 10, is quite different from the energy distribution of sunlight in near space (Air mass, ] I = 0) as well as on the earth's surface. The peak response of the cell comes at about 0.85 microns, while Vol. 6, No. 4, 1962
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the wavelength of maximum solar-radiation intensity varies from 0.48 microns in space to 0.67 microns on earth when the sun is low in the sky (M = 5). This shift in intensity distribution toward the longer wavelengths should cause the relative response of the SolA-Meter to rise with increasing air mass as more of the sun's radiation comes within the spectral range of the silicon cell. The response of the radiometer of Fig. 8, operated as a pyrheliometer with the tube in place, was compared during an entire day with the output of an Eppley normal incidence instrument (No. 4003). At 11:40 a.m., when the solar altitude was 68 degrees and the air mass was 1.075, the direct normal insolation, as determined by the Eppley pyrheliometer, was 297 Btu per hr per sq ft, and the Sol-A-Meter output was 3.62 inv. The instrument coefficient at minimum air mass was thus 82.0 Btu per hr per sq ft per my. The coefficient was determined in the same manner for air masses up to 5.6, corresponding to a solar altitude of about l0 degrees. The ratio of the coefficient at each air mass to the coefficient at minimum air mass was then computed and plotted on Fig. 11. According to this test, the increasing air mass has virtually no effect for solar altitudes above 30 degrees; for lower altitudes the departure from the true response increases to l0 percent at air mass 5.6 (10 degrees solar altitude). Using Moon's classic radiation intensity data as given in Fig. 10 (adapted from Bickler,~7), an attempt was made to calculate the relative output of a typical silicon cell for air masses from 1 to 5. The radiation intensity at each wavelength was multiplied by the cell's response at the same wavelength and the results plotted as the dash-dot lines in Fig. 10. The ratios of the areas under the dash-dot curves to the areas under the solar-radiation intensity curves were then determined. When only the wavelengths that affect the silicon cell are considered, this procedure results in a predicted increase of cell output of about 10 percent for M = 3 and 11 percent for M = 5. When the entire solar spectrum out to 1.8 microns was included in the 161
calculation, however, it appeared that the maximum output should be obtained at M = 3, which is contrary to the experimental results.
Combined Effects of Air Mass and Cosine Response-When the Sol-A-Meter is used as a pyranometer to measure the total input of radiation from sun and sky upon a horizontal surface, its coefficient, following the initial calibration against a standard thermopile instrument, can be stated in terms of the solar altitude, which determines the air mass effect, and the complement of the solar altitude, which determines the cosine effect. For high solar altitudes, as shown in Fig. 12, no correction is needed. For altitudes below 60 degrees, the air mass effect and the cosine effect oppose each other, but the latter is much more potent and so the combined response decreases quite rapidly with decreasing solar altitude. As long as the cell remains in the horizontal plane, a single combined coefficient that is a function of solar altitude alone can be used.
Day-Long Performance of Silicon Cell P y r a n o m eter The output of the pyranometer section of the instrument shown in Fig. 8 was compared throughout the entire day of April 21, 1962, with the output of an Eppley 10-junction horizontal pyrheliometer (No. 3737). Both instruments were leveled carefully and 120 A I ~ /~AS,S.N. I00
diffuse insolation as determined by a shaded Eppley horizontal pyrheliometer (No. 3737) was 35.3 Btu. The total horizontal insolation was thus 313 Btu per hr per sq ft. The noon output of the horizontal Eppley instrument was 3.25 mv, giving a coefficient of 96.4 Btu per my. The noon output of the compensated silicon cell was 3.90 mv, giving a coefficient of 80.4 Btu per my, which is slightly below the previously determined values. The minute-by-minute fluctuation of the recorder pen, even on a clear day, due to actual fluctuation of the solar radiation, is rarely less than 0.1 my, and so it is not likely that one can obtain agreement closer than 2.0 percent without taking greater precautions than are warranted by the recorder accuracy and the time lag of the thermopile instrument. When the areas of the two charts on Fig. 11 were measured by a planimeter, it was found that the area under the curve on the Eppley chart was 24.77 sq in., while the area under the Sol-A-Meter chart was 30.10 sq in. Using the noon values of the instrument outputs and the resulting coefficients, the total daily insolation as determined by the Eppley horizontal pyrheliometer was 2380 Btu per sq ft; the value as determined by the Sol-A-Meter was 2402 Btu per sq ft. This agreement leads to the conclusion that the effects of air mass and cosine deviation which have been detailed above are not of compelling importance in a day-long insolation determination. When these effects are being experienced, the total insolation is relatively small and so the errors are also comparatively small.
t"IAS8 ,.t- : COSINE
Conclusions
-%
8O
COSINE
11
\
I-.
t 20
0
t'~ •
; 0°
~o"
I ANGLE
ff
I0°
9
0[" IIVCIDK/VCE
6'0" ~,0" #0" SOLAR A L T I T U D E
3'0 °
2'o"
fo"
o"
FIG. 12--Combined effect of air mass variation and cosine law deviation upon Sol-A-Meter response at solar altitudes from 0 to 90 degrees.
