Net zero energy microalgae production: Photovoltaic integration in light emitting diode-based photobioreactors

In this work, a complete analysis of the energy requirements, sizing and productivity study of a Tetradesmus obliquus algae production system, centered around an internally illuminated LED-based photobioreactor (PBR) powered by a photovoltaic generator, is presented. Laboratory-scale experiments were carried out on a 200 mL PBR to assess the optimal growth conditions in terms of residence time, light intensity and spectral balance between red and blue light in continuous mode. A pilot-scale campaign was then carried out in a custom-designed 50 L PBR in steady-state, with a maximum productivity of 0.38 g L-1 d-1 and a photosynthetic efficiency of 14.7 % was obtained. On the basis of the results of the pilot experiments, two photovoltaic (PV) integrated case-studies were designed and analyzed: i) a compact structure, able to operate in off-grid conditions, producing 113.5 kg per year of dry-weight biomass and having a size compatible with a 20′ Conex container, and ii) a net-zero energy large-scale system, producing 50 t of biomass per year, offsetting the energy acquired from the grid with the energy produced by a utility scale PV plant, with a size of approximately 0.75–1.42 ha, depending on the location.