Reduction in Spot Size via Off-Axis Static and Dynamic Heliostat Canting

Abstract

Heliostat canting is the alignment of facets on a common frame which provides focusing of sunlight on a prescribed target. Traditionally, this alignment has been parabolic, in which the focal point of the heliostat lies on its optical axis. Two alternative off-axis canting methods are compared in this article, (i) fixed facet (static) canting in which the facet alignment is optimized for a single design day and time and then rigidly mounted to the frame and (ii) dynamic canting in which the facets are actively controlled such that the center of each facet is always perfectly focusing. For both methods, two case studies are considered: (i) a power tower with planar heliostat field (heliostat dimensions and tower height modeled after the 11 MWe plant PS10) and (ii) a hillside heliostat field which directs light down to a ground-level salt pond. In both case studies, static heliostat canting provides a small improvement in focusing by reducing the average annual insolation-weighted spot size by roughly 1–2%. Dynamic canting, in contrast, provides a 20–25% reduction in spot size.