Outdoor microalgae cultivation at different biomass concentrations - Assessment of different daily and seasonal light scenarios by modeling

An efficient microalgae outdoor cultivation is hindered by the fluctuation of the photon flux density (PFD) during one day and the natural day-night-rhythm. As the light intensity is not directly adjustable, the adjustment of biomass concentration is often used for the indirect distribution of available photons to an appropriate number of cells. For this study, repeated fed batch cultivations were conducted in 28 L flat panel airlift reactors in Stuttgart, Germany between August and October 2015. The influence of the initial biomass concentration (1.5-9 g L-1) on the productivity of Chlorella sorokiniana was investigated. Two models (basing on different underlying assumptions) were developed, which used the results of a previous laboratory study to explain the effects of different PFD courses on the productivity outdoors. Both models were validated by using the results of the outdoor cultivation. With the model based on the light yield, an adequate prediction of the productivity was achieved. This leads to an assessment of the outdoor process with paradigmatic courses of the PFD. The application of the model on recorded outdoor PFD courses proved that not only the amount of photons has an impact on the productivity, but also their distribution during the day. Longer photoperiods with lower maximum PFD values (summer) lead to a higher productivity compared to shorter photoperiods with higher maximum PFD values (spring/fall). Additionally, the biomass concentration should be at least as low as 1.5 g L-1; this leads on the one hand to an improved light yield, as there is less self-shading of the algae culture, on the other hand, there is less night biomass loss at lower biomass concentrations.