Scientists have used a simplified 2D repeated factor and advanced 3D approach to calculate energy fluxing on green roofs with PV systems. They have also built an experimental arrangement to verify their model and have discovered that without the detailed model, evapotranspiration would be underestimated by 18%.
Researchers from the University of Ljubljana of Slovenia have developed a new model for calculating energy luxury on PV -green roofs. Green roofs are roofs that are covered with vegetation, such as vegetables, and PV can benefit from their cooling effect. After making their model, the team tested against a real set -up for validation.
“By accurately determining all heat fluxing, this study aims to improve the calculation of evapotranspiration, which is important for estimating water consumption,” the group explained. “The approach to this study ensures that the findings can not only be used by researchers for further studies, but also by industrial specialists who want to implement energy -efficient urban infrastructure solutions, which effectively promotes urban sustainability goals.”
The model presupposes 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 installation parameters, it uses a simplified 2D display factor and advanced 3D approach to calculate how much sunlight and radiation the solar module blocks. With these calculations it is then able to determine energy luxury, such as short-wave and long-wave radiation, soil heat flux, sensible heat flux and latent heat flux.
“The new aspect of this study is the configurable arrangement with different shadow ratios that are used to develop and validate the models, offering robustness for various photovoltaic green roof systems,” the academics added. “The long -golf radiation model includes a developed parametric model for the surface temperature of the green roof with varying shade factors and publicly available calculation codes for shadow and display factor determination in 3D geometry.”
To verify the accuracy of their model, the team constructed an experimental attitude in Ljubljana, Slovenia. It included a green roof sample of 0.71 m at 0.71 m, consisting of sedum vegetation blanket with up to 2 cm organic material, 2 cm mineral substrate mix (lava, pumice, zeolite) and 4 cm lightweight mineral woolen substrate. Two monocrystalline modules of 0.65 m high and 0.505 m wide were placed above the green roof. They were 30 cm at the top of the bottom edge and had a tilt angle of 25 °.
“The measurements were performed in two phases: phase I is for the validation of the energy luxury and evapotranspiration models, which was performed between June 28 and August 6, 2024,” the group said. “Phase II was performed between August 21 and 28, when the long -wave radiation instrument was aimed at being directed down for the purpose of the development of the green surface temperature model.”
Comparison of the measurements of the experimental arrangement with those of the model, incoming incoming long wave radiation produced a normalized root average square error (NRMSE) of 5.1%. Modeled evapotranspiration compared to the measured gave an NRMSE of 4.4% for daily values. They emphasized that those statistics demonstrate ‘high accuracy’.
“Experimental results reveal to a difference of 100 W M -² in incoming long-wave radiation on green roof surface under photovoltaics compared to open-sky conditions, which demonstrates the significant impact on the energy balance, the results further showed. “The neglect of long -golf radiation exchange with photovoltaic modules would result in an 18 % underestimation of daily evapotranspiration.”
Their findings were presented in “Effect of photovoltaic solar energy on the energy balance of green roof and evapotranspiration“Published in Sustainable cities and society.
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