Design and experimental evaluation of an internally illuminated hybrid mixing photobioreactor for intensified microalgal biodiesel production with techno-economic and scale-up assessment

The technical and financial limitations associated with the production of microalgal biodiesel are significantly influenced by reactor structure, biomass productivity, lipid accumulation, and energy consumption. This study designed and experimentally evaluated a 50-L nominal-volume (42-L working-volume) cylindrical stainless-steel photobioreactor that combined internal LED illumination with low-shear mechanical agitation and fine-pore gas diffusion to enable culture circulation, illumination, and gas–liquid transfer. A sequential one-factor-at-a-time (OFAT) approach was used to evaluate the environmental and nutritional factors (ENF) of Crucigenia sp. OW2. The upscale cultivation was conducted under ideal regulated conditions in a specially built photobioreactor. The statistical investigation included one-way ANOVA, assumption testing, Tukey HSD multiple comparisons, effect-size calculation, and correlation/regression analyses; the correlations discovered were categorised as associative rather than causal. A six-day culture in the designed photobioreactor recorded an enhanced biomass concentration of 8.74 g L⁻1, a lipid content of 48.36%, and a biodiesel output of 85.72% after three days of growth, whereas the ENF recorded a maximum biomass concentration of 6.30 g L⁻1 and a lipid content of 36.0%. Based on equipment-rating-based power demand and annual reactor manufacturing costs, a preliminary techno-economic study estimated a biodiesel production cost of USD 31.39/L. Within the ±20% and ±50% scenario boundaries, biomass productivity, lipid content, and electricity tariff were found to be significant economic drivers by deterministic sensitivity analysis. The finding of this study reveals that the costs per unit of biodiesel produced could be reduced through an increase in the volume of the reactor but can at the same time result in limitations associated with light penetration, oxygen accumulation, mixing, gas–liquid mass transfer, and specific energy demand. Overall, the designed photobioreactor showed promising experimental-scale performance for microalgal cultivation and biodiesel production; however, additional validation of its economic and commercial potential is needed through direct energy measurements, pilot-scale operation, and scale-specific techno-economic assessment. Graphical Abstract