Etude théorique et expérimentale de l'intensification de la lumièe et de la culture dans les systèmes de culture solaires de microalgues

Le coûts de cultivation des microalgues doivent être réduits pour pouvoir lesutilisés comme une ressource renouvelable.À cette fin, deux approches sont envisagées pour augmenter la productivité des systèmes de culture de microalgues via l’intensification de la lumière et de la culture. Tout d’abord,l’utilisation de miroirs externes pour augmenter la productivité des raceways solaires a été é valuée théoriquement et expérimentalement.L’impact de l’emplacement des miroirs et les conditions solaires ont été pris en compte.Les miroirs ont augementé la productivité volumétrique de la biomasse jusqu’à 37% sous un ciel clair. Ensuite, une campagne expérimentale de preuve de concept d’un photobioreacteur intensifié à tube mince destiné à des applications de biofaçade a été réalisée.Ce concept a permis d’obtenir une productivité élevée sans la formation de biofilm. Cependant,la croissance de la culture a été limitée par la suraccumulation d’oxygène. Pour optimiser ce concept, une étude détaillée du transfert de lumière a été menée. Une approche de Monte Carlo ray-tracing a été utilisée pour prédire le champs radiative dans la culture selon la géométrie, l’inclinaison et les conditions de fonctionnement du PBR.Microalgae cultivation costs mustbe reduced to be used as a renewable, lowcarbonsource of food, energy, and valueaddedproducts. To this end, two approachesare considered to increase the productivity ofmicroalgae cultivation systems via light andculture intensification principles. First, the useof external mirrors to increase the productivityof outdoor raceway ponds was assessedtheoretically and experimentally. The impactof the mirror location, solar position, and operatingparameters such as biomass concentrationand culture depth were considered.Indeed, up to a 37 % increase in volumetricbiomass productivity was observed experimentallyunder clear skies. Then, a proof-ofconceptexperimental campaign of an intensifiedthin-tube photobioreactor targeted for biofaçadeapplications was performed. The thintubedesign enabled high biomass productivitieswith minimum biofilm formation. However,culture growth was found to be limited by oxygenover-accumulation. To optimize the thintubedesign, a detailed study of light transferwas conducted. A Monte Carlo ray-tracingapproach was used to predict the local fluencerate within the culture for a given solarPBR geometry, inclination, and operating condition

Modeling and Optimization of Light Transfer in Outdoor Microalgae Cultivation Systems

