Challenges in the pre-normative characterization of bifacial photovoltaic modules

Abstract Bifacial photovoltaic (PV) technology is receiving growing interest on the market, with several companies commercializing bifacial modules alongside their conventional products, with various c-Si structures, such as PERC, PERT, HIT, IBC, etc. Current-voltage characterization of bifacial PV modules at Standard Test Conditions (STC) is challenging, as it requires an extension of the definition of STC that includes the spectral and total irradiance on the rear side of the module. Indoor characterization in principle allows the most accurate control of these quantities: the simplest procedure involves measurements of each side of the bifacial module at a time with a single light source, while carefully keeping the other side in the dark. This procedure requires attention to minimize unintended contributions to the module power from the non-illuminated side, which is associated to the reflected irradiance from surroundings, to the optical properties of the module, and to the geometrical disposition of the cells. This work revises the pre-normative activity that has led to the draft standard IEC 60904-1-2 (due for publication in 2018) and focuses on the new norm in what concerns indoor measurement with a single-source simulator. It illustrates the metrological challenges in the selection of the non-reflective material behind the non-illuminated side, and their impact to the electrical characterization of bifacial modules.