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A system for the evaluation of solar cell samples
5,,I,r Enerl,,v, 1972.Vol. 14. pp. 43-J4 PergamonPress. Printedin Great Britain
A SYSTEM FOR THE E V A L U A T I O N OF SOLAR CELL SAMPLES H. L. SKOLNIK* (Received 10 May 1971)
A ~ t r a e t - T h e system described is a solar cell test vehicle intended primarily for education and research applications. Because of the many advantages to be derived from testing cells under real sky conditions, the sun is utilized as the system's energy source. Provisions are made for the collimation of the incident energy to simulate space conditions when desired, and also to provide a known angle of incidence when testing cells for either terrestrial or space applications, The mobile equipment console, complete with necessary instrumentation and power supplies, facilitates positioning the apparatus in an open area remote from the laboratory. The system design provides for the measurement of open circuit voltage, short circuit current, internal impedance, and output power, each as a function of incident energy. A means for the accurate angular positioning of the solar cell provides for the determination of cosine response. A specially-designed, direct read ng pyrheliometer featuring fast response provides normal incident radiation data, allowing for the accurate determination of conversion efficiency. Spectral response measurements are made by comparing the output of the cell under test. both with and without accessory band-pass filters. The temperatures of both the solar cell and the ambient air can be monitored for future data correlation. A clock drive provides for the continuous alignment of the sample holder with the sun, allowing for measurements over extended periods of time. Samples as large as 4.3 in. in diameter can be accommodated. R ~ s u m r - L e s auteurs d~crivent un vrhicule servant au cours d'exp~riences sur des cellules solaires et destin6 h 1'rducation et la recherche. Comme il y a de nombreux avantages h tester les cellules dans les conditions naturelles au dehors, le soleil est utilis6 en rant que source d'6nergie du v6hicule. Les auteurs tiennent compte de la collimation de la lumi&re incidente pour simuler les conditions spatiales ainsi que pour difinir un angle connu d'incidence lorsqu'ils testent les cellules pour des applications terrestres ou spatiales. Le support de I'appareillage mobile, comprenant les instruments necessaires et la source d'rnergie, facilite la raise en place de rappareillage dans un endroit ouvert, loin du laboratoire. Ce syst~me a 6tg con¢,u pour mesurer le voltage d'un circuit ouvert, I'imprdance interne et la puissance debitre, chacun 6tant fonction de la lumi/~re incidente. La raise en place angulaire exacte de la cellule solaire permet de drterminer la rgponse cosinusoidale. Un pyrhgliom~tre conqu sprcialement et h lecture directe fournit rapidement les donnrs de la radiation hun angle incident normal, ce qui permet d-obtenir la determination exacte de refticacit6 de la conversion, On mesure les rrponses spectrales, en comparant le rendement de la cellule, en tours d'exprrience avec et sans filtres. On enregistre les temprratures de la cellule solaire et de Fair ambiant pour des corrrlations ulterieures. Un syst~me mrcanique maintient un alignement continu de la cellule avec le soleil. I1 est possible d'inslaller ainsi des cellules ayant jusqu'~ 10,8 cm de diamgtre, Resumen- El sistema descrito es un vehiculo de comprobacion de pilas solares ideado principalmente para aplicaciones de ensehanza e investigacion. Debido alas muchas ventajas que provienen de la verification de pilas en rOgimen de cielo verdadero, se utiliza el sol como fuente energrtica del sistema. Se proveen medios pat-lt la colimacirn de la energia incidente, en orden a la simulacirn eventual de condiciones espaciales, asi c()mo para proporcionar un conocido ;ingulo de incidencia durante la comprobacit~n de pilas destinadas a aplicaciones lerrestres o espaciales, l,a consola del equipo m6vil, completa con la instrumentacirn necesaria y sEtministros de potencia, permite el emplazamiento del aparato en una zona abierta alejada del laboratorio. El sislcma prev6 la determination del voltaje en circuito abierto, corricare de cortocircuito, impedancia inlerna y pt}tencia de salida, cada uno en fi]ncirn de la energia incidente. Un medio para el posicionamiento an~ttlar exacto de la pila solar permite determinar la respuesta de coseno. Un pirhelirmetro de lectura directa, de disefio expecial con respuesta r~pida, proporciona datos sobre la radiacirn incidenle normal, Io cual permite la delerminacirn exacta de la eficiencia de la conversi6n. Se roman medidas de la respuesta espectral comparando la salida de la pila objeto de comprobacirn, con y sin filtros accesorios de paso de banda. Las temperalttras tanto de la pila solar como del aire ambiente pueden ser controladas en orden a la ft.tura correlaci(]n de los datos. Un mecanismo de reloj permite la alineacion continua del portamuestras con el sol. Io cual asegura la toma de medidas durante largos perlodos de tiempo. Se admiten muestras con di;imetro de 4, 3 pttlgadas como m~iximo. " Harris Semiconductor, Melbourne, Florida, formerly with University of Florida. 43
44
H. L. S K O L N I K INTRODUCTION
THE TEST system described in this paper is designed to provide for the measurement of the terminal characteristics of solar cells and other photo-sensitive elements when expo;ed to sunlight illumination. The problems associated with the use of the sun as the energy source are well known. However, the difficulties experienced in the interpretation of cell characteristics when measured under artificial light conditions (because of the complex nature of the Sun's spectrum) and also the high cost of a "good" sunlight simulator convinced the designers that the inconveniences associated with a sunsource system were to be tolerated. The system as shown in Fig. 1 includes: A direct reading pyrheliometer for the measurement of normal incident solar irradiance (necessary for the determination of cell conversion efficiency) A pedestal supported equatorial mount with clock drive for sun tracking collimating tubes An adjustable sample platform An adjustable source of 60 H z (120 VAC) for powering the clock drive and auxilliary measuring equipment. The motivation for the development of a solar cell test system at the University of Florida was broadly based. Fundamental to providing a sound background in the field of Direct Energy Conversion is a realistic laboratory program. The test system is intended to give the student a first-hand "feel" for real solar cell characteristics. For the advanced student or researcher who may be engaged in device fabrication, the instrument can provide essential quantitative information. The design of an instrument to be used in an educational environment must necessarily be versatile, straightforward in its operation, low in cost, and rugged. This system meets these requirements. Figure 2 indicates the major functional blocks of the solar cell test system. The equipment can Standard r" collimator ~._ . ~
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Fig. 1. Solar cell test system.
