Improved Methodology to Measure Normal Incident Solar Radiation with a Multi-pyranometer Array

An improved methodology to estimate the normal incident solar radiation based on an anisotropic clear sky model using a Multi-Pyranometer Array (MPA) is analyzed in this paper. The MPA is a static platform, which is used to measure solar energy irradiation at different tilt and azimuth angles, and that can be used to estimate the normal incident component without the tracking devices that require more detailed installation and maintenance. The array's sensors are of the photovoltaic type, which require both the spectral and incidence angle corrections. These corrections were made by empirical functions; the one for the incidence angle is generic and the spectral effect is satisfied by calibrating each of the sensors with a Precision Spectral Pyranometer (PSP) with a four parameter change- point linear function. For this study, the calibrated data was verified by calculating their corresponding theoretical value based on the beam component from solar tracking Normal Incidence Pyrheliometer (NIP) and the diffuse component from a Black and White Pyranometer (B&W), which were located on the same test bench. The improved procedure is based on grouping the measurements of two sensors and analytically look for a satisfactory solution; one for the horizontal and south sensors and one for the east and west sensors. The solution for the east and west sensors is expanded to two new solutions by mirroring their readings according to the solar noon; each real measurement and its mirrored values generate a solution. The solutions are grouped to have a more steady and robust expression. A final switching scheme provides the value that was compared with the reference NIP measurement. The analysis of the results showed that the root square mean error (RMSE) of the sample decreased by a 30% in comparison with previous results reported in the literature. The data sample used in this study correspond to clear sky conditions on the fall and winter seasons, so further comparison on other seasons and with regular solar conditions is required.