Photovoltaics and green roofs: Holistic analysis in built environments

The European Union has emphasized policies promoting photovoltaic (PV) energy generation to achieve the United Nations' Sustainable Development Goals 7 and 13. Notably, building roofs suitable for PV panels also present opportunities for passive energy-saving methods, such as green roofs. Both approaches impact beyond buildings to the urban level; PV panels intensify the urban heat island (UHI) effect, while well-irrigated green roofs mitigate it. In the Mediterranean region, where cities face challenges from extreme weather events and droughts leading to water restrictions, a comprehensive analysis of the influence of these approaches at both the building and urban levels becomes crucial. This work addresses this gap by employing dynamic simulations of a typical Mediterranean roof, an extensive green roof and a summer-irrigated green roof, all with and without PV panels, under Mediterranean climate. While both green roofs and PV systems prove beneficial at the building level, only irrigated green roofs effectively reduce the UHI impact. Unirrigated green roofs show no benefit on the UHI, whereas PV panels consistently amplify it. Combining an unirrigated green roof with PV panels has the highest UHI impact among all analyzed roof types. Summer irrigation of the extensive green roof can compensate the additional convective heat flux by PV panels, and moreover enhancing heat loss through the roof—a beneficial aspect at the building level during summer. The findings underscore the complexity of defining strategies that meet goals for renewable energy and UHI mitigation, highlighting the need for further research in this area.