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Characterisation of photovoltaic generators
Applied Energy 65 (2000) 273±284
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Characterisation of photovoltaic generators Wilhelm Durisch a,*, Dierk Tille b, A. WoÈrz b, Waltraud Plapp c a
Applied Photovoltaics, Paul Scherrer Institut, PSI, CH-5232 Villigen PSI, Switzerland b Technical College Ulm, FHU, D-89075 Ulm, Germany c Technical College Aalen, FHA, D-73428 Aalen, Germany
Abstract Reliable knowledge on the performance of dierent photovoltaic generators (as single cells, modules, laminates, shingles, car roofs, etc.) under actual operating conditions is essential for correct product selection and accurate prediction of their electricity production. For this purpose, an outdoor test facility was erected at the Paul Scherrer Institute, PSI. It consists of a sun-tracked sample holder, electronic loads and a PC-based measuring system. Insolation is measured with pyranometers, pyrheliometers and reference cells. Characterisation of a generator under given test conditions means the precise acquisition of its electrical behaviour under varying load. The generator's eciency and all the relevant electrical parameters are derived on-line from a series of measured current/voltage (I/V) values. I/V-scans at constant insolation and at dierent generator temperatures enable the temperature coecients of the eciency and the electrical parameters to be determined. Thereafter I/V-scans at dierent insolations (10±1200 W/m2) and air masses (1.1±5) yield (via temperature correction) the insolation dependence of the eciency at constant temperature. A complete scan takes about 5±15 s. Samples of size varying from 1 by 1 mm up to 1.5 by 1 m can be tested at currents up to 32 A and at voltages up to 120 V. For modelling purposes, the results are represented in the form of correlations, e.g. the eciency as a function of the operating parameters temperature, insolation and air mass. Results obtained in PSI's test facility were con®rmed by the Fraunhofer-Insitut fuÈr Solare Energiesysteme, D-79100 Freiburg, Germany. Measurements are presented from some modules and single cells as well as some eciency correlations. Results are also presented on lamination losses, on PSI's high eciency cell, on GraÈtzel cells and watch modules as well as on shading eects and of a small thermophotovoltaic generator. # 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: Photovoltaic test facility; I/V characterisation; Solar-cell eciency; Temperature coecient of eciency; Part-load eciency; Eciency correlations; Shading eects; PSI's solar cell; Watch modules; GraÈtzel's cell; Thermophotovoltaic generator
* Corresponding author. Tel.: +41-56-310-26-25; fax: +41-56-310-21-99. E-mail address: [email protected] (W. Durisch). 0306-2619/00/$ - see front matter # 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S0306-2619(99)00115-4
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1. Introduction Photovoltaics (PV) probably are the most elegant way to convert solar energy into electricity. However, the technology is not yet commercially competitive for high volume generation of electricity. Nevertheless there are more and more applications for which photovoltaics by far are the best solution [1], thus guaranteeing a steadily growing market. According to predictions by many energy experts, photovoltaic electricity will necessarily play a signi®cant role in any sustainable energy future. The future competitiveness of PV depends on many factors, such as technological advances, production volume of PV components, ecological tax on traditional energies, and also on the most judicious use of this new energy technology. The latter point in particular relates to the performance of PV-generators under site-speci®c climatic conditions. The question is: What energy output of dierent generators can be expected under actual operating conditions at climatologically dierent sites? To answer this question, the physical behaviour of solar cells and photovoltaic modules under varying solar illumination and changing climatic conditions needs to be known. Usually these data are not provided by the manufacturers and suppliers of PV products. Moreover, the data provided at Standard Test Conditions (STC) most often are taken under test conditions which never occur in practice. In the present paper, therefore, data on some selected modules and single cells under real operating conditions are provided. For this purpose, a ¯exible test stand for outdoor characterisation of single cells and modules was developed, tested, and successfully put into operation at PSI [2]. The purpose of the data furnished is to support appropriate design and optimum utilisation of PV power-supply systems, and thus to improve their economic attractiveness. This is achieved by using outdoor data together with site speci®c, time-resolved meteorological data [3,4] as inputs for simulation models which are used to calculate the speci®c daily, monthly and yearly electricity production of dierent PV generators. The use of outdoor data leads to reliable results and enables the selection of the most suitable module type for a speci®ed application at a selected site. Knowing that simulations based on Standard Test Condition data can lead to an overestimation of the production by up to 40% [5], it is important that outdoor test data are used to provide a realistic assessment of the productivity of PV generators. Outdoor test data, on the other hand, are helpful in deriving recommendations for improving PV generators and to assist PV component manufacturers, plant designers, suppliers, installers and operators in their eorts to realise successful PV systems. 2. Test facility Fig. 1 schematically shows PSI's test facility. A parallactic system is used as the sun tracker for the samples. The drive and control for this tracker have been developed at PSI and provide precise solar alignment throughout the year. Six pyranometers (CM 21, Kipp & Zonen, 305±2800 nm) and two reference cells (Siemens) are mounted on the tracking system. They are used to measure precisely the incident sunlight onto the samples. The ambient temperature is measured using a ventilated,
Fig. 1. Diagram of the photovoltaic test facility at the Paul Scherrer Institute, PSI.
