Heat absorption characteristics of gas in a solar fluidized bed receiver with carbon nanotubes

Carbon nanotubes (CNTs) have been proposed as new candidate particles to enhance the utilization efficiency of solar energy in solar fluidized bed receiver (SFBR) for solar air heating in low- and mid-temperature ranges. Heat absorption characteristics of the CNTs have been determined in a SFBR (50 mm i.d. X 150 mm high). Two types of experimental particles were used which consisted of multi-walled CNTs with different nanotube shapes, such as entangled CNTs (ENCNTs) and vertically aligned CNTs (VACNTs). The particle dynamics and heat absorption characteristics of CNTs were studied and compared with those of silicon carbide (SiC), a conventional particle. CNTs showed lower pressure fluctuation with relatively uniform particle behavior in the freeboard compared to SiC. The outlet gas temperatures of the receiver with CNTs were higher than those inside the fluidized bed above 0.10 m/s of gas velocity. The temperature increment of gas per irradiance (ΔT/I_DNI) decreased with increasing gas velocity. VACNTs, which are characterized by the coexistence of aggregates and nanotubes in the freeboard, showed a higher value of ΔT/I_DNI than ENCNTs for the same gas velocity. The relative heat absorption temperature (T*) decreased with increasing gas velocity, and dropped below 1.0 at the solid holdup of 0.04, indicating that the freeboard region’s contribution to the receiver’s heat absorption increased. VACNTs and ENCNTs showed maximum thermal efficiencies of 26.7 % and 30.5 % at gas velocities of 0.12 and 0.16 m/s, respectively, which was 33 % higher than that of SiC. Considering the particle properties and particle dynamics, the obtained thermal efficiencies in the present and previous studies were correlated with the Reynolds, Archimedes and Prandtl numbers and the ratios of the specific heat capacities of the particles to the gas.