Session: 06-02: CSP Receivers and Reactors I

Paper Number: 169834

169834 - Investigation of Flux Spreading in a Light-Trapping, Planar-Cavity Receiver for Enclosed Solar Particle Heating 

Abstract: 

Concentrating solar thermal power (CSP) technology development has recently focused on increasing the operating temperatures to accommodate high efficiency power cycles and thermochemical processes. Inert solid particles as heat transfer media enable solar receivers to operate above 700 °C resulting in increased system efficiency compared to the conventional molten salt based CSP system. An open-cavity falling-particle solar receiver that can efficiently heat particles by direct heating from concentrated solar radiation faces challenges with large particle losses from wind and is unable to support thermochemical reactions. A light-trapping, planar cavity receiver (LTPCR) where particles are indirectly heated can significantly minimize the particle losses during operation, support thermochemical reactions, and offer scalability potential.

The LTPCR features an array of vertical planar receiver/absorber panels arranged within a cavity configuration. Concentrated solar radiation from heliostats is focused onto the receiver walls, where heat is indirectly transferred to solid particles flowing inside the receiver channels. Heat transfer occurs through direct contact between the receiver panel walls and particles and is enhanced by fluidizing particles with air. This fluidization increases particle-wall contact and extends particle residence time, maximizing heat transfer efficiency. The unique vertical planar receiver structure, effectively distributing the incoming solar beam across the panel walls and trapping light. This flux spreading effect, driven by cosine projection, converts high incident solar flux into a lower, more uniform heat flux on the panel walls. This redistribution enhances heat transfer efficiency between particle-wall or reaction gases-wall, while preventing localized overheating of the receiver panel.

An experimental investigation was conducted to observe flux spreading on the receiver panel wall. A lab-scale prototype planar receiver, fabricated using Haynes 230 alloy, was tested under direct concentrated solar radiation using the high-flux solar furnace (HFSF) facility at the National Renewable Energy Laboratory (NREL). The experiment was performed under normal peak radiative heat fluxes ranging from 800 to 1900 kW/m². A temperature distribution on the panel wall was measured using a thermal imaging camera (FLIR A 6600). To prevent overheating at the receiver front tip, prism-shaped heat shields (Zircar UNIFROM C1) were placed in front of the receiver, and their influence on flux spreading was also studied. Prototype testing was conducted for two different DNI conditions (25^th January and 6^th March 2024) and different attenuator openings varying from 35% to 100%.

The absorbed flux distribution on the panel wall was modeled using SolTrace. The total solar power and flux distributions delivered from the HFSF were determined based on the heliostat mirror optical properties, direct normal irradiance (DNI) on the on-sun testing days, peak flux measurement during the on-sun testing, and shutter/attenuator settings. Due to the large incident angles of the solar beam on the panel wall, the assumed angular optical properties of Haynes 230 alloy and Zircar heat shields were incorporated into the model. This flux distribution model was then integrated into a computational fluid dynamics (CFD) simulation to predict the receiver panel wall temperature, which was compared with the experimental measurements. Both prediction and measurements identified a temperature hotspot at the back side of the panel, indicating that the incident solar beam can fully reach to the rear of the receiver. The heat shields positioned at the front of the receiver effectively reduced the excessive temperature rise at the receiver front tip. Overall, the temperature was well distributed over the panel wall, with a minor hotspot at the back of the receiver. Computational models agreed well with the overall visual trend of temperature observed over the panel walls. Upon quantitative comparison, the error for average and maximum temperature between computational and experimental results was <11% and <20%, respectively. Also, the Pearson correlation coefficient for temperature distribution varied from 0.65 to 0.82 between the computations and experiments. 

Presenting Author: Shin Young Jeong National Renewable Energy Laboratory

Presenting Author Biography: Shin Young Jeong is a postdoctoral researcher in the Thermal Energy Systems group at the National Renewable Energy Laboratory in Golden, Colorado.  He received his PhD in mechanical engineering from the Georgia Institute of Technology in 2023 on characterization of thermophysical properties and flow behaviors of bauxite particles in high-temperature for concentrated solar-thermal power application.  His current research focuses on development of particle-based concentrated solar-thermal power and high temperature thermal energy storage for power generation and industrial heats.