The use of bidirectional scattering distribution functions for solar shading modelling in dynamic thermal simulation: a results comparison

The evaluation of cooling needs and summer comfort in buildings requires an accurate quantification of solar gains through window systems (glazing and solar shading). However, in dynamic thermal simulation tools, the modelling of shading devices is usually simplified: angular dependency of properties is neglected; shading devices are considered as perfect diffuser; internal reflections between the slats of a venetian blind are neglected or considered only as diffuse. In some cases these simplifications bring significant inaccuracies on the evaluation of solar gains entering the building, and, consequently, on the building thermal behaviour. Bi-directional Scattering Distribution Functions (BSDF) allows a more detailed modelling of materials and systems which enhances the characterisation of the window behavior. The use of these functions appears to be an appropriate way to consider optical angular properties of shading devices in dynamic thermal simulations: it increases the accuracy of results without requiring excessive computation time. This paper shows and discusses the results obtained when using-directional Scattering Distribution Functions in dynamic thermal simulation, for six common configurations of solar shading devices (external screen and venetian blinds of different characteristics), in comparison with those obtained by simplified modelling methods. The Window 7 software was used to generate BSDF data that were then used as input in the EnergyPlus V8 building energy simulation program. Results focus on transmitted solar energy through windows (double low E glazing) and cooling needs of a simple office room for the different solar shading devices. In conclusion, this study shows for several solar shading types (and their characteristics), the importance of considering optical angular properties in dynamic thermal simulation and the accuracy gained obtained by BSDF characterisation