Enhanced biomass production from <i>Chlorella</i> micro-algae species: a review of new technologies towards sustainable energy development

<i>Chlorella</i> microalgae species are among the most diverse, resilient, economical and sustainable biomass strains that are presently employed in renewable biofuel production. In this systematic literature review, the necessity for micro-algae biofuel development, the pros and cons of different <i>Chlorella</i> micro-algae cultivation modes and methods and their effects on biomass and total lipids productivities were critically examined. A comprehensive comparison of <i>Chlorella</i> biofuel properties with different conventional plant biomass resources was also carried out. Moreover, the use of mixotrophy and hybrid cultivation systems as high-yield and sustainable biomass production alternatives were analyzed. The effects of surface area to volume ratio (V/S) during <i>in situ</i> micro-algae cultivation in volumetric flasks were also examined because of its critical influence in large photobioreactors design most importantly in areas of light penetration, CO₂ diffusion and nutrient availability for enhanced micro-algae growth. <i>Chlorella</i> micro-algae species have higher total lipid productivity in the range of 15-70 mg L^-1 depending on the route of nutrient metabolism with mixotrophic growth mode having the highest lipid accumulation of 45-70 mg L^-1. While photoautotrophic and heterotrophic modes have lipid contents of 15-30 mg L^-1 and 35-68 mg L^-1 respectively. This promotes sustainable energy production from the micro-algae species. Interestingly, the micro-algae biodiesel has a higher cetane index and calorific value between 50-56 and 38-43 MJ kg^-1 respectively. This results in better ignition quality, enhanced fuel combustion and performance characteristics, lower engine fuel consumption and reduced carbon oxide (Cox) and particulate matter emissions. Finally, various new technologies and potential future strategies were offered for efficient and sustainable biomass and energy production.