Comparing Glass Surface Stress in Contemporary and Historic Photovoltaic Modules as it Relates to Field Breakage

The rise of photovoltaics (PV) as a major contributor to energy generation worldwide has also coincided with major changes in module format. In the last ~5 years, utility scale modules have transitioned to a glass-glass package to take advantage of bifaciality, while at the same time decreasing the glass thickness from 3.2 mm to ~2 mm to save on cost and weight. In parallel, modules have continued their trend of increasing in size, with areas of 2.5 - 3 m2 common today. Despite top tier manufacturers testing to current IEC standards, there has been an uptick of modern glass-glass modules spontaneously breaking in the field, sometimes even before the PV plant is brought online. This is of grave concern as even a small number of cracked modules have been observed to cause ground faults and inverter down time. One of multiple possible contributing factors is a change in glass strength, which is dependent on the thickness, flaws, and stress profile from heat treatments of the glass. Our work examines the variability of glass thickness, surface stress, and compression depth in a variety of PV modules, from older glass-backsheet, to more modern glass-glass modules of various sizes. We discuss a procedure developed to get consistent stress measurements on a fully intact module. We find that while most glass panes measured pass ASTM 1048-18 as fully tempered glass, the surface stress of modules known to break in the field is overall lower compared to glass-backsheet and glass-glass modules not known to spontaneously break. Furthermore, we discuss how compression depth, inherently smaller in thinner glass, could contribute to a narrower tolerance of flaws. We discuss how the decrease of both the surface stress and compression depth in modules known to spontaneously break in the field could be one contributing factor increasing probability of failure.