Analysis of rear side irradiance non-uniformity for bifacial photovoltaic characterisation in outdoor conditions

The crystalline silicon (c-Si) bifacial photovoltaic (PV) market has grown significantly and is expected to occupy 70% of the PV market share in 2030. This is due to the increased power output of bifacial PV compared to the monofacial counterpart, as it can absorb photons from both sides. The characterisation of these devices indoors and outdoors is described by the recently published IEC TS 60904-1-2. In outdoor characterisation, the highly changeable environmental factors such as irradiance uniformity on the rear side and temperature represent a challenge. Though the technical specification describes the positioning and type of sensors to measure the irradiance uniformity on the rear side, the 10% non-uniformity limit has shown to be unachievable in outdoor conditions for some locations. This study performs an evaluation of the outdoor conditions experienced in Edinburgh and analyses the feasibility of the recommendations proposed by the technical specification. Two different module heights (0.5m and 1.0m) were tested as well as different ground surfaces (concrete and soil and white tiles) in order to measure the irradiance non-uniformity on the rear side. The results showed that the installation of a white reflective ground cover did not improve the rear side irradiance non-uniformity. The best results were found for the setup with concrete ground cover and ground clearance height of 1.0m, which showed nonuniformity levels below 10% from 9AM to 2PM. The analysis in this work describes the challenges of outdoor conditions and will serve to ensure a correct characterisation of bifacial PV modules in outdoor conditions.