Value-added products derived from photosynthetic microalgae could serve as a useful renewable resource in the face of mounting pressures on food, energy, and water systems from global climate change. In addition to acting as a carbon sink, microalgae are fast-growing organisms whose rich biodiversity is reflected in their variety of potential applications, ranging from cosmetics and pharmaceuticals to biofuels, food products, and animal feed. Light transfer plays a vital role in the productivity of outdoor microalgae cultivation systems. Indeed, in the case of optimal operating conditions such as temperature, pH, and nutrient availability, microalgae growth depends entirely on the rate of light absorption by the cells. However, large-scale microalgae cultivation typically takes place in outdoor raceway ponds where light transfer can be impacted by a variety of factors. For instance, outdoor raceway ponds may feature a transparent cover to achieve better control of the growth conditions. However, evaporation from the culture results in condensate droplets on the underside of the cover, potentially reducing the window transmittance and solar energy input to the culture. Furthermore, a variety of species of interest for value-added products readily form colonies either in the forms of aggregate-like clusters or of ordered spherical shells. Such a change in the arrangement of the cells may impact their ability to absorb the incoming photons. Finally, outdoor ponds are subject to low solar intensities and large angles of incidence in winter, mornings, and evenings. These phenomena may result in limiting light transfer conditions and low biomass productivity and remain major barriers to unlocking the potential of microalgae as a sustainable and inexpensive source of value-added products. Therefore, a comprehensive understanding of light transfer in microalgae cultivation systems is necessary to optimize their performance. This dissertation aims (1) to quantify the impact of small and large condensate droplets on the transmittance of transparent windows and on the performance of outdoor raceway ponds, (2) to assess the impact of colony formation on light absorption by microalgae cells, and (3) to investigate the use of external reflecting surfaces to increase light availability in dense cultures and increase raceway pond productivity. First, light transmittance through horizontal and tilted windows supporting large pendant droplets was predicted for various droplet volumes, contact angles, and window tilt angles. Compared to windows supporting small droplets, the transmittance of windows supporting large droplets was up to 37% smaller for horizontal windows and up to 14% larger for tilted windows. Then, light transfer through a window supporting small, cap-shaped droplets was coupled with a growth kinetics model to elucidate the impact of condensate droplets on the biomass productivity of an outdoor raceway pond. Biomass productivity was predicted to decrease by up to 18% when condensate droplets were present. Second, light transfer in aggregate-like colonies of Botryococcus braunii was also investigated both experimentally and numerically. Good agreement was found between the experimental and predicted absorption cross-sections. This approach was also applied to study light absorption in larger, ordered colonies like those observed in species of the Volvocaceae family. In both cases, mutual shading between the cells in the colonies decreases light absorption by up to 23% compared to single cells, which may decrease the algae growth rate. Third, the performance of a novel reflecting outdoor raceway pond design was predicted throughout the year for two locations and several different design configurations. A single south-facing mirror was predicted to increase biomass productivity by as much as 73% in the winter months. Overall, the biomass productivity was found to improve throughout the year thanks to the increased solar energy input provided by the additional sunlight reflected onto the culture surface. This approach could extend the growing season for outdoor cultivation of microalgae

Etude théorique et expérimentale de l'intensification de la lumièe et de la culture dans les systèmes de culture solaires de microalgues

Microalgae cultivation costs mustbe reduced to be used as a renewable, lowcarbonsource of food, energy, and valueaddedproducts. To this end, two approachesare considered to increase the productivity ofmicroalgae cultivation systems via light andculture intensification principles. First, the useof external mirrors to increase the productivityof outdoor raceway ponds was assessedtheoretically and experimentally. The impactof the mirror location, solar position, and operatingparameters such as biomass concentrationand culture depth were considered.Indeed, up to a 37 % increase in volumetricbiomass productivity was observed experimentallyunder clear skies. Then, a proof-ofconceptexperimental campaign of an intensifiedthin-tube photobioreactor targeted for biofaçadeapplications was performed. The thintubedesign enabled high biomass productivitieswith minimum biofilm formation. However,culture growth was found to be limited by oxygenover-accumulation. To optimize the thintubedesign, a detailed study of light transferwas conducted. A Monte Carlo ray-tracingapproach was used to predict the local fluencerate within the culture for a given solarPBR geometry, inclination, and operating conditionsLe coûts de cultivation des microalgues doivent être réduits pour pouvoir lesutilisés comme une ressource renouvelable.À cette fin, deux approches sont envisagées pour augmenter la productivité des systèmes de culture de microalgues via l’intensification de la lumière et de la culture. Tout d’abord,l’utilisation de miroirs externes pour augmenter la productivité des raceways solaires a été é valuée théoriquement et expérimentalement.L’impact de l’emplacement des miroirs et les conditions solaires ont été pris en compte.Les miroirs ont augementé la productivité volumétrique de la biomasse jusqu’à 37% sous un ciel clair. Ensuite, une campagne expérimentale de preuve de concept d’un photobioreacteur intensifié à tube mince destiné à des applications de biofaçade a été réalisée.Ce concept a permis d’obtenir une productivité élevée sans la formation de biofilm. Cependant,la croissance de la culture a été limitée par la suraccumulation d’oxygène. Pour optimiser ce concept, une étude détaillée du transfert de lumière a été menée. Une approche de Monte Carlo ray-tracing a été utilisée pour prédire le champs radiative dans la culture selon la géométrie, l’inclinaison et les conditions de fonctionnement du PBR

