Light/dark cycle enhancement and energy consumption of tubular microalgal photobioreactors with discrete double inclined ribs

Abstract

Tubular photobioreactors (PBRs) have a great potential in large-scale biomass cultivation and mixers in tubular PBRs have been intensively investigated to achieve high biomass productivity. However, mixers increase not only biomass yield, but also energy consumption. To evaluate performances on increasing light/dark (L/D) cycles and energy consumption of adding a mixer simultaneously, a new parameter named as efficiency of L/D cycle enhancement is introduced. Discrete double inclined ribs, intensively studied in heat transfer, are introduced to tubular PBRs in this work. The number of ribs in a cross section is discussed. These tubular PBRs are investigated in terms of the flow structure, L/D cycle frequency and efficiency of L/D cycle enhancement by computational fluid dynamics. The numerical results show that the increment of L/D cycle frequency caused by the discrete double inclined ribs is larger than the increment of energy consumption caused by the ribs under a wide range of incident light intensity. In general, the increasing of rib length ratio results in a decrease of efficiency and the PBR with two pairs of ribs performs the best. Based on the general trends, a PBR with two pairs of ribs and of which the rib length ratio is 5 is recommended for further studies.