Experimental study of different materials in fluidized beds with a beam-down solar reflector for CSP applications

Abstract

Fluidized beds are particularly suitable for integration into Concentrated Solar Power (CSP) plants with beam-down reflectors. Due to the high mixing rates and heat diffusion typical of fluidized beds, they enable the highly concentrated solar flux from the beam-down reflector to impinge directly on the particles. Depending on size, density, surface roughness, optical and mechanical properties, some granular materials are more appropriate than others to store thermal energy at high-temperatures in a fluidized bed. In this line, this work compares the experimental performance of three different granular materials: sand, carbo Accucast ID50 and SiC, considering different airflow rates, and two fluidization technologies: bubbling and spouted bed. Experimental tests were carried out in a facility specifically tailored for these type of tests that permitted different flow arrangements to apply different fluidization techniques using air as the working fluid, including a beam-down reflector with a 4 kWe lamp to simulate the concentrated solar radiation (peak radiation flux of 115 kW/m2).The temperature evolution and storage/recovery efficiencies for the different materials were compared using the two fluidization technologies, and varying the radiation level and airflow rate. Because of these tests, the influence of the airflow rate and radiation levels on the thermal efficiency of the beds was evaluated. The experimental results showed that SiC was the best candidate, as it exhibited the best thermal performance, reaching a peak storage efficiency of ηC=0.95, obtained with an airflow rate of 2.5Umf. Maximum temperatures of 250 °C were reached in a cylindrical bed with an aspect ratio of L/D≈1.