Direct sunlight method (DSM)

Ingo Kroeger

The direct sunlight method (DSM)

For the DSM method the direct component of the sunlight is taken as the light source. Therefore, 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. In order to block all diffuse light component, the DUT must be mounted in a collimator tube with a viewing angle identical to the. For secondary calibration, the reference would be another calibrated reference solar cell with an equivalent collimator tube or a pyrheliometer. For primary calibration the reference is a cavity radiometer traceable to the world radiometric reference (WRR). Additionally, a calibrated spectroradiometer with a collimator with identical viewing angle 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 within the viewing angle of the instruments.
2. Point the DUT, reference and the spectroradiometer directly to the sun. Determine solar elevation and azimuth angle. 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 via the reference, the short circuit current of the DUT, the spectral irradiance distribution and the temperature readings for DUT and reference.
5. 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.
6. Apply a temperature correction on the measured current if the DUT, if the temperature coefficient of the DUT is known.
7. Scale the measured current at given irradiance to 1000W/m² and correct for non-linearity of DUT and reference if non-linearity is known.

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. [5-9].

References

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

[5] C.R. Osterwald, K.A. Emery, D.R. Myers, R.E. Hart “Primary reference cell calibrations at SERI: History and methods” Proc. 21^st IEEE PVSC Orlando, FL, May 21-25 1990, 1062-1067.

[6] K.A. Emery, C.R. Osterwald, L.L. Kazmerski, R.E. .Hart “Calibration of primary terrestrial reference cells when compared with primary AM0 reference cells" Proc. 8th European PVSEC, Florence, Italy, May 9-12 1988 p. 64-68.

[7] C. Osterwald, K. Emery "Spectroradiometric Sun Photometry" Journal of Atmospheric and Oceanic Technology, 17 (200) 1171-1188.

[8] ASTM E 1125 “Standard test method for calibration of primary non-concentrator terrestrial photovoltaic reference cells using a tabular spectrum”.

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