Effect of colony formation on light absorption by Botryococcus braunii

International audienceThis study elucidates the effect of colony formation on light absorption by Botryococcus braunii microalgae cells. The spectral average mass absorption cross-section of suspensions of B. braunii cultures with free-floating cells or colonies was measured experimentally across the photosynthetically active radiation region. The average mass absorption cross-section was found to decrease significantly across the spectrum in the presence of colonies. This observation could be attributed to (i) reduced pigment concentrations due to nutrient limitations, (ii) mutual shading of the aggregated cells, and/or (iii) the presence of the colonies' extracellular matrix. The Monte Carlo ray tracing method was used to elucidate the contribution of each of these phenomena on the mass absorption cross-section of cells in colonies. Colonies were modeled either as fractal aggregates of monodisperse cells or as an ensemble of monodisperse cells regularly arranged at the periphery of a hollow sphere embedded in a spherical extracellular matrix or as a volume and average projected area equivalent coated sphere. The change in pigment concentrations due to nutrient limitation was found to be the most important factor. In addition, the mass absorption cross-section of cells in colonies was found to decrease due to mutual shading among cells. This effect was stronger with increasing number of cells in the colony and increasing cell absorption index. The effect of extracellular matrix on the mass absorption cross-section was found to be negligible. Finally, good agreement was found between the equivalent coated sphere approximation and the colonies modeled as fractal aggregates comprised of monodisperse cells. This study also established that the Monte Carlo ray tracing method can be used for a variety of microalgae species and colony configurations, whose absorption cross-section are not readily calculated by standard methods due to their complex geometry, inhomogeneous nature, and large size compared to the wavelength of the incident light

Fluidized particle-in-tube solar receiver and reactor: A versatile concept for particulate calcination and high efficiency thermodynamic cycles

This paper focuses on the concept of tubular Dense Particle Suspension solar receiver that consists in using solid particles transported by an air flow as heat transfer fluid. It was first developed for solar tower power plants but can also be applied for particulate thermal treatment. Experiments are being conducted on-sun at the CNRS 1 MW solar furnace in Odeillo. A first analysis of a stable experimental case is presented. A simplified model of the receiver is described and compared to the experimental case. The results show that the solar flux modeling is appropriate. The model needs to take into account the specific particle suspension flow pattern present in the absorber tube to be able to predict the temperatures of both particles and tube wall. A qualitative exploitation of the model predicts that the technology is appropriate for particulate decarbonation occurring at temperatures of 700 °C or below, but that it will be insufficient to achieve the complete decarbonation of particles reacting at 800 °C or above

Solar Reduction of Cobalt Oxide Particles: Rotary Kiln Reactor Model and Experimental Results

A solar rotary kiln reactor was analyzed numerically to determine how efficiently it utilizes concentrated solar energy to reduce Co3O4 to CoO as a function of reactor operational parameters, including the rotation rate, the feed rate of Co3O4, and the solar power. The solar thermal efficiency, defined as the fraction of solar energy used to drive the reduction reaction, is calculated using an axisymmetric, finite-volume model of the rotary kiln reactor. The model iteratively solves the nonlinear, coupled energy and species equations accounting for conduction heat transfer, volumetric and surface radiation heat transfer, and cobalt oxide reduction kinetics within cloud of cobalt oxide particles that moves through the reactor in a plug flow. Radiation is simulated using Monte Carlo Ray Tracing, and the reduction kinetics follow the shrinking core model. For a cloud of 15 micrometer-diameter particles with a volume fraction of 10-5, we show an optimum solar thermal efficiency of 27% with a Co3O4 feed rate of 3.6 kilograms per hour and 3.5 kilowatts of solar power. At this optimum operating point, we show the temperature and conversion fields within the reactor. Furthermore, the results of a preliminary experiment are shown and provide experimental evidence of the promise of the solar rotary kiln reactor: 18% of the Co3O4 was reduced to CoO

