Experimental characterization of granular materials for directly irradiated fluidized bed solar receivers

Concentrating Solar Power (CSP) systems stem out as a promising technology that has the advantage of an easy integration with thermal energy storage. The current benchmark in the CSP technology is represented by the solar tower receiver with molten salts working as heat transfer fluid. The main drawback of this system is the relatively low working temperature of the molten salts (about 565 °C), which affects the efficiency of the subsequent Rankine cycle for energy production. The use of dense suspensions of solid particles as solar receivers in CSP systems is gaining ever increasing interest, as the dense suspension can simultaneously act as receiver, heat transfer fluid and heat storage medium. Dense suspensions can work at higher temperature with respect to molten salts, even between 1000–1500 °C, without any safety, corrosion, or disposal problem. In this work, different granular materials were investigated to scrutinize their potential use as dense suspensions in directly irradiated Fluidized Bed (FB) reactors. Experimental tests were performed in a directly irradiated FB reactor exposed to a 12 kWel beam-down simulated solar furnace. The dynamics of the directly irradiated FB reactor was analyzed with specific reference to temperature distribution at the surface and in the bulk of the bed as a function of the inlet gas velocity. Material performances were compared by considering the energy required to sustain the fluidization conditions and solar absorption capacity of the granular material.