A central solar plant, based on beam-down optics, is composed of a field of heliostats, a tower reflector and a ground
receiver. The tower reflector is an optical system comprises of a quadric surface mirror (hyperboloid), where its upper
focal point coincides with the aim point of a heliostat field and its lower focal point is located at a specified height,
coinciding with the entrance plane of the ground receiver. The optics of a tower reflector requires the use of ground
secondary concentrator, composed of a cluster of CPCs, because the quadric surface mirror always magnifies the sun
image. There is an intrinsic correlation between the tower reflector position and its size on one hand, and the geometry,
dimensions and reflective area of the secondary concentrator on the other hand; both are related to the heliostat field
reflective area. Obviously, when one wishes to have a smaller tower reflector by placing it closer to the upper focal point,
the image created at the lower focus will be larger, resulting in a larger secondary ground concentrator.
The present work analyses the ways for a substantial decrease of the size of the ground concentrator cluster (and,
implicit, the concentrators area) via truncation, without significant sacrifice of the performance, although some increase
of the optical losses is inevitable. This offers a method for cost effective design of future central solar plants utilizing the
beam down optics.
Decreasing the aperture of a solar central receiver operating at high temperatures contributes significantly to the increase of the efficiency of energy absorption. However, decreasing the aperture also decreases the collection efficiency. A simple solution is using a 3D- CPC as secondary element for augmentating the energy collection, while the aperture can remain relative small. Nevertheless, the receiver aperture as well as the secondary concentrator are usually rather large. For practical considerations an approximate solution may be chosen at times, designing the concentrator by a series of truncated cones[1}. However, in particular cases, the solution of truncated cones remains expensive and unpractical and therefore we designed a CPC approximated by a series of trapezoidal planar facets. Under technical restrictions, there is an optimum for choosing the partition of the purecPc in truncated pyramids and the paper presents the method for a good solution. The design of the concentrator is such that it can accomodate two different receiver apertures (60 cm and 47 cm respectively). The final design of the concentrator provides an entrance diameter of 121 cm and consists of four truncated pyramids, each composed of 12 trapezoidal facets, when connected to the 60 cm diameter aperture of the receiver. A fifth ring of facets is added when the concentrator is used in conjunction with the second receiver. . Some considerations about the materials used in construction are presented in the final section of the paper.