Global sunlight method (GSM)

Ingo Kroeger

The global sunlight method (GSM)

For the GSM method the global component of the sunlight is taken as the light source. Also for this method, a two axis sun-tracker is needed to properly point DUT and reference directly to the sun. Both devices must be temperature controlled and kept at 25°C. For secondary calibration, the reference would be another calibrated reference solar cell or a pyranometer. Additionally, a calibrated spectroradiometer with an entrance optics for global spectral irradiance is recommended to measure the relative spectral irradiance distribution for spectral mismatch correction. The calibration procedure would be as follows:

1. Measurement should only be performed at clear sky conditions. Discard data that was taken when clouds or haze has been observed. Care should be taken to avoid strong reflecting objects in the vicinity of the measurements.
2. Point the DUT, reference and the spectroradiometer directly to the sun. Determine solar elevation, azimuth angle and atmospheric pressure. Using this data and the geographical position to determine the airmass value (AMx).
3. Wait until reference and DUT show a stable temperature reading 25°C ± 2°C.
4. Measure (in an optimum case simultaneously) the irradiance level via the reference, the short circuit current of the DUT, the spectral irradiance distribution and the temperature readings for DUT and reference.
5. Apply a temperature correction on the measured current if the DUT, if the temperature coefficient of the DUT is known.
6. Scale the measured current at given irradiance to 1000W/m² and correct for non-linearity of DUT and reference if non-linearity is known.
7. Apply the spectral mismatch correction factor on the measured current of the DUT, if the spectral responsivity of DUT and reference differ significantly or if the spectral irradiance distribution differs significantly from the AM1.5g spectrum. Alternatively, you may perform a linear regression of the measured short circuit current over the air mass value AMx and determine the short circuit current value for AM1.5.

Repeat this procedure several times across the day and repeat these measurements for at least 2 more days. Average calibration values and identify and remove outliers. More detailed information on this method can be found in Ref. [10-20].

References

[4] IEC 60904-4:2009, Photovoltaic devices - Part 4: Reference solar devices - Procedures for establishing calibration traceability

[10] K.A. Emery, C.R. Osterwald, L.L. Kazmerski, and R.E. Hart, (1988c), Calibration of Primary Terrestrial Reference Cells When Compared With Primary AM0 Reference Cells, Proceedings of the 8th PV Solar Energy Conference, Florence, pp. 64-68.

[11] K. A. Emery, C.R. Osterwald, S. Rummel, D.R. Myers, T.L. Stoffel, and D. Waddington, “A Comparison of Photovoltaic Calibration Methods,” Proc. 9th European Photovoltaic Solar Energy Conf., Freiburg, W. Germany, September 25-29, 1989, pp. 648-651.

[12] K.A. Emery, D. Waddington, S. Rummel, D.R. Myers, T.L. Stoffel, and C.R. Osterwald, “SERI Results from the PEP 1987 Summit Round Robin and a Comparison of Photovoltaic Calibration Methods,” SERI tech. rep. TR-213-3472, March 1989.

[13] Gomez, T, Garcia L, Martinez G, "Ground level sunlight calibration of space solar cells. Tenerife 99 campaign," Proc. 28th IEEE PVSC, 1332-1335, (2000).

[14] J. Metzdorf, T. Wittchen, K. Heidler, K. Dehne, R. Shimokawa, F. Nagamine, H. Ossenbrink, L. Fornarini, C. Goodbody, M. Davies, K. Emery, and R. Deblasio, “The Results of the PEP '87 Round-Robin Calibration of Reference Cells and Modules,- Final Report” PTB technical report PTB-Opt-31, Braunschweig, Germany, November 1990, ISBN 3-89429-067-6.

[15] H. Müllejans, A. Ioannides, R. Kenny, W. Zaaiman, H. A. Ossenbrink, E. D. Dunlop “Spectral mismatch in calibration of photovoltaic reference devices by global sunlight method” Measurement Science and Technology 16 (2005) 1250-1254.

[16] H. Müllejans, W. Zaaiman, E. D. Dunlop, H. A. Ossenbrink “Calibration of photovoltaic reference cells by global sunlight method”, Metrologia 42 (2005) 360-367.

[17] H. Müllejans, W. Zaaiman, F. Merli, E. D. Dunlop, H. A. Ossenbrink “Comparison of traceable calibration methods for primary photovoltaic reference cells” Progress in Photovoltaics 13 (2005) 661-671.

[18] F.C. Treble and K.H. Krebs, “Comparison of European Reference Solar Cell Calibrations”, Proc. 15th IEEE PV Spec. Conf., 1981, pp. 205-210.

[19] R. Whitaker, G. Zerlaut, and A. Purnell, “Experimental demonstration of the efficacy of global versus direct beam use in photovoltaic performance prediction of flat plate photovoltaic modules”, Proc 16th IEEE PVSC, pp. 469-474, 1982.

[20] A Fehlmann, G Kopp, W Schmutz, R Winkler, W Finsterle, N Fox, metrologia 49 (2012) S34