Diffuse solar radiation on a horizontal surface: Reviewing and categorizing the empirical models

Abstract

Accessibility to the accurate knowledge of diffuse solar radiation is a fundamental prerequisite for designing and monitoring the solar energy technologies. Nevertheless, the measured diffuse solar radiation is not available in most regions across the globe. During the past 60 years in order to estimate horizontal diffuse solar radiation on both daily and monthly mean daily basis, numerous empirical models have been developed or established for many locations around the world. For this purpose different parameters have been utilized and various functional forms employed. This study aims at providing a review on the diffuse solar radiation models developed, established and proposed thus far. To put forward a proper appraisal, the models are primarily categorized on the basis of how to correlate: (1) the diffuse fraction or cloudiness index (i.e. the ratio of diffuse solar radiation to the global solar radiation), and (2) the diffuse coefficient or diffuse transmittance index (i.e. the ratio of diffuse solar radiation to the extraterrestrial global solar radiation) with different inputs. Both categories, which in terms of the utilized inputs as well as the functional forms constitute 56 different models are re-classified into several main sub-categories and then presented in a chronological manner. This review would be profitable for solar energy researchers in terms of identifying the parameters and the functional forms widely utilized up till now as well as recognizing their significance. Accessibility to either solar and/or meteorological data varies from one location to another; thus, the appropriateness of the models to estimate the horizontal diffuse solar radiation depends on the availability of such inputs at a desired location. Additionally, to attain proper accuracy and reliability of estimation in a desired location the inputs should be selected based upon the fact that how much they can provide strong correlation with diffuse solar radiation.