Comparison of visibility measuring instruments

Abstract

The atmospheric attenuation of light between the heliostat field and receiver in solar tower plants can cause significant loss in concentrated irradiance. This phenomenon brings a limitation on the size of the heliostat field and is included in some raytracing and plant optimization tools. At this time, local atmosphere conditions near the ground are usually not evaluated for new potential sites and little reliable data is available for the improvement of standard atmosphere models near the ground. Thus different commercially available instruments that can provide the data are described, tested and intercompared. The Vaisala FS11 scattermeters display good correlation over the full measurement range and their flexible mounting and low sensitivity to soiling facilitates application at remote sites or at different heights of a wind mast or tower. The Degreane TR30 AC is in comparison inflexible in application and because of its short path length better suitable for low visibility discrimination and not for clear atmospheric conditions. The Optec LPV-4 allows with proper mounting non-horizontal measurement and measurement over long path lengths that make the instrument suitable for very clear atmospheric conditions. The hereby presented instrument tests outline the first step to improve already existing tools for tower plant design and yield analysis in future.