Hierarchical control for microalgae biomass production in photobiorreactors

• MPC and PID control are combined to optimize the biomass production.
• Nonlinear models are used to design a multilayer hierarchical control approach.
• Economic MPC is proposed based on maximizing profits and reducing production costs.

This paper addresses the development of a hierarchical control strategy for microalgal production in a tubular photobioreactor. The proposed control scheme is composed of two layers. On one hand, a lower layer is in charge of tracking a pH set-point by a PI controller plus feedforward compensator, which was used due to its recognized efficiency in industrial control processes. On the other hand, the upper layer calculates optimal pH set-points based on an economic model predictive control approach. The main aim of this control scheme is to maximize profits, computed as the difference between the incomes obtained from the final production sale and the associated production costs (including the environmental impact of the exhausted CO2 losses). For this purpose, two types of models have been used depending on the requirements of each layer. Simulated and experimental results of the proposed hierarchical control architecture are presented, as well as a comparison with a single-layer architecture with constant reference implemented by the controller used in the lower layer of the hierarchical structure.