Solar radiation and daily light integral characteristics of photovoltaic-green roof system in tropical area: A case study in Singapore

Photovoltaic roof and green roof are both sustainable building technologies. However, these two types of roofs may compete with each other in terms of space utilization. A novel solution that integrates photovoltaic panel with green roof (PVGR) has gained increasing attention in recent years, which can effectively transforms this competition into synergism. Solar radiation play a crucial role in the functioning of both types of roof, but there is limited research on the distribution characteristics of solar adiation for PVGR. Additionally, there are few studies on photosynthetically active radiation (PAR) or daily light integral (DLI) under the shading of PV arrays, which hinders the rational selection of plant species. To address this gap, a field experiment was conducted in Singapore. The results revealed that on a clear day, the maximum incident solar radiation and PAR on vegetation surface between PV arrays was up to 1136 W/m2 and 1876.6 mu mol/m2/s, respectively, whereas they were significantly lower at only 279 W/m2and 360.3 mu mol/m2/s below PV panels, respectively. Regression analysis demonstrated a highly linear relationship between DLI and solar radiation with an average ratio of 1.799. A numerical model for solar radiation transfer in PVGR was developed based on sky view factor calculated through ray tracing method. The model demonstrated good performance after comparison with observed values, and the effect of PVGR design parameters on view factors and average annual DLI of vegetated surfaces was clarified. The findings drawn herein contribute to optimizing PVGR design in terms of thermal performance and selection of plant species.