[Facing page 44]
Fig. 3. Collimator tubes.
Fig. 5. Sample homer.
A system for the evaluation o f mlar cell samples
45
be subdivided into four groups: Standard pyrheliometer Sample measuring instrumentation Sun tracking mechan sm Mobile power supply. COLLIMATION The design of the sample instrumentation is strongly influenced by the cosine response characteristic of solar cells. The cosine response makes it important to define the direction of incident energy. In space this is a trivial matter, but within earth's atmosphere sunlight appears from all directions. A simple method for collimating the system so that it will only accept normal incident energy is to prDvide a long shield to reduce non-normal components. In this equipment separate collimators are provided for the standard pyrheliometer and sample holder (See Fig. 3). The sample collimator is adjustable and can thus provide the degree of collimation required (consistent with sample size). The sample collimating tube is made of aluminum, has a 5t in. inside diameter, and is 6 ft long. Inside the tube, baffles having apertures of 5 in. help reduce non-normal radiation at the sample. The inner surface of both the sample and standard collimating tubes, as well as the sample holder, are coated with Parsons Black Optical Paint which has excellent absorption characteristics throughout the sun's spectrum. The standard collimating tube is 18 in. long and is described in greater detail below. For clarity, a distinction is made between "acceptance angle" and "view angle" (See Fig. 4). Acceptance Angle ( 0 ) - t h e angle (measured with respect to the sample-sun axis) through which light can enter the system and strike the target's surface. View Angle (t~)-the maximum angle through which the target can view incoming radiation. The collimator geometry indicates that a very high degree of collimation (small acceptance angle) dictates either a prohibitive shield length or a very small shield radius. For a given target size, there exists a minimum acceptable acceptance angle below which vignetting will result. It is important to note that the target size cannot equal the entrance aperture. Referring to Fig. 4 it is clear that R--a R tan fl = M - - L - M = tanS.
Therefore, tan 8 -~ - a - ~ (where 8max = 16 rain. 18 sec.) Thus, for the sample collimator a - d >~ 0.35 in. and for the pyrheliometer a - d ~> 0 . 0 8 5 in. If a - d is smaller than indicated above, errors due to vignetting will result. In order to arrive at a workable instrument, compromises in both the degree of collimation and the maximum sample size were made. Because the thermopile (in the pyrheliometer) is of fixed dimensions, one cannot
H. L. SKOLNIK
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Fig. 4. Collimator geometry, d = sample radius, a = entrance aperture radius, L = collimator length, m = mean Earth-Sun distance, R = sun's radius, ~ : view angle, ~i= apparent sun's radius, 0 = acceptance angle./~ : vignette angle. r e d u c e t h e e n t r a n c e a p e r t u r e w i t h o u t s h a d i n g a s e g m e n t o f the a c t i v e a r e a with a res u l t a n t l o s s o f c a l i b r a t i o n . I n c o n t r a s t , t h e s a m p l e c o l l i m a t o r is a d j u s t e d b y r e d u c i n g the e n t r a n c e a p e r t u r e . T h e a p e r t u r e is r e d u c e d b e c a u s e it is not p r a c t i c a l to e x t e n d this l a r g e t u b e b e y o n d 6 ft. T h e o p e r a t o r m u s t r e m e m b e r that the a l l o w a b l e s a m p l e size is limited by the aperture selected or vica versa. T h e a c c e p t a n c e angle o f t h e s a m p l e t u b e is d e t e r m i n e d b y the r a d i u s o f tile e n t r a n c e aperture, 0 -- a r c tan
a-0.175 36
o r o f g r e a t e r significance, d+0.175 , w h e r e a a n d d a r e in i n c h e s . 0 = a r c tan - 36 T h e a c c e p t a n c e a n g l e f o u n d a b o v e r e p r e s e n t s t h e s m a l l e s t a l l o w a b l e a n g l e (for a g i v e n s a m p l e size) t h a t will y i e l d a n o n v i g n e f t e d field. T h e b a s i c a c c e p t a n c e angle for the s a m p l e c o l l i m a t o r w i t h o u t a p e r t u r e r e d u c t i o n is 3045 ' . T o f a c i l i t a t e a c c e p t a n c e angle a d j u s t m e n t s a n u m b e r o f a p e r t u r e s h a v e b e e n c o n s t r u c t e d in s i z e s f r o m I ° to 3.5 °.