their outputs were recorded on calibrated 0-10 mv potentiometer recorders (Varian G-10). The records, shown on Fig. 13, are typical of a cloudless Arizona day. The intensity of the direct normal beam, as determined by Eppley pyrheliometer 4003, was 300 Btu per hr per sq ft at noon, when the solar altitude was 69 degrees. The direct component on a horizontal surface was thus 300 cosine (90-69) = 277.5 Btu; the 162
The short-circuit current from commercially available silicon photovoltaic cells can be used with confidence to measure the solar radiation falling on a terrestrial surface, provided that appropriate corrections are made for cosine and air mass effects and for temperature variation. The latter factor, which is unpredictable since it depends upon ambient temperature and wind velocity as well as upon intensity of solar radiation, can fortunately be eliminated by measuring the short-circuit current in terms of the millivolt drop across a metering resistance composed of a suitable thermistor in parallel with a short length of Manganin wire. The falling temperature-resistance characteristic of the thermistorManganin wire combination can be adjusted to compensate ahnost exactly for the rising current-temperature characteristic of the silicon cell. The cosine and air mass corrections can both be made in terms of the sun's altitude in the case of a horizontal silicon cell pyranometer. When the cell is used in an attitude other than horizontal, corrections must be made for both incident angle and air mass. When proper precautions are taken, and standard thermopile-type pyrheliometer~, are available for their Solar Energy
EPPLEY 180"#3r3z o
SOO
i
5
400
L ~-
I
1
300 "
~
"l . r t
2O0 PM
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i *
/I oo
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,ooo
J
,oo
~
8loo
--It
'i
~oo
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,
FIG. 13--Day-long comparison of millivolt outputs of horizontal Sol-A-Meter and Eppley horizontal pyrheliometer.
calibration, radiometers using temperature-compensated commercial silicon cells can be a valuable addition to the wide variety of instruments now available for measuring the intensity of solar radiation.
Acknowledgment The writers wish to express their appreciation to the Scientific Council of NATO and to the Faculty of Engineering of The Middle East Technical University, who enabled this work to be carried out by granting a Fellowship and a leave of absence to Mr. Sel~uk. Appendix I--Recommended Terminology for Solar Radiometers In the comprehensive treatise on solar radiation measurement 1, which was published in conjunction with the International Geophysical Year, it is recommended that the term "l)yrheliometer" be applied to radiometers that measure the intensity of the direct solar beam, the prefix "normal incidence" being implied rather than expressed. The term "pyranometer" is recommended for use with instruments which measure the total radiation from both sun and sky, while the word "photometer" should be applied to instruments used for measuring total radiation. This terminology has been used in the foregoing paper except where long usage has brought universal acceptance to some other wording, as in the case of the Eppley horizontal 180 degrees "pyrheliometer".
REFERENCES 1. l)rummond, A. J., et al: "Radiation Instruments and Measurements", Part VI, IGY Instruction Manual, Pergamon Press, New York, 1958. 2. Drummond, A. J., and Greer, H. W.: "Fundamental Pyrheliometry", Sun at Work, 3, No. 2, p. 3, June, 1958.
Vol. 6, No. ~, 1962
3. Spitzer, C. F., et al: "Solar Cell Measurement Standardization"; Lockheed Aircraft Corporation, Missiles and Space Division, Report No. LMSD-288184, 29 Feb., 1960. 4. Chapin, D., Fuller, C., and Pearson, G.: "A New Silicon p - n Junction Photocell for Converting Solar Radiation into Electrical Power"; Jnl. App. Phys., 25, p. 676, 1954. 5. Pearson, G. : "Electricity from the Sun", Proc. 1955 World Symposium on App. Solar Energy, Stanford Res. Inst., 1956. 6. Rappaport, P.: "The Photovoltaie Effect and its Utilization", Solar Energy, 3, No. 4, p. 8, Dec., 1959. (Also RCA Review, Sept., 1959). 7. Escoffery, C. A., and Luft, W. : "Optical Characteristics of Silicon Solar Cells and of Coatings for Temperature Control", Solar Energy, 4, No. 4, p. 1, Oct., 1960. 8. Ravich, L. E.: "Thin Film Photovoltaie Devices for Solar Energy Conversion", 1961. U.N. Conf. on New Sources of Energy, paper S/56. 9. Schoffer, P., Kuhn, P., and Sapsford, C. M., ibid., paper S/92. 10. Yellott, J. I., Kokoropoulos, P., and Fogle, T., "Use of Silicon Cells as Pyrheliometers", Sounion Conference, 1961. 1l. Gummel, H. K., Smits, F. M., and Froiland, A. R.: "A Method for Terrestrial Determination of Solar Cell Short Circuit Current under Outer Space Solar Illumination", 1961 West. Elec. Show and Con. (WESCON), paper 7/3. 12. Prince, M. B., and Wolf, M.: "New Developments in Silicon Photovoltaic Devices", J. British Inst. of Radio Eng., 18, 1958. 13. Kreith, F. : "Radiation Heat Transfer", Int. Textbook Co., Scranton, 1962, pp. 147-163. 14. Hoffman Electronics Corp., Semiconductor Division, 1962 Catalog, p. 22. 15. Ross, B., Hoffman Electronics Corp., personal communication of 5/18/62. 16. "Handbook of Physics and Chemistry", 23rd edition, p. 1540, 1939, Chemical Rubber Publishing Co., Cleveland. 17. Biekler, D.: "The Simulation of Solar Radiation", Solar Energy, 6, No. 2, April-June, 1962, p. 66. 18. MacDonald, T. H.: "Some Characteristics of the Eppley Pyrheliometer", Monthly Weather Review, Vol. 79, No. 8, August, 1951, p. 157.
163
John I. Yellott Director, Yellott Solar Energy Laboratory, Phoenix, Arizona, U. S. A.
