The light/dark cycle of microalgae in a thin-layer photobioreactor
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The light/dark cycle of microalgae in a thin-layer photobioreactor Alessandro Chiarini1
· Maurizio Quadrio1
Received: 13 June 2020 / Revised and accepted: 19 October 2020 © The Author(s) 2020
Abstract A numerical study of the motion of algal cells in a representative thin-layer-cascade (TLC) photobioreactor is presented. The goal is to determine the time scale associated with the light/dark (L/D) cycle seen by the cells during their turbulent motion in the liquid culture. Owing to the limited reliability of the available numerical results which deal with time-averaged quantities and thus lack time-resolved information, the present study is based upon the Direct Numerical Simulation of the Navier-Stokes equations, a reliable but consequently expensive numerical approach which does not incur in turbulence modelling errors. Indeed, the simulation is successfully validated in terms of averaged velocity with experimental data. The availability of full temporal information allows algae cells to be followed in time along their trajectories. A large number (up to a million) of tracers is placed in the flow to mimic the algae cell. Their trajectories are statistically studied and linked to the turbulent mixing. Results indicate that, in a typical TLC reactor designed to mimic an experimental setup, cells undergo an L/D cycle with a time scale in the range 0.1–2 s. Such time scale, albeit much longer than the typical time scale of the photosynthesis, significantly benefits the productivity of the algae compared to a steady illumination. Keywords Microalgae · Thin-layer-cascade photobioreactor · Light/dark cycle · Turbulence · Direct numerical simulation
Introduction A strong interest in cultivation of phototrophic microalgae is driven by their several envisaged application areas: a non-exhaustive list includes biofuels, chemicals, medicine and nutrition (Posten and Walter 2012). Microalgae hold potential in food production and in purification of contaminated water and wastewater (Aci´en et al. 2016; Bernaerts et al. 2017). They are important in the food chain as they are the main feeding source for rotifers, fishes, etc.; microalgae possess high nutritional value and contain lipids, proteins, carotene and other essential minerals (Hakalin et al. 2014). Microalgae are also used as fertilisers and in products for skin health. Over the last years, owing to the decline of fossil fuel supply and to the increasing concerns on global warming from
Alessandro Chiarini
[email protected] Maurizio Quadrio [email protected] 1
Department of Aerospace Sciences and Technologies, Politecnico di Milano, Milan, Italy
excessive carbon dioxide emissions, microalgae became interesting also as an alternative source of renewable energy (Makareviˇciene et al. 2011; Gour et al. 2016). To date, the most widely used photobioreactor (PBR) for large-scale cultivation of microalgae is of the openpond type (Grima et al. 1999). An open-pond PBR is simple and enjoys low construction costs and low power requirements. Furthermore
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