TESTING AND MODELLING OF THE ANGULAR-DEPENDENT SOLAR HEAT GAIN OF GLAZING WITH VENETIAN BLIND SHADING

As overheating problems in glassy buildings came up more and more, EMPA put a focus on the determination and modelling of the total solar energy transmittance (TSET) of multiple glazing combined with different shading systems within the framework of IEA Task 27 "Performance, durability and sustainability of solar faade components". Experimental data were produced by a calorimetric outdoor test facility near Zurich (Switzerland). In a first stage, average constant TSET values related to the incident global solar radiation were identified for a various configurations and a measuring period of a few days each. As expected the results strongly depend on the slat tilt angle as well as on the solar reflectance of the slat surface. Naturally the solar transmittance of an angle-selective component like a Venetian blind also depends on the directional distribution of the incident radiation, in particular on the sun elevation projected into the plane perpendicular to the slat rotation axis. Therefore, results from different measuring periods of the identical shading-glazing configuration are compared, and an extension of the usual average g-value identification is shown.
The experimental results are also compared with weighted average g-values from numerical modeling, which is based on a view factor method extended for circular slats in a louver type shading device. In general the agreement between measured and calculated results is good. Some deviation is explained by a significant nondiffuse Albedo component in the hemispherical radiation field. In another paper it was shown that the simplified convective heat transfer calculation used in this model may fail for mechanically ventilated double envelope faades.