Previous studies of high-temperature falling particle receiver systems have investigated both high solar-absorptive sintered-bauxite ceramic particles (solar absorptivity \~0.9) and low solar-absorptive silica sands (solar absorptivity \~0.5).  Despite the low intrinsic solar absorptivity of some silica sands, sand is an attractive media for use in concentrating solar power (CSP) systems because of its extremely low cost.  In addition, the volumetric light-trapping effect of a falling particle curtain and the blackbody effect of the cavity receiver help to offset the adverse impacts of low intrinsic particle absorptivity.  Systems modeling by Albrecht et al. \[1\] showed that lower-cost particles with lower intrinsic solar absorptivity had a larger impact on reducing the levelized cost of electricity than higher-cost particles with higher intrinsic solar absorptivity.  However, uncertainties regarding the treatment and simulation of the effective solar absorptance of a dilute particle “cloud” exist.

To date, the scattering behavior of a high-temperature particle curtain has not been rigorously determined.  Albrecht et al. \[1\] used a model describing the effective absorptivity of a particle cloud from Johnstone et al. \[2\], but the model neglected particle reflections.  Kumar et al. \[3\] investigated radiative transfer and absorptance in a free-falling particle receiver assuming the particles behaved as a continuum in a two-phase Eulerian-Eulerian model.  In this study, we perform ray-tracing simulations of discrete particles to determine the effective particle absorptance (i.e., the absorbed particle irradiance divided by the incident irradiance not directly transmitted through the particle cloud).  Alternative configurations of particles in a representative elementary volume were investigated using different values for the intrinsic particle absorptivity.  In addition, different particle volume fractions were simulated.  Strategies to reduce the number of particles required to achieve a converged solution included the use of symmetric, specularly reflecting lateral boundaries, “smart” positioning of particles to yield desired opacities, and randomization of the directional irradiance.  Different particle shapes (spherical, cubical, and revolved hexagonal) were also investigated.

Results showed that the effective absorptance decreased with decreasing particle volume fractions in the representative elementary volume.  Lower values of particle intrinsic absorptance yielded greater backscattering (toward the entrance of the incident rays) and lower effective absorptance by the particle cloud.  The effective absorptance in all cases was increased above the intrinsic particle absorptance by \~5% - 30% due to re-reflection and light-trapping effects.  These findings indicate that reflections and backscattering can play an important role in falling particle receivers, reducing the effective absorptance when particles with low intrinsic solar absorptivity are used.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

References

\[1\]        Albrecht, K., M.L. Bauer, and C.K.Ho, 2019, *Parametric Analysis of Particle CSP System Performance and Cost to Intrinsic Particle Properties and Operating Conditions*, in *Proceedings of the ASME 2019 13th International Conference on Energy Sustainability*, ES2019-3893, Bellevue, WA, July 15 - 18, 2019.

\[2\]        Johnstone, H.F., R.L. Pigford, and J.H. Chapin, 1941, Heat Transfer to Clouds of Falling Particles, University of Illinois, Engineering Experiment Station Bulletin Series 330, Vol. XXXVIII, No. 43, Urbana, Illinois. June 17, 1941

\[3\]        Kumar, A., J.S. Kim, and W. Lipinski, 2018, Radiation Absorption in a Particle Curtain Exposed to Direct High-Flux Solar Irradiation, *Journal of Solar Energy Engineering-Transactions of the Asme*, 140(6).

 


Evaluating the Effective Solar Absorptance of Dilute Particle Configurations

Division / Sponsor:
Advanced Energy Systems Division (AESD) & Solar Energy Division (SED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ES/ES2020/volume/83631

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