The effect of PV modules on green roofs
Researchers from Slovenia’s University of Ljubljana have developed a novel model for calculating energy fluxes on PV green roofs. Green roofs are rooftops that are covered with vegetation, such as vegetables, and PV might benefit from their cooling effect. Following the creation of their model, the team tested it against a real setup for validation.
“By accurately determining all heat fluxes, this study aims to improve evapotranspiration calculation, which is important for estimating the water consumption,” the group explained. “The approach of this study ensures that the findings can be utilized not only by researchers for further studies but also by industry specialists seeking to implement energy-efficient urban infrastructure solutions, effectively promoting urban sustainability goals.”
The model assumes an extensive green roof that consists of sedum vegetation, organic substrate and a mineral wool substrate layer. Based on available meteorological data and the PV setup parameters, it uses a simplified 2D view factor and advanced 3D approach to calculate how much sunlight and radiation the solar module blocks. With those calculations, it is then able to determine energy fluxes, such as shortwave and longwave radiation, soil heat flux, sensible heat flux, and latent heat flux.
“The novel aspect of this research is the configurable setup with varying shading proportions used to develop and validate the models, providing robustness for different photovoltaic green roof systems,” the academics added. “The longwave radiation model includes a developed parametric model for the green roof surface temperature with varying shading factors and publicly available calculation codes for shading and view factor determination in 3D geometry.”
To verify the accuracy of their model, the team constructed an experimental setup in Ljubljana, Slovenia. It included a green roof sample measuring 0.71 m on 0.71 m, consisting of sedum vegetation blanket with up to 2 cm of organic matter, 2 cm of mineral substrate mix (lava, pumice, zeolite), and 4 cm of lightweight mineral wool substrate. Two monocrystalline modules of 0.65 m in height and 0.505 m in width, were positioned above the green roof. They were 30 cm above it at the lower edge, and had a tilt angle of 25◦.
“The measurements were carried out in two phases: phase I is for the validation of the energy fluxes and evapotranspiration models, which was carried out between 28th June and 6th August 2024,” the group said. “Phase II was carried out between 21st and 28th August, when the longwave radiation instrument was turned facing down for the purpose of green roof surface temperature model development.”
Comparing the measurements from the experimental set-up to those of the model, incoming longwave radiation produced a normalized root mean square error (NRMSE) of 5.1%. Modeled evapotranspiration compared to the measured gave an NRMSE of 4.4% for daily values. They highlighted that those metrics demonstrate “high accuracy.”
“Experimental results reveal up to a 100 W m-² difference in incoming longwave radiation on green roof surface under photovoltaics compared to open-sky conditions, demonstrating the significant impact on the energy balance, the results further showed. “Neglecting longwave radiation exchange with photovoltaic modules would result in an 18 % underestimation of daily evapotranspiration.”
Their findings were presented in “Effect of solar photovoltaics on green roof energy balance and evapotranspiration,” published in Sustainable Cities and Society.