Analysis of the Relationship Between Photovoltaic Temperature Coefficients and Varying Solar Irradiance Levels

This work presents an experimental investigation into the temperature coefficients of aged monocrystalline photovoltaic (PV) modules operating under real outdoor conditions. The analysis is based on one year of high-resolution monitoring data from a grid-connected PV system composed of three subsystems, each consisting of 30 modules. The key electrical parameters-maximum power output (Pmax), voltage at maximum power point (Vmpp), and current at maximum power point (Impp)-were continuously recorded alongside environmental variables such as solar irradiance and wind speed. Data filtering was performed in accordance with IEC 61215 standards to ensure the reliability of results. Temperature coefficients were determined by applying linear regression to the electrical parameters as functions of module temperature, under various irradiance levels ranging from 100 W/m2 to 1200 W/m2. The results reveal that while TC_Vmpp remains relatively stable, TC_Impp displays increased variability at higher irradiance levels. Meanwhile, TC_Pmax exhibits a consistent linear decrease with increasing irradiance, supported by high correlation coefficients (R2 > 97%). These findings underscore the importance of site-specific and irradiance-aware evaluation of temperature coefficients, particularly for aged PV modules. They provide valuable insights into the determination of TC_Pmax based on a single irradiance level, allowing its estimation under other irradiance conditions-especially under standard test conditions (STC).