Characterisation of n-type bifacial silicon PV modules Juan Lopez-Garcia*, Bereket Haile, Diego Pavanello, Alberto Pozza, Tony Sample European Commission, DG JRC, Institute for Energy and Transport, Renewables and Energy Efficiency Unit, Via Fermi 2749, 21027 Ispra (VA), Italy *Tel: +39 0332783720, Fax: +39 0332789268, e-mail: juan.lopez-garcia@ec.europa.eu ABSTRACT: Bifacial PV modules have shown the potential to increase the performance of traditional photovoltaic module architectures using relatively conventional silicon manufacturing techniques. Bifacial modules can produce additional energy from the rear surface of the module, collecting not only light incident on the front surface but also sunlight scattered or reflected from the ground or environment onto the rear surface. To assess their performance and quality, PV modules are characterized using international standards to rate the modules in terms of output power and efficiency. However, at present, no specific standards exist for bifacial modules. Furthermore, these modules are often characterized by a high efficiency and an increased capacitance and the measurement with standard pulsed solar simulators may be affected by strong measurement artefacts resulting in increased or reduced maximum power. This work presents the indoor characterisation of bifacial c-Si PV modules under varying irradiance levels using different methods such as a single-sweep flash simulator, the so-called multi-flash (MF) method and a steady state sun simulator for the accurate measurement of the module parameters. The effect of different rear surface coverings, from matt black painted wood to gloss white card and the differences among the characterisation method are also investigated. Keywords: Bifacial; PV Modules; High-efficiency; Indoor testing; 1. INTRODUCTION To reduce the cost of solar power generation, the main focus areas of the photovoltaic (PV) industry are improving the solar cells and module efficiency, lifetime, cost and PV module's energy yield. Bifacial PV modules can produce additional output energy in comparison to conventional mono- facial modules because both sides of the cell/module, front and rear, can absorb solar radiation, utilizing the scattered light from the ground and surroundings. This capability of converting light from either side of the module into electricity makes these high-efficiency bifacial modules suitable to reduce the cost of PV electricity [1-3]. Replacement of a conventional free standing glass-backsheet module by a glass- glass module with bifacial cells can already result in an up to 20% annual energy gain, depending on the albedo [4]. To assess their performance and quality, PV modules are measured and characterised in a controlled environment under standard test conditions (STC) as defined by the International Electrotechnical Commission (IEC) [5]. Although the IEC is currently working on the development of a draft technical specification (IEC TS 60904-1-2) for the measurement of current-voltage characteristics of bifacial photovoltaic devices [6], there is currently no standard. Therefore, different groups and laboratories have proposed different methods [1,7]. As the PV modules are generally sold according to the nominal power output and efficiency under STC conditions, the lack of bifacial standards leads each manufacturer to claim different amounts for the "added value" of the bifaciality that makes it difficult for customers to directly compare between bifacial manufacturers. In absence of standards, many of the bifacial PV module manufacturers measure using only front-side illumination by a conventional pulsed simulator while covering the rear side with a non-reflecting cover, and vice versa, and tabulate the efficiency/power with a linear addition of the front and rear side values for particular rear side irradiance conditions [1,3,8-10]. However, most of the literature reports are on outdoor measurements where the environment and reflecting surfaces are usually not under control but they enable a "real value" of the bifacial module performance to be determined for a specific set of local environmental conditions [8-11]. Moreover, the indoor testing of high-efficiency and highly- capacitive bifacial PV c-Si modules with standard pulsed solar simulator may be affected by strong measurement artefacts and lead, depending on the pulse duration and sweep speed, to under or over-estimations of the measured power. These modules usually require solar simulators with a pulse duration longer than 100-200 ms in order to make a single sweep measurement of an I-V curve. Alternatively, the I-V curve can be obtained using the so called multi-flash (MF) method or steady-state solar simulators (continuous light) and slow sweep speeds in order to avoid measurement artefacts and obtain accurate electrical parameters [12-14]. This work reports on the indoor performance testing of highly-efficient n-type silicon bifacial modules by means of different characterisation methods and/or solar simulators and rear covers. The objective is to show the differences between the alternative methods and the influence of opaque or reflective rear covers in the electrical parameters of the c-Si bifacial PV modules. 2. EXPERIMENTAL METHOD 2.1. I-V measurement setup. A selected number of modules (ESTI codes QV61, QV62 and QV66) from a batch of 8 commercially available bifacial n-type c-Si glass/glass modules consisting of 60 cells in three sub-strings, which have been light soaked to eliminate possible light induced degradation (LID), were characterized indoors at the European Solar Test Installation (ESTI). I-V curve measurements were performed with a PASAN IIIB pulsed simulator (with sweep times or flash duration of the order of 10 ms) at different irradiances from 100 W/m 2 to 1200 W/m 2 and with a steady state solar simulator (Apollo) with single I-V sweep times of around 1 second, at a temperature of 25.0 ± 0.1 °C. The measurements using the flash simulator were performed using a standard forward sweep single flash method and a multi-flash (MF) method in which 35-40 individual points are taken to construct the final I-V curve. In this case, each point of the I-V curve was obtained by applying a constant voltage to the module which was then subjected to a flash from the solar simulator at the desired irradiance level. With this method, capacitive effects are avoided by considering only stabilized current and voltage output values for each measurement. The single I-V point is obtained from the stable portion of the measured current and voltage of the module. 32nd European Photovoltaic Solar Energy Conference and Exhibition 1724