Light Transfer Simulation Tools in Photobiological Fuel Production

Photobiological carbon dioxide fixation and fuel production have received significant attention in recent years as sustainable solutions to global warming and energy crisis. However, this technology suffers from low production rates, poor solar energy conversion efficiency, and relatively high cost compared with competing technologies. The objective of the present study is to address these limitations by developing efficient and reliable simulation tools to optimize photobiological fuel production systems.First, an efficient radiative transfer equation (RTE) solver was developed using the discontinuous Galerkin (DG) method and graphics processing units (GPUs). In this study, the RTE solver was validated with benchmark problems related to combustion systems. In addition, this study demonstrated computational benefits of GPUs computing for solving the RTE.Second, the spectral effective real and imaginary parts of the complex index of refraction of green microalgae Chlamydomonas reinhardtii were retrieved from the its experimentally measured radiation characteristics. The microalgae were considered as spherical cells with equivalent diameter distributions. Genetic algorithm and Lorentz-Mie theory were used as inverse and forward method, respectively. In addition, T-matrix was used to predict the radiation characteristics of filamentous cyanobacteria consisting of aligned and connected spheres. This study established that, from a light absorption and scattering point of views, these microorganisms can be treated as infinitely long cylinders with volume-equivalent diameter. The methodology can be used in a wide range of applications and the results can be used to predict the radiation characteristics of PBR suspensions.Finally, light transfer in photobioreactors (PBRs) containing microalgae C. reinhardtii was modeled using the previously developed RTE solver. Then, the light transfer and growth kinetics were combined to estimate the daily biomass productivity of outdoor open ponds, vertical flat-plate PBRs, and tubular PBRs. The effects of growth kinetics models and cellular respiration on daily biomass productivity were investigated. The study demonstrated that the daily productivity per unit of illuminated surface area for PBRs operated in batch mode were identical and depended uniquely on the ratio X0/a where X0 is the initial microalgae concentration and a is the illuminated surface area per unit volume of PBR. Similar results were obtained with experimental data and other simulation results reported in the literature, for different microorganisms and PBRs operated in continuous mode. The PBR optical thickness, represented by X0/a, constitutes a convenient parameter for designing (via a) and operating (via X0) these PBRs to achieve their maximum performance