Modeling and Simulation of Irradiance for Photobioreactors with Complex Geometry for Exploration of Algal Growth
Mathias Krause, Hermann Nirschl, Clemens Posten
DFG Research Grant
As part of the bio-economy strategy of the federal and state governments of Germany, the supply of oil-based materials and energy sources is being increasingly converted to bio-based products. For this purpose, the residual biomass from agriculture is not enough. Microalgae are considered to be a promising alternative for biomass production without competing with areas for which already other options are planned. However, the benefits can only be implemented in closed photobioreactors (PBR).In order to establish PBR as an (cost) alternative coverage in industry, controlled entry of light is essential, in particularly for closed PBR with vast options for the light setup. Due to the high absorption and scattering behavior of algae, a very high light gradient is established in the reactor. As a result algae close to the reactor surface are exposed to light saturation, in contrast to the underlying, shaded algae exposed to a strong limit, which increases the relative proportion of breathing. Both effects lead to a reduction in efficiency. A homogenizing of the growth on the metabolic level, driven by a flow, is very energy intensive and crucial reason why PBR cannot be operated efficiently. Until today, there remains a lack of reliable simulation tools to predict the distribution of light in moving algae suspensions to which then algae growth models are associated. Finally, with it, reliable predictions about the biomass growth are obtained.This essential gap in the modeling and simulation of growth in PBR is aimed to be closed by this project through an integral mesoscopic modeling and the development of a multiphysics Lattice Boltzmann Method (LBM). The novelty lies in solving the light transport phenomena by an LBM-based algorithm. The subsequently coupling with LBM-based multiphase flow simulations, which reflect the CO2/O2 transport in the algae suspension, and an LBM-based algae particle simulation, which calculates dark/light cycles, is very promising.The project also includes the application of the prediction tool for algal growth to innovative, highly complex spongy PBR geometries. In modern approaches increasingly photoconductive reactor elements are used as part of the reactor wall or explicitly introduced structure. Thereby, the goal is increasing the efficiency significantly by a very homogeneous illumination. The validation of the developed methodology will be based on simple test scenarios such as classical reactor geometries, but also on new and innovative sponge-like structures, so that in the end of the project both, a reliable and robust simulation tool for algae growth in arbitrarily complex PBR geometries and new insights in the interplay of light and fluid dynamics in PBR design, is expected.