Model-Based Prediction of Perceived Light Flashing in Recirculated Inclined Wavy-Bottomed Photobioreactors

Abstract

Microalgae biomass production rate in short light-path photobioreactors potentially can be improved by mixing-induced flashing light regimes. A cascade photobioreactor features a thin liquid layer flowing down a sloping, wavy-bottomed surface where liquid flow exhibits peculiar local recirculation hydrodynamics, potentially conducive to an ordered flashing light regime. This article presents a model-based analysis of the frequency distribution of perceived irradiance in said wavy-bottomed photobioreactor. The model combines a Lagrangian description of the motion of individual cells, in turn derived from the hydrodynamic parameters of the photobioreactor extracted from an experimentally validated Computational Fluid Dynamic model, with a simplified description of the irradiance field across the culture thickness, down to the spectral analysis of perceived irradiance. The main finding of the work is that the wavy bottomed photobioreactor provides a ‘robust’ spectral excitation to the circulating microalgae up to 3 Hz frequency, while in flat panels and bubble columns excitation decays evenly at a 24 db/octave rate. This analysis paves the way to improving the light flashing performance of the wavy-bottomed photobioreactor with respect to geometry (cavity size and installation inclination) and operation (flow rate).

Mass Transfer Features of Wavy-Bottomed Cascade Photobioreactors

“…Before stating that low installation angles are a win-win choice to deploy wavybottomed cascades, other aspects should be taken in consideration. The wavy-bottomed cascade photobioreactor, indeed, seems to exhibit interesting flashing light features due to the local (i.e., internal to each cavity) recirculation, and the effect of the prevailing hydrodynamics on the effective light-darkness alternation experienced by microalgae has been characterized in [14]. The authors of [14] show that the long-range recirculation (in the order of centimeters) is more effective in producing a powerful alternation frequency compared with other major photobioreactors such as the bubble column and the flat plane.…”

“…The wavy-bottomed cascade photobioreactor, indeed, seems to exhibit interesting flashing light features due to the local (i.e., internal to each cavity) recirculation, and the effect of the prevailing hydrodynamics on the effective light-darkness alternation experienced by microalgae has been characterized in [14]. The authors of [14] show that the long-range recirculation (in the order of centimeters) is more effective in producing a powerful alternation frequency compared with other major photobioreactors such as the bubble column and the flat plane. However, Moroni adopted for said study a 6 • installation angle where the transport stream flows below, and it would not be fair to conclude that, even at the same tumbling frequency, in the case of an upper-flowing transport stream, the frequency spectrum of perceived light flashing maintains the same level of sustained frequency excitation found for the bottom-flowing transport stream.…”

“…The Torzillo et al [13] prototype was aimed at inducing fast light/dark cycles and improving mass transfer without special turbulence enhancing devices and only resorting to gravity for liquid motion. This system was characterized both experimentally and computationally [14,15] and, for some operating conditions, it was found to be characterized by a particular fluid dynamics structure conducive to a flashing light regime for the circulated microalgae due to the fast and long-range ordered circulation of the microalgal suspension [14].…”