Influence of the Natural Wind Speed on the Performance of Solar Thermal Façades
Abstract
The solar thermal activation of the building envelope offers untapped potential for novel solutions based on renewable energy in multi-story buildings. Activated façades designed as uncovered collectors are particularly suitable for the support of heat pump-based supply systems as they can gain heat from both solar energy and ambient air thus achieving high yields at low temperatures. The characterisation of such façades requires the modelling of the thermal performance in dependence of the wind speed. The wind speed in front of the façade is influenced by various factors such as the surroundings, the microclimate, thermal effects as well as the size of the façade. In order to model the wind dependency more accurately than in standard tests for solar thermal collectors, we carried out detailed wind field measurements on a 26 m2 large active metal façade. The measurements showed that the flow behaviour in front of the façade is strongly affected by the wind direction. For wind directions parallel to the façade, we observed an almost linear velocity gradient in the vertical direction, while for wind directions perpendicular to the façade we reported a homogeneous distribution. The comparison of the measurements to wind data recorded by a nearby weather station suggests a scaling factor of 0.3 to 0.6. Based on our experimental results we performed an adjusted efficiency measurement of the metal façade and compared the yearly energy output calculated with both parameter sets. The difference is remarkable in low temperature ranges relevant for heat pump applications (i.e. 7.5% at 0°C) but negligible for operation temperatures above 10°C.
Keywords
Solar thermal façades, Building envelope, Solar collector measurement, Wisc, Multi-story residential buildings, Building integrated solar thermal