Evaluating the impact of urban morphology on rooftop solar radiation: a new city scale approach based on Geneva GIS data

Building rooftops represent one of the most valuable resources to harvest solar energy in cities. Nevertheless, this potential is limited by the urban morphology impacting the shading conditions. This study suggests a general methodology to assess the impact of urban form on solar harvesting. To this aim, a new GIS-based approach is developed to extract meaningful morphological parameters at a very large scale. The rooftop overall shading rate is here defined as a benchmark, and it is measured through a scaled insolation representing the ratio between the insolation of a surface within the urban context and its unshaded theoretical maximum. A set of 40 morphological features is calculated for 60,000 buildings in the Canton of Geneva (Switzerland), and the scaled solar insolation of about 350,000 roof pieces is derived from the Solar Cadaster of Geneva. The results outline the insolation distribution within the city and as a function of urban morphology. The rooftop overall shading rate shows moderate Pearson coefficients (r = 0.2/0.4) towards some parameters, namely building height, volume, and height difference with surroundings, while others seem irrelevant. Analysing the 48 Geneva municipalities one at a time, the denser downtown areas reach higher correlation levels (r = 0.4/0.6) compared to the suburban ones.