Glare potential evaluation of structured PV glass based on gonioreflectometry

Estimating glare is becoming an increasingly important step in the planning stage of PV installations. This is especially true for building-integrated photovoltaics (BIPV), where atypical orientations and tilt angles can cause reflections in unusual locations. Currently, simulation tools perform simplified ray-tracing calculations to determine specular reflections from PV installations in their environment, often based on reflectance fit functions. In this work, gonioreflectometric measurements are used to determine the bi-directional reflectance distribution function (BRDF) of PV mini-modules with different surface glasses. They show significant differences between smooth and structured glass surfaces, with satinated glass showing overall lowest reflectance. At medium to high incidence angles, however, reflections are not only higher than at normal incidence, but may also peak at different view angles than the specular reflection angles. This indicates, that the common methodology of simulating only specular reflections may underestimate glare. It furthermore suggests, that a low number of reflectance fit functions may be insufficient to describe possible glare from structured surfaces, as they can exhibit vastly different BRDFs, despite possibly similar specular reflectance fit functions. In addition, calculations based on retinal irradiance threshold values for eye damage show that even for PV modules with flat glass, temporary retinal damage in form of flash blindness is highly unlikely. In any case, retinal irradiance values are far below values required for retinal burn damage, limiting reflections from PV installations to disability or discomfort glare levels. Even for the lowest reflecting surfaces, i.e. satinated glass, reflectances at incidence angles beyond 50° are high enough to possibly cause uncomfortable reflections, requiring ray-tracing simulations for accurate estimations of glare.