Evaluation of vertical illuminance and irradiance models against data from north Bangkok

This paper presents results of a study on measurement of illuminance of daylight and solar irradiance on four cardinal orientations (N, E, S, W). Performances of mathematical models for computing value of vertical illuminance and irradiance proposed by various authors were investigated. The functional form of Perez's model was selected for developing localized models of vertical irradiance and illuminance. Sets of local coefficients of Perez's model were derived from measured data taken from a station at the Asian Institute of Technology (AIT), located north of Bangkok. Several models were examined and it was found that Perez's model gave more accurate results when its coefficients were derived from our experimental data. Accuracy of the results could be further improved when two separate models are used for a vertical surface, one model for the case when the sun is in front of the surface and another model for the case when the sun is at the back of the surface.