Development and Analysis of the Heliostat Focusing and Canting Enhancement Technique for Full Heliostat Alignments

Abstract

Central receiver power towers are regarded as a proven concentrating solar power (CSP) technology for generating utility-scale electricity. In central receiver systems, improper alignment (canting and focusing) of heliostat facets results in beam spillage at the receiver and leads to significant degradation in performance. As a result, proper alignment of heliostats is critical for increasing plant efficiency. Past tools used for analyzing and correcting heliostat alignment at the National Solar Thermal Test Facility (NSTTF) have proven to be laborious and inaccurate, sometimes taking up to six hours per heliostat. In light of these drawbacks, Sandia National Labs (SNL) and New Mexico Tech (NMT) have created the Heliostat Focusing and Canting Enhancement Technique (H-FACET). H-FACET uses a high-resolution digital camera to observe the image of a stationary target reflected by a heliostat facet. By comparing this image to a theoretical image generated via a custom software package, technicians can efficiently identify and correct undesirable deviations in facet orientation and shape. Previous tests have only proven the viability of H-FACET for canting heliostats. As a result, SNL and NMT have expanded H-FACET’s capabilities and analyzed the system’s ability to simultaneously cant and focus heliostats. Initial H-FACET focusing test results have shown improved beam sizes and shapes for single facets. Furthermore, simulations of these tests revealed an approximated system accuracy of better than 1.80 milliradians. This accuracy accounted for technician, position, and additional error sources, suggesting that H-FACET was capable of focusing facets to an even greater accuracy than those seen in the initial tests. When implemented for simultaneous canting and focusing of heliostats, H-FACET has demonstrated its capability to increase peak flux and decrease beam size. These full alignment test results demonstrated an average total system accuracy of 1.17 milliradians on five heliostats. As before, this accuracy included multiple error sources which cannot be corrected by H-FACET. Additionally, these tests revealed that H-FACET can align heliostats in about 1 hour and 30 minutes. Finally, two heliostats aligned with H-FACET maintained average accuracies 1.46 and 1.24 milliradians over a four hour window centered about solar noon. This implies that H-FACET is capable of aligning heliostats to a true off-axis alignment over NSTTF’s operating window. In light of these results, SNL has implemented both the focusing and canting portions of H-FACET at the NSTTF.