Model of the solar-driven reduction of cobalt oxide in a particle suspension reactor

We develop a model to investigate the impact of volume fraction and extent of mixing on the thermal efficiency with which a cavity-based particle suspension solar reactor reduces Co3O4 into CoO and O2. Thermal efficiency is defined as the fraction of solar energy used to drive the endothermic reduction. In the model, particles move continuously through the reactor in either mixed or plug flow—mixing conditions that give rise to isothermal and nonisothermal suspensions, respectively—and reduce at a rate governed by shrinking core kinetics, the parameters of which are determined using thermogravimetric data. Radiation is simulated using the Monte Carlo Ray Tracing technique. The model is applied to a reactor heated by 4 kW of point-focused solar radiation with a mean concentration ratio of 1400 suns containing monodisperse suspensions of 40 μm diameter particles with volume fractions between 1×10⁻⁵ and 1×10⁻². Thermal efficiency is insensitive to mixing for the two conditions considered. The maximum thermal efficiency obtained for mixed flow with an isothermal suspension is 34.1% at 102 g min⁻¹ and a volume fraction of 2×10⁻⁴. At the same volume fraction, the maximum thermal efficiency for nonisothermal plug flow is 33.2% at 94 g min⁻¹. Thermal efficiency is more sensitive to the volume fraction, but only below a threshold value of 2×10⁻⁴. Thus, from the perspective of coupling heat transfer to the chemical reaction, design and operational efforts of particle suspension reactors for the reduction of cobalt oxide should focus on generating suspensions of at least this threshold value rather than on mixing the particles within the suspension

Model of a Rotary Kiln Solar Reactor for the Reduction of Cobalt Oxide Particles in a Two-step, Hybrid Thermochemical Water Splitting Cycle

Thermochemical water splitting cycles remain a promising approach to produce hydrogen from water using concentrated sunlight, due to their high theoretical solar-to-hydrogen conversion efficiency. One promising cycle is a hybrid cycle based on cobalt oxide. Hydrogen is produced in two chemical steps. In one step, concentrated sunlight is used to reduce cobalt oxide from Co3O4 to CoO near 1000 °C. In the second step, the CoO is integrated into the anode of an electrolysis cell and oxidized back to Co3O4 during the electrolysis of water near room temperature to produce hydrogen. The Co3O4 is recycled, and a fraction of the hydrogen produced is fed to a fuel cell in order to provide the small electrical input for electrolysis, such that the net effect of the cycle is the splitting of water using concentrated sunlight. The ideal solar-to-hydrogen conversion efficiency is 38%. One advantage of this approach is that the fuel production step is decoupled from the solar step and proceeds at room temperature; it can be carried out where water is readily available. The cycle brings the sun to the water rather than the water to the sun. At Valparaiso University, we have been developing a rotary kiln solar reactor for the reduction of Co3O4 particles to CoO. The defining feature of this reactor is its ability to disperse the Co3O4 particles into a “cloud” spread over the volume of the reactor. The hypothesis is that this cloud enhances the direct absorption and distribution of the concentrated solar input to reaction sites, and thereby increases the thermal efficiency of the reactor. To determine the impact of the cloud of Co3O4 particles on reactor performance, we developed a numerical model that couples the radiative and non-radiative heat transfer within the cloud to the cobalt oxide reduction kinetics in order to calculate the reactor temperature and the rate of reduction of Co3O4. Radiation is simulated using Monte Carlo ray tracing, and the reduction kinetics follow the shrinking core model. Several cases of particle motion were investigated, including plug flow and mixed flow. In this presentation, we show the results of the modeling effort. Reactor thermal efficiency, which is defined as the fraction of solar energy used to drive the reduction reaction, is discussed as a function of the feed rate of Co3O4, the solar power, and the volume fraction of Co3O4 in the cloud