Numerical analysis of a double skin façade with integrated movable shading systems

Abstract

Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.Double skin façades have become an important and increasing architectural element in office buildings as they can provide numerous advantages such as energy saving, sound, wind and pollutant protection with open windows, solar preheating of ventilation air, night cooling and aesthetics. In the present study, preliminary results of the fluid dynamics behavior of a glass double skin façade equipped with integrated movable shading devices are presented; the aim is to optimize both winter and summer energetic performances. The model is developed for a façade oriented towards the south direction and taking into account the climatic data of Italy. The double-skin façade constitutes an optical-energetic system with several different layers; therefore, a spectral modelling is necessary, considering that materials have optical properties heavily dependent on wavelength. A spectrophotometric campaign on glasses and opaque materials has been conducted to evaluate transmittivity, reflectivity and absorptivity of each layer, investigating also their variations with light incidence angle. Optical characteristics constitute the input data for a computational fluid dynamics code, whose task is to model the façade cavity airflow that results from many simultaneous thermal, optical and fluid flow processes, which interact and are highly dynamic. The solar radiation path with its multiple reflections at the different interfaces have been taken into account, employing a ray tracing method, integrated in the CFD code. The simulation shows that the winter configuration of the proposed façade enhances the solar heat gain, in spite of the presence of shadings, placed however in horizontal position. Heat is stored in the air that flows in the gap and its motion could be driven by mechanical ventilation or by natural convection; buoyancy driven flows resulted difficult to model because of small driving forces that lead to numerical instabilities. Besides, a strong thermodynamic coupling emerged between the air flow through the naturally ventilated double skin façade and the air temperature difference between the cavity and outside. Solar gains in buildings are desirable in winter-time, but problematic in summer, as they may cause overheating and discomfort; for this reason the external layer remains open in the hot season, giving the air the possibility of escaping from the gap and blocking, at the same time, the solar irradiation by the shading devices, configured with a high tilt angle. Results from the CFD package show that the air flow in summer conditions, although limited by the absence of the stack effect, contribute significantly to reduce the heat absorbed by the inner pane, so reducing building cooling loads.