Use of airfoils for enhancement of photosynthesis rate of microalgae in raceways

Abstract

The lack of adequate vertical mixing is one of the factors limiting the productivity of open raceway microalgae reactors. The existence of large gradients of light involves the cells being mainly adapted to local irradiance instead of average irradiance, which would allow for maximizing the light utilization efficiency, thus maximizing the biomass productivity of microalgae cultures. To overcome this problem, different alternatives have been proposed, one of the more suitable being the utilization of airfoils to improve vertical mixing. In this work, numerical and experimental studies were performed to analyse the effect of the aerodynamic airfoils patented by the University of Seville (WO2020120818A1). The goal is to improve the photosynthetic efficiency, but also a better understanding of the light regime to which the microalgae cells are exposed in these systems and how to improve it. Computational Fluid Dynamics (CFD) was used to optimize the flow generated by the airfoils. A dynamic photosynthesis model of Rubio Camacho et al. (Biotechnol Bioeng 81:459–473, 2003) was used to estimate the photosynthesis rate as a function of the light regime to which the cells are exposed, including photo-adaptation and photo-inhibition phenomena, the results confirm that the use of airfoils improves the vertical mixing and the photosynthesis rate. The photosynthetic benefits were observed 10 m downstream of the airfoils, resulting in an increase in photosynthesis rate and productivity by up to 30%. These results confirm the benefits of an increase in mixing in microalgae cultures, especially when focusing on the movement of the cells between the different illuminated zones while maintaining low energy consumption and capital expenses.