Aplanatic beam-down solar towers

The pragmatic virtues of ground-level receivers in solar towers have long been recognized, but the associated beam-down optics reduce concentration, resulting in higher heat loss and cost, or the need for an actively-cooled tertiary concentrator that incurs additional optical losses. Here, we describe the concept of an aplanatic beam-down solar tower, where concentration can be increased without the need for a tertiary concentrator, while retaining a ground-level receiver. The basis for satisfying aplanatism constitutes tailoring the contour of a stationary secondary mirror atop the tower with a distinct heliostat tracking strategy. Conflating the aplanatic solar tower with the option of multiple towers, where a heliostat can be aimed at different targets depending on solar geometry, can markedly reduce shading, blocking and cosine losses. Also, a system of many mini-towers each of which is only a few meters in height could enable systems that are more modular, efficient and easily-maintained.