NCPRE has been studying the field performance and degradation in PV modules in Indian climatic conditions for over a decade. The four reports on the All-India Surveys of PV Reliability: 2013, 2014, 2016 and 2018 have received global acclaim. With every subsequent report, we have tried to improve our measurement and analysis procedures. One of the things that has a significant impact on the estimated degradation rates is the procedure used to translate the field-measured I-V data to the Standard Test Conditions (STC) of 25°C and 1000 W/m2. In all the surveys before 2020, NCPRE had used a ‘Modified IEC 60891 Procedure 1’ for STC translation. In 2020, we conducted a detailed comparison of all the major available translation procedures, including the newly published (2019) ‘Voltage Dependent Temperature Coefficient (VDTC)’ procedure. We showed that among all procedures, the VDTC procedure is the most accurate and robust against errors in the temperature coefficients given in the datasheets. Therefore, it was decided to re-process all the module I-V data included in All India Surveys 2014, 2016 and 2018 using VDTC procedure, and assess if our major conclusions are still valid. The figure below shows the histogram of the linear degradation rates in the All India Surveys using the older ‘Modified IEC 60891 Procedure 1’ and the newer ‘VDTC’ procedure. Over 2300 degradation rate samples are included in this figure. It is seen that the mean degradation rate by the new VDTC procedure (1.01%/year) is slightly less than that estimated by the earlier procedure (1.38%/year). However, this newer mean value of degradation rate (1.01%/year) is still considerably higher than that claimed by many module manufacturers (~0.6%/year), consistent with our earlier conclusion.

Another consistent conclusion that had emerged from every survey in the past was that the modules in hot climates degraded faster than the modules deployed in ‘non-hot’ climates. In order to assess the validity of this conclusion under the new VDTC procedure, we performed a similar analysis, and the results are shown below. It is seen thateven after using the VDTC procedure and combining all the 3 years’ data together, the modules in hot climates still showed higher degradation rates than those in non-hot climates. Thus, it could be concluded that though the specific values of the degradation rates have decreased slightly when we used the new and accurate VDTC procedure, the overall conclusions from the All India Surveys were still valid - even after combining all the data together and processing them using the VDTC procedure. The following paper based on this work was recently published: R. Korgaonkar, Y. R. Golive, R. Dubey, A. Kottantharayil, J. Vasi, and Narendra Shiradkar, “Longitudinal study of c-Si Photovoltaic module degradation rates in the field in India” Solar Energy, Volume 282, Pages 112908, https://doi.org/10.1016/j.solener.2024.112908