Evaluation of the bifaciality coefficient of bifacial photovoltaic modules under real operating conditions

Among the parameters that define a bifacial photovoltaic module, the bifaciality coefficients indicate the rear and front side ratio of the most representative IV curve points of a photovoltaic panel, that is, Isc, Voc and Pm. However, these parameters are defined under the ideal Standard Test Conditions (STC). Therefore, to provide a realistic insight regarding the performance of bifacial modules, it is necessary to evaluate these coefficients under real operating conditions. For such purpose, an outdoor campaign was performed to experimentally measure the maximum power bifaciality coefficient of two modified bifacial modules that resemble a rear and a front monofacial panel respectively. As a first result, if the measurements are translated to STC, using a linear approximation, the bifaciality matches the value indicated by the manufacturer. Additionally, the operating bifaciality coefficient shows a linear decrease trend, proportional to the irradiance level decrease. This result implies that on cloudy days, the average bifaciality factor is below the corresponding one from sunny days. Finally, for irradiances below 200 W/m2, there is a non-linear increase in the bifaciality, with greater values than the corresponding to the ideal STC conditions, which presumably are caused by the non-linearity performance of photovoltaic modules at low irradiances.