Decrease in light/dark cycle of microalgal cells with computational fluid dynamics simulation to improve microalgal growth in a raceway pond

Optimization and synergistic enhancement of microalgae productivity in laboratory raceway ponds via co-regulation of automated light-supplemented mixers and electric field system

Raceway pond systems face inherent challenges in achieving optimal biomass productivity due to limitations in vertical mixing efficiency and uneven light distribution, compounded by the intrinsic dilute nature of phototrophic cultures. The combination of automated light-supplemented mixers and electric field treatment introduces a promising strategy to enhance raceway pond gas‒liquid mass transfer, improve microalgae biomass production, and increase carbon fixation. Computational fluid dynamics simulations identified an optimal mixing configuration employing a 75° inclined blade rotating counterclockwise at 300 rpm, which reduced dead zones from approximately 15.5% to 1.1% and shortened the light-dark exposure of cells to 2.7 s in a laboratory-scale raceway pond (71.4 dm3). Additionally, daily one-hour electrostatic field stimulation at 0.6 V cm⁻1 during the logarithmic growth phase significantly enhanced algal growth. The novel raceway pond system achieved a 20% increase in the productivity of Limnospira fusiformis and elevated the maximum carbon fixation rate to 0.14 g L⁻1 d⁻1, representing a 43% improvement and the high-value phycocyanin increased by 14.4%. This approach enhanced mixing efficiency and light utilization, providing a scalable strategy for high-value microalgae production in controlled bioreactors.

Influence of paddlewheel geometry on hydrodynamic performance and energy consumption in microalgae cultivations in open raceway ponds

Microalgae cultivation is a field with significant potential across various industries, such as pharmaceuticals, food, cosmetics, and energy. Aiming to optimize the geometric parameters of a paddlewheel agitation system through computational fluid dynamics (CFD) and experimental validation, an investigation was conducted involving different rotation speeds, blade pitch angles, and the number of blades on a paddlewheel within a microalgae cultivation tank. The results revealed paddlewheels with 90° inclined blades exhibited higher average flow velocities. Regarding the vertical mixing index, the 8-blade paddlewheels demonstrated inferior performance compared to systems with four and six blades. A pitch angle of 60° minimized power consumption. The optimal configuration found was a 60°-angled 4-blade paddlewheel, operating at a rotation speed of 19 rpm, yielding the highest mixing performance index value at 46.12 W-1. The straight blades operated at 13 rpm could not sustain microalgae suspension. For 60° inclined blades, all tested rotations achieved a high level of suspension.

Improving light availability and creating high-frequency light-dark cycles in raceway ponds through vortex-induced vibrations for microalgae cultivation: a fluid dynamic study

Limited light availability due to insufficient vertical mixing strongly reduces the applicability of raceway ponds (RWPs). To overcome this and create light-dark (L/D) cycles for enhanced biomass production through improved vertical mixing, vortex-induced vibration (VIV) system was implemented by the authors in a previous study to an existing pilot-scale RWP. In this study, experimental characterization of fluid dynamics for VIV-implemented RWP is carried out. Particle image velocimetry (PIV) technique is applied to visualize the flow. The extents of the vertical mixing due to VIV and the characteristics of L/D cycles were examined by tracking selected particles. Pond depth was hypothetically divided into three zones, namely dark, light Iimited and light saturated for detailed analysis of cell trajectories. It has been observed that VIV cylinder oscillation can efficiently facilitate the transfer of cells from light-limited to light-saturated zones. Among the cells that were tracked, 44% initially at dark zone entered the light-limited zone and 100% of initially at light-limited zone entered the light-saturated zone. 33% of all tracked cells experienced high-frequency L/D cycles with an average frequency of 35.69 s-1 and 0.49 light fraction. The impact of VIV was not discernible in the deeper sections of the pond, due to constrained oscillation amplitudes. Our findings suggest that the approximately 20% increase in biomass production reported in our previous study can be attributed to the synergistic effects of enhanced L/D cycle frequencies and improved light availability resulting from the transfer of cells from dark to light-limited zones. To further enhance the effectiveness of VIV, design improvements were developed. It was concluded that light availability could be significantly improved with the presented method for more effective use of RWPs.

