Optimal transparent percentage in façade modules for office buildings in a central Europe climate: a case study in Frankfurt

The building enclosure plays a relevant role in the management of the energy flows in buildings and in the exploitation of the solar energy at building scale. An optimized configuration of the façade can contribute to reduce the total energy demand of the building. Traditionally, the search for the optimal façade configuration is obtained by analyzing the heating demand and/or the cooling demand only, while the implication of the façade configuration on the energy demand for artificial lighting is often not considered, especially during the first stage of the design process. A global approach (i.e. including heating, cooling and artificial lighting energy demand) is instead necessary to reduce the total energy need of the building. When considering the total energy use in building, the optimization of a façade configuration becomes not straightforward, because non-linear relationships often occur. The paper presents a methodology and the results of the search of the optimal transparent percentage of a façade module for office buildings. The investigation is carried out for the four main orientations, on three “average” office buildings (with different surface-area-to-volume ratio), and with different HVAC system’s efficiency, located in Frankfurt. The results show that the optimal configuration, regardless of the orientations and the surface-area-to-volume ratio, is achieved in an “average” office building when the transparent component of the façade module is between 35% and 45% of the total façade module surface. The north-exposed façade is the one that presents the highest difference between the “optimal configuration” and the worst one, while the south-exposed façade is the one which suffers less in case of the “worst” configuration