Investigation of temperature coefficients of PV modules through field measured data

•Temperature coefficients of PV modules are determined through long-term field data.•Spectral correction estimates temp coefficients of Isc and Pmax closer to STC values.•M-K test performed shows no monotonic trend of temp coefficients after 8-year exposure.•Temp coefficients for all parameters of the tested c-Si modules have not degraded. Varying broadband irradiance and temperature are generally known as the major factors influencing the performance of PV modules, but studies have also shown the substantial impact of spectral variations. In this work, a simple and efficient method to calculate the temperature coefficient using long term data is demonstrated. Temperature coefficients of PV modules are estimated from long term performance data following IEC 60891 standard with additional spectral correction, and are compared against the datasheet values. Significant improvement of correlation coefficient from −0.89 to −0.97 is observed during the regression for maximum power temperature coefficient of two poly-crystalline modules, after spectral correction by spectral factor (SF). Also, the standard deviation of yearly estimated values of these coefficients reduced from 5–7 % to 1–2 %. In another setup involving spectral measurements and various PV technologies, the annual mean of 1.62 eV for average photon energy in 350–1700 nm range, suggests a general blue shift of the spectrum. Higher averages than reference values of useful fraction (UF) for c-Si, CIGS and HIT technologies also validate the blue shift of spectrum. Results show SF produces maximum power temperature coefficients closer to the datasheet values compared to UF, suggesting better applicability of SF as an index for spectral correction. The coefficient values were found closer to STC values and the results from Mann and Kendall test, employed to detect any underlying monotonic trend in the development of temperature coefficients over eight years, showed no increasing or decreasing trend and hence no degradation of temperature coefficients for the long-term exposed PV modules.