Session: 18-04 HelioCon Heliostat Designs

Paper Number: 142082

142082 - Using an Equivalent Slope Error to Quantify Different Types of Optical Errors of a Heliostat 

Abstract: 

A commercial-scale Concentrated Solar Thermal (CST) system comprises thousands of heliostats, each with its own unique optical performance. Incorporating the individual heliostat performance into heliostat field design and annual performance evaluation is essential to assessing the viability of the technology as a whole. A heliostat can exhibit various errors including mirror specularity, contour, canting, and tracking inaccuracies, influenced by several factors such as material limitations, soiling, gravity sag, wind-induced vibration, manufacturing defects, and control system inaccuracies. Each type of error possesses a distinct nature and necessitates specific measurement techniques. Different measurement methods produce results in various formats. Introducing a concept of equivalent slope error (ESE), which encompasses various optical errors from measurements, is beneficial. This concept serves as an input to a Monte Carlo ray tracing (MCRT) simulation, achieving energy capture efficiency comparable to actual measurements. It allows for a quantitative comparison of error magnitudes and a holistic evaluation of the annual performance of the heliostat field. This presentation will reveal the findings from our investigation into how three types of heliostat imperfections—focal length errors, multi-facet shape errors, and tracking errors—contribute to the effective ESE. The actual errors are incorporated into the MCRT model and simulated at a specific design point as well as on an annual basis. By matching the energy capture efficiency with the actual error case at the design point, an ESE is derived, facilitating annual performance simulations and comparisons with the actual error result. Despite variances in peak flux, the differences in annual performance among evaluated cases were minimal, under 0.5%. This indicates that while the ESE may not fully replicate the flux profile of a heliostat with specific errors, it aligns closely with energy capture efficiency. Notably, a 10% focal length error corresponds to a 0.7 mrad ESE, a 20% error to 1.01 mrad ESE, and a 30% error to 1.32 mrad ESE, determined by matching 95% of energy capture. A single flat-facet heliostat introduces a 4.7–6.7 mrad ESE compared to an ideal paraboloid heliostat. However, a 5×5 flat multi-facet heliostat results in just 0.97–1.23 mrad of additional error when matching 90–98% of energy capture. It closely resembles the ideal heliostat at 99% of energy capture, a result primarily due to the heliostats being positioned at a distance from the tower. The influence of imperfection errors diminishes for heliostats positioned further from the tower, where sunshape and astigmatism effects dominate the optical performance. This indicates that strategically placing heliostats with greater errors further away does not sacrifice performance but can potentially reduce overall costs.

Presenting Author: Ye Wang Australian National University

Presenting Author Biography: Dr. Ye Wang's background is in Mechanical Engineering. She is a research fellow at the Australian National University (ANU), specializing in optical modeling, flux mapping, optical characterization of solar concentrators, design of solar thermal receivers, and techno-economic analyses of renewable energy systems.

Authors:

Ye Wang Australian National University
Joe Coventry Australian National University
John Pye Australian National University