Statistical assessment of a model for global illuminance on inclined surfaces from horizontal global illuminance

Abstract

Olmo et al. [Energy 24 (1999) 689] have recently proposed a simple model to estimate global irradiance on inclined planes, which only requires the horizontal global irradiance and the sun elevation and azimuth as input parameters. From now on, this model will be referred to as the Olmo model. Statistical assessment of this model can be considered as important, taking into account that available models for estimation of global irradiance or illuminance on an inclined surface require information of global, and direct or diffuse irradiance or illuminance on a horizontal surface. The version of the Olmo model for global illuminance is tested in the present work using mean 15 min values of global illuminance obtained with 20 sensors of different slopes (zenith angles) and azimuths. The sensors were placed on a spherical dome located at one of the corners of the roof of the experimental site and ground shielded by black mat painted honeycomb material. Assuming a value of the honeycomb albedo of 0% values of the obtained RMSD go from about 8% for surface slopes of 12° to about 30% for a vertical surface facing east. For a north facing vertical surface, receiving mostly diffuse illuminance, a value of about 52% is obtained for the RMSD. Assuming a value of the albedo of 5%, too high for our experimental set up, similar results are obtained. In general the model over estimates global illuminance on inclined surfaces in Madrid, for experimental global illuminance values higher than about 60 klux.