Determination of beam attenuation in tower plants

Atmospheric extinction between the heliostat field and receiver in solar tower plants is known to cause significant losses of reflected Direct Normal Irradiance. This phenomenon brings a limitation on the size of the heliostat field and is included in some raytracing and plant optimization tools. Usually, no detailed information about the local meteorological conditions is available for many sites that are now of interest for tower plant projects. Therefore, only standard atmospheric conditions are commonly used to describe the attenuation and also the height profiles of relative humidity and aerosol concentration. First of all the existing models are presented. The use of the Pitman and Vant-Hull model with real measurement data represents an improvement with respect to site independent calculations. Thus different commercially available instruments that can provide the input for the state of the art models are described, tested and intercompared. Also the limitations of the state of the art are discussed and methods to overcome these limitations are shown. The choice of the tested instrumentation and the evaluation of the different instruments have been performed with regard to necessary enhancements. Several months of MOR (Meteorological Optical Range) measurements from the Plataforma Solar de Almeria (PSA) are presented. These data provide a base for further evaluation of the investigated instruments. The FS11 scatter meters display satisfying accuracies on transmittance measurements and their robust composition and low sensitivity to soiling facilitate application at remote sites. The Degreane TR 30AC transmissometer is rather suitable for smaller slant ranges than those required for many solar tower plants as uncertainties for clear atmospheric conditions are high. The Optec LPV-4 transmissometer obtains high accuracies for clear conditions if large working path distances are used to exploit the preciseness of the instrument. The presented measurement methods enable improvements in tower plant design and yield analysis, but still enhancements of the existing models are required. The discussed instruments, additional sensors and modeling approaches can be used to develop such methods.