Evaluation of “Active Cavity Transition” (ACT) Facade modelling based on Measured Data of a 1:1 Scale Test Mock-up

The idea of a so-called “Active Cavity Transition” (ACT) Facade is to extract the exhaust air from the cavity between the glazing and the screen and thereby to improve both energy consumption and thermal comfort compared to a “classical façade” with internal screen. Thermal models based on ISO 15099 (ISO, 2003) are commonly used for detailed glazing modelling. Such models are available in building performance simulation software. This paper investigates the extent to which a model based on the ISO 15099 model with a BSDF optical data model is suitable for simulating the thermal behavior of ACT facades.

Detailed measurements at 1:1 test mock-up under natural weather conditions were performed to investigate the functionality of ACT facades and data for model evaluation. The air permeability of the screen and the screen perimeter play a critical role in ensuring the exhaust air rate to is adequate for ventilation requirements of the room in and avoids an undesirable backflow of heated air from the cavity into the room. The test series include internal screen with different materials and perimeter gaps in combination with a sun protection glazing. A black screen with high solar absorption was measured as an extreme case in terms of thermal conditions. In addition, a white/grey fabric with medium solar absorption and a white fabric with low solar absorption were investigated. To expand the selection of possible screen materials for ACT façades, a guide rail with brush piping was examined instead of a ZIP guide. In addition, smoke tests were used to determine the flow pattern for specific cases.

The measured data sets show a large variance e.g., a significant vertical temperature spread of 35 to 49 °C for the black screen with irradiation levels of up to 900 W/m² as well as smaller temperature increase of 25.5 and 27.5 °C for white screen with irradiation of up to 550 W/m² on the façade. In neither case was an undesirable backflow of warm air from the façade cavity into the occupiable space through the guide rail detected.

The measured boundary conditions for the different test cases are used as input for detailed dynamic simulation of the measured façade. First results of the comparison show that the model is capable to predict the trend correctly but overestimates the screen temperature. Critical modelling parameters are further investigated, and findings will be incorporating to the model.