Fundamentals of Solar PV Bolted Joint Loosening and Prevention

The photovoltaic (PV) industry has long reported anecdotal accounts of systems exhibiting intermittent or chronic fastener loosening, including joints that fail to maintain preload despite multiple re-tightening attempts. These occurrences are frequently, and often incorrectly, attributed to installer errors, vibration, or loading beyond design expectations (e.g., extreme weather events). Loose fasteners have serious implications and can significantly impact solar PV systems’ performance, reliability, and safety. However, effective bolted joint design and proper assembly practices can mitigate or eliminate loosening. Until the US Department of Energy’s Solar Energy Technologies Office (SETO) funded research on fasteners and solar PV structures, there was a notable gap in understanding the causes of loosening in solar PV systems. This Guidance document section aims to fill that gap so industry stakeholders can specify, design, procure, and install more reliable fastener strategies, bonding, and solar PV structures. As part of the SETO-funded research project titled Maturing Bolted Joint Technologies in Solar PV Structures (Prime Contract No. DE-AC02-05CH11231), a team conducted structured interviews with diverse stakeholders in the PV industry, gathering valuable insights into the characteristics, pervasiveness, and cost impacts of solar PV bolted joint failures (including loosening). The team interviewed over 28 expert respondents, providing a comprehensive understanding of the structural reliability of 17,000 systems with a combined capacity of over 94 GW. The structured interviews yielded many valuable findings, including the prevalence of fastener and module-top-down clamp loosening. Of the 80 reported solar PV bolted joint failures, nearly 44% were reported as loosening. The team assessed that ‘installer error’ accounted for only 13% of the identified causes. Less than 1% of the solar PV bolted joint failures discussed during the interviews were due to Acts of God, in which wind or snow loads exceeded the design loads. In other words, nearly all PV bolted joint failures occurred at less than the design wind and snow loads. A larger fraction (37%) came from mounting system ‘design issue’ related causes. For years, anecdotes about loose fasteners on PV mounting systems have been widely shared; the structured interviews substantiated these issues and found them to be more common than previously assumed. This issue could be a contributing factor to increased O&M costs and diminished structural reliability of solar PV mounting structures. This report illuminates the types, causes, and methods for preventing common loosening in solar PV structural and structural-bonded bolted joints. The objective is to help solar PV mounting system manufacturers design more reliable bolted joints that are less prone to loosening, and to help field engineers diagnose and address loosening issues in the field. The report does not address loosening in PV module lip clips and clamps or module frame wedges.