A system for the evaluation of solar cell samples
47
SAMPLE H O L D E R
The sample holder is arranged so that a cell (or group of cells) up to 4.3 in. in diameter can be accommodated (See Fig. 5). The mounting platform is fitted with adjustable edge hold down clamps and is scribed with concentric circles to facilitate axial mounting of the sample. Four-terminal electrical connections are made through knurled brass terminals which are also supported on the mounting platform. To facilitate cosine response measurements the platform can be rotated ± 90° from the solar axis. Measurements are aided by a calibrated angular scale and a platformqocking device to maintain angular settings for extended time periods. A hole in the center of the mounting platform is provided so that an H. P. 2801 A electronic thermometer probe may be placed in contact with the cell under test. A coarse-threaded flange permits the rapid connection of the sample holder to the collimating tube. SUN T R A C K I N G
The tracking system is comprised of a standard equatorial telescope mount equipped with a synchronous drive motor runn ng at the sidereal rate. In order to track the sun accurately, allowance must be made for the sun's annual motion on the celestial sphere (about 1° per day). Since the sun moves on the ecliptic and not the celestial equator, the tracking error will be in two directions, east-west and north-south. The sun's motion is principally eastward, and hence the sun's 1° per day movement across the sky will cause a tracking error from the sideral rate of about two arc minutes per hour. In addition, the 23½° tilt of the ecliptic from the celestial equator produces a small change in the sun's declination, which has its maximum value near the equinoxes (March and September) and is about one arc minute per hour. The maximum change in right ascension is about 1 min 45 sec. and occurs near December 21 st. This means that ideally the clock drive (sidereal) should be adjusted to lose about ten seconds per hour (maximum, depending upon time of year), and that the declination axis should be driven at about one arc minute per hour in the worst case. Hence, when extremely accurate tracking of the Sun is required, complex feedback control systems are usually utilized. In this application, however, observation periods are generally short and tracking errors due to the above variations are small. Perhaps the most significant tracking error occurs because of misalignment of the equatorial mount. Because the solar cell test system is portable and is not intended to be left in a permanent location, the problem of aligning the system before each use is apparent. The precision with which the system must track the sun is dependent upon the ratio of the entrance aperture to the sample size. Figure 6 illustrates how a tracking error (A) reduces the nonvignetted sample area. A "bull's-eye" bubble level along with adjustment screws on the pedestal mount facilitate leveling the system. The altitude of the polar axis on the equatorial mount is set simply from the site latitude. Thus the only problem confronting the operator is the alignment of the polar axis in azimuth, which must be adjusted, as nearly as possible, parallel with the spin axis of the Earth. Two simple methods have been used for adjusting the polar axis of the mount so that its azimuth is approximately 0 ° (N). A magnetic compass can be used to find "North." Despite the fact that magnetic North is not in the true direction in which one wishes to point and that there are also errors inherent in sighting along the mount, acceptable results have been noted for observation periods
48
H. L. SKOLNIK Sun
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Fig. 6. Effectof trackingerror on allowablesamplesize. up to one hour (magnetic headings should be corrected for local variations, where errors are significant). A second method which is often used to refine the alignment made with the magnetic compass is as follows: Mount an arbitrary cell (small in size) in the sample holder and connect a high impedance meter to read open circuit voltage. Adjust the collimator acceptance angle so that it is as small as possible, consistent with cell size (See Fig. 3). Level the mount and set the polar axis altitude to the site latitude. Point the polar axis approximately North (use compass or guess !). Aim the system at the s u n - m a x i m u m reading on the voltmeter (lock declination axis). Engage the drive motor and note rate of change in voltmeter reading. Rotate the mount slightly, adjust level if necessary. Repeat the last three steps until the voltmeter variations are minimized. In practice this process converges rapidly, and sufficient precision to allow tracking for several hours can be achieved in less than one half hour. (Allowance must be made for clouds and other sources of intensity variations.) Adjustable weights on the declination axis and the collimating tube provide a means fo~ balancing the driven system about the polar axis. Proper balance is essential if the drive motor is to perform properly.
A systemfor the evaluationof solar cell samples
49
SOLAR IRRADIANCE METER One of the most important characteristics of a solar cell is its conversion efficiency. In order to determine cell conversion efficiency the total incident solar irradianee must be known. Because of the spectral makeup and variability of the sun's intensity, as well as other atmospheric phenomena such as scattering, a radiation meter having the following characteristics is required: Fast response Equal sensitivity throughout the Sun's spectrum N arrow acceptance a n g l e - to reduce sky error Direct read-out in appropriate units. An instrument exhibiting these properties was not available within the budget limitations of this project. As a result a device was developed to meet the above needs. The irradiance meter is based upon an Eppley Model 8, Black and White Pyranometer. This unit was chosen because it possessed many of the characteristics mentioned above, as well as being relatively rugged. To restrict the instrument to normal incident radiation, the pyranometer was placed at the bottom of a 1.4 in. (inside diameter) collimating tube. Baffles within the tube further restrict the entrance aperture to 1.37 in. The resulting thermopile pyrheliometer has an acceptance angle defined by: . ~d ,+ a hence, 0 = 406 ,. 0 = arc tan Figure 7 shows the schematic diagram of the solar irradiance meter. The linear output of the thermopile is amplified by an operational amplifier in the potentiometric configuration. The high input impedance (100 megohms) of the Harris H A 2600 operational amplifier insures that the pyranometer is not adversely loaded. The amplifier output Eppley pyranometer
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H. L. S K O L N I K
is adjusted to drive the 1 mA meter to full scale when an input energy of 100 MW/cm 2 is incident on the thermopile. The meter itself is of taut band construction and has a mirrorbacked scale readable to within 1 per cent. Experience has shown that this pyrheliometer is extremely easy to use and has proven reliable throughout periods of thermal variation and mechanical stress. POWER SYSTEM
In order for the solar cell test system to be positioned in a direct line with the sun at different times of the day and year, it is imperative that the equipment be portable and operational at remote locations. Power requirements for the system depend upon the type of measurements to be made, but in all cases 120 V - 6 0 H z is necessary to operate the synchronous tracking motor. To provide flexibility, provisions are made to allow for the use of ordinary laboratory instruments. Generally included in the system are the following instruments: High impedance voltmeter (Fluke 8100 A) "Clip-on" current meter (H. P. 428 B) Dual probe electronic thermometer (H. P. 2801 A) X-Yrecorder (Houston H R 96 T). Most of these instruments require a.c. power. For accurate tracking a well-regulated 60 Hz source is necessary. Figure 8 shows the overall schematic diagram of the power system. The basic energy source is a 12 V, 95 A-h, lead-acid storage battery. Direct to alternating current conversion is accomplished with a Heath Model MP-14 solid state 2ACB
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Fig. 8. Mobile power supply.
