Modelling interconnections in c-SI Solar Photovoltaic modules for improved reliability in Kumasi in Sub - Saharan Africa

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The use of climate-specific temperature-cycling profile is critical in precisely quantifying the degradation rate and accurately determining the service fatigue life of the crystalline silicon photovoltaic (c-Si PV) module operating in various climates. A reliable in-situ outdoor weathering database is pivotal in generating the required climate-specific temperature cycle profile. This study concerns the prediction of the reliability of both SnPb and Pb-free solder interconnects in a c-Si PV module from a sub-Saharan Africa outdoor weathering conditions. The test site for this study is located at the College of Engineering, KNUST Ghana (latitude 6º 40" N and longitude 1º 37" W at an elevation of 250 m above sea level). The research utilizes a three-year (2012 to 2014) high-resolution data to generate temperature cycles profiles that are representative of the test site climate. Subsequently, the generated temperature cycles were used in numerical investigations to examine the impact of these temperature cycle loads on the creep damage in the solder used as cell interconnecting material. The study involved an initial determination of the accurate constitutive model of EVA (encapsulant) for thermo-mechanical analysis of the c-Si solar cell. Furthermore, the life (number of cycles to failure) of the interconnecting solders were predicted using Finite Element Analysis (FEA) software (Ansys 18.2). The Garafalo-Arrhenius creep model was used to study the creep behaviour of the interconnecting solders since creep is the main damaging mechanism in the solder. Finally, the effects of temperature dwells and ramps were investigated from the change in Accumulated Creep Energy Density (ACED) profiles at the respective load steps for temperature ramps and dwells. Analysis of the data on temperature variation and thermally induced stresses showed that the test site has a temperature profile with a ramp rate of 8.996, a hot dwell time of 228 minutes, and a cold dwell time of 369 minutes. Maximum and minimum module temperatures of 58.9 and 23.7, respectively; in a cycle time of 86400 s (24 hrs) were recorded. Results from the numerical study showed that the linear viscoelastic material model (LVMM) of EVA generated the most consistent response to the thermo-mechanical analysis. Additionally, life cycle prediction results of soldered interconnections from ACED revealed that SnPb solder interconnections are likely to last longer (23.4 years) under the sub-Saharan African test region compared with Pb-free solder interconnects (13.69 years). Finally, a study on the effects of temperature dwells and ramps on creep damage of interconnections showed that the temperature ramps (heating and cooling load steps) accounted for approximately 80% of the creep damage in the soldered interconnections.
A thesis submitted to the School of Graduate Studies of KNUST, Kumasi in partial fulfillment of the requirements for the award of the degree of Doctor of Philosophy (PhD) in Mechanical Engineering.
Modelling Interconnections, SnPb and Pb-free solder interconnects, College of Engineering, KNUST, c-SI Solar Photovoltaic Modules, Improved Reliability, Kumasi, Sub - Saharan Africa