Solar divergence collimators for collector tests

The optical characterisation of sunlight collectors represents a crucial aspect of the research on renewable energies. The information it provides is twofold. Firstly, it allows evaluating the optical characteristics of each single component, comparing them to the expected performances according to the optical project. Secondly, it allows comparatively characterising the elements of a serial production, thus giving information about production homogeneity. The use of solar divergence collimators is the key for reliable optical tests that reproduce the on-field collector operating conditions. Aiming to test small (<80 mm diameter/diagonal) and large concentrators (80-240 mm diameter/diagonal range), the collimator geometry should be evaluated because of some optical errors introduced by the need of bending the optical axis. This paper discusses the optical characteristics of the beams obtained with straight and bent collimators with solar divergence. The collimated beams are examined by ray tracing simulations, studying the aberrations as a function of bend angle, and measured in laboratory. Finally characterisation tests for sunlight concentrators of various shapes, dimensions and collection features are presented describing instrumentation, optical set-up and methodology to determine collection efficiency, image plane analysis and spot size.