Lipid production by Nannochloropsis oceanica at large scale: energy balance and techno-economic analysis

Due to the robustness of its cell, the ability to accumulate lipids when subjected to stresses and culture stability, Nannochloropsis is one of the most investigated genus for biofuel production. Another important feature of this genus that impacts on the economic viability of the process is its high content of eicosapentaenoic acid (EPA). In the EU-FP7 projects FUEL4ME and BIOFAT we have thoroughly studied the response of Nannochloropsis oceanica to nitrogen depletion. The main objective of our activity in FUEL4ME was to compare in 1-m2 GWP®-III photobioreactors the two-step process (Rodolfi et al. 2009), with the one-step process in which limiting amounts of nitrogen are regularly added to the culture according to the consumption rate. In BIOFAT the two-step process was applied at large scale: two 250 m2 GWP®-II modules were used to produce the inoculum for two 530 m2 raceway ponds which were kept under nitrogen replete conditions for optimal growth; the cultures from the ponds were then used to inoculate two 1280 m2 raceway ponds kept under nitrogen starvation for lipid accumulation. Results showed that, depending on the season and on the modality of nitrogen supply, fatty acid productivity can be higher with the one-step process, although fatty acid content is always higher in the two-step cultivation. The data obtained in the two projects were used for the calculation of NER (Net Energy Ratio) and a techno-economic analysis (TEA) of one plant entirely composed of GWP® reactors and one integrated plant composed of GWP® reactors and ponds, both at 1 ha and 100 ha scale.