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radiation-shielded Pt-100 sensor. The direct normal irradiance is measured using pyrheliometers (Eppley) mounted on separate sun trackers developed by PSI. This irradiance measurement is required for determining the diuse normal irradiance. The wind velocity and wind direction are also measured. The voltage of the sample and all meteorological signals are transmitted into the nearby photovoltaics laboratory, where they are measured using digital multimeters (Fluke and Prema). The current from the sample is measured using an appropriate precision resistor with a very low temperature-coecient. A surface-temperature probe (Pt-100) is arranged on the back of the sample for measuring its temperature. The signals are transmitted from the above-mentioned multimeters via an IEEE bus to a DEC-PC. The electrical behaviour of the sample can be scanned over the entire operating range by loading the sample with a suitable load: 100±300 current/voltage values are measured and stored over a period of 5±15 s for this purpose. They are evaluated on-line and the result is displayed on the screen immediately after measurement in the form of the characteristic curves (current and power versus voltage) and all relevant characteristic data. They are stored and printed out as a test report if desired. The characteristic data include, in particular, current, voltage and eciency at maximum power output. By plotting a series of them at constant irradiance but varying sample temperature, it is possible to determine their temperature coecients. Measurements with varying insolation ®nally result in the dependency of the eciency on the irradiance. To obtain it for a speci®c constant sample temperature, e.g. the standard test condition (STC) temperature of 25 C, the individual eciencies are converted to the desired temperature via the previously determined temperature coecient. To each I/V scan, the relative air mass AM is computed from the respective position of the sun. It is required for developing correlations describing, for example, the eciency as a function of the operating parameters' irradiance, cell temperature and air mass. Estimates of errors showed that measurements of the eciency from PSI's facility have a relative uncertainty of about 1.5%. This corresponds to internationallyaccepted accuracy. Comparisons with eciency measurements at the Fraunhofer Institute for Solar Energy Systems, FhG-ISE, D-79100 Freiburg showed excellent agreement. Further details on PSI's test facility are provided in Durisch et al. [2,7]. 3. Results 3.1. Commercial standard modules In a series of module tests the three modules of Kyocera (LA351K54S, p-Si), AstroPower (AP-1206, m-Si) and BP Solar (BP 585, m-Si) were examined in particular depth. As with all modules previously tested at PSI, they also fall below the manufacturer's speci®cation in terms of performance. The Kyocera module comes out best with a negative deviation of only 5% for the power under standard test conditions (STC). The deviations with Astro and BP were found to be minus 14% and minus 6% respectively. The STC cell eciencies were determined to be 14.4, 12.7 and 15.9% for Kyocera, Astro and BP Solar respectively. The temperature coecients of
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the STC eciencies for the corresponding modules amount to ÿ0.058, ÿ0.064 and ÿ0.069%/ C. For each module, several hundred I/V-scans were taken at various cell temperatures, insolations and air masses. They were used to derive empirical correlations for the eciency as a function of cell temperature, irradiance and air mass. A ®ve-parameter correlation could be developed for the Kyocera module, which allows the eciency to be described as a function of the cell temperature, irradiance and air mass in the entire measured range via Eq. (1): " P1
G G0
P2
P3
G G0
# 1 P4
# AM P5 #0 AM0
P1 0:28204 P2 0:39668 P3 ÿ0:44730 P4 ÿ0:092864 504G=Wmÿ2 41100 254#= C450 1:34AM43 G0 1000 W=m2
# 25 C
1 P5 0:016010
AM0 1:5
With Eq. (1), the measured eciencies could be calculated relative to those under the standard temperature of 25 C and air mass of 1.5 and plotted over the normalised insolation, Fig. 2. A simpli®ed correlation for the temperature- and AM-corrected eciency measurements is also given therein. Fig. 2 shows that the eciency over a broad insolation range exceeds 14%. This contrasts with a previously measured module [2] where the eciency begins to fall signi®cantly from 300 W/m2 downwards. It is important to bear this in mind in climatic areas with a high supply of solar radiation at relatively low intensity. The Kyocera module measurements in the ranges 7004G/ Wmÿ241000 and 1.44AM43 also demonstrate that the temperature coecient of the eciency is only very slightly dependent, if at all, on the irradiance and the relative air mass. As stated in Durisch et al. [2], this provides a simple method of converting eciency measurements under non-standard test conditions to standard conditions. The constancy of the temperature coecient of the eciency could also be found with mono-crystalline silicon cells. It, therefore, seems to apply to monoand polycrystalline cells. It remains to be checked how far it applies to other products and very low irradiances. It has also been found, with the Kyocera module, that the eciency remains high with increasing air mass to at least AM 5, whereas it drops to about half the standard value with the triple module produced by Uni-Solar. For the eciency of the AstroPower module AP-1206 versus the irradiance, the cell temperature and air mass, the following correlation was found: G; #; AM 659:3957 G=G0 1=2 ÿ 4406:8985 G=G0 1=3 7794:7337 G=G0 1=4 ÿ 4047:1102 G=G0 1=5 ÿ 0:0161 #=#0 ÿ 1 0:00606 AM=AM0 2
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where 504G=Wmÿ2 41100; 254#= C450; and 1:14AM42 The eciency falls from about 150 W/m2 as the irradiance decreases. Owing to the large cells (155155 mm), thermally-induced stresses are to be expected and could lead to cracks in the cells and interconnect in the course of time. The cell's tabbing on the front side appears to be dimensioned too closely for the high currents, up to 7 A, leading to unnecessary losses of power via series resistance. A similar correlation as Eq. (2) has been found also for the BP Solar module, BP 585, Eq. (3): G; # ÿ0:5613 G=G0 10:6579 G=G0 1=2 ÿ 45:5265 G=G0 1=3 65:5220 G=G0 1=4 ÿ 29:9336 G=G0 1=5 ÿ 0:01736 #=#0 ÿ 1
3
where 504G=Wmÿ2 41200; 254#= C450; and AM 1:3
Fig. 2. Eciency of the Kyocera module LA361K54S as a function of the normalised insolation (G0 1000 W/m2) at a cell temperature of 25 C and a relative air mass of 1.5. The measured points are ®tted well by a three-parameter correlation (the dotted line).
