Multiphysics simulations of concentric-tube internal loop airlift photobioreactors for microalgae cultivation

In this work, a comprehensive multiphysics model is developed in-house to numerically predict microalgae productivity in concentric-tube internal loop airlift photobioreactors (PBRs). The program integrates solar radiation, light attenuation in microalgae suspensions, cell growth kinetics, and hydrodynamics. The sim-ulations of light attenuation and cell growth kinetics are based on experimental data measured in a PBR. The gas-liquid bubbly flow hydrodynamics is simulated using computational fluid dynamics (CFD) (Ansys FLUENT). The program is employed to predict the annual biomass areal and volumetric productivities of PBRs with various geometrical and operating parameters. The governing equations for the model integration are numerically solved using the finite-volume method for spatial discretization and an explicit scheme for time integration with a time step of PBR liquid circulation time. The effects of plant latitude, shading, clearance between adjacent PBRs, PBR dimension, downtime, and harvesting biomass threshold concentration on the annual biomass productivities are investigated. Under the assumptions made in this study, it is found that the 2.5 m-high PBRs yield higher annual areal biomass productivity than the 1.5 m-and 2 m-high PBRs. The highest productivity is found at a plant latitude of approximately 15 degrees. The optimal harvesting threshold concentration for the highest annual areal productivity is in the range of 0.4-0.7g l-1 depending on the downtime. The optimal concentration increases with longer downtime. Small clearances between adjacent PBRs are preferred to offer higher areal productivity. For PBRs positioned with small clearances, PBRs with smaller diameter-to-height ratios are more advantageous since the higher land utilization efficiency outweighs the increased shading loss.