An optical profilometer for the characterisation of parabolic trough solar concentrators

Abstract

An optical profilometer has been developed based on the idea that the large panels that compose solar-power concentrators and a tolerance threshold for slope-errors to some milliradians, can allow the use of a geometric-optics framework and the investigation of ray-paths by means of a laser beam. The instrument scheme is discussed in detail together with the related problems and the solutions adopted. Data analysis also makes use of an innovative method, based on the iterative application of a criterion about the intersection between the tangents of adjacent points, which allows a realistic profile of the reflector-surface to be drawn. The methodology adopted allows us to call our instrument a profilometer rather than an instrument that gives only information about the tangent of the profile. The instrument accuracy on profile and arctangent deviations from the ideal parabola is 20 μm and 15 μrad, respectively. The results obtained for a reflecting panel are also reported.

Introduction

Large optical reflectors have different applications ranging from astronomy, in telescopes whose imaging capability is very relevant, to solar energy, like solar-power concentrators, where the optical requirements are much less severe. Many applications need reflectors so large that several panels compose them, each one being some meters across.

Despite the considerable differences with regard to structure, composition, shape and features dictated by specific applications, large reflectors, or each one of its sub-panels, have to be assessed to verify correspondence with project specifications. Thus, the optical assessment is concerned with the reflectance (specular and diffuse), and the geometric shape of the reflecting surface. Reflectance is mainly due to the surface quality and surface treatment, often involving a suitable optical coating; the shape dominates the focusing feature of the reflector. Whereas the experimental methods for measuring reflectance are quite homogeneous, generally based on photometric measures, the techniques for measuring geometric shape greatly differ (Malacara, 1978). The most accurate techniques use interferential methods to measure the reflected wave front. Others evaluate the image quality by considering distortions affecting the image of an object having regular features, and a final class investigates ray-paths (Briers, 1999). The choice of the most appropriate method depends on both the reflector size and the scale of the acceptable optical errors for the specific application.

In the case of solar concentrators, one or more dimensions reach some meters, and slope-errors as large as few milliradians are acceptable; consequently any interferometric method is inappropriate as they are too sensitive and expensive. For point-focus solar concentrators, good results were obtained by analysing the image of both a regular target placed at twice the focal length (2f test) (Grossman, 1994) and the sun on the focal plane (on-sun testing) (Grossman et al., 1992). Alternatively, ray-paths can be investigated by a modified Hartmann test (Malacara, 1978), where the screen is replaced with the surface scan accomplished with a tiltable laser beam (Wendelin et al., 1991).

According with the Concentrating Solar Power Project, of which Italian government has charged ENEA, a large parabolic trough mirror structure redirects solar rays onto a receiver tube. The panels composing this parabolic trough concentrator are shaped as half-parabola, with aperture 2.88 m, width 1.20 m and focal length (f) 1.81 m. These panels are rigidly connected one to each other by a modular supporting structure 12.5 m long, to produce a parabolic trough concentrator with aperture of 5.76 m; the supporting structure allows rotation of the ensemble to follow solar motion (Rubbia et al., 2001).

Different construction solutions for the panels are under investigation with the goal of realising cheap mass-production. The panel considered in the present paper is composed by a core of 2.5 cm thick honeycomb aluminium structure, sealed between two fiberglass foils; a thin glass mirror is glued on the concave surface.

Aim of this paper is to describe the optical profilometer developed at ENEA Casaccia and its use accurately test the shape of these innovative low-cost reflective panels whose production is one of the project targets.