Design considerations for a multiple tube solar reactor

A three-dimensional, steady state computational model coupling radiative transfer with fluid flow, heat transfer, mass transfer, and chemical reaction kinetics is utilized to investigate the sensitivity of the solar-to-chemical efficiency of a multiple-tube solar receiver with either a cooled reflective or insulated absorbing cavity wall to various design parameters including both operating conditions and geometric tube configurations. Geometric parameters are defined relative to a base design in which all tubes are arranged in a semicircle around the back cavity wall and allow for variations in cavity size as well as number, radius, and arrangement of tubes. For reflective cavity designs, calculations indicate that maximizing the net absorption efficiency and minimizing emission losses are not an effective means of improving receiver performance; rather, maximizing utilization of absorbed energy via minimization of tube conduction losses is essential. For the parameter ranges examined in this study, the best reflective cavity designs contain three moderately sized tubes placed near the back cavity wall while absorbing cavity designs contain three to five large tubes arranged in a semicircle near the back cavity wall, though all calculations are more sensitive to selection of operating conditions than to geometric parameters. Calculations for the application of steam gasification of acetylene black identified reflective and absorbing cavity receiver designs theoretically capable of achieving, respectively, 8% and 23% solar-to-chemical efficiency when scaled to accept 8 kW of solar power.