Photovoltaic green roofs – On the role of experimental hydrology to feature the acceptance of interdisciplinary and sustainable solutions for both climate change mitigation and adaptation

While the added value of green roofs for mitigating rainfall extremes in urban drainage systems has been addressed in numerous studies, the microscale spatial redistribution of rainfall by solar panels (photovoltaic modules) mounted on green roofs and its impact on hydrology has hardly been studied so far. However, considering both green roofs and rooftop photovoltaic installations are emerging topics relevant for decision makers, since their combination supports both climate change adaptation (transforming grey to green infrastructure in order to cope with extreme rainfall in urban areas) and climate change mitigation (energy transformation). In the framework of an experimental study, we shed light on the hydrological and hydrodynamic effects of rooftop photovoltaic installations mounted on green roofs and how this contributes to the development of sustainable solutions in an interdisciplinary setting. Since solar panels redirect rainfall to the “green” fraction of the roof not covered by solar panels, the green roof part is in effect subject to higher rainfall and hence intensified surface runoff generation. Promising results were still obtained in a first investigation, where a photovoltaic green roof has been irrigated by a 100 years design storm with 27 mm over 15 minutes: the runoff coefficient (i.e., the percentage of rainfall that becomes runoff) at the end of the rainfall event amounts to only 23%, even though surface runoff occurred after 13 minutes. Based on this first investigation, a systematic measurement campaign has been launched to scrutinize the impact of the microscale spatial rainfall redistribution by solar panels on the runoff coefficient. In this presentation, we show the results of the first investigation along with results achieved in the systematic measurement campaign, which considers different vertical layer structures as well as various flow lengths and slopes of the photovoltaic green roof. In parallel, green roofs without photovoltaic rooftop installations are investigated alongside as a benchmark. In essence, our results suggest to consider both green roofs and photovoltaic rooftop installations to support both climate change mitigation and adaptation, which are important questions that decision makers are simultaneously confronted with. This way, this presentation highlights how experimental hydrology and interdisciplinary collaboration can contribute to address policy-related emerging research. Given that an obligation to install solar panels is expected in numerous countries, this kind of research might endorse new design approaches in future green roof design guidelines relevant for practitioners.