Study of Photovoltaic System Performance Degradation Under Tropical Operating Conditions in Rivers State

Photovoltaic systems installed in the humid tropical environment of South South Nigeria are subject to accelerated degradation from high humidity, temperature cycling, salt aerosol deposition, and biological growth on module surfaces. Understanding degradation rates is critical for accurate energy yield prediction over the 25-year design life of PV installations. This study evaluates the performance degradation of 15 grid-connected and 8 off-grid PV systems installed between 2018 and 2022 in Rivers State, using periodic I-V curve tracing and visual inspection. Performance ratio, maximum power output, and fill factor are measured at six-month intervals and normalised to standard test conditions using on-site irradiance and temperature measurements. Electroluminescence imaging is applied to identify cell-level defects including micro-cracks, delamination, and hot spots. Results indicate an average annual degradation rate of 0.82 percent per year for crystalline silicon modules, somewhat higher than the manufacturer-warranted rate of 0.5 percent per year. Modules with EVA encapsulant discolouration and soiling accumulation show degradation rates exceeding 1.2 percent per year. The study recommends enhanced soiling monitoring and cleaning protocols for PV systems in the humid tropics. Keywords: photovoltaic degradation, performance ratio, tropical climate, Rivers State, I-V curve