Synergistic optimization of baffles and aeration to improve the Light/Dark cycle of microalgae photobioreactor for enhanced nitrogen removal performance: Computational fluid dynamics and experimental verification

Microalgae photobioreactor (PBR) is a kind of efficient wastewater treatment system for nitrogen removal. However, there is still an urgent need for process optimization of PBR. Especially, the synergistic effect and optimization of light and flow state poses a challenge. In this study, the computational fluid dynamics is employed for simulating the optimization of the number and length of the internal baffles, as well as the aeration rate of PBR, which in turn leads to the optimal growth of microalgae and efficient nitrogen removal. After optimization, the Light/Dark cycle of the reactor B is shortened by 51.6 %, and the biomass increases from 0.06 g/L to 3.94 g/L. In addition, the removal rate of NH4+-N increased by 106.0 % to 1.56 mg L-1 h-1. This work provides a feasible method for optimizing the design and operational parameters of PBR aiming the engineering application.

Universal and unique strategies for the production of polyunsaturated fatty acids in industrial oleaginous microorganisms

Polyunsaturated fatty acids (PUFAs), especially docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) and arachidonic acid (ARA), are beneficial for reducing blood cholesterol and enhancing memory. Traditional PUFA production relies on extraction from plants and animals, which is unsustainable. Thus, using microorganisms as lipid-producing factories holds promise as an alternative way for PUFA production. Several oleaginous microorganisms have been successfully industrialized to date. These can be divided into universal and specialized hosts according to the products range of biosynthesis. The Yarrowia lipolytica is universal oleaginous host that has been engineered to produce a variety of fatty acids, such as γ-linolenic acid (GLA), EPA, ARA and so on. By contrast, the specialized host are used to produce only certain fatty acids, such as ARA in Mortierella alpina, EPA in Nannochloropsis, and DHA in Thraustochytrids. The metabolic engineering and fermentation strategies for improving PUFA production in universal and specialized hosts are different, which is the subject of this review. In addition, the widely applicable strategies for microbial lipid production that are not specific to individual hosts were also reviewed.

Modeling and Simulation of Photobioreactors with Computational Fluid Dynamics—A Comprehensive Review

Computational Fluid Dynamics (CFD) have been frequently applied to model the growth conditions in photobioreactors, which are affected in a complex way by multiple, interacting physical processes. We review common photobioreactor types and discuss the processes occurring therein as well as how these processes have been considered in previous CFD models. The analysis reveals that CFD models of photobioreactors do often not consider state-of-the-art modeling approaches. As a comprehensive photobioreactor model consists of several sub-models, we review the most relevant models for the simulation of fluid flows, light propagation, heat and mass transfer and growth kinetics as well as state-of-the-art models for turbulence and interphase forces, revealing their strength and deficiencies. In addition, we review the population balance equation, breakage and coalescence models and discretization methods since the predicted bubble size distribution critically depends on them. This comprehensive overview of the available models provides a unique toolbox for generating CFD models of photobioreactors. Directions future research should take are also discussed, mainly consisting of an extensive experimental validation of the single models for specific photobioreactor geometries, as well as more complete and sophisticated integrated models by virtue of the constant increase of the computational capacity.

Enhancing microalgae production by installing concave walls in plate photobioreactors

In order to optimize light distribution for promoting biomass growth rate of Chlorella pyrenoidosa, concave walls were installed in plate photobioreactors (PBR) to generate rotational flow field of microalgal solution circulated from top inlets to bottom outlets. Flow vortices in four corners of concave-wall PBR resulted in decreased mixing time and increased mass transfer coefficient. The CO₂ bio-fixation by C. pyrenoidosa increased by 27% and chlorophyll-a concentration enhanced by 18.5% in concave-wall PBR compared to those in control (flat-wall) PBR. The concave walls diverge light rays to enhance frontal light exposure and supply more light photons into interior regions of PBRs. The promotion in light distribution and vortex flow field with concave walls enhanced light and nutrients utilization by microalgal cells, leading to an increased biomass growth rate by 21%.

Strengthening mass transfer with the Tesla-valve baffles to increase the biomass yield of Arthrospira platensis in a column photobioreactor

In this study, the Tesla-valve (TV) baffles were used to optimize the flow field in a column photobioreactor (PBR) in order to promote mass transfer of CO₂ gas in the solution. The TV baffles were composed of many tilted plates with central holes and curved arcs facing downwards, installed along inner rising section of the column PBR. Many clockwise and anti-clockwise vortices were generated during the rising flow while passing through proposed TV baffles. An optimum TV baffle structure (30° plate angle, 8 cm arc width) decreased mixing time by 36.4% and increased the mass transfer coefficient by 50%. The TV baffles supported the movement of the A.platensis cells between light and dark regions to enhance their photochemical efficiency ϕPSII by 24.6% and Fv/Fm by 12.7%. Therefore, the biomass yield increased by 28.1% and exhibited an increased helix pitch and trichome length in comparison with traditional column PBR without baffles.

