Cyclic Demands on Solar Structural Joints under Wind Loading

Design of solar photovoltaic (PV) support structures, especially fixed-tilt structures, is typically done using equivalent static pressures, derived from static and dynamic wind load coefficients; however, the true loading on the system from wind is dynamic, with the structure undergoing many cycles. This cyclic nature of loading can lead to deterioration of the joints, even below the design wind load. With a focus on shared top-down clamps between purlins and PV module frames, this work aims to quantify the lifetime cyclic demands on these joints for the benefit of solar designers. A combination of wind hazard analysis, computational fluid dynamics (CFD), and finite element analysis (FEA) is used. Statistical distributions of wind directions and speeds based on historical wind hazard data are used to estimate durations of typical (not hurricane) wind scenarios in the lifetime of a prototype PV structure. The CFD models are first validated against wind tunnel data and then used to obtain dynamic wind pressures for wind scenarios (i.e., a certain direction and speed) not performed in the wind tunnel tests. Wind force time series are then applied to the FEA model of the PV structure, where a nonlinear spring model is used to simulate the highly nonlinear behavior of top-down clamps, to find the dynamic structural response. The FEA results demonstrate two deformation patterns between two adjacent modules (i.e., relative bending and relative twist) that govern the performance of top-down clamps; therefore, relative rotation angles are used to determine the rotational demands on the joint. Combined with wind hazard, the rotation cycle counts and amplitudes are obtained from rainflow counting, which result in the joint’s lifetime duty cycles. The results highlight the need for consideration of dynamic loading when designing PV structures, even relatively stiff fixed-tilt structures. Simplified methods for obtaining duty cycles are also proposed to facilitate practical design.