The paper presents the results of tests carried o u t d u r i n g t h e s p r i n g o f 1962 a t t h e Y e l l o t t S o l a r Energy Laboratory to determine the suitability of commercially available silicon photovoltaic c e l l s for u s e i n s o l a r r a d i o m e t e r s . S i n c e t h e s h o r t c i r c u i t c u r r e n t o f t h e s i l i c o n cell h a s b e e n s h o w n to exhibit linear variation with varying intensity of incident radiation, this quantity was measured for six cells, s e l e c t e d a t r a n d o m , u n d e r w i d e l y varying temperatures, under both tungsten i l l u m i n a t i o n a n d s o l a r r a d i a t i o n . Over t h e r a n g e o f t e m p e r a t u r e s l i k e l y t o be e n c o u n t e r e d in s o l a r r a d i o m e t r y (40 t o 160 d e g r e e s F ) , t h e s h o r t c i r c u i t c u r r e n t varied f r o m 10 t o 15 p e r c e n t , under tungsten illumination. Under natural s u n l i g h t , t h e m a x i m u m v a r i a t i o n w a s 6.3 p e r c e n t . By measuring the millivolt drop across a small resistance, instead of determining the shortcircuit current directly with a milliammeter, t e m p e r a t u r e c o m p e n s a t i o n is s h o w n t o be a d e q u a t e l y a c c o m p l i s h e d by t h e u s e o f a t h e r m i s t o r and a Manganin wire shunting resistance, which a r e m a i n t a i n e d a t t h e t e m p e r a t u r e o f t h e cell.
T h e effect u p o n t h e cell c o e f f i c i e n t o f s o l a r altitude, in terms of deviation from the cosine r e s p o n s e a n d air m a s s effect, w a s t h e n i n v e s t i g a t e d , u s i n g a s i l i c o n cell r a d i o m e t e r w h i c h c o u l d serve b o t h as a p y r h e l i o m e t e r a n d as a p y r a n o m e t e r . It w a s f o u n d t h a t , for s o l a r a l t i t u d e s a b o v e 60 d e g r e e s , n o c o r r e c t i o n for c o s i n e d e v i a t i o n or air m a s s effect is n e c e s s a r y . F o r l o w e r s o l a r a l t i t u d e s , t h e s e f a c t o r s m u s t be t a k e n i n t o c o n s i d e r a t i o n by d e t e r m i n i n g t h e c o e f f i c i e n t at varying solar altitudes, through calibration against a standard thermopile-type instrument. A day-long comparison of the output of a s i l i c o n cell p y r a n o m e t e r w i t h t h e o u t p u t o f a n Eppley 180-degree horizontal pyrheliometer s h o w e d a n a g r e e m e n t in t o t a l r a d i a t i o n w i t h i n 1 p e r c e n t . It is c o n c l u d e d t h a t , w i t h p r o p e r c a l i b r a t i o n a n d a d e q u a t e a t t e n t i o n t o t h e effects c a u s e d by s o l a r a l t i t u d e , a t e m p e r a t u r e - c o m p e n s a t e d s i l i c o n - c e l l p y r h e l i o m e t e r c a n be u s e d with confidence to measure direct, total, and diffuse solar radiation with an accuracy of at l e a s t -4-3 p e r c e n t .
E A S U R E M E N T S of the total solar radiation reaching the earth's surface, and its two components, the direct b e a m from the sun and the diffuse radiation from the sky, are essential in most fields of solar energy research. Instantaneous values as well as day-long totals are needed to evaluate the performance of existing solar devices, and to estimate the probable effectiveness of future installations. I n other fields, such as meteorology and agriculture, where the energy of the sun is utilized b y natural forces, m e a s u r e m e n t
of the intensity of solar radiation is also vitally important. Thermopile-type pyrheliometers and pyranometers* have been in use for m a n y years for the measurement of solar radiation, and their characteristics 1, 2, 3 are too well known to require comment. However, because of the intricate and careful workmanship required to produce these instruments and the facilities and skill required in their calibration, their cost is necessarily high. M a n y laboratories have need of a versatile radiometer t h a t can be produced at low cost and used with confidence to measure solar radiation to an accuracy of =t=3 percent.
M
* See Appendix I for recommended terminology. Manuscript received June 20, 1962. Vol. 6, No. ~,, 196'2
155
The availability at relatively low cost of the silicon photovoltaic cells, which were invented in 1954 by the Bell Laboratories team' of Chapin, Fuller, and Pearson, has aroused much interest in the possibility of using these devices as the sensitive elements of radiometers. In their earliest papers), 5 the inventors of the silicon photovoltaic cell showed that, at constant temperature, the short-circuit current of the cells had a linear response to solar radiation of varying intensity. Recently, thanks to the dramatic success of silicon cells in the space program, cells of good efficiency have become commercially available in large numbers. Their cost has been reduced ten-fold since the first cells were offered for sale. A preliminary investigation in the spring of 1961 at the Yellott Laboratory 1° gave such encouraging results that a detailed study was undertaken early in 1962. The purpose of the work was to determine the effectiveness of typical, "off-the-shelf" silicon cells in measuring direct, diffuse, and total solar radiation, under a wide range of solar altitudes, angles of incidence, cell temperatures, and cell orientations. Basic measurement of the incoming radiation was made with the well-known Eppley normal incidence and horizontal pyrheliometers. These were, in turn, compared with an Abbot silver-disc instrument at the Laboratory of Solar Energy of the University of Arizona, Tucson. The results of this investigation, which are given in detail below, show that, with proper precautions, the temperature-compensated silicon cell can indeed be used with confidence as the sensing element for both pyrheliometers and pyranometers. Since the measured quantity is the short-circuit current produced by the silicon photovoltaic cells, either directly as milliamperes, or as millivolts emf across a low resistance, the name Sol-A-Meter has been coined to denote the combination of a silicon cell and a temperature-compensated resistance that is in thermal but not electrical contact with the cell.
i
25
!// I
g 2o
!r-il
/ 0
200
600
600
,
SOLAR RADIATION, W.aTTS/M2
0
40
80
120
160
; 0
TEMPERATURE, °C
FIG. 1 ( A ) - - V a r i a t i o n of short-circuit current with i n t e n s i t y of incident solar radiation, a d a p t e d from Pearson. FIG. l ( B ) - - T e m p e r a t u r e v a r i a t i o n of short-circuit c u r r e n t and open-circuit voltage for a typical silicon cell u n d e r c o n s t a n t t u n g s t e n illumination.
