Evaluating the Effective Solar Absorptance of Dilute Particle Configurations

Ray-tracing and heat-transfer simulations of discrete particles in a representative elementary volume were performed to determine the effective particle-cloud absorptance and temperature profiles as a function of intrinsic particle absorptance values (0 – 1) for dilute solids volume fractions (1 – 3%) representative of falling particle receivers used in concentrating solar power applications. Results showed that the average particle-cloud absorptance is increased above intrinsic particle absorptance values as a result of reflections and subsequent reabsorption (light trapping). The relative increase in effective particle-cloud absorptance was greater for lower values of intrinsic particle absorptance and could be as high as a factor of two. Higher values of intrinsic particle absorptance led to higher simulated steady-state particle temperatures. Significant temperature gradients within the particle cloud and within the particles themselves were also observed in the simulations. Findings indicate that dilute particle-cloud configurations within falling particle receivers can significantly enhance the apparent effective absorptance of the particle curtain, and materials with higher values of intrinsic particle absorptance will yield greater radiative absorptance and temperatures.