Should hydrodynamics be accounted for to compute the growth rate of microalgae in a photobioreactor?

Abstract

Microalgae are photosynthetic organisms cultivated in photobioreactors, where light intensity is one of the most important factors affecting their growth rate. At lower intensity, the microalgae are photolimited, since the amount of photons they received is not enough to optimally trigger the photosynthetic machinery. On the other hand, higher intensity induces damage in key proteins within the photosynthetic center of the cells. Due to the mixing in the photobioreactor, which creates a strong light gradient, the growth of the cells is affected by their light exposure history, which results from the hydrodynamics of the photobioreactor. We use the mechanistic model of Han to capture both photoinhibition and photolimitation. First, we compute the time-averaged growth rate for arbitrary continuous light signals. Our analysis reveals how mixing influences the average growth rate, assuming that hydrodynamics generate periodic light signals. Then, we tackle the challenge of computing the growth rate in photobioreactors simulated through computational fluid dynamics, considering a single-phase incompressible fluid. We conclude by examining the case of the raceway pond, and assess the error resulting from various approximations of the growth rate.