Experimental validation and comparison of direct solar shading calculations within building energy simulation tools: Polygon clipping and pixel counting techniques

This paper presents an experimental validation procedure of two solar shading calculation techniques - pixel counting (PxC) and polygon clipping (PgC) - and an inter-software comparison to highlight the capabilities and efficiency of each solar shading calculation method. For the first purpose, digital images were taken from the surfaces of small-scale mock-ups specially constructed to generate experimental data for validating simulation results obtained by cases using three different tools: EnergyPlus (PgC based), Shading II SketchUp plug-in (PxC based) and Domus (PxC based). This first task has shown, for prototypes with simple geometries, that all techniques present results in good agreement with the experimental data. However, for a prototype with a hollowed shading device, the PgC-based technique produced results far from the experimental ones since it is not appropriated to simulate multi-hollowed polygons. In order to further explore the capabilities of the two shading calculation techniques, an inter-software comparison has also been carried out for a complex case, considering different building shading solutions, including non-planar trees. The results, in general, have shown that the PxC technique is not limited to geometrical complexities and leads to an accurate and a very fast assessment of sunlit surface fraction. It has also been shown a difference as high as 10 times on the prediction of a daily-integrated solar heat gain by using the two different techniques.