Anisotropic Diffuse Radiation Model of Photovoltaic Systems

Abstract

The diffuse solar radiation is a component of the global solar radiation and may contribute a significant percentage (50% and above in some locations) of the electric output energy of Photovoltaic (PV) systems. Therefore, it is important to improve the diffuse radiation models to obtain accurate values of the incident diffuse radiation. Solar diffuse radiation models are based on isotropic and anisotropic models. The anisotropic models take into account also the circumsolar and horizon brightening. Both types of models are developed in the literature for single rows. As photovoltaic fields are deployed in multiple collector rows, the second and subsequent collector rows are partially obscured by the rows in front and thus, affecting the incident diffuse radiation on the collectors. The present article modifies an anisotropic model by two factors: (a) sky view factor of the second (and subsequent rows) collector row and (b) correction to circumsolar brightening (correction to horizon brightening in negligible, at the site of the study, and is not considered). The modified model is demonstrated on Klucher model for diffuse radiation. The study shows that the anisotropic model provides higher annual diffuse radiation by 20% than the isotropic model, higher incident global energies of about 5.5%, and the difference in the incident diffuse radiation for collectors with azimuth angles in range ${\gamma }_C=0^0$ to ${\gamma }_C={30}^0$, is negligible. The use of the modified anisotropic model allows to asses more accurately the contribution of the diffuse radiation to the generated electric energy.