Switching daylight: Performance prediction of climate adaptive ETFE foil façades

This paper reports on the daylighting performance of switchable ethylene-tetrafluoroethylene (ETFE) foil in double-skin façades (DSF). In contrast to conventional glazing or static ETFE façades, switchable ETFE moderates incident daylight and controls internal light distribution by actively responding to weather conditions and solar light intensity. To better understand the light control function of ETFE and the impact of parameters such as climate, latitude and window-to-wall ratios (WWR), a validated optical model was used to evaluate different DSF designs. ETFE façades were modelled with a Bidirectional-scattering distribution-function (BSDF) and spectral data, obtained from experimental measurements, to accurately represent specular and diffuse light transmittance. Based on the five-phase method, a parametric climate data-driven simulation of an office room with different façade designs was conducted for three climate scenarios (Oceanic, Mediterranean, Sub-Tropical). When employing switchable ETFE in façades with different WWRs (30-90%), an annual increase of useful daylight illuminance (UDI) from 11 to 69% in the range of 500-2000lx was recorded. The calculated glare probability (DGPs) declined 59% in the best-case scenarios, providing working conditions with imperceptible glare for 94% of the scheduled time. Simultaneously, the daylight uniformity ratio (UR) increased up to 19% compared to a room with a conventional double-glazed façade. Significant improvements of daylight quality were achieved for façades with large windows in climates with abundant solar light available all year long. Overall, this study contributes to expanding the knowledge on adaptive membrane façades, demonstrating their capacity to enhance the daylighting performance of indoor spaces in different climates.