Measurements and predictions of the radiation characteristics of biofuel-producing microorganisms.

摘要

Biofuel produced from photosynthetic microorganisms offers a green and sustainable alternative to fossil fuels and bioethanol for next generation transportation fuels. Their simple cell structures enable them to be more efficient in photosynthesis than higher order plants. However, large scale cultivation of these microorganisms is typically hampered by the poor light utilization of photobioreactor systems. In order to increase biomass productivities, the light transfer in photobioreactors must be characterized and optimized. The radiation characteristics of photosynthetic microorganisms are essential parameters in this analysis. This study aims to (1) experimentally measure the radiation characteristics of various photosynthetic microorganisms of different morphologies and (2) to develop models that can predict these radiation characteristics.;First, the spectral complex indices of refraction of unicellular spheroidal green algae Botryococcus braunii, Chlorella sp., and Chlorococcum littorale were retrieved from their experimentally measured average absorption and scattering cross-sections. Next, the temporal evolution of the scattering and absorbing cross-sections of marine eustigmatophycease Nannochloropsis oculata grown in flat-plate photobioreactor (PBR) was reported. The variations in cross-sections were found to be directly related to the up- and down-regulations of pigments and other intracellular components and vary significantly with time in response to changes in light and nutrients availability. Furthermore, this study demonstrates that the light transfer in the PBR could be predicted using constant radiation characteristics measured during the exponential growth phase with reasonable accuracy provided that the cultures were not nitrogen limited. In addition, this study presents experimental measurements of the absorption and scattering cross-sections and the spectral complex index of refraction of filamentous heterocystous cyanobacterium Anabaena cylindrica. Its filaments, consisting of long chains of polydisperse cells, were modeled as infinitely long and randomly oriented volume-equivalent cylinders. Finally, it was demonstrated that the absorption and scattering cross-sections and asymmetry factor of randomly oriented and optically soft bispheres, quadspheres, and circular rings of spheres, with either monodisperse or polydisperse monomers, can be approximated by an equivalent coated sphere with identical volume and average projected area. Based on this approximation, the spectral complex index of refraction of unicellular dumbbell-shaped cyanobacterium Synechocystis sp. was retrieved from experimental measurements of its average absorption and scattering cross-sections and size distribution.