Measuring the Effect of Vegetated Roofs on the Performance of Photovoltaic Panels in a Combined System

Experiments were conducted in summer using two identical photovoltaic (PV) panels at two heights using three roofing types: white, black, and green (vegetated). For experiments at an 18 cm height, the mean power output of the PV-green roof system was 1.2% and 0.8% higher than the PV-black and PV-white roofs, respectively. At a 24 cm height, the benefit of the green roof was slightly diminished with power output for the PV above a green roof being 1.0% and 0.7% higher than the black and white roof experiments, respectively. These results were consistent with measured variations in mean panel surface temperatures; the green roof systems were generally cooler by 1.5-3 °C. A unique aspect of this research is the investigation into the effect of vegetation on the convective cooling of the PV panels. Panel heat transfer coefficients for the PV-green roof were 10-20% higher than for the white and black roof configurations, suggesting a mixing benefit associated with the roughness of the plant canopy. While the best PV performance was obtained by locating PV above a green roof, the relative benefits diminish with distance between the PV and the roof.