Compensation of Gravity Induced Heliostat Deflections for Improved Optical Performance

Heliostat optical performance can be affected by both wind and gravity induced deflections in the mirror support structure. These effects can result in decreased energy collection efficiency, depending on the magnitude of structural deflections, heliostat orientation and field position, and sun position. This paper presents a coupled modeling approach to evaluate the effects of gravity loading on heliostat optical performance, considering two heliostat designs: The National Solar Thermal Test Facility (NSTTF) heliostat and the Advanced Thermal Systems (ATS) heliostat. Deflections under gravitational loading were determined using finite element analysis (FEA) in Ansys Mechanical, and the resulting deformed heliostat geometry was analyzed using Breault Apex optical engineering software to evaluate changes in beam size and shape. Optical results were validated against images of actual beams produced by each respective heliostat, measured using the Beam Characterization System (BCS) at Sandia National Laboratories. Simulated structural deflections in both heliostats were found to have visible impacts on beam shape, with small but quantifiable changes in beam power distribution. In this paper, the combined FEA and optical analysis method is described and validated, as well as a method for modeling heliostats subjected to gravitational deflection and canted in-field, for which mirror positions may not be known rigorously. Furthermore, a modified, generalized construction method is proposed and analyzed for the ATS heliostat, which was found to give consistent improvements in beam shape and up to a 4.1% increase in annual incident power weighted intercept (AIPWI).