Model-based optimization of microalgae areal productivity in flat-plate gas-lift photobioreactors

Abstract

The productivity of microalgae used for the production of biofuels is primarily influenced by the availability of exploitable light. To overcome the effects of limitation and photoinhibition, an optimized radiation profile was implemented for the cultivation of Nannochloropsis salina 40.85 and Nannochloropsis gaditana 2.99 in LED-illuminated flat-plate gas-lift photobioreactors with a depth of 2cm. For this purpose the light-dependent growth kinetics were characterized for both strains with variable incident photon flux densities up to 2750μmolm−2s−1. The mean integral photon flux density I⁎⁎ was established to describe the light every single microalgal cell is exposed to on average during exponential growth. By implementing a phototrophic growth model based on the light attenuation described by Lambert and Beer, the kinetic growth parameters have been determined including the effects of photoinhibition. Based on the defined optimal range of I⁎⁎ for both Nannochloropsis strains optimum radiation profiles were developed: Up to an identified upper limit of light, the incident irradiation is increased with rising biomass concentration while the mean integral photon flux density is kept constant at the chosen optimum. Thus, the biomass areal productivity could be increased significantly compared to constant irradiation by 113% to 10.4gm−2d−1 with Nannochloropsis salina and by 107% to 14.8gm−2d−1 with Nannochloropsis gaditana, respectively. The areal lipid productivity was likewise distinctly improved by the implementation of the radiation profiles up to 11.0gm−2d−1 (+59%) with Nannochloropsis salina and up to 6.5gm−2d−1 (+83%) with Nannochloropsis gaditana. The concept of optimized radiation profiles may be easily transferred to other microalgal strains as well as other reactor types due to the system-independence of the mean integral photon flux density.