Optical characterization of solid particle solar central receiver materials

ABSTRACT:
The angular scattering properties and scattering and absorption components of the extinction coefficient have been measured for Master BeadsR, an iron-doped A1203 spheroid manufactured by Norton Chemical Co., Worchester, MA. This material, commonly used as a proppant in oil well drilling

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operations, exhibits good optical absorption properties over the solar insolation spectrum, and favorable thermal and mechanical properties for temperatures up to 1OOO·C. These characteristics, along with its low cost, make Master BeadsR a good candidate as the absorbing medium in the solid particle solar central receiver concept. The objective of the ongoing optical properties measurement program is to obtain information to be used as input to radiation transfer models for prediction of receiver performance. Scattering and extinction measurements Were performed at 632.8 nm in a falling curtain geometry of one-particle nominal thickness. Measurements were performed over a range of mass flow rates and particle densities. Photographic documentation of curtain particle density enabled calculation of mean scattering and absorption loss components per particle in the absence of multipleparticle optical interactions. Prediction of optical extinction properties at other wavelengths is anticipated to be straightforward using hemispherical spectral reflectance measurements on bulk samples. Additional scattering and extinction data were acquired on transparent glass microspheres for comparison and as a verification of the measurement apparatus and procedures. In addition, an experimental system to measure the diffuse reflectance of a particulate sample over the wavelength range of 300 to 2500 nm and at elevated temperatures up to 1000cC was developed and implemented. A description of the experimental apparatus and measurement procedures, as well as optical reflectance data for the Master Beads, are presented. Using this high temperature measurement system, the diffuse reflectance of Master Beads R changed by less than 1% for sample temperatures from 150cC to 930cC. However, after heating a sample for 3 hours at 1000cC in air, the solar absorptance measured at room temperature decreased from initial values of 0.93 to 0.S9.

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