Light distribution in a novel photobioreactor -: modelling for optimization

The paper reports a novel photobioreactor developed to achieve homogeneous and flexible illumination inside the reactor. This is to overcome the problem of studying kinetics in standard photobioreactors, which are characterized by strong light gradients and light fluxes that cannot be controlled. The reactor is used for the study of microalgal kinetics for modelling purposes. The new reactor combines the advantages of a stirred reactor (homogeneity) and a plate reactor (short path length). The light input system consists of an external light source, a fibre-optical ring-light and a light emitting tube. Light is generated in a light source arranged externally and directed into the reactor using optical fibres. The fibres are spread in a ring-light to provide a uniform illumination in the concentrically arranged cylinder. Any focusable light source can be applied; by using a shutter module, light fluctuations can be generated in a wide range of frequencies. In order to change the light quality, spectral filters are placed between the lamp and the optical fibre. A model based approach was used to optimize the illumination: light distribution was calculated employing a Monte-Carlo simulation. Light emission characteristics, reflection, refraction, scattering in the suspension and on rough surfaces were studied numerically. Propositions were derived how to optimize the reactor, e.g. now to achieve higher light intensities and a more uniform illumination. Finally, mean photon flux densities of 100 +/- 15 mu mol m(-2) s(-1) were achieved at the illuminated surface. The simulation results revealed that the light distribution at constant biomass concentration is mainly determined by the geometrical parameters of the lightening device mentioned above and that any simplifications lead to serious misinterpretations.