Optimal optical conditions for Microalgal production in photobioreactors

TL;DR

This paper investigates how to optimize light conditions in photobioreactors to maximize microalgal productivity, introducing an optical depth productivity concept and a nonlinear control method for optimal biomass concentration.

Contribution

It introduces the concept of optical depth productivity, extends the compensation condition for light optimization, and develops a nonlinear controller for biomass regulation.

Findings

Optimal light distribution enhances productivity.

The proposed control ensures stable biomass concentration.

The model accounts for light absorption and scattering effects.

Abstract

The potential of industrial applications for microalgae has motivated their recent fast development. Their growth dynamics depends on different factors that must be optimized. Since they get their energy from photosynthesis, light is a key factor that strongly influences their productivity. Light is absorbed and scattered in the liquid medium, and irradiance exponentially decreases towards the darkest part of the photobioreactor at a rate nonlinearly depending on the biomass concentration. Maximizing productivity is then a tricky problem, especially when the growth rate is inhibited by an excess of light. Productivity optimization turns out to be highly dependent on how light is distributed along the reactor, and is therefore related to the extinction rate and the background turbidity. The concept of optical depth productivity is introduced for systems where background turbidity must be accounted for and a global optimum maximizing productivity is proposed, extending the concept of the compensation condition. This optimal condition consists in compensating the algal growth rate at the bottom of the reactor by the respiration. This condition can drive the optimization of the surface biomass productivity depending on the minimum reachable depth. We develop a nonlinear controller and prove the global asymptotic stability of the biomass concentration towards the desired optimal value.