Chapter 543 - Evaluation of Models to Estimate Average Hourly Direct From Hourly Global Solar Radiation

Abstract

This chapter discusses the experimental results that states that the choice of a correlation to estimate direct solar radiation can be dependent on both place and seasonal factors and consequently, more than one correlation can be used. Four years of solar data from the Solar Radiation Laboratory of UNESP at Botucatu/SP-Brazil were used to test correlations to estimate hourly direct from hourly global solar radiation. Global and direct radiations were measured using an Eppley PSP piranometer, and an Eppley NIP pirheliometer. Five minutes average of the solar irradiances was saved using a Campbell 21X datalogger. SIMRAS software was used to obtain hourly values by integration of the irradiances and also to generate hourly values of the estimated direct solar irradiances. The estimated values of hourly direct solar radiation were compared to the observed ones using the statistical tests: Correlation Coefficient (R), Mean Bias Error (MBE), Root Mean Square Error (RMSE), and the student test derived from both RMSE and MBE. The deviation values were separated in ranges according to sky cover conditions and in two periods in the year.