Vertical bifacial PV systems: irradiance modeling and performance analysis of a lightweight system for flat roofs Mari B. Øgaard, Vilde Stueland Nysted, Sigrid Rønneberg, Gaute Otnes, Sean Erik Foss, Trygve Mongstad and Heine N. Riise EPJ Photovolt., 15 (2024) 13 Published online: 11 April 2024 DOI: 10.1051/epjpv/2024012