A comparison of secondary optic designs for linear Fresnel collectors with a single tubular absorber

Abstract

This work presents a comparison of secondary optics for linear Fresnel collectors with a single absorber tube. The Compound Parabolic Concentrator (CPC) is indicated as the better secondary in different studies and under assessments including efficiency and flux uniformity, although it was never compared to the Compound Elliptical Concentrator (CEC) and to a recently proposed aplanatic design. Moreover, CPC geometries resulting from stochastic optimization needs to be confronted with theoretical edge-ray designs to evaluate the necessity of this optimization procedure. The ensuing ray-tracing results indicate that when the efficiency is the sole objective, CPC optimum geometry highly diverge from the edge-ray design, presenting a better efficiency but far lower performance in terms of flux uniformity and acceptance. In a two-objective problem involving efficiency and flux uniformity, optimum geometries for the CPC get closer to the edge-ray designs, particularly if the later considers the same gap size. Indeed, the gap size emerges as an important decision variable to play with the trade-off between the optical efficiency and flux uniformity. Comparison between CPC and CEC edge-ray designs shows that both optics in practice relate to a trade-off between optical efficiency and acceptance – the CPC presents a higher acceptance, the CEC has higher efficiency, whilst results of flux uniformity are practically the same. Regarding the aplanat design, it performs poorly in terms of acceptance angle compared to edge-ray designs, although outperforms them both in flux uniformity, and is outperformed by the CEC in terms of efficiency.