Glen Johnston

Photogrammetry: A Powerful Tool for Geometric Analysis of Solar Concentrators and Their Components

Digital close range photogrammetry has proven to be a precise and efficient measurement technique for the assessment of shape accuracies of solar concentrators and their components. The combination of high quality mega-pixel digital still cameras, appropriate software and calibrated reference scales in general is sufficient to provide coordinate measurements with precisions of 1:50,000 or better. The extreme flexibility of photogrammetry to provide high accuracy 3-D coordinate measurements over almost any scale makes it particularly appropriate for the measurement of solar concentrator systems. It can also provide information for the analysis of curved shapes and surfaces, which can be very difficult to achieve with conventional measurement instruments. The paper gives an overview of quality indicators for photogrammetric networks, which have to be considered during the data evaluation to augment the measurement precision. A selection of measurements done on whole solar concentrators and their components are presented. The potential of photogrammetry is demonstrated by presenting measured effects arising from thermal expansion and gravitational forces on selected components. The measured surface data can be used to calculate slope errors and undertake raytrace studies to compute intercept factors and assess concentrator qualities.

Photogrammetry : An available surface characterization tool for solar concentrators, Part I: Measurements of surfaces

Close range photogrammetry is a sensing technique that allows the three-dimensional coordinates of selected points on a surface of almost any dimension and orientation to be assessed. Surface characterisations of paraboloidal reflecting surfaces at the ANU using photogrammetry have indicated that three-dimensional coordinate precisions approaching 1:20,000 are readily achievable using this technique. This allows surface quality assessments to be made of large solar collecting devices with a precision that is difficult to achieve with other methods.

Optical performance of spherical reflecting elements for use with paraboloidal dish concentrators

While paraboloidal dishes have traditionally been used for high flux/high power solar concentration devices, the manufacture of multi-facet collectors has been complicated somewhat by the need to produce reflecting elements having different curvatures for different regions of the paraboloidal surface. This complication could be minimised by using identical spherical reflector sub-components mounted with a paraboloidal orientation on a space frame dish structure. This paper compares the optical performance and manufacturing feasibility of collectors having such a combination of surfaces.