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3.2. Single cells 3.2.1. Lamination losses The in¯uence of the quality of the front glass on the eciency of single cells was investigated on cells from Solartec, Roznov, Czechoslovakia. The cells were measured before and after lamination. It was found that the eciency of the laminate may be higher than that of the non-laminated cell if good glass is used. This is due to the fact that the sum of the glass loss and of the re¯ection losses [reduced by the ethylene/vinyl acetate (EVA)] with the laminated cell is lower than the re¯ection loss of the non-laminated cell. With a good glass, it was, therefore, possible to increase the eciency from 15.5 to 15.9%, for example, while with a bad glass the eciency of another cell fell from 16.5 to 16%. 3.2.2. In¯uence of air mass on eciency Comprehensive tests on a Saturn cell manufactured by BP Solar showed that it has its maximum eciency at AM 1:5. As AM 1:5 occurs relatively rarely in the actual operation of a module over a year, it should be asked whether the trimming of cells to the standard value AM 1:5 makes sense. In the ®nal analysis, the number of kilowatt hours produced over the year, and not a good result under standard test conditions, is what matters. Therefore, cells, which not only have a good part-load eciency but are also optimised for the air mass range with the highest solar radiation input, would be required for a maximum annual yield. For this purpose, knowledge of the spectral composition of the solar radiation supplied in typical climatic areas is indispensable for developing such cells [4]. 3.2.3. PSI's high eciency cell Extensive measurements were taken on PSI cells [6]. They yielded STC eciencies of 21.1±21.3%, based on the pyranometrically measured insolation, and of 21.5± 21.7%, based on the insolation measured with calibrated Siemens reference cells. The measurements on the 21.1% cell were con®rmed by the Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany Ð see Fig. 3. 3.2.4. GraÈtzel's dye cell Particularly extensive tests have been carried out on GraÈtzel cells [8]. In total, over 1000 I/V-scans were taken, on 20 single cells and 3 miniature modules, and were evaluated. It was found that the eciency is independent on the cell's temperature ± see Fig. 4. For reasons which have not yet been fully explained, the eciency of the best cell (PL1219/s1, Feb. 97) shown in Fig. 4 gradually dropped from an initial 8 to 6.5%. As expected, owing to the spectral sensitivity of the GraÈtzel cell, it behaves similarly to amorphous silicon cells, i.e. the eciency decreases signi®cantly with increasing air mass in the late afternoon. As chemical reactions and diusion processes take place at a ®nite rate in the GraÈtzel cell, it reacts slower than silicon cells to fast changes in irradiance. Technically, this has the advantage that transients of the insolation are transmitted somewhat smoothed to subsequent components.
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Particular attention has been paid to the modelling of current/voltage characteristics of GraÈtzel cells. Starting with simple three-parameter models, implicit-transcendent functions could be adapted to the measured values and up to ®ve parameters could be determined by a novel method. The physical parameters, in particular the series resistance, could be determined therefrom. It was found that the series resistance increases with a degrading cell. The modelling has consequently been developed from two to four independent variables, so that several characteristics at dierent irradiations and cell temperatures can be ®tted simultaneously. The
Fig. 3. Eciency of a PSI cell as a function of the cell temperature. The error bar for the FhG-ISE measurement corresponds to a relative uncertainty of 2.5%. From the linear regression, the STC eciency is found to be 21.1%.
Fig. 4. Series of eciency measurements on a GraÈtzel solar cell (PL1219/s1, February 1997) as a function of temperature and time. The eciency was 8% at the beginning of the measurement period (series 1 measurements) and 6.5% at the end (series 6 measurements). In comparison with crystalline±silicon cells, the GraÈtzel cell has the attractive property of its eciency being independent of temperature.
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advantage is that, to determine the model parameters, far more measurements can be used and their uncertainty can therefore be reduced. In this way, it was possible to ®nd more reliable values for the parallel resistance. As expected, it decreases as the degradation increases. A detailed analysis of the electrical losses in the GraÈtzel cell led to total losses of 0.3±1.7%, depending on the eciency. It can be concluded from this that the eciency can be increased by minimising these losses by a maximum of 1.7% points, for example from 8 to 9.7%. For further details, the reader is referred to Durisch et al. [8]. 3.3. Watch miniature modules A few amorphous miniature modules extracted from solar watches were measured with respect to the possibility of replacing them with PSI cells. Fig. 5 shows the comparison between the eciency of a PSI cell and a Swatch module, plotted over the irradiance. It can be seen that PSI cells would deliver about ®ve times more energy in the intensity range relevant to a watch, i.e. only one ®fth of the watch face area would be required for the supply of energy with PSI cells. With high irradiance, the eciency of the Swatch module falls to about one twentieth of the eciency of the PSI cell. This decrease may partly be due to the adhesive bonding of tabs to the module leading to a high series resistance with correspondingly high losses. 3.4. Shading eects Investigations have also been carried out concerning the shading of individual cells in modules. As an example, Fig. 6 shows the characteristics of the ``shade-tolerant'' tandem module UPM 880, manufactured by Uni-Solar, with shading of half
Fig. 5. Eciency of a PSI cell in comparison with the eciency of an amphorous miniature module from a Swatch watch.
