Comparison of characterization methods for bifacial PV devices contained within the IEC TS 60904-1-2.
Juan Lopez-Garcia, Robert P. Kenny, Ebrar Ozkalay, Laura Pinero-Prieto, David Shaw, Tony Sample
European Commission, Joint Research Centre (JRC), Ispra, Italy

Bifacial crystalline Silicon (Si) photovoltaic (PV) devices are attracting considerable interest in the market since they can enhance the performance in comparison with traditional PV devices. Since there is currently no standard for bifacial PV modules, one of the main issues that need to be addressed is the standardisation of bifacial measurements for PV devices. IEC TS 60904-1-2, close to be published includes several characterization methods for bifacial PV devices based on single-side and double-sided illumination approaches: measurement at equivalent irradiance levels, double-source simulator, rear reflectors or mirrors, outdoor measurements, etc. This paper analyses and compares the electrical performance obtained by different characterization methods used at the European Solar test Installation (ESTI) for the testing of a bifacial Si mini-module. The aim of the work is to analyse the advantages, disadvantages and the suitability of the different methods contained within the technical specification. It is observed that different approaches lead to different results with a deviation from the reference method (single-side illumination with equivalent irradiance levels method) lower than 0.7% for the double-sided illumination method using a double-source (pulsed solar simulator + LED rear bias light) and the outdoor approach. A higher overestimation of Pmax with respect to the reference method (up to +2.6%) was obtained for the double-sided method with a rear reflector. This influences the module testing, mainly increasing the uncertainty in the measurements. However, In general, an acceptable output power agreement was obtained for the different characterization methods with variations that can be included in the uncertainty calculation depending on the particular approach. Further measurements are ongoing in other devices in order to validate the methods.