Numerical investigation of the thermal performance of multistage falling particle receivers at commercial scales

Abstract

The thermal performance of commercial scale multistage falling particle receivers (MFPRs) was numerically investigated under various environmental conditions using advanced CFD models. This model accounted for spatially-varying solar radiation from the heliostats, particle dynamics, heat transfer, turbulent air flows, and the effect of wind. In this study, a 2-stage MFPR design has been explored for various trough heights and trough lengths. The final MFPR geometry provided ∼5%-points higher thermal efficiency compared to a similarly sized free-falling particle receiver (FFPR) at the same quiescent condition. The thermal performance of MFPRs was evaluated for three different commercial scales at various levels of incident solar power (Qin) to account for variation of incident solar power from changes in the direct normal irradiance (DNI) throughout a typical day. An upper bound for MFPR efficiency was obtained from scaling Qin by Ap1.2 where Ap is the open aperture area. The proposed scaling showed that the thermal efficiency asymptotes as a function of Qin/Ap1.2, and the thermal efficiency converges to ∼90% for Qin/Ap1.2 >0.75 regardless of the overall receiver scale. A parametric study was also performed to explore the effects of incident solar power, wind speed, and wind direction on the performance of MFPR at various scales. The study showed that the thermal efficiency of MFPRs is reduced with decreasing incident solar fluxes and with increasing wind speed. The degradation in thermal performance became significant for the present north-facing MFPR with the wind blowing from the northwest to west-northwest directions. It was found that for these wind directions, vortical structures existing ahead of the open aperture entrained cooler ambient air into the receiver cavity, increasing thermal convection between the ambient air and falling particles, and thus resulted in significant thermal performance degradation.