The linear variation of short-circuit current for an 11 percent efficiency silicon cell as the incident solar radiation is increased from zero to 10O0 watts per sq meter or 317 Btu per hr per sq ft is shown in Fig. I(A). Under constant illumination with tungsten light, there is a six-fold reduction in the open-circuit voltage from a typical solar cell as its temperature rises from - 2 0 to +180 degrees C, while its short-circuit current varies over a range of less than 15 percent, as shown in Fig. I(B). The relatively small variation of short-circuit current over the range of temperatures encountered in solar radiometry led to the concept of measuring the cell's output in terms of the voltage drop across a small resistance, which would have a negative temperatureresistance characteristic to offset the rising cell current 7"=mper=ture Controlled and lllumlMafeol
Environment
,OPPER 0/$C
Silicon P h o t o v o l t a i c Cell Characteristics The silicon cell has been the subject of many papers that have covered various aspects of its performance under a wide variety of operating conditions 6' 7, s Much of the later literature is devoted to the performance of these cells in space applications, where their unique ability to convert sunlight directly into electricity has made them extremely valuable as power sources for satellites. Their first cost still is far too high for terrestrial power applications, where economics must prevail, but their property of producing relatively high short-circuit currents which are directly proportional to the intensity of the radiation falling on them makes them exceptionally useful as the sensitive elements of solar radiometers. 156
=PER ISTANTAN
FIG. 2 - - D i a g r a m of a p p a r a t u s used to determine t e m p e r a t u r e v a r i a t i o n of short-circuit c u r r e n t of six silicon cells under b o t h t u n g s t e n and solar illumination.
Solar Ene~'gy
at elevated temperatures. A preliminary qualitative investigation 1° carried out with tungsten light during the spring of 1961, showed t h a t the t e m p e r a t u r e dependence of a silicon c e l l - - M a n g a n i n wire combination was small indeed. The present work began with a quantitative s t u d y of the temperature-current characteristic of a n u m b e r of silicon cells under both artificial (tungsten) illumination and under natural sunlight. The latter p a r t of the study was carried out after Dr. Bernd Ross of H o f f m a n n Electronics Corporation 15 had called the writers' attention to the substantial difference between the behavior of silicon cells under sunlight and under artificial illumination.
Temperature Compensation Six T y p e 120-C standard silicon cells were purchased from the Semiconductor Division of Hoffman Electronics Corporation. These cells, which are 1 cm wide and 2 cm long, have a nominal active surface area of 1.8 sq cm and a rated efficiency of 6.0 percent. At 82.4 degrees F (28 degrees C), their short-circuit current is rated at 40 milliamperes (ma) for a tungsten lamp illumination of 100 milliwatts per sq cm. These cells were soldered to a copper disk t h a t served both as a t e m p e r a t u r e equalizer and as a comm o n negative terminal. The positive leads from the individual cells were formed b y copper wires soldered to the thin strips of solder on the upper surfaces of the cells, and, as Fig. 2 shows, these leads were connected to one set of terminals of a double-pole multi-point r o t a r y switch. The short-circuiting resistance, a 12.1 in. (30.7 cm) length of 30-gage copper wire, was immersed in an ice b a t h to eliminate t e m p e r a t u r e variations t h a t would otherwise m a s k the changes in shortcircuit current. The voltage drop across the resistance (0.118 ohm) was measured with a calibrated 0-10 m v recorder with available chart speeds of 2 in. per hour and 2 in. per minute (Varian T y p e G-10). Cell temperatures were measured b y 30-gage copperconstantan thermocouples t h a t were cemented to, but electrically insulated from, the upper surface of each cell. The thermocouples were shielded from the incident radiation b y small pieces of black tape, which also reduced the active areas of the cells in varying degrees. All of the thermocouples were connected through a common constantan lead to the cold junction, also maintained in the ice bath. The copper (positive) leads were connected through the second pole of the rotary switch to another 0-10 m y Varian potentiometer recorder. The copper disk with the attached silicon cells was placed in a double-walled aluminum vessel t h a t was set on an electric heater. A thin Pyrex plate (92 percent normal transmissivity) was used as a cover to minimize convection currents. During the first part of the ternVol. 6, No. 4, 1962
perature-compensation study, constant illumination was provided b y a 100-watt tungsten lamp, which received its energy through a constant-voltage transformer. During the second part of the program, the double-walled vessel and the electric heater were fastened to an altazimuth mounting so t h a t t h e y could follow the sun and tests were run between l1:00 a.m. and 1:00 p.m., when the direct solar b e a m was virtually constant in its intensity. T e m p e r a t u r e control was accomplished during both series of tests b y first filling the space between the two aluminum vessels with a mixture of ice, water, and salt. The copper disk was then taped to the b o t t o m of the smaller vessel, whereupon the temperatures of the disk and the attached silicon cells immediately fell to
TABLE 1--Variation of Short-Circuit Current of Un-
compensated Silicon Cells for Temperatures from 32 to 194° F, Expressed as Departure from Current at 77° F, for Tungsten Light and Solar Radiation Cell No.
