Abstract Summary
Single axis solar trackers are constructions characterized by a series of photovoltaic modules mounted on a supporting structure, consisting of a motorized longitudinal torsional shaft and a series of vertical columns. The purpose of the underlying structure is to withstand acting loads and, by changing the inclination of the solar panels, to follow the position of the sun and maximize the energy production. In order to have competitive solutions, solar trackers need to be as cost effective as possible. Designers therefore seek to minimize the amount of material used by reducing the cross sectional area in the torsional shaft and other supporting elements, leading, consequently, to limited torsional stiffness. Combined with the low degree of torsional constraints that usually characterize solar trackers, large torsional static and dynamic deformations are expected at frequencies typically excited by wind. The dynamic response induced by turbulent wind is relevant and could cause problems for the structural integrity due to fatigue. The objective of this paper concerns the definition of a predictive model for the fatigue damage accumulation due to cyclical dynamic wind effects acting on the large tracker surfaces. In the presented approach, combining the pressure distribution time histories, and eventually information about the aeroelastic response of the tracker, both acquired through wind tunnel testing, with the structural properties derived from a FE model, it is possible to define and numerically integrate the governing problem of the system. In accordance with evidence observed in actual parks, the fatigue damage will be expected to accumulate in correspondence of the elements connecting the photovoltaic modules to the underlying structure: under this assumption, a relationship able to associate the applied loads to an internal response ought to be defined. To this end, a transfer function that relates the pressure acting on the panel surface to the stress state in a specific point is reconstructed by performing a series of analysis on a local FE model reproducing, of the whole tracker, a single PV module and its supporting beams. From the definition of the transfer function, the time history of the loading term, obtained for a specific cross section of the tracker by combining the pressure distribution and the inertial forces, is translated in the time history of the stress acting in the connections. The analysis of this last quantity, which is performed by means of the rainflow cycle count, combined with the SN curve of the structural detail and material investigated, provide useful information for the fatigue design, for example allowing the computation of the damage rate with the Miner’s rule. Finally, in order to validate the approach, a series of cyclical loads laboratory tests are carried out on real modules. From this analysis it is expected that, with minor calibration on the SN curve and transfer function, the developed procedure produces reliable results for the design. In conclusion, it is expected that the approach developed for calculating fatigue damage in PV trackers, specifically in connections, can enable designers of these structures to optimize the verification procedure.
