Efficiency limits of high-temperature transparent packed-bed solar receivers

Abstract

Gas-phase solar receivers represent one of the best near-term solutions for providing temperatures in excess of 1000 K. However, as the desired outlet temperature increases, heat losses also inexorably increase. To mitigate this trade-off, this research systematically analyses a semi-transparent packed-bed solar receiver which can enable absorption to occur within the receiver's volume rather than on the surface (where thermal emission losses occur). As such, this study aims to find the limits-and the key limiting factors-for the overall performance of this new class of the receiver. This was done by exploring a wide range of geometrical (e.g., ~0.01 to ~ 1.45 spheres per cm3 or D/dp ranging from 3 to 15) and operational parameters. This parametric study was conducted by coupling a comprehensive heat transfer model with ray tracing and a surface-to-surface radiation model. For a 1000 K outlet temperature, an effective maximum receiver efficiency of just over 70% was achieved with 18 rows of transparent spheres (i.e., D/dp = 9 or ~ 0.3 spheres per cm3). This analysis also revealed a critical dimensionless parameter, Ω, which must be kept near a value of 1 to maximise the effective efficiency. This proposed dimensionless parameter was derived from the ratio of conduction to convective heat transfer multiplied by the ratio of absorbed solar power to input pumping power. Overall, this study's results indicate that semi-transparent packed beds represent a promising near-term design for cost-effective, high-temperature solar receivers, which can be used in advanced power cycles.