Properties of solid particles as heat transfer fluid in a gravity driven moving bed solar receiver

Abstract

The directly irradiated solid particle solar receiver (SPSR) is a very useful design due to its ability to safely absorb high solar irradiance fluxes. The heat transfer fluid in the directly irradiated SPSR should consist of solid particles that do not agglomerate or fracture, have a high solar weighted absorptivity and good chemical stability at high temperature. This study analyzes the thermophysical properties, spectral absorptivity and flow stability of candidate solid particles for use in an SPSR. Most particles had excellent thermal and chemical stability following heat treatment at 1000 °C. The thermophysical analysis showed that SiO2 in the particles strongly impacted the specific heat variations with temperature. An insert was added in the quartz hopper to form a semi-annular flow channel and improve the particle flow stability during the discharge process. The measurements show how the heat treatment, operating temperature, particle size, and elemental composition influence the radiative properties. Three particles were found to have the best characteristics for the SPSR. An analysis of the energy conservation showed that the steady-state SPSR thermal efficiency can exceed 65% with the grey ceramsite sand when heated to 1073 K by an average radiant flux of 400 kW/m2.