Design tool and guidelines for outdoor photobioreactors

Author-Highlights
• Similarity behavior in biomass daily areal productivity of photobioreactors is demonstrated.
• It was valid for common PBRs in batch or continuous mode with various microorganisms.
• Productivity depended only on the ratio of initial concentration and illuminated area per unit volume.
• Results can be used as guidelines for outdoor photobioreactors design and operation.

This study provides design and operational guidelines for achieving maximum biomass productivity in outdoor photobioreactors (PBRs). Detailed simulations of coupled light transfer and growth kinetics of microalgae were performed for open ponds, vertical flat-plate, and tubular PBRs operated in batch mode and exposed to time-dependent collimated and diffuse solar irradiance. The temporal evolution of microalgae concentration was predicted by accounting for light saturation, photoinhibition, and respiration. Three-dimensional spectral light transfer simulations of collimated and diffuse solar radiation in the PBRs were performed at different times of the day. The green microalgae Chlamydomonas reinhardtii   was used for illustration purposes. The study demonstrated that the daily productivity per unit of illuminated surface area for PBRs operated in batch mode was identical and depended uniquely on the ratio X0/aX0/a where X0 is the initial microalgae concentration and a is the illuminated surface area per unit volume of PBR. A maximum daily productivity of about 0.045 kg/m2/day was achieved for X0/a=0.035kg/m2. Remarkably, similar results were obtained with experimental data and other simulation results based on different models reported in the literature, for different microorganisms and PBRs operated in continuous mode. The PBR optical thickness, represented by X0/aX0/a, constitutes a convenient parameter for designing (via a) and operating (via X0) these PBRs to achieve their maximum performance.