Design scenarios of outdoor arrayed cylindrical photobioreactors for microalgae cultivation considering solar radiation and temperature

Advancing microalgae biotechnologies requires the design of high efficiency, large scale outdoor photobioreactor systems. Here we present a predictive biomass productivity model to define system design parameters yielding high biomass productivities for a facility encompassing arrays of cylindrical photobioreactors (PBRs) in a sub-tropical location (Brisbane, Australia). The model analyses the temperature and the light distributed through the culture medium as a function of PBR height, diameter, spacing distance between reactors, biomass concentration and cultivation regime (continuous vs. batch; fixed vs. capped temperature control). Temporal changes in light and temperature were used to predict volumetric and areal productivities (P-vol and P-areal respectively) for three Chlorella strains (C. vulgaris, C. sp. 11_H5 and C. pyrenoidosa). A simple empirical relationship was derived to rapidly predict P-vol in PBR arrays based on the ratio of spacing distance and reactor height (L/H) if the P-vol of a single, unshaded PBR was known. For C. vulgaris under a continuous operation and variable temperature (within its maximum growth threshold), the highest P-vol in the range analysed was obtained at the smallest diameter (0.1 m), highest biomass concentration (1.5 g L-1) and largest L/H, (P-vol similar to 0.3 g L-1 d(-1)). In contrast, the highest P-areal (similar to 50 t ha(-1) yr(-1)) was found at higher diameters (0.15 and 0.3 m), a lower biomass concentration (0.3 g L-1) and low L/H (0.2-0.4); this was attributed to a higher overall culture volume per PBR and per area. Our predictions, based on light and temperature effects on productivity, suggest that attaining a high P-vol could reduce costs, energy and materials associated with water usage, harvest loads and PBRs; whereas attaining a P-areal toward its maxima could reduce costs associated with land. The model supports effective PBR array design and process optimisation to help minimise production cost.