Temp. OF
Illumination Source 2
32 50 68 77 86 104 122 140 158 176 194
Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight Tungsten Sunlight
4
-5.63 -2.62 --0.87 0
-6.00 -2.65 -3.28 -1.64 --1.4 -0.40 o
+1.74 +3.5 +5.65 + -+6.95 + -+8.26 + -+10.5
+11.0 +
--
-5.5 -2.68 -3.52 -1.24 --1.5 -0.62
3
6.8 2.85 3.65 1.43 1.37
~.4
o
+0.93 + +0.61 +0.62 +2.33 +3.O3 +1.42 +1.44 +2.8 +4.05 +1.63 +2.27 +3.25 +6.O5 +1.83 +2.68 +4.2 +7.1 +2.02 +3.1 +5.7 +9 +1.63 +3.1 +6.5 [ + 1 1 . 2 +1.22 I +2.27
:3:65 1.63 5.05 2.24 7.3 2.65 9.15 3.05 0.9 3.65 1.5 3.45
-6.9 -2.61 -3.47 -1.4 -0.87 -0.4 0 0 +1.15 +o.4 +2.9 +0.8 +4.05 +1.6 +5.2 +2.01 +5.5 +2.21 +5.5 +2.41 +4.87 +2.01
a b o u t 27 degrees F. Cell temperatures and shortcircuit currents were then determined at frequent intervals as the ice melted under the combined influence of the incident illumination and the electric heater. The results of typical tests with b o t h tungsten lamp illumination and natural sunlight are given in Table 1 in terms of the departure of the short-circuit current at cell temperatures from 32 to 194 degrees F from the current observed at a cell temperature of 77 degrees F. (Cell No. 1 was damaged during the soldering process and gave inconsistent results; Cell No. 2 was altered after the tungsten lamp tests b y the addition of a compensating resistance and so its results are not reported for the sunlight conditions). The variation of output in millivolts for cells No. 2, 3, and 6 is given in 157
,2o!
5
t..
I10 " i
~3
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a
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'
6
~UNLIGNT I i
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9o J
I
I
I
I
i
i f co o 20
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80
60
fOO 120 TEMPERATURE-
,
40
1 'F
60
.
i
i
80 IO0 120 TEMPERATURK
I l~O , °Y
/60
~o
200
FIG. 3--Variation of short-circuit current for typical silicon cells under constant tungsten illumination over the temperature range 40 to 195 degrees F.
FIG. 4--Percentage variation in short circuit current of typical silicon cell over the temperature range 40 to 195 degrees F for both tungsten illumination and solar radiation.
Fig. 3. The behavior of all cells follows the predicted pattern of a gradual rise in short-circuit current as the temperature increases up to about 170 degrees F, which is well above the range encountered in solar radiometry. At higher temperatures, the short-circuit current begins to diminish again. Each cell has its own particular temperature-current characteristic, however, with Cell No. 5 showing the widest variation (15.95 percent), and No. 3 the smallest variation (10.2 percent) as the temperature rose from 32 to 158 degrees F, under tungsten illumination. Under sunlight, No. 5 showed a total rise of 5.90 percent, while the short-circuit current of No. 3 rose by 4.67 percent. A comparison of the performance of Cell No. 6 under tungsten and solar illumination is presented in Fig. 4. It is evident that the temperature
coefficient is considerably higher for tungsten light than for sunlight. This is apparently caused by the increase in red response of the cell at elevated temperatures. ''~ For many practical applications in the measurement of solar radiation, acceptable accuracy can be obtained by using Manganin wire as the short-circuiting or metering resistance, since this alloy has virtually zero temperature-resistance coefficient in the range from 70 to 120 degrees F. For more accurate work, and partieularly in bright sunshine where high cell temperatures are likely to be encountered, and generally in the measurement of tungsten illumination, more effective temperature compensation is required. This can be accomplished by using a suitable low-resistance thermistor in parallel with a shunting resistance of Manganin wire. The combination can be caused to have virtually any desired negative temperatureresistance characteristic by proper selection of the thermistor. Cell No. 2 was selected for compensation since its current-temperature characteristic was typical of the other cells under consideration. Over the temperature range from 32 to 158 degrees F, its short-circuit current rose by 13.9 percent, under constant tungsten illumination. A Fenwal bead thermistor (type KD05L1, 5 ohms resistance at 77 degrees F) was chosen since its resistance-temperature characteristic, shown in Fig. 5, when connected in parallel with a 0.25-ohm Manganin wire, gave the desired reduction in total metering resistance. A test of the compensation circuit was made by cementing a thermistor and a 2 in. length of 30-gage Manganin wire to the copper disk immediately adjacent to Cell No. 2. Once again, the annular volume of the double-walled aluminum vessel was filled with crushed ice, salt, and water, and the output of the compensated cell was measured at constant ilhunination (tungsten) as its temperature rose from 36 to 195 degrees F. As Fig. 6 shows, the compensation was excellent over the
Rth
R ~h
ThermiSfor
R¢h
~huntin.g ( M onEan in)
------
C o r n b i n a t ion
co
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.90
60
90
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TE~lPERA
"C
TURE
Fro. 5--Resistance-temperature characteristics of 5-ohm (at 77 degrees F) thermistor alone and with 0.25-ohm Manganin shunt. 158
Solar E n e r g y
f.00
0.9O
.... 080
20
-s b
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¢0
80
foo,,j.
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~o ~0 Jo ,0 ~0
140 leo so
70
leo
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.
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.
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.
