A field-validated optical model of static booster reflectors for fixed photovoltaic modules: Irradiance non-uniformity, material effects, and seasonal gains at a Mediterranean site

Static reflectors below a fixed photovoltaic module raise its output at almost no cost, yet their benefit is usually reported as one percentage from a short campaign and scaled to the year, with the reflector treated as a uniform boost. This work builds a first-principles optical model of a booster reflector, validates it against a controlled field sweep, and derives design guidance transferable beyond the test site. The campaign, at a Mediterranean site, swept reflector tilt, length and material, pairing every reading with a concurrent bare-panel reference on the same meter. A height-resolved ray trace with a diffuse view-factor term predicts the irradiance added along the module. A five-parameter electrical model of the half-cut cell topology then converts that profile into power, so series-string mismatch is computed per configuration rather than assumed. The model reproduces the measured mirror response across four tilts and two lengths to 0.84 percentage points, at a geometric transfer factor of 0.81. Mismatch proves configuration dependent, holding the power gain to between 0.6 and 0.9 of the average-irradiance gain rather than to one fixed fraction. A rigid glass mirror follows flat-reflector optics; a thin aluminum sheet does not, its tilt-rising gain being consistent with mild concentration from surface non-flatness. Measured gains reach 4.7 to 10.4 percent in autumn , and the model concentrates them in the months when the Lebanese grid is most stressed, giving an annual 2.9 to 5.1 percent. Optimum reflector tilt follows 62 degrees minus site latitude.