Control and Optimization of Light Transfer in Photobioreactors Used for Biofuel Production
Microalgae are tipped as the feedstock for next generation transportation fuels due to their significantly higher photosynthetic efficiency compared to higher plants. These microorganisms can contain large amounts of triglyceride fatty-acids (TG-FAs) that can be converted to biodiesel by transesterification. However, microalgae cultivation in photobioreactors (PBRs) typically suffers from low productivity due to light transfer limitations. To optimize microalgae growth rate and productivity, radiation transfer analysis must be performed in order to optimize light availability in PBRs. Nitrogen starvation coupled with strong illumination has been used to induce large amounts of TG-FA accumulation in microalgae. However, the role of light absorption rate by the microalgae cells on TG-FA productivity is not well understood. This study aims (1) to study the interaction between light and photosynthetic microorganisms and (2) to optimize light transfer conditions in PBRs to maximize microalgal biomass and lipid productivity.
First, the complete set of radiation characteristics and optical properties of the eustegmatophycea Nannochloropsis oculata, a promising marine microalgae for biodiesel production, was obtained for cells grown under various light spectra and irradiances. Second, the radiation characteristics of aggregates and colonies of microalgae were studied theoretically. Significant differences were observed in the average absorption and scattering cross-sections of cells either free-floating or aggregated in colonies. Third, a feed-forward inversion control scheme was designed and experimentally demonstrated for maintaining an optimum incident irradiance on PBRs during batch cultivation. A data-based model-free optimization was utilized to rapidly estimate the optimum average fluence rate set-point value leading to maximum microalgae growth rate. This control scheme increased biomass productivity and reduced lag time compared to batch cultures exposed to constant irradiance throughout the cultivation process. Finally, N. oculata were grown under nitrogen starvation and were characterized in terms of their biomass, pigment, and TG-FA concentrations as well as their absorption and scattering cross-sections as a function of time for several batch cultures. The TG-FA production rate correlated to the rate of photon absorption by cells suggesting that the TG-FA production process was limited by light.