Browsing by Author "Koekemoer, JH"

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    Dynamic wind load effects in a photovoltaic single-axis tracker mounting rail.
    (Stellenbosch : Stellenbosch University, 2024-02) Koekemoer, JH; Bredell, JR; Venter, G; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: Single-axis trackers are actuated structures often used in utility-scale photovoltaic (PV) installations. These installations are sensitive to dynamic wind load effects due to their lightweight, flexible support structures and large PV module area. Significant damage to single-axis trackers have been reported in literature, despite the use of modern design methods. Design codes prescribe wind loads for a representative geometry but exclude potentially aeroelastic sensitive structures. Additionally, boundary layer wind tunnel testing and computational fluid dynamics studies are often associated with significant uncertainties. This study aimed to determine wind load effects on an existing 32 m × 25 m single-axis tracking PV array using field measurements. In this way, the uncertainties associated with wind tunnel testing and computational studies could be avoided. The project focuses on a critical component of the support structure, namely the mounting rail that attaches the PV modules to the rotating torque tube. Representative mounting rails were instrumented with strain gauges to capture dynamic wind load effects over periods of up to 109 days. The strain gauge locations were determined using results from calibrated finite element analyses of the structure. Equivalent static normal forces and moments acting on the mounting rail could be calculated using strain data and load calibration. These equivalent static loads would produce similar deformations and stresses compared to the dynamic wind loads while assuming simplified load distributions. The experimentally determined wind loads were correlated with wind speed, wind direction, and tracker tilt angle. The most critical wind directions (east and west) have a larger PV module area projected normal to the oncoming wind and subsequently showed high wind loads. Higher equivalent wind loads were also observed when the PV modules were more inclined relative to the oncoming wind. An interior located rail showed lower peak loads compared to an exterior rail, likely due to shielding from the surrounding structure. Wind load coefficients were lower compared to design codes for the range of wind conditions and tracker positions seen during the measurement period. This was expected, since design codes represent a critical combination of geometrical parameters to provide conservative estimates of wind loads. Analysis of dynamic load effects revealed contribution of torsional and bending modes of the torque tube to the normal forces and moments experienced by the mounting rails. A fatigue assessment found an insignificant fatigue damage for wind speeds below 21 m/s. Measurements suggest wind speeds above 21 m/s may be expected to cause fatigue damage. Design trends in utility-scale trackers show a decrease in mounting rail length to reduce the capital cost of new installations. The calibrated finite element model was expanded to assess the impact of a reduction in mounting rail length. Significant increases in the stress in the mounting rail and PV module were seen with a reduction in rail length, assuming all other parameters remain unchanged. Increases in stress may be non-linear, depending on the position and component.