A systemfor the evaluationof solar cell samples
51
inverter. The inverter provides an output that can be set in both amplitude and frequency. Frequency stability as a function of time and load variation has been found to be within ½ Hz for extended test intervals. Meters on the front panel of the instrument cart indicate the amplitude and frequency of the ac supply as well as the battery voltage and current (See Fig. 9). When the equipment is in storage or otherwise near a source of commercial power, a built-in charger readies the storage battery for future remote use. The Sear Model 608, 71970 battery charger used is completely automatic and does not require operator attention. Under average conditions the battery will power the system for approximately four hours without degradation in equipment performance. Recharging requires between ten and twelve hours. Periodically, it is necessary to check the battery fluid level. To facilitate this, the battery is mounted on slide-out rails (See Fig. 10). An exhaust system is activated whenever the equipment is in the battery charging mode to prevent the built-up of gas within the enclosure. Under these conditions the power to run the fan comes from the ac line. When the system is operating under remote conditions, the blower is not normally activated. However, in order to protect the power inverter from overload, thermal switches strategically located within the enclosure operate the fan when a temperature of 100°F is reached. In the event that the inside temperature of the cart reaches 125°F, all loads are disconnected from the inverter except for the power necessary to run the blower. Power output will not be restored until the temperature falls below ! 15°F. The inverter is further protected by limiting its output current with circuit breakers and fuses. Figure 10 shows the interior layout of the instrument cart. The top compartment is intended for the storage of accessiories and stowage of equipment during transportion to and from the test site. MEASUREMENT PROCEDURES The solar cell test system is designed primarily to measure the following cell characteristics: Open circuit voltage Short-circuit current Maximum output power Internal impedance Conversion efficiency Cosine response. These parameters can be measured at various levels of irradiance through the use of neutral density filters. However, for accurate determinations consideration should be given to the true neutrality of the filters used. While it is normally more desirable to measure solar cell spectral response under artificial light conditions, this test system does find some application in response measurements. The student can determine the general response of a cell by using band-pass filters (on both the sample and pyrheliometer) and by comparing the normalized cell output to a plot of solar intensity. The suitability of materials for use as "windows" over solar cells can be evaluated directly by comparing the cell's characteristics both with and without the material in the incident solar beam. As described above, the sample holder is arranged so that the plane of the cell under
52
H. L. S K O L N I K
test can be varied from 90 ° to 0 ° with respect to the sample-sun axis. Thus the cell's angular response (voltage, current, or power) can be determined by plotting the displacement angle versus cell output. Allowances can be made for variations in solar irradiance during the measurement period by simultaneously plotting irradiance versus time (with the pryheliometer and X - Y recorder). In the interest of simplicity this system does not provide for the control of sample temperature. However, the sample (as well as ambient) temperature can be constantly monitored along with the other parameters for future correlation. Figure 11 illustrates the utility of a device's current-voltage characteristic curve. Points A, B, and C yield short circuit current, maximum power output, and open circuit voltage respectively. The internal impedance at any point of interest can be calculated from the inverse slope of the curve at the point in question. The conversion efficiency of the cell under test is obtained by comparing the power output to the incident solar irradiance and cell area. The literature suggests several methods for obtaining cell I - V curves. The procedure described here is the one most commonly used and consists of varying a resistive load from open circuit to short circuit while measuring the cells output voltage and current
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Figure 12 illustrates the physical hookup of the apparatus. It is essential that four terminal connections be made to the cell under test, or wiring resistances will mask the true characteristics. The variable resistive load is arranged as shown in the figure to provide smooth control of output variations throughout the operating range. As the short circuit condition is approached, a very high degree of resolution is required to obtain an accurate curve. The impedance of the voltmeter (and/or recorder) should be sufficiently high so that it does not load the sample under test (generally Z ~> 100 K is adequate). The ammeter should be of very low impedance so that short circuit conditions can be closely approached. An ammeter voltage drop of less than 10 mV is desir-
Fig. 9. Mobile power supply control panel.
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Fig. 10. Interior view of equipment cart.
A system for the evaluation of solar cell samples
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able. The clip-on type ammeter used here offers low insertion impedance as well as a voltage output (proportional to meter deflection)suitable for driving the recorder. The high resolution requirement on load variation was mentioned above. This is due in part to the cell's characteristics and also to the dynamics of the recorder. One can approximate the required variation of resistance with time by a decreasing exponential function. Figure 13 suggests an automatic circuit that forces the cell output voltage to zero exponentially. In effect, the circuit output transistor {emitter follower) is a variable resistance that "sinks" the cell's output current. To operate the automatic sweeping load box, switch $1 is closed (to reset and hold the integrator) and the "zero" control is adjusted to set the circuits output voltage to the cell's open circuit voltage (thus lee, = 0). This condition corresponds to point C in Fig. I 1. When S, is opened, the . . . . . . . . .
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54
H. L. S K O L N I K
circuit will start to servo the output voltage towards zero according to the following relationship: E , ( t ) = Vceu exp (-2.5 × 10-4) (Rr) t.
Rr adjusts the gain of the l o o p - varying the servo time constant over the range of 0.75 to 10 sec. This unit can sweep parallel arrays with a maximum short circuit current of 2 A. However, the circuit could be modified to handle almost any series-parallel array. Thus to obtain a cell's I-V curve, one needs only to turn on the equipment, align the mount with sun, and start the automatic load box. A cknowledeement- 1 sincerely appreciate the assistance of the many people who helped during the development of this test system and in the writing of this paper. 1 am indebted to Professor Robert L. Bailey of the University of Florida, Electrical Engineering Department, for his constant encouragement and guidance during the course of this undertaking, and to William E. Vice for his great assistance throughout most of this project. Harold R. King of the Instrument Design Shop was responsible for the expert fabrication of the system's many mechanical components. His creativity was an essential ingredient.
