Angle of Incidence Measurements on Bifacial PV Modules

The current international standard EN IEC 61853-2 presents an outdoor and an indoor measurement method for the determination of angular losses [1]. Both methods determine the short circuit current Isc(θ) of the PV device in the angular range from -90° to +90° with a maximum step size of 10°. The results are used to calculate the incident angle modifier IAM(θ) and the angular response curve AR(θ) with fitting parameter ar. Angle of Incidence (AOI) measurements are usually carried out with indoor solar simulators which lead to certain problems for commercial sized modules due to light field non-uniformity when tilting the PV devices. To overcome these problems the standard allows for testing smaller-size and optically equivalent modules, to isolate electrically one cell within the module (destructive method) or to partially shade one cell in the module (non-destructive method). We show measurement results conducted with an outdoor method, utilizing direct sunlight for AOI measurements while most of the diffuse light is shaded. The advantage of this outdoor measuring method is that full-sized modules can be measured without limiting them to a particular cell, getting the most realistic values for the full module. We performed measurements on the front and rear side of seven bifacial modules of different product designs and cell technology. During rear side-measurements we observed partial cell row shading from the module frame. Although this shading effect has no influence on the output of the short-circuit current Isc(θ) it does have an influence on the output of the maximum power point Pmpp(θ). The fitting parameters ar from our rear side measurements show a factor of approximately 2 to 3 between ar rear Isc and ar rear Pmpp resulting in significantly different energy yield calculations for the rear side. The traditionally used ar rear Isc gives the wrong result. This effect cannot be characterized by the methods given by the current standard EN IEC 61853-2, since all methods rely on Isc for the calculation of IAM(θ) and using just a particular cell for those measurements only produces correct values if there is no self-shading of the module apparent. Our results lead to the conclusion that for rear side AOI measurements on bifacial modules with a frame, the self-shading effect must also be considered.