Light distribution in a novel photobioreactor – modelling for optimization

The paper reports a novel photobioreactor developed to achievehomogeneous and flexible illumination inside the reactor. This is toovercome the problem of studying kinetics in standard photobioreactors,which are characterized by strong light gradients and light fluxes that cannotbe controlled. The reactor is used for the study of microalgal kinetics formodelling purposes.The new reactor combines the advantages of a stirred reactor(homogeneity) and a plate reactor (short path length). The light inputsystem consists of an external light source, a fibre-optical ring-light and alight emitting tube. Light is generated in a light source arranged externallyand directed into the reactor using optical fibres. The fibres are spread ina ring-light to provide a uniform illumination in the concentrically arrangedcylinder. Any focusable light source can be applied; by using a shuttermodule, light fluctuations can be generated in a wide range of frequencies.In order to change the light quality, spectral filters are placed between thelamp and the optical fibre.A model based approach was used to optimize the illumination: lightdistribution was calculated employing a Monte-Carlo simulation. Lightemission characteristics, reflection, refraction, scattering in the suspensionand on rough surfaces were studied numerically. Propositions were derivedhow to optimize the reactor, e.g. now to achieve higher light intensities anda more uniform illumination. Finally, mean photon flux densities of 100± 15 μmol m-1 s-1 were achieved at theilluminated surface.The simulation results revealed that the light distribution at constantbiomass concentration is mainly determined by the geometrical parametersof the lightening device mentioned above and that any simplifications leadto serious misinterpretations.