Failure and Degradation Modes and Rates of PV Modules in a Hot-Dry Climate: Results after 16 years of field exposure K. Yedidi, S. Tatapudi, J. Mallineni, B. Knisely, J. Kutiche, G. TamizhMani Arizona State University, Photovoltaic Reliability Laboratory (ASU-PRL) ABSTRACT This study evaluates the frameless modules of same type (model B) in two 16-year old photovoltaic power (PV) systems to ascertain degradation rates, reliability failure modes and safety failure modes which occur in a hot-dry climate. Each system is composed of 1512 modules. The average degradation rate is determined to be 0.85%/year for the best modules and 1.1%/year for all the modules (excluding the safety failed modules). Primary safety failure mode is the backsheet delamination though it is small (less than 1.7%). Primary degradation mode and reliability failure mode may potentially be attributed to encapsulant browning leading to transmittance/current loss and thermo-mechanical solder bond fatigue (cell-ribbon and ribbon-ribbon) leading to series resistance increase. Under the typical 20/20 warranty terms, 0.5-1.7% of the modules qualify for the safety returns, 73-76% of the modules qualify for the warranty claims and 24-26% of the modules are meeting the warranty terms. Index Terms – reliability, safety, degradation, O&M, hot dry desert, durability. I. INTRODUCTION The reliability and lifespan of PV modules depends heavily upon module construction and the climate in which it is installed. In today’s market, large scale PV investors and project developers routinely analyze the technology risk by focusing on reliability and durability issues. Higher safety failure, reliability failure and performance degradation rates of PV modules in power plants will have serious financial impacts and O&M costs. These failure and performance degradation rates are greatly dependent on the climatic condition of the site where the power plant is located. An extensive study performed by NREL indicates that the module degradation rate can be as high as 4%/year, but the median and average degradation rates are calculated to be 0.5%/year and 0.8%/year, respectively [1]. These degradation rates have been reported for a wide and highly diversified range of global climatic conditions. Arizona State University Photovoltaic Reliability Laboratory (ASU- PRL) has been investigating a large number of power plants with several thousands of modules for a single climatic condition: hot-dry desert climate. In our previous investigation on about 1,900 modules installed in Tempe, Arizona (a hot-dry desert climate) we reported a degradation rate ranging between 0.6%/year and 2.5%/year depending on the module model/manufacturer and number of years operating in the field [2]. The major degradation modes experienced in hot-dry climates have been determined to be solder bond deterioration and encapsulant discoloration [3]. The objective of this study is to evaluate a 16-year old (7 years on 1-axis tracker and 9 years fixed horizontal) power plant which has two systems (BRO-1 and BRO-2) with 1512 modules in each. This study was conducted to identify the major PV durability (degradation rate), reliability (failures) and safety (failures) issues both at a string level and a module level. All the details related to this paper is provided on a master’s thesis submitted to Arizona State University [4]. II. METHODOLOGY All I-V measurements were taken at an irradiance level of above 800 W/m 2 . These measured curves were then normalized to STC using the temperature coefficients obtained from the baseline I-V curves right at the field. Each and every one of the 3024 modules was inspected for defects using visual, IR and diode checking tools. A diode checker instrument was used to detect failed diodes and broken interconnects on a PV module without shutting down the power plant. The I-V measurements were performed on:  144 soiled strings  42 cleaned individual modules All the investigated modules were classified as safety failures, reliability failures and durability loss as defined in Figure 1. Based on the performance tests, the best, worst and median performing strings and modules were identified using a systematic approach presented elsewhere [5]. Figure 1: ASU-PRL’s Metric definition of Failures and Degradation