A SYSTEM FOR THE E V A L U A T I O N OF SOLAR CELL SAMPLES H. L. SKOLNIK* (Received 10 May 1971)
A ~ t r a e t - T h e system described is a solar cell test vehicle intended primarily for education and research applications. Because of the many advantages to be derived from testing cells under real sky conditions, the sun is utilized as the system's energy source. Provisions are made for the collimation of the incident energy to simulate space conditions when desired, and also to provide a known angle of incidence when testing cells for either terrestrial or space applications, The mobile equipment console, complete with necessary instrumentation and power supplies, facilitates positioning the apparatus in an open area remote from the laboratory. The system design provides for the measurement of open circuit voltage, short circuit current, internal impedance, and output power, each as a function of incident energy. A means for the accurate angular positioning of the solar cell provides for the determination of cosine response. A specially-designed, direct read ng pyrheliometer featuring fast response provides normal incident radiation data, allowing for the accurate determination of conversion efficiency. Spectral response measurements are made by comparing the output of the cell under test. both with and without accessory band-pass filters. The temperatures of both the solar cell and the ambient air can be monitored for future data correlation. A clock drive provides for the continuous alignment of the sample holder with the sun, allowing for measurements over extended periods of time. Samples as large as 4.3 in. in diameter can be accommodated. R ~ s u m r - L e s auteurs d~crivent un vrhicule servant au cours d'exp~riences sur des cellules solaires et destin6 h 1'rducation et la recherche. Comme il y a de nombreux avantages h tester les cellules dans les conditions naturelles au dehors, le soleil est utilis6 en rant que source d'6nergie du v6hicule. Les auteurs tiennent compte de la collimation de la lumi&re incidente pour simuler les conditions spatiales ainsi que pour difinir un angle connu d'incidence lorsqu'ils testent les cellules pour des applications terrestres ou spatiales. Le support de I'appareillage mobile, comprenant les instruments necessaires et la source d'rnergie, facilite la raise en place de rappareillage dans un endroit ouvert, loin du laboratoire. Ce syst~me a 6tg con¢,u pour mesurer le voltage d'un circuit ouvert, I'imprdance interne et la puissance debitre, chacun 6tant fonction de la lumi/~re incidente. La raise en place angulaire exacte de la cellule solaire permet de drterminer la rgponse cosinusoidale. Un pyrhgliom~tre conqu sprcialement et h lecture directe fournit rapidement les donnrs de la radiation hun angle incident normal, ce qui permet d-obtenir la determination exacte de refticacit6 de la conversion, On mesure les rrponses spectrales, en comparant le rendement de la cellule, en tours d'exprrience avec et sans filtres. On enregistre les temprratures de la cellule solaire et de Fair ambiant pour des corrrlations ulterieures. Un syst~me mrcanique maintient un alignement continu de la cellule avec le soleil. I1 est possible d'inslaller ainsi des cellules ayant jusqu'~ 10,8 cm de diamgtre, Resumen- El sistema descrito es un vehiculo de comprobacion de pilas solares ideado principalmente para aplicaciones de ensehanza e investigacion. Debido alas muchas ventajas que provienen de la verification de pilas en rOgimen de cielo verdadero, se utiliza el sol como fuente energrtica del sistema. Se proveen medios pat-lt la colimacirn de la energia incidente, en orden a la simulacirn eventual de condiciones espaciales, asi c()mo para proporcionar un conocido ;ingulo de incidencia durante la comprobacit~n de pilas destinadas a aplicaciones lerrestres o espaciales, l,a consola del equipo m6vil, completa con la instrumentacirn necesaria y sEtministros de potencia, permite el emplazamiento del aparato en una zona abierta alejada del laboratorio. El sislcma prev6 la determination del voltaje en circuito abierto, corricare de cortocircuito, impedancia inlerna y pt}tencia de salida, cada uno en fi]ncirn de la energia incidente. Un medio para el posicionamiento an~ttlar exacto de la pila solar permite determinar la respuesta de coseno. Un pirhelirmetro de lectura directa, de disefio expecial con respuesta r~pida, proporciona datos sobre la radiacirn incidenle normal, Io cual permite la delerminacirn exacta de la eficiencia de la conversi6n. Se roman medidas de la respuesta espectral comparando la salida de la pila objeto de comprobacirn, con y sin filtros accesorios de paso de banda. Las temperalttras tanto de la pila solar como del aire ambiente pueden ser controladas en orden a la ft.tura correlaci(]n de los datos. Un mecanismo de reloj permite la alineacion continua del portamuestras con el sol. Io cual asegura la toma de medidas durante largos perlodos de tiempo. Se admiten muestras con di;imetro de 4, 3 pttlgadas como m~iximo. " Harris Semiconductor, Melbourne, Florida, formerly with University of Florida. 43
44
H. L. S K O L N I K INTRODUCTION
THE TEST system described in this paper is designed to provide for the measurement of the terminal characteristics of solar cells and other photo-sensitive elements when expo;ed to sunlight illumination. The problems associated with the use of the sun as the energy source are well known. However, the difficulties experienced in the interpretation of cell characteristics when measured under artificial light conditions (because of the complex nature of the Sun's spectrum) and also the high cost of a "good" sunlight simulator convinced the designers that the inconveniences associated with a sunsource system were to be tolerated. The system as shown in Fig. 1 includes: A direct reading pyrheliometer for the measurement of normal incident solar irradiance (necessary for the determination of cell conversion efficiency) A pedestal supported equatorial mount with clock drive for sun tracking collimating tubes An adjustable sample platform An adjustable source of 60 H z (120 VAC) for powering the clock drive and auxilliary measuring equipment. The motivation for the development of a solar cell test system at the University of Florida was broadly based. Fundamental to providing a sound background in the field of Direct Energy Conversion is a realistic laboratory program. The test system is intended to give the student a first-hand "feel" for real solar cell characteristics. For the advanced student or researcher who may be engaged in device fabrication, the instrument can provide essential quantitative information. The design of an instrument to be used in an educational environment must necessarily be versatile, straightforward in its operation, low in cost, and rugged. This system meets these requirements. Figure 2 indicates the major functional blocks of the solar cell test system. The equipment can Standard r" collimator ~._ . ~
~
Eppley
pyronometer
Irradiance meter
• .......
