Effect of solar photovoltaics on green roof energy balance and evapotranspiration
Tej Žižak, Sašo Medved, Ciril Arkar
Abstract
• Energy fluxes on photovoltaic green roof are investigated and modelled in detail. • Longwave radiation of photovoltaics significantly impacts energy balance. • Shading and view factor calculation in 3D is developed and shared. • Parametric model for photovoltaic green roof surface temperature is developed. • Without the detailed model evapotranspiration would be underestimated by 18 %. Photovoltaic green roofs represent an emerging technology that combines on-site renewable energy production with the environmental benefits of green roofs. Detailed models for calculating energy fluxes on photovoltaic green roofs are presented, relying on accessible meteorological data and setup geometry as input data. A key focus is the accurate modelling of longwave radiation, an often-overlooked component of energy balance. 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 precise modelling of short and longwave radiation is achieved through proposed shading and view factor calculation methods, as well as with a developed parametric model for green roof surface temperature under varying shading factors. Neglecting longwave radiation exchange with photovoltaic modules would result in an 18 % underestimation of daily evapotranspiration. A complete detailed model for evapotranspiration calculation is proposed, achieving a 4.4 % normalized root mean square error for daily predictions. By accurately estimating energy fluxes and evapotranspiration, this study provides tools for quantifying water needs, optimizing the synergy between green roofs and photovoltaics, and assessing their broader impacts on urban microclimates.