Further refinement of an anisotropic model for inclined surface solar irradiation

An anisotropic model for predicting inclined surface solar irradiation has previously been reported by the author. The modelling exercise was furthered by analysing one-year measured hourly irradiation data, for five tilted surfaces, for Lerwick (6P08”). The optimum values of the coeficients for Lerwick were found to be near those reported earlier for Easthampstead (51”2YN). The approach of separating sunlit surfaces from shaded ones and distinguishing between overcast and non-overcast sunlit surfaces was found to be fruitful again. A statistical test was employed for the validation of the isotropic and the presently proposed model for Lerwick. The isotropic model proved to be quite inadequate. The model now proposed was found to be adequate for the three cases although the plot between measured and computed sloped irradiation showed that the model for sunlit surfaces under non-overcast conditions could be further improved by incorporating the change of turbidity over the year. A similar plot prepared with Easthampstead data for an inclined surface reconfirmed the validity of the model established earlier. NOTATION parameter describing the radiance distribution of the sky (dimensionless) modulating function used in equation (4) (dimensionless) [ = (I,- ID)/IE] angle between the normal to a surface and the incident solar beam (rad) hourly diffuse irradiation on a horizontal surface (MJ/m2 h) hourly diffuse irradiation on a surface, tilted at an angle of P from horizontal (MJ/m2 h) hourly extraterrestrial irradiation on a horizontal surface (MJ/m2 h) hourly global irradiation on a horizontal surface (M J/m2 h) tilt factor, used in equation (4) (dimensionless) solar altitude (rad) angle of tilt of a surface from horizontal (rad)