Microalgal biofuels: Pathways towards a positive energy balance

Abstract

Current microalgae biofuel production pathways are still far from being energetically self-sufficient. Simulations scenarios (1 t·y−1 of biomass) under a purely energetic perspective, have revealed some bottlenecks mainly in the cultivation and metabolite recovery processes. Technologies such as intensified photobioreactors such as AlgoFilm©, infrared light filtering units, photovoltaic panels, solvent-free metabolite recovery processes, and high biomass concentration treatments at⩾145 kg·m−3 are suggested and discussed to also have a major contribution on the net energy ratio of the process. An energy-driven biorefinery approach has been proposed to increase the energy output by obtaining bioethanol and biodiesel liquid fuels, but also photovoltaic energy during the same production process. The latter may also compensate for the large amounts of energy consumed in typical operations of the wet-pathway processes like harvesting-concentration, cell disruption, or solvent recycling processes. This perspective increased the final volume of liquid biofuels up to 57%. It was found a maximum net energy ratio of 1.9 for a double biofuel process, and 8.5 if photovoltaic energy is also considered.