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i
80 ao
FIG. 6--Temperature variation of short-circuit current from Cell No. 2 when compensated by shunted thermistor (tungsten illumination).
range from 40 to 140 degrees F; above 150 degrees F the metering resistance falls too rapidly, and the compensation is no longer satisfactory. Fortunately, silicon cells in bright sunlight in the earth's atmosphere usually remain below 140 degrees F. Hence adequate compensation can be provided by the combination of a low-resistance thermistor and a Manganin shunting resistance. The selection of the thermistor and the shunting resistance can be accomplished with the aid of Fig. 7 when the temperature-current characteristics of a particular cell are known, and the output in millivolts is also determined. The ratio of the short-circuit currents at the lower and higher temperature limits is found, and its reciprocal is evidently the required resistance ratio. The magnitude of the Manganin shunting resistance is determined by dividing the desired output in millivolts by the short-circuit current in milliamperes. The thermistor that gives the desired resistance ratio can then be selected from Fig. 7. As an example, the successful compensation of Cell No. 2 for tungsten light, as shown in Fig. 6, was accomplished with a 5-ohm (at 77 degrees F) thermistor and a shunting Manganin resistance of 0.25 ohms. This was too drastic a compensation for use with sunlight, for which the short-circuit current ratio was 0.96, and in this case a thermistor with a resistance at 77 degrees F of 15 ohms should be used with a Manganin shunting resistance of 0.2 ohms.
o
0.o'3
of ~HUNTING
0.2 03 RE,.?I,.~TANCE , OHP'I,.~
FIG. 7--Variation of combined resistance ratio with resistances of Manganin shunt and thermistor.
The cell was soldered to a small brass disk which was mounted within but insulated from an aluminum cap. A thermistor (Fenwal type KD05L1) was inserted into a small hole drilled in the lower portion of the disk,
Th eemisfor_~ ~ , / 3 i l / c o n
Cell
Cosine Response and Air Mass Effect For use with sunshine in determining the cosine response and the dependence of cell output upon air mass, a new radiometer was constructed as shown in Fig. 8. A type l l 0 C Hoffman silicon cell (l cm square, 0.9 sq cm active area) was used as the sensitive element, since the instrument was designed to serve both as a pyranometer for measuring the total radiation from sun and sky, and as a normal incidence pyrheliometer. V o l . 6, N o . 3, 1962
7-/T e r m o c o c , p / e s Fie-. 8--Cross-seetion of temperature-compensated silicon cell pyrheliometer. ]59
k
:
~,,o!
tO °
20 °
30"
40"
0 , ANGLE
50 °
O0 °
70 °
,~0 °
90 °
OF I N C / D E N C E
blackened in order to keep it as closely as possible at the temperature of the silicon cell. The tube of the radiometer shown in Fig. 8 had a length of 5.5 in., while the diameter of the window at the upper end was 0.55 in., giving the ten-to-one ratio of length to diameter recommended for pyrheliometers. 1 The instrument was designed so that the tube was removable; thus, the instrument could also be used as a pyranometer in the next series of tests to determine the cosine response of typical silicon cells. Determination of Cosine R e s p o n s e - - T h e intensity of solar radiation reaching a surface on the earth is the sum of the direct beam component, [D~ cosine 6, and the diffuse radiation from the sky, Id. Expressed as an equation, this is:
FIG. 9 - - V a r i a t i o n of cosine response for silicon cell at, incident angles from 0 to 90 degrees.
with the upper lead of the thermistor also soldered to the disk. The lower lead was insulated and led out through a Lucite spacer ring. The Manganin shunting resistor was wound around the outer diameter of the brass disk, with one end soldered to the disk and the other end soldered to the positive lead from the upper surface of the silicon cell. For test purposes, one copper-constantan thermocouple was cemented to the thermistor, while a second thermocouple was cemented to the lower surface of the silicon cell. When the instrument was exposed to bright sunlight, the two temperatures were virtually identical. The exposed portion of the brass disk was
Ito = Ia + IoN COS
0
(1)
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Ia)/IDN
= COS 0
(2)
where 0 is the angle of incidence between the solar beam and the cell surface. When a pyranometer is used to measure total horizontal insolation, the angle of incidence between the sun's rays and the cell surface is the complement of the solar altitude, ~. In order to be able to cover the entire range of incident angles from 0 to 90 degrees, the lower portion of the radiometer shown in Fig. 8 was mounted FIG. 10--Spectral d i s t r i b u t i o n of solar r a d i a t i o n i n t e n s i t y in near space and on e a r t h for v a r y i n g air mass; spectral response of typical silicon cell.