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Fig. 6. Current and power characteristics of the ``shade-tolerant'' tandem module UPM 880, manufactured by Uni-Solar. Half of the module was shaded along its diagonal. The power characteristic has several relative peaks and the eciency falls to about 20% of its non-shaded value.
Fig. 7. Current and power characteristics of a prototype thermophotovoltaic (TPV) generator consisting of 8 silicon half-cells. The power curve has a peak at 15.2 W. Under 909 W/m2 sunlight, the generator produces 5.6 W. The variations in the horizontal region of the current curve are due to the ¯ickering of the burner light. Under burner light, the ®ll factor fell from 0.75 to 0.68.
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of the module along its diagonal. The power characteristic has several relative peaks, which may lead to problems in the power tracking of grid-inverters. 3.5. Thermophotovoltaic generator To characterise thermophotovoltaic (TPV) generators [9], which convert burner light into electric power, the solar test facility including software was modi®ed so that characteristics can also be measured with ¯ickering light. Fig. 7 shows the characteristics of a small prototype TPV generator. The ¯ickering of the light in the horizontal region of the current characteristic can be seen clearly. 4. Conclusions/outlook An outdoor test facility for photovoltaic generators has been developed and put into operation successfully. A great number of cell and module tests have proven its capability and reliability. Measurements from this facility have been con®rmed by an internationally-approved test laboratory. Regular calibration of all measuring instruments guaranties continuous high quality and reliability of the test results. Due to the possibility of performing non standard tests under realistic operating conditions, a more practical picture of the performance of the various products can be provided. The test results are published or are available as internal reports. All tests are carried out in close co-operation with the manufacturers and suppliers of the test specimens, leading to an ecient transfer of the results. In future work, more emphasis will be laid on the cell's and module's behaviour under low light levels, cloud impacts and high air masses, as well as further re®nement of the correlations to describe these eects. Furthermore, it is intended to establish a comprehensive data base for some selected modules, and make it available on the internet. It is also planned to extend the investigations towards PV systems. As a ®rst step, measurements on three small grid-connected generators were started [10]. Acknowledgements The authors would like to express their gratitude to the Industrielle Werke Basel (IWB) for generous ®nancial support of the GraÈtzel cell measurements, and to the Holinger Solar, AG, CH-4410 Liestal, the Tritec AG, CH-4123 Allschwil and the IWS Solar AG, CH-8494 Bauma for providing the standard modules and for stimulating discussions. References [1] Durisch W, BuÈhlmann M, Kesselring P, Morisod R. Measurements and operational experience with a photovoltaic plant in the Swiss Alps. Proceedings ISES Solar World Congress 1987, vol. 1, Hamburg. Oxford: Pergamon Press. 1987. p. 289±97.
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[2] Durisch W, Urban J, Smestad G. Characterisation of solar cells and modules under actual operating conditions. In: Proceedings of the World Renewable Energy Congress, vol. 1, June 1996, Denver. Oxford: Pergamon, 1996. p. 359. [3] Durisch W, Keller J, Bulgheroni W, Keller L, Fricker H. Solar irradiation measurements in Jordan and comparison with Californian and Alpine data. Applied Energy 1995;52:111±24. [4] Durisch W, Hofer B. Klimatologische Untersuchungen fuÈr Solarkraftwerke in den Alpen, am Beispiel Laj Alv, Disentis, GraubuÈnden. PSI-Bericht Nr. 96-01, 1996. ISSN 1019 Ð 0643. [5] BuÈcher K, Heidler K. Physikalisches Verhalten und energetische Bewertung von Solarzellen mit einem oder mehreren UÈbergaÈngen unter realistischen Bezugsbedingungen. BMFT Status-Seminar Photovoltaik, 1993. [6] Anon. Schweizer Solarzellen unter den weltbesten. Elektrotechnik 1997;48(1):18. [7] Durisch W, WoÈrz A, Urban J, Tille D. Charakterisierung eines Photovoltaik-Grosslaminats. Bulletin SEV/VSE 1997;88(10):35±8. [8] Durisch W, Plapp W. Charakterisierung von GraÈtzelzellen unter realklimatischen Betriebsbedingungen. Bericht zu HaÈnden Industrielle Werke Basel, IWB, International Report, August 1997. [9] Durisch W, Panitz J-C. Development and characterisation of a rare earth emitter for a thermophotovoltaic power generator. PSI Annal Report/Annex V, 1996. p. 35. [10] Durisch W, Lavric R, Struss O. Drei Netzverbund-Kleinsysteme im Vergleichstest. Nationale Photovoltaiktagung, 5 May 1998, Bern, Switzerland.
