Solar Glass Durability & Failure Modes
For the 2024 PV Module Index Report, RETC sought to better understand the unique field failure modes associated with ultra-large-format PV module designs. Here, we present our interview with Theresa Barnes, an expert in module durability.
TERESA BARNES, NREL
Teresa Barnes, Ph.D., manages the Photovoltaic Reliability and System Performance Group at NREL. She is an expert in accelerated test development, outdoor field testing, energy yield assessment, degradation rate analysis, and materials science. In addition to working with stakeholders throughout the PV value chain to develop industry standards for reliability testing and energy yield, Dr. Barnes is also the principal investigator and director of the DuraMAT Consortium for module materials research.
RETC: Has NREL noted any changes in glass breakage patterns over time?
TB: Spontaneous glass breakage is an example of a failure mode that we didn’t used to see. When I first started working on solar module reliability seven or eight years ago, we mostly heard about glass breakage when there were sloppy O&M [operations and maintenance] practices. Maybe someone is mowing for vegetation management, the mower throws a rock, and you get glass breakage. Occasionally, we would hear about glass breakage in thin film modules, which use thinner glass that is not tempered; this is known failure mode, one that we would expect to see. Outside of these known issues, we didn’t used to hear about glass breakage in silicon modules unless there was a catastrophic storm. That is no longer the case. Now we are regularly hearing about glass breakage in silicon modules. While these reports are anecdotal, there is a definite pattern. People are seeing glass breakage for no apparent reason, often before commissioning. These field reports track what we are hearing from the testing labs. It used to be the case that modules would pass the IEC 61215 static load test with a big safety factor. Today, modules are either barely passing the base static load test or they are not passing with higher safety factors. Some new module designs are simply not passing the minimum static load test.
RETC: Do you and your colleagues have any theories about why this is happening?
TB: We have started to hypothesize internally about potential root causes. At DuraMAT, for example, we funded a bunch of projects on cell cracks with the idea that researchers would take PV modules, pre-crack the cells, and then do some sort of stress test to identify the impact of the latent defect. After funding this research, I started receiving phone calls and emails saying: “Hey, I can’t break the cells without breaking the glass. It seems like cell cracking is not our problem right now. Can I look at glass breakage or something else instead?” What we believe is happening is similar to how your car windshield breaks, where a small surface defect propagates when exposed to an extreme environmental stress. Think about when you are driving down the road and something hits your windshield. The windshield probably does not shatter on impact. Instead, you might see a small stars-haped defect on the surface of the glass. Here in Colorado, where I go through more windshields than I ever did on the East Coast, that impact damage might not propagate until there is a dramatic change in the weather. But when I walk outside on the first really cold day of winter, suddenly all the windshields in my driveway have a big crack all the way across the glass. We think a similar dynamic could be a root cause of spontaneous solar glass breakage.
RETC: Why do you think this failure mode is increasing now?
TB: Solar glass is thinner today than it was in the past. Because the glass is thinner, it is not fully tempered. According to glass experts like Mike Pilliod from Central Tension, who spoke at NREL’s 2024 PV Module Reliability Workshop, any manufacturer can temper glass that is 3 mm [0.12 in.] or thicker because it is relatively easy to get the thermal differential to build the stress profile you need. However, it is more difficult to fully temper glass below a thickness of 3 mm. If you do not have a good temper on the glass, it is relatively easier for the glass to break. In other words, as solar glass gets thinner, it takes fewer defects to cause a strength-limiting flaw in the glass. Moreover, the way we specify glass in the solar industry right now does not account for strength-limiting flaws in glass that is not fully tempered. There might be a ball-drop test on a piece of glass, but people are not necessarily looking at the impacts of defects around the edges of the glass. Also, the way people are cleaning or grinding around the edges of the glass may be causing more defects rather than fewer defects, perhaps making this thinner glass even more sensitive to breakage. These are the types of things that we are starting to investigate and trying to better understand.
RETC: How does glass breakage relate to module frame and rail designs?
TB: There is undoubtedly an interaction between these different components. A module is really a whole system, often consisting of glass, a perimeter frame, and a mounting rail. When you think about ultra-large modules as a system, the glass may be getting thinner, the frame may be getting thinner, and the mounting rails may be getting closer together. The interaction between these components is where the term “big floppy modules” comes from. While glass has always been an element in the module system, it may be bearing more of the load now, which may be bad because we have made the glass weaker.
RETC: How has the industry responded to reports of spontaneous glass breakage?
TB: At this year’s PV Module Reliability Workshop, my colleagues and I started to hear about frames getting thicker again. This was somewhat surprising, given the rapid innovation in recent years focused on making modules cheaper and lighter. The problems we are seeing in the US market may be causing manufacturers to rethink product design strategies. We are starting to hear some chatter about frames getting a bit more substantial and mounting rails getting a bit wider. As people better understand how the module system interacts, they can work to optimize how loads are balanced out. The pendulum in that balancing act may already be swinging back toward the integrity of the frame and the mounting rail.
RETC: Given the catastrophic scale of recent hail loss events in Texas, do you think the industry needs hail-hardened module designs?
TB: Ten years ago, people would run you out of the meeting on a rail if you mentioned climate-specific module designs. The consensus was that this would simply be too expensive. A decade ago, module cost was really the barrier to entry. Today, solar is a terawatt-scale industry, and we are deploying hundreds of gigawatts of solar capacity annually. Now, climate-specific modules and climate-specific testing are starting to look viable because we are seeing more of an emphasis on total system costs. It is entirely possible that we could see hail-hardened modules, especially in a market like the United States, where it could be worth paying more up front for hail resilience. At the end of the day, a lot of these design decisions will come down to the business model. People flipping and selling projects will always have different priorities than those who own and operate projects for the long term. If you are the person who will be caught holding the bag when a hailstorm comes, maybe you are willing to pay a little more for hail-hardened solar modules—because that decision helps make the system pencil over a long lifetime.
This interview with Theresa Barnes originally appeared in the industry trends section of the 2024 PV Module Index Report.