"1
[
r
'I;~OV6(~Hzl i inverter I -'I20V 6i
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probe
table
Mobile power ~pply (~
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ermome er
A.C. Line Fig. 2. Solar cell test system-block diagram.
Pyrheliometer
"Clip-on" o m meter Electronic thermometer
Fig. 1. Solar cell test system.
[Facing page 44]
Fig. 3. Collimator tubes.
Fig. 5. Sample homer.
A system for the evaluation o f mlar cell samples
45
be subdivided into four groups: Standard pyrheliometer Sample measuring instrumentation Sun tracking mechan sm Mobile power supply. COLLIMATION The design of the sample instrumentation is strongly influenced by the cosine response characteristic of solar cells. The cosine response makes it important to define the direction of incident energy. In space this is a trivial matter, but within earth's atmosphere sunlight appears from all directions. A simple method for collimating the system so that it will only accept normal incident energy is to prDvide a long shield to reduce non-normal components. In this equipment separate collimators are provided for the standard pyrheliometer and sample holder (See Fig. 3). The sample collimator is adjustable and can thus provide the degree of collimation required (consistent with sample size). The sample collimating tube is made of aluminum, has a 5t in. inside diameter, and is 6 ft long. Inside the tube, baffles having apertures of 5 in. help reduce non-normal radiation at the sample. The inner surface of both the sample and standard collimating tubes, as well as the sample holder, are coated with Parsons Black Optical Paint which has excellent absorption characteristics throughout the sun's spectrum. The standard collimating tube is 18 in. long and is described in greater detail below. For clarity, a distinction is made between "acceptance angle" and "view angle" (See Fig. 4). Acceptance Angle ( 0 ) - t h e angle (measured with respect to the sample-sun axis) through which light can enter the system and strike the target's surface. View Angle (t~)-the maximum angle through which the target can view incoming radiation. The collimator geometry indicates that a very high degree of collimation (small acceptance angle) dictates either a prohibitive shield length or a very small shield radius. For a given target size, there exists a minimum acceptable acceptance angle below which vignetting will result. It is important to note that the target size cannot equal the entrance aperture. Referring to Fig. 4 it is clear that R--a R tan fl = M - - L - M = tanS.
Therefore, tan 8 -~ - a - ~ (where 8max = 16 rain. 18 sec.) Thus, for the sample collimator a - d >~ 0.35 in. and for the pyrheliometer a - d ~> 0 . 0 8 5 in. If a - d is smaller than indicated above, errors due to vignetting will result. In order to arrive at a workable instrument, compromises in both the degree of collimation and the maximum sample size were made. Because the thermopile (in the pyrheliometer) is of fixed dimensions, one cannot
H. L. SKOLNIK
/ / /
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/
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a - 2 A r c tan (-~-~) e, Atc ton
(d.~0 )
Ton~ = Ton~- 16'18" o= LTon~+d Sompte collimofor Csom= 0 3 5 4 d $~ondord collimator: ast=OOeS~d
/
Fig. 4. Collimator geometry, d = sample radius, a = entrance aperture radius, L = collimator length, m = mean Earth-Sun distance, R = sun's radius, ~ : view angle, ~i= apparent sun's radius, 0 = acceptance angle./~ : vignette angle. r e d u c e t h e e n t r a n c e a p e r t u r e w i t h o u t s h a d i n g a s e g m e n t o f the a c t i v e a r e a with a res u l t a n t l o s s o f c a l i b r a t i o n . I n c o n t r a s t , t h e s a m p l e c o l l i m a t o r is a d j u s t e d b y r e d u c i n g the e n t r a n c e a p e r t u r e . T h e a p e r t u r e is r e d u c e d b e c a u s e it is not p r a c t i c a l to e x t e n d this l a r g e t u b e b e y o n d 6 ft. T h e o p e r a t o r m u s t r e m e m b e r that the a l l o w a b l e s a m p l e size is limited by the aperture selected or vica versa. T h e a c c e p t a n c e angle o f t h e s a m p l e t u b e is d e t e r m i n e d b y the r a d i u s o f tile e n t r a n c e aperture, 0 -- a r c tan
a-0.175 36
o r o f g r e a t e r significance, d+0.175 , w h e r e a a n d d a r e in i n c h e s . 0 = a r c tan - 36 T h e a c c e p t a n c e a n g l e f o u n d a b o v e r e p r e s e n t s t h e s m a l l e s t a l l o w a b l e a n g l e (for a g i v e n s a m p l e size) t h a t will y i e l d a n o n v i g n e f t e d field. T h e b a s i c a c c e p t a n c e angle for the s a m p l e c o l l i m a t o r w i t h o u t a p e r t u r e r e d u c t i o n is 3045 ' . T o f a c i l i t a t e a c c e p t a n c e angle a d j u s t m e n t s a n u m b e r o f a p e r t u r e s h a v e b e e n c o n s t r u c t e d in s i z e s f r o m I ° to 3.5 °.
