Optical properties of solid particle receiver materials: I. Angular scattering and extinction characteristics of Norton Masterbeads ®

Abstract

As part of the Solar Thermal Technology Program, the direct absorption of sunlight by free-falling particles inside a cavity receiver is being evaluated. The objective of the on-going optical properties measurement program is to obtain information to be used as input to radiation transfer models for prediction of receiver performance. Instrumentation and data analysis techniques have been developed to determine both the angular scattering properties and the scattering and absorption components of the extinction coefficient of candidate materials. This report summarizes the measurement procedures and presents data for an iron-doped alumina spheroid, Masterbeads ®, manufactured by Norton Chemical Company. This material exhibits good optical absorption properties over the solar insolation spectrum and favorable thermal and mechanical properties for temperatures up to 1000°C. The optical scattering and extinction properties are spectral; measurements were performed at 632.8 nm for convenience. The measurements were also simplified by arranging the spheriods in a falling curtain geometry of one particle nominal thickness. Data were obtained over a range of mass flow rates and particle areal densities. Photographic documentation of curtain particle density enabled calculation of mean particle scattering and absorption loss components in the absence of multiple-particle optical interactions. Prediction of optical extinction properties at other wavelengths is anticipated to be straightforward using spectral hemispherical reflectance measurements on bulk samples. Additional scattering and extinction data were obtained on transparent glass microspheres for comparison and as a verification of the measurement apparatus and procedures.