PNNL

Abstract

Light is one of the most limiting factors in microalgae cultures. Periodic dilution is a necessary operation to maintain light availability as the culture grows dense. However, environmental conditions are constantly changing in outdoor cultivation systems, making it difficult to determine the optimal dilution rate. To address this challenge, this study evaluated a dilution approach based on light penetration to optimize illumination (OptiLum) in the culture. Biomass concentration was controlled via sensor-feedback directed dilution to ensure that light reaching the bottom of the culture was maintained above the compensation intensity. Under replicated outdoor pond conditions, by keeping the entire culture within a net-positive photosynthetic zone, the OptiLum operation improved the biomass productivities of two top-performing strains, Picochlorum celeri and Tetraselmis striata, by 97% and 86%, respectively, compared to conventional semi-continuous batch cultivation. The dilution rate varied daily and was dynamically adjusted based on the light status within the culture, which is concurrently influenced by weather, culture density, and growth rate. The techno-economic analysis showed that the OptiLum operation could reduce biomass production cost by as much as 24% and 33% for P. celeri and T. striata, respectively, assuming a low-cost sequence of dewatering steps can be maintained under lower biomass harvest densities but higher biomass productivity associated with the OptiLum strategy. However, a more costly two-stage dewatering approach may be necessary for some non-settling strains, such as P. celeri, would alternatively increase production costs by 23%.