Silicon Sensors vs. Pyranometers - Review of Deviations and Conversion of Measured Values

Many steps in a solar energy project - resource assessment, design, yield prediction, operation and maintenance - depend on accurate and reliable solar irradiance measurements. However, different types of sensors are used and have multiple applications, depending on their measurement. We have considered the main two types of sensors: thermal irradiance sensors (pyranometers) and silicon reference cells. In this study, first we present the differences in measurement results of the two sensors. This is accomplished by evaluating the global irradiance in the plane of array (POA) measured from both sensors installed in different photovoltaic systems (PV systems) around Germany. From this assessment, we enumerate the main factors that contribute to their differences: temperature dependence, angular response, spectral response and calibration deviation. Furthermore, a set of correction equations is applied to the silicon reference cell's data. These equations correlate the two sensors and the factors that contribute to their deviation (obtained from the data analysis, previous studies and the sensor's technical specifications). Finally, we evaluate the correction model with a new set of data, achieving a reduction of the differences between sensors (Root Mean Square Deviation RMSD) of around 38%.