The “beam down” optics or “solar tower reflector” has been successfully used recently for testing in different projects at the Weizmann Institute of Science. There are currently sufficient data on this technology to evaluate its upscaling for commercial uses. The sizing of a tower reflector (TR) is directly linked to the layout of the heliostat field and the geometry of the ground secondary concentrator (compound parabolic concentrator (CPC)). It depends on its position relative to the aim point of the field, amount of spillage around it, and the allowable solar flux striking the TR. Its position influences the size of the image at the entrance plane of the ground CPC and the spillage around the CPC aperture. The spillage around the CPC is also directly related to the exit diameter of the CPC (equal to the entrance opening of the solar reactor, matching the CPC exit) and therefore linked to the input energy concentration, thermal losses, and working temperature in the reactor. Restrictions on the size of the exit of the CPC can influence the entire design of the optical system. This paper provides the correlations between the main design parameters and their sensitivity analysis. These correlations are based on edge-ray methodology, which provides a quick and sufficiently accurate means for preliminary evaluating large-scale beam down solar plants without the need for detailed design of the heliostat field and considering their errors. The size of the TR and the geometry of the CPC are correlated to the size of the reflective area of the heliostats field (and the power output). Thermal modeling of the TR has been performed, showing the maximum energy flux allowed on the reflector to avoid overheating, using natural cooling to the surrounding air. The current mirrors of the TR are limited to working temperatures of $120–130°C$ to achieve reasonable lifetime. This parameter must be considered when determining the TR position. A key issue discussed in this paper is the amount of spillage around the CPC entrance. To reduce the spillage losses, one needs to increase the size of the exit aperture (although there are practical limitations to this, e.g., due to the size of the reactor’s window). This, however, reduces the concentration and increases the thermal losses from the reactor and requires optimization work.

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