Strengthening flash light effect with a pond-tubular hybrid photobioreactor to improve microalgal biomass yield

Poor light utilization efficiency and large occupied area of traditional raceway pond photobioreactors result in low areal microalgal biomass yield in industrial applications. In this study, a pond-tubular hybrid photobioreactor (PTH-PBR) comprising raceway ponds and horizontal tubes was developed to strengthen flash light effect and improve areal microalgal biomass yield. The highest flash cycle frequency (0.63 Hz) of microalgae cells along flow pathway was obtained in the raceway pond of PTH-PBR when shaded area percentage was 20% and ratio of adjacent tube interval to tube diameter was 1, which enhanced microalgal biomass yield by 31.2% than traditional raceway pond. Meanwhile, intracellular chlorophyll content increased by 33.6% and PSII maximum quantum yield (F_v/F_m) increased by 8.1% due to decreased photoinhibition stress. The areal microalgal biomass yield of PTH-PBR was 54.7% higher than that of traditional raceway pond without horizontal tubes.

Enhancing microalgal biomass productivity with an optimized flow field generated by double paddlewheels in a flat plate photoreactor with CO2 aeration based on numerical simulation

Computational fluid dynamics were used to analyze the flash light effect and CO₂ bubble behavior in an optimized flow field generated by double paddlewheels in a flat plate photoreactor to enhance microalgal biomass productivity. The increased D/w ratio significantly enhanced the average turbulent kinetic energy and flash cycle frequency. However, the effects became weak when the D/w ratio was over 0.67. Appliance of double paddlewheels increased flash cycle frequency from 0.035 to 0.121 Hz and increased light time ratio from 8.3% to 31.5%. Meanwhile, the bubble dynamic behavior was characterized using population balance model. The average bubble size reduced by 24.4% and the bubble rising velocity reduced by 10.6%, which facilitated CO₂ mixing and mass transfer in microalgal solution. Therefore, biomass accumulation of microalgae Chlorella in the photoreactor with double paddlewheels increased by 62.3% under 15% CO₂.

Simulation of a Novel Tubular Microalgae Photobioreactor with Aerated Tangent Inner Tubes: Improvements in Mixing Performance and Flashing-Light Effects

At present, large-scale and high-efficiency microalgal cultivation is the key to realizing the technology for carbon capture and storage (CCS) and bioresource recovery. Meanwhile, tubular photobioreactors (PBRs) have great potential for microalgal cultivation due to their high productivity. To improve the mixing performance and flashing-light effect, a novel tube PBR with the inner tube tangential to the outer tube was developed, whose radial aeration pores are situated along the length of the inner tube. The direction of aeration, aeration rate, light/dark cycle period (L/D), light-time ratio, average turbulent kinetic energy (TKE), and degree of synergy between the velocity and direction of the light field in the PBR were optimized by a computational fluid dynamics (CFD) simulation and field synergy theory. The results show that a downwards aeration direction of 30° and an aeration rate of 0.7 vvm are the most conducive to reducing the dead zone and improving the light/dark cycle frequency. Compared to the concentric double-tube PBR, the light/dark cycle frequency and light time of the tangent double-tube PBR increased by 78.2% and 36.2% to 1.8 Hz and 47.8%, respectively, and the TKE was enhanced by 48.1% from 54 to 80 cm²·s⁻². Meanwhile, field synergy theory can be extended and applied to the design of tubular microalgae PBRs, and the average synergy of the light and velocity gradients across the cross-section increased by 38% to 0.69. The tangential inner tube aeration structure generated symmetrical vertical vortices between the light and dark areas in the PBR, which significantly improved the mixing performance and flashing-light effect. This novel design can provide a more suitable microenvironment for microalgal cultivation and is promising for bioresource recovery applications and improving the yield of microalgae.

Application of computational fluid dynamics (CFD) on the raceway design for the cultivation of microalgae: a review

Microalgae are a potential solution to supersede fossil fuels and produce renewable energy. The major obstacle to the commercialization of microalgae-based biofuels is the high production cost, including nutritional requirements, photobioreactor design, and downstream processes. As for the photobioreactor design, open ponds have been adopted by major commercial plants for their economic advantages. Raceway is a popular type among open ponds. Nevertheless, the fluid dynamics of the raceway operation is quite complex. Software simulation based on Computational Fluid Dynamics is an upcoming strategy for optimizing raceway design. The optimization intends to affect light penetration, particle distribution, mass transfer, and biological kinetics. This review discusses how this strategy can be helpful to design a highly productive raceway pond-based microalgal culture system.

