Single- and multi-facet variable-focus adaptive-optics heliostats: A review

Abstract

The heliostat field is a critical component in the solar energy harnessing process of concentrated solar thermal (CST) central tower systems, which typically represents approximately 45% of the total capital cost of commercial plants. In commercial CST central tower systems, the heliostat field is composed of thousands of heliostats incorporating precision-engineered mirrors, which direct sunlight towards a central receiver. Typically, the mirrors maintain a fixed geometry, usually of parabolic shape, during the sun-tracking process, which is optimized to maximize the annual optical performance. However, such a design leads to subdaily energy losses stemming from alterations of the mirror geometry and from astigmatism errors. To achieve and maintain peak optical efficiency throughout the day, the adoption of variable-shape heliostats emerges as a promising solution. This review assesses the current state-of-the-art, the challenges, and the emerging trends and future directions for two types of heliostat technologies, the single facet and the multifaceted variable-focus adaptive optics heliostats. Single-facet heliostats have shown promise due to their simplified design and lower costs compared to multifaceted heliostats. However, achieving precise tracking and focusing remains a significant challenge, particularly in large-scale applications. In contrast, multifaceted heliostats provide superior accuracy and optical performance but are associated with increased design and operational complexity. While these technologies are still under development, advancements in design, materials, control systems, and tracking mechanisms highlight promising trends for the future.