Dependence on solar elevation of the performance of a light shelf as a potential daylighting device

The performance of a light shelf, with a reflectance of about 91% after excluding the specular component (reflectance about 1%), providing for solar protection during the long, hot, dry summer season, is studied at Madrid (40.4° N, 4.4° W) using two scale models (1:10) with rectangular openings facing south, one taken as a reference and the other equipped with the light shelf As a shading device, the light shelf was designed for a vertical shade angle of 500, and thus had both, internal and external parts. The models were painted inside: walls and floor with a black matt paint, and the ceiling white matt. Measurements of mean hourly illuminances were obtained in each model at points at the same distances from the openings and corresponding to the working plane. In the present work we study for the points closest to the walls opposite the openings, and for days with clear skies, the dependence on solar elevation α of the ratio η = ElEr, El and Er respectively being the mean hourly illuminances measured in the model with the lightshelf and in the reference model.