Self-rotary propellers with clockwise/counterclockwise blades create spiral flow fields to improve mass transfer and promote microalgae growth

In this work, self-rotary propellers (SRPs) with clockwise/counterclockwise blades were investigated to create spiral flow fields without external power to strengthen gas-liquid mixing and promote microalgal growth in an open raceway pond. The rotational flow around the propellers and spiral flow between the propellers generated extensive wall shear stress in three dimensions. Four-clockwise blades on the propellers exerted better mixing than three-counterclockwise blades. The bubble generation diameter was reduced by 69% and the mass transfer coefficient increased by 49% when the propeller diameter was increased from 32 to 60 mm. The photochemical efficiency (φPS_II) of Arthrospira platensis cells was enhanced by 25%, while the helix pitch and trichome lengths were enlarged by 7-16%. Self-rotary propellers (60 mm diameter) with four-clockwise blades enhanced the growth rate of A. platensis biomass by 35% compared to that in an unmodified raceway pond without propellers.

Application of computational fluid dynamics to raceways combining paddlewheel and CO2 spargers to enhance microalgae growth

The present study investigated the effect of light intensity and mixing on microalgae growth in a raceway by comparing the performance of a paddlewheel to a combination of paddlewheel and CO₂ spargers in a 20 L raceway. The increase of light intensity was known to be able to increase the microalgal growth rate. Increasing paddlewheel rotation speed from 13 to 30 rpm enhanced C. vulgaris growth by enhancing culture mixing. Simulation results using computational fluid dynamics (CFD) indicated that both the turnaround areas of the raceway and the area opposite the paddlewheel experienced very low flow velocities (dead zones) of less than 0.1 m/min, which could cause cell settling and slow down growth. The simulated CFD velocity distribution in the raceway was validated by actual velocity measurements. The installation of CO₂ spargers in the dead zones greatly increased flow velocity. The increase of paddlewheel rotation speed reduced the dead zones and hence increased algal biomass production. By complementing the raceway paddlewheel with spargers providing CO₂ at 30 mL/min, we achieved a dry cell weight of 5.2 ± 0.2 g/L, which was about 2.6 times that obtained without CO₂ sparging.

Developing a CO2 bicarbonation absorber for promoting microalgal growth rates with an improved photosynthesis pathway

In order to solve the problems of the short residence time and low utilization efficiency of carbon dioxide (CO₂) gas added directly to a raceway pond, a CO₂ bicarbonation absorber (CBA) was proposed to efficiently convert CO₂ gas and sodium carbonate (Na₂CO₃) solution to sodium bicarbonate (NaHCO₃), which was dissolved easily in the culture medium and left to promote the microalgal growth rate. The CO₂ gas reacted with the Na₂CO₃ solution (initial concentration = 200 mM L⁻¹ and volume ratio in CBA = 60%) for 90 min at 0.3 MPa to give the optimized molar proportion (92%) of NaHCO₃ product in total inorganic carbon and increase the microalgal growth rate by 5.0 times. Quantitative label-free protein analysis showed that the expression levels of the photosystem II (PSII) reaction centre protein (PsbH) and PSII cytochrome (PsbV2) in the photosynthesis pathway increased by 4.8 and 3.4 times, respectively, while that of the RuBisCO enzyme (rbcL) in the carbon fixation pathway increased by 3.5 times in Arthrospira platensis cells cultivated with the NaHCO₃ product in the CBA at 0.3 MPa.

To increase the residence time of CO₂ gas added directly to the raceway pond, a CO₂ bicarbonation absorber was proposed to convert CO₂ gas and Na₂CO₃ to NaHCO₃, which was dissolved easily in the solution and left to promote the biomass growth rate.

Numerical simulation on promoting light/dark cycle frequency to improve microalgae growth in photobioreactor with serial lantern-shaped draft tube

Computational fluid dynamics were employed to simulate microalgal cells movement with enhanced flash-light effects in a gaslift loop-current column photobioreactor (GLCP) with serial lantern-shaped draft tube (LDT). Clockwise and anticlockwise vortexes were formed in outer down-flow region of GLCP with LDT. The radial velocity, axial velocity, and turbulent kinetic energy of microalgal solution appeared periodical change around the lanterns. The average radial velocity showed a sixfold improvement from 0.003 m/s to 0.021 m/s, and average turbulent kinetic energy was enhanced by 18.2% from 22.5 × 10^-4 m²/s² to 26.6 × 10^-4 m²/s², thus increasing light/dark cycle frequency by 54%. The light/dark cycle frequency increased first and then decreased with an increase of individual lantern height. The increased lantern number promoted the light/dark cycle frequency and light time ratio. Microalgal biomass yield in the GLCP with LDT was improved by 30%, and CO₂ fixation peak rate was promoted by 35%.

