Heliostat reflective facets have traditionally been constructed with glass/silver and a metal back support. During the past year, Sandia National Laboratories evaluated low-cost materials and alternative manufacturing methods to construct facets with the goal of reducing current facet cost by at least 25% while maintaining surface slope errors at 1 milli-radians rms or below. Several companies developed prototype facet samples, which were optically evaluated at Sandia and compared to baseline facet samples using a proposed cost-to-performance metric.
A cost-performance metric for comparing facets was developed by modeling and optimizing a 200 MWe power tower plant scenario in DELSOL, a computer code for system-level modeling of power tower systems. We varied the slope error on the facets and adjusted the cost on the facets to maintain the constant plant levelized cost of energy. The result of these models provides a chart of the facet optical performance and the allowable facet cost for a constant plant LCOE.
The size of the prototype facet samples ranged from 1.4 to 3 m2. The measured optical slope errors were between 1 and 2 milli-radians rms when compared to a flat mirror design shape. Despite slope errors greater than 1 mrad rms, some of the prototype samples met the cost goals for this project using the cost-performance metric. Next steps are to work with the companies to improve the manufacturing processes and further reduce the cost and improve on the optical performance to reach DOE SunShot goal of $75/m2 for heliostats.