Urban green technologies for energy saving: numerical simulation of heat transfer between green facades and green roofs and the local outdoor environment

The last decades, a robust urbanization and climate change effects generated an increase in environmental problems in water management, energy consumption, air pollution, biodiversity, warming at urban level. According with the UN Sustainable Development Goal approach, our cities and human settlements have to become "more inclusive, safe, resilient and sustainable". In other words our cities have to become "greener", considering the help in that sense that can derive by a widespread application of urban green technologies. These technologies, as green roofs or green walls, can represent a very important tool in terms of thermal insulation, heat island effect reduction and in general energy saving at building and district scale. In recent years the mechanisms that govern exchanges between green walls (roofs and facades) and outdoor environmental air in terms of heat and mass transfer have been studied more and more in the literature. The approaches mostly used for these studies are experimental local measurements in order to characterize heat and mass transfer between the building and the environment, while theoretical studies, especially numerical studies, are still lacking. In this work, after a general overview related with environmental benefit deriving from Green Technologies at urban level, a short study results are proposed related with heat and mass transfer between green facades and green roofs and the local environment around buildings. As a case study, the transpiration cooling of vegetation as a measure to mitigate outdoor air temperatures is investigated in the case of an experimental green roof that is present over a building, in the laboratories area, inside the Campus of the Bologna University Engineering School.