Estimation of hourly, daily and monthly mean diffuse radiation based on MEO shadowring correction

A statistical estimate model for the anisotropic diffuse fraction as a function of clearness index K-T is proposed to estimate hourly, daily and monthly diffuse irradiations. Global, diffuse and direct solar irradiances were provided by the Laboratory of Solar Radiometry of Botucatu-UNESP (latitude 22.9 degrees South, longitude 48.45 degrees West, altitude 745 m). The period assigned for the study comprised the years 1996 to 2002. Global solar irradiance was measured by an Eppley PSP pyranometer, direct irradiance by an Eppley Nip pyrheliometer and diffuse irradiance by an Eppley PSP pyranometer under the Melo-Escobedo-Oliveira shadowring (radius of 40cm and width of 10cm). Isotropic and KT corrections were applied in diffuse irradiance. The proposed model was compared to classic models reported in the literature, with good results according to the MBE and RMSE statistical indicators for hourly, daily and monthly partitions, respectively. The results showed that the inclusion of KT correction improved the performance of the shadowring MEO according to MBE values for the three partitions: hourly, reduction from -7.21% to -1.74%; daily from -4.70% to 0.88% and monthly from -6.58% to -1.18%. (C) 2014 Published by Elsevier Ltd.