An experimental and modeling study of evapotranspiration from integrated green roof photovoltaic systems

Co-locating green roof (GR) systems with photovoltaic panels (PV) can allow optimal use of roof space for energy production as well as stormwater management. Models for evapotranspiration from integrated Green Roof Photovoltaic Systems (GR-PV) are needed for the design of GR-PV systems. Existing evapotranspiration models can not be used for this purpose due to complex sunlight-shading and wind-sheltering effects. PV panels partially block direct solar radiation onto underlying GR modules affecting evapotranspiration (ET) rates. Additionally, PV panels can cause wind profiles to differ from the profiles assumed in existing ET models. In this study, ET rates from an unsheltered GR module were compared with ET rates from GR modules sheltered by an overhead PV panel. ET rates were lower in the sheltered GR module than in the unsheltered GR module. A new ET model was developed incorporating reduction factors to apply to net radiation and wind speed beneath an array of PV panels. The reduction factors are based on the length, height, slope, and spacing of PV panels, as well as the diurnal characteristics of sun and wind. Predictions of ET rates from the new ET model compared well to ET rates determined from lysimeter measurements from GR modules below an array of PV panels. The sensitivity of the new GR-PV system ET model for the key design parameters, including PV height, slope, length, and row-spacing was investigated. The ET rates are most sensitive to the height above the GR and the length of PV panels in GRPV systems.