www.elsevier.com/locate/apenergy
Characterisation of photovoltaic generators Wilhelm Durisch a,*, Dierk Tille b, A. WoÈrz b, Waltraud Plapp c a
Applied Photovoltaics, Paul Scherrer Institut, PSI, CH-5232 Villigen PSI, Switzerland b Technical College Ulm, FHU, D-89075 Ulm, Germany c Technical College Aalen, FHA, D-73428 Aalen, Germany
Abstract Reliable knowledge on the performance of dierent photovoltaic generators (as single cells, modules, laminates, shingles, car roofs, etc.) under actual operating conditions is essential for correct product selection and accurate prediction of their electricity production. For this purpose, an outdoor test facility was erected at the Paul Scherrer Institute, PSI. It consists of a sun-tracked sample holder, electronic loads and a PC-based measuring system. Insolation is measured with pyranometers, pyrheliometers and reference cells. Characterisation of a generator under given test conditions means the precise acquisition of its electrical behaviour under varying load. The generator's eciency and all the relevant electrical parameters are derived on-line from a series of measured current/voltage (I/V) values. I/V-scans at constant insolation and at dierent generator temperatures enable the temperature coecients of the eciency and the electrical parameters to be determined. Thereafter I/V-scans at dierent insolations (10±1200 W/m2) and air masses (1.1±5) yield (via temperature correction) the insolation dependence of the eciency at constant temperature. A complete scan takes about 5±15 s. Samples of size varying from 1 by 1 mm up to 1.5 by 1 m can be tested at currents up to 32 A and at voltages up to 120 V. For modelling purposes, the results are represented in the form of correlations, e.g. the eciency as a function of the operating parameters temperature, insolation and air mass. Results obtained in PSI's test facility were con®rmed by the Fraunhofer-Insitut fuÈr Solare Energiesysteme, D-79100 Freiburg, Germany. Measurements are presented from some modules and single cells as well as some eciency correlations. Results are also presented on lamination losses, on PSI's high eciency cell, on GraÈtzel cells and watch modules as well as on shading eects and of a small thermophotovoltaic generator. # 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: Photovoltaic test facility; I/V characterisation; Solar-cell eciency; Temperature coecient of eciency; Part-load eciency; Eciency correlations; Shading eects; PSI's solar cell; Watch modules; GraÈtzel's cell; Thermophotovoltaic generator
* Corresponding author. Tel.: +41-56-310-26-25; fax: +41-56-310-21-99. E-mail address: [email protected] (W. Durisch). 0306-2619/00/$ - see front matter # 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S0306-2619(99)00115-4
274
W. Durisch et al. / Applied Energy 65 (2000) 273±284
1. Introduction Photovoltaics (PV) probably are the most elegant way to convert solar energy into electricity. However, the technology is not yet commercially competitive for high volume generation of electricity. Nevertheless there are more and more applications for which photovoltaics by far are the best solution [1], thus guaranteeing a steadily growing market. According to predictions by many energy experts, photovoltaic electricity will necessarily play a signi®cant role in any sustainable energy future. The future competitiveness of PV depends on many factors, such as technological advances, production volume of PV components, ecological tax on traditional energies, and also on the most judicious use of this new energy technology. The latter point in particular relates to the performance of PV-generators under site-speci®c climatic conditions. The question is: What energy output of dierent generators can be expected under actual operating conditions at climatologically dierent sites? To answer this question, the physical behaviour of solar cells and photovoltaic modules under varying solar illumination and changing climatic conditions needs to be known. Usually these data are not provided by the manufacturers and suppliers of PV products. Moreover, the data provided at Standard Test Conditions (STC) most often are taken under test conditions which never occur in practice. In the present paper, therefore, data on some selected modules and single cells under real operating conditions are provided. For this purpose, a ¯exible test stand for outdoor characterisation of single cells and modules was developed, tested, and successfully put into operation at PSI [2]. The purpose of the data furnished is to support appropriate design and optimum utilisation of PV power-supply systems, and thus to improve their economic attractiveness. This is achieved by using outdoor data together with site speci®c, time-resolved meteorological data [3,4] as inputs for simulation models which are used to calculate the speci®c daily, monthly and yearly electricity production of dierent PV generators. The use of outdoor data leads to reliable results and enables the selection of the most suitable module type for a speci®ed application at a selected site. Knowing that simulations based on Standard Test Condition data can lead to an overestimation of the production by up to 40% [5], it is important that outdoor test data are used to provide a realistic assessment of the productivity of PV generators. Outdoor test data, on the other hand, are helpful in deriving recommendations for improving PV generators and to assist PV component manufacturers, plant designers, suppliers, installers and operators in their eorts to realise successful PV systems. 2. Test facility Fig. 1 schematically shows PSI's test facility. A parallactic system is used as the sun tracker for the samples. The drive and control for this tracker have been developed at PSI and provide precise solar alignment throughout the year. Six pyranometers (CM 21, Kipp & Zonen, 305±2800 nm) and two reference cells (Siemens) are mounted on the tracking system. They are used to measure precisely the incident sunlight onto the samples. The ambient temperature is measured using a ventilated,
Fig. 1. Diagram of the photovoltaic test facility at the Paul Scherrer Institute, PSI.
