Comparison between different strategies for the realization of flashing-light effects – Pneumatic mixing and flashing illumination

Abstract

A major limitation for the phototrophic cultivation of microalgae in photobioreactors is the low culture density which results in expensive harvesting and downstream processes. It is often mentioned that intensive mixing of the culture can improve the efficiency of light utilization by realizing flashing-light effects which enhance the growth of cell cultures. Alternatively, flashing light sources could provide a way to realize this effect without the need of supplying additional energy for fast mixing. While the effects of mixing have been investigated at various reactor scales, experiments with flashing illumination have been mostly conducted in small geometries. In addition, few studies have been conducted for photobioreactors with larger light paths, being characteristic for the production scale. By means of numerical simulations, we evaluate pneumatic mixing and flashing illumination with regard to their ability to realize flashing-light effects in a 5 cm diameter bubble column filled with a suspension of Chlamydomonas reinhardtii. To the best of our knowledge, a numerical comparison between the effects of flashing illumination and pneumatic mixing on the culture growth has not been reported in literature.A thorough comparison of the two methods requires a robust numerical tool which integrates the computation of the fluid flow and the light field, as well as the growth kinetics of algal cells. In the present work, we compute the three-dimensional flow field in a bubble column photobioreactor tracking also micrometer particles in order to simulate the movement of algae. The spectral light field is computed by solving the three-dimensional Radiation Transfer Equation (RTE) at different wavelengths and biomass concentrations. The coupling of the flow and light fields enables the computation of the spatio-temporal light exposure of individual algae cells, which is used to estimate the respective dynamic photosynthesis reaction rates under different operating conditions of the bubble column reactor.We found, numerically, that the contribution of pneumatic mixing alone is negligible in comparison to flashing-light effects, for the investigated operating conditions. In contrast, illumination with flashing LED leads to an increase of the growth rate up to a factor of 2.5 at flashing frequencies higher than 50 Hz in a PBR with industrially relevant operating conditions. Thereby, a proper selection of the duty cycle is not only needed to prevent photoinhibition but also to maximize the effects of flashing light at the same time. According to these results, the utilization of flashing LED sources can provide a mostly unused way to improve photobioreactor productivity in the future.