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 degrees C. In the directly illuminated solid particle receiver, a cascade of similar to 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 degrees C-1000 degrees C. Heating the particles is shown to significantly reduce the solar weighted absorptance of as-received particles within 24 hours of exposure to air at 1000 degrees C, while heating at 700 degrees C in air has relatively little effect. In the as-received state, solar weighted absorptance can be as high as 93%, dropping to 84% after 192 hours at 1000 degrees C. Particle stability is better at 700 degrees 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% H-2 in N-2 or Ar) above 700 degrees C, providing a possible means of periodically rejuvenating degraded particles in situ. (C) 2013 The Authors. Published by Elsevier Ltd.