A system for the evaluation of solar cell samples
47
SAMPLE H O L D E R
The sample holder is arranged so that a cell (or group of cells) up to 4.3 in. in diameter can be accommodated (See Fig. 5). The mounting platform is fitted with adjustable edge hold down clamps and is scribed with concentric circles to facilitate axial mounting of the sample. Four-terminal electrical connections are made through knurled brass terminals which are also supported on the mounting platform. To facilitate cosine response measurements the platform can be rotated ± 90° from the solar axis. Measurements are aided by a calibrated angular scale and a platformqocking device to maintain angular settings for extended time periods. A hole in the center of the mounting platform is provided so that an H. P. 2801 A electronic thermometer probe may be placed in contact with the cell under test. A coarse-threaded flange permits the rapid connection of the sample holder to the collimating tube. SUN T R A C K I N G
The tracking system is comprised of a standard equatorial telescope mount equipped with a synchronous drive motor runn ng at the sidereal rate. In order to track the sun accurately, allowance must be made for the sun's annual motion on the celestial sphere (about 1° per day). Since the sun moves on the ecliptic and not the celestial equator, the tracking error will be in two directions, east-west and north-south. The sun's motion is principally eastward, and hence the sun's 1° per day movement across the sky will cause a tracking error from the sideral rate of about two arc minutes per hour. In addition, the 23½° tilt of the ecliptic from the celestial equator produces a small change in the sun's declination, which has its maximum value near the equinoxes (March and September) and is about one arc minute per hour. The maximum change in right ascension is about 1 min 45 sec. and occurs near December 21 st. This means that ideally the clock drive (sidereal) should be adjusted to lose about ten seconds per hour (maximum, depending upon time of year), and that the declination axis should be driven at about one arc minute per hour in the worst case. Hence, when extremely accurate tracking of the Sun is required, complex feedback control systems are usually utilized. In this application, however, observation periods are generally short and tracking errors due to the above variations are small. Perhaps the most significant tracking error occurs because of misalignment of the equatorial mount. Because the solar cell test system is portable and is not intended to be left in a permanent location, the problem of aligning the system before each use is apparent. The precision with which the system must track the sun is dependent upon the ratio of the entrance aperture to the sample size. Figure 6 illustrates how a tracking error (A) reduces the nonvignetted sample area. A "bull's-eye" bubble level along with adjustment screws on the pedestal mount facilitate leveling the system. The altitude of the polar axis on the equatorial mount is set simply from the site latitude. Thus the only problem confronting the operator is the alignment of the polar axis in azimuth, which must be adjusted, as nearly as possible, parallel with the spin axis of the Earth. Two simple methods have been used for adjusting the polar axis of the mount so that its azimuth is approximately 0 ° (N). A magnetic compass can be used to find "North." Despite the fact that magnetic North is not in the true direction in which one wishes to point and that there are also errors inherent in sighting along the mount, acceptable results have been noted for observation periods
48
H. L. SKOLNIK Sun
t
/i
/i /!
I I
t I
/ / |
I
/
/
B
Fig. 6. Effectof trackingerror on allowablesamplesize. up to one hour (magnetic headings should be corrected for local variations, where errors are significant). A second method which is often used to refine the alignment made with the magnetic compass is as follows: Mount an arbitrary cell (small in size) in the sample holder and connect a high impedance meter to read open circuit voltage. Adjust the collimator acceptance angle so that it is as small as possible, consistent with cell size (See Fig. 3). Level the mount and set the polar axis altitude to the site latitude. Point the polar axis approximately North (use compass or guess !). Aim the system at the s u n - m a x i m u m reading on the voltmeter (lock declination axis). Engage the drive motor and note rate of change in voltmeter reading. Rotate the mount slightly, adjust level if necessary. Repeat the last three steps until the voltmeter variations are minimized. In practice this process converges rapidly, and sufficient precision to allow tracking for several hours can be achieved in less than one half hour. (Allowance must be made for clouds and other sources of intensity variations.) Adjustable weights on the declination axis and the collimating tube provide a means fo~ balancing the driven system about the polar axis. Proper balance is essential if the drive motor is to perform properly.
A systemfor the evaluationof solar cell samples
49
SOLAR IRRADIANCE METER One of the most important characteristics of a solar cell is its conversion efficiency. In order to determine cell conversion efficiency the total incident solar irradianee must be known. Because of the spectral makeup and variability of the sun's intensity, as well as other atmospheric phenomena such as scattering, a radiation meter having the following characteristics is required: Fast response Equal sensitivity throughout the Sun's spectrum N arrow acceptance a n g l e - to reduce sky error Direct read-out in appropriate units. An instrument exhibiting these properties was not available within the budget limitations of this project. As a result a device was developed to meet the above needs. The irradiance meter is based upon an Eppley Model 8, Black and White Pyranometer. This unit was chosen because it possessed many of the characteristics mentioned above, as well as being relatively rugged. To restrict the instrument to normal incident radiation, the pyranometer was placed at the bottom of a 1.4 in. (inside diameter) collimating tube. Baffles within the tube further restrict the entrance aperture to 1.37 in. The resulting thermopile pyrheliometer has an acceptance angle defined by: . ~d ,+ a hence, 0 = 406 ,. 0 = arc tan Figure 7 shows the schematic diagram of the solar irradiance meter. The linear output of the thermopile is amplified by an operational amplifier in the potentiometric configuration. The high input impedance (100 megohms) of the Harris H A 2600 operational amplifier insures that the pyranometer is not adversely loaded. The amplifier output Eppley pyranometer
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50
H. L. S K O L N I K
is adjusted to drive the 1 mA meter to full scale when an input energy of 100 MW/cm 2 is incident on the thermopile. The meter itself is of taut band construction and has a mirrorbacked scale readable to within 1 per cent. Experience has shown that this pyrheliometer is extremely easy to use and has proven reliable throughout periods of thermal variation and mechanical stress. POWER SYSTEM
In order for the solar cell test system to be positioned in a direct line with the sun at different times of the day and year, it is imperative that the equipment be portable and operational at remote locations. Power requirements for the system depend upon the type of measurements to be made, but in all cases 120 V - 6 0 H z is necessary to operate the synchronous tracking motor. To provide flexibility, provisions are made to allow for the use of ordinary laboratory instruments. Generally included in the system are the following instruments: High impedance voltmeter (Fluke 8100 A) "Clip-on" current meter (H. P. 428 B) Dual probe electronic thermometer (H. P. 2801 A) X-Yrecorder (Houston H R 96 T). Most of these instruments require a.c. power. For accurate tracking a well-regulated 60 Hz source is necessary. Figure 8 shows the overall schematic diagram of the power system. The basic energy source is a 12 V, 95 A-h, lead-acid storage battery. Direct to alternating current conversion is accomplished with a Heath Model MP-14 solid state 2ACB
/ rQo
put
Fig. 8. Mobile power supply.
