Influence of the Length of a Photovoltaic Single-Axis Tracker Mounting Rail

Abstract

Single-axis trackers are actuated structures in utility-scale photovoltaic power plants. Wind loads on trackers often govern their structural design. A critical tracker component is the mounting rail, which forms the structural support between the rotating horizontal torque tube and the photovoltaic modules. The rails are thin gauge coldformed steel members with bolted connections. Recent design trends show decreasing mounting rail length to reduce cost. This study investigated the influence of rail length in a one-in-portrait tracker using linear static finite element analyses. The finite element models of photovoltaic modules were calibrated using experimental force-displacement data. A single glass layer of the full laminate thickness required a calibrated Young’s modulus of 56.5 GPa compared to 70 GPa nominally.  The investigated rail lengths ranged between 900 mm and 400 mm and support bifacial photovoltaic modules with dimensions 2037 mm × 1055 mm.  With the 900 mm length used as the reference case, it was shown that stresses in the photovoltaic module frame due to simplified wind loads increased by up to 124% as the rail lengths were reduced. Stresses in the solar glass and crystalline silicon cells increased by as much as 65 %. Uniform and non-uniform distributed loads were considered. Significantly higher stresses were seen for non-uniform loads with a centre of pressure eccentric to the torque tube, which more accurately represents wind loads according to design codes and field measurements. A uniform distributed load, typically used for dynamic load qualification testing on photovoltaic modules, is inadequate for calculating mounting rail deformations and stresses expected from wind loads.