A novel external reflecting raceway pond design for improved biomass productivity

Outdoor raceway ponds for microalgae cultivation suffer from low biomass productivity due in part to the low photosynthetic photon flux received by the microalgae culture in the mornings and evenings and during the winter months at middle and high latitudes. This study explores the use of external mirrors to reflect additional sunlight onto the culture and increase the incident solar flux at critical times of the day and year. Four designs cultivating Chlorella vulgaris were considered: a raceway pond without mirrors (Configuration A), a pond oriented along the north/south axis with dual east/west mirrors (Configuration B), a pond oriented along the east/west axis with a single north mirror (Configuration C), and a solar tracking rotating pond with a single mirror (Configuration D). The biomass productivity was predicted by coupling the simulated radiative field within the culture to a microalgae growth kinetics model accounting for photolimitation and biomass loss due to respiration. Two different locations were considered, namely Los Angeles, CA, USA and Saint-Nazaire, France. The use of mirrors was predicted to increase the daily culture-area-based and volumetric biomass productivity at both locations and all months of the year. Configuration D yielded the highest biomass productivity year-round. Configuration B had a higher biomass productivity than Configuration C in the summer months and vice versa in the winter months. Overall, Configuration C was considered to be the simplest and most cost-effective method to increase raceway productivity. Indeed, this configuration improved the raceway pond volumetric and culture-area-based biomass productivity by as much as 73% in the winter months compared to Configuration A. Additionally, the impact of operational parameters (initial biomass concentration and culture depth) and design parameters (pond length-to-width ratio and mirror height) were assessed to provide practical recommendations for maximizing biomass productivity.