A systemfor the evaluationof solar cell samples
51
inverter. The inverter provides an output that can be set in both amplitude and frequency. Frequency stability as a function of time and load variation has been found to be within ½ Hz for extended test intervals. Meters on the front panel of the instrument cart indicate the amplitude and frequency of the ac supply as well as the battery voltage and current (See Fig. 9). When the equipment is in storage or otherwise near a source of commercial power, a built-in charger readies the storage battery for future remote use. The Sear Model 608, 71970 battery charger used is completely automatic and does not require operator attention. Under average conditions the battery will power the system for approximately four hours without degradation in equipment performance. Recharging requires between ten and twelve hours. Periodically, it is necessary to check the battery fluid level. To facilitate this, the battery is mounted on slide-out rails (See Fig. 10). An exhaust system is activated whenever the equipment is in the battery charging mode to prevent the built-up of gas within the enclosure. Under these conditions the power to run the fan comes from the ac line. When the system is operating under remote conditions, the blower is not normally activated. However, in order to protect the power inverter from overload, thermal switches strategically located within the enclosure operate the fan when a temperature of 100°F is reached. In the event that the inside temperature of the cart reaches 125°F, all loads are disconnected from the inverter except for the power necessary to run the blower. Power output will not be restored until the temperature falls below ! 15°F. The inverter is further protected by limiting its output current with circuit breakers and fuses. Figure 10 shows the interior layout of the instrument cart. The top compartment is intended for the storage of accessiories and stowage of equipment during transportion to and from the test site. MEASUREMENT PROCEDURES The solar cell test system is designed primarily to measure the following cell characteristics: Open circuit voltage Short-circuit current Maximum output power Internal impedance Conversion efficiency Cosine response. These parameters can be measured at various levels of irradiance through the use of neutral density filters. However, for accurate determinations consideration should be given to the true neutrality of the filters used. While it is normally more desirable to measure solar cell spectral response under artificial light conditions, this test system does find some application in response measurements. The student can determine the general response of a cell by using band-pass filters (on both the sample and pyrheliometer) and by comparing the normalized cell output to a plot of solar intensity. The suitability of materials for use as "windows" over solar cells can be evaluated directly by comparing the cell's characteristics both with and without the material in the incident solar beam. As described above, the sample holder is arranged so that the plane of the cell under
52
H. L. S K O L N I K
test can be varied from 90 ° to 0 ° with respect to the sample-sun axis. Thus the cell's angular response (voltage, current, or power) can be determined by plotting the displacement angle versus cell output. Allowances can be made for variations in solar irradiance during the measurement period by simultaneously plotting irradiance versus time (with the pryheliometer and X - Y recorder). In the interest of simplicity this system does not provide for the control of sample temperature. However, the sample (as well as ambient) temperature can be constantly monitored along with the other parameters for future correlation. Figure 11 illustrates the utility of a device's current-voltage characteristic curve. Points A, B, and C yield short circuit current, maximum power output, and open circuit voltage respectively. The internal impedance at any point of interest can be calculated from the inverse slope of the curve at the point in question. The conversion efficiency of the cell under test is obtained by comparing the power output to the incident solar irradiance and cell area. The literature suggests several methods for obtaining cell I - V curves. The procedure described here is the one most commonly used and consists of varying a resistive load from open circuit to short circuit while measuring the cells output voltage and current
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Figure 12 illustrates the physical hookup of the apparatus. It is essential that four terminal connections be made to the cell under test, or wiring resistances will mask the true characteristics. The variable resistive load is arranged as shown in the figure to provide smooth control of output variations throughout the operating range. As the short circuit condition is approached, a very high degree of resolution is required to obtain an accurate curve. The impedance of the voltmeter (and/or recorder) should be sufficiently high so that it does not load the sample under test (generally Z ~> 100 K is adequate). The ammeter should be of very low impedance so that short circuit conditions can be closely approached. An ammeter voltage drop of less than 10 mV is desir-
Fig. 9. Mobile power supply control panel.
StoWage
Fig. 10. Interior view of equipment cart.
A system for the evaluation of solar cell samples
.
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53
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able. The clip-on type ammeter used here offers low insertion impedance as well as a voltage output (proportional to meter deflection)suitable for driving the recorder. The high resolution requirement on load variation was mentioned above. This is due in part to the cell's characteristics and also to the dynamics of the recorder. One can approximate the required variation of resistance with time by a decreasing exponential function. Figure 13 suggests an automatic circuit that forces the cell output voltage to zero exponentially. In effect, the circuit output transistor {emitter follower) is a variable resistance that "sinks" the cell's output current. To operate the automatic sweeping load box, switch $1 is closed (to reset and hold the integrator) and the "zero" control is adjusted to set the circuits output voltage to the cell's open circuit voltage (thus lee, = 0). This condition corresponds to point C in Fig. I 1. When S, is opened, the . . . . . . . . .
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54
H. L. S K O L N I K
circuit will start to servo the output voltage towards zero according to the following relationship: E , ( t ) = Vceu exp (-2.5 × 10-4) (Rr) t.
Rr adjusts the gain of the l o o p - varying the servo time constant over the range of 0.75 to 10 sec. This unit can sweep parallel arrays with a maximum short circuit current of 2 A. However, the circuit could be modified to handle almost any series-parallel array. Thus to obtain a cell's I-V curve, one needs only to turn on the equipment, align the mount with sun, and start the automatic load box. A cknowledeement- 1 sincerely appreciate the assistance of the many people who helped during the development of this test system and in the writing of this paper. 1 am indebted to Professor Robert L. Bailey of the University of Florida, Electrical Engineering Department, for his constant encouragement and guidance during the course of this undertaking, and to William E. Vice for his great assistance throughout most of this project. Harold R. King of the Instrument Design Shop was responsible for the expert fabrication of the system's many mechanical components. His creativity was an essential ingredient.