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radiation-shielded Pt-100 sensor. The direct normal irradiance is measured using pyrheliometers (Eppley) mounted on separate sun trackers developed by PSI. This irradiance measurement is required for determining the diuse normal irradiance. The wind velocity and wind direction are also measured. The voltage of the sample and all meteorological signals are transmitted into the nearby photovoltaics laboratory, where they are measured using digital multimeters (Fluke and Prema). The current from the sample is measured using an appropriate precision resistor with a very low temperature-coecient. A surface-temperature probe (Pt-100) is arranged on the back of the sample for measuring its temperature. The signals are transmitted from the above-mentioned multimeters via an IEEE bus to a DEC-PC. The electrical behaviour of the sample can be scanned over the entire operating range by loading the sample with a suitable load: 100±300 current/voltage values are measured and stored over a period of 5±15 s for this purpose. They are evaluated on-line and the result is displayed on the screen immediately after measurement in the form of the characteristic curves (current and power versus voltage) and all relevant characteristic data. They are stored and printed out as a test report if desired. The characteristic data include, in particular, current, voltage and eciency at maximum power output. By plotting a series of them at constant irradiance but varying sample temperature, it is possible to determine their temperature coecients. Measurements with varying insolation ®nally result in the dependency of the eciency on the irradiance. To obtain it for a speci®c constant sample temperature, e.g. the standard test condition (STC) temperature of 25 C, the individual eciencies are converted to the desired temperature via the previously determined temperature coecient. To each I/V scan, the relative air mass AM is computed from the respective position of the sun. It is required for developing correlations describing, for example, the eciency as a function of the operating parameters' irradiance, cell temperature and air mass. Estimates of errors showed that measurements of the eciency from PSI's facility have a relative uncertainty of about 1.5%. This corresponds to internationallyaccepted accuracy. Comparisons with eciency measurements at the Fraunhofer Institute for Solar Energy Systems, FhG-ISE, D-79100 Freiburg showed excellent agreement. Further details on PSI's test facility are provided in Durisch et al. [2,7]. 3. Results 3.1. Commercial standard modules In a series of module tests the three modules of Kyocera (LA351K54S, p-Si), AstroPower (AP-1206, m-Si) and BP Solar (BP 585, m-Si) were examined in particular depth. As with all modules previously tested at PSI, they also fall below the manufacturer's speci®cation in terms of performance. The Kyocera module comes out best with a negative deviation of only 5% for the power under standard test conditions (STC). The deviations with Astro and BP were found to be minus 14% and minus 6% respectively. The STC cell eciencies were determined to be 14.4, 12.7 and 15.9% for Kyocera, Astro and BP Solar respectively. The temperature coecients of
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the STC eciencies for the corresponding modules amount to ÿ0.058, ÿ0.064 and ÿ0.069%/ C. For each module, several hundred I/V-scans were taken at various cell temperatures, insolations and air masses. They were used to derive empirical correlations for the eciency as a function of cell temperature, irradiance and air mass. A ®ve-parameter correlation could be developed for the Kyocera module, which allows the eciency to be described as a function of the cell temperature, irradiance and air mass in the entire measured range via Eq. (1): " P1
G G0
P2
P3
G G0
# 1 P4
# AM P5 #0 AM0
P1 0:28204 P2 0:39668 P3 ÿ0:44730 P4 ÿ0:092864 504G=Wmÿ2 41100 254#= C450 1:34AM43 G0 1000 W=m2
# 25 C
1 P5 0:016010
AM0 1:5
With Eq. (1), the measured eciencies could be calculated relative to those under the standard temperature of 25 C and air mass of 1.5 and plotted over the normalised insolation, Fig. 2. A simpli®ed correlation for the temperature- and AM-corrected eciency measurements is also given therein. Fig. 2 shows that the eciency over a broad insolation range exceeds 14%. This contrasts with a previously measured module [2] where the eciency begins to fall signi®cantly from 300 W/m2 downwards. It is important to bear this in mind in climatic areas with a high supply of solar radiation at relatively low intensity. The Kyocera module measurements in the ranges 7004G/ Wmÿ241000 and 1.44AM43 also demonstrate that the temperature coecient of the eciency is only very slightly dependent, if at all, on the irradiance and the relative air mass. As stated in Durisch et al. [2], this provides a simple method of converting eciency measurements under non-standard test conditions to standard conditions. The constancy of the temperature coecient of the eciency could also be found with mono-crystalline silicon cells. It, therefore, seems to apply to monoand polycrystalline cells. It remains to be checked how far it applies to other products and very low irradiances. It has also been found, with the Kyocera module, that the eciency remains high with increasing air mass to at least AM 5, whereas it drops to about half the standard value with the triple module produced by Uni-Solar. For the eciency of the AstroPower module AP-1206 versus the irradiance, the cell temperature and air mass, the following correlation was found: G; #; AM 659:3957 G=G0 1=2 ÿ 4406:8985 G=G0 1=3 7794:7337 G=G0 1=4 ÿ 4047:1102 G=G0 1=5 ÿ 0:0161 #=#0 ÿ 1 0:00606 AM=AM0 2
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where 504G=Wmÿ2 41100; 254#= C450; and 1:14AM42 The eciency falls from about 150 W/m2 as the irradiance decreases. Owing to the large cells (155155 mm), thermally-induced stresses are to be expected and could lead to cracks in the cells and interconnect in the course of time. The cell's tabbing on the front side appears to be dimensioned too closely for the high currents, up to 7 A, leading to unnecessary losses of power via series resistance. A similar correlation as Eq. (2) has been found also for the BP Solar module, BP 585, Eq. (3): G; # ÿ0:5613 G=G0 10:6579 G=G0 1=2 ÿ 45:5265 G=G0 1=3 65:5220 G=G0 1=4 ÿ 29:9336 G=G0 1=5 ÿ 0:01736 #=#0 ÿ 1
3
where 504G=Wmÿ2 41200; 254#= C450; and AM 1:3
Fig. 2. Eciency of the Kyocera module LA361K54S as a function of the normalised insolation (G0 1000 W/m2) at a cell temperature of 25 C and a relative air mass of 1.5. The measured points are ®tted well by a three-parameter correlation (the dotted line).
