Comparing Impacts of Different Rooftop Technologies for Mitigating Urban Heat Islands and Reducing Building Energy Consumption in an Alpine City

In recent years, different mitigation strategies have been increasingly adopted in cities as a primary measure to reduce the undesirable effects of urbanization, being able to improve resiliency and maintain livable urban areas. In this context, this work focuses on evaluating the capability of roof photovoltaic panels (RPVP), green roofs (GR) and cool roofs (CR) to improve thermal comfort and decrease energy consumption for air conditioning. Various simulations with the Weather Research and Forecasting (WRF) model with 300-m grid spacing are performed for a clear-sky summer case study (17-20 July 2016) for the city of Trento, in the Italian Alps. The Building Effect Parameterization (BEP) and the Building Energy Model (BEM) are used in the urban environment to obtain a realistic representation of the climatic alterations induced by the city and of the buildings energy consumption. Novel parameterization schemes have been developed for GR and RPVP and implemented in the BEP model. In the RPVP parameterization the surface temperature is calculated from the energy budget of the panel, considering its radiative interaction with the underlying conventional roof. On the other hand, the GR parameterization evaluates the interactions of multi-layer green roofs with the buildings and the atmosphere. The impact of these rooftop technologies is assessed through the comparison with a conventional roof simulation that is validated against air temperature, humidity and wind measurements from urban meteorological stations. Results show that CR are the most efficient strategy in improving thermal comfort, while RPVP, due to their higher temperature with respect to conventional roofs, tend to increase air temperature during daytime. The impact of all mitigation strategies on urban canopy air temperature is greater for urban zones with large and low buildings. Results highlight that GR are an effective solution to reduce buildings energy consumption, due to the presence of insulating layers. A similar effect is shown by RPVP, whose electricity production can also be used to supply the building energy requirements. These rooftop technologies can be considered as competing ways to improve cities sustainability, especially during heat wave periods.