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Operating strategy to reduce the energy consumption of flat-panel airlift photobioreactors with respect to mixing of thermosynechococcus elongatus suspension cultures : light-specific adaptation of the superficial gas velocity
(2018) Bergmann, Peter; Trösch, Walter
Photoautotrophic microalgae mass production is limited by light availability due to effects of absorption and reflection, especially throughout outdoor cultivation prohibiting the adjustment of photon-flux density (PFD). Generating turbulence within the cultures in order to minimize photolimiting and photoinhibitive effects is the method of choice to overcome that obstacle. Then again, energy required for its generation represents one of the major drivers contributing to overall production costs of microalgae biotechnology. The present work describes the development of an advanced operating strategy for the mixing of flat-panel airlift loop photobioreactors (FPA-PBRs) that through its application decreases the specific energy consumption, thus the energy requirement per unit of biomass produced, when cultivating phototrophic microorganisms. Experiments were carried out with the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1 utilizing distinct FPA-PBRs equipped with culture-flow directing installations and illuminated by high pressure sodium (HPS) lamps. In the first paper, the impact of utilizing respective FPA-PBRs is investigated. Preliminary experiments were performed in order to eliminate any limitations beyond the sphere of influence of photobioreactor (PBR) design. Apart from the NO3- concentration which had to be retained at 2000 mg L-1 to sustain non-limited growth, special attention was paid to the administration of dissolved inorganic carbon (DIC), inter alia in the form of hydrogen carbonate as CO2 gas solubility was limited by the applied cultivation temperature of 55°C. It is for this reason, in conjunction with a short residence time of the CO2-enriched air bubbles that an increase in CO2 concentration showed only minor effects compared to increasing carbonate concentration that directly correlated to maximum productivity attaining 2.9 gDW L-1 d-1, the highest to be reported for T. elongatus BP-1, using 0.04 g L-1 Na2CO3. When comparing PBRs with and without culture flow directing installations, e.g. static mixers, it was found that the former outperformed the latter as an increase in maximum volumetric productivity and final biomass concentration by a factor of 3.4 and 2.0 was recorded, respectively, whilst the energy input in the form of superficial gas velocity remained unchanged. The enhanced growth performance was attributed to improved specific light availability due to the formation of eddies within cultures induced by static mixers. Thereby, light dependent downregulation of quantum-yield and respiratory losses were reduced, ultimately allowing for a more efficient photon-utilization towards assimilatory photochemistry when compared to randomly mixed cultures. In the second study, the joined impact of PFD, biomass concentration and superficial gas velocity is investigated and an operating strategy for FPA-PBRs deduced. Preliminary experiments were performed in order to establish a modified photosynthesis irradiance (PI) curve at default mixing settings which defined the light compensation point and the irradiance of saturation with 100 μmol m-2 s-1 and 400 μmol m-2 s-1, respectively. Cultivations were then performed at sub-, quasi-, and supra-saturating PFDs (180 .. 780 μmol m-2 s-1) utilizing multiple gas flow velocities (0.11 .. 0.83 vvm). It was found that at a given velocity, productivity and final biomass concentration increased with increasing PFD. Moreover, it was found that in comparison with default mixing settings, the superficial gas velocity during sub-saturating PFD and/or biomass concentrations < 3 gDW L-1 can be reduced to cut operational expenditures (OPEX) on mixing, whilst an increase during supra-saturating PFD and/or higher biomass concentrations enhances productivity and final biomass yield. An operating strategy based on the PFD-triggered adjustment of the superficial gas velocity is proposed and results were mathematically translated to exemplary outdoor diurnal cycles of PFD. By applying the strategy on sunny days, productivity is increased by 24%, while reducing not only energy input but also CO2-demand by 11%. On cloudy days, productivity is only slightly increased but energy input and CO2-demand reduced by 37%. Consequently, the specific energy requirement of FPA-PBRs when cultivating phototrophic microorganisms is reduced significantly, especially at locations with only stochastic light supply, e.g. in temperate latitudes.