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on an Astro Compass, or star finder, with the cell surface normal to the line of sight of the instrument. B y following the sun's motion in azimuth, and setting the altitude of the cell b y means of the vernier adjustment on the star finder, the angle of incidence between the solar beam and the cell could be varied accurately from 0 to 90 degrees by increments as small as 1 degree. If the exposed surface of the cell could absorb all of the solar radiation falling upon it, the ratio that constitutes the first part of Eq. (2) would always be exactly equal to the cosine of the incident angle. Since the reflectivity of the cell surface varies with the incident angle, a series of tests was run at mid-day when the solar altitude was approximately 68 degrees. The direct normal insolation, I . ~ , was measured with an Eppley normal incidence pyrheliometer on a suntracking mount, while the output of the silicon-cell pyranometer was read as the incident angle was varied in 10 degree increments from 0 to 90 degrees; in each setting, the output was measured with the cell both shaded and unshaded, to give both the diffuse or sky radiation, Id, and the total insolation at the prevailing incident angle, Lo. For each value of the incident angle, the agreement with the cosine law was determined by rearranging Eq. (2) to give: (Ito -- Id)
Percent true response - IDN × COS0 X 100
(3)
The results of this test are shown in Fig. 9, which gives both the cosine response and the Sol-A-Meter constant for incident angles from 0 to 83 degrees. The cell evidently follows the cosine law almost exactly for incident angles from 0 to 20 degrees; at higher angles the reflectivity of the cell's surface increases rapidly and the cell's response deviates increasingly from the true value. The performance of the Sol-A-Meter can be expressed in terms of an instrument constant or coefficient, giving the incident insolation in Btu per hr per sq ft per millivolt of output. For the instrument of Fig. 8, the coefficient at 0 = 0 degree was 82; as the incident angle increased, the coefficient also increased to a value of 90 at 0 -- 62 degrees and 100 at 0 = 74 degrees. The departure of the Sol-A-Meter response from the cosine law is considerably greater than that experienced with thermopile-type pyranometers, which generally adhere quite closely to the predicted performance until the incident angle exceeds 60 degrees. TM Effect of A i r M a s s V a r i a t i o n - - T h e spectral response of a typical silicon photovoltaic cell shown by the dashed line in Fig. 10, is quite different from the energy distribution of sunlight in near space (Air mass, ] I = 0) as well as on the earth's surface. The peak response of the cell comes at about 0.85 microns, while Vol. 6, No. 4, 1962
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the wavelength of maximum solar-radiation intensity varies from 0.48 microns in space to 0.67 microns on earth when the sun is low in the sky (M = 5). This shift in intensity distribution toward the longer wavelengths should cause the relative response of the SolA-Meter to rise with increasing air mass as more of the sun's radiation comes within the spectral range of the silicon cell. The response of the radiometer of Fig. 8, operated as a pyrheliometer with the tube in place, was compared during an entire day with the output of an Eppley normal incidence instrument (No. 4003). At 11:40 a.m., when the solar altitude was 68 degrees and the air mass was 1.075, the direct normal insolation, as determined by the Eppley pyrheliometer, was 297 Btu per hr per sq ft, and the Sol-A-Meter output was 3.62 inv. The instrument coefficient at minimum air mass was thus 82.0 Btu per hr per sq ft per my. The coefficient was determined in the same manner for air masses up to 5.6, corresponding to a solar altitude of about l0 degrees. The ratio of the coefficient at each air mass to the coefficient at minimum air mass was then computed and plotted on Fig. 11. According to this test, the increasing air mass has virtually no effect for solar altitudes above 30 degrees; for lower altitudes the departure from the true response increases to l0 percent at air mass 5.6 (10 degrees solar altitude). Using Moon's classic radiation intensity data as given in Fig. 10 (adapted from Bickler,~7), an attempt was made to calculate the relative output of a typical silicon cell for air masses from 1 to 5. The radiation intensity at each wavelength was multiplied by the cell's response at the same wavelength and the results plotted as the dash-dot lines in Fig. 10. The ratios of the areas under the dash-dot curves to the areas under the solar-radiation intensity curves were then determined. When only the wavelengths that affect the silicon cell are considered, this procedure results in a predicted increase of cell output of about 10 percent for M = 3 and 11 percent for M = 5. When the entire solar spectrum out to 1.8 microns was included in the 161
calculation, however, it appeared that the maximum output should be obtained at M = 3, which is contrary to the experimental results.
Combined Effects of Air Mass and Cosine Response-When the Sol-A-Meter is used as a pyranometer to measure the total input of radiation from sun and sky upon a horizontal surface, its coefficient, following the initial calibration against a standard thermopile instrument, can be stated in terms of the solar altitude, which determines the air mass effect, and the complement of the solar altitude, which determines the cosine effect. For high solar altitudes, as shown in Fig. 12, no correction is needed. For altitudes below 60 degrees, the air mass effect and the cosine effect oppose each other, but the latter is much more potent and so the combined response decreases quite rapidly with decreasing solar altitude. As long as the cell remains in the horizontal plane, a single combined coefficient that is a function of solar altitude alone can be used.
Day-Long Performance of Silicon Cell P y r a n o m eter The output of the pyranometer section of the instrument shown in Fig. 8 was compared throughout the entire day of April 21, 1962, with the output of an Eppley 10-junction horizontal pyrheliometer (No. 3737). Both instruments were leveled carefully and 120 A I ~ /~AS,S.N. I00
diffuse insolation as determined by a shaded Eppley horizontal pyrheliometer (No. 3737) was 35.3 Btu. The total horizontal insolation was thus 313 Btu per hr per sq ft. The noon output of the horizontal Eppley instrument was 3.25 mv, giving a coefficient of 96.4 Btu per my. The noon output of the compensated silicon cell was 3.90 mv, giving a coefficient of 80.4 Btu per my, which is slightly below the previously determined values. The minute-by-minute fluctuation of the recorder pen, even on a clear day, due to actual fluctuation of the solar radiation, is rarely less than 0.1 my, and so it is not likely that one can obtain agreement closer than 2.0 percent without taking greater precautions than are warranted by the recorder accuracy and the time lag of the thermopile instrument. When the areas of the two charts on Fig. 11 were measured by a planimeter, it was found that the area under the curve on the Eppley chart was 24.77 sq in., while the area under the Sol-A-Meter chart was 30.10 sq in. Using the noon values of the instrument outputs and the resulting coefficients, the total daily insolation as determined by the Eppley horizontal pyrheliometer was 2380 Btu per sq ft; the value as determined by the Sol-A-Meter was 2402 Btu per sq ft. This agreement leads to the conclusion that the effects of air mass and cosine deviation which have been detailed above are not of compelling importance in a day-long insolation determination. When these effects are being experienced, the total insolation is relatively small and so the errors are also comparatively small.
