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

Abstract

Novel methods for advancing reliability testing of photovoltaic (PV) modules and materials have recently been developed. Combined-accelerated stress testing (C-AST) is one such method which has demonstrated reliable reproduction of some field-failures which were not reproducible by standard certification tests. To increase confidence and assist in the development of C-AST, and other new testing protocols, it is important to validate that the failure modes observed and mechanisms induced are representative of those observed in the field, and not the product of unrealistic stress conditions. Here we outline a method using appropriate materials characterization and modelling to validate the failure mechanisms induced in C-AST such that we can increase confidence in the test protocol. The method is demonstrated by applying it to a known cracking failure of a specific polyamide (PA)-based backsheet material. We found that the failure of the PA-based backsheet was a result of a combination of stress factors. Photo-oxidation from ultra-violet (UV) radiation exposure caused a reduction in fracture toughness, which ultimately lead to the cracking failure. We show that the chemical and structural changes observed in the backsheet following C-AST aging were also observed in field-aged samples. These results increase confidence that the conditions applied in C-AST are representative of the field and demonstrates our approach to validating the failure mechanisms induced.

Wet leakage resistance development of modules with various backsheet types

“…18 First studies have been reported that bridge the gap between accelerated aging test in the lab and natural degradation in the field. 2,[19][20][21][22][23][24] These studies comprise single observations though and lack the combination and transformation of the individual results to operating conditions in the PV power stations.…”

PV modules and their backsheets - A case study of a Multi-MW PV power station

“…Gradual recognition of the fact that the degradation behavior of PV-module BSs depends strongly on the BS composition resulted in focused attention on BS failures specific for different polymer materials [1,2]. For example, many reports discuss particular issues of PAbased BS, including "chalking" (titania deposits), massive cracks in the BSs, a loss of mechanical strength and wet leakage resistance as well as potential reasons for degradation of PA-BSs [12][13][14]. In these studies, most analyses are carried out for single PV-modules in test labs.…”

An empirical investigation on the correlation between solar cell cracks and hotspots

“…To aid the transition to green energy, there has been a considerable demand to combine more photovoltaic (PV) systems into the electric grid to support the growth of renewable energy sources. As such, PV reliability and durability receives great interest from the industry as there are currently various mismatching conditions affecting these installations 1 – 3 . One of the challenges in today's PV modules is the well-known phenomenon called hotspots 4 .…”