Tubular photobioreactors illuminated with LEDs to boost microalgal biomass production

Abstract

This work proposes distinct configurations for tubular photobioreactors (PBRs) illuminated with specific and adequate light wavelength provided by LEDs. The PBRs are characterized by an involute/flat reflective surface around/below a cylindrical borosilicate glass tube that is illuminated by a LEDs panel located above. Reflectors can enlarge the absorber tube's illuminated area, enabling a uniform (spatial and temporal) light distribution across the culture vessel. Additionally, high-energy-efficient LEDs can minimize heat generation and energetic-related costs. Coupling these two factors can result in higher light utilization efficiency and photosynthetic activity. Depending on the reflector design, almost all the light arriving at the collector aperture can be collected and available for microalgal cultivation. Chlorella vulgaris (C. vulgaris) growth was evaluated as a function of the reflector geometry (flat (F), simple double parabola (SP) and traditional double parabola (DP)) and material (anodized aluminum with (MS) and without (R85) protective coating and stainless steel (SS)). C. vulgaris growth as a function of time was found to be in good agreement with the actinometric results, where the parabolic reflectors (SP and DP) made of higher specular reflectance materials (R85 and MS) were the most efficient systems. Conversely, energy-based specific growth rates slightly increased as the photon flux decreased, signaling an energetic efficiency loss due to the low transmissibility of microalgal suspensions. Additional tests using two absorber tubes (spaced between 12.5 and 75.0 mm) over the R85-F reflector were also carried out, showing that the distance of 50.0 mm led to the best compromise between the specific growth rates and biomass productivities per square meter of solar collector. Under these conditions, higher efficiency on the photonic energy usage was attained compared to the single-tube test.