Hydrothermal Performance of Partial Photovoltaic Shading on a Sedum Green Roof

Abstract

In photovoltaic green roofs, the electricity generation takes place over the vegetated surface via solar-electric process, whereas the thermal behavior is governed by evapotranspiration, substrate heat storage, and vegetation exposure under the effect of solar radiation. In the following section, a measured data set of sedum green roof in Ljubljana with elevated photovoltaics will be analyzed based on the comparison of three shade conditions of the vegetation surface: unshaded, partially shaded, and fully shaded. The Hydrothermal Constraint Number depends on evapotranspiration similarity, the vegetation-temperature correlation adjusted according to shading, photovoltaics’ correction to the evapotranspiration in the longwave range, and daytime heat-flux fraction to the heat flux reference. The measured evapotranspiration rate for 9 July 2024 is 3.98 mm day⁻¹, and the modeled value is 4.15 mm day⁻¹; for 25 July 2024, they are 4.08 mm day⁻¹ and 3.95 mm day⁻¹; and for 3 August 2024, they are 2.60 mm day⁻¹ and 2.73 mm day⁻¹. Thus, the corresponding RMSE is 0.145 mm day⁻¹, the mean absolute error is 4.15 %, and the fidelity coefficient is 0.959. The vegetation-temperature correlation, θ_v = 0.935θ_a + 0.011R_g(1 − S), implies that every reduction of the short-wave shade-sensitive irradiation by 100 W m⁻² results in the decrease of the solar thermal component of approximately 1.1 °C. Photovoltaics correction to the evapotranspiration by the long-wave exchange leads to the change in evapotranspiration.