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3.2. Single cells 3.2.1. Lamination losses The in¯uence of the quality of the front glass on the eciency of single cells was investigated on cells from Solartec, Roznov, Czechoslovakia. The cells were measured before and after lamination. It was found that the eciency of the laminate may be higher than that of the non-laminated cell if good glass is used. This is due to the fact that the sum of the glass loss and of the re¯ection losses [reduced by the ethylene/vinyl acetate (EVA)] with the laminated cell is lower than the re¯ection loss of the non-laminated cell. With a good glass, it was, therefore, possible to increase the eciency from 15.5 to 15.9%, for example, while with a bad glass the eciency of another cell fell from 16.5 to 16%. 3.2.2. In¯uence of air mass on eciency Comprehensive tests on a Saturn cell manufactured by BP Solar showed that it has its maximum eciency at AM 1:5. As AM 1:5 occurs relatively rarely in the actual operation of a module over a year, it should be asked whether the trimming of cells to the standard value AM 1:5 makes sense. In the ®nal analysis, the number of kilowatt hours produced over the year, and not a good result under standard test conditions, is what matters. Therefore, cells, which not only have a good part-load eciency but are also optimised for the air mass range with the highest solar radiation input, would be required for a maximum annual yield. For this purpose, knowledge of the spectral composition of the solar radiation supplied in typical climatic areas is indispensable for developing such cells [4]. 3.2.3. PSI's high eciency cell Extensive measurements were taken on PSI cells [6]. They yielded STC eciencies of 21.1±21.3%, based on the pyranometrically measured insolation, and of 21.5± 21.7%, based on the insolation measured with calibrated Siemens reference cells. The measurements on the 21.1% cell were con®rmed by the Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany Ð see Fig. 3. 3.2.4. GraÈtzel's dye cell Particularly extensive tests have been carried out on GraÈtzel cells [8]. In total, over 1000 I/V-scans were taken, on 20 single cells and 3 miniature modules, and were evaluated. It was found that the eciency is independent on the cell's temperature ± see Fig. 4. For reasons which have not yet been fully explained, the eciency of the best cell (PL1219/s1, Feb. 97) shown in Fig. 4 gradually dropped from an initial 8 to 6.5%. As expected, owing to the spectral sensitivity of the GraÈtzel cell, it behaves similarly to amorphous silicon cells, i.e. the eciency decreases signi®cantly with increasing air mass in the late afternoon. As chemical reactions and diusion processes take place at a ®nite rate in the GraÈtzel cell, it reacts slower than silicon cells to fast changes in irradiance. Technically, this has the advantage that transients of the insolation are transmitted somewhat smoothed to subsequent components.
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Particular attention has been paid to the modelling of current/voltage characteristics of GraÈtzel cells. Starting with simple three-parameter models, implicit-transcendent functions could be adapted to the measured values and up to ®ve parameters could be determined by a novel method. The physical parameters, in particular the series resistance, could be determined therefrom. It was found that the series resistance increases with a degrading cell. The modelling has consequently been developed from two to four independent variables, so that several characteristics at dierent irradiations and cell temperatures can be ®tted simultaneously. The
Fig. 3. Eciency of a PSI cell as a function of the cell temperature. The error bar for the FhG-ISE measurement corresponds to a relative uncertainty of 2.5%. From the linear regression, the STC eciency is found to be 21.1%.
Fig. 4. Series of eciency measurements on a GraÈtzel solar cell (PL1219/s1, February 1997) as a function of temperature and time. The eciency was 8% at the beginning of the measurement period (series 1 measurements) and 6.5% at the end (series 6 measurements). In comparison with crystalline±silicon cells, the GraÈtzel cell has the attractive property of its eciency being independent of temperature.
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advantage is that, to determine the model parameters, far more measurements can be used and their uncertainty can therefore be reduced. In this way, it was possible to ®nd more reliable values for the parallel resistance. As expected, it decreases as the degradation increases. A detailed analysis of the electrical losses in the GraÈtzel cell led to total losses of 0.3±1.7%, depending on the eciency. It can be concluded from this that the eciency can be increased by minimising these losses by a maximum of 1.7% points, for example from 8 to 9.7%. For further details, the reader is referred to Durisch et al. [8]. 3.3. Watch miniature modules A few amorphous miniature modules extracted from solar watches were measured with respect to the possibility of replacing them with PSI cells. Fig. 5 shows the comparison between the eciency of a PSI cell and a Swatch module, plotted over the irradiance. It can be seen that PSI cells would deliver about ®ve times more energy in the intensity range relevant to a watch, i.e. only one ®fth of the watch face area would be required for the supply of energy with PSI cells. With high irradiance, the eciency of the Swatch module falls to about one twentieth of the eciency of the PSI cell. This decrease may partly be due to the adhesive bonding of tabs to the module leading to a high series resistance with correspondingly high losses. 3.4. Shading eects Investigations have also been carried out concerning the shading of individual cells in modules. As an example, Fig. 6 shows the characteristics of the ``shade-tolerant'' tandem module UPM 880, manufactured by Uni-Solar, with shading of half
Fig. 5. Eciency of a PSI cell in comparison with the eciency of an amphorous miniature module from a Swatch watch.
