Uncertainty in PV Module Measurement—Part I: Calibration of Crystalline and Thin-Film Modules

This paper presents recent progress in reducing the measurement uncertainty for crystalline silicon (c-Si) and thin-film PV modules. It describes the measurement procedure and the uncertainty analysis, as applied at the CalLab PV Modules, Fraunhofer ISE´s laboratory for module measurements. The uncertainty analysis covers the complete calibration process in detail, including measurements, correction to standard testing conditions, and determination of electrical module parameters (I_SC, P_MPP, V_OC, etc.) from the I-V curve. Differences between c-Si and thin-film modules are addressed, most importantly in terms of spectral mismatch factor and short timescale stability problems. The paper outlines the importance of a comprehensive quality assurance system in a calibration laboratory as a prerequisite for accurate measurements on a daily basis. Particular attention is paid to results from a series of measurements taken every three weeks over a three-year period and conducted as part of the quality assurance system. In conclusion, this paper introduces a best-case uncertainty for c-Si module calibration of 1.6% for P_MPP and 1.3% for I_SC. This represents the lowest reported uncertainty for full size module calibration in a laboratory so far. The presented uncertainty in P_MPP of cadmium telluride and single-junction amorphous silicon modules is 2.9% and 1.8%, respectively. All mentioned uncertainties are expanded uncertainties (k = 2).