Total solar energy transmittance of glass double façades with free convection

Double façades made of glass layers with a ventilated mid-pane shading device are often used in commercial buildings. The solar-collector-like construction leads to high temperatures in the façade cavities during summertime and the possible overheating of the building. Of key significance for the cooling load of the building and thermal comfort of its occupants is the total solar energy transmittance to the interior. A procedure for modeling such façades—comprising a spectral optical and a computational fluid dynamic (CFD) model—is described and simulation results are compared with data derived from an experimental investigation of a single-story glass double façade (GDF) with free convection, incorporated in an outdoor test facility. The influence of the layer sequence and ventilation properties on the thermal behavior is discussed. It is shown that, for a given set of layers, total solar energy transmittance can easily vary by a factor greater than five. Hence, for reliable prediction of the total solar energy transmittance of a designed façade, models are needed that factor in all the relevant parameters. A spectral optical model combined with a CFD model that includes convection, conduction and radiation, is therefore recommended for analyzing and optimizing glass double façades.