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Fig. 6. Current and power characteristics of the ``shade-tolerant'' tandem module UPM 880, manufactured by Uni-Solar. Half of the module was shaded along its diagonal. The power characteristic has several relative peaks and the eciency falls to about 20% of its non-shaded value.
Fig. 7. Current and power characteristics of a prototype thermophotovoltaic (TPV) generator consisting of 8 silicon half-cells. The power curve has a peak at 15.2 W. Under 909 W/m2 sunlight, the generator produces 5.6 W. The variations in the horizontal region of the current curve are due to the ¯ickering of the burner light. Under burner light, the ®ll factor fell from 0.75 to 0.68.
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of the module along its diagonal. The power characteristic has several relative peaks, which may lead to problems in the power tracking of grid-inverters. 3.5. Thermophotovoltaic generator To characterise thermophotovoltaic (TPV) generators [9], which convert burner light into electric power, the solar test facility including software was modi®ed so that characteristics can also be measured with ¯ickering light. Fig. 7 shows the characteristics of a small prototype TPV generator. The ¯ickering of the light in the horizontal region of the current characteristic can be seen clearly. 4. Conclusions/outlook An outdoor test facility for photovoltaic generators has been developed and put into operation successfully. A great number of cell and module tests have proven its capability and reliability. Measurements from this facility have been con®rmed by an internationally-approved test laboratory. Regular calibration of all measuring instruments guaranties continuous high quality and reliability of the test results. Due to the possibility of performing non standard tests under realistic operating conditions, a more practical picture of the performance of the various products can be provided. The test results are published or are available as internal reports. All tests are carried out in close co-operation with the manufacturers and suppliers of the test specimens, leading to an ecient transfer of the results. In future work, more emphasis will be laid on the cell's and module's behaviour under low light levels, cloud impacts and high air masses, as well as further re®nement of the correlations to describe these eects. Furthermore, it is intended to establish a comprehensive data base for some selected modules, and make it available on the internet. It is also planned to extend the investigations towards PV systems. As a ®rst step, measurements on three small grid-connected generators were started [10]. Acknowledgements The authors would like to express their gratitude to the Industrielle Werke Basel (IWB) for generous ®nancial support of the GraÈtzel cell measurements, and to the Holinger Solar, AG, CH-4410 Liestal, the Tritec AG, CH-4123 Allschwil and the IWS Solar AG, CH-8494 Bauma for providing the standard modules and for stimulating discussions. References [1] Durisch W, BuÈhlmann M, Kesselring P, Morisod R. Measurements and operational experience with a photovoltaic plant in the Swiss Alps. Proceedings ISES Solar World Congress 1987, vol. 1, Hamburg. Oxford: Pergamon Press. 1987. p. 289±97.
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[2] Durisch W, Urban J, Smestad G. Characterisation of solar cells and modules under actual operating conditions. In: Proceedings of the World Renewable Energy Congress, vol. 1, June 1996, Denver. Oxford: Pergamon, 1996. p. 359. [3] Durisch W, Keller J, Bulgheroni W, Keller L, Fricker H. Solar irradiation measurements in Jordan and comparison with Californian and Alpine data. Applied Energy 1995;52:111±24. [4] Durisch W, Hofer B. Klimatologische Untersuchungen fuÈr Solarkraftwerke in den Alpen, am Beispiel Laj Alv, Disentis, GraubuÈnden. PSI-Bericht Nr. 96-01, 1996. ISSN 1019 Ð 0643. [5] BuÈcher K, Heidler K. Physikalisches Verhalten und energetische Bewertung von Solarzellen mit einem oder mehreren UÈbergaÈngen unter realistischen Bezugsbedingungen. BMFT Status-Seminar Photovoltaik, 1993. [6] Anon. Schweizer Solarzellen unter den weltbesten. Elektrotechnik 1997;48(1):18. [7] Durisch W, WoÈrz A, Urban J, Tille D. Charakterisierung eines Photovoltaik-Grosslaminats. Bulletin SEV/VSE 1997;88(10):35±8. [8] Durisch W, Plapp W. Charakterisierung von GraÈtzelzellen unter realklimatischen Betriebsbedingungen. Bericht zu HaÈnden Industrielle Werke Basel, IWB, International Report, August 1997. [9] Durisch W, Panitz J-C. Development and characterisation of a rare earth emitter for a thermophotovoltaic power generator. PSI Annal Report/Annex V, 1996. p. 35. [10] Durisch W, Lavric R, Struss O. Drei Netzverbund-Kleinsysteme im Vergleichstest. Nationale Photovoltaiktagung, 5 May 1998, Bern, Switzerland.