Lookback method for high-precision, highly flexible, low-cost heliostat optical assessment

Abstract

Heliostats reflect sunlight onto a receiver, which absorbs the light to produce heat. To achieve high temperatures, high concentration and high optical accuracy are required. This accuracy must be maintained under changing sun angle, gravity loading, temperature, wind exposure, mechanical wear, and other field conditions, motivating the need for fast, flexible, in situ optical assessment methods. These requirements drive a need to measure heliostat optical performance in situ, both at the time of installation, and periodically during field operation. There is also a similar need for measuring heliostat prototype performance during design and development, with testing spanning the same range of conditions. This paper describes progress toward a method of in situ heliostat optical characterization designed to measure heliostats across the full range of tracking configurations and actual operating temperatures, named LOOKFAST. Using one or more cameras looking back at the heliostat and the Sun or Moon as an optical target, the method is designed to simultaneously measure both high-resolution slope and pointing. There are multiple variants of the basic idea; the simplest uses only a camera on a tripod with no moving parts. We will explain the approach and present our work in progress demonstrating initial data collection methods, and example result slope and pointing maps. We conclude by describing the additional steps required to develop high-speed measurement of heliostats in parallel. Software supporting this method and example data are available on OpenCSP (https://opencsp.sandia.gov).