Abstract
In a thin-volume photobioreactor where a concentrated suspension of microalgae is circulated throughout the established spatial irradiance gradient, microalgal cells experience a time-variable irradiance. Deploying this feature is the most convenient way of obtaining the so-called “flashing light” effect, improving biomass production in high irradiance. This work investigates the light flashing features of sloping wavy photobioreactors, a recently proposed type, by introducing and validating a computational fluid dynamics (CFD) model. Two characteristic flow zones (straight top-to-bottom stream and local recirculation stream), both effective toward light flashing, have been found and characterized: a recirculation-induced frequency of 3.7 Hz and straight flow-induced frequency of 5.6 Hz were estimated. If the channel slope is increased, the recirculation area becomes less stable while the recirculation frequency is nearly constant with flow rate. The validated CFD model is a mighty tool that could be reliably used to further increase the flashing frequency by optimizing the design, dimensions, installation, and operational parameters of the sloping wavy photobioreactor.
Highlights
Microalgae are considered as one of the most promising fast-growing photosynthetic microorganisms on Earth; it is expected that they can play an important role in CO2 sequestration, food, feed, sourcing of biochemical products, commodities, biofuels, and phytodepuration.Due to significant light attenuation along the light path [1], concentrated microalgal suspensions need to be cultured at minimal thickness to avoid reducing the volume-based specific growth rate [2]so cells that circulate back and forth along the established spatially distributed irradiance gradient experience a time-varying irradiance, which is the flashing light effect
The “flashing light” effect is extremely beneficial in outdoor cultures because microalgae reach their maximum photosynthetic activity at roughly 1/10 of the maximum irradiation recorded in summer days, and photosynthetic machinery is damaged by photooxidation and photoinhibition well below the maximum sunlight irradiance values [3] if light is not alternated with darkness at a proper rate
It showed a transport stream flowing on the bottom of the channel and a recirculation zone, which steadily occupied the central part of the cavities and represented, fairly well, the hydrodynamic features obtained from the experimental investigation (Figure 4)
Summary
Microalgae are considered as one of the most promising fast-growing photosynthetic microorganisms on Earth; it is expected that they can play an important role in CO2 sequestration, food, feed, sourcing of biochemical products, commodities, biofuels, and phytodepuration.Due to significant light attenuation along the light path [1], concentrated microalgal suspensions need to be cultured at minimal thickness to avoid reducing the volume-based specific growth rate [2]so cells that circulate back and forth along the established spatially distributed irradiance gradient experience a time-varying irradiance, which is the flashing light effect. The combination between an excessively large dark volume and an excessive irradiance in the well-illuminated zone is the main cause that limits the productivity of established commercial photobioreactors (PBRs) to one order of magnitude below the theoretical limit. This depression would be partly offset if cells were exposed to a rapidly variable irradiance as an effect of hydrodynamically travelling across differently lit zones. There is quite substantial discrepancy among the quantitative estimates of the benefits of light flashing, and this is at least partly due to differences in the experimental setup or assumptions
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