Physical properties of solid particle thermal energy storage media for concentrating solar power applications
Solid ceramic particles have proven to be an effective heat transfer and thermal storage media for central receiver power production for a heat input temperature up to 1000◦C. In the directly illuminated solid particle receiver, a cascade of ~0.1-1 mm diameter particles is directly heated within a receiver cavity by concentrated solar energy. The efficiency of this approach, with respect to the energy balance on the receiver itself, is dependent on the physical properties of the particles. In this work, the radiative properties, solar weighted absorptance and thermal emittance, have been measured for several commercially available particle candidates both in the as-received state and after thermal exposure to simulate extended operation at elevated temperature in air between 700◦C-1000◦C. Heating the particles is shown to significantly reduce the solar weighted absorptance of asreceived particles within 24 hours of exposure to air at 1000◦C, while heating at 700◦C in air has relatively little effect. In the asreceived state, solar weighted absorptance can be as high as 93%, dropping to 84% after 192 hours at 1000◦C. Particle stability is better at 700◦C, and the solar absorptance remains above 92% after 192 hours of exposure. Analysis using x-ray diffraction (XRD) shows evidence of multiple chemical transformations in the sintered bauxite particle materials, which contain oxides of aluminum, silicon, titanium, and iron, following heating in air. However, the XRD spectra show only small differences between as-received and heat treated particles leaving open the possibility that the observed change in radiative properties results from a change in oxidation state without a concomitant phase change. Regardless of the specific degradation mechanism, the solar weighted absorptance of the particles can be increased beyond the as-received condition by chemically reducing the particles in forming gas (5%H2 in N2 or Ar) above 700◦C, providing a possible means of periodically rejuvenating degraded particles in situ.
Siegel, Nathan P.; Gross, M.; Ho, Clifford K.; Phan, T.; and Yuan, J.. "Physical properties of solid particle thermal energy storage media for concentrating solar power applications." Energy Procedia (2014) : 1015-1023.