Etendue-matched solar tower beam-down system for high-temperature industrial processes

The standard Concentrating Solar Thermal (CST) mono-tower technology, which uses one receiver placed on top of a tower to which all heliostats in the heliostat field aim to, is regarded as one of the best and most promising technologies for various CST-driven applications, namely CST power plants, solar metallurgical processes, thermochemical production of solar fuels and waste materials recycling. However, the technology has some technical challenges concerning optical performance/tolerances, system dimensions, operation and maintenance issues, etc. An alternative to this standard CST mono-tower technology is the so-called beam-down technology, where a special mirror is placed on the top of the tower, instead of a receiver, to redirect the incident radiation from the heliostat field onto a receiver/reactor placed closer to the ground and potentially delivering higher concentrations at the receiver than the standard CST mono-tower technology. This paper presents a new approach to improve the optics of beam-down systems, applies it to the optical design of a specific system, and shows the optical behavior of this design at two locations: Évora (Portugal) and Hurghada (Egypt). The approach uses etendue-matching between all the optical stages to minimize the optical losses between them. To analyze the optical behavior of the system designed, as an example, using the etendue-matching approach, raytracing simulations were carried out and are presented also in the paper.