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