Advancing reliability assessments of photovoltaic modules and materials using combined‐accelerated stress testing

Abstract

Previously undiscovered failure modes in photovoltaic (PV) modules continue to emerge in field installations despite passing protocols for design qualification and quality assurance. Failure to detect these modes prior to widespread use could be attributed to the limitations of present‐day standard accelerated stress tests (ASTs), which are primarily designed to identify known degradation or failure modes at the time of development by applying simultaneous or sequential stress factors (usually two at most). Here, we introduce an accelerated testing method known as the combined‐accelerated stress test (C‐AST), which simultaneously combines multiple stress factors of the natural environment. Simultaneous combination of multiple stress factors allows for improved identification of failure modes with better ability to detect modes not known a priori. A demonstration experiment was conducted that reproduced the field‐observed cracking of polyamide‐ (PA‐) and polyvinylidene fluoride (PVDF)–based backsheet films, a failure mode that was not detected by current design qualification and quality assurance testing requirements. In this work, a two‐phase testing protocol was implemented. The first cycle (“Tropical”) is a predominantly high‐humidity and high‐temperature test designed to replicate harsh tropical climates. The second cycle (“Multi‐season”) was designed to replicate drier and more temperate conditions found in continental or desert climates. Testing was conducted on 2 × 2‐cell crystalline‐silicon cell miniature modules constructed with both ultraviolet (UV)–transmitting and UV‐blocking encapsulants. Cracking failures were observed within a cumulative 120 days of the Tropical condition for one of the PA‐based backsheets and after 84 days of Tropical cycle followed by 42 days of the Multi‐season cycle for the PVDF‐based backsheet, which are both consistent with failures seen in fielded modules. In addition to backsheet cracking, degradation modes were observed including solder/interconnect fatigue, various light‐induced degradation modes, backsheet delamination, discoloration, corrosion, and cell cracking. The ability to simultaneously apply multiple stress factors may allow many of the test sequences within the standardized design qualification procedure to be performed using a single test setup.

Towards validation of combined-accelerated stress testing through failure analysis of polyamide-based photovoltaic backsheets

“…Mechanical loading up to the equivalent of a 2400 Pa load on 60-cell modules is applied through the vertical displacement of ring-shaped actuators on the front surface of the test samples. The displacement determination has been previously described 16 .…”

“…Thermoelastic material properties are implemented for EVA and backsheet based on measured values along with secant coefficients of thermal expansion 22 , while linear elastic properties are used for the glass, silicon, and metallic materials Boundary conditions include a 1.1 mm fixed displacement in the negative y-direction applied to the loading ring (Fig. S2 ), to force the mini-module to attain a similar curvature as a full module experiencing a − 2400 Pa pressure load 20 , 23 both at a 25 °C reference state and at − 20 °C and 90 °C to match C-AST Tropical cycle temperature states 16 . Simulations were run using the SIERRA/Adagio code 24 .…”

“…Conventional accelerated stress testing (AST) protocols, such as those specified in the IEC standard 61,215-series 15 , failed to reproduce the cracking failure of AAA observed in the field. Recently, more advanced accelerated-stress testing protocols, such as the combined-accelerated stress test (C-AST) 16 or the module-accelerated stress test (MAST) 17 have demonstrated the ability to reproduce the observed field-failure of AAA. C-AST and MAST employ combined and/or sequential stress protocols to better show PV module weaknesses.…”

“…When not only failure modes but failure mechanisms can be reproduced, there is greater confidence in the testing methods and conviction that the test-induced failure is not the result of unrealistic stress application. For the C-AST method, this is of particular importance as the experimental design is based on the philosophy that no stress levels are applied that could be deemed unrealistic or unobservable in the natural environment 16 .…”

Development and Analysis of a Modular LED Array Light Source

“…However, some research fields require a modular and easily adjustable array design. For example, analyzed samples in PV can range from individual polymeric material samples [22], laminates [23], or research cells [24] over mini-modules [25] to full-sized commercial modules [26]. The required irradiated areas range from a few square millimeters to several square meters.…”