Irradiance penetration distribution and flashing light frequency simultaneously affected with microalgal cell absorption and CO2 bubble scattering in a raceway pond.

In order to investigate light penetration and flashing light frequency for microalgal cell-CO2 bubble culture system in a raceway pond, user-defined function for CO2 mass transfer and bubble scattering models coupled with discrete ordinates radiation model were adopted to clarify simultaneous effects of microalgal cell absorption and CO2 bubble scattering. Light intensity along the microalgal suspension depth attenuated more rapidly with increased biomass concentration, decreased bubble generation diameter, increased CO2 gas content and incident light intensity. Ratio of light zone decreased from 81.13 % to 20.00 % when biomass concentration increased from 0 to 0.4 g/L because of light absorption and shading effects of microalgae. When bubble generation diameter increased from 0.1 to 1.6 mm, ratio of light zone increased from 37.95 % to 42.64 %, while microalgal flashing light cycle first decreased to a valley of 1.81 s at 0.8 mm and then increased. Local light intensity in the upper layers was more enhanced due to lots of CO2 bubbles gathering and reflecting more light with decreased bubble diameter and increased gas content. Light attenuated more rapidly in microalgal suspension with decreased bubble generation diameter and increased CO2 gas content because of increased bubble diffraction coefficient and contact area. When initial CO2 volume fraction increased from 0.02 to 0.2, flashing light frequency of microalgal cells decreased from 0.55 to 0.29 Hz and light zone time ratio φ decreased from 36.90 % to 18.40 %. At a biomass concentration of 0.1 g/L and a bubble flow rate of 0.1 m/s, the maximum light penetration and microalgal growth rate was achieved when bubble diameter, incident light intensity and gas content were optimally at 0.8 mm, 200 W/m2 and 0.02, respectively. This work provides data support and theoretical guidance for photobioreactor design and optimization of light energy utilization.

Evaluation of photosynthetic light integration by microalgae in a pilot-scale raceway reactor.

The improvement of photosynthetic efficiency in a 100 m2 raceway reactor by enhancement of light regime to which the cells are exposed is here reported. From Computational Fluid Dynamics it was calculated that the light exposure times ranged from 0.4 to 3.6 s while the exposure times to darkness were much longer, from 6 to 21 s. It was demonstrated that these times are too long for light integration, the cells fully adapting to local irradiances. This phenomenon was validated in the real outdoor raceway at different seasons. Simulations allows to confirm that if total light integration is achieved biomass productivity can increase up to 40 g/m2·day compared to 29 g/m2·day obtained considering local adaptation, which is close to the experimental value of 25 g/m2·day. This paper provides clear evidence of microalgae cell adaptation to local irradiance because of the unfavourable cell movement pattern in raceway reactors.