An efficient Photobioreactors/Raceway circulating system combined with alkaline-CO2 capturing medium for microalgal cultivation

High efficiency of microalgal growth and CO₂ fixation in a Photobioreactors (PBRs)/Raceway circulating (PsRC) system combined with alkaline-CO₂ capturing medium and operation was established and investigated. Compared with a pH 6 medium, the average biomass productivity of Chlorella sp. AT1 cultured in a pH 11 medium at 2 L min^-1 circulation rate for 7 days increased by about 2-fold to 0.346 g L^-1 d^-1. The maximum amount of CO₂ fixation and CO₂ utilization efficiency of Chlorella sp. AT1 could be obtained at a PBRs to Raceway ratio of 1:10 in an indoor-simulated PsRC system. A similar result was also shown in an outdoor PsRC system with a 10-ton scale for microalgal cultivation. Under the appropriate circulation rate, the stable growth performance of Chlorella sp. AT1 cultured by long-term semi-continuous operation in the 10-ton outdoor PsRC system was observed, and the total amount of CO₂ fixation was approximately 1.2 kg d^-1 with 50% CO₂ utilization efficiency.

Improving microalgal growth by strengthening the flashing light effect simulated with computational fluid dynamics in a panel bioreactor with horizontal baffles

Biological CO2 elimination by photosynthetic microalgae is a sustainable way to mitigate CO2 from flue gas and other sources. Computational fluid dynamics was used to simulate algal cell movement with an enhanced flashing light effect in a novel panel bioreactor with horizontal baffles. Calculation results showed that the light/dark (L/D) cycle period decreased by 17.5% from 17.1 s to 14.1 s and that the horizontal fluid velocity increased by 95% while horizontal baffles were used under a 0.02 vvm air aeration rate and a microalgal concentration of 0.85 g L-1. The probability of the L/D cycle period within 5-10 s increased from 27.9% to 43.6%, indicating a 56% increase when horizontal baffles existed. It was proved by experiments that the mass-transfer coefficient increased by 31% and the mixing time decreased by 13% under a 0.06 vvm air aeration rate when horizontal baffles were used, and the algal biomass yield increased by ∼51% along with the decrease in the L/D cycle period when horizontal baffles were used.

Alternatively permutated conic baffles generate vortex flow field to improve microalgal productivity in a raceway pond

Alternatively permutated conic (APC) baffles were proposed to generate vertical and horizontal vortex flow to intensify mixing and mass transfer in a raceway pond. Both clockwise vortexes were generated before and after conic baffles in the main stream to increase perpendicular velocity by 40.3% and vorticity magnitude by 1.7 times on vertical cross section. Self-rotary flow around conic baffles and vortex flow among conic baffles were generated to increase perpendicular velocity by 80.4% and vorticity magnitude by 4.2 times on horizontal cross section. The bubble generation time and diameter decreased by 25.5% and 38.7%, respectively, while bubble residence time increased by 84.3%. The solution mixing time decreased by 48.1% and mass transfer coefficient increased by 34.0% with optimized relative spacing (ε) and height (ω) of conic baffles. The biomass productivity of Spirulina increased by 39.6% under pure CO₂ with APC baffles in a raceway pond.

Experimental and modelling of Arthrospira platensis cultivation in open raceway ponds

In this study, the growth of Arthrospira platensis was studied in an open raceway pond. Furthermore, dynamic model for algae growth and CFD modelling of hydrodynamics in open raceway pond were developed. The dynamic behaviour of the algal system was developed by solving mass balance equations of various components, considering light intensity and gas-liquid mass transfer. A CFD modelling of the hydrodynamics of open raceway pond was developed by solving mass and momentum balance equations of the liquid medium. The prediction of algae concentration from the dynamic model was compared with the experimental data. The hydrodynamic behaviour of the open raceway pond was compared with the literature data for model validation. The model predictions match the experimental findings. Furthermore, the hydrodynamic behaviour and residence time distribution in our small raceway pond were predicted. These models can serve as a tool to assess the pond performance criteria.