Consideration of error sources for outdoor performance testing of photovoltaic modules

This paper gives an appraisal of the error sources associated with outdoor testing and rating of photovoltaic modules. The scope encompasses instrumentation used to measure meteorological parameters critical to the assessment of module performance such as broadband and spectral irradiance, monitoring of module operating temperature and the measurement of d.c.-side current and voltage. The impact that each measurement uncertainty has on the extraction of module performance indicators is evaluated, with a view to effect targeted improvements in the quality of outdoor PV measurements. The approach follows developmental work on the COMS (CREST Outdoor Monitoring System) project, aiming to improve overall measurement accuracy and obtain parameter uncertainties necessary for performance modeling work. Extensive work has been done on characterizing elements of the previous version of the system through error limits given in supplier literature and our own calibration methods for each instrument. Impact on the final measurement uncertainty is given by standard error propagation calculations. The effectiveness of this consideration of errors is quantified by a second error analysis, that of the upgraded system.