Estimating Hourly Direct and Diffuse Solar Radiation for the Compilation of Solar Radiation Distribution Maps

Abstract

This paper presents a new method for estimating hourly direct and diffuse solar radiation. The essence of the method is the estimation of two important factors related to solar radiation, atmospheric transmittance and a dimensionless parameter, using empirical and physical equations and data from general meteorological observation stations. An equation for atmospheric transmittance of direct solar radiation and a dimensionless parameter representing diffuse solar radiation are developed. The equation is based on multiple regression analysis and uses three parameters as explanatory variates: calculated hourly extraterrestrial solar radiation on a horizontal plane, observed hourly sunshine duration and hourly precipitation as observed at a local meteorological observatory. The dimensionless parameter for estimating a diffuse solar radiation is then determined by linear least squares using observed hourly solar radiation at a local meteorological observatory. The estimated root mean square error (RMSE) of hourly direct and diffuse solar radiation is about 0.0∼0.2 MJ·m-2·h-1 in each mean period. The RMSE of the ten-day and monthly means of these quantities is about 0.0∼0.2 MJ·m-2·h-1, based on comparisons with AMeDAS station data, located at a distance of 6 km.