Experimental analysis of micro-cavity influence on the effective solar absorptance of white sand curtain on porous obstructions of particle heating receivers

The direct particle heating receiver (DPHR) represents a critical component within particle-based central receiver tower (CRT) systems, facilitating direct exposure of solid particles to concentrated solar irradiance. Among DPHRs, the obstructed flow particle heating receiver (OF-PHR) emerges as a novel design under development at King Saud University (KSU), enabling particles to descend freely in a curtain-like manner through straight-shaped porous obstructions. This configuration effectively attenuates particle acceleration during descent, thereby prolonging their residence time within the irradiated zone. The innovative design of a porous OF-PHR significantly enhances particle heating by forming a well-dispersed and substantially thicker particle curtain. Within this curtain, micro-cavities are generated, which dramatically reduce radiation loss to the sky by facilitating multiple reflections, effectively trapping intercepted rays, and thereby maximizing solar absorptance. Achieving high solar absorptance offers substantial economic benefits by enabling the storage of thermal energy in low-cost, naturally abundant solid particles, such as silica sand, despite their inherently poor optical properties. This research studies the main aspects that influence the effective solar absorptance (ESA) of the direct porous OF-PHR. A high-flux module (HFM) was devised and constructed to experimentally assess ESA, comprising a particle-handling unit, a primary concentrator, and a secondary concentrator. Particle curtain ESA was scrutinized across various parameters, particle flow rate, and PHR structural geometry, represented by porous obstruction packing, perforation size, and arrangements. Experimental tests were conducted on white sand (WS). ESA was compared to that of another two PHR configurations: Bare, and free-fall PHRs. ESA measurements revealed a substantial enhancement in particle curtain absorptivity compared to the particle-packed bed. The WS curtain attained a peak ESA of 0.85, contrasting with the packed bed absorptance of 0.42.