t"IAS8 ,.t- : COSINE
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their outputs were recorded on calibrated 0-10 mv potentiometer recorders (Varian G-10). The records, shown on Fig. 13, are typical of a cloudless Arizona day. The intensity of the direct normal beam, as determined by Eppley pyrheliometer 4003, was 300 Btu per hr per sq ft at noon, when the solar altitude was 69 degrees. The direct component on a horizontal surface was thus 300 cosine (90-69) = 277.5 Btu; the 162
The short-circuit current from commercially available silicon photovoltaic cells can be used with confidence to measure the solar radiation falling on a terrestrial surface, provided that appropriate corrections are made for cosine and air mass effects and for temperature variation. The latter factor, which is unpredictable since it depends upon ambient temperature and wind velocity as well as upon intensity of solar radiation, can fortunately be eliminated by measuring the short-circuit current in terms of the millivolt drop across a metering resistance composed of a suitable thermistor in parallel with a short length of Manganin wire. The falling temperature-resistance characteristic of the thermistorManganin wire combination can be adjusted to compensate ahnost exactly for the rising current-temperature characteristic of the silicon cell. The cosine and air mass corrections can both be made in terms of the sun's altitude in the case of a horizontal silicon cell pyranometer. When the cell is used in an attitude other than horizontal, corrections must be made for both incident angle and air mass. When proper precautions are taken, and standard thermopile-type pyrheliometer~, are available for their Solar Energy
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calibration, radiometers using temperature-compensated commercial silicon cells can be a valuable addition to the wide variety of instruments now available for measuring the intensity of solar radiation.
Acknowledgment The writers wish to express their appreciation to the Scientific Council of NATO and to the Faculty of Engineering of The Middle East Technical University, who enabled this work to be carried out by granting a Fellowship and a leave of absence to Mr. Sel~uk. Appendix I--Recommended Terminology for Solar Radiometers In the comprehensive treatise on solar radiation measurement 1, which was published in conjunction with the International Geophysical Year, it is recommended that the term "l)yrheliometer" be applied to radiometers that measure the intensity of the direct solar beam, the prefix "normal incidence" being implied rather than expressed. The term "pyranometer" is recommended for use with instruments which measure the total radiation from both sun and sky, while the word "photometer" should be applied to instruments used for measuring total radiation. This terminology has been used in the foregoing paper except where long usage has brought universal acceptance to some other wording, as in the case of the Eppley horizontal 180 degrees "pyrheliometer".
REFERENCES 1. l)rummond, A. J., et al: "Radiation Instruments and Measurements", Part VI, IGY Instruction Manual, Pergamon Press, New York, 1958. 2. Drummond, A. J., and Greer, H. W.: "Fundamental Pyrheliometry", Sun at Work, 3, No. 2, p. 3, June, 1958.
Vol. 6, No. ~, 1962
3. Spitzer, C. F., et al: "Solar Cell Measurement Standardization"; Lockheed Aircraft Corporation, Missiles and Space Division, Report No. LMSD-288184, 29 Feb., 1960. 4. Chapin, D., Fuller, C., and Pearson, G.: "A New Silicon p - n Junction Photocell for Converting Solar Radiation into Electrical Power"; Jnl. App. Phys., 25, p. 676, 1954. 5. Pearson, G. : "Electricity from the Sun", Proc. 1955 World Symposium on App. Solar Energy, Stanford Res. Inst., 1956. 6. Rappaport, P.: "The Photovoltaie Effect and its Utilization", Solar Energy, 3, No. 4, p. 8, Dec., 1959. (Also RCA Review, Sept., 1959). 7. Escoffery, C. A., and Luft, W. : "Optical Characteristics of Silicon Solar Cells and of Coatings for Temperature Control", Solar Energy, 4, No. 4, p. 1, Oct., 1960. 8. Ravich, L. E.: "Thin Film Photovoltaie Devices for Solar Energy Conversion", 1961. U.N. Conf. on New Sources of Energy, paper S/56. 9. Schoffer, P., Kuhn, P., and Sapsford, C. M., ibid., paper S/92. 10. Yellott, J. I., Kokoropoulos, P., and Fogle, T., "Use of Silicon Cells as Pyrheliometers", Sounion Conference, 1961. 1l. Gummel, H. K., Smits, F. M., and Froiland, A. R.: "A Method for Terrestrial Determination of Solar Cell Short Circuit Current under Outer Space Solar Illumination", 1961 West. Elec. Show and Con. (WESCON), paper 7/3. 12. Prince, M. B., and Wolf, M.: "New Developments in Silicon Photovoltaic Devices", J. British Inst. of Radio Eng., 18, 1958. 13. Kreith, F. : "Radiation Heat Transfer", Int. Textbook Co., Scranton, 1962, pp. 147-163. 14. Hoffman Electronics Corp., Semiconductor Division, 1962 Catalog, p. 22. 15. Ross, B., Hoffman Electronics Corp., personal communication of 5/18/62. 16. "Handbook of Physics and Chemistry", 23rd edition, p. 1540, 1939, Chemical Rubber Publishing Co., Cleveland. 17. Biekler, D.: "The Simulation of Solar Radiation", Solar Energy, 6, No. 2, April-June, 1962, p. 66. 18. MacDonald, T. H.: "Some Characteristics of the Eppley Pyrheliometer", Monthly Weather Review, Vol. 79, No. 8, August, 1951, p. 157.
163