Structural Dynamics Testing and Analysis for Design Evaluation and Monitoring of Heliostats

Heliostat vibrations due to wind loading can degrade optical pointing accuracy while fatiguing the structural components. This paper reports the use of structural dynamic measurements for design evaluation and monitoring of heliostat vibrations. A heliostat located at the national solar thermal testing facility (NSTTF) at Sandia National Laboratories in Albuquerque, New Mexico, has been instrumented to measure its modes of vibration, strain and displacements under wind loading. The information gained from these tests will be used to evaluate and improve structural models that predict the motions/deformations of the heliostat due to gravitational and dynamic wind loadings. These deformations can cause optical errors and motions that degrade the performance of the heliostat. The main contributions of this work include: (1) demonstration of the role of structural dynamic tests (also known as modal tests) to provide a characterization of the important dynamics of the heliostat structure as they relate to durability and optical accuracy, (2) the use of structural dynamic tests to provide data to evaluate and improve the accuracy of computer-based design models, and (3) the selection of sensors and data-processing techniques that are appropriate for long-term monitoring of heliostat motions. This work also demonstrates the first measurements of rigid body modes of vibration associated with heliostat drive (azimuth and elevation) mechanisms, which are important structural dynamic response characteristics in dynamic design of heliostats.