Technical note: An investigation of possible improvements in accuracy of regressions between diffuse and global solar irradiation

Abstract

Horizontal hourly and sub-hourly diffuse and beam irradiance are required for the estimation of global irradiance on a given tilted surface. This information is used by engineers and architects for various solar energy applications and also to calculate solar heat gain in buildings. Although horizontal global irradiance is a commonly measured parameter for many sites, horizontal diffuse irradiance is not so readily obtainable. For such sites that measure global irradiation alone a simple but reasonably accurate method is required to estimate diffuse irradiance from its global counterpart. The aim of this study is to evaluate the accuracies of estimation of diffuse irradiance by developing annual, seasonal and monthly regressions between sub-hourly diffuse-ratio and clearness-index. An error analysis has been undertaken to compare these regression models. Practicalapplications: Diffuse and beam components of global irradiation are required to accurately simulate solar heat gain in buildings. The UK Meteorological office keeps records of diffuse and global irradiances for 9 stations, but there are a further 84 stations that solely record global irradiance. Hence there is a need to estimate diffuse or beam irradiance for these sites. This paper presents an investigation into the uses of annual, seasonal and monthly regressions between diffuse and global horizontal sub-hourly solar irradiation. These regressions have proven to be a simple but reasonably accurate method of estimating diffuse irradiance from its global counterpart.